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The primary goal of this study is to assess the impact of a subduction component
added to the mantle wedge beneath the Oregon Cascades to the composition and fO2 of
primitive Cascade basalts. Olivine-hosted melt inclusions from compositionally diverse
basalts across the Cascade arc (~100 km) are utilized in an effort to reduce effects of
assimilation and crystallization while retaining volatile phases lost during shallow
degassing.
Many melt inclusions from Cascade samples require rehomogenization to melt
daughter crystals formed from slow cooling of the basalts after inclusion entrapment.
Rehomogenization of crystalline inclusions documents post-entrapment modification of
inclusions from alkali-rich forearc lavas resulting in anomalously high FeO* (up to 21
wt%) from the dissolution of magnetite during rehomogenization.
Oxidation state in melt inclusions can be determined from electron microprobe
measurements of sulfur speciation, based on the sulfur Kα peak shift. This technique in
melt inclusions is largely unexplored and a series of experiments were conducted to
determine the extent to which both natural processes and rehomogenization influence
measured sulfur speciation. These experiments indicate that degassing (oxidation or
reduction) and H+ diffusion (oxidation) are the two most significant factors altering the
sulfur speciation, however, Fe-loss and crystallization will also increase fO2.
Rehomogenization has relatively minor impact on sulfur speciation provided heating
times are kept short (<10 minutes) and significant over- or under-heating has not
occurred.
Oxygen fugacity, chlorine, and incompatible trace elements in primitive melt
inclusions in Oregon Cascade basalts increase towards the trench, suggesting that the
subarc mantle is oxidized as a result of addition of a subduction component. The overall
fO2 range of Cascade basalts, <-0.25 log units (ΔFMQ) to +1.9 log units (ΔFMQ), is
consistent with previous results and is believed to be representative of the fO2 range of
the subarc mantle. Calc-alkaline basalts however appear to have re-equilibrated to lower
fO2 prior to inclusion entrapment.
A second goal of this study was to analyze phenocrysts and glass in ash from the
2004 eruption of Mount St. Helens for major- and trace-elements to identify juvenile
material and explore methods of ash generation over the course of the eruption. |
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