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

Philipp Lied,Agnes Kontny,Norbert Nowaczyk,Jan Mrlina,Horst Kämpf

doi:10.1007/s00531-020-01865-1

Abstract

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.

Highlights

Maar-diatreme volcanoes are the product of a series of small-volume subsurface phreatic, phreatomagmatic, and/ or phreato-Strombolian eruptions (Lorenz and Kurszlaukis 2007; Ross and White 2012; Németh and Kereszturi 2015; Valentine et al 2017; Ort et al 2018)
Fragments of juvenile magmatic material were microscopically not observed in these layers, which is in agreement with susceptibility measurements and geochemical studies (Flechsig et al 2015)
Our results reveal Mg–Al-rich titanomagnetites as the main ferrimagnetic oxide phases in the phreatomagmatic tephra deposits of the Pleistocene Mýtina Maar located in the western Eger Rift area

Summary

Introduction

Maar-diatreme volcanoes are the product of a series of small-volume subsurface phreatic, phreatomagmatic, and/ or phreato-Strombolian eruptions (Lorenz and Kurszlaukis 2007; Ross and White 2012; Németh and Kereszturi 2015; Valentine et al 2017; Ort et al 2018). One example is the 1977 Ukinrek Maar eruption showing a range from 805 °C in deposits of the phreato-Strombolian eruptive phase to > 100 °C in deposits of the phreatomagmatic eruptive phase (Kienle et al 1980; Ort et al 2018). This observation implies that—with a given chemistry of the erupting material—several factors like, e.g., the expansion of gases, a rapid deposition of material, or the distance of deposition to the source vent play a significant role for the emplacement temperature

Results

Discussion

Conclusion