Nephrite jade (nephrite) is a kind of valuable jade, which forms during metamorphism under greenschist facies conditions. Despite being rare in quantity, nephrite occurrences are found worldwide. Precise dating of nephrite is helpful for understanding the formation process for this economic resource. However, most investigations focus on the genesis of nephrite and the formation age of nephrite remains poorly constrained due to the lack of suitable dating methods.
Engineer Xiaoxiao Ling and her colleagues from the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), have carried out detailed studies on nephrite from Luanchuan, Henan, China. These results show that nephrite consists mainly of tremolite, minor calcite and titanite, with occasional rutile, quartz, serpentine, chlorite, pyrite, and apatite. The titanite and tremolite, which are intergrown with each other, share low-energy grain boundaries. This indicates that the two phases are in equilibrium and can be considered as cogenetic.
A total of 43 in-situ secondary ion mass spectrometer (SIMS) U–Pb dating analyses were performed on titanite using a Cameca IMS-1280 HR at IGGCAS. The titanites are low in U concentrations (2–18 ppm). All analyses yield a weighted average 206Pb/238U age of 361 ± 4 Ma (MSWD= 0.88) using the 207Pb-based method, which is the first estimate for the time of formation of the Luanchuan nephrite deposit.
Fig. 1. Back-scattered electron (BSE) images of nephrite samples.
a, nephrite sample with titanite and calcite inclusions in the tremolite crystals. b, c, and d, titanite with tremolite, rutile, and apatite inclusions. The ovals on titanite indicate the spots analyzed by SIMS. Abbreviations: Tr, tremolite; Chl, chlorite; Ttn, titanite; Cal, calcite; Rt, rutile; Ap, apatite.
Fig.2. Tera–Wasserburg inverse U–Pb concordia diagram for titanites fromthe Luanchuan nephrite deposit. Data point error bars and weighted age uncertainties are at the 2σ level.
This investigation provides a powerful in-situ dating method to unravel the age of nephrite, which could serve as a tool for future research on other nephrite deposits.
This research was published recently in the international journal Lithos (Ling et al. Age determination of nephrite by in-situ SIMS U–Pb dating syngenetic titanite: A case study of the nephrite deposit from Luanchuan, Henan, China. Lithos, 2015, 220–223: 289–299).
Dr. LING Xiaoxiao
Secondary Ion Mass Spectrometer (SIMS) Laboratory,IGGCAS