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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.

Ultramafic portions of ophiolitic fragments in the Arabian-Nubian Shield (ANS) show pervasive carbonate alteration forming various degrees of carbonated serpentinites and listvenitic rocks. Notwithstanding the extent of the alteration, little is known about the processes that caused it, the source of the CO2 or the conditions of alteration. This study investigates the mineralogy, stable (O, C) and radiogenic (Sr) isotope composition, and geochemistry of suites of variably carbonate altered ultramafics from the Meatiq area of the Central Eastern Desert (CED) of Egypt. The samples investigated include least-altered lizardite (Lz) serpentinites, antigorite (Atg) serpentinites and listvenitic rocks with associated carbonate and quartz veins. The C, O and Sr isotopes of the vein samples cluster between -8.1 parts per thousand and -6.8 parts per thousand for delta C-13, +6.4 parts per thousand and +10.5 parts per thousand for delta O-18, and Sr-87/Sr-86 of 0.7028-0.70344, and plot within the depleted mantle compositional field. The serpentinites isotopic compositions plot on a mixing trend between the depleted-mantle and sedimentary carbonate fields. The carbonate veins contain abundant carbonic (CO2 +/- CH4 +/- N-2) and aqueous-carbonic (H2O-NaCl-CO2 +/- CH4 +/- N-2) low salinity fluid, with trapping conditions of 270-300 degrees C and 0.7-1.1kbar. The serpentinites are enriched in Au, As, S and other fluid-mobile elements relative to primitive and depleted mantle. The extensively carbonated Atg-serpentinites contain significantly lower concentrations of these elements than the Lz-serpentinites suggesting that they were depleted during carbonate alteration. Fluid inclusion and stable isotope compositions of Au deposits in the CED are similar to those from the carbonate veins investigated in the study and we suggest that carbonation of ANS ophiolitic rocks due to influx of mantle-derived CO2-bearing fluids caused break down of Au-bearing minerals such as pentlandite, rele