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

Abstract

Analytical errors calculated for individual spot206Pb/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 mean206Pb/238U age of 559.0±0.2 Ma. TIMS U-Pb analyses of the SRM 610 yielded homogeneous206Pb/238U=0.2566. Errors in HR-SIMS206Pb/238U measurements are distinguished in increasing hierarchy as within-spot, within-session (internal), and external. Replicate SHRIMP measurements of208Pb+/206Pb+ and248[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 the206Pb+/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 replicate206Pb+/238U[Ox]+ analyses of z6266 zircon. That is, for both glass and zircon standards, there remained on average about ±1% (1 σ) unaccounted variation per206Pb+/238U[Ox]+ analysis (i.e., MSWDs≫1). Thus, unlike for248[ThO]+/254[UO]+, the within-session errors in206Pb+/238U [Ox]+ were necessarily higher than the within-spot errors. In contrast to208Pb+/206Pb+ and248[ThO]+/254[UO]+, the206Pb+/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 for206Pb+/238U[Ox]+. The usual methods (e.g., using254[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 in206 Pb+/238U [Ox]+ currently undetermined, and a hindrance to decreasing the uncertainties in spot206Pb/238U measurements. The minimum within-session error for an individual206Pb/238U measurement is the standard deviation (dispersion) of the discrimination-corrected206Pb+/238U[Ox]+ values for a homogeneous reference material (e.g., SRM 610). The external error in206Pb/238U additionally incorporates the standard deviation of the mean of the discrimination-corrected206Pb+/238U[Ox]+ for a natural zircon standard. The analysis of206Pb+/270[UO2]+ offers an improvement over measuring206Pb+/238U+ because variable discrimination is reduced or eliminated with respect to254[UO]+/238U+ and counting statistics are superior. Crown Show more