Isotopic fractionation of Li and B in highly evolved granitic systems

Abstract

The incompatible elements lithium (Li) and boron (B) generally show different degrees of isotopic fractionation during magmatic differentiation and fluid exsolution in highly evolved granitic pegmatite systems. We use the magmatic-hydrothermal evolution of the Qiongjiagang pegmatite-type Li deposit to demonstrate how fractionation and phase separation control the Li and B isotopic compositions of whole rock samples. The investigated samples include tourmaline-muscovite granites, unmineralized pegmatites, aplites, spodumene pegmatites and their associated wall rocks. Our data show systematic Li and B isotopic compositional variations among the studied rocks. The spodumene pegmatites have much higher Li concentrations, lower δ7Li values, and lower B concentrations and δ11B values than the tourmaline-muscovite granites, unmineralized pegmatites and aplites, which suggests phase separation into a fluid-rich melt and a fluid-poor melt. The fluid-rich melt experienced multi-stage magmatic differentiation with fluid exsolution, which eventually led to the formation of spodumene pegmatites. In contrast, the fluid-poor melt evolved through single-stage magmatic differentiation, eventually forming unmineralized pegmatites and aplites. Interactions with wall rocks modified the Li and B isotopic compositions of some aplite and unmineralized pegmatite samples. The Li and B isotopic compositions of minerals extracted from tourmaline-muscovite granite, unmineralized pegmatite, and spodumene pegmatite samples were also analyzed. The Li and B isotopic compositions of these minerals span a wide range, which is controlled by two primary factors: (i) coordination of Li and B and (ii) crystallization history. The crystallization of most magmatic minerals increases the δ7Li and δ11B values of the residual melt (except for tourmaline, whose crystallization seems not to change the δ11B values of the residual melt significantly). For minerals with the same coordination of Li and B, the δ7Li and δ11B values are higher for later crystallized minerals. As the δ7Li and δ11B values of magmatic minerals are affected by the amount of Li and B that has been removed from the melt before these minerals crystallized, bulk-rock δ7Li and δ11B values are generally more reliable tracers of the source and evolution of melts than the δ7Li and δ11B values of minerals that crystallized from such a melt. Only if the budgets of Li or B are dominated by one single phase, δ7Li and δ11B values of minerals and bulk rocks are the same.