"Although initial oxygen isotopic compositions are consistent with an origin in the Earth-Moon system, numerous observations appear to eliminate both bodies," Shearer and his team write in an abstract presented at this week's meeting.
Doug Rumble from the Carnegie Institution and his colleagues will also present an analysis of GRA 06128 and 06129 at the conference.
He said the New Mexico researchers initially studied a "tiny crumb that fell off the outside" of GRA 06129, but they didn't wash the outer layer to remove any Earthly weathering agents that might alter the findings.
"The UNM analytical results reflect only imperfectly the actual extraterrestrial composition of the meteorite, because UNM also analyzed rust that formed as the meteorite lay exposed to Earth's atmosphere," he said.
Rumble's own analysis puts the meteorites "right in the range of values expected for rare meteorites called brachinites," he said.Brachinites are believed to be the remnants of planets that once orbited between Mars and Jupiter
"So now we are all meeting in Houston to hang our heads a little and admit to one another that maybe we were a little too hasty," Rumble said, "and got too excited over an unwashed meteorite."
Unsettled Debate David Kring, a visiting scientist at Houston's Lunar and Planetary Institute and a co-author on Treiman's paper, said there's still work to be done to nail down the exact nature of the meteorite pair.
The chemical and mineral compositions of the rocks are similar to brachinites and another type of meteorite called chondrites, he said.
Chondrites, leftovers from the formation of asteroids, are the most common meteorites. But the pair doesn't match either category exactly, Kring said.
The rocks are partially melted, which is not characteristic of chondrites.
And they contain high levels of the mineral feldspar, which is not typically associated with the short-lived planets between Mars and Jupiter.
"Thus there are two mysteries," Kring said. "What is the parent planet for the rock, and what type of geologic activity on that planet produced the unusual mineral assemblage?"
There is no shortage of new ideas to account for the disparity.
"My preferred working hypothesis is that [the meteorites] actually represent a piece of crust from an asteroidal body—the surface of another planet, if you like," offered James Day, a geologist at the University of Maryland.
Day has joined the other researchers at this week's meeting to puzzle over the rocks.
Treiman added that it would take a very large asteroid or a dwarf planet to cause the melting seen in the meteorites.
Whether or not its home is discovered, GRA 06128 is a "pretty puzzle, and will help us understand how asteroids form and how they evolve to become planets," he said.