Rita Lorenzini

Katel Fiselier

Maurus Biedermann

Martino Barbanera

Ilaria Braschi

Konrad Grob
email: konrad.grob@klzh.ch

SATURATED AND AROMATIC MINERAL OIL HYDROCARBONS FROM PAPERBOARD FOOD PACKAGING: ESTIMATION OF LONG-TERM MIGRATION FROM CONTENTS IN THE PAPERBOARD & DATA ON BOXES FROM THE MARKET

Keywords:

HAL is a

migration into the food by testing at 40°C for 10 days is difficult, it seems preferable (and easier) to 20 use the mineral oil content in the paperboard. Evaporation experiments showed that hydrocarbons 21 eluted up to about n-C 24 are sufficiently volatile for relevant migration into dry food: in worst case 22 situations about 80% migrate into the packed food. The extraction of the paperboard was optimized 23 to give good recovery of the relevant hydrocarbons, but discriminate against those of high 24 molecular mass which tend to disturb gas chromatographic analysis in on-line coupled normal 25 phase HPLC-GC-FID. Even though some of the relevant hydrocarbons had already evaporated, the 26 average concentration of <C 24 mineral oil saturated hydrocarbons (MOSH) in the paperboard boxes 27 of 102 products from the Swiss and Italian market was 626 mg/kg. Nearly 15% of investigated 28 boxes still contained more than 1000 mg kg -1 <C 24 MOSH up to over 3000 mg kg -1 (maximum, 29 3500 mg kg -1 ). This amount of MOSH in the board have the potential of contaminating the packed 30 food at a level exceeding the limit, derived from the JECFA ADI, hundreds of times. 31 2002/72), the use of mineral oil products free of MOAH and with a high molecular mass (average 15 of at least 480 Da, i.e. n-C 34 , and a carbon number at the 5% distillation point of at least 25) is 16 authorized without a specific migration limit. The mineral oils which are subject of this paper are of 17 smaller molecular mass (Biedermann and Grob 2010) and fall into the categories for which JECFA 18 specified a "temporary" ADI of 0-0.01 mg kg -1 body weight (JECFA 2002). Assuming generally 19 applied conventions, i.e. 60 kg body weight and 1 kg of food contaminated with the given substance 20 being daily consumed, the Specific Migration Limit (SML) for these oils is calculated as 0.6 mg kg - 21 1 food. This JECFA evaluation was based on white mineral oils, refined to eliminate MOAH, 22 whereas the mineral oils in recycled board and most printing inks are of technical grade, thus also 23 contain MOAH. 24 25

The main sources of mineral oil in paperboard are offset printing inks, either directly applied for 26 decorating the food packaging material or entering via recycling of fibers contaminated by mineral-27 oil-containing inks, primarily from newspaper (Biedermann et al. 2010). These mineral oils 28 typically contain 15-20%, sometimes 50% MOAH (Biedermann and Grob 2010). For the transfer 29 into dry food, direct contact is negligible. Migration is restricted to components of sufficient 30 volatility to evaporate from the packaging material and recondense in the food [START_REF] Shepherd | Trace contamination of foods by migration from plastics packaging -A 19 review[END_REF]31 Droz and Grob 1997; [START_REF] Jickells | Migration of contaminants by gas 4 phase transfer from carton board and corrugated board box secondary packaging into foods[END_REF]). The process depends on the vapor pressure (determining 32 migration rate) and the partitioning between the packaging material and the food, but also on 33 3 removed into ambient air, whereas this is not possible for boxes packed into larger units and stacked 1 on pallets. In the latter case, vapors are primarily transferred into the packed food unless there is an 2 internal bag of a material stopping this migration. 3 4 For a worst case assumption, escape of vapors should be assumed to be negligible, i.e. the vapor 5 pressure is likely to approach saturation in the air within the packs and between the packs. 6

Migration is driven towards equilibrium with the food. Triantafyllou and coworkers (2005) 7 investigated this mechanism by determining the partitioning between paperboard and air and the 8 uptake into model foods as a function of temperature. The partitioning coefficient depends on the 9 properties of these materials. However, since the mass of the food exceeds that of the packaging 10 material by a factor typically ranging from 5 to 25 (Fiselier et al. 2010), most of the hydrocarbons 11 may end up in the food fairly independently of this partitioning coefficient. 12 13 For producers as well as enforcement authorities it is important to predict long-term migration from 14 paperboard into foods (many products have shelf lives of 1-3 years). Migration from paper and 15 board is often tested using modified polyphenylene oxide (MPPO -Tenax ® ) placed on the material 16 during 10 d at 40°C (e.g. [START_REF] Aurela | Phthalates in paper and board packaging and their 7 migration into Tenax and sugar[END_REF]). MPPO may be considered as an adsorbent adequately 17 simulating food, but the 10 d at 40°C do not reflect migration over up to several years at room 18 temperature (RT). Such testing becomes even more equivocal if there is a barrier between the 19 paperboard and the food -such as a plastic lamination on the paperboard or an internal bag acting as 20 a weak functional barrier -slowing the transfer. In fact this type of barrier introduces a lag time 21 with low migration until the migrant passed through this layer (Franz et al. 1996 and1997;22 Piergiovanni et al. 1999;Feigenbaum et al. 2005). A low migration into MPPO after 10 d may then 23 be misleading, as more substantial migration may only start later. 24

25

Here an alternative approach is proposed, based on the content in the paper or board. Prolonged 26 storage under conditions preventing the escape of vapors to ambient atmosphere and the absence of 27 an efficient barrier protecting the food causes a high proportion of the volatile hydrocarbons to be 28 transferred to the food. The upper limit of molecular mass permitting relevant gas phase transfer is 29 determined experimentally. This method is rapid and provides reliable worst case data. 30

31

In previous work, the upper limit of the mineral oil hydrocarbons migrating from paperboard into 32 foods varied between C 20 and C 25 (partly published in Droz and Grob, 1997). Little was known 33 about the age of the products. Since the more volatile components are transferred faster, the hydrocarbons found in young products range to lower carbon numbers than in aged packs, which 1 was probably the main reason for the variation observed. Similar transfer was observed also in the 2 hydrocarbons evaporation process from jute bags made with mineral batching oil (often used to 3 contain raw foodstuff like nuts and beans). Comparing a new jute bag to an old one, the 4 concentration of the hydrocarbons was diminished up to n-C 24 and the contaminants found in 5 hazelnuts, chocolate, coffee and rice ranged to n-C 21 , n-C 31 , n-C 24 and n-C 21 , respectively (Grob et 6 al., 1991b). There was no certainty, however, that the migration from the jute bag was the only 7 source of mineral oil contamination. 8 9 Summerfield and Cooper (2001) investigated the migration of hydrocarbons and phthalates from 10 paper products intended for food contact, using a mixture of dichloromethane and ethanol for the 11 Soxhlet extraction of paperboard. Triantafyllou and coworkers (2002Triantafyllou and coworkers ( , 2005Triantafyllou and coworkers ( and 2007) ) White mineral oil centered on n-C 23 , in the past used as release agent by a candy manufacturer, was 4 used as external standard for recovery tests. MOSH and MOAH internal standard solutions were 5 prepared as described by Biedermann et al. (2009). 6

Extraction of paper-based packaging 7

Chopped paperboard (1 g) was weighed into a 20 mL amber vial with PTFE-lined screw cap. After 8 adding 10 µL of internal standard solution and 10 mL solvent, the vial was shaken on a vortex 9 (Haidolph, Germany) and allowed to stand during various periods of time. Before injection, 10 ethanol-if present -was removed from the extract by adding ca. 10 mL of water to 5 mL extract, 11 vortexing and centrifuging. To prevent contamination, samples were handled without gloves; hand 12 creams were avoided. Working up virgin fiber paperboard free of mineral oil verified the absence of 13 sample contamination during manipulations. 14

Chromatographic analysis 15

MOSH and MOAH contents were analyzed by on-line coupled HPLC-GC-FID chromatography as 16 described by Biedermann et al. (2009). Briefly, 20 or 50 µL extract (for MOSH and MOAH 17 analysis, respectively) was injected onto a 25 cm x 2 mm i.d. HPLC silica gel column and 18 chromatographed at 300 µL min -1 using a gradient starting with hexane and reaching 30% 19 For the migration into dry foods, only the hydrocarbons of some volatility are relevant, since 6 (wetting) direct contact is negligible and a minimum vapor pressure is required to support a 7 transport through the gas phase. The minimum volatility could be determined by the hydrocarbons 8 detected in food, but, as mentioned in the introduction, variations were noted, since the transfer also 9 depends on other factors. Such influences can be avoided by observing the evaporation from the 10 board into ambient air, i.e. by focusing on the first step of the migration process. A paperboard was 11 hung up in the laboratory and sections were analyzed after various periods of time. The paperboard 12 was exposed to air on all sides; there was neither air conditioning nor ventilation. 13 width corresponding to a single carbon atom and cuts at a point immediately after the elution of the 2 n-alkane designating the fraction. As most branched and cyclic hydrocarbons are eluted before the 3 n-alkane of the same carbon number, the majority of the components in the MOSH fractions have 4 this carbon number. However, the fractions are likely to include also hydrocarbons with one more 5 or one less carbon atom. 6 7 For the C 17 section of the MOSH (b.p. n-C 17 , 302°C) it is observed that 58% was evaporated during 8 the first day and 80% within the first 4 days. Of the C 19 section, only 38% evaporated during the 9 first 4 days, but 85% during two weeks. Some 44% of the C 23 evaporated in 180 days but rather 10 little of the C 24 section. A small decrease was noticed for all hydrocarbons (up to about C 55 ), which 11 was probably due to oxidation. It was concluded that n-C 24 is the upper end of the MOSH with a 12 significant potential to evaporate and migrate into food. 13 14 Figure 3 shows the MOSH data before and after exposure of the paperboard to air at about 20°C 15 (autumn/winter) during 14 and 180 days. During 2 weeks in summer, with an average temperature 16 in the laboratory close to 30°C, the evaporation of the MOSH C 19 to C 23 was substantially faster 17 (about a factor of 2 for C 19 and C 20 ), pointing out that temperature plays a significant role. The 18 figure also includes data from a rice box made of the same paperboard and stored for about 6 19 months (described in Biedermann and Grob, 2010). Of the hydrocarbons C 16 to C 24 roughly 20% 20 less material was lost than of the board exposed to air for 180 days. This might be due to the 21 partitioning process with the rice: instead of being carried away by convection, the vapors were in 22 stagnant air in contact with the rice and the transport was governed by the absorption into the rice. 23

Since migration into the solid rice is slow, even after 6 months partitioning with the rice only 24 involved a fraction of the rice particle. This confirms that the results observed for the board exposed 25 to air correspond to a worst case of migration into foods readily absorbing hydrocarbons. 26

