Background
Carbonaceous chondrite (CC) meteorites recorded conditions during the formation of our Solar System. In 2021, I discovered the mineral tremolite in a CC meteorite called Almahata Sitta 202 (AhS 202). This discovery was sufficiently noteworthy to be published in Nature Astronomy because tremolite forms only under prolonged conditions of elevated temperature and pressure not possible inside the small (<100 km diameter) parent asteroids of CC meteorites, implying a previously unknown parent asteroid 100s to >1000 km in diameter. Few large asteroids exist today; and none are viable parents for AhS 202. From AhS 202, however, we were able to infer that its parent asteroid must have been capable of a process, known on Earth as prograde metamorphism, which had not been previously recognized in early Solar System history from the meteorite record.
In late 2022, I discovered tremolite in a second CC meteorite called Miller Range (MIL) 090292. This discovery either represents another large asteroidal parent body, or it is a second sample from the same parent asteroid as AhS 202. It also leads to the question of how many other instances of tremolite in CC meteorites may have gone unrecognized. The project consisted of five tasks aimed at (1) determining whether AhS202 and MIL 090292 might share a common asteroid parent body that hosted prograde metamorphism, and (2) obtaining preliminary statistics on the occurrence of amphibole minerals in CC to determine how rare or common they may be.
Approach
Using an infrared (IR) microscope, I measured 100 µm/pixel maps of MIL 090292 to assess the spatial distribution of tremolite in MIL 090292. With a colleague from the Lunar and Planetary Institute, we measured detailed bulk chemical compositions for tremolite in MIL 090292 at micron scales for comparison with the chemistry of tremolite amphibole in AhS 202. I reviewed the literature on oxygen isotopes measured in MIL 090292 to compare with AhS 202, as another means of assessing any genetic relationship. Using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), we examined the texture of tremolite in MIL 090292. I also searched my library of over 35,500 IR spectra measured on more than 100 CC meteorites to assess whether other occurrences of tremolite might exist. Finally, I applied the results of the IR spectral library search to determine what additional meteorites might need to be searched for tremolite.
Accomplishments
Infrared spectral mapping showed that tremolite in MIL 090292 is preferentially located in a large, aqueously altered chondrule. Chemical analyses indicated that the tremolite in MIL 090292 has a composition like that of the tremolite in AhS 202. Published oxygen isotope data for both meteorites are similar, also suggesting that they may share the same parent body. SEM and EDS analyses revealed that tremolite is intergrown with the mineral diopside, helping to constrain the chemical reactions that took place. The spectral library search revealed no other detections of tremolite, helping to place better statistical constraints on their representation in CC meteorite collections and indicating that they may be preferentially destroyed upon impact with Earth. I presented the results of this work at the Meteoritical Society meeting in August 2023. I am now in the process of writing a manuscript, with my colleagues, for peer-reviewed publication. The results of this project will be used in support of a proposal to NASA’s Emerging Worlds program.