‘Louise Michel’

Martian Shergottite
olivine-phyric ∗
(enriched, permafric, oxidized)

standby for nwa 1068 photo
Found April 2001
no coordinates recorded

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 (~22 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 both the olivine-phyric shergottite LAR 06319 and the olivine-basaltic 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 crystallization ages for both NWA 1068 and Los Angeles are concordant at ~180 m.y., and they might be derived from a common primary magma source—with Los Angeles crystallizing after ~40% fractionation, and NWA 1068 after addition of 22% olivine (Treiman and Filiberto, 2014).

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 argument 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. Thereafter, 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 that have magnesian cores (up to Fo72), and rims (as well as other smaller phenocrysts) that are more ferroan (Fo49). 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 poikilitic (formerly lherzolitic) shergottites, and it is considered by many investigators, based on the disparate redox conditions under which core and rim crystallized (i.e., increasing oxidation from core to rim) as well as on textural and other petrographic evidence, that the megacrysts are in fact xenocrysts that were accumulated into the magma flow from different melt reservoirs (Herd, 2006; Shearer et al., 2012). The zoning in the rims of these olivine megacrysts could 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 appear to 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 compared to the groundmass, and evidence indicates that they crystallized under more oxidizing conditions.

In an alternative view, it has been argued that the olivine megacrysts represent phenocrysts (co-genetic), as demonstrated by the equilibrium between cumulate olivine and the olivine megacryst cores (Filiberto et al., 2010). 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. The observed zoning in NWA 1068 olivine megacrysts could be interpreted as reworked phenocrysts that were 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, as well as in other olivine-phyric shergottites, have a similar composition to that of the bulk rock, indicating a derivation from a common parental source magma. Since the olivine megacrysts and the bulk rock are in chemical equilibrium, there is a high likelihood that the megacrysts represent phenocrysts derived from a common parental source melt. Moreover, the formation of spinel and high-Ca pyroxene in both the megacrysts and the matrix was concurrent. As such, it was undertaken by Filiberto et al. (2013) to compare the composition of the primary trapped melt in the magnesian megacrysts to that of the calculated parental melt prior to incorporation of excess olivine; their results were most consistent with a xenocrystic origin for the megacrysts.

Cooling rate studies place the crystallization of the low-Ti/Al pyroxene in NWA 1068 at a depth of ~85 km (~10 kbar), near the base of the crust, whereas the pyroxenes with a higher Ti/Al crystallized near the surface (<4.3 kbar), possibly upon eruption. in contrast, geochemical modeling conducted by Filiberto and Dasgupta (2012) suggest that formation occurred at a depth of ~150 km at a temperature of ~1520°C. This temperature is within the range of the calculated average mantle temperature of 1450 (±80) °C for basalt formation during the Noachian period, 4.5–3.6 b.y. ago. A scenario for the complex petrogenesis of this meteorite was constructed as follows:

Following their crystallization at depth, the cumulate olivine megacrysts were incorporated into an ascending, enriched, oxidized magma plume that originated at the upper-mantle (at depths of ~250–400 km; Kiefer, 2003), envisioned to be similar to lunar ur-KREEP that crystallized as a late-stage residual liquid of the martian magma ocean (Borg et al., 2012). The magma ponded in or near the base of the crust where olivine crystallized and accumulated from either the same (phenocrysts or antecrysts) 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 might 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).

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 that which 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, and more-oxidized cumulate component close to lherzolitic composition. Thereafter, olivine crystallized and was accumulated, perhaps as phenocrysts (Shearer et al., 2008).

Studies of NWA 1068 have continued in an effort to characterize the true nature of the olivine megacrysts and to better resolve the petrogenesis of the meteorite. Through advanced Fe–Mg isotope and major, minor, and trace element analyses of NWA 1068 bulk rock and olivine megacrysts, Collinet et al., (2017) determined that the meteorite is most consistent with a near-primary magma composition. In their formation model the olivine megacrysts and the groundmass of the meteorite are co-genetic. Subsequent to olivine megacryst formation, which they ascertained occurred over a time period of ~2–6 years under relatively slow cooling conditions within a deep pluton (the final span of ~100 days involved rim and groundmass crystallization at much faster cooling rates during/after magma ascent), the megacryst cores experienced simultaneous diffusion and growth of outer rims along with crystallization of the groundmass pyroxene and olivine, as evidenced by the fractionated Fe–Mg isotope and element profiles which are observed. This diffusion process reduced the Fo content by ~3.2 mol% from its original value to values as magnesian as Fo77; these current Fo values had previously been attributed to Fe–Mg equilibrium conditions attained through olivine accumulation. Therefore, the bulk rock composition of NWA 1068, and in a similar way that of LAR 06319, is likely representative of a primary magma, derived from a refractory mantle source, having an intermediate composition of at least ~Fo80 (see diagram below).

standby for equilibrium diagram
Diagram credit: Collinet et al., GCA, vol. 207, p. 294 (2017)
'Crystallization history of enriched shergottites from Fe and Mg isotope fractionation in olivine megacrysts'

Isotopic analyses 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 480/1460. Cosmic ray exposure ages have now been determined for many martian meteorites, and Mahajan (2015, #1166) 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 1068/1110 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 1068/1110 might have been ejected with the large group of at least 26 enriched shergottites, or alternatively, it could represent a unique ejection event because of its disparate texture.

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 originated 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.

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

standby for nwa 1068 photo
Photo courtesy of B. Fectay and C. Bidaut—