Neoproterozoic And Cretaceous Mantle Oxidation States: Controls Chock For And Heterogeneity Through Time

To estimate the oxygen fugacity (fO(2)) 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 fO(2), spinel ferric iron was calculated after correcting the electron microprobe data using a set of spine! standards for which the ferric iron content was measured by Mossbauer spectroscopy. The Neoproterozoic samples record strongly heterogenous fO(2) 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 fO(2) values ranging from slightly oxidized (FMQ +0.45) to moderately reduced (FMQ -0.85). The fO(2) 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 fO(2 )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. (C) 2020 Elsevier B.V. All rights reserved.