ACFER 214

In the initial discovery in 1990, two pieces of an Fe-rich, fine-grained carbonaceous chondrite, weighing together 166 g, were found in the Algerian Sahara Desert and given the name Acfer 182. In 1991, additional pieces paired with Acfer 182 were found, including a 105 g piece named Acfer 207 and two pieces with a combined weight of 612 g named Acfer 214. This meteorite does not belong to any of the established chondrite groups but does have chemical, mineralogical, and textural similarities to the unique 11.9 g chondritic breccia ALH 85085, and has close affinities to the CR chondrites and the newly designated CB chondrites; these groups considered together have been called the CR-clan. In light of their high bulk iron and metal content, Acfer 182 and ALH 85085 were initially designated as HH chondrites (Bischoff et al., 1992). However, due to their many similarities to carbonaceous chondrites they were designated as CH chondrites (Bischoff et al, 1993). More recently, several separate finds from Antarctica were included in this rare CH group (EET 96238, PAT 91546, PCA 91467, and RKP 92435), as well as additional finds from Northwest Africa (NWA 470, NWA 739, NWA 770, and NWA 4781) and Oman (SaU 290).

This meteorite is composed primarily of chondrules and chondrule fragments (~70 vol%). Interestingly, it has a much lower abundance of complete chondrules than chondrule fragments, with some fragments being derived from larger chondrules than those now present. Most chondrules in Acfer 214 and other CH members are significantly smaller than those in other chondrite groups. The majority of the chondrules are of the much rarer cryptocrystalline texture rather than porphyritic, and they have a mean diameter of 0.03–0.15 mm, the largest measuring 1.1 mm in diameter. Most magnesian and ferroan cryptocrystalline chondrules in CH chondrites have identical chemical and O-isotopic values to those of CB cryptocrystalline chondrules, and the two groups are considered to be genetically related (Nakashima et al., 2010). However, other cryptocrystalline and porphyritic chondrules in CH chondrites have anomalous isotopic values compared to other carbonaceous chondrite groups, inferring that these CH chondrules originated in a separate nebular region and/or during a different time period. An extremely 16O-rich, cryptocrystalline chondrule has been identified in Acfer 214 (Kobayashi et al., 2003). This chondrule, which condensed as a liquid directly from a nebular reservoir, contains a lighter O signature than even refractory inclusions, and is the most 16O-enriched component discovered in a meteorite thus far.

Rare carbonaceous chondrite fragments are also present in Acfer 214 that contain silica-rich spherules composed of nanocrystalline quartz that formed at very high temperatures. They subsequently underwent supercooling until rapid crystallization ensued. The consistently small size of this and all components within this meteorite probably reflects aerodynamic size sorting in the nebula prior to accretion, possibly through size-dependent interactions between gas drag pulling inward and photophoresis and radiation pressure pushing outward (Haack et al., 2006). According to thermal models of Scott et al. (2007), the accretion of CH chondrites is consistent with late accretion ~3–5 m.y. after CAI formation, at a time when radiogenic heating by 26Al was minimal. The formation location was likely in the outer region of the asteroid belt.

FeNi-metal is present in higher concentrations (~20 vol%) than in other carbonaceous chondrites, which, taken together with the volatile- and sulfide-depleted grain cores, indicates an early accretion through condensation in a very hot (~1000°C at 10 Pa) nebular environment, sustained by one or more transient heating events such as that caused by shock. A nebular fractional condensation model is suggested by the widely varied patterns of zoning observed in some of these sub-mm-sized metal grains for the siderophile elements Ni, Co, Cr, P, Si, Au, Os, Ir, Ru, and Pd. To account for the preservation of these primitive zoned metal grains, as well as their virtual lack of Ga and Ge, it is necessary that these grains were isolated from the residual hot nebular gas before temperatures dropped below ~527°C. After condensing near 1 AU, they were possibly radially transported to cooler nebular regions where oxidation, sulfidization, and thermal metamorphism effects were minimal, and cooling was rapid—measured in hours or days. Although a primary martensitic structure was retained in most of the FeNi-metal grains (Kimura et al., 2008), exsolution of Ni-rich taenite has been observed in some of the zoned metal grains, attesting to a brief period of reheating (hours to a day) and subsequent cooling at a reduced rate (Goldstein et al., 2007).

