Heterogeneous pumice populations in the 2.08-Ma Cerro Galán Ignimbrite: implications for magma recharge and ascent preceding a large-volume silicic eruption

Abstract

Triggering mechanisms of large silicic eruptions remain a critical unsolved problem. We address this question for the~2.08-Ma caldera-forming eruption of Cerro Galán volcano, Argentina, which produced distinct pumice populations of two colors: grey (5%) and white (95%) that we believe may hold clues to the onset of eruptive activity. We demonstrate that the color variations correspond to both textural and compositional variations between the clast types. Both pumice types have bulk compositions of high-K, high-silica dacite to low-silica rhyolite, but there are sufficient compositional differences (e.g.,~150 ppm lower Ba at equivalent SiO 2 content and 0.03 wt.% higher TiO 2 in white pumice than grey) to suggest that the two pumice populations are not related by simple fractionation. Trace element concentrations in crystals mimic bulk variations between clast types, with grey pumice containing elevated Ba, Cu, Pb, and Zn concentrations in both bulk samples (average Cu, Pb, and Zn concentrations are 27, 35, and 82 in grey pumice vs. 11, 19, and 60 in white pumice) and biotite phenocrysts and white pumice showing elevated Li concentrations in biotite and plagioclase phenocrysts. White and grey clasts are also texturally distinct: White pumice clasts contain abundant phenocrysts (44-57%), lack microlites, and have highly evolved groundmass glass compositions (76.4-79.6 wt.% SiO 2 ), whereas grey pumice clasts contain a lower percentage of phenocrysts/microphenocrysts (35-49%), have abundant microlites, and have less evolved groundmass glass compositions (69.4-73.8 wt.% SiO 2 ). There is also evidence for crystal transfer between magma producing white and grey pumice. Thin highly evolved melt rims surround some fragmental crystals in grey pumice clasts and appear to have come from magma that produced white pumice. Furthermore, based on crystal compositions, white bands within banded pumice contain crystals originating in grey magma. Finally, only grey pumice clasts form breadcrusted surface textures. We interpret these compositional and textural variations to indicate distinct magma batches, where grey pumice originated from an originally deeper, more volatile-rich dacite recharge magma that ascended through and mingled with the volumetrically dominant, more highly crystalline chamber that produced white pumice. Shortly before eruption, the grey pumice magma stalled within shallow fractures, forming a vanguard magma phase whose ascent may have provided a trigger for eruption of the highly crystalline rhyodacite magma. We suggest that in the case of the Cerro Galán eruption, grey pumice provides evidence not only for Editorial responsibility: J. Stix This paper constitutes part of a special issue: Cas RAF, Cashman K (eds) The Cerro Galan Ignimbrite and Caldera: characteristics and origins of a very large-volume ignimbrite and its magma system. Electronic supplementary materialThe online version of this article (cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.

Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina

“…For example, in the phase diagram for the dacitic Fish Canyon Tuff, sanidine defines a negative Clapeyron curve such that it will begin to crystallize as pressure and temperature decrease (Johnson and Rutherford 1989b). Preliminary data from melt inclusions trapped within quartz crystals show no systematic difference in water contents across the temporal history of the Galán ignimbrites (all ignimbrites generally contain a range of 2-5 wt.% H 2 O; Wright et al 2011;H. Wright, unpublished data).…”

“…Wright et al (2011) propose that the deeper, grey pumice magma in the CGI provides evidence of a silicic recharge body. Rising magma, potentially "recharge" grey magma from intermediate depths, could have also provided the heat and volatiles necessary to trigger eruption from these shallow chambers at Cerro Galán.…”

Causes of fragmented crystals in ignimbrites: a case study of the Cardones ignimbrite, Northern Chile

“…Klug and Cashman 1996;Rust and Cashman 2011). In crystal-rich ignimbrites, the melt-based vesicularity is 50-75% (Bachmann et al 2000;Lindsay et al 2001;Wright et al 2011). Although the upper boundary is comparable to that of crystal-poor pumice, the lower end suggests that high crystal contents may alter critical vesicularity thresholds for fragmentation.…”

“…Unusually dense pumice clasts might be expected in shallower parts of the reservoir, where magma experienced the smallest pressure gradient and also by analogy with other large ignimbrite deposits, some pre-eruptive degassing (and thus the least bubble growth) (e.g. Wright et al 2011). …”

“…ignimbrites; ash-flow tuffs) associated with large calderaforming eruptions. Ignimbrites with broken crystals include: many in the Great Basin, USA (Best and Christiansen 1997 and references therein); the Fish Canyon Tuff, USA (Lipman et al 1997); the Cerro Galan ignimbrite, Argentina (Wright et al 2011); Bishop Tuff, USA (Pamukcu et al 2012;Bindeman 2005); the Huckleberry Ridge Tuff, USA (Bindeman 2005); the Ora ignimbrite, Italy (Willcock et al 2015); and the Cardones ignimbrite, Chile (van Zalinge et al 2016a). Broken crystals have also been documented in smaller explosive eruptions (Bindeman 2005;Williamson et al 2010) and lavas (e.g., Allen and McPhie 2003;Cordonnier et al 2009).…”