EP13B-0958
Investigation of Mg isotope fractionation during low temperature precipitation of dolomite analogue minerals BaMg(CO3)2 and PbMg(CO3)2 PbMg(CO3)2
Abstract:
Stable Mg isotopes are an emerging geochemical tool for studying dolomite-related problems. Currently available Mg isotope fractionation factors for dolomite are calibrated at hydrothermal conditions. Direct measurement of low temperature Mg isotope fractionation between dolomite and aqueous solution is hampered by the difficulty of synthesis of dolomite at ambient abiotic conditions. Dolomite analogue minerals including PbMg(CO3)2 and BaMg(CO3)2, however, can be readily synthesized at low temperatures. Therefore study of fractionation factors of Mg isotopes during PbMg(CO3)2 and BaMg(CO3)2 precipitation at low temperature conditions provides insights on Mg isotope behavior during dolomitization in geological processes..We successfully synthesized PbMg(CO3)2 and BaMg(CO3)2 at 30-80ºC and characterized the products using scanning electron microscopy (SEM). In general, the crystallinity of PbMg(CO3)2 and BaMg(CO3)2 correlated with temperature. PbMg(CO3)2 synthesized at below 40℃ formed aggregates of nanocrystals. The aggregates became fewer in experiments at higher temperatures, and fine-grained (200~600nm in size), euhedral rhombohedrons of PbMg(CO3)2 were produced in the experiment at 80℃. In BaMg(CO3)2 synthesis experiments, morphology of BaMg(CO3)2 was mainly granular but the crystal size increased from 20 to 100nm with increasing temperature, and 0.5-1.1µm sized euhedral to subhedral crystals were produced at 70 and 80℃. Preliminary Mg isotope analyses for the experimental products show that Δ26Mg fractionation between PbMg(CO3)2 and aqueous solution at 40℃ is -1.87‰, whereas Δ26Mg fractionation between BaMg(CO3)2 and aqueous solution at 40℃ is -1.40‰. The result is consistent with the idea that heavy Mg isotopes favor shorter and stronger Mg-O bonds in minerals.