Cédric Hamelin

Laure Dosso

Barry B Hanan

Manuel Moreira

Andrew Kositsky

Marion Y Thomas

Geochemical portray of the Pacific Ridge: New isotopic data and statistical techniques

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Introduction

Mid-oceanic ridge basalts (MORB) are the result of continuous melting of the ambient upper mantle beneath oceanic ridges. The MORB-source mantle is generally thought to have been depleted ~2 Gy ago by extraction of the continents. Although the range of geochemical variations in oceanic basalts is mostly attributed to the influence of Ocean Island Basalt (OIB), significant geochemical heterogeneity in MORB has been recognized (Hoffman et al., 2003 and reference therein;[START_REF] Rudge | A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt[END_REF]. This heterogeneity has been revealed using radiogenic isotope ratios (Sr, Nd, Pb and Hf) in areas devoid of plume influence. Numerous studies have attempted to model the mixing relationship between enriched and depleted domains within the mantle to reproduce the range of isotopic variations observed in oceanic basalts [START_REF] Albarède | Radiogenic ingrowth in systems with multiple reservoirs: applications to the differentiation of the mantle-crust system[END_REF][START_REF] Meibom | The statistical upper mantle assemblage[END_REF][START_REF] Rudge | A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt[END_REF][START_REF] Kellogg | Modeling lead isotopic heterogeneity in mid-ocean ridge basalts[END_REF]. The range of Sr, Nd and Pb isotopic compositions depends not only on the endmember compositions but also on the volume of mantle sampled during the melting relative to the length scale of heterogeneities [START_REF] Kellogg | Modeling lead isotopic heterogeneity in mid-ocean ridge basalts[END_REF]. Therefore, the range of MORB geochemical variations reflects the size, the spatial distribution, and the difference of fusibility of heterogeneities within the upper mantle. In addition to this intrinsic heterogeneity of the MORB mantle, radiogenic isotope studies have led to the definition of distinct broad mantle isotopic domains, such as the archetypal DUPAL anomaly located in the south hemisphere [START_REF] Dupré | Pb-Sr isotope variation in Indian Ocean basalts and mixing phenomena[END_REF]Castillo, 1988). Boundaries between these domains can be (i) extremely sharp as seen at the Antarctic Australian-Antarctic Discordance (ADD), with isotopic ratios (Sr, Nd, Pb and Hf) abruptly changing from Indian to Pacific values [START_REF] Pyle | Resolving an isotopic boundary within the Australian-Antarctic Discordance[END_REF][START_REF] Hanan | Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge[END_REF][START_REF] Meyzen | Isotopic portrayal of the Earth/'s upper mantle flow field[END_REF][START_REF] Cooper | Distribution of recycled crust within the upper mantle: Insights from the oxygen isotope composition of MORB from the Australian-Antarctic Discordance[END_REF] or (ii) more gradual as seen in the transition from Southwest Indian Ridge to South Atlantic MORB [START_REF] Meyzen | New insights into the origin and distribution of the DUPAL isotope anomaly in the Indian Ocean mantle from MORB of the Southwest Indian Ridge[END_REF].

Another example of these large geochemical provinces is given by the two sub-pacific mantle domains [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. The distinctive isotopic properties of these large-scale geochemical domains suggest a long-term isolation of these mantle provinces, each with their own convective histories involving various amounts of melting residues and recycled components. On the basis of an incomplete dataset, a boundary between these two pacific provinces has been located at the latitude of the Easter Island microplate [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. But in order to have a complete geochemical view of the southern Pacific Ridge from 66 to 10°S, it was necessary to fill the sampling gap between 53°S and 41°S. This became one of the main objectives of the PACANTARCTIC2 cruise which took place in 2004-05 [START_REF] Dosso | The Pacific-Antarctic Ridge between 41°15'S and 52°45'S: Survey and sampling during the PACANTARCTIC 2 cruise[END_REF][START_REF] Klingelhoefer | New structural and geochemical observations from the Pacific-Antarctic Ridge 52°45'S and 41°15[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF]. Analyses of Sr, Nd, Pb, Hf and He isotopic compositions of these new samples from the Pacific Antarctic Ridge (PAR) are compiled here together with published data from 66°S to 53°S and from the East Pacific Rise (EPR). Therefore it becomes possible to have a picture of the geochemical variations along the entire Pacific Ridge from 66°S to the 10°S. Adapting a Principal Component Analysis (PCA) to incomplete datasets, we show a detailed portray of the geochemical variability of the Pacific Ridge.

Data selection and analytical methods.

The Pacific Ridge database from 10 to 66°S.

Along the Pacific Ridge, the Chile Triple Junction at 35°S/110°W separates the PAR from the EPR. The two pacific ridges show different geological settings: the full spreading rate increases along the PAR from 54 mm/yr at 65°S to 100 mm/yr at 35°S whereas it decreases along the EPR from 158 mm/yr at 35°S to 146 mm/yr at 10°S. In conjunction, the ridge axis morphology changes from a valley to a dome north of 60°S along the PAR [START_REF] Ondréas | Variations in axial morphology, segmentation, and seafloor roughness along the Pacific-Antarctic Ridge between 56°S and 66°S[END_REF], whereas the EPR is characterized by a uniform dome shaped morphology. Three plume-ridge interactions generate abnormal morphological structures along this section: Foundation (~38°S), Easter Island (~26°S) and 17°S. They are recognized by a high ridge cross-sectional area (Klingelhöfer et al., 2006) and the presence of intense off-axis volcanic activity. Compared to the EPR, the Pacific Antarctic Ridge still remains geochemically poorly known. A large portion of this plate boundary has been previously surveyed [START_REF] Lonsdale | Geomorphology and structural segmentation of the crest of the southern (Pacific-Antarctic) East Pacific Rise[END_REF][START_REF] Cande | Geophysics of the Pitman fracture zone and Pacific-Antarctic plate motions during the Cenozoic[END_REF] but the northern part has only recently been sampled [START_REF] Dosso | The Pacific-Antarctic Ridge between 41°15'S and 52°45'S: Survey and sampling during the PACANTARCTIC 2 cruise[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

To generate a coherent database along the pacific ridges, on-axis samples analyzed for one or more isotopic ratios (Sr-Nd-Pb-Hf-He) are selected from 66°S to 10°S using our new isotope data (table 1) completed with the petrological database of the Lamont-Doherty Earth Observatory (http://www.petdb.org). In most cases when the reference values of the standards are reported, the data are normalized to the values of NBS987, 0.71025 for Sr, of JNdi-1, 0.512104 and La Jolla, 0.511852 for Nd, NBS981, 206 Pb/ 204 Pb=16.9373, 207 Pb/ 204 Pb=15.4925, 208 Pb/ 204 Pb=36.7054, for Pb and JMC475, 0.282162 for Hf (see http://georem.mpchmainz.gwdg.de/ for details and references).:

Analytical methods.

Double Spike Pb analyses along the PAR

New high-resolution Pb analyses were carried out for samples collected during PACANTARCTIC1 (PAC1) and PACANTARCTIC2 (PAC2) cruises along PAR segments from 66°S to 56°S and from 53°S to 41°S respectively (Fig. 1, Table 1). Small chips from the inner part of the pillow lavas were handpicked to avoid altered surfaces that could be a potential source of Pb contamination. Powdered samples were leached with 6M HCl at 140°C

for an hour and then rinsed up to 6 times with ultrapure water prior to dissolution. Lead separation was then performed on an anionic exchange resin. Pb analyses were performed at Ifremer (Centre de Brest) on a Finnigan MAT 26x multicollector instrument (MAT261 upgraded by Spectromat), using the double spike technique with the calibrated Southampton-Brest 207/204 spike [START_REF] Ishizuka | Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes[END_REF]. Replicate analyses of the Pb isotope standard NBS981 gave an average of 16.9432±0.0027 and 15.5004±0.0029 and 36.7326±0.0086 for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, respectively. Pb blanks measured using this procedure were < 100 pg, and thus negligible relative to the amount of sample analyzed.

