At 5:00 A.M. in Rio de Janeiro, Brazil, a stone weighing ~1.5 kg was seen to fall. The meteorite left a smoke trail as it plunged into the bay to a depth of about 2 meters. Two small pieces were recovered by a diver the following day. An unmatched fresh surface on one of the fragments indicates that a third piece was not recovered. One fragment was described at the time as weighing 444.5 g, but no reference was made to the other piece. Unfortunately, there is only ~150 g of Angra dos Reis (AdoR) accounted for in collections today. More than one hundred years passed since the fall and classification of Angra dos Reis until other angrites were found, primarily in the cold and hot deserts of the world. The relatively small number of unique angrites represented in our collections today have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample (NWA 8535).
Angra dos Reis is the only pyroxenite among the known angrites, composed of 93 vol% clinopyroxene in the rare form of Al,Tidiopside-hedenbergite, formerly known as fassaite. This fassaite is present in two textures: 1) poikilitic megacrysts up to 3 mm in size, possibly representing phenocrysts or relict cumulus grains, and 2) groundmass grains ~100 µm in size, possibly derived from devitrified melt or by an annealing process (Treiman, 2011). Historically, Angra dos Reis had been thought to have crystallized as a cumulate, or possibly from a fractionated melt, but uniquely, Angra dos Reis contains minor calcic ferroan olivine incorporating magnesian kirschsteinite which exsolved from the olivine during slow cooling or annealing (Fittipaldo et al., 2003, 2005). Kirschsteinite also occurs between grains in olivine aggregates, often associated with troilite, which suggests an origin from a melt residue. Rare kirschsteinite lamellae also occur within some olivine grains in olivine aggregates. The Al,Tidiopside-hedenbergite, olivine, and kirschsteinite mineral components each have homogeneous major, minor, and trace element compositions consistent with extended equilibration. Based on studies of how kirschsteinite lamellae profiles relate to cooling rates, the burial depth of the angrites as they crystallized in a lava field is inferred to have been 1575 m; the quench-textured angrites (e.g., D'Orbigny and Sahara 99555) could have crystallized within a meter of the surface.
Minor constituents of AdoR include FeNi-metal, spinel, and whitlockite, along with rare Timagnetite, plagioclase, celsian, and baddelyite. Angra dos Reis is highly depleted in volatiles such as Na, and highly enriched in oxidized elements such as FeO, TiO and CaO, characteristics which distinguish this meteorite from those of other groups. The angrite source region can be modelled as an incomplete mixing of an alkali- and metal-depleted primitive chondrite with high-Ca, high-temperature condensates similar to CAIs, but containing excess melilite.
Angra dos Reis is an extremely ancient basaltic meteorite, and extensive isotopic studies have established that it is an early planetary differentiate undisturbed since its crystallization ~4,556.6 m.y. ago. Among the limited suite of angrites, it remains the best representative of the original mantle isotopic composition (Abernethy et al., 2013). Another group of angrites have isochrons reflecting a more rapid cooling history, crystallizing up to 7 m.y. earlier than AdoR. The relatively late crystallized AdoR has a minor δ26Mg content that might reflect the Mg isotopic composition of the APB after 26Al decay (Schiller et al., 2010). Two radically divergent models for the formation of the angrites have been presented. One was proposed by Kurat et al. (2004), a brief synopsis of which can be found on the D'Orbigny page. They present evidence for a non-igneous origin of angrites on a very early-formed parent body which was composed primarily of refractory material. Another scenario was proposed by King and Henley (2016), in which angrites formed within a small dust/gas clump that existed in the proto-planetary disc, rather than on a large differentiated object.
The decay products of extinct radionuclides in AdoR suggest that the entire sequence from nebular condensation through parent body accretion, partial melting of the parent body, metallic core formation, formation of clinopyroxene rock, cumulate/crystallization processes, and final cooling to temperatures low enough to retain fission tracks and noble gases took an incredibly short 18 m.y. Crystallization of AdoR proceeded as a two-stage process beginning with partial melting from a source composed of olivine, orthopyroxene, and clinopyroxene at low pressure, followed by an extended period of slow cooling and annealing to ~650°C, after which time it was rapidly quenched during a severe impact event. Vigorous outgassing, element fractionation (e.g., Si), and evaporation of volatiles likely occurred during the planetesimal accretionary stage (Pringle et al., 2015), as well as during severe impact events; impact-induced devolatilization was possibly hastened by the reduced strength of the gravitational field following asteroid fragmentation. Despite the extreme volatile depletions in angrites, a high water content has been measured in silicates (2060 ppm) and phosphates (>400 ppm) in both AdoR and D'Orbigny, and this water is highly enriched in deuterium (δD >500) compared to the Earth (Sarafian et al., 2015). The volatile-depleted nature of angrites may reflect accretion from volatile-poor precursor material, followed by early accretion of either D-rich water or water that subsequently experienced strong degassing and fractionation; however, other scenarios are also possible.
