An Analysis of Sulfur Content and Multiple Sulfur Isotope Fractionation of Tephra Deposits at Valles Caldera, New Mexico: Variations over the Course of Two Caldera-Forming Eruptions
Abstract
Caldera-forming eruptions leave behind a record of their tempo and mode in their associated tephra deposits. Interpreting this record requires monitors of the pre-eruptive state of the host magma chamber, potential triggers of the eruption, as well as a proxy for volatile evolution during the eruption. Because of the coexistence of multiple S-bearing species in a magma-gas system, S isotopes stand as a potentially powerful tool to interrogate the eruptive records of caldera-forming events. To date, few S isotopic profiles have been measured through the full eruptive cycle of an explosive silicic complex. Here, we present the first multiple S isotope dataset of the complete evolution of a large, silicic, caldera-forming system. High-silica rhyolite samples from Valles Caldera, New Mexico, were gathered from formations spanning two entire caldera cycles: the Lower Bandelier Tuff (LBT, which formed Toledo caldera at 1.6 Ma), the Cerro Toledo Rhyolite, the Upper Bandelier Tuff (UBT, which formed Valles caldera at 1.2 Ma), and the Valles Rhyolite. Whole-rock S was chemically extracted to Ag2S from samples collected from various units within these formations, and S isotopic compositions were subsequently evaluated by fluorination of the Ag2S and mass spectrometry of the resulting SF6. All S isotopic measurements are reported relative to the V-CDT scale. Concurrent analyses of in-house standard materials demonstrate 1 sigma uncertainties for the full analytical procedure on Δ34S, Δ33S, and Δ36S of better than 0.1‰, 0.01‰, and 0.1‰ respectively. Measured δ34S values range from 0.6 to 8.3‰, while Δ33S and Δ36S values vary from -0.06 to 0.02‰ and -0.5 to 0.3‰, respectively. The lowest δ34S values are from units occurring within or immediately before the earliest-erupted plinian Pumice Beds of the LBT and UBT (0.6 to 2.6‰), while the most elevated δ34S values are from the latest-erupted UBT plinian Pumice Bed unit that we sampled (7.0 and 8.3‰). δ34S analyses for the LBT and UBT high-silica rhyolite ignimbrite samples are relatively moderate, varying from 3.5 to 4.3‰. Whole rock sulfur concentrations range from only 4 mg S/kg rock to 393 mg S/kg rock. Measured δ34S values and their corresponding whole rock sulfur concentrations are broadly inversely correlated, and are most simply interpreted as a series of degassing events that enriched the residual Valles magma in isotopically-heavy S. The S profiles of the LBT and UBT erupted products show similar trends: from the early plinian through to the late plinian units, S contents decrease and δ34S values increase, then plateau at the ignimbrite. Both profiles start out with δ34S values near 0‰, indicative of a relatively primitive, mafic source for the S in the Valles complex, and perhaps for the caldera-forming eruptions themselves.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V34C..03C
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 8430 VOLCANOLOGY / Volcanic gases;
- 8440 VOLCANOLOGY / Calderas;
- 9350 GEOGRAPHIC LOCATION / North America