Cr-spinel-hosted inclusions provide evidence for reactive melt transport in layered gabbros from the lower oceanic crust at Hess Deep

Abstract The lower oceanic crust beneath fast-spreading ridges is commonly attributed to fractional crystallization of basaltic melts, but reactive melt transport and crystal-mush reorganization may also be important. We investigated spinel-bearing layered gabbros from Hole U1415P, Hess Deep Rift (IODP Expedition 345), focusing on Cr-spinel-hosted silicate inclusions. Heterogeneous spinel-bearing olivine gabbros are sharply juxtaposed against relatively homogeneous, spinel-poor troctolite. They contain plagioclase chadacrysts enclosed by olivine and clinopyroxene oikocrysts, orthopyroxene rims and veinlets associated with olivine, and locally abundant Cr-spinel, indicating localized melt-mush interaction. Cr-spinel hosts fully crystallized inclusions with contrasting mineral assemblages. Cpx + Pl-type multiphase solid inclusions (MSIs) have host-gabbro-like assemblages and basaltic to locally picritic bulk compositions, consistent with a resident basaltic component. In contrast, rounded to subrounded Opx-type MSIs contain voids and accessory phases absent from the host gabbro and define a distinct SiO 2 -rich, Mg-rich bulk-compositional group. Opx-bearing MSIs and single-phase Opx inclusions indicate that orthopyroxene-bearing components were present during Cr-spinel growth. We infer localized interaction between a troctolitic mush and an infiltrating SiO 2 -rich reactive melt. Reaction with olivine may have generated locally Mg-enriched reaction-zone melts, whereas interaction or mixing with resident basaltic interstitial melt likely promoted Cr-spinel saturation. The U1415P rocks therefore record melt infiltration, hybridization, and reactive reworking of preexisting troctolitic cumulates, processes that may contribute to chemical and textural heterogeneity within the lower oceanic crust.