Abstract Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The 40Ar/39Ar system of unbrecciated eucrites should be in a relatively pristine state, and thus can inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ∼300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase. We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the 40Ar/39Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (∼10–15 km deep), cooled below ∼300 °C at a rate of 17.3 ± 3.6 °C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. 40Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by 40Ar/39Ar.

中文翻译：

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分化小行星灶神星4号岩浆活动时间和上地壳冷却

摘要 Eucrites 是外星玄武岩和累积辉长岩，在 HED（Howardite-Eucrite-Diogenite）母体的地壳水平上形成，随后或多或少地变质，被认为是小行星 4 Vesta。未碎屑晶石提供了最好的方式来了解灶神星地壳吸积后发生的火成岩、变质和冷却过程，因为它们基本上没有受到二次撞击过程的影响/改变。40Ar/39Ar 未碎屑晶辉石系统应该处于相对原始状态，因此可以告诉我们 HED 母体的早期火山和热史，特别是 300 ℃以下的各个地壳部分的冷却史。 °C 等温线，表示斜长石中 Ar 扩散的平均闭合温度。我们分析了斜长石和辉石（± 地块）分离的两个累积（摩尔县和莫阿马）和五个（Caldera，BTN 00300，EET 90020，GRA 98098，QUE 97053）使用热力学平衡的玄武质晶石和 40Ar/3© 技术ARGUS VI 多收集质谱仪。两个累积的不平衡辉长岩的冷却年龄也分别为 4531±11 Ma 和 4533±12 Ma，并结合从薄片厚度估计的快速冷却速率，表明岩浆活动持续到 4533±11 和 4535±12 Ma，并且岩体被侵入到地壳相对较浅的部分，在变质区之上。四个平衡的 eucrites 产生了一个明确的年龄群，介于 4523 ± 8 Ma 到 4514 ± 6 Ma 之间。这些年龄表明上地壳的一部分，这些 eurite 可能居住的地方（约 10-15 公里深），以 17.3 ± 3.6 °C/Ma (2σ) 的速率冷却到 300 °C 以下。这种缓慢的冷却速度与可用的全球热模型相结合，支持了全球地壳变质作用的假设，即熔岩流的掩埋和再加热。最后，获得了变质晶质 EET 90020 的年龄为 4531 ± 5 Ma，并结合岩相学观察，通过撞击指示了一次主要地壳挖掘事件的年龄。40Ar 扩散模型表明，如果通过 40Ar/39Ar 测量足够数量的辉石，则可以区分撞击与地壳冷却。获得了变质晶质 EET 90020 的年龄为 4531 ± 5 Ma，结合岩石学观察，通过撞击表明了一次主要的地壳挖掘事件的年龄。40Ar 扩散模型表明，如果通过 40Ar/39Ar 测量足够数量的辉石，则可以区分撞击与地壳冷却。对于变质的 EET 90020，获得了 4531 ± 5 Ma 的年龄，并且结合岩相学观察，通过撞击表明了一次主要的地壳挖掘事件的年龄。40Ar 扩散模型表明，如果通过 40Ar/39Ar 测量足够数量的辉石，则可以区分撞击与地壳冷却。