Guo-Qiang Tang

Precise U–Pb and Pb–Pb dating of Phanerozoic baddeleyite by SIMS with oxygen flooding technique

Baddeleyite has long been recognized as one of the most important U-bearing minerals for dating silica undersaturated igneous rocks. Age determination of baddeleyite calls for analysis within small volumes using high-resolution secondary ion mass spectrometry (SIMS) because of its minuscule grain size as well as potential altered domains or micro-inclusions. However, precise SIMS U–Pb dating has been hampered for baddeleyite owing to crystal orientation effects that bias Pb/U ratio measured in baddeleyite. In this study we carried out a series of tests of U–Pb and Pb–Pb measurements on Phanerozoic baddeleyite using a multi-collector Cameca 1280 IMS with oxygen flooding technique. Our results demonstrate that the oxygen flooding can not only enhance secondary Pb+ ion yield by a fact of 7 for baddeleyite, but also depress the baddeleyite U/Pb orientation effect down to ∼2% (1 RSD). Therefore, Phanerozoic (as young as Cenozoic) baddeleyite can be precisely dated by SIMS Pb–Pb and/or U–Pb measurements with precision of 1–3% (2 RSE).

Chelyabinsk airburst, damage assessment, meteorite recovery, and characterization

Deep Impact? On 15 February 2013, the Russian district of Chelyabinsk, with a population of more than 1 million, suffered the impact and atmospheric explosion of a 20-meter-wide asteroid—the largest impact on Earth by an asteroid since 1908. Popova et al. (p. 1069, published online 7 November; see the Perspective by Chapman) provide a comprehensive description of this event and of the body that caused it, including detailed information on the asteroid orbit and atmospheric trajectory, damage assessment, and meteorite recovery and characterization. A detailed study of a recent asteroid impact provides an opportunity to calibrate the damage caused by these rare events. [Also see Perspective by Chapman] The asteroid impact near the Russian city of Chelyabinsk on 15 February 2013 was the largest airburst on Earth since the 1908 Tunguska event, causing a natural disaster in an area with a population exceeding one million. Because it occurred in an era with modern consumer electronics, field sensors, and laboratory techniques, unprecedented measurements were made of the impact event and the meteoroid that caused it. Here, we document the account of what happened, as understood now, using comprehensive data obtained from astronomy, planetary science, geophysics, meteorology, meteoritics, and cosmochemistry and from social science surveys. A good understanding of the Chelyabinsk incident provides an opportunity to calibrate the event, with implications for the study of near-Earth objects and developing hazard mitigation strategies for planetary protection.

Nonglacial origin for low-δ18O Neoproterozoic magmas in the South China Block: Evidence from new in-situ oxygen isotope analyses using SIMS

Low-δ 18 O signatures in supracrustal rocks have been used as geochemical proxies for cold paleoclimates, e.g., glaciations. Unusual low-δ 18 O values found in Neoproterozoic igneous rocks in parts of the South China Block have thus been genetically linked to Neoproterozoic glaciation events. However, we report here new oxygen isotope compositions from Neoproterozoic magmatic zircons in central southern China using in-situ techniques that argue against such an interpretation. Our results show that (1) low-δ 18 O magmatic zircons started to appear in the South China Block from ca. 870 Ma, coinciding with the tectonic switching from Sibao orogenesis to postorogenic extension, which occurred more than 150 m.y. prior to the first glaciation event. The most abundant low-δ 18 O magmatic zircons have ages of 800–700 Ma. (2) The 830–700 Ma magmatic zircons are characterized by their bimodal nature of oxygen isotope compositions, i.e., mantle-like δ 18 O values (+4.4‰ to +5.8‰) and high-δ 18 O values (+9.3‰ to +10.8‰). (3) A sharp temporal change in maximum zircon δ 18 O values in the South China Block coincided with the onset of continental rifting and the possible arrival of a plume head. (4) No negative δ 18 O zircons have been identified in this study, contrary to previous studies. These features strongly argue against a glaciation origin for low to negative δ 18 O values in Neoproterozoic magmatic zircons from southern China. We propose that two stages of high-temperature water-magma interaction during plume-driven magmatism and continental rifting best explain the low-δ 18 O magmas. The most important implication of this study is that formation of such low-δ 18 O magmatic zircons was not necessarily related to glacial events and should not be used as a geochemical proxy for a cold paleoclimate.

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.

Precise U–Pb zircon dating at a scale of <5 micron by the CAMECA 1280 SIMS using a Gaussian illumination probe

Zircon is arguably the best, certainly the most commonly used mineral for U–Pb geochronology. Modern large-geometry secondary ion mass spectrometry (SIMS) has been routinely utilized for precise U–Pb zircon age determination at a lateral resolution of 10–30 μm. However, in situ U–Pb dating at a scale of ca. 5 μm scale or less for fine-grained zircons and/or zircon crystals with complex structural and chemical features is still a challenge to the geoscience community. Here we describe a method of precise U–Pb dating for zircons as young as the Jurassic age at a scale of up to <5 μm using the CAMECA ims-1280 SIMS. Gaussian mode primary O2− and O−probes of ca. 5.2 μm and ca. 4.5 μm in diameter with beam intensities of ∼100 pA were obtained, respectively, by optimizing the primary column. Secondary ion optics was optimized to ensure a high Pb+ sensitivity in zircons, producing ∼21 cps/ppm/nA using O2− and ∼13 cps/ppm/nA using O− (with oxygen flooding technique). As a demonstration of this method, three well-characterized zircon standards with a range of ages, AS3 (1099 Ma), Plesovice (377 Ma) and Qinghu (159.5 Ma), were analyzed. We demonstrate with these zircon standards that their ages could be determined with precision and accuracy of 1–2% using a spot <5 micron. The O− primary beam is preferred over the O2− beam for small-spot U–Pb zircon geochronology, because it has higher density and produces smaller craters on the target surface, with insignificant trade off in precision and accuracy of the final U–Pb ages. For U-poor minerals of younger ages, O2− might be preferred in order to generate sufficient Pb+ ions for measurement with minimal loss of spatial resolution.

Further characterization of M257 zircon standard: A working reference for SIMS analysis of Li isotopes

Zircon is the most useful mineral for studies in U–Pb geochronology, Hf and O isotope geochemistry, trace element geochemistry, and increasingly geothermometry. In situSIMS zircon Li isotope analysis shows potential for studying the genesis of crustal magmas and evolution of the continental crust, but its application has been hampered due to lack of well-characterized zircon Li isotope standards. Reconnaissance SIMS measurement of Li isotope ratio and concentration for several commonly-used zircon U–Pb age standards, including M257, BR266, Plesovice, 91500 and TEMORA 2 zircons are reported here. Of these, the M257 zircon is demonstrated to be homogeneous in Li isotopic composition, with δ7Li = 2.1 ± 1.0‰ (2SD). It is also relatively homogeneous in Li concentration, with Li concentration = 0.86 ± 0.18 ppm (2SD). Therefore, we recommend the new M257 zircon standard as a working reference for SIMS Li isotope and concentration measurements in zircons.

Towards higher precision SIMS U–Pb zircon geochronology via dynamic multi-collector analysis

The U–Pb dating system has been widely used in geochronology because the system contains two independent parent/daughter pairs yielding three ages (i.e., 238U/206Pb, 235U/207Pb and 207Pb/206Pb ages) to internally check self-consistency. Among numerous U-bearing minerals, zircon has been recognized as the best mineral for U–Pb geochronology owing to its moderate U content, negligible initial unradiogenic Pb (or common Pb) and occurrence in a wide range of rock types. With the development of Secondary Ion Mass Spectrometry (SIMS) and in situ dating methods, the 238U/206Pb zircon age uncertainty could be achieved at an ∼1% level. However, the 207Pb/206Pb age uncertainty of Phanerozoic zircon is always very poor, when single-collector SIMS is used. The low level precision often hampers effective examination of concordance of young zircon between U–Pb and Pb/Pb ages, which is crucial to the data quality evaluation and chronological interpretations. In this study, we developed a hybrid “dynamic multi-collector U–Pb dating technique”. It takes advantage of both the static multi-collector mode and peak-hopping mono-collector mode. The technique is able to simultaneously measure with high-precision the 207Pb/206Pb ratio as in the static multi-collector mode without trade off in the analytical precision of the 238U/206Pb ratio of the conventional peak-hopping mono-collector mode. Four zircon reference materials (91500, M257, Temora and Plesovice) were measured to demonstrate that this new analytical protocol is able to achieve a higher precision for the 207Pb/206Pb age by a factor of two than the conventional mono-collector mode within the same working time. It is possible to simultaneously obtain the 207Pb/206Pb age and 238U/206Pb age with comparable quality to effectively evaluate the concordance of the U–Pb system for Phanerozoic samples.

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.