Although water controls the biology and geology of the surface, hydrogen is perhaps the most poorly constrained compositional variable in the bulk Earth. Its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases, olivine, pyroxene, garnet, wadsleyite, and ringwoodite. Here we describe a series of experiments showing that the solubility of H 2 O in olivine at 12 GPa increases with temperature to 8900 ppm by weight at 1250°C and decreases at higher temperature with the onset of melting. Sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2H + for Mg 2+ . Similar results obtained from samples coexisting with clinohumite (low‐silica) and with clinoenstatite (high‐silica) indicate that silica activity has minimal effect on hydration under these conditions. Single‐crystal X‐ray diffraction measurements constrain the volume of hydration and indicate significant M‐site vacancies. Hydrogen thus appears to become a geochemically compatible element as depths approach 400 km.
M. A. Demange 2012. Mineralogy for Petrologists: Optics, Chemistry and Occurrences of Rock-Forming Minerals. xvi + 201 pp. CRC Press. Price £49.99, US$79.95 (HB+CD). ISBN 9780415684217. - Volume 150 Issue 2
Abstract Impact ejected rocks are targets for life detection missions to Mars. The Martian subsurface is more favourable to organic preservation than the surface owing to an attenuation of radiation and physical separation from oxidising materials with increasing depth. Impact events bring materials to the surface where they may be accessed without complicated drilling procedures. On Earth, different assemblages of organic matter types are derived from varying depositional environments. Here we assess whether these different types of organic materials can survive impact events without corruption. We subjected four terrestrial organic matter types to elevated pressures and temperatures in piston-cylinder experiments followed by chemical characterisation using whole-rock pyrolysis-gas chromatography-mass spectrometry. Our data reveal that long chain hydrocarbon-dominated organic matter (types I and II; mainly microbial or algal) are unresistant to pressure whereas aromatic hydrocarbon-dominated organic matter types (types III and IV; mainly land plant, metamorphosed or degraded, displaying some superficial chemical similarities to abiotic meteoritic organic matter) are relatively resistant. This suggests that the impact excavated record of potential biology on Mars will be unavoidably biased, with microbial organic matter underrepresented while metamorphosed, degraded or abiotic meteoritic organic matter types will be selectively preserved.
Abstract Manganese incorporation in synthetic hercynite, and partitioning between hercynite and silicate melt synthesized at 1.0 GPa, 1250°C, and at an f O 2 buffered by Fe–FeO, has been studied by X-ray absorption spectroscopy and single-crystal X-ray structure refinement. Spectra indicate the presence of both Mn 2+ and Mn 3+ (and possibly also Mn 4+ ) in synthetic hercynite and partitioning of Mn 2+ into the melt phase, and Mn 3+ into hercynite, respectively, under run conditions. X-ray refinement is consistent with partial disorder of Fe and Al across tetrahedral and octahedral sites. A higher than expected degree of Fe-Al disorder in the Mn-bearing hercynite can be explained by preferential incorporation of Mn 2+ onto the tetrahedral site, and indicates that Fe-Al disorder in pure, stoichiometric hercynite cannot necessarily be used to determine closure temperatures in natural spinel. However, partitioning of Mn 2+ and Mn 3+ between melt and hercynite suggests that Mn incorporation in hercynite could be used as a measure of f O 2 conditions in magmas during spinel crystallization.
Abstract The solubility and incorporation mechanisms of H and various trivalent and divalent cations in synthetic rutile have been investigated. Experiments performed using different bulk Fe 2 O 3 contents demonstrate that Fe 3+ substitutes onto the main Ti site, charge-balanced by oxygen vacancies. Under more reducing conditions in Fe-poor systems, the concentration of Ti interstitials in rutile is increased, resulting in a decrease in H solubility. Variation in the solubility of different oxides in rutile as a function of ionic radius implies substitution onto the main Ti site, probably charge-balanced by oxygen vacancies. To a lesser degree, substitution of trivalent and divalent cations is locally charge-balanced by H incorporation. Variation in OH-stretching frequencies in infrared spectra as a function of composition implies that octahedral defects and structurally-incorporated H are coupled. However, in all samples, some of the H is also decoupled from substitutional impurities, as is evident from an OH-absorption band at 3279 cm –1 . This band corresponds to the main OH band seen in spectra of many natural rutiles, implying that in most rutiles, H defects are decoupled from substitutional defects.
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