Geostandards and Geoanalytical Research

Cộng đồng quốc tế về địa chất tuổi U‐(Th‐)Pb bằng phương pháp LA‐ICP‐MS đã xác định các tiêu chuẩn mới cho việc xác định tuổi U‐(Th‐)Pb. Một quy trình làm việc mới xác định sự truyền đạt phù hợp về các không chắc chắn cho các dữ liệu này, xác định các thành phần ngẫu nhiên và hệ thống. Chỉ những dữ liệu có các không chắc chắn liên quan đến lỗi ngẫu nhiên mới nên được sử dụng trong các phép tính trung bình trọng số của tuổi dân số; các thành phần không chắc chắn cho lỗi hệ thống được truyền đạt sau giai đoạn này, ngăn chặn sự giảm thiểu sai lầm của chúng. Theo quy trình truyền đạt không chắc chắn được cải thiện này, dữ liệu có thể được so sánh ở các mức độ không chắc chắn khác nhau để giải quyết tốt hơn sự khác biệt về tuổi. Các giá trị tham chiếu mới cho các vật liệu tham chiếu zircon, monazite và titanite thường dùng được xác định (dựa trên ID‐TIMS) sau khi loại bỏ các chỉnh sửa cho chì chung và các tác động của thorium dư thừa ²³⁰Th. Những giá trị này phản ánh chính xác hơn vật liệu được lấy mẫu trong quá trình xác định các yếu tố chuẩn hóa bằng phân tích LA‐ICP‐MS. Các khuyến nghị được đưa ra để biểu diễn dữ liệu một cách đồ họa chỉ với các elip không chắc chắn tại 2s và để gửi hoặc trích dẫn dữ liệu xác thực cùng với dữ liệu mẫu khi gửi dữ liệu để xuất bản. Các tiêu chuẩn báo cáo dữ liệu mới được xác định để giúp cải thiện quy trình phản biện. Với những cải tiến này, dữ liệu U‐(Th‐)Pb bằng phương pháp LA‐ICP‐MS có thể được coi là vững chắc hơn, chính xác hơn, được tài liệu hoá tốt hơn và được định lượng, góp phần trực tiếp vào việc cải thiện diễn giải khoa học của chúng.

VizualAge, một công cụ phần mềm máy tính mới để phân tích dữ liệu U‐Pb thu được bằng phương pháp ICP‐MS hấp dẫn laser, đã được phát triển. Nó bao gồm một sơ đồ giảm dữ liệu (DRS) cho Iolite (một công cụ phân tích dữ liệu khối phổ chung) cũng như các quy trình trực quan hóa. Ngoài các tuổi U/Pb và Th/Pb được tính toán bởi DRS địa sinh học U‐Pb của Iolite, VizualAge cũng tính toán tuổi ²⁰⁷Pb/²⁰⁶Pb và các hiệu chỉnh Pb chung cho mỗi mảnh thời gian của dữ liệu thô. Quan trọng là VizualAge cho phép hiển thị một sơ đồ concordia trực tiếp để trực quan hóa dữ liệu trên sơ đồ này khi một khoảng thời gian tích hợp đang được điều chỉnh. Điều này cung cấp phản hồi ngay lập tức về sự không đồng nhất, không chắc chắn, tương quan sai số và Pb chung. Một số bộ dữ liệu zircon đã được sử dụng để minh họa cách sơ đồ concordia trực tiếp có thể được sử dụng như một công cụ kiểm tra mạnh mẽ, cho thấy một phân tích đơn lẻ bao gồm các vùng đồng thuận, các khu vực biến chất cũng như các lõi thừa kế hoặc các lớp phủ trẻ hơn. VizualAge cũng xây dựng các biểu đồ hình chữ nhật, các sơ đồ concordia kiểu cổ điển và Tera‐Wasserburg, cũng như các sơ đồ concordia 3D U‐Th‐Pb và tổng hợp U‐Pb. Độ chính xác và độ tin cậy của dữ liệu được giảm với VizualAge được chứng minh bằng các ví dụ về vật liệu tham chiếu zircon Plešovice, Temora‐2 và Penglai. Dữ liệu cho zircon từ Batholith Long Lake (cấu tạo Wyoming) đã được sử dụng để minh họa cách VizualAge tính toán các hiệu chỉnh Pb chung và giúp làm sáng tỏ những khó khăn chưa được giải thích với việc xác định chính xác ²⁰⁴Pb.

