Graphite in the Bishop Tuff and its effect on postcaldera oxygen fugacity

Abstract

Several cubic kilometers of Paleozoic graphite-bearing argillitic country rocks are present as lithic fragments in Bishop Tuff ignimbrite and fallout. The lithics were entrained by the 650 km 3 of rhyolite magma that vented during the 5-to 6-day-long, caldera-forming eruption at Long Valley, California. The caldera is floored by a 350 km 2 roof plate that collapsed during the eruption and consists in large part of the Paleozoic strata that provided the abundant hornfelsed metapelitic lithic clasts in the tuff. Graphite has been identified by Raman spectroscopy, electron-dispersive spectroscopy, and X-ray diffraction as an irregularly dispersed component in the small fraction of Bishop Tuff pumice that is dark-colored. Carbon concentration has been determined in pumice, lithics, and wall rocks. Values of δ 13 C range from -21‰ to -29‰ Vienna Peedee Belemnite (VPDB) for pumice, lithics, and argillitic wall rocks, reflecting the biogenic origin of the reduced carbon in oxygen-limited black Paleozoic marine mudrocks. Carbonate contents, measured separately, are negligible in fresh pumice and lithics. Microprobe analyses of titanomagnetite-ilmenite pairs show that oxygen-fugacity values of numerous batches of postcaldera Early Rhyolite (750-640 ka; ~100 km 3 ) are up to one log unit more reduced than those of the temperature-oxygen fugacity (T-fO 2 ) array of the Bishop Tuff (767 ka), despite similar major-element compositions and Fe-Tioxide temperature ranges. All of the many batches of Early Rhyolite, which erupted episodically over an interval of ~125,000 years, yield the reduced fO 2 values, indicating that reaction with graphite lowered magmatic fO 2 after the caldera-forming eruption but before the first eruption of Early Rhyolite. It is inferred that reaction of postcaldera rhyolite magma with the reduced carbon in a great mass of subsided roof rocks lowered its fO 2 . It is suggested that comparable effects could have attended caldera collapse of other magma chambers hosted in continental sedimentary rocks.

Hydrogen Isotope Composition of a Large Silicic Magma Reservoir Preserved in Quartz‐Hosted Glass Inclusions of the Bishop Tuff Plinian Eruption

“…Although some previous work suggests that assimilation of crustal material did not play a role in contributing to the compositional variability observed between the early‐ and late‐erupted units of the Bishop Tuff (e.g., Hildreth & Wilson, 2007), contamination of the magma by surrounding country rock is likely to be significant during the assembly of any large‐volume magmatic system in the upper crust. Here, the contributions are wall rocks of Mesozoic Sierran granitoids and reservoir roof of Paleozoic marine shales (Hildreth et al., 2018). Masi et al.…”

Comparative Rhyolite Systems: Inferences From Vent Patterns and Eruptive Episodicities: Eastern California and Laguna del Maule

“…For the 650 km 3 Bishop Tuff eruption, the average diameter of the subcaldera magma volume evacuated would be 20 km if 2 km thick or 16.6 km if 3 km thick. These estimates bracket the area of deep collapse and known depth of subsidence (Hildreth, Ryan-Davis, & Harlow, 2017). I speculate that it took hundreds of melt batches over the course of >80,000 years to accumulate such a chamber.…”

“…Similarities in composition (Figure 9) and temperature (Hildreth, Fierstein, & Calvert, 2017; Hildreth, Ryan‐Davis, & Harlow, 2017) between ER and late‐erupted parts of the Bishop Tuff suggest that mixing with hotter deeper magma was limited, whereas the relatively elevated Ba content of ER (Figure 9) suggests contributions either from unobserved trachydacitic magma or from melt extracted from sanidine‐rich Bishop Tuff cumulate residue. Mafic enclaves (50%–58% SiO 2 ) that reflect early postcaldera recharge have been found in two ER lavas and rare mafic blebs in two more.…”

“…For most of the zoned Bishop Tuff, oxide pairs equilibrated at 700°C-780°C, whereas late pre-eruptive rhyolitic recharge heated a fraction of the magma volume released to 780°C-820°C (Evans et al, 2016). Only this climactic rhyolite (withdrawn from deeper levels in the subcaldera reservoir) and the postcaldera Early Rhyolites (750°C-840°C; Hildreth, Fierstein, & Calvert, 2017;Hildreth, Ryan-Davis, & Harlow, 2017) yield temperatures as high as the ranges typical of rhyolites at LdM. The latter may reflect the proximity of mafic recharge batches to LdM rhyolite lenses that lack the thick mush-zone buffer typical of Long Valley.…”

“…FeTi‐oxide geothermometry yields 752°C–844°C for many ER samples, compared to the 700°C–820°C continuous array for the zoned Bishop Tuff. However, ER values of fO 2 are up to one log unit more reduced than those of the T–fO 2 array for Bishop Tuff (Hildreth, Ryan‐Davis, & Harlow, 2017). It is inferred that the postcaldera reduction was caused by the reaction between rhyolite melt and the reduced carbon in the great mass of Paleozoic graphitic metapelite that collapsed into the chamber and is so abundant as black lithics in Bishop Tuff.…”