|Title:||Mass‐spectrometric mining of Hadean zircons by automated SHRIMP multi‐collector and single‐collector U/Pb zircon age dating: The first 100,000 grains|
|Authors:||P. Holden, P. Lanc, T.R. Ireland, T.M. Harrison, J.J. Foster, and Z. Bruce|
|Publication:||Int. Jour. Mass Spectrom., v. 286, p. 53‐63.|
The identification and retrieval of a large population of ancient zircons (>4 Ga; Hadean) is of utmost priority if models of the early evolution of Earth are to be rigorously tested. We have developed a rapid and accurate U‐Pb zircon age determination protocol utilizing a fully automated multi‐collector ion microprobe, the ANU SHRIMP II, to screen and date these zircons. Unattended data acquisition relies on the calibration of a digitized sample map to the Sensitive High Resolution Ion MicroProbe (SHRIMP) sample‐stage co‐ordinate system. High precision positioning of individual grains can be produced through optical image processing of a specified mount location. The focal position of the mount can be optimized through a correlation between secondary‐ion steering and the spot position on the target. For the Hadean zircon project, sample mounts are photographed and sample locations (normally grain centers) are determined off‐line. The sample is loaded, reference points calibrated, and the target positions are then visited sequentially. In SHRIMP II multiple‐collector mode, zircons are initially screened (ca. 5 s data acquisition) through their 204Pb corrected 207Pb/206Pb ratio; suitable candidates are then analyzed in a longer routine to obtain better measurement statistics, U/Pb, and concentration data. In SHRIMP I and SHRIMP RG, we have incorporated the automated analysis protocol to single−collector measurements. These routines have been used to analyze over 100,000 zircons from the Jack Hills quartzite. Of these, ca. 7%, have an age greater than 3.8 Ga, the oldest grain being 4372 ± 6 Ma (2σ), and this age is part of a group of analyses around 4350 Ma which we interpret as the age when continental crust first began to coalesce in this region. In multi‐collector mode, the analytical time taken for a single mount with 400 zircons is approximately 6 h; whereas in single‐collector mode, the analytical time is ca. 17 h. With this productivity, we can produce significant numbers of zircons for statistically limited studies including correlations between age and morphology, mineral‐inclusion paragenesis, as well as isotopic studies including Hf and O isotopic compositions, Pu‐Xe, and Sm‐Nd isotopes.