H3–6
Fell August 4 or 5, 1998
~27° 20' N., 9° 20' W.
This fall was observed from a mountain near Zag, Morocco. A large quantity was imported by meteorite dealer A. Lang under the name Kem Kem, and about 175 kg has been distributed under the names Tan-Tan, Sagd, and Zag. Zag is a gas-rich regolith breccia composed of both light and dark clasts (H5–6) within a gray matrix (H3–4), and exhibits shock features ranging from S2 to S4. Zag contains bubbles of salty fluid inclusions within crystals of halite (NaCl) similar to that found in the Monahans (1998) H5 chondrite. Halite has only been found in the H4 matrix component of Zag. A xenolithic clast found in Zag has been identified as a thermally metamorphosed CV (Kebukawa et al., 2011).
The formation of halite may have proceeded through aqueous alteration processes in which dehydration of phyllosilicates by impact events produced the aqueous fluids. However, unlike the sylvite-containing halite crystals found in Monahans (1998), the purity of the NaCl brine in Zag suggests an origin from the evaporation and concentration of asteroidal impact-accumulated ices at a depth of a few km. As the halide brine became supersaturated, precipitation of halite occurred, trapping fluid inclusions at temperatures of <70°C. Secondary fluid inclusions were trapped along fractures (Busfield et al., 2004). Subsequent impact gardening mixed the halite, the H5–6 clasts, and the H3–4 matrix material and emplaced it near the surface. Notably, a carbonaceous chondrite clast that was identified in Zag has O-isotopes that plot within the CI1 group. This clast contains a carbonate component that is enriched in Na, which suggests a possible source for the halite (Zolensky et al., 2003).
An alternative scenario for the production of halite, proposed by Jones et al. (2011), begins with degassing from increased heating at depth (petrologic type 6) and the production of a halogen-rich, water-poor fluid. Next, this fluid reacted with merrillite to form fluor-chlor-apatite leading to the consolidation of F in apatite and the enrichment of Cl in the fluid. As the Cl-rich fluid ascended into petrologic type 4 material, it infiltrated merrillite, forming Cl-rich apatite and becoming enriched in Na. Finally, the fluid precipitated halite at the H4 horizon.
The halite crystals in Zag (see photo below) attained their blue-to-purple coloration through cosmic-ray-induced electron trapping in Cl ions. The halite was formed after the silicates underwent thermal metamorphism but before brecciation of the matrix in the outer regolith 4.25 b.y. ago. Recent efforts to date the salt crystals in Zag have utilized radioisotope chronometry employing 129Xe found within the halite. This isotope is produced by the decay of 129I, which was only present in the early Solar System. From the fixed rate of decay of 129I into 129Xe, and the proportions of each isotope present in the halite, the age of the halite was calculated (relative to other isotopic dating systems). An ancient age in the range of 4.561–4.559 b.y. was found, evidence that water was available on some asteroids only ~2 m.y. after the birth of the Solar System. Radioisotope studies also indicate that the I–Xe system was reset ~4.546 b.y. ago, likely by shock or aqueous alteration, in an event that followed the deposition of halite (Ebisawa and Nagao, 2005).
Zag consists of four lithologies, representing types H4 through H6, that are present in these approximate abundances by volume: 65% light-colored, chondrule-bearing, angular clasts (S2–4); 25% gray-colored, chondrule-bearing, clastic matrix (S2–3); 10% dark-colored, chondrule-bearing, angular clasts (S4); and <1% impact-melt-rock clasts. If any original H3 material is still present, it is rare. Most lithologies exhibit various degrees of silicate darkening, produced from curvilinear blebs and veinlets of impact-mobilized FeNi-metal, troilite, and chromite within and around the silicates. The dark-blue halite crystals have been found only within the clastic matrix, which is also the site of solar noble gas concentrations. Many Zag fragments display slickensides, produced by the shearing motion of adjacent fault faces. This shearing motion is lubricated through the accumulation of very fine particles, which results in a polished surface. Consistent with the fact that Zag is a recent witnessed fall, it has a weathering grade of W0/1.
The S(IV)-type asteroid 6 Hebe is thought to be the probable parent body of the H-type ordinary chondrites and possibly the IIE iron meteorites as well. Hebe is a 116-mile-diameter asteroid located next to both the v6 and 3:1 resonances providing an efficient and rapid transfer mechanism into Earth-crossing orbit and a significant source of meteorites to Earth. The average CRE age of Zag, based on 3He, 21Ne, and 38Ar, is calculated to be 5.1 (±0.5) m.y., close to the peak of the latest of the three breakup events determined for H chondrites (Eugster et al., 2007). It has been estimated that 6 Hebe could contribute ~10% of the meteorite flux to Earth and that it may be the source of one of the major ordinary chondrite groups. Models show that by mixing a component of 40% FeNi-metal with 60% H5 chondrite, an exact match to the spectra of 6 Hebe is produced. The IIE irons could then be created through impact melting on the metal-rich H chondrite parent body to produce melt sheets or pods near the surface. Read more about the formation of IIE irons on the Miles page.
However, hydrocode models show inconsistencies exist between expected and observed CRE ages based on the scenario of direct injection into resonances. The steady delivery of H chondrite material from 6 Hebe to Earth also remains unexplained. Current studies by Rubin and Bottke (2009) have led to the conclusion that family-forming events resulting in large meteoroid reservoirs having homogenous compositions which are located near dynamical resonances such as the Jupiter 3:1 mean motion resonance are the likely source of the most prevalent falls, including the H chondrites. See further details on the NWA 2898 page. The specimen of Zag shown above is a 1.0 g fragment displaying signs of brecciation.
