TESTING THE ACCURACY OF DETRITAL ZIRCON AGE PROVENANCE – NATURAL AND EXPERIMENTAL APPROACH

Several factors affecting the accuracy of detrital zircon age provenance studies have been investigated. We have aimed for quantifying the effects of individual factors that deviate the measured detrital age spectra from the real zircon age distribution in the sediment and in the sediment source. The effects of variable redistribution of zircons from source rocks into the stream have been studied in a natural catchment in Scotland that represent simple two-component source system. The factors affecting reproducibility of the age spectra of the sample were investigated on synthetic sediment prepared on purpose for this study using zircon-free quartz sand and known number of zircon grains of known age distribution.

Our results show that the zircon fertility of the source rocks and physical parameters of zircon grains represent the most important factors affecting the distribution of zircon age populations in the stream sediments. It can account for a several-fold difference between the ratio of the rocks in the source area and abundance of zircon in the sediment. Additional age biases are introduced during sample preparation and data processing. The sample preparation and grain picking result in loss of small grains and errorneous amplification of the age component represented by larger grains. This can, together with the preference for larger grains during handpicking, cause several-fold difference compared to the real age distribution in the sediment sample. These factors are more important for the reproducibility of zircon age spectra than is the number of zircon grains analyzed per sample. Even the most abundant age population in the sample may deviate by tens of percent from its real content in the sediment after hundred or more analyses have been done. It is therefore difficult to relate the peak intensity in the age spectra to the sediment quantity contributed from different sources. The analytical limits of the dating techniques must always be considered when evaluating potential overlap of zircon populations that are closely spaced in time. Although the visualization of U-Pb data in probability density plots is commonly used for comparison between samples, the detrital zircon age spectra must always be interpreted relative to the volume of individual age populations, not to the intensities of the age peaks.