In order to understand the effects of the earliest stages of hydrothermal alteration and fluid‐assisted metamorphism on the matrices of unequilibrated ordinary chondrites (UOCs), we have investigated the fine‐grained matrix of MET 00526 (L3.05) using multiple electron microscope techniques. Iron‐rich olivines (Fa_50‐100) are present in all four representative fine‐grained matrix regions analyzed in this study. This study shows for the first time the occurrence of FeO‐rich olivines in distinct submicron veins that crosscut regions of matrix consisting of amorphous silicates and phyllosilicates, providing evidence for elemental mass transport in a hydrothermal fluid. Our detailed transmission electron microscopy study reinforces the idea that FeO‐rich olivines are formed on asteroidal parent bodies by the interaction between a hydrothermal fluid and the pristine solar nebular materials that may be the product of condensation processes in the protoplanetary disk, that is, amorphous silicates. We propose that the FeO‐rich olivines currently observed in MET 00526 matrix are the products of three possible reaction mechanisms, (1) replacement of amorphous silicates, (2) precipitation from an SiO‐rich fluid, and (3) replacement of phyllosilicates; all these mechanisms take place in the presence of an iron‐rich fluid. The chemical evolution of the hydrothermal fluid could trigger the formation of either fayalite or phyllosilicates depending on the Si/Fe ratios. A low Si/Fe ratio is required to form FeO‐rich olivines, rather than phyllosilicates, which form at high Si/Fe ratio. Although MET 00526 records the effects of secondary alteration processes, its fine‐grained matrix still preserves some evidence of its pristine, solar nebular characteristics.

中文翻译：

---

未平衡的普通球粒陨石中的富铁橄榄石，MET 00526：最早形成

为了了解水热蚀变和流体辅助变质作用的早期阶段对不平衡普通球粒晶体（UOC）的影响，我们使用多电子显微镜技术研究了MET 00526（L3.05）的细颗粒基质。富铁橄榄石（Fa _50-100）出现在本研究分析的所有四个代表性细粒度矩阵区域中。这项研究首次显示出富FeO橄榄石出现在不同的亚微米静脉中，这些橄榄横切了由无定形硅酸盐和页硅酸盐组成的基质区域，为水热流体中元素的质量传递提供了证据。我们详细的透射电子显微镜研究强化了这样的想法，即富含FeO的橄榄石是通过水热流体与原始太阳状星云材料之间的相互作用而形成的，这些流体可能是原行星盘中凝结过程的产物，即非晶质。硅酸盐。我们认为，目前在MET 00526基质中观察到的富含FeO的橄榄石是三种可能的反应机理的产物，（1）替代无定形硅酸盐，（2）从富含SiO的流体中沉淀出来，以及（3）更换页硅酸盐；所有这些机制都是在富铁流体的存在下发生的。根据Si / Fe的比例，热液的化学演化可能触发铁铝石或页硅酸盐的形成。需要低Si / Fe比才能形成富含FeO的橄榄石，而不是形成高Si / Fe比的层状硅酸盐。尽管MET 00526记录了次生蚀变过程的影响，但其细颗粒基质仍保留了其原始的太阳日针状特征的一些证据。根据Si / Fe的比例，热液的化学演化可能触发铁铝石或页硅酸盐的形成。需要低Si / Fe比才能形成富含FeO的橄榄石，而不是形成高Si / Fe比的层状硅酸盐。尽管MET 00526记录了次生蚀变过程的影响，但其细颗粒基质仍保留了其原始的，太阳状星云特征的一些证据。根据Si / Fe的比例，热液的化学演化可能触发铁铝石或页硅酸盐的形成。需要低Si / Fe比才能形成富含FeO的橄榄石，而不是形成高Si / Fe比的层状硅酸盐。尽管MET 00526记录了次生蚀变过程的影响，但其细颗粒基质仍保留了其原始的，太阳状星云特征的一些证据。