ORIGINAL RESEARCH article
Front. Earth Sci.
Sec. Geochemistry
Crustal assimilation in arc magmas controlled by overriding plate thickness
Provisionally accepted
- 1University of Geneva, Geneva, Switzerland
- 2Universite de Geneve Faculte des Sciences, Geneva, Switzerland
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Magmas erupting in arcs range in composition from MgO-rich basalt to MgO-poor rhyolite. This broad compositional range is due to the sequential crystallization and separation from the cooling magma (a process known as fractional crystallization) of minerals having different chemical compositions, to mixing between magmas that have undergone different degrees of fractional crystallization and to assimilation of rocks where high temperatures allow magmas to partially melt and assimilate the surrounding rocks. Whereas the roles of fractional crystallization and mixing in arc magmas have been addressed on a large scale, the role of crustal assimilation has been assessed mostly on local to regional scales and not on a global scale. Using published whole rock geochemical data on 18 modern arcs of variable crustal thickness (~10 to ~65 km), this study highlights that correlations of elements (MgO, Co), which are indices of fractional crystallization, with Nd and Sr isotopes, which are tracers of assimilation, change systematically in magmatic rocks with the crustal thickness of the arc. These correlations indicate the occurrence of assimilation-fractional crystallization (AFC) processes in arcs of different crustal thickness. Based on the results of geochemical modelling, the systematics of the correlations between Sr-Nd isotopes and MgO-Co suggest that the rate of crustal assimilation during fractional crystallization of the magmas increases as the thickness of the arc crust becomes greater, which is a consequence of both these processes occurring at deeper and hotter crustal levels in a thick crust compared to a thin crust. In addition, the rocks that are assimilated by arc magmas in increasingly thick arcs are isotopically more evolved, suggesting that the process of crustal growth, refining and maturation in arcs results from a continuous reworking of previously formed crust through time by subsequent arc magmatic events.
Keywords: Arc magmas, Fractional crystallisation and crustal assimilation, Nd isotopes, Sr isotopes, Crust thickness, subduction
Received: 21 Aug 2025; Accepted: 13 Oct 2025.
Copyright: © 2025 Chiaradia. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Massimo Chiaradia, massimo.chiaradia@unige.ch
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