A population of unzoned metal grains which are depleted in Ni and refractory siderophile elements are present, possibly forming a continuum with zoned metal grains. These grains condensed at a later, lower-temperature stage than the zoned metal grains from a gas previously depleted in refractory elements. Moreover, they may have remained longer within the gas environment (> ~10 weeks) and undergone diffusive equilibration of metal (Campbell and Humayun, 2004). Some of these grains were plastically deformed and experienced a brief period of reheating (hours to a day), as evidenced by a recrystallized structure and Ni-rich taenite exsolution phases (Goldstein et al., 2007). It was considered likely that this reheating/exsolution stage occurred after the metal grain was incorporated within the precursor aggregate of a chondrule, and where the heating is attributed to impact events. Finally, brecciation on the CH parent body brought together the various metal grains into the meteorites that we observe.

A silica-rich component (>65 wt% silica), which comprises <0.1 vol% of the meteorite, has recently been studied by Hezel et al. (2003). Based on the chemical compositions of these silica-rich objects, as well as on the sequence of layers that is recorded in one object, an origin through fractional condensation from an evaporated nebular gas was proposed. Initially, Ca–Al–Ti-rich minerals were isolated from the residual gas, followed in the condensation sequence by forsterite and enstatite, the formation of which left the residual gas enriched in SiO and infused with precursory silica-rich material. This precursor material was then reheated to temperatures above 1695°C, at which point several different silica-rich phases condensed—quartz, cristobalite, glass, and an unidentified polymorph—followed by rapid cooling to form the glassy objects, or slower cooling to form the crystalline objects. While the porphyritic chondrules may have been produced during this transient high-temperature reheating phase, evidence suggests that the cryptocrystalline chondrules were likely formed and isolated earlier during fractional condensation processes. Finally, each of these types of objects were accreted into the CH chondrite body.

A small component within Acfer 214 consists of dark, fine-grained inclusions with phyllosilicate-rich clasts. Since the other CH components did not experience alteration, aqueous alteration of these dark hydrated clasts occurred prior to their being incorporated into the CH parent body. Also present are very small (up to 0.45 mm), extremely refractory, rimmed CAIs that are high in grossite, melilite, hibonite, and perovskite occur throughout (~0.1 vol%); some CAIs are significantly less altered than those from other carbonaceous chondrites. An exceedingly rare phase, Ca-monoaluminate, has been identified in the CH group member NWA 470, the first time that this phase has been found in nature. This Ca-monoaluminate is thought to have condensed from a dust-enriched region of the nebula. Since these highly refractory CAIs are depleted in 26Mg, they probably condensed at a very early stage, prior to the injection of 26Al into the nascent solar nebula; alternatively, production of this radionuclide may have been a more localized process that left it absent in the CAI condensation region. The remaining component of Acfer 214 consists of a sparse, fine-grained, chondritic matrix (~5 vol%), not present in other CH members, which has been terrestrially altered to a large degree.

The presence of solar noble gases and the fragmental nature of the components indicates that the CH chondrites were once located in a brecciated surface regolith. Similar to bencubbinites, CH chondrites contain heavy 15N thought to have been accreted from interstellar molecular clouds. Although the actual source of the heavy N remains a source of study, it is thought to have been initially located within carbon-silicate aggregates, and then subsequently redistributed to other phases through shock heating or hydrous alteration. An alternative scenario proposed by Perron and Mostefaoui (2007) calls for the 15N to be delivered by a lagging portion of a hydrated cometary object impacting onto the CH parent body after some degree of cooling of the initial impact plume. They further argued that the data support an origin for the 15N within N-rich molecules rather than from meteoritic, carbonaceous material. In their in-depth study of Bencubbin, Perron et al. (2007) proposed that water and 15N-bearing organics were degassed from the hydrated clasts during the impact of a chondritic object(s). These hydrated clasts were agglomerated onto the Bencubbin parent body during its initial accretionary stages.

Acfer 182, 207, 214, and ALH 85085, along with several meteorites found more recently are designated as CH chondrites. They resolve a group of volatile-poor, high-metal, carbonaceous chondrites that represent the most pristine nebular condensates known. Notably, Acfer 214 and NWA 739 share some anomalous features compared to the other CH chondrites, including larger sized chondrules and O-isotopic compositions that plot outside of the CH field (they are also not concordant with each other). These two meteorites may represent a parent body that is similar to, but separate from, the CH parent body.

The formation of Mercury from similar metal-rich chondritic material has been hypothesized to account for its high density and large core, just as have the inner planets to explain their volatile element depletions. The specimen shown above is a 1.5 g partial slice exhibiting an abundance of tiny metal grains throughout.