Neoproterozoic and Cretaceous mantle oxidation states: Controls and heterogeneity through time

Abstract

To estimate the oxygen fugacity (fO2) of the Neoproterozoic and Cretaceous suprasubduction zone mantle, and to evaluate the possible secular changes in the upper mantle oxidation state, the compositions of spinel, olivine and orthopyroxene of Neoproterozoic (Egypt and Saudi Arabia) and late Cretaceous (Iran) mantle rocks were determined. For accurate estimation of fO2, spinel ferric iron was calculated after correcting the electron microprobe data using a set of spinel standards for which the ferric iron content was measured by Mӧssbauer spectroscopy. The Neoproterozoic samples record strongly heterogenous fO2 values ranging from moderately oxidized (FMQ +0.54) to ultra-reduced (FMQ -4.73) for harzburgites, from highly oxidized (FMQ +1.49) to moderately reduced (FMQ -0.60) for dunites as well as one highly reduced (FMQ -1.61) value for chromitite. Such heterogeneity is not apparent in the late Cretaceous harzburgites that record fO2 values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The fO2 of the Neoproterozoic forearc mantle is most easily explained by melt-mantle interaction and deep-mantle recycling, while that of the late Cretaceous forearc mantle can be attributed to variable degrees of melt-mantle interaction. The estimated fO2 values of Neoproterozoic/Cretaceous mantle unaffected by melt-rock interaction and deep-mantle recycling, and published values of Precambrian and Modern mantle suggest a consistent upper mantle oxidation state from Proterozoic to present day.