High‐precision calcium isotopic compositions of a set of geological reference materials from the IAG (OU‐6), ANRT (UB‐N), MPI‐DING, USGS and GSJ, relative to NIST SRM 915a, are reported here. Measurements were performed by thermal ionisation mass spectrometry (Triton instrument) using a ⁴²Ca–⁴³Ca double spike. δ^44/40Ca values of selected reference materials, mainly felsic rocks, are reported for the first time. Felsic rock values of δ^44/40Ca ranged from 0.13‰ to 1.17‰, probably implying Ca isotopic fractionation could occur during magma evolution. δ^44/40Ca values of ultramafic rocks, ranging from 0.74‰ to 1.51‰, were positively correlated with MgO and negatively with CaO contents, possibly owing to Ca isotopic fractionation during partial melting. δ^44/40Ca of intermediate‐mafic rocks were around 0.78‰ and displayed limited variation, suggesting Ca isotopic fractionation is insignificant during magma evolution processes. As expected, δ^44/40Ca of sedimentary and metamorphic rocks varied widely due to complex geological processes.

Various zircons of Proterozoic to Oligocene ages (1060‐31 Ma) were analysed by laser ablation‐inductively coupled plasma‐mass spectrometry. Calibration was performed using Harvard reference zircon 91500 or Australian National University reference zircon TEMORA 1 as external calibrant. The results agree with those obtained by SIMS within 2s error. Twenty‐four trace and rare earth elements (P, Ti, Cr, Y, Nb, fourteen REE, Hf, Ta, Pb, Th and U) were analysed on four fragments of zircon 91500. NIST SRM 610 was used as the reference material and ²⁹Si was used as internal calibrant. Based on determinations of four fragments, this zircon shows significant intra‐and inter‐fragment variations in the range from 10% to 85% on a scale of 120 μm, with the variation of REE concentrations up to 38.7%, although the chondrite‐normalised REE distributions are very similar. In contrast, the determined age values for zircon 91500 agree with TIMS data and are homogeneous within 8.7 Ma (2s). A two‐stage ablation strategy was developed for optimising U‐Pb age determinations with satisfactory trace element and REE results. The first cycle of ablation was used to collect data for age determination only, which was followed by continuous ablation on the same spot to determine REE and trace element concentrations. Based on this procedure, it was possible to measure zircon ages as low as 30.37 0.39 Ma (MSWD = 1.4; 2s). Other examples for older zircons are also given.

Geochemical studies of geological samples require the precise determination of their major and trace element contents and, when measured, of their isotopic compositions. It is now commonly accepted that the accuracy and precision of geochemical analyses are best estimated by the concomitant analysis of international reference materials run as unknown samples. Although the composition of a wide selection of basalts is relatively well constrained, this is far from being the case for sedimentary materials. We present here a comprehensive set of major and trace element data as well as Nd, Hf, Sr and Pb isotopic compositions for thirteen commonly used international reference materials – eight magmatic rocks (BHVO‐2, BR, BE‐N, BR 24, AGV‐1, BIR‐1, UB‐N, RGM‐1) and five sediments (JLk‐1, JSd‐1, JSd‐2, JSd‐3, LKSD‐1). We determined the concentrations of over forty elements in the magmatic rocks together with Sr, Nd, Hf and Pb isotopic compositions. Our trace element results were both accurate (difference ≤ 3%) and precise (reproducibility at 1s ≤ 3%) and the isotopic results were very similar to other published values. In contrast, we observed a significant chemical and isotopic variability in the sedimentary materials, which we attribute to mineral heterogeneities in the powders. Despite the limitation imposed by this heterogeneity, our work presents a complete set of data determined with a precision not yet achieved in the literature for sedimentary material. We also provide the first Nd, Hf and Pb isotopic measurements for the five sediments, which are commonly used by the geochemical community. Our study of both basalt and sediment reference materials represents a comprehensive and self‐consistent set of geochemical data and can therefore be considered as a reference database for the community.

The NanoSIMS ion probe is a new‐generation SIMS instrument, characterised by superior spatial resolution, high sensitivity and multi‐collection capability. Isotope studies of certain elements can be conducted with 50–100 nm resolution, making the NanoSIMS an indispensable tool in many research fields. We review technical aspects of the NanoSIMS ion probe and present examples of applications in cosmochemistry and biological geochemistry. This includes isotope studies of presolar (stardust) grains from primitive meteorites and of extraterrestrial organics, the search for extinct radioactive nuclides in meteoritic materials, the study of lunar samples, as well as applications in environmental microbiology, cell biology, plant and soil science, and biomineralisation.

