Zircon is a strong mineral capable of withstanding extreme geologic pressures and temperatures. It is able to retain geochemical signatures. which allow the timing of geologic events to be determined. The Western Gneiss Region of Norway is one of the largest exposed complexes of ultrahigh-pressure metamorphic rocks in the world. By determining the ages of the grains, we can understand when in time the rocks containing the zircons were at temperatures and pressures high enough to cause partial melting and resetting of the U-Pb ratio. Several of our zircon samples show an original core dated at ~1600 million years old with rims dated to ~400- 420 million years.
As a mineral grows, it uses elements in the vicinity; these elements then become depleted in the surrounding area and cause chemical zoning in the mineral. Oscillatory zones are extremely common in minerals, especially those with an igneous history. Sector zoning is less common, as such, it is not well understood. The presence of sector and oscillatory zoning in zircon grains is often used to interpret the geologic history. The study of sector zoning in other minerals is used to understand sector zoning in zircon in this paper. The zircon grains were imaged by cathodoluminescence (CL) to compare the pattern of zonation of each grain with the spot age location within the grain.
Zircon grains were picked from nine different rock samples. Sector zoning and oscillatory zoning often occur together; when they are together they are often in the grain core. The samples that contained the most sector-zoned grains were the migmatitic gneiss and the granitic gneiss. In the migmatitic gneiss, 13 grains contained sector zones. Of those grains, nine had oscillatory zoning as well. The granitic gneiss contained 25 zoned grains. The basement rocks (basalts) contain few zircons and with the exception of a few outliers contain no sector zoning. Although there is a relationship between sector and oscillatory zoning, the two types generally occur in different parts of the grain. The core is usually oscillatory zoned while the rim is sector zoned. Sector zones are not found to be correlative with age, except where the sector zones are located within the cores of the grains.
Megan Regel, ’08 Aurora, IL
Emily Walsh, Ph.D.
Sponsor: Emily Walsh