Alteration of a komatiite flow from Alexo, Ontario, Canada

Abstract

To investigate the ability of komatiites to preserve during alteration a record of the geochemical and isotopic composition of the Archean mantle, we studied the petrology and geochemistry of a komatiite flow from Alexo, Ontario. Although this flow is relatively well preserved, two main types of alteration are recognized: (1) hydration of variable intensity throughout the whole flow; (2) local rodingitization. In samples only slightly affected by hydration, the alteration was essentially isochemical; where more intense, hydration was accompanied by the mobility of TiO2, Zr and the light rare-earth elements (LREE). The mobility of these reputedly immobile trace elements is influenced by texture and secondary mineral assemblages. Rodingitization was accompanied by still greater mobility. An Nd isochron with an age of 2726±98 Ma and εNd(T) = + 2·5 is defined by whole-rock samples. Because the spread of Sm-Nd ratios is far greater than can be explained by magmatic processes, the age is interpreted as that of the main alteration event during or soon after emplacement. Scattered initial Sr isotopic ratios and εNd(T) (+ 3·6 to + 0·7) provide evidence for several younger events. The εNd(T) of magmatic clinopyroxene (∼ + 3·8) is higher than that of mafic to felsic rocks and pyroxenes from the Abibiti belt (εNd(T) ∼ + 2·5). This result implies: (1) the komatiites exchanged Nd with surrounding rocks during waterrock interaction; (2) the isotopic composition of komatiites and their mantle source differed from that of the more common basalts of the Abitibi belt.