Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems
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2022-01
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Journal Article
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Abstract
Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO₂) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO₂ corresponding to the Re-ReO₂ buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO₂ exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO₂ conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.
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177 (1)
Pages / Article No.
3
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Springer
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Subject
Arc magmatism; Magma differentiation; Fractional crystallisation; Calc-alkaline rocks; Polybaric differentiation; Liquid line of descent
Organisational unit
03958 - Bachmann, Olivier / Bachmann, Olivier
08786 - Ulmer, Peter (Tit.-Prof.) (em.) / Ulmer, Peter (Tit.-Prof.) (em.)
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Funding
ETH-14 16-1 - Polybaric fractional crystallization of arc magmas: An experimental study (ETHZ)