Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin

R. James Roberts, Keabetswe D. Lehong, Andries E.J. Botha, Gelu Costin, Frikkie C. De Beer, Willem J. Hoffman, Callum J. Hetherington

Research output: Contribution to journalArticlepeer-review

4 Scopus citations


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.

Original languageEnglish
Pages (from-to)155-167
Number of pages13
JournalMineralogy and Petrology
Issue number2
StatePublished - Apr 4 2019


  • Ankaramites
  • Clinopyroxene megacrysts
  • Late stage eruption
  • Marion Island


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