Pressure and concentration effects on intermineral calcium isotope fractionation involving garnet

To better understand the behaviors of calcium (Ca) isotopes during igneous and metamorphic processes that differentiate the Earth and other rocky planets, we investigated the effects of pressure and Ca/Fe concentration on the average Ca-O bond length and the reduced partition function ratio of Ca-44/Ca-40 (10(3)ln(44/40Ca)beta) in garnet using first-principles calculations. Our calculations show that (1) the average Ca-O bond length increases and 10(3)ln(44/40Ca)beta decreasing (approximately 0.41 parts per thousand at 1000 K from a Ca/(Fe + Ca + Mg) of 1/24 to 12/12) with increasing Ca concentrations. In contrast to orthopyroxene and forsterite, whose Ca-O bond lengths and 10(3)ln(44/40Ca)beta are sensitive to Ca concentrations only in narrow range, the garnet Ca-O bond length and 10(3)ln(44/40Ca)beta vary with Ca concentrations in wide ranges. (2) The average Ca-O bond length increases and 10(3)ln(44/40Ca)beta decreases with increasing Fe concentrations (by 0.12 parts per thousand at 1000 K, FeO content of 12.4 wt%). (3) The average Ca-O bond length decreases by similar to 0.014 angstrom, and 10(3)ln(44/40Ca)beta increases by 0.1 parts per thousand-0.2 parts per thousand at 1000 K when the pressure increases from 0 to 3 GPa. (4) However, the pressure effect on 10(3)ln(44/40Ca)alpha(garnet-clinopyroxene) is insignificant. Using our new results of the effects of pressure and Ca/Fe concentration on 10(3)ln(44/40Ca)alpha(garnet-clinopyroxene), we evaluated the observed difference in Ca isotope compositions between garnet and clinopyroxene in natural samples. Most have reached equilibrium, whereas some samples exceed our prediction, which might reflect the majorite effect or kinetic fractionation.

Automated summary

In this study, the effects of pressure and Ca/Fe concentration on calcium isotopes in garnet were investigated using first-principles calculations. The results show that increasing Ca and Fe concentrations lead to an increase in the average Ca-O bond length and a decrease in the reduced partition function ratio of Ca-44/Ca-40. Pressure also affects the Ca-O bond length and partition function ratio, with an increase in pressure leading to a decrease in the bond length and an increase in the ratio.