Evidence for mantle heterogeneity from platinum-group-element abundances in Indian Ocean basalts

Abstract

Oceanic-island tholeiitic basalts recovered from four sunken oceanic islands along the Reunion hot-spot trace show trace-element and mineralogical characteristics ranging from typical oceanic-island tholeiites to incompatible-element-depleted tholeiites resembling mid-ocean-ridge basalts. There are also variable degrees of magma evolution at each island. Noble metal (Au, Pd, Pt, Rh, Ru, Ir) abundances tend to decrease with magma evolution and with magma "alkalinity", indicating that the metals behave as compatible elements during crystal fractionation processes and during mantle melting processes. Palladium-to-iridium ratios also decrease with increasing alkalinity. Absolute abundances of elements such as Pd are higher than those in typical mid-ocean-ridge basalts, by factors up to 30, despite many major-element similarities with the latter. Comparison with other types of mafic rocks shows that Pd/Ir ratios increase with decreasing alkalinity in basaltic rocks but plunge to alkali-basalt values in komatiites. A model involving retention of low-melting-point Au, Pd, and Rh in mantle sulphides, which completely dissolve by intermediate percentages of melting, and the high-melting-point metals Ir and Ru in late-melting mantle alloys explains increasing Pd/Ir ratios with decreasing alkalinity (increasing melting percentages) in oceanic basalts and the low Pd/Ir ratios of high-percentage melt komatiites.The high noble metal concentrations in Indian Ocean basalts compared with basalts from many other ocean basins are most easily explained by higher concentrations in their source regions. This may be related to incomplete mixing of a post-core-formation meteoritic component of the upper mantle, or deep mantle plume-derived blebs of core material that either failed to reach the core, during core–mantle differentiation, or were plucked from the core by a convecting lower mantle. The latter is tentatively favoured due to the apparently higher noble metal concentrations in oceanic-island (plume) basalts.