Mg isotope fractionation during continental weathering and low temperature carbonation of ultramafic rocks

摘要

The Mg-isotope systematics of peridotite weathering and low-temperature carbonation have not yet been thoroughly investigated, despite their potential to provide insights into reaction pathways and mechanisms of lithosphere-hydrosphere transfer of Mg and sequestration of CO2 in carbonate minerals. Here, we present new observations of the evolution of Mg isotope ratios during subtropical ultramafic rock weathering and associated magnesite formation, including the lowest delta Mg-26 of magnesite reported so far. At the investigated field sites in eastern Australia, the proximity of the ultramafic Mg source rocks and associated magnesite deposits provides boundary conditions that constrain Mg isotope fractionation during low-temperature alteration. Saprolite samples from Attunga, New South Wales, show that weathering of serpentinite is accompanied by Mg loss and formation of secondary Mg-bearing clay minerals. Furthermore, Mg isotope ratios increase systematically with weathering intensity, indicating that incorporation of Mg-26 into clay mineral structures controls Mg isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a delta Mg-26 value of similar to 0.35 parts per thousand, which is up to similar to 0.6 parts per thousand heavier than the ultramafic precursor. In contrast, nodular magnesite hosted in ultramafic rock shows delta Mg-26 values between -3.26 parts per thousand and -2.55 parts per thousand that are significantly lower than those of magnesite and dolomite formed by hydrothermal alteration of peridotite at higher temperature (delta Mg-26 = -0.69 parts per thousand and -0.62 parts per thousand). The strong enrichment of Mg-24 in nodular magnesite does not reconcile with simple fractionation during direct precipitation from ultramafic host rock buffered meteoric fluids and instead suggests multiple formation steps involving dissolution and reprecipitation of pre-existing carbonate a