ANGRA DOS REIS


Angrite
Pyroxenite
Plutonic/Metamorphic

angra dos reis

angra dos reis
click on photos for a magnified view

Fell January 20, 1869
22° 58' S., 44° 19' W.

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.

Angra dos Reis is the only pyroxenite among the known angrites, composed of 93 vol% clinopyroxene in the rare form of Al,Ti–diopside-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,Ti–diopside-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 15–75 m; the quench-textured angrites (e.g., D'Orbigny and Sahara 99555) may have crystallized within a meter of the surface.

Minor constituents of AdoR include FeNi-metal, spinel, and whitlockite, along with rare Ti–magnetite, 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 meteorite, with an absolute age only slightly younger (within ~2 m.y.) than the CAIs found in Allende. Extensive isotopic studies establish AdoR as an early planetary differentiate undisturbed since its crystallization ~4,557.6 m.y. ago, and among the 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 late crystallized AdoR has a minor δ26Mg* content that may reflect the Mg isotopic composition of the APB after 26Al decay (Schiller et al., 2010). A radically divergent model for the formation of the angrites has been presented 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.

The decay products of extinct radionuclides in AdoR suggest 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 of volatiles occurred at this time, possibly hastened by the reduced strength of the gravitational field of the fragmented asteroid.

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 catastophic 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, evidence of an ancient core dynamo, and possible high-pressure intergrowth phases are consistent with a large parent body.

In addition to Angra dos Reis, a small number of other angrites compose our current sampling of the angrite parent body: LEW 86010 (6.9 g), LEW 87051 (0.6 g), Asuka 881371 (11.2 g), Sahara 99555 (2,710 g), D'Orbigny (16.55 kg), NWA 1296 (810 g), NWA 1670 (29 g), NWA 2999 (392 g plus 928 g of paired stones designated NWA 3164), NWA 4590 (212.8 g), and NWA 4801 (252 g). 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 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 Rb–Sr 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 (700–1200°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 between ~2.8 and 2.9 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. Portions of the 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.

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 40–160 km occurring within 1.5 m.y. was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums.

Precise U–Pb 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 Mn–Cr systematics, was established for the angrite NWA 2999—it 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. 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. A comparison of cosmic-ray exposure ages among the angrites reveals a wide spread exists (0.2–60 m.y.), suggesting that each may represent a unique ejection event from a different site on the parent body (the CRE ages of A-881371 and Sah 99555 are within statistical uncertainty limits).

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.


angra dos reis main mass
click on photo for a magnified view