Melt-Rock Reaction in the Mantle: Mantle Troctolites from the Parece Vela Ancient Back-Arc Spreading Center
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The Godzilla Megamullion is a giant oceanic core complex located in an extinct slow-spreading segment of the Parece Vela Basin (Philippine Sea). It exposes lower crust and mantle, offering an opportunity to unravel the architecture, composition, and magmatic processes operating in the shallow mantle of a back-arc basin. Here we present data on primitive troctolites and associated olivine-gabbros close to the breakaway zone of the megamullion. The troctolites are subdivided into olivine (olivine > 60 vol. %) and plagioclase (plagioclase > 60 vol. %) sub-types. Both have textures and mineral compositions suggesting formation through melt-rock reaction. We suggest that the Ol-troctolites formed by dissolution and disaggregation of dunite with crystallization of new plagioclase and diopside by a mid-ocean ridge basalt (MORB)-like melt, producing a high-Mg melt. The Pl-troctolites probably formed by local dissolution of an anorthositic cumulate and crystallization of olivine, diopside and new plagioclase owing to mixing the high-Mg melt residual to the formation of the Ol-troctolites with melt crystallizing an anorthositic cumulate. We show that the Ol- and Pl-troctolites and associated Ol-gabbros were strongly influenced by melt-rock reaction with the enclosing peridotite, reflecting solidification by reaction and conductive cooling of ascending melts in the mantle. The effects of the melt-rock reaction account for a sharp decrease in plagioclase anorthite content with respect to diopside Mg# and olivine forsterite content. Contrasting olivine nickel contents can constrain the environment of formation of olivine-rich rocks in the lower crust and mantle, the extent and nature of the processes by which they form, and the melt compositions involved in the reaction process. Comparison of olivine nickel contents in troctolite and gabbro occurrences in oceanic and ophiolite settings reveals a set of relationships consistent with this. Olivine gabbros from the 1508 m Hole 735B Atlantis Bank gabbro section have low nickel contents, whereas those from the 1400 m Hole U1309D Atlantis Massif gabbro section have high nickel. This is consistent with less-reacted infiltrating melt at the latter, and suggests that U1309D represents a deeper section of the lower crust near the crust-mantle transition, whereas the former has been shown to have crystallized near the dike-gabbro transition. Godzilla troctolites and gabbros bear clear similarities to East Pacific Rise (EPR) troctolites and gabbroic segregations drilled in dunites crosscutting residual mantle peridotite at Hess Deep Site 895. The latter represent multi-stage crystallization of stagnant melts in conduits beneath the EPR. Thus, it appears that the processes that formed the Godzilla troctolites occur beneath ocean ridges across the spreading rate spectrum.