Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

Chalcophile element partitioning among base-metal sulfide, oxide and silicate phases during magmatic processes is poorly constrained in part because there are very few studies of reliable sulfide melt–silicate melt partition coefficients (Ds) and because the crystallization history of the sulfide liquid is ignored in most studies. Here we present laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of sulfide droplets and their host fresh mid-ocean-ridge basalt (MORB) glasses. The compositions of the sulfide droplets suggest that they formed in magma chambers beneath the mid-ocean ridges and equilibrated with their host silicate melt, enabling us to calculate new Ds for chalcophile elements. These are Co (45 ± 4.5), Ni (776 ± 98), Cu (1334 ± 210), Zn (3.5 ± 0.9), Se (345 ± 37), Ag (1138 ± 245), Cd (107 ± 47), Sn (11 ± 1.6), Te (4478 ± 1146), Pb (57 ± 10) and Bi (316 ± 38). The contents of the highly siderophile elements in the glasses were too low to be determined using LA-ICP-MS. The whole rock values were used as a proxy for the glass and these allow an estimate of minimum Ds in the 10 000 to 40 000 range for the platinum-group elements (PGE) and 870 for Re. Partition coefficient values are affected by oxygen fugacity; comparison of our values with those from experimental studies suggests that oxygen fugacities of the MORBs considered here were between FMQ and FMQ-1. From the determined D values we calculate the contribution of the sulfides to bulk partition coefficient during mantle melting. Considering these values in combination with what is known about the behavior of the chalcophile elements during mantle partial melting, we suggest that Co, Ni, Zn, Te, PGE and Au behave as compatible elements during mantle melting, with Ni, Co and Zn being controlled mainly by the silicate and oxide minerals and the PGE, Au and Te being controlled mainly by the sulfides (or other discrete metallic phases). Copper, Se, Ag, Cd, Sn, Re, Pb and Bi are slightly to strongly incompatible during mantle melting. MORB sulfide droplets consist mainly of monosulfide solid solution (MSS), which is the first mineral to crystallize from a sulfide liquid, and intermediate solid solution (ISS), which crystallizes from the remaining liquid. The distribution of the chalcophile elements within the sulfide droplets shows that Co and Re have a slight preference for MSS, whereas the Cu, Zn, Ag, Cd, Sn, Te, Au, Bi and Pb partition into ISS. Selenium is present in approximately equal amounts in both MSS and ISS, and Pt and Pd are also present in both phases. Previous experimental and empirical studies have shown that Os, Ir, Ru and Rh partition into MSS and a portion of these elements exsolve from the sulfides as platinum-group minerals (PGM) during cooling. The same studies show that most of the Pt and Pd partition into the liquid and eventually crystallize from the late fractionated liquid as PGM. We examined our MORB sulfide droplets closely for platinum group-minerals (PGM) but none were found. We suggest that because the rocks sustained rapid cooling PGM were unable to exsolve from the sulfide minerals and the liquid did not fractionate sufficiently to permit the crystallization of the Pt or Pd minerals. Thus the PGE are present in MSS and ISS.