Geochemical and isotopic characterisation of trench sediment at the Hikurangi Margin from IODP sites U1518 and U1520

Abstract

ABSTRACT The composition of trench sediment at convergent margins exhibits strong compositional links to the corresponding arc magmas. To test for the existence of such links at the Hikurangi margin, we provide a first geochemical characterisation of the IODP sites U1518 and U1520 drilled at the upper and incoming plate respectively. The identification of a décollement in the trench sediments allows distinction of the accreting clastic sediment above the décollement and the subducting pelagic and volcaniclastic material below it. The accreted material and that located above the décollement show homogenous Pb, Sr and Nd isotope ratios with bulk compositions of 206Pb/204Pb: 18.8953, 207Pb/204Pb: 15.6552, 208Pb/204Pb: 38.8371, 87Sr/86Sr: 0.70935, 143Nd/144Nd: 0.51241. In contrast, the material located below the décollement shows more heterogeneous Pb-Sr-Nd isotope ratios and bulk compositions of 206Pb/204Pb: 20.4826, 207Pb/204Pb: 15.7355, 208Pb/204Pb: 39.9833, 87Sr/86Sr: 0.70752, and 143Nd/144Nd: 0.51286. Tectonic erosion in the Hikurangi margin implies that accreted material could subsequently be subducted from the underside of the upper plate. Therefore, this characterisation is of prime importance for future work to constrain the role that subducted materials play in the compositional variations of the arc magmatism associated with subduction along the Hikurangi margin.