Figure 4 shows the results expressed in percent of the hydrocarbons present in the new board. It 27 highlights the rapid loss of the C 18 fraction (87% in 20 days) and the far slower evaporation of the 28 hydrocarbons with just 3 more carbon atoms. It also points out that the evaporation of MOAH 29 (dashed lines) in segments eluted at the same retention window as the MOSH (solid lines) are 30 similar: for MOAH C 18 , the evaporation is somewhat slower, for the MOAH C 21 slightly faster (at 31 least in the beginning) and for the sum of the MOSH and MOAH up to C 24 it is virtually identical 32 (bold lines). This means that the retention time of the n-C 24 can also be considered as the upper end 33 of the MOAH potentially being transferred to dry food to a significant extent. <C 24 fraction to evaporate. It remains to be checked whether this is also an estimate for the worst 2 case migration into food. The migration into rice packed during 6 months in the same type of board 3 reached 86% of the MOSH <C 24 . Despite the two evaporation percentages are similar, a direct 4 comparison is not advisable because the two analyzed boards were from different batches. Although 5 the variability in recycling row material composition can not be considered negligible, it is 6 interesting to note that the two results are in the same order of magnitude. 7 8 Figure 5 shows the transfer of the mineral oil hydrocarbons for two products from the market: egg 9 noodles and cocoa powder. On the left are the MOSH chromatograms of noodles (bottom) 10 embedded in a paperboard tray of recycled fibers (top) and both wrapped into an outer plastic film. 11

The example illustrates how the volatile components were largely transferred to the noodles 12 (bottom left), whereas the high molecular mass hydrocarbons remained on the board (top left). For 13 this rather fresh product (ca. 2 months on the market), the transfer of the C 20 to C 24 hydrocarbons 14 was still modest. Transfer beyond n-C 24 (bold bar) was negligible. 15 For this same product also the MOAH chromatograms are added (top and bottom, right). They also [START_REF] Binderup | 23 Toxicity testing and chemical analysis of recycled fibre-based paper for food contact[END_REF] show migration for the hydrocarbons eluted up to the retention time of n-C 24 . Again DIPN is 23 transferred to a surprisingly small proportion compared to the MOAH of similar volatility. 24

Optimization of the extraction 25

The on-line HPLC-GC-FID analysis applied involved transfer by an on-column technique, which 26 means that all material in the transferred HPLC fraction is deposited into the uncoated precolumn. 27 Non-evaporating material may be a problem, since it accumulates there and builds up retention 28 power. As a result, the reconcentration efficiency by the retention gap effect is reduced and peaks 29 are broadened. Printed paper and board as well as recycled board often contain hydrocarbons of 30 such a high molecular mass (e.g. from petroleum resins) that they cannot be eluted from GC. For 31 instance, the use of MTBE for extracting paperboard, despite not being a particularly good solvent 32 for high molecular mass compounds, resulted in a rapid deterioration of the chromatographic 33 performance. Since these high molecular mass compounds are not of concern for migration into dry 34 sufficient, as shown by the large proportion of these compounds in the second extract. Recoveries 3 were checked on four different printed paperboards made of recycled fibers; no relevant differences 4 were noted. The discrimination against the high molecular mass material disturbing GC was 5 confirmed by several hundred analyses which could be performed before the uncoated precolumn 6 needed being exchanged (compared to hardly 10 when using hexane alone). 7

Data on paperboard boxes of products from the market 8

Totally 102 products were sampled from the Swiss and Italian market, 66 in paperboard mainly of 9 recycled fibers and 36 in board of fresh fibers (chemical or mechanical pulp). The printed boards 10 were analyzed for MOSH and MOAH up to the retention time of n-C 24 . 11

Figure 8 shows the results for the <C 24 MOSH fraction measured in the printed recycled 12 paperboard. Concentrations ranged from 50 to 3800 mg kg -1 , with an average of 626 mg kg -1 and a 13 median of 350 mg kg -1 . MOAH concentrations (not reported in figure) related to the sum of MOSH 14 and MOAH contained in mineral oil varied from 16 to 28% with mean value of 19%. For the 15 interpretation of these results, two factors must be taken into consideration: firstly, the 16 hydrocarbons of concern were already partly transferred to the packed food or evaporated into the 17 atmosphere, with the effect that the concentrations were substantially reduced. Secondly, the boxes 18 were printed and for a substantial proportion of the samples it must be assumed that still offset 19 printing inks based on mineral oil were used, i.e. that the values measured represent the sum of the 20 contributions from the recycled board and the printing ink. Since the MOSH concentrations below 21 C 24 or C 28 in unprinted paperboard were usually below 1000 mg kg -1 (Biedermann and Grob, 2010), [START_REF] Binderup | 23 Toxicity testing and chemical analysis of recycled fibre-based paper for food contact[END_REF] it is assumed that the highest results were predominantly determined by the diluent in the printing 23 inks. Conversely, there seems to be little room for contributions of mineral oil from printing inks 24 for about half of the samples with a rather low <C 24 MOSH content. 25 26 Nine of the 36 printed paperboards (Figure 9) consisting fresh fibers contained less than 10 mg kg -1 27 MOSH. However, the concentrations reached up to 1900 mg kg -1 (median, 50 mg kg -1 ). It is 28 assumed that the printing inks were the major source of this contamination (with a molecular mass 29 distribution indeed centered on C 18 -C 21 ), but there were also samples containing higher molecular 30 mass oils. Since such oils were also detected in unprinted fresh fiber board (Biedermann and Grob, 31 2010), printing inks are not the only source. MOAH concentrations ranged from 5 to 22% with an 32 average of 15%. 33 To safely meet the 0.6 mg kg -1 limit for MOSH in food derived from the ADI of JECFA, the 1 maximum concentration of the MOSH <C 24 in the paperboard is calculated as follows: if the mass 2 ratio of the food to the paperboard is 5 (the average is around 10; Fiselier et al., 2010) and 70% of 3 the MOSH <C 24 migrate into food, the limit is reached at 4.3 mg kg -1 in the paperboard. The data 4 shown in Figures 7 and8 do not enable to calculate the expected migration into food, since the 5 process may already have progressed, but the data show that almost all paperboards still contained 6 amounts of MOSH <C 24 far in excess of 4.3 mg kg -1 . 7 8 Conclusions 9

Mineral oil may migrate from printed paperboard into packed food at concentrations far exceeding 10 the limit derived from the ADI specified by the WHO-JECFA (Droz and Grob, 1997; Biedermann 11 and Grob, 2010). Both the design of migration tests and the analytical set-up for these contaminants 12 and food packaging are often controversial. Reliable analytical tools are thus required to solve these 13 difficulties. 14 15 This paper deals with two elements. Since standard testing with MPPO (Tenax®) at 40°C for 10 d 16 does not reflect migration over several years, estimation of long term migration based on contents in 17 the paperboard is proposed. To this end, the upper limit of the mass of the hydrocarbons capable of 18 migrating through the gas phase was determined by focusing on the first step of the process, i.e. 19 evaporation. Migration ends quite sharply at C 24 both for MOSH and MOAH. It is estimated that 20 60-80% of the <C 24 MOSH and MOAH may end up being transferred into the packed food when 21 there is no internal functional barrier (e.g. a bag with a layer of aluminum or PET) and the boxes are 22 stored in such a way that evaporation into ambient air is hindered. This approach involves a rapid 23 determination and provides a robust estimate of the potential migration. Some first applications 24 validated the approach (Biedermann and Grob, 2010), but more work is on-going to compare real 25 migration with such estimates. 26 27 On-column introduction into GC (in this case by on-line HPLC-GC) rules out distortion of the 28 sample composition, but is sensitive to loading of the uncoated precolumn with non-evaporating 29 material. The extraction of (printed or unprinted) paperboard with ethanol/hexane 1:1 was shown to 30 provide virtually complete extraction of the hydrocarbons of interest, but to discriminate against 31 those disturbing the analysis. 32 The <C 24 MOSH found in the paperboard boxes from the market do not enable to estimate the long-1 term migration into the packed food, since a substantial part of the mineral oil may already have 2 been transferred, but they show a frightening remaining potential for migration: the highest 3 concentrations found has the potential to exceed the limit in food almost 1000 times.

  food packaging materials tend to contain contaminants (e.g. Binderup et al. 2002, 6 Ozaki et al. 2004, Sturaro et al. 2006, de Fàtima Poças and Hogg 2007, EU Biosafepaper project), 7 among them mineral oils. Here mineral oil saturated hydrocarbons (MOSH), including paraffins 8 (straight chain and branched alkanes) and cyclic naphthenes are distinguished from mineral oil 9 aromatic hydrocarbons (MOAH). In contrast to the polyaromatic hydrocarbons (PAH) commonly 10 analyzed, the MOAH are largely alkylated (Grob et al. 1991a, Biedermann and Grob 2009, 11 Biedermann et al. 2009). 12 13 No European legal limits for MOSH and MOAH have been established. For plastics (EU Directive 14

3and discussion 4

 4 dichloromethane. The column was backflushed with dichloromethane. HPLC-GC transfer occurred 20 by the retention gap technique and partially concurrent eluent evaporation. A 10 m x 0.53 mm i.d. 21 uncoated, deactivated pre-column was followed by a steel T-piece union connecting to the solvent 22 vapor exit and a 15 m x 0.25 mm i.d. separation column coated in the laboratory with PS-255, a 23 dimethyl polysiloxane. From HPLC, the MOSH were eluted between 2.0 and 3.5 min, the MOAH 24 between 4.0 and 5.5 min. The oven temperature was programmed from 65°C (6 min) to 350°C at 25 the rate of 20°C min -1 . White mineral paraffin oil served as external standard. The area representing 26 the MOSH or MOAH were either integrated or determined manually by drawing one or several 27 triangles into the chromatogram adjusted to the hump of the mineral oil hydrocarbons, as described 28 by Concin et al. (2008) and Fiselier et al. (2009). 29 30The detection limits were below 5 mg kg -1 . Results from repeated analyses of samples containing 31 around 50 mg kg -1 MOSH had a standard deviation below 10%. The uncertainty of the data was 32 primarily from positioning the baseline and the upper contour of the hump and must be estimated 33 chromatogram, but was below 25% for all except the lowest results presented here. 1 Concentrations given are averages of two or more independent analyses. 2ResultsDetermination of the hydrocarbon fraction relevant for migration into dry foods 5

