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This project has been focused on using fission-track ages of radiation-damaged zircons to reveal subtle low-temperature (c. 200°C) thermal events in basin sequences.  Much of this effort has been aimed at understanding and quantifying radiation damage in zircon, and dating these damaged crystals from different settings with distinct thermal histories.  The scientific focus of this project has been the glue that has held our research group together and has provided an important framework for these undergraduate and graduate students to operate in.  The effort has been lead by A. Marsellos (post-Doc) and M. Montario (PhD) who have done a wonderful job doing analytical work and innovating in the lab as well as mentoring undergraduate thesis students.  The research done by the undergraduates has focused on specific studies on basin strata that have been heated.  These students include D. Merkert (Paleocene-Eocene Chickaloon Fm., Alaska), L. Ancuta (Paleocene Kootzanahoo Fm., Alaska), J. Wold (Paleocene-Eocene strata of the Power River basin), and ongoing studies by T. Izykowski (Paleocene Orca Group, Alaska) and E. Milde (Campanian-Maastrictian Valdez Group, Alaska).   For the undergraduate students this research effort has been a defining experience in their academic career. 

Zircon undergoes a long-term crystalline-to-amorphous transition by internal radiation damage, which causes internal disorder and lowers retention and stability of fission tracks.  This work uses radiation damage and track stability as sensitive thermochronometer that can date heating in the upper bounds of the oil and gas window (c. 180-200°C).

1.0 Laboratory annealing.  Our laboratory experiments are aimed at understanding the relationship between track stability and radiation damage.  Our analytical approach has been to use chemical susceptibility of zircon and micro-Raman spectroscopy as proxies for evaluating the amount of internal damage.   Annealing experiments were conducted on pegmatitic zircons from the Precambrian Grenville basement of NY State.  Zircons were heated to determine the point of annealing and then progressive annealing of different aliquots was accomplished by incrementally heating to 1050°C.  Radiation damage is progressively removed at higher temperatures and a significant fraction of alpha damage is more resistant to thermal annealing compared to fission damage. 

2.0 Field-based studies.   Samples from basin sequences have been used to further develop techniques in using zircon as a provenance discriminator and to date low-temperature thermal resetting. To better understand the provenance of sedimentary rocks, our work has expanded the traditional bounds of what can be dated by ZFT because we have developed a technique to date old zircon grains with high track densities using a SEM (we call this approach SEM HD FT Dating, see Montario and Garver, 2008).

One example is from the Cambrian Potsdam Formation has ZFT ages that are essentially unreset and the provenance of this unit can be directly tied to the underlying Grenville basement. Approximately 90% of the U-Pb ages fall between 950 and 1200 Ma. Zircon FT (ZFT) ages from the same suite of samples have component populations of ∼540, ∼780, and ∼1200 Ma, with single-grain cooling ages as old as 2.1 Ga. The FT data indicate that there is not widespread resetting around the Adirondack massif and a clear thermal event related to the rifting of the Iapetus ocean is revealed in this cover strata.

Another provenance study involves understanding the characteristics of detrital zircon deposited in the Powder River Basin (PRB) during unroofing of the Bighorn Mountains.  U/PB ages of zircons from the Lance and Fort Union formations are dominated by Cretaceous grains, but zircons in the Eocene Wasatch Formation are dramatically almost exclusively Precambrian. SEM HDFT ages from these zircons show that the basement experienced a dramatic thermal event at c. 800 Ma, which is previously undocumented.

We have made important progress in dating the annealing of detrital zircon in different settings.  Several of our studies have been on rocks from different field conditions that have zircons with varying amounts of internal radiation damage that have been heated to the lower part of the critical thermal zone for fission track annealing (180-250°C).  We are developing techniques based on radiation damage, Raman spectroscopy, and statistics to isolate reset grains for non-reset grains or partly reset grains.

The Cambrian Postdam Formation in eastern NY was buried heated in the Paleozoic. ZFT Grain ages from the Potsdam Formation in the eastern Adirondacks are over-dispersed with a large fraction of grains younger than deposition. Peak fitting analysis of the grain above the inferred alpha dose threshold (α-DT) yields two component populations at 270 Ma (-30.6/+34.4) and 460 Ma (-22.0/+23.1), which likely corresponds to the Alleghenian and Taconic orogenies (Montario, in prep.).  

In Greece, the Phyllite-Quartzite unit (PQU) rocks were derived from Paleozoic-cooled rocks in the Cretaceous, subducted, heated and exhumed in the Tertiary. FT dating of zircon from quartzites of the PQU in the Hellenic fore-arc reveal common and typical over-dispersion of the age distribution. We separate fully annealed grains from others using mineral properties revealed by Raman spectroscopy. This “Raman discrimination” allows calculation of a ZFT-LR age (“low retentive”), which allows for a dramatic reduction in grain-age dispersion, and gives greater insight into the tectonic significance of the cooling ages (Marsellos et al., 2010; Marsellos and Garver, 2010).

The Chugach Accretionary Complex and flanking basins (Alaska) provide us with an excellent opportunity to see and study large-scale partial annealing of zircon. The Chugach-Prince William(CPW) terrane is a Campanian-Eocene accretionary complex, dominated by turbidites, that has a remarkable record of widespread thermal resetting and we now see this as an ideal test case for our developing methodology of dating strata that have been heated to c. 200°C.  We have evaluated strata in flanking basins that have been inferred to have been derived from the Chugach (Matanuska basin – Merkert, 2009 and the Kootzanahoo in SE AK – Ancuta, 2010; Ancuta and Garver, 2010).  Our work now is focused on directly dating resetting of this thick marine turbiditic section in Valdez (Milde, in prep.), Cordova (Izykowski, in prep.) and Garver et al., 2010.