Rocks Reveal Secrets of Ancient Untouched Mantle Reservoir

A diamond-bearing kimberlite rock, taken from a mine in the United States. The diamonds are not visible in this photo. (Woudloper via Wikipedia)

(CN) – Volcanic rocks that sometimes carry diamonds on their trip to Earth’s surface are reliable roadmaps of the planet’s evolution over time – drawing from a reservoir in Earth’s mantle likely untouched by other forms of shifting tectonic plates for billions of years, scientists said in a study Wednesday.

Kimberlites, igneous rocks that form about 100 miles below Earth’s surface, include minerals that can only be found at great depths. But all originated from the same primordial source, according to the study published in the scientific journal Nature.

Researchers say kimberlites have been surfacing for at least 2.85 billion years and provide a unique window into the chemical evolution of deep-mantle regions throughout the planet’s history.

The igneous rocks that sometimes carry diamonds and come to the Earth’s surface through volcanic explosions are believed to be bits of the planet’s primitive mantle.

According to researchers, the deep-mantle reservoir containing the kimberlites has existed for at least half as long as Earth has and probably longer. The ancient source is also likely isolated since it shows no signs of contamination from billions of years of crashing tectonic plates.

The preservation of primitive mantle and what exists deep below Earth’s crust remains a topic of debate and the study’s authors agree it’s a difficult question to answer. Most samples are isolated and provide snapshots in time that may be part coincidence, but data from kimberlites are unique in that they represent a long-term evolutionary process of a relatively different source from other samples.

Kimberlites are the best source to estimate a primitive-mantle model, according to the study’s authors. While other events beneath the planet’s surface may have broken into the pristine mantle’s space, primitive mantel remained relatively intact. Researchers say an event that took place 200 million years ago marked a turning point in the evolution of Earth’s mantle and could explain the formation of Pangea, the first supercontinent which eventually broke apart to form the continents we see today.

Figure of the supercontinent Pangea, produced using PALEOMAP53 and GPlates 2.0. White circles provide indicative locations for primitive kimberlites, and gold circles indicate anomalous-kimberlite localities. Red lines indicate subduction zones at the western edge of Pangea. (C.R. Scotese / PALEOMAP via Nature)

 

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