SAHARA 99555
Angrite
Found May 1999
y° 13' 53" N., x° 32' 01" W.
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 silico-phosphate phase 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 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. Moreover, this revised age is now consistent with that of D'Orbigny. This very early period 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 which 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–~200 km diameter (Nyquist and Bogard, 2003).
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 they represent plutonic igneous intrusions onto the regolith (Irving and Kuehner, 2007).
To complicate matters, results of CRE age studies utilizing cosmogenic nuclides have determined that the CRE age of D'Orbigny (11.0 ±0.8 m.y.) is significantly different from that of both Sah 99555 (6.1 m.y.) and Asuka 881371 (5.3 m.y.); these latter two angrites have similar CRE ages within uncertainties and likely represent a common ejection event. Cosmic ray exposure age data show that other angrites—Angra dos Reis (55.5 m.y.), LEW 86010 (17.6 m.y.), and LEW 87051 (~0.2 m.y.)—each represent a unique ejection event on the angrite parent body.
The Sah 99555 specimen pictured above is a 1.97 g partial slice measuring 20 x 10 x 3 mm. A tiny vug reflecting the incident light can be seen just left of center. However, much larger vugs are present within this meteorite, which are apparent in the following photo:
