This is a preprint of I paper intended for publication in a Journal or proceedlnEs. Since channes mav be made before Dubliatbn. thb DreDrint is made avrilable with the un--. . dentmndhg that it will not be cited or reproduced without the prmisaion of the author.

No abstract

Drilling at Obsidian Dome has provided continuous core samples of the distal and proximal portions of Obsidian Dome, its conduit, and an associated feeder dike. Both the dome and conduit are chemically and mineralogically zoned and consist of a finely porphyritic, high‐Ba, low‐silica rhyolite occurring in the basal portion of the dome and margins of the conduit and a finely porphyritic, low‐Ba, higher‐silica rhyolite occurring in the upper portion of the dome and center of the conduit. The high‐Ba rhyolite contains two distinct phenocrysts assemblages with two distinct compositions and represents mingled magmas. The low‐Ba rhyolite in the dome and conduit contains significantly fewer disequilibrium phenocrysts and is only slightly mingled. The dike, sampled at 600 m depth, as well as a related tephra fall from Obsidian Dome vent, are entirely low‐Ba rhyolite that contain no evidence of magma mingling. End‐members of the mingled magma, calculated using two different methods, are a 63% silica end‐member and a silicic end‐member identical in composition to the dike and tephra fall from Obsidian Dome vent. This silicic end‐member was the first magma emplaced in the dike and comprised much or all of the first magma vented to the surface during formation of the Obsidian Dome vent when eruption rates were high. Magma mingling of mafic and rhyolite magmas occurred during formation of the conduit. A comparison of the magma types at Obsidian Dome with the other 600‐year‐old magma types in the Inyo chain demonstrates that the finely porphyritic rhyolites at Obsidian Dome are similar to the finely porphyritic rhyolites at the other domes. All of these originated by mingling of two end‐members. The coarsely porphyritic rhyolites that occur at Glass Creek and Deadman domes are distinctly different from the finely porphyritic rhyolites. The two rhyolites can not be related by any fractionation scheme and could not have been in thermal or chemical equilibrium with each other. These observations require considerable heterogeneity in the Inyo Domes magmatic system. Two possible interpretations of the chemical variation and emplacement sequence are explored. In the first interpretation the chemical patterns observed in the surface and subsurface samples at Obsidian Dome are explained by differential draw‐up of magma from a stratified dikelike reservoir. This differential draw‐up is the result of changing flow rates during the course of the eruption. When the magma initially vented to the surface during the main part of the explosive phase, flow rates were very high, and the deepest layer of the reservoir was drawn into the conduit where magma mingling occurred. During the subsequent effusive phase, flow rates were low, draw‐up depths were reduced, and only magma from the uppermost layer in the reservoir erupted, only slightly contaminated by earlier hybrid magma drawn up. Similar processes most likely occurred at the other Inyo domes; however, the low‐Ba rhyolite was less abundant, and highly viscous, coarsely prophyritic rhyolite ...

No abstract

During the 1984 field season, two research holes were cored into the Inyo chain of young rhyolite domes, which cuts across the northwest margin of Long Valley Caldera, Calif. (Figure 1). One hole slanted under the vent of Obsidian Dome and completely penetrated the conduit of the volcano. It was spudded on August 16 and reached its final depth on September 9. The second hole was sited along Glass Creek between Obsidian Dome and the next dome to the south; it intersected the rhyolite dike which underlies the chain. It was spudded on September 18 and reached final depth on October 28.