Lead isotope characteristics of volcanic rocks and hydrothermal sulfide deposits from the Eastern Manus Basis and Franklin Seamount, Papua New Guinea: evidence for the Pacific - Indian mantle boundary

Full text: Submarine hydrothermal activity in the Eastern Manus Basin (EMB) is hosted by a suite of cosanguinous andesitic and dacitic to ryhodacitic lavas and associated volcaniclastic deposits in a back-arc environment to the north of the New Britain island arc. The igneous activity is related to northward subduction of the Solomon Sea Plate at the New Britain Trench. Seafloor hydrothermal activity has been observed at three major fields: PACMANUS, DESMOS and SuSu Knolls. Here we present Pb isotope data for volcanic rocks from the EMB, including the host rocks to these hydrothermal fields, for sulfides from PACMANUS and SuSu Knolls, and for barite chimneys from Franklin Seamount in the Woodlark Basin south of New Britain. Lead isotope data for volcanic rocks and hydrothermal sulfide deposits from the EMB plot at the radiogenic end of a spectrum of data for hydrothermal centres in the SW Pacific (EMB data range: 206Pb/204Pb 18.66 18.78; 207Pb/204Pb 15.51 - 15.56; 208Pb/204Pb 38.25 - 38.44, n = 35). The volcanic rocks have similar isotopic characteristics to those from other volcanic provinces nearby, including New Britain and the Tabar-LihirTanga-Feni island chain, and cluster at the edge of the Pacific MORB field. These data suggest that the lavas in all these areas are sourced from subduction-modified Pacific mantle, in contrast to the basaltic central Manus spreading zone which has Indian mantle-like affinities. Sulfides have Pb isotope values that lie within a restricted range (206Pb/204Pb 18.74 - 18.78; 207 Pb/204Pb 15.51 - 15.54; 208Pb/204Pb 38.29 - 38.44, n = 22) and are slightly more radiogenic than values for the volcanic rocks. Between hydrothermal fields, Pb from SuSu Knolls is slightly more radiogenic than Pb from PACMANUS, which may reflect local isotopic variations between magma chambers and/or a larger crustal component at SuSu Knolls. On a regional scale, the sulfides are slightly more radiogenic than those from the Hine Hina Field in the Lau Basin. A small number of barite chimneys from Franklin Seamount yield somewhat different Pb isotope ratios (206Pb/204Pb 18.54 - 18.58; 207Pb/204Pb 15.56 - 15.57; 208Pb/204Pb 38.39 - 38.44, n = 4). These data are quite unusual compared with other hydrothermal systems in the SW Pacific although they have some affinity with sulfides from the southern Lau Basin. The boundary between the Indian and Pacific Mantle reservoirs is believed to extend beneath parts of the SW Pacific (e.g. Woodhead et al., 1997). Lead isotope ratios for the EMB suggest that the Pb was derived from the Pacific Mantle rather than the Indian Mantle, whereas Pb at Franklin Seamount is consistent with an Indian Mantle source. These data, together with data from the Central Manus Basin (Agapova et al., 1994) and New Britain (Woodhead and Johnson, 1993), suggest that the boundary between the Indian and Pacific Mantles runs to the southwest of the EMB and NE New Britain, and to the northeast of the Central Manus Basin and Franklin Seamount. Pb isotope data for EMB volcanic rocks are also comparable to those for basalts from the southern Lau Basin, but are very different from those of the northern Lau Basin. Probably this too reflects Pacific versus Indian subduction-modified mantle sources for the lavas and indicates the complexity of the boundary between these two mantle components in the region. Copyright (1999) Geological Society of Australia