Richfield, A New Stone Meteorite

by Russell W. Kempton, New England Meteoritical Services

I have a confession to make, albeit a small one. I have always wondered about the beginning of things, especially of the Sun and the Earth. What events caused the darkness and cold of space to be driven back by our Sun as it began to burn? During the infall period of the solar nebula as dust and gas rushed inward, did the friction of large gaseous cells rubbing together produce static charges so great that lightning bolts millions of kilometers in length were common? And, what of chondrules - those incredibly complex little pieces of melt glass that we find in chondritic meteorites? Were they really the very first tiny lumps of matter to form in our solar system - the precursor of planets, moons and asteroids?

Well, happily, I'm not alone in trying to stretch my comprehension of things back four and a half billion years. Understanding how the Sun and planets of our solar system formed is one of the most profound questions in astronomy and planetary science, and after looking at a new meteorite found in Richfield, Kansas, USA, I have no doubt that it will add new information to the puzzle.

Richfield, Kansas

Terracing a field is a time-honored means of erosion control. It is not, however, the method of choice if one is looking for meteorites. In 1983, a farmer was terracing a tract of land for agricultural use about 4 miles southeast of Richfield, Morton County, Kansas (37° 13' 20" N, 101° 40' 53 " W) when he uncovered a 40.8 kg stone mass. Finding a rock of any size in this field was so unusual that he brought it home, where it stayed for the next ten years. Finally in 1995, curiosity won out. A piece of the meteorite was sent to Dr. Alan Rubin at UCLA. Rubin, who is currently conducting a thorough analysis of the Richfield meteorite, recently classified Richfield as an LL3 - a rare class of chondrite. Excluding recovered Antarctic finds, Richfield is only the sixteenth LL3 chondrite to be found.

The First Rocks

Chondrites are stone meteorites with an astounding age of 4.5 billion years. Their age has been determined by studying the gradual decay of their constituent radioactive atoms. They are the oldest known matter and may represent intact samples of first generation mineralogy from the solar nebula.

Chondrites are a simple mixture of two very complex ingredients - chondrules and a fine-grained material called matrix. Four and a half billion years ago, the newly ignited Sun was enveloped in a nebula of interstellar dust and gas. Chondrules are thought to have formed by the melting of this interstellar dust somewhere in the nebula. They are igneous masses of high temperature silicates, primarily olivine and pyroxene. Matrix material has been theorized to be additional dust vaporized and then recondensed into new minerals.

The Richfield Meteorite

One of the more important characteristics of LL3 chondrites is that the chondrules are sharply defined. Unlike most other classes of chondrites, they have undergone a minimum of change (metamorphism or textural recrystallization from heat) since their formation within the solar nebula. This pristine material is clearly visible in prepared sections of Richfield.

The preliminary data indicates that Richfield is highly shocked. Based upon the mosaicism of the pyroxenes and olivines, it is shock stage S4. Studies to determine if it is rich in solar-wind-implanted rare gases have not yet been completed. The terrestrial age of Richfield is unknown, but it is not a fresh fall. The exterior surface is covered with a calcareous type of crust - caliche. However, oxidized fusion crust is visible in small patches.

There appear to be three distinct lithologies in Richfield: a light-gray colored chondrule-rich lithology, a homogenous dark-gray lithology, and an angular dark-gray to black metal-veined clastic lithology. Chondrules are abundant throughout. They range from submillimeter in size to 5 millimeter diameters. Many well-defined, metal-rimmed chondrules are present in both the light-gray and dark-gray structures. Additionally, there is an even distribution of fine-grained Ni/Fe inclusions as well as several larger (6 mm x 5mm) Ni/Fe inclusions. The light and dark gray structures are mixed randomly throughout the specimen with diffuse boundaries between the two. Curiously, embedded within all lithologies are 1 to 2 mm diameter jet-black, glassy inclusions.

The Question

If one looks at a prepared section of the Richfield meteorite, one can see the many areas of interest it offers to researchers. It appears to be a rather primitive chondrite that has survived a variety of "geologic processes" or mixing events. As chondrites are the only rocks that can be traced back through time to the birth of our solar system, they carry a record of the conditions present during their formation. It is this record carried within the minimally altered structures of Richfield, that may shed new light on one of the more perplexing questions in meteoritics - the formation of chondrules.

Dr. Harry McSween at the University of Tennessee in Knoxville has described chondrules as "the most important and exciting matter available for scientific scrutiny." These tiny, glasseous spherules show a wide variety of sizes, textures, composition, and mineral abundances. Their properties provide data on the nature of their formative environment - the solar nebula. Thousands of research papers have been written about chondrules - we know a lot about them. But, the problem is that we don't know how, when and where they formed. Without this crucial bit of information we do not know where to apply all that we have learned.

The Astrophysicist and the Geologist

Theories of chondrule formation are constrained by chondrule composition, the nature of the energy source that melted the precursor matter, and the cooling process that resulted in rapid congealment. Currently, theories of chondrule formation involve two possible settings: an astrophysical one - in which some form of melting and condensation occurs in the dust environment of the solar nebula precipitated either through grain to grain collisions, lightning, or some other high-energy event, and a geological setting melting by impacts between small and large bodies within the nebula.

If they formed on primitive asteroids then the information they carry pertains to the pressure, temperature, and composition of those parent bodies, and if not, then the data applies to a high-energy, gaseous environment.

The protosolar nebula is believed to have gone through two distinct stages of evolution: a brief half million year active period of accretion and a second, much longer (10 million years), relatively quiescent period. Recently Dr. John Wood at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts proposed that as chondrule and chondrite formation is a high-energy process, the "most promising" time for their formation was during the brief or initial half million year period of the protosolar nebula when 99.9 percent of the mechanical energy available was dissipated as heat instead of the second 10 million year period.

Whatever the energy source was, it had to be extremely efficient to produce the vast amounts of material that was to become our planetary system. If the process was an astrophysical one, then perhaps, there really were lightning bolts millions of kilometers long, vaporizing everything in their path, turning any material adjacent to the discharge column into tiny bits of matter that visually resemble tiny bits of terrestrial fulgurite. Similarly, could turbulence created during discharge sweep fine dust into the column concentrating it into chondrules?

On Earth, lightning flashing across the sky is an exciting weather event. It is an impressive display of electrostatic charges and masses seeking equilibrium. Are the chondrules in Richfield and other chondrites the product of astrophysical processes on a truly grand scale or the result of some form of geological "impact gardening?" An accurate and thorough understanding of our solar system's evolution depends upon which is correct. I wonder.....

Acknowledgments - The author expresses his sincere thanks to Dr. John Wood, Dr. Ursula B. Marvin, Dr. Michael Petaev, Harvard - Smithsonian Center for Astrophysics, and to Dr. Timothy Grove, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, for their valuable observations and discussions.

Russell W. Kempton is the Director of New England Meteoritical Services based in Mendon, Massachusetts, USA.

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