A single meteorite weighing 136 g was found in the desert region of Northwest Africa, and subsequently purchased by S. Ralew. The stone was analyzed and classified at Humboldt University, Berlin (A. Greshake), and was determined to be a completely recrystallized H chondrite lacking any relict chondrulesan H7 chondrite. The olivine Fa (17.7) and pyroxene Fs (15.6) values are consistent with the H chondrite group. On an oxygen 3-isotope diagram, the plot of NWA 2898 is also consistent with an origin on the H-chondrite parent body (I. Franchi and R. C. Greenwood, OU).
ORDINARY CHONDRITE COMPOSITIONS
Northwest Africa 2898 is one of a very few H7 chondrites to be classified, five of which were found in Antarctica, a few others being found in Northwest Africa (NWA 2835, 4226, and 4229). While these meteorites presently constitute the currently accepted H7 petrologic grouping, it should be noted that certain highly metamorphosed meteorites with very close chemical and isotopic similarities to the H-group chondrites have been assigned to the recently proposed metachondrite classification (Irving et al., 2005); e.g., NWA 2353, NWA 2635, and 3145, all three of which are thought to be paired with the H7 NWA 2835.
The thermal history of the H chondrite parent body was calculated by Harrison and Grimm (2010) based on cooling rate data and closure times. They found that the object accreted over a short time period of 2.2 m.y. In an alternative viewpoint, Monnereau et al. (2012) determined a more rapid accretion time period of 0.10.2 m.y. while 26Al was still extant. Moreover, Sokol et al. (2007) concluded that accretion of the ~150 km H-chondrite parent body occurred relatively late after most radioactive 26Al had decayed, at least 2 m.y. after CAI formation; it was probably heated by continuing impacts. It is generally considered likely that the asteroid eventually formed an insulated "onion-shell" structure with a diameter of 150260 km. The H-chondrite parent body was composed approximately (by volume) of 84%, 10%, and 6% of type 6, type 4/5, and type 3 material, respectively.
Amelin et al. (2005) utilized thermal models to calculate the progressive increase in petrologic types from the core to the surface as follows: from the core outward to a distance of 44.9 km is type 6 material: between 44.9 km and 48.9 km is type 5 material; between 48.9 km and 56.9 km is type 4 material; and from 56.9 km to the surface at 92.5 km is type 3 material. Peak temperatures were determined to be 8651000°C, 675865°C, and <675°C for type 6, type 4/5, and type 3 material, respectively. The higher petrologic types were excavated at depth by impact, forming craters measuring tens of km-wide and reaching depths of 5.611.2 km on their 200 km diameter model. Fission track thermochronometry indicates that type 7 chondrites cooled more slowly at greater depths than did those with lower petrologic types (Trieloff et al., 2003). Consequently, type 7 chondrites experienced a longer period of thermal metamorphism within this interior layer, and now they exhibit extensively recrystallized textures that are transitional to an achondrite classification.
Importantly, a complex cooling history for the higher petrologic type H chondrites (5/6) was suggested from thermometric studies conducted by Ganguly et al. (2012). They reconciled data from calculations of two-pyroxene thermometers with the ArAr, PbPb, and HfW closure temperatures of select minerals to determine a cooling history consistent with very rapid cooling between ~800°C and 450°C, followed by a very slow cooling stage, and then another rapid cooling stage. By contrast, those H chondrites with lower petrologic types experienced a steady state of very rapid cooling. It was proposed that this scenario was more consistent with a collisional disruption and re-accretion of the parent body as opposed to a smoothly transitional "onion shell" model.
Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphic criteria to distinguish between petrographic types 6 and 7:
the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
low-Ca pyroxenes contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo) in type 6, but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
feldspar grains gradually coarsen to reach a size of at least 0.1 mm in type 7
In the intervening years since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. As a result of more recent studies, it was proposed by Wittke and Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules, but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This would lump those meteorites containing "poorly defined" chondrules and "relict" chondrules into the type 6 category.
In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all of the silicates, instead of only the feldspar grains, would provide a better gauge of a petrographic type 7 since silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of plagioclase occurs only in type 7, and suggest that this could be utilized as an additional parameter. Beyond that, it was revealed that modal metal contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce olivine, resulting in only small amounts of Ni-rich metal along with lower amounts of orthopyroxene and clinopyroxene compared to those amounts present in lower metamorphic grades.
H7 chondrites have an uneven distribution of metal and silicates, and a heterogeneous grain size distribution. The coarse silicates might be remnants of the original chondrules. Research has been published which identifies specific characteristics that distinguish type-7 chondrites from metachondrites (primitive achondrites). The following characteristics are typically observed in metachondrites (Ford et al., 2004):
an equigranular (igneous) texture with no extensive segregation
experienced temperatures to levels necessary for FeNi-metal, FeS, and silicate partial melting (~1200°C, perhaps through shock-melting)
migration of free metal from olivine fayalite and chromite as a result of reduction processes (i.e., by reaction with graphite), resulting in Mg-rich olivine and chromite and low-Ni metal
Cr acting as a chalcophile element during reduction leading to its incorporation into troilite
close to chondritic bulk composition
Those meteorites which have undergone more extensive thermal processing and have lost their original geochemical and isotopic features (e.g., members of the HED suite) are then characterized as achondrites.
It has been determined that the H-chondrite parent body recently suffered three distinct collisional events at ~7.0, 22, and 33 m.y. ago, which are distinguished by a lower than normal 3He/38Ar ratio in the metal of those fragments ejected during the earlier events. These ejection events produced only weak shock effects (S1S2) and radiogenic gas loss, but injected abundant fragments into Earth-crossing resonances.
From spectrographic data, the S(IV)-type asteroid 6 Hebe is thought to be a candidate for the parent body of the H-type ordinary chondrites. 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. 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 have been created through impact-melting on the metal-rich H chondrite parent body to produce melt sheets or pods near the surface.
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, which have homogenous compositions and locations near dynamical resonances such as the Jupiter 3:1 mean motion resonance, are the likely source of the most prevalent falls including H chondrites and HED achondrites (especially howardites). As a matter of fact, a number of asteroids having H-like mineralogies have been observed near the 3:1 and 5:2 resonances at 2.82 AU (Burbine et al., 2015 and references therein). See further details on the Abbott page.
Northwest Africa 2898 shows evidence of having been very weakly shocked (stage S2, peak pressure of 510 GPa), and having experienced a low degree of weathering since its fall (grade W1/2). The specimen of NWA 2898 shown above is a 1.6 g partial slice exhibiting a coarse-grained, recrystallized texture with many grains exhibiting 120° triple junctions. The photo below shows a magnified view of this H7 depicting the completely recrystallized texture.