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−1). 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 206Pb/204Pb values (to 16.58) yet found among oceanic islands and spreading centers worldwide and confirm their extremely anomalously high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb and low 143Nd/144Nd. This extreme Indian signature does not bear any isotopic affinities with recent products of the nearby Marion hot spot. Furthermore, the presence of old subducted marine sediment as a source for the low 206Pb/204Pb component is excluded, as no sediment group has the appropriate parent-daughter Th/Pb and U/Pb characteristics to yield such compositions, not even when dewatering subduction processes are considered. Incorporation of old subduction-modified mantle into the MORB source does not yield high enough 207Pb/204Pb and 208Pb/204Pb to explain the features of lavas from the 39–41°E section. The unusual isotopic attributes of this section (low 206Pb/204Pb and 143Nd/144Nd and high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb) are best explained by the presence of stranded lower continental crust embedded in the upper mantle. This component is also inferred to be present in MORB from other spreading centers in the Indian Ocean.