At least forty-two conjoint stone meteorite fragments, mostly devoid of fusion crust, were found in Algeria in 2010. They were subsequently purchased by G. Hupé in February 2011 during the Tucson Gem and Mineral Show. Associated fragments were tracked down and acquired from various Moroccan dealers by G. Hupé over the succeeding four months. The total combined weight of these paired fragments was 8,387 g. An additional 5,100 g of fragments, designated NWA 6693, were acquired in March 2011 by E. Thompson and are considered likely paired with NWA 6704. One other group of paired stones previously obtained by G. Fujihara in January 2011 received a separate designation as NWA 6926. A portion of both NWA 6704 and NWA 6926 were submitted for analysis and classification to the University of Washington in Seattle (A. Irving and S. Kuehner), while a portion of NWA 6693 was submitted to the University of California in Los Angeles (P. Warren).
The meteorite is an igneous cumulate that originated on a large partially differentiated parent body distinct from all others known to date. As seen in the high-res photo above (courtesy of G. Hupé), NWA 6704 is composed primarily of orthopyroxene (~70 vol%), yellowish-green in color, occurring as both mm-sized grains and larger cm-sized megacrysts that enclose other smaller grains such as olivine and plagioclase (Irving et al., 2011; Warren et al., 2013; Hibiya et al., 2018). The Fe-rich olivine grains (~14 vol%) are likely xenocrysts similar to those present in some quenched angrites (Hibiya et al., 2014). These olivine grains contain relatively high NiO (ave. 0.9 wt%). Other components include highly sodic plagioclase (~10 vol%, as albite), pigeonite (~7 vol%, as rims on orthopyroxene megacrysts), FeNi-metal (~0.3 vol%, as awaruite), and chromite (~0.15 vol%), along with trace amounts of merrillite and sulfides (heazlewoodite and pentlandite). Plagioclase forms a continuous 3-D framework within the orthopyroxene megacrysts, and is inferred to have crystallized in situ from an undercooled melt. The plagioclase contains tiny grains of the Ni-rich (~80 wt%) metal awaruite. Curvilinear trains of micro-inclusions (mostly oxides) along with empty, smooth-walled bubbles are present in the orthopyroxene megacrysts, reflecting shock mobilization/injection (Irving et al., 2011; Warren et al., 2013; high-res image from Hibiya et al., 2018). Local compositional and textural variation, including an olivine-rich enclave, was observed in the NWA 6693 meteorite.
This meteorite is unique because of its great abundance of highly-ferroan mafic silicates (low-Ca pyroxene: Mg# = 5760; olivine: Fa5053), its highly sodic plagioclase (Ab9193), and its Ni-rich phases (olivine: ave. 0.9 wt% NiO; metal: ave. 81 wt% NiO; and sulfide: ~0.002 wt% NiO). These Ni-rich phases are indicative of formation in a highly oxidizing environment (FMQ 2.6). Consistent with oxidation conditions is the fact that the bulk rock and siderophile element compositions are nearly chondritic, although the meteorite does show depletions of S and highly volatile elements. Elemental ratios (Mg/Si = 0.48 by wt.) indicate that very little fractionation has occurred in the parent melt, and the compositional trends exclude this rock from being a refractory residue (restite) of a partial melt (Warren et al., 2011; 2013). The FeO/MnO ratios for low-Ca pyroxene in this meteorite (81106; Irving et al., 2011) are distinctly higher than those of both HED and martian meteorites, and it was concluded that the parental source materal for the NWA 6704 pairing group was extrememly oxidized (FeO-rich).
Iizuka et al. (2013) report high abundances of highly siderophile elements (HSE) for this meteorite similar to those of some brachinite-like achondrites such as GRA 06128/9 and Zag (b). In their study of HSE abundances and Os-isotopic systematics in brachinites and other FeO-rich brachinite-like achondrites, Day and Warren (2015) found that NWA 6693 has a high Pt/Os ratio indicative of a cumulate derived from a low-degree partial melt infused with a residual metal melt component. In addition, they reasoned that the variability observed in the Pd/Os ratios of NWA 6693 and other brachinite-like achondrites reflects variable degrees of fractionation of such a metal melt prior to its incorporation. The high abundance of HSEs was attributed by Warren et al. (2013) to the highly oxidizing formation conditions which inhibited sequestration and removal of HSEs by FeNi-metal during differentiation. In a study of NWA 6704 conducted by Archer et al. (2016), it was determined that the bulk HSE abundances are nearly chondritic and exhibit only a low degree of fractionation, consistent with a scenario in which only minor, localized silicate partial melting occurred with some loss of sulfides. Their results indicate that core formation had not proceeded to any great extent on the parent body by the time the meteorite was formed. It was further argued that the NWA 6704 chondritic precursor lithology was instantaneously melted during an impact event, and then rapidly cooled at ~1100°C/hr under super-saturation conditions which promoted coarse dendritic growth of orthopyroxene (Hibiya et al., 2018). As temperatures fell below ~1100°C cooling proceeded much more slowly, consistent with burial ~100 m deep under an ejecta blanket (Hibiya et al., 2017, 2018).
