Jean-Alix J-A Barrat
email: barrat@univ-brest.fr.

Akira Yamaguchi

Richard C Greenwood

Claire Bollinger

Marcel Bohn

Ian A Franchi

Trace element geochemistry of K-rich impact spherules from howardites

published or not. The documents may come

Introduction

4-Vesta, the second largest object by mass in the asteroid belt, is probably the parent body of the most abundant group of achondrites, namely the howardite, eucrite, and diogenite (HED) suite (e.g., [START_REF] Mccord | Asteroid Vesta: spectral reflectivity and compositional implications[END_REF][START_REF] Drake | The eucrite/Vesta story[END_REF]. Eucrites are basaltic or gabbroic rocks that formed as lava flows or intrusions. They are generally regarded as being samples of the upper crustal lithologies of their parent body. Diogenites are orthopyroxene-rich cumulates, and are generally believed to sample deeper structural levels, but this is presently a matter of discussion [START_REF] Barrat | Petrology and geochemistry of the fine-grained, unbrecciatted diogenite Northwest Africa 4215[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]. Most eucrites and diogenites record a complex post crystallization history. These rocks were extensively brecciated and locally melted by meteorite impacts. Therefore, eucrites and diogenites are generally breccias consisting of a mixture of crystal debris and rock fragments. Howardites are more complex breccias, composed predominantly of both eucritic and diogenitic clasts [START_REF] Mittlefehldt | Non-chondritic meteorites from asteroidal bodies[END_REF].

Howardites and polymict eucrites commonly contain impact melt clasts (e.g., [START_REF] Noonan | Glass particles and shock features in the Bununu howardite[END_REF][START_REF] Labotka | Howardites: samples of the regolith of the eucrite parent-body: petrology of Frankfort, Pavlovka, Yurtuk, Malvern, and ALHA 77302[END_REF][START_REF] Fuhrman | Howardites and polymict eucrites! Regolith samples from the eucrite parent body. Petrology of Bholgati, Bununu, Kapoeta, and ALHA76005[END_REF][START_REF] Mittlefehldt | Magnesian basalt clasts from the EET 92014 and Kapoeta howardites and a discussion of alleged primary magnesian HED basalts[END_REF], and less frequently impact glass beads (fig. 1). The petrography and the geochemistry of these glasses have recently been reinvestigated, with at least two types of mafic spherules identified in howardite breccias (Barrat et al., 2009a). Firstly, low-K impact glasses (Mg# (=100 x Mg/(Mg+Fe), atomic) from 41 to 72, with K 2 O generally less than 0.1 wt%) have been analyzed in Bununu, Kapoeta, Yamato (Y-) 7308 and Y-791208. They are chemically similar to howardites or eucrites, and are formed by melting of an ordinary HED target with negligible effects of impact induced vapor fractionation (e.g., [START_REF] Noonan | Glass particles and shock features in the Bununu howardite[END_REF][START_REF] Yagi | Mineralogical and petrological studies of the Yamato meteorites, Yamato-7301(j), -7305(k), -7308(l) and -7303(m) from Antarctica[END_REF]Klein andHewins, 1979: Noonan et al., 1980;[START_REF] Delaney | Partial melt genesis for glassy clasts in basaltic achondrites[END_REF][START_REF] Ikeda | Petrography and mineral compositions of the Yamato 7308 howardite[END_REF]. Secondly, mafic glasses found in Malvern [START_REF] Noonan | Glass particles and shock features in the Bununu howardite[END_REF][START_REF] Desnoyers | The Malvern howardite: a petrological and chemical discussion[END_REF], Macibini [START_REF] Buchanan | The South African polymict eucrite Macibini[END_REF], Northwest Africa (NWA) 1664 (e.g., [START_REF] Kurat | Major, minor and trace elements in some glasses from the NWA 1664 howardite[END_REF], NWA 1769 and LAP 04838 (Barrat et al., 2009a) display a range of Mg#-values similar to the previous K-poor glasses (Mg# from 32 to 73), but are unusually K-rich, with K 2 O concentrations ranging from 0.15 to 2.33 wt%. These K-abundances are much higher than the values generally found in HED meteorites, which in most cases contain significantly less than 0.1 wt% K 2 O. Such high K abundances indicate that these impact spherules could not have formed from any known HED lithology. Barrat et al. (2009a) have shown that the behavior of K in these glasses cannot be explained by a projectile contribution, selective alkali vaporization/condensation processes, or by terrestrial contamination. More likely, the high K abundances measured in some impact spherules indicate that some K-rich lithologies were present in the fused target materials. The occurrence of granitic (or felsic) glasses in the Yamato 791073 breccia [START_REF] Takeda | Mineralogy of Yamato 791073 with reference to crystal fractionation of the howardite parent body[END_REF] and in a NWA 1664 spherule (Barrat et al., 2009a) strongly strengthens this interpretation.

The high-K glasses found in howardites are particularly attractive from a geochemical point of view. Since impact glasses record some of the geochemical features of their source materials and in some instances could have been ballistically transported over large distances, they can potentially provide a complementary view of the chemical composition of the rocks exposed on Vesta. In this paper, we report on the trace element abundances of the unusual high-K glasses found in NWA 1664 and NWA 1769. In addition to NWA 1664 and NWA 1769, we have studied a further two howardites and two polymict eucrites in order to assess whether there are any geochemical features unique to the two spherule-bearing samples.

Analytical procedures

Bulk rock compositions

Meteorite fragments were powdered using a boron carbide mortar and pestle. Minor and trace element concentrations were measured at the Institut Universitaire Europeen de la Mer (IUEM), Plouzané, by ICP-MS (inductively coupled plasma-mass spectrometry) using a Thermo Element 2 spectrometer following the procedures described by [START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF].

Based on standard measurements and sample duplicates, concentration reproducibility is generally much better than 5 %.

Oxygen isotopes

Oxygen isotope analyses were carried out using an infrared laser fluorination system at The Open University [START_REF] Miller | High precision 17 O measurements of oxygen from silicates and other oxides: method and applications[END_REF]. Aliquots (~2 mg), taken from larger batches of homogenized powdered samples (section 3.1), were analyzed both untreated and after leaching in 6M HCl for 30 minutes. To maximize yields and decrease the risk of crosscontamination the powdered samples were fused in vacuum to form a glass bead prior to fluorination. O 2 was liberated by heating the glass beads using an infrared CO 2 laser (10.6 μm) in the presence of 210 torr of BrF 5 . After fluorination, the O 2 released was purified by passing it through two cryogenic nitrogen traps and over a bed of heated KBr. O 2 was analyzed using a Micromass Prism III dual inlet mass spectrometer. Published system precision (1σ) [START_REF] Miller | High precision 17 O measurements of oxygen from silicates and other oxides: method and applications[END_REF], based on replicate analyses of international (NBS-28 quartz, UWG-2 garnet) and internal standards, is approximately ±0.04‰ for 17 O; ±0.08‰ for 18 O; ±0.02‰ for 17 O. More recent replicate analyses of the UWG-2 garnet standard gave the following values for system precision (1 ): ±0.03‰ for 17 O; ±0.07‰ for 18 O; ±0.01‰ for 17 O. The quoted precision (1σ) for the howardite samples is based on replicate analyses. Oxygen isotope analyses are reported in standard δ-notation where 18 O has been calculated as: 18 O = (( 18 O/ 16 O sample / 18 O/ 16 O ref )-1) x 1000 and similarly for 17 O using 17 O/ 16 O ratio. 17 O has been calculated using the linearized format of [START_REF] Miller | Isotopic fractionation and the quantification of 17 O anomalies in the oxygen threeisotope system: an appraisal and geochemical significance[END_REF]: 17 O = 1000 ln (1+ ( 17 O /1000)) -λ1000 ln(1+ (δ 18 O /1000)) where λ = 0.5247.

In-situ trace element abundances

Glass spherules located within polished thick-sections were first analyzed for major and minor elements by electron microprobe at Ifremer, Plouzané, using a Cameca SX100 instrument. The results of this work were previously reported by Barrat et al. (2009a). Trace elements were subsequently determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) at Institut Universitaire Européen de la Mer, Plouzané. The analyses were performed under a He atmosphere using an Excimer (193 nm wavelength) laser ablation system (Geolas Pro102), connected to a Thermo Element 2 spectrometer operated in low resolution mode (m/ m=300). Analytical and data reduction procedures generally followed those described by Agranier and Lee (2007). Concentrations were determined on individual spots using a 44 µm-diameter laser beam (fig. 1) and a laser repetition rate of 10 Hz. The power output of the laser was approximately 15 J/cm 2 . Transmission was typically estimated at about 10 5 cps/ppm of La in the BCR2 glass standard. Results were normalized to TiO 2 abundances measured by electron microprobe as an internal standard to account for variable ablation yield. For all data, the NIST 612 and BCR-2G glass standards were both used for external calibration of relative element sensitivities, using values given by [START_REF] Jochum | Chemical characterisation of the USGS reference glasses GSA-1G, GSC-1G, GSD-1G, GSE-1G, BCR2-G, BHVO2-G, and BIR1-G using EPMA[END_REF]. Replicate analyses of the USGS basaltic-glass standards BIR-1G and BHVO-2G run at intervals during the analytical session, yielded an external reproducibility generally better than 5 % (1 relative standard deviation) at abundances similar to those found in impact spherules (Table 1).

