ACFER 214

CH3 chondrite
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Found September 30, 1991
27° 41.40' N., 4° 22.72' E.

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 designated Acfer 207 and two pieces with a combined weight of 612 g designated Acfer 214.

This meteorite has chemical, mineralogical, and textural similarities to the unique 11.9 g chondritic breccia ALH 85085, and also has close affinities to the CR and CB chondrites. In light of their high bulk iron and metal content, both Acfer 182 (and pairings) and ALH 85085 were initially designated as 'HH chondrites' (Bischoff et al., 1992). However, due to their many similarities to carbonaceous chondrites they were given the new designation of 'CH chondrites' (Bischoff et al, 1993). Several separate finds from Antarctica have been 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). All of these carbonaceous chondrite groups considered together have been termed the CR clan.

Acfer 214 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 (~80%) 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).

Some 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. Volatile depletions in these cryptocrystalline chondrules suggest they were formed in high temperature conditions and cooled rapidly (Nakashima et al., 2011). An extremely 16O-rich cryptocrystalline chondrule has been identified in Acfer 214 (Kobayashi et al., 2003). This chondrule, among others, condensed as a liquid directly from a nebular reservoir (Varela et al., 2011). It contains a lighter O signature than even refractory inclusions, and is the most 16O-enriched component discovered in a meteorite thus far.

Leitner et al. (2018) identified two presolar silicate grains (O-rich stardust grains) and four presolar SiC grains in the fine-grained material of the hydrated lithic clasts from the paired Acfer 182, representing a bulk abundance of 4 (+5/–2) ppm and 21 (+16/–10) ppm, respectively. Both of the silicate dust grains belong to group 1 of Nittler et al. (1997), which were derived from low-mass (1.2–2.2 M) AGB-stage red giant stars. They note that presolar silicate grains are susceptible to destruction by aqueous alteration while SiC grains are not, and infer that the lithic clasts are consistent with petrologic type 2.

Rare carbonaceous chondrite fragments present in Acfer 214 contain silica-rich spherules composed of nanocrystalline quartz formed at very high temperatures. The spherules subsequently underwent supercooling until rapid crystallization ensued. The consistently small size of these spherules and all other components in this meteorite probably reflects aerodynamic size sorting in the nebula region prior to accretion, possibly through size-dependent interactions between gas drag pulling inward and photophoresis and radiation pressure pushing outward (Haack et al., 2006). Alternatively, size-sorting could have been controlled by the abundance of dust in the nebular region and by the number of chondrule remelting episodes that occurred (Rubin, 2010). 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 Hf–W isochron age for the paired Acfer 182 was determined by Wölfer et al. (2020) to be 3.77 (±1.18) m.y. after CAIs, which is identical within error of that calculated for CB and CR chondrites, the latter considered to be one of the impacting bodies. They also showed that the Mo nucleosynthetic anomalies present in the metal component of both the CB and CH groups are indistinguishable, consistent with condensation from a common impact-generated vapor plume (see diagram below).

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Diagram credit: Wölfer et al., 51st LPSC, #2445 (2020)

FeNi-metal is present in higher concentrations (~20 vol%) than in most other carbonaceous chondrites, which, taken together with the volatile- and sulfide-depletions observed, is indicative of an early accretion through condensation in a very hot (~1000°C at 10 Pa) nebular environment; these high temperatures are sustained by transient heating events associated with impact shocks. 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, these grains could have been radially transported to cooler nebular regions where oxidation, sulfurization, 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 that 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 could 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 is considered likely that this reheating/exsolution stage occurred after the metal grains were incorporated into the precursor aggregate of the chondrules, where the heating is attributed to impact events. Ultimately, brecciation on the CH parent body brought together the various metal grains into the meteorites that we now have for study.

A silica-rich component (bulk SiO2 >65 wt% and up to 86.7 wt% in Acfer 182) that comprises <0.1 vol% of CH meteorites has been investigated by several researchers (e.g., Petaev et al. [2001], Hezel et al. [2003], M. Varela, 2019). Based on the chemical compositions of these silica-rich objects, as well as on the sequence of layers recorded in one of the objects, a formation process involving fractional condensation from an evaporated nebular gas along with liquid immiscibility was considered the most likely scenario. Such nebula regions would have a high dust/gas ratio to support the fractional condensation process. In this scenario, Ca–Al–Ti-rich minerals were initially isolated from the residual gas, followed in the condensation sequence by forsterite and enstatite chondrules (barred olivine [BO], skeletal olivine [SO], Mg-rich cryptocrystalline [CC], ferrous radiating pyroxene [RP]), ultimately leaving the residual gas depleted in Mg and trace elements and enriched in SiO liquid 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 possibly other polymorphs. This stage was followed by either rapid cooling to form the glassy objects or slower cooling to form the crystalline objects. While the porphyritic chondrules (porphyritic olivine [PO], porphyritic olivine pyroxene [POP]) may have been produced during this transient high-temperature reheating phase, evidence suggests that the cryptocrystalline chondrules were formed and isolated earlier during fractional condensation processes. Finally, each of these types of objects were accreted into the growing CH chondrite parent body.

