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, National Science Foundation-funded paper paints an even more intricate picture of the mantle as a geochemically diverse mosaic, far different than the relatively uniform lavas that eventually reach the surface. Even more importantly, a copy of this mosaic is hidden deep in the crust. 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 floor 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 a continent and sinks back into the mantle. What happens after that is somewhat unclear, but if both the mantle and the old crust melt, there should be some variation in the chemical composition of the magmas. That means that the mantle is far from well-mixed and that this variability is preserved in the cumulates.
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