14Figure 1 Figure 2

 12 Figure 1 shows the MOSH and MOAH chromatograms from a 300 g m -2 paperboard of recycled 15 fibers before it was hung up as well as after 4 and 180 d. The vertical lines shown in the top 16 chromatogram of the MOSH (extract of the paperboard before airing) indicate the integrated 17 sections of a width corresponding to one carbon atom (every second section being labeled). The 18 areas of these sections provided the data for Figures 2-4. In the early part of the chromatograms, the 19 internal standards are eluted, assigned in italics in the middle MOSH and MOAH chromatograms. 20 Before airing, a maximum concentration was found for the MOSH n-C 18 /C 19 (top chromatogram). 21 After 4 d, losses are visible in the early eluted part, being substantial up to about n-C 19 . After 180 22 days, n-C 20 was largely evaporated and losses reached up to about n-C 24 . Evaporation seemed to 23 proceed by volatility, removing the hump of unresolved hydrocarbons from the left to the right. 24 Vertical lines in the top MOAH chromatogram show the two fractions integrated and used for 25 Figure 4, with retention times corresponding to the MOSH fractions n-C 18 and n-C 21 . These 26 retention windows do not mean that the MOAHs comprised a corresponding numbers of carbon 27atoms, but they reflect same volatilities in a nonpolar GC stationary phase. It is interesting to note 28 that only about a third of the diisopropylnaphthalenes (DIPN) evaporated during 180 days, despite a 29 volatility according to GC retention time which corresponded to MOAH evaporated to a large 30 extent already after 4 days. This suggests that most of DIPN is still encapsulated. 31 32

Figure 4

 4 Figure 4 shows that exposure to indoor air during 180 d caused 77% of the MOSH and MOAH 1

16

  The cocoa powder (right, MOSH and MOAH) was purchased packed in an internal polyethylene 17 bag and an external paperboard largely consisting of recycled fibers. The paperboard (bottom, 18 centre) contained a broad spectrum of MOSH with a strong maximum at n-C 24 . Again the more 19 volatile MOSH (from C 13 to ca. C 20 ) were largely transferred to the cocoa powder (top centre), with 20 a migration fraction reaching to about n-C 24 . The n-alkanes C 23 and C 25 are from the cocoa powder. 21

3 Figure 6

 36 Figure 6 shows MOSH and MOAH content from a printed recycled board extracted with hexane or 4 ethanol for 2 h at RT. The chromatograms of the MOSH show a broad hump of unresolved 5 hydrocarbons topped by some n-and iso-alkanes ranging from about C 14 to about C 50 , with n-6 alkanes from about C 22 to C 45 representing paraffinic wax. The MOAH fraction comprised a hump 7 of material corresponding to the early eluted MOSH, presumably representing the MOAH present 8 in this mineral oil. It is topped by signals from diisopropyl naphthalene (DIPN) and components 9 from wool oils. At higher elution temperature, a second hump is observed which is not 10 accompanied by MOSH of similar molecular mass and which is assumed to represent petroleum 11 resins, e.g. used as binder for printing inks. This hump grew with increasing extraction time (data 12 not shown) and is assumed to extend into a range of molecular masses which are no longer eluted 13 from GC. It is such material the extraction should discriminate against. 14 15 Ethanol and hexane showed complementary extraction properties: ethanol more efficiently 16 extracted the low molecular mass hydrocarbons, which is probably due to its ability to swell 17 cellulose polymeric chains. However, its extracting power is limited for the higher molecular 18 weight hydrocarbons, particularly shown for the n-alkanes of the wax by the extracted segments of 19 the upper chromatogram at the right. Hexane, on the other hand, because of chemical similarity, is 20 the ideal solvent for hydrocarbons. Both properties (ability to swollen cellulose fibers and 21 hydrocarbons extracting power) are needed for this application: the improved extraction of the 22 hydrocarbons of interest as well as the discrimination against high molecular mass hydrocarbons, 23 such as resins, waxes and hot melts. 24 25 Mixtures of ethanol and hexane combined the advantages of the two solvents. Different proportions 26 were tested heading for high extraction efficiency up to at least C 24 and discrimination beyond this. 27 Extraction during 2 h at RT was sufficient to quantitatively extract the MOSH and MOAH of 28 interest; longer durations primarily increased the extraction of the high molecular mass 29 hydrocarbons. Yields were tested by re-extraction of the paperboard at more drastic conditions 30 (overnight at RT). The extraction performance of 50:50 (v/v) hexane/ethanol mixture was superior 31 to 90:10, 80:20, 20:80 and 10:90. As shown in Figure 7 for a printed and an unprinted recycled 32 paperboard, extraction of the iso-paraffins and the naphthenes (forming the unresolved hump) was 33 quite complete up to beyond C 40 , whereas the n-alkanes started to show up in the second extract 34

6 FeigenbaumFigure 1 . 6 Figure 2 . 9 Figure 3 .Figure 4 .Figure 5 .Figure 6 .Figure 7 .FoodFigure 9 .Food

 61629345679 Figure captions. 1 2 Figure 1. On-line HPLC-GC-FID analysis of MOSH and MOAH fractions from recycled 3 paperboard before it was hung up in the laboratory air ("new") as well as 4 and 180 days later. 4 Peaks of internal standards labeled in italics. DIPN: diisopropyl naphthalenes. 5 6 Figure 2. MOSH concentrations in recycled unprinted paperboard hung up in ambient air 7 determined for fractions incremented by steps of one carbon atom at the time (x-axis). 8 9 Figure 3. MOSH concentrations in the board shown in Figure 2, including results for 14 days at 10 about 30°C (substantially faster evaporation) and for the same type of board from 180 days old pack 11 of rice (highest losses compared to other time and temperature conditions). 12 13 Figure 4. Evaporation of MOSH (solid lines) and MOAH (dashed lines) C 18 and C 21 fractions, 14 diisopropyl naphthalenes (DIPN, dotted line) and the sum of MOSH and MOAH <C 24 (Σ, bold 15 lines) from paperboard exposed to ambient indoor air during different periods of time. 16 17 Figure 5. Online HPLC-GC-FID analysis of MOSH and MOAH migrating from recycling 18 paperboard (top chromatograms) into packed egg noodles and cocoa powder (bottom 19 chromatograms). Migration beyond n-C 24 fraction is negligible. DIPN: diisopropyl naphthalenes. 20 21 Figure 6. Online HPLC-GC-FID chromatograms of MOSH and MOAH fractions obtained by 22 extraction of recycled paperboard with different solvents during 2 h at RT: ethanol is the better 23 solvent for the lower molecular mass hydrocarbons and discriminates against those of higher mass. 24 DIPN: diisopropyl naphthalenes. 25 26 Figure 7. Online HPLC-GC-FID hydrocarbons recovery tests for extraction of printed and unprinted 27 recycled paperboard with a 50:50 hexane/ethanol mixture during 2 h at RT. For each 28 chromatogram, the overnight re-extraction is overlapped as the lower trace. DIPN: diisopropyl 29 naphthalenes (high amount left in the board after 2 h at RT extraction). 30 31 Figure 8. MOSH n<C 24 concentrations in printed paperboard primarily consisting of recovered 32 fibers from packed foods sampled on the market, sorted by increasing concentration. 33 34

tending to disturb GC analysis. The paperboard from 102 products 22 sampled from the Italian and the Swiss market were analyzed for the potentially relevant MOSH 23 and MOAH content. 24 25 Materials and Methods 26

	1	distilled before use. Technical grade methyl tert-butyl ether (MTBE) from Brenntag (Germany) was
	2	distilled before use. HPLC grade methanol, ethanol and dichloromethane were from J.T. Baker
	3	(Deventer, Holland). The dimethyl polysiloxane PS-255 was from Fluka (Buchs, Switzerland).
		F o r
	12 13 14 15	used ethanol at RT (5 min double extraction, 30 min, and 1 h, respectively). Schaefer et al. (2003) analyzed P lubricants in food packaging materials, among which paraffins and wax esters, using iso-octane as e extraent. e r
	16 17	This paper reports results with regard to the determination of the concentration of relevant MOSH R
	18	and MOAH in paperboard. The range of hydrocarbons potentially migrating into dry food was e
	19 20 21 27	investigated focusing on the first step of the transfer process, i.e. evaporation: the loss of hydrocarbons from paperboard was monitored during 180 d. The extraction of the paperboard was optimized to yield high recovery of these relevant hydrocarbons, but to discriminate against high v i e w O n l molecular mass components Materials y
	28	Food samples packed in paperboard boxes were collected from the Italian and the Swiss retail
	29	market in spring and summer 2009. Only products without any kind of aluminum internal bag
	30	(aluminum foil or metalized plastic) were analyzed.
	31
	32	Hexane (10 L) from Brenntag (Schweizerhall AG, Basel, Switzerland) was filtered through 400 g
	33	silica gel activated at 400°C (silica gel 60, 0.063-0.200 mm; Merck, Darmstadt, Germany) and

Page 4 of 26 http://mc.manuscriptcentral.com/tfac Email: fac@tandf.co.uk Food Additives and Contaminants

http://mc.manuscriptcentral.com/tfac Email: fac@tandf.co.ukFood Additives and Contaminants

Page 8 of 26 http://mc.manuscriptcentral.com/tfac Email: fac@tandf.co.uk Food Additives and Contaminants

Page 10 of 26 http://mc.manuscriptcentral.com/tfac Email: fac@tandf.co.ukFood Additives and Contaminants

Page 13 of 26 http://mc.manuscriptcentral.com/tfac Email: fac@tandf.co.ukFood Additives and Contaminants


Keywords: Cratons, mantle plumes, numerical modelling, ore deposits

Plume headlithosphere interactions around cratonic blocks result in thermo-mechanical disturbances that lead to heating and burial phases of crustal rocks. We present results from numerical models of plume head -cratonic blocks interactions where a free upper surface condition and realistic rheologies are accounted for. These models include distinct cratonic blocks embedded within a continental lithosphere and separated by several hundreds of kilometres. Surface topography, thermal field and effective viscosity values are tracked for 20 Myr of interactions. The modelled dynamic interaction of a plume head around cratonic blocks results in two main types of instabilities, each of them resulting in a distinct P-T-t path.

The "slab-like" instability, focused on cratonic edges when plume head is away from the craton centre, shows a near-isothermal burial phase, while the "drip-like" instability occurring above plume head material results in a near-isobaric heating phase. Consequently, both clockwise and counterclockwise P-T-t paths can be expected around cratons, as actually observed around the Tanzanian craton and other cratonic areas. Metallogenic data from gemstone-bearing rocks in south-east Africa and data from ultrahigh temperature and ultrahigh pressure metamorphism are compatible with our model. It appears that vertical mantle dynamics around cratons may also explain thermobarometric signatures that are often attributed to horizontal tectonics.

