A widespread compositionally bimodal tephra sourced from Volcán Melimoyu (44°S, Northern Patagonian Andes):Insights into magmatic reservoir processes and opportunities for regional correlation

Authors Organisations
  • C. A. Geoffroy(Author)
    Universidad de Chile
  • B. V. Alloway(Author)
    University of Auckland
    University of Wollongong
  • À. Amigo(Author)
    Universidad de Chile
    Red Nacional de Vigilancia Volcánica
  • M. A. Parada(Author)
    Universidad de Chile
  • F. Gutierrez(Author)
  • A. Castruccio(Author)
    Universidad de Chile
  • Nick Pearce(Author)
  • E. Morgado(Author)
    Universidad de Chile
    University of Leeds
  • P. I. Moreno(Author)
    Universidad de Chile
Type Article
Original languageEnglish
Pages (from-to)141-159
Number of pages17
JournalQuaternary Science Reviews
Early online date08 Oct 2018
Publication statusPublished - 15 Nov 2018
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We describe the stratigraphy, age, constituent geochemistry and phenocryst thermobarometry of a closely spaced Holocene tephra couplet from Volcán Melimoyu (VMm), located in the Northern Patagonian Andes. The lower tephra unit (La Junta Tephra, Mm-1) is distinctly banded comprising a dominant lower layer of rhyodacitic (∼70% SiO2) pumiceous ash and lapilli (Mm-1p) that abruptly transitions to a subordinate upper scoriaceous layer (Mm-1s) of basaltic andesite composition (∼53% SiO2). This bimodality within Mm-1 contrasts significantly with the closely overlying Santa Ana Tephra (Mm-2) that has a homogeneous trachyte-dacite (∼63% SiO2) composition and is intermediate between the two magmatic end members of Mm-1. We propose a genetic affiliation between Mm-1 and Mm-2, and that the latter event likely represents a hybridised-remnant of those discrete magmas involved in the earlier Mm-1 eruption. To test this hypothesis we applied whole rock elemental mixing and fractional crystallisation model to reproduce the composition and crystallinity of Mm-2. Results indicate that Mm-2 can be reproduced by mixing ∼70% Mm-1p with ∼30% Mm-1s, with subsequent ∼13% fractional crystallisation of plagioclase, and minor amphibole, orthopyroxene, magnetite and biotite.

Equilibrium P-T conditions calculated from Mm-1p phenocrysts point towards magma residency at moderately shallow depths (200–290 MPa, ∼7–10-km depth, 850–1000 °C), whereas Mm-1s phenocrysts indicate higher overall P-T conditions (240–480 MPa, ∼8.5–17-km depth, 1080–1150 °C). P-T conditions determined for Mm-2 (∼290 MPa, ∼10-km depth, 930–1000 °C) are similar to those of Mm-1p. There is no physical and/or geochemical evidence of mafic magma involvement in the Mm-2 eruption.

Similar compositionally bimodal tephra are known from other Northern Patagonian Andean centres (i.e. Playas Blanca-Negra Tephra, Antillanca; Lepue Tephra, Michimahuida; Ho and H3 eruptions of Hudson) suggests that the intrusion of mafic magma into more silicic magma bodies is a common occurrence throughout this Andean sector. These widely dispersed, compositionally bimodal tephra not only provide key insights into pre-eruptive magmatic conditions and triggering processes, but can also be readily identified geochemically, and thereby be more fully utilised within future hazard- and paleoenvironmental-related studies


  • Volcán Melimoyu, Andean southern volcanic zone, Northern Patagonia, biomodal tephra, magma bodies, zonation, magma mingling, eruption triggering