Complex zoning in clinopyroxenes from Marion Island

Abstract

Euhedral clinopyroxene mega-crystals have been retrieved from Marion Island, a volcanic island situated on an inactive transform fault near the mid oceanic ridge on Antarctic tectonic plate and part of the Prince Edward Island group. The island is considered to be the product of hotspot- related volcanism. Clinopyroxene megacrysts were sampled from the southern side of the island on a scoria cone named Pyroxene Hill. Several analytical methods including Energy Dispersive Spectrometry (EDS), Electron Probe Microanalysis (EPMA) and EPMA-mapping were utilized in investigating the zoning found within these 5-10 cm large crystals. The zoning found was “patchy”, in that it did not conform to commonly described zoning such as normal, reverse, sector or oscillatory, but rather consisted of chemically distinct areas with either diffuse or sharp boundaries, not orientated parallel to the grain boundaries or crystal lattice. The chemistry of the crystals indicates that they have formed from an evolved basaltic melt, and are likely to have crystallised at a depth of 15- 30 km. A model is postulated for the formation of these crystals in which the megacrysts crystallise rapidly from a supersaturated melt. Pre-existing crystalline material undergoes imperfect diffusion at high temperature to create a patchwork of compositional zones. Supersaturation likely requires a volatile-rich melt, which undergoes rapid degassing owing to an external trigger. The presence of a transform fault directly below Marion Island may provide a seismic trigger for such a degassing event.