Using Chromium to Catch the Culprit: Which Meteorite Formed Lonar Lake?

Lonar Lake
Lonar Crater Lake, Aurangabad, India
Image credit: Akshay Charegaonkar, licensed under CC BY-NC-SA 2.0

Lonar Lake is located in the Buldhana District of Maharashtra, India. Though small, (the Great Salt Lake covers approximately 760 times more surface area!), it first attracted scientists because of its near-perfect circular shape. Until the late 1970’s and early 1980’s, it was thought that Lonar Lake was the remains of an ancient volcano. However, the discovery of minerals shocked by high pressure events confirmed that Lonar Lake formed due to a meteor impact nearly 570,000 years ago. While scientists agree that a chondrite, a primitive meteorite made up of tiny spheres called chondrules, was the body that impacted Earth and formed the crater now known as Lonar Lake, there has been a lot of debate as to the type of chondrite that caused the impact.

Chondrites can be sorted into many groups based on characteristics such as the minerals they contain, the composition of the chondrite, and the degree of alteration the chondrite experiences; in other words, did the sample react with fluids or get reheated? Unfortunately, because the chondrite that formed Lonar Lake disintegrated on impact, scientists cannot rely on the physical appearance of a sample to provide evidence that would allow them to determine the chondrite’s group. Instead, scientists have to look at the chemical signatures left over from the impact event.

A very useful chemical signature to consider is the presence (or absence) of certain elemental isotopes. Frequently, scientists use isotopes, which are atoms of an element that have different atomic masses, as a way to classify different planetary materials, For example, the relative proportions of oxygen isotopes has historically been used to classify chondrite groups.

A study recently published in Meteoritics and Planetary Science used isotopes of chromium to determine which type of chondrite formed Lonar Lake. Scientists collected samples from around the lake. They identified samples of interest by looking for features such as glasses, melt structures, and fragmented rock, all of which are indicators of impact. Samples were brought back to the Institut de Physique du Globe de Paris, where the chromium was extracted through a series of chemistry techniques and then its isotopic abundances were measured using a mass spectrometer.

The results suggest that the chondrite that formed Lonar Lake was part of the CM chondrite group. In the chondrite classification scheme, the C means carbonaceous, or carbon bearing, and these chondrites bring organic material to Earth when they impact it. This group of chondrites is thought to form in the outer solar system and represents some of the most primitive material scientists have studied. Hopefully, new isotope studies will continue to increase our understanding of early solar system material. READ MORE