NORTHWEST AFRICA 11577


Iron, IAB complex, sHL subgroup
standby for nwa 11577 photo
Found December 2016
no coordinates recorded

Two iron masses with a combined weight of 86 g were found in Algeria by nomads. The mass was sold to a Moroccan merchant in Ouarzazate who sent a sample to the University of Washington in Seattle (A. Irving) for study; based on a preliminary examination, the iron was determined to be a meteorite. The main mass was subsequently purchased by J. Shea, and the initial sample was sent to the University of Alberta (C. Herd) for an in-depth analysis and official classification. Ultimately, NWA 11577 was determined to be a new member of the rare sHL (high-Au, low-Ni) subgroup of the IAB iron complex as defined by Wasson and Kallemeyn (2002). Upon etching, NWA 11577 exhibits a Thomson (Widmanstätten) structure of finest octahedrite (Off) texture due to its Ni content of 12.5 wt%.

In an investigation of the Mo and W isotope compositions for the IAB complex, Worsham et al. (2017) determined that the Mo isotopic composition of the sHL and sHH subgroups are identical, but differ from that of the sLL, sLM, and sLH subgroups. Moreover, they calculated the formation ages of these irons based on W isotopes and determined that the sHL and sHH subgroups experienced metal–silicate segregation 0.3–1.6 m.y. after CAIs, which is significantly earlier than the MG, sLL, and sLM subgroups, and before the short-lived radiogenic isotope 26Al had become extinct (half-life of 700 t.y.). From this study they concluded that the MG and the sLL, sLM, and sLH subgroups formed in distinct impact-melt pools on at least two distinct asteroids, whereas the sHL and sHH subgroups formed by fractional crystallization associated with core formation resulting from internal heating on one or more separate parent bodies (see schematic diagram below).

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Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 164 (2017)
'Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex'
(https://doi.org/10.1016/j.epsl.2017.02.044)

Employing precise Mo, W, and Os isotope data along with HSE and other literature data, Worsham et al. (2017) ascertained that the two sHL irons in the study (Chebankol and Quarat al Hanish) have indistinguishable µ97Mo isotopic compositions with an average value of 23 (±9). They also demonstrated on a coupled µ97Mo vs. µ189Os diagram that the sHL and sHH subgroups formed in a common reservoir in a spatial and/or temporal aspect. While this reservoir was separated from the other IAB complex irons, all of these irons accreted in the inner region of the protoplanetary cloud. In a broader view, two distinct reservoirs existed in the early protoplanetary disk—carbonaceous chondrite (CC) and non-carbonaceous (NC). These reservoirs were segregated by the rapid accretion of proto-Jupiter and reflect differences in the contribution (i.e., susceptibility to thermal processing) of p-, r-, and s-process isotopes inherited as dust ejecta from explosive stellar nucleosynthesis (Poole et al., 2017; Bermingham et al., 2018). See diagrams below, where µ notation denotes deviation from terrestrial standards in parts per million.

CRE-corrected Mo Isotope Compositions of Meteorite Groups
(µ notation denotes deviation from terrestrial standards in parts per million)
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Diagram credit: Worsham et al., Earth and Planetary Science Letters, vol. 467, p. 165 (2017)
'Characterizing cosmochemical materials with genetic affinities to the Earth: Genetic and chronological diversity within the IAB iron meteorite complex'
(https://doi.org/10.1016/j.epsl.2017.02.044)

In another analysis of siderophile elements in IAB irons, Worsham et al. (2016) revealed that the two sHL subgroup irons included in their study, Chebankol and Quarat al Hanish, have HSE patterns broadly similar to late-crystallized IIAB and IIIAB magmatic irons (see diagram below, where Grant (IIIB) is a dashed line and Thurlow (IIIB) is a dash-dotted line). However, based on the dissimilar Pd, Ni, Au, and Ga abundances in these two sHL irons, they concluded that each probably derives from separate parental source melts. It has been demonstrated through Mo- and Os-isotopic analyses that the ungrouped iron Sombrerete, once considered a tentative member of the sHL subgroup, actually formed in the carbonaceous reservoir closely allied with group IVB irons (Worsham et al., 2017).

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Diagram credit: Worsham et al., GCA, vol. 188, p. 268 (2016)
'Siderophile element systematics of IAB complex iron meteorites: New insights into the formation of an enigmatic group'
(https://doi.org/10.1016/j.gca.2016.05.019)

To learn more about the relationships that exist among irons of the IAB complex, as well as those for other iron chemical groups, see the Appendix, Part III. The photo shown above is a 13.5 g partial slice of NWA 11577, with both sides beautifully photographed by John Shea. Below is a photo of the two masses of this rare iron.

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Photo courtesy of John Shea—Big John Meteorites
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