NWA 998

A 456 g nakhlite, said to have been found by Berber nomads in western Algeria or possibly eastern Morocco, was brought to the attention of Tucson meteorite dealers M. Farmer and J. Strope in September 2001. The acquisition of the meteorite was finalized at the 2002 Tucson Gem and Mineral Show, after which a type specimen was provided to the Field Museum of Natural History in Chicago. The main mass was subsequently acquired by A. and G. Hupé. The stone, designated NWA 998, was analyzed at the University of Washington and determined to be the first orthopyroxene-bearing nakhlite, exhibiting several unique features.

Northwest Africa 998 exhibits many similarities to Nakhla and the other nakhlites, including its mineral compositional range and its O-isotopic composition. Modally, it consists primarily of cumulus, olive-green, zoned, magnesian augite (~75%) and ferroan olivine (~9%), along with plagioclase (~7%), low-Ca pyroxene (~3.5%), and titanomagnetite (~1%), plus minor amounts of alkali feldspar, chlorapatite, and pyrrhotite, all forming a loosely consolidated, friable texture. The fact that NWA 998 has the lowest REE abundance of any nakhlite may be attributable to its low retention of intercumulus magma, which is reflected in its small volume of mesostasis (~2%). This virtual lack of mesostasis makes NWA 998 the only actual cumulate among the nakhlites. The abundance of low-Ca pyroxenes (orthopyroxene and pigeonite) in NWA 998, mostly associated with olivine grains, is greater than that in other nakhlites (Treiman and Irving, 2008). Although veinlets of an orange-colored alteration product ("iddingsite") observed within olivine grains are thought to be pre-terrestrial in origin, an extensive 6,000 (±1,000) year terrestrial residence has resulted in some minor Fe staining and veinlets of other alteration products.

A Sm–Nd isotopic plot for NWA 998 establishes an isochron at 1.29 (±0.05) b.y., while a refined Ar–Ar age was calculated at 1.334 (±0.011) b.y., ages which are virtually identical to the six other nakhlites studied thus far (Nakhla, Lafayette, Governador Valadares, NWA 817, Y-000593 pairing group, and MIL 03346). Northwest Africa 5790 age data is forthcoming. Based on all available age data for all studied nakhlites (excluding NWA 817), Park et al. (2009) presented a combined isochron reflecting an age of 1.325 (±0.018) b.y. The shielding-corrected CRE age for NWA 998 (14.5 m.y. and 13.0 m.y. based on 21Ne and 3He, respectively) is also identical to that of the other studied nakhlites, suggesting all were ejected from Mars in a single impact event from a common source. Notably, this impact produced features commensurate with relatively low shock.

A significant difference between Nakhla and NWA 998 can be seen in the rims around augite grains. In NWA 998, they consist of orthopyroxene rather than the pigeonite in Nakhla rims. Also unique to NWA 998 are miniscule melt inclusions along fractures in pyroxene consisting of alkali-bearing silica glass and rare Fe-bearing ("ankeritic") carbonate. Other minerals which are present in fractures and along grain boundaries of augite include Fe–Mg–Ca-bearing carbonates, K-feldspar, and phyllosilicates.

Olivines in nakhlites typically contain lamellar inclusions of magnetite and augite called symplectites, which were formed through exsolution processes, precipitated by the contractive cooling (from above 900°C) of Fe(3+)-bearing olivine grains. Symplectites in Nakhla constitute ~1 vol% of the mass, and they are found both along olivine grain boundaries and within the olivine grains. In contrast, symplectites in NWA 998 are found only at olivine grain boundaries. Furthermore, the magnetite in NWA 998 symplectites contains titanium, while none has been observed in Nakhla. Nearly identical symplectites have been identified in the Governador Valadares nakhlite, as well as in the possibly genetically-related Chassigny, but none have been found in the Lafayette nakhlite. Hydrogen isotope studies of melt inclusions, olivine, and clinopyroxene in NWA 998 show an extraterrestrial signature with an enriched D value (Boctor et al., 2005). However, studies of NWA 998 apatite derived from late-stage, open system magma movement suggest that very little water was present in its parent magma (Treiman and Irving, 2008). On the contrary, results of analyses of both apatite and amphibole in martian meteorites are consistent with a magma volatile content high in chlorine (Filiberto and Treiman, 2009). Still, McCubbin et al. (2009) found that kaersutite and Ti-biotite in Chassigny melt inclusions contain higher abundances of water than previously measured, a value which correlates to a parental source magma water content of ~460–840 ppm (0.5–0.8 wt%), while lower abundances of Cl and F were observed.

There are several competing scenarios to explain the formation of nakhlites. One scenario places their origin in a deep plutonic environment, while another places it close to the surface in a lava flow or shallow intrusion, such as a dike or sill. Following are details of two of the major competing scenarios.

