Posted on Leave a comment

NWA 2635

Meta-H
(Achondrite-ung in MetBull 89)
standby for nwa 2635 photo
Found 2004
no coordinates recorded A large fragmented stone weighing 4,085 g was found in Morocco during the Fall of 2004. The stone was analyzed and classified at Northern Arizona University (T. Bunch and J. Wittke), and NWA 2635 was initially considered to be a highly equilibrated and recrystallized H chondrite lacking any relict chondrules. The olivine Fa (18.9) and pyroxene Fs (16.8) values are consistent with the H chondrite group.

ORDINARY CHONDRITE COMPOSITIONS
Fa Fs
H 16–20.4 14.5–18.1
H/L 19.5–21.8 17.2–21.2
L 22–26 18.7–22
L/LL 25.5–26.5
LL 26–33 22–26

Based on the completed O-isotope analyses, it is evident that the ratios plot on an oxygen three-isotope diagram slightly outside of the main field of H chondrites while still overlapping the fields that resolve the H chondrite–IIE iron parent body. Further studies will be needed to examine the possibility that the O-isotope ratios might have been shifted due to open system aqueous flows on the H chondrite parent body. Since NWA 2635 is a recrystallized, texturally evolved chondrite with elemental ratios and an O-isotopic composition showing affinities to the H chondrite group, it is appropriately assigned to the newly proposed group of metachondrites (Irving et al., 2005).

Northwest Africa 2635 is unshocked (S1, peak shock pressure <5 GPa), and has experienced moderate terrestrial weathering (W2). The numerous small vugs that occur throughout this meteorite may be indicative of a very high-temperature environment during formation, consistent with greater depth within the asteroid. Despite some petrographic differences, e.g., differences in grain size, differences in O-isotopic compositions, and the absence of clinopyroxene in NWA 2635, there exist very close similarities between NWA 2635 and the paired metachondrites NWA 2353 and NWA 3145, and in all likelihood they constitute a pairing group. Furthermore, there is a strong possibility that the H7 chondrite NWA 2835 also belongs to this pairing group (Irving and Kuehner, UWS; Bunch, NAU). The specimen of NWA 2635 shown above is a 6.45 g partial slice. <!– This meteorite is the sixth H7 chondrite to be classified, a group which includes NWA 2353 and four Antarctic samples—Y-75008, Y-790120, Y-790960, and ALH 88119. It is presumed by some scientists that the unusually low abundance of H7 finds compared to L7 and LL7 finds is a result of the higher intrinsic metal content of H chondrites. This high metal content may lead to more extensive terrestrial weathering and pronounced oxidation, the appearance of which precludes such a meteorite from being submitted for classification.

It is generally accepted that following accretion, the ~100 km H chondrite parent body was heated by radioactive decay of 26Al, as well as by continuing impacts. Fission track thermochronometry indicates that type 7 chondrites cooled more slowly at greater depths than did those of 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. 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 petrologic types 6 and 7:

  1. the presence of poorly defined chondrules in type 6, but only relict chondrules in type 7
  2. 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
  3. 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 may be remnants of the original chondrules. Research has been published which identifies specific characteristics that distinguish type 7 chondrites from primitive achondrites. The following characteristics are typically observed in primitive achondrites (Ford et al., 2004):

  1. an equigranular (igneous) texture with no extensive segregation
  2. experienced temperatures to levels necessary for FeNi-metal, FeS, and silicate partial melting (~1200°C—perhaps by shock melting)
  3. 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
  4. Cr acting as a chalcophile element during reduction leading to its incorporation into troilite
  5. 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) would be called achondrites.

It has been determined that the H chondrite parent body recently suffered two distinct collisional events at ~7.0 and ~7.6 m.y. ago, which are distinguished by a lower than normal 3He/38Ar ratio in the metal of those fragments ejected during the earlier event. These ejection events produced only weak shock effects (S1–S2) 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 the probable parent body of the H-type ordinary chondrites, as well as the IIE iron meteorites. Hebe is a 116-mile-diameter asteroid located next to both the ν6 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 were likely created through impact melting on the metal-rich H chondrite parent body to produce melt sheets or pods near the surface.

–> standby for o-isotopic diagram
Diagram credit: Greenwood et al., Chemie der Erde, vol. 77, p. 24 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)


Leave a Reply