| Summary: | The Southeast Indian Ridge (SEIR) runs approximately west to east and separates the Indo-Australian plate from the Antarctic plate. It terminates in the west at the Rodrigues Triple Point and in the east at the Macquarie Triple Junction. The SEIR spreads at a moderate rate (~7.2 cm/ yr, full spreading rate) and has a region characterized by closely spaced offsetting fractures between about 115 and 130E known as the Australian-Antarctic discordance. This section is hypothesized to mark the boundary between the mantle source of magmatism in the Indian Ocean and that in the Pacific Ocean. We compiled trace element data for 120 samples of lava erupted along this ridge from Gale et al., (2013). The samples were collected from near the Rodrigues Triple point through the discordance. The samples were normalized to the composition of average Normal Mid Ocean Ridge Basalt (NMORB) to aid in characterizing geochemical variations along the ridge. This method allows identification of samples that are enriched or depleted in incompatible trace elements relative to NMORB, and provides insight into the evolutionary history of magmas as well as into the mantle source regions of the magmas. The results show that samples from the western part of the ridge have similar patterns on the normalized plots, with variable degrees of enrichment or depletion relative to NMORB. This range of values implies that a heterogeneous mantle source underlies the western part of the ridge. In contrast, samples from east of and within the discordance are depleted in incompatible trace elements, suggesting that a depleted mantle source exists beneath the discordance. Variations in Na8 along the ridge suggest that the mantle beneath the discordance is cooler than elsewhere along the ridge, confirming inferences made by previous workers. A five-year embargo was granted for this item. Academic Major: Earth Sciences
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