NORTHWEST AFRICA 725

Eleven pieces of a Moroccan meteorite having a combined weight of 3,824 g were recovered by a French team under the organization of B. Fectay and C. Bidaut. Although the coordinates of the recovery location in Tissemoumine, Morocco were recorded by GPS equipment, a NWA-series designation was accepted as the name for this meteorite by the Meteoritical Society NomCom. This primitive meteorite is of great interest due to its abundance of relict chondrules.

Although NWA 725 was initially classified as an acapulcoite, an O-isotopic analysis of a portion of the main mass (from the collection of S. Turecki) by the Open University, UK resolves the material within the winonaite field: "In particular, the Δ17O value of –0.431 is in reasonable agreement with the mean value of –0.48 for the winonaite–IAB complex group determined by Clayton and Mayeda (1996)." In addition, when plotted on a diagram comparing Δ17O vs. Fa mol% in olivine (Rumble III et al, 2005), NWA 725 (Fa6.1) with the winonaites NWA 1463 (Fa7.4; Δ17O = –0.45‰), NWA 1457 (Fa5; Δ17O = –0.40 [±0.03]‰), and NWA 1058 (Fa6.5; Δ17O = –0.53‰), all plot very close together within the winonaite field. The inference could thus be made that NWA 725 is actually a winonaite, considered by Irving and Rumble III (2006 #5288) to be likely paired with the primitive winonaites NWA 1463 and 1058 (NWA 1052 and NWA 1054 may also be members of this pairing group).

Northwest Africa 725 and possible pairings contain relict intact chondrules, and accordingly it was grouped among the most primitive winonaites with a near chondritic composition. However, the meteorite has experienced extensive heating and possibly a low degree of partial melting resulting in a depletion of certain trace elements. The features of this meteorite suggest that winonaites are actually metamorphosed chondrites. It was suggested by Irving et al. (2005) that the term 'metachondrites' be used for such metamorphosed chondrites, including several newly recognized chondrule-free, texturally evolved meteorites with elemental ratios and O-isotopic compositions showing affinities to existing chondrite groups. It was also suggested that the term 'W-chondrites' be used for those winonaites like NWA 725 that contain relict chondrules.

Oxygen isotopic data for IAB silicate inclusions, along with observed volatile element depletions, led Hunt et al. (2012) to infer that the winonaite precursor had a volatile-depleted carbonaceous chondrite-like composition. However, analyses of metal in the primitive winonaites NWA 725 and Y-8005, as well as in the primitive acapulcoite Y-74063 studied by Hidaka et al. (2019), suggest instead that the precursor material to these meteorites was compositionally similar to H and EL chondrites; however, both groups derive from otherwise unsampled parent bodies with unique O-isotopic compositions.

It has been demonstrated that two distinct reservoirs existed in the early protoplanetary disk—carbonaceous chondrite (CC) and non-carbonaceous (NC). These reservoirs were likely segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution (i.e., susceptibility to thermal processing) of proton (p-) process, rapid neutron capture (r-) process, and slow neutron capture (s-) process isotopes inherited as dust ejecta from explosive stellar nucleosynthesis (Poole et al., 2017; Bermingham et al., 2018). Subsequent studies employing Mo and W isotope data (e.g., Kruijer et al., 2017) reveal that the IAB complex irons, and thus the genetically-related winonaites, accreted in the non-carbonaceous reservoir (see the Protoplanetary Disk page for further details). Preliminary data based on Al–Mg chronometry show that NWA 725 was formed ~1.4 m.y. after CAIs. This age is consistent with Hf–W ages in the range of 1.5–5 m.y. that were calculated for more highly metamorphosed winonaites (Hidaka et al., 2014).

