Posted by: magmatist | January 16, 2014

MBVRC research grant report: May Sas, WWU Geology: Mount Baker andesite geochemistry

May on Ptarmigan Ridge, east of Mount Baker.

May on Ptarmigan Ridge, east flank of Mount Baker.

MBVRC provides grants to support geologic research on Mount Baker. Reports from grant recipients are periodically posted so you, who are the principal contributors to our research fund, will know how the money is being spent. Here is a report from May Sas, one of our three 2013 grant recipients.  May is a geology grad student at Western Washington University. Last spring we posted the proposals from our 2013 grantees; scroll down  here to read May’s full study plan. In late December we posted a final report from Ian Delaney. A report from Ricardo Escobar, the third award winner, will come your way soon.

It’s really easy to contribute via paypal, or by mail. All contributions are tax-deductible.

High-Magnesium andesites from the northern Cascade Arc: Using mineral chemistry to distinguish between hypotheses for their origin

by May Sas, WWU.

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Geologic map of Mount Baker volcanic field, modified after Hildreth et al. (2003), showing May’s study areas in the insets. (A) Glacier Creek andesite (agc). (B) Tarn Plateau basaltic-andesite (atp).

The origin of high-Mg andesites (HMAs) poses a fundamental geochemical paradox given their unusually high Mg# (‘magnesium number’, or the ratio of Mg to the sum of Mg and Fe), indicating equilibrium with mantle olivine, and at the same time their overall andesitic SiO2 contents, which are more typical of crustal magmas. Trace element chemistry of HMAs is also distinctive; they exhibit a steep rare earth element (REE) pattern with high La/Yb ratios, notable depletion in heavy REE, and Ni and Cr enrichment. In this project I am trying to discern between variable hypotheses for the origin of HMAs in the northern Cascade Arc using rocks from Mount Baker and Glacier Peak.

Because samples from lavas in the Mount Baker volcanic field have been collected during previous studies and whole-rock chemistry completed, this project represents an excellent opportunity to explore the information carried in large crystals (phenocrysts) to answer questions of andesite magma origin. An investigation of phenocryst geochemistry, in combination with whole-rock geochemical data and modeling, will allow me to test previous hypotheses in order to constrain the origin and evolution of these magmas prior to eruption. My methods include analysis of crystal textures (chiefly disequilibrium textures), mineral chemistry (major, minor, trace element), and modeling.

Progress Report: Grant funding from the Mount Baker Volcano Research Center, totaling $667, was used toward electron microprobe (EMP) analysis completed at University of Washington in September 2013. Data collection during this session included major and minor element concentration in olivine, clinopyroxene, and plagioclase mineral phases. Three different high-Mg flows were analyzed, Glacier Peak’s Lightning Creek basaltic andesite and Mount Baker’s Tarn Plateau basaltic andesite and Glacier Creek andesite. Over 15 crystals of each phase have been examined per unit. These data are being worked on and reviewed in detail, and will be used to constrain crucial magmatic conditions as well as provide the first clue regarding the origin of these controversial lava flows.

Image of a 100μ thick (that’s 0.004 inch) plagioclase crystal as seen with the optical microscope. Please click to enlarge this beautiful tiny crystal.

Additional work that has been completed includes preparation of 100 micron(μm) thick sections of selected samples, extensive petrographic examination for identification of mineral phases and textural relationships, and extensive scanning electron microscope imaging of ideal crystals using the back-scatter electron (SEM-BSE) technique. Future work, in addition to data processing, includes more EMP analysis, specifically deciphering oxide chemistry which will be used to derive magma oxygen fugacity levels (ƒO2). Laser ablation inductively-coupled plasma mass spectrometry (LA-ICPMS) will follow EMP work, and will be used to determine trace element concentrations in the selected samples to calculate parental magma compositions. Finally, modeling will be used to conclude the probable source of these HMAs by amalgamating all of the constraints, such as H2O contents and ƒO2 conditions, in addition to mineral and whole rock major, minor, and trace element concentrations.


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