Stable isotope geochemistry and alteration of Cretaceous komatiitic and associated rocks from Gorgona Island, Colombia

Abstract

δ18O values of unaltered, primary minerals from two Cretaceous rocks of komatiitic affinity and from associated rocks from Gorgona Island, Colombia, indicate that the komatiitie samples, haveδ18O values ≥6.0 whereas the associated tholeiitic igneous rocks have greater (Ce/Sm)Nratios and δlsO values near 5.5. 5jbO values of the komatiitic rocks, in conjunction with their low LREE contents and high eNdvalues, suggest derivation from sources that were extensively depleted by prior melting events and different from those of some of the associated rocks. The divergence between theδ180 values of primary chromite, olivine and pyroxene in the komatiitic rocks is in accordance with petrographic relations which suggest substantial undercooling took place during their emplacement. 6D and 61sO values of chrysotile from the komatiitic samples are characteristic of those for serpentines formed from meteoric water at Gorgona rather than from seawater. This post emplacement alteration has also affected the 87Sr/86Sr ratios. © 1987, GEOCHEMICAL SOCIETY OF JAPAN. All rights reserved.