The Comagmat-5: Modeling the Effect of Fe-Ni Sulfide Immiscibility in Crystallizing Magmas and Cumulatesстатья

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[1] The comagmat-5: Modeling the effect of fe-ni sulfide immiscibility in crystallizing magmas and cumulates / A. Alexey, B. Kirill, N. Georgy, B. Galina // Journal of Petrology. — 2018. — Vol. 59, no. 2. — P. 283–298. Details of the calibration and testing of the first ‘sulfide version’ of the COMAGMAT magma crystallization model (version 5.2, 2012-2014) are presented. The model’s updated empirical basis includes new mineral – melt geothermometers for olivine, plagioclase, high-Ca pyroxene, pigeonite, and orthopyroxene (calibrated at 1 atm. pressure), which are combined with a recently proposed Fe-Ni sulfide solubility model. This allows COMAGMAT-5 to be used for calculations of equilibrium and fractional crystallization of S-saturated and S-undersaturated magmas, including changes in the Fe/Ni ratio in silicate melts, femic minerals, and coexisting sulfides, as well as sulfide-silicate (±Fe-Ti oxides) proportions for multiply-saturated mineral assemblages. Based on our experience in tests of experimental data and modeling crystallization of mafic magmas, the possible range of application of COMAGMAT-5 may be extended up to 1-2 kbar pressure. The new model suggests a strong dependence of sulfide liquid immiscibility on the Ni content of the melt, as the increase of Ni is shown to decrease sulfide solubility, thus stabilizing the sulfide liquid in crystallizing mineral assemblages. This effect was tested for a Ni-rich sulfide-saturated glass dredged from the the southern Mid-Atlantic Ridge. The sulfide COMAGMAT was found to accurately predict the low S content observed in the glass (∼600 ppm); other existing ‘FeS’ solubility models yield higher values, mostly > 1000 ppm. In addition, using an experimentally studied high magnesia andesite composition, the results of calculations with two versions of COMAGMAT (3.72 & 5.2) were compared with those produced by MELTS family models. Application examples of COMAGMAT-5 include the modeling Fe-Ni sulfide saturation during equilibrium and fractional crystallization of ultramafic systems, approximating the most primitive magmas and cumulates from the Bushveld Complex in South Africa. [ DOI ]

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