D'ORBIGNY


Angrite
Basaltic/Quenched
standby for d'orbigny photo
Found July 1979
37° 40' S., 61° 39' W.

This relatively fresh, 16.55 kg, shield-shaped, regmaglypted meteorite is by far the largest of the angrites found so far. The mass was found in Buenos Aires Province, Argentina by a farm worker who struck it with a plow. Thinking he had unearthed an Indian artifact, possibly an old mortar, he gave it to the landowner who set it by his house for the next ~20 years. Not until 1998, after reading an article on meteorites, did the owner seek to have the stone analyzed. In September of 2000, Dr. Gero Kurat of the Naturhistorisches Museum of Vienna, Austria made the determination that it was an angrite.

D'Orbigny is an unusual achondrite that shows no evidence of brecciation, shock metamorphism, or significant thermal metamorphism, and some believe that it may not have an igneous origin. It consists predominantly of Ca-bearing olivine and anorthite, which compose an intergrowth of plates and networks (Kurat et al., 2004). The other major constituent is Ti–Al-augite, which likely formed later than the olivine–anorthite intergrowths. A late oxidizing event produced strongly zoned grains of the clinopyroxene Al,Ti–diopside-hedenbergite, previously known as fassaite, which now fill most of the intergranular spaces. Large (up to 1 cm) clear to milky, green to greenish-white, magnesian olivine megacrysts and polycrystalline olivinites represent one of the earliest phases of the host rock. Also representing very early constituents, Cr-bearing Al-spinel and Fe-bearing spinel occur within some olivine and anorthite grains. Minor kirschsteinite, ulvöspinel, and troilite (and other sulfides) are present, along with rare awaruite, calcium silico-phosphate (a late-stage crystallization phase similar to silicocarnotite which only crystallizes from Fe-, Ca-, and P-enriched basalt; Mikouchi et al, 2010), and an unidentified Fe–Al–Ti silicate. Rare, cm-sized, Mg- and Cr-rich olivine and spinel xenocrysts with granoblastic textures have been identified, similar to those found in greater abundance in other angrites (except Sah 99555). The xenocrysts are indicative of a rapid ascension of magma.

D'Orbigny has a heterogeneous composition consisting of alternating layers of a dense, coarse-grained texture, and a porous texture containing abundant round vugs or hollow shells up to 2.5 cm, along with plates and druses composed primarily of augite (diopside-hedenbergite) crystals and some anorthite crystals. Formation of these vesicles is consistent with bubble growth involving significant CO and CO2 concentrations of 10–20 ppm (up to 25 ppm C) in a magma as it ascended within a dike, undergoing decompression and coalescence of smaller bubbles, eventually solidifying near the surface (McCoy et al., 2003, 2006). It was ascertained that the druse pyroxenes formed under oxidizing conditions (near the QFM buffer), then underwent rapid cooling from ~1000°C, perhaps as a result of the dissipation of a hot vapor (Abdu et al., 2009).

Conversely, instead of formation of the vesicles through an igneous process, it was proposed by Kurat et al. (2002) that they were originally solid spheres composed of one of the earliest and most reduced phases, possibly CaS, which was covered by anorthite-olivine rims and plates. The cores were subsequently lost through an oxidizing Fe–Mn–Cr metasomatism process incorporating water, with the calcium being utilized in the formation of the augite, kirschsteinite, and diopside-hedenbergite. This druse formation scenario is consistent with a pneumatolytic formation process. Some of the vugs are now filled with glass.

Ubiquitous primary glasses present in D'Orbigny have unfractionated chondritic relative abundances of refractory lithophiles, indicating a possible origin through bulk rock melting, but excluding an origin as residues of a partial melt. Solar-like trapped noble gases present in these glasses are thought to have originated from primordial reservoirs of solar wind gases which accumulated very early in Solar System history. These noble gases were subsequently implanted within the glasses by way of an ascending deep magma (Busemann et al., 2006). The presence of vesicles in Sahara 99555 and D'Orbigny angrites attests to this rapid ascent and cooling of volatile-enriched magma. Schiller et al. (2010) argue that angrites experienced such volatile depletion associated with accretion within a short time after CAI formation, which was ~1 m.y. before volatile depletion occurred on the HED parent body. Two further episodes of volatile depletion on the angrite parent body are considered likely impact-related events. Formation of the glasses was contemporaneous with formation of the bulk of D'Orbigny.

An alternative formation mechanism for the glass phase has been proposed by Varela et al. (2003). Based upon the finding that some elemental abundances, such as FeO and MnO, as well as some elemental ratios, such as CaO/TiO and FeO/MnO, are similar to those in CI chondrites, and because the glass shares many characteristics with glass inclusions in olivine grains from carbonaceous chondrites (e.g., the presence of volatile elements such as C and N, thought to have been incorporated as refractory material, which were subsequently volatilized through oxidation reactions and the depletion of volatile lithophile elements), they proposed that the glass crystals grew from a vapor phase (conceivably in the solar nebula) upon moist surfaces within interstitial spaces during olivine formation. Later, an oxidizing metasomatic alteration event that was intrinsically chondritic affected the glass and bulk rock.

