Date of Award

2010

Document Type

Campus Access Thesis

Department

Earth and Ocean Sciences

Sub-Department

Geology

First Advisor

Gene Yogodzinski

Abstract

Minor element abundances in olivine phenocrysts from primitive, high-Mg# lavas (Mg/Mg+Fe >0.60) are used to explore the nature of magma genesis in the Cascade subduction system. Electron-microprobe analyses were obtained for olivine phenocrysts in primitive basalt from central Oregon and northern California, primitive basaltic andesite from central and southern Oregon, and for primitive andesite from northern California and southern British Columbia. Phenocrysts in nearly all samples show normal core-to-rim compositional zonation, with no evidence for significant crystal-melt disequilibria during phenocryst growth. The samples can be divided into two groups based on whole-rock and olivine phenocryst compositions. Basalts contain olivine phenocrysts up to forsterite 88, which have low Ni (0.25 wt%), and high Mn (0.13 wt%), Ca (0.10 wt%), and Cr (.035 wt%). Intermediate-composition lavas contain olivine phenocrysts up to forsterite 90, which have high Ni (up to 0.6 wt%), and low Ca (.05 wt%), Cr (.03 wt%), and Mn (.09 wt%). The most Mg-rich phenocryst core compositions in most samples are in Fe-Mg exchange equilibrium with their whole-rock `melt' compositions. Minor element abundances in olivine phenocrysts are in agreement with predicted values assuming compositional-dependant partition coefficients. Consistent with published trace element results, the observed major and minor element trends in olivine phenocrysts provide clear evidence for the widespread presence of separate basaltic and andesitic liquid lines of descent among Cascade lavas. These distinctive melt evolutionary pathways are inferred to reflect different pre-eruptive water contents, which are higher in the intermediate-composition melts, based on published experimental and geochemical studies. Primitive intermediate melts probably exist at relatively lower temperatures and with generally lower incompatible trace element abundances when compared to basalt. Given the primitive nature of the lavas chosen for this study, and the simple record of melt evolution recorded in their phenocryst populations, it is likely that both of the parental melt types were produced by processes occurring in the Cascade mantle wedge, and were not substantially affected by magma mixing and crustal contamination in shallow volcanic plumbing systems. The propinquity of these distinctive primitive lava types along the length of the arc suggests that a thermally and compositionally heterogeneous mantle wedge probably exists beneath most of the Cascade Range. Additionally, the large volume of primitive basaltic andesite in central Oregon appears to challenge the widely held idea that volcanism there is derived primarily from basaltic parental melts that are unusually hot and dry compared to those in other arc systems.

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