Anthology of quartz-hosted embayment textures
Abstract
The faces of volcanic phenocrysts may be marked by defects that occur as irregular holes intruding into the crystal interior. These features, called embayments, have been leveraged as a powerful recorder of magmatic ascent rate. Embayment speedometry relies on a record of disequilibrium preserved in diffusion-limited volatile concentration gradients in glassy embayments. Volatile gradients are produced during magmatic decompression as volatiles diffusively leak from the pockets of imperfectly entrapped melt in response to reduced solubility. Clear, glassy embayments are carefully selected for speedometry. As such, descriptions of pristine embayments may be overrepresented in the literature. Here, we provide a textural analysis of the abundance, morphology, and filling characteristics of quartz-hosted embayments. We target a collection of rhyolitic tuffs, including the Bandelier, Bishop, Lava Creek, Bluff Point, and Younger Toba. For each unit ~1000 quartz were picked from gently crushed pumice. From that population, the total number of embayment-hosting crystals were counted and categorized into 4 classifications based on the vesicularity of the entrapped glass: dense glassy, single intruding bubble, discrete interior bubbles, or empty and void. We observed significant variability in embayment abundance and vesicularity across different eruptions. At Yellowstone, embayments are present in 73% and 41% of quartz from the Lava Creek and Bluff Point, respectively. Tuff of Bluff Point contains brown, discrete and partially decrepitated (29%) embayments with lesser glassy embayments (23%), and intruding bubbles (< 2%). ~30% of quartz from the Bishop Tuff have embayments; most are glassy (39%) but intruding (32%) and discrete (28%) bubbles are also common. Only 4% of quartz from the Younger Toba Tuff are embayment-bearing, with many of those containing an intruding bubble. ~82% of quartz from the Bandelier Tuff contain embayments, from which the glassy texture dominates (64%), followed by discrete (47%), and intruding (13%). Embayments are diverse. Continued research is exploring how those differences may be exploited to better understand magmatic processes.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV003.0002R
- Keywords:
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- 1033 Intra-plate processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY;
- 8440 Calderas;
- VOLCANOLOGY