The geochemistry of olivine-hosted melt inclusions in a FAMOUS basalt ALV519-4-1

Abstract

Some 22 melt inclusions hosted by high-Mg olivines (Fo 88–91) from a single basalt lava from the FAMOUS area of the Mid-Atlantic Ridge were analyzed for major and trace elements. The results demonstrate that large chemical variabilities are present in melts that existed in close temporal and spatial proximity at the site of host olivine growth. Major element compositions after corrections for growth of host olivine after entrapment show that they are very primitive (MgO ranging from 9 to 14 wt.%; Mg# around 0.73), and that variations of CaO, FeO and SiO 2 as a function of MgO are much greater than expected from fractionation of olivine from the most magnesian melt. Exactly 14 of the melt inclusions are consistent with equilibration with mantle minerals at 8.7±2 kbar and 1272±18°C. Abundance patterns of rare earth elements (REE) show that light REE (LREE)-enriched transitional mid-ocean ridge basalts (MORB) and LREE-depleted N-type MORB existed in close temporal and spatial proximity. It is argued that co-variations of trace element ratios, especially [La/Sm] N and Ti/Zr can be modeled by critical melting of a depleted MORB source at depths where garnet is stable to form LREE-enriched melts, and at depths of spinel lherzolite residue to form LREE-depleted melts. Abundance patterns of middle to heavy REE (M∼HREE) indicate, however, that important chemical modification has occurred by reactions between deep melts and shallow mantle in an AFC-type mechanism during upward migration of melts. This mechanism also appears to explain some of the major element characteristics, and suggests general importance of melt/mantle reaction during migration of MORB.