Processes and temperatures of FGR formation in chondrites

Abstract

In order to understand the nature of the dust that accreted onto chondrules in the nebula and to unravel the conditions of formation of fine grained rims (FGRs), we studied three of the least altered chondrites from different chondrite groups (LL3.00 Semarkona, CO3.0 DOM 08006, CR2.8 QUE 99177) and compared the results with our previous work on the Paris CM chondrite (Zanetta et al., 2021). For each sample, we selected representative rimmed chondrules showing minimal traces of aqueous alteration. We performed high-resolution SEM X-ray chemical mapping to obtain relevant phase abundances and grain size distributions. Four FIB sections were then extracted from each meteorite, two in the rims and two in their adjacent matrix for quantitative TEM analysis. At the microscale, texture, modal abundances and grain size differ depending on the chondrite but also between FGRs and their adjacent matrix. At the nanoscale (i.e. TEM observations), matrices of the four chondrites consist mostly of domains of amorphous silicate associated with Fe-sulfides, Fe-Ni metal, Mg-rich anhydrous silicates and an abundant porosity. The related FGRs in Semarkona (LL) and DOM 08006 (CO) exhibit more compact textures with a lower porosity while FGRs in QUE99177 (CR) are similar to the matrix in terms of porosity. In the three chondrites, FGRs are made of smooth and chemically homogeneous amorphous (or nanocrystalline) silicate with no porosity that encloses domains of porous amorphous silicate bearing Mg-rich anhydrous silicates, Fe-sulfides, Fe-oxides and sometimes metal and Fe-rich olivines. The average compositions in major elements of the amorphous regions are similar for the FGRs and the matrix within a given chondrite (but differ between chondrites). The texture and the chemical homogeneity of the smooth silicate and the fact that it encloses domains of porous amorphous silicate bearing other mineral phases similar to matrix-like material suggests a formation by condensation. Areas that are enclosed in this smooth silicate exhibit Fe-rich olivine formed through Fe interdiffusion that also suggest a thermal modification of the dust accreted to form FGRs. These characteristics indicate a transformation process for the modification of the FGR material similar to the one proposed in our previous work on Paris. We conclude that matrix and FGRs accreted a similar type of dust but FGR material was affected by thermal modification and compaction contemporary with their accretion. For each chondrite, dust accreted onto chondrules under different conditions (dust density, temperature) which led to diverse degrees of compaction/thermal modification of the sub-domains and explain the textural differences observed in FGRs. They accreted on chondrules in a warm environment related to the chondrule formation episode, whereas matrix accreted later in a cooler environment.

Introduction

Fine-grained rims (FGRs) are located at the interface between chondrules (and refractory inclusions and metal grains) and the matrix. They are present in carbonaceous (CR, CM, CO, CV), ordinary chondrite (OC) and enstatite chondrites (Brearley and Geiger, 1991, Metzler et al., 1992, Zolensky et al., 1993, Lauretta et al., 2000, Zega and Buseck, 2003, Metzler, 2004, Krot et al., 2014, Scott and Krot, 2014, Zanetta et al., 2021). They consist of an unequilibrated assemblage of submicrometric phases but in comparison to the nearby matrix, large (>5 µm) crystalline anhydrous phases such as Mg-rich olivine, pyroxenes and sulfides are absent (Brearley, 1993, Zolensky et al., 1993, Zega and Buseck, 2003, Metzler, 2004, Chizmadia and Brearley, 2008, Zanetta et al., 2021). Rims are chemically comparable to the matrix but are more compact (Metzler et al., 1992, Zolensky et al., 1993, Lauretta et al., 2000, Zega and Buseck, 2003, Greshake et al., 2005, Hanna and Ketcham, 2018, Vollmer et al., 2020, Zanetta et al., 2021).

The favored scenario to explain the origin of FGRs is that dust first accreted onto chondrules in a nebular environment before they were all assembled with the matrix to form parent bodies. Chondrules and their attached FGRs then evolved in a primary parent-body as a function of hydrothermal and metamorphic conditions (Brearley and Geiger, 1991, Metzler et al., 1992, Zolensky et al., 1993, Hanowski and Brearley, 2000, Hua et al., 2002, Leitner et al., 2016, Haenecour et al., 2018, Vollmer et al., 2020). Another scenario invoking a nebular environment is the Kinetic Dust Aggregation (KDA) model, where FGRs are formed through relatively high-speed (order of meters per second to kilometers per second) collisions (Liffman and Toscano, 2000, Liffman, 2019). Scenarios considering parent body aqueous alteration and metamorphism of FGRs initially made of anhydrous material have also been proposed (Brearley and Geiger, 1991, Zolensky et al., 1993, Hanowski and Brearley, 2001, Hua et al., 2002). Zanetta et al. (2021) recently proposed an alternative nebular scenario based on the study of the Paris (CM) chondrite. They observed, in FGRs but not in the matrix, a characteristic micro-texture made of sulfide-rich and sulfide-poor compacted domains of amorphous silicate (as also observed in the CM2 chondrite Y-791198 (Chizmadia and Brearley, 2008), and two CR2 chondrites MIL 07525, and GRA 95229 (Vollmer et al., 2020)). They interpreted this micro-texture as the result of a thermal process, which occurred in the nebula during FGR accretion onto the chondrule and before incorporation into the matrix (i.e. the parent body accretion).

