New Mexico Geological Society Annual Spring Meeting — Abstracts

Over the past decade, the U-Pb geochronology of individual detrital-zircon (DZ) grains has become a standard methodology for inferring the provenance of sand and sandstone because: (1) zircon grains resistant to weathering and abrasion are persistent in the sedimentary environment; (2) zircon and more abundant quartz are both derived dominantly from felsic igneous rocks, and zircon ages serve as a proxy for quartz ages; (3) the ages of detrital zircons faithfully reflect the ages of source rocks because the U-Pb isotopic system is not reset in sedimentary rocks, or even in metamorphic rocks at temperatures below granulite grade; (4) the double decay scheme of the U-Pb isotopic system (²³⁸U to ²⁰⁶Pb; ²³⁵U to ²⁰⁷Pb) allows disturbed ages to be detected during isotopic analysis (²⁰⁶Pb*/²³⁸U and ²⁰⁷Pb*/²³⁵U ages for a zircon grain are then not concordant, and the grain plots off the standard concordia curve for ²⁰⁶Pb*/²³⁸U vs ²⁰⁷Pb*/²³⁵U ages).

U-Pb ages for individual zircon grains as small as coarse silt are conveniently determined rapidly by laser ablation–multicollector–inductively coupled plasma–mass spectrometry (Gehrels et al., 2008 G3 , v. 9, no. 1), thereby allowing ~100 U-Pb grain ages to be obtained routinely from each DZ sample at acceptable cost. In practice, 206Pb/238U, ²⁰⁶Pb/²⁰⁷Pb, and ²⁰⁶Pb/²⁰⁴Pb ratios are measured directly, and the ²⁰⁷Pb/²³⁵U ratio is calculated from knowledge that all uranium in the world today has a ²³⁸U/²³⁵U ratio of 137.88 (²⁰⁶Pb/²⁰⁷Pb can be measured more accurately than ²⁰⁷Pb/²³⁵U). Determination of ²⁰⁶Pb/²⁰⁴Pb is required to correct for non-radiogenic common lead (dominantly but not entirely ²⁰⁴Pb) to define relevant isotopic ratios involving radiogenic ²⁰⁶Pb* and ²⁰⁷Pb*. Both the hardware and the software of analytical procedures are demanding. Best age estimates are provided by ²⁰⁶Pb*/²³⁸U ages for younger grains and ²⁰⁶Pb*/²⁰⁷Pb* ages for older grains (with the transition age near 1.2 Ga) because age uncertainties for the former increase with age whereas age uncertainties for the latter decrease with age. Uncertainties in grain age are inherently greatest near 1.0-1.3 Ga (Grenvillean).

Grains discordant for ²⁰⁶Pb*/²³⁸U and ²⁰⁷Pb*/²³⁵U ages by more than a specified percentage are rejected for provenance analysis. Age spectra of the DZ grains retained are displayed as age-distribution curves (probability-density plots) generated by incorporating each U-Pb age and its analytical uncertainty as a normal distribution, and stacking the individual normal distributions into a compound curve. For comparison of DZ age spectra, visual impressions are supplemented by KolmogorovSmirnoff (K-S) statistical analysis. The K-S analysis calculates a probability P that two age spectra are comparable, and where P>0.05 there is <95% confidence that the two age spectra are not defined by grains selected at random from the same parent population (with P=1.0 indicating statistical identity).

Several caveats apply to provenance interpretations from U-Pb ages: (1) the range of U-Pb ages of zircons in potential source rocks must be known independently from geochronological studies for inferences of provenance to be made from DZ populations; (2) proportions of zircon grains of various ages in DZ populations do not necessarily equate to proportions of total detritus from different source rocks because the zircon contents of igneous rock assemblages vary (e.g., when arc plutons of North America are assigned a zircon fertility factor ZFF of 1.0, ZFF~2.5 for rift and anorogenic plutons, and ZFF~3.5 for plutons along the collisional Grenville orogen); (3) DZ ages reflect the ages of ultimate igneous sources for DZ grains, but cannot detect the nature of proximate sources in sedimentary strata from which durable DZ grains have been recycled.