Magnesium isotopic composition of continental arc andesites and the implications: a case study from the El Laco volcanic complex, Chile

Abstract

Continental crust can dramatically modify the geochemical and isotopic compositions (e.g., Sr-Nd, Pb) of mantle-derived lavas, and has important implications in understanding magmatic processes in continental arcs which involve subducted materials. In this paper, we report the Mg isotopic compositions of continental arc andesites from El Laco in northern Chile, and evaluate the contribution of the subducted slab to the formation of continental arc lavas. The andesites in the El Laco volcanic complex (ELVC) display relatively high (⁸⁷Sr/⁸⁶Sr)i ratios and negative age-corrected εNd(t) (t = 1.6 Ma) values. The δ²⁶Mg values of the ELVC andesites range from −0.26 ± 0.05‰ to −0.15 ± 0.04‰ (average δ²⁶Mg = −0.18 ± 0.05‰), slightly heavier than that of the primitive mantle. The δ²⁶Mg values of clinopyroxene, orthopyroxene and magnetite grains separated from andesites are −0.27 ± 0.03‰ to −0.20 ± 0.04‰, −0.24 ± 0.06‰ to −0.18 ± 0.03‰ and +0.09 ± 0.06‰ to +0.33 ± 0.06‰, respectively. Our results suggest that Mg isotope fractionation occurred during the fractional crystallization of El Laco andesite, and imply complex Mg isotopic fractionation at high-temperature conditions than previously inferred. Combing the Mg isotopes with the geochemical and Sr-Nd isotopic data, we confirm that neither the deep process of partial melting nor the shallow process of crust contamination contributed to the heavier Mg isotopes of the andesites. The heavier Mg isotopic features of these lavas were mainly inherited from aqueous fluids derived from the subducted slab, which reacted with the primitive mantle wedge to produce a metasomatic mantle source for the continental arc lavas.