11B-rich fluids in subduction zones: the role of antigorite dehydration in subducting slabs and boron isotope heterogeneity in the mantle

Serpentinites form by hydration of mantle peridotite and constitute the largest potential reservoir of fluid-mobile elements entering subduction zones. Isotope ratios of one such element, boron, distinguish fluid contributions from crustal versus serpentinite sources. Despite 85% of boron hosted within abyssal peridotite being lost at the onset of subduction at the lizardite-to-antigorite transition, a sufficient cargo of boron to account for the composition of island arc magma is retained (c. 7 μg g− 1, with a δ11B of + 22‰) until the down-going slab reaches the antigorite-out isograd. At this point a 11B-rich fluid, capable of providing the distinctive δ11B signature of island arc basalts, is released. Beyond the uniquely preserved antigorite-out isograd in serpentinites from Cerro del Almirez, Betic Cordillera, Spain, the prograde lithologies (antigorite–chlorite–orthopyroxene–olivine serpentinite, granofels-texture chlorite-harzburgite and spinifex-texture chlorite-harzburgite) have very different boron isotope signatures (δ11B = − 3 to + 6‰), but with no significant difference in boron concentration compared to the antigorite-serpentinite on the low P–T side of the isograd. 11B-rich fluid, which at least partly equilibrated with pelagic sediments, is implicated in the composition of these prograde lithologies, which dehydrated under open-system conditions. Serpentinite-hosted boron lost during the early stages of dehydration is readily incorporated into forearc peridotite. This, in turn, may be dragged to sub-arc depths as a result of subduction erosion and incorporated in a mélange comprising forearc serpentinite, altered oceanic crust and pelagic sediment. At the antigorite-out isograd it dehydrates, thus potentially providing an additional source of 11B-rich fluids

New constraints on the abundances of silicate and oxide stardust from supernovae in the Acfer 094 meteorite

We studied about 5000 mu m(2) of fine-grained matrix material in the Acfer 094 meteorite by high-resolution (nominal 50 nm) NanoSIMS ion imaging for the presence of O-rich presolar (stardust) grains. This approach permits identifying presolar grains down to 150 nm in lower-resolution (nominal 100 nm) ion imaging surveys. The number density of identified presolar grains is a about a factor of two to three higher than what was found by lower-resolution ion imaging studies. The abundances of grains of O isotope Group 3 and 4 are higher than previously found. None of the presolar grains shows the strong enrichments in O-16 expected from model predictions for the majority of supernova (SN) grains. Other potential O-rich SN grains, the Group 4 and some of the Group 3 grains, make up 33% by number and 19% by mass. This is clearly higher than the similar to 10% (by number) inferred before and the 5% (by mass) estimated by a model for stellar dust in the interstellar medium. Our work shows that O-rich SN grains might be more abundant among the population of presolar grains in primitive solar system materials than currently thought, even without the O-16-rich grains as predominantly expected from SN models

New Insights into the Galactic Chemical Evolution of Magnesium and Silicon Isotopes from Studies of Silicate Stardust

We report high-resolution (<100 nm) Mg and Si isotope data of 12 presolar silicate grains (230–440 nm) from red giant and/or asymptotic giant branch stars that were previously identified based on their anomalous O-isotopic compositions (11 Group 1 grains and one Group 2 grain) in five primitive meteorites. The data were acquired by NanoSIMS ion imaging with the new Hyperion ion source that permits Mg and Si isotope measurements of presolar silicates with higher precision than was possible before. For a subset of five Group 1 ("category A") grains, 25Mg/24Mg and 29Si/28Si ratios correlate with the inferred initial 18O/16O ratios of their parent stars, a measure of stellar metallicity. The Mg and Si isotope data of category A grains show positive correlations in the δ 25Mg–δ 26Mg, δ 29Si–δ 30Si, and δ 25Mg–δ 29Si spaces. The correlations between O-, Mg, and Si-isotopic compositions are best explained by Galactic chemical evolution (GCE), with only minor imprints of nucleosynthetic and mixing processes in the grains' parent stars. Six Group 1 silicate ("category B") grains have close-to-normal Mg and Si isotopic compositions, possibly the result of isotope exchange in interstellar space or the meteorite parent bodies. For Si in category A grains, we find, with ~2σ significance, a slightly shallower slope in the δ 29Si–δ 30Si space for the GCE than inferred from presolar SiC mainstream grains. The 2σ upper limit on the slope for the linear trend in the δ 25Mg–δ 26Mg space of category A grains is slightly lower than the slope-1 predicted by GCE models around solar metallicity

