Marc Chaussidon

H2O CONCENTRATIONS IN PRIMARY MELTS FROM SUPRA-SUBDUCTION ZONES AND MID-OCEAN RIDGES : IMPLICATIONS FOR H2O STORAGE AND RECYCLING IN THE MANTLE

A total of 145 inclusions, trapped and isolated in Mg-rich olivine phenocrysts (Fo0.87–0.94) from basalts and ultramafic lavas, and representing the most primitive mantle melts known, have been analysed by ion microprobe for their H2O contents. This approach allows us to conduct a general survey of the distribution of water in primary melts derived from the mantle beneath mid-ocean ridges and above subduction zones. The primitive melts included in MORB olivines have low H2O contents (mean at 0.12 wt% for N-MORB (14 samples), 0.17 wt% for T-MORB (9 samples) and 0.51 wt% for E-MORB (14 samples)). A strong decoupling between H2O and K2O has been found in some MORB primary melts which might well be explained by the presence of a H2O-bearing CO2-rich fluid. In contrast with mid-ocean ridges, primitive melts of subduction zones basalts are very rich in H2O (between 1.0 and 2.9 wt% (mean at 1.7 wt%, 84 samples) for boninites and between 1.2 and 2.5 wt% (mean at 1.6 wt%, 24 samples) for island arc tholeiites). In addition, most of these melts have high H2O/K2O ratios which are consistent with a transfer of H2O as a fluid phase from the subducted slab to the mantle wedge. For boninites and island arc primary melts, the present H2O contents are ≈ 2.5 × higher than commonly assumed, which suggests that the amount of H2O released to the surface in arc magmatism has been previously underestimated.

Isotopic Compositions of Cometary Matter Returned by Stardust

Hydrogen, carbon, nitrogen, and oxygen isotopic compositions are heterogeneous among comet 81P/Wild 2 particle fragments; however, extreme isotopic anomalies are rare, indicating that the comet is not a pristine aggregate of presolar materials. Nonterrestrial nitrogen and neon isotope ratios suggest that indigenous organic matter and highly volatile materials were successfully collected. Except for a single 17O-enriched circumstellar stardust grain, silicate and oxide minerals have oxygen isotopic compositions consistent with solar system origin. One refractory grain is 16O-enriched, like refractory inclusions in meteorites, suggesting that Wild 2 contains material formed at high temperature in the inner solar system and transported to the Kuiper belt before comet accretion.

A palaeotemperature curve for the Precambrian oceans based on silicon isotopes in cherts

The terrestrial sediment record indicates that the Earth’s climate varied drastically in the Precambrian era (before 550 million years ago), ranging from surface temperatures similar to or higher than today’s to global glaciation events. The most continuous record of sea surface temperatures of that time has been derived from variations in oxygen isotope ratios of cherts (siliceous sediments), but the long-term cooling of the oceans inferred from those data has been questioned because the oxygen isotope signature could have been reset through the exchange with hydrothermal fluids after deposition of the sediments. Here we show that the silicon isotopic composition of cherts more than 550 million years old shows systematic variations with age that support the earlier conclusion of long-term ocean cooling and exclude post-depositional exchange as the main source of the isotopic variations. In agreement with other lines of evidence, a model of the silicon cycle in the Precambrian era shows that the observed silicon isotope variations imply seawater temperature changes from about 70 °C 3,500 million years ago to about 20 °C 800 million years ago.

Enriched and depleted primitive melts included in olivine from Icelandic tholeiites: origin by continuous melting of a single mantle column

