Polished NWA 869 meteorite slice, weighing 30 g
Name: NWA 869
Classification: L3–6 ordinary chondrite, regolith breccia
Shock stage: S3
Weathering grade: W1
Parent body: L-chondrite parent body
Total known mass: at least 2 t; estimated total recovered material approximately 7 t
Find location: Northwest Africa, Sahara
Find date: 2000–2001
Northwest Africa 869 (NWA 869) is one of the most remarkable and widely distributed ordinary chondrites from Northwest Africa. The meteorite consists of thousands of individual stones ranging in mass from less than 1 g to more than 20 kg. The total amount of recovered material has been estimated at approximately 7 metric tons.
NWA 869 was initially classified as an L4–6 fragmental breccia. Subsequent detailed petrographic and noble-gas investigations demonstrated that the meteorite also contains unequilibrated type 3 material and significant evidence for exposure to solar cosmic rays. It was therefore reclassified as an L3–6 chondritic regolith breccia.
The unusual combination of material from different petrologic types, shock-darkened lithologies, impact-melt-rock clasts, and regolith-derived components makes NWA 869 an important archive of the geological history of the L-chondrite parent body.
NWA 869 specimen – Microscopic view of the cut and polished side of an endcut
Mineralogy
The mineral assemblage of NWA 869 is characteristic of L-type ordinary chondrites. The principal silicate phases are olivine and low-Ca pyroxene, accompanied by plagioclase, chromite, troilite, pentlandite, and Fe-Ni metal.
The chemical compositions of the silicates show a remarkably broad range, reflecting the mixture of material with different degrees of thermal metamorphism and equilibration within the breccia.
In lithic clasts, olivine ranges from approximately Fa0.2 to Fa49.2, whereas matrix olivine ranges from Fa10.3 to Fa29.3. Low-Ca pyroxene in lithic clasts ranges from approximately Fs2.1 to Fs35.9, while matrix low-Ca pyroxene ranges from Fs7.0 to Fs23.9.
The Fe-Ni metal occurs principally as kamacite and taenite, accompanied by sulfide phases such as troilite and pentlandite. These metal–sulfide assemblages preserve microstructural evidence of the thermal and impact history experienced by the meteorite.
Petrography and Breccia Structure
NWA 869 is a complex, heterogeneous breccia dominated by a fine-grained clastic matrix. Point-counting of a large representative section yielded approximately 73.6 vol% clastic matrix.
The remaining material consists predominantly of lithic clasts belonging to different petrologic types:
- Type 4–6 chondrite clasts: approximately 20.3 vol%
- Shock-darkened clasts: approximately 3.9 vol%
- Impact melt rock clasts: approximately 0.9 vol%
- Type 3 chondrite clasts: approximately 0.8 vol%
- Fe-Ni metal grains larger than 2 mm: approximately 0.3 vol%
- Sulfide grains larger than 2 mm: approximately 0.2 vol%
Individual clasts can reach several centimetres in size, with the largest documented clasts measuring approximately 5.5 cm.
The matrix is particularly significant. It cannot simply be regarded as fine-grained debris derived from the larger lithic clasts. A substantial portion of the matrix appears to represent an additional, relatively low-petrologic-type lithology. One interpretation is that this material originated in a weakly consolidated region of the parent-body regolith.
The breccia therefore records the mechanical mixing and lithification of material that experienced substantially different thermal and impact histories before being incorporated into the same rock.
NWA 869 specimen – Microscopic view of the uncut side of an endcut
Regolith Breccia and Parent-Body History
The presence of solar-wind-derived noble gases provides strong evidence that NWA 869 formed, at least in part, within the regolith of its L-chondrite parent body. This makes the meteorite different from a simple fragmental breccia formed entirely at depth.
The regolith component contains material that had previously been exposed near the surface of the parent body and irradiated by solar and galactic cosmic rays. Individual components subsequently became incorporated into a breccia together with more strongly metamorphosed L-chondrite material.
NWA 869 consequently preserves material representing a broad range of geological conditions—from relatively unequilibrated type 3 material to thermally metamorphosed type 6 material.
Shock Metamorphism
NWA 869 has an overall shock stage of S3.
This classification is based on the shock features observed in the investigated material. At the same time, the breccia contains individual lithic components that had already experienced substantially stronger shock events before they were incorporated into the NWA 869 breccia.
Rare shock veins occur within the meteorite, generally on the order of tens to approximately 100 μm in thickness, while an older generation of shock veins can reach approximately 1 mm.
The presence of shock-darkened clasts and impact-melt-rock fragments demonstrates that impact processes played an important role in the geological history of the material before its final assembly.
Impact-Melt-Rock Clasts
NWA 869 contains small amounts of impact melt rock, accounting for approximately 0.9 vol% of the investigated material.
These clasts are depleted in Fe-Ni metal and sulfide relative to the surrounding chondritic material. Their presence provides direct evidence for impact melting on the L-chondrite parent body.
The impact-melt components therefore represent material that experienced considerably higher temperatures than the surrounding matrix and ordinary chondritic clasts.
Cosmic-Ray Exposure History
The noble-gas and cosmogenic-nuclide record of NWA 869 reveals a complex exposure history.
For the actual meteoroid that ultimately reached Earth, the best estimate for its cosmic-ray exposure in space is approximately 5 ± 1 million years. This value is based primarily on the combined cosmogenic 10Be and 21Ne data and is interpreted as the principal 4π exposure age of the meteoroid after its ejection from the parent body.
However, individual components within the regolith breccia preserve evidence of substantially older cosmic-ray exposure. Some components yield apparent exposure ages of approximately 10–30 million years. These older ages are interpreted as reflecting previous irradiation of individual regolith components while they were still located near the surface of the parent body.
Thus, the older exposure ages should not be interpreted as the exposure time of the NWA 869 meteoroid itself in interplanetary space.
This distinction is important because the meteorite preserves both an earlier regolith exposure history and the much later cosmic-ray exposure associated with its final ejection from the parent body.
Evidence for a Relatively Recent Impact Event
Two investigated NWA 869 fragments exhibit anomalous cosmogenic-nuclide ratios that indicate a relatively recent change in shielding.
The most plausible interpretation is that an impact excavated material from beneath the surface of the meteoroid, exposing previously shielded material to a substantially higher cosmic-ray flux. The excavation may have removed as much as approximately 80 cm of material.
The exposure associated with this event is estimated to have occurred roughly 20,000–120,000 years before the meteorite’s fall to Earth.
Importantly, this interpretation applies to the two investigated fragments and should not be regarded as evidence that every piece of NWA 869 experienced the same event. The observation nevertheless provides evidence that the meteoroid underwent a relatively recent collisional modification before entering Earth’s atmosphere.
Radiometric and Thermal History
Argon–argon dating provides evidence for several major thermal and impact events recorded by different components of NWA 869.
A type 4 chondrite clast yielded an 40Ar–39Ar plateau age of approximately 4402 ± 7 Ma, interpreted as the result of impact-related heating.
An impact-melt-rock clast yielded an age of approximately 1790 ± 36 Ma, while a shock-darkened clast yielded approximately 2216 ± 40 Ma.
These ages demonstrate that different components of the breccia experienced distinct thermal events at different times. NWA 869 therefore preserves a multi-stage record of impact processing rather than the signature of a single geological event.
Noble-gas data additionally indicate major thermal disturbance and gas loss around approximately 2.8 billion years ago, while some components retain evidence of older thermal histories extending to approximately 3.6 billion years.
Lustrous Fe-Ni metal grains exposed on the polished surface of NWA 869
Oxygen Isotopes
The bulk oxygen-isotope composition of NWA 869 is:
- δ17O: +3.52‰
- δ18O: +4.67‰
- Δ17O: +1.09‰
These values are consistent with an ordinary L-chondrite affinity and support the classification of NWA 869 as an L-chondritic meteorite.
The oxygen-isotope data are particularly useful because they provide an independent chemical and isotopic criterion for relating the meteorite to the L-chondrite parent-body reservoir.
Terrestrial Weathering and Age
NWA 869 has a weathering grade of W1, indicating relatively limited terrestrial alteration.
Despite its low weathering grade, iron-bearing phases at exposed surfaces may undergo oxidation after the meteorite reaches Earth. This can produce reddish-brown iron oxides and contribute to the characteristic weathered appearance of some specimens.
The terrestrial residence time of NWA 869 has been determined from cosmogenic radionuclides to be approximately 4.4 ± 0.7 thousand years.
This indicates that the meteorite material remained on Earth’s surface for several thousand years before being recovered.
Physical Appearance
Individual NWA 869 stones commonly display a grey to greenish-grey interior with contrasting light and dark brecciation. Darker regions may correspond to shock-darkened material, whereas lighter areas contain more typical chondritic lithologies.
Freshly exposed surfaces can reveal a highly heterogeneous breccia texture, with lithic clasts embedded within a fine-grained matrix.
Many specimens show little or no well-preserved fusion crust. Wind erosion in the Saharan environment has removed fusion crust from numerous stones and has contributed to the rounded, abraded appearance of many recovered fragments.
Surface alteration may include reddish iron oxides and caliche deposits associated with prolonged residence in the terrestrial environment.
A 68 g NWA 869 meteorite specimen from the Sahara
Find History and Material Distribution
NWA 869 was discovered in Northwest Africa around 2000–2001 and subsequently became widely distributed as individual stones.
The material occurs as thousands of fragments ranging from less than one gram to more than 20 kg. Although the Meteoritical Bulletin database records at least approximately 2 metric tons of classified material, the total recovered amount has been estimated at around 7 metric tons.
Cosmogenic-nuclide data indicate that the original meteoroid entering Earth’s atmosphere was substantially larger. Its pre-atmospheric radius has been estimated at approximately 225 ± 25 cm, corresponding to a pre-atmospheric mass of roughly 120–230 metric tons.
The difference between this estimated pre-atmospheric mass and the recovered mass indicates that the vast majority of the meteoroid was lost during atmospheric entry and subsequent fragmentation and ablation.
Provenance and Identification of Individual Specimens
An important consideration when working with NWA 869 is the uncertain provenance of individual specimens.
The original strewn field has not been documented in sufficient detail, and a substantial amount of material was distributed without secure geographic provenance. Consequently, the Meteoritical Bulletin specifically recommends that the classification of individual specimens be confirmed rather than relying solely on their macroscopic appearance or commercial designation.
This is particularly relevant because material from the extensive Northwest African meteorite trade can occasionally be incorrectly paired or assigned.
For scientific work, individual specimens should therefore be confirmed by appropriate petrographic, mineral-chemical, and, where necessary, isotopic investigations.
Scientific Significance
NWA 869 is scientifically important because it combines several geological records within a single meteorite.
It contains:
- unequilibrated type 3 material,
- metamorphosed type 4–6 material,
- shock-darkened lithologies,
- impact-melt-rock clasts,
- a substantial regolith-derived matrix,
- Fe-Ni metal and sulfide assemblages,
- evidence for solar-wind irradiation,
- evidence for multiple impact and thermal events,
- and a well-constrained cosmic-ray exposure history.
The meteorite therefore provides an unusually detailed record of the geological evolution of the L-chondrite parent body, including regolith formation, impact processing, thermal metamorphism, mixing, lithification, and eventual ejection into interplanetary space.
Its complex breccia structure also makes NWA 869 particularly valuable for studying how material from different depths and geological environments can become incorporated into a single parent-body regolith.
References
Meteoritical Bulletin Database. Northwest Africa 869 (NWA 869), official Meteoritical Bulletin Database entry. Classification history: L4–6; revised to L3–6.
Metzler, K., Bischoff, A., Scott, E. R. D., et al. (2011). The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (I) Petrology, chemistry, oxygen isotopes, and Ar–Ar age determinations. Meteoritics & Planetary Science, 46, 652–680.
Welten, K. C., Wieler, R., Nishiizumi, K., et al. (2011). The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides. Meteoritics & Planetary Science, 46, 970–988.
Skirak, M., et al. (2025). Microtextures and structures in metal–sulfide grains in NWA 869 meteorite and their relation to thermal history of L-chondrite parent body. Meteoritics & Planetary Science.