Mincy mesosiderite metallic nodules analyzed byEBSD: An approach to understanding their thermal history

Abstract

Meteorites carry information about the most common processes that have been active in the early solar system. In particular, mesosiderites are meteorites with a structure considered to be composed of equal parts of iron–nickel metal and silicates. A natural delimitation in the study of such complex systems is the discrimination of the iron–nickel metallic and silicate domains. In this work, we focus on the metallic phases of the Mincy mesosiderite, a specimen available at the Instituto de Ciencias Astronómicas, de la Tierray y del Espacio repository. In Mincy, the metallic phases are iron–nickel–carbon alloys that are distributed forming metallic lumps or pebbles (referred to as metallic nodules in the article) in which kamacite and taenite are present, and taenite is found both at the kamacite/silicate interface and surrounded by kamacite, that is, isolated from the silicates. We made use of the electron backscattered diffraction technique to determine the crystallographic orientation relationships along the taenite/kamacite boundaries as well as for characterizing the (hkl)‐specific grain boundaries regarding the underlying tilt, twist, or twinning mechanism to assist the interpretation of the phase transformations and mechanisms that could explain the formation of these metallic nodules. From the results, each of the metallic nodules has a unique temperature–pressure history and kinetics to undergo phase transformations (mainly partial melting, heterogeneous nucleation‐controlled solidification, and possible evaporation–condensation) as well as liquid‐phase sintering and recrystallization in its own way.

The Vaca Muerta mesosiderite: The path under which Fe‐Ni alloy ±C phases could have formed

“…The γ‐α core‐shell structure, in the form of nodule or space‐limited microstructure, has a rather high Ni content which increases continuously outward γ‐core edge with drastic Fe/Ni ratio changes across the γ/α interface to reach local equilibrium with 50 atom% Ni for γ versus 4.6–6 atom% Ni for α at approximately 350°C according to the Fe‐Ni phase diagram (Yang et al., 1996). This is consistent with a directional diffusion‐controlled L → L′ + γ solidification presumably as the result of a shock event affecting the WP (Garcia et al., 2022), and then grain‐boundary nucleation‐controlled precipitation in the solid state. Then, rather rapid cooling is accomplished in the freezing temperature range of ~1200–600°C to have Ni content increasing toward the γ‐crystal edge in local equilibrium with α shell.…”

“…Then, the α shell can be regarded as the so‐called grain boundary allotriomorph (GBA) (cf. Porter et al., 2009), as has been recently shown by an EBSD study of γ‐α core‐shell structures in the Mincy mesosiderite (Garcia et al., 2022). Accordingly, the presence of polycrystalline taenite situated at the kamacite/silicates boundaries provides support to the fact that phase boundaries are the preferred nucleation sites for partial melting and then recrystallization as GBA in contrast with WP formation via the solid‐state precipitation process.…”

“…Silicate aggregates are cemented by troilite, and metal fills the large and small spaces between these aggregates (Figure 1(a)A,B). The most interesting metal microstructures in the Vaca Muerta DR sample are the large volume percentage of the kamacite domains (probably continuous in 3D and named as “nodules” by Garcia et al., 2022) filling the interstitials of silicates. Each separated kamacite domain in the 2D section was noted to always contain one, or occasionally two, irregularly shaped, or semirounded taenite domains at its interior, which are never in direct contact with silicates (Figure 1(a)A,B).…”