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. Northwest Africa 998 was analyzed at the University of Washington, and among other unique features, it is distinguished as the first orthopyroxene-bearing nakhlite.
Northwest Africa 998 shares many characteristics with Nakhla and the other nakhlites, including a similar mineral compositional range and O-isotopic composition. Modally, it consists primarily of olive-green, chemically-zoned, magnesian augite (~75%) and ferroan olivine (~9%), along with plagioclase (~7%), orthopyroxene (~3.5%), and titanomagnetite (~1%), plus minor amounts of alkali feldspar, chlorapatite, and pyrrhotite, all forming a loosely consolidated, friable texture. The formation of orthopyroxene in NWA 998 is attributed to sub-solidus changes along olivine-plagioclase grain boundaries related to temperature or redox state, or to silicate melt infusion (Breton and Lee, 2015). The fact that NWA 998 has the lowest REE abundance of any nakhlite might be attributable to its low retention of intercumulus magma, which is reflected in its small volume of mesostasis (~2%). This virtual lack of mesostasis defines NWA 998 in the strict sense as the only cumulate among the nakhlites. The occurrence of orthopyroxene in NWA 998, mostly associated with olivine-plagioclase grain boundaries, has also been identified in Lafayette (McCubbin et al., 2013). Although veinlets of orange-colored alteration product ("iddingsite") observed within olivine grains are considered to be pre-terrestrial in origin, an extensive 6,000 (±1,000) year terrestrial residence has resulted in some minor Fe staining and the formation of veinlets composed of other alteration products.
A SmNd isotopic plot for NWA 998 establishes an isochron at 1.29 (±0.05) b.y., while a refined ArAr age was calculated at 1.334 (±0.011) b.y., ages which are virtually identical to six other nakhlites studied (Nakhla, Lafayette, Governador Valadares, NWA 817, Y-000593 pairing group, and MIL 03346 pairing group). 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 surrounding augite grains. In NWA 998, they consist of orthopyroxene rather than the pigeonite in the 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 FeMgCa-bearing carbonates, K-feldspar, and phyllosilicate; the phyllosilicate phase has been shown to be a replacement for carbonate (siderite), which had previously been formed as a replacement for olivine during periods of increased CO2 and water (Tomkinson et al., 2013).
Olivines in nakhlites typically contain lamellar inclusions of magnetite and augite known as symplectites, which were formed through exsolution processes, precipitated by the contractive cooling (from above 900°C) of Fe3+-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. By 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 Governador Valadares, as well as in the genetically-related Chassigny, but none have been found in Lafayette. 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, ~0.070.11 wt% (Treiman and Irving, 2008; Channon et al., 2011). Conversely, 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 ~460840 ppm (0.50.8 wt%), while lower abundances of Cl and F were observed. However, it is now thought that dehydration of the magma resulting from the infiltration of an exogenous Cl-rich fluid occurred after the crystallization of the chassignites (McCubbin et al., 2013).
Several competing scenarios have been presented to explain the formation of nakhlites. Some scenarios place their origin in a plutonic environment, while others place it close to the surface in a thick lava flow or a shallow intrusion, such as a dike or sill. Following are details of three of the major competing scenarios:
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.
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.
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.
the rapid growth of augite and olivine was followed by a period of slow cooling, gravitational settling, and equilibration within a plutonic environment, forming a cumulus-textured solid.
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.
lastly, a more rapid final cooling phase occurred, with solidification possibly taking place nearer the surface, perhaps in a lava flow or sill.
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.
initially a very mafic magma flow tens of meters thick began to pool.
pyroxene nucleation began in a cooler zone underneath a quenched crustal layer which began to link into clusters.
these pyroxene clusters then sank through the low-viscosity magma only to be carried back up in convection currents.
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.
when the clusters became too heavy for convective forces, they settled out onto the cumulus pile below, where the grains developed a preferred orientation.
lastly, plagioclase crystallized from trapped low-Al melt, along with a possible unsampled gabbroic layer.
