U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb

Abstract

Precise and accurate U-Pb LA-ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA-ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but particularly into the reference material. We present here a general approach to common Pb correction in U-Pb LA-ICP-MS dating using a modified version of the VizualAge U-Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U-Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the 204 Pb, 207 Pb or 208 Pb (no Th) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTVizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide "model" U-Pb fractionation curves to the time-resolved U-Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is intercept or isochron methods on co-genetic analyses) can be performed offline.The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U-Pb crystallization age) using a Thermo Scientific iCAP-Qc Ma rutile which exhibits variable incorporation of common Pb. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTA range of LA-ICPMS U-Pb dating applications are presented and include U-Pb dating of apatite from >3.8 Ga gneisses from Akilia, SW Greenland. These apatites host 13 C-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U-Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core-rim age differences of up to 300 Ma.These data explain the scatter and poor precision of earlier U-Pb apatite age determinations on Akilia apatite. Other LA-ICPMS dating applications include U-Pb apatite dating as a rapid method for determining the age of mafic intrusions, U-Pb titanite and apatite dating of ash fall tuffs, determining temperature-time histories using multiple U-Pb thermochronometers and improving concordance in LA-ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage.

Chemical Abrasion Applied to LA-ICP-MS U–Pb Zircon Geochronology

“…The resultant error of our calculated mean 206 Pb/ 238 U age for Penglai zircons (4.7% 2σ, including decay constant uncertainties) could likely be reduced with a larger spot size. The concordant nature of our data is in contrast to all published U-Pb data for this reference material [28,29,37]. This demonstrates that chemical abrasion has preferentially and effectively removed common Pb from these zircons and opens the possibility of using Penglai as a primary reference material for micro-beam U-Pb zircon geochronology, especially when analysing young (<10 Ma) material.…”

“…Penglai zircons however, define a linear discordant array on a Tera-Wasserburg diagram with a "preferred" 206 Pb/ 238 U age of 4.4 ± 0.1 Ma [28]. This discordance makes Penglai unsuitable for use as a primary reference material for U-Pb geochronology, unless a common Pb correction is undertaken [29]. As with 91500, slight pitting due to chemical abrasion is evident in the SEM image.…”

Multiple Stage Ore Formation in the Chadormalu Iron Deposit, Bafq Metallogenic Province, Central Iran: Evidence from BSE Imaging and Apatite EPMA and LA-ICP-MS U-Pb Geochronology

“…The measured age for Phalaborwa of 2031 ± 43 Ma (n = 12; MSWD = 0.42) confirms the overall accuracy of the U-Pb apatite ages for Chadormalu. For both trace-element and U-Pb dating off-line selection and integration of background and analyzed signals, and time-drift correction and quantitative calibration for U-Pb dating and trace element concentrations were performed with Iolite 3.5 using the VizualAge_UcomPbine [44] and trace-element (Internally Standardized) data reduction schemes, respectively. Details of the error propagation are given in Paton (2010) [45].…”

Standardless fission-track dating of the Durango apatite age standard

“…The actinide concentrations are ca. 7-22 ppm U and 150-380 ppm Th (Abdullin et al, 2014;Boyce and Hodges, 2005;Chew et al, 2014;Johnstone et al, 2013;Kimura et al, 2000;Morishita et al, 2008;Soares et al, 2014;Young et al, 1969). Table 2 summarizes some geochronological data relating to the Durango apatite; classical fission-track ages are not included.…”