TRACE ELEMENT PARTITION COEFFICIENTS FOR CLINOPYROXENE AND PHLOGOPITE IN AN ALKALINE LAMPROPHYRE FROM NEWFOUNDLAND BY LAM-ICP-MS

摘要

Mineral/rock matrix partition coefficients have been determined for clinopyroxene (Cpx) and phlogopite from a Mesozoic alkaline lamprophyre from Newfoundland, Canada, by Laser Ablation Microprobe (LAM-ICP-MS). Values for twenty-one elements were obtained for Cpx, whereas only eleven were possible for phlogopite due to REE abundances below detection Limits (much less than 1 ppm). Ablation pits of 40-50 mu m diameter enabled investigation of zonation in phenocryst phases. In general, phenocrysts exhibit little trace element zonation except in the outermost overgrowths of Cpx. In these, a fourfold to fivefold increase in many trace element abundances correlates strongly with increasing Ti and Al contents, in agreement with recent experimental studies. Only Li shows appreciable zonation in phlogopite, being enriched in the rims. Comparison of the partition coefficients determined by in-situ laser analysis with those obtained from apparently pure mineral separates by solution ICP-MS indicates that, for several elements, considerable differences exist, e.g., D-Ba 0.0006 vs. 0.0255. These differences are attributable to the inclusion of trace element-rich overgrowths, and zones of trace element enrichment and micro-inclusions, in addition to the possibility of small amounts of matrix or glass (1-2%) in the nominally clean clinopyroxene mineral separates. Partition coefficients for Cpx are lower than most published values for basaltic rocks, but are comparable to experimental values for basalt determined by SIMS. Comparison with experimental values from lamproite discounts a strong bulk compositional effect, so that the discrepancy with earlier values for basalt should probably be attributed to problems with mineral separates, emphasizing the need for high quality partitioning determinations with in-situ microbeam methods. The phlogopite data extend considerably the published range; the new values are also generally lower than published values, although the discrepancy here may be due to bulk compositional effects, as many published values are from more silicic systems. The lamprophyre values are within the range of sparse experimentally determined values. [References: 58]