ADVANCES IN RUTILE PETROCHRONOLOGY (Invited Presentation)

Rutile petrochronology has become an increasingly significant tool for deciphering the timing and conditions of petrological processes. Rutile provides a reliable single-mineral thermometer – the Zr-in-rutile thermometer – capable of retaining temperature information during ultra-high temperature metamorphism. It also exhibits high U/Pb and enables U-Pb thermochronology in the intermediate temperature range through laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS).

Spatial resolution is key in rutile U-Pb thermochronology by LA-ICPMS, especially for cases of rapid cooling. So far, this has been limited to c. 35 μm, due to the amount of rutile needed for analysis and the use of circular ablation spots. Modern laser ablation systems enable the analysis of rectangular spots to target narrow compositional zones in samples. We applied this method to rutile U-Pb microanalysis as part of a new approach that integrates in situ U-Pb thermochronology by LA-multi-collector-ICPMS with diffusion zoning analysis to determine thermal histories from single rutile grains.

The new analytical approach was applied to rutile grains from: 1) a slowly cooled granulite from the Saglek Block (SB), Nain Province, Labrador, Canada, and 2) a tectonically exhumed eclogite from the UHP zone of the Western Gneiss Complex (WGC), Norway. The grains show age gradients over 140 Ma (SB) and 30 Ma (WGC) over a distance of 40-50 μm (SB) and 200 μm (WGC), both of which could be resolved at a precision of 1.5-2.0 % (2 s.d.) and spatial resolution of ~14 μm. Diffusion zoning length was used with well-established Pb diffusion parameters [1] to determine the thermal histories of both terranes over a temperature range of up to ~300 °C.

The data demonstrate that rutile U-Pb microanalysis yields reliable and precise temperature and age information that can be combined to resolve cooling histories from single crystals. This novel application of rutile petrochronology has great potential for research in lithosphere dynamics and tectonics.

[1] Cherniak (2000) Contrib. Mineral. Petrol. 139. 198-207.