Experimental evidence for the preservation of U-Pb isotope ratios 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 natural zircon 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.This paper has been financed by the Spanish Grants CGL2013-40785-P and CGL2017-84469-P

Paleomagnetism of the Cretaceous rocks from Cape Kronotskiy, East Kamchatka and reconstruction of terrane trajectories in the NE Pacific area

Abstract. The Kamchatka Peninsula of northeastern Russia is located along the northwestern margin of the Bering Sea and consists of zones of complexly deformed accreted terranes. Paleomagnetic samples were collected for study from a Late Cretaceous aged locality at Cape Kronotskiy (λ=54°44.8´ N, φ=162°1.29´ E). Two components of magnetization were observed. During stepwise thermal demagnetization, the B-magnetic component was observed up to 600°C having a direction and associated uncertainty in stratigraphic coordinates of Ds=300.7°, Is=48.7°, α95=10.9°, k-value=11.8, n=17. The B component paleolatitude calculated from the Fisher mean in stratigraphic coordinates and associated statistics are λobs=30.4° N or S, λ95=8.9°, n=17 (sites), k-value=11. Our overall study paleolatitude result is similar to a previously reported paleomagnetic study completed within this unit. Terrane trajectories calculated using the finite rotation poles of Engebretson et al. (1985), which are corrected for either Pacific-hotspot drift or True Polar Wander hotspot-spin axis relative motion, show that the sampled unit represents a far traveled tectonostratigraphic terrane and support a model in which accretion (docking) events of this composite or superterrane with the North America plate occur at approximately 40 Ma. </jats:p