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

Abstract

Carbonate is a common sedimentary rock throughout the earth history and as a significant hydrocarbon reservoir and dutiful recorder for information on sedimentary environment and diagenesis process it has attracted a lot of attention of geologists. Conventionally, element contents and traditional stable isotope ratios such as carbon and oxygen isotope serve as prime proxies in previous studies. Magnesium isotope is 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, particularly calcite, remains poorly constrained. Here we systematically investigated the dolomitized Middle Triassic Geshan carbonate section in eastern China. Multiple lines of evidence consistently indicate that post-depositional diagenesis of carbonate minerals occurred in the carbonate rocks. Magnesium isotope compositions of the carbonate rocks closely follow a mixing trend between a high δ26Mg dolomite end member and a low δ26Mg calcite end member, irrespective of sample positions in the section and calcite/dolomite ratio in the samples. Based on fitting of the measured Mg isotope data using a two-end member mixing model, an inter-mineral Δ26Mgdolomite-calcite fractionation of 0.72‰ was obtained through an optimization approach. Using the experimentally derived Mg isotope fractionation factors for dolomite and calcite, a temperature of 150-190 degrees centigrade was calculated to correspond the 0.72‰ Δ26Mgdolomite-calcite fractionation. Such temperature range matches well 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 equilibrated with a common fluid during burial diagenesis, and based on mass balance such fluid should have been buffered by dolomite from the section. Therefore, burial diagenesis may reset Mg isotope signature of calcite, and Mg isotope compositions in calcite should be dealt with caution in studies of carbonate rocks with thermal history. By contrast, Mg isotopes of dolomite are less prone to diagenetic resetting due to its high abundance in Mg, and Mg isotopes in dolomite may be a more robust recorder for original precipitates.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017