ORIGIN OF E-MORB IN A FOSSIL SPREADING CENTER: THE ANTARCTIC-PHOENIX RIDGE, DRAKE PASSAGE, ANTARCTICA

Abstract

The fossilized Antarctic-Phoenix Ridge (APR) with three segments (P1, P2, and P3), Drake Passage, is distant from the known hotspots, and consists of older N-MORB formed prior to the extinction of spreading and younger E-MORB after extinction. The older N-MORB (3.5–6.4 Ma) occur in the southeastern flank of the P3 segment (PR3) and the younger E-MORB (1.4–3.1 Ma) comprise a huge seamount at the former ridge axis of the P3 segment (SPR) and a big volcanic edifice at the northwestern flank of the P2 segment (PR2). The PR3 basalts have higher Mg#, K/Ba, and CaO/Al2O3 and lower Zr/Y, Sr, and Na80 (fractionation-corrected Na2O to 8.0% MgO) with slight enrichment in incompatible elements and almost flat REE patterns. The SPR and PR2 basalts are highly enriched in incompatible elements and LREE. The extinction of spreading at 3.3 Ma seems to have led to a temporary magma oversupply with E-MORB signatures. Geochemical signatures such as Ba/TiO2, Ba/La, and Sm/La suggest the heterogeneity of upper mantle and formation of E-MORB by higher contribution of enriched materials (e.g., metasomatized veins) to mantle melting than the N-MORB environment. E-MORB magmas beneath the APR seen to have been produced by low-degree melting at deeper regime, where enriched materials have preferentially participated in the melting. The occurrence of E-MORB at the APR is a good example to better understand what kinds of magmatism would occur in association with extinction of the ridge spreading.