Hf measurements along the PAR

Hafnium isotopic compositions were analyzed along the PAC1 ridge segments at SDSU on splits from the same samples [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF]. Hf was separated using the protocol of Blichert-Toft et al. 1997 with a negligible blank of less than 25 pg. Hf isotope ratios were measured at SDSU using the Nu Plasma. The 176 Hf/ 177 Hf was normalized for mass fractionation relative to 179 Hf/ 177 Hf=0.7325. The JMC-475 Hf standard 176 Hf/ 177 Hf gave 0.282160 ±0.000010 (2σ) during this study. The standard was run alternately with samples to monitor machine performance.

He measurements along the PAR

Helium isotopic compositions have been measured on PAC1 and PAC2 samples at IPGP (Institut de Physique du Globe de Paris). Fresh pieces of glass were cleaned with distilled water, ethanol and acetone using an ultrasonic bath. Some samples were also cleaned with hydrogen peroxide in order to remove some Mg crust. Analytical procedure is identical to previous studies from our laboratory and can be found in [START_REF] Moreira | A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51-52°S)[END_REF]. Samples were crushed under vacuum with analytical blanks of 7±1 nccSTP 4 He. This corresponds to 0.02 to 0.4% of the samples. Helium concentrations and isotopic composition were measured using the ARESIBOII mass spectrometer [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF].

New results compared to published MORB data along this ridge section.

These results include samples from 66°S to 56°S (PAC1) and from 53°S to 41°S (PAC2) (Fig. 1). New high resolution Pb analyses as well as Hf and He analyses are presented here in Table 1. The Sr, Nd and Hf analyses on the same samples are found in [START_REF] Vlastélic | Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56°S and 66°S[END_REF] and [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

Binary isotopic correlations

With 7 isotopic ratios, the number of possible binary isotope diagrams is 21. As an example, we choose here to describe 4 such binary plots, selecting some of the most commonly discussed (Fig. 2A-D). In all diagrams, pacific MORB samples devoid of plume influence define linear correlations that in most cases have been previously described in the literature. This result is in good agreement with the expected coherence in behavior between Rb-Sr, U-Pb, Th-Pb, Sm-Nd and Lu-Hf isotopic systems compared to each other during magmatic processes. When plume-ridge interaction samples are included in our dataset, they define elongated fields overlapping PAR-EPR array except in Figure 2C where He-Ɛ Nd isotopic ratios define sub-parallel trends which emerge from the PAR-EPR array and point towards higher 3 He/ 4 He (R/Ra) ratios. More than 30 years of He isotopes systematic of oceanic basalts worldwide have shown that MORB samples are characterized by a narrow range of composition compared to OIB samples. However even within this limited range, our data show a linear correlation between 3 He/ 4 He (R/Ra) values and Ɛ Nd (Fig. 2C). The correlation between He and other isotopic systems will be discussed further below ( §4.4.2).

In contrast with the decoupling of Hf and Nd isotope compositions previously documented in MORB (e.g., [START_REF] Patchett | Hafnium isotope variations in oceanic basalts[END_REF][START_REF] Salters | Hf isotope constraints on mantle evolution[END_REF][START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF][START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF], a rather good correlation between these two isotopic systems is observed along our studied are (Fig. 2B). [START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF] have suggested that a distinctive behavior of Hf during disequilibrium melting along their studied ride area (Atlantic ridge 22-35°N) could explain a decoupled behavior of 176 Hf/ 177 Hf with respect to other isotopic ratios. The linear correlations observed in our dataset, show that the hypothesis of a specific behavior of Hf does not apply along the pacific ridges. Recently, good correlations between Hf and Nd isotopic ratios have been reported from other ridge segments such as Mohns Ridge [START_REF] Blichert-Toft | Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic[END_REF] and the entire mid-Atlantic Ridge [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. Helium isotopes do not fit this Sr-Pb/Hf-Nd coherent behavior. At the latitude of the Foundation-Ridge intersection (38°S), the bell-shaped He curve shows radiogenic 3 He/ 4 He ratios. But on the other hand, hot-spot influenced samples form negative "plume anomalies" with very unradiogenic signatures (high 3 He/ 4 He ratios, up to 12 R/Ra). The amplitudes of the PIB and the anomalies attributed to the plume effect are of the same order of magnitude for Nd, Hf and Pb. For He and Sr, the PIB has much smaller amplitude than the hotspot anomalies. Additionally, local MORB variability is expressed as spikes, which are likely related to transform faults (Eltanin System).

MORB variability along the south

Discussion

Statistical definition of mantle reference lines

In most binary plots of radiogenic isotopic ratios, representative points of mantle derived material define linear trends. Historically, these trends have been used to define reference lines such as the Northern Hemisphere Reference Line (NHRL) in Pb-Pb plot [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF] or the "Mantle Array" in Sr-Nd plot [START_REF] Depaolo | Sm-Nd age of the Stillwater complex and the mantle evolution curve for neodymium[END_REF] and the Nd-Hf plot [START_REF] Vervoort | Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time[END_REF]. These reference lines are arbitrary and depend on the abundance of data available at the time of their definition. However, the data colinearity in these plots justifies these convenient choices. The NHRL has been convenient to quantify the Dupal anomaly using the Δ7/4 and Δ8/4 [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF]. In 1999, new MORB data from the south Pacific lead to the definition of the Pacific Reference Line (PRL) which proved convenient to compare two sub-pacific mantle domains, using the δ(Nd-Sr) and δ(Sr-Pb) notations [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. With the recent advance in analytical MC-ICPMS techniques, the number of isotopic data increases dramatically. It becomes possible to look at these reference lines in multidimensional space from a statistical point of view. Principal Component Analysis (PCA) is a powerful mathematical tool to study data sets such as a geochemical database including Sr-Nd-Pb-Hf analyses. The purpose of PCA analysis is to reduce the number of dimensions in a data set by keeping those characteristics that contribute most to its variance. This technique has been used in previous mantle heterogeneity studies (e.g. [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF][START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF] The PCA method used with our geochemical dataset has been initially developed for low-rank matrix approximations [START_REF] Srebro | Weighted low-rank approximations[END_REF] and was recently adapted for tectonic problems using incomplete geodetic times series [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. The main difference with traditional PCA methods is that the singular value decomposition is replaced by a more sophisticated decomposition, which appropriately takes into account samples with a missing isotope measurement. This technique is particularly suitable to geochemical data as it allows computing the principal components using the whole dataset, increasing therefore the accuracy of the calculation (see supplementary material for a more detailed discussion about the concept and the limits of our calculation). Although more recently ICA (Independent Component Analysis) has been chosen by some authors (Iwamori andAlbarède, 2008, Iwamori et al., 2010), the preferred PCA method used here has the advantage of dealing with missing isotopic data, assuming that decorrelation is still a good assumption of independence in our dataset.

