前處理在配備100級操作臺的千級超凈室完成。樣品消解：（1）將200目樣品置于105℃烘箱中烘干12小時；（2）準確稱取粉末樣品50-200mg置于Teflon溶樣彈中；（3）依次緩慢加入1-3ml高純HNO3和1-3ml高純HF；（4）將Teflon溶樣彈放入鋼套，擰緊后置于190℃烘箱中加熱24小時以上；（5）待溶樣彈冷卻，開蓋后置于140℃電熱板上蒸干，然后加入1ml HNO3 并再次蒸干；（6）用1.5ml的HCl（2.5M）溶解蒸干樣品，待上柱分離?；瘜W分離：用離心機將樣品離心后，取上清液上柱。柱子填充AG50W樹脂。用2.5M HCl淋洗去除基體元素。最終用2.5M HCl將Sr從柱上洗脫并收集。收集的Sr溶液蒸干后等待上機測試。樹脂殘留物質通過4.0M HCl淋洗可獲得REE溶液。接收的REE溶液蒸干后以0.18M HCl提取，用于Nd同位素分離。
一次分離獲得的溶液首先轉換為3M HNO3介質，然后樣品上柱。柱子填充Sr特效樹脂。采用3M HNO3淋洗去除干擾元素。最終用MQ H2O將Sr從柱上洗脫并收集。收集的Sr溶液蒸干后等待上機測試。
Sr同位素分析采用德國Thermo Fisher Scientific 公司的MC-ICP-MS（Neptune Plus）。儀器配備9個法拉第杯接收器。83Kr+、167Er++、84Sr+、85Rb+、86Sr+、173Yb++、87Sr+、88Sr+同時被L4、L3、L2、L1、C、H1、H2、H3等8個接收器接收。其中83Kr+、85Rb+、167Er++、173Yb++被用于監控并校正Kr、Rb、Er和Yb對Sr同位素的同質異位素干擾。MC-ICP-MS采用了H+S錐組合和干泵以提高儀器靈敏度。根據樣品中的Sr含量，50 µl/min-100 µl/min兩種微量霧化器被選擇使用。Alfa公司的Sr單元素溶液被用于優化儀器操作參數。Sr國際標準溶液（NIST 987，200 µg/L）的88Sr信號一般高于7V。數據采集由8個blocks組成，每個block含10個cycles，每個cycle為4.194秒。
Sr同位素的儀器質量分餾采用內標指數法則校正（Russell et al. 1978）：
公式中i和j指示同位素質量數，Rm和RT分別代表樣品的測試比值和參考值（推薦值），f指儀器質量分餾因子。88Sr/86Sr被用于計算Sr的質量分餾因子（8.375209，Lin et al. 2016）。由于前期有效的樣品分離富集處理，干擾元素Ca、Rb、Er、Yb被分離干凈。殘余的83Kr+、85Rb+、167Er++、173Yb++等干擾校正采用Lin et al.（2016）校正方法實驗流程采用兩個Sr同位素標樣（NIST 987和AlfaSr）之間插入7個樣品進行分析。全部分析數據采用專業同位素數據處理軟件“Iso-Compass”進行數據處理（Zhang et al., 2020）。NIST 987的87Sr/86Sr分析測試值為0.710242±14（2SD, n=345），與推薦值0.710248±12（Zhang and Hu, 2020）在誤差范圍內一致，表明本儀器的穩定性和校正策略的可靠性滿足高精度的Sr同位素分析。
BCR-2（玄武巖）和RGM-2（流紋巖）（USGS）被選擇作為流程監控標樣。兩個樣品分別代表了基性巖和酸性巖，具有顯著的物理化學差異。RGM-2的具有較高的Rb含量（149 µg/g）和適中的Sr含量（108 µg/g）,可以有效監控Rb的分離過程和測試結果。BCR-2的87Sr/86Sr分析測試值為0.705012±22 （2SD, n=63），與推薦值0.705012±20（Zhang and Hu, 2020）在誤差范圍內一致。RGM-2的87Sr/86Sr分析測試值為0.704173±20 （2SD, n=20），與推薦值0.704184±10（Li et al. 2012）在誤差范圍內一致。數據表明，本實驗流程可以對樣品進行有效分離，分析準確度和精密度滿足高精度的Sr同位素分析。
10.2. Scheme for Sr isotope ratio analyses using MC-ICP-MS
All chemical preparations were performed on class 100 work benches within a class 1000 over-pressured clean laboratory. Sample digestion: (1) Sample powder (200 mesh) were placed in an oven at 105 ℃ for drying of 12 hours; (2) 50-200 mg sample powder was accurately weighed and placed in an Teflon bomb; (3) 1-3 ml HNO3 and 1-3 ml HF were added into the Teflon bomb; (4) Teflon bomb was putted in a stainless steel pressure jacket and heated to 190 ℃ in an oven for >24 hours; (5) After cooling, the Teflon bomb was opened and placed on a hotplate at 140 ℃ and evaporated to incipient dryness, and then 1 ml HNO3 was added and evaporated to dryness again; (6) The sample was dissolved in 1.5 mL of 2.5 M HCl. Column chemistry: After centrifugation, the supernatant solution was loaded into an ion-exchange column packed with AG50W resin. After complete draining of the sample solution, columns were rinsed with 2.5 M HCl to remove undesirable matrix elements. Finally, the Sr fraction was eluted using 2.5 M HCl and gently evaporated to dryness prior to mass-spectrometric measurement. The residue was rinsed with 10 mL of 4.0 M HCl and then the REE fraction was eluted using 10 ml of 4.0 M HCl. The REE solution was used to separate the Nd fraction by the Nd-column method.
