NWA 1068
"Louise Michel"

One large mass of 522 g along with twenty-two additional fragments, all together weighing 576.77 g, were found by a French team in the Maarir region near the border of Morocco and Algeria. This meteorite was designated Northwest Africa 1068, and a sample was submitted to École Normale Supérieure de Lyon and other institutions for analysis and classification. Unaware of the analyses being conducted by the French institutions, additional paired fragments purchased in Morocco having a combined weight of 118 g, were submitted to the University of Washington (Irving and Kuehner, 2002). Since the name NWA 1110 had been previously reserved from the Nomenclature Committee for these additional fragments, this meteorite will be recognized as a pairing under both names. Subsequent to this, other individual fragments were recovered in the strewnfield, some of which were submitted to the NomCom under unique NWA numbers (e.g., NWA 1183, NWA 1775, NWA 2373, NWA 2969).

Northwest Africa 1068 is considered to represent a distinct olivine-phyric subgroup of shergottites, characterized by an abundance of olivine megacrysts (~27 vol%) embedded within a primarily low-Ca pyroxene groundmass (~42 vol%). This subgroup would comprise those meteorites derived from a primary magma associated with an ascending mantle plume. They are ultramafic rocks, enriched in Mg (>12%), Ti, and other incompatibles, and were formed at greater depths under higher pressures than the basaltic subgroup. Despite some important compositional differences, close petrological and geochemical similarities exist between NWA 1068 and the olivine-phyric shergottite LAR 06319 and the olivine-basalt shergottite NWA 4468 (Sarbadhikari et al., 2009). According to MELTS program modeling, Marks et al. (2010) found that both NWA 1068 and 4468 have compositions that are consistent with the hypothesized parental melt for LA 001; in particular, the REE patterns and initial Sr–Nd isotopic compositions are consistent with such a relationship, and the major element compositions reflect a mantle-associated fractionation.

The occurrence of plagioclase in the form of maskelynite (~15 vol%), undulose extinction of pyroxene and olivine, impact melt pockets, and shock veins attest to high shock metamorphism (29–55 GPa) for this meteorite. Minor amounts of Ca-phosphates, K-feldspar, FeS, ulvöspinel, and chromite are also present. Despite its lack of fusion crust, NWA 1068 is relatively fresh, with only minor calcite and clay minerals present in cracks and along grain boundaries. Using the typical increases in the Sr, Ba, and Pb abundances observed in hot desert meteorites as a barometer, NWA 1068 has not been greatly affected by weathering. (Barrat et al., 2002)

Similar to other highly shocked martian meteorites, NWA 1068 contains a significant concentration of martian atmospheric Ar within melt pockets (ave. 8.6 ppb), with a minor component present within shock veins (ave. 0.9 ppb). The favored scenario for the existence of this trapped gas component within melt pockets is based on the arguement that martian atmospheric gas was initially introduced into pre-existing cracks and pores. Following the passage of a shock wave, sudden decompression and pressure release created bubbles within sub-mm- to mm-sized localized melt pockets. Finally, as pressures became equilibrated, the trapped atmospheric gases migrated into the vesicles of the melt phase from the surrounding cracks and pores (Walton et al., 2007).

Northwest Africa 1068 is composed of abundant olivine megacrysts up to 2 mm in size which have magnesian cores (Fa28) and ferroan (Fa51) rims. These olivine grains usually occur as single crystals, but many are polycrystalline and contain magmatic inclusions. They show almost identical chemical compositions to the olivines in martian lherzolitic meteorites, and it is considered by many investigators from their textures and other petrographic evidence that they are xenocrysts which accumulated into the flow from different melt reservoirs. The zoning in the rims of these olivine megacrysts can be attributed to diffusion between the olivine and the magma that ensued following their incorporation. In keeping with this diffusion process is the fact that these large olivines have only equilibrated with the groundmass along their rims, a feature which further supports a xenocrystic origin. Moreover, they are enriched in Co and incompatible elements, and they crystallized under more oxidizing conditions.

