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 850μm×250μm) derived from a sedimentary layer in the Meishucunian Yuhucun Formation, Yunnan Province, southern China, 23 spots provide a 238U/206Pb isochron age of 547±43Ma (2σ, MSWD=1.9), which is consistent with the depositional age, 536.5±2.5Ma estimated using zircon U-Pb dating of interbedded tuffs. However, five spots on a small region (approximately 250μm×100μm) in the same protoconodont yield an isochron age of 417±74Ma (2σ, 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 420-440Ma. We also measured 87Sr/86Sr ratios of the protoconodont by NanoSIMS. In the older domain, 19 spots give the 87Sr/86Sr ratio of 0.71032±0.00023 (2σ), 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).
ASJC Scopus subject areas
- Earth-Surface Processes