Experimental Geochemistry Facilities
The Watson Lab
Humans are unable to directly observe most Earth processes. We can neither travel substantially beneath the Earth’s surface, nor are we able to turn back the clock and witness the numerous changes within the planet over the last 4.5 billion years. Despite our limits, we know the approximate range of physical conditions in the Earth, both now and throughout time. Earth scientists are able to characterize the deep Earth by monitoring the behavior of seismic waves, and their evaluations constrain the nature of the materials and processes at depth. And we understand the chemical composition of few deep earth materials brought to the surface by Earth’s dynamic lithosphere. With this knowledge, it is therefore reasonable to create experiments which examine the effect of relevant physical conditions on specific chemical systems. This method of investigation is known as experimental geochemistry, and it provides tremendous insight to those processes at work in our planet.
The experimental facilities here at RPI were assembled under the direction of E. Bruce Watson. These are used to examine a wide-range of processes in geologically relevant materials. Mostly, the lab efforts characterize transport phenomena: the transfer of material or energy within naturally occurring chemical systems.
Important Variables: Temperature and Pressure
The Earth is hottest at its core, and cools as one approaches the surface. Additionally, as one goes deeper into the earth, the pressure increases because of the accumulated weight of the overlying rocks. The cartoon to the left shows a thin slice through the upper part of a continent (the distance is not even half of a percent of the Earth's radius). Even over this relatively short distance once sees a dramatic increase in temperature and pressure. As one goes 20 km into the Earth, the temperature increases from near 0oC to 250oC, and the pressure increases from 0.1 MPa (atmospheric) to 660 MPa (roughly 6600 times atmospheric).
Experimental geochemistry seeks to examine processes by recreating the conditions of the deep Earth at its surface. It is therefore necessary to replicate a range of pressures and temperatures within the lab. This may be done somewhat easily using pumps and furnaces configured to reach the conditions of interest.
This piece of equipment is similar to an electric kitchen oven: a heating element (resistant wire) converts electrical input into thermal energy, which heats a volume of air, the temperature of which is monitored and the electrical input is adjusted to achieve or maintain a preset temperature. The materials to be heated may be open or sealed, but no external pressure is applied - in other words, the sample experiences near-atmospheric pressure. However, unlike the your average household oven, this furnace is configured to heat its center very evenly to temperatures up to 1200oC (2192oF). Other furnaces in the lab with U-shaped elements made of MoSi2 achieve temperatures up to 1800oC (3272oF). These devices are particularly useful for examining how temperature influences chemical processes likely to be at work in the Earth.
This device is similar to the one-atmosphere furnace. However, in the cold-seal apparatus the sample is placed inside a vessel of a strong alloy. The bomb, as the alloy vessel is called, is sealed outside the furnace (hence the name) and may be pressurized with water to 400 MPa (56,000 PSI). Because very high temperatures will weaken the bomb, temperatures may not exceed 1000oC. Despite this limitation, the device is extremely useful, as this range of pressure and temperature well matches that of the upper and middle crust of the Earth.
Although greatly different in appearance, this device utilizes a furnace and pump just like the cold-seal apparatus. The heating element is different; instead of a high-temperature wire, it uses a disposable cylinder of graphite. And instead of water pressure on the sample, the pump forces a piston against the starting components, which are packed in crushable solid materials which are placed snugly into a thick-walled cylinder of rigid tungsten carbide and steel. With sufficient pumping, the machine produces uniform (isostatic) pressure around the sample. This apparatus reaches a wide range of temperatures, from 600-1700oC (1112-3092oF), and pressures, from 500 MPa to 3500 MPa (70,000-490,000 PSI), covering the extent of conditions relevant to processes in the lower crust and upper mantle of the Earth.