This paper reports the results from a second characterisation of the 91500 zircon, including data from electron probe microanalysis, laser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS), secondary ion mass spectrometry (SIMS) and laser fluorination analyses. The focus of this initiative was to establish the suitability of this large single zircon crystal for calibrating in situ analyses of the rare earth elements and oxygen isotopes, as well as to provide working values for key geochemical systems. In addition to extensive testing of the chemical and structural homogeneity of this sample, the occurrence of banding in 91500 in both backscattered electron and cathodoluminescence images is described in detail. Blind intercomparison data reported by both LA‐ICP‐MS and SIMS laboratories indicate that only small systematic differences exist between the data sets provided by these two techniques. Furthermore, the use of NIST SRM 610 glass as the calibrant for SIMS analyses was found to introduce little or no systematic error into the results for zircon. Based on both laser fluorination and SIMS data, zircon 91500 seems to be very well suited for calibrating in situ oxygen isotopic analyses.

We introduce a potential new working reference material – natural zircon megacrysts from an Early Pliocene alkaline basalt (from Penglai, northern Hainan Island, southern China) – for the microbeam determination of O and Hf isotopes, and U–Pb age dating. The Penglai zircon megacrysts were found to be fairly homogeneous in Hf and O isotopes based on large numbers of measurements by LA‐multiple collector (MC)‐ICP‐MS and SIMS, respectively. Precise determinations of O isotopes by isotope ratio mass spectrometry (IRMS) and Hf isotopes by solution MC‐ICP‐MS were in good agreement with the statistical mean of microbeam measurements. The mean δ¹⁸O value of 5.31 ± 0.10‰ (2s) by IRMS and the mean ¹⁷⁶Hf/¹⁷⁷Hf value of 0.282906 ± 0.0000010 (2s) by solution MC‐ICP‐MS are the best reference values for the Penglai zircons. SIMS and isotope dilution‐TIMS measurements yielded consistent ²⁰⁶Pb/²³⁸U ages within analytical uncertainties, and the preferred ²⁰⁶Pb/²³⁸U age was found to be 4.4 ± 0.1 Ma (95% confidence interval). The young age and variably high common Pb content make the Penglai zircons unsuitable as a primary U–Pb age reference material for calibration of unknown samples by microbeam analysis; however, they can be used as a secondary working reference material for quality control of U–Pb age determination for young (particularly < 10 Ma) zircon samples.

Isotope dilution determinations of Lu, Hf, Zr, Ta and W are reported for nine test portions (five for W) of NIST SRM 610 and 612 glass wafers. Additionally, all test portions were analysed for their Hf isotope compositions. In general, high field strength elemental (HFSE) distributions in NIST SRM 610 and 612 were reproducible to ∼± 1%, except for Zr (± 5%) in NIST SRM 612, and absolute reported concentrations agreed with previously published values, but with higher precision. The slightly worse reproducibility of Zr in NIST SRM 612 compared to other HFSE is interpreted to result from analytical scatter, rather than sample inhomogeneity. The analyses demonstrated elemental homogeneity for both glass wafers for samples of 1–2 mg with respect to the precision of the method, i.e., ± 1% or better. Average Hf isotope compositions for both glass wafers agreed within uncertainty and the weighted average of all determinations yielded a mean ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.282111 ± 0.000009 (95% confidence level). However, although mean values for NIST SRM 610 and 612 agreed within analytical limits, NIST SRM 610 test portions showed a tendency of systematically elevated isotope composition of ∼ 0.5 _ɛHf units when compared to NIST SRM 612, which may indicate a slightly more radiogenic Hf isotope composition of NIST SRM 610. The results of this study suggest that NIST SRM 610 and 612 are valuable calibrators for HFSE in situ analyses within the given uncertainties.

There is a growing need for new zircon reference materials for in situ Hf‐isotope analysis by laser ablation‐multicollector inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS). In this contribution we document the results of a preliminary investigation of seven natural zircons, conducted in order to test their suitability in this regard. Solution MC‐ICP‐MS data on separated Lu and Hf fractions provided reference compositional data while the results of ca. 750 in situ LA‐MC‐ICP‐MS analyses allowed assessment of potential micrometre‐scale heterogeneity. On the basis of these analyses and additional relevant considerations such as availability, size and (Lu)Yb/Hf ratio, we suggest that, of the currently available zircons, Temora‐2 and Mud Tank are most likely to provide robust reference materials for Hf isotope determinations both at the present time and into the future. The former has the advantage of also being well‐characterised for U‐Th‐Pb systematics and suitable for in situ age determination, while the latter is the most readily available and is of very large grain size. Additional materials such as BR266, and 91500, although limited in supply, show more consistent Lu/Hf ratios and are thus of use in monitoring elemental fractionation during ICP‐MS analysis.