High-precision Mg-isotope measurements of terrestrial and extraterrestrial material by HR-MC-ICPMS-implications for the relative and absolute Mg-isotope composition of the bulk silicate Earth
High-precision Mg-isotope measurements of terrestrial and extraterrestrial material by HR-MC-ICPMS-implications for the relative and absolute Mg-isotope composition of the bulk silicate Earth
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DOI：
10.1039/c0ja00190b
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发表时间：
2011-01-01
影响因子：
3.4
通讯作者：
Ulfbeck, David
中科院分区：
文献类型：
--
作者：
Bizzarro, Martin;Paton, Chad;Ulfbeck, David
我们报告了新的方法，从硅酸盐岩石的化学纯化镁离子交换色谱，高精度分析镁同位素的高分辨率多接收电感耦合等离子体源质谱（HR-MC-ICPMS）。使用这些方法，我们已经测量了一些地球和地球外的材料，包括国际参考岩石标准以及纯镁标准，橄榄石晶体分离的幔源尖晶石二辉橄榄岩（J12橄榄石），顽火辉石球粒陨石，火星shergottite和海水样品的相对和绝对镁同位素组成。对地球和地球外样品的重复分析表明，可以常规测量硅酸盐材料的相对镁同位素组成，μ Mg-26* 和μ Mg-25值的外部再现性分别为2.5和20 ppm（μ符号是与参考材料的每10（6）偏差）。对大块地幔衍生岩石以及火星shergottite和顽火辉石球粒陨石的分析确定了相对于DSM-3参考标准（mu Mg-25，Mg-26 = 0）的mu Mg-25的限制范围为-120 +/- 28 ppm（2sd），这表明太阳系内大块行星材料的Mg同位素组成在我们分析的分辨率范围内是均匀的。我们已经确定了绝对镁同位素组成的J12橄榄石，两个CI的钙钛矿以及DSM-3和剑桥-1参考标准使用混合Mg-26-Mg-24双加标。相对于DSM-3标准品分析的各种材料的绝对Mg-25/Mg-24比率之间的差异与通过样品-标准品交叉法获得的结果非常一致。根据J12橄榄石分离物的平均值，我们估计地幔的绝对镁同位素组成-因此，块状硅酸盐地球-为Mg-25/Mg-24 = 0.126896 +/- 0.000025和Mg-26/Mg-24 = 0.139652 +/- 0.000033。鉴于有限的范围内的亩镁-25获得的散装行星材料的样品标准括号技术和相对和绝对的方法获得的数据之间的优良协议，我们建议，这些新的值代表绝对镁同位素组成的散装内太阳系。使用J12橄榄石的绝对Mg同位素组成，我们计算出Mg的同位素丰度为Mg-24 = 0.789548 +/- 0.000026，Mg-25 = 0.100190 +/- 0.000018，Mg-26 = 0.110261 +/- 0.000023。基于这一结果，我们计算出Mg的原子量为24.305565 +/- 0.000045，比之前的估计略重，但精确度高出10倍。
We report novel methods for the chemical purification of Mg from silicate rocks by ion-exchange chromatography, and high-precision analysis of Mg-isotopes by high-resolution multiple collector inductively coupled plasma source mass spectrometry (HR-MC-ICPMS). Using these methods, we have measured the relative and absolute Mg-isotope composition of a number of terrestrial and extraterrestrial materials, including international reference rock standards as well as pure Mg standards, olivine crystals separated from a mantle-derived spinel lherzolite (J12 olivine), one enstatite chondrite, a martian shergottite and sea water samples. Repeated analyses of terrestrial and extraterrestrial samples demonstrate that it is possible to routinely measure the relative Mg-isotope composition of silicate materials with an external reproducibility of 2.5 and 20 ppm for the mu Mg-26* and mu Mg-25 values, respectively (mu notation is the per 10(6) deviation from a reference material). Analyses of bulk mantle-derived rocks as well as a martian shergottite and an enstatite chondrite define a restricted range in mu Mg-25 of -120 +/- 28 ppm (2sd) relative to the DSM-3 reference standard (mu Mg-25,Mg-26 = 0), suggesting that the Mg-isotope composition of inner solar system bulk planetary materials is uniform within the resolution of our analyses. We have determined the absolute Mg-isotope composition of the J12 olivine, two CI chondrites as well as the DSM-3 and Cambridge-1 reference standards using a mixed Mg-26-Mg-24 double-spike. The differences between the absolute Mg-25/Mg-24 ratios of the various materials analyzed relative to the DSM-3 standard are in excellent agreement with results obtained by the sample-standard bracketing method. Based on the averages obtained for the J12 olivine separates, we estimate the absolute Mg-isotope composition for Earth's mantle - and hence that of the bulk silicate Earth - to be Mg-25/Mg-24 = 0.126896 +/- 0.000025 and Mg-26/Mg-24 = 0.139652 +/- 0.000033. Given the restricted range of mu Mg-25 obtained for bulk planetary material by the sample-standard bracketing technique and the excellent agreement between the data obtained by the relative and absolute methods, we propose that these new values represent the absolute Mg-isotope composition of the bulk inner solar system. Using the absolute Mg-isotope composition of the J12 olivine, we calculate the isotopic abundances of Mg as Mg-24 = 0.789548 +/- 0.000026, Mg-25 = 0.100190 +/- 0.000018, and Mg-26 = 0.110261 +/- 0.000023. Based on this result, we have calculated an atomic weight for Mg of 24.305565 +/- 0.000045, which is marginally heavier than previous estimates but a factor of 10 more precise.