UC Santa Cruz

Abstract

Our understanding of the chemical and physical structure of the mantle is driven by connections between seismologic observations and experimental results. The mantle, which makes up the largest portion of the planet by volume, is heterogeneous, as suggested by seismic discontinuities. Sources of heterogeneity are largely from the surface of the Earth: both the basalt that forms subducted slabs and the volatiles within (and on) the slab provide chemical and thermal heterogeneity to the deep Earth. Four of the chapters in this dissertation concern volatile stability (carbon and nitrogen) and the last chapter is focused on the physical and chemical differences in slab strength and deformation.

To understand the properties of materials in the deep Earth, we need to simulate high pressure conditions that occur at depth. The technique used to generate ultra-high pressures is a diamond anvil cell (where a sample is placed between two diamonds). These cells operate due to the relation of pressure = force/area, such that with a small area (from the tip of the diamonds) large pressures can be generated with relatively little force. Since the diamonds are optically transparent, we are able to probe spectroscopically with light (Raman and infrared spectroscopy) to detect changes in local bonding environments. Additionally, we are able to conduct X-ray diffraction in situ to measure density changes to the crystals and interatomic distances within the high pressure crystal structures. We can reliably generate pressures above transition zone pressures (25 GPa) all the way up to the core mantle boundary pressure (135 GPa).

Nitrogen and carbon are ubiquitous on the surface of the Earth and are essential for life; their cycling on the surface of the Earth is well constrained, however their concentration transport and stability in the mantle are still debated. It is generally accepted that volatiles are transported into the mantle via subducting slabs and expelled at mid ocean ridges and volcanoes. Understanding the amount of nitrogen in the deep Earth and fluxes of nitrogen into and out of can help us understand evolution of the Earth and the formation of a habitable atmosphere. Volatiles, such as carbon dioxide, in the deep Earth affect the Earth by (1) lowering the melting temperature of peridotite (2) having local effects on the elastic moduli, thus lowering vs and vp in the mantle (3) lowering the viscosity of the mantle, (4) controlling the amount of CO2 in the atmosphere, and (5) producing metasomatic fluids and magmas (such as carbonatites). To better understand volatile stability, I studied the high-pressure behavior of these four minerals.

Chapter 1. Buddingtonite ((NH4)AlSi3O8) to help understand nitrogen’s subduction as ammonium in a silicate framework (in this case, sanidine structure).

Chapter 2. Shortite (Na2Ca2(CO3)3), an alkali carbonate which is commonly found in carbonatite alkali rich eruptions to understand carbonatite magmas at depth.

Chapter 3. Dolomite (CaMg(CO3)2), a carbonate mineral commonly subducted to study carbonate bonding in the lower mantle.

Chapter 4. Bastnäsite ((Ce,Nd,La,Pr)CO3F), a rare earth fluorocarbonate was studied to understand rare earth elements behavior in a carbonate matrix.

The fifth chapter concerns the silicate material that basalt transforms into after being subducted—a majoritic garnet assemblage. We investigate the deformation of garnet to understand (1) it’s strength relative to other mantle phases; (2) it’s plastic deformation mechanism; and (3) it’s influence on the seismic anisotropy of the upper mantle

Chapter 5. Natural pyrope (Py2Al1) was studied to understand the strength of the subducted slab at depth.