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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf in eucrites and the differentiation of the HED parent body
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~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf in eucrites and the differentiation of the HED parent body

机译：体中〜（147）Sm-〜（143）Nd和〜（176）Lu-〜（176）Hf与HED母体的分化

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摘要

The ~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf systematics of 21 whole-rock eucrites, including five cumulates, have been investigated by MC-ICP-MS. A statistically significant Sm-Nd isochron was obtained on 18 samples with an age of 4464±75 Ma (MSWD=1.26) and an intercept of 0.50680±0.00010. This age clearly is controlled by the cumulate eucrites. The 21 basaltic and cumulate eucrites together do not form a statistically significant Lu-Hf isochron. Basaltic eucrites, however, form an errorchron with an age of 4604±39 Ma (MSWD=4.52), which becomes an acceptable isochron if the error on the Lu/Hf ratio is doubled with respect to the laboratory estimate. The initial ~(176)Hf/~(177)Hf is 0.27966±0.0002. Three of the cumulate eucrites regressed together further yield a statistically significant age of 4470±22 Ma, indistinguishable from their Sm-Nd age. We therefore conclude that cumulate eucrites are ～100 Myr younger than basaltic eucrites and basaltic eucrites are close in age to the Solar System. There is no evidence in the present data to support a decay constant of ~(176)Lu significantly different from 1.93 10~(-11) yr~(-1) [Sguigna, Can. J. Phys., 60 (1982) 361-364]. The broad range of variation and strong correlation of Lu/Hf and Sm/Nd ratios require that eucrites are partial melts and not residual magmas. The relative Lu/hf and Sm/Nd fractionation is controlled by the plagioclase to mafic mineral ratio in the residue. The steep correlation between these two ratios is explained by the enhancement of plagioclase saturation in the low-gravity field of the eucrite parent body. Basaltic eucrites probably formed as large-degree melts of a chondritic source. The strong Lu/Hf and Sm/Hf fractionation observed in cumulates clearly reflects the presence of residual ilmenite during melting: our preferred interpretation is that cumulates were impregnated by small-degree melts of ilmenite-bearing gabbros. Since pressure on the eucrite parent body never reaches the garnet stability field, this observation questions the ubiquitous presence of residual garnet i nthe mantle sources of magmas formed on larger planets such as the Moon, Mars, and possibly Earth.
机译：通过MC-ICP-MS研究了21种全岩块玉岩的〜（147）Sm-〜（143）Nd和〜（176）Lu-〜（176）Hf系统。在年龄为4464±75 Ma（MSWD = 1.26），截距为0.50680±0.00010的18个样品上获得了具有统计学意义的Sm-Nd等时线。这个年龄显然是由累积的玉石控制的。 21个玄武岩和累计玉石一起没有形成统计上显着的Lu-Hf等时线。但是，玄武岩的欧亚岩形成了一个年龄为4604±39 Ma（MSWD = 4.52）的误差钟，如果Lu / Hf比的误差相对于实验室估计值加倍，则该误差钟成为可接受的等时线。初始〜（176）Hf /〜（177）Hf为0.27966±0.0002。累积在一起的三个三聚体玉石进一步产生了统计学上显着的年龄4470±22 Ma，与它们的Sm-Nd年龄没有区别。因此，我们得出的结论是，累积的欧氏体比玄武质的幼体约少100 Myr，并且玄武质的幼体的年龄接近太阳系。在目前的数据中，没有证据支持〜（176）Lu的衰减常数与1.93 10〜（-11）yr〜（-1）明显不同[Sguigna，Can。 J. Phys。，60（1982）361-364]。 Lu / Hf和Sm / Nd比率的变化范围广且相关性强，因此要求欧氏体是部分熔体，而不是残余岩浆。 Lu / hf和Sm / Nd的相对分馏由残留物中斜长石与镁铁矿矿物的比率控制。这两个比率之间的陡峭相关性是由于在欧氏岩母体的低重力场中斜长石饱和度的增强所致。玄武质的珠光体很可能是软骨来源的高度熔体形成的。在堆积物中观察到的强烈的Lu / Hf和Sm / Hf分离清晰地反映了熔融过程中残留的钛铁矿的存在：我们的首选解释是，堆积在含钛铁矿的辉长岩的小规模熔液中会浸渍这些堆积物。由于对欧氏体母体的压力从未达到石榴石稳定性场，这一观察结果质疑了残留石榴石在大行星（例如月球，火星以及可能是地球）上形成的岩浆的地幔源中普遍存在。

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