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 U-Pb dating. Cassiterite has extremely low Th/U ratios (Th/U < 0.01) and its Pb-208 is dominantly common Pb. This is particularly helpful as there is significant interference of tungsten oxides on Hg-202 and Pb-204. The feasibility of the Pb-208 correction procedure is discussed in detail. The Pb-208 corrected LA-SF-ICP-MS data are in good agreement with intercept ages in the Tera-Wasserburg diagram and Pb-207 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 U-Pb ages of cassiterite from the same deposits. We present an ID-TIMS U-Pb 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.