Bulletin of Volcanology

, Volume 73, Issue 10, pp 1513–1533 | Cite as

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

  • Heather M. N. WrightEmail author
  • Chris B. Folkes
  • Raymond A. F. Cas
  • Katharine V. Cashman
Research Article


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 SiO2 content and 0.03 wt.% higher TiO2 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.% SiO2), 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.% SiO2). 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 cryptic silicic recharge in a large caldera system but also a probable trigger for the eruption.


Caldera Pumice textures Shear Phenocryst fragments Magma ascent Silicic recharge 



This work was funded by ARC grant DP0663560 to Cas. The authors would like to thank Jake Lowenstern, editor John Stix, and an anonymous reviewer for helpful suggestions to improve the manuscript.

Supplementary material

445_2011_525_MOESM1_ESM.pdf (7 kb)
Supplementary Figure 1 Table of analyzed melt inclusion dimensions, bubble and crystal contents within melt inclusions, and measured volatile contents. (PDF 6 kb)
445_2011_525_MOESM2_ESM.xls (58 kb)
Supplementary Table 1 All analyzed bulk compositions of white and grey pumice, including XRF and ICPMS results. Major elements are normalized to 100% on a volatile-free basis. (XLS 57 kb)


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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Heather M. N. Wright
    • 1
    • 2
    Email author
  • Chris B. Folkes
    • 1
  • Raymond A. F. Cas
    • 1
  • Katharine V. Cashman
    • 3
  1. 1.School of GeosciencesMonash UniversityClaytonAustralia
  2. 2.U.S. Geological SurveyMenlo ParkUSA
  3. 3.Department of Geological SciencesUniversity of OregonEugeneUSA

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