Results and discussion

Bulk howardite and polymict eucrite samples

Howardites and polymict eucrites are extremely heterogeneous breccias. Obtaining a representative sample of a breccia would require crushing and homogenization of several grams of sample, depending on grain size and the proportion of clasts to matrix. It is generally not feasible to obtain such large amounts of a meteorite for bulk rock analysis. The major and trace element compositional variation displayed by howardites has been extensively reported in the literature (e.g., [START_REF] Mccarthy | Further evidence in support of the mixing model for howardite origin[END_REF][START_REF] Chou | Trace element evidence regarding a chondritic component in howardite meteorites[END_REF][START_REF] Fukuoka | Genesis of howardites, diogenites and eucrites[END_REF][START_REF] Mittlefehldt | Mesosiderites and howardites; igneous formation and possible genetic relationships[END_REF][START_REF] Palme | New data on lunar and achondrites and acomparaison of the least fractionated samples from the Earth, the Moon, and the eucrite parent body[END_REF], and the reader is referred to these previous studies for detailed discussion of the eucrite/diogenite mixing model and chondritic contribution. In this study, we have prepared "large" matrix samples (fine-grained portion devoid of large clasts (> 2-4 mm) of orthopyroxene or eucrite) weighing about 1 g. Four Saharan howardites were studied (NWA 1664, NWA 1769, NWA 5306, NWA 5614) and two polymict eucrites (NWA 5616 and 5618). Our aim was first to better characterize NWA 1664 and NWA 1769 using minor and trace element abundances (including the alkalis), oxygen isotope analysis, and then to compare these high-K glass-bearing samples with the other howardites .

Minor and trace elements

Bulk compositions of the six analyzed matrix samples of Saharan howardites and polymict eucrites are given in Table 2. In all cases the concentrations are in the range of previously analyzed HED polymict breccias (Mittlefehldt et al., 1998 and references therein).

Ni abundances range from 12 to 125 µg/g and indicate a chondritic contribution of less than 1 wt%.

The six samples display flat CI-normalized rare earth (REE) patterns (La n /Yb n =1.1 to 1.2, fig. 2). Four of them (NWA 1664, 1769, 5306 and5614) exhibit a similar negative Eu anomaly (Eu/Eu*=0.77 to 0.85). Interestingly, NWA 1664 and NWA 1769 matrices display virtually identical REE patterns and these resemblances extend to all the other analyzed elements (Table 2). Their Juvinas-normalized patterns are flat except for Cs and Rb enrichments, a feature which is well known in eucrites, in particular some main group-Nuevo Laredo eucrites (fig. 3). The patterns show no trace element enrichment that can be ascribed to hot desert secondary processes such as positive Ba or Sr anomalies (e.g., [START_REF] Barrat | The Tatahouine diogenite: Mineralogical and chemical effects of sixty-three years of terrestrial residence[END_REF][START_REF] Barrat | Bulk chemistry of Saharan shergottite Dar al Gani 476[END_REF][START_REF] Barrat | Petrology and geochemistry of the unbrecciatted achondrite North West Africa 1240 (NWA 1240): an HED parent body impact melt[END_REF][START_REF] Crozaz | The terrestrial alteration of Saharan shergottite Dar al Gani 476 and 489: a case study of weathering in a hot desert environment[END_REF]. Indeed, these two samples display Ba/La, Sr/Nd, and Th/U ratios within the range measured in eucrite and howardite falls. This indicates, in agreement with petrographical observations, that these two samples are rather fresh, and the contribution from secondary phases to the trace element budget is negligible. Furthermore, the chemical differences between the two samples are so subtle, that they strongly suggest that these two howardites are paired. This inference is strengthened by the examination of the polished thick sections and the chemical composition of their fusion crust (Barrat et al., 2009a).

The other polymict breccias analyzed here exhibit the same type of Juvinasnormalized patterns as NWA 1664 and NWA 1769 but two of them (NWA 5306 and 5614) display marked positive Ba anomalies (Ba/La= 22 and 74, compared to approximately 10 in eucrite and howardite falls) which suggests that these two samples are much more weathered than the others examined in this study. Furthermore, with 27 ng/g Cs and 1 µg/g Rb, NWA 5614 is the Cs and Rb-richest sample studied here, and such an enrichment is most likely the result of terrestrial weathering. However, desert weathering is not the only explanation for these elevated values, as similarly high Rb and Cs abundances have previously been reported in the matrix of the polymict eucrite Macibini [START_REF] Buchanan | The South African polymict eucrite Macibini[END_REF], an observed fall.

Notice that in both cases Li and Na abundances are unaffected by hot desert secondary processes.

Oxygen isotopes

Oxygen isotope results obtained in this study are given in Table 3 and plotted on figure 4a along with the previous HED results of [START_REF] Greenwood | Widespread magma oceans on asteroidal bodies in the early solar system[END_REF]. All of the analyzed samples, plot on or within 3 of the Eucrite Fractionation Line (EFL) ( 17 O = -0.239 ± 0.007‰ (1 ), [START_REF] Greenwood | Widespread magma oceans on asteroidal bodies in the early solar system[END_REF]), consistent with them being isotopically normal HED meteorites (fig. 4a). Analyses of untreated polymict breccias form a relatively broad cluster that overlaps both the diogenite and eucrite fields on Fig. 4a; a feature that is consistent with the fact that howardites are a mixture of these two lithologies. The howardite samples show a slightly greater spread about the EFL than either eucrites or diogenites, which in part may reflect the fact that they sometimes contain non-HED materials [START_REF] Mittlefehldt | Non-chondritic meteorites from asteroidal bodies[END_REF].

Deviation from the EFL could also be the result of terrestrial weathering. To test this possibility all samples were acid leached in 6M HCl for 30 minutes. The results for both leached and untreated samples are plotted on Fig. 4b. The untreated samples form a relatively broad cluster straddling the EFL, whereas those that have been acid leached form a more linear array, slightly displaced to higher 18 O values compared to the untreated ones.

Weathering in hot desert environments tends to shift samples to higher 18 O values compared to their primary compositions [START_REF] Greenwood | Hot and cold weathering: determining the oxygen isotope composition of achondrites Workshop on Antarctic meteorites abstract #4020[END_REF]. This is the opposite trend to that seen in figure 4b, where the leached samples, which should have a lower content of secondary minerals compared to the untreated samples, in fact have the highest 18 O values. This suggests that the untreated howardites are relatively fresh and do not contain significant amounts of secondary minerals. In addition, the 6M HCl treatment is relatively harsh compared to other leaching techniques commonly used used to remove secondary weathering products prior to oxygen isotope analysis [START_REF] Greenwood | Hot and cold weathering: determining the oxygen isotope composition of achondrites Workshop on Antarctic meteorites abstract #4020[END_REF]. Consequently, the shift to higher 18 O values seen in the acid leached samples may reflect some slight preferential removal of a primary low 18 O phase such as orthopyroxene at the expense of plagioclase, which generally has slightly higher 18 O values [START_REF] Clayton | Oxygen isotopes in meteorites[END_REF].

High-K impact spherules

Among the score of spherules we have previously identified in NWA 1664 and 1769, only 17 were large and thick enough to be analyzed using our laser procedure. Depending on their size, we performed between one and three analyses per spherule-core, with the average for each spherule given in Tables 4 and5. The spherules analyzed here cover the entire compositional range as established before (e.g., K 2 O= 0.28 to 2.33 wt%, Barrat et al., 2009a), and display significant trace element variations.

Trace element abundances

High K glasses show an impressive range of Ni and Co abundances: from less than 5 to 31 µg/g for Co, and from 4 µg/g to 173 µg/g for Ni. These abundances are not correlated with MgO content, or any differentiation index. They are clearly related to small metal grains dispersed within the glasses. These concentrations make it possible to estimate the projectile contribution to the chemistry of the glasses. Assuming a chondritic composition, the projectile fractions never exceed 2 wt% in the glasses and hence contribute only a very negligible fraction to the major and trace element (apart from Ni and Co) inventories determined in this study.

CI-normalized REE patterns for the high-K impact spherules from NWA 1664 and NWA 1769 are presented in figure 5, and are in agreement with previous results [START_REF] Kurat | Major, minor and trace elements in some glasses from the NWA 1664 howardite[END_REF]. The glasses display a significant range of REE abundances (e.g., La abundances vary from 1.7 to 4.9 µg/g, Yb from 1 to 2.6 µg/g, Tables 4 and5). Their REE patterns are flat (La n /Yb n =1.05 -1.56) and display negative to positive Eu anomalies (Eu/Eu*=0.77 -1.17).

