Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites
Effect of parent body evolution on equilibrium and kinetic isotope fractionation: a combined Ni and Fe isotope study of iron and stony-iron meteorites
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DOI：
10.1016/j.gca.2016.04.050
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发表时间：
2016-08-01
影响因子：
5
通讯作者：
Vanhaecke, Frank
中科院分区：
文献类型：
--
作者：
Chernonozhkin, Stepan M.;Goderis, Steven;Vanhaecke, Frank
在样品消化和色谱分离目标元素后，利用多接收器电感耦合等离子体质谱仪（MC-ICP-MS）对各种铁和石铁陨石的镍和铁同位素组成进行了表征，试图进一步限制改变这些同位素比率的行星分化过程，并揭示选定的无球粒陨石母体小行星的形成历史和演化。重点放在铁陨石的空间分辨同位素分析，已知是不均匀的μ m至mm尺度，并在主族钯（PMG），mesosiderite，和IIE和IAB复杂的硅酸盐轴承铁陨石的相邻金属和硅酸盐相的同位素表征。在Ni-60/58与Ni-62/58的3同位素图中，通过铁陨石的横向分辨Ni同位素比数据的最佳拟合直线的斜率揭示了动力学控制的同位素分馏（beta（exper）= 1.981 +/- 0.039，1 SD），主要由亚固相线扩散引起（分馏指数β连接同位素分馏因子，如α（62/58）= α（β）（60/58））。所观察到的三角洲Fe-56/54和Ir浓度之间的关系，在PMGs的金属馏分和IIIAB铁陨石表明依赖于散装Fe同位素组成的小行星金属核心的结晶分离。没有这样的分离结晶趋势被发现相应的镍同位素比率或其他铁陨石群，如IIAB。在IIE和IAB硅酸盐铁陨石的情况下，铁和镍同位素的签名可能反映了影响过程的影响，扩散控制的镍同位素分馏的程度更接近于铁相比，什么是观察到的岩浆铁陨石类型。在钯矿的金属和橄榄石对应物之间，Fe和Ni同位素组成显示出明显的可分辨差异，大小相似但符号相反（Delta Fe-56/54（met-oliv）为+0.178 +/- 0.092%，Delta Ni-60/58（met-oliv）为-0.212 +/-0.082%，2SD）。因此，较重的铁同位素比值的金属（δ Fe-56/54 = +0.023%至+0.247%）和较轻的值相应的橄榄石（δ Fe-56/54 = -0.155%至-0.075%）被解释为反映后期阶段的铁同位素再平衡这些阶段之间，而不是一个原始记录的幔核分化。在中菱铁矿的情况下，同样较轻的镍和铁同位素签名发现的硅酸盐相，（对于δ Ni-60/58为-0.149%至+0.023%，对于δ Fe-56/54为-0.214%至-0.149%）（δ Ni-60/58为+0.168%至+0.191%，δ Fe-56/54为+0.018%至+0.120%）可能由Fe和Ni扩散引起。总的来说，富铁陨石的Fe和Ni同位素组成反映了多个，往往是叠加的，平衡或动力学性质的过程，说明错综复杂的母体历史和早期形成的微行星储层之间的后期相互作用。(C)2016爱思唯尔有限公司版权所有。
Various iron and stony-iron meteorites have been characterized for their Ni and Fe isotopic compositions using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) after sample digestion and chromatographic separation of the target elements in an attempt to further constrain the planetary differentiation processes that shifted these isotope ratios and to shed light on the formational history and evolution of selected achondrite parent body asteroids. Emphasis was placed on spatially resolved isotopic analysis of iron meteorites, known to be inhomogeneous at the mu m to mm scale, and on the isotopic characterization of adjacent metal and silicate phases in main group pallasites (PMG), mesosiderites, and the IIE and IAB complex silicate-bearing iron meteorites. In a 3-isotope plot of Ni-60/58 versus Ni-62/58, the slope of the best-fitting straight line through the laterally resolved Ni isotope ratio data for iron meteorites reveals kinetically controlled isotope fractionation (beta(exper) = 1.981 +/- 0.039, 1 SD), predominantly resulting from sub-solidus diffusion (with the fractionation exponent beta connecting the isotope fractionation factors, as alpha(62/58) = alpha(beta)(60/58)). The observed relation between delta Fe-56/54 and Ir concentration in the metal fractions of PMGs and in IIIAB iron meteorites indicates a dependence of the bulk Fe isotopic composition on the fractional crystallization of an asteroidal metal core. No such fractional crystallization trends were found for the corresponding Ni isotope ratios or for other iron meteorite groups, such as the IIABs. In the case of the IIE and IAB silicate-bearing iron meteorites, the Fe and Ni isotopic signatures potentially reflect the influence of impact processes, as the degree of diffusion-controlled Ni isotope fractionation is closer to that of Fe compared to what is observed for magmatic iron meteorite types. Between the metal and olivine counterparts of pallasites, the Fe and Ni isotopic compositions show clearly resolvable differences, similar in magnitude but opposite in sign (Delta Fe-56/54(met-oliv) of +0.178 +/- 0.092% and Delta Ni-60/58(met-oliv) of -0.212 +/- 0.082%, 2SD). As such, the heavier Fe isotope ratios for the metal (delta Fe-56/54 = +0.023% to +0.247%) and lighter values for the corresponding olivines (delta Fe-56/54 = -0.155% to -0.075%) are interpreted to reflect later-stage Fe isotopic re-equilibration between these phases, rather than a pristine record of mantle-core differentiation. In the case of mesosiderites, the similarly lighter Ni and Fe isotopic signatures found for the silicate phase (-0.149% to + 0.023% for delta Ni-60/58, -0.214% to -0.149% for delta Fe-56/54) compared to the metal phase (+0.168% to +0.191% for delta Ni-60/58, +0.018% to +0.120% for delta Fe-56/54) likely result from Fe and Ni diffusion. Overall, the Fe and Ni isotopic compositions of iron-rich meteorites reflect multiple, often super-imposed, processes of equilibrium or kinetic nature, illustrating convoluted parent body histories and late-stage interaction between early-formed planetesimal reservoirs. (C) 2016 Elsevier Ltd. All rights reserved.