Anomalously high delta D values in an Archean ferropicritic melt; implications for magma degassing

The in situ isotope analysis of igneous amphibole from the Boston Creek ferropicrite, in the Abitibi belt, Ontario, a 100-m-thick mafic-ultramafic unit, provides insights into the evolution of Archaean mantle-derived hydrous melts in the crust. The magnesiohastingsite has a wide range of delta D values, from -47 to +54 0/00. The contrast with the uniform major-element and trace element compositions and H (sub 2) O content of the amphibole, the negative delta D values for whole-rock samples and the negative delta D values for igneous amphiboles from similarly altered ultramafic sills in the area, indicates that the heterogeneous and high values for the ferropicrite unit can best be attributed to magmatic processes. The anomalously high delta D values, in conjunction with the development of extensive zones of vesiculation are explained by outgassing of hydrogen during emplacement of the ferropicrite melt at high levels in the crust. The outgassing of hydrogen via dissociation of H (sub 2) O resulted in oxidation and probably cooling of the degassed residual melt in the interior of the ferropicrite unit. The resulting crystallization of Fe-Ti oxide reduced the FeO content of the melt and triggered Cu-PGE-Ag sulphide mineralization.