Effect of chemical abrasion of zircon on SHRIMP U/Pb, δ¹⁸O, Trace element, and LA-ICPMS trace element and Lu-Hf isotopic analyses

Abstract

Abstract. Chemical abrasion improves the U/Pb systematics of SHRIMP analyses of reference zircons, while leaving other isotopic systems largely unchanged. SHRIMP 206Pb/238U ages of chemically abraded reference materials TEMORA 2, 91500, QGNG, and OG1 are precise to within 0.25 to 0.4 %, and are within uncertainty of chemically abraded TIMS reference ages, while SHRIMP 206Pb/238U ages of untreated zircons are within uncertainty of TIMS ages of zircons which are untreated by chemical abrasion. Chemically abraded and untreated zircons appear to cross-calibrate within uncertainty using all but one possible permutations of reference materials, provided that the corresponding chemically abraded or untreated reference age is used for the appropriate material. In the case of reference zircons QGNG and OG1, which are slightly discordant, the SHRIMP U-Pb ages of chemically abraded and untreated material differ beyond their respective 95 % confidence intervals. SHRIMP U/Pb analysis of chemically abraded zircons with multiple growth stages are more difficult to interpret. Treated igneous rims on zircons from the S-type Mount Painter Volcanics are much lower in common Pb than the rims on untreated zircons. However, the analyses of chemically abraded material show excess scatter. Chemical abrasion also changes the relative abundance of the ages of zircon cores inherited from the sedimentary protolith, presumably due to some populations being more likely to survive the chemical abrasion process than others. We consider these results from inherited S-type zircon cores to be indicative of results for detrital zircons from unmelted sediments. Trace element, δ18O, and εHf analyses were also performed on these zircons. None of these systems showed substantial changes as a result of chemical abrasion. The most discordant reference material, OG1, showed a loss of OH as a result of chemical abrasion, presumably due to dissolution of hydrous metamict domains, or thermal dehydration during the annealing step of chemical abrasion. In no case did zircons gain fluorine due to exchange of lattice-bound substituted OH or other anions with fluorine during the HF partial dissolution phase of the chemical abrasion process. As the OG1, QGNG, and TEMORA 2 zircons are known to be compositionally inhomogenous in trace element composition, spot-to-spot differences dominated the trace element results. Even the 91500 megacrystic zircon exhibited substantial chip-to-chip variation. The LREE in chemically abraded OG1 and TEMORA 2 were lower than in the untreated samples. Ti and phosphorus saturation ((Y+REE)/P) were generally unchanged in all samples.