The Sr fraction was separated again by the Sr-specific resin. The solution was first converted to the HNO3 media (3 M HNO3). Then the solution was loaded into the Sr-specific resin and pre-conditioned with 6 M HCl and 3 M HNO3. After complete draining of the sample solution, columns were rinsed with 3 M HNO3 to remove undesirable matrix elements. Finally, Sr was eluted using MQ H2O and gently evaporated to dryness prior to mass-spectrometric measurement.
Sr isotope analyses were performed on a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Dreieich, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China. The Neptune Plus, a double focusing MC-ICP- MS, was equipped with seven fixed electron multiplier ICs, and nine Faraday cups fitted with 1011 Ω resistors. The faraday collector configuration of the mass system was composed of an array from L4 to H3 to monitor 83Kr+、167Er++、84Sr+、85Rb+、86Sr+、173Yb++、87Sr+、88Sr+. The large dry interface pump (120 m3 hr-1 pumping speed) and newly designed H skimmer cone and the standard sample cone were used to increase the instrumental sensitivity. Sr single element solution from Alfa (Alfa Aesar, Karlsruhe, Germany) was used to optimize instrument operating parameters. An aliquot of the international standard solution of 200 μg L−1 NIST SRM 987 was used regularly for uating the reproducibility and accuracy of the instrument. Typically, the signal intensities of 88Sr in NIST 987 were > ~7.0 V. The Sr isotopic data were acquired in the static mode at low resolution. The routine data acquisition consisted of ten blocks of 10 cycles (4.194 s integration time per cycle). The total time of one measurement lasted about 7 minutes.
The exponential law, which initially was developed for TIMS measurement (Russell et al. 1978) and remains the most widely accepted and utilized with MC-ICP-MS, was used to assess the instrumental mass discrimination in this study. Mass discrimination correction was carried out via internal normalization to a 88Sr/86Sr ratio of 8.375209 (Lin et al. 2016). The interference elements Ca, Rb, Er, Yb have been completely separated by the exchange resin process. The remaining interferences of 83Kr+、85Rb+、167Er++、173Yb++ were corrected based on the mothed described by Lin et al. (2016). One international NIST 987 standard was measured every seven samples analyzed. All data reduction for the MC-ICP-MS analysis of Sr isotope ratios was conducted using “Iso-Compass” software (Zhang et al. 2020). Analyses of the NIST 987 standard solution yielded 87Sr/86Sr ratio of 0.710242±14（2SD, n=345）, which is identical within error to their published values 0.710248±12（Zhang and Hu, 2020）). In addition, the USGS reference materials BCR-2 (basalt) and RGM-2 (rhyolite) yielded results of 0.705012±22 （2SD, n=63） and 0.704173±20 （2SD, n=20） for 87Sr/86Sr, respectively, which is identical within error to their published values (Zhang and Hu, 2020； Li et al. 2012).
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