ALTERATION OF A KOMATIITE FLOW FROM ALEXO, ONTARIO, CANADA

To investigate the ability of komatiites to preserve during alteration a record of the geochemical and isotopic composition of the Archean m antle, we studied the petrology and geochemistry of a komatiite flow f rom Alexo, Ontario. Although this flow is relatively well preserved, t wo main types of alteration are recognized: (1) hydration of variable intensity throughout the whole flow; (2) local rodingitization. In sam ples only slightly affected by hydration, the alteration was essential ly isochemical; where more intense, hydration was accompanied by the m obility of TiO2, Zr and the light rare-earth elements (LREE). The mobi lity of these reputedly immobile trace elements is influenced by textu re and secondary mineral assemblages. Rodingitization was accompanied by still greater mobility. An Nd isochron with an age of 2726+/-98 Ma and epsilon Nd-(T) = +2.5 is defined by whole-rock samples. Because th e spread of Sm-Nd ratios is far greater than can be explained by magma tic processes, the age is interpreted as that of the main alteration e vent during or soon after emplacement. Scattered initial Sr isotopic r atios and epsilon Nd-(T) of magmatic clinopyroxene (similar to+3.8) is higher than that of mafic to felsic rocks and pyroxenes from the Abib iti belt (epsilon Nd-(T) similar to+2.5). This result implies: (1) the komatiites exchanged Nd with surrounding rocks during water-rock inte raction; (2) the isotopic composition of komatiites and their mantle s ource differed from that of the more common basalts of the Abitibi bel t.