Mantle to surface degassing of alkalic magmas at Erebus volcano, Antarctica
Research highlights
► A conceptual model to explain magma differentiation and degassing at Erebus volcano. ► Model based on melt inclusion analyses and FTIR measurements of the plume. ► Isobaric fractionation of magmas results from deep CO2 fluxing. ► Redox signatures discriminate deep versus shallow magmatic processes. ► Results of broad relevance to understanding of intraplate alkaline volcanism.
Introduction
Erebus, the world's southernmost active volcano, offers an exceptional opportunity to examine the complexities that accompany degassing and evolution of magmas from deep to shallow levels. These include generic aspects of magmatic differentiation, redox evolution and eruptive transitions that are widely relevant to understanding other volcanic systems. Its long-lived anorthoclase phonolite lava lake has sustained emission of magmatic gases to the atmosphere for decades (Giggenbach et al., 1973) providing especially favourable circumstances for direct measurement of the magmatic system. Emissions rates of SO2 have been measured by ultraviolet absorption spectroscopy since 1983 (Sweeney et al., 2008) and the relative abundances of many constituents of the plume (including gas and aerosol phases) have been measured using sampling equipment sited on the crater rim (Zreda-Gostynska et al., 1997) and open-path Fourier transform infrared (FTIR) spectroscopy (Oppenheimer and Kyle, 2008).
High emission rates of CO2 from both the crater and a number of fumarolic ice towers scattered around the summit flanks have also been measured (Wardell et al., 2004, Werner and Brantley, 2003), revealing the presence of a CO2-rich magma reservoir. The adjacent basaltic volcanic centres of Mt. Bird, Mt. Terror and Hut Point Peninsula on Ross Island are radially distributed at 120° around Erebus (Kyle et al., 1992), indicating generation of large volumes of parental basanite beneath Erebus, at the base of the thin (~ 20 km; Bannister et al., 2003) crust on which it is built. This supports the hypothesis that large quantities of mantle are required to account for the voluminous production of anorthoclase phonolite by differentiation (Kyle et al., 1992) and requires mantle upwelling, either as a ≥ 40-km wide plume (Kyle et al., 1992) or via decompression melting of (metasomatised) mantle during intraplate strike-slip tectonics (Rocchi et al., 2002, Rocchi et al., 2003, Rocchi et al., 2005).
One feature that makes Erebus particularly interesting from a degassing perspective is the evidence for extensive differentiation of magmas. Lavas on the upper parts of Erebus volcano are composed predominantly of anorthoclase-phyric tephriphonolite and phonolite compositions (Kyle et al., 1992). Indeed, those erupted during the last 17 ka have a remarkably stable bulk phonolitic composition (Kelly et al., 2008). However, basanite, phonotephrite, and plagioclase-bearing tephriphonolite lavas crop out in eroded volcanic cones that form islands and sea cliffs on the southwest side of Erebus volcano and Hut Point Peninsula (Kyle et al., 1992, Moore and Kyle, 1987). Collectively, the lavas define a continuous fractionation series (referred to as the Erebus Lineage) from basanite to phonolite (Kyle et al., 1992).
Our principal aim here is to develop a coherent model for the magmatic plumbing system at Erebus. We analyse two principal datasets: surface observations of explosive versus passive degassing from the active lava lake, and the compositions and volatile contents of melt inclusions from a suite of Erebus Lineage and related rocks ranging from basanite to recently erupted anorthoclase phonolite bombs. The inclusion compositions mimic their associated whole rock chemistry, and document the evolution of the magmas. In light of the magmatic origin of CO2 emitted from Erebus (Werner and Brantley, 2003), we use the pre-eruptive volatile content of this fractional-crystallisation sequence to build a framework for understanding the CO2-dominated degassing of the magmatic system, and to interpret the composition of the emitted gas plume. Coupling of both deep and shallow signatures enables us to track volatile release right down to the lower lithospheric source of Erebus and its related volcanic centres. This provides valuable insights into the extent of degassing and its relationship with parental magmas at one of the world's most active intraplate alkaline volcanoes.
Section snippets
Samples
Samples for melt inclusion study were collected from Dry Valley Drilling Project (DVDP) cores extracted at Hut Point Peninsula (basanite), and from exposures on Inaccessible and Tent Islands (tephriphonolite), Turks Head (basanite, phonotephrite, tephriphonolite) and the summit of Erebus volcano (phonolite) (Moore and Kyle, 1987). A map of locations can be found in the Supplementary Material (Fig. 1). Samples DVDP 3–283, DVDP 3–295, AW82033, 7713, 97009, 97010, and 97011 are rapidly-quenched
Lava lake degassing
The complex degassing of Erebus is characterised by passive emissions from its persistent lava lake, which are sporadically perturbed by Strombolian eruptions. The explosions are associated with low frequency seismic signals whose source centroid lies less than ~ 400 m below, and several hundred metres west and north, of the lava lake surface (Aster et al., 2008). Passive degassing has been interpreted as a two-component mixing of a CO2-rich (~ 40 mol%; mass CO2gas/H2Ogas = 1.8) “conduit gas” that
Discussion
Here we evaluate and synthesise the evidence from both melt inclusions and surface degassing observations to build a new conceptual model for the magmatic plumbing system at Erebus.
Summary and conclusions
We have reconciled volatile data representing the entire plumbing system of Erebus volcano, and of its associated older and peripheral volcanic centres, with observed gas emissions from the active lava lake. We have shown that deep gas transfer characterises the shallow activity, possibly explaining the observed eruptive style, switching from passive degassing to intermittent Strombolian explosions in which the deep and more oxidised gas component is discharged. Two mechanisms (bubble-only
Acknowledgements
Fieldwork was supported by grantsANT-0538414 and ANT-0838817 from the Office of Polar Programs (National Science Foundation). CO thanks the Leverhulme Trust for a Fellowship, and the European Research Council (DEMONS project) and NERC National Centre for Earth Observation for funding. Le Studium receives support from the European Regional Development Fund. RM acknowledges financial support from PRIN 2007 funds of the Ministry of University and Research of the Italian Government. We thank Bruno
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