176Lu–176Hf geochronology of garnet I: experimental determination of the diffusion kinetics of Lu3+ and Hf4+ in garnet, closure temperatures and geochronological implications

Elias Bloch, Jibamitra Ganguly, Richard Hervig, Weiji Cheng

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55 Scopus citations

Abstract

The 176Lu–176Hf and 147Sm–143Nd decay systems are routinely used to determine garnet (Grt)–whole-rock (WR) ages; however, the 176Lu–176Hf age of garnet is typically older than the 147Sm–143Nd age determined from the same aliquots. Here we present experimental data for Lu3+ and Hf4+ diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of Hf4+ in almandine/spessartine garnet is significantly slower than that of Lu3+. The diffusive closure temperature (TC) of Hf4+ is significantly higher than that of Nd3+, and although this property is partly responsible for the observed 176Lu–176Hf and 147Sm–143Nd Grt–WR age discrepancies, the difference between the TC-s of Lu3+ and Hf4+ could lead to apparent Grt–WR 176Lu–176Hf ages that are skewed from the age of Hf4+ closure in garnet. In addition, the slow diffusivity of Hf4+ indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic 176Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of 176Lu–176Hf garnet ages and invalidates the use of analytical TC expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385–2398, 2005), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of 147Sm–143Nd garnet ages to diffusive resetting.

Original languageEnglish (US)
JournalContributions to Mineralogy and Petrology
Volume169
Issue number2
DOIs
StatePublished - 2015

Keywords

  • Closure temperature
  • Garnet
  • Garnet diffusion
  • Geochronology
  • Isochron rotation
  • Lu–Hf

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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