Although the Earth’s mantle layer makes up 84% of the planet’s volume, mantle rocks are rare at the surface. However, at mid-ocean ridges, tectonic plates pull apart, and deep material from the mantle melts and reaches the Earth’s surface.
The moment that molten rock hardens into volcanic basalts, the isotopic composition of the mantle region from which it welled up gets preserved. The characteristic isotope ratios for fresh basalt samples collected from all over the world, coupled with knowledge of known decay rates, allow scientists to determine the origin and history of the mantle materials from which magmas are extracted.
Through conducting such analyses and using a different sampling strategy, Mougel et al. found that the mantle is even more heterogeneous than decades of mid-ocean ridge basalt investigations had previously predicted. The team collected samples of basaltic glass with the French submersible Nautile from a small stretch of the East Pacific Rise—a mid-ocean ridge off the western coast of Mexico.
They cleaned the basalt chips with water and ethanol and heated them until all solids turned into gas. The gas was extracted, purified, and siphoned off into a mass spectrometer to determine the helium isotopic composition of the rocks.
Despite the small study area, the authors found a wide range of both helium and lead isotopes in the sampled rocks, indicating that the mantle is highly heterogeneous beneath this 15-kilometer stretch of mid-ocean ridge. This result was not expected because isotopic compositions of basalts tend to be quite homogeneous over small spatial scales on a single fast-spreading ridge segment.
The authors note that the isotopic ratio of helium to lead that they detected in some of the samples describes a mantle component that had never been observed before at other mid-ocean ridges, highlighting how the composition of source material for ridges across the world can vary from place to place.
On the basis of these isotope analysis, the authors speculate that the study area formed from two mantle sources: a typical upper mantle reservoir and an ancient reservoir with a deep continental origin. (Geophysical Research Letters, doi:10.1002/2014GL062921, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2015), New insights into the composition of inner Earth, Eos, 96, doi:10.1029/2015EO036343. Published on 2 October 2015.