240-6 Chlorine-rich Metasomatism was Widespread in the Inner Solar System >70 Ma after Accretion

Session: From Atoms to Asteroids and Habitable Planets: Coordinated Analysis of Planetary Samples and Their Terrestrial Analogues

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Abstract:

Apatite is a common accessory mineral in many terrestrial environments. Importantly among major minerals, apatite contains stochiometric amounts of halogens (Cl and F) and water (as OH), making it an important tool for evaluating the volatile contents of parental magmas [1]. In contrast to the Earth, apatite is not the most common phosphate mineral on many other Solar System bodies, with the anhydrous merrillite being prevalent on the Moon and 4 Vesta, largely due to their low volatile element concentrations [2]. Apatite and merrillite are ubiquitous minerals in martian meteorites. Apatite in martian rocks is extremely Cl-rich relative to terrestrial apatite, and likely results from interactions with metasomatic Cl-rich fluids [3]. Cl-rich apatite (Cl/F ~10) is also found in equilibrated ordinary chondrites and likely formed from phosphide oxidized during metamorphic reactions with Cl-rich metasomatic fluids [4]. Finally, large amounts of very Cl-rich apatite (Cl/F = 25.6) have recently been recognized in brachinite-related meteorites [5]. Such large amounts of apatite should not be present in brachinites due to the large degrees of silicate melting they likely experienced.

The compatibility of U in apatite makes it amenable to dating using the U-Pb concordia method [6]. Therefore, U-Pb dating of apatite can constrain the timing of mobilization of halogen-rich fluids on a variety of meteorite parent bodies. A compilation of U-Pb ages of apatite in planetary materials is presented, showing apatite formation ranging from 169 to 4578 Ma. However, primitive achondrites and chondrites only show U-Pb ages ranging from 4452 to 4578 Ma. Notably, among undifferentiated bodies young apatite (<4500 Ma) is only found on ordinary chondrites, carbonaceous chondrites, and brachinites. Apatite that formed <4500 Ma is unlikely to be the result of metamorphism associated with heating from short-lived radionuclides, but instead must reflect impact events. The many instances of <4500 Ma Cl-rich apatite indicates that impact-mobilization of volatile-rich fluids was a widespread process well over 60 Myr into Solar System history. Dating of apatite in understudied meteorite groups would serve to identify whether this late impact event was widespread and anomalous.                 

[1] Harlov (2015) Elements 11 [2] McCubbin (2015) Elements 11 [3] Hausrath (2024) Minerals 14 [4] Lewis and Jones (2016) MAPS 51 [5] Nicklas (2025) MAPS (in press) [6] Chew (2011) Chem Geol 280  

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5075

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