Nitrogen in D'Orbigny is scarce and exhibits an enrichment in δ15N (Abernethy et al., 2013). Futhermore, D'Orbigny has a low abundance of C that is also enriched in the heavier isotopes (δ18O) compared to other angrites, demonstrating a preferential loss of lighter isotopes during degassing. This enrichment in δ18O is unique from all other angrites, but similar to CI/CM chondrites. Although they could not determine a specific correlation between the C and N based either on their abundances or isotopic compositions, it was demonstrated that much of the C and N was likely 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 CO and 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.

Prior studies have shown that the close textural and compositional trends present in the angrites D'Orbigny, Sahara 99555, Asuka 881371, LEW 87051, NWA 1670, and possibly NWA 1296 provide evidence for their crystallization from a common magma source. It was suggested that this D'Orbigny group of angrites underwent rapid cooling and crystallization at depths of less than 0.5 m. However, since D'Orbigny contains no solar implanted gases, it could not have been exposed to the surface environment of the parent body. Angra dos Reis, LEW 86010, and NWA 2999 show evidence of a slower cooling history than the angrite grouping above, and they are probably not co-magmatic with them. Furthermore, precise U–Pb ages obtained for these three slowly cooled angrites indicate that they crystallized at least 0.9 m.y. apart, inferring an independent source magma for some or all of them (Amelin, 2007).

Trace element data argues for a more complex history for D'Orbigny and most angrites, including a non-igneous formation from a refractory condensate of a chondritic nature. Late phases of D'Orbigny are enriched in moderately volatile elements compared to early phases, and the two phases were formed under very different redox conditions—the early phases grew under highly reducing conditions while the late phases grew under highly oxidizing conditions (Varela et al., 2005). Moreover, highly incompatible elements in all olivine phases are far out of equilibrium (highly enriched) with the parental melt that formed the bulk rock. Curiously, plagioclase, which formed together with the olivine, contains very different abundances of incompatible elements, suggesting that the olivine and plagioclase formed from melts of dissimilar compositions. It is thought that the solid spheres (possibly CaS), enriched in Ca and unfractionated trace elements, were subsequently decomposed under the highly oxidizing conditions. By this process the previously bound trace elements were converted into a vapor phase and became available for late phase metasomatism and augite formation.

Angrites are extremely ancient meteorites, with some such as D'Orbigny having accretion ages as old as ~0.5 m.y. after the first nebular condensates (CAIs) were formed (Sugiura and Fijiya, 2012). Other angrites such as LEW 86010 and Angra dos Reis demonstrate that basaltic extrusion on the angrite parent body continued for ~7 m.y. longer. The early thermal history of the angrite parent body is most consistent with a relatively large sized planetesimal of at least 100 km in diameter (Sahijpal et al., 2007). One scenario for the formation of angrites involves an igneous history.

Another possible petrogenetic history involves a non-igneous formation:

A number of whole-rock and mineral isochrons have been calculated for the angrites. A U–Pb age of 4,555.3 (±1.7) m.y. was previously reported for D'Orbigny (Jotter et al., 2002), an age that is slightly younger than that determined for other angrites using this isotopic system (4,557.8 ±0.5 m.y. for LEW 86010 and AdoR). Other studies of matrix and druse pyroxenes from D'Orbigny have yielded a range of U–Pb ages between 4,549 (±2) m.y. and 4,563 (±1) m.y (Jagoutz et al., 2003), with a mean age of 4,563.9 (±0.6) m.y. (Zartman et al., 2006). A more precisely determined measurememnt of the Pb–Pb isotopic ages yielded an even older age for D'Orbigny of 4,564.42 (±0.12) m.y. (Amelin, 2007). These ages are consistent with the Pb–Pb age determined for the A-881371 angrite (4,562.4 [±1.6] m.y.). Recently, a more precisely determined initial 238U/235U ratio of 137.822 (۪.028) was brought to light by Brennecka et al. (2010) and substituted for the previously accepted ratio of 137.88; the Pb–Pb anchor age of D'Orbigny is now slightly younger at 4,563.8 (±0.5) m.y. The U-corrected Pb–Pb ages of D'Orbigny and Sahara 99555 are identical within uncertainties, calculated to be 4,563.37 (±0.25) m.y. and 4,563.53 (±0.14) m.y., respectively (Tang and Dauphas, 2014, and references therein).

In addition, Glavin et al. (2004) calculated an absolute Mn–Cr isotopic age for D'Orbigny of 4,562.9 (±0.6) m.y., which is concordant with the Al–Mg age calculated by Spivak–Birndorf et al (2005, 2009) for both D'Orbigny and Sahara 99555, and with the calculated Mn–Cr ages and Hf–W ages determined for both angrites (Nyquist et al., 2003; Spivak–Birndorf et al, 2009). These extinct radionuclides provide ages that are slightly younger than the measured Pb–Pb ages.

