Most of the processes that influence the composition and properties of the continental crust are not accessible to direct observation because they occur too deep in the Earth or too slowly. Alternative strategies are thus needed to investigate the deep crust; one of these is high temperature-pressure experimentation. Laboratory re-creation of deep-crustal conditions allows direct investigation of phenomena such as impurity uptake during mineral growth and the rates at which atoms move around in crystals. Insight gained from these laboratory experiments can, in turn, be used to make deductions about processes occurring in the natural environment over geologic time. This project involves implementation of experimental strategies in combination with theoretical models to provide tools for "reading the record" of crustal evolution over geologic time. The project focuses specifically upon "accessory" minerals -- minerals of only modest abundance in the crust, but which are of widespread occurrence and which tend to concentrate key trace elements and radioactive isotopes. Rare crystals of one of these accessory minerals (zircons from Australia) are the oldest known terrestrial materials, dating back 97% of the age of the Earth.
Ongoing research involves three principal sub-projects, the first of which is calibration and development of "geothermometers" for estimating the crystallization temperatures of accessory minerals recovered from crustal rocks. Temperature history is a vital component of the overall geologic history of a crustal sample; experimental studies at RPI are providing detailed knowledge of the systematic variation with temperature of impurity incorporation into zircon, rutile and other titanium-bearing minerals (in zircon, the impurity of interest is Titanium; in rutile, it is Zicronium). The results allow researchers to determine the crystallization temperature of zircon and rutile crystals of unknown origin ("provenance") simply by analyzing them for their titanium and zirconium contents, respectively. This study has already revealed that the world's oldest zircons (formed 4.4 billion years ago) crystallized at temperatures typical of granite-forming processes under present-day geologic conditions. This finding suggests that crustal processes of the very early Earth were not very different from what they are today.
The second sub-project addresses the migration (diffusion) rates of atoms in accessory mineral crystals, focusing upon the possible consequences of rapid atomic movements via extended crystal defects such as sub-grain boundaries and dislocations. Experiments at high pressures and temperatures are used in combination with theoretical models to determine the circumstances under which isotopic age information might be perturbed by unexpected loss of daughter (radiogenic) isotopes unusually fast atomic migrations. The motivation of this work is to identify circumstances under which the significance and usefulness of isotopic measurements on natural samples might be compromised by diffusion-enhancing defects in crystal structures.
The third sub-project focuses on the behavior of atoms of the noble gas helium in accessory minerals. Helium is produced mainly by radioactive decay of uranium and thorium, which are concentrated in most accessory minerals relative to the major rock-forming minerals. Laboratory heating of accessory minerals causes degassing of helium, and the pattern of degassing can be used to constrain the geologically recent temperature histories of rock bodies. This thermal history provides, in turn, information on local erosion rates. Underpinning this "helium thermochronometry" approach is knowledge of the migration rates of helium atoms in accessory minerals, most importantly apatite, zircon and titanite. Much has been learned about He diffusion properties over the past decade by thermal degassing of natural samples, but a few key questions can be more definitively answered through direct laboratory studies of helium atomic migration rates. The experimental program at RPI includes investigations of the effects of crystal orientation and isotope mass (helium-3 vs. helium-4) on helium migration.