New analyses were conducted by Worsham et al. (2017) for IAB complex irons along with two winonaites (Winona and HaH 193), a lodranite (GRA 95209), the primitive achondrite NWA 725, and other selected meteorite groups. Employing precise Mo, W, and Os isotope data along with HSE and other literature data, they ascertained that the IAB complex irons represent at least three distinct parent bodies and at least three impact-generated metal–silicate segregation events (see top schematic diagram below). Moreover, they determined that the Mo isotope data and the chemical and mineralogical data attest to a common parent body for the winonaites and the MG/sLL irons. Analysis of metal in the winonaite Y-8005 was shown by Hidaka et al. (2019) to be consistent with that of IAB sLL subgroup irons, which supports such a genetic relationship. Contrariwise, Zhang et al. (2023 #1174) showed that HSE abundances for Winona metal and sulfides are not similar to those of IAB main group irons (Worsham et al., 2016), but instead are similar to those of the sLM subgroup irons (see diagram below).

Importantly, Worsham et al. (2017) demonstrated that the Mo isotope values for NWA 725 do not plot with the IAB MG/sLL/winonaites, and that the values are all higher than those of the lodranite in their study. Notably, the Mo-isotopic values for NWA 725 plot within the field of the magmatic sHL and sHH irons, which are not genetically related to the other IAB parent bodies (see bottom diagram below). Oxygen isotopic data for the sHL and sHH irons could help resolve whether any potential genetic relationship exists with NWA 725.

In their chemical composition analysis of metal in primitive achondrites, which included the two winonaites NWA 725 and Y-8005, Hidaka et al. (2019) demonstrated that these two meteorites have significant differences in abundances of the moderately volatile siderophile elements Cu, Sb, Ga, Ge, and Sn. Moreover, utilizing coupled diagrams for various elemental abundance ratios in metal, they demonstrated that NWA 725 does not plot together with Y-8005 in the sLL subgroup field, but rather it generally plots in a unique space between the sLL and the overlapping sHL and sLM fields. It is notable that in the Ga/Ge coupled diagram NWA 725 plots at the very border of the sHL and sLM fields (see diagrams below). It was concluded that the metal in NWA 725 and Y-8005 had separate origins, and that NWA 725 possibly represents a reassembled daughter body that was contaminated by metal from the projectile that disrupted the IAB parent body.

Schmitz et al. (2016) conducted a Cr-isotopic analysis on a xenolithic inclusion from the L6 chondrite Villalbeto de la Peña, as well as on chrome-spinel grains from both the winonaite NWA 725 and the L6 chondrite Lundsgard. They demonstrated through a coupled Δ17O vs. ε54Cr diagram (shown below) that the Villalbeto de la Peña clast plots with NWA 725 in the winonaite field.

It was previously considered that NWA 725 is likely paired with NWA 1052, NWA 1054, NWA 1058 and NWA 1463 (Irving and Rumble III, 2006); a further possible pairing was found in 2007 and designated NWA 4835 (T. Bunch, NAU). However, new analyses by Stephant et al. (2023) indicate that only NWA 1052 and NWA 1054 are fall paired, and that based on variability in their modal abundances (see Fig. 3) the other samples represent distinct ejection events from a common parent body.

Stephant et al. (2023) investigated NWA 725 and seven very similar meteorites (Dhofar 1222, NWA 090, NWA 1052, NWA 1054, NWA 1058, NWA 1463, and NWA 8614) using newly acquired data for NWA 090 and literature data for each of the other meteorites. The authors went to great depths with regard to the petrography, mineralogy, isotopy, geothermometry, and especially the geochemistry of these samples in an effort to resolve the subtle differences that exist between them and the winonaite and acapulcoite groups. They were able to show through various means, but especially utilizing an oxygen three-isotope diagram and a ternary diagram combining olivine fayalite content, olivine FeO/MnO ratio, and plagioclase Or content, that the tissemouminite group is resolved from the winonaite and acapulcoite groups (see diagrams below).

Other potential members of the new tissemouminite group (e.g., NWA 4816 [ACA], NWA 4937 [WIN], NWA 6448 [WIN], SuV 014 [ACA], Y-983237 [ACA]) share some of the same characteristics as the eight known members, but each will need further work to distinguish to which of the three groups it belongs. The NWA 725 specimen pictured above is a 1.3 g partial slice showing many distinct chondrules, while that below is an excellent petrographic thin section micrograph of NWA 725 shown courtesy of Peter Marmet.