The ~4,564 m.y. Pb–Pb age for D'Orbigny is ~7 m.y. older than some other angrites such as AdoR, LEW 86010 and NWA 2999 (~4,557.8, ~4,558, and ~4,557.9 m.y., respectively), and attests to very early accretion, igneous activity, differentiation, partial melting, and production of basaltic magma on the planetesimal. Attainment of isotopic equilibrium and crystallization will have occurred very soon thereafter (Shukolyukov and Lugmair, 2007). The decay products of extinct radionuclides such as 53Mn, 146Sm, 244Pu, and 182Hf suggest that the entire sequence from nebular condensation through parent body accretion, partial melting, siderophile–lithophile element fractionation, multiple metasomatic alteration events, and final cooling to temperatures low enough to retain fission tracks and noble gases was on the order of only a few m.y.

Trace and major element compositions and textures of D'Orbigny and Sah 99555 are almost identical (Nyquist et al., 2003; Floss et al., 2003), suggesting a possible genetic relationship (i.e., same parent body). In addition, Asuka 881371 and LEW 87051 have trace element trends similar to D'Orbigny and Sah 99555, suggesting that they may all share a common origin, or at least experienced similar petrologic histories. Trace element trends for LEW 86010 and AdoR are significantly different from each other and from all other angrites, and they represent distinct lithological sources.

To complicate matters, the results of CRE age studies (Eugster et al., 2002) based on cosmogenic nuclide data infer a CRE age for D'Orbigny (12.3 ±0.9 m.y.) which is significantly different from all other angrites studied: Sah 99555 (6.6 ±0.8 m.y.), Asuka 881371 (5.4 ±0.7 m.y.), Angra dos Reis (55.5 ±1.2 m.y.), LEW 86010 (17.6 ±1.0 m.y.), and LEW 87051 (~0.2 m.y.). All or most of these angrites represent unique ejection events on the angrite parent body, suggesting that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 55 m.y.

Spectral data from studies of these new angrites, especially D'Orbigny, have yielded two possible spectral analogs among main-belt asteroids: the A-type 289 Nenetta and the Sr-type 3819 Robinson. Both asteroids exhibit the strong spectral reddening characteristic of the Al,Ti–diopside-hedenbergite component of angrites. However, important differences exist—the spectra of 289 Nenetta and 3819 Robinson contain distinct olivine bands which are absent in that of D'Orbigny, and the spectra of 3819 Robinson matches that of D'Orbigny in the visible but not in the near-infrared.

It was inferred by Nyquist and Bogard (2003) that since D'Orbigny was spectroscopically similar to these two asteroids, both located between ~2.8 and 2.9 AU, 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 radiogenic 26Al to cause global melting and differentiation before attaining a diameter greater than ~200 km; i.e., a body larger than ~200 km in diameter would not have produced enough radiogenic heat to melt and differentiate an object of this size. By this line of reasoning, it may be concluded that the differentiated angrite PB 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 that 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 in the Solar System, 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) reasoned 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 accretion to a diameter of 40–160 km within 1.5 m.y. after CAI formation was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums. Sanders and Scott (2011) later revised that to suggest radiogenic melting proceeded in bodies >20 km in diameter when accreted within 1.5 m.y. after CAI formation, while bodies accreting later than 1.5 m.y. after CAIs were heated but not melted. Furthermore, they found that bodies which accreted later than 2.2 m.y. would not have melted at all. Be that as it may, it can still be represented that at large heliocentric distances (>~2.8 AU), accretion would proceed too slowly for sufficient 26Al to accumulate and initiate global melting prior to a body growing too large (~200 km diameter) for melting to be possible (Nyquist and Bogard, 2003).

The spectrum of asteroid 3628 Boznemcová has also been studied and compared to those of the angrite meteorites (Cloutis et al., 2006). Boznemcová is thought to have experienced partial melting and fractional crystallization under oxidizing conditions, and is considered to have a surface composition akin to an angritic crust (i.e., a composition of ~55–75 wt% clinopyroxene, ~20–33 wt% plagioclase feldspar, and 0–20 wt% olivine plus kirschteinite). It is a spectral type A asteroid containing an (Fe³+)-free clinopyroxene phase known only from angrites. However, despite its similarities in reflectance spectra, and thus mineralogy, to that of angrite meteorites, the latter typically contain more olivine than is observed on Boznemcová. On the other hand, studies of the orbits of the LL6 ordinary chondrites Bensour and Kilabo (Alexeev et al., 2009) suggest that these meteorites cross the orbit of 3628 Boznemcova, which is located in the inner asteroid belt (~2.2 AU). This location is associated with two efficient resonances responsible for transferring material into Earth-crossing trajectories.

The relatively small number of meteorites classified as angrites constitute a very limited sampling of the angrite parent body. Another recent find from Antarctica, Y-1154, is an anomalous meteorite containing Al,Ti–diopside-hedenbergite that is compositionally similar to angrites, but it has a unique fine-grained, dendritic texture. An excellent petrographic thin section micrograph of D'Orbigny can be seen on John Kashuba's webpage. The specimen of D'Orbigny pictured above is a 1.6 g partial slice.