Through the impact glass: Insight into the evolution of melt at the Mistastin Lake impact structure

Abstract

By analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast-rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least-squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt-poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late-stage melt flows (group 1 glasses, occurring in meltbearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater). P. J. A. Hill et al. Impact glass at Mistastin Lake

The Mesoproterozoic Stac Fada Member, NW Scotland: an impact origin confirmed but refined

“…Semi-automatic digital image analysis was conducted to quantify the geometry of clasts in hand samples to compare them to previously collected and published data on hand samples of other glass-bearing breccias from confirmed and well-studied impact ejecta deposits at the Ries (Germany) and Mistastin Lake (Canada) impact structures, together with similar glassbearing breccias from the Sudbury (Canada; i.e., the Onaping Formation) and Chicxulub (Mexico) impact structures (Osinski et al 2016(Osinski et al , 2020Hill et al 2020). Following the methods of Chanou et al (2014), particles of interest (POI) (i.e., vitric particles) were segmented and measured using the National Instituted of Health's digital imaging freeware, ImageJ.…”

“…The MFCI model is consistent with the fact that Sudbury occurred in a marine setting and has been tested via comparison with volcanic MFCI deposits (Osinski et al 2016). Chicxulub impact meltbearing breccias share many of the same attributes as the Onaping Formation and an MFCI origin for those has also been recently proposed (Osinski et al 2020). It is worth mentioning that various origins have been proposed for suevite at the Ries impact structure, including the suggestion that MFCI played a role (Artemieva et al 2013;Stöffler et al 2013).…”

“…Here we provide additional documentation of shock features, further reinforcing the impact origin of material contained within the Stac Fada rocks. We offer, though, an alternative hypothesis for their genesis, one based on recent understanding of impactites associated with the Sudbury (Canada) and Chixulub (Mexico) impact structures, both of whose origins are attributed to molten-fuel-coolant interaction analogous to what occurs during phreatomagmatic volcanic eruptions (Grieve et al 2010;Osinski et al 2020).…”