Achondrite, ungrouped

nwa 5400
click on photo for a magnified view

Found 2008
no coordinates recorded

A single stone partially covered with remnant fusion crust and weighing 4,818 g was found in Northwest Africa. Upon close examination, meteorite dealer Greg Hupé noticed that the meteorite had an unusual appearance with similarities to primitive achondrites, and he purchased the meteorite in June 2008 in Er Rachidia, Morocco. A portion of the meteorite was sent to the University of Washington in Seattle (A. Irving and S. Kuehner) for analysis and classification, and NWA 5400 was determined to be a unique ungrouped achondrite. Thirty-one possibly paired stones were found by Mbarek Ait el Caid in February 2008 (NWA 5363, 2,455 g). Samples of these stones and a sample of NWA 5400 were studied by Albert Jambon (Université Pierre et Marie Curie, Paris), including O-analyses, and they were declared to constitute a pairing group. Two other stones, NWA 6077 (1,010 g) and NWA 6292 (725 g), were found in 2008 and 2010, respectively; these have also been demonstrated by O-isotopic analysis (Carnegie Institution in Washington D.C.) to be paired with NWA 5400.

The four recognized members of the NWA 5400 pairing group are plotted on two different oxygen three-isotope diagrams: (1) (courtesy of Bernd Pauli) and (2) (courtesy of Achim Raphael). The first diagram utilizes a less precise slope for the TFL (Terrestrial Fractionation Line) of ~0.5. Recent O-isotopic studies conducted by the Carnegie Institute, the Open University, and the University of Groningen have determined that a slope of 0.526 represents a more precise TFL slope (Rumble et al., 2007). When constructed with updated coordinates (A. Irving), the adjusted TFL slope passes very close to the points determined for the NWA 5400 pairing group. The slope also passes through the CI chondrite field and probably the martian field as well. The second diagram compares the NWA 5400 pairing group to the brachinites and brachinite-like meteorites Divnoe, Zag (b), and NWA 4042. It is evident that the NWA 5400 pairing group is not genetically related to either the brachinites or the brachinite-like meteorites, but probably formed by analogous processes on a separate parent body. See also a linearized O-isotope plot (3) showing a slope similar to the TFL, based on data from Miller (2002).

In-depth analyses have revealed that NWA 5400 has petrographic, mineralogical, and isotopic similarities to brachinites, including a fine- to coarse-grained protogranular texture (0.1–1.5 mm) and a high olivine content (79–80.5 vol%). Pyroxene constitutes 16.4–19.4 vol% (approx. 1:1 opx to cpx), and it also contains minor chromite, chlorapatite, troilite, FeNi-metal (altered), taenite, and rare plagioclase. The meteorite is considered to represent a restite from a chondritic parent body that experienced a low degree of partial melting and silicate melt extraction evidenced by the lack of plagioclase and lithophile elements (Burkhardt et al., 2015). As shown above, the O-isotopic composition of NWA 5400 (D. Rumble, Carnegie Institution in Washington) has values that plot on the TFL, which up until now was only true for E chondrites, E achondrites, CI chondrites, and the Earth–Moon system.

Although minor terrestrial weathering has produced iron hydroxides and calcite veining, the presence of minor amounts of free metal are indicative of little to no differentiation having occurred on the parent asteroid at the time this rock was formed. The difference in Fe/Mn ratios in olivine and pyroxene in NWA 5400 compared to those in lunar and Earth samples, along with the presence of FeNi-metal in NWA 5400, eliminates both objects as the possible source parent body. That said, it was initially proposed that this unique achondrite might possibly represent a primitive terrene meteorite ejected prior to the collision which spalled the Moon. Such a petrogenetic history is based on the higher abundances of highly siderophile elements and the Os isotope composition of NWA 5400 (Shukolyukov et al., 2010). In such a scenario, NWA 5400 could have been stored in the asteroid belt until recent impacts ejected material into an Earth-crossing orbit.

Further speculations about NWA 5400 consider an origin on 'Theia' itself prior to its collision with Earth 4.52 b.y. ago, consistent with the O-isotopic composition of the meteorite being indistinguishable from that of the Earth and Moon. An asteroid-sized fragment could have experienced similar storage mechanisms in the asteroid belt as described above. However, this theory is inconsistent with the 'Icy Impactor Model' proposed by Wolbeck and Connolly (2010). They envision the impacting planetessimal 'Theia' as originating near the frost line close to Jupiter, and therefore it's composition would have been predominantly ice. The impact with proto-Earth is thought to have introduced vast quantities of steam/water which helped to stabilize the orbit of the proto-Moon and create the oceans on proto-Earth. Another possible scenario for the origin of NWA 5400 was propounded by Alan Rubin (UCLA), in which NWA 5400 could represent a unique, undifferentiated or partially differentiated E-type asteroid—as distinguished from the Shallowater, aubrite, EH, and EL parent bodies—and which had a chondritic bulk composition. He considered that it may have experienced extensive metasomatic oxidation processes similar to the E chondrite Galim. However, this model has been refuted on the basis of the high olivine content in NWA 5400 and on the inconsistencies in its age.

