Comparative Re [sbnd]Os, Sm [sbnd]Nd and Rb [sbnd]Sr isotope and trace element systematics for Archean komatiite flows from Munro Township, Abitibi Belt, Ontario

Abstract

A series of komatiite flows from Munro Township, Ontario, have Re and Os concentrations that range from 0.545 to 1.27 ppb and 1.00 to 2.20 ppb, respectively. The 187Re/ 186Os in the suite range from 14.05 to 52.37 and the 187Os/ 186Os range from 1.49 to 3.22, giving a crystallization age of2726 ± 93Ma and an of initial 187Os/ 186Os of0.873 ± 0.035. This age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems. The Re Os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks. The initial Os isotopic composition is consistent with the derivation of the melts from a mantle source(s) that originated with a chrondritic ( 187Os/ 186Os) i and evolved with an approximately chondriticRe/Os. Sm Nd data for the same suite cluster about anε Nd value at 2.72 Ga of +2 to +3. Rb Sr isotope data indicate significant mobilization of these elements by the greenschist-grade metamorphism that has altered the rocks. Comparisons of Re and Os concentrations with those of MgO, FeO, Ni, Cr and S suggests that Re was incompatible ( D < 0.01) with the mantle residues and also with the predominantly olivine assemblages that crystallized from the komatiite melts. A lack of correlation between Os and MgO indicates that Os concentrations were not controlled by the presence of olivine in the mantle residue or olivine crystallization within the flows. Os was, however, moderately compatible with the mantle residues ( D > 1). Os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization, and not by the modally predominant silicate minerals.