Perhaps the most plausible scenario for the petrogenesis and comagmatic origin of both the chassignites and nakhlites was proposed by McCubbin et al. (2013), which is based in large part on advanced analytical techniques used to investigate the volatile-bearing minerals present in these two groups.
a very mafic, depleted partial melt intruded into the near-surface crust.
olivine and chromite were the first cumulus minerals to crystallize within this chamber, composing the most slowly cooled, lowest stratigraphic horizon to become the dunitic chassignites.
as cooling proceeded, a Cl-rich LREE-enriched fluid infiltrated the chamber leaving the magma enriched in Cl and LREE, and depleted in water.
the next horizon to crystallize is posited to have been composed of olivine and pigeonite, but a representative sample has not yet been found.
after the stratographic unit represented by NWA 998 was accumulated, the melt reached the point of Cl saturation, and a low-density, Cl-rich fluid was exsolved/degassed; this fluid migrated upwards initiating an eruptive phase.
this escaping Cl-rich fluid removed proportionately more Fe2+ than Fe3+, thereby increasing the oxygen state in the later-crystallizing upper horizons.
as clinopyroxene and olivine crystallization continued, Cl-rich liquid became trapped as inclusions, subsequently forming apatite and amphibole.
following crystallization of the cumulate pile and lava flow, alteration by low-temperature aqueous fluids produced secondary mineral phases such as iddingsite.
Based on textural and mineralogical characteristics, including mesostasis (REE) abundance, plagioclase size, olivine Fa composition, intercumulus porosity, closure temperature, oxygen fugacity, pyroxene composition (Mg#), FeMg and Ca-zoning profiles in olivine and pyroxene, Li zoning profiles in augite, Al-content in pyroxene cores (see diagram below), crystal size distribution analysis, and volatile history, a comparative burial depth within the ~100 m nakhlite pile (both magma chamber and lava flow) for each of the nakhlites and the two chassignites is inferred as follows (Mikouchi et al., 2003, 2005, 2006, 2012, 2014; Lentz and McSween, 2003; Macrì et al., 2004; Imae et al., 2005; Day et al., 2006; McKay et al., 2006; Treiman and Irving, 2008; Jambon et al., 2010; Szymanski et al., 2010; McCubbin et al., 2013; Corrigan et al., 2014; Richter et al., 2015; Hewins et al., 2015; Giesting et al., (2015); Richter et al., 2016; Mikouchi et al., 2017):
NWA 5790/6148: cooling 0.35 to 4.5 °C/hr; 34% mesostasis; olivine Fa94; top of the nakhlite pile, <2 m deep
MIL 03346 pairing group: cooling 0.04 to 11.0 °C/hr; 22% mesostasis; olivine Fa92; near top, <2 m deep
NWA 817: cooling 0.5 to 2.2 °C/hr; 24% mesostasis; olivine Fa90; near top, <2 m deep
Y-000593/749/802: cooling 0.015 to 0.03 °C/hr; 10% mesostasis; olivine Fa85; 7 m deep
Nakhla: cooling 0.01 to 0.04 °C/hr; 8% mesostasis; olivine Fa72; ~10 m deep
Governador Valadares: cooling 0.01 to 0.085 °C/hr; 7% mesostasis; olivine Fa70; ~10 m deep
NWA 10153: cooling ~0.01 °C/hr; ~25% mesostasis; ~10 m deep
Lafayette: cooling <0.001 to <0.015 °C/hr; 7% mesostasis; olivine Fa67; up to >30 m deep
NWA 998: cooling <0.0009 to <0.015 °C/hr; 710% mesostasis; olivine Fa62; up to >30 m deep
unsampled olivinepigeonite horizon
NWA 8694: cooling 0.003 to 0.1 °C/hr; olivine Fa46
Chassigny: cooling 0.003 to 0.1 °C/hr; 7% mesostasis; olivine Fa31
NWA 2737: cooling 0.003 to 0.1 °C/hr; 6% mesostasis; olivine Fa21; greatest depth (<200 m)
Diagram credit: McKay et al., LPSC 37, #2435 (2006)
To account for the difference in trace element ratios that is observed between the chassignites and the nakhlites, McCubbin et al. (2013) suggested that an exogenous Cl-rich, LREE-enriched fluid was introduced during initial crystallization and accumulation of olivine and chromite, as sampled by the chassignites, and prior to crystallization of clinopyroxene and olivine (±orthopyroxene), as sampled by the nakhlites. Subsequent draining/degassing of this Cl-rich fluid from the accumulating nakhlite pile was accompanied by preferential removal of Fe2+ over Fe3+ from the residual magma, which concurrently increased the oxidation of the system. McCubbin et al. (2013) also reasoned that such an infiltration of metasomatic fluid into the chassignitenakhlite cumulate pile would produce an olivinepigeonite layer between the chassignite and nakhlite horizons, although such a lithology is not yet represented in our meteorite collections. Interestingly however, Krzesińska et al. (2017) have identified cumulate pigeonite grains in two small pyroxene-bearing fragments in Chassigny which they infer were derived from this hypothesized olivinepigeonite layer prior to its final crystallization. Moreover, they found unique µm-scale aggregates associated with these pyroxene-bearing fragments that consist primarily of sulfide phases incorporating µm-sized metallic grains of Pb, Hg, Ag, Au, and Sn. It is considered likely that these metal grains were introduced to the cumulate pile through the Cl-rich metasomatic fluid.
