Tiny melt traces shed light on the Colorado Plateau's explosive past

Deep beneath Earth's surface, some minerals hold clues to processes that happened millions of years ago. One such mineral is lawsonite, which forms under high pressure and relatively low temperatures. What makes it significant is its ability to store water within its crystal structure and carry it deep into Earth as tectonic plates move beneath one another.

When exposed to high temperatures, lawsonite can break down and release this water, potentially causing surrounding rocks to partially melt. Understanding this process can help scientists learn more about how rocks change underground and how magma forms.

Now, researchers in Japan and Germany have found evidence of this process in an ancient rock from the Colorado Plateau. Microscopic traces preserved in the rock reveal a previously overlooked episode in its journey from deep underground to the surface.

"What makes this discovery particularly interesting is that we found what we believe to be the first evidence of lawsonite releasing water and causing partial melting in a natural rock," says professor Tatsuki Tsujimori of Tohoku University's Graduate School of Science. "These tiny traces of melting can help us piece together what happened deep beneath the Colorado Plateau millions of years ago."

The findings are published in the Journal of Petrology.

An ancient pulse of water

Around 28 million years ago, the Farallon Plate, an ancient tectonic plate sinking beneath North America, released water into the surrounding mantle. This water spread through the rocks deep underground in an event known as the Great Hydration. Scientists believe this event contributed to the region's uplift and powerful explosions that brought rocks from deep underground to the surface.

However, one question remained unanswered: How were these rocks transported from deep underground to the surface?

Microscopic traces of rapid heating

To investigate this, the researchers studied an ancient eclogite xenolith, a fragment of rock that formed under extremely high pressure deep inside Earth.

By examining the rock under high-resolution microscopes, the researchers discovered unusual changes in omphacite, one of the main minerals in eclogite, along with tiny pockets of silica-rich glass. These findings suggest that the rock was heated to temperatures above 800°C (1,472 degrees Fahrenheit), causing parts of it to melt.

Although the original lawsonite was no longer present, it had left behind distinctive mineral structures and traces of melted material, suggesting that it had broken down and released water during heating deep beneath the Colorado Plateau.

The researchers also found evidence that this heating occurred relatively quickly. Their calculations suggest that it could have lasted less than 10 years if the rock came into direct contact with hot magma at around 1100°C (2,012 degrees Fahrenheit).

A two-stage journey to the surface

Together, these findings suggest that the rock reached the surface in two stages. First, hot magma carried it upward from deep inside Earth, heating it along the way. The rock then cooled at a depth of less than about 60 kilometers (37 miles) before a later explosive event associated with volcanic activity brought it to the surface.

This provides new insights into how water, magma and rocks interact deep inside Earth, helping scientists better understand the geological history of the Colorado Plateau.