The region of earth’s interior known as the lower mantle may be more chemically heterogeneous than previously thought, suggests new research in PNAS Online Early Edition.
The interior of the sphere we call home is a veritable layer cake of hot, pressurized materials. At the center is the solid inner core, surrounded by the liquid outer core. Both are believed to be chemically homogenous, consisting mainly of iron and a nickel-iron alloy, respectively. Floating atop this metal interior is the mantle: a more heterogeneous mix of materials than the core, but the bulk of which is made up of silicate perovskite (MgSiO3) arranged in a perovskite crystal structure. The core and the mantle are separated by the thin core-mantle boundary or CMB.
In the lowermost mantle, resting directly on the CMB, is the mountainous D” region, which averages between 100 and 300 km thick (60 to 180 miles). For decades, seismic studies have identified a very distinct interface between D” and the rest of the lowermost mantle. In 2004, scientists demonstrated that this interface could be explained as the location where the MgSiO3 perovskite transitions into the high-pressure phase known as post-perovskite (Pv to pPv), which would therefore make up most of the D” region.
The new research, led by Robert van der Hilst of MIT’s department of earth, atmospheric and planetary science, identifies the Pv to pPv phase change occurring 400 km to 500 km above the CMB, or roughly 200 km above the observed interface of the D” layer. But regions that high above the CMB do not have sufficient pressure to allow for a phase change of MgSiO3 perovskite. The results therefore indicate that another compound with a perovskite crystal structure is responsible for the phase transition occurring high above the D” interface. This lends support to recent chemical studies searching for other materials found in the mantle region that could explain a Pv to pPv phase change that depth.
Xuefeng Shang and colleagues analyzed seismic data from a 1.5 million km2 region under Central America and a 4.5 million km2 region under east Asia. The team used the massive, publicly available data bank provided by the Incorporated Research Institutions for Seismology (IRIS), which includes data from thousands of seismographic stations.
The D” interface creates a discontinuity in the way seismic waves travel through the lowermost mantle, creating a “seismic reflector.” Current maps of the lower mantle use model extrapolation to predict the location of the D” region above the CMB, in areas where direct data analysis has not been done. The new results, which rely entirely on data analysis and use what Shang calls a “novel” analysis methodology, provide more precise measurements of the location of the D” interface. The results seem to agree with the theory that the D” interface is created by the post-perovskite phase transition of MgSiO3.
The researchers also identified secondary “seismic reflectors” in both geographic regions, between 400 and 500 km above the CMB. Each of those reflectors extends continuously for at least 1,000 km horizontally through the lowermost mantle.
Other studies have revealed possible chemical compositions arranged in the perovskite crystal structure that could produce post-perovskite transitions at 400-500 km above the CMB, including midoceanic ridge basalt or MORB. While Shang and his colleagues cannot confirm that hypothesis, Shang says “it is very convincing based on our seismic observation.”
Shang said the findings indicate that the lower-most mantle is “more complicated than previously thought,” but that this analysis is “just a start point to look again at what happens there.”