NORTHWEST AFRICA 1670


Angrite
Basaltic/Quenched, Picritic
standby for nwa 1670 photo
Purchased January 1, 2001
no coordinates recorded

A single 30.6 g stone that was found in 2001 in Morocco, possibly in Attamina, was subsequently sold in Erfoud, Morocco to Bruno Fectay and Carine Bidaut. Northwest Africa 1670 was classified at two French institutions, the Université Pierre & Marie Curie (A. Jambon and O. Boudouma) and the Université d'Angers (J-A. Barrat), and has been described as a highly shocked angrite representative of an impact melt.

Northwest Africa 1670 has been described (Mikouchi et al., 2003; Jambon et al., 2008) as having a porphyritic texture, primarily consisting of a very fine-grained groundmass (82 vol%) composed of lath-shaped grains up to 1 mm in size consisting primarily of fassaite and plagioclase. Embedded within the groundmass are highly-magnesian (Fo9688) olivine xenocrysts (~18–20 vol%) measuring 0.5–3 mm in size, but it is considered likely they were all in the larger size range before sectioning (Mikouchi, 2014). Along the rims of the olivine xenocrysts in NWA 1670 are <1 mm-sized euhedral olivine phenocrysts that crystallized from the groundmass melt. As reported in D'Orbigny and A-881371, xenocrysts in NWA 1670 contain ~5–10 µm-sized inclusions consisting of FeNi-metal and sulfides, while some contain traces of fluid inclusions (Mikouchi et al., 2011; Mikouchi, 2014). These magnesian olivine xenocrysts were formed under reducing conditions before they were incorporated into the oxidizing parental melt of the groundmass (Mikouchi et al., 2015). The olivine xenocrysts are proposed to be zoned mantle material which was incorporated into an ascending magma and was subsequently quenched upon eruption onto the surface of a relatively large angrite protoplanet. However, an alternative formation scenario through a severe impact melting event is still under consideration (Mikouchi, 2014). Either way, angrites represent the earliest known differentiated material from a solar system object, and with a U-corrected Pb–Pb age of 4.56437 (±0.00019) b.y., NWA 1670 is the oldest known angrite (Bizzarro et al., 2013).

As in other angrites, the plagioclase is nearly chemically pure anorthite (An99100), but is more Fe-enriched. Lesser amounts of calcic olivine are incorporated as patches within the fassaite. Accessory phases include spinel (both xenocrystic and groundmass types), FeS, kirschsteinite, Ti-magnetite, and Ca-silicophoshate. Ca-carbonate droplets (up to 5 µm) are trapped in pyroxene. Alkalies such as Na and K are lacking, possibly as a result of loss during impact events. Trace element and REE data for NWA 1670 are similar to that for the other quenched angrites, and along with the similar mineralogies, indicates a common magmatic origin (Sanborn and Wadhwa, 2010; Mikouchi et al., 2011).

Northwest Africa 1670 is typical in many respects to other angrites, being derived from a primary angritic source melt—the apparent differences among them can be attributed in large part to the accumulation of xenocrystic, highly magnesian olivine and to pyroxene accumulation. The groundmass texture and olivine zoning profiles in NWA 1670 are consistent with that of a more rapidly quenched melt located at very shallow depths. The low Si content and the overabundance of Ca in many mineral phases of NWA 1670 attests to melting in the presence of carbonate (Jambon et al., 2005). This is a process unique to angrites, which might illustrate one of the earliest stages of Solar System evolution.

It was proposed by Mikouchi et al (2001) that a rapidly cooling magma (~10–50°C/hour) entrained locally variable amounts of magnesian olivine xenocrysts derived from the mantle into the groundmass melt. Cooling rate data acquired with respect to chemical zoning of olivine xenocrysts gave consistent rates of 7–13°C/hour (Mikouchi et al., 2008). The lower Mn/Cr ratios obtained by Sugiura et al. (2003) are also consistent with rapid cooling within a thin lava flow at a depth of ~0.5–2 m. In further contrast to other angrites (with the exception of moderately shocked NWA 7203), NWA 1670 exhibits signs of a severe shock event, as evidenced by mosaicism and undulose extinction in olivine xenocrysts, and by the presence of cracks and impact-melt veins. In view of the shock deformation features present in NWA 1670 (and other quenched angrites) olivine xenocrysts, late metamorphism associated with an impact-shock event is considered a possibility (Jambon et al., 2008; Mikouchi et al., 2015, 2017).

