Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents
Abstract
Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This ques-tion is explored here using Th/La, which is low in oceanic basalts (025) and varies in arc basalts and marine sediments (009–034). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractiona-tion in Th/La within the continental crust, caused by the preferen-tial partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of ‘fractionation & foun-dering’, whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60 % of its mafic residues to foundering. KEY WORDS: arc magmatism; continental crust; delamination; thorium; sediment subduction
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