SAHARA 99555

standby for sah 99555 photo
Found May 1999
y° 13′ 53′ N., x° 32′ 01′ W. A single fusion-crusted angrite meteorite weighing 2.71 kg was found by the Labenne Family during their 1999 meteorite expedition in the Sahara Desert. The trace element and mineral composition and texture of Sah 99555 is very similar to that of D’Orbigny and the groundmass component of Asuka 881371. However, where Asuka 881371 contains only small vugs, Sah 99555 contains large mm-sized vugs within a greenish-gray, coarse-grained matrix. Sah 99555 also lacks the olivine xenocrysts that the other two contain.

Prior studies based on a somewhat limited sampling of the angrite parent body have shown them to be igneous rocks composed of mostly clinopyroxene in the rare form of Al,Ti–diopside-hedenbergite, formerly known as fassaite. Sah 99555 has a higher content of anorthite (33 vol%) than clinopyroxene (24 vol%), together with significant amounts of Mg-rich olivine (23 vol%), Ca,Fe-rich olivine (19 vol%), and low-Ca kirschsteinite (8.5 vol%). In addition, minor high-Ca kirschsteinite, titano-magnetite, troilite, and a late-stage Ca silico-phosphate (determined to be silico-apatite by Mikouchi et al., 2015) are present. As with other angrites, Sah 99555 is highly depleted in volatiles such as Na and K and highly enriched in oxidized elements such as FeO, TiO and CaO—characteristics which separate this class from all others, and suggest a precursor that was extremely CAI-rich, probably similar to the CV-type chondrites. In fact, in a study of the least metamorphosed members D’Orbigny and Sah 99555, it was demonstrated by Jurewicz et al. (2004) that these angrites were compositionally similar to, though not identical to, devolatilized Allende chondrite melts formed under low pressures at elevated oxygen levels.

Angrites are extremely ancient meteorites, with absolute ages ranging from ~4.557 b.y. to ~4.564 b.y., only slightly younger than CAIs in Allende (~4.5685 b.y.; Burkhardt et al., 2007). Angrite core formation occurred 1.7–2.8 m.y. after these first nebular condensates (Markowski et al., 2006). Various radioactive isotope chronometers have been employed to establish the date for the formation of angrites. These extensive isotopic studies establish angrites as an early planetary differentiate undisturbed since their formation. Based on the Pb–Pb chronometer, an age of 4.5662 (±0.0001) b.y. was derived for Sah 99555 and NWA 1296 by Baker et al. (2005), while a slightly younger Pb–Pb age of 4.56441 (±0.00065) b.y. was determined for Sah 99555 by Amelin (2007). A highly precise progressive dissolution technique, which successfully accounts for three Pb components, was recently conducted by Connelly et al. (2008) and Amelin (2008). A revised Pb–Pb age of 4.56458 (±0.00014) b.y. was determined to be the best estimate for the crystallization age of Sah 99555. This revised Pb–Pb age is now consistent with that of D’Orbigny. On the other hand, a number of other radionuclide chronometers reveal an age ~2 m.y. younger than the Pb–Pb age, and this discrepancy has not been resolved thus far.

High precision measurements conducted on the Al–Mg system have established a crystallization age for Sah 99555 of ~4.11 m.y. after CAIs (given that CAIs formed 4.5683 [±0.0007] b.y. ago). More specifically, a magma ocean was formed 3.0–3.5 m.y. after CAI formation, followed by ~1.5–2.0 m.y. of magma ocean evolution prior to eruption and crystallization (Schiller et al., 2010). This corresponds to an absolute age of 4.5635 (±0.0005) b.y. Relative to Efremovka CAIs, an Al–Mg age of 4.5624 (±0.0002) b.y. was determined by Spivak-Birndorf et al (2009). Almost within error margins, a Mn–Cr age for Sah 99555 was determined to be 4.5637 (±0.0004) b.y., while a Hf–W age was determined to be 4.5628 (±0.0008) b.y., which is concordant with other extinct radionuclide chronometers; however, all ages are slightly younger than the Pb–Pb age. This very early period of Solar System history corresponds to a time when the short-lived isotopes 26Al and 60Fe were still extant and could have initiated parent body melting. In their studies of the 176Hf excess in Sah 99555, Thrane et al. (2007) demonstrated that it was derived from the rapid decay of 176Lu, the nuclei of which were excited by cosmic rays generated from a supernova explosion that occurred after the crystallization of the angrite PB.

