Ancient magma reveals signs of life in zircons from ancient Earth

Zircon crystals, like a time capsule, can preserve traces of life hundreds of millions of years old in the form of bioactive carbon. Using new methods, geoscientists at the University of Heidelberg have succeeded in tracking down very old and rare examples of the mineral zircon that contain inclusions of graphite in which the light carbon can be identified as a remnant of previous life.

According to the researchers, this opens up new possibilities for research into the early period of our planet in which neither fossils nor sediments were preserved in their original form.

The mineral zircon grains are formed from magma, that is, molten rock, in an extremely hot and hostile environment in its core. However, the superheated organismal remains were converted to carbon dioxide and methane gases and deposited as graphite in the metallic zircon at about 700 °C.

“The particular isotopic signature of biological carbon remains largely preserved in most inclusions and leaves behind a kind of imprint of past life forms,” ​​explains lead author of the study Dr. Manfred Vogt from the Institute of Earth Sciences at the University of Heidelberg.

The researchers emphasized that taking measurements is very difficult. First, the intact graphite inclusions, some of which measure only a few micrometers in size and thus 100 times finer than a human hair, must be found and identified within the zircon crystals. To exclude carbon contamination from the environment, non-destructive Raman spectroscopy is used to examine in situ encapsulated inclusions in zircon. Next, the zircons are bombarded with an ion beam to expose the graphite inclusions so that their carbon isotopic composition can be analyzed.

“In this process, we can remove just a few nanometers of thick carbon layers and measure them individually, thus obtaining many data points for a single inclusion to detect potential differences,” explains Dr. Winfried Schwartz, one of the study’s co-authors. The search results are published in Geochimica et Cosmochimica Acta.

Zircon is among the oldest minerals on Earth, some of which are more than four billion years old. “They can teach us upwards of 96% of Earth’s history. For the first 100 million years, these crystals represent the only known record that holds information about very early conditions on the planet. The inclusions in these oldest zircons have already revealed that water and was Oceans existed on early Earth, as did continental plate movements,” explains Dr Vogt.