Mineral recorders of pegmatite internal evolution: REE contents of tourmaline from the Bob Ingersoll pegmatite, South Dakota

Trace rare earth elements (REE) have been determined by radiochemical neutron activation analysis for tourmaline samples from an internally zoned, rare-element, granitic pegmatite, located in the Black Hills, South Dakota. The total REE concentrations range from 40 ppm-0.2 ppm, and are highest in tourmaline from the exomorphic halo (country rock) and pegmatite border zone. Chondrite-normalized patterns are highly fractionated from light REE to heavy REE; and REE concentrations decrease in tourmaline from the outer wall zone and first intermediate zone, through the inner wall zone and third intermediate zone, to lowest levels in the pegmatite core. The REEs, as recorded by tourmaline, appear to behave compatibly in this pegmatite system due to early crystallization of apatite and other possible "REE-sink" minerals. The large range of REE concentrations and differences in slopes of chondrite-normalized patterns probably also reflect significant changes in the structural state of the pegmatite melt, caused by changes in pH₂O and other volatiles (B, F, P) as crystallization progressed. Tourmaline samples that appear to have been fluid-derived are HREE-depleted relative to coexisting silicate-melt-derived tourmaline. Tourmaline does not exhibit any strong preference for specific REEs, rather its REE content appears to reflect the REE content of the medium from which the tourmaline crystallized.