Mantle Provinciality of the Western Pacific - Evidence from Pb Isotopes

David A. Gust1, Richard J. Arculus2, and Annie B. Kersting3

1School of Geology, Queensland University of Technology, QLD 4001, AUSTRALIA

2GEMOC, Department of Geology, Australian National University ACT 0200, AUSTRALIA

3Lawrence Livermore National Laboratory, Livermore, CA 91125, USA

The existence of unequivocal provinciality of MORB geochemistry together with preservation of distinctive long-term isotopic variations entrained in the global hot-spot sample (e.g., Pacific vs. Indian vs. Atlantic MORB, HIMU, EM1/2, FOZO etc.) has been established through detailed geochemical and isotopic studies (e.g., Sun and McDonough, 1989; Hauri et al., 1994). The origins of these regionally and isotopically distinctive geochemical characteristics are controversial. Recent studies in the western Pacific and Southern Ocean provide examples of spatial and temporal changes in mantle affinities, apparently linked in part to tectonic developments:- 1. the rift-to-drift transition in the Lau Basin and the collision of the d'Entrecasteaux Ridge with the Vanuatu arc (Pacific-to-Indian MORB affinity in both cases - e.g., Hickey-Vargas et al., 1995; Crawford et al., 1995); 2. the sharp juxtaposition of Indian and Pacific MORB sources at the Australia-Antarctic discordance (Klein et al., 1988); 3. the separation of the mantle sources by the chain of arcs that extend southward from the Japanese archipelago and divide backarc basins to the west (e.g., Sea of Japan, Shikoku, Parece Vela, Mariana Trough) of Indian MORB affinity from the Pacific domain to the east (Hickey-Vargas et al., 1995).

Establishment of the boundaries between Indian MORB mantle and Pacific MORB mantle is highly dependant upon Pb isotope ratios, with Indian MORB signatures being more enriched in 208Pb/204Pb and 207Pb/204Pb for similar 206Pb/204Pb than are Pacific MORB leads. Note however, that these enrichments may be generated by a number of different processes including recent contamination with old, continent-derived sediment, entrainment of various "plume" components, or interaction with 'aged' subcontinental lithosphere.

Comparison of recently acquired Pb isotopic data for Quaternary basalts from Honshu Japan, with published isotopic values for basalts from the southwestern Pacific, East Asian, and Australian regions stimulates the examination of the ingress of Indian MORB mantle into Pacific MORB mantle with time. Pb isotopic values of Quaternary Honshu basalts are Indian MORB in character and overlap with Pb isotopic data for similarly aged basalts erupted in the Sea of Japan, Korea, southeastern China, and eastern Australia. While published Pb isotopic data for arc basalts from the Philippines and Sunda arcs are also of Indian MORB affinity, the intra-oceanic arcs between Japan and the Philippines are Pacific in character.

A striking feature of the combined Pb-Sr-Nd isotopic systematics is the provincial constancy of Pb coupled with substantial variations of 87Sr/86Sr and 143Nd/144Nd, suggestive of the decoupling of Pb from processes that control the other isotopic systems. We suggest that the variation observed in Pb signatures in Australasian-China-Japan region reflect the interaction of Indian MORB asthenosphere with subcontinental lithosphere thickened and 'aged' during the growth of Pangea/Gondawana. The breakup of Gondawana and dispersal of its fragments in the southwestern Pacific is important in controlling aspects of mantle magmatism in this region..

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Crawford, A.J., Briqueu, L., Laporte, C.,& Hasenaka, T., 1995. Coexistence of Indian and Pacific oceanic upper mantle reservoirs beneath the central New Hebrides island arc. Active Margins and Marginal Basins, Geophysical Monograph 88, AGU, 199-217.

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