Pd-Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets
Pd-Ag chronometry of iron meteorites: Correction of neutron capture-effects and application to the cooling history of differentiated protoplanets
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DOI：
10.1016/j.gca.2015.07.027
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发表时间：
2015-11-15
影响因子：
5
通讯作者：
Kleine, Thorsten
中科院分区：
文献类型：
--
作者：
Matthes, Maximilian;Fischer-Goedde, Mario;Kleine, Thorsten
短寿命的Pd-107-Ag-107系统是测定铁陨石年龄的通用工具，但宇宙射线暴露期间的中子捕获反应可能会改变Ag同位素组成。这些宇宙射线引起的效应将因样品的暴露时间及其在母流星体中的位置而异，因此可能使从钯银同位素系统学推断的年龄信息产生偏差。我们新的铁陨石的Pd-Ag和Pt同位素数据结合模型计算揭示了大的宇宙射线引起的Ag-107/Ag-109的向下移动，这排除了根据测量的Ag同位素组成确定Pd-Ag等时线。对于强辐射的铁陨石安斯沃思（IIAB）和卡波（IID），这些变化类似于甚至大于放射性向内生长的影响，由Pd-107衰变。对于辐射强度较低的IIIAB铁陨石Boxhole，Grant和亨伯里，宇宙射线引起的位移小于放射成因的Ag-107过量，但仍然很重要。我们已经开发了一种方法来量化宇宙射线引起的银同位素位移使用中子俘获模型和Pt同位素组成的中子剂量监测器。校正后，钯银等时线得到所有调查的铁陨石，即使是最强烈的辐照样品。从等时线推断的Pd-Ag年龄与铁陨石的其他年代学数据吻合得很好，表明我们的中子捕获模型提供了一个可靠的校正方法，用于量化宇宙射线引起的测量Ag同位素组成的变化。铁陨石的Pd-Ag年龄在这个和以前的研究表明，快速结晶和冷却的母体金属核心在几个Ma核心形成和太阳系形成后。这种快速冷却可以归因于小的母体尺寸或绝缘硅酸盐地幔从较大的机构碰撞侵蚀。碰撞将促进在Pd-Ag同位素闭合以下的快速冷却，因此在这种情况下，Pd-Ag年龄将有效地确定碰撞的时间。(C)2015爱思唯尔有限公司版权所有。
The short-lived Pd-107-Ag-107 system is a versatile tool for dating iron meteorites, but neutron capture reactions during cosmic ray-exposure might have modified Ag isotope compositions. These cosmic ray-induced effects would vary depending on the exposure time of a sample and its location within the parent meteoroid and, therefore, could bias the age information inferred from Pd-Ag isotope systematics. Our new combined Pd-Ag and Pt isotope data for iron meteorites in conjunction with model calculations reveal large cosmic ray-induced downward shifts of Ag-107/Ag-109, which preclude the determination of Pd-Ag isochrons based on measured Ag isotope compositions. For the strongly irradiated iron meteorites Ainsworth ( IIAB) and Carbo ( IID) these shifts are similar to or even larger than the effects from radiogenic ingrowth resulting from Pd-107-decay. For the less strongly irradiated IIIAB iron meteorites Boxhole, Grant and Henbury, the cosmic ray-induced shifts are smaller than the radiogenic Ag-107 excesses, but are nevertheless significant. We have developed a method to quantify the cosmic ray-induced Ag isotope shifts using a neutron capture model and Pt isotope compositions as the neutron dose monitor. After correction, Pd-Ag isochrons are obtained for all investigated iron meteorites, even for the most strongly irradiated samples. The Pd-Ag ages inferred from the isochrons are in good agreement with other chronological data for iron meteorites, indicating that our neutron capture model provides a reliable correction method for quantifying cosmic ray-induced shifts on measured Ag isotope compositions. The Pd-Ag ages for iron meteorites obtained in this and previous studies indicate rapid crystallization and cooling of the parental metal cores within a few Ma after core formation and solar system formation. Such rapid cooling can be attributed to either small parent body sizes or collisional erosion of the insulating silicate mantle from larger bodies. The collisions would have facilitated rapid cooling below Pd-Ag isotopic closure and so in this case the Pd-Ag ages would effectively date the time of the collisions. (C) 2015 Elsevier Ltd. All rights reserved.