NWA 3132

standby for northwest africa 3132 photo
no coordinates recorded A 125.39 g meteorite was found in the Sahara Desert and later purchased by American collector N. Oakes. A sample was submitted to Arizona State University, Center for Meteorite Studies (G. Huss), and NWA 3132 was determined to be a rare EL3 chondrite composed almost entirely of orthopyroxene. Although almost no metal or sulfide is present, vestigial signs of metal can be seen throughout the meteorite. This enstatite chondrite has a shock stage of S2 and a weathering grade of W4. An unequilibrated petrologic type 3 sample is quite rare among EL-group members, and is thus far represented primarily by Antarctic meteorites including ALH 85119, A-881314, A-882067, EET 90299/90992, MAC 88136 pairing group, MAC 02635, MAC 02837/02839, QUE 93351 pairing group, LAP 03930, and PCA 91020, along with EL3 xenoliths identified in the carbonaceous chondrite Kaidun (designated Kaidun IV) and in the ureilite Almahata Sitta.

With the exception of the transitional EH/L chondrite Y-793225, E chondrites have been historically assigned to one of two distinct groups—a high-Fe, high-siderophile (EH) group, and a low-Fe, low-siderophile (EL) group. Surprisingly, it has been demonstrated by Macke et al. (2009) that these two groups do not actually differ in their iron content, and that they are indistinguishable in density, porosity, and magnetic susceptibility as well. However, differences in siderophile, chalcophile, and other mineralogical abundances can be employed to distinguish the two groups. From comparisons of elemental abundance ratios between the EH and EL groups, it has been demonstrated that values for all elements except the refractory siderophiles are consistently lower in the EL group than in the EH group. Certain elemental ratios (e.g. La/Sm, Sb/Ir) easily resolve the two groups. Furthermore, the following mineralogical relationships are diagnostic of their distinct parent bodies:

  • EH group has a higher Si content in kamacite (EH: 1.9–3.8 wt%; EL: 0.3–2.1 wt%)
  • EH group has a lower Mn content in daubreelite (EH: 0.4–1.1%; EL: 1.4–4.0%)
  • EH group has a lower Ni content in schreibersite (EH: <20 wt%; EL: >20 wt%)
  • EH group has a lower Ti content in troilite (EH: <4.8 wt%; EL: >5.5 wt%)
  • EH group has a lower An content in plagioclase (EH: <3 mol%; EL: 13–17 mol%)
  • EH group sulfides are enriched in alkali elements (e.g., Na in caswellsilverite, K in djerfisherite), and chondrule mesostasis is enriched in Na relative to EL group
  • EH group chondrites contain niningerite [(Mg,Fe)S] or keilite [(Fe,Mg)S], the Mg-rich end member of the monosulfide series having the formula [(Mg,Mn,Fe)S]; EL group chondrites contain the Mn-rich end member alabandite [(Mn,Fe)S]
  • EH group chondrites have higher abundances of the siderophile elements Ni, Fe, Au, and Co
  • EH group chondrites contain an average of 15 times the abundance of the volatile element Zn
  • Other good discriminators are Ga, As, Se, and Sb, each of which are found in greater abundances in EH group chondrites

In addition, both Fe- and Zn-isotope compositions are fractionated to different degrees between EL and EH chondrites; EL chondrites are isotopically heavier than EH chondrites, indicating EL chondrites experienced higher volatilization due to its formation closer to the Sun (Mullane et al., 2005), or alternatively, due to elemental fractionation during impact-shock events (Rubin et al., 2009). Studies into the origins of EL chondrites conducted by Goresy et al. (2012) determined that petrologic evidence, including the occurrence within FeNi-metal nodules of repeated sinoite–graphite condensation events associated with oldhamite (CaS) in the sequence CaS ⇒ sinoite ⇒ graphite, was indicative of a nebular condensate origin for these chondrites rather than their formation as an impact-melt breccia of preexisting proto-asteroids. Still, later incidences of impact-shock for EL chondrites are attested by the higher prevalence of impact-melt breccias among the more metamorphosed members, as well as by the occurrence of sinoite crystallized from a melt.

Other comparisons demonstrate that EH group chondrites have smaller average chondrule diameters (220 µm vs. 550 µm; Rubin et al., 2000) and smaller average metal diameters than EL-group members as shown in the table below:

Comparison of EH and EL Chondrule and Metal Diameters
Chondrule Diameter 0.045–1.313 mm
(EH3 average 0.278 ±0.229 mm)
0.085–2.125 mm
(EL3 average 0.476 ±0.357 mm)
Metal Diameter 0.008–0.492 mm 0.002–1.107 mm

The disparity in the size and Na content of chondrules within the EH and EL groups can be reconciled by several possible scenarios, including the one proposed by Schneider et al., 2002:

Chondrules from both groups were formed from similar precursor material in the same nebular region. During accretion, the chondrules underwent a size sorting process induced by volatile flows within the regolith, or alternatively, by the abundance of dust in the prospective accretion regions and by the number of chondrule remelting episodes (Rubin, 2010). Photophoresis, utilizing pressure and particle-size dependence, was also a likely size sorting mechanism of chondrules (Hesse et al.,2011). This resulted in the larger EL chondrules becoming more deeply buried than the smaller EH chondrules on their respective parent bodies. It has been reported that EL3 chondrites usually exhibit a preferred orientation of chondrules and other constituents. The deep burial conditions would support such a foliation as the result of continued impact deformation processes. After burial, lithification of the chondrules into bulk rock was quickly achieved. The shallower EH material experienced more rapid cooling, and thus retained more of its volatile component such as Na, while volatiles were lost during a more extended cooling period in the more deeply buried EL material. In an attempt to model the precursor material of Earth, it was calculated that approximately 55% of the precursor component could have been of EL chondrite composition, which is the meteoritic material that provides the best match to the Earth in O-isotope composition, bulk Fe/Al weight ratio, and bulk FeO concentration (T. Burbine and K. O’Brien, 2004). However, it was not possible to model the Earth’s precursor based solely on known chondrites. Alternatively, the Earth could have been formed from chondritic material that subsequently underwent differentiation and loss of a basaltic component, or that had a significant Si component sequestered into the core or lower mantle.

Based on isotopic studies, the meteorites of the EL3 group, as opposed to the EL6 group, are thought to be the best candidates for the building blocks of Earth (Boyet et al., 2018). Further information on the classification and petrogenesis of the E chondrites can be found on the Saint-Sauveur page. The specimen of NWA 3132 shown above is a 0.8 g partial slice. The photo below shows the main mass. standby for northwest africa 3132 photo
Photo courtesy of Nelson Oakes—Meteorites–R–Us

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