How Earth's mantle is like a Jackson Pollock painting

In countless grade-school science textbooks, the Earth's mantle is a yellow-to-orange gradient, a nebulously defined layer between the crust and the core.

To geologists, the mantle is so much more than that. It's a region that lives somewhere between the cold of the crust and the bright heat of the core. It's where the ocean floor is born and where tectonic plates die.

A new paper published today in Nature Geoscience paints an even more intricate picture of the mantle as a geochemically diverse mosaic, far different than the relative uniform lavas that eventually reach the surface. Even more importantly, a copy of this mosaic is hidden deep in the crust. The study is led by Sarah Lambart, Assistant Professor of Geology at the University of Utah, and is funded by the European Union's Horizon 2020 research and innovation program and the National Science Foundation.

If you look at a painting from Jackson Pollock, "Lambart says. "These colors represent different mantle components and the lines are produced by these components and transported to the surface.You look at the yellow line, it's not going to mix much with the red or black."

Primitive minerals

Our best access to the mantle comes in the form of lava that erupts at mid-ocean ridges. These ridges are at the middle of the ocean and generate new ocean crust. Samples of this lava show that it's chemically mostly the same anywhere on the planet.

But that's at odds with what happens at the other end of the crust's life cycle. Old ocean crust spreads away from mid-ocean ridges until it's shoved beneath has continent and sinks back into the mantle. What happens after that is somewhat unclear, but both the mantle and the old crust melt, there should be some variation in the chemical composition of the magmas.

So Lambart and her colleagues from Wales and the Netherlands, sought to discover what the mantle looks like before it rises a lava at a mid-ocean ridge. They studied cores, drilled through the ocean crust, to look at cumulate minerals: the first minerals to crystallize when the magmas enter the crust.

"We are looking at the most primitive part of these minerals," Lambart says, adding that they have located the primary minerals they have analyzed only the chemical composition of those minerals. "If you are not really looking at the most primitive part of the world, you may have the message that it is the origin of our work."

They analyzed the samples in different forms of neodymium and strontium, which may be different from those of different types of rock. "If you have isotopic variability in your cumulates, that isotopic variability in the mantle too," Lambart says.

When the blender turns on

That's exactly what the team found. The amount of isotope variability in the cumulates was greater than that in the mid-ocean ridge lavas. That means that the mantle is far from well-mixed and that this variability is preserved in the cumulates.

The likely reason, Lambart says, is that different rocks at different temperatures. The first rock to melt, for the old crust, can create channels that can transport magma up to the crust. Melting of another type of rock can do the same. The end result is one of the many networks of convergences towards the mid-ocean ridge but do not mix-hearkening back to the streaks of paint Jackson Pollock painting.

To get at this finding for geology, a smoothie. No-go farther back than that and picture the blender decanter full of fruit, ice cream, milk and other ingredients. That's like the mantle-discrete ingredients, a different strawberry is from a blueberry. The fully blended smoothie is like the mid-ocean ridge lava. It's fully mixed. At some point between the deep mantle and the mid-ocean ridge, Earth turns on the blender. Lambart says that it has been produced in the past. The blender, it turns out, does not turn into the crust.

Lambart's work helps her and other geologists redefine their idea of ​​how to move material through the surface.

"The problem is a need to find a way to model the geodynamic earth," said Tectonics, "to actually reproduce what is recorded in the rock today," she says. "So far this link is missing."

Now Lambart is setting up a new experimental petrology lab to study the conditions for their chemical compositions during their journey through the mantle and the crust.