Separate O-isotopic analyses were conducted for NWA 6704 and NWA 6693 initially at Okayama University in Japan, and at Seoul National University in the Republic of Korea, respectively. It was ascertained by both institutions that this ungrouped achondrite plots within the field of the acapulcoitelodranite clan (see oxygen three-isotope plot). However, mafic silicates in these paired meteorites are significantly more ferroan and the feldspar significantly more sodic than members of the acapulcoitelodranite clan. While this diagnostic method suggests a connection might exist with the acapulcoitelodranite clan, its O-isotopic composition also plots within the CR/CB/CH field, presenting yet another possibility for a connection (see oxygen three-isotope plot). The O-isotopic composition of NWA 6704 also plots close to the ungrouped LEW 88763, and although their mineralogy is completely different it is notable that they have complementary HSE patterns (Day and Warren, 2015). Notably, new analyses of LEW 88763 by Day et al. (2015) led them to propose its reclassification as an anomalous achondrite, possibly related to NWA 6704 and pairings. A diagram depicting the O-isotopic plot for NWA 6704 and NWA 6693, the acapulcoitelodranite clan, and brachinites is shown below courtesy of Achim Raphael.
Northwest Africa 6704 Oxygen isotopes (R. Tanaka, OkaU): replicate
analyses by laser-fluorination produced δ17O =
1.015, 0.880; δ18O = 3.922, 3.613; Δ17O = -1.048, -1.020
Northwest Africa 6693 Oxygen isotopes (B-G. Choi and I. Ahn, Seoul-NU): replicate analyses by laser-fluorination produced δ17O = 1.19; δ18O = 4.32; Δ17O = -1.08
Additional constraints on the origin of this meteorite were established through studies of the Cr-isotopic systematics (Sanborn et al., 2013). The resulting ε54Cr value of +1.69 (±0.07) resolves NWA 6704 from the acapulcoitelodranite clan (ε54Cr = 0.75; Göpel and Birck, 2010); discrimination between these groups had not been attained through the use of O-isotopic values alone. Continued efforts to better resolve the relationship that exists among the numerous anomalous meteorites has been ongoing (e.g., Bunch et al., 2005, [#2308]; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites. The diagrams below include the paired stones NWA 6704 and 6693, and it is apparent that they plot within the CR chondrite field.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)
However, a plot of Δ17O vs. olivine Mg# (Mg/[Mg+Fe]) resolves the NWA 6704 pairing group from other meteorite groups as well as ungrouped meteorites with similar O-isotopic compositions, as it displays a significantly more ferroan composition (Warren et al., 2013). Further resolution of the parental source group for NWA 6704 was obtained by Hibiya et al. (2017, 2018) through a titanium isotope analysis. It is demonstrated on both Δ17O vs. ε50Ti and ε54Cr vs. ε50Ti coupled diagrams that the meteorite plots within the field for carbonaceous chondrites (see diagrams below). Based on the results of their petrologic, geochemical, and isotopic analyses of NWA 6704, Hibiya et al. (2017, 2018) concluded that an undifferentiated asteroid experienced an impact-generated melting event followed by rapid cooling. Presumably, subsequent burial beneath an ejecta blanket ushered an extended period of slow cooling.
