MIXING? HEATING? ZONING? UNRAVELING THE MAGMATIC HISTORY OF PLEISTOCENE RHYOLITE AT AUGUSTINE VOLCANO

Despite intermediate composition lavas and relatively small explosions dominating recent activity at Augustine Volcano, Alaska, a thick (~30 m) rhyolite fall was erupted earlier in the volcano’s history, ca. 25 ka. Multiple studies have found evidence of magma mixing in the recent intermediate eruptive products. Given that the rhyolite deposit contains at least three distinct lithologies, we speculate that mixing may also have played a role in the Pleistocene magmatic system. Here we attempt to evaluate thermal or mixing events that may have affected the 25 ka rhyolitic magma prior to its eruption.

It is clear that at least two magmas were present at the same or nearly the same time in Augustine’s plumbing system around the time of the rhyolite eruption; basaltic to basaltic-andesitic deposits are found interbedded with the rhyolite. Xenocrysts of olivine and clinopyroxene are also present in the rhyolite, each with mafic melt inclusions, and add credence to the likelihood of multiple magmas interacting. Two of the three rhyolitic lithologies also contain high-aluminum amphiboles, which appear chemically equivalent to amphiboles erupted in mafic inclusions in the 2006 andesites.

To investigate possible mixing, we examine relationships between trace elements in the silicate melt inclusions from multiple phenocryst types to determine if there is evidence for magmatic input with different compositions. We also investigate possible heating by mixing, replenishment, or underplating by using the titanium-in-quartz geothermometer (TitaniQ) on titanium zonation in quartz phenocrysts. Most quartz has a distinct 3-zone pattern, though one lithology shows a few instances of complex zoning, perhaps indicating a xenocrystic origin. Additionally, we evaluate the mineralogy of a high-phosphorus dacite that stratigraphically overlies the rhyolite to assess their similarity and the degree of mixing, if any, that may have led to the transition from rhyolitic to dacitic magma.