Lithium and its isotopes in tourmaline as indicators of the crystallization process in the San Diego County pegmatites, California, USA

In the lithium-cesium-tantalum-type pegmatite dikes of San Diego County, California, USA, tourmaline is the main reservoir for Li, except in the cores and the pockets of the dikes where other Li-bearing minerals also occur. Tourmaline from three subhorizontal dikes was analyzed for bulk Li concentrations and Li isotope ratios. The bottom portion of each dike includes rhythmically layered aplite called line-rock. Above the aplite is the lower pegmatite zone that crystallized upward whereas the hanging pegmatite zone crystallized downward. The lower and hanging pegmatite zones are joined at the core zone. Pockets that were once fluid-filled occur in the core zone. Tourmaline in the line-rocks and the upper border zones has 22-70 ppm Li and in the pegmatite zones 53-450 ppm Li. Large tourmaline blades in the cores have 174-663 ppm Li. Elbaite rims on prismatic tourmaline in the pockets have up to 5075 ppm Li. The progressive enrichment in Li from the wall-zones to the pockets is attributed to inward fractional crystallization of the dikes. The line-rock in each dike appears to have crystallized until the melt reached fluid saturation, at which point the melt and the fluid began to unmix to form the pegmatite zones and the pockets. The estimated initial Li concentration in the magma that produced the dikes is ∼630 ppm. At this low concentration, Li has had much smaller effect on crystallization of the dikes than H₂O. δ⁷Li in tourmaline in the line-rocks, the cores, and the pockets ranges from + 11.2 to + 16.1‰ with no systematic difference between these textural zones. However, in radial tourmalines δ⁷Li is > 19 ‰. The very elevated δ ⁷Li may reflect Li isotope fractionation between the melt and the exsolving fluid at the time of crystallization of these tourmalines, with ⁷Li preferring the more strongly-bonded occupancy in the silicate melt over a hydrated ion occupancy in the fluid. Alternatively, the elevated δ⁷Li may also have been caused by preferential accumulation of the slower-diffusing ⁷Li ahead of the rapidly-growing radial tourmalines. The overall elevated δ⁷Li values of the dikes may have been acquired by Li isotope exchange with wall-rock during passage of the pegmatite melts from their sources.