Ion Microprobe U-Pb Dating and Sr Isotope Measurement of Conodont

Abstract:

We have developed a method of in situ ion microprobe U–Pb dating and Sr isotope measurement of biogenic apatite using NanoSIMS. This was applied to a protoconodont, an early Cambrian phosphate microfossil [1]. On a single fragment of a fossil derived from a sedimentary layer in the Meishucunian Yuhucun Formation, southern China [2], 23 spots provide a ²³⁸U/²⁰⁶Pb isochron age of 547 ± 43 Ma (2sigma), which is consistent with the depositional age, 536.5 ± 2.5 Ma estimated using zircon U–Pb dating of interbedded tuffs [3]. However, five spots on a small region in the same protoconodont yield an isochron age of 417 ± 74 Ma (2sigma), apparently younger than the formation age. The younger age might be attributable to a later hydrothermal event, perhaps associated with Caledonian orogenic activity recorded in younger zircon with an age of 420–440 Ma [4]. We measured Sr isotopic ratios of the protoconodont by NanoSIMS. In the older domain, 19 spots give the ratio of 0.71032 ± 0.00023 (2sigma), although seven spots on the younger region provide the ratio of 0.70862 ± 0.00045; which is significantly less radiogenic than the older domain. We also measured U-Pb age and Sr isotopes of a Carboniferous conodont derived from the Kinderhookian stage from the Illinois Basin region in North America. 20 spots yield a ²³⁸U/²⁰⁶Pb isochron age of 291 ± 56 Ma (2sigma), which is markedly younger than the depositional age of the fossil of 350-363 Ma. On the other hand, 9 spots give a Sr isotopic ratio of 0.70784 ± 0.00030, less radiogenic than the older domain of protoconodont. These data together with other isotopes such as Cl may provide a constraint on the model for chemical evolution of seawater. [1] Sano et al. (2014) J. Asian Earth Sci. 92, 10-17. [2] Condon et al. (2005) Science 308, 95-98. [3] Sawaki et al. (2008) Gondwana Res. 14, 148-158. [4] Guo et al. (2009) Geochem. J. 43, 101-122.