Abstract
We present the results of phase equilibrium experiments carried out on basanite and phonotephrite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P–T–X–f O 2 conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000–1150°C) and pressures (200–400 MPa), under oxidized conditions (NNO to NNO + 3, where NNO is the nickel–nickel oxide buffer), with XH2O of the H 2 O–CO 2 mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM – 1, where QFM is the quartz–fayalite–magnetite buffer) in the Erebus lava lake. In basanite, crystallization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below ∼1050°C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content ( X H 2 O fluid < 0·2) where it is replaced by clinopyroxene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050°C at 200–400 MPa and NNO + 1·5 to NNO + 2. Olivine- and clinopyroxene-bearing EL lavas are consistent with experiments performed above 1050°C and pressures around 200 MPa. The plagioclase liquidus at <1–2 wt % H 2 O suggests extremely dry conditions for both lineages ( X H 2 O fluid approaching zero for EL, ∼0·25 for DVDP), probably facilitated by dehydration induced by a CO 2 -rich fluid phase. Our results agree with previous studies that suggest a single plumbing system beneath Ross Island in which DVDP lavas (and probably other peripheral volcanic products) were erupted through radial fractures associated with the ascent of parental magma into the lower crust. The longer travel time of the DVDP lavas through the crust owing to lateral movement along fractures and the lack of a direct, sustained connection to the continuous CO 2 -rich gas flux that characterizes the main central Erebus conduit is probably responsible for the lower temperatures and slightly wetter conditions and hence the change in mineralogy observed.
Highlights
Ross Island, Antarctica (7732’S, 16710’E) is part of the Erebus Volcanic Province (EVP) and comprises five predominant volcanic centers
Our results are consistent with the presence of a CO2-rich fluid in the Erebus Lineage (EL) plumbing system but not in the Dry Valley Drilling Project (DVDP). Given that both lineages are thought to be derived from the same parental melt (Kyle et al, 1992), we suggest that a sustained connection of the EL plumbing system to its deep mantle source and a subsequent CO2-rich fluid flux that characterizes the EL is responsible for the differences in storage conditions and mineralogy of the two lava suites
The appearance of kaersutitic amphibole in natural intermediate and evolved DVDP lavas indicates an elevated water activity for the DVDP plumbing system compared with the EL, which does not contain kaersutite
Summary
Ross Island, Antarctica (7732’S, 16710’E) is part of the Erebus Volcanic Province (EVP) and comprises five predominant volcanic centers. Recent investigations employing experimental petrological work (Moussallam et al, 2013) and seismic tomography (Zandomeneghi et al, 2013) have placed tight constraints on the physical and petrological properties of the shallow plumbing system and phonolitic lava lake including its geometry, temperature, pressure, and oxygen fugacity. These studies have the potential to refine the results of numerical models developed for the shallow Erebus system, such as those of Burgisser et al (2012) and Molina et al (2012). Understanding the deep system at Erebus is crucial because the constant supply of deeply derived CO2-rich gases combined with a sustained energy and mass input into the lava lake suggests a direct link between the phonolitic lava lake and the mantle source of the parental magmas via a deep mafic plumbing system (Kyle et al, 1992; Oppenheimer & Kyle, 2008; Oppenheimer et al, 2011)
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