A high field strength element perspective on early lunar differentiation
A high field strength element perspective on early lunar differentiation
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高场强元素视角对早期月球分异的影响
DOI：
10.1016/j.gca.2010.09.021
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发表时间：
2010
影响因子：
5
通讯作者：
C. Münker
中科院分区：
文献类型：
--
作者：
C. Münker
据推测，月球岩石是在月球岩浆海洋（LMO）结晶过程中形成的不同化石堆积层。通过同位素稀释获得的相干数据集，包括Zr同位素数据和高精度HFSE （W,Nb,Ta,Zr,Hf）和REE （Nd,Sm,Lu）数据，现在可以为月球岩浆在LMO结晶和岩石成因过程中活跃的过程提供新的见解。测得的92zr和91zr丰度在0.2 ε-单位范围内与地面值一致。来自Procellarum KREEP Terrane （PKT）的不相容微量元素富集岩石显示Nb/Ta和Zr/Hf高于月体值（约17），而海玄武岩显示较低的比值，一般证实了月球上存在互补的富集和亏缺地幔储层。然而，月球玄武岩的完整成分谱也需要与月幔中富含钛铁矿的层相互作用。值得注意的是，高钛海玄武岩在所有月球岩石中表现出最低的Nb/Ta和Zr/Hf，并且在相似的Lu/Hf下，Sm/Nd高于低钛玄武岩。在相似的W含量下，高ti玄武岩的Ta/W（高达25）和Hf/W（高达140）也表现出较高且强相关的特征，这与正辉石和斜辉石控制的熔融难以调和。总的来说，这些模式可以通过吸收高达约20%的富含钛铁矿和斜辉石的LMO累积物来解释，这些LMO是由来自下月幔的更耗尽的熔体形成的。富钛矿堆积物的直接熔化或月幔中可能存在的残余金属都不能轻易解释观测到的Ta/W和Hf/W模式。累积同化也是一种可行的机制，可以部分缓冲海玄武岩相对较低的Lu/Hf值，同时产生可变的Sm/Nd。由此可以解释月球玄武岩低Lu/Hf与高时间积分源Lu/Hf的二元对立。提出的同化模型对地月系统的短寿命核素年代学也有重要意义。新的Hf/W和Ta/W数据，以及对现有月球岩石W- th - u数据的汇编表明，陆地和月球地幔在Hf/W上是不可区分的。地球和月球地幔中几乎相同的εW和Hf/W表明，地球最终的核幔平衡与月球形成巨大撞击之间存在着密切的联系。以前，月球样品在182w与Hf/W和142nd与Sm/Nd空间的线性阵列被解释为等时线，认为LMO结晶晚至太阳系形成后250Myrs。基于所提出的同化模型，许多月球岩浆中的182w和142nd可以显示与其周围Hf/W和Sm/Nd源组成解耦。因此，182w vs. Hf/W和142nd vs. Sm/Nd阵列将构成混合线而不是等时线。因此，lunar182Hf-182W和146sm - 142数据将完全符合LMO的“早期”结晶年龄，甚至早在太阳系形成后50Myrs，月球可能形成。
Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W,Nb,Ta,Zr,Hf) and REE (Nd,Sm,Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured92Zr and91Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in182W vs. Hf/W and142Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250Myrs after solar system formation. Based on the proposed assimilation model, the182W and142Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the182W vs. Hf/W and142Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar182Hf–182W and146Sm–142Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50Myrs after solar system formation when the Moon was probably formed.