Storage of intermediate magmas erupted from Novarupta

The 1912 Novarupta eruption in the Valley of Ten Thousand Smokes (VTTS) is remarkable in many respects, not the least of which has been its impact on diverse areas of volcanological research, such as large volume ignimbrite emplacement, caldera collapse dynamics, and vapor transport of ore metals from volcanic systems. Chief among the outstanding puzzles of this eruption are the striking compositional variety of juvenile erupted material and the subsurface plumbing system that allowed caldera collapse at Mt. Katmai to occur 10 km from the Novarupta vent. During a 60-hour period, 13 km³ magma (dense rock equivalent) was erupted, comprised of 7.5 km³ high silica rhyolite and an only slightly smaller amount (5.5 km³) of intermediate composition material spanning the range from basaltic andesite to dacite. Highly evolved rhyolite is not only extremely rare in the Aleutian arc, but completely absent from the Pleistocene record of the local Katmai group of stratovolcanoes (Griggs, Katmai, Trident, Mageik, and Martin). Intermediate compositions are typical of the Katmai group, and the 1912 magmas are compositionally most similar to material erupted from Mt. Katmai itself (Hildreth and Fierstein, 2000). Determining the pre-eruptive conditions at which the 1912 magmas last equilibrated is critical to addressing the issues of their relative crustal storage depths, conduit geometry, and ultimately the petrogenesis of the high silica rhyolite.

Phase relations at H₂O+CO₂ fluid saturation were determined for an andesite (58.7 wt.% SiO₂) and a dacite (67.7 wt.%) erupted during the 1912 eruption in the Valley of Ten Thousand Smokes, Alaska. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally at H₂O-saturated conditions. If H₂O-saturated, these magmas equilibrated at (and above) the level where co-erupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally at H₂O-undersaturated conditions for the same range in P_H2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H₂O-saturated rhyolite is consistent with experimental findings only if the intermediates are H₂O-undersaturated, having X_H2O^fl= 0.7 and 0.9 for the extreme compositions, respectively.

Hammer, J.E., M.J. Rutherford, and W. Hildreth, Magma storage prior to the 1912 eruption at Novarupta, Alaska, Contributions to Mineralogy and Petrology, 144, 144-162, 2002.