They are similar to the patterns displayed by the howardites and eucrites, both in abundances and shapes. Indeed, REE abundances are correlated with MgO concentrations (fig. 6), and the spherule analyses straddle the howardite and eucrite fields. An immediate interpretation could be that the composition of the spherules can be explained by mixing of eucritic and diogenitic components, as expected for a regolith from Vesta. Although a similar conclusion is apparently suggested by the behavior of most of the refractory trace elements we have analyzed (REE, Zr, Hf, Sr, Nb, Ta, Th, U, and Ti), the reality is certainly more complex. The Juvinas-normalized patterns of high-K spherules are unlike those of howardites and eucrites (fig. 7), and display very distinct Ba, Li, K, Rb, Cs enrichments and deep Na anomalies, features which are unknown in either howardites or eucrites.

Are trace element abundances in high-K spherules pristine?

Ba in unweathered eucrites, being an incompatible, refractory element behaves like a light REE and as a consequence, the Ba/La ratio in this lithology is nearly constant and close to 10. Because diogenites display very low Ba and light REE abundances (e.g., [START_REF] Fukuoka | Genesis of howardites, diogenites and eucrites[END_REF][START_REF] Palme | New data on lunar and achondrites and acomparaison of the least fractionated samples from the Earth, the Moon, and the eucrite parent body[END_REF][START_REF] Mittlefehldt | The genesis of diogenites and HED parent body petrogenesis[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF], the Ba/La ratio in howardites is controlled by their eucritic components, and consequently is also close to 10 (fig. 8). High-K spherules are Ba-rich and their Ba/La ratios range from 16 to 112. It could be argued that these unusual values are not pristine but are the result of terrestrial weathering. As pointed out above (section 3.1.1), Ba is a good indicator of hot-desert weathering, and many Saharan finds display high Ba concentrations. Indeed, [START_REF] Crozaz | The terrestrial alteration of Saharan shergottite Dar al Gani 476 and 489: a case study of weathering in a hot desert environment[END_REF] have shown that glasses, as exemplified by some impact melt pockets in the highly weathered shergottite Dar al Gani (DAG) 476/489, were prone to enrichment in Ba by hot desert secondary processes. Nevertheless, this interpretation is not relevant to this study because:

(1) the howardites NWA 1664 and NWA 1769 are relatively fresh, and certainly much less weathered than DAG 476/489;

(2) unlike the DAG 476/489 impact melts the glasses do not show traces of weathering; their Sr abundances and Th/U ratios, which are other possible weathering indicators (e.g., [START_REF] Barrat | Petrology and geochemistry of the unbrecciatted achondrite North West Africa 1240 (NWA 1240): an HED parent body impact melt[END_REF], are not anomalous;

(3) and finally, [START_REF] Boesenberg | The driest glass in the Solar System: glass spherules in howardites[END_REF] have tried to determine the H 2 O contents of two high-K spherules from NWA 1664 by Fourier transform infrared spectrometry (FTIR), and found no evidence of molecular H 2 O, or OH. Although the behavior of water in impact glasses during hot desert weathering has yet to be investigated, it would seem highly likely that any significant interaction with terrestrial fluids during weathering would result in the presence of detectable traces of water, which is not the case... High Li (29 to 93 µg/g), Rb (4.1 to 24.7 µg/g) and Cs (0.09 to 0.6 µg/g) abundances have been measured in high K spherules. As is the case for K (Barrat et al., 2009a), these high concentrations, are not affected by hot desert weathering. Li enrichments have yet to be observed in hot desert finds, and if some Rb can be introduced by alteration in hot desert meteorites, such a process could only account for a few tenths of a µg/g in the most weathered stones (e.g., DAG 476 [START_REF] Barrat | Bulk chemistry of Saharan shergottite Dar al Gani 476[END_REF], or Dhofar 019 [START_REF] Taylor | Martian meteorite Dhofar 019: A new shergottite[END_REF]) and certainly not for several µg/g. In contrast to other alkali elements and Ba, Na abundances in K-rich spherules are particularly low. Despite the large number of hot desert finds which have been studied in detail, some of which are strongly weathered, none show a depletion in Na abundance. Thus, these low abundances are certainly not a consequence of hot desert alteration.

We conclude that the remarkable patterns displayed by the high-K spherules found in NWA 1664 and NWA 1769 (fig. 7) are pristine features and not the result of hot desert secondary processes.

Origin of the unusual trace element patterns of the high-K spherules

The contrasting alkali concentrations seen in high-K spherules and HEDs might lead one to suggest that they originated from distinct parent bodies. Howardites often contain exotic fragments (e.g., Gounelle et al., 2003, and references therein), and NWA 1664 or NWA 1769 are no exceptions [START_REF] Lorenz | Foreign meteoritic material in howardites and polymict eucrites[END_REF]. Unfortunately, due to their relatively small size (less than 0.03 mg for a typical spherule 200 µm in diameter) oxygen isotope analysis of individual spherules by laser fluorination is currently not possible. Furthermore, the current levels of precision available by ion-probe analysis are insufficient to resolve the very small differences in 17 O that separate many achondrite groups (i.e. isotopically normal and anomalous eucrites, angrites, brachinites, winonaites, pallasites and mesosiderites). Despite the present lack of oxygen isotope information, many observations enable us to infer that the high-K spherules formed on the same parent body as HEDs:

-it is now well established that basaltic achondrites derived from a number of parent bodies (Yamaguchi et al., 2001;[START_REF] Scott | Oxygen isotopic constraints on the number and origin of basaltic achondrite parent bodies[END_REF][START_REF] Gounelle | A unique basaltic micrometeorite expands the inventory of solar system planetary crusts[END_REF]; the trace element abundances and the O isotopic compositions presented above confirm that NWA 1664 and NWA 1769 are true howardites, and are not related to the same parent body as one of the isotopically anomalous eucrites, such as NWA 011, Pasamonte or Ibitira; -the Fe/Mn ratios of the spherules are consistent with a link to the HEDs; moreover, the occurrence of an unmelted diogenitic remnant in a high K-spherule strongly strengthens this conclusion (Barrat et al., 2009a).

If the high-K spherules formed on Vesta, then two alternative formation scenarios need to be considered: firstly, they formed from normal HED targets but their compositions are not representative of their parent rocks, or have been modified by impact melting processes or during their flight; alternatively, they formed from targets whose compositions are unlike typical HEDs.

In terms of processes that might significantly alter the composition of spherules, alkali enrichment during incomplete melting of the target, and projectile contamination can be excluded (Barrat et al., 2009a), however, two opposing processes, evaporation and recondensation of volatile elements might be important.

In eucrites, Na abundances are controlled by plagioclase, and display the same behavior as Eu. Most eucrites, including the cumulate ones, have Na/Eu ratios around 5500 (when the concentrations are expressed in µg/g). Because of the low concentrations of both of these elements in diogenites, the eucritic fraction in howardites controls the Na and Eu budget.

Thus, howardites plot on the same Na/Eu trend as eucrites (fig. 9). A few eucrites, however, display lower Na/Eu ratios. For example, ALHA 81001 exhibits a Na/Eu ratio as low as 2530 [START_REF] Jerde | Composition and origin of Nuevo Laredo trend eucrites[END_REF][START_REF] Scott | Oxygen isotopic constraints on the number and origin of basaltic achondrite parent bodies[END_REF], but such very low ratios are exceptions and not representative of the whole HED suite. The high-K spherules generally have a Na/Eu ratio close to 1500. Taken at face value, this suggests that if the spherules were formed from HED targets, they have lost about 75 % of their Na through evaporation. If this inference is correct, elements more volatile or with the same volatility as Na, should have been affected in a similar manner. Instead, high-K spherules are not just significantly richer in K than any putative HED target, they have higher Rb and Cs values (fig. 7), which are much more volatile than Na (e.g., [START_REF] Gibson | Thermal volatilization studies on lunar samples[END_REF].

The discovery of the high Al, Si-poor (HASP) glasses in the Apollo 16 drill core have demonstrated that impacts can generate melts with bulk compositions very different to those of the melted targets, especially for the most volatile elements [START_REF] Naney | The Apollo 16 drill core: Statistical analysis of glass chemistry and the characterization of a high alumina-silica poor (HASP) glass[END_REF][START_REF] Papike | Trace-element fractionation by impact-induced volatilization: SIMS study of lunar HASP samples[END_REF]. Thus, it has often been assumed that the abundance of Na and other alkali elements in impact melts will not always closely match the composition of the target material (e.g., [START_REF] Delano | Glasses of impact origin from Apollo 11, 12, 15, and 16: evidence for fractional vaporization and mare/highland mixing[END_REF]. Whereas we do not dispute this argument, it is important to note that the behavior of alkalis during impact melting is often complex, and that significant losses of these elements do not follow a simple pattern, as shown by many examples of terrestrial (e.g., [START_REF] Humayun | Potassium isotopic composition of Australasian tektites[END_REF] or lunar impact glasses (e.g., [START_REF] Wentworth | Apollo 12 ropy glasses revisited[END_REF].

