Ion microprobe U-Pb dating and Sr isotope measurement of a protoconodont

This report describes in situ ion microprobe U-Pb dating of a protoconodont, an early Cambrian phosphate microfossil, using laterally high-resolution secondary ion mass spectrometry (NanoSIMS). On a single fragment of a fossil (approximately 850 mu m x 250 mu m) derived from a sedimentary layer in the Meishucunian Yuhucun Formation, Yunnan Province, southern China, 23 spots provide a U-238/Pb-206 isochron age of 547 +/- 43 Ma (2 sigma, MSWD = 1.9), which is consistent with the depositional age, 536.5 +/- 2.5 Ma estimated using zircon U-Pb dating of interbedded tuffs. However, five spots on a small region (approximately 250 mu m x 100 mu m) in the same protoconodont yield an isochron age of 417 +/- 74 Ma (2 sigma, MSWD = 0.31), 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. We also measured Sr-87/Sr-86 ratios of the protoconodont by NanoSIMS. In the older domain, 19 spots give the Sr-87/Sr-86 ratio of 0.71032 +/- 0.00023 (2 sigma), although seven spots on the younger region provide the ratio of 0.70862 +/- 0.00045; this is significantly less radiogenic than the older domain. This is the first report of U-Pb age and Sr isotope heterogeneity within a single fragment of micro-fossil (215). (C) 2014 Elsevier Ltd. All rights reserved.