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[1] We report new Sr, Nd, and Pb isotope data for Mid-Ocean Ridge Basalts (MORB) dredged between 35 and 69°E along a $4100 km section of the Southwest Indian Ridge (SWIR), which is one of the slowest spreading ridges of the global mid-ocean ridge system (full rate of 16 mm yr À1 ). The SWIR appears isotopically more heterogeneous than other mid-ocean ridges over similar length scales and comparable sampling density. Isotopic variations are generally independent of ridge segmentation, and the degree of heterogeneity decreases from west to east. This decrease in heterogeneity correlates with the observed increase in axial depth and decrease in crustal thickness, commonly attributed to decreasing mantle potential temperature. Data for the easternmost SWIR in the vicinity of the Indian Triple Junction confirm that these lavas are isotopically distinct from those of the Triple Junction, the Southeast Indian Ridge, and the Central Indian Ridge, reflecting the presence of an isotopic boundary over an along-axis distance of less than 78 km. Results for lavas from the 39-41°E section extend the isotopic range of MORB to the lowest

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