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.

中文翻译：

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墨西哥科利马地堑地下交代地幔减压熔融的实验证据

原始俯冲带岩浆提供了有关下伏地幔楔的成分和热结构的信息。在墨西哥的科利马地堑，从煤渣锥喷发的原始熔岩范围从高镁钙碱性玄武岩到高 K 粗面玄武岩。这种化学多样性表明它们源自的亚弧地幔楔是异质的。为了探索 Colima 下方楔形体中岩浆生成的条件，我们使用逆向实验方法来约束原始高 K 巴桑石 (COM-1) 液相线表面上的多个饱和点。在含水（1.8–3.8 wt% H2O）和氧化（ƒO2 = − 0.5 到 4.3 log 单位相对于 NNO）条件下，在 1.0 到 2.4 GPa 之间进行平衡活塞-气缸实验。COM-1 + 3。在 1.9–2.4 GPa 和 1300 ºC 下，8 wt% 的 H2O 与含金云母的尖晶石辉石岩组合 (cpx + opx + phl + sp) 接近其液相线。液相线表面的实验映射揭示了多饱和点 (MSP)，其中 ol + cpx + opx + sp + phl 的锂沸石相组合共存。MSP 的拓扑结构表示形式为 cpx + opx + phl + sp = 液体 + ol 的包晶。四个括号实验将 COM-1 的 MSP 定义为 1300 ± 10 °C、1.7 ± 0.1 GPa、∆NNO = 3.4 ± 0.5 log 单位，对于含有 3.6 ± 0.4 wt% H2O 的熔体。MSP 橄榄石富含镁橄榄石 (Fo92-94)，无法与地幔玄武岩平衡，但与天然玄武岩中的斑晶核心成分相匹配。因此，实验结果表明 COM-1 是由无橄榄石的非一致熔化产生的，金云母辉石源，其本身是地幔楔被板片衍生流体交代作用的结果。这些条件为科利马岛下方地幔楔的热结构和化学成分提供了有价值的约束。