A U.S.-German research team led by Stony Brook’s Lars Ehm and including other members of the Department of Geosciences in the College of Arts and Sciences at Stony Brook University has simulated meteorite impacts in the lab and tracked the resulting structural changes with X-rays. The team’s findings are published in the February 1 issue of Earth and Planetary Science Letters.
Associate Professor Ehm, who holds a joint appointment with the National Synchrotron Light Source II (NSLS-II) at Brookhaven National Laboratory, is principle investigator of the project, and alumna Melissa Sims, PhD, is the lead author. Professor Timothy Glotch and alumni Steven Jaret, PhD, and Brandon Rhymer, MS, are also part of the team.
Meteorite impacts play an important part in the formation and evolution of Earth and other planets. But the impact conditions — impactor size, velocity, pressure and temperature — are usually determined long after the impact occurred by studying changes in the minerals within the impact crater. Scientists use a classification scheme that ties impact conditions to pressure- and temperature-induced changes in rock-forming minerals that can be found in typical rocks in impact craters.
The research team’s experiments, performed at DESY in Germany and Argonne National Laboratory in the US, show that structural changes can occur at very different pressures depending on the compression rate. The team studied feldspar minerals, which are found in planetary crusts. Changes in these minerals with respect to pressure and temperature — such as the structural transformations or amorphisation, that is the loss of ordered crystal structure — are used as indicators for very large impacts.
However, for the feldspar minerals, the reported values for the pressure conditions of the amorphisation transition differ greatly if static or dynamic compression techniques are used. “These differences point to large gaps in our understanding of compression rate induced processes in minerals,” said Ehm. This has far-reaching implications for the interpretation of natural impact events based on the rock record with respect to the velocity, size and other properties of the meteorite.
“In our experiment we used gas- or actuator-controlled Diamond Anvil Cells to rapidly compress our samples, while we continuously collect X-ray diffraction patterns,” explained Sims. “This allows us to monitor the changes in the atomic structure during the complete compression and decompression cycle, and not only at the start and end of the experiment as in previous so-called recovery experiments.”
“The results show that, depending on the rate of compression, the minerals undergo the amorphisation transition at very different pressures,” added Ehm. “The increase in compression rate leads to a lowering of the observed amorphisation pressure.”
Further investigations are needed to fully understand the behavior of these minerals and to determine if impact conditions can be measured against the structure of rock minerals.
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