Because the PCA method is an orthogonal linear transformation, it assumes the linearity of the data co-variations. In most binary isotopic diagrams, mixing processes are expressed by hyperboles whose curvatures depend on the elemental concentration ratios of the involved end-members. But in the case of MORB, pseudo-linear correlations are observed (Fig. 2) indicating that denominator elements are in approximately constant proportions in the mixing components. It is very difficult to evaluate the extent of non-linear relationships concealed within the analytical noise. However, it is worth noting that the geographical variations of the components calculated with our method are approximately the same as those computed in the 3-dimensional space of Pb isotopes, in which relationships are linear. In order to minimize the correlations induced by the 204 Pb analytical noise (it represents only about 1.4% of the total Pb), the computation has been made in the 204 Pb/ 206 Pb, 207 Pb/ 206 Pb and 208 Pb/ 206 Pb space.

Considering the problematic of our study, this observation suggests that the curvature can be neglected. One drawback of PCA in general stems from the fact that PCA is a projection method, and sometimes low-dimensional visualization can lead to erroneous interpretations.

Distribution of the variance among the principal components.

The most striking result of the PCA is that the first principal component accounts for more than 70% of the total variance. This result is remarkable considering the number of geochemical parameters involved in the very large number of samples used in this analysis. It indicates the strong coherence of all isotopic ratios within the depleted mantle domain. These correlations in binary plots of isotopic systems are the result of two antagonistic processes that took place over time: chemical fractionation events leading to the existence of enriched and depleted mantle reservoirs and mechanical mixing of theses reservoirs during mantle convection. The chemical fractionation between parent and daughter isotopes is controlled by distribution coefficients. Even if these coefficients can be modified by multiple parameters, they remain consistent from one element to another. Since these processes are approximately linear, the resulting dispersion is located along a line corresponding to PC1 (the first Principal Component).

While the importance of the first component is indisputable, one challenge with PCA is to establish the number of significant components needed to explain the observed data variance.

A classical way to illustrate the number of relevant components is to study the residual variance (χ 2 ) obtained for each component (e.g. [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. This value drops abruptly after the third component (see supplementary material). At first, we can infer that the information brought by the fourth to the sixth principal components is statistically insignificant and can safely be ignored. Using a PCA computed in the 3-dimensional space of Pb isotopes, the principal components account for 91.7%, 8.2% and 0.1% of the variance for PC1, PC2 and PC3 respectively. This observation suggests that the variance expressed by principal components over the order of 2 is the result of a randomly distributed error. Also the absence of coherent geographical variation for the third component suggests that this variance could either be a very local variation or an artifact of our data compilation (i.e. sampling bias, data and error normalization between different laboratories). This justifies to limit the discussion of the south pacific mantle heterogeneity to the PC1 and PC2 characteristics.

Mapping geochemical heterogeneities in the mantle using a PCA method.

Using the dataset available at the time, [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF] identified a large geochemical variation in the south Pacific depleted mantle. Based on an incomplete sampling of the ridge, they suggested the existence of a sharp boundary located at the latitude of the Easter Island Microplate (27°S). As an interpretation, they proposed that the Pacific Superswell divided the mantle into two domains each with their own convective histories, producing slight differences in their average isotopic signatures. More recently, the origin of these two domains has been challenged by a model based on plate kinetics [START_REF] Small | Plate-kinematic explanation for mid-oceanic-ridge depth discontinuities[END_REF]. Small and Danyushevsky proposed that geochemical discontinuities result from variations of the asthenosphere consumption, which corresponds to the ratio between the accretion rate and the spreading center migration relative to plumes. Their model predicted that the fast spreading, slowly migrating East Pacific Rise should have higher average melting degrees compared to the slower spreading, rapidly migrating Pacific-Antarctic Rise. In order to identify the two mantle domains along the Pacific Ridge, [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF] used δ(Sr-Pb) and δ(Nd-Sr), which are defined as the vertical deviations from references lines in Pb vs Sr and Sr vs Nd isotopic ratio plots respectively. These reference lines (see §4.1) were drawn intuitively in the greatest variance of their dataset. They can be directly compared to our first principal component. By construction, whatever the number of dimensions considered in the PCA, the projection of PC1 in a binary diagram resembles the relevant reference line (Fig. 2). Because PC2 is orthogonal to the greatest variance, sample values along this component are correlated to delta notations such as δ(Sr-Pb), δ(Nd-Sr) or Δ8/4Pb. But compared to delta notations, PC2 has the advantage of being rigorously and statistically determined in a multidimensional isotopic space. The application of the PCA method to our data compilation (excluding He isotopes) reveals a geochemical profile of the Pacific Ridge. The plot of PC1 versus latitude summarizes all the characteristics noted with the different isotopic systems (Fig. 4). The short scale geochemical variations associated with hotspots as well as the large-scale variation are clearly visible. 17°S and 25°S (Easter Island) are well defined by sharp anomalies superimposed on the bell shaped curve. A plot of PC2 versus latitude shows a very different picture (Fig. 4): only the large scale variation is expressed by this component, the geochemical variations associated with hotspots are flattened and the corresponding samples are projected along the bell shape curve defining the Pacific Isotopic Bump. This PIB is also shown on plots of delta versus latitude, illustrating the equivalence of these parameters and PC2 (Fig. 4). At the Juan Fernandez/Foundation latitude (36°S), the isotopic variation curve reaches an extreme which corresponds to a less "depleted" isotopic signature. In contrast with the conclusion of [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF], we proposed that geochemical variations along the Pacific Ridge are not the result of two separated mantle domains but should rather be seen as a progressive variation of the isotopic composition of the sub-Pacific depleted mantle. Since the asthenosphere consumption varies abruptly at the Chile triple junction and is almost constant along the PAR, the plate kinematic model of [START_REF] Small | Plate-kinematic explanation for mid-oceanic-ridge depth discontinuities[END_REF] is also inconsistent with the observed progressive variation of the MORB depleted matrix.

4.4.

Mixing relationship within the depleted mantle compared to ridge-hotspot interactions.

Asthenospheric versus hotspots signals as illustrated by Sr-Nd-Pb-Hf.

Except in figures involving He isotopes, geochemical variations related to ridge/hotspot interactions are consistent with variations of samples devoid of plume influence. At first, it is difficult to distinguish the two types of variations in binary isotopic plots (Fig. 2).

Nevertheless, a PCA computed with all heavy radiogenic isotopes clearly illustrate a difference: hotspot signatures are exclusively visible with PC1, while the variance related to the large scale variation (PIB) is illustrated by both PC1 and PC2 versus latitude (Fig. 4). In PC1 versus PC2 space, the MORB field extends from the depleted end member of the mantle (DMM) toward a recycled oceanic crust end member with a HIMU affinity (Fig. 5). The samples identified as resulting from a plume-ridge interaction are not part of this "depleted trend". Rather their data field extends from the depleted trend towards more enriched-type end members such as C and/or EM. This observation supports the idea that the large scale variation in the depleted Pacific mantle is unrelated to ridge/hotspot interactions. This variation is therefore equivalent to the intrinsic variability of MORB recognized in other areas (e.g. [START_REF] Dosso | The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31-41°N)[END_REF], Donnelly et al., 2004;[START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF][START_REF] Hémond | Origin of MORB enrichment and relative trace element compatibilities along the Mid-Atlantic Ridge between 10° and 24[END_REF]. It has to be noted that PCA calculations assume a linearity criteria which is not satisfied when dealing with isotopic compositions of the mantle end-members. It is thus not possible to determine more precisely the nature of the different mantle components responsible for the observed trends. Nevertheless, the PCA calculations establish that the progressive geochemical change of the depleted matrix of the Pacific mantle is not the result of hotspot material being diluted into the depleted mantle. Using a different approach, [START_REF] Meyzen | Isotopic portrayal of the Earth/'s upper mantle flow field[END_REF] successfully unscrambled the hotspot and asthenospheric signals. These authors have proposed that the geochemical variations unrelated to ridge/hotspot interactions along the South West Indian ridge and the South Atlantic ridge are related to a broad lower mantle upwelling in this area. Similarly, a broad lower mantle input could be a plausible cause of the Pacific Isotopic Bump associated with the high spreading rates in the vicinity of the Chile Ridge Triple junction.

