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Volume 31 Issue 2
Apr.  2020
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Lu Chen, Zhian Bao, Honglin Yuan, Kaiyun Chen, Chunlei Zong. Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks. Journal of Earth Science, 2020, 31(2): 249-253. doi: 10.1007/s12583-019-1261-6
Citation: Lu Chen, Zhian Bao, Honglin Yuan, Kaiyun Chen, Chunlei Zong. Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks. Journal of Earth Science, 2020, 31(2): 249-253. doi: 10.1007/s12583-019-1261-6

Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks

doi: 10.1007/s12583-019-1261-6
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Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks

doi: 10.1007/s12583-019-1261-6
    Corresponding author: Honglin Yuan

Abstract: In sample preparation and mass spectrometry analysis, sample dissolution, column chemistry, concentration mismatches, and matrix effects have significant potential for introducing analytical artifacts during Mg isotope analysis. Based on the low MgO content and undesirable matrix elements in felsic rocks, the development of well-characterized felsic standards is essential to reduce interlaboratory mass bias, enable the assessment of data accuracy, and facilitate the comparison of chemical separation procedures in different laboratories. In this work, the homogeneity and long-term stability of two felsic rock standards, GSR-1 and RGM-2, were evaluated due to their low MgO contents. Furthermore, synthetic solutions with doped matrix elements were used to evaluate potential Mg isotope analytical artifacts using multi-collector inductively coupled plasma mass spectrometry. The accuracy and precision of Mg isotopic compositions in GSR-1 and RGM-2 were assessed by repeated measurements over twelve months. The long-term tests show that the Mg isotopic compositions of the two low MgO felsic rocks (GSR-1 and RGM-2) are homogenous among batches and can be used as low MgO reference materials for accuracy assessments of Mg isotopic analyses. The Mg isotopic compositions (δ26Mg) of GSR-1 and RGM-2 were marked as -0.223‰±0.053‰ (2s, n=50) and -0.184‰±0.058‰ (2s, n=50) respectively.

Lu Chen, Zhian Bao, Honglin Yuan, Kaiyun Chen, Chunlei Zong. Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks. Journal of Earth Science, 2020, 31(2): 249-253. doi: 10.1007/s12583-019-1261-6
Citation: Lu Chen, Zhian Bao, Honglin Yuan, Kaiyun Chen, Chunlei Zong. Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks. Journal of Earth Science, 2020, 31(2): 249-253. doi: 10.1007/s12583-019-1261-6
  • Magnesium (Mg) is a major element of the silicate Earth. It has three stable isotopes, 24Mg, 25Mg, and 26Mg, with abundances of 78.99%, 10.00%, and 11.01%, respectively. 24Mg and 26Mg have a large mass difference of ~8%, which enables them to function as a powerful tool to trace a variety of geological processes, such as weathering, magmatic differentiation, and metamorphic processes (Su et al., 2018; Hu et al., 2017; Teng, 2017; Huang et al., 2012; Liu et al., 2010; Teng et al., 2007). As little Mg isotope fractionation occurs during high-temperature processes (Teng, 2017), high-precision analysis is necessary for Mg isotope studies of igneous rocks.

