Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico
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ORIGINAL PAPER
Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico Eduardo Becerra‑Torres1 · Elena Melekhova1 · Jon D. Blundy1,2 · Richard A. Brooker1 Received: 6 January 2020 / Accepted: 19 September 2020 © The Author(s) 2020
Abstract Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H 2O. The MSP olivine is too forsterite-rich ( Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima. Keywords Phlogopite peridotite · Mantle heterogeneity · High-potassium basalt · Colima graben · Multiple saturation · Incongruent melting
Introduction Arc magmatism begins with the generation of primary magmas in the hot part of the mantle wedge between the subducted slab and the sub-arc crust. It is widely accepted that Communicated by Timothy L. Grove. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00410-020-01740-x) contains supplementary material, which is available to authorized users. * Eduardo Becerra‑Torres [email protected] 1
School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
2
most, but not all, primary arc magmas are basaltic (Gaetani and Grove 2003) and that fl
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