Chemical Evolution during Undercooled Crystallization of Granitic Pegmatites: Boron, Lithium and Lithium Isotopes

Mineral textures, chemical disequilibria, inclusion microthermometry, and heat-transfer modeling suggest that undercooled pegmatite magmas rich in H₂O, B, and F crystallize rapidly. Fluxes affect magmatic nucleation/crystal growth, therefore igneous texture. Thin, inwardly crystallized dikes from San Diego County, California display consistent textural sequencing: 1) layered aplite in their wall zone, 2) coarser pegmatite with radiating, acicular tourmaline, and graphic intergrowths in their intermediate zones, and 3) an extremely coarse massive core including crystal-lined pockets once filled with aqueous fluid.

Fluid inclusion chemistry can constrain the evolution of the inward crystallizing system. Although probably magma remained fluid undersaturated until intermediate zones crystallized, inclusion trapping started in the outer zones, in conditions of local saturation, at the crystal-melt interface. Quartz is the preferred inclusion host due to its potential to arrest magmatic fluid compositions. Crush-leach ion chromatography analysis yielded ionic ratios, including B and Li. MC-ICP-MS analyses of inclusion leachates yielded Li isotopic ratios (δ⁷Li) of the magmatic fluid, to complement existing tourmaline δ⁷Li values (Maloney et al., Eur.J.Min, in press).

Boron and lithium concentrations in fluid inclusions increase towards inner zones. Preliminary LA-ICP-MS analyses of pocket fluid inclusions indicate that B (3000±200 ppm) and Li (1200±200) are the dominant dissolved species. In gem-mineralized pockets, F is also important (F/Cl=0.2-5). The widely ranging δ⁷Li_fluid (+10.1 to +24.1‰; 24 samples) can be attributed to 1) isotope fractionation among phases and 2) kinetic effects caused by differential diffusion rates in transient thermal and chemical gradients. In general δ⁷Li_fluid > δ⁷Li_tourmaline, in agreement with expected mineral-fluid lithium fractionation. However, intermediate-zone radiating tourmaline is consistently heavier than coeval fluid by 3-6.5‰, possibly because of ⁷Li enrichment in the melt boundary layer incorporated by rapidly growing crystals. The pocket fluids have high δ⁷Li_fluid values probably because of preferential partitioning of ⁷Li in the fluid phase.