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NWA 4801

Angrite
Plutonic/Metamorphic
standby for nwa 4801 photo
Found May 2007
coordinates not recorded Four fragments comprising a single 252 g individual stone meteorite was found in Algeria and purchased by G. Hupé in Erfoud, Morocco. This is a friable meteorite in which the fusion crust has been eroded away by prolonged terrestrial weathering processes, leaving only oxidation products on its surface. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and it was ascertained that NWA 4801 is a plutonic igneous cumulate angrite.

This angrite has a metamorphosed texture being that it is a coarse-grained rock (0.1–1.2 mm) with 180° triple junctions (Irving and Kuehner, 2007). It is composed of a variety of multi-colored grains, primarily Al–Ti-rich clinopyroxene, and contains a high abundance of pure anorthite in the form of white crystals and aggregates. Other grains are composed of Cr-pleonaste, Ca-rich olivine, pleonaste, and merrillite. Minor troilite is present in association with FeNi-metal and small oxide grains (Riches et al., 2016). While kirschsteinite is present in most angrites, it has not been observed in this one. Notably, NWA 4801 has a greater abundance of merrillite than in most other angrites.

The crystallization age of NWA 4801 based on Pb isotopes is barely resolvable from that of the youngest angrite, Angra dos Reis (Amelin and Irving, 2007). This young age is also very close to that of the plutonic angrites LEW 86010 and NWA 4590—NWA 4801 is ~1.2 m.y. younger than LEW 86010. The crystallization/isotope closure age for NWA 4801 based on Mn–Cr systematics is 4.5643 (±0.0005) b.y. When anchored to the absolute Pb–Pb chronometer, which has now been accurately determined for NWA 4801 to be 4.558 (±0.013) b.y., these ages provide the best agreement between these two chronometers yet obtained for angrites (Shukolyukov et al., 2009). The Sm–Nd-based age is concordant with the Pb–Pb-based age, and is identical within error to the angrite NWA 4590 (Sanborn et al., 2011). A Lu–Hf isochron for NWA 4801 was determined by Bouvier et al. (2015) to be 4.563 (±0.05) b.y. The time of the last mantle fractionation as determined by Mn–Cr and tied to the new NWA 4801 Pb–Pb anchor is consistent with the crystallization age of the oldest known angrites at 4.5646 (±0.0005) b.y.

With the steadily increasing number of unique angrite samples available for study, new models of their formation are now emerging. In an abstract from the Workshop on Chronology of Meteorites 2007, A. Irving and S. Kuehner (UWS) conceive of a rapid progression of events on the angrite parent body following its accretion within ~2 m.y. after CAI formation. Immediately thereafter, the onset of internal heating by 26Al decay, along with significant impact heating (John T. Wasson, 2016), resulted in differentiation of the mantle and formation of a small core (core mass fraction of 0.08; Shirai et al., 2009). Subsequent to core formation, plutonic and volcanic magmatism, metasomatism, metamorphism, and impact-generated regolith formation occurred within ~4–11 m.y. after CAIs.

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 metal–silicate 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 0.12–0.29 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 ΔIW–1.5 (±0.45).

In-depth studies of the diverse angrite samples collected thus far are bringing to light a scenario in which a large planetary body accreted and crystallized over an extended period of time, perhaps as long as 7 m.y., beginning only a couple of m.y. after the formation of the earliest nebular condensates. The refractory bulk composition of this body, along with features such as a high abundance of trapped solar noble gases, attest to an origin in close proximity to the Sun. The oldest angritic material is recognized in the form of early crustal vesicular rocks represented by such meteorites as Sahara 99555, D’Orbigny, and NWA 1296. Younger angritic material, occurring in the form of impact-mixed extrusive and intrusive magmatic rocks combined with regolith material, is represented by A-881371, LEW 87051, and NWA 1670. The youngest angritic rocks known, represented by the meteorites Angra dos Reis, LEW 86010, NWA 2999, NWA 4590, and NWA 4801, are composed of annealed regolith and late intrusive plutonic lithologies.

