Local Diversity of MORB Parent Magmas: Evidence from Melt Inclusions in High-An Feldspar from the Gorda Ridge

Abstract

A primary reason to study MORBs is to constrain the chemical and physical properties and processes of the oceanic crust and mantle. In order for our interpretations to be accurate, we must use samples that have seen a minimum of modification since they were produced. One means by which primitive magmas can be preserved is as melt inclusions. However, even though trapped magmas are not subjected to the same differentiation processes as the magma body as a whole, they are not unaltered. This is because the melts were not quenched at the time of entrapment. Subsequent cooling in more evolved magmas, plus the development of quench crystals at the time of eruption both have a significant effect on the composition of the observed glass inclusions. Our investigation centered on melt inclusions hosted in anorthitic plagioclase (>An90) megacryst from a single sample dredged from the Gorda Ridge (D9-2-1 Davis and Clague, 1987). In our study we focused on two basic problems: 1) Could we develop an experimental technique for re-homogenizing melt inclusions? 2) What was the diversity of melt inclusion compositions at any single location, and what did that tell us about igneous processes in the crust and mantle?