Critical assessment of pressure estimates in volcanic plumbing systems: the case study of Popocatépetl volcano, Mexico

Most geobarometers use chemical compositions of minerals and their host melt to estimate crystallization pressures. Crystal structural parameters such as cell and site volumes are not usually considered despite their known sensitivity to pressure. Here, we compare two clinopyroxene geobarometers based upon electron microprobe analysis alone and coupled with single-crystal X-ray diffraction data. The case study is the plumbing system of Popocatépetl volcano (Mexico), which consists of three distinct magma reservoirs in upper, middle and lower crustal depths, represented by three compositionally and texturally distinct clinopyroxene populations (T1, T2, and low-Ca). These clinopyroxenes are augites of limited compositional variability, although yielding a significant increase in cell (V cell) and M1 site (V M1) volumes from low-Ca and T2 core to T1 (core and rim) and T2 (rim) clinopyroxenes. This variation is not due to chemical or temperature effects but is linked to their depth of crystallization. The application of the geobarometer based on chemical composition alone is unable to distinguish the three different reservoirs postulated on volcanological and petrological grounds. In contrast, the application of the geobarometer based on both structural parameters and chemical composition yields a remarkable correlation between the calculated cell volume and the estimated depth of crystallization of the different clinopyroxenes, including core to rim differences.

These results have twofold implications. First, the determination of the structural parameters of clinopyroxenes is the only method to resolve the actual distribution of Mg, Fe²⁺, Fe³⁺ in the M1 and M2 structural sites and, given the sensitivity of cell and site volumes to pressure, permits to improve geobarometric estimates in volcanic plumbing systems. Second, the quantitative determination of the crystallization depth of the different clinopyroxenes has permitted to rescale the depth of the three different reservoirs in the plumbing system of the Popocatépetl Volcanic Complex located from ~30 km b.s.l. (low-Ca clinopyroxene) to ~18 km b.s.l. (T2 clinopyroxene core) and ~10-0 km b.s.l. (T1 clinopyroxene core and rim, T2 clinopyroxene rim) within the crustal structure of the Morelos platform. This provides further support to the complex plumbing system of the Popocatépetl Volcanic Complex consisting of polybaric storage layers of variable interconnected and interacting transient magma reservoirs.

From Tommasini et al. (2021), Lithos, https://doi.org/10.1016/j.lithos.2021.106540