A C C E P T E D

M A N U S C R I P T

Introduction

Thermal convection in the Earth's mantle involves large-scale horizontal motions of continental and oceanic plates at the Earth's surface. Continental drift illustrates one of the dynamic consequences of the underlying mantle convection, and subduction of oceanic plates attests to the vertical motions induced by mantle convection. In addition to horizontal and downward motions of the lithosphere, geological evidence of crustal-scale vertical motions within continents has generally been associated with horizontal tectonic forces (e.g. [START_REF] England | Pressure-temperature-time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened continental crust[END_REF]. At a larger scale, lithospheric thickening events due to horizontal shortening could trigger Rayleigh-Taylor instabilities at the base of the lithosphere [START_REF] Houseman | Convective instability of a thickened boundary layer and its relevance for the thermal evolution of continental convergent belts[END_REF]Platt and England, 1994;[START_REF] Houseman | Gravitational (Rayleigh-Taylor) instability of a layer with non-linear viscosity and convective thinning of continental lithosphere[END_REF], and thus vertical motions of crustal rocks. Although variations in surface topography have often been related to mantle upwellings (e.g. [START_REF] Crough | Thermal origin of mid-plate hot-spot swells[END_REF][START_REF] Olson | Hot spots, swells and mantle plumes[END_REF][START_REF] Şengör | East Anatolian high plateau as a mantle-supported, north-south shortened domal structure[END_REF], most previous studies have considered the lithosphere as a stagnant lid which surface undulations followed local isostatic compensation level. However, [START_REF] Gurnis | Constraining mantle density structure using geological evidence of surface uplift rates: the case of the African superplume[END_REF] demonstrated how variations of dynamic topography can be used to constrain mantle density and viscosity values. [START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF] and [START_REF] Guillou-Frottier | Deciphering plume-lithosphere interactions beneath Europe from topographic signatures[END_REF], have later shown that due to the layered structure of the continental lithosphere, surface topography generated by plume-lithosphere interactions (PLI) may significantly differ from conventional isostatic estimations. However, vertical motions of crustal rocks resulting from PLI have not been extensively investigated.

Vertical motions of crustal rocks depend on their buoyancy and rheological behaviour and thus on thermo-mechanical constraints allowing or preventing burial and exhumation processes. At the scale of the continental lithosphere, the first order thermo-mechanical heterogeneities correspond to cratonic areas, defined by old thick rigid blocks embedded in between younger, thinner and weaker lithosphere. [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF] demonstrated that the craton edge, defined by a vertical thermo-mechanical contrast concentrates stresses and

A C C E P T E D M A N U S C R I P T

strains beneath the younger lithosphere adjacent to the cratonic border. After impingement of mantle plume head at the base of the lithosphere, lateral spreading of plume head was hindered by the steep vertical craton edge while plume head spread on the other side. In this study, we examine a more common geological setting, where two cratonic blocks, separated by a few hundreds of kilometres, are inserted within a younger continental lithosphere.

When heat and mass transfer occurs in the crust, thermobarometric data provide records of possible mechanisms leading to burial and exhumation events. In a P-T diagram where pressure increases upwards, a clockwise P-T-t path describes a burial phase during which the peak pressure precedes the peak temperature, thus leading to a near-isobaric heating phase. In contrast, a counterclockwise P-T-t path shows that the peak temperature is reached before the peak pressure, defining a near-isothermal burial phase. In terms of source mechanisms, fast subduction can explain clockwise P-T-t paths (i.e. fast burial episode; e.g. [START_REF] Agard | Exhumation of oceanic blueschists and eclogites in subduction zones: timing and mechanisms[END_REF], while mantle underplating may give rise to counterclockwise P-T-t paths (e.g. [START_REF] Warren | Mantle underplating, granite tectonics, and metamorphic P-T-t paths[END_REF]. Within granulite terrains, the coexistence of clockwise and counterclockwise P-T-t paths has been explained by different conflicting models calling for either slab rollback or significant mantle upwelling (e.g. [START_REF] Collins | Hot orogens, tectonic swithching, and creation of continental crust[END_REF][START_REF] Brown | From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens[END_REF][START_REF] Collins | Hot orogens, tectonic swithching, and creation of continental crust: reply[END_REF]. Recent analysis of ultra-high temperature (UHT) metamorphism in mafic granulites from southern India revealed the important role of heat input from the mantle [START_REF] Prakash | Petrological evolution of the high pressure and ultrahightemperature mafic granulites from Karur, southern india: evidence for decompressive and cooling retrograde trajectories[END_REF]. For thrusting events, both patterns may also coexist depending on burial and erosion rates, and initial thermal structure (e.g. [START_REF] Ruppel | Pressure-temperature-time paths from two-dimensional thermal models: prograde, retrograde, and inverted metamorphism[END_REF].

Following up on numerous studies of the effects of tectonic stacking or downward subduction on P-T-t paths, we investigate the impact of mantle upwellings on thermomechanical processes affecting crustal rocks around cratonic areas. Our approach is partly motivated by the spatial distribution of distinct types of ore deposits in east Africa, near the Tanzanian craton. In addition, an apparent asymmetrical behaviour seems to occur between one side of the craton and the other, as recently suggested by geochemical signatures of erupted lavas [START_REF] Chakrabarti | Isotopic and geochemical evidence for a heterogeneous mantle plume origin of the Virunga volcanics, Western rift, East African Rift system[END_REF]. In this context, geological, thermobarometric and

A C C E P T E D M A N U S C R I P T
metallogenic data around cratonic areas, and in particular around the Tanzanian craton, are compared with the modelling results.

Mantle plumes beneath cratons

The presence of continents, their sizes, and their distribution at the Earth's surface strongly controls the dynamic regime of the mantle. It has been demonstrated that plate velocities, convective wavelengths, and mantle temperatures are to a large extent affected by the heterogeneous character of the Earth's surface [START_REF] Elder | Convective self-propulsion of continents[END_REF][START_REF] Hewitt | Large aspect ratio cells in two-dimensional thermal convection[END_REF][START_REF] Guillou | On the effect of continents on mantle convection[END_REF][START_REF] Trubitsyn | Cold cratonic roots and thermal blankets: how continents affect mantle convection[END_REF][START_REF] Lenardic | Continental insulation, mantle cooling, and the surface area of oceans and continents[END_REF][START_REF] Coltice | Global warming of the mantle at the origin of flood basalts over supercontinents[END_REF][START_REF] Phillips | Temperature beneath continents as a function of continental cover and convective wavelength[END_REF]. From the Earth's mantle, the upper thermal boundary condition is heterogeneous (presence of oceans and continents) and depends, for example, on the amount of continental masses, whose spatial distribution varied through geological time history. Laboratory experiments by [START_REF] Guillou | On the effect of continents on mantle convection[END_REF], where a heterogeneous thermal condition at the top surface mimicked the presence of a continent, showed that within the convecting fluid, a vertical upwelling of deep hot material was always locked beneath the central part of the "continent". Since Archean cratonic zones are generally surrounded by Proterozoic belts, they can be considered as inner parts of continents, from which underlying mantle flow tend to diverge, thus facilitating the arrival of an underlying thermal plume.

Upwelling currents of hot and deep mantle material (here referred to "mantle plumes") stand for perturbations acting on the lithosphere that could lead to various deformation modes (e.g. [START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF]. Numerous studies suggest that several tens of mantle plumes are present in the current convective pattern of the Earth's mantle (e.g. [START_REF] Jellinek | Links between long-lived hotspots, mantle plumes, D" and plate tectonics[END_REF][START_REF] Montelli | Finite frequency tomography reveals a variety of plumes in the mantle[END_REF]. In the present-day Earth, seismic tomography has revealed anomalously hot zones hundreds of kilometers wide [START_REF] Zhao | Global tomographic images of mantle plumes and subducting slabs: insights into deep Earth dynamics[END_REF][START_REF] Koulakov | P-and S-velocity anomalies in the upper mantle beneath Europe from tomographic inversion of ISC data[END_REF]. Depending on their size, various types of upwelling plumes have been described, from "baby-plumes" beneath western Europe (e.g. [START_REF] Ritter | A mantle plume below the Eifel volcanic fields, Germany[END_REF] to "superplume" events in the past [START_REF] Condie | Continental growth during a 1.9 Ga superplume event[END_REF] or present-day "mega-plumes" in the lower

A C C E P T E D M A N U S C R I P T
mantle, such as large-scale upwellings beneath Africa and Pacific [START_REF] Forte | Deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data[END_REF][START_REF] Romanowicz | Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux[END_REF][START_REF] Dziewonski | Mantle Anchor Structure: an argument for bottom up tectonics[END_REF]. Successive formations and destructions of supercontinents have probably induced several episodes of large-scale thermal blanketing and consequent superplume (or plume clusters) events. Therefore, through the Earth's history, cratonic roots of continents which were either isolated or embedded within a supercontinent were probably pounded by heads of small and large-scale mantle plumes. This scenario stands for the basic hypothesis of our study, where mantle plumes regularly impinged on the base of cratons. In section 3, numerical experiments show details about a single plume-craton interaction, on a short time scale of 20-30 Myr, but dynamic consequences are here supposed to occur regularly through several episodes of plume headcratonic block interactions.

Present-day active interaction between mantle convection and continental lithosphere probably occurs in east Africa, where surface tectonics and volcanism around the Tanzanian craton illustrate the role of cratonic zones. Recent research focused on this area showed spatial distribution of the structural features related to craton-mantle interactions, emphasizing the crucial role of the Archean craton stiffness when compared to the weak adjacent Proterozoic belts [START_REF] Ebinger | Rifting Archean lithosphere: the Eyasi-Manyara-Natron rifts, East Africa[END_REF][START_REF] Petit | Flexure and mechanical behaviour of cratonic lithosphere: gravity models of the East Africa and Baikal rifts[END_REF][START_REF] Corti | Tectonic inheritance and continental rift architecture: numerical and analogue models of the East African Rift system[END_REF].

Tectonic inheritance, differences in rheological properties, but also location of lithospheric weakness zones can explain spatial organisation of continental rifts around the Tanzanian craton. It is likely that concentration of shear zones and location of rift structures around the stiff craton also promotes vertical motions of crustal rocks whereas stabilized cratonic zones are not expected to record such thermo-mechanical disturbances. Surface topography, which presumably reflects underlying vertical motions of crustal rocks, appears to be significantly affected at cratonic margins, whereas flat topographic signatures are observed above cratons (e.g. [START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF][START_REF] Wallner | Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains' extreme elevation ?[END_REF]. However, dynamic signatures also depend on rheological properties of the continental lithosphere, and it is thus important to investigate distinct thermo-mechanical regimes of the lithosphere when it is impinged by a mantle plume.

