Jianchao Zhang

Precise micrometre-sized Pb-Pb and U-Pb dating with NanoSIMS

We report a new method of Pb-Pb dating for zircon and baddeleyite with a lateral resolution of <2 μm and U-Pb dating for zircon with a lateral resolution of <5 μm using CAMECA NanoSIMS 50L. The O− primary beam was optimized to a current of ∼500 pA with a diameter of 1.7 μm. The zircon standard M257 and baddeleyite standard Phalaborwa were dated by the Pb-Pb method, yielding 207Pb/206Pb ages of 563 ± 14 Ma and 2058 ± 6 Ma, respectively. These results agree well with the recommended ages within analytical uncertainties. Four zircon standards, including Qinghu, Plesovice, Temora, and 91500 were dated by the U-Pb method. The samples were measured in a scanning mode by rastering 3 × 3 μm2 areas in order to eliminate pit depth-dependent U-Pb fractionation. The weighted average 206Pb/238U ages obtained by this method for the four zircon standards are 158 ± 3 Ma, 337 ± 7 Ma, 427 ± 10 Ma, and 1076 ± 14 Ma, respectively. These results are consistent with the reported ID-TIMS ages of these samples within error. Our study demonstrates that Pb-Pb ages of zircon and baddeleyite can be determined with a lateral resolution of <2 μm, and U-Pb ages of zircon can be measured with a lateral resolution of <5 μm by CAMECA NanoSIMS. This technique will have important applications to high lateral resolution dating, such as thin layers of zoned zircons, small grains of lunar zircon, and micron-sized baddeleyites in Martian meteorites and in various achondrites from differentiated asteroids.

Improved precision and spatial resolution of sulfur isotope analysis using NanoSIMS

High precision analyses of all four sulfur isotopes in four pyrite and three sphalerite standards and in working reference samples were carried out using a CAMECA NanoSIMS 50L instrument. The measurements were made using three different settings of the Faraday cup (FC) and/or electron multiplier (EM) detectors, which meet different requirements for spatial resolution. The effects of EM aging and quasi-simultaneous arrival were corrected before the calibration of instrumental mass fractionation by a standard–sample–standard bracket method using the standards measured together with the samples. High analytical precision was achieved by counting 32S, 33S and 34S with the FCs and 36S with the EM (i.e. the FC–FC–FC–EM mode) using a 0.7 μm diameter ∼350 pA Cs+ primary beam and scanning over areas of 5 × 5 μm2. The standard deviations of spot-to-spot and grain-to-grain (external reproducibility 1 SD) measurements were less than 0.3, 0.3 and 0.7‰ for δ33S, δ34S and δ36S, respectively. To achieve a higher lateral resolution of ≤2 × 2 μm2, the Cs+ beam was reduced to 7–10 pA with a diameter of ∼200 nm; 32S was measured with the FC and the other signals were measured with the EMs. The external reproducibility (1 SD) was better than 0.5‰ for both δ33S and δ34S and was 3‰ for δ36S. To achieve the highest lateral resolution for the analysis of submicron-sized sulfides, a ∼0.7 pA Cs+ beam of ∼100 nm diameter was used, scanning over areas of 0.5 × 0.5 μm2, and all 32S, 33S and 34S were counted with the EMs. The external reproducibility (1 SD) was better than 1.5‰ for both δ33S and δ34S. These three modes have important applications in the isotope analysis of micron-sized sulfur samples, such as pyrite framboids and areas of complex zoning in sulfide minerals.

Measurements of water content and D/H ratio in apatite and silicate glasses using a NanoSIMS 50L

