Trans-Lithospheric Ascent Processes of the Deep-Rooted Magma Plumbing System Underneath the Ultraslow-Spreading SW Indian Ridge

Processes of magma generation and transportation in global mid-ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end-member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow-spreading ridges. Here we present detailed mineralogical studies of high-Mg and low-Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45 degrees E. High-Mg basalts (MgO = similar to 7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85-89) and high-An plagioclase (Pl, An = 66-83) as phenocrysts, whereas low-Mg basalts contain low-Mg# Ol and low-An Pl (Fo = 75-78, An = 50-62) as phenocrysts or glomerocrysts. One low-Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high-Mg and low-Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high-Mg melts started to crystallize at similar to 32 +/- 7.8 km, close to the base of the lithosphere. The low-Mg melts may have evolved from the high-Mg melts in an AMC at a depth of similar to 13 +/- 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow-spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of similar to 0.002-0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum. Mid-ocean ridges are divergent plate boundaries where asthenospheric mantle undergoes decompressional melting to form basaltic melts and lithospheric mantle. The southwest Indian ridge (SWIR) represents the slowest spreading class of ocean ridges. It is characterized by intermittent volcanism resulting in thin or missing oceanic crust. To better understand the magma dynamics of such specific ridges and their correlation with the spreading rates, we calculated the depth of mineral crystallization in high-Mg and low-Mg basalts collected together and constrained the residence time of individual crystals in the melt through Fe-Mg diffusion profiles. We show that the high-Mg melts started crystallization close to the base of the tectonic plate, and likely partially crystallized in a deep magma chamber in the mantle to form low-Mg magmas prior to eruption to the seafloor. This study determined that the rate at which the melts flowed up through the mantle to the crust was the slowest yet found for any ocean ridges. The mineral dynamics of the ultraslow-spreading SWIR therefore illustrate the critical role of the velocity at which the tectonic plates separate on the formation of magmas at mid-ocean ridges. Deep crystallization depths (similar to 32 +/- 7.8 km) and a deep axial magma chamber (AMC) (similar to 13 +/- 7.8 km) may exist under the ultraslow-spreading Southwest Indian Ridge (SWIR)The ascent rate from the AMC to seafloor of the SWIR is similar to 0.002-0.01 m/s, which is the slowest in global mid-ocean ridge systemsMagma ascent rates correlate positively with the spreading rates in the global mid-ocean ridge systems