A C C E P T E D M A N U S C R I P T

The fate of plume heads impinging on the base of -or near -cratonic roots has been the subject of several scenarios, from ponding of mantle material beneath cratons (e.g. [START_REF] Silver | Understanding cratonic flood basalts[END_REF] to lateral flow towards the craton edges (e.g. [START_REF] Ebinger | Cenozoic magmatism in central and east Africa resulting from impact of a one large plume[END_REF]. In addition, there is growing evidence that removal of lower lithosphere occurred around cratonic blocks in the past (e.g. North China craton, [START_REF] Gao | Recycling deep cratonic lithosphere and generation of intraplate magmatism in the North China craton[END_REF]. Ultrahigh temperature metamorphism occurs near cratonic margins (see [START_REF] Kelsey | On ultrahigh-temperature crustal metamorphism[END_REF] for a review) and contrasting (clockwise and anticlockwise) P-T-t paths have been detected in the vicinity of cratonic areas (e.g. east Antarctica, [START_REF] Halpin | Contrasting P-T-t paths for Neoproterozoic metamorphism in McRobertson and Kemp Lands, east Antartica[END_REF].

Amongst the numerical models simulating plume head -cratonic block interactions, few incorporate realistic formulations for the lithosphere and the mantle. For example, most models use a rigid boundary on the top of the model (see, however, [START_REF] Pysklywec | Time-dependent surface topography in a coupled crust-mantle convection model[END_REF][START_REF] Lenardic | Longevity and stability of cratonic lithosphere: insights from numerical simulations of coupled mantle convection and continental tectonics[END_REF], i.e. the upper surface is not deformable (e.g. [START_REF] Trubitsyn | Cold cratonic roots and thermal blankets: how continents affect mantle convection[END_REF][START_REF] Van Thienen | Interaction between small-scale mantle diapirs and a continental root[END_REF]. In addition, lithosphere and mantle rheologies are often too simplified for adequate studies on plume-lithosphere interactions:

constant viscosities for the lithospheric mantle [START_REF] Pysklywec | Time-dependent surface topography in a coupled crust-mantle convection model[END_REF] or Newtonian laws (Elkins-Tanton, 2007) implicitly ignore the rheological stratification of the lithosphere. An important objective of this study is, therefore, to explore possible convective instabilities around cratonic blocks with a thermo-mechanically coupled numerical approach that has been previously used to model mantle-lithosphere interactions for scenarios invoking a laterally homogeneous lithosphere (d'Acremont et al., 2003;[START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF] or a single lateral heterogeneity [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF], as illustrated in Figure 1. Modeling results presented here centre on lithospheric instabilities occurring near cratonic margins, and their consequences for the dynamic topography and associated P-T-t paths.

Numerical models

Basic approach

A C C E P T E D M A N U S C R I P T

The need for including lateral heterogeneities in models of plume headcontinental lithosphere interactions has been demonstrated in previous studies [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF], which were limited to the influence of one cratonic margin. Main differences between other previous numerical modelling studies and our thermo-mechanical modelling approach are in the account of (i) a free upper surface condition; (ii) an explicit elastic-viscous-plastic rheology; and (iii) a stratified structure of continental lithosphere (see details in [START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF] and in the electronic supplementary material). In contrast to convective models that are commonly adopted to study mantle-lithosphere interactions, our model permits to investigate the transient evolution of dynamic topography and of deep (lithospheric) dynamical instabilities resulting from rheological changes induced by plumecratons interactions. In addition, the implementation of particle-in-cell technique combined with a consistent account for pressure (solution of motion equations for full stress tensor instead of that for shear stress tensor as in most convection models) allows us to trace P-T-t paths of selected particles and compare them with thermobarometric data. For the sake of simplicity, we used Boussinesq approximation for density. Yet we also made some tests accounting for density variations due to full-scale phase changes through PERPLEX (Conolly, 2005;[START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF], see also supplementary material). Comparisons with the results based on conventional (constant thermal expansion coefficient) Boussinesq approximation, revealed some differences (see also [START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF] that do not, however affect the behavior of the system as a whole in the context of the present study.

The majority of previous studies have not addressed the most common geological configurations, where two or more cratonic blocks with a small size are separated by areas of dimensions of hundreds to a few thousands of kilometres-long younger continental crust.

Examples can be found in several areas (see the map of present-day Archean cratons by [START_REF] Kusky | Growth of granite-greenstone terranes at convergent margins, and stabilization of Archean cratons[END_REF], including the Yilgarn-Pilbara Archean (Australia), Guyana-Sao Francisco (South America), Slave-Hearne (North America), Aldan-Anabar (Siberia), Zimbabwe-Kaapvaal (South Africa), and Tanzanian-Madagascar blocks (East Africa).

A C C E P T E D M A N U S C R I P T

Model setup and description of the experiments

The numerical model and its applications were described in previous studies [START_REF] Burov | Gravity Anomalies over the Ferghana Valley (central Asia) and intracontinental Deformation[END_REF][START_REF] Burov | Ascent and emplacement of buoyant magma bodies in brittle-ductile upper crust[END_REF]d'Acremont et al., 2003;[START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF][START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF][START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF], see also supplementary electronic material). Below, we therefore limit ourselves to specific features of the new numerical experiments.

The numerical box is vertically limited to the upper mantle (650 km depth) and has horizontal dimension of 1500 km (Figure 1). The initial plume diameter (spherical density contrast associated with 150°C temperature difference) has a diameter of 100-200 km (200 km if not mentioned otherwise) and is located at the centre of the bottom boundary, except in the first experiments where only one cratonic margin is present [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF]. Apart from this case, the continental lithosphere comprises two 300 km-long and 150 km-thick "cratonic" (actually simply older) blocks, or terrains, separated by a younger continental belt, 100 km-thick in reference case, 300 km large. Although the dimensions of the blocks at first sight may appear small, the presence of a lateral heterogeneity is more important than the width of the heterogeneity itself [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF][START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF]. Here, the chosen distance between the two cratons allows to examine the case where a plume head could be locked in between the two thick blocks, reflecting the typical configuration encountered in African continent. The thickness of the non-cratonic continental lithosphere that surrounds the blocks has been varied from the cold thick case (200 km) to a thin weak case (100 km). In Figures 1a-1c, black, grey and white stripes illustrate rheological layering of the lithosphere. The chosen rheological laws and initial temperature profiles are shown in Figure 1d (see caption and details in Table 2). Thermal and mechanical boundary conditions are detailed in caption of Figure 1. The "cratonic" blocks have the same thermo-mechanical structure as in [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF] with a thickness of 50 km in excess of the thickness of the surrounding lithosphere. To investigate the role of distinct convective regimes, low (10 5 ) and high (10 7 ) plume Rayleigh number experiments have been performed. The plume Rayleigh number is defined in [START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF] and use maximal driving density contrast and effective viscosity (see supplementary material, A4).

One lateral heterogeneity

The results shown in Figure 2 illustrate the predictions from models involving a single lateral heterogeneity (see also [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF]. For each case, the craton (800 Ma age) is embedded within a thin weak lithosphere (150 Ma age).These models show the development of three distinct lithospheric downwellings leading to the burial of mantle lithosphere. These downwellings are thermally revealed by distinct geometries. In the first case where the plume is initiated below the thick craton (dotted semi-circle, Figure 2a), return flow from the left edge of plume head entrains a large amount of lithosphere (blue zones in the phase field) within the mantle. When the plume starts below the lateral heterogeneity (Figure 2b), a "driplike" instability, labelled "2" in the thermal field, occurs at shallow depths after plume head material has softened the lower crust. This instability occurs at a certain distance of the cratonic boundary since the plume head centre is laterally deviated towards the thin lithosphere. When the plume starts below the thin lithosphere, at a certain distance from the cratonic edge (Figure 2c), plume head spreading is still hindered by the vertical cratonic margin, but the return flow around plume edges affects the cratonic root margin and mechanically entrained part of it downwards (see the green-yellow area labelled "3", sinking into the mantle). This "slab-like" instability is also incipient in the previous case, but appears to be more efficiently initiated when plume starts away from beneath the cratonic margin.

Delamination of the mantle lithosphere by drip-like instabilities, resulting in compressive stresses at the surface, as well as topographic signatures of plumelithosphere interactions, are useful results to interpret geological processes explaining structural and geochemical records of the Columbia River flood basalts [START_REF] Camp | A plume-triggered delamination origin for the Columbia River Basalt Group[END_REF]. In particular, compressive zones at cratonic edges (see circular diagrams in Figure 2) correspond to the highest topography gradient (2000-3000m of elevation difference over less

A C C E P T E D M A N U S C R I P T
than 100 km), whereas extensive zones show only small-amplitude and short-wavelength topographic variations.

Temporal variations of topography correspond, at depth, to burial and exhumation processes. The right column of Figure 2 suggests contrasting P-T-t paths followed by crustal material buried by the three distinct downwellings. Crustal material can be heated up to 1000-1200°C before being buried (instabilities 1 and 2), or can be pushed downward at temperatures below 900°C by plume edges before being heated (instability 3, slab-like). The consequent P-T-t paths illustrate both clockwise and anticlockwise trajectories. These P-T-t trajectories being obtained qualitatively, a more rigorous investigation (numerically deduced)

in the case of two cratonic blocks is presented in section 3.5.

Results with two cratonic blocks

3.4.1. Thick continental lithosphere. Figure 3 shows four snapshots of the ascent of a mantle plume beneath a 200 km-thick strong continental lithosphere, where two 250 km-thick cold blocks are embedded. The rheological stratification of the lithosphere is reflected in the middle column (effective viscosity) where the ductile lower crust is outlined in non-cratonic areas (yellow region between the strong upper crust and the upper mantle), and where thick and cold blocks can be mechanically defined by a stiff cratonic keel. 1.5 Myr after plume initiation, the plume head has already reached the base of the lithosphere, between the two cratonic blocks. The plume head spreading phase is symmetric (see thermal field) and plume material erodes the base of the lithosphere (see the effective viscosity column), resulting in a smoother topography at the base of cratons. Surface topography reacts as if the crust was mechanically coupled to the lithospheric mantle since a single large-scale uplift is observed.

It can be noted that at the final stage (16 Myr) two (small) drip-like instabilities are incipient at the base of the central part of the lithosphere (see the temperature field).

This numerical experiment demonstrates that the thermo-mechanical stability of a thick strong lithosphere is not affected by plume head dynamics, although small-scale driplike instabilities can be triggered at the base of the lithosphere.

A C C E P T E D M A N U S C R I P T

regimes of the mantle, described by low and high plume Rayleigh numbers (Ra p ).

At low Ra p (Figure 4a), the head of the large-scale plume entrains shallow mantle material down to the plume sides, implying a whole negative subsidence of the lithosphere.

