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Crystal fragmentation inducing euhedral crystal habits in volcanic rocks: the fracture history of crystals from various tectonomagmatic settings

Georg F. Zellmer1 , Yoshiyuki Iizuka2

  • Affiliations: 1Volcanic Risk Solutions, SAE, Massey University, Palmerston North, New Zealand; 2Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 226

  • Lightning talk:

  • Programme No: 1.1.12

  • Theme 1 > Session 1

Abstract

Fracturing of crystals in volcanic rocks is a phenomenon that has been widely recognized. However, the history of repeated crystal fragmentation as recorded by the phenocrysts carried in volcanic rocks is yet to be considered. We provide examples from hot spot, arc, back-arc and ocean ridge settings indicating that crystals often display linear fractures, some following cleavage planes, and show that elemental mapping is useful to identify such fractures. We provide evidence of cryptic zoning and fracture annealing. Crystal fragmentation appears to be a fundamental and recurrent process operating in magmatic systems, impacting both internal crystal zoning patterns as well as their final crystal habit in erupted rock samples. Euhedral crystal habits can be the result of edge fracturing along cleavage planes, rather than unhindered crystal growth within a melt phase. How common such edge fractures are in generating euhedral crystals will require a larger dataset and statistical evaluations that are beyond the scope of this pilot study. Such studies will be required to evaluate if crystal fracturing impacts the application of chronological methods, including geospeedometric and crystal size distribution studies directed at volcanic hazard mitigation. Either way, the use of advanced imaging techniques to decipher pre-eruptive processes remains paramount.