[meteorite-list] Meteorites on the moon

From: Randy Korotev <korotev_at_meteoritecentral.com>
Date: Wed Sep 6 16:40:13 2006
Message-ID: <6.2.3.4.0.20060901153511.01ff81d0_at_levee.wustl.edu>

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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