Xiao-Xiao Ling

Deciphering the physical mechanism of the topography effect for oxygen isotope measurements using a Cameca IMS-1280 SIMS

The surface condition of a sample mount is an important factor influencing the precision of SIMS isotope analysis. The phenomenon that the sample topography affects the analytical precision is called the topography effect. We carried out a systematic experiment of O-isotope analyses using a Cameca IMS-1280 SIMS to quantitatively characterize the topography effect with the aim of better understanding its physical mechanism underlying such an artifact and ultimately improving the analytical precision. Our results indicate that within a mineral grain, the topography effect is obvious in the X-direction (horizontal direction) of the sample stage but insignificant in the Y-direction (vertical direction). In addition, within a single mineral grain, the topography effect creates analytical spots on the left rim (lower X-coordinates) yielding higher measured δ18O values than those on the right rim (higher X-coordinates) in our instrument. The physical reason that the topography effect compromises the analytical reproducibility is attributed to lateral energy dispersion of secondary ions caused by surface topography changing the ion position in the entrance slit plane. By increasing the transfer optics magnification, the topography effect could be significantly reduced. Beam centering parameters could be used to quantitatively assess the topography effect and improve the data quality.

In situ SIMS Th–Pb dating of bastnaesite: constraint on the mineralization time of the Himalayan Mianning–Dechang rare earth element deposits

Bastnaesite, a major economic rare earth element (REE) mineral, has been considered to be a promising geochronological tool for determining mineralization time by U–Pb dating. However, it has been found that this mineral often favours the incorporation of Th relative to U, with Th/U ratios even larger than 1000. Young samples contain considerable amounts of excess 206Pb from initial 230Th that is incorporated during formation, which could have a significant effect on U–Pb dating. In this study, we present analytical and data calibration protocols for in situ Th–Pb analyses of bastnaesite using a Cameca IMS 1280 HR SIMS. Six bastnaesite samples from the Himalayan Mianning–Dechang REE belt were analysed in this study to test our analytical procedures. The zircon from the host syenites are dated by the SIMS U–Pb method at 12.7 ± 0.2 Ma for the Dalucao deposit, 28.8 ± 0.3 Ma for the Lizhuang deposit, 28.0 ± 0.2 Ma for the Muluozhai deposit, and 26.6 ± 0.3 Ma for the Maoniuping deposit, which are interpreted as the intrusion time of the syenites. The SIMS bastnaesite Th–Pb dating yielded ages of 11.9 ± 0.2 Ma and 11.8 ± 0.2 Ma for the Dalucao deposit, 28.4 ± 0.2 Ma for the Lizhuang deposit, 26.7 ± 0.2 Ma and 26.9 ± 0.2 Ma for the Muluozhai, and 25.7 ± 0.2 Ma for the Maoniuping deposits. These bastnaesite ages are interpreted as the mineralization time. To compare the zircon age and corresponding bastnaesite age, the time span from intrusion of the host magma to REE mineralization is approximately 1 Ma or less. This work demonstrated that the Th–Pb dating method for high Th bastnaesite yields more precise ages than the U–Pb system, as the former can avoid the excess 206Pb problem.

Speleothem annual layers revealed by seasonal SIMS δ~(18)O measurements

In-situ seasonal δ18O measurements of section 236.3–235.6 cm of speleothem HS4, from Qingjiang Valley of the Middle reaches of Yangtze River, China, were performed by an Secondary Ionization Mass Spectrometry (SIMS) with Oka (Chinese primary calcite standard GBW04481) and UWC-3 (international calcite standard from University of Wisconsin). The potential of using SIMS δ18O measurements to establish speleothem time series has been explored and the differences between conventional and SIMS δ18O values have been discussed. During a 3-day period, UWC-3 δ18O has been measured on Cameca IMS 1280 Ion Microprobe Mass Spectrometer against “Oka” external standard. The measured mean value of UWC-3 (δ18OVPDB= −17.85‰±0.22‰, 1SD) matches well with its recommended value (δ18OVPDB=−17.83‰±0.08‰), suggesting that the instrument was stable. The same method applied on HS4 produced δ18O measurements at seasonal resolution with distinct annual cycles and the total cycle number in agreement with that from Mg/Ca cycles and lamination layer counting of the same section, so it offers an alternative for accessing speleothem time series. However, compared with conventional δ18O values of HS4, SIMS δ18O values are more negative by 0.90‰ with larger seasonal variation. The main reasons might come from the micro-cracks, micro-pores or liquid inclusions existing in HS4, and organic materials in the speleothem might be another factor affecting the SIMS δ18O values, indicating that to obtain reliable speleothem SIMS δ18O values, both compaction and purity of samples are crucial.

Zircon Th–Pb dating by secondary ion mass spectrometry

Zircon has been widely used as a geochronometer with the U–Pb decay system but rarely with the Th–Pb system. However, zircon from carbonatite contains very little U and has a high Th/U ratio, making Th–Pb dating preferable. As a one-dimensional system, a series of consistent Th–Pb ages can be used to date a geological event. In contrast, a wide variation in Th–Pb ages could result from Pb loss or multiple growth events, making it difficult to link to specific geological events. In this study, we described Th–Pb dating protocols on zircon and carried out Th–Pb measurements on seven zircon reference materials using a Cameca IMS 1280HR ion microprobe. The results demonstrated that these seven U–Pb zircon standards have similar absolute concentrations of common lead. The radiogenic 208Pb concentrations (depending on the Th content and age) determine the proportion of common lead and define the extent of variation in the Th–Pb system under certain analytical conditions. This relationship could be used as a criterion to evaluate whether it is a single population or not based on Th–Pb dating results of unknown zircons. By comparison, M257 and Qinghu zircons are suggested as the most suitable Th–Pb dating standards, with ID-TIMS U–Pb ages representing the best estimate of the Th–Pb reference ages. A zircon sample from the Wu dyke, an extremely rare earth element (REE)-rich carbonatite dyke in the Bayan Obo area, was dated with the established Th–Pb dating procedure and yielded an age of 1327 ± 20 Ma as the emplacement time. A pronounced relationship between apparent Th–Pb ages and corresponding Th contents (>1500 ppm) indicates that zircons with accumulated radiogenic doses of >1.7 × 1018 α per g may tend to lose radiogenic 208Pb.