BACKGROUNDMagnesium isotope fractionation effect during low-temperature geochemical processes is the foundation of tracing supergene evolution and material cycle of the earth.

OBJECTIVESTo summarize the magnesium isotope composition of different geological reservoirs and investigate the controlling factors of the magnesium isotope fractionation during the formation of carbonate minerals.

METHODSSystematic collection and summary of previous research results.

RESULTSMagnesium isotope compositions of igneous rocks were relatively homogeneous. The weathering products were relatively enriched in heavy isotopes of magnesium with significant variation. Carbonate rocks showed enrichment in light isotopes of magnesium in general. The large variation of magnesium isotope composition of river water was affected by lithology, weathering degree and vegetation. Magnesium isotope composition of seawater was homogeneous with an average of -0.83‰. At low temperature, the factors controlling the fractionation of magnesium isotopes in the inorganic process of carbonate minerals were mineral phase, precipitation rate and temperature, of which mineral phase was the main controlling factor. The factors influencing the magnesium isotope composition of biogenic carbonate minerals were forms of utilization of magnesium carbonate minerals by organisms. In addition to considering the mechanisms that were similar to inorganic carbonate precipitation, the selective absorption of light and heavy magnesium isotopes by different species should be considered. Almost all biogenic marine carbonate minerals were transformed from the original amorphous phase carbonate precursor, and their original magnesium isotope composition was masked by the later magnesium isotope composition during transformation. Therefore, the magnesium isotope composition of biogenic marine carbonate minerals cannot represent the fluid isotope composition from which the original amorphous carbonate precursor formed.

CONCLUSIONSMagnesium isotopes have a good fractionation effect at low temperature. With the development of analytical technology and the accumulation and improvement of magnesium isotope composition data in different geological reservoirs, many problems related to the mechanism of magnesium isotope fractionation in the supergene environment will be solved gradually. Magnesium isotopes will play a greater role in revealing the evolution of the supergene environment and the material cycle of the earth.