Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias
Extreme early solar system chemical fractionation recorded by alkali-rich clasts contained in ordinary chondrite breccias
复制标题
普通球粒陨石角砾中含有的富碱碎屑记录的极早期太阳系化学分馏
DOI：
10.1016/j.epsl.2016.10.037
复制
发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Yoneda S
中科院分区：
文献类型：
--
作者：
Yokoyama T.;Misawa K.;Okano O.;Shih C.-Y.;Nyquist L.E.;Simon J.I.;Tappa M.J.;Yoneda S
对Yamato（Y）-74442和Bhola LL-榴辉角砾岩中的富碱火成岩碎屑进行了新的K-Ca和Rb-Sr同位素分析，以更好地了解早期太阳系中碱富集的程度和时间。Y-74442碎片的K-Ca年龄为4.41±0.28 Ga，λ（40 K）= 0.5543 Ga− 1，初始40 Ca/44 Ca比值为47.1618±0.0032。用Rb-Sr同位素系统研究相同的碎片，得到的年龄为4.420±0.031 Ga，λ（87 Rb）= 0.01402 Ga− 1，初始比值为87 Sr/86 Sr= 0.7203±0.0044。Bhola中的火成岩碎片显示出与Y-74442碎片相似的碱分馏模式，但没有绘制Y-74442碎片的K-Ca或Rb-Sr等时线。钙同位素组成的全岩样品的钙黄长石和钙黄长石是原始的，从地幔衍生的陆地岩石，并在这里被认为是代表的初始组成散装硅酸盐地球。Y-74442中碱性碎屑源的初始ε 40 Ca值比太阳系值高出约0.5 ε单位，表明碱性碎屑源早期富集。对这些富碱碎片的多同位素研究表明，Y-74442碎片的源物质的元素比为K/Ca= 0.43±0.18，Rb/Sr= 3.45±0.66，K/Rb = 170，可能是由富含碱的成分（可能是由早期星云冷凝物分馏产生的富含碱的气体储层）和闪烁的成分，4.42 Ga前在LL球粒陨石母体上的撞击事件期间加热。钾和铷相对于钙和锶的进一步富集，以及相互的碱分馏（K/Rb> 50和更重的碱富集）可能在该熔体随后的冷却和分化过程中发生。波拉的碱性碎片可能经历了与Y-74442碎片类似的固-汽分馏过程，但似乎是通过一个独特的撞击熔融事件形成的。
Abstract New K–Ca and Rb–Sr isotopic analyses have been performed on alkali-rich igneous rock fragments in the Yamato (Y)-74442 and Bhola LL-chondritic breccias to better understand the extent and timing of alkali enrichments in the early solar system. The Y-74442 fragments yield a K–Ca age of 4.41±0.28 Ga for λ (40 K)= 0.5543 Ga− 1 with an initial 40 Ca/44 Ca ratio of 47.1618±0.0032. Studying the same fragments with the Rb–Sr isotope system yields an age of 4.420±0.031 Ga for λ (87 Rb)= 0.01402 Ga− 1 with an initial ratio of 87 Sr/86 Sr= 0.7203±0.0044. An igneous rock fragment contained in Bhola shows a similar alkali fractionation pattern to those of Y-74442 fragments but does not plot on the K–Ca or Rb–Sr isochron of the Y-74442 fragments. Calcium isotopic compositions of whole-rock samples of angrite and chondrites are primordial, indistinguishable from mantle-derived terrestrial rocks, and here considered to represent the initial composition of bulk silicate Earth. The initial ε 40 Ca value determined for the source of the alkali clasts in Y-74442 that is∼ 0.5 ε-units higher than the solar system value implies an early alkali enrichment. Multi-isotopic studies on these alkali-rich fragments reveal that the source material of Y-74442 fragments had elemental ratios of K/Ca= 0.43±0.18, Rb/Sr= 3.45±0.66 and K/Rb∼ 170, that may have formed from mixtures of an alkali-rich component (possibly an alkali-enriched gaseous reservoir produced by fractionation of early nebular condensates) and chondritic components that were flash-heated during an impact event on the LL-chondrite parent body∼ 4.42 Ga ago. Further enrichments of potassium and rubidium relative to calcium and strontium as well as a mutual alkali-fractionation (K/Rb∼ 50 and heavier alkali-enrichment) would have likely occurred during subsequent cooling and differentiation of this melt. Alkali fragments in Bhola might have undergone similar solid–vapor fractionation processes to those of Y-74442 fragments but appear to have formed via a distinct impact melting event.