According to Sanders and Scott (2007), any body that accreted to a diameter >60 km (i.e., large enough to minimize heat loss from the surface through conduction) within ~2 m.y. of CAI formation (the oldest objects dating to 4.567 b.y. ago) as the angrites did, must contain enough 26Al to produce global melting and differentiation. In contrast, Senshu and Matsui (2007) determined that accretion to a diameter of only ~14 km occurring within 2 m.y. of CAI formation was all that was required for global differentiation to occur, while a diameter of 40–160 km occurring within 1.5 m.y. was cited by Hevey and Sanders (2006) and Sanders and Taylor (2005) as the minimums. Only at large heliocentric distances (>~2.8 AU) would accretion proceed too slowly for sufficient 26Al to accumulate and initiate global melting prior to a body growing too large to melt, considered to be ~200 km diameter (Nyquist and Bogard, 2003).

Be that as it may, John T. Wasson (2016) presented evidence that the slow heating generated entirely by the decay of 26Al is insufficient to melt asteroids, and that an additional heat source would have been required; e.g., the rapid heating incurred from major impact events. He determined that the canonical 26Al/27Al ratio of 0.000052 is much too low to cause any significant melting, and that a minimum ratio of 0.00001 would be required to produce a 20% melt fraction on a well-insulated body having a significant concentration of 26Al. The initial ratio of 0.0000004–0.0000005 calculated for the angrites Sah 99555 and D’Orbigny based on their 26Al–26Mg isochrons is too low to have generated any significant melting without an additional heat source.

Kurat et al. (2004) have conducted an extensive study of D’Orbigny and other angrites, in which they utilized mutiple sources of data (i.e., structural, textural, chemical, and redox evidence). They concluded that the angrites are most consistent with a non-igneous origin from refractory solar nebula condensates—basically an asteroid-sized version of a CAI—which record unusual circumstances in the early foundation of the solar system. Some further details on their proposed angrite petrogenesis can be found on the D’Orbigny page.

Sahara 99555 has a K–Ar age of 3.54 (±0.15) b.y., reflecting a late isotopic disturbance. Interestingly, the D’Orbigny plagioclase Sm–Nd data show a disturbance at 3.08 (±0.05) b.y. Trace and major element compositions, textures, and crystallization ages of Sah 99555 and D’Orbigny are almost identical (Nyquist et al., 2003; Floss et al., 2003), suggesting a possible genetic relationship. They are considered to represent the earlier formed crustal lithology on the angrite parent body. In addition, Asuka 881371 and LEW 87051 have trace element trends similar to D’Orbigny and Sah 99555, suggesting that they may all share a common origin, or at least have experienced similar petrographic histories. Trace element trends for LEW 86010 and AdoR are significantly different from each other and from the other angrites, which suggests that they represent distinct lithological sources and that they, along with NWA 4590 and NWA 4801, crystallized a few m.y. later than the oldest angrites. It has been suggested that these latter angrites represent plutonic igneous intrusions into the regolith (Irving and Kuehner, 2007).

The results of CRE age studies based on cosmogenic nuclide data infer a CRE age for Sah 99555 of 6.6 (±0.8) m.y. (Bischoff et al., 2000). This age is similar to that calculated for Asuka 881371 of 5.3 m.y., and these two angrites might share a common ejection event. Portions of the angrite asteroid must be in a stable orbit (planetary or asteroid belt) from which spallation has continued to occur over the past ~56 m.y. as indicated by the broad range in angrite CRE ages. The number of unique angrites represented in our collections today is very limited, and they have been grouped by some as basaltic/quenched, sub-volcanic/metamorphic, or plutonic/metamorphic, along with a single dunitic sample in NWA 8535 (photo courtesy of Habib Naji). The specimen of Sah 99555 pictured above is a 1.97 g partial slice measuring 20 × 10 × 3 mm. A tiny vug reflecting the incident light can be seen just left of center. Much larger vugs are present in this angrite, which are apparent in the following photo shown courtesy of Labenne Meteorites:

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The top photo below shows Marc Labenne as he removed the angrite meteorite from its shallow depression. The bottom photo below shows a 470 g end section in the collection of the University of New Mexico. standby for sah 99555 photo
standby for sah 99555 photo
Photos courtesy of Labenne Meteorites

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