Barium isotope fractionation during granitic magmatism and potential of δ138/134Ba for distinguishing magmatic-hydrothermal transition in granitic magma systems

The geochemistry of Ba is commonly used to study igneous processes and fluid-melt interaction in subduction zone and granitic environments. Recent advances in analytical techniques have also indicated that Ba isotope geochemistry has the potential to significantly augment these studies. Understanding the Ba isotope fractionation mechanisms is thus critical if this potential is to be fully realized. Hence, we have used the first-principles calculations based on the density functional theory (DFT) and long-time ab initio molecular dynamics (AIMD) simulation to calculate Ba isotope fractionation during felsic magma evolution. In fluids, the reduced partition function ratio (beta factor) of aqueous Ba2+ is highly variable, depending on the coordination number (n) of H2O in the first hydration shell. At ambient surface conditions on the Earth, n = 8 best represents the aqueous Ba2+ species in natural fluid, and the temperature-dependent equilibrium Ba isotope fractionation between barite/witherite and fluids can be described by the relationships: 10(3)ln alpha(witherite-fluid) = -0.00009x(10(6)/T-2)(2) + 0.0603x(10(6)/T-2) + 0.0003 and 10(3)ln alpha(barite-fluid) = -0.0001x(10(6)/T-2)(2) + 0.0648x(10(6)/T-2) + 0.0004, which is consistent with previous theoretical and experimental studies. In hydrothermal fluids, the hydration number of Ba2+ varies from 4 to 6 at high P-T, and the cumulative average of 10(3)ln beta is derived to be 0.0798 +/- 0.005 parts per thousand in 923.15 K and 0.2 GPa. In silicate minerals, the enrichment of heavy Ba isotopes decreases in the sequence of muscovite > microcline approximate to celsian approximate to sanbornite > barylite > phlogopite. Variations in Ba concentrations and the degree of Al-Si disordering only induce limited Ba isotope fractionation, which would be hard to distinguish analytically. At high temperatures (> 600 degrees C), the melt phase is enriched in heavy Ba isotopes relative to any coexisting aqueous fluid and crystallizing minerals. During the early stages of felsic magma differentiation involving plagioclase crystallization as the main mineral, the delta Ba-138/134 and concentration in the residual melt remain close to their initial values, but any fluid exsolved from the melt will have light delta Ba-138/134 values. The high Ba distribution coefficient of K-feldspar means that the resultant K-feldspar granite inherits > 90% of the Ba contained in the initial magma reservoir. Hence, the delta Ba-138/134 of K-feldspar is close to that of the initial melt. During the final stage of felsic magma evolution, mixing of exsolved fluids from deeper reservoirs may significantly reduce the delta Ba-138/134 of the now Ba-depleted melt, and can account for the low delta Ba-138/134 values measured in highly differentiated leucogranites and pegmatites.

Automated summary

The geochemistry of Ba plays a significant role in studying igneous processes and fluid-melt interaction. Understanding Ba isotope fractionation mechanisms is crucial for fully utilizing its potential.