Abstract
The faces of volcanic phenocrysts may be marked by imperfections occurring as holes that penetrate the crystal interior. When filled with glass these features, called embayments or reentrants, have been used to petrologically constrain magmatic ascent rate. Embayment ascent speedometry relies on the record of disequilibrium preserved as diffusion-limited volatile concentration gradients in the embayment glass. Clear, glassy embayments are carefully selected for speedometry studies. The use and subsequent descriptions of pristine embayments overrepresent their actual abundance. Here, we provide a textural analysis of the number, morphology, and filling characteristics of quartz-hosted embayments. We target a collection of large (i.e., >20 km3 erupted volume) silicic eruptions, including the Bishop Tuff, Tuff of Bluff Point, Bandelier Tuff, Mesa Falls Tuff, and Huckleberry Ridge Tuff in the United States, Oruanui Tuff in New Zealand, Younger Toba Tuff in Indonesia, the Kos Plateau Tuff in Greece, and the Giant Pumice from La Primavera caldera in Mexico. For each unit, hundreds of quartz crystals were picked and the total number of embayment-hosting crystals were counted and categorized into classifications based on the vesicularity and morphology. We observed significant variability in embayment abundance, form, and vesicularity across different eruptions. Simple, cylindrical forms are the most common, as are dense glassy embayments. Increasingly complex shapes and a range of bubble textures are also common. Embayments may crosscut or deflect prominent internal cathodoluminescence banding in the host quartz, indicating that embayments form by both dissolution and growth. We propose potential additional timescales recorded by embayment disequilibrium textures, namely, faceting, bubbles, and the lack thereof. Embayment formation likely occurs tens to hundreds of years before eruption because embayment surfaces are rounded instead of faceted. Bubble textures in embayments are far from those predicted by equilibrium solubility. Homogenous nucleation conditions likely allow preservation of pressures much greater than magmastatic inside embayments. Our textural observations lend insight into embayment occurrence and formation and guide further embayment studies.
Highlights
Volcanic rocks preserve textures that reflect the dynamic magmatic and volcanic processes which lead to their formation, eruption, and preservation
We present the range of embayment morphologies and bubble textures within embayment glass
Pumice samples were acquired from the Bishop Tuff of Long Valley caldera, Tuff of Bluff Point, Mesa Falls Tuff, and Huckleberry Ridge Tuff from Yellowstone caldera, Tsankawi Pumice of the Bandelier Tuff from Valles caldera, each in the United States, Oruanui Tuff from Taupo caldera in New Zealand, Younger Toba Tuff from Toba caldera in Indonesia, the Kos Plateau Tuff from the island of Kos in Greece, and the Giant Pumice from La Primavera caldera in Mexico (Table 1)
Summary
Volcanic rocks preserve textures that reflect the dynamic magmatic and volcanic processes which lead to their formation, eruption, and preservation. The textures can be interpreted to infer processes in the subsurface that are not directly observable, including pre-eruptive magmatic storage, magmatic ascent, and fragmentation (e.g., Toramaru, 2006, 2008; Rutherford, 2008; Gardner et al, 2017). Some textures can be used as geospeedometers because they preserve information about the rates and timescales over which processes occur (e.g., Cashman, 1992; Pamukcu et al, 2015; Hajimirza et al, 2021). Glass-filled pockets in volcanic phenocrysts, called embayments or reentrants, have been exploited as a compositional, petrologic record of magmatic ascent, and have been increasingly applied as a geospeedometer for silicic and mafic systems (e.g., Liu et al, 2007; Humphreys et al, 2008; Lloyd et al, 2014; Ferguson et al, 2016; Myers et al, 2016, 2018, 2021; Moussallam et al, 2019; Newcombe et al, 2020). Embayments commonly occur in many minerals but most studies focus on quartz- or olivine-hosted embayments
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
