Single Spot Rb‐Sr Isochron Dating of Biotite by LA‐MC‐ICP‐MS/MS

Abstract

Laser ablation tandem mass spectrometry is a burgeoning field for in situ Rb‐Sr geochronology. Here, we determined simultaneous isotope ratios of 87Sr/86Sr and 87Rb/86Sr in metamorphic biotite from western Maine, using an ESL™ imageGEO™193 excimer laser ablation system coupled to a Thermo Scientific™ Neoma™ MC‐ICP‐MS/MS. Measurements were made on Faraday cups with Rb+ at mass 87; Sr isotopes were reacted with SF6 gas and measured as SrF+ at masses 103–107. Twenty‐two laser spots in biotite from a single sample yield a "traditional" Rb‐Sr isochron date of 289 ± 6 Ma. Time‐resolved signals reveal significant zoning in 87Sr/86Sr and 87Rb/86Sr within single spot analyses, which were used to construct single spot isochrons. Individual laser spots contain multiple isochronous subpopulations; some spots contain up to three distinct Rb‐Sr isochrons that are decoupled from variations in Rb/Sr. Thirty‐five isochron dates were determined using this "sub‐spot" approach, with 87Sr/86Sr intercepts that systematically vary with Rb‐Sr date; two‐point isochrons were calculated for individual integrations (n = 780) based on these variable intercepts. Both methods yield age peaks at 303, 270 and 240 Ma. These data suggest that the Rb‐Sr system has the potential to record multiple heating, cooling or fluid‐alteration events spanning ~ 100 My within small domains in single biotite crystals.

LA-ICP-MS/MS-based Rb–Sr isotope mapping for geochronology

“…This observation supports earlier conclusions on the general matrix mismatch between crystalline mica and pressed mica nanopowders. 21,27,29…”

“…These μm-scale age variations correspond to the identification of multiple age populations in natural biotite, as observed in the time-resolved signal of a single spot ablation interval. 21 While increasing the size of a particular age population segment improves the weighted mean uncertainty, the obtainable total uncertainty of the Rb–Sr age is limited by the propagation of the uncertainty of the initial Sr correction and other systematic uncertainties. In this respect, the availability of matrix-matched, homogeneous, and well characterized RM as well as an accurate and precise determination of the initial 87 Sr/ 86 Sr is crucial.…”

“…Analyzing these lighter elements alongside Rb/Sr poses challenges, particularly when employing a Wien field to restrict the mass range transmitted to the CRC. 21 Rb–Sr age maps are also highly promising for thermochronology applications, as they allow to constrain diffusion profiles and to distinguish diffusion (at static conditions) from fluid-induced age resetting or dynamic recrystallization. This capability is pivotal for the much-needed re-evaluation of true diffusion-related closure temperatures in natural biotite and muscovite.…”

“…For in situ Rb-Sr dating, NIST 610 glass 25 and MicaMg-NP 9,26 are the most widely used. 15,20,21,27,28 The latter is a nely ground pressed powder of phlogopite from a pyroxenite from southern Madagascar that displays a homogeneous age signal (519.4 ± 6.5 Ma). 9 Although these RM are frequently used in geochronological applications, none provides a proper matrix match to single-crystal mica, which is also the case for the recently tested fused mica mineral glasses.…”

“…20 Most recently, the isochron approach was applied to single spot dating using high-precision LA-MC-ICP-MS/MS analyses, which allows precise single spot dating (2–4%) without prior knowledge of the initial 87 Sr/ 86 Sr ratio. 21 So far, in situ Rb–Sr dating relies on placing rather large single laser spots (50–100 μm) 20,21 on the phase of interest. To obtain a reasonable level of uncertainty for 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios (typically around 1–3% and 0.5–1% when using quadrupole instruments), the ablation interval is chosen long enough to enable the recording of multiple mass sweeps ( n > 100).…”