Abstract
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.
Highlights
The discovery of older-than-host zircons of crustal origin in modern Mid-Atlantic Ridge oceanic gabbros by Pilot et al.[1] caused a significant impact in the Earth Science community - it caused skepticism
Zircon grains heated at 1300 °C for 30 days perceptibly spread up and down along concordia, this effect increases with time and temperature to reach a maximum in the 1500 °C/30 days experiment
The experimental results do, fit well with the simulation that considers a non-diffusive environment (Fig. 3B) except for a small fraction of the experimental data with younger 206U/238Pb ages between 250 Ma to 300 Ma peaking at about 285 Ma (Fig. 3C). These slightly discordant ages always correspond to spots analyzed in the region of large inclusions of albitic glass (Fig. 3D) presumably representing molten inclusions of albite, that are very abundant in the pristine SAB51 zircon grains
Summary
The discovery of older-than-host zircons of crustal origin in modern Mid-Atlantic Ridge oceanic gabbros by Pilot et al.[1] caused a significant impact in the Earth Science community - it caused skepticism. Ongoing oxygen isotope work fully confirms that conclusion Following these initial findings, crustal zircons often accompanied by other crustal minerals have been found in a wide variety of mantle-derived rocks. Watson et al.[19] proposed that it is Pb2+ but Kramers et al.[20], to explain the scarcity of observable strong diffusion effects in pre-magmatic zircons (see discussion in21), suggested that it is tetravalent. The latter, found no support from XANES measurements because these revealed that Pb2+ is far more abundant than Pb4+ 22. The U4+ and Th4+ diffusion coefficients are about 5 to 7 orders of magnitude lower, implying that diffusion can substantially disturb the U-Th-Pb isotope ratios of zircon xenocrysts, especially those recycled through the mantle because of the long timescales and high temperatures involved
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