Mineralogy and petrology of listvenites from the Oman ophiolite: effects of fluid-rock interactions and carbonation on ultramafic rocks

Abstract

Mineral replacement reactions during carbonation of ultramafic rocks lead to the formation of carbonate-bearing reaction product assemblages. These include ophicarbonate (assemblage of serpentine and carbonate), soapstone (consists mostly of talc) and listvenite (carbonate, quartz and fuchsite assemblage). Samples were collected from serpentinites, ophicarbonates and listvenites from the Semail ophiolite during the Oman Drilling Project (ODP), which has been exposed to extensive hydrothermal alteration. The objective of this research is to study the chemical composition and alteration textures of carbonated ultramafic rocks in order to determine the effects of fluid-rock interactions and the evolution of carbonation, with emphasis on the onset of carbonation. Optical and electron microscopy show that the mineral assemblage of ophicarbonates consists of serpentine, magnesite, dolomite, magnetite, chromite, Cr-spinel and relicts of orthopyroxene. Listvenites contain magnesite, dolomite, quartz, magnetite, hematite (in the upper listvenites) and fuchsite (in the lower listvenites). Both ophicarbonates and listvenites have been subjected to extensive veining, reflecting the presence of pathways for CO2-rich fluids that led to carbonation. The timing of vein formation is determined by cross-cutting relationships; serpentine veining is followed by formation of magnesite, quartz and dolomite veins, respectively. Carbonate spherules observed in ophicarbonates and listvenites represent the initial stage of carbonation in serpentinite, and can be traced into strongly carbonated samples. The features have a magnesite core and dolomite (in ophicarbonates) or Ca-rich magnesite (in listvenites) rim. Optical and electron microscopy, X-ray micro CT scanning and Tescan Integrated Mineral Analysis (TIMA) prove that spherules first appear as separate features, mostly concentrated around carbonate veins, but evolve into aggregates and finally into euhedrally overgrown textures. The presence of compositionally varying veins and zoned carbonate spherules demonstrate that carbonation occurred under the influence of a changing fluid composition. Serpentinization of peridotites was followed by carbonation, leading to the production of magnesite veins and carbonate spherules. The breakdown of magnetite, chromite and other oxides provided Fe for zoned spherules and veins. Continued carbonation led to the formation of quartz, and Ca-rich fluids ultimately produced dolomite and Ca-rich carbonate veins and spherules.