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 Geocheminar

Abstract: The ability to measure fractionation of Fe isotopes in natural substances, developed within the past 5 to 10 years, has raised questions about the interpretation of iron isotopes in the geological record. Is the isotopic signature indicative of life or can such fractionation also occur abiotically? Is the fractionation due mainly to redox effects or to the effect of the ligands to which the iron is bonded?

We modeled equilibrium iron isotope fractionation from first principles for a sequence of ferric chloride complexes and for ferrous hexahydrate. We studied the effects on isotopic fractionation due to the progressive subsitution of Cl­ ligands for OH2 ligands in the complexes, compared them with the effects of Fe3+/Fe2+ fractionation, and examined other factors that influence the magnitude of the fractionation, such as coordination number, bond length, and bond stiffness.

Abstract: Hydroxyl solubility in rutile was determined as a function of pressure, temperature and oxygen fugacity (buffered at nickel-nickel oxide and hematite-magnetite). (OH) in rutile increases with increasing T and f(O2); the effect of pressure is less pronounced. Over the 25 seconds that it takes to quench the experiments, loss of (OH) from rutile along the c crystallographic axis is significant. Measurements of (OH) should be obtained in the centers of rutile grains at least 0.5 mm in diameter to obtain equilibrium values using our methods.

Abstract: Methane may have provided sufficient greenhouse warmth to prevent the early Earth from freezing despite the low luminosity of the young Sun. While biological activity could have generated a significant methane flux to the atmopshere, hydrothermal alteration of ultramafic rocks such as komatiites remains an important alternative mechanism. To test this hypothesis, natural and synthetic komatiites were reacted for two months at 300 degC and 350 bars in the presence of a carbon-bearing fluid. Our results suggest that the flux of methane generated by komatiites could have at least been on par with that by biology.

Abstract: In 2006, we published work showing that Ti3+ abundances in the fassaite of Wark-Lovering Rims around the CAI Leoville 144A was largely zero, indicating a rapidly increasing oxygen fugacity in the CAI environment consistent with that of chondrule formation. Subsequent investigators have been critical of this data, and have used XANES measurements to call the low-Ti3+ of WL rim fassaite into question. They explain our results as a mixture of fassaite + spinel.

Here I will present our original study, their criticisms, and recent work that confirms the original data and conclusions.

Abstract: The history of oxygen in the Earth's atmosphere and oceans is intimately connected to the evolution of oxygenic photosynthesis and other key metabolisms and hence is a topic of broad interest. However, we have only a rudimentary understanding of this history. Although it is widely accepted that PO2 was low before the "Great Oxidation Event" (GOE) ca. 2.4 Ga, we do not know if O2 was essentially absent or if Archean oxygen oases and transient, non-steady-state atmospheric oxygenation events were the norm. After the GOE, it is unclear if the oceans became fully oxygenated right away or if sulfidic conditions were common in large areas of the oceans until the Neoproterozoic. This talk will review new insights into these questions emerging from studies of redox sensitive metals and their stable isotopes in the geologic record.

Abstract: Solubility experiments involving natural fluids and important, rock forming minerals allow insight into the chemistry of high P-T fluids. The solubility of diopside (CaMgSi2O6) in H2O and H2O-NaCl solutions has been measured at 650-900°C, 0.7 to 1.5 GPa in a piston cylinder apparatus. Diopside was found to dissolve incongruently to forsterite plus dissolved species at all conditions investigated. The data indicate that the solubility of diopside + forsterite in pure H2O increases with increasing pressure and temperature.

Abstract: Secondary carbonates in CM chondrites provide evidence of aqueous alteration that occurred in the CM parent body. In this study, we focus on Mn-rich calcite (CaCO3) and dolomite (CaMg(CO3)2) grains that are widespread throughout the fine-grained matrix. Alteration products such as carbonates can potentially provide a record of the timing of aqueous alteration on the parent body. Here we describe chemical characteristics of the carbonates in the highly altered QUE 93005 CM chondrite and initial results of 53Mn-53Cr dating of these carbonates using the high-resolution ims 1270 ion microprobe. Our data and previous studies imply that the degree of aqueous alteration is correlated with the age of carbonate formation and that alteration processes occurred for an extended period of time on carbonaceous chondrite parent bodies.

Abstract: Supercritical fluids with compositions intermediate between H2O and silicate are widely invoked as important transport agents in subduction zones. This proposal is in part motivated by the expectation that such fluids might have greater ability to dissolve and transport key trace elements at high P and T. As a test of this hypothesis, we measured the solubility of rutile (TiO2) in supercritical albite (ab, NaAlSi3O8)-H2O at 900°C, 1.5 GPa, from Xab = 0 to 0.4. At this P and T, rutile has very low solubility in H2O and there is full miscibility between H2O and ab melt. Experiments were conducted in a piston-cylinder apparatus with NaCl-graphite furnaces. In each, a 1.6 mm OD Pt inner capsule with a synthetic rutile crystal was lightly crimped and placed in a 3.5 OD Pt capsule with ultra pure H2O and powdered Amelia albite. Equilibrium was achieved after 4 hrs. Solubility was determined by the weight loss of the rutile grain. Quench textures consistent with supercritical behavior were observed in all runs. Residual corundum is present in the H2O-rich runs, but it decreases with increasing ab concentration. Results show that rutile solubility initially rises sharply with increasing ab concentration from 90 ppm in pure H2O to 739 ppm at Xab =0.05 (44 wt%). With further increase in ab, rutile solubility increases only slightly, to 922 ppm at Xab =0.25 (83 wt%). No significant solubility increase was noted near the critical composition (~50 wt% ab). Our results show that intermediate fluids do not significantly enhance Ti solubility above dilute silicate- bearing solutions. The presence of residual Al2O3 and the sharp initial rise in rutile solubility at low Xab imply that, by analogy with silicate melts, Ti is present in solution as Na-Ti-O complexes (e.g., Dickenson and Hess, 1985, GCA, 49, 2289). However, the lack of residual corundum at high Xab suggests a transition to different Ti species, perhaps aqueous NaAlSi3O8-like complexes. Our results give insight into rutile in high- P veins. Low Ti solubility requires that these features result from channelized flow and/or high fluid fluxes. For example, at our experimental conditions, growth of a 1 mm3 crystal from a fluid produced by 2 wt% dehydration from metabasalt and containing 5 wt% ab would require 61 m3 of rock, assuming the fluid precipitates all dissolved Ti. If the supercritical ab-H2O system adequately approximates the possible range of natural fluids, it is evident that supercritical fluids may not have the ability to dissolve and mobilize significant concentrations of nominally insoluble trace elements.