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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.
Highlights
Calc-alkaline magmatism is characteristic for active convergent plate margins and is related to the formation and evolution of continental and island arc crust
To resolve the open question of the predominant pressure level or crustal depth of crystallisation differentiation, we experimentally investigated the process of polybaric fractional crystallisation, where hydrous basaltic liquids extracted from the mantle differentiate at various levels in the crust (e.g. Grove et al 2003; Almeev et al 2013; Melekhova et al 2015)
We explored different polybaric fractionation ascent trajectories and conducted a limited number of additional experiments to evaluate the effect of oxygen fugacity (fO2) and initial starting material composition on the resultant liquid lines of descent
Summary
Calc-alkaline magmatism is characteristic for active convergent plate margins and is related to the formation and evolution of continental and island arc crust. Bowen 1915, 1928; Grove et al 2003; Nandedkar et al 2014; Ulmer et al 2018). Since the fundamental work of Bowen (1928), a large number of studies have been conducted supporting as well as objecting the dominant control of fractional crystallisation on calcalkaline differentiation trends Green and Ringwood 1968; Grove et al 2003; Ulmer 2007; Turner and Langmuir 2015; Clemens et al 2021). The most commonly invoked fractionating mineral phases controlling calc-alkaline differentiation include magnetite (e.g. Osborn 1959), amphibole The most commonly invoked fractionating mineral phases controlling calc-alkaline differentiation include magnetite (e.g. Osborn 1959), amphibole (e.g. Cawthorn and O'Hara 1976; Foden and Green 1992; Sisson and Grove 1993a; Davidson et al 2007; Dessimoz et al 2012; Blatter et al 2013; Nandedkar et al 2014; Goltz et al 2020), or garnet (e.g. Macpherson et al 2006; Müntener and Ulmer 2006; Tang et al 2018)
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