Dr. Todd A. Bianco has been granted an NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at Brown University. Using computer simulations of mantle dynamics, rock melting and melt transport, this project will test how different size distributions in mantle heterogeneity affect crust composition. Further, the project will address how realistic processes of melt migration amplify or dampen the signature of mantle composition and mantle dynamics in crust composition. The computer simulations will solve conservation of mass, momentum, and energy in a three dimensional space with properties that capture the rheologic, temperature, and density variations in the upper mantle. In addition, the simulation will include discrete domains of compositional heterogeneity of varying sizes. Predictions from computational simulations will be compared to observations of crust composition at a variety of tectonic settings such as a mid-ocean ridge, and anomalously thick seafloor.
Mantle convection controls much of the evolution of earth, and therefore is an essential piece to understanding observations of the current state of the earth. The size and distribution of compositional heterogeneity in the mantle gives a picture of the state of mantle convection over time. Therefore, by quantifying the relationship between observed compositional variation in the crust with inferred variation in the mantle, this project will, in a broader sense, improve our understanding of how the earth has evolved. In addition the PI will provide undergraduate research opportunities at Brown University, the host institution for the project, as well as research experience to local teachers through Brown University's NSF-funded GK-12 program.
The composition of the crust varies on different length scales, from variations within a single rock, to the differences between the average composition of the continents and the oceanic crust. Regardless of the length scale one considers, the compositional heterogeneity in the crust is certainly caused by compositional heterogeneity in the mantle source of the crust. It is not yet certain, however, how the length scale of compositional variation in the mantle relates to the compositional variation observed at the surface. One hypothesis is that there is a nearly one-to-one relationship between mantle and crustal compositional variation. For example, one might hypothesize that kilometer scale variations in the crust are caused by kilometer scale variations in the mantle. This project focused on understanding the causes of the compositional patterns observed within a single island group, such as Hawaii, Samoa, or Galapagos. The basic pattern and scale at these types of locations is typically that one (or more) extreme composition is observed near the center of volcanic activity with a gradual change (over 10 to 100 km length scales) to a composition that is more similar to normal oceanic crust. The project tested the hypothesis that the island-wide compositional trends observed at the surface need not have a one-to-one relationship to variations in mantle composition because the length scale of mantle dynamics plays an important role in forming patterns in crust composition. There are two mantle dynamic processes that the work explored, and each might have a different effect on crust composition. The first was solid mantle dynamics, and the project explored how different size distributions of mantle heterogeneity affect crust composition. The second was mantle melt dynamics, and the project tested how processes of melt migration amplify or dampen the signature of mantle composition and solid dynamics in crust composition. The work completed included models to test the likelihood of sampling different scales of heterogeneity, a model to track mantle compositional heterogeneity (blobs of different rock types) through a mantle melting zone, and a model to predict the effects of variable melting rates and melt transport on uranium and thorium isotope content in crust rocks. One basic prediction is that if the blobs with different composition are much smaller than the mantle melting zone they pass through, solid mantle dynamics are important to patterns in the crust composition. Another is that variations in melting rate and melt transport rate can affect uranium and thorium isotope content in the crust if melt percolates to the surface quickly compared to the half-life of the isotopes. This project helped improve our understanding of compositional variation in the mantle and the dynamics that should be considered when relating crust composition to mantle composition. It is important to understand how composition varies in the mantle because the characteristic length scale of compositional heterogeneity in the mantle is fundamentally linked to the large-scale dynamics of the solid earth. For example, vigorous mantle convection might have led to small blobs of compositional variation in the mantle, whereas sluggish convection might have led to compositional layering in the mantle. Understanding large-scale dynamics of the earth will improve our understanding of how the earth developed over time and resulted in relatively unique characteristics such as a hydrosphere, a biosphere, continents, and plate tectonics. In addition to the scientific knowledge and models that can be shared with the researching community, this project also supported collaboration with graduate students at Brown University in Providence, RI, and mentorship and research experience for an undergraduate student at Brown. The PI also participated as a visiting science presenter at Vartan Gregorian Elementary School in Providence, RI, which enhanced the school's existing science curriculum.
