U-Pb LA-(MC)-ICP-MS dating of rutile: New reference materials and applications to sedimentary provenance

In response to the general lack of sufficiently abundant and high quality rutile U-Pb reference materials for in situ geochronology, we have characterised two new potential rutile similar to 1.8 Ga reference materials (Sugluk-4 and PCA-S207) from granulite fades belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). Characterisation includes ID-TIMS and LA-ICP-MS U-Pb dating, imaging, and trace element analysis. We compare these materials with existing rutiles used already (R19 and R10; Luvizotto et al., 2009; Zack et al., 2011) and show that the measured U-Pb compositions (i.e. including any common Pb) of our rutiles are considerably more homogeneous. This makes possible a U-Pb normalisation procedure (not reliant upon a common Pb correction) that results in a significant decrease in the uncertainty contribution from the common Pb correction and better reproducibility of reference materials and unknowns for provenance analysis and other applications. The reproducibility is 2-4% (2RSD) for (206)pb/238u and (207)pb/(206) PD only slightly greater than long-term data for zircon reference materials. We show in a rutile provenance study from young orogens (Bhutan Himalaya and Canadian Cordillera) that the sensitivity of our analytical set-up allows dating of similar to 90% of rutiles in a sediment using a 50 mu m laser ablation spotsize within samples containing rutile as young as 10-20 Ma, and obviates the requirement for U concentration pre-screening, thus reducing or eliminating rutile selection bias. Unsuccessful analyses are due to poor quality rutiles with predominant common Pb, Pb-207 signal below detection, or U content below similar to 1-2 ppm. We have used the Pb-207-method' (using the Tera-Wasserburg diagram) to correct for substantial common Pb in very young and/or very low-U rutiles, rather than developing an on-line correction. Since rutile ages reflect mainly the time of cooling, rutile is a sensitive recorder of metamorphic thermochronological information and therefore is an excellent complement to detrital zircon (UPb)-Pb-207 data. The contrast between zircon and rutile signatures in Himalayan samples with rutile as young as 10 Ma is shown to be very dramatic (most zircons from the same sample are >480 Ma, with only a few grains or metamorphic rims reflecting Miocene metamorphism); as such rutile provides complementary information about the thermal events within the source regions of the grains. Rutile U-Pb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies. (C) 2013 Elsevier B.V. All rights reserved.