Depths of Melting and Melt Extraction Beneath Mid-Ocean Ridges as Recorded in Melt Inclusions

Abstract

Recent studies have shown that chemical compositions of primitive melt inclusions in olivine and plagioclase phenocrysts in mid-ocean ridge basalts (MORB) carry important information of process of melting and melt extraction beneath mid-ocean ridges (e.g., Sobolev and Shimizu, 1993). It is generally observed that variations of trace element abundances and abundance ratios in melt inclusions on small sampling scales (e.g., single dredge, single pillow) are very large, and can be adequately explained by melting models involving efficient extraction of small-degree melt fractions. This paper reports chemical variations in two primitive melt inclusion suites (33.7~ MidAtlantic Ridge and 36.5~ Mid-Atlantic Ridge, FAMOUS area) with distinct trace element characteristics, representing melting and melt extraction occurred at different depths. Major element compositions in melt inclusions and host minerals were determined with a JEOL 733 at MIT, and trace element abundances (Ti, V, Cr, Sr, Y, Zr, REE) were determined with a Cameca IMS 3f at WHOI. Only perfectly glassy primary inclusions were analyzed. Effects of postentrapment growth of host minerals were found to be typically 5 ~ 15%. Olivine (s9-91)- and plagioclase (Ansl-ss)