Thermal storage conditions and origin of compositional diversity of plagioclase crystals in magmas from the 1980 and 2004-2005 eruptions of Mount Saint Helens
Authors: Schlieder Tyler D , Cooper Kari M , Kent Adam J R , Bradshaw Richard , Huber Christian 
Published: July 2022

Abstract
The thermal histories of upper crustal magma reservoirs place important constraints on the formation, evolution, and remobilization of crustal magmas, yet quantifying thermal histories remains challenging. We report new in-situ plagioclase trace-element data, Sr in plagioclase diffusion timescales, and plagioclase 238U-230Th-226Ra disequilibria data from Mount Saint Helens (MSH) 1980 cryptodome and 2004-2005 dacite domes to evaluate the thermal storage conditions and compositional diversity of recent MSH magmas. This approach allows us to more directly link the thermal (and therefore physical) conditions within the MSH magma reservoir to timescales of storage, thereby constraining the fraction of time 1980-2005 MSH magmas have spent in a mobile state. Plagioclase trace-element data and U-series characteristics reveal a compositionally heterogenous magmatic system beneath MSH and also require multi-stage plagioclase growth histories. The data also show that 2004-05 dacites contain a different plagioclase population relative to the 1980 cryptodome dacite, comprised of either a compositionally (and possibly temporally) distinct plagioclase component or composed of the same plagioclase components but in significantly different proportions. Discordant plagioclase 238U-230Th and 230Th-226Ra apparent ages require a mixture of young (likely eruption related) and old (&gt;20-40 ka) plagioclase crystals in both 1980 and 2004-05 dacites. The low (230Th)/(232Th) in all measured 1980 and 2004-05 plagioclase requires a significant fraction (&gt;10-30%) of the plagioclase to be old (&gt;10s kyr). Maximum modeled Sr diffusion timescales at 750oC range from decades to centuries for both eruptions, with a maximum of ~600 years found in 1980 cryptodome plagioclase. However, partially equilibrated Sr in the innermost exposed parts of some crystals found in both 1980 and 2004-05 plagioclase indicate that a fraction (&gt;20%) of plagioclase may have experienced a total of &gt;10 kyrs at temperatures ≥750oC. Coupling Sr diffusion timescales with U-series measurements indicates that significant fraction (at least ~40%) of modeled MSH plagioclase spent &lt;5% of their storage time at temperatures ≥750oC and thus in an easily mobilized rheological state. In contrast, the fraction of partially equilibrated plagioclase possibly spent &gt;25-50% of their storage time at hotter conditions. Our data combined with data from other arc magmatic systems (e.g., Mt. Hood) imply that the process of remobilizing magma in arc systems towards successive eruptions requires thermally rejuvenating largely crystalline material, as opposed to sequential tapping of a persistent liquid-dominated magma body, even if successive eruptions are spaced closely in time (decades). In addition, rejuvenation events responsible for successive eruptions may sample spatially localized, but potentially overlapping, portions of the broader magma reservoir.