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The timing of lunar solidification and mantle overturn recorded in ferroan anorthosite 62237
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.epsl.2020.116219 C.K. Sio , L.E. Borg , W.S. Cassata
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2020-05-01 , DOI: 10.1016/j.epsl.2020.116219 C.K. Sio , L.E. Borg , W.S. Cassata
Abstract Ferroan anorthosite suite (FAS) rocks are widely interpreted to represent primordial lunar crust. Despite their importance in pinpointing the timing of lunar crust formation, robust chronological investigations for this rock type are scarce. Here, we report the Ar-Ar, Rb-Sr, and Sm-Nd isotopic systematics for the FAS troctolitic anorthosite 62237. The Ar-Ar isotopic system has been reset by a thermal event at 3710 ± 48 Ma, and the Rb-Sr isotopic systematics has been disturbed such that a Rb-Sr isochron age cannot be determined. However, an internal isochron for the Sm-Nd isotopic system has yielded an age of 4350 ± 73 Ma (MSWD = 2.0) with an initial ϵ 143 NdCHUR of −0.53 ± 0.26. The mineral and whole-rock fractions of 62237 plot on the same internal isochron as FAS sample 60025. The combined datasets define an age of 4372 ± 35 Ma (MSWD = 4.0) with an initial ϵ 143 NdCHUR of −0.17 ± 0.22. Literature Sm-Nd data for FAS and Mg-suite whole-rocks also plot on the 60025-62237 isochron. The coherence of data from both FAS and Mg-suite rocks examined thus far suggests that both rock suites formed contemporaneously from identical, or nearly identical, sources. In addition, the concordance of FAS and Mg-suite ages suggests that primordial crust solidification either involved both magmatic suites, or that Mg-suite magmatism was contemporaneous with FAS magmatism within resolution of the Sm-Nd chronometer. The ages for FAS and Mg-suite also coincide with the formation ages of the mare basalt source regions and urKREEP. Ferroan anorthosite suite rocks and urKREEP are thought to represent primordial LMO solidification products, whereas Mg-suite and the mare basalt source regions are argued to represent mixtures of various LMO crystallization products that were formed during density-driven overturn of the LMO. The concordance of ages implies that the 4372 ± 35 Ma Sm-Nd isochron records the age of mantle overturn, and that overturn occurred during, or shortly after, solidification of the LMO.
中文翻译:
在铁质斜长岩 62237 中记录的月球凝固和地幔翻转的时间
摘要 铁质斜长岩套 (FAS) 岩石被广泛解释为代表原始月球地壳。尽管它们在确定月壳形成的时间方面很重要,但对这种岩石类型的可靠的年代调查却很少。在这里,我们报告了 FAS 褶皱长斜长岩 62237 的 Ar-Ar、Rb-Sr 和 Sm-Nd 同位素系统。Ar-Ar 同位素系统已被 3710 ± 48 Ma 的热事件重置,Rb-Sr同位素系统学受到干扰,因此无法确定 Rb-Sr 等时线年龄。然而,Sm-Nd 同位素系统的内部等时线产生的年龄为 4350 ± 73 Ma(MSWD = 2.0),初始 ϵ 143 NdCHUR 为 -0.53 ± 0.26。62237 的矿物和全岩部分绘制在与 FAS 样品 60025 相同的内部等时线上。组合数据集定义的年龄为 4372 ± 35 Ma(MSWD = 4.0),初始 ϵ 143 NdCHUR 为 -0.17 ± 0.22。FAS 和镁组全岩的文献 Sm-Nd 数据也绘制在 60025-62237 等时线上。迄今为止所检查的 FAS 和镁套件岩石数据的一致性表明,这两个岩石套件是由相同或几乎相同的来源同时形成的。此外,FAS 和镁套件年龄的一致性表明原始地壳凝固要么涉及两个岩浆套件,要么镁套件岩浆活动与 Sm-Nd 天文钟分辨率内的 FAS 岩浆活动同时期。FAS和Mg-suite的年龄也与母海玄武岩源区和urKREEP的形成年龄一致。Ferroan 斜长岩套件岩石和 urKREEP 被认为代表原始 LMO 固化产物,而镁组和母玄武岩源区被认为代表了在密度驱动的 LMO 翻转过程中形成的各种 LMO 结晶产物的混合物。年龄的一致性意味着 4372 ± 35 Ma Sm-Nd 等时线记录了地幔翻转的年龄,而这种翻转发生在改性活生物体凝固期间或凝固后不久。
更新日期:2020-05-01
中文翻译:
在铁质斜长岩 62237 中记录的月球凝固和地幔翻转的时间
摘要 铁质斜长岩套 (FAS) 岩石被广泛解释为代表原始月球地壳。尽管它们在确定月壳形成的时间方面很重要,但对这种岩石类型的可靠的年代调查却很少。在这里,我们报告了 FAS 褶皱长斜长岩 62237 的 Ar-Ar、Rb-Sr 和 Sm-Nd 同位素系统。Ar-Ar 同位素系统已被 3710 ± 48 Ma 的热事件重置,Rb-Sr同位素系统学受到干扰,因此无法确定 Rb-Sr 等时线年龄。然而,Sm-Nd 同位素系统的内部等时线产生的年龄为 4350 ± 73 Ma(MSWD = 2.0),初始 ϵ 143 NdCHUR 为 -0.53 ± 0.26。62237 的矿物和全岩部分绘制在与 FAS 样品 60025 相同的内部等时线上。组合数据集定义的年龄为 4372 ± 35 Ma(MSWD = 4.0),初始 ϵ 143 NdCHUR 为 -0.17 ± 0.22。FAS 和镁组全岩的文献 Sm-Nd 数据也绘制在 60025-62237 等时线上。迄今为止所检查的 FAS 和镁套件岩石数据的一致性表明,这两个岩石套件是由相同或几乎相同的来源同时形成的。此外,FAS 和镁套件年龄的一致性表明原始地壳凝固要么涉及两个岩浆套件,要么镁套件岩浆活动与 Sm-Nd 天文钟分辨率内的 FAS 岩浆活动同时期。FAS和Mg-suite的年龄也与母海玄武岩源区和urKREEP的形成年龄一致。Ferroan 斜长岩套件岩石和 urKREEP 被认为代表原始 LMO 固化产物,而镁组和母玄武岩源区被认为代表了在密度驱动的 LMO 翻转过程中形成的各种 LMO 结晶产物的混合物。年龄的一致性意味着 4372 ± 35 Ma Sm-Nd 等时线记录了地幔翻转的年龄,而这种翻转发生在改性活生物体凝固期间或凝固后不久。