PREFERENTIAL DISSOLUTION OF 234U AND RADIOGENIC PB FROM α-RECOIL-DAMAGED LATTICE SITES IN ZIRCON: IMPLICATIONS FOR THERMAL HISTORIES AND PB ISOTOPIC FRACTIONATION IN THE NEAR SURFACE ENVIRONMENT

Abstract

Zircons from two Archean samples, a 2732-Ma trondhjemite and a 2728-Ma dacite, both from the western Superior province, were washed in hydrofluoric acid (HF) at moderate temperatures. Zircon residues and HF wash solutions were analyzed for U/Pb isotope systematics and 234U/238U ratios in an attempt to study the effect of HF washing on selective dissolution of radiogenic isotopes in areas damaged by α recoil. On a conventional concordia diagram, the datum from one analyzed wash solution plots below concordia, near the array of discordant data from magnetic, unabraded zircon fractions, which record a lower concordia intersection of about 230 Ma. Data points from HF wash residues of two different zircon fractions from the trondhjemite are also discordant and collinear with near-concordant data from abraded, unleached fractions, which record the true age of crystallization. However, this array has a negative lower concordia intercept of -749 Ma. The residue and wash from HF treatment of a single zircon fraction from the dacite are collinear with the crystallization age of the sample, as defined by near-concordant data from unleached fractions. This line has a lower concordia intersection of -1388 Ma, also negative but distinct from the trondhjemite residue line. The discordance of strongly-washed residues is proportional to the calculated α-recoil dosage of the grains. Leach residues show negative 234U/238U anomalies, while the leaches themselves show 234U excess on the order of 10%. These results can be explained by the bulk dissolution of altered and metamict crystal domains during the HF wash accompanied by leaching of 234U and radiogenic Pb from α-recoil-damaged lattice sites in alteration-free zircon residues. Studies were also carried out on zircons washed in nitric acid. These show small (ca. 0.5%) negative 234U anomalies. This suggests that small amounts of radiogenic Pb can be leached from damaged lattice sites by nitric acid.The negative-age lower concordia intercepts of Pb loss lines defined by HF leach residues can be explained if early radiation damage has been annealed so that only more recently formed radiogenic Pb atoms are in damaged crystal lattice sites and are susceptible to leaching. Since this Pb fraction is relatively young, the residue Pb is biased toward anomalously old values. A model is proposed for early burial and heating of the rocks above the annealing temperature for α-recoil damage, followed by uplift and cooling several hundred million years after crystallization. This model predicts a cooling time of 2500+/-20 Ma for the trondhjemite and 2295+/-35 Ma for the rhyolite. Comparison with metamorphic and cooling histories from Ar/Ar studies suggest that the long-term annealing temperature for low levels of α-recoil damage in zircon is in the range 250-300°C, approximately the same as for fission tracks.These results demonstrate that HF, and possibly to a small extent HNO3, can preferentially leach radioactive daughter elements from damaged zircon lattice sites, in addition to dissolving metamict domains. The threshold α-recoil dosage for dissolution of damaged zircon appears to be slightly in excess of 1.5x1015 α decay events/mg. The isotopic composition of the leached Pb is sensitive to the annealing history of the zircon. Thus, in addition to supplying the magmatic crystallization age of the sample, the HF leaching of zircon might be useful for constraining its later low-temperature thermal history. Leaching from α-recoil-damaged sites also occurs in the natural environment and this may lead to significant enhancement of 206Pb/204Pb in groundwater.