An alternative model invoking moderate degrees of partial melting (13–30%) on a volatile-rich, oxidized, chondritic asteroid was proposed by Day et al. (2011). Based on results of petrological, geochemical, and isotopic analyses, they suggest that a genetic link (common parent body) may exist between brachinite-like GRA 06128/9 and other brachinites. Newly acquired highly-siderophile element (HSE) data reflect variable extraction of metals and sulfides producing residues having a range of compositions. In light of this, they hypothesize that one complementary metal-rich residue possibly derived from the brachinite parent body might be NWA 5400. However, through studies of HSE abundances, and upon examination of potential metal-sulfide segregation processes, it was determined by Day et al. (2012) that NWA 5400 and similar brachinite-like achondrites are not likely genetically related to brachinites, but rather originated on similar volatile-rich, oxidized, chondritic precursor asteroids which experienced similar petrologic processes. The measured HSE abundances for NWA 5400 are consistent with a partially melted parent body in which heating from short-lived radionuclides (26Al) came to a halt before significant core formation could occur. The precursor material from which the brachinites (and possibly NWA 5400) derive has been determined by Gardner-Vandy et al. (2013) to be R chondrite-like (see the NWA 3151 page).

Burkhardt et al. (2015) determined the potential timing of metal–silicate segregation based on the Hf–W chronometer to be 1.2 (±0.5) m.y. after CAI formation. Burkhardt et al. (2017) determined a revised two-stage model Hf–W age of 2.2 (±0.8) m.y. after CAIs, which they consider likely reflects the approximate timing of silicate melt extraction with the addition of W occurring shortly thereafter. Isotopic studies for NWA 5400 also indicate NWA 5400 formed very early in Solar System history, as attested by its chondritic Xe similar to that of brachinites. An I–Xe analysis conducted for an olivine/apatite sample yielded a very early absolute age of 4.5689 (±0.0006) b.y. based on the Shallowater standard, coeval with CAI formation (Pravdivtseva, et al., 2015). Amelin and Irving (2011) calculated a Pb–Pb based isochron of 4.478 (±0.055) b.y., but that very young age was attributed to a terrestrial Pb contribution due to weathering. The U–Pb systematics indicate a more primitive Pb-isotopic composition than that determined for the Earth, and therefore rules out a terrestrial origin for NWA 5400. Crystallization age studies for NWA 5400 based on Mn–Cr systematics were conducted by both Shukolyukov et al. (2010) and Sanborn et al. (2016), and they determined an age of ≤4.541 b.y. and ≤4.552 b.y., respectively. This crystallization age is somewhat younger than that determined for Brachina, but older than the proposed collision of 'Theia' and proto-Earth. An Ar–Ar dating study was conducted for the paired meteorite NWA 6077 by Beard et al. (2016), which shows complete resetting of this chronometer ~600 m.y. ago with another possible event ~200 m.y. ago.

The initial measurement of 54Cr abundances in NWA 5400 was found to be indistinguishable from that of both terrestrial samples and E chondrites; this lack of a Cr isotope anomaly indicates an old crystallization age, but reveals that NWA 5400 is significantly younger than Brachina (A. Irving). New Cr-isotopic analyses conducted by Sanborn et al. (2016) for NWA 5400 and paired stone NWA 6077 provided different results; the δ54Cr values are actually well resolved from terrestrial and E chondrite values (see Δ17O vs. δ54Cr coupled diagram below [top]). They also demonstrated that NWA 5400 is well resolved from other meteorite groups which are located in relatively close proximity in isotopic space, such as the anomalous eucrites and brachinites (see Δ17O vs. δ54Cr coupled diagram below [bottom]). The initial consideration of a potential genetic relationship between NWA 5400 and the brachinite and brachinite-like meteorites is refuted by this comparitive technique. However, utilizing this diagram does reveal a close similarity in δ54Cr values among NWA 5400, the brachinite and brachinite-like meteorites, and the anomalous eucrites, which suggests their parent bodies formed in a similar nebular reservoir.

standby for o–cr diagram
Diagram credit: Sanborn et al., 47th LPSC, #2309 (2016)
standby for o–cr diagram
Diagram credit: Sanborn et al., 47th LPSC, #2309 (2016)

Burkhardt et al. (2015, 2017) have conducted in-depth petrographic, geochemical, and isotopic (O, Cr, Ca, Ti, Ni, Mo, Ru and Nd) analyses for the paired meteorites NWA 5363 and 5400. Results of their studies indicate that while NWA 5363/5400 has Δ17O values indistinguishable from Earth, other isotopic anomalies (ε54Cr, ε48Ca, ε50Ti, ε92Ni, ε92Mo, ε100Ru, ε145Nd) show significant differences from the Earth. The isotopic anomalies together with the chronological and petrographic data demonstrate that this meteorite does not represent precursor material of Earth, early impact ejecta from Earth, or remnants from 'Theia'. Instead, Burkhardt et al. (2017) concluded that its parent body was a relatively small asteroid that originated in a unique nebula isotopic reservoir most similar to enstatite and ordinary chondrites, and experienced a petrogenetic history similar to that of brachinites. It is interesting to note that, with the exception of ε48Ca (no angrite data for ε100Ru), NWA 5363/5400 and angrites have values for each of these isotopic anomalies which are nearly the same or overlap within uncertainties. It was proposed by Amelin and Irving (2011) that NWA 5400 originated from a parent body associated with one of the iron meteorite groups containing primitive Pb.

The CRE age based on 3He and 21Ne was determined to be ~29 m.y. The specimen of NWA 5400 shown above is a 5.3 g partial slice. Shown below is a composite image of both sides of the complete mass as found.

Photos courtesy of Aziz Habibi