Origin of arc magmas and xenoliths from the Aleutians and Kamchatka

Origin of arc magmas and xenoliths from the Aleutians and Kamchatka
Bryant, J. A.
2008
University of South Carolina, Columbia
180 p.
This dissertation research involved three projects, all directed at improving our understanding of the sources and processes contributing to the formation of subduction-related volcanic rocks from Kamchatka, Russia and the Aleutian Islands, Alaska. The first project was a petrologic and geochemical study of rare peridotite xenoliths from Shiveluch volcano, the northernmost active composite cone of the Kamchatka arc. The results from this study indicate that interaction between ascending magmas and peridotite wall rock at shallow mantle depths plays a significant role in creating and modifying the uppermost mantle and deepest crust in some subduction settings. The second project used whole-rock and mineral composition data from high Mg# (Mg/Mg+Fe) basalt and andesite lavas to investigate primitive arc magmatism and mantle wedge processes in the Kamchatka-Kurile subduction system. These data suggest that primitive andesites from the Shisheisky complex in northern Kamchatka form under physical and chemical conditions in the mantle wedge that are distinct from those that form primitive basalts in this region. This implies that andesite, not basalt, may be a primary, mantle-derived end-member magma type in northern Kamchatka. The third project involved measuring the Fe 3+ /Fe2+ ratio in clinopyroxene grains from Aleutian xenoliths on a high-resolution transmission electron microscope (HRTEM) using electron-energy loss spectroscopy (EELS). The purpose of this study was to characterize the Fe-redox conditions under-which arc magmas crystallize. These results emphasize that the oxidation state of mantle-derived melts forming beneath the Aleutian arc is highly variable and may be an important parameter that gives rise to the distinct igneous differentiation trends (calc-alkaline and thoeliitic) that are observed at volcanoes here and elsewhere.
Ph. D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)