Scenario 1

1. a two-stage evolution of nakhlites began with the formation of pyroxene-rich cumulates during an early Mars differentiation period ~4.56 b.y. ago.
2. a late magmatic event ~1.3 b.y. ago resulted in the partial melting of the cumulate mantle and the crystallization of augite from the parent magma.
3. basaltic melt inclusions were then entrapped within olivine crystals, showing that the intercumulus magma was rich in Fe and Ca, poor in Al and Ti, enriched in K relative to Na, and similar to alkaline basalts on Earth.
4. the rapid growth of augite and olivine was followed by a period of slow cooling and gravitational settling in a plutonic environment forming a cumulus-textured solid.
5. continued percolation of intercumulus magma resulted in extensive late-magmatic re-equilibration at temperatures of ~750°C under increasingly more oxidizing conditions, which promoted the replacement of olivine by low-Ca pyroxene + magnetite.
6. finally, a more rapid final cooling phase occurred, with solidification possibly taking place nearer the surface.

Scenario 2

This scenario utilizes the 2.7 b.y. old, differentiated, surface lava flow in Ontario, Canada known as Theo's Flow as an analog for the nakhlites. In particular, the differentiated, 60 m thick pyroxenite layer is so similar to the nakhlites that it is proposed they share the same formation processes.

1. initially a very mafic magma flow tens of meters thick began to pool.
2. pyroxene nucleation began in a cooler zone underneath a quenched crustal layer which began to link into clusters.
3. these pyroxene clusters then sank through the low-viscosity magma only to be carried back up in convection currents.
4. during the few days the pyroxene clusters were in this convective cycle, each cycle lasting for a few hours, they grew larger each time they reached the cooler nucleation zone.
5. when the clusters became too heavy for convective forces, they settled out onto the cumulus pile below, where the grains developed a preferred orientation.
6. finally, plagioclase crystallized from trapped low-Al melt, along with a possible unsampled gabbroic layer.

Based on texture and mineralogy (mesostasis [REE] abundance, plagioclase size, olivine Fa composition, intercumulus porosity, closure temperatures, oxygen fugacities, and pyroxene composition), Fe–Mg and Ca zoning profiles in olivine and pyroxene (reflecting cooling rates), and crystal size distribution analysis, a comparative burial depth within the ~100 m cumulate pile (lava flow or magma chamber) for each of these nakhlites has been proposed (Mikouchi et al., 2003, 2005, 2006; Lentz and McSween, 2003; Macrì et al., 2004; Imae et al., 2005; Day et al., 2006; Treiman and Irving, 2008; Jambon et al., 2010; Szymanski et al., 2010; Mikouchi et al., 2012):

1. NWA 5790—top of cumulate pile; <2 m
2. MIL 03346/090030/090032/090136—near top; <2 m
3. NWA 817—near top; <2 m
4. Y-000593/749/802—7 m
5. Nakhla—~10 m
6. Governador Valadares—~10 m
7. Lafayette—>30 m
8. NWA 998—bottom of the cumulate pile; >30 m

The hypothesis that NWA 998 was formed at the bottom of the cumulate pile or lava flow, where assimilation of sulfur-rich regolith material took place, is consistent with the observation that it contains the highest sulfur concentration among the nakhlites (Chevrier et al., 2011).

A study which proposes a different stratigraphic ordering of the nakhlites has been published by Grady et al. (2007). Without regard to the later recovery of the nakhlite NWA 5790, they argued that although MIL 03346 contains the least equilibrated olivine cores and has experienced the most rapid cooling of all the known nakhlites, both factors which are consistent with a shallow emplacement, it also contains the lowest abundance of carbonates with intermediate 13C isotopic composition; factors more consistent with minimal aqueous alteration at greater depth. Therefore, they envisage MIL 03346 as forming at the lowest zone of the flow and experiencing cooling by circulating groundwater derived from melted ices. The source of the isotopically-light carbonates is considered to be from this groundwater.

From cooling rate studies of the nakhlites conducted by Mikouchi et al. (2012), utilizing Fe–Mg and Ca zoning of olivine along with secondary mineralogy, they determined burial depths of the suite of nakhlites. Although the exact burial depth among NWA 5790, MIL 03346 and pairings, and NWA 817 could not be resolved, they were all emplaced within 2 m of the surface. Notably, NWA 5790 exhibits some important differences compared to the other nakhlites. It exhibits sector-like zoning in Al, Ti, and Cr within augite that is more similar to that in Nakhla and Y-000593/749/802 instead of like that present in NWA 817 and MIL 03346. For these mineralogical reasons, NWA 5790 is considered to have been derived from a more evolved portion of a common flow, or possibly from a separate flow.

In a contrasting view, Lentz et al (2005) find a lack of correlation between the olivine texture/distribution and the stratigraphic order, but rather, propose that the olivine texture/distribution is related to formation in different lava flows of variable composition. They suggest that at least two flows generated the various nakhlites—one which produced NWA 817, Nakhla, and Governador Valadares, and another which produced Y-000593/749/802, Lafayette, and NWA 998. A third flow may have generated MIL 03346. To account for the postulated gabbroic layer, which should have overlain the nakhlite unit and slowed its rapid cooling rate, they proposed that these rapidly cooled nakhlites were extruded late in evolved lava flows, but before plagioclase formation began.

Further information and excellent photos of NWA 998 can be found on the website catchafallingstar.com. A transmitted light view of a petrographic thin section of NWA 998 can be seen on J. Kashuba's page. The specimen shown above is a 0.65 g crusted fragment of NWA 998.

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