南非开普敦大学Geoffrey H. Howarth和 Beth Kahle团队研究了大陆岩石圈中金刚石的生长与破坏。2023年3月29日出版的《地质学》发表了他们的研究新成果。

课题组人员使用X射线计算机断层扫描了24个含金刚石榴辉岩中金刚石，展现了其前所未有的形态和分布的细节。这些扫描清晰的捕捉到在被金伯利岩夹带之前，次大陆岩石圈地幔内发生的金刚石的生长和破坏的细节。研究发现，这些榴辉岩中的自行金刚石主要呈现台阶面的八面体形态。即使当金刚石暴露在榴辉岩表明时，这种形态仍然得以保留，表明金伯利岩诱导的再吸收并不显著。

六种榴辉岩仅含圆形金刚石，没有明显的的晶面，它们在捕掳体外部的暴露面非常不规则，表明次大陆岩石圈地幔的金刚石被破坏。在三个例子中，单个捕虏体既含有再吸收的金刚石，又含有台阶面八面体的金刚石，这表明发生了多次交代事件，其中一些是侵蚀金刚石的，另一些是有利于金刚石的。因此，这些含金刚石的捕掳体提供了次大陆岩石圈地幔金刚石生长和破坏的快照。

据介绍，金刚石可以在次大陆岩石圈地幔(SCLM)的榴辉岩(古代俯冲洋壳残余物)中形成，其碎片可以作为金伯利岩中的捕虏体被搬运到地表。这些含金刚石的捕虏体提供了一个难得的机会，来研究其宿主岩性中的金刚石。金刚石的形态和表面特征记录了被金刚石侵蚀性熔体/流体侵蚀的证据，这导致其八面体的生长形式的再吸收、变圆和破坏。这种金刚石破坏过程的位置和时机仍有争议，金伯利岩岩浆本身通常被认为是主要的原因。

附：英文原文

Title: Caught in the act: Diamond growth and destruction in the continental lithosphere

Author: Geoffrey H. Howarth, Beth Kahle, Philip E. Janney, Richard Kahle, Anton Du Plessis, Stephen H. Richardson, John J. Gurney

Issue&Volume: 2023-03-29

Abstract: Diamonds can form within eclogite (remnants of ancient subducted oceanic crust) in the subcontinental lithospheric mantle (SCLM), pieces of which may then be transported to the surface as xenoliths in kimberlites. These diamond-bearing xenoliths offer a rare opportunity to study diamonds in their host lithology. The morphology and surface features of diamonds record evidence of attack by diamond-aggressive melts/fluids, which results in resorption, rounding, and destruction of octahedral growth forms. The location and timing of such diamond-destruction processes remain controversial, and the kimberlite magma itself is often considered to be the primary culprit. We used X-ray computed tomography scanning to present a view of diamond morphology and distribution within 24 diamondiferous eclogites in unprecedented detail. These scans clearly capture diamond growth and destruction that occurred within the SCLM, prior to kimberlite entrainment. We show that euhedral diamonds in these eclogites are predominantly step-faced octahedra. This morphology is preserved even when the diamonds are exposed at the surface of the eclogites, indicating that kimberlite-induced resorption was not significant. Six eclogites contain only rounded diamonds with no distinct crystal faces, and their exposed surfaces on the exterior of the xenoliths are highly irregular, indicating diamond-destruction in the SCLM. In three cases, single xenoliths host both resorbed diamonds and step-faced octahedra, indicating multiple metasomatic events, some of which were diamond-aggressive and others diamond-friendly. These diamondiferous xenoliths thus provide snapshots of diamond growth and destruction in the SCLM, caught in the act.

DOI: 10.1130/G51013.1

Source: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G51013.1/621490/Caught-in-the-act-Diamond-growth-and-destructionredirectedFrom=fulltext