It is interesting to compare the results of our PCA calculation along the Pacific ridge with the spectral analysis performed along the Atlantic ridge by [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. These authors found two contrasting types of spectra along their study area. The first type is associated with ridge-hotspot interactions and is seen in the first principal component. The second type is illustrated by the continuous power decrease with the decreasing wavelength of the second principal component. [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF] have interpreted this second type of spectra, unrelated to hotspots, as being the result of the continuous size reduction of mantle heterogeneities upon stretching and refolding of the convecting mantle. Despite the differences in geological settings between the Pacific ridge and the Atlantic ridge, our observations are in good agreement with the statistical analysis of [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. We view the PIB as the consequence of a progressive change in the relative proportions of the marble-cake components present in the Pacific upper mantle. In this model, hotspot anomalies are superimposed on the intrinsic mantle heterogeneity expressed in MORB.

He isotopes in the depleted mantle compared to hotspot signals.

Unlike other isotopic systems, helium shows a first order discrepancy between the hotspot and the depleted mantle signals in our studied area (Fig. 3 and Fig. 6): the large scale variation associated to the depleted mantle is characterized by an increase towards more radiogenic (low 3 He/ 4 He) compositions, whereas the short scale variation associated to the mantle plumes points towards high 3 He/ 4 He values as previously noted in [START_REF] Hanan | Lead and Helium Isotope Evidence from Oceanic Basalts for a Common Deep Source of Mantle Plumes[END_REF]. The origin of the large range of elevated 3 He/ 4 He values in OIB lavas is a long-lived controversial question [START_REF] Kurz | Helium isotopic systematics of oceanic islands and mantle heterogeneity[END_REF][START_REF] Allègre | Constraints on evolution of the earth's mantle from rare gas systematic[END_REF][START_REF] Meibom | Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components? Earth Planet[END_REF]Moreira et al., 2004).

Historically, the requirement of a high 3 He/ 4 He reservoir for the OIB lead to the idea that plumes are tapping a deep, undegassed lower mantle, isolated from the upper mantle convection [START_REF] O'nions | Phase-transition modulated mixing in the mantle of the Earth -Discussion[END_REF][START_REF] Allègre | Limitation on the mass exchange between the upper and lower mantle: the evolving convection regime of the Earth[END_REF]. Numerous alternative models have tried to solve the apparent inconsistency between the high 3 He/ 4 He ratio in plumes and the requirement of a source previously processed through partial melting (e.g. [START_REF] Parman | Helium solubility in olivine and implications for high 3He/4He in ocean island basalts[END_REF][START_REF] Class | Evolution of helium isotopes in the Earth's mantle[END_REF]Purtika, 2008;[START_REF] Albarède | Rogue mantle Helium and Neon[END_REF]Davies, 2010).

Compared to plume-influenced samples, systematics of He isotopes in depleted MORB samples have shown a very restricted range of 3 He/ 4 He with a peak of distribution around 8±1 (R/Ra) [START_REF] Moreira | A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51-52°S)[END_REF]. Within the uncontaminated depleted mantle sampled along our studied area, 3 He/ 4 He varies from 6 to 9.5 (R/Ra). Samples devoid of plume influence fall along a negative trend toward low 3 He/ 4 He values and high 206 Pb/ 204 Pb. This variation of He isotopes in our samples devoid of plume influence is clearly related to the PIB identified along the pacific ridge using PCA with Sr, Nd, Pb and Hf isotopes. It is interesting to note that the most enriched samples from this correlation show the lowest 3 He/ 4 He values (more radiogenic). In order to reconcile the 3 He/ 4 He variations with variations of Sr, Pb, Nd and Hf isotopes, new models take into account the physical specificities of He compared to heavy radiogenic isotopes: its diffusivity in mantle conditions and its capacity to be outgassed from melts at sub-surface pressure (e.g. [START_REF] Hart | Scale length of mantle heterogeneities: Constraints from helium diffusion[END_REF][START_REF] Albarède | Rogue mantle Helium and Neon[END_REF][START_REF] Gonnermann | Preserving noble gases in a convecting mantle[END_REF]. We propose that the melting of a marble-cake upper mantle, unpolluted by plumes, produces the good correlation observed between He and Pb isotopes in our depleted samples. The less depleted component of the mantle assemblage is characterized by a high Pb isotopic ratio and a low 3 He/ 4 He ratio. We suggest that this component was derived from the recycling of an extensively outgassed oceanic crust. When the oceanic crust is subducted back into the mantle, it contains negligible concentration of mantle-derived He [START_REF] Staudacher | Recycling of oceanic crust and sediments: the noble gas subduction barrier[END_REF]. With time, this very low 3 He/(U+Th) in subducted slabs is expected to produce the end-member with high Pb isotopes value and low 3 He/ 4 He ratio (Fig. 6).

In our model, the variation of He isotopes along the pacific ridge could be interpreted as:

(1) a smaller contribution of the refractory layers (less radiogenic), corresponding to a lower 3 He/ 4 He ratio at the PIB but because the melting rate is expected to be higher near the Chile Ridge Triple Junction, this hypothesis seems very unlikely, or (2) a higher volume of the recycled end-member in this area. This hypothesis is in agreement with the conclusions derived from the PCA based on heavy radiogenic isotopes ( §4.4.1). The intrinsic geochemical variation of the depleted upper mantle comforts the image of a marble cake mantle composed of a refractory component and a recycled oceanic crust component. A broad volume of recycled component associated with the high spreading rates in the vicinity of the Chile Ridge Triple junction is therefore a plausible cause of the Pacific Isotopic Bump. The correlation between He and Pb isotopes revealed by our new data (Fig. 6) suggests that the depleted mantle is the outcome of a mixing of a different nature that the one involved in the plumeridge interaction.

Conclusion

Analyses of this new sampling of the PAR complete the dataset of the Pacific Ridge.

Therefore it becomes possible to have a picture of the geochemical variations from 10 to 70°S. Our data show a clear geographical evolution of isotopic characteristics along the Pacific Ridge. In binary isotopic plots, this large-scale variation is expressed by correlations between each isotopic dimension. This observation holds true even for the Nd-Hf system previously reported as decoupled for MORB samples [START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF]. Even more significant in this study is the linear correlation shown in the isotopic He-Nd and 3 He/ 4 He -206 Pb/ 204 Pb plots. In these isotopic spaces, the samples affected by plume-ridge interactions depart clearly from the linear correlations displayed by the ridge samples coming from the depleted mantle.

