Using Mg isotopes to constrain the formation temperature of dolomite

The "dolomite problem" has long been a significant challenge in sedimentology, particularly regarding the determination of dolomite formation temperatures, a subject that remains highly debated. Magnesium (Mg) isotopes, due to their stability in dolomite during diagenesis and the strong relationship between isotopic equilibrium fractionation and temperature, present a promising tool for estimating the formation temperatures of dolomite. In this study, we analyzed the Mg isotope composition (δ²⁶Mg) of various dolomite samples from the Ediacaran to Ordovician in the Tarim Basin, China, to assess the potential of Mg isotopes as a thermometer for dolomite formation. The δ²⁶Mg values of micro-fine crystalline dolomites (D1) ranged from - 1.98 to - 1.69‰, fine-medium crystalline dolomites (D2) from - 1.68 to - 1.33‰, and medium-coarse crystalline dolomites (D3) from - 2.35 to - 1.99‰. Based on the Mg isotope temperature equilibrium fractionation equations, Δ²⁶Mg_dol-fluid = - 0.1554(± 0.0096) × 10⁶/T² or Δ²⁶Mg_dol-cal=0.1453(± 0.0106)×10⁶/T², the calculated formation temperatures for D1 ranged from 45.0 to 65.4 °C, for D2 from 53.1 to 73.9 °C, and for D3 from 142.4 to 173.9 °C or 156.4 to 189.0 °C. These temperature estimates align closely with those derived from fluid inclusions and clumped isotopes in previous literature studies of the same formations, supporting the reliability of Mg isotopes as a method for determining dolomite formation temperatures. This study introduces an innovative approach for assessing dolomite formation temperatures using Mg isotopes, offering crucial insights into the resolution of the "dolomite problem."