Dating an impact

Some boulders are exposed in an impact melt sheet. How can these boulders help geologists understand more about the timing of impact? LROC NAC M1120322807L, image width is 1600 m [NASA/GSFC/Arizona State University].

The impact process produces unimaginable amounts of energy! Some of this energy goes into melting rock, resulting is spectacular landforms. But not all of the rocks melt, and some are just heated up! Such is probably the case for the boulders we see in today's Featured Image. But how can these rocks be of use for geologists? On Earth, some enterprising geologists use boulders like these to date impact structures!

Context image of today's Featured Image of boulders located on a terrace in Bridgman F crater (44.053° E, 141.825° E). Image width is 100 km [NASA/GSFC/Arizona State University].

Geochronologists are a brand of geologists who study the ages of rocks. One way to get at a rock's absolute age is to measure its gas content. This method is particularly useful for igneous (and impact) settings. Rocks will accumulate gas over time as a result of radioactive decay of different elements, but these gases don't want to be there. If a rock is later warmed up past a specific temperature (which we term the closure temperature) the gases will start to escape. So if an impact event has heated some material above the closure temperature, the gas content of the rock will be 'reset'.

Several gas systems are currently in use to obtain absolute dates for rocks, but there are two important ones for impact cratering. One measures the ratio of Argon 40 to Argon 39 (40Ar/39Ar dating). The other uses the Uranium, Thorium, Helium system ((U-Th)/He)). However, both utilize separate materials. 40Ar/39Ar dating benefits from having impact melt to sample. This is one of the methods used in the 1970's that dated Apollo samples and helped scientists understand lunar geologic time. But what if you have no impact melt? On Earth that might be more of a concern since impact melt might not last as long as boulders, and the (U-Th)/He might be the answer.

ASU graduate student Kelsey Young stands next to a boulder in Mistastin crater, CA. The red box outlines an impact melt zone between two boulders, which we can imagine are contextually very similar to the boulders in our Featured Image [Image credit Kelsey Young].

So Kelsey Young (and her colleagues) at ASU have come up with a new way to date a crater. They date boulders at impact sites using the (U-Th)/He method because the system has a lower closure temperature. This method has a few advantages. One is that boulders are easier to find than melt on Earth's surface. The other is that the system's lower closure temperature means that it is easier to reset the ages of these boulders.

The canonical age for Mistastin crater is 36 +/- 4 Ma, and the (U-Th)/He system came up with 32.7 +/- 1.2 which is within the error of previous estimates. So far this technique shows promise in matching the dates found using 40Ar/39Ar and adds another tool to those of us trying to understand impact cratering! 

Landsat image of Mistastin crater, CA, located at 55.83° N, 66.3° W. The impact structure is ~28 km wide, shown here by the red ellipse [NASA/USGS].

Now that we have a better idea of how best to date impact craters, how might you find the absolute age of Bridgeman F crater in today's Featured Image? Of course, you'll have to get there first!

Look for more boulders and melt in the full LROC NAC!

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Published by Drew Enns on 2 May 2013