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In basaltic volcanic ash recovered from a seamount at 3000 m water depth, we discovered marcasite and pyrite precipitation within cavities that formed by partial to complete dissolution of olivine. In places, these cavities are reminiscent of negative crystal shapes; elsewhere they apparently continue along cracks. In strong contrast, adjacent volcanic glass shows little, if any, evidence for dissolution. The FeS2 precipitates were commonly found to be conjoined and planar aggregates, occurring in the center of the voids. Their maximum volume fraction in relation to the void space as determined by 2D and 3D imaging techniques corresponds to the amount of iron released by olivine dissolution. Almost all occurrences of FeS2 precipitation are related to Cr-spinel inclusions in the former olivine. We propose that rapid olivine dissolution was initiated by reduced H2S-bearing fluids at olivine grain boundaries or surfaces exposed by cracks. Many of these cracks are connected to spinel grains, where the iron liberated from olivine is mineralized as FeS2, initially facilitated by heterogeneous nucleation. Subsequent pyrite and/or marcasite precipitation occurred as overgrowths on existing FeS2 aggregates. The particular chemical environment of low-pH, hydrogen sulfide-bearing fluids may have enhanced olivine dissolution by (1) keeping Fe in solution and (2) sequestering important quantities of Fe as FeS2. The in situ oxidation of ferrous Fe and precipitation of ferric hydroxides at the olivine surface commonly observed in oxic environments were obviously impeded. It would have slowed down olivine dissolution to rates more similar to the dissolution of basaltic glass. We have no direct indication that the process of rapid olivine dissolution was aided by subseafloor life. However, the presence of fibrous structures with small sulfide particles could indicate late colonization of sulfate-reducing bacteria that may add an additional path of iron fixation. [ABSTRACT FROM AUTHOR]

Graph HT ht (12) Finally, a feature observed at both temperatures is the inward increase in the width of chlorite coronas around olivine. [Extracted from the article]

Molar enthalpies of formation of natural, near end-member tremolite and talc have been calculated from high temperature lead borate drop-solution calorimetry. Using three mutually consistent reaction cycles for each phase, the critical volatile component [H.sub.2]O was treated in fundamentally different ways, namely as the simple 'oxide' [i.e., as the enthalpy difference ([H.sub.TCal] - [H.sub.298.15]) only], or as the (OH)-component in both low and high water-content phases, respectively. The molar enthalpies of drop solution of all solid phases participating in the various reaction cycles (brucite, diopside, enstatite, calcite, magnesite, quartz) were independently measured. We corroborate that the dynamic gas flow techniques introduced by Navrotsky et al. (1994) are critically important for very hydrous phases such as brucite, but that this dynamic technique yields the same results as traditional static drop-solution techniques for phases with relatively lower water contents such as tremolite and talc. When these critical differences in [H.sub.2]O behavior are taken into account, it can be shown that drop-solution measurements on tremolite and talc over the last twenty years actually lead to similar and consistent results on the enthalpy of formation of these phases; reported differences are due to incorrect assumptions on the final state of [H.sub.2]O. Our refined values for the enthalpy of formation from the elements for ideal end-member tremolite and talc range from -12299.2 to -12308.9 kJ/mol and -5892.1 to -5900.2 kJ/mol, respectively, depending on the internally consistent data set used for the reaction cycle components. These values are identical within error to the results refined from various phase-equilibrium experiments and show that these two fundamentally different techniques can lead to a consistent picture on the thermodynamic properties of hydrous minerals, a conclusion that has been strongly questioned in the past. Heat capacity data have been obtained for the same samples of tremolite and talc by differential scanning calorimetry in step-scanning mode in the range 50-500 [degrees] C and 50-400 [degrees] C, respectively. The following best-fit equations apply to the ideal end-members [T in Kelvin, [C.sub.p] in in J/(mol [multiplied by] K)] in the range 298.15 to 800 K and 298.15 to 650 K, respectively: [C.sub.p] (tremolite) = 1335.77586 - 0.02378 T- 1.00400 [10.sup.7] [T.sup.-2] - 9678.23152 [T.sup.-0.5]. [C.sub.p] (talc) = 1300.66304 - 0.03007 T - 1.31510 [10.sup.7] [T.sup.-2] - 8778.5468 [T.sup.-0.5].

This paper presents studies of samples from different localities of the Garnet-Amphibole-Magnetite (GAM)-Quartzite Member of the Ammonoosuc Volcanics from southern New England, USA. Based on stratigraphic and geochemical evidence, earlier workers interpreted the Ammonoosuc Volcanics as part of a volcanic island-arc complex. As such, these rocks have a highly varied stratigraphy with a wide spectrum of chemical compositions. During subsequent metamorphism, these rocks reached amphibolite-facies conditions (6 kbar and 580-640 [degrees] C). Within the Ammonoosuc Volcanics, a fine-grained, garnet-, magnetite-, and amphibole-bearing quartzite forms a distinct stratigraphic horizon. The quartzite commonly shows delicate millimeter- to centimeter-thick layering, which is interpreted as an original depositional feature that could have formed by precipitation from solutions enriched in silica, iron, and manganese. Volcanic detritus that was rich in Al, Ca, Mg, and Na could account for the variation in concentration of these elements, which are present along with Fe, Mn, and Si in some laminae of these quartzites. The mineralogy of layers within these quartzites can vary greatly on the scale of several centimeter. The most mineralogically complex layers contain, in addition to quartz, garnet, and magnetite, and one or more of the amphiboles cummingtonite, anthophyllite, or hornblende. In the most recent investigations of these rocks, both orthopyroxene and clinopyroxene have been identified in a few samples. A comparison of Mn contents of minerals that are common to different assemblages indicates that the pyroxenes are present in bulk compositions rich in Mn. The Mn content of the bulk composition is the most important factor controlling the mineralogy, and increasing the Mn content causes changes in the mineralogy that mimic the same progression of assemblages that is seen in mafic rocks undergoing the amphibolite- to granulite-facies transition.

Serpentinization of mantle peridotite generates molecular hydrogen that can be exploited by microorganisms to gain metabolic energy; however, the mechanisms that control hydrogen generation and magnetite formation during serpentinization remain poorly understood. We have examined partly to completely serpentinized peridotites recovered during the Ocean Drilling Program and find a remarkable variation in the abundance of magnetite. Some completely serpentinized peridotites have as much as 6.15 wt% magnetite, whereas others are nearly magnetite free (<0.04 wt%). Using isotopic, magnetic, and thermodynamic constraints, our study reveals a simple link between the abundance of magnetite, Fe content of brucite, and serpentinization temperature. Samples with abundant magnetite have Fe-poor brucite and were formed at temperatures of 200-300 °C, whereas magnetite-poor samples are associated with Fe-rich brucite and were formed at temperatures <~200 °C. We demonstrate that, despite the small amounts of magnetite, abundant hydrogen is generated through Fe(III) hosted in serpentine, lending support to the idea that low- to moderate-temperature serpentinization can generate abundant hydrogen within the temperature limits of life. [ABSTRACT FROM AUTHOR]