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A Highly Unradiogenic Lead Isotopic Signature Revealed by Volcanic Rocks from the East Pacific Rise

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A Highly Unradiogenic Lead Isotopic Signature Revealed by Volcanic Rocks from the East Pacific Rise ARTICLE Received 19 Jun 2013 | Accepted 20 Jun 2014 | Published 16 Jul 2014 DOI: 10.1038/ncomms5474 A highly unradiogenic lead isotopic signature revealed by volcanic rocks from the East Pacific Rise Berengere Mougel1, Arnaud Agranier1, Christophe Hemond1 & Pascal Gente1 Radiogenic isotopes in oceanic basalts provide a window into the different geochemical components defining the composition of Earth’s mantle. Here we report the discovery of a novel geochemical signature in volcanic glasses sampled at a sub-kilometre scale along the East Pacific Rise between 15°370N and 15°470N. The most striking aspect of this signature is its unradiogenic lead (206Pb/204Pb ¼ 17.49, 207Pb/204Pb ¼ 15.46 and 208Pb/204Pb ¼ 36.83). In conjunction with enriched Sr, Nd and Hf signatures, Pb isotopes depict mixing lines that trend away from any known mantle end-members. We suggest that this unradiogenic lead component sampled by magmatic melts corresponds to a novel upper mantle reservoir that should be considered in the Pb isotope budget of the bulk silicate Earth. Major, trace element and isotope compositions are suggestive of an ancient and lower continental origin for this unradiogenic lead component, possibly sulphide-bearing pyroxenites that were preserved even after prolonged stirring within the ambient upper mantle. 1 Laboratoire Domaines Oce´aniques, UMR6538, IUEM, 29280 Plouzane´, France. Correspondence and requests for materials should be addressed to B.M. (email: [email protected]). NATURE COMMUNICATIONS | 5:4474 | DOI: 10.1038/ncomms5474 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5474 ver time, melting processes have exhausted Earth’s upper system after core segregation, all crustal and mantle material mantle of its most incompatible elements, resulting in a should be equally distributed around a 4.53 Ga Geochron line in a Odepleted residual reservoir (depleted mid-ocean-ridge- 207Pb/204Pb versus 206Pb/204Pb diagram. However, MORB and basalt mantle) striated by geochemical heterogeneities inherited OIB are mostly found to the right of this line; this apparent from the recycling of contrasting ocean and continental material1. Pb paradox7 suggests that a low U/Pb (father/daughter ratio) Basalts from mid ocean ridges (MORB) and ocean islands (OIB) reservoir is hidden somewhere in the mantle. Ancient peridotites are both extracted from the asthenospheric mantle, and carry found in the Horoman orogenic massif (Japan) have recently within their geochemical make-up (concentrations and isotopes) shed light on the existence and properties of such a discrete the fingerprints of these ‘enriched’ re-injected materials. In reservoir8. Very low Pb isotope ratios were reported in these rocks particular, Sr, Pb, Nd, Hf and noble gas isotopic compositions of and provide physical evidence that unradiogenic Pb could be MORB and OIB, have been successfully used to describe the trapped within the upper mantle as widespread and extremely global geochemical complexity of the mantle and reduce it to a refractory heterogeneous domains8–11 largely untapped by multi-component mixture of isotopic end-members2,3. Isotopic basaltic melts. data have also demonstrated the presence of spatial mantle However, most of the MORB samples used in the global mantle domains separated by geochemical boundaries, such as the geochemical scheme were dredged tens to hundreds of kilometres Australian–Antarctic discordance4. On the eastern side of this apart over decades of seafloor exploration. In this study, we take limit, the Pacific upper mantle appears only enriched by advantage of submersible in situ access, to adopt a sampling subducted oceanic lithosphere, while on the other side, the strategy specifically adapted to the scale of volcanic edifices and Indian upper mantle is also polluted by lower continental inputs5. covering 15 km of a single East Pacific Rise (EPR) segment. Our The geochemical properties of the latter, dubbed the ‘DUPAL results reveal the presence of an unradiogenic Pb signal never (Dupre´-Alle`gre) anomaly4,6’, include lower 206Pb/204Pb, 207Pb/ before reported for Pacific MORB and that constitutes a 204Pb and 208Pb/204Pb relative to Pacific MORB. Such magmatic witness for the existence of an unradiogenic Pb upper unradiogenic Pb signatures are unusually rare in MORB relative mantle reservoir. Together with trace elements and Sr, Nd, to expectations from global Pb isotopes mass balance of the bulk Hf and He isotopes, these results suggest a high degree of silicate Earth (BSE). Indeed, if the BSE has evolved as a closed heterogeneity below this region of the EPR, and reveal a novel abHIMU 40.0 37.7 MAR 39.5 37.5 C 39.0 EM2M2 37.3 SEIR Pb 38.5 37.1 204 PUB16-05 Pb/ 38.0 SWIR 36.9 208 EPR 37.5 36.7 90–10 DMMMMM 80–20 