After a gentle negative deflection at 2.4 Myr, subsidence at locations between the two cratonic blocks is accelerated yielding a 2000m topographic low. Due to the presence of stiffer -and thus less deformable -cratonic blocks, subsidence is focused and restricted to this weak internal zone (called "crumple zone" by [START_REF] Lenardic | Longevity and stability of cratonic lithosphere: insights from numerical simulations of coupled mantle convection and continental tectonics[END_REF], much more deformable than the surrounding cratons. ). This allows for computation of mineralogically and thermo-dynamically consistent densities and elastic parameters of rocks that are directly updated during the thermo-mechanical computations (see [START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF]. Figure 5a shows that for the same initial plume diameter as in the previous case (low Ra p ), the plume head rises faster. In contrast with the previous case, the central zone embedded between cratons undergoes a significant uplift (Figure 5a, bottom right). With time, the stiffer cratonic blocks

A C C E P T E D M A N U S C R I P T
are progressively uplifted while the central zone subsides (time 15 Myr; Figure 5a). The plume head spreading is slowed down by the cratonic roots. The phase field shows that plume head is split in two parts which dynamically support the cratonic blocks and maintain the topography above the cratons. As shown in the thermal field, heating of the lithospheric mantle induces Rayleigh-Taylor-like instabilities at plume head centre and beneath cratons.

At time > 3 Myr, small-scale "slab" or "blob" -like instabilities are observed at the very edges of plume head, at both cratonic edges (see burial of cratonic material in the phase field). At that time, the plume head has only begun its flattening stage and lateral heterogeneities are entrained downwards. The flattening stage then prevents subduction of subcontinental lithospheric mantle to proceed further (time step of 15 Myr). On the base of experiments with a single heterogeneity [START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF] it can be predicted that in the case when two cratonic blocks are separated by a distance greater than the plume head diameter, the lateral edge of craton will hinder plume head flattening, promoting development of slab or blob-like instabilities and shear stresses concentration at cratonic margins.

For comparison, Figure 5b shows an experiment where cratonic blocks are only slightly thicker (only 25 km) than the embedding lithosphere. In this case plume spreading is not hindered and plume head removes almost all mantle lithosphere in less than 12 Myr. This experiment highlights the importance of the cratonic blocks, and lateral lithospheric heterogeneities in general, as localizing factors for plume-related deformation. Finally, a "cold" experiment shown in Figure 5c demonstrates plume-lithosphere interaction in case of colder background geotherm in the upper mantle (1750°C at 650 depth). This condition corresponds to the hypothesis of whole mantle convection [START_REF] Schubert | Mantle convection in the Earth and planets[END_REF]. As can be seen, the assumption of a colder background geotherm does not mitigate the consequences of plume impact (see also discussion in [START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF].

3.5 P-T-t paths around cratonic margins affected by mantle plumes.

Figure 6 shows a series of retrograde P-T-t paths deduced from the simulation shown in Figure 5. The followed particles are located near the inner cratonic margins where Rayleigh-

A C C E P T E D M A N U S C R I P T

Taylor like instabilities develop (see black dots in Figure 5a). For the deeper lithospheric material ascending towards the surface (path number 1), the clockwise P-T-t path includes a near-isothermal decompression phase around 800-900°C. For material coming from Moho depths (paths number 2), the clockwise P-T-t paths exhibit near-isothermal decompression episodes at lower temperatures (~ 700-750°C). However, for material coming from middle crust, the four particles follow anti-clockwise P-T-t paths with maximum (P,T) values reaching 920°C and 1.3 GPa (green dashed-dotted lines). It is noteworthy that paths number 2 reach almost zero pressure levels, that some would interpret as exhumation to the surface, whereas the corresponding phase field in Figure 5 does not always reveal exposure of initially buried lower crustal units to the surface (pink layer in Figure 5a). This is because the grid elements have dimensions of 5x5 km; their colors in Figure 5a correspond to an average dominant phase in the element, while the isolated particles (markers) used for PT tracing may be several km above the center of the mass of the element. In the model, there are about 20 phase markers per grid element yielding sub-grid resolution on the order of 1x1 km. On the other hand, the main grid resolution of 5 km implies that pressure, referred to the centers of the cells, is resolved with accuracy on the order of 75 MPa (150 MPa change over the cell). Extension at the surface creates dynamic (tectonic) pressure on the order 20-100 MPa, involving that conversion of pressure to depth may have an uncertainty on the order of 1-2 km at shallow depths, 5 km between 10 and 20 km depths, and 10-15 km below 20 km depth (Figure 8). This circumstance is specific for the lithosphere, since its high strength can support important non-lithostatic pressures. This issue has been previously discussed by [START_REF] Petrini | Lithospheric pressure-depth relationship in compressive regions of thickened crust[END_REF]. It is noteworthy that such pressure-depth conversion ambiguities are specific to the areas of non-localized deformation. For example, P-T-t paths from faults and

P-T-t paths shown in

A C C E P T E D M A N U S C R I P T

subduction channels are likely to be lithostatic, while in other cases important deviations may be expected. In any case, such deviations do not exceed 5-10% of the maximum values shown in Figure 6.

Mineral resources near cratonic margins

Most studies dealing with mantle plumes and mineral resources signatures around Archean cratons were focused on diamond genesis and kimberlitic eruptions (e.g. [START_REF] England | On the geodynamic setting of kimberlite genesis[END_REF]. The recent study by [START_REF] O'neill | Geodynamic controls on diamond deposits: implications for Australian occurrences[END_REF] emphasizes the role of intracontinental heterogeneities where abrupt changes in lithospheric thickness occur, promoting favorable conditions for diamond exhumation. Our study accounts for additional metallogenic data including gemstones and plutonic-related tin (Sn ± W-Ta-Li-Be) deposits (e.g. [START_REF] Milesi | Géologie et principaux gisements d'Afrique[END_REF]. Relationships between gemstones, cratons and mantle plumes are briefly presented below and specific data from east Africa are presented in section 5.

Gemstones

The formation and the exhumation of metamorphic gemstones require distinct geodynamic processes occurring at high pressures and high temperatures, thus involving the dynamics of the lithospheric mantle. Diamonds are stable at depths greater than 180 km [START_REF] Boyd | Evidence for a 150-200 km thick Archaean lithosphere from diamond inclusion thermobarometry[END_REF], and can thus remain stagnant beneath the thickest parts of Archean cratons, until kimberlitic explosive volcanism induces their fast exhumation. Other gemstones such as rubies and sapphires derived from metamorphic and metamorphic-metasomatic processes, have also recorded high-pressure and high-temperature conditions (typically between 650 and 800°C and between 0.6 and 1GPa) at the time of their formation (e.g. [START_REF] Sutherland | Models of corundum origin from alkali basalts terrains: a reappraisal[END_REF][START_REF] Shaw | A corundum-quartz assemblage from the Eastern Ghats Granulite belt, India: evidence for high P-T metamorphism?[END_REF][START_REF] Santosh | Gemstone mineralization in the Palghat-Cauvery shear zone system (Karur-Kangayam belt), southern India[END_REF][START_REF] Simonet | The Dusi (Garba tula) sapphire deposit, central Kenya -a unique Pan-African corundum-bearing monzonite[END_REF]) but contrary to diamonds, they are mainly found at cratonic margins or within the neighbouring Proterozoic mobile belts (Le [START_REF] Goff | The Tanzanian "Ruby Belt": structural, petrological and geochronological constraints within the pan-African orogenyexamples from the Morogoro and Mahenge districts[END_REF][START_REF] Giuliani | Oxygen isotope systematics of gem corundum deposits in Madagascar: relevance for their geological origin[END_REF][START_REF] Goff | Tectonic implications of new single zircon Pb-Pb evaporation data in the Lossogonoi and Longido ruby-districts, Mozambican metamorphic Belt of northeastern Tanzania[END_REF].

Rubies and sapphires associated with magmatic processes are often related to mafic-

A C C E P T E D M A N U S C R I P T

alkaline rocks (e.g. [START_REF] Yui | Oxygen isotope composition of the Denchai sapphire, Thailand: a clue to its enigmatic origin[END_REF] and gem-bearing basaltic volcanism has already been suggested to derive from a mantle plume activity [START_REF] Sutherland | Gem-bearing basaltic volcanism, Barrington, New South Wales: Cenozoic evolution, based on basalt K-Ar ages and zircon fission track and U-Pb isotope dating[END_REF].

Other rocks and ore deposits

A number of authors have advocated mantle plume events to explain geological and metallogenic features related to cratonic areas. For example, in Precambrian times, several superplume events have been suggested to account for apparent pulses of continental growth and metallogenic crises (e.g. [START_REF] Condie | Continental growth during a 1.9 Ga superplume event[END_REF][START_REF] Pirajno | Hotspots and mantle plumes: global intraplate tectonics, magmatism and ore deposits[END_REF]. Reconstructions of paleosupercontinents through the Earth's history suggest that major metallogenic crises occurred during periods of superplume activity leading to breakup of supercontinents [START_REF] Teixeira | Supercontinent evolution and the Proterozoic metallogeny of South America[END_REF]. In addition, world-class Ni-Cu-PGE ore deposits have been proposed to be related to ultramafic volcanism derived from mantle plumes and located on the edges of cratonic blocks (e.g. [START_REF] Arndt | Ore deposits associated with mafic magmas in the Kaapvaal craton[END_REF][START_REF] Zhang | Flood basalts and metallogeny: the lithospheric mantle connection[END_REF].

At first sight it was expected that plume head -continental lithosphere interactions would result in uplift phases and melting episodes. However, so far these signatures have been usually interpreted in terms of other tectonic processes, and only occasionally, mantle upwelling has been suggested as a possible source for burial of lower crustal material [START_REF] Saleeby | Production and loss of high-density batholithic root, southern Sierra Nevada, California[END_REF][START_REF] Smithies | Formation of Earth's early Archean continental crust[END_REF]. The presence of eclogites is often associated with the oceanic subduction process, and some authors used eclogites as evidence of past subduction [START_REF] Möller | Evidence for a 2 Ga subduction zone: eclogites in the Usagaran belt of Tanzania[END_REF]. However, as suggested by our experiments, eclogites could have formed through interaction of a mantle upwelling and the consequent downward dynamic instability. The latter conceptual model is corroborated by P-T-t data recorded on mineral assemblages from rocks of east Antarctica, that show both clockwise and counterclockwise P-T-t paths from rocks located near cratonic margins. To resolve this paradox, [START_REF] Halpin | Contrasting P-T-t paths for Neoproterozoic metamorphism in McRobertson and Kemp Lands, east Antartica[END_REF] have proposed a complex scenario involving two apparently opposite dynamic processes. Namely, records of both clockwise and counterclockwise P-T-t paths would suggest that two distinct exhumation histories could coexist near cratonic

A C C E P T E D M A N U S C R I P T

margins. However, our numerical experiments suggest that these observations can also be explained with a single mantle-lithosphere interaction model.