Water plays an important role in evolution of the Earth, Mars, Moon and other planets, with H isotopes used as a crucial tracer for fractionation processes and water reservoirs. In order to accurately and precisely measure water contents and D/H ratios of apatite and silicate glass with high lateral resolution, we carried out a long term measurement with a NanoSIMS 50L, with special consideration for the H background, calibration of water content and instrumental mass fractionation. A detection limit of <10 ppm of water content has been achieved mainly by reducing the level of the H background, via improving vacuum and using a high primary beam current of up to 1 nA and a blanking technique. The measurements were carried out in three modes of detector configuration. In multicollection isotope mode, all 1H−, 2D−, 12C− and 18O− were measured simultaneously. Apatite and silicate glasses with water contents of <1.2 wt% were plotted on the same water content calibration curve with a slope of 0.704 ± 0.037 (2SD). In peak jump isotope mode, 1H−, 2D− and 12C− were first measured simultaneously at a magnetic field BF1, and then 18O− and other elements if needed at BF2 by switching the magnetic field. In this mode, apatite and MORB glass standards also share the same water content calibration curve with a slightly higher slope (0.786 ± 0.054, 2SD) relative to that of the multicollection isotope mode. In these two isotope modes, apatite and silicate glass standards have similar instrumental mass fractionation of H isotopes within the analytical uncertainty (45‰, 2SD) and similar precisions on water contents, however, the peak jump isotope mode can determine the volatile element contents and chlorine isotopes. In multicollection element mode, 16O1H− (for water content) and 18O− were measured simultaneously, accompanied usually by other volatile elements. The slope of the water content calibration curve of apatite (3.727 ± 0.112, 2SD) significantly differs from that of silicate glass (0.873 ± 0.049, 2SD). Multicollection element mode can only determine the water and volatile element contents with two times higher sensitivity than that of two isotope modes.

NanoSIMS analytical technique and its applications in earth sciences

Despite the significant improvement on spatial resolution, NanoSIMS still preserves relatively high mass resolution, sensitivity, and analytical precision. It has become an important analytical platform to determine chemical compositions of solid materials, and has been widely used in space, earth, life, and materials sciences, etc. By using a Cs+ ion beam with a size as small as 50 nm scanning over sample surfaces, we are able to obtain high spatial resolution images of up to 7 species simultaneously. When utilizing Faraday cup, high analytical precision of 0.3‰–0.5‰ (1SD) for C, O and S isotopic analysis can be achieved. Although this precision level is still lower than that of conventional SIMS, it already meets the major requirements of Earth Sciences. In 2011, the first NanoSIMS of China (Cameca NanoSIMS 50L) was installed at Institute of Geology and Geophysics, Chinese Academy of Sciences. Based on the working mechanism and analytical modes of the instrument, this paper will systematically introduce the analytical methods established with the NanoSIMS and their potential applications in earth sciences. These methods include trace element distribution images in mineral zoning, high spatial resolution (2–5 μm) Pb-Pb and U-Pb dating, water content and H isotopic analysis for silicate glass and apatite, C isotopic analysis for diamond and graphite, O isotopic analysis for carbonate, S isotopic analysis for sulfides. In addition, the specific requirements for sample preparation will also be introduced in order to facilitate domestic earth scientists’ use.

Sintering nano-crystalline calcite: a new method of synthesizing homogeneous reference materials for SIMS analysis

Reference materials play a crucial role in high accurate isotope and trace element analysis by secondary ion mass spectrometry, and they are used to calibrate instrumental fractionation of samples with various compositions. However, it is often a challenge to prepare natural or synthetic reference materials that are homogenous on a micron or even sub-micron scale. We report a novel method to synthesize homogeneous reference materials via sintering nano-crystals. Coarse-grained crystals (100–500 μm) of calcite have been grown from a starting material of 20–40 nm calcite powder at 1000 °C and 1 GPa for 24 h, which is ∼400 °C below the liquidus line of calcite under the same pressure. The non-molten sintering process and high temperature of the experiments ensure a homogeneous composition of the synthesized calcite. In situ analysis of 18O/16O ratios of the synthetic calcite with a Cameca NanoSIMS 50L revealed no detectable variation within the analytical uncertainty (0.5‰, 1SD), which was further confirmed using a Cameca IMS 1280 with a higher precision (0.1‰, 1SD). In addition, trace elements, Sr and P, are also homogeneous in the synthetic calcite. Another important advantage of this method is that the compositions of synthetic reference materials can be accurately and conveniently determined from the bulk starting materials using independent analytical techniques.

Simultaneous determination of sulfur isotopes and trace elements in pyrite with a NanoSIMS 50L

Pyrite is common sulfide mineral involved in the formation of various ores and hydrothermal and biogenetic activities, and its S isotopic ratios and trace element contents and their spatial distribution have been recorded in the processes of these events. We established simultaneous analyses of 34S/32S ratios and trace element contents of pyrite using nanometer-scale secondary ion mass spectrometry (NanoSIMS). Firstly, the images of S (34S−), As (75As−), Se (80Se−), Cu (63Cu32S−), Au (197Au−) and Pb (208Pb32S−) of pyrite were acquired by rastering areas ranging from 20 × 20 μm2 to 40 × 40 μm2 using a Cs+ beam of 7–10 pA with a diameter of ∼250 nm. Then, the 34S/32S ratios (32S− measured by using a Faraday cup and 34S− measured by using an electron multiplier) and the concentrations of these trace elements of the individual layers of the zoned pyrite grains were simultaneously measured in spot analysis mode, by rastering the same current over an area of 2 × 2 μm2. The 34S/32S ratios were calibrated for matrix effects with pyrite standard Balmat or Py1117, and the external reproducibility (1SD) is <0.5‰. The concentrations of trace elements were calibrated using the relative sensitivity factors (RSFs) of As, Se, Cu, Au and Pb, which were determined from pyrite grains with zoning layers. The determined RSFs of As, CuS, Au and PbS are 4.43 ± 0.28, 0.36 ± 0.04, 0.18 ± 0.03 and 38.0 ± 15.1, respectively. Pyrite from Lannigou Carlin type gold deposits was analyzed as an example, and the results revealed three main episodes of its formation, with each superposed by micron-width oscillation zonings of trace elements. This method has important potential applications in isotopic and elemental investigation of thin layers of pyrite and other sulfides.

THE FIRST DISCOVERY OF PRESOLAR GRAPHITE GRAINS FROM THE HIGHLY REDUCING QINGZHEN (EH3) METEORITE

Presolar graphite grains have been extensively studied, but are limited in carbonaceous chondrites, particularly in Murchison (CM2) and Orgueil (CI1), which sampled materials from the oxidizing regions in the solar nebula. Here, we report the first discovery of presolar graphite grains from the Qingzhen (EH3) enstatite chondrite which formed under a highly reducing condition. Eighteen presolar graphite grains were identified by C-isotope mapping of the low-density fraction (1.75–1.85 g cm−3) from Qingzhen acid residue. Another 58 graphite spherules were found in different areas of the same sample mount using a scanning electron microscope and were classified into three morphologies, including cauliflower, onion, and cauliflower–onion. The Raman spectra of these spherules vary from ordered, disordered, and glassy to kerogen-like, suggestive of a wide range of thermal metamorphisms. NanoSIMS analysis of the C- and Si-isotopes of these graphite spherules confirmed 23 presolar grains. The other 35 graphite spherules have no significant isotopic anomalies, but they share similar morphologies and Raman spectra with the presolar ones. Another three grains were identified during NanoSIMS analysis. Of all the 44 presolar graphite grains identified, six grains show 28Si-excesses, suggestive of supernovae origins, and four grains are 12C- and 29,30Si-rich, consistent with low-metallicity asymptotic giant branch star origins. Another two graphite spherules have extremely low 12C/13C ratios with marginal solar Si-isotopes. The morphologies, Raman spectra, and C- and Si-isotopic distributions of the presolar graphite grains from the Qingzhen enstatite chondrite are similar to those of the low-density fractions from Murchison carbonaceous chondrites. This study suggests a homogeneous distribution of presolar graphite grains in the solar nebula.

NanoSIMS imaging method of zircon U-Pb dating

We report an imaging method of zircon U-Pb dating with NanoSIMS 50L, which overcomes the significant U-Pb fractionation as the pit was sputtered deeper during conventional spot mode analysis and can be applied to irregular small grains or heterogeneous areas of zircon. The U-Pb and Pb-Pb ages can be acquired simultaneously for 2 μm×2 μm (for small grains) or 1 μm×9 μm (for zoned grains), together with Zr, Y and other trace elements distributions. Using zircon M257 as standard, the U-Pb ages of other zircon standards, including Qinghu, Plesovice, Temora and 91500, were measured to (2σ) as 158.8±0.8, 335.9±3.4, 412.0±12 and 1067±12 Ma, respectively, consistent with the recommended values within the analytical uncertainties. Tiny zircon grains in the impact melt breccia of the lunar meteorite SaU 169 were also measured in this study, with a Pb-Pb age of 3912±14 Ma and a U-Pb age of 3917±17 Ma, similar to previous results reported for the same meteorite. The imaging method was also applied to determine U-Pb age of the thin overgrowth rims of Longtan metamorphic zircon, with a Pb-Pb age of 1933±27 Ma and a U-Pb age of 1935±25 Ma, clearly distinct from the Pb-Pb age of 2098±61 Ma and the U-Pb age of 2054±40 Ma for detrital cores.