It was proposed by Irving and Kuehner (2007) that one or more severe collisional impacts onto the angrite parent body resulted in the stripping of a significant fraction of its crust and upper mantle, with the dissemination of large sections of this material into a stable orbit that has been maintained for the past 4+ b.y. The source of the delivery of angrite material to Earth might lie within the main asteroid belt, or it could remain associated with the original collisionally-stripped parent body postulated by some to be the planet Mercury (see schematic diagram below). The disparity in FeO content that exists between the angrite group of meteorites (up to 25 wt%) and that which is observed on the surface of Mercury (~5 wt%) may reflect the existence of a redox gradient in which the lower mantle region, now the present surface of Mercury, has a more magnesian composition. standby for angrite schematic diagram
click on image for a magnified view

Diagram credit: A. Irving and S. Kuehner, Workshop on Chronology of Meteorites, #4050 (2007) While angrites could possibly be original fragments from ‘Maia’, mother of Hermes (Mercury), they may instead derive from ‘Theia’, mother of Selene (the Moon goddess). In a new study of the Fe/Mn ratio in olivine grains for a number of angrites, Papike et al. (2017) determined that these meteorites plot along a trend line between the Earth and Moon, which indicates a possible location for the angrite parent body (see diagram below). standby for angrite fe/mn diagram
Diagram credit: Papike et al., 48th LPSC, #2688 (2017) In connection with their in-depth study of NWA 5363/5400, Burkhardt et al. (2017) published comparative data for nucleosynthetic anomalies among parent bodies for O, Cr, Ca, Ti, Ni, Mo, Ru and Nd. It is interesting to note that with the exception of ε48Ca (no angrite data is available for ε100Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies that are nearly the same or overlap within uncertainties. Results of their studies indicate that while both angrites and NWA 5363/5400 have Δ17O values indistinguishable from Earth, and that other anomaly values for angrites overlap with Earth within uncertainties (ε92Ni, ε92Mo, ε145Nd), the ε54Cr and ε50Ti values for angrites are distinct from Earth. Based on their studies, Burkhardt et al. (2017) concluded that the parent body of NWA 5363/5400, and perhaps by extention that of angrites, originated in a unique nebular isotopic reservoir most similar to that of enstatite and ordinary chondrites.

The CRE age calculated for NWA 4801 is 31.6 (±1.5) m.y. (Nakashima et al., 2008). A more precise noble gas analysis conducted by Nakashima et al. (2018) established a CRE age for NWA 4801 of 26.4 (±6.1) m.y. Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below). Cosmic-ray Exposure Ages of Angrites
standby for angrite cre age diagram
Diagram credit: Nakashima et al., MAPS, Early View, p. 14 (2018)
‘Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039)
Although NWA 4801 has been remagnetized by hand magnets, a study by Weiss et al. (2008) of remanent magnetism in angrites revealed that a magnetic field with a strength of ~10 µT, ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization (as observed particularly from the angrite D’Orbigny). This magnetic field could be attributed to a number of possible causes such as accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more likely, to a magnetic field generated by an internal core-dynamo mechanism.

Small fine-grained basalt clasts exhibiting textures and mineralogy generally consistent with a quenched angrite-like impactor are preserved in impact melt glass fragments recovered from a significant impact event that occurred ~5.28 m.y. ago near Bahía Blanca, Argentina (Schultz et al., 2006; Harris and Schultz, 2009, 2017; see photo below). This impactor is considered to have been very large, perhaps at least one km³, and its source object could plausibly reside near the Earth–Moon system. Interestingly, analyses of other grains obtained from Bahía Blanca impact melt glass have a geochemistry similar to the Moon (Harris and Schultz, 2017). standby for bahia blanca angritic fragments photo
Photo credit (left): Schultz et al., MAPS, vol. 41, #5, p. 755 (2006) (http://dx.doi.org/10.1111/j.1945-5100.2006.tb00990.x)
Diagram credit (right): Harris and Schultz, 40th LPSC, #2453 (2009)
The number of unique angrites identified today is quite limited, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo courtesy of Habib Naji). In a recent study 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. The unique metamorphic NWA 2999 pairing group was not included in the Bouvier et al. (2015) study. The specimen of NWA 4801 shown above is a 0.98 g partial slice. The photo below is an excellent petrographic thin section micrograph of NWA 4801, shown courtesy of Peter Marmet. standby for nwa 4801 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet


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NWA 2999

Angrite
Plutonic/Metamorphic
standby for nwa 2999 photo
Purchased August 2004
no coordinates recorded Twelve individual fragments constituting a single meteorite, with a combined total weight of 392 g, were visually distinguished by Greg Hupé from an assortment of meteorites he had purchased in Morocco. Samples from different stones were sent for analysis to Northern Arizona University (T. Bunch and J. Wittke) and the University of Washington in Seattle (A. Irving and S. Kuehner). A preliminary analysis found similarities to known angrites, and a sample was sent to the Carnegie Institute, Washington D.C. (D. Rumble III) for O-isotopic analysis. By this method it was verified that these meteorites were in fact a new sampling of the angrite parent body. Because of the importance and uniqueness of this find, a sample from each of the twelve fragments was submitted for analysis. Numerous other pairings have been independently analyzed and given separate NWA series numbers, with the total combined weight of this pairing group being ~7.8 kg.

Only a small number of unique angrites are currently represented in our collections, which some investigators have resolved into four subgroups: basaltic/quenched, sub-volcanic/metamorphic, plutonic/metamorphic, and dunitic. In a study based on a comparison of Hf/Sm ratios for a diverse sampling of both angrites and eucrites, Bouvier et al. (2015) inferred that these meteorite subgroups 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 sub-volcanic and plutonic angrites (NWA 4590 and NWA 4801, respectively) and the unusual cumulate eucrite Binda. The metamorphic NWA 2999 pairing group was not included in the Bouvier et al. (2015) study.

In contrast to other angrites, NWA 2999 exhibits a polygonal-granular texture consistent with a relatively slowly-cooled and annealed lithology, more similar to the sub-volcanic and plutonic angrites than to the quenched angrites. Evidence in support of a plutonic origin for NWA 2999 can be found in the homogenous pyroxene compositions compared to the wider compositional range that exists in some other angrites (Kuehner et al., 2006). However, evidence also exists for an extended residence within a regolith—large (up to 6 mm) anorthite, spinel, and olivine rock fragments are present within the fine-grained groundmass. Moreover, while other angrites contain only minor FeNi-metal (<2 vol%), the NWA 2999 pairing group contains up to 9 vol% (NWA 3164 pairing) coarse FeS and FeNi-metal having chondritic abundance patterns (Baghdadi et al., 2015). The FeNi-metal in NWA 2999 has elemental ratios that are inconsistent with what would be expected from incomplete core separation (Jambon et al., 2012), and neither could this high abundance of metal have been derived through partial reduction of iron. Instead, it is considered more plausible that the FeNi-metal was incorporated from an exogenous source during an impact event on the angrite parent body. The impactor was most likely a metallic object unrelated to any known chondritic or iron chemical group (Humayun et al., 2007; Jambon et al., 2012).

Consistent with this finding, an increased level of other siderophile elements such as Co, Ir, and Au support the presence of a significant meteoritic component. However, it is unknown if this exogenous FeNi-metal source can also explain the increased Mg content and the reduced concentration of refractory elements (e.g., Ca, Al, and Ti) observed in this angrite. Since a chondritic impactor would also have necessarily carried an O-isotopic composition close to that of the TFL, an alternate scenario was proposed by Gellissen et al. (2007) and then by Irving and Kuehner (2007) to account for the observed anomalous elemental abundances. They suggest that a large impact onto the angrite parent body occurred, perhaps by an evolved iron object, which created a mixture of diverse lithologies from within the angrite target body. These diverse lithologies which constitute NWA 2999 were then deeply buried (~120 cm based on depth profiles of 22Ne/21Ne ratios; Nakashima et al., 2018) where they underwent thermal metamorphism and annealing to produce the observed granulitic texture. The chemical composition of the NWA 2999 pairing group shows that it derives from a picritic source magma, which thereafter experienced further fractional melting, metamorphism, and annealing, along with incorporation of an exogenous metal component (Baghdadi et al., 2015).

Northwest Africa 2999 preserves some unique metamorphic features (previously observed in some terrestrial metamorphic rocks) which initially were thought to reflect a decompression stage followed by rapid cooling. Investigators presumed that these events were initiated during an extensive multi-km-deep thrust faulting event on a large parent body, postulated by some to be Mercury (Irving et al., 2005). These metamorphic features include the presence of clinopyroxene–spinel symplectites between plagioclase and olivine clasts (reflecting a decompression phase), and plagioclase coronas surrounding portions of spinel grains (reflecting a rapid cooling phase).

