Magmatic evolution of the post-caldera magma reservoir at Yellowstone revealed by compositional zoning in the crystals of Central Plateau Member Rhyolites

The voluminous (>900km3) high-silica rhyolites composing the Central Plateau Member (CPM) of the Plateau Rhyolite at Yellowstone caldera erupted between 160 ka and 70 ka, and sample one of the youngest magma reservoirs in the Yellowstone magmatic system. An older lava erupted at 200 ka, the Scaup Lake Flow of the Upper Basin Member, may represent the oldest CPM lava based on its Nd and Sr isotopic affinity to the CPM lavas. In order to determine the magmatic history in the prelude to eruption of CPM rhyolites, we analyzed the compositions and zonation patterns in single sanidines and pyroxenes from four lavas erupted at 160 ka (Dry Creek flow), 120 ka (West Yellowstone flow), 110 ka (Solfatara Plateau flow), and 70 ka (Pitchstone Plateau flow), as well as the Scaup Lake flow, which serve as sequential "snapshots" of the evolving CPM reservoir. Crystallization temperatures obtained using QUILF equilibria and zircon/glass thermometry range from 800°C to 820°C, at pressures of ~1.1 - 1.2 kilobars. Clinopyroxenes from older (160-200 ka) CPM lavas have higher Mg and lower Fe concentrations than those from the younger lavas, consistent with crystallization from a progressively differentiating reservoir of rhyolitic magma. Clinopyroxenes and sanidines from all the analyzed CPM lavas, except Solfatara Plateau flow, contain reverse zoning. Reverse zoning of clinopyroxenes and sanidine from the Dry Creek and Pitchstone Plateau flows occur within 20 micrometers of individual crystal rims, suggesting a near-eruption episode of mixing with hotter, less-evolved magma. A core from one normally zoned clinopyroxene from the Dry Creek flow has a composition identical to the rims of Scaup Lake clinopyroxenes, suggesting some recycling of crystals.