Abstract
The Bishop Tuff is a giant silicic ignimbrite erupted at 0.76 Ma in eastern California, USA. Five pumice clasts from the late-erupted Bishop Tuff (Aeolian Buttes) were studied in an effort to better understand the pre- and syn-eruptive history of the Bishop magma body and place constraints on the timescales of its existence. This study complements and expands on a previous study that focused on early-erupted Bishop Tuff pumice clasts. Bulk densities of pumice clasts were measured using an immersion method, and phenocryst crystal contents were determined using a sieving and winnowing procedure. X-ray tomography was used to obtain qualitative and quantitative textural information, particularly crystal size distributions (CSDs). We have determined CSDs for crystals ranging in size from {approx}10 to {approx}1000 {micro}m for three groups of mineral phases: magnetite ({+-}ilmenite), pyroxene + biotite, quartz + feldspar. Similar to early-erupted pumice, late-erupted pumice bulk density and crystal contents are positively correlated, and comparison of crystal fraction vs size trends suggests that the proportion of large crystals is the primary control on crystallinity. Porosity is negatively correlated with crystal content, which is difficult to reconcile with closed-system crystallization. Magnetite and pyroxene + biotite size distributions are fractal in nature, often attributedmore » to fragmentation; however, crystals are mostly whole and euhedral, such that an alternative mechanism is necessary to explain these distributions. Quartz + feldspar size distributions are kinked, with a shallow-sloped log-linear section describing large crystals (> 140 {micro}m) and a steep-sloped log-linear section describing small crystals (< 140 {micro}m). We interpret these two crystal populations as resulting from a shift in crystallization regime. We suggest that the shallow-sloped section describes a pre-eruptive quartz + feldspar growth-dominated regime, whereas the steep-sloped section represents a population that grew during a nucleation-dominated regime that began as a result of decompression at the onset of eruption. Timescales of quartz growth calculated from the slopes of these two segments of the size distributions indicate that the pre-eruptive crystal population grew on timescales on the order of millennia and may describe the timescale of crystallization of the Bishop magma body. The syn-eruptive population gives timescales of < 1-2 years (but possibly much less) and probably marks the onset of eruptive decompression.« less
Highlights
The existence of giant magma reservoirs within the Earth’s crust is evidenced most strikingly by huge pyroclastic deposits, often comprising hundreds to thousands of km3 (Self, 2006), inferred to have erupted in a matter of days to weeks (Smith & Bailey, 1966; Ledbetter & Sparks, 1979; Wilson & Hildreth, 1997)
Intermediate-density clasts are the more common class; for presentation purposes, we describe first the intermediate-density quartz þ feldspar size distributions and discuss how high- and low-density pumice clasts deviate from this pattern
The early-erupted pumice clasts studied by Gualda et al (2004) and the late-erupted clasts studied here span much of the spectrum of pumice bulk density, crystallinity and porosity reported for the quickly cooled Bishop Tuff pumice clasts studied by Skirius et al (1990), Wallace et al (1999), and Anderson et al (2000)
Summary
The existence of giant magma reservoirs within the Earth’s crust is evidenced most strikingly by huge pyroclastic deposits, often comprising hundreds to thousands of km (Self, 2006), inferred to have erupted in a matter of days to weeks (Smith & Bailey, 1966; Ledbetter & Sparks, 1979; Wilson & Hildreth, 1997). Compositional zonation of the Bishop magma body has been inferred based on a number of lines of evidence, the stratigraphic zonation of the deposit as a whole (Hildreth, 1977; Hildreth & Wilson, 2007), major, trace and O isotope compositions of phenocrysts (Hildreth, 1979; Bindeman & Valley, 2002), and volatile and trace-element compositions of melt inclusions (Hervig & Dunbar, 1992; Wallace et al, 1995, 1999; Anderson et al, 2000) This compositional stratigraphy of the Bishop Tuff mainly reflects variation that developed prior to the onset of the eruption, as indicated by the pressures of formation (!1000 atm) of melt inclusions in phenocrysts. These prior studies provide a framework to interpret quantitative textural information as it pertains to the pre- and syn-eruptive crystallization of the Bishop magma
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