METEORITE OR METEORWRONG?

vesicles in meteorites - vesicular fusion crusts

Most stony meteorites have vesicular fusion crusts (for example, see Cynthiana). As the surface melts when the meteorite passes through the atmosphere, gases in the meteorite is are released. Some of that gas becomes trapped in the glassy melt when the melt cools.

Many lunar meteorites are regolith breccias, and many of these have fusion crusts that are thick and highly vesicular. The best examples are QUE 03069 and PCA 02007, although the effect can be also seen in ALHA 81005 and Calcalong Creek. At one time, all of the material of a regolith breccia was fine grained "soil" on the surface of the Moon. Soil grains exposed at the very surface of the Moon absorbed ions emitted by the sun as solar wind. Most of the ions were of gaseous elements like hydrogen, helium, and nitrogen. Impacts of small meteoroids on the Moon mixed and stirred the upper part of the regolith. In a location where there has not been a recent large impact, nearly all the grains in the upper few meters of the regolith will contain solar-wind implanted gases because over millions of years all grains spend some time at the surface. On Earth, the solar wind is absorbed by the atmosphere, so there are no Earth rocks with solar-wind implanted ions. Some meteorites from the asteroid belt have solar-wind gases, but none have the high levels found in lunar meteorites because the Moon is closer to the sun. When the exterior of the meteoroid is heated by the friction of the atmosphere, it melts and the gases are released, forming gas bubbles that get trapped in the glass when the glass cools.


Backscattered-electron image of lunar meteorite PCA 02007, a regolith breccia. At the top and right is the glassy, vesicular fusion crust that occurs on the outside of the meteorite in this photo; the vesicles are black in the image. At the bottom is the brecciated interior of the meteorite. It was never molten, so there are no vesicles. Longest dimension: 1.3 cm. The vesicles are all smaller than 1 mm in diameter. Image courtesy of Ryan Zeigler.

  

This is a photo of tiny lunar meteorite QUE 94281. It also has a highly vesicular fusion crust because it is a regolith breccia.

Many people have contacted me saying, "My rock looks just like QUE 94281." Above is a photo that one such person sent me. Superficially, the rocks do resemble QUE 94281, but not in detail. QUE 94281 is a fragment broken from a larger stone, so it has some rough edges, like the rocks above. The fusion crust on QUE 94281 coats only part of the stone. As in PCA 02007 above, the interior of QUE94281 does not contain vesicles (although that's hard to see in this photo) because it was never molten. The rocks above are not regolith breccias, but terrestrial basalts. Notice in the upper right stone that there are fewer vesicles in the chilled top than in the interior - the opposite of what is seen in meteorites.

A piece of vesicular basalt from New Mexico showing the smooth top that was exposed to the atmosphere as the molten lava cooled.  


Bottom line: If your rock has a vesicles on the inside, it's probably not a meteorite. Such rocks are very common on Earth but are exceedingly rare among meteorites.

vesicles and amygdules

vesicles in meteorites


www.catchafallingstar.com
www.catchafallingstar.com


Prepared by:

Randy L. Korotev


Department of Earth and Planetary Sciences
Washington University in St. Louis


Please don't contact me about the meteorite you think you’ve found until you read this and this.

e-mailkorotev@wustl.edu