NWA 1058

Chondrite, ungrouped (type 5+)
previously Achondrite, ungrouped
originally Primitive Achondrite (MetBull 87)

standby for northwest africa 1058 photo
Purchased 2001
no coordinates recorded A 180 g stone was purchased in Erfoud, Morocco and subsequently sold to a meteorite dealer. The meteorite was submitted to the Institut für Planetologie in Münster and given the designation NWA 1058. Although initially considered to be an acapulcoite, NWA 1058 was classified in a broad terminology as a primitive achondrite by Russell et al. (2003) due to its anomalous O-isotopic composition. On an O-isotope plot it falls on the terrestrial fractionation line similar to the winonaites, but with O-isotopes that are more 16O-rich than most winonaites.

Further petrographic and compositional analyses have determined that NWA 1058 is very similar to the primitive acapulcoite GRA 98028 (Patzer et al., 2004), and that the anomalous O-isotopic composition might actually be due to terrestrial weathering effects (W2–3). A high abundance of relict chondrules are observed in NWA 1058, which is also a feature of some primitive acapulcoites. However, while most acapulcoites have similar CRE ages of ~6 m.y. (16.8 m.y. for TIL 99002), that calculated for NWA 1058 is significantly higher at 38.2 m.y. (Patzer et al., 2003). Nevertheless, the noble gas abundances of NWA 1058 remain high with 36Ar/132Xe ratios that are similar to the ‘Q’ component, characteristics which are more consistent with acapulcoites.

A study was undertaken by Eugster and Lorenzetti (2005) in which they determined a possible structure for the acapulcoite parent body. They specifically analyzed a number of new acapulcoites, as well as NWA 1058. They found that the data on the whole were more consistent with NWA 1058 being a winonaite rather than an acapulcoite: 1) NWA 1058 is more Mg-poor and Fe-rich than any acapulcoite; 2) its O-isotope composition plots outside the acapulcoite field; 3) its CRE age is much higher than any other acapulcoite (38.9 ±4.0 m.y.), with all except one acapulcoite having almost identical CRE ages of ~6 m.y.

Therefore, this meteorite appears to be neither a typical winonaite nor a typical acapulcoite. Utilizing a diagram that compares the Δ17O to the Fa content of olivine, Rumble III et al (2005) found that the winonaites and the acapulcoite–lodranite clan meteorites were readily resolved, and that NWA 1058 plots within the winonaite field. In addition, Moggi-Cecchi et al (2011) published a diagram (42nd LPSC, #1398 [2011]) that plots the Cr content of diopside vs. Mg# of olivine, and the acapulcoite–lodranite clan is clearly distinguished from the winonaite group.

standby for aca/lod/win diagram photo

Moggi-Cecchi et al (2011) also published a diagram showing the reduction state of the samples (Fe# in orthopyroxene vs. Mg# of olivine), and it distinguishes three separate clusters: the acapulcoites, lodranites, and winonaites. To that end, Irving et al. (2005) described NWA 1058 as a metamorphosed chondrite probably representing the regolith of the winonaite parent body. Furthermore, they argued that the occurrence of distinct chondrules precludes the use of the term achondrite to describe this meteorite, and suggest that the term metachondrite or ‘W chondrite’ would be a more appropriate term to describe this texturally-evolved meteorite pairing group (Irving et al., 2005; Irving and Rumble III, 69th MetSoc #5288 [2006]).

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 (considered a pairing to NWA 1058), 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 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. standby for iab iron formation diagram

CRE-corrected Mo Isotopic Compositions of Meteorite Groups
(µ notation denotes deviation from terrestrial standards in parts per million)
standby for chinga mo diagram
click on photo for a magnified view

Diagrams credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, pp. 157–166 (2017)
‘Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex’
There is convincing evidence that NWA 1058 is paired with NWA 725, NWA 1052, NWA 1054, and NWA 1463 (Irving and Rumble III, 2006); a further pairing was found in 2007 and designated NWA 4835 (T. Bunch, NAU). The partial slice of NWA 1058 shown above weighs 0.9 g.

Leave a Reply