HR: 11:05h
AN: V22B-04    [Abstracts]
TI: Disequilibrium Experiments and Micro-XANES Analysis: Novel Tools to Unravel the Speciation of Sulfur in Silicate Melts.
AU: * Jugo, P J
EM: pjugo@Laurentian.ca
AF: Laurentian University, 935 Ramsey Lake Rd., Sudbury, ON P3E 2C6, Canada
AU: Wilke, M
AF: Universitaet Potsdam, Karl-Liebknecht-Str. 24, Potsdam-Golm, D-14476, Germany
AU: Susini, J
AF: ESRF, 6 rue Jules Horowitz, Grenoble, F-38043, France
AB: Sulfur is an element of interest in magmatic processes for several reasons, some of which are related to the oxidation state of S during magma generation and evolution. For example, S as sulfide (S^2-) controls the behavior of chalcophile and highly siderophile elements, whereas S as sulfate (S⁶⁺) is responsible for high- S explosive volcanic eruptions, which can cause global cooling by increasing the Earth's albedo. An adequate understanding of the speciation of S in magmatic systems and the transition from S^2- to S⁶⁺ is therefore needed to understand these processes. Data from natural samples is incomplete and experimental data are required to link natural data with oxygen fugacity (fO₂). However, the change in speciation from sulfide to sulfate in silicate melts is difficult to simulate experimentally because: (a) common capsule materials react with S (e.g. Pt) or have low melting points (e.g. Au); (b) the change in speciation occurs over a very narrow fO₂ interval (FMQ to FMQ+2) and common buffering techniques (e.g. "double capsule" technique with FMQ or NNO buffers) are not sufficient to investigate a wide-enough range in fO₂; (c) sulfur solubility in silicate melts in the fO₂ range of interest is too low at atmospheric pressures, limiting the use of gas-mixing furnaces to either very oxidized or very reduced conditions. We have used disequilibrium experiments in which sulfate-saturated (i.e. oxidized) basaltic and andesitic melts were reacted with graphite (a reductant) and quenched before the system reached equilibrium. Quenching of the experiments before complete re-equilibration (i.e. complete reduction by graphite) preserved reduction profiles in which sulfate-saturated glass (in the center of the capsule) coexisted with sulfide-saturated glass (at the edge of graphite capsule). We used the ID-21 beamline at the European Synchrotron Radiation Facility (ESRF) to perform micro-XANES analysis at the S K- edge to determine the speciation of sulfur along the reduction profile. Tests with a combination of broad (50 μm to 200 μm) and narrow beams (0.8 μm) showed that beam-sample interaction caused partial reduction of S⁶⁺ to S⁴⁺ (sulfite) in the glass. However, sulfate reduction under the beam is time- dependent and we modified our analytical protocol to avoid beam damage. By performing rapid scans across the samples by resonant excitation at the energy position of each species of interest we were able to record the transition from sulfide to sulfate in a single experiment over a reduction profile of approximately 120 μm in length.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3618 Magma chamber processes (1036)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3694 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting