TY - JOUR
T1 - Ion microprobe U-Pb dating and Sr isotope measurement of a protoconodont
AU - Sano, Yuji
AU - Toyoshima, Kosaku
AU - Ishida, Akizumi
AU - Shirai, Kotaro
AU - Takahata, Naoto
AU - Sato, Tomohiko
AU - Komiya, Tsuyoshi
N1 - Funding Information:
We thank Stephen Mozsis and anonymous referee for their constructive comments. Strontium isotopic composition of the standard was measured by TIMS under the guidance of Hiroshi Amakawa. This work was partly supported by Grant-in-Aid for Scientific Research Program No. 24221002 from the Ministry of Education, Science and Culture of Japan.
PY - 2014/10/1
Y1 - 2014/10/1
N2 - 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).
AB - 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).
KW - NanoSIMS
KW - Protoconodont
KW - U-Pb dating
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U2 - 10.1016/j.jseaes.2014.05.024
DO - 10.1016/j.jseaes.2014.05.024
M3 - Article
AN - SCOPUS:84903463385
SN - 1367-9120
VL - 92
SP - 10
EP - 17
JO - Journal of Asian Earth Sciences
JF - Journal of Asian Earth Sciences
ER -