Effects of Fe³⁺ in sillimanite on mineral stabilities and parageneses in ultrahigh‐temperature metapelites

Abstract

Based on the nature of ferric sillimanite, an activity model for sillimanite containing Fe3+ is constructed, tested and adopted to calculate phase equilibria of pelitic compositions under ultrahigh‐temperature (UHT) conditions. The calculated P–T projections and pseudosections suggest that the incorporation of Fe3+ into sillimanite can fairly solve the current imperfectly topological match between thermodynamic calculations and synthetic experiments, especially at high oxygen fugacity. Fe3+ in sillimanite remarkably elevates the temperature to switch the parageneses of orthopyroxene + sillimanite (Opx + Sil) and sapphirine + quartz (Spr + Qz) in oxidized metapelites, with an increment around 50–70°C. The calculated compatibility diagrams show that the widely approbatory UHT‐diagnostic mineral assemblages of Opx + Sil and Spr + Qz usually occur in metapelites with high Mg/Fe2+ ratios, which depend on both bulk‐rock MgO and oxygen fugacity, whereas the metapelites with low Mg/Fe2+ ratios are characterized by the assemblage of Garnet + sillimanite (Grt + Sil) with or without spinel (Spl) in UHT conditions. Moreover, comprehensive comparisons suggest that the essential petrogenetic framework of natural UHT metapelites is mostly governed by the two metamorphic reactions of Opx + Sil = Spr + Grt and Grt + Sil = Spl + Spr.

Ultrahigh‐Temperature Anatexis of Metapelites Generates Near‐Anhydrous Garnet Granitoids

“…To model the crystallization of garnet granitoid magmas, H 2 O content in the system was adjusted using a T − M (H 2 O) pseudosection to ensure that the observed mineral assemblage occurs precisely above the solidus (e.g., Korhonen et al., 2013) and corresponds to the observed proportions. The Fe 3+ /Fe T ratio was assumed to be ∼0.2 as the mineral assemblages in the garnet granitoids likely indicate a relatively low oxygen fugacity condition with a bulk‐rock Fe 3+ /Fe T range around 0.1–0.25 (B. Wang & Wei, 2022). The bulk‐rock compositions used for calculations were normalized to molar proportions in the NCKFMASHTO system and presented in Table S4 of Supporting Information S1.…”

“…The Fe 3+ /Fe T ratio was assumed to be ∼0.2 as the mineral assemblages in the garnet granitoids likely indicate a relatively low oxygen fugacity condition with a bulk-rock Fe 3+ /Fe T range around 0.1-0.25 (B. Wang & Wei, 2022). The bulk-rock compositions used for calculations were normalized to molar proportions in the NCKFMASHTO system and presented in Table S4 of Supporting Information S1.…”