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