LA-ICP-MS U-Pb Dating of Zircon

There are several different approaches to LA-ICP-MS in terms of equipment used and standardization, but most will start with the separation of zircons from a rock sample by crushing and sieving, followed by separation of the non-magnetic heavy fraction with a Frantz isomagnetic separator and heavy liquids such as sodium or lithium polytungstate, bromoform, tetrabromoethane, and/or methylene iodide. 

As with any analysis, the precision with which the isotope ratios of interest can be determined is related to the intensity of signal of the least abundant isotope. During laser ablation, this intensity is strongly related to the volume of material ablated smaller laser pits will yield smaller amounts of material and therefore 

Apart from a reduced intensity, small-diameter ablation craters generally display another disadvantage, namely that they cause the elements to fractionate from each other with depth. A clear correlation exists between the diameter of the crater and the amount of fractionation with depth [25], which can also be expressed as a fundion of the depth/width ratio of the crater [26]. This laser-induced fractionation effect is seen in glass (such as the widely used NIST 610 borate glass) in addition to minerals such as zircon. 

Laser-induced elemental fractionation is the norm for the most widely used types of lasers, namely 266-193 nm lasers with pulse lengths in the nanosecond range the effect is absent in a new generation of lasers with shorter pulse lengths, the femtosecond lasers [27]. Until these have replaced the current generation of lasers, strategies need to be adopted to correct for laser-induced elemental fractionation. This is especially important for zircon age dating, where the age is calculated on the basis of and ratios, that is, ratios of two 

A solution to the problem of laser-induced elemental fractionation is to use very wide laser beams ( 90 pm) or raster patterns [28]. Marginal downhole fractionation generally still occurs, even with wide laser beams, but it is assumed that the 206pb/238u. (.jjg. (.jjg analysis (the intercept ratio ) is only influenced 

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