Isotope Systematics of Presolar Silicate Grains: New Insights from Magnesium and Silicon

We report on Mg and Si isotope data of 86 presolar silicate grains identified through NanoSIMS oxygen ion imaging in thin sections of carbonaceous and ordinary chondrites. The O, Mg, and Si isotope data of 106 presolar silicates (including grains studied previously by our group) suggest division of O isotope Group 1 grains into four subpopulations: (i) "normal," (ii) 25Mg-rich, (iii) 26Mg-rich, and (iv) 25Mg-poor. Normal Group 1 grains (~60% of Group 1 grains) formed in the winds of low-mass asymptotic giant branch (AGB) stars, with Mg and Si defining linear arrays with slopes of ~0.9 and 1.37, respectively, in three-isotope representations, most likely representing Galactic chemical evolution (GCE). The 25Mg-rich grains (~25%) show enrichments in 25Mg of up to a factor 2.4 relative to solar composition and most likely formed in supernova (SN) ejecta or the winds of intermediate-mass AGB stars. The 26Mg-rich and 25Mg-poor Group 1 grains lie below the Mg GCE line and their isotopic compositions favor origins from supergiants or SNe. The O isotope Group 2 grains show a wide range of Mg-isotopic compositions, similar to Group 1 grains, with likely origins from massive AGB stars, super-AGB stars, supergiants, and SNe. The Mg- and Si-isotopic compositions of Group 4 grains are compatible with previously proposed SN origins. Our results suggest that >30% of presolar silicates formed in the winds of supergiants and in SN ejecta, and that low-mass AGB stars appear to have contributed only some 50% to presolar silicates, less than previously thought

From ocean to mantle: new evidence for U-cycling with implications for the HIMU source and the secular Pb isotope evolution of Earth's mantle

Of the isotopically distinctive mantle domains, the so-called HIMU (“high-μ”; μ = 238U/204Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206Pb/204Pb (reaching 56) but unchanged 208Pb/204Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208Pb/204Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum

Structural Investigation of Silicon Carbide X Grains: Constraints on Condensation in Supernova Ejecta

We analyzed seven presolar SiC grains of supernova origin (average diameter: 1–2 μm) with transmission electron microscopy. Five grains are polycrystalline, whereas two grains are single crystals. Individual crystal domains of polycrystalline grains are in epitaxial relationship, with two grains consisting almost entirely of twinned crystal domains. Most grains are free of inclusions (only one TiC inclusion and one iron- and nickel-rich inclusion were found in two separate grains). Almost all crystals have cubic symmetry (3C polytype), but we found hexagonal SiC (6H polytype) in two grains. The large range of crystal domain sizes (average diameter: 50–970 nm), as well as the larger fraction of noncubic SiC polytypes in supernova grains relative to SiC grains that crystallized in the winds of asymptotic giant branch (AGB) stars, suggest that SiC condensation in supernova ejecta occurs at a larger range of chemical and physical conditions, including supersaturation, than in the winds of AGB stars. Modeling condensation of SiC struggles to produce SiC grains as large as, or bigger than, observed here, if condensation of large (i.e., several μm in diameter) graphite grains is to precede that of SiC, which is suggested by the presolar grain record and published equilibrium condensation models. We propose that future models of graphite and SiC condensation in SN ejecta explore higher ejecta densities than before, as well as gas compositions that are more silicon- and carbon-rich. Furthermore, we infer that some supernova SiC grains may have formed without prior condensation of graphite from their parent gas