Processes of magma generation and transportation in global mid-ocean ridges are key tounderstanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependenton spreading rate and melt flux, where the SW Indian Ridge represents an end-member. The vertical extentof ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particularat ultraslow-spreading ridges. Here we present detailed mineralogical studies of high-Mg and low-Mg basaltsfrom a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High-Mg basalts (MgO = ∼7.1 wt.%) containhigh Mg# olivine (Ol, Fo = 85–89) and high-An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low-Mgbasalts contain low-Mg# Ol and low-An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. Onelow-Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similarto those in high-Mg and low-Mg basalts, respectively. Mineral barometers and MELTS simulation indicatethat the high-Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low-Mgmelts may have evolved from the high-Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths ofmagma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow-spreadingridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC toseafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to dateamong global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreadingspectrum.