Resetting of Mg isotopes between calcite and dolomite during burial metamorphism: Outlook of Mg isotopes as geothermometer and seawater proxy

Abstract

Magnesium isotopes are an emerging tool to study the geological processes recorded in carbonates. Calcite, due to its ubiquitous occurrence and the large Mg isotope fractionation associated with the mineral, has attracted great interests in applications of Mg isotope geochemistry. However, the fidelity of Mg isotopes in geological records of carbonate minerals (e.g., calcite and dolomite) against burial metamorphism remains poorly constrained. Here we report our investigation on the Mg isotope systematics of a dolomitized Middle Triassic Geshan carbonate section in eastern China. Magnesium isotope analysis was complemented by analyses of Sr-CO isotopic compositions, major and trace element concentrations, and petrographic and mineralogical features. Multiple lines of evidence consistently indicated that post-depositional diagenesis of carbonate minerals occurred to the carbonate rocks. Magnesium isotope compositions of the carbonate rocks closely follow a mixing trend between a high δ 26 Mg dolomite end member and a low δ 26 Mg calcite end member, irrespective of sample positions in the section and calcite/dolomite ratio in the samples. By fitting the measured Mg isotope data using a two-end member mixing model, an inter-mineral Δ 26 Mg dolomite-calcite fractionation of 0.72‰ was obtained. Based on the experimentally derived Mg isotope fractionation factors for dolomite and calcite, a temperature of 150-190ºC was calculated to correspond to the 0.72‰ Δ 26 Mg dolomite-calcite fractionation. Such temperature range matches with the burial-thermal history of the local strata, making a successful case of Mg isotope geothermometry. Our results indicate that both calcite and dolomite had been re-equilibrated during burial metamorphism, and based on isotope mass

Isotopic responses of magnesium to two types of dissolution-reprecipitation processes for the growth of the double-carbonate mineral norsethite

“…It is a common case for carbon isotopes in carbonates (Swart, 2015) but also for Mg isotopes in dolostone during the burial metamorphism (Azmy et al, 2013;Geske et al, 2012;Hu et al, 2017) or hydrothermal alteration (Geske et al, 2015;Hu et al, 2019).…”

“…This operation was repeated three times to convert the salts to nitrate form for the column procedure. Then Mg of the solid and aqueous phases was purified following a well-established two-stage column procedure at the State Key Lab for Mineral Deposits Research, Nanjing University (Hu et al, 2017;Liu and Li, 2020). Magnesium isotope analysis was performed using a Thermo Scientific Neptune Plus multi-collector inductively coupled mass spectrometer (MC-ICP-MS) that operated on medium-mass-resolution, wet plasma mode, the details of instrumental parameters are listed in Appendix Table S5.…”

“…Magnesium isotope analysis was performed using a Thermo Scientific Neptune Plus multi-collector inductively coupled mass spectrometer (MC-ICP-MS) that operated on medium-mass-resolution, wet plasma mode, the details of instrumental parameters are listed in Appendix Table S5. Instrument drift and mass bias were corrected using a standard-sample bracketing routine with an in-house Mg solution (HPS909104) as the bracketing standard (Hu et al, 2017;. Magnesium isotope data are reported using the standard delta notation (‰) of δ 26 Mg and δ 25 Mg for the 26 Mg/ 24 Mg and 25 Mg/ 24 Mg ratios relative to the DSM3 Mg isotope standard, where…”

Mineralogical and Geochemical Characteristics of Triassic Lithium-Rich K-Bentonite Deposits in Xiejiacao Section, South China

“…Strontium was separated twice using a cation exchange procedure using 100-200 mesh AG58X8 resin [23] and measured using a Finnigan Triton Thermo ionization mass spectrometer (TIMS) at Nanjing University for isotopic research. The analytical procedure of strontium isotope were described by Hu et al (2017) [24]. 87 Sr/ 86 Sr ratios were corrected for mass fractionation by normalizing to 86 Sr/ 88 Sr = 0.1194 with exponential law.…”

Sabkha dolomite as an archive for the magnesium isotope composition of seawater

“…However, the scarcity of such record data, and differences between existing datasets, currently limit this approach. Dolomite, a common sedimentary rock constituted by the homonymous mineral CaMg(CO3)2, has been suggested as a promising archive for seawater δ 26 Mg, because Mg is a major element in dolomite and, therefore, it is less sensitive to post-depositional alteration than other substrates (e.g., Geske et al, 2012;Hu et al, 2017). However, the use of dolomite is more complicated than Ca-carbonate archives.…”