Mechanism of solid-state clumped isotope reordering in carbonate minerals from aragonite heating experiments

Abstract

The clumped isotope compositions of carbonate minerals are subject to alteration at elevated temperatures. Understanding the mechanism of solid-state reordering in carbonate minerals is important in our interpretations of past climates and the thermal history of rocks. The kinetics of solid-state isotope reordering has been previously studied through controlled heating experiments of calcite, dolomite and apatite. Here we further explore this issue through controlled heating experiments on aragonite. We find that Δ47 values generally decrease during heating of aragonite, but increase by 0.05–0.15‰ as aragonite starts to transform into calcite. We argue that this finding is consistent with the presence of an intermediate pool of immediately adjacent singly-substituted carbonate ion isotopologues (‘pairs’), which back-react to form clumped isotopologues during aragonite to calcite transformation, revealing the existence of kinetically preferred isotope exchange pathways. Our results reinforce the ‘reaction-diffusion’ model as the mechanism for solid-state clumped isotope reordering in carbonate minerals. Our experiments also reveal that the reordering kinetics in aragonite is faster than in calcite and dolomite, making its clumped isotope composition highly susceptible to alteration during early diagenesis, even before conversion to calcite.