Hamilton V E, Goodrich C A, Treiman A H, Connolly H C, Zolensky M E, Shaddad M H
Department of Space Studies, Southwest Research Institute, 1050 Walnut St., #300, Boulder, CO 80302 USA.
Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX 77058 USA.
Nat Astron. 2020;2020. doi: 10.1038/s41550-020-01274-z. Epub 2020 Dec 21.
Carbonaceous chondrite meteorites record the earliest stages of Solar System geo-logical activities and provide insight into their parent bodies' histories. Some carbonaceous chondrites are volumetrically dominated by hydrated minerals, providing evidence for low temperature and pressure aqueous alteration. Others are dominated by anhydrous minerals and textures that indicate high temperature metamorphism in the absence of aqueous fluids. Evidence of hydrous metamorphism at intermediate pressures and temperatures in carbonaceous chondrite parent bodies has been virtually absent. Here we show that an ungrouped, aqueously altered carbonaceous chondrite fragment (numbered 202) from the Almahata Sitta (AhS) meteorite contains an assemblage of minerals, including amphibole, that reflect fluid-assisted metamorphism at intermediate temperatures and pressures on the parent asteroid. Amphiboles are rare in carbonaceous chondrites, having only been identified previously as a trace component in Allende (CV3) chondrules. Formation of these minerals requires prolonged metamorphism in a large (~640-1800 km diameter), unknown asteroid. Because Allende and AhS 202 represent different asteroidal parent bodies, intermediate conditions may have been more widespread in the early Solar System than recognized from known carbonaceous chondrite meteorites, which are likely a biased sampling.
碳质球粒陨石记录了太阳系地质活动的最早阶段，并为了解其母体的历史提供了线索。一些碳质球粒陨石在体积上以水合矿物为主，这为低温低压下的水岩蚀变提供了证据。其他陨石则以无水矿物和结构为主，这表明在没有水流体的情况下发生了高温变质作用。在碳质球粒陨石母体中，中压和中温下的水合变质作用的证据实际上并不存在。在这里，我们表明，来自阿尔马塔·西塔（AhS）陨石的一块未分类的、经水蚀变的碳质球粒陨石碎片（编号202）包含一组矿物，包括角闪石，这些矿物反映了母体小行星在中温和中压下的流体辅助变质作用。角闪石在碳质球粒陨石中很罕见，此前仅在阿伦德（CV3）球粒陨石中被鉴定为微量成分。这些矿物的形成需要在一个直径约640 - 1800公里的未知小行星中进行长时间的变质作用。由于阿伦德陨石和AhS 202代表不同的小行星母体，早期太阳系中的中间条件可能比从已知的碳质球粒陨石中认识到的更为普遍，而这些已知的碳质球粒陨石可能是有偏差的样本。
