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

Mineral/rock matrix partition coefficients have been determined for cl inopyroxene (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 ele ven were possible for phlogopite due to REE abundances below detection Limits (much less than 1 ppm). Ablation pits of 40-50 mu m diameter e nabled investigation of zonation in phenocryst phases. In general, phe nocrysts 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 A l contents, in agreement with recent experimental studies. Only Li sho ws appreciable zonation in phlogopite, being enriched in the rims. Com parison of the partition coefficients determined by in-situ laser anal ysis with those obtained from apparently pure mineral separates by sol ution ICP-MS indicates that, for several elements, considerable differ ences exist, e.g., D-Ba 0.0006 vs. 0.0255. These differences are attri butable to the inclusion of trace element-rich overgrowths, and zones of trace element enrichment and micro-inclusions, in addition to the p ossibility 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 comp arable to experimental values for basalt determined by SIMS. Compariso n with experimental values from lamproite discounts a strong bulk comp ositional effect, so that the discrepancy with earlier values for basa lt should probably be attributed to problems with mineral separates, e mphasizing 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 publish ed values, although the discrepancy here may be due to bulk compositio nal effects, as many published values are from more silicic systems. T he lamprophyre values are within the range of sparse experimentally de termined values.