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 (M/ΔM=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.
1.1. Reagents and Samples
1.2. Chemical Separation and Mass Spectrometry
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.
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.
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) (δ26MgGSR-1= -0.23‰±0.02‰) and Teng et al. (2015b) (δ26MgGSR-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
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.