The GSC Sensitive High Resolution Ion Microprobe (SHRIMP): analytical techniques of zircon U-Th-Pb age determinations and performance evaluation

Abstract

Analytical methods of zircon U-Pb-Th age determinations using the newly acquired Sensitive High Resolution Ion Microprobe (SHRIMP) have been established and are reported in detail. In a SHRIMP analysis, an attempt is made to sample the smallest possible amount of zircon that can give useful age information, usually a spot selectively variable from about 5 to 30 µm diameter and <1 µm deep. Only 1-10 ng of zircon is sampled, three orders of magnitude smaller than a typical isotope dilution thermal ionization mass spectrometry (ID-TIMS) analysis. Causes and mitigation of the variable relative yields of Pb, U, and Th due to the sputtering process are discussed. Megacrystic zircon from the Kipawa syenite complex has been adopted as the current Pb/U and Pb/Th standard; the Pb/U age of the standard has been independently determined by ID-TIMS analysis at 993.4 ± 0.7 Ma. SHRIMP data reduction and error propogation procedures are summarized. The overall performance of the SHRIMP was evaluated by analyzing zircons of varying ages (0.5 to 2.7 Ga) using the new zircon standard to calibrate the Pb/U and Pb/Th ratios. Both 207Pb/206Pb and 206Pb/238U ages agree within error with the ID-TIMS ages. The 208Pb/232Th are systematically younger and indicate unknown analytical problems. Under routine conditions, 2s age uncertainties for single spots in average zircons decrease from typically ±4% at 0.5-1.5 Ga, to ±2% at 1.9 Ga, and ±0.5% at 2.7 Ga. The 2s errors on pooled analyses of relatively simple zircon samples decrease from ±1.5% at 0.5-1.5 Ga, ±0.5% at 1.9 Ga, and ±0.2% at 2.7 Ga. For zircons <1.5 Ga, precise age determinations rely mainly on pooling of 206Pb/238U ratios or regression on a Tera-Wasserberg diagram, whereas for zircons >1.5 Ga, ages are derived mainly from the pooled 207Pb/206Pb ratios of concordant zircons. As a rule of thumb, it is generally possible to derive a SHRIMP zircon age that has an error of about five times that of a precise ID-TIMS analysis.