We report highly variable mid-ocean ridge basalt (MORB) major element and water concentrations from a single 1050-km first-order spreading segment on the ultraslow spreading Southwest Indian Ridge, consisting of two supersegments with strikingly different spreading geometry and ridge morphology. To the east, the 630 km long orthogonal supersegment (<10° obliquity) dominantly erupts normal MORB with progressive K/Ti enrichment from east to west. To the west is the 400 km long oblique supersegment (up to 56° obliquity) with two robust volcanic centers erupting enriched MORB and three intervening amagmatic accretionary segments erupting both N-MORB and E-MORB. The systematic nature of the orthogonal supersegments' ridge morphology and MORB composition ends at 16°E, where ridge physiography, lithologic abundance, crustal structure, and basalt chemistry all change dramatically. We attribute this discontinuity and the contrasting characteristics of the supersegments to localized differences in the upper mantle thermal structure brought on by variable spreading geometry. The influence of these differences on the erupted composition of MORB appears to be more significant at ultraslow spreading rates where the overall degree of melting is lower. In contrast to the moderate and rather constant degrees of partial melting along the orthogonal supersegment, suppression of mantle melting on the oblique supersegment due to thickened lithosphere means that the bulk source is not uniformly sampled, as is the former. On the oblique supersegment, more abundant mafic lithologies melt deeper thereby dominating the more enriched aggregate melt composition. While much of the local major element heterogeneity can be explained by polybaric fractional crystallization with variable H2O contents, elevated K2O and K/Ti cannot. On the basis of the chemical and tectonic relationship of these enriched and depleted basalts, their occurrence requires a multilithology mantle source. The diversity and distribution of MORB compositions, especially here at ultraslow spreading rates, is controlled not only by the heterogeneity of the underlying mantle, but also more directly by the local thermal structure of the lithosphere (i.e., spreading geometry) and its influence on melting processes. Thus at ultraslow spreading rates, process rather than source may be the principle determiner of MORB composition.