Biased Witnesses: Crystal Thermal Records May Give Conflicting Accounts of Magma Cooling. Issue 5 (4th May 2022)

• Record Type: Journal Article
• Authors: Culha, C.; Keller, T.; Suckale, J.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-05-04
• Subjects: reactive -- crystal‐driven convection -- crystal records -- fluid dynamics -- thermodynamics -- downwelling; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023530
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Crystals retain an imprint of the dynamic changes within a magma reservoir and hence contain invaluable information about the evolving conditions inside volcanic plumbing systems. However, instead of telling a single, simple story, they comprise overprinted evidence of numerous processes relating to temperature, pressure and composition that drive crystal precipitation and dissolution in magmatic systems. To decipher these different elements in the story that crystals tell, we attempt to identify the observational signatures of a simple, yet ubiquitous process: crystal precipitation and dissolution during magma cooling. To isolate this process in a complex magmatic system with intricate dynamic feedbacks, we assume that synthetic crystals precipitate and dissolve rapidly in response to deviations from thermodynamic equilibrium. In our crystalline‐scale simulations, synthetic crystals drag along the cooler‐than‐ambient melt in which they precipitated and can drive a temperature‐dependent, crystal‐driven convection. We analyze the non‐dimensional conditions for this coupled convection and record the heterogeneous thermal histories that synthetic crystals in this flow regime experience. We show that many synthetic crystals dissolve, loosing their thermal record of the convection. Based on our findings, we suggest that heterogeneity in the thermal history of crystals is more indicative of local, crystal‐scale processes than the overall, system‐wide cooling trend. Plain Language Summary: Similar to tree rings, crystals tell a story of their past through their sequential growth records. However, unlike trees, crystals are dynamic and can travel throughout the magmatic domain. We hypothesize that this mobility may result in a bias in the crystal record and prevent it from accurately recording the magma reservoir history. In order to understand what patterns natural sample crystals would record during cooling, we built a simulator that tracks individual crystals and the thermal environment they experience. We simulate how crystals drive flow, but also precipitate and dissolve in response to temperature change. Our results show that each synthetic crystal has a unique record of its past that is dependent on the temperature distribution within its vicinity. Additionally, most of the synthetic crystals dissolve in the convection, so we lose their record. Overall, the synthetic crystals in our simulation can reproduce some of the variable patterns that are often found in natural samples, suggesting that real crystals are imperfect witnesses of the magmatic dynamics they are exposed to. Key Points: Crystals in convection preserve the least information about the most dynamic flow domain Crystals in convection show signs of dissolution, suggesting a slower than actual cooling rate Crystals can record cooling, heating, and complex thermal histories even during steady cooling