Moreover, in the case of low-K impact spherules found in other howardites, Na abundances are well accounted for by mixing of selected eucrite and diogenite end members, suggesting that there has been no significant loss of this element from the majority of such spherules (Barrat et al., 2009a). We have generated many chemical maps and profiles across high-K spherules in order to detect evidence of alkali evaporation. In a few cases only, rims exhibit K depletion suggesting that some loss has taken place (fig. 10). Where such loss has taken place, it is restricted to the outer 30 µm (or less) of the spherules, and in consequence has not been analyzed for trace elements during the course of this study. Barrat et al. (2009a) have shown that apart from these K-depleted rims, the K abundance in the glassy objects from howardites was not seriously affected by impact processes. The composition of the high-K glasses found in the howardites have been plotted on a Na 2 O/MgO vs. CaO/MgO plot (fig. 11). Mg and Ca are both refractory elements (e.g., [START_REF] Delano | Glasses of impact origin from Apollo 11, 12, 15, and 16: evidence for fractional vaporization and mare/highland mixing[END_REF]. Hence, the CaO/MgO ratio should not be significantly fractionated by impact melting. On the other hand, the Na 2 O/MgO ratio is highly sensitive to possible loss of Na during melting, or enrichment in Na via volatilisation/condensation processes. A selective Na-depletion should result in an erratic behavior of Na, and a strong decoupling of the refractory elements. A striking correlation for the NWA 1664 and NWA 1769 glasses is displayed on the diagram. This relationship suggests that Na has not been severely lost by the spherules during their formation. Similarly, this conclusion can be extended to the other alkalis.

It has been previously proposed that interaction between vapor and melt/glass can substantially enrich the latter in alkali metals. [START_REF] Delano | Apollo 14 high-Ti picritic glass: oxidation/reduction by condensation of alkali metals[END_REF] has shown that some high-Ti picritic spherules found in Apollo 14 soils display substantial Na and K enrichments, and he ascribed this enrichment to a possible condensation of alkalis from the associated volcanic gases. However, such a process is unlikely to have operated in the case of the high-K impact spherules. It is an acknowledged fact that HEDs are K-poor. If vapors had been produced during impact into such targets, they will not have contained significant amounts of K. Unlike lunar volcanic glass beads [START_REF] Meyer | The source of sublimates on the Apollo 15 green and Apollo 17 orange glass samples[END_REF], the impact spherules found in howardites are not coated by condensates rich in alkali elements. The depletion in alkalis displayed by the rims of some of them strongly argue against any addition of K during their flight or subsequent cooling (fig. 10, and Barrat et al., 2009a).

A more complex process has been suggested for the alkali-rich clasts found in the Krähenberg and Bohla LL-chondritic breccias [START_REF] Wlotzka | Alkali differentiation in LL-chondrites[END_REF]. These clasts display high enrichments of K (about 10 x), Rb (about 45 x) and Cs (about 70 x) relative to typical LL chondrites, while Na, Sr and Eu are strongly depleted (about 0.5 x), and the REE abundances are normal (except for a pronounced negative Eu anomaly). [START_REF] Wlotzka | Alkali differentiation in LL-chondrites[END_REF] proposed that these clasts formed from melts previously enriched in K, Rb, Cs, and that these features occurred as an exchange for Na in feldspars via a vapor phase. Although, these clasts share some unusual features with the high-K glasses found in howardites (e.g., high K/Na ratios), this mechanism is unlikely to be applicable to this study. Firstly, from an alkali content perspective, LL-chondrites and HEDs are hardly comparable. LL-chondrites contain twice as much Na as eucrites, and are thus much more prone to such exchange processes. Moreover, as pointed out by [START_REF] Wlotzka | Alkali differentiation in LL-chondrites[END_REF], this process does not severely affect the sum of Na + K atoms (Rb and Cs are insignificant here). Thus, a negative correlation between Na and the other alkalis should be observed. This is clearly not the situation for the high-K spherules: firstly, the most K-rich ones contain a much greater abundance of alkalis than even the most alkalis-rich potential HED target (e.g., the spherule A,2S found in NWA 1769 contain 3 x more alkali atoms than a eucrite); secondly, if this exchange process was operative, a strong negative correlation between Na and the other alkali elements would be expected; instead, Na abundances are positively correlated with Rb and other alkali abundances (fig. 12), ruling out this hypothesis.

Hence, the low-Na and high Li, K, Rb and Cs abundances displayed by the high-K impact spherules are not the fingerprints of complex processes that took place during the genesis of impact melts. They are more likely inherited from the targets. Indeed, the high Ba abundances and Ba/La ratios shown by all the high-K spherules cannot be achieved by impact processes, and strengthen this inference. An important question is the extent to which typical HED lithologies were present in the target material and how many end members are required to explain the data? In agreement with the previous study (Barrat et al., 2009a), our trace element data suggest the involvement of specific lithologies.

The occurrence of a diogenite remnant in a K-rich impact spherule indicates unambiguously that this lithology was present in the target(s) (Barrat et al., 2009a). 11). In order to evaluate the possible role of eucritic components, the analyses of the spherules have been compared to eucrites and howardites in a Na/Eu vs. Rb/La plot (fig. 13). (Note that the dispersion seen in this diagram is insensitive to the participation of diogenites because these rocks have very low contents of Na, Rb, La and Eu compared to any "basaltic" component). The high-K spherules define a well-developed trend clearly distinct from the known eucrites and howardites. Moreover, this relationship points to the participation of at least two end members in addition to diogenites: a component with very low Na/Eu and Rb/La ratios (component A) and a second one with a very high Rb/La ratio (component B). The trend strongly suggests that typical eucrites which display Na/Eu>4000 are not required to explain the composition of the spherules. However, it should be noted, that the number of analyzed spherules is still low, and probably not sufficient to conclude definitively that this kind of rock is missing from the target(s) materials.

Unfortunately, we can only speculate about the possible parent lithologies of these two components, and their connections with diogenites. The first component could be a type of eucrite with extremely low Na abundances. It is entirely possible that such rocks could be present on Vesta. As pointed out above, ALHA 81001 an unusual eucrite with a low Na concentration has been identified, although its Na/Eu ratio (about 2530, Warren and [START_REF] Jerde | Composition and origin of Nuevo Laredo trend eucrites[END_REF] is higher than most high-K spherules. The second component could be a very high-K lithology. Indeed, a high-K felsic glass has been found in a spherule from NWA 1664 (SiO 2 = 66.4 wt%, Al 2 O 3 =17.9 wt%, CaO=5.0 wt%, Na 2 O=0.56 wt%, K 2 O=5.8 wt%, Barrat et al., 2009a), but unfortunately was too small to be analyzed using our laser-ICP-MS procedure for trace elements. It should be noted that this Si-rich glass displays the same K 2 O/Na 2 O ratio (=10.4) as the spherule NWA1769A,2S which shows the highest K 2 O abundance (K 2 O/Na 2 O=10.6), and this convergence is surely not fortuitous. Although such a component was certainly present in the target(s), its composition cannot explain satisfactorily the range of chemical compositions of the spherules (e.g., the K 2 O abundances of the spherules are not correlated with the SiO 2 abundances), and requires a complex model. As an alternative, we suggest the following explanation. Assuming that lithologies of granitic composition are locally present in the crust, interactions between very-low Na basic melts (component A) and these "granitic" rocks are feasible, and can potentially generate melts with a "basaltic" composition and high Cs, Rb, K and Li abundances. A similar model has already been proposed for some lunar rocks. Small "granite" fragments have been recovered from lunar regoliths and breccias, and demonstrate that such rocks, although scarce, crop out on the Moon (see review in [START_REF] Papike | Lunar samples[END_REF]. Debris of very high K basalts has been found in Apollo 14 breccias. These rocks could be generated from low-K basalts contaminated by these "granites" [START_REF] Shervais | Very high potassium (VHK) basalts: complications in mare basalt petrogenesis[END_REF][START_REF] Neal | High alumina (HA) and very high potassium (VHK) basalt clasts from Apollo14 breccias, part 2: Whole rock chemistry: Further evidence for combined assimilation and fractional crystallization within the lunar crust[END_REF]. Interestingly, these very high K basalts are not only K rich and characterized by high K/Na ratios, but also like the lunar "granites" display high Rb abundances and high Ba/La ratios, a feature shared with the high K spherules found in the howardites. Interestingly, small amounts of a new high-K lithology have been found in the howardites NWA 1664 and NWA 1769 [START_REF] Lorenz | Two possible sources of potassium bearing inclusions in the NWA 1664 howardite. The 48-th Vernadsky/Brown Microsymposium on Comparative Planetology[END_REF]Barrat et al., 2009b), and these could be remnants of the assumed high-K lithology (component B). These small fine-grained clasts are Mg-poor and made of a pyroxenoid breakdown product (fayalitic olivine, hedenbergitic pyroxene, and silica), high-Ca pyroxene with exsolution lamellas, plagioclase, silica, hyalophane (K-Ba feldspar, with sometimes up to 10 wt% BaO), ilmenite, merrillite, apatite, zircon, baddeleyite, and troilite. Unfortunately, these rare clasts are very small (< 1 mm), and the studied surfaces cannot provide a correct estimate of their modes, nor of their chemical compositions. A more rigorous discussion of the origin of the Cs, Rb, K, Li, Ba enrichments and Na depletions shown by the high-K spherules must await the discovery of larger unmelted fragments of the target(s) ejected with the spherules.

