5. How mantle convection controls redox status of Earth's mantle? more here

Working with mineral physicists, we find that redox of mineral affects density of lower mantle minerals. We find for similar composition, more oxidized material is less dense than more reduced material. This redox-induced density contrast can cause a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser reduced material to the core-mantle boundary. We suggest that the resulting heterogeneous redox conditions in Earth's interior can contribute to the large low-shear velocity provinces in the lower mantle and the evolution of atmospheric oxygen. Below is a figure of snapshots from geodynamical calculations showing the process of segregation between reduced and oxidized material caused by intrinsic density differences. Red and blue parts represent reduced and oxidized compositions, respectively.

4. What causes seismic anisotropy in Earth's lowermost mantle? more here

I work with seismologists and mineral physicists to understand the seismic anisotropy in Earth's lowermost mantle. Below is a snapshot from geodynamic modeling, in which we use tracers to track the deformation of slabs. The deformation information is latter fed into mineral physics models to calculate seismic anisotropy. We find that observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core-mantle boundary.

3. How much melt is produced and volatiles are released at mid-ocean ridges? more here

The Earth's surface and deep mantle interacts. Here, we develop methods to quantify melt production and degassing rate from global 3D mantle convection models. With this method, we build up the bridge between deep mantle convection and surface volcanism. Our results show that melt production at mid-ocean ridges is mainly controlled by surface plate motion history, and that changes in plate tectonic motion, including plate reorganizations, may lead to significant deviation of melt production from the expected scaling with seafloor production rate. We also find a good correlation between melt production and degassing rate beneath mid-ocean ridges. The calculated global melt production and CO2 degassing rate at mid-ocean ridges varies by as much as a factor of 3 over the past 200 Myr. We show that mid-ocean ridge melt production and degassing rate would be much larger in the Cretaceous, and reached maximum values at ~150-120 Ma. Our results raise the possibility that warmer climate in the Cretaceous could be due in part to high magmatic productivity and correspondingly high outgassing rates at mid-ocean ridges during that time.

(a-d)melt flux at mid-ocean ridges at 200, 150, 120 and 0 Ma (e-f)temperature at 45 km depth at 200, 150, 120 and 0 Ma

This video shows the amount of partial melting produced at mid-ocean ridges in the past 200 million years. The gray color shows the locations of continents from past plate motion model in (Seton et al., 2012). For more information, visit: ciei.colorado.edu/~mli or http://onlinelibrary.wiley.com/doi/10.1002/2016GC006439/

2. Why is the geochemistry of hotspot basalts so complex? more here

Below we show that as the oceanic crust is subducted to the lowermost mantle, it takes multiple pathways. Some oceanic crust is directly entrained into mantle plumes, but a significant fraction enters the primordial piles. As a result, plumes forming on top of piles entrain a variable combination of relatively young oceanic crust directly from the subducting slab, older oceanic crust that has been stirred with ancient more primitive material and background, depleted mantle. Cycling of oceanic crust through mantle reservoirs can therefore reconcile observations of different recycled oceanic crustal ages and explain the chemical complexity of hotspot lavas.

(a)oceanic crust is directly entrain into mantle plumes (b)some oceanic crust accumulates on top of the primordial piles (c)the accumulation of oceanic crust sinks into the piles. At the same time, plumes entrain variables components. (d-e)the oceanic crust within the piles is stirred with mantle flow

This video shows a numerical simulation of Earth's deep mantle. The top panel is temperature and the bottom panel is composition which includes three components: the more-primitive reservoir at the lowermost mantle (cyan), the subducted oceanic crust (yellow) and the depleted background mantle (black).

1. Where is subducted oceanic crust? more here

One hypothesis for the cause of LLSVPs is that they are thermochemical piles caused by accumulation of subducted oceanic crust at the CMB. However, although subducted oceanic crust is denser than its surroundings, it was unclear whether thin oceanic crust could provide enough negative buoyancy to overcome viscous stresses that act to stir the crust into the mantle. Our results find that viscous forces caused by mantle plume regions are stronger than the negative buoyancy of subducted oceanic crust, so the crust is easily stirred into the background mantle. A small amount of crustal material may collect at the base of plumes, but it is sufficiently entrained away into the plume and does not accumulate into larger-scale thermochemical structures. Therefore, it is difficult for a thin subducted oceanic crust (~6 km) to accumulate into large piles at the CMB with the same size as LLSVPs.

(a) Snapshot (at 1.0 Gyr) of the nondimensional temperature superimposed with oceanic crust superimposed (shown in green). (b) Logarithm of nondimensional viscosity at 1.0 Gyr. The black lines are contours of viscosity with an interval of 0.5. (c) Nondimensional temperature and oceanic crust at 2.0 Gyr. (d) Nondimensional temperature and oceanic crust at 2.8 Gyr

The results of our numerical melding shows that, under present-day Earth-like conditions, it is difficult for the subducted oceanic crust to accumulate into large thermochemical piles at the core-mantle boundary. The three panels are temperature, viscosity and composition respectively. We see that the major parts of the subducted oceanic crust are carried up by upwelling plumes and mixed with ambient mantle, instead of settle at the core-mantle boundary.