Abstract The late heavy bombardment (LHB) hypothesis, wherein the terrestrial planets are thought to have suffered intense collisions ca. 3.9 billion years ago, is under debate. Coupled with new dynamical calculations, re-examination of geochronological data seem to support an earlier solar system instability and a smooth monotonic decline in impacts, as opposed to a “cataclysm.” To better understand this collisional history, records from the asteroidal meteorites are required. Here, we report a uranium–lead (U–Pb) chronological dataset for eucrite meteorites thought to originate from the asteroid 4 Vesta; this dataset indicates to a continuous history of collisions prior to 4.15 Ga. Our 207Pb⁎/206Pb⁎ model ages of apatite [Ca5(PO4)3(F,Cl,OH)] and merrillite [Ca9NaMg(PO4)7] from three brecciated basaltic eucrites—Juvinas (4150.3 ± 11.6 million years ago (Ma); merrillite only), Camel Donga (disturbed around 4570–4370 Ma), and Stannern (4143.0 ± 12.5 Ma)—record multiple thermal metamorphic events during the period of ∼4.4–4.15 Ga. We interpret this to mean that Vesta or the vestoid cluster underwent multiple impacts and moderate high-temperature reheating during this time. The ages of ∼4.4–4.15 Ga are distinctly younger than the initial magmatic process on Vesta (>4.5 Ga) but are significantly older than a later “impact peak” based on some interpretations of 40Ar–39Ar chronologies (∼3.9–3.5 Ga). The intense collisions prior to 4.15 Ga on Vesta are at odds with the conventional LHB hypothesis but not inconsistent with the much earlier bombardment and monotonic decline scenario. Different radiometric chronologies of the asteroid likely represent the different stages of a continual collisional process. Conversely, the model 207Pb⁎/206Pb⁎ ages of apatite in the unbrecciated basaltic eucrite, Agoult, returned an age of 4524.8 ± 9.6 Ma. This may represent slow cooling from an earlier global reheating of the crust on Vesta at 4.55 Ga, as documented by other radiometric chronologies. The apatite in Juvinas recorded a coincident timing of 4516.9 ± 10.4 Ma, which could be due to either slow crustal cooling or impact.

中文翻译：

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来自玄武质磷辉石磷酸盐 U-Pb 年代学的 4.15 Ga 之前早期小行星碰撞的证据

摘要 后期重轰击 (LHB) 假说，其中类地行星被认为在大约 39亿年前，正在争论中。再加上新的动力学计算，对地质年代学数据的重新检查似乎支持较早的太阳系不稳定和影响的平稳单调下降，而不是“灾难”。为了更好地了解这一碰撞历史，需要小行星陨石的记录。在这里，我们报告了一个铀-铅 (U-Pb) 年代学数据集，用于被认为起源于小行星 4 Vesta 的 eurite 陨石；该数据集表明 4.15 Ga 之前的连续碰撞历史。我们的 207Pb⁎/206Pb⁎ 模型年龄的磷灰石 [Ca5(PO4)3(F,Cl,OH)] 和磷灰石 [Ca9NaMg(PO4)7] 来自三个角砾岩玄武质 eurites-Juvinas (4150.3 ± 11. 600万年前（Ma）；merrillite only）、Camel Donga（在 4570-4370 Ma 左右受到干扰）和 Stannern（4143.0 ± 12.5 Ma）——记录了 ∼4.4-4.15 Ga 期间的多次热变质事件。我们解释这意味着灶神星或前庭星团在此期间经历了多次冲击和适度高温再加热。∼4.4–4.15 Ga 的年龄明显比灶神星上的初始岩浆过程 (>4.5 Ga) 年轻，但比基于 40Ar–39Ar 年代学 (∼3.9–3.5 Ga) 的一些解释的后来的“撞击峰”要早得多. 灶神星上 4.15 Ga 之前的强烈碰撞与传统的 LHB 假设不一致，但与更早的轰击和单调下降情景并不矛盾。小行星的不同辐射年表可能代表了连续碰撞过程的不同阶段。相反，未角砾质玄武岩 Agoult 中磷灰石的模型 207Pb⁎/206Pb⁎ 年龄返回 4524.8 ± 9.6 Ma。正如其他辐射测量年表所记录的那样，这可能代表灶神星上地壳早期全球重新加热温度为 4.55 Ga 的缓慢冷却。Juvinas 的磷灰石记录了 4516.9 ± 10.4 Ma 的重合时间，这可能是由于缓慢的地壳冷却或撞击造成的。