U-Pb isotopic dating of cassiterite: Development of reference materials and in situ applications by LA-SF-ICP-MS

Cassiterite, the economically most important tin mineral, typically has moderate U and variable common Pb contents, making it amenable for UPb dating. Cassiterite has extremely low Th/U ratios (Th/U < 0.01) and its 208Pb is dominantly common Pb. This is particularly helpful as there is significant interference of tungsten oxides on 202Hg and 204Pb. The feasibility of the 208Pb correction procedure is discussed in detail. The 208Pb corrected LA-SF-ICP-MS data are in good agreement with intercept ages in the Tera-Wasserburg diagram and 207Pb corrected ages. Twelve cassiterite samples were investigated using the ID-TIMS and LA-SF-ICP-MS methods. The ID-TIMS results of Pit-AB, Rond-A, RG-114, BB#7 and 19GX cassiterite are reported for the first time in this study. RG-114, BB#7 and 19GX cassiterite have very low common Pb contents and are recommended for use as primary reference materials for in situ cassiterite. Pit-AB, Rond-A and Yankee cassiterite contain a small amount of common Pb, produce reliable and consistent ages and are suitable as primary reference materials. The remaining five cassiterite samples (Kard, Zinnwald, Els, XBD-W and Y724) were only investigated using the LA-SF-ICP-MS method and produce ages consistent with published age data from the host rocks associated with the tin deposits and with published UPb ages of cassiterite from the same deposits. We present an ID-TIMS UPb of 154.3 ± 0.7 Ma for the commonly used cassiterite reference material AY-4. This age differs from previously reported ID-TIMS ages. This age discrepancy is caused by different initial common Pb compositions rather than age heterogeneity.