Geochemistry Of Igneous Rocks And Material Contribution Of Cu To Seafloor Hydrothermal System In Eastern Manus Basin

A semi-continuous series volcanic rocks ranging from basaltic andesite to dacite were dredge from eastern Manus Basin. Mineralogy and element geochemistry are studied to explore the origin of the intermediate lavas and its subduction components in the eastern Manus Basin as well as the source of Cu in the hydrothermal system by numerical simulation.Mid-ocean ridge are dominated by basalts and basaltic andesites, and lack of silicic magma. The eastern Manus Basin is an immature back-arc basin with characteristic both subduction zone and spreading center and the magma there is distinc different from that of Manus Spreading Center and MORB-like lava. Geochemical data indicate that fractional crystallization plays a dominant role in generating lavas with subduction features in eastern Manus Basin. Furthermore, low H2 O contents, positive relationships of La vs. Si O2 and Yb vs. Si O2, the relative high oxygen isotope data,and the indistinct concentration differences of trace element（Fe, Mg and Sr） with the change of An in oscillatory zoning of plagioclase phenocrysts, rule out models in which silicic lavas result from direct melting of hydrous mantle, partial melting of altered oceanic crust or gabbros, and magma mixing, respectively.⁸⁷/Sr/⁸⁶Sr, Ba/La, B/Be and 206Pb/204 Pb ratios decrease along the latitude, from the south to the north, indicating subduction components are origin from Solomon subduction zone chiefly. Furthermore, a majority of 230Th/238 U ratios of volcanic rocks from eastern Manus Basin suggested the addition of Solomon subduction component.We estimated the chemical budget of sulfide deposited on the seafloor surrounding the PACMANUS hydrothermal field to contain 25,219.4 t of Cu. The total Cu tonnage associated with the high-temperature static reaction zone（0.144km3） amounts only to 10,281.6 t which cannot satisfy the Cu requirement of sulfides even under a 100 % leaching, transport and precipitating efficiency. Furthermore, more than 17.7 km3 of volcanic rocks need to be leached to explain the total Cu concentrations found in the PACMANUS hydrothermal field sulfides using a more realistic leaching scenario. Thus, leaching of the oceanic crust is insufficient to explain the tonnage of Cu in PACMANUS hydrothermal field and a second source of Cu is required. Each vesiculation event generates at least 70,000 to 672,000 t of magmatic Cu that would be transferred to the PACMANUS hydrothermal system when active bringing the total amount contributed from 2.5 to 3.5 Mt of Cu. Only 0.72 to 1.0 % of magmatic-derived fluids would meet the demand for Cu sulfides deposited on the seafloor surrounding the PACMANUS hydrothermal field.