HR: 15:25h
AN: V32F-08    [PDF]
TI: Generation of Sulfur-rich, Sulfur-undersaturated Basaltic Melts in Oxidized Arc Sources.
AU: * Jugo, P J
EM: pjugo@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Luth, R W
EM: robert.luth@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada
AU: Richards, J P
EM: jeremy.richards@ualberta.ca
AF: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada
AB: Although sulfur is a minor element in the Earth, it has a disproportionate impact because it commonly occurs as sulfide. Sulfides largely control the behavior of chalcophile (e.g., Cu, Ni) and highly siderophile elements (Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) that are of interest because either they are economically important or because they provide valuable information about geochemical processes. Island arc basalts are more oxidized than basalts from other tectonic settings and therefore, in these settings, sulfur maybe present not as sulfide but as sulfate. In addition to the impact on the behavior of chalcophile and siderophile elements, sulfur speciation as sulfate may have a role on the occurrence of sulfur-rich explosive volcanism, which has been linked to significant short-term variations in global climate. However, little is known about the range in oxygen fugacity for the transition from solubility as sulfide to solubility as sulfate. We used experimental data on the solubility of sulfur in basaltic melts saturated with either sulfide or sulfate at different oxygen fugacities to model this transition. Our model shows that the ten-fold increase in the solubility of sulfur (from 0.14 wt.% to 1.5 wt.%) observed experimentally occurs at oxygen fugacities between $\sim$FMQ+1 and $\sim$FMQ+2, conditions under which many arc magmas are thought to be generated. The increase in the solubility of sulfur with increasing oxygen fugacity implies that in oxidized arc sources very low degrees of partial melting are sufficient to generate basaltic melts that are simultaneously sulfur-rich and sulfur-undersaturated. In the absence of sulfides, oxides and metallic alloys may influence the behavior of some (but not all) the highly siderophile elements whereas the chalcophile and some siderophile elements become incompatible. As a consequence, melting of oxidized sources in which sulfides are not stable would favor incorporation of metals such as Cu, Ni, Au and Pd in the melts and increase the ability of those melts to generate Au-rich and Pd-rich ore deposits. In addition, partitioning of sulfur into a hydrous phase upon decompression and mixing of a sulfur-rich basaltic melt with water-rich felsic magmas provides a feasible mechanism to explain the sulfur-rich degassing observed in some volcanoes. We emphasize the importance of accounting for the oxidation state of sulfur when modeling magmatic processes in which sulfur is a key component, specially in arc magmas where sulfur is likely to be present both as sulfide and sulfate.
DE: 1025 Composition of the mantle
DE: 3630 Experimental mineralogy and petrology
DE: 8409 Atmospheric effects (0370)
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting