Formation of spinel-bearing layered gabbros by the reaction between the basaltic crystal mush and high-Mg andesitic melts in the lower oceanic crust, East Pacific Rise (IODP Exp. 345 U1415P)

Petrographic studies of the multi-textured layered gabbro series of the lower crustal section recovered from U1415P were conducted to determine the mechanism of orthopyroxene formation in lower crustal gabbros drilled in the Hess Deep rift near the East Pacific Rise. In a particular horizon of the series (U1415P 6R-1 to 6R-2), homogeneous medium-grained troctolite and fine-grained spinel-bearing heterogeneous olivine gabbro occur in proximity.

The samples from the Hole1415P 6R are divided into homogeneous troctolite parts with medium-grained equigranular textures and spinel-bearing heterogeneous olivine gabbro parts with poikilitic textures (irregularly shaped fine-grained plagioclase characrysts enclosed in Ol-oikocrysts and Cpx-oikocrysts). The wide variation in TiO₂ content against nearly constant Mg# and Cr₂O₃ of clinopyroxene in these rocks is quite different from the compositional trend of clinopyroxene in layered mafic intrusions explained by the fractional crystallization of magma.

Recently, Yang et al. (2019) conducted a reaction experiment between MORB melts and troctolite and reported microtextures and mineral chemical compositions of the experimental products. In the experiment, fine-grained plagioclase was found to be poikilitically enclosed in olivine and clinopyroxene in the melt infiltration zone within troctolite near the boundary between troctolite and melt. The poikilitic textures and the oikocristic clinopyroxene compositional variations in the Hess Deep spinel-bearing olivine gabbro are very similar to those in the reaction experiment by Yang et al. (2019). These indicate that the heterogeneous olivine gabbro of the Hess Deep was formed by a melt-rock (or crystal mush) reaction.

              In addition, the multiphase solid inclusions of Cpx-Pl assemblage with basaltic magma composition and Opx-bearing multiphase solid inclusions with composition of high-Mg basaltic-andesite to andesite are enclosed in Cr-spinel from the spinel-bearing heterogeneous olivine gabbro. The presence of Opx-bearing multiphase solid inclusions indicate that the injection of SiO2-rich melt different in composition from the troctolite-forming magma has occurred. The SiO₂-rich melt reacted with the troctolite to form spinel-bearing heterogeneous olivine gabbro. Our results may suggest that the sheeted sills model, in which magma of various compositions intrudes into the crust, is a more appropriate mechanism for the formation of the lower oceanic crust.