Samuel E. Swanson

Crystallization history of Obsidian dome, Inyo domes, California

Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (ΔT) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low ΔT. A drastic increase in ΔT, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H2O) magma crystallized spherulites and fine-grained rhyolite at high ΔT. A granophyric texture, representing crystallization at a moderate ΔT, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H2O) rhyolitic magma at shallow depths.

Magmatism in the eastern Aleutian Arc: temporal characteristic of igneous activity on Akutan Island

Lavas from Akutan Island, located in the eastern Aleutian arc at the transition between continental and oceanic crust, show a gradual change in their petrologic and chemical characteristics over the last 4 million years. The oldest lavas exposed on the island, the Hot Springs Bay Volcanics (HSBV), range from magnesian basalt to dacite (45%–62% SiO2). The most mafic basalts contain salitic clinopyroxene, Cr- and Al-rich spinel, and pargasitic amphibole suggesting that they were derived from relatively hydrous magmas at greater pressures than lavas from the younger Akutan Volcanics (AKV) and the modern volcano (MOD). AKV lavas also range between basalt and dacite (46%–63% SiO2), but contain no hydrous phenocrysts and seem to have fractionated within a shallow level magma chamber. Lavas from the modern volcano are andesitic (52%–57% SiO2) and have a mineral assemblage similar to that of AKV lavas of similar composition. With the exception of clinopyroxene and spinel in the most mafic lavas, the compositions of plagioclase (An92−45), olivine (Fo88−51), orthopyroxene (En69−56), and titanomagnetite (15%–21% TiO2) phenocrysts found in these lavas are within the range observed in lavas from other Aleutian volcanoes. Variations in the major element chemistry of the older lavas can be reproduced by fractional crystallization of the observed mineral assemblages, however closed system crystal fractionation models are inadequate to explain the trace element variations. During the last 4 million years, La/Yb ratios have decreased (6.5–3.3 for HSBV lavas and 2.9–1.9 for MOD lavas) whereas Ba/La ratios appear to have increased slightly (37–43 for HSBV and AKV, and 41–45 of MOD). The lower La/Yb ratios of MOD lavas correspond with lower total abundances of the REE and slightly higher Sr and Pb isotopic ratios. The increased87Sr/86Sr ratios and Pb isotopic ratios in the MOD lavas, the less enriched LREE, and the higher Ba/La ratios may result from partial melting of an arc source which has experienced previous melting events but has continued to be contaminated by a component from the subducting slab. It may also indicate a change in the degree of partial melting of the underlying mantle, which corresponds to a different percentage of a slab derived component being incorporated into the overlying mantle.

Geochemistry of the 1989-1990 eruption of redoubt volcano: Part I. Whole-rock major- and trace-element chemistry

Abstract The 1989–1990 eruption of Redoubt Volcano produced medium-K calc-alkaline andesite and dacite of limited compositional range (58.2–63.4% SiO 2 ) and entrained quenched andesitic inclusions (55% SiO 2 ) which bear chemical similarities to the rest of the ejecta. The earliest (December 15) magmas are pumiceous, often compositionally banded, and the majority is relatively mafic ( 2 ). The most silicic magmas of the eruption are the late December to early January domes (up to 63.4% SiO 2 ). Subsequent magmas formed domes and rare pumices which converge on 60% SiO 2 . Chemical variations among ejecta comprise tight, linear, two-component arrays for all elements for which the analytical uncertainty is much less than the compositional range. The two-component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent. Additionally, the felsic endmember cannot be easily related to the mafic endmember by normal high-temperature igneous processes (e.g., the silicic endmember has higher Zr yet lower Hf than the mafic endmember). Also relative enrichments of highly incompatible elements are dramatically different across the arrays. The mixing event must have preceded eruption by a significant, yet unspecified amount of time because groundmass glass compositions are homogeneous for all post-December samples (Swanson et al., 1994-this volume), in spite of the whole-rock chemical diversity. This implies time for additional crystallization after the mixing event. Swanson et al. (1994-this volume) discuss evidence for a potentially different mixing event recorded only in December 15 magmas. Cognate cumulate xenoliths composed of pl+cpx+opx+hb+mt+melt were recovered from January and April deposits. These blocks differ from local batholithic country rock in their low concentrations of incompatible elements (e.g., Rb vs 20–90 ppm, Ba vs 300–2000 ppm) and low SiO 2 ( 60 wt.%). They have Mg, Cr, Ni, Sc, and V contents higher than the andesites, but lower than Redoubt basalts and basaltic andesites. Thus, they may be crystallization products of andesites, but do not represent the cumulate residue of basalt fractionation. The xenoliths were probably derived from a shallow or intermediate crustal chamber.

