Resolving an isotopic boundary within the Australian-Antarctic discordance

Abstract

New Sr, Nd and Pb isotopic analyses of MORB glasses from the Australian-Antarctic Discordance (AAD) confirm the presence of an abrupt boundary between ‘Indian’ type and ‘Pacific’ type MORB mantle. The transition between these two upper mantle reservoirs is gradational along ∼ 40 km of the easternmost AAD spreading center (i.e. segment B5W) and terminates at its western ridge-transform intersection. Axial lavas dredged immediately west of the B4/B5 transform are unequivocally derived from an ‘Indian’ MORB source, whereas axial lavas dredged east of this transform have a ‘Pacific’ type signature with evidence of an ‘Indian’ MORB imprint. Off-axis sampling of the easternmost AAD spreading segment show these lavas to have been derived from an ‘Indian’ type source; indicating that the isotopic boundary has migrated westward into the AAD in the last 3–4 Myr. Pacific mantle must flow beneath the B5 spreading axis at a rate of ∼ 25 mm/yr in order for it to displace Indian mantle as the source of melt for this segment. The gradational character of the boundary suggests that, as the westward migration of ‘Pacific’ mantle progresses, an increasing memory of a remnant ‘Indian’ signature is incorporated into lavas near the leading edge of the boundary zone. At present, approximately equal proportions of ‘Pacific’ and ‘Indian’ mantle contribute to the isotopic signature of lavas erupted 10 km east of the B4/B5 spreading axis offset at ∼ 126°E. Migration of the boundary could reflect a a continuous, large-scale, westward outflow of upper mantle which has recently arrived beneath the AAD from a shrinking Pacific basin, or, alternatively, the displacement may reflect a small-scale perturbation of a long-term isotopic discontinuity, created and maintained by the mantle dynamics producing the AAD.