Shards of rock from deep below Mongolia are a window into Earth’s primordial composition.

Earth’s inner heat continuously melts and changes the chemistry of the planet’s crust, and this makes it difficult for scientists to find samples of Earth’s early chemistry. Richard Carlson at the Carnegie Institution for Science in Washington DC and Dmitri Ionov at the University of Montpellier in France scoured Mongolia’s Tariat region and found rocks studded with unusual-looking fragments known as xenoliths.

Chemical analysis of 97 xenoliths shows that they were born beneath the Mongolian crust. The xenoliths are similar in composition to material that, when melted, produces the lava that erupts from seafloor features called mid-ocean ridges. Together these ridges form Earth’s biggest volcanic chain.

This similarity suggests that the Mongolian rocks are a remarkably pristine sample of the large portion of Earth’s interior that melts to feed the mid-ocean ridges, the authors say. The motion of plates of Earth’s crust likely shoved the xenolith source beneath Mongolia hundreds of millions of years ago.