Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

Akihiro Tamura,Yusuke Soda,Chuanzhou Liu,Christopher MacLeod,Yumiko Harigane,Tomoaki Morishita,Du Nguyen,Peter Blum,Henry Dick,Biswajit Ghosh,Lydéric France,Alessio Sanfilippo,James Natland

doi:10.3390/min8120583

Akihiro Tamura, Yusuke Soda + Show 11 more

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https://doi.org/10.3390/min8120583

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Abstract

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.

Highlights

The lower oceanic crust is dominated by gabbroic sequences with compositions ranging from troctolite, olivine gabbro to evolved oxide gabbros
Hole U1473A (32◦ 42.36220 S; 057◦ 16.68800 E), located on the summit of Atlantis Bank at the Southwest Indian Ridge was drilled to 789.7 m below seafloor during Indian Ridge duringInternational Ocean Discovery Program (IODP) Exp. 360 into massive gabbros that are locally cut by isolated dikes (Figure 1)
The main results of this investigation can be summarized as follows: (1) IODP-Hole U1473A was drilled on the summit of Atlantis Bank, two kilometers far from

Summary

Introduction

The lower oceanic crust is dominated by gabbroic sequences with compositions ranging from troctolite, olivine gabbro to evolved oxide gabbros. The formation of the lower crust is generally believed to be an integrated process from generation to crystallization of basaltic magmas derived from partial melting of adiabatically upwelled asthenospheric mantle at the mid-ocean ridge. It has been, documented that reaction between the gabbroic mineral assemblage in the earlier stages of crystallization and migrating melts is an important process in melt evolution beneath mid-ocean ridge [1,2,3].

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