In further study of the silica-rich objects in Acfer 182, Varela (2020) recognized that the trace element abundances for Yb and La remained unfractionated in the silica-rich objects. Therefore, she reasoned that a scenario involving nebular condensation (gas-to-liquid) from a dust- and Si-enriched, Mg-depleted gas under variable redox conditions was likely. The silica-rich objects could have formed as a final condensate in the CH chondrule-forming reservoir following the formation and removal of Mg-rich CC and RP chondrules. The silica-rich objects show evidence they were formed under lower temperature conditions (~1200 K) and lower cooling rates than considered previously; notably, it has been demonstrated experimentally that such silica polymorphs can form through irradiation and annealing of amorphous silica condensates under similar conditions (Varela, 2020 and references therein). In addition, Varela (2020) suggests that multiple chondrule-forming regions are necessary to account for the significant differences observed in bulk trace element abundances and refractory element fractionation between emulsion-type and amoeboid-type silica-rich objects.

A small component within Acfer 214 consists of dark, fine-grained inclusions with phyllosilicate-rich clasts. Given that the other CH components did not experience alteration, aqueous alteration of these dark hydrated clasts must have occurred prior to their incorporation into the CH parent body. Also present are very small (up to 0.45 mm), extremely refractory rimmed CAIs which are high in grossite, melilite, hibonite, and perovskite that are throughout (~0.1 vol%); some CAIs are significantly less altered than those in other carbonaceous chondrites. An exceedingly rare phase, Ca-monoaluminate, has been identified in the CH3 NWA 470 and is the first time this phase has been found in nature. This Ca-monoaluminate is thought to have condensed from a dust-enriched region of the nebula. Given that 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. Due to extended weathering of the large metal component in Acfer 214 and its conversion to pore-filling iron oxide, the porosity was calculated to be zero (Macke et al., 2011).

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 considered 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. In addition, they 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 one or more chondritic objects. These hydrated clasts were agglomerated onto the Bencubbin parent body during its initial accretionary stages.

Extraterrestrial amino acids (13–16 ppm) were found to be present in a sampling of CH chondrites studied by Burton et al. (2013), abundances of which are similar to those found among CM2 chondrites. The types of amino acids are different from those identified in other carbonaceous chondrite groups and were likely synthesized through different chemical pathways under different environmental conditions (e.g., degree of aqueous alteration).

The relatively late formation of the CH chondrites is considered to have been concordant with, and related to the highly-energetic event that produced the CB chondrites, likely within an impact-generated plume. Mineral components studied in Acfer 214 and Acfer 182 are consistent with an accretionary origin from such an event (Krot et al., 2014). Further information about this collisional event can be found on the Bencubbin page. Also, see the HaH 237 page for a detailed scenario of the CB group formation process ascertained by Fedkin et al. (2015) through kinetic condensation modeling.

Acfer 182, 207, 214, along with ALH 85085 and several meteorites found more recently, are designated CH chondrites. They constitute a group of volatile-poor, high-metal, carbonaceous chondrites that represent the most pristine nebular condensates known, or more likely, a late-stage condensate origin in a relatively high molecular weight gas such as a debris plume produced by a high-energy protoplanetary collision (Richter et al., 2014). It is notable that both 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 daughter parent body that is similar to, but separate from that of other CH chondrites.

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 all of the inner planets to explain their volatile element depletions. Reflectance spectra of asteroid (21) Lutetia (Xk or Xc type in Bus-DeMeo taxonomy, M-type in Tholen taxonomy) obtained by ESA's Rosetta spacecraft is a very close match to laboratory spectra of CH3 chondrite PCA 91467 across a wide range of wavelengths for a variety of parameters, and Lutetia is considered to be a good candidate source body for this meteorite group (Trigo-Rodriguez et al., 2012; Moyano-Cambero et al., 2013, 2014, 2016). The data indicate that Lutetia may have a partially differentiated structure with a metallic core, a silicate mantle, and a primitive chondritic crust. The degree of aqueous alteration on Lutetia is higher than for PCA 91467, which could reflect collisional exposure of deeper crustal regions on the asteroid subsequent to the impact ejection of CH chondrites (Moyano-Cambero et al., 2016). A density estimate for Lutetia is similar to the bulk density calculated for the metal-rich CH group (Moyano-Cambero et al., 2016 and references therein). It was inferred that the specific source location for the CH chondrites could be near the northern equatorial region of this heterogeneous asteroid or on an asteroid with similar properties (Moyano-Cambero et al., 2014).

The specimen of Acfer 214 shown above and in the top photo below is a 1.5 g partial slice exhibiting an abundance of metal grains throughout. The bottom photo shows the main mass, courtesy of Luc Labenne.

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