STEP-HEATING ANALYSIS OF ZIRCONS EXPERIMENTALLY SATURATED WITH HE

To better understand He diffusion kinetics in zircon, we have analyzed fragments of Mud Tank zircon through step heating. We started with a ~1.5 cm piece of gem-quality Mud Tank zircon. We cut a ~ 1.5 mm thick slab with a low-speed diamond saw. We then crushed this into ~3-5-mm pieces and select for absence of inclusions, oxide staining, etc., then handpicked around 200 fragments of 1-3 mm in length and 0.2 to 0.5 mm wide. These were annealed in air at 1250°C for one week to eliminate in-grown ⁴He and repair possible mechanical damage caused by crushing and damage due to radioactive decay. Next, these crystals were pressurized with 1.04 kbar of tank helium in a cold-seal pressure vessel and heated under pressure at 750±1°C for 21 days. The material was then quenched more or less isobarically; cooling to room temperature took ~20 minutes. Arrhenius diagrams for these treated zircons show evidence of a wide range of sizes of diffusion domains (by a factor of at least 160,000 assuming the same activation energy). The relative contribution from small domains is significant with perhaps as much as 40% (we are checking this value) of the ⁴He being present in domains with an apparent closure temperature less than 40°C. If these small domains are not an artifact of the treatment we used to saturate the crystals with He, then natural zircons may have large volumes empty of He due to the low retentivity over geologic timescales. In other words, saturating the zircons seems to have revealed domains ~20 times smaller than have been modeled for untreated zircons. Alternatively, these domains may be the result of structural modification in the lab or enhanced concentration of He in the near surface (1-2 nm) of our material. We have also step heated unsaturated fragments approximately 500x2000 µm (i.e., larger than other Mud Tank fragments previously studied); our Arrhenius results show a larger range in apparent domain sizes than previously published data but not nearly as large as our saturated samples.