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- Title
TEM ANALYSYS OF FEO-RICH OLIVINES IN KABA: IMPLICATIONS FOR UNDERSTANDING THE ROLE OF FLUIDS IN CV CHONDRITES.
- Authors
Dobrică, E.; Maxwell, J. L.; Krot, A. N.
- Abstract
Introduction: In order to understand the effects of the earliest stages of hydrothermal alteration and fluid-assisted metamorphism on the matrices of CVOxB chondrites, we have investigated the fine-grained matrix of Kaba (CV3.1) using multiple electron microscope techniques. According to previous studies, Kaba has experienced a moderate degree of alteration and minor thermal metamorphism resulting in the formation of secondary ferrous olivine (Fa50-100) through fluid-assisted metasomatic alteration at 200-300°C [e.g., 1-2]. The matrix of this meteorite contains both finegrained ferroan olivine ((Fe,Mg)2SiO4, Fa~50) associated with nearly pure, euhedral, coarse-grained fayalite (Fe2SiO4, Fa90-100) [1]. The formation of pure fayalite olivine during hydrothermal alteration has been demonstrated to be viable based on hydrothermal experiments [3]. However, no ferroan olivine was formed during similar experiments, although it has been demonstrated to be thermodynamically viable based on the calculations [4-5]. This study aims to understand the hydrothermal alteration and fluid-assisted metamorphic growth of ferroan olivine associated with nearly pure fayalite on the parent bodies of CV chondrites through a detailed analysis of the mineralogy, texture, chemical composition, and crystallography. The research has important implications for understanding the chemical reactions and the role of fluids in the fluid-assisted metamorphism that affected the matrices of CV chondrites. Samples and Methods: We selected a region that contains both ferroan olivine and nearly pure fayalite for the focused ion beam/transmission electron microscopy (FIB/TEM) study (Fig. 1a). The electron transparent section was prepared using the Helios 660 dual-beam focused ion beam SEM (FIB-SEM) instrument at the University of Hawai'i at Mānoa and examined by transmission electron microscopy (TEM) using the JEOL NEOARM 200CF at the University of New Mexico. Results and discussion: Our detailed TEM study shows the presence of coarse-grained, zoned, euhedral olivine (2 × 7 µm in size) associated with fine-grained, elongated, ferroan olivine (20-90 nm in length and 10-20 nm in width, Fig. 1). The coarse-grained olivine has a ferroan olivine (Fa20) core and FeO-rich rim (Fa74, Fig. 1b). A diffuse compositional boundary is observed between the core and the rim. The groundmass of this FIB section is largely composed of elongated, ferroan olivine (Fa43-58, avg. Fa50), associated with sulfide and magnetite with Al-rich rims (Fig. 1c). Our observations are in agreement with [6], suggesting that the coarse- and fine-grained olivines in Kaba form both through diffusion/replacement (i.e., zoned, coarse-grained olivine) and precipitation (i.e., fine-grained olivine groundmass) mechanisms. The mineral texture of these olivines indicates the pseudomorphic replacement of forsteritic olivine by FeO-rich olivines, which requires extensive mobilization of Mg. This Mg-rich fluid will subsequently precipitate as ferroan olivine in the fine-grained matrix. Additionally, we show that an Al-rich fluid circulated through the CV parent body after the precipitation of ferroan olivines considering that the Al-rich rims are only present on the rims of magnetite and sulfides and are lacking at the contact between ferroan olivine and magnetite (Fig. 1c). Alumina is one of the least soluble major rock-forming elements in H2O. However, the complete leaching of chondrule mesostasis and the transport and reprecipitation of the Al-rich rims around the secondary phases clearly demonstrates that alumina was remarkably mobile in Kaba.
- Subjects
OLIVINE; UNIVERSITY of Hawaii at Manoa; UNIVERSITY of New Mexico; ELECTRON microscope techniques; CHONDRITES; FOCUSED ion beams; HYDROTHERMAL alteration; MAGNETITE; TRANSMISSION electron microscopy
- Publication
Meteoritics & Planetary Science, 2022, Vol 57, p6445
- ISSN
1086-9379
- Publication type
Article