Investigating Compositional Links Between Arc Magmas And The Subducted Altered Oceanic Crust

Abstract:

Arc magmatism is causally related to the recycling of materials from the subducting plate. Numerous studies showed that the recycled material flux is dominated by recycled continental crust (oceanic sediment, eroded crust) and altered oceanic igneous crust (AOC). The crustal component is highly enriched, and thus its signal in arc magmas can readily be distinguished from mantle wedge contributions. In contrast, the impact of the AOC flux is much more difficult to detect, since the AOC isotopically resembles the mantle. Mass balance studies of arc input and output suggest that the recycled flux from the thick (6000 meter on average) AOC may buffer the flux of the recycled continental crust to the point of concealment in arc settings where the latter is volumetrically minor. In particular, highly fluid- mobile elements Sr and Pb in arc magmas are strongly influenced by the AOC, implying that the arc chemistry may allow for inferring the Sr and Pb isotopic composition of the subducted AOC.

This hypothesis is being tested by a compilation of published data of high-quality trace element and isotope compositions from global arcs. In agreement with previous studies, our results confirm that the Sr-rich fluids released from the AOC control the arc Sr isotopes, whereby the slightly elevated ⁸⁷Sr/⁸⁶Sr (up to 0.705) of many arcs may principally reflect the similarly elevated Sr isotope ratios of the AOC rather than a recycled crustal component. In contrast, the arc Pb isotope ratios are influenced by both the AOC and the recycled crustal component which create the typical binary mixing arrays. These arrays should then point to the Pb isotope composition of the AOC and the recycled crust, respectively. However, as the proportions of these end members may strongly vary in arc magmas, the exact ²⁰⁶Pb/²⁰⁴Pb of the subducted AOC in a given setting is challenging. Remarkably, the Pb isotope systematics from well-constrained western Aleutian (minimal sediment subduction) and central Mexican (dominated by recycling of eroded crust) arcs imply a similar unradiogenic Pb of the recycled AOC at ²⁰⁶Pb/²⁰⁴Pb ~18.1. Tentatively, these results suggest that the AOC subducted in the past may be more unradiogenic on average than mean composition inferred from the erupted magmas along the active mid-oceanic ridges.