The origin of the fractionation of platinum-group elements in terrestrial magmas

The platinum-group elements (PGE's), when chondrite normalized, have been found to be fractionated in order of descending melting point (Os, Ir, Ru, Rh, Pt, Pd and Au). Mantle-derived material (garnet lherzolite and spinel lherzolite xenoliths and alphine peridotites) have essentially unfractionated PGE patterns. Periotitic komatiites have mildly fractionated patterns (PdIr = 10), pyroxenitic komatiites are slightly more fractionated (PdIr = 30). Both continental and ocean-floor basalts are highly fractionated (PdIr = 100). Data from intrusive rocks show a large range in PGE fractionation from Pd-depleted chromities of ophiolites (PdIr = 0.1) to the extreme Pd enrichment in the JM Reef of the Stillwater Complex (PdIr = 865).\ud Some possible mechanisms for the origin of this fractionation are: alteration, partial melting and crystal fractionation. Carbonate alteration affects Au and Pt and hydrothermal alteration mobilizes Pd. Solid\ud substitution of Ir (and associated Os and Ru) into olivine and chromite, during crystal fractionation or\ud partial melting is rejected as a mechanism of fractionating the PGE's. It is suggested; that the major factor in PGE fractionation is the differences in solubility of the PGE's in a silicate magma, that Pd, Pt and Rh are more soluble than Os and Ir, which form an alloy and Ru which forms laurite. These differences in PGE solubility could fractionate the PGE's during partial melting or crystal fractionation. During crystal fractionation prior to Fe--Ni--Cu sulphur saturation the low solubility of Os, Ir and Ru leads to the formation\ud of Os-Ir alloys and RuS 2 in the magma. These may then be settled out of the magma by whatever phase is crystallizing and the remaining magma becomes fractionated in PGE's