Nickel isotope fractionation patterns in continental ultramafic environments generally show a depletion of delta Ni-60 in weathered rocks and an enrichment in bedrock samples. The present study focuses on stable Ni isotope fractionation patterns in carbonate-rich, ultramafic ophiolite samples with concomitant fluids at an active serpentinization site in southwestern Turkey, with a comparison to results from an inactive serpentinization site in the Eastern Desert of Egypt with carbonate-rich samples. All solid phase data from the inactive serpentinization area are consistent with previously reported values from serpentinites, whereas the solid precipitates in the active area (SW Turkey) give values slightly heavier than previously reported data. However, the Ni isotopic signatures in the active serpentinization system likely reflect the scavenging of light Ni by iron oxide and carbonate precipitation, as has been previously demonstrated in laboratory coprecipitation experiments. It is also possible that the active system results resemble previous laboratory experimental results that show a relatively strong initial fractionation between fluids and solids, which then diminishes with time due to aging of the precipitates.

Late Cretaceous mantle peridotite of the Birjand ophiolite (eastern Iran) contains variably serpentinized and carbonated/listvenitized rocks. Transformation from harzburgite protolith to final listvenite (quartz + magnesite/+/- dolomite + relict Cr-spinel) reflects successive fluid-driven reactions, the products of which are preserved in outcrop. Transformation of harzburgite to listvenite starts with lizardite serpentinization, followed by contemporaneous carbonation and antigorite serpentinization, antigorite-talc-magnesite alteration, finally producing listvenite where alteration is most pervasive. The spectrum of listvenitic assemblages includes silica-carbonate, carbonate and silica listvenites with the latter (also known as birbirite) being the youngest, based on crosscutting relationships. The petrological observations and mineral assemblages suggest hydrothermal fluids responsible for the lizardite serpentinization had low aCO(2), oxygen and sulfur fugacities, distinct from those causing antigorite serpentinization and carbonation/listvenitization, which had higher aCO(2), aSiO(2), and oxygen and sulfur fugacities. The carbonate and silica listvenite end-members indicate variations in aSiO(2) and aCO(2) of the percolating hydrothermal fluids, most likely driven by local variations in pH and temperature. Beyond the addition of H2O, serpentinization did not significantly redistribute major elements. Progressive infiltration of CO2-rich fluids and consequent carbonation segregated Mg into carbonate and Si into silica listvenites. Trace element mobility resulted in different enrichments of fluid-mobile, high field strength, and light rare earth elements in listvenites, indicating a listvenite mobility sequence. The delta C-13, delta O-18 and Sr-87/Sr-88 values of magnesite and dolomite in carbonated lithologies and veins point to sedimentary carbonate as the main C source. Fluid-mobile element (e.g., As and Sb) patterns in carbonated lithologies are consistent with contribution of subducted sediments in a forearc setting, suggesting sediment-derived fluids. Such fluids were produced by expulsion of pore fluids and release of structurally bound fluid from carbonate-bearing sediments in the Sistan Suture Zone (SsSZ) accretionary complex at shallow parts of mantle wedge. The CO2 -bearing fluids migrated up along the slab-mantle interface and circulated through the suture zone faults to be sequestered in mantle peridotites with marked element mobility signatures.

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, releasing Au and S to the hydrothermal fluids that later formed the Au-deposits. This is the first time that gold has been observed to be remobilized from rocks during the lizardite-antigorite transition.