Development of high precision Rb–Sr phlogopite and biotite geochronology; an alternative to 40Ar/ 39Ar tri-octahedral mica dating

Abstract

Potassium–Ar and Rb–Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K–Ar and, in particular, 40Ar/ 39Ar dating of micas is still widely applied, Rb–Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb–Sr ages than the K–Ar or 40Ar/ 39Ar system. Moreover, a reduction of uncertainties typically reported for Rb–Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb–Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% ( n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/ 39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb–Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb–Sr and 40Ar/ 39Ar ages demonstrates the robustness of the Rb–Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/ 39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb–Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb–Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma. Combining the mica Rb–Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb–Sr system in 1–2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.