Microscopic Melt Traces in an Ancient Rock Reveal How Water Shaped the Colorado Plateau's Past

A fragment of rock from the Colorado Plateau, carried up from deep inside the Earth, has turned out to hold a record of something researchers had not seen in a natural sample before. Under high-resolution microscopes, the rock showed signs that a mineral called lawsonite had given up its water and caused the rock around it to partly melt.

Lawsonite forms under high pressure and relatively low temperatures. Its useful trait is that it can lock water into its crystal structure and haul it deep into the Earth as one tectonic plate slides beneath another. If that mineral is later exposed to high temperatures, it can break down and release the water, which can cause surrounding rock to partially melt. That mechanism bears on how rocks change underground and how magma forms.

The new work comes from researchers in Japan and Germany, from Tohoku University, JAMSTEC, the University of Bayreuth, and Ehime University. Professor Tatsuki Tsujimori of Tohoku University's Graduate School of Science said the team believes it has found the first evidence of lawsonite releasing water and causing partial melting in a natural rock. He said these tiny traces of melting help reconstruct what happened beneath the plateau millions of years ago.

The setting matters. About 28 million years ago, the Farallon Plate, an ancient plate sinking beneath North America, released water into the mantle above it. That water spread through the rocks at depth in what is known as the Great Hydration. Scientists believe the event contributed to the uplift of the region and to powerful explosions that carried deep rocks to the surface. What remained unclear was how those rocks made the trip.

To probe that, the team studied an ancient eclogite xenolith, a fragment of rock formed under extreme pressure deep in the Earth and later carried up inside other material. Under the microscope they found unusual changes in omphacite, one of eclogite's main minerals, along with tiny pockets of silica-rich glass. Together these suggest the rock was heated above 800°C, enough for parts of it to melt.

The lawsonite itself was gone. But it left behind distinctive mineral structures and traces of melted material, which the researchers read as evidence that it broke down and released water as the rock heated deep beneath the plateau.

The heating also appears to have been quick. Their calculations indicate it could have lasted less than 10 years if the rock touched hot magma at around 1100°C.

From this the team infers a two-stage ascent. First, hot magma carried the rock upward from deep within the Earth, heating it on the way. The rock then cooled at a depth of less than about 60 kilometers. Later, an explosive event associated with volcanic activity brought it the rest of the way to the surface.

The study was published in the Journal of Petrology and chosen as an Editor's Choice article. Its conclusions rest on one xenolith and on calculations, so the next question is whether similar traces turn up in other samples from the plateau.