Using high precision titanite data to examine U/Pb zircon age data spread on concordia (2013)

Undergraduate: Laura Pianowski

Faculty Advisor: Drew Coleman
Department: Geology

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With recent advancements in high precision zircon dating, we are now able to determine the crystallization age of a zircon with an error of around 0.1% by measuring uranium and lead isotope ratios with a mass spectrometer. While such precision has allowed us to more accurately pinpoint the crystallization age of an igneous rock, it has also revealed spread in the zircon ages on the U/Pb concordia within a group of zircon grains from a single rock sample. Obtaining an age higher than the actual crystallization age of the magma is possible if part of the zircon grain was inherited from an earlier crystallization event, and obtaining an age lower than the actual age is possible if lead has escaped from the crystal lattice of the zircon grain since crystallization. In order to determine which age is accurate, a secondary dating method can be implemented. This study measured U/Pb isotope data from titanite grains extracted from four samples that had already been dated using zircon grains, each of which had a spread of about one million years in the zircon ages. The measured titanite ages did not consistently line up with either the higher or lower zircon ages, but instead varied with each sample, indicating that zircons are affected by both inheritance and lead loss, or a combination of the two, and that high precision zircon data should be combined with a secondary dating method to determine an accurate crystallization age.