Deciphering the thermal and mixing history of the Pleistocene rhyolite magma chamber at Augustine Volcano

Abstract:

Recent activity at Augustine Volcano, located in Cook Inlet, Alaska, has been dominated by intermediate composition lavas and relatively small explosions. Earlier in Augustine’s history, however, a thick (~30 m) rhyolite fall was erupted ca. 25 ka, containing at least three distinct rhyolite lithologies. Numerous studies have documented evidence of magma mixing in the more recently-erupted material. Here we attempt to evaluate similar mixing events that may have affected the 25 ka rhyolitic magma prior to its eruption.

Basaltic to basaltic-andesitic deposits are found interbedded with the rhyolite at Augustine, so at least two magmas were present in Augustine’s plumbing system at the same or nearly the same time. Hints at interactions between two or more magmas are also evident on a smaller scale. Xenocrysts of olivine and clinopyroxene are present in the rhyolite, each with mafic melt inclusions. Additionally, two of the three rhyolitic lithologies studied contain high-aluminum amphiboles that are compositionally similar to amphiboles from mafic enclaves entrained during the 2006 eruption and thus may be xenocrystic.

To further investigate possible heating by secondary melts and the history of mixing, we use the titanium-in-quartz geothermometer (TitaniQ) on chemical zonation in quartz phenocrysts. We find that most quartz has a distinct 3-zone pattern, though one lithology also contains some complex zoning patterns in phenocryst cores, perhaps suggesting a xenocrystic origin. Additionally, we examine relationships between trace elements in the silicate melt inclusions from a variety of phenocryst types to determine if there is evidence for input of additional magma of different compositions. Finally, we apply results of a preliminary investigation of 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.