Conclusions

The high-K impact spherules found in the NWA 1664 and NWA 1769 howardites are characterized by a high abundance of Cs, Rb, K, Li, and Ba, distinctive negative Na anomalies, while the REE and other refractory elements are unfractionated when normalized to the Juvinas eucrite (fig. 7). These remarkable patterns are unlike those of the host howardites, nor of other putative targets from Vesta. Because such features are pristine, and can hardly be the result of impact melting processes, we suggest that the high-K spherules formed on Vesta from unusual targets and were ballistically transported into their host breccias. The exact composition of the target is a matter of debate and cannot be fully constrained with the available data. They certainly contained a diogenitic component, but other end members are required: the minimum being a strongly-Na depleted eucrite-like component and a Cs, K, Rb, Li, and possibly Ba rich mafic component. Paradoxically, normal eucritic materials are not required to explain the compositional variation displayed by the Krich impact spherules.. It is commonly accepted that the HED collection gives a fairly representative picture of the rocks exposed on the surface of Vesta. However, while our HED meteorite collection is extensive (today it is on the order of 1000 different meteorites), it may be biased. Two lines of evidence point to this being the case. Firstly, cosmic-ray exposure ages for the HEDs suggest that all these meteorites are associated with only five impact events [START_REF] Eugster | Common break-up events of eucrites, diogenites, and howardites and cosmic-ray production rates for noble gases in achondrites[END_REF][START_REF] Welten | Cosmic-ray exposure ages of diogenites and the recent collisional history of the howardite, eucrite and diogenite parent body/bodies[END_REF], a number that is probably insufficient to sample all the geological diversity of the entire surface of the body. Secondly, despite the abundance of diogenites, and breccias that contain diogenitic debris (more than 40 % of the HED population), the parental melts of these orthopyroxenites have still not been recognized in the HED collection (e.g., [START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]. The high-K glasses found in some howardites which are formed from unusual targets, is a third line of evidence indicating that the lithological diversity of the Vesta's surface is much greater than commonly thought.

It is difficult to estimate the extent of the surface heterogeneity revealed by the K-rich impact spherules. K-rich glasses are now known from 5 unpaired breccias (LAP 04838, Luotolax, Macibini, Malvern, and NWA 1664/1769), and are not necessarily related to the same impact event. This suggests that the high-K areas on Vesta from which the glasses originated may be relatively extensive. The remote sensing of Vesta that will be performed from 2011 with the Dawn spacecraft, should allow the identification of chemically different areas on Vesta, and may help pinpoint the source regions of the K-rich impact glasses [START_REF] Russell | Dawn mission to Vesta and Ceres[END_REF].

Table 1. trace element compositions of international standards BIR-1G and BHVO-2G obtained by laser ICP-MS using a 44 µm-diameter laser beam (in µg/g). Literature data are from [START_REF] Jochum | Chemical characterisation of the USGS reference glasses GSA-1G, GSC-1G, GSD-1G, GSE-1G, BCR2-G, BHVO2-G, and BIR1-G using EPMA[END_REF] et al. (1998) and [START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]). et al. (1998) and [START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]). (1974), [START_REF] Desnoyers | The Malvern howardite: a petrological and chemical discussion[END_REF], [START_REF] Yagi | Mineralogical and petrological studies of the Yamato meteorites, Yamato-7301(j), -7305(k), -7308(l) and -7303(m) from Antarctica[END_REF], [START_REF] Hewins | Provenance of metal and melt rock textures in the Malvern howardite[END_REF], [START_REF] Klein | Origin of impact melt rocks in the Bununu howardite[END_REF], [START_REF] Noonan | Chondrules in the Kapoeta and Bununu howardites[END_REF], [START_REF] Delaney | Partial melt genesis for glassy clasts in basaltic achondrites[END_REF], [START_REF] Ikeda | Petrography and mineral compositions of the Yamato 7308 howardite[END_REF]), and Barrat et al., 2009). The fields for eucrites (E), cumulate eucrites (CE), diogenites (D), and howardites are drawn from a compilation of literature data (see references in [START_REF] Mittlefehldt | Non-chondritic meteorites from asteroidal bodies[END_REF] and [START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]). (1970), and [START_REF] Barrat | The differentiation of eucrites: the role of in-situ crystallization[END_REF][START_REF] Barrat | Petrology and geochemistry of the unbrecciatted achondrite North West Africa 1240 (NWA 1240): an HED parent body impact melt[END_REF][START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF]).

Figures captions

Furthermore

  , the involvement of a diogenitic-like component can adequately explain the spread of the data in the Yb vs. MgO diagram (fig. 6), or in the Na 2 O/MgO vs. CaO/MgO diagram (fig.

Fig. 1 .

 1 Fig. 1. Backscattered electron image of a glass spherule from the NWA 1664 howardite. Also shown are pits from two laser ICP-MS analyses.

Fig. 2 .

 2 Fig.2. REE patterns of the Saharan howardites and polymict eucrites. The reference chondrite is from[START_REF] Evensen | Rare Earth abundances in chondritic meteorites[END_REF].

Fig. 3 .

 3 Fig. 3. Juvinas normalized element patterns of the Saharan howardites. Data for Juvinas, Nuevo Laredo and Stannern are from[START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF].

Fig. 4 .

 4 Fig. 4. (a) Oxygen isotope variation in treated and untreated howardites and polymict eucrites (this study) shown in relation to the HED, angrite and Main-Group pallasite data of Greenwood et al. (2005, 2006). D = diogenites. E = eucrites. H = howardites. TFL = terrestrial fractionation line. EFL = eucrite fractionation line ( 17 O = -0.239±0.007(1 ) (Greenwood et al. (2005). (b) Oxygen isotope variation in untreated and acid leached howardites (this study). Untreated samples form a cluster centred on the EFL. Acid leached samples are shifted to higher 18 O values indicating preferential removal of a low 18 O phase.

Fig. 5 .

 5 Fig. 5. REE patterns of the high-K impact spherules found in howardites NWA 1664 and NWA 1769. The reference chondrite is from[START_REF] Evensen | Rare Earth abundances in chondritic meteorites[END_REF].

Fig. 6 .

 6 Fig.6. Yb (µg/g) vs. MgO (wt%) plot for high K impact glasses from howardites NWA 1664 and NWA 1769. The fields for eucrites (E), cumulate eucrites (CE), diogenites (D), and howardites (H) are drawn from a compilation of literature data (see references in[START_REF] Mittlefehldt | Non-chondritic meteorites from asteroidal bodies[END_REF] and[START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF][START_REF] Barrat | Geochemistry of diogenites : Still more diversity in their parental melts[END_REF]).

Fig. 7 .

 7 Fig. 7. Juvinas normalized element patterns of the high-K impact spherules found in howardites NWA 1664 and NWA 1769. Data for Juvinas are from[START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF].

Fig. 8 .

 8 Fig. 8. Ba vs. La plot for high K impact glasses from howardites NWA 1664 and NWA 1769. The fields for unweathered eucrites (E), cumulate eucrites (CE), diogenites (D), and

Fig. 9 .

 9 Fig. 9. Na 2 O (wt%) vs. Eu (µg/g) plot for high K impact glasses from howardites NWA 1664 and NWA 1769. The fields for eucrites (E), cumulate eucrites (CE), diogenites (D), and howardites (H) are drawn from a compilation of literature data (see references in Mittlefehldt

Fig. 10 .

 10 Fig.10. Backscattered electron image and maps of Na, K, S and P of a spherule found in NWA 1769 that contains a rim devoid of K and a coating made of fine sulfide and phosphate grains.

Fig. 11 .

 11 Fig. 11. Na 2 O/MgO vs. CaO/ MgO (wt%/wt%) for impact glasses from howardites (Noonan

Fig. 12 .

 12 Fig. 12. Li, Cs, K, Na vs. Rb plots for high K impact glasses from NWA 1664 and NWA 1769. The fields for HED are drawn from a compilation of literature data (mainly Tera et al.

Fig. 13 .

 13 Fig. 13. Na/Eu vs. Rb/La (concentrations expressed in µg/g) plot for high K impact glasses from NWA 1664 and NWA 1769. The fields for howardites and eucrites are drawn from a compilation of literature data (see references in figure11).

Table 2 .

 2 excepted Li in BHVO-2 from[START_REF] Barrat | The Stannern trend eucrites: contamination of main group eucritic magmas by crustal partial melts[END_REF] Minor and trace element compositions of six Saharan howardites (*) and polymict eucrites (oxides in wt%, trace elements in µg/g).

