Browne, B.L., 2006, Investigating magma withdrawal dynamics during Plinian eruptions through mineral and eruptive stratigraphies: Examples from the chemically zoned 400 yr BP eruption of Half Cone Volcano, Aniakchak National Park, Alaska: American Geophysical Union Fall Meeting 2006, Eos, Transactions v. 87, no. 52, supplement, abstract id: V53E-05.
Half Cone volcano is the eviscerated remnant of a post-caldera composite volcano on the northwest floor of Aniakchak caldera (Alaska). The pyroclastic deposit produced during the 400 yr BP cataclysmic eruption of Half Cone is divisible in two volumetrically subequal fall units as well as a late-stage, lithic-rich, near-vent series of pyroclastic density current deposits and lava flow. The base of the deposit, known as the Pink Pumice, is composed of highly vesicular, crystal poor, and oxidized dacite pumice clasts (63-67% SiO2). Two stratigraphic horizons exist within this unit represented in outcrop by distinctly coarser-grained pumice and lithic clasts compared to the Pink Pumice's uppermost, middle, and lowermost levels. Overlying the Pink Pumice is the Brown Pumice layer, a brown-colored crystal-rich andesitic pumice fall (58-62% SiO2) with an abruptly coarse-grained base that normally grades upward. Although the contact between the Pink and Brown Pumice layers is abrupt in outcrop due to changes in color, vesicularity, and crystal content, whole-rock compositions plot along a linear continuum that ranges from andesite to rhyodacite. Isopach and isopleth mapping of the Pink Pumice indicate eruption column heights of 22-26 km for the coarser horizons, compared to only 12-15 km for uppermost, middle, and lowermost levels. Similar mapping of the upper and lower Brown Pumice indicate a declining plume from 20-24 km at the base to less than 15 km at the top. Granulometric analysis of lithic clasts coupled with electron microbeam analyses of individual phenocrysts and glass from the Pink and Brown Pumice indicate diverse lithic and mineral populations at horizons indicative of greater mass flux (plume height) compared to other levels in the deposit. Coarser-grained horizons contain granitic (quartz, feldspar, and mica) lithic fragments, oscillatory zoned plagioclase with An58-78 cores and An57-64 rims, and Fe-Ti oxide pairs yielding temperatures of 850- 890C. In contrast, levels indicative of lower eruption energy contain few granitic lithics, nearly unzoned plagioclase ranging from An58 to An62, and Fe-Ti oxide pairs that yield temperatures of 850-865C. Interestingly, glass compositions linger from 69-74% SiO2 throughout the Pink and Brown Pumice regardless of stratigraphic horizon. These observations suggest that (1) the magma chamber feeding the 400 yr BP eruption of Half Cone resulted via differentiation crystallization of an andesite liquid as evidenced by uniform glass compositions through the Pink and Brown Pumice stratigraphy coupled by increased crystal content in the Brown Pumice; (2) the magma chamber was intermittently tapped at deeper or shallower depths due to fluctuating mass flux, resulting in the eruption of heterogeneous mineral and lithic populations during periods characterized by higher mass flux compared to more uniform mineral and lithic populations during periods of lower mass flux; and (3) the combination of microbeam analyses of individual phenocrysts with granulometric and stratigraphic analyses is an effective tool in the investigation of magma withdrawal dynamics during Plinian eruptions.
