NORTHWEST AFRICA 1670

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. 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 consisting of ~18 vol% highly-magnesian olivine xenocrysts (Fo96) (up to 5 mm), which are embedded in a very fine-grained groundmass constituting ~82 vol%. Euhedral olivine phenocrysts (<1 mm), which crystallized from the groundmass melt, are present on the rims of the olivine xenocrysts. The groundmass is composed of lath-shaped grains (up to 1 mm) consisting primarily of fassaite and plagioclase. The olivine xenocrysts contain inclusions (~10 µm) consisting of FeNi-metal and sulfides, which were likely formed under more oxidizing conditions than the groundmass olivine (Mikouchi et al., 2011). These xenocrysts are proposed to be the zoned mantle material from a quenched angrite protoplanet representing the earliest known (≥4.564 m.y.) differentiated material from a body in the solar system.

As in other angrites, the plagioclase is nearly chemically pure anorthite (An99–100), 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 may 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 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, 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.

The Mn–Cr ages of NWA 1670, Asuka 881371, 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 quenched angrites LEW 87051 and Asuka 881371. By inferring the amount of dissolved olivine xenocrysts each of these angrites should contain, it was shown that they, along with NWA 1296, have similar bulk elemental compositions supporting a common magma source. However, NWA 1670 contains the most magnesian olivine xenocrysts of any angrite (or achondrite) known (Fo96), and it has been suggested that this is an indication of a large, reduced, angrite parent body with a significant metallic core.

In a study of remanent magnetism in angrites, Weiss et al. (2008) discovered that a magnetic field with a strength of ~10 µteslas, ~20% of that of present-day Earth, was imparted to the angrite PB during its earliest phase of crystallization (as observed particularly from the angrite D'Orbigny). 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. The specimen of NWA 1670 pictured above is a 0.25 g partial slice.

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