236-15 Chemical Evolution during Undercooled Crystallization of Granitic Pegmatites: Boron, Lithium and Lithium Isotopes

See more from this Division: Topical Sessions
See more from this Session: Evolution of Simple Granite Systems (Haplogranites) and Rhyolites: A 50th Anniversary Perspective of the Tuttle and Bowen Studies

Tuesday, 7 October 2008: 11:45 AM
George R. Brown Convention Center, 330B

Mona-Liza Sirbescu, Dept. of Geology, Central Michigan Univ, Mt. Pleasant, MI, Peter Nabelek, Geological Sciences, Univ of Missouri-Columbia, Columbia, MO and Jennifer Maloney, Newfield Exploration Company, Houston, TX
Abstract:
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 δ7Lifluid (+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 δ7Lifluid > δ7Litourmaline, 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 δ7Lifluid values probably because of preferential partitioning of 7Li in the fluid phase.

See more from this Division: Topical Sessions
See more from this Session: Evolution of Simple Granite Systems (Haplogranites) and Rhyolites: A 50th Anniversary Perspective of the Tuttle and Bowen Studies

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