Evaluating the Relationship Between Melt Inclusions and Host for high Anorthite Plagioclase: Prevalence and Significance of Trace element Diversity in MORB Plagioclase

Oregon State University, Corvallis OR

Investigators

Abstract

ABSTRACT (Nielsen - 0453090) One of the primary ways the Earth gives up its inner heat is by eruption of mid-ocean ridge basalts. This research studies the origin of the large high Ca-rich plagioclase crystals characteristic of many suites of these ocean floor lavas to examine the exchange of material and heat between the mantle and crust. These minerals are unusual because they have never been found to be in equilibrium with the glass in which they occur. In this study, plagioclase crystals and glass inclusions within them, which are thought to represent the magmas from which the plagioclase crystallized, will be analyzed for trace elements to obtain information on mantle and crystallization processes as well as with magma interaction with the lower crust. Because of difficulties analyzing chemical gradients and trace elements that vary over small distances, the initial stage of the research focuses on development of micro-analytical techniques for measuring small differences, chemically and spatially in basaltic rocks using laser ICP mass spectrometry and the electron microprobe. The second stage involves making geochemical traverses of high Ca-plagioclase crystals and their glass inclusions, as well as their host glasses. Trace elements to be measured and their gradients in the glasses and minerals include Ti, Zr, REE, Sr, Ba. Results of these analyses will be used to assess the degree to which melt inclusions in the crystals and the host glasses have achieved equilibrium through use of partition coefficients obtained from laboratory experiments. The final objective will be to put high Ca plagioclase into a framework of crustal and magmatic evolution that permits better understanding of formation of the ocean crust.