Journal of Petrology Advance Access originally published online on February 4, 2005
Journal of Petrology 2005 46(5):921-944; doi:10.1093/petrology/egi005
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Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents
DEPARTMENT OF EARTH SCIENCES, BOSTON UNIVERSITY, BOSTON, MA 02215, USA
RECEIVED MARCH 17, 2003; ACCEPTED DECEMBER 1, 2004
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 question is explored here using Th/La, which is low in oceanic basalts (<0·2), elevated in the continents (>0·25) and varies in arc basalts and marine sediments (0·09–0·34). 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 fractionation in Th/La within the continental crust, caused by the preferential partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of ‘fractionation & foundering’, 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
* E-mail: tplank{at}bu.edu
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