Despite the paucity of combined Sr-Nd-Pb-Hf isotope data on individual samples, the PCA algorithms used here allow us to portray the variation along a huge section of the Pacific Ridge from a statistical point of view. Compared to previous studies of the sub-Pacific mantle, the application of PCA reveals a Pacific Isotopic Bump, which can be seen as a progressive geochemical variation of the depleted upper-mantle matrix rather than a sharp frontier between two mantle domains. Combining PCA results with the information given by He isotopes, we suggest that the Pacific Isotopic Bump is unrelated to plume-ridge interactions. This geochemical variation in the upper mantle reservoir is the result of a marble-cake mantle assemblage composed of a residual mantle component and a recycled oceanic crust component. [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF], in the Nd/Hf plot (B), the mantle array [START_REF] Vervoort | Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time[END_REF], in the Pb/Pb plot (D), the North Hemisphere Reference Line (NHRL) [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF]. Since no reference line had yet been recognized in plots involving He isotopes, we show here a regression line ( 3 He/ 4 He = 0.63  Nd + 1.74, r = 0.89) (C). The projection of the two first principal components (PC1 and PC2) calculated in our study are shown (solid black and grey lines, see text for additional information). Data references can be found in supplementary materials. samples. In italic font, previously published data [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF][START_REF] Vlastélic | Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56°S and 66°S[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF]. [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF][START_REF] Vlastélic | Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56°S and 66°S[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

Figure captions

  Pacific Ridge. Latitudinal isotopic variations of south Pacific Ridge basalts are shown in Figure 3. An important geochemical feature from 66 to 10°S is a large scale (spanning approximately 5000km) and coherent variation of all isotopes shown by a bell shape grey line. The extremum of this variation defining the Pacific Isotopic Bump (PIB), is located at the latitude of Foundation (38°S). It reveals a less depleted component, characterized by more radiogenic values for Sr, Pb isotopic ratios, and correlatively less radiogenic values for Nd and Hf. Three shorter wavelength variations of the order of 200 to 700km are seen as isotopic anomalies superimposed on the otherwise bell shaped curve of the isotope signature along the ridge. In a way similar to the Pacific Isotopic Bump, these variations are towards more radiogenic Sr and Pb values and coherently less radiogenic Nd and Hf values. They indicate the influence of enriched materials due to plume-ridge interactions.

Figure 1 :

 1 Figure 1: Map of the south pacific basin showing the location of the PACANTARCTIC1 and

Figure 2 :

 2 Figure 2: Examples of binary plots showing correlations within the MORB field. Samples

Figure 3 :

 3 Figure 3: Geochemical variations along the PAR and the EPR from 66°S to 10°S. Dashed

Figure 4 :

 4 Figure 4: Plot of the two first principal components along the Pacific Ridge. The similarity

Figure 5 :

 5 Figure 5: Plot of PC2 versus PC1 for a principal component analysis computed using Sr, Nd,

Figure 6 :

 6 Figure 6: Binary plot of 3 He/ 4 He vs 206 Pb/ 204 Pb. The dashed regression line ( 3 He/ 4 He = -2.33 206 Pb/ 204 Pb + 51.0) is defined by samples devoid of plume influence. Plume-ridge interactions

Table 1 :

 1 Complete Sr, Nd, Pb, Hf and He isotope data table for PACANTARCTIC 1 and 2

Table 1 :

 1 Complete Sr, Nd, Pb, Hf and He isotope data table for PACANTARCTIC 1 and 2 samples. In italic font, previously published data

Acknowledgements

The Pb isotope work was funded by CNRS/INSU. The Hf isotope work was supported by NSF grants to B.B. Hanan. We thank Joan Miller for technical assistance at SDSU. We acknowledge Rick Carlson for the editorial handling, Francis Albarède and an anonymous reviewer for constructive comments. Cedric Hamelin thanks the Caltech Tectonic Observatory for its hospitality during his stay in Pasadena. We gratefully acknowledge Neus Sabater for her valuable comments.

Supplementary Material

New algorithm to compute a PCA:

The PCA method is a linear transformation that converts the data into a new coordinate system in which the direction along which the greatest data variance is expressed, becomes the first axis (PC1).

The direction along which the greatest data variance is expressed orthogonal to PC1 becomes the second axis (PC2), and so on. The first step, called the whitening, consists in normalizing the data. This is done by subtracting the mean value from each data point and dividing the result by the standard deviation of each variable. The correlation matrix (Pearson's correlation matrix) of the reduced variables can then be calculated. The next step consists into diagonalizing the correlation matrix in order to find the eigenvalues, which correspond to the dimensions that have the strongest correlation in the data set. The principal components are the coordinates of the data points in the eigenvector referential.

The PCA method used with our geochemical dataset has been initially developed for low-rank matrix approximations [START_REF] Srebro | Weighted low-rank approximations[END_REF] and was recently adapted for tectonic problems using incomplete geodetic times series [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. The algorithm used to compute our principal components has been written specifically to address the major problem of a geochemical database: missing data in the matrix. Traditionally, for a given sample, it is necessary to acquire as many different isotope measurements as the number of dimensions used in the PCA. The paucity of combined Sr-Nd-Pb-Hf isotope data in individual samples is therefore an important issue for standard PCA algorithms. The more dimensions we use in the PCA, the fewer samples meet the required conditions: from 210 samples for 2-dimension (Sr-Nd) space to only 99 samples for a 6-dimension space. The main difference with a classical PCA is that we have replaced the standard Singular Value Decomposition (SVD) with a more sophisticated decomposition proposed by [START_REF] Srebro | Weighted low-rank approximations[END_REF]. In this approach the data matrix X is decomposed into components U, S, V, with each data point weighted according to the square of its standard error. This decomposition is particularly adapted to take into account individual measurement errors and deal with missing data points. It allows us to compute the Principal Components on the entire data set, even if some isotopic ratios are missing.

Each missing data point is weighted with infinite standard error. We want to emphasize that, consequently, no interpolation is made, which could erase some local effect. This technique is particularly suitable to geochemical data as it allows us to complete the principal components using the whole dataset, increasing therefore the accuracy of the calculation.

Limits of PCA:

Classical limits related to PCA calculation:

Because the PCA method is an orthogonal linear transformation, it assumes the linearity of the data co-variations. In most binary isotopic diagrams, mixing processes are expressed by hyperboles whose curvatures depend on the elemental concentration ratios of the involved end-members. But in the case of MORB, pseudo-linear correlations are observed (Fig. 2) indicating that denominator elements are in approximately constant proportions in the mixing components. It is very difficult to evaluate the extent of non-linear relationships concealed within the analytical noise. However, it is interesting to note that the geographical variations of the components calculated with our method are approximately the same as those computed in the 3-dimensional space of Pb isotopes, in which relationships are linear. In order to minimize the correlations induced by the 204 Pb analytical noise (it represents only about 1.4% of the total Pb), the computation has been made in the 204 Pb/ 206 Pb, 207 Pb/ 206 Pb and 208 Pb/ 206 Pb space.

Considering the problematic of our study, this observation suggests that the curvature can be neglected.

Limits related to our new algorithm:

Compared to traditional PCA algorithms, principal components are computed here altogether rather than separately, because the number of components affects the subspace in which these components reside. A limit to this method appears if one variable is represented by fewer samples than compared to other variables. In this particular case, a sample with a value corresponding to this "rare" variable is artificially given more weight. In return, each variable is also weighed in proportion to its number of samples. Because the number of He isotope analyses along the studied ridge section is too low compared to other isotopes, the information provided by this parameter has not been included in the PCA calculation.

Distribution of the variance among the principal components:


Pascal Bador
email: pascal.bador@univ-lyon1.fr

Thierry Lafouge
email: thierry.lafouge@univ-lyon1.fr

Comparative analysis between impact factor and h-index for psychiatry journals

Keywords: Journal Impact Factor, Hirsch index, journal ranking, psychiatry journals

Objective: Journal Impact Factor (IF) is well known for being the document measure of scientific journal impact, despite several recognised limitations. Our study tried to propose a new rating system (journal h-index) applied to a sample of psychiatry journals and compared to IF. Method: In order to strictly compare their IF and h-index using the same data, we wanted to work on all the citations obtained in 2006 by the articles published in 2004-2005 in the 50 journals of our Web of Science sample of psychiatry. We studied the statistical correlation between the IF 2006 and hindex 2006. Results: The rankings of the 50 psychiatry journals were different when we took into account respectively the descending order of IF 2006 and h-index 2006 which revealed 16 steps and therefore 16 groups of journals. We noted that two journals were up 21 places (record) in h-index 2006 ranking and one journal lost 17 places. Nevertheless we obtained a high correlation coefficient well illustrated by the group of the seven first journals whose the two rankings were very close. We noted that our sample had only two journals really specialised in the publication of reviews. Conclusions: The rating of journals starting from the h-index may represent an interesting and complementary alternative to the well-known rating based on the IF. The h-index rating proposes a categorization of journals making it possible to create classes of journals with the same h-index. This type of ranking by classes is often appreciated and used by experts and scientific committees of evaluation.

