Cooling rates of pyroclastic deposits inferred from mineral magnetic investigations: a case study from the Pleistocene Mýtina Maar (Czech Republic)

0524297 - GFÚ 2021 RIV DE eng J - Journal Article
Lied, P. - Kontny, A. - Nowaczyk, N. - Mrlina, Jan - Kämpf, H.
Cooling rates of pyroclastic deposits inferred from mineral magnetic investigations: a case study from the Pleistocene Mýtina Maar (Czech Republic).
International Journal of Earth Sciences. Roč. 109, č. 5 (2020), s. 1707-1725. ISSN 1437-3254. E-ISSN 1437-3262
R&D Projects: GA MŠMT(CZ) LM2015079
Institutional support: RVO:67985530
Keywords : Eger rift system * maar-diatreme volcano * titanomagnetite * Curie temperature * cation ordering effect
OECD category: Volcanology
Impact factor: 2.523, year: 2020
Method of publishing: Open access
https://link.springer.com/article/10.1007/s00531-020-01865-1

Tephra layers of the Mýtina Maar, Czech Republic, contain ferrimagnetic Mg–Al-rich titanomagnetite, which is suggested to originate from a fractionated alkaline CO2-rich lithospheric mantle melt. We investigated the magnetic mineralogy and Curie temperature (TC) from tephra deposits of two drill cores (< 9 m depth). TC calculated (208 ± 14 °C) from chemical composition (Fe2+ 0.8Mg0.5Fe3+ 1.1Al0.3Ti0.3O4) is in accordance with TC retrieved from cooling curves of temperature-dependent magnetic susceptibility measurements (195–232 °C). However, thermomagnetic curves are irreversible either with lower (type I) or higher (type II) TC in the heating curve. All curves show transition temperatures above ca. 390 °C, indicating maghemitization. We interpret the irreversibility of TC (∆TC) in terms of different degrees of cation ordering, overprinted or masked by different degrees of maghemitization, which is a low-temperature phenomenon. Negative ∆TC indicates that original deposited titanomagnetite has cooled faster and, therefore, has stored a lower degree of cation ordering compared to heating/cooling rate of 11 °C/min in the Kappabridge. Type II with positive ∆TC indicates higher degree of cation ordering, and, therefore, slower cooling rate. The central part of this deposit shows most severe maghemitization, indicating rather wet emplacement. We, therefore, suggest different eruption styles for deposition of type I pyroclastics with more phreatomagmatic and type II pyroclastics with more phreato-Strombolian eruption styles. Our study is a new approach to discriminate different cooling histories in maar deposits using the Curie temperature of titanomagnetite. We suggest that this method has the potential to discriminate different emplacement modes resulting from different eruption styles.
Permanent Link: http://hdl.handle.net/11104/0308666