Storage and evolution of mafic and intermediate alkaline magmas beneath ross Island, Antarctica

• Main Authors: Iacovino, K, Oppenheimer, C, Scaillet, B, Kyle, P
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press 2016
• Subjects: alkaline volcanism; basanite; CO₂; phase equilibria; phonotephrite; Ross Island; Lava Lake; Hut Point; Hut Point Peninsula; Antarc*; Antarctica
• Online Access: https://www.repository.cam.ac.uk/handle/1810/261735; https://doi.org/10.17863/CAM.6946

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-fO₂ 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 X_Η2O of the H₂O-CO₂ 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 (XH₂O^fluid <0·2) where it is replaced by clinopyroxene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050°C at 200-400MPa 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 % H₂O suggests extremely dry conditions for both lineages (XH₂O^fluid approaching zero for EL,∼0·25 for DVDP), probably facilitated by dehydration induced by a CO₂-rich fluid phase. Our results agree ...