Résumé

Objectif : Le Facteur d'Impact (FI) est bien connu pour être le principal indice bibliométrique permettant d'évaluer les journaux scientifique, malgré certaines limites maintenant reconnues. Notre étude propose un nouvel indice bibliométrique basé sur le h-index et testé sur un échantillon de journaux de psychiatrie, comparativement au FI. Méthode : Afin de comparer rigoureusement le FI et le h-index en utilisant strictement les mêmes données, nous avons travaillé sur l'ensemble des citations reçues en 2006 par les articles publiés en 2004-2005 par les 50 journaux issus d'un échantillon de la section « Psychiatry » du Web of Science. Nous avons étudié la corrélation entre le FI 2006 et le h-index 2006. Résultats : Les classements des 50 journaux de psychiatrie étaient différents lorsque nous prenions en compte respectivement l'ordre décroissant du FI 2006 et l'ordre décroissant du h-index 2006, ce dernier révélant 16 paliers et donc 16 groupes de journaux. Nous avons mis en évidence que deux journaux ont gagné 21 places (le record) dans le classement h-index 2006 et qu'un journal a perdu 17 places. Néanmoins nous avons obtenu un coefficient de corrélation élevé, ce qu'illustre bien le groupe des sept premiers journaux dont les deux classements sont très proches. Nous avons noté que dans notre échantillon seuls deux journaux étaient réellement spécialisés dans la publication de revues de synthèse. Conclusion : L'évaluation des journaux basée sur le h-index peut représenter une alternative intéressante et complémentaire au classement fondé sur le FI bien connu. Le classement basé sur le h-index propose une catégorisation des journaux par classes de journaux présentant le même hindex. Ce type de classement par classes est souvent apprécié et utilisé par les experts et les comités scientifiques d'évaluation.

Mots clés : Facteur d'Impact, h-index, classement des journaux, journaux de psychiatrie

The Thomson Scientific journal Impact Factor (IF) of Garfield is well known for being the document measure of journal impact [START_REF] Garfield | Citation indexes to science: a new dimension in documentation through association of ideas[END_REF]. IF is often used to rank scientific journals, despite several recognised limitations well summarized by [START_REF] Curtis | What the impact factor means for surgery journals[END_REF], [START_REF] Delavalle | Refining dermatology journal impact factors using pagerank[END_REF], [START_REF] Dong | The "impact factor" revisited[END_REF], [START_REF] Hecht | The journal impact factor: A misnamed, misleading, misused measure[END_REF]. First, IF only looks two years retrospectively. Secondly, review articles are summaries of the field and are much more frequently cited. Third, the denominator of the equation relates to the number of citable items, including original manuscripts, letters, case reports, and literature reviews published; the journal that publishes very few citable items annually will have an advantage despite infrequent citation. Fourth, some journals encourage self-citation. Fifth, different specialties may indeed have different standards for the number of articles cited per manuscript. The ISI recognizes the shortcomings of their methodology and have agreed that it is but one measure of a journal's quality.

Hirsch recently suggested a new research performance indicator that is designed for application at the micro level [START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF]. The Hirsch-Index, or h-index, quantifies as a single-number criterion the scientific output of a single researcher. The h-index is a very simple new measure incorporating both quantity and visibility of publications [START_REF] Bornmann | What do we know about the h index?[END_REF]: "A scientist has index h if h of his or her N p papers have at least h citations each and the other (N p -h) papers have fewer than ≤ h citations each" [START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF]. For example h-index of 20 means that the scientist has published 20 papers that each had at least 20 citations. [START_REF] Braun | A Hirsch-type index for journals[END_REF] proposed that the h-index could be usefully applied to the citation analysis of journals, as well. The h-index for evaluating the scientific impact of journals as a robust alternative indicator can be an advantageous complement to journal IF. The journal h-index is calculated as follows: "Retrieving all source items of a given journal from a given year and sorting them by the number of times cited, it is easy to find the highest rank number which is still lower than the corresponding 'Times Cited' value. This is exactly the h-index of the journals for the given year".

In order to progress in the IF/h-index comparison analysis, this study compares IF and h-index using exactly and strictly the same parameters (identical two publication years (2004)(2005) and identical one-year citation window ( 2006)). Hence, we propose here to compare IF 2006 and what we call hindex 2006 for one sample of 50 psychiatry journals taken from the Journal Citation Reports (JCR) 2006. Moreover the field of psychiatry (using both social and scientific methods) would be interesting to compare to one more classical medical field such as pharmacology (that publishes many literature reviews) recently studied [START_REF] Bador | Analyse comparative du facteur d'impact et du h-index pour les journaux de pharmacologie[END_REF].

Methods

Constitution of the sample

We ranked the 94 journals of the "Psychiatry" section of the JCR 2006 drawn from the Web of Science in descending order of IF and we took the first 50 journals to constitute our sample.

In order to strictly compare their IF and h-index using the same data, we wanted to work on all the citations obtained in 2006 by the articles published in 2004-2005 in the 50 journals of our sample of Psychiatry. So we had to calculate a Hirsch-type index for journals that agrees with the Braun et al.'s (2006)

Calculation of h-index 2006

The IF was easily extracted from the JCR 2006 whereas h-index 2006 was calculated manually for each of the 50 journals of our sample in the following way:

-1) Search for articles of 2004 -2) We displayed the references citing each article obtained ("Times Cited" link)

-3) Using the window obtained and the "Refine Results" function, we extracted the number of articles of 2006 (Citations 2006) from the "Publication Years" menu -4) The same procedure for the articles of 2005 Thus, for each of the 50 journals, we compiled a table similar to the one shown in table 1 for the journal "Journal of Psychiatry and Neuroscience". We then identified the h-index 2006 corresponding to the number h of articles published in 2004-2005 and cited at least h times during 2006.

Please insert Table 1: Calculation of the h-index 2006

Also, for all the articles of 2004-2005 published in the 50 journals, we identified the number of "Reviews" using the "Refine Results" function and the "Document Types" menu. We thus calculated the percentage of Reviews compared with the total number of articles published in [2004][2005].

Correlation between the h-index 2006 and the impact factor 2006

We studied the statistical correlation between the IF 2006 and h-index 2006. For this, we calculated Pearson's correlation coefficient.

Results

Table 2 presents the data obtained [START_REF] Rousseau | A case study: Evolution of JASIS' h-index[END_REF], h-index 2006, IF ranking, h-index For the Psychiatry journals, we obtained a high Pearson's correlation coefficient of 0.88. In figure 2, we show the scatter plot and the associated linear regression line. 2007) is based on strictly the same parameters as well, but particularly on one publication year and on a three-year citation window beginning with the publication year for both the journal impact measure and the h-index.