Volcanism in the eastern Aleutian arc: Late quaternary and holocene centers, tectonic setting and petrology

Abstract Cale-alkaline volcanism and oceanic plate subduction are intimately linked in the eastern Aleutian arc. The volcanic arc is segmented: larger caldera-forming volcanic centers tend to be located near segment boundaries. Intrasegment volcanoes form smaller stratocones. Ten of the 22 volcanoes that make up the 540 km long volcanic front in the eastern Aleutian are have erupted in recorded history and another six show hydothermal activity. The geometry of the Benioff zone in the eastern Aleutian arc has been defined by earthquake data from a local, high-gain short-period seismograph network. The Benioff zone dips at an angle of about 45° beneath the volcanic arc and reaches a maximum depth of 200 km. Based on the alignment of volcanoes, the eastern Aleutain arc can be subdivided into two main segments, the Cook and Katmai segments. A misorientation of 35° of the two segments reflects a change in strike of the underlying Benioff zone and implies a lateral warping of the subducting plate. The Cook segment volcanoes line up closely on the 100 km isobath of the Benioff zone. The Katmai segment volcanoes lie on a cross-cutting trend with respect to the strike of the underlying Benioff zone. Depths to the dipping seismic zone beneath volcanoes of the Katmai segment vary by 25% from 100 to 75 km. In the Katmai segment there is also good geophysical evidence that crustal tectonics plays an important role in localizing volcanism. Narrowly spaced linear groups of volcanoes appear to be positioned over a deep crustal fault that underlies the volcanic front. Transverse arc elements divide the arc into subsegments and localize larger magma reservoirs at shallow levels in the crust. Intrasegment volcanoes in both the Cook and Katmai segments erupt andesite and minor dacite of remarkably uniform composition despite differences in depths to the Benioff zone. Segment boundary volcanoes erupt lavas with a wider range of compositions (basalt to rhyolite) but are still calc-alkaline, in contrast to volcanoes in similar tectonic settings near segment boundaries in the central Aleutains. Greater crustal thickness in the eastern Aleutian arc, coupled with structural traps in the crust, allow magma ponding at shallow crustal levels. Differentiation at shallow depths yields dacite and even rhyolite.

Geochemistry of the 1989–1990 eruption of redoubt volcano: Part II. Evidence from mineral and glass chemistry

Abstract Early stages (December 1989) of the 1989–1990 eruption of Redoubt Volcano produced two distinct lavas. Both lavas are high-silica andesites with a narrow range of bulk composition (58–64 wt.%) and similar mineralogies (phenocrysts of plagioclase, hornblende, augite, hypersthene and FeTi oxides in a groundmass of the same phases plus glass). The two lavas are distinguished by groundmass glass compositions, one is dacitic and the other rhyolitic. Sharp boundaries between the two glasses in compositionally banded pumices, lack of extensive coronas on hornblende phenocrysts, and seismic data suggest that a magma-mixing event immediately preceeded the eruption in December 1989. Textural disequilibrium in the phenocrysts suggests both magmas (dacitic and rhyolitic glasses) had a mixing history prior to their interaction and eruption in 1989. Sievey plagioclase and overgrowths of magnetite on ilmenite are textures that are at least consistent with magma mixing. The presence of two hornblende compositions (one a high-Al pargasitic hornblende and one a low-Al magnesiohornblende) in both the dacitic and rhyolitic groundmasses indicates a mixing event to yield these two amphibole populations prior to the magma mixing in December 1989. The pargasitic hornblende and the presence of Ca-rich overgrowths in the sievey zones of the plagioclase together indicate at least one component of this earlier mixing event was a mafic magma, either a basalt or a basaltic andesite. Eruptions in 1990 produced only andesite with a rhyolitic groundmass glass. Glass compositions in the 1990 andesite are identical to the rhyolitic glass in the 1989 andesite. Cognate xenoliths from the magma chamber (or conduit) are also found in the 1990 lavas. Magma mixing probably triggered the eruption in 1989. The eruption ended when this rather viscous (rhyolitic groundmass glass, magma capable of entraining sidewall xenoliths) magma stabalized within the conduit.