The alteration history of the Jbilet Winselwan CM carbonaceous chondrite: An analog for C‐type asteroid sample return

Abstract

Jbilet Winselwan is one of the largest CM carbonaceous chondrites available for study. Its light, major, and trace elemental compositions are within the range of other CM chondrites. Chondrules are surrounded by dusty rims and set within a matrix of phyllosilicates, oxides, and sulfides. Calcium‐ and aluminum‐rich inclusions (CAIs) are present at ≤1 vol% and at least one contains melilite. Jbilet Winselwan is a breccia containing diverse lithologies that experienced varying degrees of aqueous alteration. In most lithologies, the chondrules and CAIs are partially altered and the metal abundance is low (<1 vol%), consistent with petrologic subtypes 2.7–2.4 on the Rubin et al. () scale. However, chondrules and CAIs in some lithologies are completely altered suggesting more extensive hydration to petrologic subtypes ≤2.3. Following hydration, some lithologies suffered thermal metamorphism at 400–500 °C. Bulk X‐ray diffraction shows that Jbilet Winselwan consists of a highly disordered and/or very fine‐grained phase (73 vol%), which we infer was originally phyllosilicates prior to dehydration during a thermal metamorphic event(s). Some aliquots of Jbilet Winselwan also show significant depletions in volatile elements such as He and Cd. The heating was probably short‐lived and caused by impacts. Jbilet Winselwan samples a mixture of hydrated and dehydrated materials from a primitive water‐rich asteroid. It may therefore be a good analog for the types of materials that will be encountered by the Hayabusa‐2 and OSIRIS‐REx asteroid sample‐return missions.

Terrestrial modification of the Ivuna meteorite and a reassessment of the chemical composition of the CI type specimen

“…This "nugget effect" has been observed and discussed in other aqueously altered carbonaceous chondrites (e.g. Dauphas and Pourmand 2015;King et al 2019a).…”

“…Gibson and Bogard 1978;Jull et al 1988;Ikeda and Kojima 1991;Velbel et al 1991;Wlotzka 1993;Bland et al 1998aBland et al , 1998bStelzner et al 2011;Velbel 2014) but few have investigated the carbonaceous chondrite meteorites (e.g. Zolensky and Gooding 1986;Gounelle and Zolensky 2001;Velbel and Palmer 2011;Walker et al 2018;King et al 2019a). Such studies are crucial, however, as carbonaceous chondrites are scientifically valuable -as pristine samples of the early solar system and potential sources of water and organics to the terrestrial planets -but relatively scarce meteorites that due to their fragile, porous and chemically volatile nature are highly vulnerable to terrestrial alteration.…”

CM carbonaceous chondrite falls and their terrestrial alteration

“…2018; King et al. 2019). Antarctic CMs are less affected, although some are depleted in highly fluid mobile elements including K, Na, and Rb (Braukmüller et al.…”

Alkali‐halogen metasomatism of theCMcarbonaceous chondrites

“…), and Jbilet Winselwan (CM2.7–2.4; King et al. ). MET 01075 by contrast is classified as CM2.2 and so has been much more highly altered.…”

“…This reaction could have taken place before or after aqueous alteration of MT75-1. A prehydration timing is preferred because melilite is highly reactive during aqueous alteration and has survived only in the most pristine CMs, namely Murchison (CM2.5; Armstrong et al 1982), Paris (CM2.7;Hewins et al 2014;Marrocchi et al 2014), EET 96029 (CM2.7;Lee et al 2016), and Jbilet Winselwan (CM2.7-2.4;King et al 2018). MET 01075 by contrast is classified as CM2.2 and so has been much more highly altered.…”