Carbonate Alteration of Ophiolitic Rocks : Sources of Fluids, Conditions of Alteration, and Element Mobility

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2020-05

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Abstract

Ultramafic portions of ophiolitic sequences commonly show evidence of carbonate alteration reflecting geological process with implications including carbon sequestration, fluxing of CO₂ to the Earth’s atmosphere, the carbon cycle and chemical exchange between shallow and deep reservoirs. The forearc mantle wedge is one setting where natural carbon mobility and sequestration occurs due to release of CO₂-bearing fluids during dehydration and devolatilization of subducting rock. Carbonation in subduction zones preserves evidence for transfer of carbon between shallow and deep reservoirs as well as the mobility of other elements that are affected by such alteration. This PhD research aims at better understanding the significance of a series of serpentinized and carbonated ultramafic rocks from ophiolitic fragments in the Eastern Desert (ED) of Egypt and the Sistan Suture Zone (SsSZ) in eastern Iran. It focuses on the changes mineral assemblages and texture, on chemical reactions, on mineral chemistry and on element mobility during serpentinization and carbonation. It also attempts to constrain the sources of carbonating fluids and the environmental conditions where the carbonation occurred. Field and petrological observations indicate that lizardite-serpentinite, carbonated antigoriteserpentinite, talc-rich rocks, and associated quartz-carbonate veins in the Meatiq ophiolite in the ED result from interaction of aqueous and CO₂-rich fluids with a peridotite protolith. Fluid inclusion data and bulk rock geochemistry indicate carbonation occurred at 270-300 °C and 0.7- 1.1 kbar. The stable and radiogenic isotope compositions of carbonate veins indicate influx of mantle-derived CO₂-bearing fluid that mixed with surficial fluid such as sedimentary pore fluid. Hydration and carbonation reactions are also preserved in the next ophiolite locality in Hangaran, eastern Iran, where variably serpentinized and carbonated ophiolitic rocks preserve successive fluid-driven reactions. Transformation of harzburgite to listvenite starts with lizardite serpentinization, followed by contemporaneous carbonation and antigorite serpentinization, finally producing listvenite when alteration is most pervasive. The petrological observations and mineral assemblages suggest that hydrothermal fluids responsible for the lizardite serpentinization had low aCO₂, oxygen and sulfur fugacities, distinct from those causing antigorite serpentinization and listvenitization, which had higher aCO₂, aSiO₂, and oxygen and sulfur fugacities. The stable and radiogenic isotope composition of carbonate in carbonated lithologies and veins point to sedimentary pore fluids as the main carbon source. Trace element patterns in carbonated lithologies are consistent with contribution of subducted sediments in a forearc setting, suggesting sediment-derived fluids. Such fluids were produced by sediment dehydration in the accretionary complex at shallow parts of the mantle wedge. Cr-spinels in the aforementioned suite of Hangaran ultramafics are the only mantle mineral likely to survive during the transformation from harzburgite to listvenite; however, their compositional variability suggests progressive modification. Modification is linked to Mg and Al loss via MgFe²⁺ and Al-Fe³⁺ exchanges with surrounding fluids, silicates and carbonates, that in turn, affected Cr# and Mg# of the Cr-spinel cores. Therefore, significant compositional modifications of Crspinel cores observed during transformation of harzburgite protolith to Lz- and Atg- serpentinites and listvenite lithologies show that reliance on their composition may lead to erroneous petrogenetic and tectonic setting interpretations.

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Peridotite, Geological carbon sequestration, Listwanite, Carbon -- Isotopes, Oxygen -- Isotopes, Strontium -- Isotopes, Ophiolites, Arabian-Nubian Shield Element mobility C, O and Sr isotopes

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©2020 Ahmad Boskabadi. All rights reserved.

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