Between a Rock and a Hard Place

Experimental petrology

PhD reflections: Sorcha

Between a Rock and a Hard Place began as an Earth Science PhD blog in February 2013, as a place to ramble on about PhD life and general science topics. Almost two years later, some of the contributors have finished, others have submitted, and the rest are nearing the end. Over the next few weeks, the BaR contributors will be sharing some reflections on their PhD experiences. Taken from an original post on the Bristol Doctoral College blog.

First up, the newly crowned Dr Sorcha McMahon!

PhD highlight:

Working in the lab was both the most exciting, and most frustrating, aspect of my PhD. Rather than jetting off to exotic field locations, I spent most of my days heading downstairs to the basement to carry out experiments on a piston cylinder apparatus to provide insights into deep mantle melting.

Despite shedding blood, sweat and tears down there, the satisfaction of deliberately ending a successful experiment is hard to beat! Lab work was made all the more fun when shared with fellow experimentalists – discussing similar experiences, particularly failures (unfortunately rather common!) proved to be incredibly useful in planning future experiments and trying different approaches to improve methods.

Sorcha using the piston cylinder apparatus in the Petrology labs at the University of Bristol. She is manually topping up the pressure (to 30 kbar, equivalent to ~100 km depth!) whilst checking the run temperature recorded by the thermocouple – lots of multitasking! Photo credit: University of Bristol

Top tip:

My top tip is to talk to lots of people in the lab, and attend lots of seminars/discussion groups, about different techniques that you could possibly try out on your samples. Most lab-based PhDs tend to be a case of trial-and-error for the appropriate method so the more options that you are aware of, the better!

Science Snap (6): SEM images of a high-pressure experiment

Sorcha McMahon is a third year PhD student in the School of Earth Sciences at the University of Bristol. Sorcha is investigating how strange igneous rocks called carbonatites may have formed, using both natural samples and high-pressure experiments.

Sorcha's SEM SS

These back-scattered electron (BSE) images are a typical view of one of the high-pressure experiments that I run on the piston-cylinder apparatus, here in the BEEST labs at the University of Bristol. Such photographs are taken using the Scanning Electron Microscope (SEM), and are an essential stage in the analysis of run products as the different shades, textures and compositions are used to identify different mineral and melt phases.

The image on the right shows an entire capsule (a metal container that holds the powdered sample) and its contents after it has experienced conditions of 1375oC and 30 kbar (equivalent to ~100 km depth) for 24 hours. The AuPd capsule (an alloy that can withstand up to ~1400oC before melting at this pressure) appears brighter than the phases produced because this material has a higher atomic mass than the minerals (more information in Charly’s post).

The two images on the left show closer shots of the same experiment, labelled with the different minerals. In varying shades of grey; garnet, olivine, clinopyroxene (cpx) and orthopyroxene (opx) are typical minerals found in lherzolite (‘normal’ mantle) assemblages. As I am working in a synthetic carbonate-bearing system (CMAS-K2O-CO2), my run products contain an abundance of carbonate minerals, such as dolomite. At higher temperatures, melt may be observed, and is identified by its ‘streaky’ quenched texture.