[meteorite-list] Meteorites on the moon

[Randy Korotev]

Regarding meteorites on the Moon...

There is a great deal of "meteoritic matter" on the Moon, but very 
few meteorites.  The two miniscule fragments that Martin Altmann 
mentioned are the best known ones from the Apollo collection, but I'm 
aware of  2 others even smaller.

Virtually all meteoroids that strike the Moon either melt or vaporize 
on impact.  If they melt, they mix with the melted silicates of the 
lunar target rocks.  So either way, they become unidentifiable as 
meteorites.  Lunar impact-melt rocks and breccias do contain blebs of 
meteoritic metal - metal the melted during the impact but as a liquid 
was immiscible with the molten silicates.

All lunar soils and breccias contain meteoritic material.  In any 
handful of lunar soil, 1-4% of the mass is "extralunar" 
stuff.  Except for blebs of metal, most of which were melted and 
resolidified, "meteorites" are virtually absent, however.  In the 
lunar soil, most of the meteoritic material arrives as 
micrometeorites. By one estimate, approximately 80 grams per square 
kilometer of micrometeoroids accrete to the Moon (and Earth's 
atmosphere) each year.

We know the meteoritic material (melted and mixed, recondensed from 
vapor) exists in lunar regolith (soil) and breccias because both are 
loaded with "siderophile" (iron-loving) elements like iridium, gold, 
and platinum in ratios characteristic of chondrites.  In contrast, 
the unbrecciated igneous rocks of the lunar crust - the basalts and 
anorthosites - have almost immeasurably low concentrations of these 
elements, as do igneous rocks on Earth.

So, every one of the lunar meteorite that is a breccia (which is 
nearly all of them) contains "regular" meteoritic material.  Those 
lunar meteorites that are regolith breccias (like NWA 3136 that Adam 
Hupe mentioned) tend to contain the most.  Those that are impact-melt 
breccias tend to contain the least, judged on the basis of 
concentrations or, say, iridium.

Here's a quote from a paper I've submitted on PCA 02007, a lunar 
meteorite regolith breccia with a high proportion of chondritic material:

"The mean Ir concentration of PCA 02007 is equivalent to a component 
of 2.7% ordinary chondrite or 2.8% CM chondrite. This means that 14% 
of the Fe and 9% of the Mg and Cr in PCA 02007 derive from extralunar 
sources (Figs. 8, 9). Day et al. (2006) report an actual meteorite 
fragment in their thin section of PCA 02007."

To my knowledge, the chondrite fragment in a lunar meteorite reported 
by Day et al. is a first.

Randy Korotev 
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