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Lassiter, J.C., 1995

Geochemical investigations of plume-related lavas: Constraints on the structure of mantle plumes and the nature of plume/lithosphere interactions

Bibliographic Reference

Lassiter, J.C., 1995, Geochemical investigations of plume-related lavas: Constraints on the structure of mantle plumes and the nature of plume/lithosphere interactions: University of California, Berkeley, Ph.D. dissertation, 231 p.

Abstract

To evaluate plume/lithosphere interaction during the generation of plume-related lava suites, chemical and isotopic systematics of lavas from the Wrangellia flood basalts and Mauna Kea volcano, Hawaii, are examined. These suites represent two primary types of plume-generated volcanism, flood basalts and ocean island basalts. Depth of melt generation and extent of partial melting are constrained for the Wrangellia flood basalts using major and trace element data. The geologic history of the Wrangellia terrane and composition of the basalts indicate derivation from a large plume head beneath a thick lithospheric lid. Trace element signatures of arc material (e.g. depletion of high field strength elements) are used to constrain the extent of melt/lithosphere interaction. The composition of Wrangellia lavas have been significantly modified by their passage through arc lithosphere, particularly during the early stages of volcanism. Nevertheless, chemical and isotopic characteristics of the Wrangellia plume are still apparent in less contaminated lavas. Chemical and isotopic variations in Mauna Kea lavas over approximately 200 Ka record changes in source composition and depth and degree of melting during the waning stages of shield volcanism. In contrast with the Wrangellia flood basalts, chemical interaction between plume-derived melts and lithospheric mantle or crust was minimal. Systematic differences in the isotopic composition of shield lavas from various Hawaiian volcanoes are explained by radial compositional and thermal zoning of the Hawaiian plume, possibly due to entrainment of lower mantle by the ascending plume. The results of these case studies are generalized through a comparison of continental and oceanic flood basalts and ocean island basalts. The thickness of the lithosphere is found to strongly influence the extent of partial melting in plumes. However, the extent of melting within the continental lithosphere is significantly greater than predicted by simple models of anhydrous mantle. It is proposed that melting of hydrous domains within continental lithosphere during the early stages of flood basalt volcanism produces the observed 'continental' chemical signatures. This also weakens the lithospheric mantle and allows starting plume 'heads' to ascend to shallower depths and undergo increased partial melting. Narrower plume conduits are unable to significantly erode anhydrous oceanic lithosphere.

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