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Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths, and as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0.94, Mg# olivine = 0.88), high NiO in olivine (∼0.26 wt.%) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs. SiO2 wt.%, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure-temperature at 2.6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE & Ce enrichments and strong Nb & Zr depletions. The trace elements and the pressure-temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

High pressure-temperature aqueous fluids are essential to melt generation, element cycling, and fluid-melt-rock reactions occurring in subduction zones. Recent advances in theoretical thermodynamic modeling help facilitate calculations at a range of pressure conditions relevant to subduction zones. Here we explore stable and metastable equilibrium speciation of C 1 and C 2 aqueous carbon species along a theoretical slab surface pressure-temperature path. These calculations reveal a thermodynamic drive to stabilize small organic compounds at elevated pressures and temperatures, with pH buffered by the diopside-antigorite-forsterite mineral assemblage. At stable equilibrium, oxidized forms of aqueous carbon dominate the speciation at and above oxidation conditions set by the fayalitemagnetite-quartz (FMQ) assemblage. Under conditions more reduced than FMQ, a larger variety of aqueous carbon species are stabilized. If metastability were to persist along the path targeted by this study, it is predicted that a plethora of C 1 and C 2 aqueous species would be stabilized, especially under reduced conditions. These results point the way for theoretical geochemical modeling in the pressure-temperature-composition space of sub duction zone fluids and provide new constraints on forms of deep carbon.

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