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Calibrating the pTRM and charcoal reflectance (Ro%) methods to determine the emplacement temperature of ignimbrites: Fogo A sequence, São Miguel, Azores, Portugal, as a case study

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Abstract

The emplacement temperatures of three ignimbrites belonging to the 4.6-ka Fogo A plinian eruption sequence in São Miguel Island (Azores, Portugal) were determined using partial thermal remanent magnetization (pTRM) of lithic clasts and reflectance (Ro%) of charcoal fragments embedded within the deposits and collected at the same localities close to each other. The Fogo A sequence is characterised by a complex stratigraphy consisting of a thick plinian deposit interbedded with two intraplinian ignimbrites (here named “pink” and “black” intraplinian ignimbrite, respectively) and capped by a final ignimbrite (here named “dark brown” ignimbrite). A total of 140 oriented lithic clasts from the three ignimbrites were collected from 15 localities distributed along the northern and southern flanks of the volcano. The pTRM analyses show different paleomagnetic behaviours, which correspond to different emplacement temperatures of the ignimbrites. The emplacement temperatures of the pink and black intraplinian ignimbrites inferred from pTRM analysis were respectively ≥400 and ≥600 °C; the temperatures of the dark brown ignimbrite are lower, estimated between 300 and 350 °C. Thermal estimations of three key sites were compared with the results of the analysis of reflectance (Ro%) measured on eight specimens derived from charcoal fragments collected from the pink intraplinian ignimbrite and the dark brown ignimbrite. Results indicate Ro% values between 1.61 and 1.37 for the pink intraplinian ignimbrite, whereas fragments collected from the dark brown ignimbrite show Ro% values between 0.85 and 0.50. No charred wood was found in the black intraplinian ignimbrite. Ro% values indicate that charcoal fragments in the pink intraplinian ignimbrite reached temperatures of 380–460 °C, whereas the Ro% values of the dark brown ignimbrite indicate slightly lower temperatures of 330–350 °C. TRM and Ro% results are comparable and validate the use of both methods. Greatest accuracy in the determination of emplacement temperatures of ignimbrites is achieved when both methods can be applied at the same locations.

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Acknowledgments

We would like to thank Roma Tre University for use of the paleomagnetic and reflectance facilities and CVARG Centre, University of Azores, São Miguel, for the kind hospitality during the field work. We particularly thank Prof. N. Wallenstein for useful indications about the stratigraphy of Fogo A and Prof. M. Mattei and Dr. F. Cifelli for their support for paleomagnetic measurements. We are also grateful to the reviewers of this manuscript (M. Ort, J. Hower and Anonymous Reviewer) as well as the editor (S. Fagents) for the detailed and constructive comments, which have resulted in a much improved paper. This research forms part of the PhD research of A. Pensa at Monash University, supported by discretionary research funds of Emeritus Professor R.A.F. Cas.

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Authors and Affiliations

  1. Monash University, Wellington road, 3800, Clayton, VIC, Australia

    Alessandra Pensa & Raymond Cas

  2. Department of Physics and Geology, University of Perugia, Via Innamorati, 7, 06123, Perugia, Italy

    Massimiliano Porreca

  3. Roma Tre University, Largo L. Murialdo 1, 00146, Rome, Italy

    Sveva Corrado & Guido Giordano

  4. Centro de Vulcanologia e Avaliação de Riscos Geológicos (CVARG), Departamento de Geociências, Universidade dos Açores, 9500-801, Ponta Delgada, Portugal

    Massimiliano Porreca

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  1. Alessandra Pensa
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Correspondence to Alessandra Pensa.

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Editorial responsibility: S. A. Fagents

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Pensa, A., Porreca, M., Corrado, S. et al. Calibrating the pTRM and charcoal reflectance (Ro%) methods to determine the emplacement temperature of ignimbrites: Fogo A sequence, São Miguel, Azores, Portugal, as a case study. Bull Volcanol 77, 18 (2015). https://doi.org/10.1007/s00445-015-0904-4

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