A re-analysis of the NWA 5790 nakhlite was conducted and published by Jambon et al. (2016). Compared to other nakhlites, the augites in NWA 5790/6148 exhibit unusual zoning features involving FeO/MgO variations, including the presence of micron-scale oscillatory zoning which reflects rapid growth (~1 mm/10 hours) followed by rapid cooling in order to preserve this zoning. Several other features that were observed in this nakhlite, such as discontinuous zoning on augites and rounded augite cores enclosed in olivine, are indicative of a more complex petrogenetic history than has been previously described. To account for the petrography, mineralogy, and geochemistry across the entire nakhlite suite, a new scenario for their petrogenesis was proposed which involves a multi-stage growth history:
Augite crystallized from an evolved mantle melt and accumulated at the bottom of a sub-surface magma chamber. Multiple melt batches involving variable degrees of fractionation produced the compositional variation observed among the nakhlites.
Subsequently these augite cumulus crystals (glomerocrysts) were disrupted and entrained by a more evolved magma flow. As a consequence the augites sustained crystal damage and partial resorption.
This ascending magma erupted to the surface where lower pressures promoted crystallization of olivine, incorporating the augite cores as xenocrysts.
Growth of olivine continued for a time followed by a final overgrowth of augite, all of which halted when the melt drained away and a rapid cooling stage ensued.
Multiple successive lava flows ultimately formed a layered cumulate pile with nakhlites within different strata exhibiting compositional variability, as illustrated by Jambon et al. (2016) in the diagram below:
Diagram credit: Jambon et al., GCA, vol. 190, p. 209 (2016)
'Northwest Africa 5790: Revisiting nakhlite petrogenesis' (http://dx.doi.org/10.1016/j.gca.2016.06.032)
In their scenario, Jambon et al. envision NWA 5790/6148 at the top of the earliest lava flow where the rock experienced very rapid cooling, thereby limiting compaction (reflected in the lowest degree of preferred orientation of augite crystals among nakhlites) and equilibration through diffusion, and enabling the retention of a relatively large volume of mesostasis. They argue that the final position of NWA 5790/6148 was at the bottom of a stratified cumulate pile, a location which prevented the occurrence of secondary low-temperature hydrous alteration consistent with the absence of "iddingsite" in this nakhlite. In their diagram of the nakhlite cumulate pile, NWA 998 and NWA 817 are located within the intermediate flow zone based on several features they have in common; e.g., they have similar augite core compositions which are distinct from all other nakhlites, they have high incompatible element abundances (similar also to NWA 5790/6148), and they have similar magnetite abundances. The top flow contains those nakhlites with similar augite core compositions as well as those with a high abundance of secondary aqueous alteration minerals; Nakhla is considered to reside near the bottom of this stratum. Chevrier et al. (2011) observed that NWA 998 contains the highest sulfur concentration among the nakhlites, which they found to be consistent with the hypothesis that NWA 998 formed near the bottom of the nakhlite pile where assimilation of sulfur-rich regolith material could take place; this correlation needs to be reconciled with the new scenario proposed by Jambon et al. (2016).