Diagram credit: Hibiya et al., 48th LPSC, #1317 (2017)
An ArAr and noble gas study was conducted by Fernandes et al. (2013) to constrain the petrologic history and to determine the CRE age of this meteorite. The oldest possible crystallization age was determined to be 4.56 (±0.29) b.y., or 45 m.y. after CAIs. This is in agreement with the UPb age of 4.56280 (±0.00046) b.y. determined by Iizuka et al. (2013), and the most precise and accurate age of 4.56238 (±0.00049) b.y. determined by P. Koefoed (2017). A refined PbPb isochron age of 4.56278 (±0.00018) b.y. was calculated by combining data obtained from both the Australian National University and UC Davis (Huyskens et al. (2017). In a subsequent investigation, Amelin et al. (2019) employed the 238U/235U value directly measured for NWA 6704 of 137.7784 to calculate the most precise and accurate PbPb isochron age of 4.56276 (+0.00022/-0.00030) b.y. In a similar manner, the best age estimate for NWA 6693 was determined to be 4.56263 (+0.00029/-0.00021) b.y. Other isotopic chronometers (e.g., AlMg, MnCr [Sanborn et al., 2019], RbSr, ReOs) give ages that are consistent with the PbPb age and attest to rapid cooling, crystallization, and isotopic closure. Based on initial Sr chronometry data, Amelin et al. (2019) determined that accretion of the NWA 6693/6704 parent body occurred within 3.6 m.y. of the formation of CAIs.
The occurrence of an impact-resetting event prior to complete cooling of the planetesimal is indicated by the loss of fluid from micro-inclusion bubble trains present in orthopyroxene crystals. This event may be related to a resetting event that occurred 4.199 (±0.032) b.y. ago as revealed by Amelin et al. (2018) through ArAr chronometry. In addition, they provided further evidence of another thermal event that occurred ≤2.12 b.y. ago. The CRE age was calculated based on Ne and Ar systematics to be 30 (±3) m.y. (Fernandes et al., 2013). Furthermore, the pre-atmospheric size of the meteoroid was calculated to have been at least 100 cm in diameter.
Further studies have been conducted by Sanborn et al. (2018) of new anomalous ungrouped meteorites recovered in Northwest Africa. Utilizing a coupled Δ17O vs. ε54Cr diagram, they demonstrated that NWA 6704 and pairings, NWA 011 and pairings, and NWA 6962/7680 all plot within the CR/CH carbonaceous chondrite field represented by CR2 Renazzo and CH3 NWA 2210, which suggests a possible genetic linkage among them (see diagram below).
Chromium vs. Oxygen-isotope Plot
click on image for a magnified view
Diagram credit: Sanborn et al., 49th LPSC, #2296 (2018)
The reflectance spectra of NWA 6704 was acquired and compared to that of known asteroid groups (Le Corre et al., 2014). The analysis of a large-sized sample was comparable to spectra from the S(VI) asteroid group, which is thought to represent formation as a partial melt residue (e.g., winonaites). The spectra of a smaller, grain-sized sample plotted between the S(V) and S(VI) asteroid fields, the former thought to represent formation as a metamorphosed H chondrite or a primitive achondrite (e.g., acapulcoites/lodranites). However, direct spectral comparisons of NWA 6704 to known asteroids only produced a relatively close match to V-type asteroids (e.g., Vesta). Moreover, direct spectral comparisons of NWA 6704 to HED suite meteorites demonstrated a close similarity to the basaltic and cumulate eucrites as well.
A separate 940 g stone designated NWA 10132 was found at a location different from the NWA 6704 pairing group, but it has a similar mineralogy, an almost identical O-isotopic composition, and a matching UPb age (Irving et al., 2015; Koefoed et al., 2015). Based on these facts, NWA 10132 is thought to be a possible genetic relative, likely co-magmatic and/or launch paired. Consistent with this hypothesis, Y. Amelin (2017) determined that the RbSr data for NWA 10132, NWA 6693, and NWA 6704 are similar and establish an isochron age of 4.543 (±0.035) b.y. In addition, the ε54Cr value for NWA 10132 determined by Sanborn et al. (2018) matches that of the NWA 6704 pairing group. Moreover, they calculated an age for NWA 10132 based on MnCr systematics and anchored to D'Orbigny of 4.56276 (±0.00041) b.y., and an age based on UPb systematics of 4.56271 (±0.00019) b.y., both of which are concordant with the ages previously calculated for the NWA 6704 pairing group. Employing the 238U/235U value of 137.7784 (Huyskens et al.), a similar PbPb isochron age of 4.56212 (±0.00067) b.y. was calculated for NWA 10132 by P. Koefoed (2017).
The features of this unique meteorite pairing group were found to be consistent with a low weathering grade, and it has been described as generally unshocked (S1) after significant annealing (Warren et al., 2013). The specimen of NWA 6704 framed above is a 1.56 g fragment.