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 ε_Nd 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.