11-3 Volatiles and Plumbing Systems in Explosive Cinder Cone Eruptions: Melt Inclusion Study of Tecolote’s Tephras, Pinacate Volcanic Field, Mexico

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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Distributed volcanic fields are relatively common across western North America. These fields produce a range of eruptive behaviors from gentle flows to violent sub-Plinian eruptions. Explosive mafic volcanic eruptions are poorly characterized, yet due to the ubiquity of volcanic fields they pose a considerable hazard to the population, infrastructure, and economies of their regions. Tecolote (27 ± 6 ka)—a cinder cone in the Pinacate volcanic field of northern Sonora, MX and southwestern Arizona, USA—produced several lava flows and two significant tephra deposits from violent explosive events.

We are studying olivine-hosted melt inclusions from Tecolote’s tephra fall to understand the magma plumbing system and the role of volatiles in explosive events within large volcanic fields. Selected free olivine crystals and their associated melt inclusions were analyzed via electron microprobe (major oxides, S, and Cl), Raman (CO₂ in vapor bubbles), and FTIR (C-O-H). Olivine morphology from both deposits varies widely and all crystal habits commonly contain embayments and several to dozens of melt inclusions. Tecolote magma is alkali-rich basalt, and the melt inclusion compositions show a broad range of K₂O content (0.32-2.11 wt. %) though the majority of melt inclusions range between 0.6 and 1.0 wt. % K₂O. Most olivine core compositions are normally zoned and range from Fo₈₀ to Fo₈₂, but reversely zoned crystals are also present. Both phases overlap compositionally, but the latter, weaker phase displays a more evolved trend with lower MgO and higher K₂O than the earlier sub-Plinian phase.

We are determining the complete volatile budget of melt inclusion glasses using FTIR and Raman analysis. Although single-vapor bubble melt inclusions are typical, Tecolote melt inclusions contain multiple vapor bubbles (range 1-12; average 3.7). Melt inclusion vapor bubbles have a wide range (0.05-0.49 g/cm³) and high average (~0.30 g/cm³) CO₂ density. CO₂ density does not correlate with bubble volume or total bubble volume percentage. Vapor bubbles within the same melt inclusion have similar CO₂ density (2𝜎). Initial FTIR results indicate the Tecolote magma was relatively water poor (~1.5 wt. %), but CO₂-rich (>3000 ppm). Volatile saturation pressures from reconstructed melt inclusions will allow us to constrain the depth and geometry of the magma plumbing system prior to eruption of the sub-Plinian and violent Strombolian events.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10088

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