Structural Geology Projects IV

Rheological properties of the Mojave mantle

by Rachel Bernard (Ph.D. student)

Rheology is the study of flow in liquid and solid materials.  In the Earth’s lithospheric mantle, the rheological properties of minerals control processes such as thermal convection, localization into narrow zones (e.g. plates), and alignment of minerals and fabrics that can be detected seismically. To investigate the rheological properties of minerals in the mantle, and their effects on these geological processes, we are studying the microstructures of rocks sourced from the mantle that have made their way to the Earth’s surface. Peridotites – an ultramafic rock type representative of the composition of the mantle – can come to the surface as xenoliths, which are rock fragments that get entrained and brought the surface during volcanic eruptions.

Using deformed peridotite xenoliths found in the Cima volcanic field in the Mojave Desert of southern California, we are characterizing the rheology of the lithospheric mantle in the region.  A suite of analytical techniques are being used to investigate these rocks and the minerals within them.

Figure 1. CT scanning reveals a foliation and lineation in this piece of deformed peridotite. Elongation of the mineral spinel (blue color) is what defines the fabric. Other minerals (pyroxene and olivine) are colored purple. Zones of alteration in the rock appear pink.

First, a CT scanner, often used in medicine, uses x-rays to produce tomographic images (or “slices”) of the rock, allowing us to see the fabric within it without cutting it open (Figure 1).  This is necessary because linear fabrics are sometimes not clear to the eye, and the next step – electron backscattered diffraction (EBSD) – requires thin sections be cut parallel to lineation for meaningful analysis.

Once we’ve identified the lineation and foliation in the rock and cut it properly, EBSD is then used to investigate the fabrics in the material – i.e. how minerals are deformed and oriented relative to stress on the rock.

Understanding the lattice preferred orientation (LPO) of olivine (a major mineral in peridotites) in particular has important implications for the study of seismic anisotropy, which is used to understand mantle convection, flow, and structure. This is because olivine LPO is thought to be the biggest factor affecting seismic anisotropy in the upper mantle.

A related aspect of this project focuses on understanding the role water and stress content have on LPO development in olivine. EBSD is being used to measure LPO, and Fourier infrared technology (FTIR) and secondary ion mass spectrometry (SIMS) is being used to measure the water content of the peridotites. Grain size paleopiezometry is used to estimate the stress magnitude undergone by the rock. Looking at these factors in the diverse peridotite xenoliths of the Cima volcanic field helps reveal whether the effects of water and stress on LPO observed in experimentally deformed rocks are also observed and applicable to nature.

Figure 2. Phase map of peridotite sample obtained from EBSD. Map created using HKL Channel5 software.

Figure 3. EBSD Inverse Pole Figure (IPF) map of olivine plotted with respect to the x-axis (lineation direction).

Figure 4. Pole figures of olivine orientation obtained from the EBSD maps. The horizontal axis represents the rock lineaiton. This rock has a moderate lattice preferred orientation in olivine.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Publications/Abstracts related to this research project:

Behr & Hirth, 2014.  Rheological properties of the mantle lid beneath the Mojave region in southern California.  Earth and Planetary Science Letters, 393, p. 60-72.