In an alternative view, it has been argued that the olivine megacrysts represent phenocrysts, as demonstrated by the equilibrium between cumulate olivine and the olivine megacryst cores (Filiberto et al., 2010). Furthermore, the trace element compositions of the NWA 1068 olivine megacrysts are similar to the bulk rock, indicating a derivation from a common parental source magma. Magmatic inclusions within the olivine megacrysts of other olivine-phyric shergottites is similar to their bulk parental melt, indicating that the olivines crystallized from the same magma source, and therefore likely represent phenocrysts. Another interpretation for the observed zoning in NWA 1068 olivine megacrysts is that they are reworked phenocrysts subjected to a short period of gravitational settling and/or convective transport before accumulation (Shearer et al., 2008). In further support of a phenocrystic origin, it was argued that the melt inclusions within the olivine megacrysts of NWA 1068 and other olivine-phyric shergottites have a similar composition to that of the bulk rock, indicating a derivation from a common parental source magma. They point out the chemical equilibrium that exists between the olivine megacrysts and the bulk rock, and the likelihood that the megacrysts represent phenocrysts. They also suggest that the formation of spinel and high-Ca pyroxene in both megacrysts and matrix were concomitant. Another observation in support of a phenocrystic origin for the olivine megacrysts is that the olivine appears to be in isotopic equilibrium with the other mineral components of the rock.

Cooling rate studies place the crystallization of the low-Ti/Al pyroxene at a depth of ~85 km (~10 kbar), near the base of the crust. The pyroxenes with a higher Ti/Al crystallized near the surface (<4.3 kbar), possibly upon eruption. A scenario for the complex petrogenesis of this meteorite has been constructed as follows:

Following their crystallization at depth, the cumulate olivine megacrysts were incorporated into an ascending, enriched, oxidized, magma plume which originated at the mantle. The magma ponded in or near the base of the crust where olivine crystallized and accumulated from either the same (phenocrysts) or neighboring (xenocrysts) magma plumes. Low-Ti/Al pyroxene then crystallized and erupted onto or near the surface together with the olivine megacrysts. Cooling occurred rapidly close to the surface where shock metamorphic effects became significant. This scenario is consistent with the finding that NWA 1068 has a REE pattern that is similar to other basaltic shergottites, while other olivine-containing shergottites such as DaG 476, Dhofar 019, and SaU 005 do not. Xenocrystic olivines in EETA79001A may have a similar origin. Northwest Africa 1068 is a relatively primitive shergottite with a magnesian bulk composition, but is not as magnesian as experiments indicate it should be if it represented a primary liquid composition. Because it has incorporated a high abundance of olivine megacrysts, it no longer represents a primary magma composition (Bunch et al., 2009).

Isotopic analysis using Sm–Nd and Rb–Sr data have determined a crystallization age for this shergottite of 185 (±11) m.y., and its CRE age has been calculated to be 2.2 (±0.2) m.y. (2.5–3.1 m.y. based on 10Be [Nishiizumi and Caffee, 2006], and 2.0 ±0.5 m.y. based on Ar systematics [Walton et al., 2007]). This CRE age is similar to several other martian meteorites, including NWA 2646, LAR 06319, and NWA 1460.

Trace element data confirm that Northwest Africa 1068/1110 is unpaired with any previously found martian meteorites. In contrast to the depleted LREE evident in most all other olivine-phyric shergottites, NWA 1068 is enriched in incompatible elements similar to what is found in the basaltic shergottites Shergotty, Zagami, and Los Angeles; incompatible element ratios are consistent with these basaltic shergottites as well. This suggests a parental magma for NWA 1068 of basaltic shergottite composition which had assimilated a late-stage, enriched, more-oxidized, cumulate component close to lherzolitic composition. Thereafter, olivine crystallized and was accumulated, perhaps as phenocrysts (Shearer et al., 2008).

Interestingly, a determination of the Pb-isotopic composition of the original source of the olivine-phyric shergottites shows a similar plot to that of the nakhlites, and these diverse martian meteorites may have been originally derived from the same mantle reservoir (Emil et al., 2006). The specimen of NWA 1068 pictured above is a 1.19 g partial slice with a thin, black impact-shock vein along the left side and a natural edge along two sides. The photo below shows the main mass of NWA 1068.

Photo courtesy of B. Fectay and C. Bidaut—www.Meteorite.fr