Search

In this study, we developed two new SIMS (secondary ion mass spectrometry) analytical protocols to simultaneously measure oxygen–hydrogen (O–H) isotopic compositions and water content for hydrous geological samples. These two protocols involve the measurement of two sets of secondary ion contents: (1) 1H, 2H, 16O, 18O; and (2) 16O, 16O1H, 18O, 17O1H, 16O2H. Both measurements utilize a hybrid dynamic multi-collector system of CAMECA IMS 1280-HR, which benefits from both the static multi-collector mode and peak-hopping mono-collector mode. These new methods can simultaneously measure (with high-precision) the 18O/16O ratio in static multi-collector mode without trading off its analytical precision, and 1H/16O (or 16O1H/16O) and 2H/1H (or 16O2H/16O1H) ratios in conventional peak-hopping mono-collector mode. Three glass samples (LBS7H, LBS5H and LBS6H-) with known water contents and two apatite samples (Kovdor, Durango) with known oxygen–hydrogen isotopes and water content were measured to verify the protocols' reliability. The olivine crystal San Carlos with ∼1 ppmw water content was used for background monitoring. For the apatite samples, the external precision (spot-to-spot reproducibility) for δ18O and δD is better than 0.56‰ (2SD) and 54‰ (2SD), respectively. After eliminating the outlier (beyond 3SD error), the external precision of 16O1H/16O or 1H/16O ratio improves to 10.27% (2SD). For the glass samples, the water content calibration curves, which were constructed by comparison of the known water content with the SIMS measured 16O1H/16O or 1H/16O ratios, yielded good correlations. It is noteworthy that the apatite and glass samples can have a uniform water content calibration curve for protocol 1, but not for protocol 2, indicating different matrix effects for the two protocols.

In 40Ar/39Ar dating, 39ArK generated is dependent on irradiation conditions, and the accumulation fast neutron fluence is a critical parameter for 40Ar/39Ar dating. With technological advancement, especially in the ultra-low background and multicollector technology, the boundary conditions to choose irradiation conditions have been greatly improved. Based on the detected and processed signal, the maximum boundary condition of 40Ar*/39Ar can be up to 5750, meaning that very low 39Ar signals can yield considerable analytical accuracy. In this study, one biotite sample (ZBH-25) was irradiated for 4 hours to assess the modified irradiation conditions, and the high 40Ar*/39Ar ratio up to 4100 could yield ∼1% analytical uncertainty. This reveals that the new boundary condition is useful to choose fast neutron fluence, especially for very old or low K/Ca ratio samples.

In this study, a high-precision secondary ion mass spectrometry (SIMS) zircon oxygen isotope ratio analytical protocol is described. The analysis is conducted using a Cameca IMS 1280-HR at the SIMS Laboratory of Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS). Four reference zircons (91,500, Qinghu, TEMORA 2 and Penglai) have been chosen to evaluate the reproducibility (precision) and accuracy. An identical protocol is used for the analyses, with unchanged instrumental parameters and with ion beam intensities kept as identical as possible. The Penglai zircon is used as an external reference material to calibrate the instrumental mass fractionation. The 91,500, Qinghu and TEMORA 2 zircons are used as the unknown samples, which yielded 10.15 ± 0.26‰ (2SD), 5.46 ± 0.24‰ (2SD) and 8.33 ± 0.29‰ (2SD), respectively, all consistent with the recommended values within error (2SD). A half-year long-term precision of 0.44‰ (2SD) is achieved using the Qinghu zircon as a monitoring sample for the routine measurement of zircon 18O/16O ratio, which demonstrates the reliability of this analytical protocol.

First comprehensive investigations of the Cuonadong leucogranite exposed in North Himalayan gneiss dome of southern Tibet are presented in this study. The SIMS U–Pb ages of oscillatory zircon rims scatter in a wide range from 34.1 to 16.0 Ma, and the Cuonadong leucogranite probably emplaced at 16.0 Ma. High-precision 40Ar/39Ar dating on a muscovite sample yields an essentially flat age spectrum with consistent plateau and isochron ages, indicating that the Cuonadong leucogranite cooled below 450 °C at 14 Ma. Based on the youngest zircon U–Pb age and muscovite 40Ar/39Ar age, the Cuonadong leucogranite experienced rapid cooling with a rate of 119 °C/Myr from 16 to 14 Ma. The geochronological data of this undeformed leucogranite also suggest that the ductile extension of the South Tibetan Detachment System in the eastern Himalaya ceased by ca. 14 Ma. Furthermore, the initial Sr–Nd isotopic compositions and Nd model ages demonstrate that the leucogranite was derived from metapelitic source within the Greater Himalayan Crystalline Complex. The distinct Ba depletion with high Rb/Sr ratios and negative Eu anomalies make it clear that the leucogranite melts were generated by breakdown of muscovite under fluid-absent conditions.

Secondary ion mass spectrometry (SIMS) has been applied to analyze a wide range of materials for earth science research due to its high sensitivity, high precision and capacity for in situ micro-analysis. This technique operates under ultra-high vacuum (UHV) conditions, especially for water content measurements on nominally anhydrous minerals (NAMs). However, UHV conditions are hard to achieve when using epoxy mounts, which degas readily during the vacuum processes. In this study, we developed a novel sample preparation method using a tin-based alloy instead of epoxy resin. Using a tin alloy mount coupled with an automatic liquid nitrogen refilling system, the results show that the vacuum conditions have been significantly improved from ∼1.2 × 10−8 mbar to ∼1.9 × 10−9 mbar. The background for mineral water content, measured with a CAMECA IMS 1280-HR SIMS installed at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS), is lowered to

Identifying whether and when the southern continental margin of the South China Sea (SCS) was part of the South China Block (SCB) is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia. In this paper, we present geochronological and geochemical data for dacites from Well NK-1 in the Nansha Block (Dangerous Grounds) of the southern SCS margin to investigate their petrogenesis and tectonic affinity. Zircon U Pb and 40Ar/39Ar datings show that these dacites formed during the Late Triassic (218–217 Ma). The dacites have high total alkali (K2O + Na2O = 7.39–8.42 wt%) and relatively low CaO contents (1.87–3.38 wt%), and show geochemical affinities with A-type granites, with high Zr saturation temperatures (806–855 °C), FeOTot/MgO ratios (5.22–7.48), Zr + Nb + Ce + Y contents (530–680 ppm) and 10,000 × Ga/Al values (2.40–2.94), and relatively low A/CNK values (0.86–1.04). The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials. Late Triassic dacites in the Nansha Block and Triassic A-type granites in the SCB formed in an extensional setting at the continental margin of the SCB. This setting was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic. Therefore, the southern SCS margin was part of the SCB continental margin during the early Mesozoic, which provides more detailed insights into the paleogeography of SE Asia since the Mesozoic.