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 komatiitic samples, have δ18O values ≥ 6.0 whereas the associated tholeiitic igneous rocks have greater (Ce/Sm)N ratios and δ18O values near 5.5. δ18O values of the komatiitic rocks, in conjunction with their low LREE contents and high eNd values, 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 the δ18O 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. δD and δ18O values of chrysotile from the komatlitic 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.