    The combined application of multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) and ion exchange chromatographic separation has achieved a precision of better than 0.10‰ for Mg isotope analysis of igneous rock samples (Hu et al., 2016; Teng et al., 2015a, b, 2010, 2007; An et al., 2014; An and Huang, 2014; Wang et al., 2011; Liu et al., 2010; Bolou-Bi et al., 2009; Huang et al., 2009). This excellent analytical capability is useful for determining small mass- dependent Mg isotope fractionation at high temperatures for terrestrial samples (Teng, 2017). However, accurate determinations of δ26Mg in diverse igneous rocks remain challenging due to the significant risk of bias introduced during ion exchange chromatographic separation, large instrument mass bias caused by matrix effects, and the effects of acidity and concentration mismatch during standard-sample bracketing measurements (An et al., 2014; Teng and Yang, 2014; Wang et al., 2011; Huang et al., 2009; Chang et al., 2003; Galy et al., 2001). For example, analyses of basalt and olivine in different laboratories yield significantly different Mg isotopic compositions, e.g., δ26Mg of BCR-2 ranged from 0 to -0.45‰; δ26Mg of BHVO-2 ranged from 0 to -0.34‰ (Jochum et al., 2005); δ26Mg of San Carlos olivine ranged from -0.06‰ to -0.73‰ (Hu et al., 2016; Wang et al., 2016; Sio et al., 2013; Pogge von Strandmann et al., 2011; Chakrabarti and Jacobsen, 2010; Wimpenny et al., 2010; Handler et al., 2009; Huang et al., 2009; Young et al., 2009; Teng et al., 2007; Wiechert and Halliday, 2007; Pearson et al., 2006). Hu et al. (2016) indicated that the highly variable Mg isotopic compositions of olivine were the result of analytical artifacts during chemical separation and instrumental analysis. Teng et al. (2015b) determined δ26Mg values of 12 silicate standards ranging from ultramafic to felsic in five laboratories using three types of MC-ICP-MS instruments. Small interlaboratory differences observed were likely caused by incomplete sample digestion or chemical separation, such as incomplete separation of the matrix elements, low Mg yields, contamination during sample preparation, and fluoride precipitate during sample digestion. Hence, some well-characterized reference materials (RMs) matching sample matrices are required to validate analytical results. Previous studies have established diverse RMs with varied matrices, such as olivine, seawater, dolomite, limestone, soil, river water, and apple leaves. San Carlos olivine has a homogenous Mg isotopic composition, making it a suitable standard for Mg isotopic analysis of olivine samples (Hu et al., 2016). Mg isotopic compositions of seawater sampled form the Gulf of Mexico, both vertical and horizontal, are homogeneous and identical to the published values. Collectively, global seawater exhibits a homogeneous δ26Mg value of -0.83‰±0.09‰ (2s, n=90) and is an excellent solution standard for the accuracy assessment of chemical separation (Ling et al., 2011). A wide range of Earth-surface materials from low-temperature environments, including river water, spring water, Dead Sea brine, dolomite, limestone, soil, and vegetation, has been prepared by Shalev et al. (2018) for δ26Mg determination validation in Earth-surface geochemical studies.

    More attention should be paid when dealing with felsic rocks because of their low MgO contents and undesired matrix elements. Li et al. (2010) performed at least four types of column chemistry for A-type granites to obtain a pure Mg solution and ensure data accuracy. An et al. (2014) adopted a newly calibrated HNO3+HF step to remove matrix elements in low MgO rock standards. Bao et al. (2019) developed a precipitation procedure to remove K and column chemistry to remove Fe, Ca, Al, Ti, and Na in diverse rocks. The long-term reproducibility of δ26Mg for standard solutions and rock samples is ±0.06‰. Although different sample preparation methods have been certified to determine accurate Mg isotopic compositions in felsic rocks, there are still some matrix elements remained in the purified Mg solutions. Therefore, the effects of acidity and concentration mismatch must be re-evaluated using solutions with reduced undesired matrix elements. Furthermore, well-characterized low MgO standard rocks should be used to reduce interlaboratory mass bias, enable data accuracy assessments, and facilitate comparison of chemical separation procedures between laboratories.

    In this study, a synthetic solution doped with little matrix elements was used to investigate the effects of acidity and concentration mismatches during mass spectrometry analyses. Furthermore, the Mg isotope compositions of low MgO felsic rocks (granite GSR-1 and rhyolite RGM-2) in different batches were determined over a period of twelve months. The obtained results were evaluated to determine if the two rocks can be used as low MgO reference materials for Mg isotope assessments.

  • The reference granite GSR-1 and rhyolite RGM-2 were purchased from the Chinese National Research Center for Certified Reference Materials and the United States Geological Survey (USGS), respectively. The batch numbers on the GSR-1 bottles were 564401, 551709, 552808, 444508, 552803, 490689, 430492, 490473, 420125, 310022, 580063, and 490720. The batch numbers on the RGM-2 bottles were 1632, 0903. The MgO contents of GSR-1 and RGM-2 were 0.42 wt.% and 0.28 wt.%, respectively. All samples were prepared directly from 200 mesh powders which were dried at 105 ℃ in an oven for 1 h prior to acid digestion. Twelve bottles of GSR-1 and two bottles of RGM-2 in different batches were used to determine the homogeneity of Mg isotopic compositions in the references. Commercially available hydrofluoric acid (HF, 40% v/v, GR grade), nitric acid (HNO3, 68% v/v, GR grade), and hydrochloric acid (HCl, 36% v/v, GR grade) were further distilled using a sub-boiling distillation system (Savillex DST-1000). Ultrapure water with a resistivity of 18.2 MΩ·cm was obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA). A purified and concentrated GSB-Mg solution was purchased from China Iron & Steel Research Institute Group (CISRI). DSM3 and Cambridge-1 standard solutions were provided by Professor Uwe H. Wiechert and stored in 100 mL FEP bottles at 20 μg/g in 10% HNO3. Synthetic GSB-Mg-1 solutions with little undesired matrix elements (Mg : Na : K : Ca : Al : Fe : Ti : Mn : Cu : Zn=1 : 0.005 : 0.005 : 0.005 : 0.005 : 0.005 : 0.001 : 0.001 : 0.001 : 0.001) were used to investigate the effects of acidity and concentration mismatches during MC-ICP-MS analyses.

