EXPERIMENTAL EVIDENCE FOR THE BEHAVIOUR OF PB AND O ISOTOPES IN MANTLE-RECYCLED CRUSTAL ZIRCON GRAINS

Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics. Consideration of this requires understanding of how mantle temperatures, notably higher than zircon crystallization temperatures, affected the recycled zircon grains, particularly their isotopic clocks. Since Pb²⁺ diffuses faster than U⁴⁺ and Th⁺⁴, it is generally believed that recycled zircon grains lose all radiogenic Pb after a few million years, thus limiting the time range over which they can be detected. Nonetheless, this might not be the case for zircon included in mantle minerals with low Pb²⁺ diffusivity and partitioning such as olivine and orthopyroxene because these may act as zircon sealants. Annealing experiments with zircon concentrates from two different rocks —a peraluminous ca. 485 Ma granite in which most zircon grains contain ca. 605 Ma cores, and a metaluminous calc-alkaline 320 Ma tonalite with very uniform zircons— embedded in cristobalite (an effective zircon sealant) show that zircon grains do not lose Pb to their surroundings, although they may lose some Pb to molten inclusions. Diffusion tends to homogenize the Pb concentration in each grain changing the U-Pb and Th-Pb isotope ratios proportionally to the initial ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb concentration gradients (no gradient-no change) but in most cases the original age is still recognizable. It seems, therefore, that recycled crustal zircon grains can be detected, and even accurately dated, no matter how long they have dwelled in the mantle. We found that zircons from the peraluminous granite resist heating better than the calk-alkaline tonalite. We also found that the δ¹⁸O of cores and rims of zircons from the peraluminous granites, peaking at 6 and 8 respectively, changed and converged to one single value around 9. The highest temperature experiments also reveal some oxygen isotopes interchange with the enclosing cristobalite. This may cause wrong provenance estimations based on oxygen isotopes in mantle-recycled crustal zircons.