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

摘要

In response to the general lack of sufficiently abundant and high quality rutile UPb reference materials for in situ geochronology, we have characterised two new potential rutile ~1.8Ga reference materials (Sugluk-4 and PCA-S207) from granulite facies 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 UPb 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 UPb compositions (i.e. including any common Pb) of our rutiles are considerably more homogeneous. This makes possible a UPb 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/~(238)U and ~(207)Pb/~(206)Pb, 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 ~90% of rutiles in a sediment using a 50μm laser ablation spotsize within samples containing rutile as young as 10-20Ma, 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, ~(207)Pb signal below detection, or U content below ~1-2ppm. We have used the ~('207)Pb-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 U~(207)Pb data. The contrast between zircon and rutile signatures in Himalayan samples with rutile as young as 10Ma is shown to be very dramatic (most zircons from the same sample are >480Ma, 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 UPb dating is an underexploited provenance method with wide applicability to sedimentary provenance studies.