  • All chemical procedures were performed in an ultra-clean lab at the State Key Laboratory of Continental Dynamics (SKLCD), Northwest University, China. First, 19.8 and 27.78 mg of sample powders were weighed for GSR-1 and RGM-2 to obtain approximately 50 μg of Mg for chemical purification. The detailed sample digestion procedure and chemical separation procedure were described elsewhere (Bao et al., 2019; Yuan et al., 2018). The Mg isotope ratios were determined using a Nu Plasma II MC-ICP-MS, which is a double-focusing mass spectrometer with 16 Faraday cups and five full-size discrete dynode multipliers. For the analyses, L5, Ax, and H5 Faraday cups were used to collect 24Mg, 25Mg, and 26Mg, respectively. In this study, all Mg isotope analyses were performed using Faraday cups in static mode. A 'wet' plasma with wet cones and a GE 100 μL/min quartz nebulizer were used to determine the Mg isotopes. The sample-standard bracketing (SSB) technique was used to correct instrumental mass bias during analysis. The detailed instrumental parameters of Nu Plasma II are listed in Table 1.

    Instrument parameters Nu Plasma II
    RF powder 1 300 W
    Cooling gas 13.0 L·min-1
    Auxiliary gas 0.8 L·min-1
    Accelerating voltage 6 000 V
    Nebulizer ~38 Psi
    24Mg sensitivity 8 V·ppm-1
    Background of 24Mg < 1.5 mV
    Cones Ni orifice, wet cone set
    Resolution mode Low resolution (MM=400)
    Sample uptake rate 100 μL·min-1

    Table 1.  Instrument operating parameters for the Mg isotope measurements

    The results were expressed as millesimal deviation of the DSM3 standard isotopic composition

    where X refers to mass 25 or 26. Because all samples analyzed in this study follow a mass-dependent fractionation, δ26Mg was exclusively used in the following discussion.

  • Acidity and concentration mismatches in the SSB method can cause metal stable isotopic deviation, as has been reported previously (Yuan et al., 2017; An et al., 2014; Teng and Yang, 2014; Huang et al., 2009). Different deviation degrees have been observed using different analytical instruments in different laboratories. Purified solutions with varied acidities and concentrations are often used to investigate the effects of acidity and concentration mismatches. Based on the undesired matrix elements in the final Mg solutions after chemical separation, a series of 0.5 μg/g GSB solutions with matrix elements (Mg : Na : K : Ca : Al : Fe : Ti : Mn : Cu : Zn=1 : 0.005: 0.005 : 0.005 : 0.005 : 0.005 : 0.001 : 0.001 : 0.001 : 0.001) were diluted with 0.5% to 5.0% (v/v) HNO3. These solutions were used to investigate the effects of acidity, whereas a pure 0.5 μg/g GSB solution was diluted with 2% (v/v) HNO3 used as the medium. The results show that the differences in acidity between the standard and samples significantly influenced the Mg isotopic compositions measured in wet plasma mode (Fig. 1). When the acidity ranged from 2% to 3.5% (v/v), the Mg isotopic compositions remained within the uncertainty intervals (two standard deviation, 2s=0.06‰). The δ26Mg values were 0.27‰ heavier and 0.19% lighter than those measured in 2% (v/v) HNO3 at acidities of 0.5% and 4.5%, respectively. A strongly negative correlation was observed between the δ26Mg values and acidity, whereas it did not significantly influence the Mg isotopic compositions in purified GSB Mg solutions in previous studies (Bao et al., 2019). The same analytical conditions with different results using the same instrument indicate that the effect of acidity is more significant when undesired matrix elements remain in the final solution. To obtain accurate δ26Mg ratios, all solutions in the measurement session were prepared using the same batch of 2% HNO3.

    Figure 1.  δ26Mg variations during the measurement of GSB-Mg-1 solutions diluted using different HNO3 acid strengths. The error bars (2s) were obtained from four replicates.

  • The effects of concentration mismatch between the standards and samples in the Mg isotopic analysis were re-evaluated using a GSB-Mg-1 solution with little matrix elements, while a series of pure GSB solutions with varied concentrations served as the bracketed standards. The δ26Mg values agreed well within the 2s, when the Csample/Cstandard ratios ranged from 0.6 to 1.6 (Fig. 2). A strongly positive correlation was observed with Csample/Cstandard ratios ranging from 0.3 to 0.6. Because the effect of concentration mismatch significantly influenced the GSB-Mg-1 solution measurement, a rigorous concentration match between the standard and samples was recommended to obtain accurate Mg isotopic ratios.

