Internal distribution of Li and B in serpentinites from the Feather River Ophiolite, California based on laser ablation ICP-MS

International audienceLaser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) analyses of B and Li in serpentinized peridotites from the Feather River Ophiolite (California) indicates that B is enriched in serpentine minerals compared to the whole-rock and less altered olivine grains while Li in serpentine is depleted or comparable to whole-rock Li. The high B contents of serpentine minerals correlate with the relatively enriched whole-rock B contents. The low Li contents of serpentine minerals are consistent with the relatively low Li whole-rock contents and suggest that only small amounts of Li were added during serpentinization or that some Li was even leached out. A simple model of partial melting shows that Li/Yb increases with increasing melt depletion (and clinopyroxene depletion) in the peridotitic residue because Li is most compatible in olivine while Yb is most compatible in clinopyroxene. Thus, high Li/Yb ratios in peridotites by themselves do not indicate secondary enrichments in Li. However, Li/Yb and Yb contents of many of the Feather River Ophiolites plot above the melt depletion curve in Li/Yb versus Yb space, indicating that these serpentinites experienced subtle and preferential enrichments in Li during serpentinization. If serpentinized oceanic lithospheric mantle, as represented by the Feather River Ophiolite, is important in subduction recycling, then recycled mantle domains having a serpentinite protolith might be characterized by strong B enrichments but only small Li enrichments

A " high 4 He/ 3 He " mantle material detected under the East Pacific Rise (15°4′N)

International audienceWe investigate in details helium isotope data reported in Mougel et al. (2014) for 14 basaltic samples collected on the East Pacific Rise by submersible (15°4′N) where the ridge interacts with the Mathematician seamounts. Samples locations are separated by only few hundred meters across a 15 km along-axis profile. The data reveal a strong geochemical variability that has never been observed at such high spatial resolution for helium isotope compositions. Moreover, they reveal an unusually high 4 He/ 3 He mantle component also characterized by unradiogenic lead, atypical in oceanic basalts. He-Pb systematics suggests a mixture between a nonradiogenic lead and radiogenic helium pyroxenitic component, recycled from the deep continental lithosphere and the ambient peridotitic mantle. The He isotope difference between these two end-members can be interpreted as a time evolution of two distinct mantle sources after a slight (U + Th)/ 3 He fractionation, likely due to some ancient degassing during the formation of deep continental pyroxenites

Origin of Light Noble Gases (He, Ne, and Ar) on Earth: A Review

We review the different scenarios for the origin of light noble gases (He, Ne, and Ar) on Earth. Several sources could have contributed to the Earth's noble gas budget: implanted solar wind, solar nebula gas, chondrites, and comets. Although there is evidence for solar-like neon in the Earth's mantle, questions remain as to its origin. A new compilation of noble gas data in lunar soils, interplanetary dust particles, micrometeorites, and solar wind allows examination of the implanted solar wind composition, which is key to understanding the solar-like mantle neon isotope composition. We show that lunar soils that reflect this solar-wind-implanted signature have a Ne-20/Ne-22 ratio very close to that of ocean island basalts. New data and calculations illustrate that the measured plume source Ne-20/Ne-22 ratio is close to the primitive mantle ratio, when taking into account mixing with the upper mantle (that has lower Ne-20/Ne-22 ratio). This favors early solar wind implantation to account for the origin of light volatiles (He, Ne, and possibly H) in the Earth's mantle: they were incorporated by solar wind irradiation into the Earth's precursor grains during the first few Myr of the solar system's formation. These grains must have partially survived accretion processes (only a few percent are needed to satisfy the Earth's budget of light volatiles). As for the atmosphere, the neon isotope composition can be explained by mixing 36% of mantle gases having this solar-wind-implanted signature and 64% of chondritic gases delivered in a late veneer phase

Quantifying trace element disequilibria in mantle xenoliths and abyssal peridotites

We apply a tool based on element distribution between orthopyroxene and clinopyroxene for quantifying rare earth element (REE) disequilibria in ultramafic rocks in the subsolidus state. We present case studies of the REE contents of mineral cores in mantle xenoliths and abyssal peridotites using in situ analytical tools. Even when only mineral cores are measured (to avoid enriched rims), equilibrium is not always achieved on the mineral scale. Mineral cores in mantle xenoliths are closer to equilibrium than those in abyssal peridotites even though mantle xenoliths are known to be light REE-contaminated from the host lava. In the case of the abyssal peridotites, 13 out of 14 are out of equilibrium with the least metasomatized most in disequilibrium and the most metasomatized closest to equilibrium. We discuss hypotheses for these observations, but regardless of what caused the disequilibria, this tool allows one to “see through” the effects of secondary processes, such as infiltration by fluid inclusions via cracks and diffusive exchange between minerals and melts/fluids along grain boundaries. The ease of making in situ REE measurements makes this tool formidable in identifying different generations of clinopyroxenes in ultramafic lithologies. Such data will complement the interpretation of isotopic and petrographic studies of continental and oceanic lithospheric mantle

