by Matt Morgan of Mile High Meteorites

The Chico, New Mexico Impact Melt Breccia: Observations

The 231-pound Chico meteorite was found in January of 1954 on the northeastern plains of New Mexico.  Originally classified as an ordinary L6 chondrite, Chico has undergone scrutiny once again and is now classified as an L6 impact melt breccia.  While this May not seem like much of a change, one look at a polished section of the meteorite and you will see something very unique.

 Meteorites classified as impact melt breccias are very unique, not only because they are few in number (others notables are Cat Mountain, Caldwell, Paranaiba), but also because they offer us a snapshot of the impact crater producing processes that take place on the surfaces of asteroids.  Chico is actually divided into two separate lthologies, one that is a brecciated L6 and the other, more interesting, impact melt.  A clear line of melt separates the two.  It appears to me that Chico actually represents a crater fill deposit since it strongly resembles the crater fills that are found here on earth. 

A complete 250g slice of the Chico impact melt breccia. Photo by the author.


A complete 250g slice of the Chico impact melt breccia. The image above is a 250 gram complete slice of Chico.  Several different colors and divisions are apparent and represent distinct surfaces where the impact melt has flowed.  This indicates that the original rock material was plasticized, and began to flow along a boundary of differential cooling.  You can also see vesicle trains, where the gas-filled vesicles were moving along with the melt.

Close-up of vesicles.  Photo by the author.

Close-up of sulfide filled vesicles.  Photo by the author.


The above images essentially show similar features, vesicles and vesicle trains that formed while the melt was still plastic enough to move. The vesicles contain sulfide minerals, which also happen to be preferentially growing on one side (the bottom in this case) of the vesicle.  This preferential growth could be used as a "way-up" indicator, meaning that the image is right side up and represents the surface of the asteroid.  Maybe the parent body had a slight gravitational field or was rotating and creating a simulated gravitational field.


Vertical gas escape fissures?

The image above shows some vertical features that rise perpendicular to the flow direction, which in this case is horizontal.  I interpret these as gas or fluid escape fissures, possibly related to boiling of water vapor or a density contrast between the gas and the overlying melt material. Notice how they also connect several vesicles together. These types of features are quite common in terrestrial lava flows and impact melts.

While Chico May be classified as an “ordinary” chondrite, it is everything except ordinary.  With only a handful of these meteorites available for study, scientists can only speculate on the complexity of events that created them.  This cosmic interloper gives us a glimpse, albeit small, into the processes that shape and form the asteroids and even our own world.