Iceland represents a type locality for mid-ocean ridge and plume-related magmatism. The petrogenesis of Icelandic lavas, however, is complicated by high-level crustal fractionation enhanced by the thick Icelandic crust, as well as assimilation of crustal material. Here we present results of major and trace elements studies of melt inclusions in high-magnesium olivines (Fo85.6–90.6) taken from the most primitive tholeiitic lavas found on Iceland. The compositions of the melt inclusions indicate that they represent very primitive trapped melts. Two populations of inclusions, enriched melt inclusions (EM: 0.07–0.23 wt% K2O, 0.07–0.52 wt% P2O5, and 0.54–1.78 wt% TiO2) and depleted melt inclusions (DM: 0.01–0.05 wt% K2O, 0.02–0.07 wt% P2O5, and 0.27–1.02 wt% (TiO2) can readily be distinguished on the basis of major and trace elements (e.g., (La/Sm)n ratios ranging between 0.14 and 1.89, (Sm/Yb)n ratios between 0.62 and 2.59, and Zr/Y ratios between 0.69 and 5.85). The compositions bracket the range of Icelandic primitive magmas, which we believe to be the result of mixing between these two endmembers. These two primary melt populations can be produced by critical (continuous) melting of a single mantle column, without the addition of material from the crust. In this model, the EM inclusions represent mixtures between enriched and depleted instantaneous melts in the ratios 0.6:0.4, respectively, where the first was formed in equilibrium with a garnet-bearing (up to 6 wt% of garnet) mantle source of primitive composition at a low degree of melting (F = 5.5%) and with 2.7 wt% of critical melt retained in residue. The most depleted inclusions represent unmixed instantaneous melts produced by 17–18% melting of a spinel lherzolite (either primitive or depleted composition) with slightly different amounts of critical melt (3.0 to 3.5 wt%) depending on the type of the source. In addition, Ba and Sr concentration anomalies noted in previous studies of Icelandic lavas are also present in the DM inclusions. They have been variously ascribed to assimilation-contamination processes, but can be explained by the presence of plagioclase in the source of the high level melt fractions. Thus, the complete range of Icelandic primary compositions can be produced by mineralogical variations in the mantle column in the framework of a dynamic melting model.

Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications

Ion microprobe determination of boron content and δ11B values has been performed for a set of 40 oceanic basalt glasses (N-MORB, E-MORB, BABB and OIB) whose chemical characteristics (major and trace elements and isotopic ratios) are well documented. Boron contents, determined at ±10% relative, range from 0.34 to 0.74 ppm in N-MORB, whereas E-MORB, BABB and OIB extend to higher concentrations (0.5–2.4 ppm). After correction for crystal fractionation, this range is reduced to 0.5–1.3 ppm. N-MORB and E-MORB also exhibit different B/K ratios, 1.0 ± 0.3 × 10−3 and 0.2 to 1.4 × 10−3 respectively. This can be interpreted as resulting from the incorporation into the upper mantle of a K-rich and B-poor component (e.g., subducted oceanic crust having lost most of its initial boron). δ11B values range between −7.40 ± 2 and +0.6 ± 2‰, with no significant difference between N-MORB, E-MORB, OIB or BABB. The Hawaiian samples define a strong linear correlation between boron contents, δ11B values, MgO and water contents and δD values. This is interpreted as resulting from assimilation-fractionation processes which occurred within a water-rich oceanic crust, and which produced high δ11B values associated with high δD values. The low level of11B enrichment in the upper mantle constrains the amount of boron reinjected by subduction to a maximum of about 2% of the boron present in the subducted slab. This is turn corresponds to a maximum net Boron transfer of about 3 x 1010 g/a towards the surface reservoirs. Finally, a boron content of 0.25 ± 0.1 ppm is estimated for the bulk silicate Earth (i.e., primitive mantle), corresponding to a depletion factor relative to C1 chondrites of about 0.15 and suggesting that B was moderately volatile upon terrestrial accretion.

Homogeneous Distribution of 26Al in the Solar System from the Mg Isotopic Composition of Chondrules

Solar Chronometer The use of the short-lived radioactive isotope 26Al as a precise chronometer of early solar system processes relies on the assumption that it was uniformly distributed in the initial solar accretion disk. Villeneuve et al. (p. 985; see the Perspective by Davis) validate this assumption on the basis of high-precision isotopic analyses of primitive meteoritic materials. Furthermore, chondrules—constituents of the most common type of meteorites and among the first materials to have formed in the solar system—formed episodically over a period of more than one million years. High-precision isotopic analyses in chondrule minerals validate the use of 26Al as an early solar system chronometer. The timing of the formation of the first solids in the solar system remains poorly constrained. Micrometer-scale, high-precision magnesium (Mg) isotopic analyses demonstrate that Earth, refractory inclusions, and chondrules from primitive meteorites formed from a reservoir in which short-lived aluminum-26 (26Al) and Mg isotopes were homogeneously distributed at ±10%. This level of homogeneity validates the use of 26Al as a precise chronometer for early solar system events. High-precision chondrule 26Al isochrons show that several distinct chondrule melting events took place from ~1.2 million years (My) to ~4 My after the first solids condensed from the solar nebula, with peaks between ~1.5 and ~3 My, and that chondrule precursors formed as early as 0.87-0.16+0.19 My after.

