Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin
Clinopyroxene megacrysts (up to 5 cm) from a scoria cone on Marion Island, Antarctic Ocean are zoned, with compositionally distinct low (Al + Ti) and high (Al + Ti) patches arranged haphazardly throughout crystals. Inclusions of olivine, pyrrhotite, oxides, sulphides, and rounded inclusions with euhedral micro-crystals interpreted as former melt inclusions are observed. Olivine inclusions have variable compositions, ranging from primary Ti-poor crystals to Ti-rich crystals hosting secondary haematite crystals formed by hydrogenation. The crystals contain voids that are concentrated in the middle of each crystal indicating that the initial crystal growth was skeletal. Subsequent crystallisation filled in the skeletal framework creating the patchy zoning in the crystals. The Marion Island megacrysts are not homogenous, but the combination of crustal clinopyroxene compositions, primary and hydrogenated olivine, and the mode of eruption in scoria eruptions indicates that these crystals most likely formed in a shallow magma chamber. Primary olivines crystallised from a mafic magma and secondary altered olivines were incorporated into a rapidly growing megacryst in a super-saturated, fluid-rich environment, prior to being ejected onto surface in a scoria eruption.
KeywordsMarion Island Clinopyroxene megacrysts Late stage eruption Ankaramites
This paper represents the culmination of several years’ work, and has relied on the generosity of numerous people for analytical time and interpretation, especially the late Peter Gräser who performed the original EPMA analysis. David Evans, an anonymous reviewer, and associate editor Xisheng Xu are thanked for their comments and revisions on drafts of the paper. Thanks is also given to David Dixon for redrafting the map.
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