Please insert

The results given in table 2 show that, for the Psychiatry journals, the two rankings are quite different. The IF allows a ranking using a customary descending order starting from the values of the JCR given to the nearest thousandth. However, the h-index offers a decreasing ranking starting from values that are integers. Therefore, the h-index ranking is much less fine and precise and reveals 16 steps (figure 1) each corresponding to a group of journals with the same h-index. Also, the amplitude of the 50 h-index values (amplitude = 22) is higher than that for the 50 IF values (amplitude = 12). Furthermore, we must note that for a given journal of our sample the h-index is always higher than the IF (up to 4 times higher).

The "Psychiatry" section has very few journals specializing in the publication of reviews, which makes this type of data not very important for comparing the IF and h-index. So for our sample the rankings based on the IF and the h-index are not very sensitive to the percentage of reviews published. This is not what we observed with the "Pharmacology and Pharmacy" section which has the characteristic of having a very big percentage of journals specializing in the publication of reviews. These journals are those that very often present the best IF, which is logical since their review articles are more often cited than the original articles. However, ranking based on the h-index is not very sensitive to the percentage of reviews published [START_REF] Bador | Analyse comparative du facteur d'impact et du h-index pour les journaux de pharmacologie[END_REF].

If we study the relative ranking (in table 2) of our 50 psychiatry journals in the 2 types of ranking analysed here, we can see that 6 journals have identical rank: "Archives of General Psychiatry" 1 rst , "Neuropsychopharmacology" 5 th , "British Journal of Psychiatry" 7 th , "European Neuropsychopharmacology" 21 rst , "Psychiatry Research-Neuroimaging" 33 rd , "Human Psychopharmacology-Clinical and Experimental" 42 nd . Among the 44 other journals, 23 journals won between one and five places, 17 journals won between 10 and 21 places. Two journals having the same h-index 2006 ranking have the same [START_REF] Rousseau | A case study: Evolution of JASIS' h-index[END_REF](IF 2006 = 3.857) which is very rare in a given section of the JCR, they have of course an identical IF 2006 ranking. For a given journal, the possibly high difference between the two rankings may be explained by the fact that the high IF (ant then the good IF ranking) is the result from one or several outstandingly highly cited articles for which the journal h-index is not sensitive.

The second part of this comparative study of the IF and h-index concerned the analysis of their Pearson's correlation coefficient. Thus, we noted a high Pearson's correlation coefficient (0.88) for the Psychiatry sample. This is well illustrated by the group of the seven first journals whose the two rankings are very close as seen in table 2. This was not what we observed with the Pharmacology and Pharmacy section which obtained a low Pearson's correlation coefficient (0.59) [START_REF] Bador | Analyse comparative du facteur d'impact et du h-index pour les journaux de pharmacologie[END_REF].

More generally, and as shown in our study of one very small sample, the rating of journals starting from the h-index may represent an interesting and complementary alternative to the well-known rating based on the IF. In fact, the h-index rating proposes a categorization of journals (several journals capable of having the same h-index) making it possible to create classes of journals with the same h-index: e.g. class 5, class 6, class 7...class 19, class 25. It is evident that, to have a meaning, this ranking must be made as for IF, within a collection of comparable journals of the same well-identified scientific discipline. In order to put this new type of ranking into perspective, we could certainly propose to display beside the value of each class the maximum value found for the journal obtaining the best h-index for the discipline studied, using the following model: journal X  h-index=6/25 journal Y h-index=19/25 meaning that journal X is characterized by an h-index of 6 and journal Y by an h-index of 19, with the note that the journals rated first of the discipline studied have an h-index of 25. This type of ranking by classes of journal is often appreciated and used by experts and scientific committees of evaluation as shown by Vanclay in the study proposing a ranking of forestry journals based on an evaluation of the journals by experts and also on their h-index [START_REF] Rousseau | A case study: Evolution of JASIS' h-index[END_REF].

The use of ranking by classes of journal based on the h-index is interesting in disciplines in which the amplitude of h-index is high in order to have the maximum number of classes to compare. This is the case of scientific, technical and medical fields where authors cite a lot of articles. Nevertheless in other types of disciplines such as social sciences, arts and humanities in which scientific collaboration and citations of articles by authors are less developed, the method of journal h-index may be less significant and more difficult to use.

As shown by [START_REF] Braun | A Hirsch-type index for journals[END_REF], for a given journal the h-index presents different and useful characteristics compared with the IF. Firstly, h-index is insensitive to an accidental excess of uncited papers and also to one or several outstandingly highly cited papers; secondly, it combines the effects of "quantity" (number of publications) and "quality" (citation rate) in a rather specific balanced way that should reduce the apparent "overrating" of some review journals.

The h-index could be very interesting and a complementary tool of IF if it would not be calculated for a "life-time contribution" as suggested by [START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF] for individual scientists, but for a definite period as we did in this study with the same parameters as [START_REF] Rousseau | A case study: Evolution of JASIS' h-index[END_REF]IF (articles published in 2004[START_REF] Hirsch | An index to quantify an individual's scientific research output[END_REF]IF and cited in 2006)). In our study, IF and h-index were exactly and strictly comparable and thus complementary in the rating of journals of the same discipline.

Nevertheless we must point out a limitation in the use of the journal h-index. The journal h-index cannot be higher than the number of articles published, so it disadvantages journals that may have a high IF but with a smaller journal h-index as a result. [START_REF] Braun | A Hirsch-type index for journals[END_REF], who worked on 2001 as source year (one publication year), had to eliminate the first and second journals of the 2001 IF list. Since these journals published 24 and 23 papers, respectively, in 2001, they had no chance to compete with the chart toppers (obviously the h-index cannot be larger than the number of papers it is based on). So if we take a two publication year period as in our study, all journals will have published enough articles (probably at least fifty) and this will avoid having to possibly eliminate some journals having very high IF because they published a very low number of articles.

  definition of the h-index of a journal for a given year. Our given year being 2006, we wanted to compare IF 2006 and what we logically called h-index 2006.

Figure 2 :

 2 Correlation of IF 2006 and h-index 2006 for Psychiatry journals Discussion The interest and originality of our study were to compare rankings based on IF 2006 and h-index 2006, using strictly the same data based on the usual definition of IF (identical two publication years 2004-2005 and identical one-year citation window 2006) for one sample of journals of the health field such as Psychiatry. The study by Schubert et al. (

Figure 2 :

 2 Figure 2: Pearson's correlation of IF 2006 and h-index 2006 for Psychiatry journals

Please insert Table 2: h-index 2006 ranked list of the 50 first Psychiatry journals

  ranking, IF/h-index ranking difference, number of articles published in 2004-2005, percentage of articles that are reviews) for the 50 psychiatry journals ranked in descending order of h-index 2006 and compared to the ranking based on IF 2006. Except the group of the seven first journals for which the two rankings are very close, table 2 shows significant differences between the two rankings, for example : Table 2 also shows the saving or loss of places in the two rankings. So we can see that for example the journals "Progress in Neuro-Psychopharmacology and Biological Psychiatry" and "Psychiatry Research" are up 21 places (which is the record) in h-index 2006 ranking, the journal "International Journal of Neuropsychopharmacology" loses 17 places in h-index 2006 ranking. 27 to 5. The h-index 2006 is always, and without exception, much higher than the IF 2006 (up to 4 times higher for the journals "Psychopharmacology" and "Progress in Neuro-Psychopharmacology and Biological Psychiatry").Overall, our sample of Psychiatry journals publishes quite few articles in the form of reviews. Only two journals (4%) are really specialised in the publication of this type of article: "Mental Retardation and Developmental Disabilities Research Reviews" and "Progress in Neuro-psychopharmacology and Biological Psychiatry", respectively published 100% and 94% of reviews.

