Constraints from thorium/lanthanum on sediment recycling at subduction zones and the evolution of the continents

Publication Type  Journal Article
Year of Publication  2005
Authors  Plank, T.
Journal Title  Journal of Petrology
Volume  46
Issue  5
Pages  921-944
Journal Date  May
ISBN Number  0022-3530
Accession Number  ISI:000228777500003
Key Words  arc magmatism; continental crust; delamination; thorium; sediment subduction; lower crustal xenoliths; trace-element fractionation; mantle wedge composition; island-arc magmatism; rare-earth-elements; lesser antilles; chemical-composition; volcanic-rocks;

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.


921PTTimes Cited:45Cited References Count:172

URL  <Go to ISI>://000228777500003
DOI  DOI 10.1093/petrology/egi005