The findings about the earlier impact support previous evidence that there was a lot of high-energy rock-smashing in space between 4.48 to 4.44 billion years ago. It was also, they believe, likely the impact that broke the meteorite off its larger parent body, and sent it on a collision course with Earth. The team's results suggest that this impact occurred less than 50 million years ago. The later impact seemed lesser, with lower pressures and temperatures. They found that the earlier impact, occurring 4.5 billion years ago, shattered phosphate minerals into small pieces, and subjected them to high temperatures.
Taking new uranium-lead dating for a point of comparison, the researchers studied the microscopic details of how the phosphate minerals had shattered, and the effect of impact-induced heating on the crystal structure. "The phosphates in most primitive meteorites are fantastic targets for dating the shock events experienced by the meteorites on their parent bodies," said geophysicist Sen Hu of the Chinese Academy of Sciences in China. Walton and his colleagues wanted to corroborate these dates by studying the way phosphate minerals in the meteorite had shattered over successive impacts. With this tool in hand, scientists had previously found that the Chelyabinsk meteorite had undergone two impact events, one around 4.5 billion years ago, and the other around 50 million years ago. In addition, an impact can even partially or entirely 'reset' the radioisotope mineral ages. We know how long uranium takes to decay, therefore we can infer the age of the zircon from the lead component. Therefore, any lead that is found in zircon has to be the product of the radioactive decay of uranium. When it is forming, zircon incorporates uranium, but strongly rejects lead. One of these is through uranium-lead dating in zircon crystals.
We do have some means of dating ancient collisions in the minerals found in meteorites.
0 kommentar(er)
