NORTHWEST AFRICA 1460

Martian Shergottite
low-Al basalt ∗
(intermediate, mafic, diabase)

standby for northwest africa 1460 photo standby for northwest africa 1460 photo
Purchased June, 2002
no coordinates recorded

A single, very fresh, individual stone meteorite covered by shiny black fusion crust was found by a nomad in the Moroccan Sahara. This meteorite weighed 70.2 g and had dimensions of 47 × 34 × 27 mm. It was subsequently acquired by a Moroccan meteorite dealer who provided a sample of the stone to the University of Washington (A. Irving and S. Kuehner) for petrographic analysis. It was determined that NWA 1460 is a martian mafic, basaltic, or more properly, diabasic shergottite that is texturally and compositionally identical to the 28 g shergottite NWA 480 (Barrat et al., 2002) previously found by the same nomad; the two stones are considered to be paired. Sometime thereafter, the mass of NWA 1460, carrying the name "Black Beauty", was purchased by N. Oakes.

This is a coarse-grained shergottite composed primarily of large (typically 2 mm, but up to 5 mm), highly zoned, light yellow-green, low-Ca pyroxene crystals along with maskelynite glass. Minor constituents include the phosphate minerals merrillite and chlorapatite, the opaque minerals titanomagnetite, ilmenite, chromite, and pyrrhotite, and it is completely lacking in olivine. Symplectitic intergrowths consisting of varying amounts of fayalite+hedenbergite+silica occur along some grain boundaries. These are thought to represent breakdown products of silicates. Baddeleyite (zirconium dioxide) has been identified associated with the symplectites. Pyroxenes are compositionally similar to those in QUE 94201 and Los Angeles, which suggests that they all experienced similar crystallization histories. However, it is noteworthy that the NWA 1460 pyroxenes have cores with higher magnesium contents than the others. The texture of these shergottites is consistent with a slow, single-stage cooling process consistent with fractional crystallization.

Although major and trace element abundances in NWA 1460 correspond to those of other basaltic shergottites such as Zagami and Shergotty, it has a REE abundance pattern that is more similar to the martian poikilitic (formerly "lherzolitic") shergottites. Redox studies have demonstrated that a positive correlation exists between the degree of oxidation and the LREE abundance in basaltic shergottites (Herd, 2002). However, this correlation is not supported by NWA 480/1460. Although its LREE pattern corresponds to a more oxidized rock like Zagami, the mineralogy of NWA 480/1460 appears to have a redox state that is intermediate to reduced, like that of EETA79001 and QUE 94201, respectively; QUE 94201 represents a depleted mantle component. A widespread mixing of separate source parent magmas could explain these findings (see the Zagami page for current formation scenarios).

Trace element and isotopic studies of NWA 480/1460 indicate that this meteorite might be derived from the same general source region as the poikilitic shergottites, but that its parent magma experienced more extreme fractionation (Bunch et al., 2005). The Rb–Sr and Sm–Nd ratios calculated for the source regions of NWA 480/1460 and the poikilitic shergottites are very similar to each other, and similar to the ratio determined for bulk Mars, but they are very different from these elemental ratios in other SNC members (Nyquist et al, 2009). However, the large discrepency in age that exists between NWA 480/1460 (~346 m.y.) and the poikilitic shergottites (~185 m.y.) likely rules out any parental source relationship. Notably, the unique ol-phyric shergottite NWA 6234/2990/5960/6710 has similar intermediate incompatible trace element values and isotopic compositions to the intermediate basaltic shergottite NWA 480/1460. This raises the possibility that they are genetically related and derive from a common mantle source region (Filiberto et al., 2012).

A cosmogenic-corrected Ar–Ar age of 345 (±22) m.y. was determined for NWA 1460, while a preferred Ar–Ar age of 351 (±15) m.y. was determined by Nyquist et al. (2009). The Sm–Nd and Rb–Sr ages are concordant with this age within error margins at ~346 m.y. (350 ±16 and 336 ±15 m.y., respectively), an age similar to some martian poikilitic shergottites (ALHA77005 and LEW 88516) and basaltic shergottites (QUE 94201 and EETA79001B) (Nyquist et al., 2005, 2006; Nyquist, 2006). The Sr-isotopic sytematics are intermediate to those of both the enriched and depleted shergottites.

