Two moderately weathered friable stones having a combined weight of 145 g were found by nomads in Mauritania and subsequently sold to A. Habibi in Erfoud, Morocco. A portion of the meteorite was submitted for analysis and classification to the Université Pierre & Marie Curie in France (A. Jambon, O. Boudouma, D. Badia) and the Université de Bretagne Occidentale in France (J-A. Barrat). Oxygen-isotopic values were determined at the Open University in the UK (R. Greenwood and I. Franchi) to be consistent with a martian origin, and NWA 5790 was classified as a unique unpaired member of the nakhlite meteorite suite. During the same time period, two more conjoint stones weighing together 270 g were found and sold in Erfoud, Morocco to A. Aaronson. Analysis of these stones was conducted at Northern Arizona University (T. Bunch and J. Wittke), and NWA 6148 was determined to be paired with NWA 5790.
Samples of NWA 5790/6148 have been studied at several different institutions through various techniques, and it was determined to be a loosely consolidated cumulate having a somewhat variable abundance consisting of olive-colored, strongly zoned augite (~5161 vol%; 0.32 mm in size) and olivine (~29 vol%; 13 mm in size), along with accessory titanomagnetite (<12.4 vol%). The augite was found to have initially crystallized at depth over an average time period of ~54.5 (±19.5) years from a late-stage magma following 30% fractional crystallization (Udry et al., 2012). By comparison, a slightly longer crystallization period of 1001,000 years was calculated for the MIL 03346 nakhlite by Day et al. (2006). The olivine and pyroxene phenocrysts are embedded in an interstitial mesostasis present in a higher abundance than in any other nakhlite, with measurements ranging from 33.6 vol% (Mikouchi et al., 2012) to 42 vol% (up to ~57 vol% in some samples; Corrigan et al., 2013, 2014). A more accurate measurement technique involving X-ray computed tomography was utilized by Tomkinson et al. (2015) for their sample, and a mesostasis abundance of 38.1 (±3.6) vol% was obtained. A mesostasis abundance of 44 vol% was determined for the NWA 5790 thin section analyzed by Balta et al. (2016) which indicates a degree of compositional heterogeneity exists in the meteorite.
The fine-grained mesostasis in NWA 5790/6148 is comprised of mostly dendritic oxide phases, augite, and fayalitic olivine within a silica-bearing feldspathic glass (basaltic-trachy-andesite to trachy-andesite in composition). Merrillite and Cl-apatite are also present in the mesostasis. No secondary alteration products ("iddingsite") were reported from initial studies of this meteorite, but Giesting and Filiberto (2015) subsequently identified a chloro-amphibole within melt inclusions in augite. This potassicchlorohastingsite is thought to have formed through very late-stage alteration processes involving a solute-saturatedFe, K, and Cl-rich, as well as LREE-richmetasomatic fluid which ultimately became trapped in silicate-hosted melt inclusions, perhaps following eruption onto the surface (Jambon, 2014). The process of Fe2+ concentration into this chloride-dominant fluid over time left the source magma more oxidized and the final silicate hosting this fluid more reduced (Giesting and Filiberto, 2016 and references therein). A similar Cl- and Al-bearing amphibole associated with a smectite alteration product was identified in fractures within augite and olivine grains by Jambon et al. (2016). Importantly, the NWA 5790 thin section utilized by Balta et al. (2016) contains a mesostasis-hosted pocket measuring 300 × 200 µm that is associated with FeS and which contains alteration phases such as water or oxidized iron; a similar alteration texture was described in MIL 03346 by Day et al. (2006). Leaving aside the contribution of the mesostasis component in the modal composition of bulk NWA 5790/6148, it can be seen that the augite/olivine ratio would then be the same as that in Nakhla; however, the texture and mesostasis composition of NWA 5790/6148 is similar to that of NWA 817 and the MIL pairing group.
Given the scenario that all nakhlites formed within a common magma structure (sill/lava flow), petrographic evidence indicates that NWA 5790/6148 likely formed nearest the top of the nakhlite pile (or at the rapidly-cooled margins). For instance, the mesostasis in NWA 5790/6148, which is derived from intercumulus melt displaced from below, is present in the highest proportion of any nakhlite, while the MIL pairing group (the most highly oxidized nakhlite exhibiting the lowest degree of equilibration), which is considered to have also formed near the top (or margins) of the pile, contains the next highest proportion of mesostasis (as high as 35 vol% reported by Rutherford et al., 2005). Lesser mesostasis abundances are present in NWA 817 (~20 vol%; Sautter et al., 2002) and the other nakhlites (~710 vol%; Mikouchi et al., 2014), corresponding to a descending sequence from the top. The nakhlites with phenocrysts present in the lowest proportions are those located nearest the top (or at the margins) of the pile, which is consistent with the rapid quenching observed (Sautter et al., 2002). Futhermore, the presence of rare filaments of both Ti-magnetite and sulfide in the upper-most nakhlites is also indicative of rapid cooling.
