Mary Keskinen

Rhyolite intrusions in the intracaldera Bishop Tuff, Long Valley Caldera, California

Abstract Drilling of the Long Valley Exploratory Well on the resurgent dome in the Long Valley Caldera revealed > 300 m cumulative thickness of granophyric intrusions within the 1180-m-thick, 760 ka intracaldera Bishop Tuff. The intrusions are aphyric to sparsely plagioclase-phyric, high-silica, high-barium and low-strontium rhyolites. They resemble the lavas of the Early Rhyolite, the first phase of post-caldera volcanism. A mean 40 Ar/ 39 Ar age of 590 ± 17 ka from a part of a shallow intrusion is coeval with Early Rhyolite volcanism. A second mean age of 454 ± 17 ka from the same intrusion may reflect either younger Early Rhyolite activity with no external equivalent or hydrothermal resetting of the argon system. Hydrothermal alteration of the intrusions is characterized by introduction of quartz, calcite and pyrite and formation of illite/smectite. High CO 2 content of fluids apparently inhibited zeolite formation. Alteration varies locally within intrusions and intrusive groups and does not vary systematically with depth. Oxygen shows consistent depletion of the 18 O isotope from an initial magmatic composition of + 6.0 to + 8.5‰ to values ranging from +1.4 to −0.4‰. The constant oxygen isotope depletion most likely reflects alteration of intrusions due to local emplacement-induced hydrothermal circulation rather than a caldera-scale hydrothermal system. In contrast, 18 O depletion of the host Bishop Tuff increases regularly with depth (except at an intrusive contact). A pre-Early Rhyolite geothermal gradient of approximately 70 °C/km was inferred. This is substantially higher than the current gradient but substantially lower than expected for the case of a conductive regime over a shallow residual magma chamber. Either the intrusions were fed from a deep chamber, or a cool hydrologic recharge regime was established early in caldera history. The age, thickness and suspected lateral extent of these shallow intrusions are such that emplacement of the intrusions, rather than inflation of a shallow chamber, is responsible for resurgence of the central Long Valley Caldera. Similar intrusions occur in another well on the resurgent dome ( LV13-21 ) but not in wells located off the dome.

Trace-element geochemistry of individual glass shards of the Old Crow tephra and the age of the Delta glaciation, central Alaska

Abstract Two widespread tephra deposits constrain the age of the Delta Glaciation in central Alaska. The Old Crow tephra (ca. 140,000 ± 10,000 yr), identified by electron microprobe and ion microprobe analyses of individual glass shards, overlies an outwash terrace coeval with the Delta glaciation. The Sheep Creek tephra (ca. 190,000 yr) is reworked in alluvium of Delta age. The upper and lower limiting tephra dates indicate that the Delta glaciation occurred during marine oxygen isotope stage 6. We hypothesize that glaciers in the Delta River Valley reached their maximum Pleistocene extent during this cold interval because of significant mid-Pleistocene tectonic uplift of the east-central Alaska Range.

Old Crow tephra found at the Palisades of the Yukon, Alaska

Abstract A 20-cm-thick volcanic ash layer at the Palisades along the Yukon River is geochemically correlated with the Old Crow tephra. A buried organic and wood-rich layer above the Old Crow tephra probably records the last interglaciation. The recognition of the Old Crow tephra provides the first chronologic information on the age of the thick sequence of Pleistocene sediments at the Palisades, an important ice-age mammal site in central Alaska and a key locakity for the reconstruction of the history of the Yukon River and the last interglaciation in central Alaska.

Fumarole distribution, morphology, and encrustation mineralogy associated with the 1986 eruptive deposits of mount St. Augustine, Alaska

Numerous rootless fumaroles were developed on pyroclastic flows and a lava flow generated during the March 1986 eruptive cycle of Mount St. Augustine. Gases issued from fumarole vents with four different shapes: fissure, phreatic explosion crater, single/multiple ovoid opening, and diffuse, multiple opening. Fumarole distribution and morphology were controlled by preeruption drainage and topography, as well as by the thickness, compaction, and settling of the flow deposits. Fumarole temperatures measured in June and July 1986 ranged from 75°–394°C. Varying amounts of colorful and often roughly zoned encrustations are associated with all fumarole vent shapes. Only six types of crystalline phases were detected by X-ray diffraction, with gypsum the most abundant mineral, followed by anhydrite, sulfur, tridymite, halite, and soda alum. Scanning electron microscopy and energy dispersive X-ray analysis revealed a number of amorphous phases, mainly halogen-rich, as well as other minor crystalline phases. The mineral assemblages in the encrustations suggest formation conditions for these deposits within a general range of 25°–250°C in an oxidizing environment. Many of the amorphous phases are metastable and upon cooling of the fumarole lose nonstructural water and crystallize to more stable forms. The high halogen contents of the fumarole condensates and the mineralogy, chemistry, and morphology of the encrustations support leaching of the andesitic ash and lava flow by condensed acid vapors as the primary source for the chemical components contained in the encrustations. Comparison of traceelement (Sr, Ba, V, Co, Ni, and Cr) contents in unaltered and altered ash suggests that trace-element distribution follows a pattern of isomorphic substitution in the encrustation phases.