Carbon Isotope Fractionation during Metal–Carbonate Interaction at the Mantle Pressures and Temperatures

Abstract

—Subduction of marine carbonates is accompanied by numerous transformations and interactions, including reactions with reduced mantle rocks. At depths of 250–300 km, carbonates enter mantle zones where metallic iron can be stable. The interaction of carbonates with metals is one of the mechanisms of the release of elemental carbon and the formation of diamond. These processes are also accompanied by carbon isotope fractionation and can result in a significant isotopic heterogeneity of mantle carbon. In this work we study the partitioning of carbon isotopes between carbon and carbon-bearing phases obtained in experiments on the interaction of FeNi alloy with (Mg,Ca)CO3, which simulates mantle–crust redox reactions in the temperature range 800–1550 °C and at a pressure of 6.3 GPa. It has been established that at 800–1000 °C, the carbon of carbonate is reduced at the metal/carbonate interface and dissolves in the FeNi alloy. This process leads to a 17–20‰ depletion of the metal in the heavy carbon isotope. At temperatures above 1330 °C, the fractionation of carbon isotopes between carbonate and metal–carbon melts is reduced to 8.5‰, approaching the thermodynamic calcite–cohenite isotope equilibrium. At temperatures above 1400 °C, diamond crystallizes from metal–carbon and carbonate melts, which leads to isotopic depletion of the metal–carbon melt. As a result, the measured carbon isotope fractionation between the carbonate and metal–carbon melts increases and moves away from the thermodynamic CaCO3–Fe3C equilibrium line. The carbonate–metal redox interaction is supposed to be one of the probable mechanisms of the formation of isotopically light carbon in the mantle at the expense of the marine carbonate sediments subducted into the mantle. This mechanism also provides the formation of anomalous isotopically heavy carbonates found in kimberlites of the Siberian Platform.