Prolonged, episodic evacuation of discrete magma bodies at the onset of the Huckleberry Ridge Tuff supereruption

Abstract:

Integration of geochemical data with time breaks inferred from physical characteristics in early-erupted fall deposits shed light on the triggering mechanisms and initial episodic behavior¹ of the Huckleberry Ridge Tuff (HRT: 2500 km³) supereruption². In each layer sampled in the basal 2 m of fall deposits at Mount Everts, wide H₂O variations (1.0-4.7 wt.%) in co-erupted, fully enclosed, quartz-hosted rhyolitic melt inclusions (MI) imply <1 day to a week of diffusive loss. These data indicate highly variable and surprisingly slow ascent conditions during the opening stages of the eruption. The second largest Quaternary eruption on Earth² started hesitatingly, with magma slowly ascending to feed periodic explosive activity, with time breaks manifested by contemporaneous reworking in the fall deposits¹. Importantly, this behavior requires low degrees of overpressure in the feeding magma body to permit such slow ascent⁴, and we thus propose that external rather than internal (i.e. chamber overpressure) controls were central to initiation of the HRT eruption. In addition, multi-variant cluster analysis on trace elements for all MI reveals that the fall deposit contains six statistically distinct host-quartz populations. CO₂ vs. restored H₂O data show that the first erupted, most evolved compositions crystallized deeper (150-200 MPa), whereas the later-erupted, least evolved compositions crystallized shallower (100-140 MPa). This diversity indicates that the quartz populations represent magma bodies that were spatially separated to some extent during crystallization and evolution from a chemically similar parent. However, trace element analysis of reentrants and co-erupted obsidian clasts, which represent the compositions of melts at the time of eruption, although clustered, have values that correspond to only three of the six quartz populations. Taken all together, we conclude that the HRT eruption onset saw several vents active simultaneously and sequentially, supporting the notion that an external trigger, such as rifting or fault motion, caused destabilization, and accompanied the initial evacuation, of the HRT magma body.

¹ Wilson (2009), AGU, #V23C-2085. ²Christiansen (2001) USGS Prof. Pap. 729G, 1-143. ³Mason et al. (2004), Bull. Volc. 66, 735-748. ⁴ Melnik & Sparks (1999), Nature 402, 37-41.