Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Abstract

To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ⁷Li varying from -3.1 to +6.6. The δ⁷Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from -3.1 to +2.5 and overlap with post- Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ⁷Li and elements indicative of degrees of granite differentiation (SiO₂, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to reflect the source composition.

Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may reflect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100.1100 ppm), quartz (30.140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ⁷Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ⁷Li showing an inverse correlation with Li concentration. This correlation reflects the mixing of isotopically heavy Li in quartz and lighter Li in fluid inclusions, as documented by fluid inclusion compositions (δ⁷Li = +8.1 to +13.4 and Li of 280 to 3960 ppm). Extrapolation of this trend to an estimated intrinsic Li concentration in quartz of <30 ppm, yields an inferred δ⁷Li for fluid inclusion-free quartz of >+21. The large difference in δ⁷Li between quartz and other minerals may reflect ⁷Li preference for less highly coordinated sites, which have higher bond-energies (i.e., the two- or fourfold site in quartz vs. higher coordination number sites in other minerals). Comparison of the Li isotopic composition of fluid inclusions with that of the wall zone of the Tin Mountain pegmatite suggests ~4‰ isotopic fractionation during fluid exsolution, which agrees with the results derived from studies of hydrothermal alteration of basalts.