Petrology and emplacement dynamics of intrusive and extrusive rhyolites of Obsidian Dome, Inyo Craters Volcanic Chain, eastern California

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 ...

Crystallization history of rhyolites at Long Valley, California, inferred from combined U-series and Rb-Sr isotope systematics

“…Calculated zircon saturation temperatures are well below 800°C (Heumann, 1999) and indicate a clear temperature decrease with increasing fractionation from LSR to HSR. These temperatures confirm results obtained from Fe-Ti oxides (Vogel et al, 1989). Currently, no solubility data exist for allanite, but by analogy to monazite, we predict allanite crystallization temperatures would be ϳ50°C lower than zircon (Montel, 1993).…”

Evaluation of magma mixing and fractional crystallization using whole‐rock chemical analyses: Polytopic vector analyses

“…PVA extracts end‐member compositions and the proportion of end‐members in each sample in the database. Although one of the original researchers that led to the development of PVA was a petrologist/geochemist [ Miesch , 1976b], there have been few investigations that have applied this technique (or its earlier versions) to mixing problems in petrology [ Horkowitz et al , 1989; Vogel et al , 1989]. However, recently Tefend et al [2007] used PVA to evaluate the complex mixing/fractionation processes in the Topopah Springs–Pah Canyon ash‐flow sheets in the Southwest Nevada Volcanic Field.…”

The unique radar properties of silicic lava domes

“…The finely porphyritic rhyolite shows both chemical and mineralogical zonation [Bailey et al, 1976], suggesting that mingling of two different magma types occurred during emplacement [Vogel et al, 1989]. According to detailed surface mapping by Sampson [1987], Deadman Dome consists primarily of coarsely porphyritic lava, Glass Creek Dome contains roughly equal amounts of coarsely and finely porphyritic varieties, and Obsidian Dome is entirely finely porphyritic rhyolite.…”