An alternate explanation for these unique metamorphic features has been proposed by Ruzicka and Hutson (2006), who argue that under low-pressure oxidizing conditions at various degrees of melt formation, both plagioclase coronas and clinopyroxene–spinel symplectites can be produced as cooling proceeds. Improved models of these symplectite and corona textures by Irving and Kuehner (2007) led them to conclude these features are more likely the result of the percolation of a S-bearing fluid during a metasomatic phase. These unique corona microstructures have been further interpreted by Baghdadi et al. (2012, 2013), who reason that a granulitic peridotitic lithology, which originated either as a slowly-cooled pluton or possibly as an annealed brecciated rock at depth (P <0.9 GPa), was intruded by a hot magma that increased the temperature to 1000–1200°C. This thermal event resulted in the formation of solid state metamorphic coronas at mineral contacts (‘contact metamorphism’) over an extended time interval through the following reaction: clinopyroxene + spinel ⇒ olivine + anorthite (with the reverse reaction occurring upon re-cooling). Eventual ejection of this angrite from its likely planetary-sized parent body produced the fracturing observed within the coronas and throughout this meteorite.

Other features consistent with a very rapid melting and cooling event on the angrite PB have been identified in the angrite NWA 4590. Glass present along mineral grain boundaries attests to a late mobilization of primary phases consistent with a decompression event (Kuehner and Irving, 2007). It has been postulated that the angrite meteorites might represent the impact-related dissemination of a more FeO-rich outer layer during the early history of Mercury, thereby explaining the chemical and mineralogical differences observed on Mercury compared to the angrites; e.g., the higher FeO-abundance of angrites compared to that on the present surface of Mercury, and the reversed Fe/Mn values for both olivine and pyroxene as compared to those of other planetary bodies. Nevertheless, even accepting the occurrence of collisional-stripping of a hypothetical FeO-rich basaltic (angritic) crust on Mercury, Hutson et al. (2007) find it implausible that Mercury initially differentiated under oxidizing conditions to form the angritic crust, and then subsequently differentiated under reducing conditions to form the surface that we observe today. They have also argued that other mineralogical features identified in angrites (e.g., reaction coronas), which on one hand may be attributed to rapid decompresion on a planetary-sized body such as Mercury, may just as well be consistent with the typical cooling processes that occurred during crystallization of a melt.

In contrast to some other angrites, neither kirschsteinite nor orthopyroxene has been found in NWA 2999, and vesicles are absent. Based on Hf–W systematics, NWA 2999 formed ~5 m.y. later than Sahara 99555 and D’Orbigny (Markowski et al., 2007). However, Jambon et al. (2012, #1758) contend that due to the exogenous FeNi-metal present in this meteorite, the Hf–W chronometer is not reliable. It was concluded by Kleine et al. (2009) that both NWA 2999 and AdoR were derived from a parental source magma that had higher Hf–W than other angrites, likely the result of extended differentiation after core formation. A precise crystalization age based on the Mn–Cr system indicates an age for NWA 2999 of 4.5579 (±0.0011) b.y., indistinguishable from that of AdoR and LEW 86010. As deduced by Shukolyukov and Lugmair (2008), two age clusters encompass all of the angrites studied thus far, and this attests to a very early period of magmatic activity.

A CRE age of 73.4 (±6.6) m.y. was calculated for NWA 2999 by Nakashima et al. (2008), while an age of 69.6 (±11.2) m.y. was calculated for the paired NWA 4931. A more precise noble gas analysis conducted by Nakashima et al. (2018) established a CRE age for NWA 2999 and NWA 4931 of 47.2 (±6.1) m.y. and 51.7 (±6.4) m.y., respectively. Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below). Cosmic-ray Exposure Ages of Angrites
standby for angrite cre age diagram
Diagram credit: Nakashima et al., MAPS, vol. 53, #5, p. 965 (2018)
Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039)
Taken together, all of the anomalous characteristics observed for the NWA 2999 pairing group could be attributed to contamination through exotic impact event(s), or as speculated by some, it could be that the NWA 2999 pairing group might even represent a unique parent body with similar O-isotopic values to those measured for the angrite PB. The specimen of NWA 2999 shown above is a 0.228 g partial slice. The photo below is an excellent petrographic thin section micrograph of the pairing NWA 6291, shown courtesy of Peter Marmet. standby for nwa 6291 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet


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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. 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, and plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo courtesy of Habib Naji).

