MORB Melt Transport through Atlantis Bank Oceanic Batholith (SW Indian Ridge)

Abstract

\n The Atlantis Bank Oceanic Batholith is a 660\u2009km2 gabbro massif representing the plutonic foundation of a major ridge magmatic center. It was continuously accreted, emplaced, and exposed in the rift mountains of the paleo-SW Indian Ridge from 13 to 10·3\u2009Ma. Ocean Drilling Program (ODP) Hole 735B, drilled to 1508\u2009m at Atlantis Bank, recovered evolved intercalated olivine and oxide gabbros representing the upper levels of the lower ocean crust. Within this section, ∼5·6\u2009m of primitive chromian-spinel-bearing troctolites (0·1–2\u2009m thick), with sharp modal contacts with the host gabbros, were cored between 410 and 500\u2009m depth. Here we present new mineral (chromian spinel, clinopyroxene, olivine, and plagioclase) major and trace element and petrographic data for the troctolite suites (i.e. troctolites, clinopyroxene-rich troctolites, troctolitic gabbros) from 410 to 528\u2009mbsf (meters below seafloor) and olivine gabbros in the 0–274\u2009mbsf and 410–500\u2009mbsf intervals, and examine the origin of these troctolite layers. Equilibrium melts for the troctolites are primitive to moderately evolved, with Mg# [= 100Mg/(Mg + Fe), 48–68\u2009mol%] comparable with those of Atlantis Bank basalts. By contrast, equilibrium melts for both the 0–274\u2009mbsf and 410–500\u2009mbsf host olivine gabbros are highly to moderately evolved (Mg# 27–63\u2009mol%). The 410–500\u2009mbsf troctolite suites maintain high clinopyroxene Mg# (81–89\u2009mol%) and mineral concentrations of compatible elements (i.e. clinopyroxene Cr2O3 0·1–1·2\u2009wt%, Ni 151–330\u2009μg\u2009g–1 and olivine Ni 1055–1559\u2009μg\u2009g–1) with the increase in incompatible trace element abundances (i.e. clinopyroxene TiO2 0·3–2·1\u2009wt%, Zr 5·8–112\u2009μg\u2009g–1 and olivine Ti up to 293\u2009μg\u2009g–1). Combined with abundant dissolution–reprecipitation textures, our results indicate that the troctolites were formed by the reaction between a spinel-bearing, troctolitic mush, from which they inherited the high Mg#, Ni and Cr, and percolating melts adding incompatible trace elements. Moreover, the spinel (NiO versus TiO2) and olivine (Ti versus Y) trace element compositions indicate that the troctolites were affected by low-degree Fe–Ti-rich melt metasomatism. In contrast, both the 0–274\u2009mbsf and 410–500\u2009mbsf olivine gabbros display a prominent decrease in clinopyroxene Mg# (from 88 to 66) and mineral compatible element concentrations (i.e. clinopyroxene Cr2O3 from 1·1 to ∼0\u2009wt%, Ni from 208 to 34\u2009μg\u2009g–1 and olivine Ni from 1200 to 136\u2009μg\u2009g–1) with increasing incompatible trace element abundances (e.g. clinopyroxene Zr from 4·8 to 157\u2009μg\u2009g–1). These features are compatible with the reactive porous flow of slightly to highly evolved melts through the cooling crystal mush zone. Our results, combined with the literature data, indicate that most olivine gabbros between 410 and 500\u2009mbsf were formed prior to the troctolite layers. We document that the troctolites represent conduits for mid-ocean ridge basalt melt transport through the lower oceanic crust, whereas the olivine gabbros represent crystallization of a large crystal mush, recording initial gabbro emplacement, hyper- and sub-solidus deformation, and melt–rock reaction owing to upward penetrative flow of intercumulus melt.