A 15N-Poor Isotopic Composition for the Solar System As Shown by Genesis Solar Wind Samples

The solar atmosphere is about 40% enriched in the heavy nitrogen-15 isotope compared with the Sun and Jupiter. The Genesis mission sampled solar wind ions to document the elemental and isotopic compositions of the Sun and, by inference, of the protosolar nebula. Nitrogen was a key target element because the extent and origin of its isotopic variations in solar system materials remain unknown. Isotopic analysis of a Genesis Solar Wind Concentrator target material shows that implanted solar wind nitrogen has a 15N/14N ratio of 2.18 ± 0.02 × 10−3 (that is, ≈40% poorer in 15N relative to terrestrial atmosphere). The 15N/14N ratio of the protosolar nebula was 2.27 ± 0.03 × 10−3, which is the lowest 15N/14N ratio known for solar system objects. This result demonstrates the extreme nitrogen isotopic heterogeneity of the nascent solar system and accounts for the 15N-depleted components observed in solar system reservoirs.

Primitive boron isotope composition of the mantle.

Boron isotope ratios are homogeneous in volcanic glasses of oceanic island basalts [–9.9 � 1.3 per mil, relative to standard NBS 951 (defined by the National Bureau of Standards)], whereas mid-oceanic ridge basalts (MORBs) and back-arc basin basalts (BABBs) show generally higher and more variable ratios. Melts that have assimilated even small amounts of altered basaltic crust show significant variations in the boron isotope ratios. Assimilation may thus account for the higher boron ratios of MORBs and BABBs. A budget of boron between mantle and crust implies that the primitive mantle had a boron isotope ratio of –10 � 2 per mil and that this ratio was not fractionated significantly during the differentiation of the mantle.

pH control on oxygen isotopic composition of symbiotic corals

Abstract Boron, carbon and oxygen isotopic compositions were determined at the micrometre scale by high-resolution ion microprobe in a sample of modern coral (massive hermatypic coral, Porites lutea). The ion probe data show for B and O much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques: δ18OPDB values range from −10.6±0.9‰ to −0.2±0.5‰ and δ11B values range from +18.6±1.5‰ to +30.6±1.6‰. By contrast, δ13C values show the same range of variations, from −4.6±0.65‰ to −2.2±0.67‰ at the micrometre and millimetre scales. The range of δ11B values indicates that significant pH variations, from ≈7.1 to ≈9.0, are present at the sites of calcification. The largest δ18O variations correspond to the highest δ11B values, i.e. to the highest pHs. This measurement of pH allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation. Taking into account the rate of O isotopic equilibrium between dissolved carbonate species (H2CO3, HCO3− and CO32−) and water via the two reactions of hydration and hydroxylation, the full range of δ18O values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between ≈1 h and at maximum ≈12 h. The pH controls the δ18O of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation. The so-called ‘vital effect’ systematically observed for δ18O in corals can thus be understood as representing an average of rapid pH variations due to coral biology during coral growth. Selectively measuring δ18O values in the zones of coral skeletons that have low δ11B values (i.e. formed at low pH) should significantly improve the quality of palaeoclimatic reconstructions based on δ18O values.

Sulphur isotope variations in the mantle from ion microprobe analyses of micro-sulphide inclusions

Abstract 21 samples of sulphide trapped either as liquid globules or grains in various minerals (olivine, pyroxenes, ilmenite and garnet) or rocks (basalt glasses, peridotites, eclogites and kimberlites) of mantle origin, have been analysed for their sulphur isotope, and their Cu, Ni, Fe compositions by ion microprobe. The results show a wide range of δ 34 S values between −4.9 ± 1 and +8 ± 1‰. Sulphides with high nickel contents (up to 40% pentlandite), corresponding mostly to residual peridotites, have δ 34 S values ranging from −3.2‰ to +3.6‰ with a mode of +3 ± 1‰, compared to low Ni content sulphides, mostly contained in pyroxenites, OIB and kimberlites, ranging from −3.6‰ to +8‰ with a mode of +1 ± 1‰. The δ 34 S of sulphides originating from within the mantle are variable. The sulphide globules with high Ni contents and δ 34 S values close to +3‰, are probably produced by 10–20% partial melting of a mantle source containing 300 ppm sulphur as an upper limit and having a δ 34 S value of +0.5 ± 0.5‰. This difference in δ 34 S values suggests a high-temperature S-isotope fractionation of ≈ +3‰ between liquid sulphide and the sulphur dissolved in the silicate liquid. The sulphur isotopes balance in the system upper mantle + oceanic crust + continental crust + seawater requires a mean δ 34 S value of the primitive upper mantle of +0.5‰, slightly but significantly different from that of chondrites (+0.2 ± 0.2‰) [1].