ALBERT

Description: The recent discovery in high-pressure experiments of compounds stable to 24-26 GPa with Fe4O5, Fe5O6, Fe7O9, and Fe9O11 stoichiometry has raised questions about their existence within the Earth's mantle. Incorporating both ferric and ferrous iron in their structures, these oxides if present within the Earth could also provide insight into diamond-forming processes at depth in the planet. Here we report the discovery of metallic particles, dominantly of FeNi (Fe0.71Ni0.24Cu0.05), in close spatial relation with nearly pure magnetite grains from a so-called superdeep diamond from the Earth's mantle. The microstructural relation of magnetite within a ferropericlase (Mg0.60Fe0.40)O matrix suggests exsolution of the former. Taking into account the bulk chemistry reconstructed from the FeNi(Cu) alloy, we propose that it formed by decomposition of a complex metal M oxide (M4O5) with a stoichiometry of (Fe3+2.15Fe2+1.59Ni2+0.17Cu+0.04)Σ=3.95O5 We further suggest a possible link between this phase and variably oxidized ferropericlase that is commonly trapped in superdeep diamond. The observation of FeNi(Cu) metal in relation to magnetite exsolved from ferropericlase is interpreted as arising from a multistage process that starts from diamond encapsulation of ferropericlase followed by decompression and cooling under oxidized conditions, leading to the formation of complex oxides such as Fe4O5 that subsequently decompose at shallower P-T conditions.

Description: Published

Description: 21088-21094

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: Platinum-group elements (PGE), rhenium and osmium isotope data are reported for basalts from Deep Sea Drilling Project cores in the Philippine Sea Plate (PSP). Lithophile trace element and isotopic characteristics indicate a range of source components including DMM, EMII and subduction-enriched mantle. MORB-like basalts possess smooth, inclined chondrite-normalised PGE patterns with high palladium-PGE/iridium-PGE ratios, consistent with previously published data for MORB, and with the inferred compatibility of PGE. In contrast, while basalts with EMII-type lithophile element chemistry possess high Pt/Ir ratios, many have much lower Pd/Ir and unusually high Ru/Ir of 〉10. Similarly, back-arc samples from the Shikoku and Parece-Vela basins have very high Ru/Ir ratios (〉30) and Pd/Ir as low as 1.1. Such extreme Pd/Ir and Ru/Ir ratios have not been previously reported in mafic volcanic suites and cannot be easily explained by variable degrees of melting, fractional crystallisation or by a shallow-level process such as alteration or degassing. The data appear most consistent with sampling of at least two mantle components with distinct PGE compositions. Peridotites with the required PGE characteristics (i.e. low Pd, but relatively high Ru and Re) have not been documented in oceanic mantle, but have been found in sub-continental mantle lithosphere and are the result of considerable melt depletion and selective metasomatic enrichment (mainly Re). The long-term presence of subduction zones surrounding the Philippine Sea Plate makes this a prime location for metasomatic enrichment of mantle, either through fluid enrichment or infiltration by small melt fractions. The Re-Os isotope data are difficult to interpret with confidence due to low Os concentrations in most samples and the uncertainty in sample age. Data for Site 444A (Shikoku Basin) give an age of 17.7+/-1.3 Ma (MSWD = 14), consistent with the proposed age of basement at the site and thus provides the first robust radiometric age for these samples. The initial 187Os/188Os of 0.1298+/-0.0069 is consistent with global MORB, and precludes significant metasomatic enrichment of Os by radiogenic slab fluids. Re-Os data for Sites 446A (two suites, Daito Basin) and 450 (Parece-Vela Basin) indicate ages of 73, 68 and 43 Ma, which are respectively, 30, 17 and 〉12 Ma older than previously proposed ages. The alkalic and tholeiitic suites from Site 446A define regression lines with different 187Os/188Osinitial (0.170+/-0.033 and 0.112+/-0.024, respectively) which could perhaps be explained by preferential sampling of interstitial, metasomatic sulphides (with higher time-integrated Re/Os ratios) by smaller percentage alkalic melts. One sample, with lithophile elements indistinguishable from MORB, is Os-rich (146 pg/g) and has an initial 187Os/188Os of 0.1594, which is at the upper limit of the accepted OIB range. Given the Os-rich nature of this sample and the lack of evidence for subduction or recycled crust inputs, this osmium isotope ratio likely reflects heterogeneity in the DMM. The dataset as a whole is a striking indication of the possible PGE and Os isotope variability within a region of mantle that has experienced a complex tectonic history.

Description: Leg 165 of the Ocean Drilling Program afforded a unique opportunity to investigate organic and inorganic geochemistry across a wide gradient of sediment compositions and corresponding chemical pathways. The solid fractions at Sites 998, 999, 1000, and 1001 reveal varying proportions of reactive carbonate species, a labile volcanic ash fraction occurring in discrete layers and as a dispersed component, and detrital fluxes that derive from continental weathering. The relative proportions and reactivities of these end-members strongly dictate the character of the diagenetic profiles observed during the pore-water work of Leg 165. In addition, alteration of the well-characterized basaltic basement at Site 1001 has provided a strong signal that is reflected in many of the dissolved components. The relative effects of basement alteration and diagenesis within the sediment column are discussed in terms of downcore relationships for dissolved calcium and magnesium. With the exception of Site 1002 in the Cariaco Basin, the sediments encountered during Leg 165 were uniformly deficient in organic carbon (typically 〈0.1 wt%). Consequently, rates of organic oxidation were generally low and dominated by suboxic pathways with subordinate levels of bacterial sulfate reduction and methanogenesis. The low rates of organic remineralization are supported by modeled rates of sulfate reduction. Site 1000 provided an exception to the generally low levels of microbially mediated redox cycling. At this site the sediment is slightly more enriched in organic phases, and externally derived thermogenic hydrocarbons appear to aid in driving enhanced levels of redox diagenesis at great depths below the seafloor. The entrapment of these volatiles corresponds with a permeability seal defined by a pronounced Miocene minimum in calcium carbonate concentration recognized throughout the basin and with a dramatic downcore increase in the magnitude of limestone lithification. The latter has been tentatively linked to increases in alkalinity associated with microbial oxidation of organic matter and gaseous hydrocarbons. Recognition and quantification of previously unconstrained large volumes and frequencies of Eocene and Miocene silicic volcanic ash within the Caribbean Basin is one of the major findings of Leg 165. High frequencies of volcanic ash layers manifest as varied but often dominant controls on pore-water chemistry. Sulfur isotope results are presented that speak to secondary metal and sulfur enrichments observed in ash layers sampled during Leg 165. Ultimately, a better mechanistic understanding of these processes and the extent to which they have varied spatially and temporally may bear on the global mass balances for a range of major and minor dissolved components of seawater.