It has been demonstrated by four independent dating methods that all nakhlites and the chassignites have the same igneous crystallization age of approximately 1.3 b.y., and also that they share similar CRE ages of ~913 m.y., consistent with a common ejection event from Mars (Nyquist et al., 2001). In their CRE-age analysis of NWA 5790, Huber et al. (2011) determined an age of 9.6 m.y. based on the more reliable 21Ne, and an age of 8.9 m.y. was determined using the method of Eugster and Michel (1995), supporting the hypothesis for a common ejection event for all nakhlites. Weiler et al. (2016) conducted noble gas analyses on several meteorites including NWA 5790, with consideration of the shielding parameter (i.e., meteoroid size and sample depth) that affects the calculation of the CRE age. Based on the amount of shielding indicated by the galactic cosmic ray (GCR) 21Ne/22Ne ratio (0.83), an ambiguous result was obtained for the coupled meteoroid size vs. sample depth; the data are consistent with either a 1 m-sized meteoroid and sample depth of 20 cm, or a 14 cm-sized meteoroid and sample depth of 3 cm. Utilizing their preferred model for determination of CRE age based on the mean of calculated 3He, 21Ne, and 38Ar production rates, they derived an age of 7.3 m.y. While this relatively young CRE age might reflect a separate ejection event on Mars, it is still possible that NWA 5790 experienced unique shielding conditions following its ejection with the other nakhlites.
It is notable that NWA 5790/6148 is the most evolved nakhlite known, containing the highest Th, U and REE concentrations reported thus far in a nakhlite as well as a having a strong LREE enrichment. The enrichment of Fe and LREE in the alteration phase of NWA 817 has been attributed to the infiltration of fluids, which had incorporated a dissolved plagioclase component, into the local pre-existing rock. By the same token, since NWA 998 has the lowest REE abundance of any nakhlite, it might be attributed to its low retention of intercumulus melt reflected in its relatively small volume of mesostasis (~2%). Although this same process which affected the NWA 817 REE concentration has been considered as a possible explaination for the high REE concentration in NWA 5790/6148, the lack of "iddingsite" in most studies of this meteorite suggests that fluid infiltration was not prevalent at its location, or alternatively, the scarcity of olivine in this meteorite was not conducive to the development of significant permeability (Tomkinson et al. 2015). Studies have revealed that NWA 5790/6148 contains a very low abundance of water (119 ppm in select pyroxene grains; A. Peslier, 2013). Through comparisons with nakhlites derived from other locations within the pile, it was determined that water abundances are not correlated with either the location or the cooling rate. McKay et al. (2006) observed that complex correlations exist between the Al content and the cooling rate among the nakhlites. For instance, those containing the most rapidly cooled pyroxenes contain both the highest content of Al in augite cores and the greatest homogeneity, while those that experienced slower cooling rates exhibit complex Al zoning and a core Al content with a pronounced bimodal distribution. NWA 5790/6148 clearly has the highest Al value among the nakhlites and is inferred to have crystallized closest to the top or margins of the pile or lava flow(s); curiously, NWA 998 does not follow this trend.
Mikouchi et al. (2014) conducted an examination of all known nakhlites (with the exception of the most recently recognized nakhlites NWA 5790/6148 and NWA 10153), and have identified major element chemical zoning in the olivine grains of each member indicative of rapid cooling. Across the respective zoning profiles, they found that the range of variation decreased in the following order: MIL pairing group and NWA 817 (Fa5493) ⇒ Y-000593 pairing group (Fa5885) ⇒ Nakhla and Governador Valadares (Fa5872) ⇒ Lafayette and NWA 998, the latter two approaching homogeneity (Fa6667 and Fa6162, respectively). The fact that this chemical zoning in olivine reflects differences in cooling rates, as opposed to the possibility that the gradients were inherited from the magma, was demonstrated by Sio et al. (2014). They measured core to rim Mg-isotopic diffusion profiles in the olivine grains of certain members of the nakhlite suite, and concluded that the existence of such isotopic fractionation was consistent with chemical diffusion during an extended cooling period.
The fact that NWA 5790/6148 is the sole member of the nakhlite group which has retained complex primary zoning features on augite grains indicates that it cooled the fastest among the nakhlites, and this is consistent with a position at the top of the nakhlite pile or specific lava flow. Based on calculations of the growth rate of oscillatory zoning profiles in olivine and augite, A. Jambon (2014) concluded that simultaneous crystallization of both minerals occurred over a period of ~1 hour. Further evidence for rapid cooling at this upper-most (or margin) location can be seen in the Cl contamination previously identified in the MIL pairing group, in the high proportion of evolved mesostasis, and in the lack of equilibrium among the mineral phases. 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 (magma chamber and/or lava flow) for each of the nakhlites and the chassignites has been proposed 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):
NWA 5790/6148: cooling 4.5°C/hr; top of the nakhlite pile, <2 m deep
MIL 03346 pairing group: cooling 1°C/hr; near top, <2 m deep
NWA 817: cooling 0.2°C/hr; near top, <2 m deep
Y-000593/749/802: 7 m deep
Nakhla: cooling 0.04°C/hr; ~10 m deep
Governador Valadares: cooling 0.08°C/hr; ~10 m deep
Lafayette: cooling 0.015°C/hr; >30 m deep
NWA 10153: near the bottom of the nakhlite pile
NWA 998: >30 m deep
unsampled olivinepigeonite horizon
NWA 2737: greatest depth (<200 m)
Diagram credit: McKay et al., LPSC 37, #2435 (2006)
A re-analysis of NWA 5790 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'
In their scenario, Jambon et al. (2016) 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 observed absence of "iddingsite" in their study of 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 have resided near the bottom of this stratum.
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, p. 21 (2016) and references therein
Color background adapted from Jambon et al., GCA, vol. 190, p. 209 (2016)
Analyses of the most recent nakhlite find, NWA 10153 and pairings, has been published by investigators including Mikouchi et al. (2016). 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 is similar to that in NWA 998, 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 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 (lava flow) in the nakhlite cumulate pile has not yet been determined.
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. The specimen of NWA 6148 shown above is a 0.092 g partial slice. The two intact masses composing NWA 5790 are shown below, courtesy of Aziz Habibi.