From peridotite to fuchsite bearing quartzite via carbonation and weathering: with implications for the Pb budget of continental crust

Abstract

Extensive carbonation of peridotite results in listvenite, a rock composed of magnesite and quartz. At Gråberget, Røros, SE-Norway, a variably serpentinized peridotite body, surrounded by the Røros schists, a former abyssal sediment displays all stages of transformation of peridotite to quartzite. In this paper we record the sequence of steps in this process by combining the observation of mineral assemblages, textural relationships and geochemistry, and variations in Pb isotopic compositions. Initial serpentinization, a stage that also involved an enrichment in fluid-mobile elements (Pb, Sb and As), was followed by carbonation through CO2 fluids that formed soapstone, and eventually listvenite. The listvenite grades by decreasing amounts of carbonates into fuchsite bearing quartzite. The carbonates dissolved during supergene alteration and formed pores coated with oxides of Fe, Mn and Ni resulting in a brown rock color. The quartzite displays porous stylolites enriched in Pb, As and Sb and fuchsite with porous chromite grains as the only relicts of the original mineralogy in the peridotite. The dissolution of the carbonate occurred at oxidizing conditions at temperatures below 150 °C, where the solubility of magnesite is higher than that of quartz. Formation of quartzite from peridotite is supported by low REE contents and lack of zircons in the two rock types. The transformation involved enrichment of Pb, coupled with the elimination of Mg and enrichment of Si. This chemical fractionation and selective transfer of elements to the continents is an important mechanism and needs to be taken into account in models of continental evolution.