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% H2O. 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.
Highlights
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
Of the 28 experiments where glass H2O was measured by secondary ion mass spectrometry (SIMS), in 23 experiments H2O change is less than ± 0.6 wt%, including those used to define the liquidus multiple saturation point (MSP)
High-potassium, primitive Colima Volcanic Complex (CVC) magmas have been shown experimentally to form by peritectic melting of phlogopitepyroxenite lithologies at depths of ≥ 59 km
Summary
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. Slab-derived fluids supply H2O and other components, notably silica and alkalis, that both depress the peridotite solidus and act as a metasomatic agent capable of generating non-lherzolite ultramafic lithologies, such as pyroxenite or wherlite (e.g., Bouihol et al 2009). Beyond this simple petrological framework, we still know surprisingly little about the temperature at which melting occurs in the wedge, the depth at which mantle wedge-derived melts segregate from their source, the style and degree of mantle melting, and even the composition and mineralogy of the source lithology
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