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

Abstract

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 δ 26 Mg 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 26 Mg into clay mineral structures controls Mg-isotope fractionation during ultramafic rock weathering. The Mg-bearing clay formed by decomposition of serpentine minerals has a δ 26 Mg value of ~ 0.35 ‰, which is up to ~ 0.6 ‰ heavier than the ultramafic precursor. In contrast, nodular magnesite hosted in ultramafic rock shows δ 26 Mg values between -3.26 ‰ and -2.55 ‰ that are significantly lower than those of magnesite and dolomite that formed by hydrothermal alteration of peridotite at higher temperature (δ 26 Mg = -0.69 ‰ and -0.62 ‰). The strong enrichment of 24 Mg 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 accompanied by fractionation between species of dissolved Mg. Our data highlight the potential of Mg isotope studies for distinguishing the formation pathways of low temperature magnesite and for tracing Mg in low temperature alteration processes based on the distinct signatures of secondary silicate and carbonate minerals.

A Mg Isotopic Perspective on the Mobility of Magnesium During Serpentinization and Carbonation of the Oman Ophiolite

“…Following a molecular dynamics approach, Wang et al (2019) also concluded that lizardite crystallization preferentially removes 26 Mg from the fluid. These experimental results contrast with fractionation estimates based on natural samples that concluded that serpentinization does not fractionate Mg isotopes (Beinlich et al, 2014;Liu et al, 2017;Oskierski et al, 2019). Studies of natural samples also suggest that talc and Mg-rich clays formed during alteration are enriched in 26 Mg (Beinlich et al, 2014;Liu et al, 2017) in contrast with recent experimental studies that showed preferential incorporation of 24 Mg in stevensite and saponite (Hindshaw et al, 2020).…”

“… Bulk‐rock MgO/SiO 2 * vs. δ 26 Mg for Oman samples, abyssal peridotites (Liu et al., 2017) and serpentinite (Oskierski et al., 2019). Black solid line is mantle average and dashed black lines delineate the range of variability of mantle compositions (Teng, 2017; Teng et al., 2010).…”

Formation of Magnesium Carbonates on Earth and Implications for Mars

“…Mg has three stable isotopes of 24 Mg, 25 Mg, and 26 Mg. In general, magnesite minerals are depleted in 26 Mg compared with their source fluid, with the offset determined by a temperature‐dependent fractionation factor (de Obeso et al., 2020; Oskierski et al., 2019; Pearce et al., 2012). As such, the Mg isotope composition of carbonates can be used to trace the fluid/rock source of Mg (Tipper et al., 2006).…”

Mineralisation of atmospheric CO2 in hydromagnesite in ultramafic mine tailings – Insights from Mg isotopes

“…This agrees with previous descriptions of the bedrock lithologies and mineralogy at Woodsreef (Glen and Butt, 1981;O'Hanley and Offler, 1992). The nodular bedrock carbonate from the mine pit consists of magnesite, with traces of lizardite and associated pyroaurite (WOMP-M1; Oskierski et al, 2019). Lizardite dominates the bulk tailings sample (WO-T2top) but trace amounts of pyroaurite, magnetite, quartz and hydromagnesite are present.…”

“…Teng et al, 2007;Yang et al, 2009;Teng et al, 2010). Similarly, the Mg isotopic composition of bulk peridotite, and of primary minerals in peridotite, is not significantly affected by serpentinisation but by weathering and hydrothermal carbonate alteration (Beinlich et al, 2014;Liu et al, 2017;Oelkers et al, 2019;Oskierski et al, 2019). As demonstrated by Oelkers et al, (2019) for injection of CO2bearing fluids into basalt, Mg isotopic compositions can thus be used to assess the fate of Mg during weathering and carbonation reactions.…”