New Robust Reference Materials for In Situ Single Grain Rutile U-Pb Geochronology and Method Refinements for Detrital Rutile Analysis by LA-MC-ICP-MS
Abstract
While rutile (TiO2) occurs in the heavy mineral suite of detrital sediments and originates mainly in medium- to high-grade metamorphic and some igneous rocks, there are very few applications of U-Pb dating of rutile to provenance studies; this is due to an overreliance on zircon, low U content of rutile limiting measurement quality by in situ methods, a higher proportion of common Pb relative to zircon, and a lack of widely available good quality reference materials. We have addressed these issues and characterized two ~ 1.8 Ga rutile reference materials by SEM, trace elements, U-Pb ID-TIMS, and intra-grain and inter-grain U-Pb LA-MC-ICP-MS analysis using mixed faraday and multiple ion counting detectors with high sensitivity. We have assessed U-Pb discordance and in situ variations in relative common Pb and age and their bearing on the quality of the reference materials for in situ U-Pb dating. The rutiles (Sugluk-4 and PCA-S207) come from granulite facies belts of the Canadian Shield, namely the northern Cape Smith Belt of Quebec and the Snowbird Tectonic Zone (Sasatchewan). The ID-TIMS data are slightly discordant due to variable common Pb and limited Pb loss; the variation in 6 single grains of Sugluk-4, that we use as the primary reference material, is <1% in 206Pb/238U, and <2% for 207Pb/206Pb (95 % conf.); after common Pb correction these variations are <1%. The measured variations are smaller than in existing reference materials (i.e. R10) in current use. LA-ICP-MC-MS data (n ~ 500 for each) have a reproducibility of 206Pb/238U and 207Pb/206Pb of ~2-4% (at the 2S level), which is only modestly worse than long-term data for multiple zircon standards, this being due to the real variation in measured values arising from limited Pb loss, age variation, and common Pb variability [1]. We have applied our refined method to the provenance of rutile from drainages from British Columbia, Bhutan, and the Brahmaputra River of NE India (predominant rutile ages ~ 50, 15, and 2 Ma, respectively; Bracciali et al., this meeting). Our method successfully dates >75% of all rutile grains in a sediment; unsuccessful analyses are due to poor quality rutiles with massive common Pb and/or U contents < ~1ppm. While some analyses are therefore unusable, unlike zircon age zoning is rare to absent in rutile and there is little need to image grains to identify 'inheritance' to arrive at a correct interpretation of measured ages. Rutile has a ~ 500°C closure temperature and thus records mainly the time of cooling; it is therefore a sensitive recorder of metamorphic thermochronological information, and an excellent complement to detrital zircon analysis. There appears to be huge scope of in situ application of U-Pb dating to detrital rutile in provenance studies in the future. [1] Bracciali L., Parrish R.R., Condon D., Horstwood M.S.A., Najman,Y., Two new rutile reference materials for in situ U-Pb LA-MC-ICP-MS dating and applications to sedimentary provenance, submitted to Chem. Geol.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V23B2805P
- Keywords:
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- 1140 GEOCHRONOLOGY / Thermochronology;
- 1165 GEOCHRONOLOGY / Sedimentary geochronology;
- 1194 GEOCHRONOLOGY / Instruments and techniques;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution