On the stability of magmatic cordierite and new thermobarometric equations for cordierite-saturated liquids

Abstract

In this work, we have reviewed a large compositional dataset (571 analyses) for natural and experimental glasses to understand the physico-chemical and compositional conditions of magmatic cordierite crystallization. Cordierite crystallizes in peraluminous liquids (A/CNK ≥1) at temperatures ≥750 °C, pressures ≤700 MPa, variable H₂O activity (0.1–1.0) and relatively low fO₂ conditions (≤NNO − 0.5). In addition to A/CNK ratio ≥1, a required condition for cordierite crystallization is a Si + Al cation value of the rhyolite liquid of 4 p8O (i.e. calculated on the 8 oxygen anhydrous basis), which is consistent with low Fe³⁺ contents and the absence or low content of non-bridging oxygens (NBO). This geochemical condition is strongly supported by the rare, if not unique, structure of cordierite where the tetrahedral framework is composed almost exclusively of Si and Al cations the sum of which is equal to 4 p8O [i.e. (Mg,Fe)_8/9Al_16/9Si_20/9O₈], indicating that aluminium (and cordierite) saturation is limited by rhyolite liquids with Al = 4 − Si. Indeed, synthetic or natural systems with Al > 4 − Si always show metastable glass-in-glass separation or crystallization of refractory minerals such as corundum (Al_16/3O₈) and aluminosilicates (Al_16/5Si_8/5O₈). Multivariate regression analyses of literature data for experimental glasses coexisting with magmatic cordierite produced two empirical equations to independently calculate the T (±13 °C; ME, maximum error = 29 °C) and P (±16 %; ME% = 27 %) conditions of cordierite saturation. The greatest influence on the two equations is exerted by H₂O_melt and Al concentrations, respectively. Testing of these equations with other thermobarometric constraints (e.g. feldspar-liquid, GASP, Grt–Bt and Grt–Crd equilibria) and thermodynamic models (NCKFMASHTO and NCKFMASH systems) was successfully performed for Crd-bearing rhyolites and residual enclaves from San Vincenzo (Tuscany, Italy), Morococala Field (Bolivia) and El Hoyazo (Spain). The reliability of each calculated P–T pair was graphically evaluated using the minimum and maximum P–T–H₂O relationships for peraluminous rhyolite liquids modified after the metaluminous relationships in this work. Both P–T calculations and checking can be easily performed with the attached user-friendly spreadsheet (i.e. Crd-sat_TB).

Appendix: On the stability of magmatic cordierite and new thermobarometric equations for cordierite-saturated liquids. Contribution to Mineralogy and Petrology

Electron (BSE) images in Fig. 8 show representative equilibrium textures of MI-phenocryst pairs in the San Vincenzo rhyolite lavas. Figure 9 shows the main compositional differences between Crd-bearing experimental and natural glasses where the compositions of these glasses are calculated on both the anhydrous (a–d) and hydrous (e–h) bases. The fluid-saturated melting experiments produced glasses showing higher H₂O and lower Si than natural glasses, which have compositions similar to those of the fluid-absent experimental glasses, consistently with the fact that all the selected natural glasses are inferred to be derived from the partial melting of H₂O-poor crustal rocks (e.g. Ferrara et al. 1989; Morgan et al. 1998; Poli and Perugini 2003; Cesare et al. 1997; Acosta-Vigil et al. 2007).

Fig. 8

Representative BSE images of melt inclusions (MI) in phenocrysts of Crd-bearing San Vincenzo rhyolites (Tuscan Magmatic Province, Italy). Mineral abbreviations from Whitney and Evans (2010): Crd cordierite, Bt biotite, Qz quartz, Sa sanidine, Ilm ilmenite

The diagrams in Fig. 9 also show that the Crd-bearing experimental glasses behave in a complex manner. Indeed, their anhydrous compositions calculated on the basis of 8 oxygens show an M²⁺ (Fe + Mn + Mg + Ca) − Si correlation (Fig. 9a, e). Hydrous compositions do not show such correlations (Fig. 9e). In contrast to the good Al vs. Si linear relationship in Fig. 3b, no Al–Si correlation is evident for the hydrous compositions calculated on the 8 oxygens basis. The hydrous cation compositions show an approximately positive correlation between (Na +K) and Si (Fig. 9f, R ² ~ 0.5) and a good negative correlation between H₂O and Si (Fig. 9g, R ² = 0.91). These correlations are not evident for the anhydrous compositions (Fig. 9b). The compositions of experimental glasses calculated on both a hydrous and an anhydrous basis show a roughly negative correlation between Ca and K (Fig. 9d, h).

Fig. 9

Representative cation sum, cation and H₂O versus cation diagrams for both natural and experimental Crd-bearing glass compositions calculated on the anhydrous (a–d; on the left) and hydrous (e–h; on the right) basis. M²⁺ = Fe + Mn + Mg + Ca. Determination coefficient (R ²) values for experimental glass correlations with R ² higher or equal to 0.5 (solid curve or lines) are reported