			BIR-1G			BHVO-2G
		n=4	RSD (%)	literature	n=4	RSD (%)	literature
	Li	-	-		5.53	2.8	4.7
	Ti	5874	(fixed)	5874	16663	(fixed)	16663
	Co	56.3	3.4	52.5	45.8	1.1	43
	Ni	192	0.7	179	132	2.0	112
	Rb	0.31	15.4	0.217	9.58	2.7	9.183
	Sr	115	3.6	110	412	1.4	396
	Y	15.22	1.5	16	25.42	2.8	29.2
	Zr	14.54	4.0	17.8	173	2.3	172
	Nb	0.57	4.0	0.535	19.10	1.9	18.2
	Cs	0.01	9.9	0.005	0.12	4.0	0.102
	Ba	6.86	4.7	6.8	138	1.9	132
	La	0.63	5.4	0.607	15.70	2.5	15.25
	Ce	2.02	5.8	1.88	39.69	2.2	37.67
	Pr	0.38	5.8	0.376	5.38	2.4	5.33
	Nd	2.53	4.4	2.4	25.30	2.6	24.55
	Sm	1.14	3.6	1.09	6.26	2.2	6.119
	Eu	0.54	3.6	0.524	2.10	2.6	2.077
	Gd	1.84	2.9	1.85	6.03	1.8	6.125
	Dy	2.49	3.3	2.64	5.35	2.4	5.281
	Er	1.63	3.2	1.74	2.52	4.0	2.566
	Yb	1.63	1.9	1.67	1.99	2.4	1.995
	Lu	0.23	1.8	0.248	0.27	3.7	0.2766
	Hf	0.52	3.4	0.62	4.03	2.9	4.22
	Ta	0.04	11.1	0.038	1.20	3.4	1.08
	Th	0.03	8.9	0.03	1.20	2.4	1.14
	U	0.02	18.5	0.028	0.45	3.4	0.403
	(La/Sm)n	0.35	2.3	0.35	1.58	1.3	1.57
	(La/Yb)n	0.26	3.7	0.25	5.33	1.2	5.16
	Eu/Eu*	1.14	0.5	1.13	1.05	1.0	1.04
	Ba/La	11.0	0.8	11.2	8.80	1.0	8.66

Table 3 .

 3 Oxygen isotopic compositions of six Saharan howardites (*) and polymict eucrites.

	SAMPLE	n	17 O‰		18 O‰		17 O‰
	UNTREATED SAMPLES
	NWA 1664*	1	1.629		3.596		-0.256
	NWA 1769*	1	1.689		3.649		-0.224
	NWA 5306*	2	1.571	0.065	3.494	0.085	-0.260	0.021
	NWA 5614*	2	1.720	0.005	3.712	0.012	-0.226	0.011
	NWA 5616	1	1.661		3.614		-0.233
	NWA 5618	1	1.679		3.678		-0.249
	ACID LEACHED SAMPLES (6M HCl)
	NWA 1664*	1	1.753		3.819		-0.249
	NWA 1769*	1	1.762		3.832		-0.246
	NWA 5306*	2	1.644	0.019	3.600	0.040	-0.243	0.002
	NWA 5614*	2	1.795	0.074	3.882	0.131	-0.240	0.006
	NWA 5616	1	1.889		4.072		-0.245
	NWA 5618	1	1.690		3.715		-0.257

Table 4 .

 4 Major and trace element compositions of 8 high-K spherules from howardite NWA 1664 (oxides in wt%, trace elements in µg/g).

		NWA1664 NWA1664 NWA1664 NWA1664 NWA1664 NWA1664 NWA1664 NWA1664
		P,1S	P,3S	B1, 1S	B4,2S	B5,1S	B5,2S	B5,3S	B6,1S
	TiO2	0.60	0.33	0.73	0.58	0.54	0.65	0.57	0.40
	MgO	10.61	16.39	8.34	10.28	11.75	8.93	13.18	16.89
	Na2O	0.09	0.10	0.20	0.14	0.09	0.18	0.08	0.14
	K2O	0.75	0.76	1.41	0.54	0.93	0.78	0.46	1.06
	Li	35	43	81	45	43	50	39	69
	Co	9.3	<5	<5	27.6	11.2	28.9	13.2	<5
	Ni	39	12	4	111	54	108	39	4
	Rb	7.55	5.36	14.66	10.63	7.70	9.65	5.46	12.28
	Sr	54.7	54.0	120	84.2	63.9	81.6	67.6	60.1
	Y	15.10	9.68	19.90	16.05	15.72	16.10	15.99	11.00
	Zr	43.7	29.8	56.7	41.8	47.8	47.2	50.6	35.4
	Nb	4.66	2.06	4.42	3.41	3.71	3.87	3.81	2.74
	Cs	0.18	0.12	0.25	0.28	0.18	0.23	0.09	0.14
	Ba	300	163	217	156	179	189	82.7	64.0
	La	2.67	1.74	3.55	2.72	2.84	2.58	3.14	2.19
	Ce	6.55	4.49	9.59	7.38	6.98	6.39	8.02	5.91
	Pr	0.95	0.67	1.36	1.04	1.04	0.94	1.16	0.82
	Nd	5.14	3.47	6.86	5.33	5.50	5.13	6.10	4.28
	Sm	1.65	1.14	2.23	1.72	1.72	1.66	1.89	1.31
	Eu	0.52	0.49	0.97	0.63	0.59	0.70	0.67	0.45
	Gd	2.20	1.45	2.92	2.24	2.33	2.26	2.40	1.65
	Dy	2.64	1.77	3.45	2.67	2.73	2.74	2.85	1.91
	Er	1.54	1.07	2.17	1.67	1.62	1.70	1.69	1.15
	Yb	1.41	0.99	2.07	1.64	1.50	1.66	1.60	1.10
	Lu	0.21	0.15	0.30	0.23	0.23	0.24	0.23	0.16
	Hf	1.01	0.78	1.51	1.05	1.18	1.20	1.25	0.85
	Ta	0.22	0.11	0.22	0.15	0.20	0.21	0.20	0.13
	Th	0.32	0.24	0.43	0.30	0.36	0.33	0.41	0.27
	U	0.09	0.06	0.16	0.11	0.10	0.09	0.11	0.08
	(La/Sm)n	1.02	0.96	1.00	0.99	1.04	0.98	1.05	1.05
	(La/Yb)n	1.28	1.19	1.16	1.12	1.28	1.05	1.33	1.34
	Eu/Eu*	0.84	1.16	1.17	0.98	0.90	1.10	0.96	0.93
	Ba/La	112.16	93.42	61.07	57.39	62.85	73.12	26.36	29.17

Table 5 .

 5 Major and trace element compositions of 9 high-K spherules from howardite NWA 1769 (oxides in wt%, trace elements in µg/g).

		NWA1769 NWA1769 NWA1769 NWA1769 NWA1769 NWA1769 NWA1769 NWA1769 NWA1769
		A,1S	A,2S	A,3S	A,4S	A, 5S	A, 6S	A, 8S	C, 1S	C, 2S
	TiO2	0.63	0.46	0.50	0.63	0.55	0.67	0.91	0.56	0.62
	MgO	11.63	15.43	16.03	11.84	13.11	8.45	5.95	12.99	9.53
	Na2O	0.14	0.22	0.05	0.29	0.10	0.20	0.27	0.11	0.16
	K2O	0.28	2.33	0.37	1.79	0.65	1.48	0.60	0.78	1.21
	Li	33	81	29	93	37	73	58	51	62
	Co	23.5	<5	30.7	16.0	15.7	9.8	<5	16.0	15.7
	Ni	62	4	173	12	55	72	17	82	51
	Rb	5.03	24.68	4.10	16.89	6.25	15.22	17.60	8.10	10.38
	Sr	76.8	70.3	56.6	102.4	65.4	98.6	133.0	61.0	71.0
	Y	17.55	13.02	14.84	16.66	16.06	19.24	26.44	14.02	15.69
	Zr	49.55	42.41	47.19	50.51	52.50	55.39	73.14	44.57	53.01
	Nb	3.87	3.35	3.91	4.14	4.42	4.24	5.66	3.76	4.55
	Cs	0.11	0.60	0.09	-	0.12	0.28	0.47	0.18	0.27
	Ba	92.4	205	66.3	53.6	87.3	139	171	118	131
	La	3.13	2.79	3.09	3.22	3.43	3.49	4.94	2.96	3.47
	Ce	8.51	7.46	8.48	8.98	9.39	9.41	13.33	8.15	9.81
	Pr	1.18	1.03	1.14	1.23	1.28	1.28	1.85	1.14	1.32
	Nd	6.07	5.21	5.81	6.25	6.53	6.62	9.46	5.85	6.95
	Sm	1.90	1.46	1.74	1.93	1.92	2.07	2.95	1.79	1.99
	Eu	0.71	0.43	0.53	0.65	0.58	0.77	0.99	0.57	0.60
	Gd	2.50	2.02	2.21	2.45	2.45	2.73	3.80	2.18	2.44
	Dy	3.01	2.27	2.54	2.87	2.80	3.26	4.47	2.60	2.76
	Er	1.84	1.37	1.53	1.74	1.66	1.98	2.69	1.51	1.67
	Yb	1.80	1.21	1.44	1.71	1.58	1.91	2.57	1.46	1.52
	Lu	0.25	0.17	0.20	0.24	0.22	0.27	0.36	0.20	0.23
	Hf	1.18	0.98	1.09	1.21	1.22	1.32	1.72	1.12	1.23
	Ta	0.17	0.16	0.18	0.19	0.20	0.20	0.25	0.19	0.21
	Th	0.33	0.31	0.34	0.36	0.39	0.38	0.51	0.38	0.43
	U	0.10	0.10	0.09	0.09	0.10	0.13	0.16	0.11	0.15
	(La/Sm)n	1.03	1.20	1.12	1.05	1.13	1.06	1.06	1.04	1.10
	(La/Yb)n	1.17	1.56	1.45	1.27	1.46	1.23	1.30	1.37	1.54
	Eu/Eu*	0.99	0.77	0.82	0.92	0.82	0.99	0.91	0.88	0.83
	Ba/La	29.55	73.52	21.44	16.61	25.43	39.96	34.57	39.79	37.89

