This study employs the single grain pyrite Rb-Sr technique to date mineralization,using the example of the Lin...
Qiu-Li Li~(a,b),Fukun Chen~(a,b,*),Jin-Hui Yang~a,Hong-Rui Fan~a a Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,100029,People’s Republic of China b Laboratory for Radiogenic Isotope Geochemistry,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing,100029,People’s Republic of China
Trace element analysis was made by an electron microprobe on rutiles from ultrahigh-pressure ec-logite at Jinheqiao in the Dabie orogen. The results show Zr contents of 37 to 118 μg·g?1, with signifi-cant variations between grains within the same thin section. Zr-in-rutile temperatures were calculated by the pressure-dependent thermometer, yielding 595 to 678℃ at 4.1 GPa for the upper limit of meta-morphic pressures. The temperatures are about 200 to 250℃ lower than temperatures of 846±70℃ calculated from garnet-omphacite Fe-Mg partition geothermometry, but 150 to 200℃ higher than ~460℃ from quartz-rutile O isotope temperatures and rutile Pb diffusion closure temperature for the same specimen. This indicates that the diffusion rate of Zr in rutile is remarkably slower than that of Pb and O in rutile, and that the temperature of Zr-in-rutile is higher than the closure temperature corre-sponding to cooling age as dated by rutile U-Pb radiometric system. The latter is consistent with ex-perimental result of Zr diffusion in rutile. Diffusion and re-crystallization with intensive retrograde fluid during exhumation of UHP metamorphic rocks may be the major reason why the Zr contents in rutile decreased and thus did not provide the record of the peak metamorphic temperatures. The remarkable difference in Zr content between rutile grains suggests that the distribution of Zr in rutile was in the state of disequilibrium due to differential diffusion and retrograde resetting.
The Sr isotopic composition of foraminifera has been used in studies of chronstratigraphy and sedi- mentary geochemistry. However, it is difficult to sample foraminifera without overprinting of later diagenesis, which contains enough amount of Sr for isotopic measurement. Analysis of single-grain foraminifera was successively conducted by a combination of low Sr procedural blank and high precision determination of 87Sr/86Sr ratio. This combi- nation of single-grain foraminifera can be significant for the study and application of Sr isotopic composition of fo- raminifera. Measured foraminifera of this study, having a size between 250 μm and 400 μm, were sampled from the carbonate basement of a ferromanganese crust in western Pacific. Mean value of 87Sr/86Sr ratios of the analyzed sin- gle-grain foraminifera is 0.709150 ±0.000013 with internal precision between 4 ppm and 8 ppm for each measurement run. Average value of total procedural blank of Sr is 14 pg. It is significant but difficult to date foraminifera in carbonate basements of ferromanganese crusts. Based on the ob- tained value of 87Sr/86Sr ratio, the analyzed foraminifera can be dated at 0.91 +0.33/?0.39 Ma.
