Evaluate Qinghu and Other Known Apatite Samples as the Reference Materials for In-situ U-Pb Dating

Apatite has been widely applied as a reliable chronometer and geochemical tool for studies on genesis and provenance.Due to small quantity, most apatite standards for laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating were exhausted.Therefore, apatite reference materials with homogeneous age and abundant reserves are still necessary.The famous Qinghu (QH) monzonite in the western Nanling region, South of China was dated well at 159.5 0.2 Ma (Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) zircon U-Pb method) and 159.5 0.7 Ma (Secondary Ion Mass Spectrometry (SIMS) zircon U-Pb method).The QH apatite grains selected from this monzonite are characterized by abundant reserves, euhedral crystals with sizes up to 1mm, high U and Pb contents of 137.30 and 7.82 g/g, respectively, and a small Th/U value of 2.6.The QH apatite were analyzed as a quality control over five years in the SKLCMRE lab and yielded an accurate U-Pb age at 161.3 0.8 Ma (N=110, MSWD= 1.1, calibrated by OD306), which agrees well with the published U-Pb ages within uncertainties.In combination of these age results and microscopic observations on cogenetic minerals in this rock, the zircon ID-TIMS U-Pb age of 159.5 Ma was accepted as a reference age for QH apatite.Up to now, it is the most suitable reference material for apatite samples from the Jurassic to Cretaceous periods owing to its high U and Pb content and formation age.An initial 207 Pb/ 206 Pb ratio of 0.837 0.008, which was derived from LA-SF-ICP-MS analyses on intergrown feldspars, is consistent with the calculated value (0.836) in Stacey and Kramers (1975).One hundred and sixty analyses on QH apatite grains have a common lead percentage (f206) value ranging from 4.2% to 38.4% and form a good linear trend on the Tera-Wasserburg diagram.Using QH apatite as a primary standard in a two-stage calibration method, four known apatite standards, including OD306, 401, MAD1, and MM, have been accurately determined with age offsets of -0.7% to 1.2% by LA-ICP-MS.These age results are the same as those calibrated by apatite OD306 (-1.1% -0.1%), MAD1 (-1.2% --0.1%), and MM (-1.1% -0.1%), and much improved than those calibrated by apatite 401 (0.3% -1.4%) and Otter Lake (-3.4% --2.2%).Moreover, in a method application, MRC-1 apatite yielded a lower intercept 206 Pb/ 238 U ages of 154.1 5.3 Ma, which agrees well with the published ID-TIMS U-Pb age of 153.3 0.2 Ma within uncertainty.