Carbonation of ophiolitic ultramafic rocks: Listvenite formation in the Late Cretaceous ophiolites of eastern Iran

Abstract

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 aCO2, oxygen and sulfur fugacities, distinct from those causing antigorite serpentinization and carbonation/listvenitization, which had higher aCO2, aSiO2, and oxygen and sulfur fugacities. The carbonate and silica listvenite end-members indicate variations in aSiO2 and aCO2 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 δ13C, δ18O and 87Sr/86Sr 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.