Large-volume rhyolite genesis in caldera complexes of the Snake River Plain
Watts, Kathryn Erin, 1983-
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Watts, Kathryn Erin, 1983-
Caldera-forming eruptions are dramatic and destructive natural phenomena, causing severe and sustained consequences to society. This dissertation presents new geochemical and geochronologic data for caldera-forming tuffs and pre- and post-caldera rhyolites of the two youngest caldera complexes in the Snake River Plain (SRP) in the western USA: Heise (6.6-4.5 Ma) and Yellowstone (2.1-0.6 Ma). Caldera complex evolution at Heise and Yellowstone can be described by formation of 3-4 spatially overlapping "nested" calderas, successive collapse of intracaldera fill, and development of a large hydrothermal system. Comparison between Heise and Yellowstone reveals that late-stage rhyolite eruptions have drastic depletions in 18 O that require remelting of large volumes (1,000's of km 3 ) of hydrothermally altered rock. Archean xenoliths and Phanerozoic rocks of the crustal basement beneath the SRP province are not depleted in 18 O and therefore cannot be a source of these rhyolites. Isotopic mixing models indicate that early large-volume rhyolites are produced by melting and hybridization of the crust by mantle-derived basalt, and late-stage rhyolites tap hydrothermally altered portions of intracaldera rocks from previous eruptions. Caldera-forming eruptions at Heise culminated 4.45 Ma with eruption of the 1,800 km 3 Kilgore Tuff, the most voluminous 18 O-depleted rhyolite in the SRP and worldwide. O, Sr, and Nd isotope geochemistry, zircon ages, mineral and whole-rock geochemistry, and liquidus temperatures for Kilgore Tuff samples erupted >100 km apart are similar and/or overlapping within error, indicating derivation from a remarkably homogeneous low-δ 18 O magma reservoir (δ 18 O=3.4[per thousand]). Caldera-wide batch assembly and homogenization of variably 18 O-depleted melt pockets with diverse zircon populations can explain the Kilgore Tuff's genesis. Central Plateau Member (CPM) rhyolites at Yellowstone have the same timing (∼2 million years after the initiation of volcanism), magnitude of δ 18 O depletion (∼3[per thousand] depleted relative to normal rhyolites), and cumulative eruptive volume (∼4,000-4,500 km 3 ) as the Kilgore Tuff of the Heise volcanic field. Isotopic, age, and geochemical data for CPM rhyolites show that they become progressively more homogeneous and evolved from 260 ka to 75 ka. Whereas the Kilgore Tuff erupted climactically as an explosive caldera-forming tuff, CPM rhyolite eruptions record sequential, predominantly effusive, "snapshots" of magma assembly, homogenization, and differentiation. This dissertation includes co-authored materials both previously published and submitted for publication.