Rare zircon samples give new clues on earth’s ancient dwellers

Opened graphite inclusion in zircon after ion bombardment. (Image by Winfried Schwarz, Heidelberg University, Institute of Earth Sciences).

Geoscientists at Heidelberg University have succeeded in tracing very old and rare samples of the mineral zircon that host graphite inclusions in which light carbon is identifiable as a remnant of ancient life.

According to the scientists, this opens up new possibilities for research into our planet’s early period for which neither fossils nor sediments have been preserved in their original form.

In a paper published in the journal Geochimica et Cosmochimica Acta, the researchers explain that zircon mineral grains form from magma or melted rock in an extremely hot and intrinsically hostile environment. Heated remnants of organisms were converted to carbon dioxide and methane gases and deposited as graphite in zircon at approximately 700°C.

“The special isotopic signature of biogenic carbon remains largely preserved in most inclusions and leaves behind a kind of fingerprint of earlier life forms,” the study’s lead author Manfred Vogt said in a media statement.

Vogt noted that taking measurements is extremely demanding. First, intact graphite inclusions, some measuring just a few micrometres and thus a hundred times finer than a human hair, must be found and identified within zircon crystals.

To exclude contamination with carbon from the environment, non-destructive Raman microspectroscopy is used to examine the encapsulated inclusions in place in the zircons. 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 only a few nanometers of thick carbon layers and measure them individually, thus obtaining many data points for a single inclusion to detect possible variations,” Winfried Schwarz, co-author of the study, said.

Zircons are among the oldest minerals on the planet, some older than four billion years. Thus, the researchers believe the recently-analyzed samples can “say” a lot about well over 96% of the earth’s history.

“For the first hundred million years, these crystals represent the only known record holding information on very early conditions on the planet. Inclusions in these oldest zircons have already revealed that water and oceans existed on earth early on, as well as movements of the continental plates,” Vogt said.