FRESH BASALTS FROM THE PACIFIC ANTARCTIC RIDGE EXTEND THE PACIFIC GEOCHEMICAL PROVINCE

OVER the past several decades, the examination of volcanic rocks recovered from mid-ocean ridges and ocean islands worldwide has led to the identification of large-scale geochemical provinces reflecting compositionally distinct domains in the Earth's mantle1,2. The spatial distribution of these domains may reveal global patterns of upper-mantle convection and mixing. Previous studies have shown, for example, that the distinct Indian Ocean isotope province1 has a relatively sharp eastern boundary within the Australian-Antarctic Discordance (AAD) south of Australia3,4 (Fig. 1). The juxtaposition of Indian Ocean basalt compositions west of this boundary and Pacific compositions to the east suggests that the AAD may overlie a zone of convergence between ocean-basin-scale upper-mantle convection regimes: if this is so, Pacific isotope compositions should occur continuously along the length of the Pacific-Antarctic Ridge (PAR). Fresh basaltic glasses have now been recovered from the southernmost portion of this previously unsampled ridge axis, and we report their major element, trace element and isotopic compositions. The chemical systematics of these rocks suggest that the Pacific Ocean geochemical province includes the PAR, extends to the AAD south of Australia, and thus is one of the largest chemically coherent mantle domains on the Earth.