T95 The How, When, Where, and Why of Open-System Magma Processes (Posters)

Monday, 11 October 2021
9:00 AM - 1:00 PM
Exhibit Hall A

REVEALING THE CONTRASTING HISTORIES OF MAFIC MAGMAS IN THE CENTRAL OREGON CASCADES USING DIFFUSION CHRONOMETRY AND MINERAL CHEMISTRY

Paper number: T95-5
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

Basaltic volcanoes are common in the Central Oregon Cascades and include small shields, flank eruptions and thousands of cinder cones. It has been suggested that small shields can have a shallow plumbing system where magmas are stored prior to eruption, which may enable complex magma interaction (e.g. magma mixing) to take place. In contrast, monogenetic, small-volume “simple, single-eruption” volcanoes have been suggested to have less complex magma interaction prior to eruption. This view of monogenetic-style centres has been challenged in recent years.

Detailed eruption timescales for the basaltic volcanoes in the Cascades are rare in the regional dataset. We present petrological data and diffusion modelling timescales from Belknap (a small shield volcano) and Four in One (a small, monogenetic-style fissure eruption) located west of Sisters, OR.

Both volcanic centres contain olivine and plagioclase crystals that are compositionally zoned. Belknap olivine are equant and normally zoned with core compositions of Fo84-80 and rims of Fo72. Four in One olivines are both normally zoned (core compositions of Fo84-81, rim compositions of Fo82-70) and reversely zoned (core compositions of Fo74 and 82 and rim compositions of Fo78 and 83) with dendritic overgrowths. Plagioclase from Four in One have two core populations -An45 and An80. An45 cores are surrounded by a fritted inner rim, followed by a thin An65 outer rim. An80 cores are only surrounded by the An65 rim. Plagioclase from Belknap have core compositions of ~An81 with rims ~An70-65 and they do not display any fritted textures. Mg-Fe diffusion timescales are on the order of weeks to months for the Belknap olivine, and days to weeks for the Four in One olivine. Mg diffusion timescales in Four in One plagioclase cores are ~1 month. These plagioclase timescales and textures indicate short storage before magma mixing, with ascent from depth prior to eruption recorded by the olivines. Diffusion timescales for Belknap olivines indicate longer storage timescales prior to eruption.

These data will be important to shape responses and for mitigation during future geophysical unrest in areas where basaltic volcanism has been prevalent. The results also support the notion that complex magmatic interaction also occurs beneath both shield and monogenetic-style centres.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-369680

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.