Crystal-Melt Elemental Partitioning in Silicic Magmatic Systems: an Example From the Peach Spring Tuff High-Silica Rhyolite, Southwest USA

Abstract:

Partition coefficients (K_d) are critical in quantitative models of magmatic evolution. High-SiO₂ rhyolites (HSR) are characterized by saturation in numerous phases, thus requiring a comprehensive K_d dataset, including both major and accessory minerals. While a large body of published K_d exists, most studies are limited to small suites of elements (e.g. REE) in few (2-3) minerals, using various methods, bulk compositions, and conditions. We use SEM-EDS and LA-ICP-MS analysis to determine crystal rim and unaltered glass compositions in a single sample of Peach Spring Tuff HSR, which reflect equilibration between crystals and melt at or near the time of eruption. We present K_d for 45 elements in 8 minerals: sanidine, plagioclase, biotite, amphibole, titanite, apatite, zircon, and chevkinite.

We show that titanite strongly partitions REE from melt, with significant preference for middle (MREE) over light (LREE) and heavy (HREE) REE. Apatite and amphibole share a similar MREE-enriched pattern but with absolute K_d up to ~2 orders of magnitude lower than titanite; while apatite displays a slight preference for LREE over HREE, amphibole shows a higher affinity for HREE than LREE. Zircon strongly partitions HREE, Hf, and U, with little affinity for LREE. Chevkinite concentrates LREE and Th significantly more than any other phase. Biotite K_d are generally <10, the highest being for transition metals and Ba. Both feldspars show strong preference for Sr, Ba, and Eu.

We find that our dataset is largely consistent with published studies that use similar (in-situ) techniques and compositions (HSR). We observe a large variation in other studies that we partly attribute to contamination from inclusions, particularly for compositions measured by whole-crystal methods. Our K_d suggest that accessory minerals play a dominant role in partitioning trace elements, and in particular they collectively control the distribution of REE and high field strength elements. Large ion lithophile elements (LILE), in contrast, are most dominantly influenced by feldspars, but if abundant, phases such as biotite and apatite may also influence LILE partitioning.