2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 1
Presentation Time: 8:00 AM-6:00 PM

The Pulse of the Earth: Implications of U-Pb and Hf Ages from Detrital Zircons

VOICE, Peter J., Department of Geosciences, Virginia Tech, Blacksburg, VA 24060, ERIKSSON, Kenneth A., Department of Geosciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061 and KOWALEWSKI, Michal, Department of Geological Sciences, Virginia Polytechnic Institute and State Univ, Blacksburg, VA 24061, voicep@vt.edu

The Global Detrital Zircon Database (GDZDb) has been compiled over the past three years by the Virginia Tech Sedimentary Geology Group and includes approximately 104,000 individually dated detrital zircon grains (U-Pb) cited in peer-reviewed literature, with approximately 3,500 having associated Hf isotope model ages. This abstract presents new Hf data that adds support for using our dataset to understand the processes of crustal growth and recycling.

Using Hf isotope model ages, it was possible to refine our crustal growth model. The pooled frequency distribution of Hf model ages from the GDZDb is multi-modal and similar in shape to the global pooled detrital zircon U-Pb age distribution with strong peaks present at 2.7-2.5, 2.0-1.7, 1.2-1.0, and 0.3-0.1 Ga and a weak peak at 0.7-0.5 Ga.

One approach is to observe the Hf model age distribution of a single age class (or peak) from the U-Pb database. For example, the 2.5-2.7 Ga peak in the U-Pb database exhibits a peak in Hf at 2.9-2.8 Ga. This suggests that the majority of grains observed formed during a 2.9-2.8 Ga event and were subsequently recycled and reset to 2.7-2.5 Ga (U-Pb age). The 2.0-1.7 peak from the U-Pb distribution exhibits two peaks in the Hf distribution (1.8 and 2.3-2.4 Ga). These two peaks suggest that zircons making up the 1.8 U-Pb peak were derived from two sources: recycled pre-existing grains (formed at 2.3-2.4 Ga) and younger grains that formed during the 1.8 Ga event.

Combining Hf data with the U-Pb age data provides constraints on whether crustal growth was juvenile (newly formed), recycled from pre-existing crustal materials, or a combination of the two processes. This dataset suggests that the prominent U-Pb peaks are dominated by grains derived from crustal recycling.