Geoscience Frontiers (Sep 2020)
An origin of the along-arc compositional variation in the Izu-Bonin arc system
Abstract
The Izu-Bonin arc system is sediment-poor (~400 m thick with no accretionary prism) and, therefore, the influence of the altered oceanic crust (AOC) is most likely the source of the documented along-arc lava compositional variations, especially in Pb isotopes. Izu-Bonin arc lava geochemistry suggests an influx of subduction component from an Indian-type AOC. However, samples drilled from the western Pacific geochemical reference site at Integrated Ocean Drilling Program Site 1149 implies subduction of a Pacific-type AOC. To solve the apparent discrepancy of slab input versus arc output in this arc system, samples of the AOC were dredged from vertical fault scarps of the subducting Pacific Plate along a transect from 27.5°N to 34.5°N. Samples range from tholeiitic to mildly alkalic mid-ocean ridge basalts as well as trachybasalts, basaltic trachyandesites, tephrites, and phono-tephrites. Isotope ratios also exhibit a range of values (87Sr/86Sr = 0.70282–0.70673, 143Nd/144Nd = 0.512552–0.513174, 206Pb/204Pb = 18.43–20.00, 207Pb/204Pb = 15.40–15.67, 208Pb/204Pb = 37.75–39.55). Our results suggest that there is a geochemical variation in the AOC that is neither completely due to seawater or hydrothermal alteration, nor to petrogenetic processes. Rather, this variation is the result of the Pacific-Izanagi Ridge system tapping into a heterogeneous, plume-polluted mantle source during the Mid-Cretaceous volcanic event. The observed Pacific-type AOC is not responsible for the Indian-type Pb isotopic signature of Izu-Bonin arc lavas. This leads us to propose an alternative scenario where the Izu-Bonin arc lava Indian-type Pb isotopic signature originates from slab-derived fluids interacting and adsorbing Pb from an Indian-type mantle wedge through zone-refining.
