High CO2 content in magmas of the explosive andesitic Enco eruption of Mocho-Choshuenco volcano (Chile)

Abstract

Mocho-Choshuenco volcano has produced several highly explosive eruptions during its history, which make it one of the most hazardous volcanoes in the southern volcanic zone of Chile, although it is still relatively little studied to date. We present a geochemical study of the products of the sub-Plinian, andesitic, Enco eruption that occurred about 1600 years ago. We determined the major and trace elements compositions, as well as the volatile (H2O, CO2, Cl, and S) contents of melt inclusions trapped in minerals (olivine, plagioclase, and pyroxene) using electron microprobe, ion microprobe (SIMS), and 3D confocal Raman mapping. Though the whole-rock composition of the Enco magma is andesitic (60.2 ± 1.1 wt.% SiO2), the melt inclusions have SiO2 contents ranging from 50.3 to 67.3 wt.%, following the magmatic series of Mocho-Choshuenco, and the compositions of the most mafic melt inclusions are close to those of the most mafic erupted magmas. Geochemical modeling indicates that mixing occurred between a mafic magma and an andesitic-to-dacitic magma. Glass analysis revealed typical parental arc magma values for H2O (2.6–3.8 wt.%), S (116–1936 ppm), and Cl (620–1439 ppm). However, CO2 contents are very high in some melt inclusions with concentrations above 4000 ppm (measured in the glass), suggesting trapping depths > ~ 17–22 km. Presence of solid carbonates inside inclusion-hosted bubbles clearly indicates that the CO2 contents measured in the glass phase were minimum values. We conclude that a CO2-rich basaltic magma ascended and mixed with a shallower andesitic magma. The magma cooled and exsolved high amounts of CO2, which may have dramatically increased the pressure and triggered the highly explosive Enco eruption.

3D Raman mapping combined with FIB‐SEM on multiphase fluid inclusions: A tool to unravel complex phase assemblages

“…One of the most important advantages of 3D Raman mapping of fluid and melt inclusions is to offer the detection and distribution of the small phases (<1 μm), which would likely be overlooked because their size hinders their optical observation. Such small phases can also be overlooked during regular Raman point measurement, or even during 2D Raman mapping as well, this likewise points out the significance of 3D Raman mapping (e.g., previous works 14,19 ).…”

“…Correcting the spherical aberration with the refractive index of the host mineral (real depth ≈ optically measured depth × n, where n is the refractive index of the mineral) is although good, but not an exact method, 14 because in minerals with high refractive indices, the difference is even larger 34 . Similarly, most studied inclusions in the literature are assumed to be close to a spherical shape, calculating the mean of the two visible axes should give a reliable result 19,35 . However, none of these assumptions can replace the exact measurement of the horizontal size of an inclusion exposed either by grinding and polishing 36 or by FIB‐SEM (e.g., previous works 18,37–40 ).…”

“…34 Similarly, most studied inclusions in the literature are assumed to be close to a spherical shape, calculating the mean of the two visible axes should give a reliable result. 19,35 However, none of these assumptions can replace the exact measurement of the horizontal size of an inclusion exposed either by grinding and polishing 36 or by FIB-SEM (e.g., previous works 18,[37][38][39][40] ). In our study, we corrected the Z dimensions (i.e., the thickness of the slices) of the mapped inclusions on the 3D Raman models based on complementary FIB-SEM analyses (Table 1).…”

The Andean Southern Volcanic Zone: a review on the legacy of the latest volcanic eruptions

“…The existing magmatic reservoirs can be affected by occasional mafic recharge events, which can sometimes drive Plinian eruptions, as in the Quizapu (Ruprecht, et al, 2012) and Mocho-Choshuenco (Feignon et al, 2022) volcanoes via "recharge filtering", a model usually applied to arc settings (Kent et al, 2010;Kent, 2014). The fractionation of parent magmas, assimilation of crustal rocks, and mixing processes within multiple shallow crustal reservoirs can also produce andesites and dacites, as in the Tatara-San Pedro (Davidson et al, 1987(Davidson et al, , 1988Dungan et al, 2001;Ferguson et al, 1992;Singer et al, 1997) and Lonquimay (Gilbert et al, 2014) volcanoes.…”