Vanadium Isotope Composition of Rock Reference Materials by MC-ICP-MS

Significance: With the development of analytical methods and the improvement of analytical accuracy, vanadium isotopes have been increasingly used in the study of various geological processes. In the analysis process of vanadium isotopes, using reference materials similar to the sample matrix can reduce experimental bias, which is conducive to obtaining more accurate and precise vanadium isotope data. Previous studies have reported the vanadium isotopic composition of some common rock reference materials provided by the United States Geological Survey (USGS), with δ⁵¹V values ranging from −1.65‰ to −0.61‰^{[12,15,28,42]}. However, many USGS reference materials with reported vanadium isotopic composition are facing issues such as insufficient inventory (some are already sold out). In order to provide continuous support for research in related fields with high-precision vanadium isotope data, there is an urgency to calibrate the vanadium isotopes of more new geological reference materials, so as to better monitor the precise measurement of vanadium isotopes and enable data comparison between laboratories internationally. A series of international and national geological reference materials with unreported vanadium isotope composition have been selected from the Geological Survey of Japan (GSJ) and the Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences (IGGE) for measuring vanadium isotopic composition. Their composition are widely distributed, with vanadium content ranging from 77g/g to 635g/g, SiO₂ from 43.44% to 64.43%, and TiO₂ from 0.64% to 2.94%, which cover the components of most natural samples. The vanadium isotopic composition of these reference materials is intended to supplement the reference materials database of vanadium isotope research and also to provide more options for the comparison of vanadium isotope data between different laboratories.

Methods: The detailed information of the reference materials (GSP-2, BIR-1, GBW07454, JA-1, JB-3, JB-1b, GBW07105, JGb-1, and GBW07123) and isotope standard solutions (AA, USTC-V, BDH, and NIST-3165) used in the experiment is shown in Table 1. The samples are rock or soil standard materials. The rock standard materials were digested using the acid dissolution method on an electric heating plate at ordinary pressure, while the soil standard materials were digested using the dissolution method with bomb. Chemical purification used the four-column combined chemical separation method with AG50W-X12 cation exchange resin and AG1-X8 anion exchange resin described by Wu et al (2016)^[15]. The vanadium isotopes were determined using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), and the main working conditions of the instrument are shown in Table 2. The mass bias effect produced by the instrument during the test process was corrected using the sample standard bracket method. Although the majority of matrix elements Ti and Cr were removed during the resin purification, trace amounts of Ti and Cr can also affect the measurement of ⁵⁰V during mass spectrometry analysis. Therefore, it is necessary to accurately correct the interference of residual ⁵⁰Ti and ⁵⁰Cr on ⁵⁰V. Experimental condition testing shows that the sample solution must meet the conditions of ⁴⁹Ti/⁵¹V<0.00004 and ⁵³Cr/⁵¹V<0.00004. At this point, the interference of ⁵⁰Ti and ⁵⁰Cr on ⁵⁰V can be corrected, and the test data is considered reliable^[15].

　　Three methods were used to monitor the precision and accuracy of vanadium isotope measurements, including (1) internal laboratory standard monitoring; (2) monitoring of reference samples with recommended values; (3) monitoring of replicate samples. The vanadium isotopic composition of the laboratory internal standard solutions BDH and NIST-3165 during this test process is consistent with the long-term test results of this laboratory within the error range (Fig.2). The vanadium isotopic composition of the petrologic reference materials BIR-1 and GSP-2 is consistent with the data reported in previous literature within the error range. The vanadium isotopic composition of all standard materials in multiple replicate samples is consistent within the error range, and the measurement precision of δ⁵¹V is greater than 0.08‰ (2SD). The above results ensure the precision and accuracy of the test.

Date and Results: The δ⁵¹V value of the soil reference material GBW07454 is −0.78‰±0.06‰ (2SD, n=12), gabbro reference material JGb-1 is −1.05‰±0.08‰ (2SD, n=9), diabase reference material GBW07123 is −0.72‰±0.06‰ (2SD, n=15), basalt reference material JB-3 is −0.81‰±0.09‰ (2SD, n=9), basalt reference material JB-1b is −0.79‰±0.08‰ (2SD, n=9), basalt reference material GBW07105 is −0.80‰±0.06‰ (2SD, n=15), and andesite reference material JA-1 is −0.34‰±0.06‰ (2SD, n=9) (see Table 4 and Fig.3).

　　The δ⁵¹V value of the gabbro reference material JGb-1 is currently the lowest in reported igneous rock samples, lower than the average vanadium isotope composition of MORB (−0.84‰±0.10‰). Its lighter V vanadium isotope composition may be influenced by high-temperature hydrothermal alteration, but the specific genesis requires further research^[27]. The vanadium isotopes of the diabase standard material and three basalt standard materials GBW07105, JB-3, and JB-1b are relatively homogeneous, all falling within the MORB range. The δ⁵¹V value of the andesite reference material JA-1 is the highest, indicating a significant enrichment of vanadium isotopes compared to basalt, indicating the possible presence of vanadium isotope fractionation during magma evolution^{[26-28,32-33]}. The δ⁵¹V value of the soil reference material GBW07454 is consistent with previous reports (−0.74‰±0.08‰) and is close to the composition of MORB. It also indicates that continental weathering does not cause significant vanadium isotope fractionation, which is consistent with previous research findings on the weathering product of basalt, Zhanjiang laterite^[34].