COMBINED RUTILE U-PB THERMOCHRONOLOGY AND TRACE ELEMENT SYSTEMATICS REVEALED BY LA-ICPMS DEPTH-PROFILING

Rutile is a powerful chronometer of the thermal signature imparted to the lower—middle crust by a spectrum of geophysical processes. Traditionally, single-spot micro-beam techniques are employed to extract complementary U-Pb ages and Zr-in-rutile temperatures. However, this approach overlooks the high-resolution thermochronometric record preserved by intragrain concentration profiles of Zr, Pb and HFSEs. We have developed a method to extract such concentration profiles from single rutile grains using LA-ICPMS operated in continuous ablation mode. ²⁰⁶Pb/²³⁸U-age depth-profiles obtained from lower-crustal granulite facies rutiles of the Ivrea Zone, Central Alps, exhibit a systematic decrease in ²⁰⁶Pb/²³⁸U age, from 170—200 Ma in core domains to 120—130 Ma in grain rims, consistent with diffusive loss of Pb during cooling. In addition to constraining the cooling history of the Ivrea Zone, these profiles provide an independent means to assess D₀ and E_a for Pb diffusion in rutile over geological timescales. Grain-center Zr concentrations typically vary between 2000—3000 ppm (800—850ºC). Notably, profiles are observed that are consistent with both diffusive loss and gain of Zr, emphasizing the control of the diffusivity and solubility of Zr in the adjacent matrix phase on the interpretation of Zr-in-rutile temperature estimates. Similar diffusion topologies are observed for Nb, Ta and Hf, implying that HFSE equilibrium was not attained between coexisting melt and rutile during lower-crustal anatexis. This has important implications for chemical models of crustal formation. Depth-profiling results are corroborated by concentrations measured by EPMA.