Abstract Ordinary chondrites (OCs) are variably thermally metamorphosed meteorites thought to originate from at least three different parent bodies (H, L, and LL) in the Main Belt of asteroids. The thermal evolutions of OC parent bodies are frequently explained by the onion shell model; however, a competing hypothesis is the fragmentation-reassembly model. The onion shell model proposes undisrupted, internally heated parent bodies with concentrically stratified thermal structure, and posits that OC petrologic types (i.e., 3–6) develop with increasing temperature and burial depth. In this model, petrologic types are inversely correlated with cooling rate in the parent body. The alternative fragmentation-reassembly model invokes catastrophic collisional disruption of parent bodies that initially possessed onion shell structures, followed by rapid reaccretion of hot fragments, forming rubble pile bodies. Fragmentation would result in fast cooling (quenching) of collisional fragments from the temperature experienced by the parent body at the time of collision. Discrimination between these two models may be possible via investigation of the thermal histories of OCs by application of geothermometry and geospeedometry, which are used to constrain the temperatures and rates through which igneous and metamorphic rock samples cool. Most published cooling rate data for OC parent bodies are based on methods that record rates through low closure temperatures (∼500–200 °C) rather than from peak metamorphic temperatures. Recently, a rare earth element (REE)-in-two-pyroxene thermometer has been shown to establish peak or magmatic temperatures (TREE; Liang et al. [2013]) for rocks that cooled at moderate to fast geologic rates. We applied the REE-in-two-pyroxene method to determine peak temperatures for 18 OC samples (mostly type 6), in conjunction with conventional two-pyroxene thermometry (TBKN; Brey and Kohler [1990]) and Ca-in-olivine thermometry (TCa-Ol; Kohler and Brey [1990]), to determine closure temperatures and estimate cooling rates for OC parent bodies. Inconsistent with slow cooling rates expected within an onion shell structure, we obtain fast cooling at rates ≳0.3 °C/y from peak temperatures of ∼900 °C. Corroborating the TREE and TBKN measurements, TCa-Ol suggests that the OCs cooled through TCa-Ol closure temperatures (∼700 to 800 °C) at ∼10−2 to 10−1 °C/y. These cooling rates are three to six orders of magnitude faster than rates determined using methods sensitive to low temperature (≤500 °C) cooling (e.g., metallography, 40Ar–39Ar ages, 244Pu fission track). We developed a novel numerical thermal model that incorporates fragmentation of an initial onion shell body and reassembly into a rubble pile body that reproduces both the fast cooling from high temperatures and the slow cooling through low temperatures observed in chondritic meteorites. We hypothesize that OC parent bodies initially possessed onion shell thermal structures, but later experienced collisional breakup, then reaccreted rapidly to form thermally stable rubble-pile asteroids.

中文翻译：

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普通球粒陨石（H、L 和 LL）母体早期碎裂-重组的证据来自双辉石中的 REE 温度测定法

摘要 普通球粒陨石 (OCs) 是经过不同程度热变质的陨石，被认为起源于小行星主带中至少三个不同的母体（H、L 和 LL）。OC 母体的热演化经常用洋葱壳模型来解释；然而，一个相互竞争的假设是碎片重组模型。洋葱壳模型提出了具有同心分层热结构的不间断、内部加热母体，并假定 OC 岩石类型（即 3-6）随着温度和埋藏深度的增加而发展。在这个模型中，岩石类型与母体的冷却速率成反比。另一种破碎-重组模型引发了最初具有洋葱壳结构的母体的灾难性碰撞破坏，随后热碎片迅速再堆积，形成碎石堆体。碎片会导致碰撞碎片从碰撞时母体所经历的温度快速冷却（淬火）。通过应用地热测量法和地球速度测量法研究 OC 的热历史，可以区分这两种模型，这些方法用于限制火成岩和变质岩样品冷却的温度和速率。大多数已发表的 OC 母体冷却速率数据基于通过低闭合温度（~500-200°C）而不是峰值变质温度记录速率的方法。最近，一种稀土元素 (REE)-二辉石温度计已被证明可以建立峰值或岩浆温度（TREE；Liang 等人，2017 年）。[2013]) 用于以中等至快速地质速率冷却的岩石。我们应用 REE-in-two-pyroxene 方法结合传统的双辉石温度测定法（TBKN；Brey and Kohler [1990]）和 Ca-in-橄榄石温度测定法测定 18 个 OC 样品（主要是 6 型）的峰值温度（TCa-Ol；Kohler 和 Brey [1990]），以确定闭合温度并估计 OC 母体的冷却速率。与洋葱壳结构内预期的缓慢冷却速率不一致，我们从约 900°C 的峰值温度以≳0.3°C/y 的速率获得快速冷却。证实 TREE 和 TBKN 测量结果，TCa-Ol 表明 OC 以~10-2 至 10-1°C/y 通过 TCa-Ol 闭合温度（~700 至 800°C）冷却。这些冷却速率比使用对低温（≤500 °C）冷却敏感的方法（例如，金相学、40Ar–39Ar 年龄、244Pu 裂变径迹）确定的速率快三到六个数量级。我们开发了一种新颖的数值热模型，该模型将初始洋葱壳体的破碎和重新组装成碎石堆体，该模型再现了在球粒陨石中观察到的高温快速冷却和低温缓慢冷却。我们假设 OC 母体最初具有洋葱壳热结构，但后来经历了碰撞破裂，然后迅速重新吸积形成热稳定的碎石堆小行星。我们开发了一种新颖的数值热模型，该模型将初始洋葱壳体的破碎和重新组装成碎石堆体，该模型再现了在球粒陨石中观察到的高温快速冷却和低温缓慢冷却。我们假设 OC 母体最初具有洋葱壳热结构，但后来经历了碰撞破裂，然后迅速重新吸积形成热稳定的碎石堆小行星。我们开发了一种新颖的数值热模型，该模型将初始洋葱壳体的破碎和重新组装成碎石堆体，该模型再现了在球粒陨石中观察到的高温快速冷却和低温缓慢冷却。我们假设 OC 母体最初具有洋葱壳热结构，但后来经历了碰撞破裂，然后迅速重新吸积形成热稳定的碎石堆小行星。