Assessment of errors in SIMS zircon U–Pb geochronology using a natural zircon standard and NIST SRM 610 glass

Analytical errors calculated for individual spot 206Pb/238U measurements of zircon analyzed using high mass resolution secondary ion mass spectrometry (HR-SIMS, e.g., SHRIMP II) were assessed using natural zircon (z6266) and synthetic glass standards (NIST SRM 610). Evidence for U/Pb homogeneity of these materials includes new thermal ionization mass spectrometry (TIMS) U–Pb analyses of 22 fragments of z6266 zircon from two laboratories, which are identical within error and yield a weighted mean 206Pb/238U age of 559.0±0.2 Ma. TIMS U–Pb analyses of the SRM 610 yielded homogeneous 206Pb/238U=0.2566.Errors in HR-SIMS 206Pb/238U measurements are distinguished in increasing hierarchy as within-spot, within-session (internal), and external. Replicate SHRIMP measurements of 208Pb+/206Pb+ and 248[ThO]+/254[UO]+ in SRM 610 demonstrate that within-spot analytical errors are sufficient (i.e., MSWD''s∼1) to account for the dispersion of data. In contrast, it was not possible with SRM 610 to reproduce the 206Pb+/238U[Ox=0,1,2]+ ratios from spot-to-spot within the limits of the within-spot errors, after correcting the data for obvious systematic variations in the discrimination of the Pb and U isotopes. Identical findings were made following replicate 206Pb+/238U[Ox]+ analyses of z6266 zircon. That is, for both glass and zircon standards, there remained on average about ±1% (1σ) unaccounted variation per 206Pb+/238U[Ox]+ analysis (i.e., MSWDs≫1). Thus, unlike for 248[ThO]+/254[UO]+, the within-session errors in 206Pb+/238U[Ox]+ were necessarily higher than the within-spot errors.In contrast to 208Pb+/206Pb+ and 248[ThO]+/254[UO]+, the 206Pb+/238U[Ox]+ ratios in glass and zircon are significantly fractionated relative to accepted values, and it seems probable that this difference underlies the larger within-session errors for 206Pb+/238U[Ox]+. The usual methods (e.g., using 254[UO]+/238U+) to correct for the inherent discrimination are imperfect and sensitive to small differences in analytical conditions between spots. Matrix variations are ruled out, leaving the precise cause(s) of the additional variation in 206Pb+/238U[Ox]+ currently undetermined, and a hindrance to decreasing the uncertainties in spot 206Pb/238U measurements. The minimum within-session error for an individual 206Pb/238U measurement is the standard deviation (dispersion) of the discrimination-corrected 206Pb+/238U[Ox]+ values for a homogeneous reference material (e.g., SRM 610). The external error in 206Pb/238U additionally incorporates the standard deviation of the mean of the discrimination-corrected 206Pb+/238U[Ox]+ for a natural zircon standard. The analysis of 206Pb+/270[UO2]+ offers an improvement over measuring 206Pb+/238U+ because variable discrimination is reduced or eliminated with respect to 254[UO]+/238U+ and counting statistics are superior. [Copyright &y& Elsevier]