The Evolution of Oceanic Gabbros: In-situ and Ancient Examples

Astri Kvassnes, Ph.D., 2004
Henry J.B. Dick, Advisor

This study is a geochemical investigation into the accretion of lower oceanic crust and processes of shallow melt-rock reaction at mid-ocean ridges. Geochemical signatures from the 480-My Lyngen Gabbro indicate the complex formed from hydrous supra-subduction zone magmas.

Minerals in gabbros from Atlantis Bank, Southwest Indian Ridge, an in-situ example of lower ocean-crust, formed by near-fractional crystallization at mid-crustal pressures, modeled by the MELTS algorithm. The lower crust is more evolved than the lavas, representing mantle-derived melts fractionated 50-95%. >770-m of additional primitive cumulates remains below the 1500-m deep Hole 735B or within the underlying mantle.

The Atlantis Bank gabbros minerals have internal disequilibrium. Melt-rock interaction may have caused this. Dissolution-experiments for plagioclase-olivine and plagioclase-augite pairs were performed at 1180º-1330ºC and 20-min-24hrs. Dissolution occurs rapidly and out of equilibrium, the dissolution rates dependent on the ?T above the solidus. Coarse-grained rocks will heat internally when enclosed in hot magma, melt and disaggregate. The hybrid magma crystallizes more refractory-looking minerals than the melts that precipitated the original gabbroic rocks. Assimilation increases the Na content and decreases the Fe content of the melt that digests gabbro, thus hybrid basaltic glasses will falsely reflect a lower degree and pressure of mantle melting.