Elsevier

Chemical Geology

Volume 141, Issues 1–2, 29 August 1997, Pages 1-18
Chemical Geology

Research paper
RbSr and 40Ar39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

https://doi.org/10.1016/S0009-2541(97)00052-1Get rights and content

Abstract

The combined use of the RbSr and 40Ar39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar39Ar plateau ages. 40Ar39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

References (43)

  • S Scaillet

    Excess 40Ar transport scale and mechanism in high-pressure phengites: a case study from an eclogitized metabasite of the Dora Maira nappe, Western Alps

    Geochim. Cosmochim. Acta

    (1996)
  • S Scaillet et al.

    MgFe and [(Mg,Fe)SiAl2] compositional control on argon behaviour in high-pressure white micas: a 40Ar39Ar continuous laser probe from the Dora Maira nappe of the internal Western Alps, Italy

    Geochim. Cosmochim. Acta

    (1992)
  • R.H Steiger et al.

    Subcommission on Geochronology: convention on the use of decay constants in geo- and cosmochronology

    Earth Planet. Sci. Lett.

    (1977)
  • Y Yu et al.

    Age and cooling history of the Kiglapait intrusion from an 40Ar39Ar study

    Geochim. Cosmochim. Acta

    (1992)
  • N.O Arnaud et al.

    Evidence for excess argon during high pressure metamorphism in the Dora Maira Massif (Western Alps, Italy), using an ultra-violet laser ablation microprobe 40Ar39Ar technique

    Contrib. Mineral. Petrol.

    (1995)
  • S.A Bowtell et al.

    SmNd isotopic evidence on the age of eclogitization in the Zermatt-Saas ophiolite

    J. Metamorph. Geol.

    (1994)
  • R Compagnoni

    The Sesia-lanzo zone: high pressure-low temperature metamorphism in the Austroalpine continental margin

    Rend. Soc. Ital. Miner. Petrol.

    (1977)
  • P.S Dahl

    The crystal-chemical basis for Ar retention in micas: inferences from interlayer partitioning and implications for geochronology

    Contrib. Mineral. Petrol.

    (1996)
  • J.S Delaney et al.

    Chemistry of micas from kimberlites and xenoliths, II. Primary and secondary textured micas from peridotite xenoliths

    Geochim. Cosmochim. Acta

    (1980)
  • D Gebauer et al.

    A 35 Ma old ultrahigh-pressure metamorphism in the Dora Maira Massif and its geodynamic implications for the Pennine zone of the Central and Western Alps

    Bochumer Geol. Geotech. Arb.

    (1995)
  • M.A Geyh et al.

    Absolute Age Determination: Physical and Chemical Dating Methods and Their Application

  • Cited by (116)

    • Cryptic excess argon in metamorphic biotite: Anomalously old <sup>40</sup>Ar/<sup>39</sup>Ar plateau dates tested with Rb/Sr thermochronology and Ar diffusion modelling

      2021, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      This study uses Rb/Sr biotite geochronology to test for the presence of cryptic excess 40Ar in biotites of the Albany-Fraser Orogen of Western Australia. Because Ar and Sr have similar closure temperatures in biotite, the 40Ar/39Ar and Rb/Sr geochronometers should yield similar dates; if the 40Ar/39Ar dates are significantly older than the Rb/Sr dates, cryptic excess 40Ar may be present (e.g. Ruffet et al., 1997; Batt et al., 2000; Willigers et al., 2004). Subsequent diffusion modelling of 40Ar/39Ar step-heating plateaus is used to test different models for the incorporation and distribution of excess 40Ar in the biotite crystal lattice.

    • Supergene manganese ore records 75 Myr-long Campanian to Pleistocene geodynamic evolution and weathering history of the Central African Great Lakes Region – Tectonics drives, climate assists

      2020, Gondwana Research
      Citation Excerpt :

      Two batches of Mn oxide samples from Kasekelesa (RGM series) and Mwene-Ditu, Kasai (RG series), were analyzed by the 40Ar/39Ar method in step-heating using a CO2 laser probe coupled with a MAP 215 mass spectrometer. The procedure was described by Ruffet et al. (1991, 1995, 1997). Irradiation of samples was performed at Mac Master Nuclear Reactor (Hamilton, Ontario, Canada) in the 8C facility.

    View all citing articles on Scopus
    View full text