The isotopic composition of magnesium in the inner Solar System
The isotopic composition of magnesium in the inner Solar System
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DOI：
10.1016/j.epsl.2010.03.001
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发表时间：
2010-05-01
影响因子：
5.3
通讯作者：
Jacobsen, Stein B.
中科院分区：
文献类型：
--
作者：
Chakrabarti, Ramananda;Jacobsen, Stein B.
本文采用MC-ICPMS对14块原始和分异陨石、5块月球玄武岩和角砾岩、5块陆地玄武岩、1块泥质岩和3块橄榄岩进行了25 mg/ 24 mg和26 mg/ 24 mg同位素比的高精度测量。我们估计大块硅酸盐土（BSE）的δ (26)Mg值为-0.54 +/- 0.04 (2SE) w.r.t.dsm3。碳质和普通球粒陨石的δ (26)Mg平均值为-0.52 +/- 0.04 (2SE)，火星陨石δ (26)Mg的范围较窄，δ (26)Mg平均值为-0.57 +/- 0.02 (2SE)， pallasite橄榄石δ (26)Mg平均值为-0.54 +/- 0.04 (2SE)，而月球样品（包括钛铁矿玄武岩、橄榄石玄武岩和一个结晶角砾岩）δ (26)Mg平均值为-0.51 +/- 0.03 （2SE）。我们的数据表明，（26）Mg /(24)Mg的内太阳系整体稳定的Mg同位素组成(平均δ (25)Mg = -0.273, δ (26)Mg = -0.535)是均匀的，在+/- 0.04‰以内。该值与BSE和月球样品的值难以区分，表明地月系统的Mg同位素组成是球粒质的。精确测定岩石行星的Mg同位素组成对于地球表面过程的Mg同位素研究以及利用Mg同位素数据了解太阳星云中的蒸发和凝结效应具有重要意义。与最近的一份报告一致，我们发现斜辉石比共存的橄榄石和正辉石有更高的δ (26)Mg，尽管这种差异的幅度不同（0.08到0.2千分之一）。如此小的矿物分异以及原始陨石（钙、球粒和基质）成分之间的差异，使小于0.05‰的质量相关分异无法分辨。在太阳系天体之间我们证明，我们的结果不支持最近的说法：(i)地月系统具有不同于球粒陨石的独特的Mg同位素组成，或（ii）较高的三角洲（26）Mg值（高达0.0）报道的球粒陨石，无球粒陨石，陆生玄武岩和橄榄岩。为了解决实验室间在硅酸盐岩石Mg同位素组成上的差异问题，我们准备了3个纯Mg标准，其范围很广，δ (26)Mg（相对于DSM3为-3.75至-0.69），以及来自San Carlos橄榄岩的大量均质橄榄石，这些都将提供给科学界，用于未来的比较研究。(C) 2010 Elsevier BM。版权所有。
We report high-precision MC-ICPMS measurements of (25)mg/(24)mg and (26)mg/(24)mg isotope ratios for 14 meteorites, both primitive and differentiated, 5 lunar basalts and breccia samples, 5 terrestrial basalts, 1 dunite and 3 peridotites. We estimate the delta(26)Mg value of the bulk silicate Earth (BSE) at -0.54 +/- 0.04 (2SE) w.r.t. DSM3. Carbonaceous and ordinary chondrites show an average delta(26)Mg of -0.52 +/- 0.04 (2SE), martian meteorites show a narrow range in delta(26)Mg with an average delta(26)Mg of -0.57 +/- 0.02 (2SE), average delta(26)Mg of pallasite olivines is -0.54 +/- 0.04 (2SE) whereas that of lunar samples, including ilmenite basalts, olivine basalt and one crystalline breccia is -0.51 +/- 0.03 (2SE). Our data demonstrate that the overall stable Mg isotopic composition of the inner Solar System (average delta(25)Mg = -0.273, delta(26)Mg = -0.535) is homogeneous to within +/- 0.04 parts per thousand for (26)mg/(24)Mg. This value is indistinguishable from that of the BSE and lunar samples demonstrating that the Mg isotopic composition of the Earth-Moon system is chondritic. Accurate determination of the Mg isotopic composition of the rocky planets is important for Mg isotope studies of surface processes on Earth as well as for using Mg isotope data to understand evaporation and condensation effects in the solar nebula.Consistent with a recent report we find that clinopyroxenes have higher delta(26)Mg than co-existing olivines and orthopyroxenes in mantle peridotites, although the magnitude of this difference varies (0.08 to 0.2 parts per thousand). Such small mineral fractionations along with differences between components in primitive meteorites (CAls, chondrules, and matrix) preclude the resolution of mass-dependent fractionation differences smaller than about 0.05 parts per thousand. between Solar System objects. We demonstrate that our results do not support the recent claims that: (i) the Earth-Moon system has a unique Mg isotope composition different from chondrites, or (ii) the higher delta(26)Mg values (up to 0.0) reported for chondrites, achondrites, terrestrial basalts and peridotites. To address the issue of inter-laboratory discrepancies on the Mg isotopic composition of silicate rocks, we have prepared 3 pure Mg standards with wide ranging delta(26)Mg (-3.75 to -0.69 with respect to DSM3) as well as a large volume of homogenized olivine from the San Carlos peridotite, which will be available to the scientific community, for future comparative studies. (C) 2010 Elsevier BM. All rights reserved.