    Figure 2.  δ26Mg variations during the measurement of GSB-Mg-1 solutions with different concentrations. The Mg concentration of the bracketing standard was 0.5 μg/g. The error bars (2s) were obtained from four replicates.

  • The long-term precision and accuracy of GSR-1 and RGM-2 measurements were assessed by determining independent digestions of different batches of rock powders over a period of twelve months. The δ26Mg ratios of GSR-1 and RGM-2 after independent digestion are provided in Fig. 3. The twelve batches of GSR-1 after the first digestion section yield δ26Mg values of -0.202‰±0.054‰, -0.238‰±0.028‰, -0.213‰±0.053‰, -0.225‰±0.033‰, -0.202‰±0.043‰, -0.253‰±0.055‰, -0.248‰±0.020‰, -0.206‰±0.035‰, -0.211‰±0.044‰, -0.231‰±0.035‰, -0.215‰±0.021‰, and -0.259‰±0.068‰ (Table 2, bottles 1–12). Within the analytical precision, no heterogeneity of the Mg isotope compositions in different bottles of GSR-1 was observed. These values are identical to the previously published values by An et al. (2014)26MgGSR-1= -0.23‰±0.02‰) and Teng et al. (2015b)26MgGSR-1= -0.24‰±0.02‰). The twelve bottles of GSR-1 were also randomly digested and the δ26Mg values were determined over twelve months to perform Grubbs' test and identify outliers with respect to the mean values. As all residuals were smaller than the critical value (|vp < λ(0.95, 50)|), no data were discarded. Thus, the Mg isotopic compositions of GSR-1 were marked as δ26Mg= -0.223‰±0.053‰ (n=50; Fig. 3). The good agreements between three labs show that the GSR-1 is a suitable candidate as a low MgO reference material for assessment of accuracy in felsic rocks.

    Figure 3.  Mg isotope compositions of GSR-1 and RGM-2. The δ26Mg mean values were obtained from 50 independent tests (n). Each value and error bar (2s) was calculated from four replicates in one independent test. The numbers near the points corresponded to batch numbers in Table 2.

    Sample Batch number This work Reported values
    δ26Mg (‰) 2s (‰) n δ26Mg (‰) 2s (‰) Sources
    GSR-1-1 564401 -0.202 0.054 5 -0.23
    -0.24
    0.02
    0.02
    An et al. (2014)
    Teng et al. (2015b)
    GSR-1-2 551709 -0.238 0.028 5
    GSR-1-3 552808 -0.213 0.053 5
    GSR-1-4 444508 -0.225 0.033 5
    GSR-1-5 552803 -0.202 0.043 5
    GSR-1-6 490689 -0.253 0.055 5
    GSR-1-7 430492 -0.248 0.020 5
    GSR-1-8 490473 -0.206 0.035 5
    GSR-1-9 420125 -0.211 0.044 5
    GSR-1-10 310022 -0.231 0.035 5
    GSR-1-11 580063 -0.215 0.021 5
    GSR-1-12 490720 -0.259 0.068 5
    GSR-1 Mean -0.223 0.043
    GSR-1 and GBW07103 in Teng et al. (2015b) are the same granite standard.

    Table 2.  Mg isotope composition of 12 bottles of GSR-1 independently digested

    Similarly, the Mg isotopic compositions of RGM-2 of two batches were homogeneous and agree with previous work (An et al., 2014). After long-term testing, the Mg isotopic compositions of RGM-2 were marked as δ26Mg= -0.184‰±0.058‰ (n=50; Fig. 3). Hence, the RGM-2 can be also used as a low MgO reference material for Mg isotope tests of felsic rocks.

  • In this study, the effects of acidity and concentration mismatches were re-evaluated strictly using GSB-Mg-1 solutions with few matrix elements. The acidity and concentration of the samples should be identical to those of the standards, which was essential for eliminating Mg isotopic composition deviation. Moreover, the long-term tests show that the Mg isotopic compositions of the two low MgO felsic rocks (GSR-1 and RGM-2) are homogenous between batches and can be used as low MgO reference materials for accuracy assessments of Mg isotopic analyses.

  • This study was co-supported by the National Science Foundation of China (Nos. 41803040, 41825007, 41421002, and 41427804), and the MOST Research Foundation from the State Key Laboratory of Continental Dynamics. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1261-6.

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