Geospeedometry in an ancient subducting slab
Gray Bebout and his international colleagues have discovered that a geological event involving the fracturing of rocks more than 40 miles beneath the earth’s surface occurred in the relative blink of an eye.
“We usually think of geological processes as occurring over many thousands to millions, or even billions, of years,” says Bebout, a professor of earth and environmental science.
“Our group has determined that the formation of a mineralized fracture, or vein, in the Tianshan [‘celestial mountains’] of western China could have formed over a time period as short as 200 years, which for geologists is very short.”
The provocative results were published last year in Nature Geoscience, a multidisciplinary monthly that brings together top-quality research from across the spectrum of earth sciences and is one of the journals of the Nature Publishing Group, which publishes Nature Magazine.
Bebout and researchers from Germany, Switzerland and the Czech Republic have been studying samples of metamorphic rock collected in the Tianshan. Using these samples, they examined how water-rich fluids were released at a depth of 70 kilometers (about 42 miles) within a subducting oceanic lithospheric slab.
Subduction zones are the sites of the underthrusting of one tectonic plate beneath another, where the plates converge. Subducting plates are known to sink to great depths in the Earth, in some cases perhaps as deep as about 2900 km (the base of the mantle).
Geologists have long been dating geological events using radioactive isotopes such as rubidium or strontium, but these methods are unable to resolve the durations of geological events of the type that Bebout’s group examined. One new approach in the field of “geospeedometry” is to determine the duration of a particular geological event by examining the distances over which multiple isotopes of the same element have diffused during that event.
Using a combination of rubidium-strontium and lithium isotopes, Bebout and his team have determined that the formation of a vein (representing a fluid flow event) approximately 317 million years ago (give or take 5 million years) lasted for only a very short period of time, perhaps as short as 200 years.
The assessment of the duration of the vein formation event is based on the measured diffusion distances of the two stable isotopes of lithium (7Li and 6Li). The lighter isotope is expected to diffuse more rapidly than the heavier one and should therefore diffuse over a larger distance during the same period of time.
Bebout’s role in the study was to help set up and evaluate the team’s calculations and to provide measurements of nitrogen isotope compositions for the same vein.
The global nature of modern geological research enabled Bebout to make significant contributions working with European colleagues who have collected samples in China and measured their geochemistry in analytical laboratories in Europe and at Lehigh.
The group plans further research to continue to explore characteristics of the ancient oceanic subduction in the Tianshan, partly through additional attempts at geospeedometry.
Bebout’s laboratory at Lehigh is especially designed to study nitrogen isotopes, which are expected to shed further light on the fluid flow in the Tianshan. Study of the rates and durations of processes such as these sheds light on the mechanisms of material cycling among Earth’s major reservoirs and, more specifically, can help us understand how volcanoes are formed in subduction zones (e.g., around the Pacific Ocean “Ring of Fire”).
In separate projects, Bebout often visits a laboratory in Japan to use instruments to analyze the geochemistry of rocks collected in the Italian Alps, California and Cyprus.
Posted on Monday, April 29, 2013