THE RELATIONSHIP BETWEEN BASALTINC MELT INCLUSIONS AND THEIR HOST HIGH AN PLAGIOCLASE: THE SIGNIFICANCE OF TRACE ELEMENT DIVERSITY IN MORB PLAGIOCLASE

A persistent question regarding the petrogenesis of mid-ocean ridge basalts (MORB) pertains to the significance of anorthitic plagioclase phenocrysts (>An90) characteristic of many primitive lavas of such suites. Specifically, no recovered volcanic glass compositions are near to being in equilibrium with those phenocrysts. However, many of these phenocrysts contain literally hundreds of primitive (high MgO) melt inclusions. The composition of those inclusions, taken together with the host crystal can potentially provide valuable information on the processes that govern the mass and heat balance in the oceanic crust.

The geochemical signal represented in the phenocrysts and their melt inclusions is not simple. It appears to include information on the array of primary mantle melts, variable degrees of fractional crystallization and variable degrees of interaction with the lower crust, all superimposed on one another. In addition, interpretation of the melt inclusion signal has been complicated by our level of confidence of the degree the melt inclusions represent the magmas in the environment at the time of crystal formation. Recent experimental data has addressed many of these questions, permitting this study to concentrate on the critical issue of composition of both phenocrysts and the melts from which they crystallized.

Our goals in this study were to 1) further develop laser ICP-MS techniques for the analysis of Ti, Zr, REE, Sr, Ba in plagioclase and basaltic glass 2) to evaluate the degree to which the melt inclusions and the host have achieved equilibrium using new partitioning information and our analyses and 3) using these analyses as the framework, evaluate the degree to which the magmas that produce anorthitic plagioclase belong to the “main stream” of MORB magmas.

We will report on data collected from high anorthite phenocrysts that contain high magnesium melt inclusions in ultra-phyric basalt from Gorda Ridge and Southwest Indian Ridge. The magnesium number of individual melt inclusions range from Mg#65 to Mg#75, with one as high as Mg#83. The within crystal range of concentration of Ti in the melt inclusions for individual phenocrysts varies from near zero (e.g. homogeneous) to variation of a factor of two at a given Mg #.