The petrology of a hazardous volcano: Calbuco (Central Southern Volcanic Zone, Chile)

Abstract

The recurrent explosive eruptions of Calbuco (Andean Southern Volcanic Zone (SVZ)) threat a rapidly expanding touristic and economic region of Chile. Providing tighter constraints on its magmatic system is therefore important for better monitoring its activity. Calbuco is also distinguished by hornblende-bearing assemblages that contrast with the anhydrous parageneses of most Central SVZ volcanoes. Here we build on previous work to propose a detailed petrological model of the magmatic system beneath Calbuco. Geochemical data acquired on a hundred samples collected in the four units of the volcano show no secular compositional change indicating a steady magmatic system since ~ 300 ka. A tholeiitic Al 2 O 3 -rich (20 wt. %) basalt (Mg# = 0.59) is the parent magma of a differentiation trend straddling the tholeiitic/calc-alkaline fields and displaying a narrow compositional Daly gap. Amphibole crystallization was enabled by the higher H 2 O content of the basalt (3-3.5 wt. % H 2 O at 50 wt. % SiO 2 ) compared to neighboring volcanoes. This characteristic is inherited from the primary mantle melt and possibly results from a lower degree of partial melting induced by the mantle wedge thermal structure. Although macrocrysts are not all in chemical equilibrium with their host rocks and were thus presumably unlocked from the zoned crystal mush and transported in the carrier melt, the bulk-rock trend follows both experimental liquid lines of descent and the chemical trend of calculated melts in equilibrium with amphibole (AEMs). These contradictory observations can be reconciled if minerals are transported in near cotectic proportions. The AEMs overlap the Daly gap revealing that the missing liquid compositions were present in the storage region. Geothermobarometers all indicate that the chemical diversity from basalt to dacite was acquired at a shallow depth (210-460 MPa). We suggest that differentiation from the primary magma to the parental basalt took place either in the same storage region or at the MOHO.

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

“…Despite the thinning of the continental crust towards the south, crustal assimilation, particularly at lower crustal depths, is still a relevant mechanism for intermediate magma production (e.g., Hildreth and Moorbath, 1988;McMillan et al, 1989). Calbuco volcano represents an unusual case, as it presents a stationary storage zone at intracrustal discontinuity depths (after Tassara and Echaurren, 2012), where the primary basaltic magmas fractionate to form andesites and eventually dacites prior to volatile saturation and eruption (Vander Auwera et al, 2021).…”