The effects of retrograde reactions and of diffusion on 39Ar-40Ar ages of micas

Publikation: Bidrag til tidsskriftTidsskriftartikelfagfællebedømt

  • Julien Allaz
  • Martin Engi
  • Alfons Berger
  • Igor Villa
Effects of metamorphic reactions occurring during decompression were explored to understand their influence on 39Ar-40Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) were studied. Collected samples reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, in order to focus on samples that record a minimum of retrogression.
XRD-data indicated that some mineral separates prepared for Ar-Ar stepwise heating analysis were monomineralic, whereas others were composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37Ar/39Ar and 38Ar/39Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar-reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age.
In a chlorite-margarite-biotite-calcschist equilibrated near 560 °C and 0.65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560-580 °C, 0.8-0.9 GPa), the assemblage muscovite-paragonite-plagioclase is at equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16.5 Ma. Above 590 °C, paragonite was mostly consumed to form plagioclase (>590 °C), whereby the relic mica yields an age up to 5.6 Ma younger than muscovite. This is interpreted to reflect plagioclase growth during decompression, which in turn led to partial or total resetting of the Ar clock in paragonite. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite: by 0.5 to 2 Ma. However, these biotites all show small amounts of retrograde chlorite formation.
We conclude that even very minor chloritisation of biotite is apparently a more effective parameter than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is necessary to ensure that equilibrium in investigated samples is achieved, and helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar-ages of micas. A more robust interpretation of such 40Ar-39Ar results may require use of a second geochronometer, such as U-Pb on monazite
OriginalsprogEngelsk
TidsskriftJournal of Petrology
Vol/bind52
Udgave nummer4
Sider (fra-til)691-716
Antal sider27
ISSN0022-3530
DOI
StatusUdgivet - apr. 2011

ID: 32325510