Noble metal abundances in komatiite suites from Alexo, Ontario and Gorgona Island, Colombia

Details Noble metal abundances in komatiite suites from Alexo, Ontario and Gorgona Island, Colombia Noble metal abundances in komatiite suites from Alexo, Ontario and Gorgona Island, Colombia

Aug 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.2159b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 51, Issue 8, pp.2159-2169

Computer Science

29

Scientific paper

The distribution of the chalcophile and siderophile metals Cu, Ni, Au, Pd, Ir, Os and Ru in an Archaean komatiite flow from Alexo, Ontario and in a Phanerozoic komatiitic suite of Gorgona Island, Colombia, provides new information about the geochemical behaviour of these elements. Copper, Au and Pd behave as incompatible elements during the crystallization of these ultramafic magmas. In contrast, Ni, Ir, Os and Ru concentrations systematically decrease with decreasing MgO contents, a pattern characteristic of compatible elements. These trends are most probably controlled by olivine crystallization, which implies that Ir, Os and Ru are compatible in olivine. Calculated partition coefficients for Ir, Os and Ru between olivine and the melt are about 1.8. Compared to primitive mantle, parental komatiitic liquids are enriched in (incompatible) Cu, Au and Pd and depleted in (compatible) Ir, Os and Ru. Within both Archaean and Phanerozoic komatiites, noble metal ratios such as Au / Pd , Ir / Os , Os / Ru and Ru / Ir and ratios of lithophile and siderophile elements such as Ti/Pd, Ti/Au are constant and similar to primitive mantle values. This implies that Au and Pd are moderately incompatible elements and that there has been no significant fractionation of siderophile and lithophile elements since the Archaean. Platinum-group element abundances of normal MORB are highly variable and always much lower than in komatiites, because MORB magma is saturated with sulfur and a variable but minor amount of sulfide segregated during mantle melting or during the ascent of magma to the surface. Sulfide deposits associated with komatiites display similar chalcophile element patterns to those of komatiites. Noble metal ratios such as Pd / Ir , Au / Ir , Pd / Os and Pd / Ru can be used to determine the composition of the host komatiite at the time of sulfide segregation.

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