A COMPARISON OF DETRITAL ZIRCON PROVENANCE TECHNIQUES: SHRIMP VERSUS LA-ICP-MS
Detrital zircon provenance studies have become indispensable in reconstructing sediment dispersal patterns, paleogeographic configurations and unroofing histories and have become increasingly feasible due to the availability of the sensitive high resolution ion microprobe (SHRIMP) and laser-ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS). In order to test the advantages and limitations of these two methods, 70 detrital zircon grains from two different samples composed primarily (~90%) of Paleozoic zircons were first analyzed using the SHRIMP-RG at Stanford University; the same spots were subsequently analyzed using the LA-ICP-MS at the University of Arizona. SHRIMP analysis was conducted using an ion beam spot size of 20μ diameter that excavated pits 2μ deep. LA-ICP-MS analysis was conducted using a 35μ diameter laser beam that excavated pits 20μ deep. Histograms and probability curves plotted using 20 Ma bins produced similar detrital zircon fingerprints for SHRIMP and ICP-MS data sets, exhibiting peaks at either the same age range or within 1 bin of each other. Comparing 206Pb/238U ages of the two different methods revealed ~70% of the ages correlated within error and ~15% of the ages varied by >100 Ma. Analyses produced results with mean uncertainties for the two samples of 3.9 Ma and 3.8 Ma for SHRIMP age determinations and 13.5 Ma and 16.6 Ma for ICP-MS age determinations. Thus, SHRIMP analyses produce ages that allow for discrimination between sources that vary by only 5-15 Ma in age and can better determine reliable ages of grains that are <35μ in diameter or have complex internal structure (inherited cores/overgrowth rims, fractures, inclusions). On the other hand, ICP-MS analyses are much faster, allowing a 600% increase in productivity compared to SHRIMP analyses, and can produce overall similar age spectra, making it feasible to date the 117 grains (Vermeesch, in press) needed to achieve 95% confidence of dating at least one grain of an age range that represents ≥5% of the total population. As each technique provides unique advantages, a more thorough comparison will help establish rationales for choosing an approach using one or both methods based on specific research goals.
Vermeesch, P., in press, How many grains are needed for a provenance study?, Earth and Planetary Science Letters.