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

Yuji Sano, Kosaku Toyoshima, Akizumi Ishida, Kotaro Shirai, Naoto Takahata, Tomohiko Sato, Tsuyoshi Komiya

Research output: Contribution to journalArticle

2 Citations (Scopus)

Abstract

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).

Original languageEnglish
Pages (from-to)10-17
Number of pages8
JournalJournal of Asian Earth Sciences
Volume92
DOIs
Publication statusPublished - 2014 Oct 1
Externally publishedYes

Keywords

  • NanoSIMS
  • Protoconodont
  • U-Pb dating

ASJC Scopus subject areas

  • Geology
  • Earth-Surface Processes

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