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Constraints from thorium/lanthanum on sediment recycling at subduction zones and the evolution of continents
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| Venue: | J. Petrol |
| Citations: | 46 - 1 self |
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@ARTICLE{Plank_constraintsfrom,
author = {T. Plank},
title = {Constraints from thorium/lanthanum on sediment recycling at subduction zones and the evolution of continents},
journal = {J. Petrol},
year = {},
pages = {921--944}
}
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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 (<02), elevated in the continents (>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
Keyphrases
th la subduction zone thorium lanthanum arc magma continental crust preferen-tial partitioning marine sediment fractionation foun-dering arc magmatism key word major question bulk continental crust sediment subduction different arc varies granitic upper crust volcanic arc form archaean magmatism large fractiona-tion oceanic basalt arc crust differentiates high th la sedimentary end-member sm la accessory mineral local trench sediment mafic residue arc basalt