The CRE age of NWA 1460 based on 10Be production by local irradiation from the proto-Sun is 2.5–3.1 m.y. (2.6 ±0.2 m.y. determined by Nishiizumi), which is consistent with the 2.4 (±0.2) m.y. CRE age calculated for the paired meteorite NWA 480. As elucidated by Nyquist et al (2009), this CRE age also falls near the lower range of that of several other martian meteorites, including the enriched shergottites Zagami (2.8 [±0.3] m.y.) and Shergotty, the depleted shergottite QUE 94201 (2.7 [±0.2] m.y.), and the primitive basaltic shergottite Y-980459 (2.1–2.8 m.y., adjusted for terrestrial age). Moreover, this CRE age is within statistical uncertainty of the ejection age of the poikilitic shergottites such as ALHA77005, provided there was pre-exposure on Mars for this group. This CRE age could also be consistent with that of EET 79001, a meteorite with an identical crystallization age to that of both the enriched shergottites and the poikilitic shergottites, provided that there was an in-space breakup of the large EET 79001 parent meteoroid. Cosmogenic 21Ne/22Ne ratios are consistent with such an extended period of shielding within a large body. In addition, the CRE age of the enriched shergottite NWA 1068 could also be consistent with this ejection event given its highly variable CRE age measurements, although it may instead belong with the DaG 476 ejection event. These possibly concordant CRE ages suggest that all of these meteorites may have been ejected in a common impact event (Marty et al., 2001).

Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015) compiled a chart based on the reported CRE ages for 53 of them. He concluded that together these 53 meteorites represent 10 distinct impact events which occurred 0.92 m.y., 2.12 m.y., 2.77 m.y., 4.05 m.y., 7.3 m.y., 9.6 m.y., 11.07 m.y., 12.27 m.y., 15 m.y., and 16.73 m.y.—see his chart here. It was argued that NWA 480/1460 was launched from Mars during the 2.12 m.y.-old impact event. In a subsequent review based on multiple criteria, Irving et al. (2017 [#2068]) made a new determination of the number of separate launch events associated with the known (101) martian meteorites. They speculate that the number could be as few as twenty, and suggest that NWA 1460 and the intermediate diabasic shergottite NWA 5029 were ejected in a common impact event unique from the others.

In a study of the Li systematics in NWA 480, Beck et al. (2004) found that Li in the pyroxenes reveals a large isotopic variation from core to rim, but that it also maintains a constant concentration. They suggest these compositional trends for Li reflect the loss of Li (i.e., mass-fractionation) from the crystallizing melt through the degassing of water-rich fluids, a process similar to that proposed to have occurred in the basaltic shergottites Shergotty and Zagami.

A study of the degree of shock experienced by the paired shergottite NWA 480 (a study which included several other shergottites) was undertaken by Aoudjehane et al. (2005). Utilizing cathdoluminescence and Raman spectroscopy, they found that stishovite always coexists with high-pressure silica glass and occurs throughout. They also found that the highly unstable post-stishovite was very likely a common component, although one which could only be indirectly observed given the techniques that were used. In addition, they described for the first time stishovite needles which occur near shock-melt pockets. In light of the presence of all these particular silica phases, they estimated that a shock intensity in excess of 60 GPa affected this shergottite and the others in their study. However, they found that the shock intensities were strongly heterogeneous in all of the shergottites studied.

Although the 14.67 g basaltic diabase NWA 5029 is probably not paired to NWA 480/1460, it shares a CRE age of ~2.5 m.y., and it exhibits some close textural and mineralogical similarities. While it is plausible that these meteorites originated from the same parental source melt on Mars, they crystallized under different conditions—NWA 480 experienced uniformly rapid cooling and crystallization, while NWA 5029 experienced initial slow cooling followed by more rapid crystallization during later stages (Mikouchi and Barrat, 2009). Nagao and Mikouchi (2010) determined that NWA 480 is less contaminated by terrestrial heavy noble gases than NWA 5029, indicating significant differences in their terrestrial residence as a result of separate trajectories from Mars. In a study of the high-temperature, oxidized titano-magnetite in NWA 480, NWA 1460, and NWA 5029, Hoffmann et al. (2011) found that there is a similar magnetic signature and similar textures among them, which attests to the likelihood that they are launch paired. The natural remanent magnetization of NWA 5029 may be attributed to the ejection event on Mars.

The photo above shows both sides of a 0.47 g crusted cut fragment of NWA 1460. A photo of the cut face of the main mass of NWA 1460 is shown below, along with a cut section of NWA 480 for comparison (scale approximated). The 25 g main mass of NWA 480 is now curated at the Centre National d'Etudes Spatiales (CNES) in France.

∗ Recent geochemical research on the martian basalts has led to new petrogenetic models and classification schemes.  read more >>

standby for northwest africa 1460 photo
Photo courtesy of N. Oakes—Meteorites–R–Us
standby for northwest africa 480 photo
Photo courtesy of B. Fectay and C. Bidaut—The Earth's Memory