In contrast to the unshocked, unbrecciated nature of other angrites, Angra dos Reis is an unbrecciated meteorite that has experienced a significant shock event or thermal metamorphism. Scott and Bottke (2011) proposed that the non-shocked appearance of AdoR is most consistent with a long-term storage residence of several b.y. following a catastrophic impact ~4.5 b.y. ago. This storage period commenced after angrite material was ejected and accreted into one or more small, secondary angritic bodies ~10 km in diameter. They reason that an original parent body <200 km in diameter would have resulted in a loss of basalts through explosive volcanism, and that the presence of trapped solar-type gases, presence of possible high-pressure intergrowth phases, and evidence of an ancient core dynamo, are factors consistent with a large parent body.
Paleomagnetic intensity studies conducted for Angra dos Reis by Wang et al. (2015) have established a natural remanent magnetization value of ~15 µT (microteslas), demonstrating that this lithology formed under the influence of a significant core dynamo which existed ~11 m.y. after CAIs. By comparison, no natural remanent magnetization (paleointensity) > ~1 µT was detected for the earlier formed angrites D'Orbigny and Sahara 99555, which constrains the onset of the APB core dynamo to later than ~4 m.y. after CAI formation. It was also recognized that the strong solar nebula-generated magnetic field which had existed ~1.23 m.y. after CAIs (~50 µT, measured in Semarkona chondrules) had virtually disappeared by the time the earliest angrites were formed, indicating that the solar nebula had already been largely dissipated.
Diagram credit: Wang et al., 46th LPSC, #2516 (2015)
Since Angra dos Reis is anomalous in its mineralogy and has aberrant major and trace elemental compositions compared to other angrites, it has been proposed that AdoR represents either a separate source magma on the angrite parent body that experienced a unique thermal history, or that it represents an entirely distinct parent body. It was concluded by Kleine et al. (2009) that both AdoR and the plutonic/metamorphic angrite NWA 2999 were derived from a parental source magma which had higher Hf/W than other angrites, likely the result of extended differentiation after core formation.
Recent investigations by Tonui et al. (2003) into the initial 87Sr/86Sr in Angra dos Reis and D'Orbigny have determined that their parent sources were similar, and they have provided actual evidence that AdoR and D'Orbigny, and probably the other angrites, share a common parent body. Moreover, O-isotope analyses conducted for AdoR and several other angrites clearly indicate that all angrites studied originated from a single parent body (Greenwood et al., 2003). This O-isotope study also included diverse members of the HED suite (thought to originate on the asteroid 4 Vesta), and it was concluded that HED meteorites represent a single parent body unique from the angrite parent body. The RbSr chronometry of angrites as it relates to CAIs indicates that a possible late volatile depletion occurred, which is difficult to reconcile with very early accretion and differentiation (Hans et al., 2010). Differences in Mo isotopic compositions between angrites and IVB irons exclude a genetic link (Burkhardt et al., 2011).
Utilizing stepped combustion analyses to study the indigenous carbon and nitrogen component of five of the angrites, including Angra dos Reis, Abernethy et al. (2013) found that both of these light elements were released at
similar temperatures (7001200°C). Although they could not determine a specific correlation between the two elements based on their abundances or their isotopic compositions, the team did demonstrate the likelihood that much of the C and N was incorporated as atoms within the silicate lattice, probably attained through metasomatic processes involving sulfur-rich fluids. It was further hypothesized that the atomic C originated from graphite, itself being an earlier product of a carbonate reduction process, or that it was a result of dissociation of CO2. In a similar manner, it was shown that atomic N was likely dissociated at high temperatures and then became bound within the silicate lattice. There remains a speculation at this point that some of the C and/or N was originally an organic component of a carbonaceous phase similar to that found in CM-type carbonaceous chondrites.