Signatures of plume-cratons interactions in East Africa

Recurrent plume activity

According to recent seismic tomography studies [START_REF] Ritsema | Upper mantle seismic velocity structure beneath Tanzania, east Africa: implications for the stability of cratonic lithosphere[END_REF][START_REF] Weeraratne | Evidence for an upper mantle plume beneath the Tanzanian craton from Rayleigh wave tomography[END_REF][START_REF] Walker | On the relationship between extension and anisotropy: constraints from shear wave splitting across the East African plateau[END_REF][START_REF] Montagner | Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath east Africa[END_REF], the Tanzanian craton is probably at least 150 km thick with a surrounding asymmetric mantle flow. The present-day plume head which would be present under the lithospheric keel corresponds to a 200-400 km wide thermal anomaly [START_REF] Nyblade | Seismic evidence for a deep upper mantle thermal anomaly beneath east Africa[END_REF]. It should also be noted that numerous episodes of riftingrelated magmatism have been inferred for the past geological evolution around Tanzania. In Kangankunde-Nathace in Malawi) [START_REF] Milesi | Géologie et principaux gisements d'Afrique[END_REF][START_REF] Milesi | An overview of the geology and major ore deposits of Central Africa : explanatory note for the 1 :4,000,000 map « geology and major ore deposits of Central Africa[END_REF][START_REF] Goff | Tectonic implications of new single zircon Pb-Pb evaporation data in the Lossogonoi and Longido ruby-districts, Mozambican metamorphic Belt of northeastern Tanzania[END_REF]. This heterogeneity and the distribution of alkaline-peralkaline plutonic activity around organized centres, suggest the operation of mantle plumes at four different periods [START_REF] Pouradier | Sig Afrique: magmatisme alcalin et carbonatitique et minéralisations en métaux rares associés[END_REF], spanning at least 1 Ga. One recent study also concludes that the intracratonic Congo basin reveals a hidden rift structure which was periodically affected and weakened during a long period of time [START_REF] Kabongo | A Proterozoic-rift origin for the structure and the evolution of the cratonic Congo basin[END_REF]

Diamonds

As well known, a striking correlation appears between primary diamond deposits and Archean cratons. It is now commonly accepted that the P-T conditions for stability field of diamond implies that they are stored beneath thick (>150-180 km) Archean cratons. Figure 7 A C C E P T E D M A N U S C R I P T (top row) shows primary diamond deposits of south and east Africa together with contours of Archean cratons. Diamonds of south and east Africa have formed at 3.2, 2.9, 1.5 Ga, and were exhumed through kimberlitic magmatism at three main periods in the last 1150 Ma [START_REF] Richardson | Episodic diamond genesis at Jwaneng, Botswana, and implications for Kaapvaal craton evolution[END_REF].

Other gemstones

The famous "Gemstone belt of East Africa" [START_REF] Malisa | Tectonic setting of gemstone mineralization in the Proterozoic metamorphic terrane of the Mozambique belt of Tanzania[END_REF]) is located in between Tanzanian and Madagascar cratonic areas, i.e. within the Neoproterozoic Mozambique belt. It includes numerous gemstone deposits yielding ruby, sapphire, garnet, tourmaline and various other gems [START_REF] Simonet | Geologie des gisements de saphir et de rubis[END_REF]. Contrary to diamonds, formation of synmetamorphic gemstones occurs at depths of a few tens of kilometers, and their exhumation does not require explosive eruptions, except in the case of the so-called "basaltic sapphires" related to the Neogene alkaline rift volcanism. Thermobarometric data from ruby-bearing rocks from Madagascar and Kenya indicate that P-T conditions of ~1 GPa and 800°C were reached (Mercier et al., 1999a(Mercier et al., , 1999b)). According to several other studies, the stability field for corundum-quartz and corundum-garnet-clinopyroxene assemblages in other ruby-bearing rocks involve more extreme P-T conditions, such as P > 1.5 GPa and T >1100°C [START_REF] Shaw | A corundum-quartz assemblage from the Eastern Ghats Granulite belt, India: evidence for high P-T metamorphism?[END_REF][START_REF] Rakotondrazafy | Petrographie des roches à rubis et à saphir du socle cristallin métamorphique et des basaltes alcalins de Madagascar[END_REF][START_REF] Schenk | Ubendian ultrahigh-temperature metamorphism of metapelitic migmatites in the Songea-Mbamba bay area, southern Tanzania[END_REF].

Figure 7 (bottom left) illustrates the spatial distribution of ruby and sapphire deposits around cratonic areas. The high P-T conditions which are required for their formation are reached between cratonic areas, but surprisingly, the distribution is not symmetric. Actually, no gemstone deposit appears on the western side of the Tanzanian craton. The continuity of the gemstone belt from Kenya to south Madagascar confirms that this particular intercratonic area underwent severe burial and exhumation processes, contrary to the western part of Tanzania.

Tin

A C C E P T E D M A N U S C R I P T

The bottom right of Figure 7 shows the spatial distribution of felsic intrusion-related tin deposits (Sn ± W, Ta, Li, Be). Classically, felsic intrusion-related tin concentrations derive from pneumatolitic (late-magmatic) tin-bearing fluids (400-600°C) related to specialized (peraluminous) felsic magmas. Focused outside cratonic areas, the numerous tin districts evidence periods of crustal melting, in particular within the Mesoproterozoic Kibaran orogenic belt produced by the collision of the Congo and Tanzanian cratons. The Mesoproterozoic cycle concluded around 1000-950 Ma with a plutonic to volcano-plutonic event, and with the emplacement of gneissic complexes in the internal domain. This Lurian late-orogenic crustal magmatic event, distinguished by a peraluminous signature, is expressed by numerous tintantalum granites and pegmatites in Rwanda, Burundi, southern Uganda, western Tanzania, Kivu and Maniema, eastern Congo [START_REF] Milesi | Géologie et principaux gisements d'Afrique[END_REF][START_REF] Milesi | An overview of the geology and major ore deposits of Central Africa : explanatory note for the 1 :4,000,000 map « geology and major ore deposits of Central Africa[END_REF].

The notion that no tin deposits are found within cratons is not surprising since crustal melting can be induced by post-tectonic events or by heat input at crustal depths, two scenarios which can be ruled out in a tectonically stable and thick cratonic area.

Nevertheless, the absence of intrusion-related tin deposits at the eastern side of the Tanzanian craton area suggests that either crustal melting in this area was quantitatively limited, or that another type of dynamic interaction between the mantle and the lithosphere occurred.

Eclogites and granulites

A series of additional thermobarometric data demonstrate that extreme P-T conditions were present near the Tanzanian craton borders. Among them, the Paleoproterozoic (2.0 Ga) eclogites found at both south-east and south-west margins of the craton (Usagaran and Ubendian belts, respectively) indicate an exhumation process starting with conditions of ~1.8

GPa and 700°C and following a clockwise P-T-t path [START_REF] Möller | Evidence for a 2 Ga subduction zone: eclogites in the Usagaran belt of Tanzania[END_REF][START_REF] Collins | Temporal constraints on Palaeoproterozoic eclogites formation and exhumation (Usagaran Orogen, Tanzania)[END_REF].

In addition, metagabbros from the Ubendian terrane were also affected by metamorphic conditions of about 840-900°C and pressures of 1.2-1.4 GPa [START_REF] Muhungo | High-pressure granulite-facies metagabbros in the Ubendian belt of SW Tanzania: preliminary petrography and P-T estimates[END_REF].

A C C E P T E D M A N U S C R I P T

During the Paleoproterozoic orogenic cycle, high temperature metamorphism (850-900°C; 0.8 GPa) occurred in the southern margin of the Tanzanian craton [START_REF] Schenk | Ubendian ultrahigh-temperature metamorphism of metapelitic migmatites in the Songea-Mbamba bay area, southern Tanzania[END_REF]. On the other side (northern margin), ultrahigh temperature metamorphism (~1000°C and 0.7-0.9 GPa) has been recognized in Labwor hills, Uganda [START_REF] Sandiford | Metamorphic evolution of aluminous granulites from Labwor Hills, Uganda[END_REF].

Close to the eastern margin of the Tanzanian craton, Neoproterozoic metamorphism of high pressure granulite facies was reported [START_REF] Muhongo | Pan-African high pressure isobaric cooling: evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru mountains, eastern Tanzania[END_REF][START_REF] Appel | High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P-T-t evidence against granulite formation by continent collision[END_REF][START_REF] Möller | U-Pb dating of metamorphic minerals: Pan-African metamorphism and prolonged slow cooling of high pressure granulites in Tanzania, east Africa[END_REF][START_REF] Cutten | Protolith ages and timing of metasomatism related to the formation of whiteschists at Mautia Hill, Tanzania: implications for the assembly of Gondwana[END_REF][START_REF] Sommer | Isothermal decompression history in the "Western Granulite" terrane, central Tanzania: evidence from reaction textures and trapped fluids in metapelites[END_REF]. Thermobarometric results and isotopic dating revealed anticlockwise P-T-t paths, where conditions of peak metamorphism give a pressure range between 0.9 and 1.2 GPa at 800°C. However, a recent analysis by [START_REF] Sommer | Isothermal decompression history in the "Western Granulite" terrane, central Tanzania: evidence from reaction textures and trapped fluids in metapelites[END_REF] reveal clockwise P-T-t paths (with an isothermal decompression phase) for the "Western Granulite" terrane, with P-T conditions of 1.3-1.4

GPa and 760-800°C.

Summary of geological observations

Figure 8 summarizes the above described thermobarometric data from the Tanzanian area.

Estimates of P-T data and inferred P-T-t paths suggest that burial and exhumation episodes probably occurred through distinct processes, since clockwise and anti-clockwise P-T-t paths are both recorded.

The asymmetry in geological signatures around Tanzania (e.g. location of gemstone and tin deposits, contrasting P-T-t paths) points to distinct thermo-mechanical mechanisms at both sides of the craton. Even if gemstones and tin deposits are not coeval, the comparison can be considered valid since the craton is supposed to have undergone a similar thermal history through more than 1 Gyr: successive mantle plumes have probably followed similar mantle paths before impinging on the base of the craton.

Discussion

A C C E P T E D M A N U S C R I P T

Models of plume headcratonic blocks interactions have shown that contrasting P-T-t paths can be retrieved from rocks at cratonic margins initially emplaced at different depths (Figure 6). General shapes of numerical P-T-t paths (clockwise and counterclockwise paths), absolute values of pressure and temperature, as well as cooling rates can be quantitatively compared to thermobarometric data recorded around the Tanzanian craton. In addition, thermo-mechanical instabilities induced by plume headcratonic block interactions show two distinct processes leading either to isobaric heating before burial (the "drip-like" instability) or to isothermal burial before heating (the "slab-like" instability). Drip-like instabilities would trigger counterclockwise P-T-t paths whereas slab-like instabilities would promote clockwise P-T-t paths. In addition to these modelling results, distribution of geological and metallogenical signatures in east Africa suggests that distinct deep processes occurred and still occur on both sides of cratonic areas.

