Chondrite, ungrouped (type 5+)
previously Winonaite (primitive) or 'W Chondrite'
originally Acapulcoite (MetBull 85)
Found July 4, 2000
30° 36' N., 5° 3' E.
Eleven pieces of a Moroccan meteorite, having a combined weight of 3,824 g (total weight of the entire pairing group is ~5.1 kg), were recovered by a French team under the organization of Bruno Fectay and Carine 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.
*Previously, Floss (2000) and Patzer et al. (2003) proposed that the acapulcoite/lodranite meteorites should be divided based on metamorphic stage:
primitive acapulcoites: near-chondritic (Se >1213 ppm [degree of sulfide extraction])
typical acapulcoites: FeNiFeS melting and some loss of sulfide (Se ~512 ppm)
transitional acapulcoites: sulfide depletion and some loss of plagioclase (Se <5 ppm)
lodranites: sulfide, metal, and plagioclase depletion (K <200 ppm [degree of plagioclase extraction])
enriched acapulcoites (addition of feldspar-rich melt component)
A similar distinction could be made among the winonaites in our collections, although there is not yet an analog of the IAB complex irons for the acapulcoite/lodranite PB. Northwest Africa 1463 (and pairing group) ranks as the most primitive member of the winonaites, containing intact chondrules comparable to a petrologic type 5 chondrite (Benedix et al., 2003). However, most winonaites experienced extensive thermal metamorphism involving incipient sulfide melting and exhibit highly recrystallized textures, characteristics analogous to the "typical" acapulcoites. Metamorphic progression in other winonaites led to partial loss of the low-melting phases FeS and plagioclase, and these are designated as a "transitional" stage in the acapulcoite/lodranite metamorphic continuum. Those winonaites which experienced the highest temperatures ultimately crystallized from residual melt material, and they exhibit significant depletions in FeS, FeNi-metal, and plagioclase (including plagiophile trace elements). Samples representing this advanced metamorphic stage are known as lodranites in the acapulcoite/lodranite metamorphic sequence, while the term "evolved" could be used to represent a similar metamorphic stage in the winonaite group (e.g., Tierra Blanca; Hunt et al., 2017).
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 winonaiteIAB 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) and 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 can thus be made that NWA 725 is actually a winonaite, likely paired with the primitive winonaites NWA 1463 and 1058 (NWA 1052 and NWA 1054 are also possible members of this pairing group; see Irving and Rumble III, 69th MetSoc, #5288 ).
Northwest Africa 725 and pairings contain relict intact chondrules, and accordingly it was grouped among the most primitive winonaites with 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 isotope data for IAB silicate inclusions, along with observed volatile element depletions, led to the inferrence that the winonaite precursor had a volatile-depleted carbonaceous chondrite-like composition (Hunt et al., 2012). It has been demonstrated that two distinct reservoirs existed in the early protoplanetary diskcarbonaceous chondrite (CC) and non-carbonaceous (NC). These were segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution of p-, r-, and s-process material (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 Appendix, Part III for further details). Preliminary data based on AlMg chronometry show that NWA 725 was formed ~1.4 m.y. after CAIs. This age is consistent with HfW ages in the range of 1.55 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 metalsilicate segregation events (see top schematic diagram below). Moreover, they ascertained that the Mo isotope data, as well as the chemical and mineralogical data, attest to a common parent body for the winonaites and the MG/sLL irons. Importantly, they demonstrated that the Mo isotope values of 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 isotope values of 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 isotope data for the sHL and sHH irons could help resolve whether any potential genetic relationship exists with the NWA 725 pairing group.
CRE-corrected Mo Isotopic Compositions of Meteorite Groups
(µ notation denotes deviation from terrestrial standards in parts per million)
click on photo for a magnified view
Diagrams credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, pp. 157166 (2017)
'Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex'
Schmitz et al. (2016) conducted a Cr-isotopic analysis on a xenolithic inclusion from the L6 chondrite Villalbeto de la Peña, and 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.
Northwest Africa 725 Chromium vs. Oxygen Isotope Plot
click on diagram for a magnified view
Diagram credit: Schmitz, B. et al., Nature Communications, vol. 7, p. 4 (2016, open accesslink) 'A new type of solar-system material recovered from Ordovician marine limestone' (https://doi.org/10.1038/ncomms11851)'
Further information about the classification of this meteorite and its probable pairings can be found on the NWA 1058 page. The NWA 725 specimen pictured above is a 1.3 g partial slice showing many distinct, intact chondrules. The photo below is an excellent petrographic thin section micrograph of NWA 725, shown courtesy of Peter Marmet.
click on photo for a magnified view
Photo courtesy of Peter Marmet