MINERAL-MELT PARTITION COEFFICIENTS AS A TOOL FOR INVESTIGATING FRACTIONAL CRYSTALLIZATION, MELT SEGREGATION AND LATE-STAGE CRYSTAL GROWTH IN YELLOWSTONE RHYOLITIC MAGMAS

Laser ablation ICP-MS major and trace element analyses on fused whole rock powders, glass and minerals provide new insights on magma evolution processes of the ~600-480 ka Upper Basin member (UBM) and ~255-70 ka Central Plateau member (CPM) post-caldera rhyolitic lava flows of Yellowstone. Subsequent whole rock and glass decreases in Mg and Sr (from ~0.44 to ~0.020 wt% MgO and ~120 to ~3 ppm Sr, whole rock), later accompanied by decrease in Ba (from ~1000 to ~40 ppm, whole rock) suggest that, together, the UBM and CPM rhyolites form a fractionation series in which pyroxene and plagioclase crystallizes first, later accompanied by sanidine ± quartz. Lava flows have low crystallinity (typically <10%) and their mineralogy is not strictly consistent with what predicted by that fractional crystallization trend, as many units frequently lack at least one of these phases. Throughout most samples, minerals have homogeneous major and trace element compositions with no obvious zoning or xenocrystic cores, but commonly have sieve texture and glass embayments, arguing for late-stage rapid crystal growth. By contrast with large variations of compatible elements compositions in glass, mineral-melt partition coefficients of trace elements in plagioclase (K_DSr~23±2 (1σ), K_DBa~ 3.3±0.9, K_DEu~ 8.4±1.0), sanidine (K_DSr~20±2, K_DBa~30±6, K_DEu~9.0±1.0), clinopyroxene, and of Ti in quartz (K_D~0.16±0.014) are remarkably invariant among lava flows over time and degree of differentiation. These observations together argue for (1) a long-lived crystallizing magma body, (2) episodic melt segregation, and (3) in-situ rapid crystallization in the extracted melts. Three of the most primitive UBM rhyolites also exhibit small (usually <2 mm) and abundant clusters (glomerocrysts) of plagioclase-oxides ± orthopyroxene ± clinopyroxene that coexist with individually occurring larger crystals of same nature. Clustered crystals are more primitive with higher pyroxene Mg# and plagioclase Ca contents. Therefore, these might represent earlier formed and aggregated crystals which were entrained in the extracted melts. In younger lavas, such crystal clusters would be entirely segregated from the melt, and may now be forming a mush or cumulates under Yellowstone caldera.