The major goal of our project was to determine what caused North American granitic magmas, that are approximately 1 billion years old, to have considerably higher concentrations of the element zirconium (Zr) compared to most average granite bodies.  One possibility that had been suggested was that the granites contained an unusually large amount of the mineral zircon, not from direct crystallization from the magma but ‘inherited’ from the source region of the magma.  We tested this hypothesis be determining the age of a statistically significant number of zircon crystals in several high zirconium granites using the uranium-lead radiometric geochronological clock.  Several hundred analyses were performed per sample using a rapid analysis method.  If a high proportion of the zircon crystals were inherited from the magma’s source they would be older than the native grains that had crystallized directly from the magma. Our results showed that only a small proportion of the zircon crystals were of the inherited type and thus this could not be the explanation for the extremely high Zr concentrations in the granites.  A second hypothesis we tested was the possibility that extreme amounts of sedimentary material were incorporated into the magmas, thus providing a zircon-rich source, which would lead to high Zr concentrations in the granites as zircon is the primary source of Zr.  If this hypothesis was valid then there would be a record of such sediment involvement based on the oxygen isotopic composition of the zircon (a reflection of the oxygen isotopic composition of the magma).  Magmas that incorporate significant amounts of sediment have unusually high ratios of ¹⁸O to ¹⁶O.  We tested this hypothesis using an in situ method of determining the ratio of ¹⁸O/¹⁶O in individual zircon crystals from several high Zr granites as well as a control group of low to moderate Zr granites.  The oxygen isotope ratios indicate that only a minimal amount of sedimentary material could have been involved in the genesis of the magma.  Our third major test was to see if unusually high magmatic temperatures could explain the high Zr abundance – hotter magmas allow for more Zr to dissolve into the magma. To estimate the temperatures of the magmas we measured the amount of the element titanium (Ti) in the zircon crystals. As it has been shown that the hotter the magma the higher amount of Ti will be incorporated in the zircon crystal structure.  Most zircon crystals have a Ti concentration of only a few parts per million (which translate to temperatures of roughly 700 – 750 degrees Celsius).  For our Zr rich granites we found that many of the zircon crystals contain four to five times as much Ti as most granites which translates to magmatic temperatures close to 1,000 degrees.  Thus we have demonstrated that these granites are not only some of the most Zr rich large granite bodies in the world but were also some of hottest granitic magmatic bodies in the world.  The research was also extremely beneficial to expand the scientific literacy of students as it formed part of the basis for the research projects of two Masters degree students and two PhD candidates.

Last Modified: 07/30/2020
Modified by: Scott D Samson