NORTHWEST AFRICA 1296


Angrite
Basaltic/Quenched
standby for nwa 1296 photo
Found spring 2001
no coordinates recorded

A single 810 g stone was found in the Moroccan Sahara and subsequently sold to a dealer in Bouarfa, Morocco. The dark-gray, shiny fusion crust is very thin but well-preserved, suggesting a relatively recent fall. Northwest Africa 1296 was analyzed at multiple laboratories in France (Université d'Angers and Université Pierre & Marie Curie) and determined to be a member of the rare angrite group (Jambon et al., 2002).

An account of the mineralogy and petrology of NWA 1296 was published by Jambon et al. (2005). The major minerals in NWA 1296 include anorthitic plagioclase (32 vol%), strongly zoned grains of the clinopyroxene Al,Ti–diopside-hedenbergite, formerly known as fassaite (34 vol%), strongly zoned Ca-rich olivine (28 vol%), and kirschsteinite (3 vol%)—a characteristic mineral present in most angrites. Other minor constituents include pyrrhotite, F-bearing apatite, ulvöspinel, and possibly Ca silico-apatite similar to that found in D'Orbigny, Sah 99555, NWA 4590, and Asuka 881371. Calcium enrichment is a common feature of this group. Trace element and REE data for NWA 1296 are similar to that for the other quenched angrites, indicating a common magmatic origin (Sanborn and Wadhwa, 2010). The average Pb–Pb date of 4,564.20 (±0.45) m.y. for NWA 1296 provides a minimum estimate for its crystallization age, which is also consistent with the ages derived for D'Orbigny and Sahara 99555 (Amelin and Irving, 2011).

Although it has a mineralogy that is typical of the angrite group, this angrite exhibits a very fine-grained texture that is different from the other angrites, one which is consistent with rapid cooling. Northwest Africa 1296 consists of µm-sized, branched olivine crystals that form the cores of mineral chains, with anorthite crystals associated with these olivines. These chains are incorporated into later crystallizing clinopyroxene crystals. Other polycrystalline olivines, associated with kirschsteinite, formed outside of the mineral chains as the crystallization sequence culminated with clinopyroxene and accessory phases such as the opaques. Some pyrrhotite globules contain exsolved metal, while others contain inclusions of quenched Ca-carbonate melt. Small vugs are present among these final crystallization phases, some of which contain this primary Ca-carbonate, similar to that which is present in D'Orbigny vesicles.

The composition and texture of NWA 1296 is consistent with an origin involving ~15% partial melting of a peridotite influenced by a high carbonate content, which resulted in low silica contents and high Ca to Al ratios characteristic of angrites. Fractional crystallization occurred followed by secondary impact-melting, loss of volatiles, and mixing of olivine xenocrysts into some angrite members (rare or absent in NWA 1296). Thereafter, rapid cooling ensued.

Jambon et al. (2005) argue that since the end path of crystallization is kirschsteinite rather than silica, angrites cannot be considered to be basaltic rocks. In addition, they demonstrate that derivation from a CV- or CM-like source cannot be reconciled with angrite compositional factors, such as core formation, the presence of highly magnesian olivines in some members, low silica and alkali contents, the presence of carbonates in the melt, and unique oxygen isotope ratios. A contrary conclusion based on partial-melting experiments and Rb–Sr systematics supports the derivation of angrites from CV3-type precursor material. A different scenario for the petrogenesis of the angrites has been presented by Kurat et al. (2004), a brief synopsis of which can be found on the D'Orbigny page. They provide compelling evidence for a non-igneous origin of the angrites on a very early-formed parent body, one which was composed primarily of refractory condensate material.

A limited number of unique angrites are represented in our collections today, and they have been grouped as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample NWA 8535 (photo courtesy of Habib Naji). Another quenched angrite, NWA 7203 (photo courtesy of Labenne Meteorites), exhibits a striking variolitic texture. Interestingly, small fine-grained basalt clasts exhibiting textures and mineralogy generally consistent with a quenched angrite-like impactor are preserved in impact melt glass fragments recovered from the significant impact event that occurred ~5.28 m.y. ago near Bahía Blanca, Argentina (Schultz et al., 2006; Harris and Schultz, 2009, 2017; see top photo below). The photo of NWA 1296 shown above is a 1.1 g partial slice, while those below show the main mass.

standby for bahia blanca angritic fragments photo
Photo and diagram credit: (left) Schultz et al., MAPS, vol. 41, #5, p. 755 (2006); (right) Harris and Schultz, 40th LPSC, #2453 (2009)

standby for nwa 1296 photo
Photo credit: A. Jambon
standby for nwa 1296 photo
Photo credit: Jambon et al., MAPS, vol. 40, #3, p. 362 (2005)