Igneous rims are also observed in different types of chondrite groups (Krot et al., 2014). It has been proposed that these rims can provide insight into chondrule/rim relationship as their properties recorded conditions during chondrule formation (density of dust, temperature, composition of the gas e.g, Krot and Wasson. (1995)). They have distinct textures in comparison to FGRs with more abundant and larger crystalline grain sizes (<40 μm). They are interpreted as the result of melting and crystallization of a previously accreted dust rim through a flash heating event in a nebular environment (Krot and Wasson, 1995, Rubin and Krot, 1996). The possibility that FGRs and igneous rims have a genetic link is thus worth investigating.

The presence of FGRs and igneous rims in different chondrite groups suggests they formed at various times and locations during Solar System formation. Among the different chondrite groups, chondrule and matrix abundances vary significantly, as well as oxidation state and isotopic signatures (Hewins, 1997, Krot et al., 2014, Scott and Krot, 2014, Russell et al., 2018, Hellmann et al., 2020). These particularities reflect the diversity of reservoirs and physical conditions in the disk at their origin and should also affect the nature of the FGRs. The presolar grain abundance and the noble gas signature also indicate that rims were formed under specific conditions in the disk and experienced diverse pathways of aqueous alteration on their parent body(ies) according to the petrologic group and to the chondrite (Metzler et al., 1992, Leitner et al., 2016, Haenecour et al., 2018).

So far, FGRs have been mainly described in altered chondrites. A few recent studies have investigated weakly altered chondrites (Chizmadia and Brearley, 2008, Vollmer et al., 2020, Zanetta et al., 2021), which are the best candidates to have preserved information about the accretion conditions. The analysis of FGRs is challenging because of their small grain size (<5 µm) and potential secondary overprints. Thus, in order to minimize the overprints of secondary parent body processes, we selected three of the most pristine objects in the collections. These chondrites were chosen from various groups (QUE 99177 (CR), DOM 08006 (CO) and Semarkona (LL)) and complete the recent work of Zanetta et al. 2021 on the Paris (CM) chondrite. These chondrites all exhibit a high abundance of amorphous silicate in the matrix (Abreu and Brearley, 2010, Dobrică et al., 2019, Davidson et al., 2019, Dobrică and Brearley, 2020a), a low structural order of the polyaromatic carbonaceous matter (Burton et al., 2012, Bonal et al., 2016, Alexander et al., 2018, Quirico et al., 2018) and a high abundance of presolar grains (Floss and Stadermann, 2009, Nguyen et al., 2010, Haenecour et al., 2018, Nittler et al., 2018). They also belong to the least heated samples based on the chromium standard deviation scale (Grossman and Brearley, 2005, Schrader and Davidson, 2017, Davidson et al., 2019). However, despite their pristine characteristics, they exhibit some low degree of alteration (Rubin et al., 1997, Howard et al., 2009, Howard et al., 2011, Howard et al., 2014, Hewins et al., 2014, Le Guillou et al., 2015b, Dobrică et al., 2019, Dobrică and Brearley, 2020a). The matrix of the CR chondrite QUE 99177 contains minor phyllosilicate, carbonate, and magnetite and was therefore classified as 2.8 by Harju et al., 2014. The LL chondrite Semarkona also shows evidence of aqueous alteration in the matrix with its significant amount of smectite (Hutchison et al., 1987, Alexander et al., 1989a, Alexander et al., 1989b, Krot et al., 1997a, Keller, 1998, Dobrică et al., 2019, Dobrică and Brearley, 2020a). The Antarctic CO chondrite, DOM 08006 is characterized by the highest known matrix-normalized abundance of O-rich presolar grains (Nittler et al., 2018). Nonetheless, Davidson et al. (2019) reported some magnetite grains in DOM 08006 (CO) replacing metal, which could originate from a limited aqueous alteration, either in a parent body environment or during terrestrial weathering. We investigated the least altered area of the selected samples at high spatial resolution, using a high-resolution methodology named ACADEMY in order to compare FGRs and their adjacent matrix (Zanetta et al., 2019). This method consists of integrating results of scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM) to quantify the mineralogy and composition of sub-micrometric assemblages. It was developed to obtain a complete petrological description at the submicron scale and over large areas, in order to obtain representative analyses. It also provides quantitative mineralogical characteristics (modal abundances, density variation, grain size and shape). The ACADEMY method is therefore ideal for analyzing large objects with fine granulometry and complex mineralogy such as FGRs or matrix in chondrites.

The questions that arose from previous studies and are addressed here are the following: Is the thermal process advocated for Paris (CM) by Zanetta et al. 2021 also observed in other groups? If chondrite groups form at different times and/or places in the disk, were FGR processed under similar conditions? How does the difference between matrix and rims vary among groups? Lastly, since many chondrules from ordinary chondrites and CR chondrites are surrounded by igneous rims (Krot and Wasson, 1995, Krot et al., 2004a), we expect to make new observations in Semarkona (LL) and QUE 99177 (CR) that could answer another fundamental question: are the formation of igneous rims and of FGRs related?