Clare Warren on Divining the History of a Rock
Clare Warren with a petrological microscope for looking at thin sections of rocks with polarized light, which reveals optical properties that can distinguish minerals clearly.
Clare Warren is Senior Lecturer in the School of Environment, Earth and Ecosystem Sciences at The Open University. She investigates how, when and why metamorphic rocks record their history, and, specifically, when different geological clocks start and stop 'ticking'. Using such clocks, together with detailed geochemistry, she uncovers the temperatures and pressures experienced throughout a rock’s history. She applies this knowledge to work out just what happens when continents collide and mountains form.
Photo courtesy of The Open University
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Podcast Illustrations
Masang Kang Valley in northwest Bhutan where Clare and her team collected the rocks discussed in the podcast. The glacially carved valley is 4,000 meters high, and sits at the base of Masang Kang, Bhutan’s highest mountain.
Photo courtesy of Clare Warren
An outcrop of the high-pressure rock discussed in the podcast in the Masang Kang valley, Bhutan. This rock was formed as a basalt that intruded into the Indian continent about 800 million years ago, long before the collision with Asia. That collision, which started about 50 million years ago, metamorphosed this basalt at great depths, turning it into a metamorphic rock called eclogite.
Photo courtesy of Clare Warren
Closeup of the eclogite shown in the picture above. The red mineral is garnet, the white mineral is plagioclase, and the grey/green/blue mineral is hornblende. This rock tells us about the lower crust of the Himalaya. It was metamorphosed at temperatures of around 650 C and pressures corresponding to a depth of 42 km about 15 million years ago. How it was brought back to the surface so quickly, geologically speaking, is still unclear.
Photo courtesy of Clare Warren
When rocks are cut into slices about 30 microns thick, they become transparent. In this thin section of the eclogite shown in the previous photos, garnet (pink), quartz (colourless), iron oxide (black) and hornblende (green) can be identified. There are other minerals in the rock too small to be seen at this scale. The relative abundance and juxtaposition of the garnet, hornblende (which has formed by replacing another mineral called pyroxene) and quartz tells us about the high pressure history of this rock.
The field of view is 1 cm across.
Photo courtesy of Eleni Wood
Scanning electron microscope image of the rock shown in the previous images. Minerals are colored by density, with the darker green minerals the least dense, going through blue, pink, orange, and red to the densest.
The image is 80 mm across.
Photo courtesy of Eleni Wood
The lower-pressure rock (a garnet-bearing gneiss) discussed in the podcast, found about a kilometre away from the high-pressure eclogite shown above. Clare Warren and her team proposed a way in which rocks with such diverse histories could end up so close to each other - see below.
Photo courtesy of Clare Warren
This diagram shows how the low-pressure rock (red star) and the eclogite (green star) came from different levels of the Indian continental margin and were metamorphosed in different parts of the Himalayan system at different times. During their transport towards the surface, they ended up next to each other. The red hashed zone is the partial melting zone, where rocks get so hot they partially melt to form the big granite plutons that are dotted around the Himalaya. Although the low-pressure rock revealed a history spanning 34 million years, it was never pushed down deep enough to be converted into eclogite, remaining stuck in the middle Himalayan crust. (The blue star indicates a rock with yet another, even shallower, history that is also juxtaposed with the other two rocks today.)
Courtesy of Eleni Wood
Images through the cores of two different zircon crystals showing tree-ring-like growth rings. The cores are 800 million years old, but the brighter outer rims of these grains record growth during metamorphism accompanying the formation of the Himalaya 20 million years ago. Zircon ages are obtained from the decay of uranium to lead. These grains were extracted from rocks similar to those shown above, but in Sikkim, about 100 km to the southwest of the Masang Kang Valley in Bhutan.
Photo courtesy of Catherine Mottram
One of the instruments used to measure the elemental and isotopic composition of a mineral to very high accuracy. A laser in the black instrument to the left acts like a tiny drill that excavates 10 micron wide pits in the sample, which sits in the yellow box. The excavated particles are sucked into the mass spectrometer (white box on right). A plasma torch ionizes the particles and breaks them down even further. The ions then pass between powerful magnets that separate them by mass, and the different masses are measured on a sensitive detector.
Photo courtesy of Barbara Kunz