Lunar Regolith Breccias and Fragmental Breccias

Regolith is the name for the layer of unconsolidated material at the surface of a planet - the loose stuff that overlies the solid rock.  On Earth, soil is part of the regolith, so lunar regolith is consequently often called "soil."

The lunar regolith.  This picture was taken on July 20, 1969 by Apollo 11 astronaut Buzz Aldrin
(courtesy NASA: AS11-40-5878).

 

Lunar regolith is composed in part of rock and mineral fragments that were broken apart from underlying bedrock by the impact of meteorites.  A rock composed of bits and pieces of older rocks is called a breccia.

These are small rock fragments from the Apollo 11 regolith. All are from the 2-4-mm grain-size fraction. There are also two impact-glass spherules in the image. Click on image for enlargement. (Photo by Randy Korotev)
 

  

Photomicrograph of grains of regolith from the Apollo 17 landing site in a thin section. All the grains in this photo are about the same size, ~0.1 mm, because the regolith (“soilâ€) sample was passed through various sieves, and these grains are from the 0.090–0.150-mm grain-size fraction.  Grain sizes in lunar regolith range from much smaller than this (<1 micrometer) to pieces the size of a house.
 

Two special kinds of lithologies occur in the lunar regolith. (Lithology means "rock type," but its a more general term.  Neither lithology discussed here is really a type of rock.)  One is called agglutinate.  Agglutinates are small glassy breccias formed when a micrometeorite strikes the lunar regolith.  Micrometeorites are a millimeter or less in size.  Millions of micrometeorites strike the Moon every day. (Millions strike Earth's atmosphere every day, too.)  When a micrometeorite strikes the lunar surface, some of the impacted regolith melts and some doesn't, so the product is a glass with mineral and rock fragments entrained.  The glass often shows flow features. Agglutinates are typically tens of micrometers to a few millimeters in size.

Photomicrograph of a thin section of a large agglutinate fragment from the Apollo 17 regolith.  The fragment is about 3 mm across.  (Photo by Brad Jolliff)
 

Agglutinates contain holes called vesicles. The vesicles frozen gas bubbles in the glass.  The bubbles occur for the following reason.  Rock and mineral fragments at the lunar surface are exposed to the solar wind, ions of mostly-light elements like hydrogen and helium that are emitted from the sun at exceedingly high speeds, several hundred kilometers per second.  Because the Moon has no atmosphere and the solar wind ions are moving fast, they are imbedded or implanted into the surface material of the Moon.  They do not penetrate very deep into a rock or mineral grain, only a few hundredths of a micrometer, so all the solar-wind- implanted atoms are at the very surface of lunar regolith grains.  Meteorite impacts stir the surface regolith so that the upper few meters of regolith are rich in implanted ions of hydrogen and helium.  The amount of solar-wind implanted ions is greater in the very finest material because the fine material has more surface area than the coarser material.  

When a micrometeorite strikes fine-grained material at the surface, some of the material gets hot enough to melt and form the glass of an agglutinate.  It also gets hot enough to liberate the solar-wind-implanted hydrogen and helium, causing bubbles in the glass.    

Photomicrograph of a thin section of a volcanic spherule from the Apollo 17 regolith.  The dark area is where some of the glass has crystallized (probably the mineral ilmenite).  The spherule is about 0.1 mm in diameter.
 

The other special lithology in the lunar regolith is glass spherules.  Glass spherules are formed in two ways.  Some are formed when a meteorite impact melts material, the melt is ejected from a crater, and small globs of the melt solidify before they land.  Such melt bombs are usually spherical and range in size from much less than a millimeter to about a centimeter.  Other glassy spherules derive from magma that is violently erupted from volcanoes - a "fire fountain."  On Earth, we'd call such material volcanic ash.  On the Moon, it's usually called pyroclastic glass.  In both cases, molten rock cools and solidifies above the Moon's surface, leading to glassy spherules.

The reason agglutinates and glass spherules are special is that both lithologies can only be produced at or above the Moon's surface. 

Photograph of a petrographic thin section of Apollo 16 regolith breccia 60016. The longest dimension is 16 mm. A few glass spherules and are evident. Agglutinates are hard to find in regolith breccuas and there may be none in this section.

  
Meteorite impacts both break rocks apart and glue rock fragments back together again.  During impacts of meteorites that form craters of hundreds of meters in diameter or larger, the material just below the point of impact melts and some even vaporizes.  Material that is deeper may just shatter in place, creating rock fragments.  When the shock wave associated with the impact passes through fine grained surface material, the material can be compressed into a rock, something like making a snowball by squeezing snow in your hands.   If the resulting rock contains glass spherules or agglutinates, it is called a regolith breccia.  If it consistent only of fragmental material with no glass spherules or agglutinates, it is called a fragmental breccia.  Regolith breccias consist of fine-grained material from the upper few meters of the Moon; fragmental breccias consist of material that was deeper.  Some regolith breccias are partially melted, but many show little sign of melting, except perhaps at sharp grain boundaries. 

 

Lunar meteorite NWA 2995 is a fragmental breccia. (We are looking at a sawn face.) Again, note that the clasts are shades of gray, not colored. Also, there are several different kinds (colors and textures) of clasts, which we would expect in a lunar regolith or fragmental breccia considering the diversity of rocks in the Apollo 11 regolith sample, above. In contrast, many terrestrial breccias (angular clasts), conglomerates (rounded clasts), and basalts (igneous) consist of clasts or phenocrysts of a single rock or mineral type (see meteorwrong numbers 086, 102, 196, and 216.) In lunar breccias, it is common to see clasts that are themselves breccias - "breccias within breccias." In this photo, there is a gray breccia containing white clasts on the right. Notice that there is no preferred orientation of the clasts. In terrestrial sedimentary rocks, clasts often are aligned in the same direction because the Earth has more gravity than the Moon. The clasts are not rounded. In terrestrial sedimentary rocks, clasts are often rounded because the pebbles from which they form were rounded by abrasion against each other in water or ice before they were cemented into a new rock. (See meteorwrong number 124.) Note also that aspect ratio (length to width) is short, almost always less than 3. Elongated clasts and phenocrysts are more typical of terrestrial rocks (e.g., meteorwrong number 148). In most, but not all, lunar regolith and fragmental breccias, the matrix is darker than the clasts. (Photo by Randy Korotev)
 

It is a fascinating and curious observation that many lunar meteorites are regolith breccias.  Regolith breccias were collected by the Apollo astronauts, but another type of breccia - impact-melt breccia - is more common in the Apollo collection, however. Thin regoliths occur on the surface of asteroids, and some "regular" (asteroidal) meteorites are regolith breccias.  Because the Moon is closer to the sun than are the asteroids and because the lunar regolith is thicker than asteroidal regoliths, lunar regolith breccias are much richer in solar-wind implanted gases than are asteroidal regolith breccias.  

When a lunar meteoroid that is a regolith breccia is heated and the surface melts as it passes through the Earth's atmosphere, the solar-wind implanted gases are driven off.  This leads to a vesicular fusion crusts - a fusion crusts with bubbles. If a meteorite has a vesicular fusion crust, it's likely to be a lunar meteorite

Vesicular fusion crust on lunar meteorite QUE 94281.
 
 

 

Vesicular fusion crust on lunar meteorite QUE 93069.
 

 

See some photos of regolith breccias from the Apollo collection.


meteorites
lunar meteorites


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

Last revised07-Mar-2012