A highly unradiogenic lead isotopic signature revealed by volcanic rocks from the East Pacific Rise

International audienceRadiogenic 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°37′N and 15°47′N. The most striking aspect of this signature is its unradiogenic lead (206Pb/204Pb=17.49, 207Pb/204Pb=15.46 and 208Pb/204Pb=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

320,000 years of interaction between a fast-spreading ridge and nearby seamounts monitored using major, trace and isotope composition data from oceanic basalts: Zoom at 15.6°N on the East Pacific Rise

Basaltic samples were collected by the French submersible "Nautile" during the “Parisub” cruise (2010, R/V L'Atalante, Ifremer) along a 25 km long sampling profile crossing perpendicularly the current axis of the East Pacific Rise at 15.6°N, as well as the trace of its two former parallel axes located further east. The total length of the profile corresponds to an approximate time interval of ∼ 320,000 years. The corresponding dataset documents the geochemical response of Mid-Ocean Ridge Basalts (MORB) related to the progressive convergence between the ridge segment and a nearby hotspot. It also represents one of the highest-sampling (and in turn geochemical) resolution efforts to date. The major, trace element and isotopic compositions determined through optical and mass spectrometry analysis of 52 samples are presented and compared to other previous data obtained from the same area. The data obtained strictly follow the conventions used in rock geochemistry in terms of data acquisition, reduction, and format, so that they can be compared to similar data from other regions. The different figures present (i) The geological context of study area, (ii) A classification of the samples according to their geochemical composition and geological context for a better legibility of the dataset, (iii) A comparison with data from other oceanic rises, (iv) A detailed method explaining the foundations of the chronology between samples established, and (v) A chronological representation of the geochemical composition of the basalts collected. These data can be useful for anyone interested in marine geosciences and more specifically scientists studying mantle geochemistry, oceanic lithosphere formation, and hotspot-ridge interactions. These data can also be used to model magmatic processes, crust-mantle interactions, and can be integrated in geophysical and geological models of seafloor accretion

High-resolution isotopic variability across EPR segment 16°N: A chronological interpretation of source composition and ridge-seamount interaction

International audienceA high-resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub-marine eruptions at a fast spreading ridge over a time interval of ~320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice towards it. Combining previous geochemical studies and bathymetry, we established a first-order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ~125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two-step hotspot-ridge interaction including a first stage (≥ 600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (< hundreds ya) as erupted lavas are exclusively recorded in a cluster proximal to the axial zone of the ridge

Isotope fractionation of palladium in chemical exchange reaction

consultable à l'adresse suivante : http://www.oldenbourg-link.com/doi/abs/10.1524/rcpr.2011.0060International audiencePalladium isotopes were fractionated by the solvent extraction technique with a crown ether. After purification by ion-exchange chemistry, the isotopic ratios of 105Pd/106Pd, 108Pd/106Pd, and 110Pd/106Pd were determined by multiple-collector inductively coupled plasma mass spectrometry. Isotope fractionations between the two phases were found to be larger than 0.1‰. The isotope fractionation of the odd atomic mass isotope (105Pd) showed a deviation from that estimated from the even atomic mass isotopes (106Pd, 108Pd, and 110Pd). The mass-independent isotope fractionation found was attributable to the nuclear field shift effect. The quantum chemical calculations for the different Pd species supported the validity of the isotope fractionation factors obtained

Sr stable isotope composition of Earth, the Moon, Mars, Vesta and meteorites

International audienceHigh-precision stable Sr isotopic variations (88Sr/86Sr) are reported in a variety of terrestrial samples, martian and lunar meteorites, HED, undifferentiated primitive meteorites, chondrules and refractory inclusions. Almost all the whole-rock samples are isotopically indistinguishable at a 50 parts per million (ppm) level. The exceptions are CV and CO chondrites which are isotopically light and for which we believe that their isotopic composition is controlled by the proportion of refractory material. Five separated chondrules and one refractory inclusion from Allende are isotopically light, with δ88/86Sr fractionations up to −1.73‰, whereas the matrix is enriched in the heavy isotopes (δ88/86Sr = + 0.66‰). The depletion in heavy isotopes observed in chondrules and refractory inclusions could be attributed to the condensation of a material already depleted in Sr, however, in that case more than 60% of the original material would be unaccounted. We propose instead that isotopic fractionation by electromagnetic sorting of ionized heavy Sr from neutral Sr in the early solar system for the origin of the fractionation observed in refractory inclusions and redistribution of Sr by aqueous alteration for the origin of the fractionation observed in chondrules and matrix. We conclude that CV and CO chondrites are not the primary building blocks for Earth and Mars. Research Highlights ► The Sr of most planetary materials is isotopically indistinguishable at a 50 ppm level. ► Exceptions are CV and CO chondrites which are isotopically light. ► Their isotopic composition is controlled by the proportion of refractory material. ► Chondrules and refractory inclusion from Allende are isotopically light. ► The Sr isotopic composition of CAIs can be explained by electromagnetic separation in the early solar system. ► The Sr isotopic composition of chondrules and matrix can be explained by aqueous alteration