HR: 12:05h
AN: V41D-08 [PDF]
TI: Pre-Eruptive Exsolution of Chlorine-Enriched Volatile Phases at Augustine Volcano, Alaska: Evidence
from Silicate Melt Inclusions and Cl Solubility Modeling
AU: * Webster, J D
EM: jdw@amnh.org
AF: Department of Earth and Planetary Sciences, AMNH, Central Park West at 79th St., New York, NY
10024-5192 United States
AU: Tappen, C
EM: ctappen@amnh.org
AF: Department of Earth and Planetary Sciences, AMNH, Central Park West at 79th St., New York, NY
10024-5192 United States
AU: Mandeville, C
EM: cmandy@amnh.org
AF: Department of Earth and Planetary Sciences, AMNH, Central Park West at 79th St., New York, NY
10024-5192 United States
AU: Harms, C
EM: charmsou@hotmail.com
AF: School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019-0628 United States
AB:
Augustine volcano has experienced 6 explosive eruptions in historic time and is located in Cook Inlet, Alaska, approximately
330 km from Anchorage. Most lavas and tephra range from primitive andesite to dacite, but minor basalt flows crop out on the
island. The matrix and silicate melt inclusion glasses are more felsic, however. Volcanic gases collected from the
Augustine crater during and immediately after the most recent eruptions in 1976 and 1986 were strongly enriched in HCl
(Symonds et al., 1990). The presence of elevated Cl levels in magmatic gases is consistent with high concentrations of Cl in
silicate melt inclusions in plagioclase and pyroxene phenocrysts in materials erupted from Augustine in 1976 and with the
conclusion that the 1976 magma was saturated in a Cl-enriched fluid/vapor prior to eruption (Johnston, 1978).
To determine the role of Cl and other volatiles in magmatic and volcanic processes at Augustine volcano, we have begun a
systematic study of silicate melt inclusions in tephra erupted at Augustine in 1986 and also erupted $\sim$ 1000, $\sim$
1400, $\sim$ 1700, $\sim$ 2100 years ago. We analyzed glassy silicate melt inclusions in plagioclase and pyroxene
phenocrysts for major, minor, and some trace elements (including Cl, S, and F) by electron microprobe and for H$_{2}$O and
CO$_{2}$ by FTIR.
Preliminary results show that each of these magmas was strongly but variably enriched in H$_{2}$O, S, and Cl. Sulfur ranges
from 100-700 ppm, and Cl varies from 2000 to more than 8000 ppm. Trends involving Cl, H$_{2}$O, S, and major elements in
tephra from all 5 of the eruptions that we studied are consistent with the exsolution of aqueous-carbonic, S- and Cl-charged
volatile phases well before eruption. Moreover, the Cl contents of most melt inclusions are quite high, and the most
recently erupted (e.g., 1976 [Johnston, 1978] and 1986) magmas were particularly enriched in Cl. In fact, the Cl abundances
of some inclusions approach that of the 2000 bar chloride solubility limit for felsic magmas. Thus, some fractions of the
late-stage, dacitic Augustine magmas, erupted in 1976 and 1986, would have exsolved a S-bearing, hydrosaline chloride liquid
with or without a S- and Cl-bearing, aqueous-carbonic vapor at 2000 bars before eruption. In summary, the geochemistry of
melt inclusions of the eruptive units studied so far indicates that the exsolution of Cl-charged volatile phases is a common
process that preceeds most of Augustine's explosive eruptions.
(D. Johnston, 1978, Unpublished Ph.D. Diss., Univ. of Washington)
(Symonds et al., 1990, Bull. Volcanol. 52:355-374)
DE: 3640 Igneous petrology
DE: 3670 Minor and trace element composition
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting