In situ location and U‐Pb dating of small zircon grains in igneous rocks using laser ablation–inductively coupled plasma–quadrupole mass spectrometry

Abstract

A new U‐Pb zircon dating protocol for small (10–50 μm) zircons has been developed using an automated searching method to locate zircon grains in a polished rock mount. The scanning electron microscope‐energy‐dispersive X ray spectrum‐based automated searching method can routinely find in situ zircon grains larger than 5 μm across. A selection of these grains was ablated using a 10 μm laser spot and analyzed in an inductively coupled plasma‐quadrupole mass spectrometer (ICP‐QMS). The technique has lower precision (∼6% uncertainty at 95% confidence on individual spot analyses) than typical laser ablation ICP‐MS (∼2%), secondary ion mass spectrometry (<1%), and isotope dilution‐thermal ionization mass spectrometry (∼0.4%) methods. However, it is accurate and has been used successfully on fine‐grained lithologies, including mafic rocks from island arcs, ocean basins, and ophiolites, which have traditionally been considered devoid of dateable zircons. This technique is particularly well suited for medium‐ to fine‐grained mafic volcanic rocks where zircon separation is challenging and can also be used to date rocks where only small amounts of sample are available (clasts, xenoliths, dredge rocks). The most significant problem with dating small in situ zircon grains is Pb loss. In our study, many of the small zircons analyzed have high U contents, and the isotopic compositions of these grains are consistent with Pb loss resulting from internal α radiation damage. This problem is not significant in very young rocks and can be minimized in older rocks by avoiding high‐U zircon grains.