Feldspathic clasts in Yamato-86032: Remnants of the lunar crust with implications for its formation and impact history
Feldspathic clasts in Yamato-86032: Remnants of the lunar crust with implications for its formation and impact history
复制标题
DOI：
10.1016/j.gca.2006.07.042
复制
发表时间：
2006-12
影响因子：
5
通讯作者：
L. Nyquist;D. Bogard;A. Yamaguchi;C. Shih;Y. Karouji;M. Ebihara;Y. Reese;D. Garrison;G. Mckay;H. Takeda
中科院分区：
文献类型：
--
作者：
L. Nyquist;D. Bogard;A. Yamaguchi;C. Shih;Y. Karouji;M. Ebihara;Y. Reese;D. Garrison;G. Mckay;H. Takeda
低浓度的Th和Fe的大和（Y）-86032大块陨石支持早期的建议，Y-86032来自一个地区的月球远离Procellarum KREEP地形（PKT），可能从月球farside.39Ar-40 Ar，Rb-Sr，Sm-Nd，和Sm-同位素研究的特点Y-86032的年代学和它的前体在大风化层。浅灰色角砾岩岩性中的一种岩石类型是斜长岩，其特征是斜长石An ≥ 93，即，比月球的FAN更富钠质，但87 Rb/86 Sr和87 Sr/86 Sr与FAN相似。(FAN代表Ferroan Anorthosite）。这一“An 93斜长岩”的Nd同位素系统学与近侧的诺长岩相似。一个类似于FAN的“An 97斜长岩”存在于第二个浅色斜长角砾岩碎屑中，并且具有更负的ε Nd值，这与月球岩浆海洋（LMO）上早期斜长石浮选地壳提供的LREE富集环境中的居住一致。这一结果与阿波罗16号FAN的ε Nd值通常为正值形成对比，表明LMO可能是不对称发展的。文中还提出了ε Nd值二分法的其他可能解释。Y-86032原岩形成于至少4.43±0.03 Ga前，由角砾岩碎屑的矿物碎片的Sm-Nd等时线确定，角砾岩碎屑主要由An 93斜长岩和成分更多样的第二碎屑组成。我们认为矿物碎片主要来自一套同生岩。以An 97斜长岩为主的第三个碎屑的~（39）Ar-~（40）Ar年龄为4.36-4.41±0.035Ga，支持原岩的古老年龄。碎屑中的初始143 Nd/144 Nd比具有南极Sm/Nd比值的储层中的143 Nd/144 Nd低−0.64±0.13 ε-单位，这与LMO中镁铁质堆积物的先前分馏相一致。第二个样品的4.23±0.03Ga的39 Ar-40 Ar年龄谱的最大值可能反映了一些扩散的40 Ar损失。浅灰色角砾岩碎屑中缺乏太阳风和月球大气注入的Ar，测得39 Ar/40 Ar年龄为4.10±0.02Ga，这是该岩石学的初始角砾岩时代。经注入的月球大气~（40）Ar校正后，撞击熔体和暗风化碎屑的Ar年龄为3.8±0.1Ga，表明熔体形成和角砾岩组合的形成时间为3.8Ga。一些角砾岩岩石暴露于102 × 1015 n/cm 2的热中子注量，只有其他一些月球高地陨石所经历的注量的1%左右。其他岩性的中子注量为101 × 1015 n/cm 2。因此，Y-86032在最后角砾岩化之后的大部分时间都深埋在镁铁质岩中。中子注量数据与宇宙成因38 Arcosmic射线暴露年龄为10 Ma一致。不同岩性中几种风化层暴露指标的变化，包括宇宙成因惰性气体丰度、中子俘获引起的Sm同位素丰度变化和Ir含量，与早期（> 103.8Ga前）月球风化层暴露、随后深埋在> 105 m深度以及107 - 10 Ma前从月球喷出的时期相一致。
Low concentrations of Th and Fe in the Yamato (Y)-86032 bulk meteorite support earlier suggestions that Y-86032 comes from a region of the moon far distant from the Procellarum KREEP Terrain (PKT), probably from the lunar farside.39Ar–40Ar, Rb–Sr, Sm–Nd, and Sm-isotopic studies characterize the chronology of Y-86032 and its precursors in the mega regolith. One of the rock types present in a light gray breccia lithology is an anorthosite characterized by plagioclase with An ∼93, i.e., more sodic than lunar FANs, but with very low87Rb/86Sr and87Sr/86Sr similar to those of FANs. (FAN stands for Ferroan Anorthosite). This “An93 anorthosite” has Nd-isotopic systematics similar to those of nearside norites. A FAN-like “An97 anorthosite” is present in a second light-colored feldspathic breccia clast and has a more negative εNdvalue consistent with residence in a LREE-enriched environment as would be provided by an early plagioclase flotation crust on the Lunar Magma Ocean (LMO). This result contrasts with generally positive values of εNdfor Apollo 16 FANs suggesting the possibility of assymetric development of the LMO. Other possible explanations for the dichotomy in εNdvalues are advanced in the text. The Y-86032 protolith formed at least 4.43±0.03 Ga ago as determined from a Sm–Nd isochron for mineral fragments from the breccia clast composed predominantly of An93 anorthosite and a second clast of more varied composition. We interpret the mineral fragments as being predominatly from a cogenetic rock suite. An39Ar–40Ar age of 4.36–4.41±0.035Ga for a third clast composed predominantly of An97 anorthosite supports an old age for the protolith. Initial143Nd/144Nd in that clast was −0.64±0.13 ε-units below143Nd/144Nd in reservoirs having chondritic Sm/Nd ratios, consistent with prior fractionation of mafic cumulates from the LMO. A maximum in the39Ar–40Ar age spectrum of 4.23±0.03Ga for a second sample of the same feldspathic breccia clast probably reflects some diffusive40Ar loss. Lack of solar wind and lunar atmosphere implanted Ar in the light gray breccia clast allows determination of an39Ar/40Ar age of 4.10±0.02Ga, which is interpreted as the time of initial brecciation of this litholgy. After correction for implanted lunar atmosphere40Ar, impact melt and dark regolith clasts give Ar ages of 3.8±0.1Ga implying melt formation and final breccia assembly ∼3.8Ga ago. Some breccia lithologies were exposed to thermal neutron fluences of ∼2×1015n/cm2, only about 1% of the fluence experienced by some other lunar highlands meteorites. Other lithologies experienced neutron fluences of ∼1×1015n/cm2. Thus, Y-86032 spent most of the time following final brecciation deeply buried in the megaregolith. The neutron fluence data are consistent with cosmogenic38Arcoscosmic ray exposure ages of ∼10Ma. Variations among differing lithologies in the amount of several regolith exposure indicators, including cosmogenic noble gas abundances, neutron capture induced variations in Sm isotopic abundances, and Ir contents, are consistent with a period of early (>∼3.8Ga ago) lunar regolith exposure, subsequent deep burial at >∼5m depth, and ejection from the moon ∼7–10Ma ago.