	-The two journals ranked 11 th with h-index 2006 (h-index 2006 = 13) were ranked
	respectively 9 th (IF 2006 = 4.767) and 23 rd (IF 2006 = 3.630) with IF 2006.
	-Among the seven journals ranked 14 th with h-index 2006 (h-index 2006 = 11), the best IF
	2006 ranking was 15 th and the worst was 35 th . However two journals had exactly the same
	IF 2006 (IF 2006 = 3.857) and the same h-index 2006 (h-index 2006 = 11), and therefore the
	same rankings!
	As a complement, to illustrate the data in table 2 in graph form, we present figure 1 which shows the
	ranking of the sample in descending order IF 2006.

-The journal "Psychopharmacology" ranked 24 th with IF 2006 (IF 2006 = 3.625), was ranked 7 th with h-index 2006 (h-index 2006 = 15), equal to the journals "British Journal of Psychiatry" (whose IF ranking, 7 th , did not change, IF 2006 = 5.436) and "Schizophrenia Research" (itself ranked 14 th in the IF 2006 ranking, IF 2006 = 4.264).

Please insert Figure 1: Comparison of IF 2006 and h-index 2006 for Psychiatry journals (IF 2006 ranking)

Figure

1

shows an overall decrease in h-index 2006, revealing 16 steps (each step having an identical h-index 2006, as also shown in table 2) and therefore 16 groups of psychiatry journals whose h-index varies from

Table 1 : Calculation of the h-index 2006 Journal of Psychiatry and Neuroscience

 1

	Articles 2004* Citations 2006 Articles 2005* Citations 2006
	1	1	1	0
	2	13	2	0
	3	0	3	2
	4	13	4	1
	5	1	5	0
	6	4	6	1
	7	3	7	0
	8	3	8	2
	9	0	9	0
	10	3	10	1
	11	0	11	1
	12	7	12	0
	13	1	13	0
	14	0	14	0
	15	15	15	0
	16	17	16	1
	17	5	17	0
	18	2	18	1
	19	5	19	4
	20	15	20	0
	21	8	21	0
	22	1	22	0
	23	11	23	0
	24	15	24	4
	25	0	25	2
	26	19	26	5
	27	1	27	0
	28	2	28	1
	29	7	29	1
	30	1	30	3
	31	3	31	1
	32	1	32	1
	33	0	33	6
	34	0	34	3
	35	1	35	1
	36	2	36	5
	37	0	37	5
	38	5	38	1
	39	12	39	12
			40	0
			41	0
			42	1
			43	0
			44	5
			45	3
			46	0
			47	2
			48	1
			49	0
			50	1
			51	5
			52	1
			53	0
			54	1
			55	2
			56	3
	9 papers cited at least 10 times 1 paper cited at least 10 times
	Total: 10 papers 2004-2005 cited at least 10 times
	9 papers cited at least 11 times 1 paper cited at least 11 times
	Total: 10 papers 2004-2005 cited at least 11 times
		h-index = 10
	*Order of the articles given by the Web of Science

Table 2 : h-index 2006 ranked list of the 50 first Psychiatry journals h-index 2006 ranking

 2

		Abbreviated Journal Title	IF	h-index	IF	h-index	IF/h-index	NB articles	%
			2006	2006	ranking	ranking	ranking	2004-2005	Reviews
							difference
	1	ARCH GEN PSYCHIAT	13.936	27	1	1	0	233	3
	2	AM J PSYCHIAT	8.250	25	3	2	+1	592	49
	3	BIOL PSYCHIAT	7,154	22	4	3	+1	635	5
	4	MOL PSYCHIATR	11.804	21	2	4	-2	204	49
	5	NEUROPSYCHOPHARMACOL	5.889	16	5	5	0	468	6
		J CLIN PSYCHIAT	5.533	16	6	5	+1	552	19
	7	BRIT J PSYCHIAT	5.436	15	7	7	0	326	25
		SCHIZOPHR RES	4.264	15	14	7	+7	503	3
		PSYCHOPHARMACOLOGY	3.625	15	24	7	+17	801	5
	10	AM J MED GENET B	4.463	14	11	10	+1	298	1
	11	J AM ACAD CHILD PSY	4.767	13	9	11	-2	287	41
		J NEUROL NEUROSUR PS	3.630	13	23	11	+12	700	3
	13	J AFFECT DISORDERS	3.138	12	28	13	+15	456	4
	14	CNS DRUGS	4.210	11	15	14	+1	157	50
		ADDICTION	4.088	11	17	14	+3	319	11
		ACTA PSYCHIAT SCAND	3.857	11	18	14	+4	258	13
		PSYCHOSOM MED	3.857	11	18	14	+4	287	6
		PSYCHOL MED	3.816	11	20	14	+6	299	5
		BIPOLAR DISORD	3.494	11	25	14	+11	156	23
		PROG NEURO-PSYCHOPH	2.584	11	35	14	+21	320	94
	21	J CLIN PSYCHOPHARM	4.561	10	10	21	-11	171	4
		J PSYCHIATR NEUROSCI	4.100	10	16	21	-5	70	26
		EUR NEUROPSYCHOPHARM	3.794	10	21	21	0	160	6
		DRUG ALCOHOL DEPEN	3.213	10	27	21	+6	300	8
	25	INT J NEUROPSYCHOPH	5.184	9	8	25	-17	114	10
		SCHIZOPHRENIA BULL	4.352	9	12	25	-13	122	11
		PSYCHOTHER PSYCHOSOM	4.333	9	13	25	-12	93	4
		J PSYCHIATR RES	3.700	9	22	25	-3	140	2
		DEMENT GERIATR COGN	2.511	9	38	25	+13	223	3
		PSYCHIAT SERV	2.430	9	41	25	+16	316	0
		J INT NEUROPSYCH SOC	2.367	9	43	25	+18	188	7
		PSYCHIAT RES	2.310	9	46	25	+21	303	1
	33	J PSYCHOPHARMACOL	3.255	8	26	33	-7	149	14
		INT CLIN PSYCHOPHARM	3.080	8	29	33	-4	113	5
		EUR ARCH PSY CLIN N	3.042	8	30	33	-3	120	5
		AM J GERIAT PSYCHIAT	2.894	8	31	33	-2	198	5
		PSYCHIAT RES-NEUROIM	2.755	8	33	33	0	151	2
		MENT RETARD DEV D R	2.671	8	34	33	+1	82	100
		DEPRESS ANXIETY	2.549	8	36	33	+3	113	5
		J PSYCHOSOM RES	2.322	8	45	33	+12	273	0
		EPILEPSY BEHAV	2.026	8	50	33	+17	345	14
	42	GEN HOSP PSYCHIAT	2.500	7	39	42	-3	118	0
		J CHILD ADOL PSYCHOP	2.486	7	40	42	-2	148	8
		HUM PSYCHOPHARM CLIN	2.386	7	42	42	0	127	19
		NEUROPSYCHOBIOLOGY	2.367	7	43	42	+1	158	2
		COMPR PSYCHIAT	2.181	7	47	42	+5	138	2
		CNS SPECTRUMS	2.051	7	49	42	+7	175	53
	48	PHARMACOPSYCHIATRY	2.849	6	32	48	-16	119	7
		CAN J PSYCHIAT	2.531	6	37	48	-11	192	23
	50	WORLD J BIOL PSYCHIA	2.094	5	48	50	-2	53	30

As suggested by [START_REF] Rousseau | A case study: Evolution of JASIS' h-index[END_REF], one might also consider calculating a relative h-index by dividing it by the yearly number of articles of the journal, which could be another research lead for the assessment of the different ranking methods of scientific journals.