73 Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes (Posters)

This session invites presentations that leverage volcanic deposits to reconstruct magmatic and eruptive processes. We welcome contributions from field and experimental volcanology, petrology, geochemistry, thermodynamics, and fluid mechanics, ranging from the landform to nanoscale.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Remnants of the deep: a preliminary map of mafic enclaves near the vent of Big Obsidian Flow, Newberry Caldera, Central Oregon

Paper number: 73-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

A renewed interest in magma mixing during the late 20th century gave us a better understanding of how mafic recharge magmas transport the heat often necessary to remobilize magmatic mushes to an eruptible state. Recharge magmas are often preserved as variably-mixed mafic enclaves and can provide a record of magma storage conditions beneath a volcano. At Newberry Volcano, a rear-arc shield volcano located in central Oregon near the cities of Bend and Redmond (combined population: 150,000), the conditions of magma storage have yet to be petrologically constrained. Since the ~75 Ka caldera-forming event, erupted Newberry lavas are bimodal in character, with the most recently extruded being the 1,300 year old, aptly named Big Obsidian Flow (BOF).

Linneman and Myers (1990) studied BOF’s mafic enclaves to better understand their petrogenesis. Because GPS was not widely available at the time of fieldwork, precise spatial coordinates were not recorded in their dataset. We were also unable to locate the hand samples collected by Linneman and Myers (1990), and few viable thin sections remain. Here we report a new set of mafic enclaves recently collected from BOF.

We present a map outlining the variation of enclave characteristics related to location in the vent area. Mafic enclaves were present outside of the previously studied area (a 200 m2 area surrounding the BOF vent). Enclave types range from rarer, more fine-grained (and rarely vesiculated) basaltic enclaves to commonplace plagioclase+amphibole-bearing enclaves up to 28.5±0.65 cm, alongside rare metamorphic and igneous xenoliths. Enclave volumes increase with proximity to the vent, and fine-grained, visibly-vesiculated enclaves were not observed outside the vent region. Although fine-grained enclaves are not exclusively present at the BOF vent, our preliminary map suggests that enclaves representing liquid (or nearly-liquid) recharge magmas are more likely to be present in the vent than elsewhere in the flow.  Enclaves from this dataset will be used to constrain the most recent conditions of magma storage at Newberry Volcano.

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

doi: 10.1130/abs/2025AM-413520

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