Balta et al. (2016) conducted a geochemical and petrographic study of NWA 5790 which included major, minor, and trace element analyses along with crystal size distribution (CSD) and spatial distribution pattern analyses. Based on this study, a new scenario was presented to explain the formation history of the known nakhlites (excluding the most recently discovered nakhlite NWA 10153 and pairings) and to further characterize the stratigraphic relationship that existed among them, with an emphasis on the three nakhlites generally considered to have crystallized nearest the top of the cumulate pile or lava flow(s)NWA 5790/6148, MIL 03346 and pairings, and NWA 817. Although previous studies have concluded that NWA 5790/6148 likely formed above MIL 03346 in a common lava flow, Balta et al. (2016) noted that the mesostasis in NWA 5790 and NWA 817 contains a significant abundance of crystalline phases compared to that in MIL 03346; this is consistent with NWA 5790 and NWA 817 cooling more slowly at a deeper location than MIL 03346. Another inconsistency with the location of NWA 5790/6148 at the top of a common cumulate pile or lava flow was demonstrated through their REE analyses. The measured REE abundances in augite rims are higher in MIL 03346 and NWA 817 compared to NWA 5790, which would correspond to a higher degree of fractionation and a higher position in the cumulate pile or lava flow for MIL 03346 and NWA 817. Furthermore, based on both its CSD pattern and small olivine diffusion rims, MIL 03346 appears to have experienced a higher cooling rate nearer the top as compared to NWA 5790. In addition, major and minor element chemistry and petrographic characteristics tend to link NWA 5790 with NWA 817 and distinguish both of them from MIL 03346, making the hypothesis of a single verticle stratigraphic sequence untenable. In consideration of these and other findings (e.g., a sizable "iddingsite" alteration pocket in NWA 5790), Balta et al. (2016) have proposed that these three nakhlites represent separate breakout lobes from a common parental lava flow (see schematic diagram below).
Diagram credit: Balta et al., MAPS, vol. 52, #1, p. 56 (2017) and references therein
'Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile' (http://dx.doi.org/10.1111/maps.12744)
Color background adapted from Jambon et al., GCA, vol. 190, p. 209 (2016)
'Northwest Africa 5790: Revisiting nakhlite petrogenesis' (http://dx.doi.org/10.1016/j.gca.2016.06.032)
Analyses of the most recent nakhlite find, NWA 10153 and pairings, has been published by investigators including Mikouchi et al. (2016, 2017). They report that this nakhlite contains ~2530% interstitial mesostasis similar to the abundances in NWA 817 and MIL 03346 which are considered to have cooled rapidly near the top of the cumulate pile. However, the coarse nature of the feldspar in NWA 10153 (0.20.3 mm laths) is similar to that in NWA 998 (0.5 mm laths) which is considered to have cooled slowly near the bottom of the pile. Augite core compositions in NWA 10153 are similar to those of all other nakhlites, consistent with their crystallization from a common parental melt. This nakhlite exhibits complex chemical zoning in olivine and augitethe augite zoning is unlike that in any other nakhlite. Based on the Ca-Fe-Mg zoning profile, the cooling rate was determined to be 0.010.05 °C/hr; this is similar to that for Nakhla, GV, and Y-000593, which are considered to have crystallized at an intermediate depth within the pile. While Nakhla and other nakhlites from intermediate depths contain carbonates and halides, these are absent in NWA 10153. Low-temperature secondary aqueous alteration products (silicate gel phase) have been observed in both mesostasis and olivine fractures, a mineralogy common to the nakhlites near the top of the pile (Hicks et al., 2016). Another unique feature of this nakhlite is its significantly higher initial Nd-isotopic values and lower initial Hf-isotopic values compared to other nakhlites (Righter et al., 2016). Because of the ambiguous nature of NWA 10153, its position within a specific stratum in the nakhlite cumulate pile or lava flow(s) has not yet been determined; however, the combined data suggest a separate flow or lobe.
Importantly, Mikouchi et al. (2016) found that significant ambiguities also exist among the three known chassignites. Although each chassignite exhibits a similar cooling rate (0.0030.1 °C/hr), olivine compositions between them show large variations: NWA 8694 is Fa46, Chassigny is Fa31, and NWA 2737 is Fa21. Moreover, each chassignite exhibits a distinct shock history. Therefore, they suggest that each of the chassignites is more likely associated with a separate flow or lobe (possibly within a common extensive igneous unit) rather than a single sequential accumulation (see above diagram illustrating separate lobes as proposed by Balta et al. ).
Based on data from the Infrared Mineralogical Mapping Spectrometer aboard the Mars Express orbiter, olivine-enriched craters in the region of Thaumasia Planum were found to be the best match to the nakhlites (Ody et al., 2013). Additional details on the petrogenetic history of the nakhlite group, including the possible presence of martian bacterial micro-fossils, can be found on the Nakhla page. More information and excellent photos of NWA 998 can be found on J. Strope's 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.