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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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.
KeywordsIgnimbrite emplacement temperature Thermal remanent magnetization Charcoal reflectance Fogo A eruption sequence
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.
- Burden RE, Chen L, Phillips JC (2013) A statistical method for determining the volume of volcanic fall deposits. Bulletin of Volcanology 75(6):1–10Google Scholar
- Capaccioni B, Forjaz VH, Martini M (1994) Pyroclastic flow hazard at Agua de Pau Volcano (São Miguel Island, Azores Archipelago) inferred from the Fogo A eruptive unit. Acta Vulcanol 5:41–48Google Scholar
- Chadima M, Hrouda F (2006) Remasoft 3.0 a user-friendly paleomagnetic data browser and analyser. Travaux Geophysique XXVII:20-21Google Scholar
- Cioni R, Gurioli L, Zannella E (2004) Temperatures of the A.D. 79 pyroclastic density current deposits (Vesuvius, Italy). J Geophys Res 109(B02):1–18Google Scholar
- Correia M, Maury R, Arai F (1974) Mesure par leur Pouvoir Refecteur, des temperatures de carbonisation des bois fossilises dans les formations volcaniques. Bull du Centre de Recherches de Pau 8(2):527–536Google Scholar
- Di Vito MA, Zanella E, Gurioli L, Lanza R, Sulpizio R, Bishop J, Tema E, Boenzi G, Laforgia E (2009) The Afragola settlement near Vesuvius, Italy: the destruction and abandonment of a Bronze Age village revealed by archaeology, volcanology and rock-magnetism. Earth Planet Sci Lett 277(3–4):408–421CrossRefGoogle Scholar
- Jones TP, Scott AC, Cope MJ (1991) Reflectance measurements against temperature of formation for modern charcoals and their implications for the study of fusain. Bull Soc Géol Fr 162:193–200Google Scholar
- McClelland EA, Thomas R (1990) A palaeomagnetic study of Minoan age tephra from Thera. In: Hardy D (ed) Thera and the Aegean World III. The Theran Foundation, London, pp 129–138Google Scholar
- Ort MH, Rosi M, Anderson CD (1999) Correlation of deposits and vent locations of the proximal Campanian Ignimbrite deposits, Campi Flegrei, Italy, based on natural remanent magnetization and anisotropy of magnetic susceptibility characteristics. J Volcanol and Geotherm Res 91(2–4):167–178CrossRefGoogle Scholar
- Wallenstein N (1999) Estudo da história recente e do comportamento eruptivo do vulcão do Fogo (S.Miguel, Açores). A valiação preliminar do hazard. PHD thesis (not published) Departamento de Geociências Universidades dos Açores, São Miguel Island (Portugal)Google Scholar