Time resolved rutile U/Pb data derived from LA-ICPMS – a case study from the North Pamir

Rutile is frequently found in metamorphic and less commonly in igneous rocks, as well as sediments derived from the former rock types. It may contain enough U (typically up to ~100ppm) to be dated by U/Pb geochronology. In detrital studies, rutile U/Pb ages supplement zircon U/Pb data, as zircon age peaks often reflect magmatic activity, while rutile U/Pb age peaks can be connected to metamorphic events. Using Zr-in-rutile thermometry, one could also estimate metamorphic facies of the terrane, from which detrital rutile grains are derived. Zircon U/Pb dating provides usually a crystallization age, while rutile gives cooling ages that are dependent on the size of the diffusion domain and its cooling rate. The closure temperature has been estimated at ca. 600°C. A major challenge of rutile U/Pb geochronology is the variable amount of common Pb present and most rutile dating requires the correction for common Pb. A widely used method is the Stacey & Kramers approach, which estimates a formation age for a group of rutile grains and assigns them an age-dependent initial Pb isotope composition from the terrestrial Pb evolution curve (Stacey and Kramers, 1975). We present detrital rutile U/Pb data measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) from Mesozoic and Cenozoic units in the North Pamir in Central Asia. The laser ablation system obtains a time resolved signal of all required isotopes. Using data reduction schemes in Iolite (Paton et al., 2011) and VizualAge (Petrus and Kamber, 2012), the signal is routinely integrated to a single spot age for each ablation pit. Following a similar approach for apatite (Stockli et al., 2017), we subdivided the signal of each single spot into several time-slices and obtained data that crosses diffusion domains or compositional zones within a single rutile grain. Time slices in most cases are aligned along a Discordia in the Tera-Wasserburg diagram, enabling us to calculate a lower intercept age and initial 207Pb/206Pb ratio. We also discuss similarities and differences between these internally corrected ages and the Stacey & Kramers approach-corrected ages.

 

Paton, C., Hellstrom, J., Paul, B., Woodhead, J., Hergt, J., 2011. Iolite: Freeware for the visualisation and processing of mass spectrometric data. Journal of Analytical Atomic Spectrometry 26 (12), 2508–2518.

Petrus, J.A., Kamber, B.S., 2012. VizualAge: A Novel Approach to Laser Ablation ICP-MS U-Pb Geochronology Data Reduction. Geostandards and Geoanalytical Research 36 (3), 247–270.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26 (2), 207–221.

Stockli, D.F., Boyd, P., Galster, F., 2017. Intra-grain common Pb correction in apatite by LA-ICP-MS depth profiling and implications for detrital apatite U-Pb dating. EGU General Assembly Abstract Volume.