Previous hypotheses to account for the observed variety of P-T-t paths were generally associated with horizontal tectonics and downward motions induced by subduction dynamics. Our modelling study suggests mantle upwellings as a driving mechanism for crustal metamorphism.

The subcontinental mantle upwelling may represent the missing heat source that seems to be required when ultra-high temperature metamorphism is investigated. For example, [START_REF] Schmitz | Ultrahigh-temperature metamorphism in the lower crust during Neoarchean Ventersdorp rifting and magmatism, Kaapvaal craton, southern Africa[END_REF] providing the necessary heat input, drip-like instabilities would also represent an efficient mechanism to remove the lithospheric mantle and replace it by mantle-derived magmas. It must be noted that such deep processes involving lithospheric instabilities have already been invoked to explain particular strain patterns identified around the Dharwar craton (Bouhallier et al., 1995;[START_REF] Chardon | Sinking of the Dharwar basin (south India): implications for Archaean tectonics[END_REF]. Archean and Proterozoic tectonics might have been

A C C E P T E D M A N U S C R I P T

significantly affected -if not controlled -by Rayleigh-Taylor like instabilities which easily occurred at cratonic boundaries beneath hot and weak lithospheres.

As far as ultra-high pressure metamorphism (UHP) is concerned, most models invoke continental subduction to explain nearly isothermal burial down to depths of 80-100 km (e.g. [START_REF] Brown | From microscope to mountain belt: 150 years of petrology and its contribution to understanding geodynamics, particularly the tectonics of orogens[END_REF]. Here again, edges of mantle plume heads can mechanically entrain mantle lithosphere down to several hundreds of kilometres, while temperature does not necessarily increase much. The resulting clockwise P-T-t path shows peak pressures in excess of 1 GPa (Figure 6). As for subduction-triggered burial of crustal rocks, the subsequent exhumation process would then be driven by buoyancy forces. lithosphere (see [START_REF] Burov | The plume headcontinental lithosphere interaction using a tectonically realistic formulation for the lithosphere[END_REF] with no pre-imposed horizontal velocity at horizontally "permeable" (see Burov and Cloetigh, 2009) Myr, all the affected area is thinned). c) "Cold" experiment similar to one shown in Figure 5a but assuming whole mantle convection geotherm: temperature boundary condition at 650 km is 1750°C [START_REF] Burov | Controls of mantle plumes and lithospheric folding on modes of intraplate continental tectonics: differences and similarities[END_REF]. Paths coming from the middle crust. Note that some HP-HT paths from the base of the lithosphere (P=2.5 GPa) have arrived near to middle-crustal depth (see text for uncertainties and order of magnitudes indicated on the right). In all cases only the paths that ascent to the surface (to at least middle crustal depths) were retained. We also show one of the "rotating" paths starting from near-keel zone (1) in the mantle lithosphere. Rotating paths were also observed in the other units (2,3). The existence of such paths hints that in some cases the interpretation of petrology data might be not straightforward as the usual assumption of the petrological analysis is based on the hypothesis of simple trajectories. dotted curve shows a typical geotherm in a stable cratonic area (e.g. [START_REF] Jaupart | The thermal structure and thickness of continental roots[END_REF]. A: P-T-t path (dotted curve) for eclogites from the Usagaran and Ubende belts [START_REF] Möller | Evidence for a 2 Ga subduction zone: eclogites in the Usagaran belt of Tanzania[END_REF][START_REF] Collins | Temporal constraints on Palaeoproterozoic eclogites formation and exhumation (Usagaran Orogen, Tanzania)[END_REF]; B: P-T-t path for granulites from the western Mozambique belt [START_REF] Cutten | Protolith ages and timing of metasomatism related to the formation of whiteschists at Mautia Hill, Tanzania: implications for the assembly of Gondwana[END_REF]; C: P-T-t path (dashed curve) for granulites from the eastern Mozambique belt [START_REF] Appel | High-pressure granulite facies metamorphism in the Pan-African belt of eastern Tanzania: P-T-t evidence against granulite formation by continent collision[END_REF]; D: P-T-t path for metapelites from eastern Mozambique belt [START_REF] Sommer | Isothermal decompression history in the "Western Granulite" terrane, central Tanzania: evidence from reaction textures and trapped fluids in metapelites[END_REF]; E: Ultramafic granulites from Uluguru mountains, eastern Tanzania [START_REF] Muhongo | Pan-African high pressure isobaric cooling: evidence from the mineralogy and thermobarometry of the granulite-facies rocks from the Uluguru mountains, eastern Tanzania[END_REF]; F: Metagabbros from the Ubendian belt, south-western Tanzania (Muhongo et al., 2002); G: Ruby-bearing rocks from Southwest Madagascar (Mercier et al., 1999b); H: Ruby-bearing rocks from SE Kenya (Mercier et al., 1999a). I: Stability of Crn+Qtz [START_REF] Shaw | A corundum-quartz assemblage from the Eastern Ghats Granulite belt, India: evidence for high P-T metamorphism?[END_REF][START_REF] Schenk | Ubendian ultrahigh-temperature metamorphism of metapelitic migmatites in the Songea-Mbamba bay area, southern Tanzania[END_REF] and of Crn+Grt+Cpx in ruby-bearing rocks of Madagascar [START_REF] Rakotondrazafy | Petrographie des roches à rubis et à saphir du socle cristallin métamorphique et des basaltes alcalins de Madagascar[END_REF]; J and K: ultrahigh temperature metamorphism at southern and northern edges of Tanzanian craton [START_REF] Sandiford | Metamorphic evolution of aluminous granulites from Labwor Hills, Uganda[END_REF]

Figure

  Figure 4b is a close-up of this area at time 15 Myr, and shows lateral segmentation of

Figure

  Figure 5a to 5c show experiments characterized by high Ra p ~10 6 , based on thermodynamically consistent computations. For that we have coupled the

  Figure 6 represent 20-30 Myr of metamorphic evolution. The slow cooling phases identified in paths number 2 (blue and orange plain lines, from maximum temperatures around 700°C to depths of 0.4 GPa where T=650°C) are equivalent to cooling rates of about 5-9°C/Ma.

  particular, the east African rift area has been affected by at least four alkaline-peralkaline magmatic and carbonatitic events, beginning at the end of the Mesoproterozoic (around 1 Ga, Meponda in Mozambique and Ngualla in Tanzania), and subsequently continuing in successive spurts with displacement of the magmatic centres in the Neoproterozoic around 730-655 Ma (Lueshe district), in the Early Paleozoic (Lueshe, Matongo), and then in the Late Jurassic -Late Cretaceous in the southern segment of the rift (Panda Hill in Tanzania,

  suggested transient removal of lithospheric mantle to explain thermobarometric data showing UHT metamorphism around the Kaapvaal craton, south Africa. Similarly, Prakash et al. (2010) invoked accretion of mantle-derived magma to explain the additional heating required by P-T data around the Dharwar craton. Besides

Figure 9 Figure 1 :

 91 Figure 9 illustrates the basic concepts developed in this study: (a) a large cratonic

Figure 2 :

 2 Figure 2: Experiments for a case with a single lithosphere heterogeneity for three distinct initial plume locations (dotted half-circle). A 800 Myr old craton is inserted in the half-right part of the 150 Myr old lithosphere (dotted line shows the initial configuration). From top to bottom, time steps correspond to 7.2 Myrs, 11.3 Myrs, and 16.5 Myrs. Color code for the phase field (left column): purple: crust, blue:mantle lithosphere, green: upper mantle, yellow: plume, orange: marker layer at the bottom. Note the three kinds of lithospheric instabilities (black arrows and white numbers): 1: subduction-like instability (a), 2: drip-like instability (b) and 3: slab-like instability (c). Third column illustrates three qualitative distinct P-T-t paths followed by associated instabilities, as inferred from the temperature field in the second column. Based on[START_REF] Burov | Plume headlithosphere interactions near intra-continental plate boundaries[END_REF].

Figure 3

 3 Figure 3 Case of a thick (200 km) lithosphere with two cold blocks. Temperature in °C, log (effective viscosity) in Pa.s, and surface topography in m. Note that surface topography consists mainly in one

Figure

  Figure 4: a) Low (10 5 ) plume Rayleigh number experiment, with two cratonic blocks and a thin weak lithosphere. The low plume Rayleigh number promotes the formation of a large plume head with a large plume tail. The large-scale return flow induces a negative topographic signature (i.e. subsidence) above the plume head. At the final stage, alternating zones of extension and compression are present between cratonic areas; b) Close-up of the last time step, where thermo-mechanical interactions between the plume head and the non-cratonic lithosphere imply a lateral segmentation of crustal rheology.

Figure 5 :)

 5 Figure 5: Thermodynamically consistent model (density computed from PERPLEX (see A3) as

Figure 6 :

 6 Figure 6: Some characteristic return (retrograde) P-T-t paths extracted from the model near the inner borders of the cratonic blocks (see sketch on top, and black dots in Figure 5a, temperature field at

Figure 7 :

 7 Figure 7: Spatial distribution of south and east African cratons (upper left), primary diamond deposits (upper right), ruby and sapphire deposits (lower left) and tin (+/-Ta, W, Li, Be) deposits (lower right). Initials of cratons: WC=West central African craton; WN=West Nile craton; Tz=Tanzanian craton; K=Kasai craton; CS(WA)=Central Shield (West Angola) craton; Z-K=Zimbabwe-Kaapvaal craton; A-A=Antananarivo-Antongil craton. See text for a more detailed description.

Figure 8 :

 8 Figure 8: Thermobarometric data from rocks and gemstones bordering the Tanzanian craton. Dash-

  ; Schenk et A C C E P T E D M A N U S C R I P T al., 2004). Localities of rock samples bordering the craton are indicated in the inset (data D, G and I are located out of the inset).

Figure 9 :

 9 Figure 9: Model for plume head -cratonic blocks interactions in east Africa, from an initial superplume event (a) leading to Gondwana breakup and dispersal of the three cratonic blocks (Tanzanian, Madagascar and Dharwar); (b and c): continuous plume activity results in partial erosion of the base of cratonic blocks, either focusing plume pathways along the sloping base (left) or blocking plume heads at cratonic border (right); (d) favourable conditions for partial melting, ultrahigh temperature metamorphism and granite-related (tin) deposits establish in the left case, while gemstones, eclogites and ultrahigh pressure metamorphism are favoured in the right case; (e) drip-like instabilities result in counterclockwise P-T-t paths whereas slab-like instabilities are associated with clockwise P-T-t paths.

  Fig. 2

  Fig. 3

  Fig. 4

  Fig. 5

  Fig. 6

  Fig. 7

Fig. 9

 9 Fig. 9

Acknowledgements. This study was partly supported by BRGM, UPMC and ISES. We thank F. Neubauer, R. Oberhänsli and previous anonymous reviewers including P. Rey for helpful comments than greatly improved the final version of this paper.