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, it 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; by comparison, 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 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 basaltic meteorite, and extensive isotopic studies have established that it is an early planetary differentiate undisturbed since its crystallization ~4.5566 b.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 protoplanetary 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 (20–60 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 unshocked 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 ~1.2–3 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. standby for angrite dynamo timeline diagram
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 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).

The similarity in Δ17O values between angrites and the ungrouped iron Tishomingo (based on anaysis of a stishovite grain) suggests that a genetic relationship might exist (Corrigan et al., 2005, 2017 [see diagram below]). Furthermore, both angrites and Tishomingo formed from a volatile-depleted precursor under oxidizing conditions. Investigators have also explored the possibility of a genetic relationship between angrites and IVB irons (e.g., Campbell and Humayun, 2005), as well as between Tishomingo and IVB irons (e.g., Corrigan et al., 2005). Based on O-isotopic analyses utilizing chromite grains from IVB irons Warburton Range and Hoba, Corrigan et al. (2017) concluded that IVB irons are not genetically related to either angrites or to Tishomingo, but that their respective parent bodies experienced similar petrogenetic histories. Beyond that, Burkhardt et al. (2011) determined that differences in both O- and Mo-isotopic compositions between angrites and IVB irons exclude a genetic linkage. standby for o-isotopic relationship between groups diagram
Diagram credit: Corrigan et al., 48h LPSC, #2556 (2017) 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 ~2.8–2.9 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.

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.

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 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg 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 metal–silicate 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 0.12–0.29 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 ΔIW–1.5 (±0.45).

Precise U–Pb ages have been calculated for AdoR and LEW 86010 to be 4.55765 (±0.00013) b.y. and 4.55855 (±0.00015) b.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 4.5579 (±0.0011) b.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 angrites—<0.2–56 m.y. for thirteen angrites measured to date, possibly representing as many ejection events (Nakashima et al., 2008; Wieler et al., 2016; Nakashima et al., 2018). This range is consistent with a single large parent body enduring multiple impacts over a very long period of time, which would suggest that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 56 m.y. Alternatively, Nakashima et al. (2018) consider it plausible that there is currently at least two angrite (daughter) objects occupying distinct orbits: one representing the fine-grained (quenched) angrites with the shorter CRE age range of <0.2–22 m.y., and another representing the coarse-grained (plutonic) angrites with the longer CRE age range of 18–56 m.y. (see diagram below). Cosmic-ray Exposure Ages of Angrites
standby for angrite cre age diagram
Diagram credit: Nakashima et al., MAPS, Early View, p. 14 (2018)
‘Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures’
(http://dx.doi.org/10.1111/maps.13039)
Notably, Rivkin et al. (2007) have determined that the largest known co-orbiting ‘Trojan’ asteroid of Mars, the 1.3 km-diameter 5261 Eureka located at a trailing Lagrangian point, is a potentially good spectral analog to the angrites (as measured by Burbine et al., 2006) (see diagrams below). They suggest that 5261 Eureka could represent a captured fragment of the disrupted angrite parent body now in a stable orbit around Mars. standby for eureka–angrite diagramstandby for eureka–angrite diagram
Diagrams credit: Rivkin et al., Icarus, vol. 192, #2, (2007)
‘Composition of the L5 Mars Trojans: Neighbors, not siblings’
(https://doi.org/10.1016/j.icarus.2007.06.026; open access link)
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 magnified 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 photo
click on photo for a magnified view Tragedy struck the 200-year-old National Museum of Brazil in Rio de Janeiro during the evening of September 2, 2018, when a fire quickly spread through the structure. According to curator María Elizabeth Zucolotto, 30 out of 33 meteorites that were on display have been recovered, and the 70 g main mass of Angra dos Reis was eventually recovered from the office safe. The angrite meteorite is reported to have an estimated value of approximately $750,000. standby for angra dos reis main mass photo
standby for national museum of brazil fire photo
standby for national museum of brazil fire photo
Photos courtesy of National Museum of Brazil