Shallow magmatic processes with hydrothermal circulation at slow-spreading mid-ocean ridges: Revisiting clinopyroxene microcrysts from the Tianxiu segment, Carlsberg Ridge

There is increasing recognition that groundmass clinopyroxene microcrysts in alkaline basaltic magma reveal the final magmatic pathways within active continental volcanic systems. However, little attention is focused on microcrysts from the mid-ocean-ridge volcanic system. Here, we revisit groundmass clinopyroxene microcryst chemistry and textures in mid-ocean-ridge basalt from the Tianxiu segment, Carlsberg Ridge. The Tianxiu clinopyroxene microcrysts predominantly display skeletal, dendritic, and blocky textures with crystallographic branching because of undercooling (ΔT = 12−119 °C). The decompression and undercooling thermobarometry used in this study yield pressure-temperature (P-T) results for the Tianxiu clinopyroxene microcrysts of 1082 °C to ∼1136 °C and 0.78 kbar to ∼3.41 kbar. These values are lower than those obtained from both isothermal−isobaric thermobarometry and enhanced machine learning thermobarometry, but are plausibly closer to the true P-T conditions, showing shallow magmatic processes (dominantly 5.0 km to ∼8.1 km below the seafloor) near the Moho and within the overlying oceanic crust. The Tianxiu clinopyroxene microcrysts have higher calculated pressures than those of continental basaltic eruptions, while ΔT decreases following magma ascent, which is different from the latter ones. It might result from extra heat loss in the deep crust and extra heating in the shallow crust due to deep hydrothermal circulation. We emphasize that groundmass clinopyroxene microcryst is an excellent material for understanding the thermal conditions of the shallow magmatic conduit with hydrothermal circulation at slow-spreading mid-ocean ridges.