NEW ZIRCON-MELT PARTITION COEFFICIENTS FROM NATURAL ZIRCON-GLASS PAIRS APPLIED TO MOUNT ST. HELENS, USA: UNDERSTANDING MSH MELT COMPOSITIONS THROUGH TIME

We present new, robust zircon-melt Kds based on natural mineral/host glass pairs, and use them to model melt compositions through time at Mount St. Helens (MSH) volcano, WA, USA. We analyzed zircon rims (outer ~15 microns of polished grain interiors) or surfaces (1-2 micron deep pits on crystal faces) by SHRIMP-RG and glasses by LA-ICPMS in tholeiitic, calc-alkaline, and alkaline dacites and rhyolites from diverse settings (continental extension, AZ-NV, USA; hot spot/spreading center, Iceland; continental arc, MSH). Kds for all elements span more than an order of magnitude but are highly coherent. REE Kds fit lattice strain model parabolas and show strong negative correlations with T indicators (e.g., Ti_zirc). Kds for zircon can be estimated from Ti_zirc-Kd_element correlations, and we quantify this relationship to allow calculation of Ti-dependent Kds for natural systems. MSH Kds were based on eruption-age surfaces with adhering glass, so should approach true crystal-melt equilibrium. MSH Kds based on surfaces are consistent with those from rim analysis in the MSH and other samples, suggesting that rim analysis of zircon is sufficient for estimating Kds.

Compositions of MSH melts through time modeled using 55 zircon analyses from 3 samples resemble measured MSH glass compositions, suggesting that zircon Kds and modeled melts are reasonably reliable. Uranium (median=3.0) and Th (med=6.7) are positively correlated, with Th/U from 0.6-4 (med=2.6). Model melts exhibit REE patterns typical of felsic magmas: enriched in LREE, depleted in HREE, median similar to MSH glasses. HREE are more robustly quantified than LREE due to relative unreliability of LREE Kds. MSH model melt compositions (U, Th, REE, Th/U, Zr/Hf) are diverse at any given time, suggesting that discrete pockets of melt were crystallizing zircon simultaneously, as they cooled and solidified in isolation from one another. Variability increases during the last 60 kyr (higher Th/U, Zr/Hf, MREE/HREE); we infer that this marks replenishment by less evolved melts. These results strengthen our confidence that magmas intruding the active plutonic system diversified at around 60 ka, ~40 kyr before hotter, drier, less evolved magmas began erupting from MSH.