Polybaric differentiation for arc magma genesis: a perspective from alumina saturation index

• Published in: Lithos Vol. 512-513; p. 108136
• Main Authors: Gao, Peng, García-Arias, Marcos, Collins, William J., Chen, Long, Xiang, Hua, Gu, Hai-Ou, Zhao, Zi-Fu
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2025
• Subjects: Alumina saturation index; Arc magmas; Phase equilibrium modeling; Polybaric fractional crystallization; Arc magmas; Phase equilibrium modeling; Polybaric fractional crystallization; Alumina saturation index
• ISSN: 0024-4937
• DOI: 10.1016/j.lithos.2025.108136

The crustal level at which arc magma differentiation predominantly occurs remains a subject of debate. Experimental petrology has demonstrated that the SiO2-ASI (alumina saturation index) relationship of arc magmas could address this issue because it strongly depends on pressure, but these results are far from conclusive due to either insufficient evaluation or inconsistency with geological observations. Using compiled data from arc volcanics and crystallization experiments, this study classifies the arcs as thick (40–63 km), normal (25–35 km) and thin (15–20 km) according to crustal thickness. We find that experimental melts produced at the corresponding arc root pressures can only account for the most aluminous compositions at intermediate SiO2 content (55–65 wt%), with the typical lesser aluminous compositions requiring lower pressure crystallization. To further evaluate the pressure at which crystallization in arcs occurs, phase equilibrium modeling is used here to model the crystallization processes from primitive arc rocks on different modes (isobaric and polybaric, equilibrium and fractional) in the range of 1.5 to 0.2 GPa. Modeled results verify the experimental findings and highlight that the liquid lines of descent (LLD) from 0.2 GPa overlap the lesser aluminous parts of the SiO2-ASI plots for arc rocks. LLD from H2O-poor systems are deviated to lower ASI at constant SiO2 relative to the H2O-rich ones due to more proportion of plagioclase in the H2O-poor condition. The dominant phase to increase melt ASI is clinopyroxene, whereas garnet at high-pressure and plagioclase at low-pressure (especially H2O-poor) play an opposite role. This study underscores that polybaric differentiation is more relevant to the genesis of arc volcanics than deep crustal differentiation, irrespective of crustal thickness. •Crystallization of arc magma is modeled by phase equilibrium.•Pressure plays the primary role in controlling the LLD in the SiO2-ASI plot.•Polybaric FC can account for the entire compositional range of global arc data.