No eyewitness recorded the fall of one of Arizona’s official meteorite sites. The unique “Gold Basin” meteorite exploded over more than 130 square kilometer of Mohave County in northwest Arizona at the end of the last Ice Age.
“As far as I know, this is the first ‘fossil’ strewn field found outside of Antarctica,” said David A. Kring, geologist and senior research associate with the Lunar and Planetary Laboratory at the University of Arizona in Tucson. That the meteorite fragments survived exposure to the elements for 15 millennia says that conditions even during the last of the Ice Age must have been fairly mild, he added. Kring, who directs the Lunar Lab’s Meteorite Recovery Program, did a series of analyzes to classify the meteorite. Classification is requisite for a meteorite to be officially recognized by the Meteorite Nomenclature Committee, the international body of scientists who assess meteorite finds. The Gold Basin meteorite brings the number of officially approved Arizona meteorites to 31.
Radiocarbon tests at the University initially dated the Gold Basin meteorite at approximately 15,000 years. This date was confirmed in 2001 as being 15,000 years plus/minus 600 years. Kring theorized that 15,000 years ago, a small asteroid hit Earth’s upper atmosphere with an energy of between ten to one thousand tons of TNT. Preliminary evidence suggests the asteroid may have been two to three meters, in diameter. It lost energy as it plowed through Earth’s ocean of air, then it exploded, probably 10 to 30 kilometers above the ground.
All other known strewn fields of this type of asteroid, called a type L4 ordinary chondrite, are “witnessed falls,” or those seen to explode and fall to Earth, Kring said. L4 ordinary chondrites are relics of the original debris that orbited the sun when it coalesced. The debris accreted to form a small planetary body about 4.56 billion years ago, probably in an orbit between the planets Mars and Jupiter, the region now known as the asteroid belt. “Gold Basin is also special because it is one of the most numerous collections of fragments ever found,” Kring said. “We’ve found more than 3,000 fragments so far, and it wouldn’t surprise me if we found another 10,000 fragments, in addition.”
James D. Kriegh of Oro Valley discovered the first two fragments of the Gold Basin meteorite on Nov. 24, 1995. Kriegh is a retired UA civil engineering professor and member of the Desert Gold Diggers, a group whose members spend their spare time gold prospecting. A few years ago, Kriegh heard a talk by Kring on how to identify meteorites. Kriegh soon began successful searches for meteorites, including the Greaterville meteorite he found in November 1994. Kriegh said he and the others garnered between one and 140 pieces of the meteorite on later field trips. Collecting meteorites that fell to Earth 20,000 years ago after sitting in space for more than 4 billion years “is every bit as exciting as searching for gold,” he added.
Kring, Kriegh, John Blennert and Ingrid (Twink) Monrad, also of the Oro Valley-Tucson area, collaborate in collecting and mapping the Gold Basin meteorite fragments, which range in size from a peanut to a 3-pound softball that Blennert recovered. So far, the collection weighs more than 34 pounds. The strewn field covers private and federal land, so the Gold Basin meteorite recovery team has been coordinating the project with the relevant federal authorities.
“It’s really a joy to have a person like Kriegh involved in this,” Kring said. “I told him these are the things we need to do to preserve the scientific integrity of the site, and he did absolutely everything I asked him to do. The team mapped the location of every fragment as it was found, and they recorded how deep it was in the soil or if it was found right at the surface.”
“We have not yet hit the edge of the Gold Basin strewn field in any direction,” Kring said. “We don’t know how big this is going to be, eventually. Every time we go a little farther, we find more meteorites. The goal has been to find as many of these fragments as possible. We wanted to find the limits of this field before making it public. The problem is, the field is just too big. We may be collecting samples for another decade.”
The largest collection of stones from a single meteorite is also in Arizona, Kring added. The community of Holbrook was pelted with 14,000 fragments of a meteorite that exploded in the early evening sky of July 19, 1912. One fragment severed the branch of a tree when it fell, witnesses reported. Fragments from the meteor, an estimated half-meter in diameter, showered to Earth over an ellipse roughly 1.5 square miles — a much smaller area than the fall site of the Gold Basin meteorite, which is estimated to be two-to-four times larger than the Holbrook asteroid.
An important distinction between the Holbrook fall and the Gold Basin meteorite strewn field is that Holbrook is a classic case in which important information on the distribution of the fragments was lost. A mineral collector in Philadelphia paid Holbrook residents to collect the pieces and ship them to him on the train, Kring said. In the Gold Basin case, by contrast, he added, “I can tell you precisely where this sample was collected, thanks to the efforts of the great field team.”
Mapping exactly how meteorite fragments are strewn across the impact site is no trivial academic exercise. Mapping the strewn field to reconstruct how the meteorite fragmented should help scientists understand what causes meteorites to break apart or survive intact as they blast through the atmosphere, Kring noted. This is of great interest to scientists trying to understand the hazards of asteroid impacts. Given that the world’s growing population is expanding over more of our planet’s surface, relatively common collisions with small asteroids like the Gold Basin meteorite and the Holbrook meteorite become growing hazards, Kring added. Ask the astronomers who search the skies for near-Earth crossing asteroids: They will tell you a future significant collision is not a matter of if, but of when.
(NOTE — Kriegh and Monrad have donated five fragments of the Gold Basin meteorite to the UA Mineral Museum, Flandrau Science Center, where they are on display.)
New Chondrite in Gold Basin Found
Twink Monrad, one of the original discoverers of the Gold Basin strewn-field, found a new chondrite within the field in mid-February of 1999. She was searching for meteorites when she noticed a dark rock resting on the surface. Her metal detector gave off a strong signal as she passed it over the rock and the rock was strongly attracted to a magnet. The 797.6 g specimen exhibits a fresh-appearing almost complete black fusion crust, unlike the very weathered exteriors of the Gold Basin meteorites. Dr.Jim Kreigh cut off a small slice which revealed a highly recrystallized light gray interior with abundant metal but few chondrules. It is reminiscent of L6 chondrites like Holbrook or Bruderheim. Dr. David Kring of the University of Arizona Lunar and PlanSetary Laboratory confirmed that it was a meteorite and tests are under way to determine its petrologic type. The Gold Basin meteorites are type L4. It is not unusual to find other unrelated meteorites within a strewn- field as large as Gold Basin. Monrad capped the find with a philosophical understatement: “Just goes to show you that patience and perseverance pay off sometimes!”