Inclusions in clinopyroxene megacrysts from Marion Island

Abstract

Marion Island is a sub-Antarctic island which lies 300 km south of the South West Indian Ridge. The island is extensively affected by two periods of volcanism with the oldest erupting during the Pleistocene and the younger during the Holocene period. This study focused on inclusions present within several enormous megacrysts retrieved from a scoria cone on Marion, named Pyroxene Hill. Microfocus CT, electron microprobe, and Raman spectroscopy are used to identify all the mineral phases present within the clinopyroxene megacrysts as well as to determine their textural relationships in order to visualise their internal features without obliterating any of these rare megacrysts. Inclusions of olivine, Fe-Ti-Al-Cr oxides (magnetite, chromite, and ilmenite), rhomb oxides (hematite) and sulphides were identified. Amphibole was also identified as secondary product. Their textural relationships are used to determine the major magmatic processes and formation mechanisms influencing their formation and preservation. Dehydrogenation-oxidation process was found to be the primary magmatic process driving the formation of mineral phases present. Water was also identified as a crucial phase required in the magmatic system to accelerate rapid growth of the clinopyroxene megacryst.