Acknowledgements

The samples analyzed during the course of this study were kindly provided by the Museum National d'Histoire Naturelle de Paris, Ahmed Pani, Philippe Thomas, Frederic Béroud and Ali Hmani. We thank Christian Koeberl for the editorial handling, Abhijit Basu and David Mittlefehldt for constructive comments, Philippe Gillet for fruitful discussions, and Pascale Barrat for her help. Gilles Chazot and Arnaud Agranier are thanked for their help with laser analyses. Jenny Gibson is thanked for her help with oxygen isotope analyses. We gratefully acknowledge the Programme National de Planétologie (CNRS-INSU) for financial support. IAF and RCG acknowledge support from STFC through a Rolling Grant to PSSRI. This research has made use of NASA's Astrophysics Data System Abstract Service.

Open Research Online

The Open University's repository of research publications and other research outputs Trace element geochemistry of K-rich impact spherules from howardites Journal Item

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Suresh Gatla

Daniel Aubert

G D'agostini

Olivier Mathon

Sakura Pascarelli

Thomas Lunkenbein

Marc Georg Willinger

Helena Kaper

Giovanni Agostini

Room-Temperature CO Oxidation Catalyst: Low-Temperature Metal-Support Interaction between Platinum Nanoparticles and Nanosized Ceria

Keywords: CO oxidation, Metal-support interaction, in-situ x-ray absorption spectroscopy, platinum cerium oxide catalyst, heterogeneous catalysis

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Introduction

Catalysts operating effectively at room temperature are a challenging class of materials covering applications as varied as the cold start of engines to indoor air quality. Room temperature activity is most often reported for CO or formaldehyde oxidation. [1][2][3][4] Most non-noble metal systems, such as Hopcalites (copper manganese oxides) 5,6 or Co3O4 7 lack stability with time and/or in presence of moisture. More stable systems are often based on platinum nanoparticles deposited on reducible supports, 2,3,8 demonstrating the importance of tuning both, the metallic phase and the support material.

In general, the catalytic performance of an active phase can be strongly influenced by interaction with the support material. As early as the seventies, Tauster reported on the so-called SMSI (strong metal-support interaction) for noble metals supported onto TiO2, 9 however, this important effect is observed at higher temperatures. Recently, attention has focused on the local environment between the active species and the support [10][11][12][13] , e.g. on the interactions of support atoms in close contact to the active phase. The group of Illas reported on platinum-cerium oxide interactions on the nanoscale. 14,15 The authors observe two main characteristics of this catalyst: an electron transfer from platinum to ceria and an oxygen transfer from ceria to platinum at the interface between nanosized ceria and platinum nanoparticles.

The application of ceria in catalysis has been recently reviewed by Montini et al. 16 Among the vast literature on ceria, nanosized ceria plays an important role as catalyst support due to the increased amount of surface oxygen defects as compared to microcrystalline ceria. [17][18][19] The role of ceria supports and in peculiar oxygen ion vacancies has been described for numerous reactions, such as CO oxidation, 4,[20][21][22] water-gas shift reaction, 17,[23][24][START_REF] Fu | Flytzani-stephanopoulos[END_REF][26] NOx abatement 27,28 preferential oxidation of CO (PROX). 29,30 Next to the acknowledged effect of oxygen ion vacancies on the catalytic performance, also the presence of noble metals can change the ceria surface properties. It has been shown more than thirty years ago that the Ce-O bond is weakened by the presence of metals and thus renders the surface more reducible. 31 This concept usually lowers the reaction temperature. However, while the effect is wellestablished, the behavior of this system at low temperature (close to room temperature) has rarely been studied using operando characterization techniques. One recent example is the work of Ke et al. on PtOx clusters deposited onto CeO2 nanowires. 22 In their work, the authors show strong interaction between the PtOx clusters and the ceria (110) surface using in-situ spectroscopic techniques coupled with DFT calculation. The catalytic performance of the PtOx clusters is strongly influenced by oxygen coordination around platinum: a too high coordination and thus stabilization of the Pt inactivates the nanoclusters. The present study focusses on platinum nanoparticles deposited on nanosized ceria active at room temperature. Insights into the working principles of these Pt/CeO2 catalysts during low temperature CO oxidation are reported using HAADF-STEM analysis and operando XAFS measurements combined with simulation.

Results and Discussion

Figure 1 shows CO oxidation light-off curves for as-prepared and in-situ reduced 2.5 wt-%Pt/CeO2 samples. While oxidation takes place only at relatively high temperature for the as-prepared samples, in-situ reduced samples are already active at room temperature (10 % conversion of CO, e.g. 600 ppm). Upon cycling, room temperature conversion of CO diminishes, but the catalyst remains active and stabilizes after the third cycle. In comparison to recent advances in Pt-based CO catalysts, the present catalyst works at lower platinum loading (2.5 wt-% as compared to 12 wt-% loading) 3 and converts more CO (600 ppm at room temperature compared to 100 ppm for a Pt/TiO2 system). 32 This result is somewhat surprising considering the simplicity of the system: we use pure cerium oxide without additional dopants, often introduced to improve the catalytic activity due to the creation of oxygen ion vacancies.

