TY - JOUR
T1 - Solubility of Os and Ir in sulfide melt
T2 - Implications for Re/Os fractionation during mantle melting
AU - Fonseca, Raúl O.C.
AU - Mallmann, Guilherme
AU - O'Neill, Hugh St C.
AU - Campbell, Ian H.
AU - Laurenz, Vera
N1 - Funding Information:
We are grateful to James Brenan and Steve Barnes for their comments, which greatly improved the manuscript, and to Richard Carlson for his editorial handling. We would also like to thank Klaus Peter Jochum and Brigitte Stoll (Max Planck, Uni-Mainz), Michael Shelley, Charlotte Allen and Charles Magee (RSES, ANU) for providing invaluable assistance during the LA-ICP-MS work. We are indebted to Chris Ballhaus and Ambre Luguet for their insightful comments and discussion. We are also grateful to Carsten Münker for generously providing access to the experimental facilities of the University of Cologne. Raúl Fonseca would also like to acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG — grant FO 698/1-1 ) and the Portuguese Foundation for Science and Technology (FCT). Guilherme Mallmann acknowledges financial support from FAPESP (grant 2010/05512-1 ).
PY - 2011/11/15
Y1 - 2011/11/15
N2 - Although both rhenium (Re) and osmium (Os) are highly siderophile elements (HSE), they show contrasting geochemical behaviors during partial melting of the mantle - Re is mildly incompatible whereas Os appears to be compatible. This fundamental difference, unique among commonly used isotopic chronometers, causes large variations of Re/Os in oceanic basalts. However, which mantle phases control the geochemical behavior of these elements during partial melting is controversial. Sulfide is typically regarded as a major host for these elements, but recent studies have shown that silicate phases and spinel may also play a role. Here we report the results of an experimental study on the solubilities of Os and Ir in sulfide melts (or mattes) over a large range of oxygen (fO2) and sulfur (fS2) fugacities at 1300 °C. Our experiments indicate that the solubilities of Os and Ir in mattes increase with increasing fS2, with both Os and Ir dissolving as trivalent species at high fS2 and metallic species at low fS2. The effect of fO2 on Os and Ir solubilities appears to be related to oxygen being dissolved into the matte at more oxidizing conditions. Our results coupled with solubility data for Os and Ir in silicate melts have enabled matte/silicate melt partition coefficients for these elements (Dimatte/sil) to be calculated. Assuming a relative oxygen fugacity equal to the quartz-fayalite-magnetite redox equilibrium (i.e. QFM) and a sulfur fugacity of 10-0.5 bar, calculated DOsmatte/sil is ~104 and DIrmatte/sil is ~106. The low solubilities of Os and Ir in silicate melts, coupled with their high matte/silicate melt partition coefficients, suggest that Os and Ir in fertile mantle with ~200 ppm S are held in the mantle matte phase. However, we show that the empirical range of Re/Os in mid-ocean ridge basalts (MORB) can only be reproduced when sulfide is exhausted by high degrees of partial melting, leaving Os-Ir-rich metal alloy in the mantle residue.
AB - Although both rhenium (Re) and osmium (Os) are highly siderophile elements (HSE), they show contrasting geochemical behaviors during partial melting of the mantle - Re is mildly incompatible whereas Os appears to be compatible. This fundamental difference, unique among commonly used isotopic chronometers, causes large variations of Re/Os in oceanic basalts. However, which mantle phases control the geochemical behavior of these elements during partial melting is controversial. Sulfide is typically regarded as a major host for these elements, but recent studies have shown that silicate phases and spinel may also play a role. Here we report the results of an experimental study on the solubilities of Os and Ir in sulfide melts (or mattes) over a large range of oxygen (fO2) and sulfur (fS2) fugacities at 1300 °C. Our experiments indicate that the solubilities of Os and Ir in mattes increase with increasing fS2, with both Os and Ir dissolving as trivalent species at high fS2 and metallic species at low fS2. The effect of fO2 on Os and Ir solubilities appears to be related to oxygen being dissolved into the matte at more oxidizing conditions. Our results coupled with solubility data for Os and Ir in silicate melts have enabled matte/silicate melt partition coefficients for these elements (Dimatte/sil) to be calculated. Assuming a relative oxygen fugacity equal to the quartz-fayalite-magnetite redox equilibrium (i.e. QFM) and a sulfur fugacity of 10-0.5 bar, calculated DOsmatte/sil is ~104 and DIrmatte/sil is ~106. The low solubilities of Os and Ir in silicate melts, coupled with their high matte/silicate melt partition coefficients, suggest that Os and Ir in fertile mantle with ~200 ppm S are held in the mantle matte phase. However, we show that the empirical range of Re/Os in mid-ocean ridge basalts (MORB) can only be reproduced when sulfide is exhausted by high degrees of partial melting, leaving Os-Ir-rich metal alloy in the mantle residue.
KW - Highly siderophile element
KW - Partitioning
KW - Re/Os fractionation
KW - Sulfide melt
UR - http://www.scopus.com/inward/record.url?scp=80054701412&partnerID=8YFLogxK
U2 - 10.1016/j.epsl.2011.09.035
DO - 10.1016/j.epsl.2011.09.035
M3 - Article
AN - SCOPUS:80054701412
SN - 0012-821X
VL - 311
SP - 339
EP - 350
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -