Meteorite weathering

Two children are sitting within the corrosion grooves of the Willamette meteorite (14 tons). It has lost probably about 6 tons by terrestrial weathering.

A Sikhote-Alin meteorite with visible rust.

Meteorite weathering is the terrestrial alteration of a meteorite. Most meteorites date from the oldest times in the Solar System and are by far the oldest material available on our planet. However, despite their age, they are pretty vulnerable to terrestrial environment: water, chlorine, oxygen attack the meteorites as soon they reach the ground.

Weathering scales

In order to quantify the degree of alteration that a meteorite experienced, several qualitative weathering indices have been applied to antarctic and desertic samples.^[1]

The most known weathering scale is based upon the effects seen in polished thin sections and it ranges from W0 (pristine) to W6 (heavy alteration). It was proposed by Jull A. J. T. et al.^[2] (1991) and updated by Wlotzka^[3][4](1993) and Al-Kathiri et al.^[5](2005).

• W0: no visible oxidation of metal or troilite, but may be noticeable in transmitted light a limonitic staining. Usually recently fallen meteorites are of this grade, although some are already W1.
• W1: small oxide rims around metal and troilite, small oxide veins.
• W2: moderate oxidation of metal (about 20-60% replaced).
• W3: heavy oxidation of metal and troilite (60-95% replaced).
• W4: complete oxidation of metal and troilite (>95% replaced), but no alteration of silicates.
• W5: beginning alteration of mafic silicates, mainly along cracks.
• W6: heavy replacement of silicates by clay minerals and oxides.

The Meteorite Working Group at the Johnson Space Center uses weathering categories A B, C and E to denote the alteration of antarctic meteorites.^[1] Here are the official definitions^[6]:

• A: Minor rustiness; rust haloes on metal particles and rust stains along fractures are minor.
• B: Moderate rustiness; large rust haloes occur on metal particles and rust stains on internal fractures are extensive.
• C: Severe rustiness; metal particles have been mostly stained by rust throughout.
• E: Evaporite minerals visible to the naked eye.

References

1. ^ ^{a b} P. A. Bland, M. E. Zolensky, G. K. Benedix, M. A. Sephton. "Weathering of Chondritic Meteorites"
2. ^ Jull, A. J. T.; Wlotzka, F.; Donahue, D. J. (1991). "Terrestrial Ages and Petrologic Description of Roosevelt County Meteorites". Abstracts of the Lunar and Planetary Science Conference 22: 667. Bibcode 1991LPI....22..667J. 
3. ^ Wlotzka, F. (1993). "A Weathering Scale for the Ordinary Chondrites". Meteoritics 28: 460. Bibcode 1993Metic..28Q.460W. 
4. ^ Wlotzka, F.; Jull, A. J. T.; Donahue, D. J. (1995). "Carbon-14 Terrestrial Ages of Meteorites from Acfer, Algeria". Workshop on Meteorites from Cold and Hot Deserts: 72. Bibcode 1995mfch.work...72W. 
5. ^ Al-Kathiri, A.; Hofmann, B. A.; Jull, A. J. T.; Gnos, E. (2005). "Weathering of meteorites from Oman: Correlation of chemical and mineralogical weathering proxies with14C terrestrial ages and the influence of soil chemistry". Meteoritics & Planetary Science 40 (8): 1215–1239. Bibcode 2005M&PS...40.1215A. doi:10.1111/j.1945-5100.2005.tb00185.x. 
6. ^ http://curator.jsc.nasa.gov/antmet/amn/weather.htm

See also

• Weathering
• Meteorite classification