Sequential Lonsdaleite to Diamond Formation in Ureilite Meteorites via In Situ Chemical Fluid/Vapor Deposition

Abstract

Ureilite meteorites are arguably our only large suite of samples from the mantle of a dwarf planet and typically contain greater abundances of diamond than any known rock. Some also contain lonsdaleite, which may be harder than diamond. Here, we use electron microscopy to map the relative distribution of coexisting lonsdaleite, diamond, and graphite in ureilites. These maps show that lonsdaleite tends to occur as polycrystalline grains, sometimes with distinctive fold morphologies, partially replaced by diamond + graphite in rims and cross-cutting veins. These observations provide strong evidence for how the carbon phases formed in ureilites, which, despite much conjecture and seemingly conflicting observations, has not been resolved. We suggest that lonsdaleite formed by pseudomorphic replacement of primary graphite shapes, facilitated by a supercritical C-H-O-S fluid during rapid decompression and cooling. Diamond + graphite formed after lonsdaleite via ongoing reaction with C-H-O-S gas. This graphite > lonsdaleite > diamond + graphite formation process is akin to industrial chemical vapor deposition but operates at higher pressure (∼1–100 bar) and provides a pathway toward manufacture of shaped lonsdaleite for industrial application. It also provides a unique model for ureilites that can reconcile all conflicting observations relating to diamond formation.

Formation of diamond and lonsdaleite in ureilites by impact shock processing of graphite

“…Recently, Tomkins et al. (2022) showed direct evidence for the presence of crystallites of lonsdaleite within two ureilites. Using real‐time in situ synchrotron XRD measurements during impact experiments on high‐order pyrolitic graphite, Turneaure et al.…”

“…Observations of diamond in some weakly shocked ureilites (Miyamoto et al, 1988;Nakamuta & Aoki, 2000;Takeda et al, 2001) also support a vapor-growth mechanism for the origin of diamond. Recently, Tomkins et al (2022) observed the presence of polycrystalline aggregates of lonsdaleite with folded texture within two ureilites. They concluded that the lonsdaleite, diamond, and graphite assemblage formed through a CVD process by reaction between a C-H-O-S fluid/gas with graphite after the impact disruption of the ureilite parent body.…”

“…But, lonsdaleite has been reported to have formed in various experiments: It has been formed from glass-like carbon or graphite under static pressure (Bundy & Kasper, 1967;Isobe et al, 2013;Le Guillou et al, 2007;Utsumi & Yagi, 1991), by shock compression of graphite and diamond (DeCarli & Jamieson, 1961;Erskine & Nellis, 1991;Frondel & Marvin, 1967;He et al, 2002;Kurdyumov et al, 2012), and by chemical vapor deposition (Misra et al, 2006). Recently, Tomkins et al (2022) showed direct evidence for the presence of crystallites of lonsdaleite within two ureilites. Using real-time in situ synchrotron XRD measurements during impact experiments on high-order pyrolitic graphite, Turneaure et al (2017) also showed that hexagonal diamond forms within several nanoseconds at 50 GPa.…”

“…Recently, Tomkins et al. (2022) observed the presence of polycrystalline aggregates of lonsdaleite with folded texture within two ureilites. They concluded that the lonsdaleite, diamond, and graphite assemblage formed through a CVD process by reaction between a C‐H‐O‐S fluid/gas with graphite after the impact disruption of the ureilite parent body.…”

“…Lonsdaleite is a metastable sp 3 ‐bonded allotrope of carbon and shows features that point to a hexagonal symmetry. Lonsdaleite was first identified in the Canyon Diablo meteorite, and subsequent studies have shown that it can be produced by various processes that leads to distinct signatures (Bhargava et al., 1995; Bundy & Kasper, 1967; DeCarli & Jamieson, 1961; Erskine & Nellis, 1991; Frondel & Marvin, 1967; He et al., 2002; Isobe et al., 2013; Jones et al., 2016; Kraus et al., 2016; Kurdyumov et al., 2012; Le Guillou et al., 2007; Németh et al., 2014; Németh & Garvie, 2020; Tomkins et al., 2022; Turneaure et al., 2017; Utsumi & Yagi, 1991).…”