The Mn–Cr ages of NWA 1670, Asuka 881371/12209, D'Orbigny, and Sah 99555 are identical and represent the oldest angrite crystallization ages. Despite the fact that D'Orbigny and Sah 99555 lack olivine xenocrysts, NWA 1670 likely originated from a common magma source, as did the two other olivine xenocryst-bearing (picritic) quenched angrites LEW 87051 and Asuka 881371/12209. By inferring the amount of dissolved olivine xenocrysts each of these angrites should contain, it was ascertained that they, along with NWA 1296, have similar bulk elemental compositions supporting a common magma source controlled by fractional crystallization with or without addition and resorption of Mg-rich olivine xenocrysts (Mikouchi and Bizzarro, 2012). Furthermore, the chemical composition of the NWA 2999 pairing group shows that it also derives from a picritic source magma, which thereafter experienced further fractional melting, metamorphism, and annealing, along with incorporation of an exogenous metal component (Baghdadi et al., 2015). NWA 1670 contains the most magnesian (Fo96) olivine xenocrysts of any angrite (or achondrite) known and also contains FeNi-metal grains, which suggests that it originated on a large, reduced angrite parent body having a significant metallic core; the xenocrysts subsequently experienced a period of oxidation prior to incorporation into the parental melt (Mikouchi et al., 2017).

In order to better constrain the properties of the differentiated angrite parent body core, van Westrenen et al. (2016) conducted a study modeling siderophile element depletions along with their metal–silicate partitioning behavior for the hypothesized angrite parental melt composition. A CV chondrite mantle composition was used for their calculations, along with a temperature and pressure (0.1 GPa) appropriate for a solidifying melt on a small planetesimal. Their results indicate that the observed siderophile element depletions of angrites are consistent with a core mass fraction of 0.12–0.29 composed of Fe and Ni in a ratio of ~80:20 (with a low S content), and that it was formed under redox conditions (oxygen fugacity) of ΔIW–1.5 (±0.45).

Multiple episodes of impact, disruption, and dissemination of the crust can be inferred by the wide range of CRE ages determined for the angrites—0.6–73 m.y. for eleven angrites measured, representing as many as nine ejection events (Nakashima et al., 2008; Wieler et al., 2016). This range is consistent with a large parent body enduring multiple impacts over a very long period of time, and suggests that the parent object resides in a stable orbit (planetary or asteroid belt) permitting continuous sampling over at least the past 73 m.y. A CRE age of ~16–18 m.y. was calculated for the LEW 86010 and NWA 1670 angrites, possibly representing a single ejection event (Eugster et al., 1991; Herzog and Caffee, 2014). This event might also include the angrite NWA 7812 with a CRE age of 20–21 m.y., since this age should be considered an upper limit based on the possibility that it contains a solar cosmic ray Ne component (Wieler et al., 2016).

In a study of remanent magnetism in angrites, Weiss et al. (2008) discovered that a magnetic field with a strength of ~10 µT (microteslas), ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization. This magnetic field may be attributable to a number of possible causes; e.g., accretion to an orbit in close proximity to the early T-Tauri phase solar field, or perhaps more plausible, a magnetic field generated through an internal core-dynamo mechanism. Subsequent paleomagnetic intensity studies conducted for D'Orbigny, Sahara 99555, and Angra dos Reis by Wang et al. (2015) have established a natural remanent magnetization value for Angra dos Reis of ~15 µT (microteslas), demonstrating that this lithology formed under the influence of a significant core dynamo which existed ~11 m.y. after CAIs. By comparison, no natural remanent magnetization (paleointensity) > ~1 µT was detected for the earlier formed angrites D'Orbigny and Sahara 99555, which constrains the onset of the APB core dynamo to later than ~4 m.y. after CAI formation. It was also recognized that the strong solar nebula-generated magnetic field which had existed ~1.2–3 m.y. after CAIs (~50 µT, measured in Semarkona chondrules) had virtually disappeared by the time the earliest angrites were formed, indicating that the solar nebula had already been largely dissipated.

standby for angrite dynamo timeline diagram
Diagram credit: Wang et al., 46th LPSC, #2516 (2015)

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 photo below). The specimen of NWA 1670 pictured above is a 0.25 g partial slice.

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)