The Brunauer-Emmett-Teller surface area decreases from 97 m 2 /g (CeO2 support) to 82 m 2 /g upon impregnation (see Table S1). After hydrogen treatment, the specific surface area remains stable (81 m 2 /g), thus showing that the catalytic performance is related to structural changes of Pt and not the surface area. The X-ray diffraction (XRD) patterns contain solely diffraction peaks of the fluorite phase CeO2, no other phases from platinum metal or platinum oxide are visible. The coherent scattering domain size of CeO2 as derived from Rietveld analysis is 9 nm for all samples (see Table S1). The structure of the catalyst can be best described by imaging the catalysts with electron microscopic techniques such as HRTEM and/or high angle annular darkfield -scanning TEM (HAADF-STEM). The corresponding micrographs of the Pt/CeO2 catalysts after reductive activation in hydrogen and after CO oxidation are shown in Figure 2. The HRTEM image of Figure 2a displays a highly crystalline ceria support, which is terminated by stepped surfaces. No amorphous layer was observed at the ceria surface. In combination with the HAADF-STEM image presented in Figure 2b, these images indicate that the Pt nanoparticles are well-dispersed over the ceria support and exhibit a particle size of around 1 nm. After reductive activation in hydrogen some of the ill-defined Pt nanoparticles (Figure 2c) spread along the ceria surface, after stimulation by the electron beam within the first 37 s (see supporting information movie M1). In addition, an increased lattice spacing (3.37 Å) of the (111) planes of ceria is observed. During the prevailing time of the catalytic reaction the (111) lattice spacing is reduced to 3.21 Å, which is within the error range of the expected (111) lattice spacing (3.18 Å) for ceria. After CO oxidation, some of the Pt nanoparticles are slightly increased, as seen by the HAADF-STEM images in Figure 2e and 2f. In addition, some atomic Pt clusters are observed. No spreading of the Pt nanoparticles along the surface of CeO2 was found even after the exposure to the electron beam for 302 s (Figure 2f, see supporting information movie M2). The different behavior under the electron beam of the reduced and spend samples suggests a structural modification of the platinum species during the catalytic test reaction. After hydrogen treatment and the catalytic testing, the Pt species are not covered by CeO2. However, the spreading of the reduced Pt nanoparticles on the ceria surface suggests the presence of the so called two dimensional strong metal support interaction as introduced by van Delft et al. 33 Mobility and spreading of noble metal nanoparticles on ceria due to strong particle-surface interaction has been described in the literature, [34][35][36] but to the best of our knowledge has not been described in the form of spreading under the electron beam. Further investigations under in-situ conditions will be needed to understand the role of the spreadable particles on the catalytic reactions, which is beyond the scope of the current work. However, since the spend catalyst does not spread, the initially present very small particles seem to be less stable. The structural changes observed might also explain the decrease in catalytic activity after the first cycle. In order to draw a more detailed picture of the electronic and structural properties of the platinum and cerium spe-cies during reduction and CO oxidation, we used operando XAFS coupled with mass spectrometry (MS). Details on the experimental setup (Figure S2), Pt L3 and Ce L3 XAFS measurements, in-situ treatment conditions, CO activity profiles (Figure S3) and data analysis procedures are summarized in the supporting information. The main observation from Ce L3 edge data (Figure S4) is that the amount of Ce 3+ increases from 15% to 28% during hydrogen treatment. The sample reoxidizes immediately under CO oxidation conditions, being in agreement with HRTEM observations. Presumably, the catalytic performance is not determined by the overall Ce 3+ concentration, but by Ce 3+ located in vicinity to the noble metal, which is coherent with recent publication on the role of Ce 3+ in CO oxidation. 22,37,38 These active Ce 3+ -species are short lived. 38 Therefore, the concentration and its role cannot be observed using our technique and is beyond the scope of this work. Significant changes during reduction and CO oxidation are observed in the Pt L3 EXAFS spectra. Figure 3 shows the amplitude of the Fourier Transformation (FT) EXAFS spectra of the as-prepared and in-situ reduced samples together with the best fit and the contributions from Pt-O and Pt-Pt. S2, S3) shows that the local environment of Pt in the as-prepared sample is very similar to that in platinum oxide (only oxygen nearest neighbors), with a Pt-O bond distance of 2.01 ± 0.01 Å (PtO2: 2.00 ± 0.01 Å). The main difference between our sample and PtO2 is in the coordination number, which is 2.9± 0.4 instead of 4.8± 0.6 for PtO2, This lower coordination number indicates a higher dispersion of platinum particles on the surface of the cerium oxide support. After insitu reduction, the local environment of platinum is composed of both oxygen and platinum atoms (Figure 3b and Table S3). The Pt-O bond distance is considerably longer than the Pt-O bond distance in PtO2 (2.10 ± 0.05 Å compared to 2.00 ± 0.01 Å in PtO2), while the Pt-Pt bond distance is shorter than the Pt-Pt bond in Pt metal (2.72 ± 0.01 Å compared to 2.76 ± 0.002 Å in Pt metal). This shorter Pt-Pt bond distance compared to the metal can be explained by rehybridization of the spd orbitals in metal nanoparticles, 39 which leads to an increase in the local electron density between two adjacent metal atoms and consequently to a smaller bond distance. [40][41][42] The effect is more pronounced for smaller particles. Upon increasing the reaction temperature, the Pt-Pt metal bond distance changes slightly to 2.72 ± 0.01 Å (50°C), 2.70 ± 0.04 Å (100°C) and 2.69 ± 0.03 Å (150°C). The elongated Pt-O bond distance of 2.10 Å after in-situ reduction remains stable during CO oxidation at 50 °C. At higher reaction temperatures, the Pt-O bond distance shifts again to lower values that are comparable to those in platinum oxide: 2.03 ± 0.03 Å at 100 °C and 2.01 ± 0.04 Å at 150 °C. The contraction of this bond with temperature is consistent with Pt L3 XANES spectral shapes (Figure S5), which also resemble more those of the oxide phase (increasing white line) with increasing temperature, indicating reoxidation of the platinum species on the surface under reaction conditions at higher temperature.

EXAFS analysis (Table

In order to interpret the anomalously large Pt-O bond distance of 2.10 Å in the in-situ reduced sample at ambient temperature, we performed full multiple scattering ab initio simulations of the Pt L3 XANES spectra corresponding to different local atomic configurations around Pt (see supporting information for details). We choose interaction of the platinum species with the {111} facet, since this orientation is most dominant in ceria nanoparticles. 43 Best results were obtained for the atomic configuration shown in Figure 4 depicting a Pt4 cluster lying on top of the CeO2(111) surface, with the three base Pt atoms bonded to oxygen atoms at a distance of 2.10 Å. The PtO model was used to simulate the spectrum relative to the as-prepared sample, where Pt-O distance is 2.01 Å. Figure 4 compares the experimental and simulated XANES spectra of the as-prepared, the in-situ reduced 2.5 wt-%Pt/CeO2 catalysts and the PtO2 reference. The simulations reproduce well the main features of the spectra. This is a strong indication that the local atomic configuration around Pt is close to that of the chosen models. Vaarkamp et.al observed such elongated Pt-O bonds, as noticed in present in-situ reduced sample, in a Pt/H-LTL zeolite catalyst after treatment at 500 °C in H2. 44 The authors propose this distance as a characteristic of zero valent Pt in contact with oxygen from the support. Concerning the Pt/CeO2 system, interaction between platinum metal and ceriumoxide via the formation of Pt-O-Ce bonds has been discussed in the literature as favorable for the catalytic activity, 45,46 but to the best of our knowledge, has not been reported under operando conditions using EXAFS. In this context, our direct observation also sheds light on the oxidation state of platinum on cerium oxide. It has been observed that oxidized noble metal species can indeed be more active than the completely reduced particle. [START_REF] Fu | Flytzani-stephanopoulos[END_REF]46 In the case of strong interaction with the support via oxygen bonds, the noble metal species is naturally in an oxidized state. DFT calculations show that the oxygen between a cerium atom and platinum is situated in a rather unusual configuration: the energy to withdraw this oxygen is strongly reduced, which might allow the participation of this oxygen in the reaction and thus encounter for the low temperature activity. 45,46 Tang et al. propose this mechanism for the oxidation of methane. 46 The calculated Pt-O bond distance in Ce-O-Pt linkage using DFT is 2.12 Å, i.e. quite similar to the bond distance of 2.10 ± 0.05 Å that we observe. Our catalyst might be in a similar favorable situation as described by DFT calculations, namely with easily removable oxygen shared by ceria and platinum.

Conclusion

We have shown that room temperature activity of Pt/CeO2 catalysts can be achieved by a hydrogen pretreatment of nanosized platinum species. HAADF-STEM analysis clearly show the presence of well-dispersed nanoparticles that are not covered by ceria after hydrogen treatment, as it would be the case in a classic SMSI situation. The reduced platinum particles spread under the electron beam, suggesting strong interaction with the support. After the first catalytic cycle, no spreading is observed, which in in coherence with loss in catalytic activity after the first cycle. In-situ EXAFS show that the platinum particles are not fully reduced in the active state. The active catalyst is characterized by an elongated Pt-O bond distance of 2.10 ± 0.05 Å. Combination of EXAFS analyses and FEFF simulation suggests that the low temperature catalytic activity can be related to platinum species-ceria interaction via these elongated Pt-O bonds. Elongated Pt-O-Ce bond distances between platinum particles and ceria have been discussed in the literature. To the best of our knowledge, this is the first time that the bond distance of 2.10 Å has been observed in-situ. The present study contributes thus to the general understanding of support-catalyst interaction at low temperature, which opens the field for optimizing catalysts active in this temperature regime.

ASSOCIATED CONTENT

Supporting Information. The supporting information contains information on materials preparation, characterization, catalytic test, X-Ray diffraction, HAADF-STEM analysis and movies of the Pt particles under the electron beam, XASexperimental set-up, XAS spectra of Pt-edge and Ce-edge, and details on the FEFF9 simulation. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 1 .

 1 Figure 1. Light-off curves for CO oxidation of the as-prepared, in-situ reduced and cycled 2.5 wt-%Pt/CeO2 catalysts.

Figure 2 .

 2 Figure 2. (S)TEM images of Pt/CeO2 catalysts. a) HRTEM image of Pt nanoparticles supported on ceria. The arrow in a) points to a Pt nanoparticle spread on the surface of ceria. The inset shows the power spectrum of the ceria support. b) to d) HAADF-STEM images of Pt/CeO2 catalyst after reductive activation in H2 at different magnifications. c) Atomic resolution HAADF-STEM image and (d) corresponding HAADF-STEM image after exposure to the electron beam for 37 s. e) HAADF-STEM image after CO oxidation and f) HAADF-STEM image of the same position after exposure to the electron beam for 302 s. The ceria support is highlighted in blue, whereas the Pt nanoparticles are depicted in red. The original images are given in the supporting information, Figure S2.

Figure 3 .

 3 Figure 3. FT of EXAFS spectra of a) as-prepared and b) in-situ reduced 2.5 wt-% Pt/CeO2 catalysts together with the best fits and the ccontributions of Pt-O, Pt-Pt.

Figure 4 .

 4 Figure 4. Experimental PtL3-edge XANES patterns of the as-prepared as well as in-situ reduced 2.5wt-%/CeO2 (left side) and the simulated ones with FEFF9 code (right side) along with the proposed model structures (middle panel).

AUTHOR INFORMATION

Corresponding Author *E-mail:helena.kaper@saint-gobain.com *E-mail: sakura@esrf.fr

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.