Chronology of formation of early solar system solids from bulk Mg isotope analyses of CV3 chondrules
Chronology of formation of early solar system solids from bulk Mg isotope analyses of CV3 chondrules
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根据 CV3 球粒的大量镁同位素分析得出早期太阳系固体的形成年代
DOI：
10.1016/j.gca.2018.02.011
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发表时间：
2018
影响因子：
5
通讯作者：
Chen H
中科院分区：
文献类型：
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作者：
Chen H
我们分析了来自CV3球粒陨石Allende、Mokoia和Vigarano的19个铁镁球粒陨石的岩相学、主量元素丰度和整体铝镁同位素体系，以及来自Mokoia的富铝球粒陨石和难熔橄榄石。块状球粒陨石中Al/Mg、Na/Mg和Ti/Mg的协变表明，它们的组成主要受不同比例的球粒组分(如镁铁质矿物和中晶石)的改造所控制，因此它们的前体是前几代球粒陨石的碎片。我们的样品显示的分馏校正范围为26 mg/24 mg(Δ‘26 mg) ∼ 60 ppm，相对于精密度<；±5 ppm(2SE)，这些值与27Al/24 mg大致相同。这些数据可以用来计算球粒前驱体的初始模型26Al/27Al或(26Al/27Al)0。我们的可分辨放射成因球粒屈服模型(26Al/27Al)0∼ 1-2 × 10−5，相当于前驱形成≦1 Ma后的“年龄”模型。然而，我们的许多球粒显示接近太阳Δ的26毫克，没有变化，尽管范围在27Al/24毫克。这表明它们要么来自较年轻的前体球粒陨石，要么来自曾经有效灭绝的开放系统行为26Al((26Al/27Al)0<； 0.8 × 10−5，在这里的分辨率下)。斜方辉石取代橄榄石的明显边缘为后一种解释提供了证据，表明球粒陨石与周围的硅酸盐蒸气发生了反应。后期改造过程中镁与近球粒陨石蒸气的同步同位素交换可以解释它们的同位素系统学。一个铁镁质天体以高镁橄榄石为主，其钛和钙丰度较高。这块难熔橄榄石的Δ‘26 mg = −16 ± 3 ppm(2SE)明显为负，这反映了它从一个演化为 ’26 mg的储集层中的早期清除(模型年龄为<；0.5 Ma)。如果代表球粒陨石形成区域，该颗粒定义了与(26Al/27Al)0>； 3.4 ± 0.6 × 10−5相称的CV球粒陨石的最小放射成因生长间隔。总体而言，我们的样品记录了从CaI 0.5 Ma内铁镁体的形成到CaI后球粒陨石和硅酸盐蒸气重新平衡的一系列事件，假定初始为均一的26Al/27Al。小行星母体上的变质作用可能在随后影响镁同位素组成方面发挥了作用，但我们认为这对这里的观测结果影响很小。
We have analysed the petrography, major element abundances and bulk Al-Mg isotope systematics of 19 ferromagnesian chondrules from the CV3 chondrites Allende, Mokoia, and Vigarano, together with an Al-rich chondrule and refractory olivine from Mokoia. Co-variations of Al/Mg with Na/Mg and Ti/Mg in our bulk chondrules suggest their compositions are dominantly controlled by reworking of different proportions of chondrule components (e.g. mafic minerals and mesostatis); their precursors are thus fragments from prior generations of chondrules. Our samples show a range in fractionation corrected26Mg/24Mg (Δ′26Mg) ∼ 60 ppm, relative to precisions <±5 ppm (2se) and these values broadly covary with27Al/24Mg. The data can be used to calculate model initial26Al/27Al, or (26Al/27Al)0, of the chondrule precursors. Our resolvably radiogenic chondrules yield model (26Al/27Al)0∼ 1–2 × 10−5, equivalent to model “ages” of precursor formation ≦1 Ma post CAI. However, many of our chondrules show near solar Δ′26Mg and no variability despite a range in27Al/24Mg. This suggests their derivation either from younger precursor chondrules or open system behaviour once26Al was effectively extinct ((26Al/27Al)0< 0.8 × 10−5, given the resolution here). Evidence for the latter explanation is provided by marked rims of orthopyroxene replacing olivine, indicating reaction of chondrules with a surrounding silicate vapour. Concurrent isotopic exchange of Mg with a near chondritic vapour during late reworking could explain their isotopic systematics. One ferromagnesian object is dominated by a high Mg# olivine with elevated Ti and Ca abundances. This refractory olivine has a markedly negative Δ′26Mg = −16 ± 3 ppm (2se), reflecting its early removal (model age of <0.5 Ma post CAI), from a reservoir with evolving Δ′26Mg. If representative of the chondrule forming region, this grain defines a minimum interval of radiogenic ingrowth for CV chondrites commensurate with (26Al/27Al)0> 3.4 ± 0.6 × 10−5. Overall, our samples record a sequence of events from the formation of ferromagnesian objects within 0.5 Ma of CAI to re-equilibration of chondrules and silicate vapour >2 Ma post CAI, assuming an initially homogeneous26Al/27Al. Metamorphism on the asteroid parent body may have played a subsequent role in affecting Mg isotope composition, but we argue this had a minor influence on the observations here.