| Summary: | We present data for glassy basalts from ~37 localities along the spreading axis of the Southeast Indian Ridge (SEIR) between 126°-140°E, eastward of the Australian-Antarctic Discordance (AAD). Each of the five ridge segments (A1 to A5, west to east) show well-defined major element trends. An isotopic and negative axial depth anomaly is present, centered on the overlapping tips of segments A3 and A4 at ~135°E. Segment A4 basalts have distinct radiogenic Pb and He isotopes plus enriched MORB-like εHf, relative to segments to the west and east. Crystal fractionation is more extensive at the A3 and A5 overlapping segment tips adjacent to A4, and decreases both to the west and east. The along axis pattern suggests a mantle heterogeneity located beneath the A3-A4 segments. Pb-Pb isotopic co-variations for the 5 segments define two linear arrays, with a western trend (A1-A3) and an eastern trend (A4-A5) that intersects it at the composition of the anomalous A4 segment, at a 206Pb/204Pb ~ 19. The western trend has higher 208Pb/204Pb for a given 206Pb/204Pb, revealing a gradient in the asthenosphere, with ∆208Pb/204Pb decreasing to the east away from the AAD. Overall, 206,207,208Pb/204Pb and 4He/3He of the A4 anomaly define trends that vector toward the fields for Cenozoic lavas from west Antarctica (Marie Byrd Land and Balleny Islands). West Antarctica has a history of mantle plume underplating and lithosphere modification by subduction [1,2], and there is a broad seismic anomaly below 250 km underlying the West Antarctic Rift system [3]. Our data supports a model in which flow of underplated material plus lithosphere may be guided by the underside topography of the lithosphere beneath the Transantarctic mountains. This flow emerges from beneath east Antarctica, where it leads to volcanism in the Balleny Islands [4]. The material apparently continues to flow northward to the SEIR at ~135°E. The geochemical anomaly beneath Zone A is potentially explained by the presence of this residual plume/mobilized lithospheric ...
|
|---|