A high-resolution in situ X-ray diffraction study of mineral transitions due to post-hydration heating in CM chondrite meteorites

Abstract

Abstract: The effects of post-hydration heating over a broad range of temperatures are evident in many Mighei-like carbonaceous (CM) chondrites as a variety of mineral transitions. To better understand these processes and how a CM chondrite's starting composition may have affected them, we experimentally heated two meteorites with different degrees of aqueous alteration, Allan Hills 83100 and Murchison, at 25 °C temperature steps from 200 °C to 950 °C and 300 °C to 750 °C, respectively. During heating, synchrotron in situ X-ray diffraction patterns were collected. With the exception of calcite decomposition and its products, most mineral transitions were unaffected by starting composition. Key observations include: (1) partial decomposition of tochilinite at 200 °C, which indicates that tochilinite breakdown might be a two-stage process due to its intergrown layers of brucite/amakinite and mackinawite; (2) the breakdown of serpentine occurring at 300 °C with transitional phases appearing at 525 °C and 575-600 °C, while secondary olivine formed at 600 °C; (3) cronstedtite decomposing faster than lizardite, (4) the formation of secondary enstatite at 750 °C, and (5) calcite decomposition temperature differing significantly between meteorites, occurring at 725 °C and 575 °C in ALH 83100 and Murchison, respectively. The results for calcite are likely controlled by differences in its microstructure and chemical composition, related to the meteorite's impact history and degree of aqueous alteration. The difference in calcite decomposition temperature also explains the contrasts in the observed breakdown products, with clinopyroxene occurring in both meteorites, and oldhamite only in ALH 83100. Mineral transitions due to post-hydration heating have been characterized with a high resolution XRD method, enabling a better understanding of processes occurring on the parent asteroids of CM chondrites.

Supplementary information: The online version contains supplementary material available at 10.1186/s40623-024-02116-2.

Keywords: Carbonaceous chondrites; Laboratory experiments; Meteorites; Post-hydration heating; Thermal metamorphism; X-ray diffraction.

Fig. 1

XRD patterns showing changes in ALH 83100 sulfides due to heating. Labelled minerals are tochilinite (Thi), lizardite (Lz), cronstedtite (Cro), pyrrhotite (Pyh), enstatite (En), pentlandite (Pn), forsterite (Fo), fayalite (Fa), magnetite (Mag), and troilite (Tro). a Thi decomposition. At 200 °C, many Thi peaks weaken or disappear. At 300 °C, nearly all Thi peaks are gone except for a single peak at d = 6.02 Å (13.3° 2ϴ). At 375 °C this last peak disappears. b Tro crystallization. At 275 °C, Tro peaks appear. c At 675 °C, Pn peaks weaken, with many disappearing

Fig. 2

XRD patterns showing changes experienced by silicates in ALH 83100 due to heating. Labelled minerals are tochilinite (Thi), lizardite (Lz), cronstedtite (Cro), forsterite (Fo), diopside (Di), an unmatched phase (I), enstatite (En), fayalite (Fa), troilite (Tro), pentlandite (Pn), pyrrhotite (Pyh), calcite (Cal), and magnetite (Mag). a Serpentine decomposition. Lz and Cro peaks begin weakening at 300 °C and disappear by 700 °C. Shifting of Lz and Cro peaks between 28 °C and 300 °C is due the thermal expansion of their crystal lattices. b Changes experienced by unmatched phase and Di. A peak at d = 3.56 Å (13.3° 2ϴ) begins strengthening at 525 °C until 750 °C is reached, wherein it begins weaking. Di is also shown appearing at 725 °C. c Crystallization of secondary En, Fo, and Fa. Some but not all En peaks begin strengthening at 575 °C. All En peaks strengthen later at 750 °C. Fo peaks strengthen at 600 °C and Fa peaks appear at 650 °C

Fig. 3

XRD patterns showing changes experienced by Ca-bearing minerals in ALH 83100 due to heating. Labelled minerals are diopside (Di), oldhamite (Old), pyrrhotite (Pyh), calcite (Cal), tochilinite (Thi), pentlandite (Pn), fayalite (Fa), forsterite (Fo), enstatite, (En), magnetite (Mag), and lizardite (Lz). a Cal decomposition. Cal peaks begin weakening at 725 °C and disappear by 800 °C. b Old crystallization. Old appears at 725 °C

Fig. 4

XRD patterns showing changes experienced by sulfides in Murchison. Minerals labelled are troilite (Tro), enstatite (En), calcite (Cal), forsterite (Fo), magnetite (Mag), tochilinite (Thi), lizardite (Lz), cronstedtite (Cro), pentlandite (Pn), and augite (Aug). a Decomposition of Thi and crystallization of secondary Tro and Mag. Thi peaks disappear at 300 °C, while at the same time Tro peaks appear. Mag peaks (e.g., 15.8°, 18.7°, and 19.1° 2ϴ) start strengthening at 350 °C. b Pn decomposition. Pn peaks start weakening at 675 °C, with peak weakening accelerating during the cooldown to room temperature. Tro peaks also start strengthening significantly with Pn decomposition

Fig. 5

XRD patterns showing changes as a result of serpentine decomposition in Murchison. Minerals labelled are cronstedtite (Cro), lizardite (Lz), tochilinite (Thi), forsterite (Fo), augite (Aug), enstatite (En), fayalite (Fa), and an unmatched phase (I). a Serpentine decomposition and crystallization of secondary silicates. Both Cro and Lz (e.g., see peaks at 9.1°, 12.0°, 12.2°, 13.2°, and 13.3° 2ϴ) begin decomposition at 300 °C, with peaks associated with Cro (e.g., 13.2° 2ϴ) weakening at a faster rate. All serpentine peaks disappear at 525 °C. At 525 °C, peaks associated with an unmatched phase appear and strengthen. At 600 °C, Fo peaks strengthen, and Fa peaks appear. En peaks strengthen at 750 °C. b Weakening of the peaks associated with the unmatched phase, which becomes most apparent during cool down to room temperature

Fig. 6

XRD patterns showing changes experienced by Ca-bearing phases in Murchison during heating. Labelled minerals are forsterite (Fo), augite (Aug), an unmatched phase (I), troilite (Tro), enstatite (En), calcite (Cal), cronstedtite (Cro), and lizardite (Lz). At 575 °C, Cal peaks start weakening and peaks associated with Aug appear

Fig. 7

Mineral transitions observed in ALH 83100 and Murchison during experimental post-hydration heating