Inter- and intra-crystal REE variations in apatite from the Bob Ingersoll pegmatite, Black Hills, South Dakota

Abstract

Concentrations of rare earth elements (REE) have been measured on a suite of apatite crystals from an internally zoned granitic pegmatite enriched in Li, B, Be, F, Nb, Ta, Sn and U with a Cameca IMS 3f ion microprobe using energy filtering. An apatite specimen from the Tin Mountain pegmatite, analyzed previously by isotope dilution, was used as a standard. The chondrite-normalized pattern determined with the ion microprobe closely matches the pattern determined by isotope dilution, with maxima at Sm and Dy, and minima at Nd and Er. Apatite samples from the Bob Ingersoll pegmatite show a large range of REE patterns and concentrations. In one case, apatite crystals within millimeters show differences in REE concentrations and pattern shapes, including a switch from positive to negative Eu anomalies. Samples from several mineral assemblages show patchy individual crystal zoning with respect to the REE that is not mirrored by major element zoning. This indicates disequilibrium conditions on a small scale, consistent with rapid growth from a melt/fluid system that was either of locally heterogeneous melt structure, or in which the melt structure fluctuated rapidly as volatile rich minerals such as tourmaline crystallized. These effects may be coupled with non-ideal partitioning of REE in a heterogeneous mixture of melt, aqueous fluid and crystals. REE concentrations in apatite samples from the different pegmatite zones indicate a large variation in outer zones (10–500X chondrite), high concentrations (100–1000X chondrite) near the pegmatite core, and very low concentration in the core (2–20X chondrite). Patterns are flat to slightly inclined (Ce/Yb: 1 to 5), and most samples have positive Eu anomalies. The magnitude of positive Eu anomalies decreases with inward position in the pegmatite, possibly indicating a progressive increase in ƒ O 2, and a sharp increase may be indicated by systematic Ce depletion in apatite from the pegmatite core. REE-specific volatile complexes may contribute to variations, including unusual kinks, observed in REE patterns of apatite from mineral assemblages in upper parts of the pegmatite.