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

Alessandra Pensa, Massimiliano Porreca, Sveva Corrado, Guido Giordano, Raymond Cas

Research output: Contribution to journalArticleResearchpeer-review

10 Citations (Scopus)

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.
Original languageEnglish
Article number18
Pages (from-to)1-19
Number of pages19
JournalBulletin of Volcanology
Volume77
Issue number3
DOIs
Publication statusPublished - 2015

Keywords

  • Charcoal reflectance
  • Fogo A eruption sequence
  • Ignimbrite emplacement temperature
  • Thermal remanent magnetization

Cite this

@article{9efbea76ed9d4d7297ce75ecc557a352,
title = "Calibrating the pTRM and charcoal reflectance (Ro{\%}) methods to determine the emplacement temperature of ignimbrites: Fogo A sequence, Sao Miguel, Azores, Portugal, as a case study",
abstract = "The emplacement temperatures of three ignimbrites belonging to the 4.6-ka Fogo A plinian eruption sequence in S{\~a}o 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.",
keywords = "Charcoal reflectance, Fogo A eruption sequence, Ignimbrite emplacement temperature, Thermal remanent magnetization",
author = "Alessandra Pensa and Massimiliano Porreca and Sveva Corrado and Guido Giordano and Raymond Cas",
year = "2015",
doi = "10.1007/s00445-015-0904-4",
language = "English",
volume = "77",
pages = "1--19",
journal = "Bulletin of Volcanology",
issn = "0258-8900",
publisher = "Springer-Verlag London Ltd.",
number = "3",

}

Calibrating the pTRM and charcoal reflectance (Ro%) methods to determine the emplacement temperature of ignimbrites: Fogo A sequence, Sao Miguel, Azores, Portugal, as a case study. / Pensa, Alessandra; Porreca, Massimiliano; Corrado, Sveva; Giordano, Guido; Cas, Raymond.

In: Bulletin of Volcanology, Vol. 77, No. 3, 18, 2015, p. 1-19.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Calibrating the pTRM and charcoal reflectance (Ro%) methods to determine the emplacement temperature of ignimbrites: Fogo A sequence, Sao Miguel, Azores, Portugal, as a case study

AU - Pensa, Alessandra

AU - Porreca, Massimiliano

AU - Corrado, Sveva

AU - Giordano, Guido

AU - Cas, Raymond

PY - 2015

Y1 - 2015

N2 - 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.

AB - 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.

KW - Charcoal reflectance

KW - Fogo A eruption sequence

KW - Ignimbrite emplacement temperature

KW - Thermal remanent magnetization

UR - http://link.springer.com.ezproxy.lib.monash.edu.au/content/pdf/10.1007%2Fs00445-015-0904-4.pdf

U2 - 10.1007/s00445-015-0904-4

DO - 10.1007/s00445-015-0904-4

M3 - Article

VL - 77

SP - 1

EP - 19

JO - Bulletin of Volcanology

JF - Bulletin of Volcanology

SN - 0258-8900

IS - 3

M1 - 18

ER -