It was inferred by Nyquist and Bogard (2003) that since the angrite D'Orbigny was spectroscopically similar to two asteroids located ~2.82.9 AU (289 Nenetta and 3819 Robinson), then it was also probable that the angrite parent body formed in this same region. They argued that asteroids at this heliocentric distance accreted too slowly to permit the accumulation of enough 26Al to cause global melting and differentiation before a diameter greater than ~200 km would have been attained; i.e., given a body with a diameter larger than ~200 km, there would not have been enough heat necessary to melt and differentiate this body. By this line of reasoning, it may be concluded that the differentiated angrite parent body was either not as large as 200 km in diameter, or that it formed at a smaller heliocentric distance than ~2.8 AU.
Without regard to heliocentric distance, Sanders and Scott (2007) argued that any body which accreted to a diameter >60 km (i.e., large enough to minimize heat loss from the surface through conduction) within ~2 m.y. of CAI formation (the oldest objects dating to 4.567 b.y. ago) as the angrites did, must contain enough 26Al to produce global melting and differentiation. In contrast, Senshu and Matsui (2007) determined that accretion to a diameter of only ~14 km occurring within 2 m.y. of CAI formation was all that was required for global differentiation to occur, while a diameter of 40160 km occurring within 1.5 m.y. was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums.
Be that as it may, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. For example, the initial ratio of 0.00000040.0000005 calculated for the angrites Sah 99555 and D'Orbigny based on their 26Al26Mg isochrons is too low to have generated any significant melting without an additional heat source. Therefore, impacts were a major source of heating in early solar system history. It has also been suggested by some that relatively small planetesimals might have been just the required size to allow heating by induction in the plasma environment of the T Tauri Sun.
In order to better constrain the properties of the differentiated angrite parent body core, van Westrenen et al. (2016) conducted a study modeling siderophile element depletions along with their metalsilicate partitioning behavior for the hypothesized angrite parental melt composition. A CV chondrite mantle composition was used for their calculations, along with a temperature and pressure (0.1 GPa) appropriate for a solidifying melt on a small planetesimal. Their results indicate that the observed siderophile element depletions of angrites are consistent with a core mass fraction of 0.120.29 composed of Fe and Ni in a ratio of ~80:20 (with a low S content), and that it was formed under redox conditions (oxygen fugacity) of ΔIW1.5 (±0.45).
Precise UPb ages have been calculated for AdoR and LEW 86010 to be 4,557.65 (±0.13) m.y. and 4,558.55 (±0.15) m.y., respectively (Y. Amelin, 2007). An identical age within error, based on MnCr systematics, was established for the angrite NWA 2999it was determined to be 4,557.9 (±1.1) m.y. old. Although these three angrites are slowly-cooled basalt-type rocks exhibiting unzoned minerals, they crystallized over an extended period of at least 0.90 (±0.19) m.y., and therefore, were likely derived from independent magma sources. Based on a comparison of Hf/Sm ratios for a diverse sampling of both angrites and eucrites, Bouvier et al. (2015) inferred that these two meteorite groups reflect the existence of three distinct crustal reservoirs on their respective parent bodies. These three reservoirs reflect similar chemical differentiation processes on both parent bodies: 1) subchondritic Hf/Sm ratios for the Angra dos Reis angrite and the cumulate eucrites (such as Moama); 2) chondritic Hf/Sm ratios for the quenched angrites (such as D'Orbigny and Sahara 99555) and the basaltic eucrites; 3) superchondritic Hf/Sm ratios for the plutonic angrites (NWA 4590 and NWA 4801) and the unusual cumulate eucrite Binda.
Cosmic-ray track densities place the pre-atmospheric mass of AdoR at ~80 kg with an exposure age of 55.5 (±1.2) m.y. (Lugmair and Marti, 1977). Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites0.673 m.y. for eleven angrites measured, representing as many as nine ejection events (e.g., Nakashima et al., 2008; Wieler et al., 2016). This range is consistent with a large parent body enduring multiple impacts over a very long period of time, and suggests that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 73 m.y.
Portions of the angrite asteroid must be in a stable orbit (planetary or asteroid belt) from which spallation has continued to occur over the past ~55 m.y. as indicated by the wide variation in angrite CRE ages. The photos of AdoR shown above depict both sides of a 0.34 g fragment with a small patch of glossy black fusion crust visible in the top picture; click on the photos for a detailed view. A photo of the main mass of Angra dos Reis, curated at the National Museum of Brazil, is shown below courtesy of Andre Moutinho. This photo exhibits clearly the extensive area of rippled, glossy black fusion crust.