EM1M11 LC 37.0 36.5 U 70–30 * 36.5 36.3 cd 15.8 15.54 MAR HIMU 4.53 Ga 15.7 GeochronEM2 15.52 C 15.50 Pb 15.6 SEIR 204 Pb/ 15.48 15.5 PUB16-05 207 EM1 15.46 15.4 SWIR DMMDMDMMM 90–10 EPR 80–20 ULC 15.44 70–30 * 15.3 16 17 18 19 20 21 17.1 17.3 17.5 17.7 17.9 18.1 18.3 18.5 206Pb/204Pb 206Pb/204Pb Figure 1 | Pb isotope compositions. Pb isotope compositions of basaltic glasses from this study (circles) and of MORB from various oceanic domains (coloured fields, SEIR: South East Indian Ridge51, SWIR: South West Indian Ridge51, MAR: Mid-Atlantic Ridge52 and EPR: East Pacific Rise51). The blue and green circles in all diagrams demonstrate two different mixing trends, one between ambient mantle and unradiogenic lead component (ULC, blue circles), and one between ambient mantle and a secondary Mathematician seamount component (green circles). The mantle end-members: DMM (depleted MORB mantle), EM1 (enriched mantle type-1), EM2 (enriched mantle type-2) and HIMU (high-m) are reported from ref. 2. The common component C composition is from ref. 15. (a) 208Pb/204Pb versus 206Pb/204Pb, (b) enlargement of a. Both regression lines have a R2 ¼ 0.99, (c) 208Pb/204Pb versus 206Pb/204Pb, with 4.53 Ga Geochron as reported in ref. 53, (d) enlargement of c. The black crosses correspond to theoretical compositions of the ULC component using a 70–30%, 80–20% (*preferred value) and 90 À 10% source mixtures for sample 10-PUB16-05 (see Supplementary Table 2). The orange square represents the mean composition of the study samples. Error bars correspond to the external reproducibility of Pb isotope compositions (only visible on 207Pb/204Pb, and smaller than the symbols for all other ratios). 2 NATURE COMMUNICATIONS | 5:4474 | DOI: 10.1038/ncomms5474 | www.nature.com/naturecommunications & 2014 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms5474 ARTICLE MORB signature suggestive of the recycling of lower continental and Sr anomalies, as well as Th and U depletions (Fig. 3b; material (meta-gabbroic pyroxenites) into the Pacific upper Supplementary Fig. 3). mantle. Discussion Results The low LREE concentrations in the ULC samples are consistent Data description. Here we present data from EPR on-axis basalts with relatively high degrees of partial melting and/or very that display unradiogenic Pb compositions (associated with depleted sources. Moreover, the association of coinciding Eu, relatively enriched Sr-Nd-Hf isotopic compositions) indicative of Sr and Ba enrichments can be directly attributed to the a novel and previously cryptic upper mantle reservoir. Basaltic involvement of cumulative plagioclase in the source, a common glasses sampled on discrete lava flows during submersible dives feature of gabbros10,16 and of some rare East African rift basalts17. between 15°370 and 15°470N along the EPR covered a span of This primarily gabbroic imprint on the ULC source is illustrated only 15 km, which represented one of the highest spatial resolu- by Fig. 4, where clear correlations can be observed between Pb tion sampling efforts to date (Supplementary Fig. 1). The data isotopes (proxy for the ULC component) and geochemical ratios depict two sharp linear trends that are likely to reflect two binary that serve as typical plagioclase markers: (CaO/Al2O3), (Eu/Eu*) mixtures (or pseudo-binary mixtures12) and reveal the presence and (Sr/Eu*). The involvement of gabbroic (plagioclase of high amplitude, short length-scale mantle heterogeneities in cumulates) material has been previously proposed for plume this area. While Pb isotopes are neatly correlated (Fig. 1), Sr-Nd- melts18–20, however, plagioclase is not stable at the pressure and Hf-Pb plots (Fig. 2) are moderately scattered. Such dispersion in temperature range of mantle melting under mid ocean ridges. geochemical diagrams associated with mixing trends, may also be The material involved in this source is therefore more likely meta- explained by non-uniformed mixing of fractional melts extracted gabbroic pyroxenites retaining a signature of ‘ghost plagioclase’20 from different sources, where depth controls the degree of than actual gabbros. Indeed, the hump-shaped REE patterns homogenization13. observed here are also common features of clinopyroxenes and One of these trends points towards very low 208Pb/204Pb and pyroxenites21. 206Pb/204Pb (36.83 and 17.49, respectively) and a moderately low The absence of depletion in the heaviest REE (Yb and Lu) in 207Pb/204Pb (15.46) component that we define here as the Fig. 3 (and Supplementary Fig. 3) indicates that residual garnet unradiogenic lead component (ULC) (Figs 1 and 2). Samples that was not present during ULC melt extraction. This is rather are most imprinted by the ULC signature are located at the unexpected since, because of their alumina content, the junction of the EPR and the Mathematician seamount chain metamorphic transformation of plagioclase cumulates into (Supplementary Fig. 1). These constitute the lowest 208Pb/204Pb pyroxenites should involve garnet formation.
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