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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 to serve as sensitive thermochronometer that can date heating in the upper bounds of the oil and gas window. Damaged zircon grains have relatively low annealing temperatures when subjected to moderate heating (c. 180-200°C and higher) under geological conditions. We are developing this dating technique on several well studies zircon-bearing clastic rocks.

In addition to studies of zircons annealed in geological conditions, we initiated laboratory experiments to address the relationship between track stability and radiation damage. These laboratory experiments focused on well-characterized samples that had considerable radiation damage. Our analytical approach has been to use chemical susceptibility of zircon and micro-Raman spectroscopy. These experiments were designed to understand the annealing of alpha damage in the context of fission-track annealing (both types of radiation damage).

The overall goal of our research has been to understand how radiation damage in zircon affects annealing of fission tracks. Annealing and resetting of fission tracks in damaged grains will allow use of damaged zircons as sensitive low-temperature thermochronometer. Most of our experimental work is completed and the main part of the project is being written up at this time. The following sections outline some of our primary findings to date:

1.0 The Cambrian Potsdam Formation has ZFT ages that are esentiailly unreset and the provenance of this unit can be tied to the underlying Grenville basement. We separated zircon from seven samples of the Cambrian Potsdam Formation dated these detrital grains using SEM HDFT dating (ZFT) and LA-MC-ICPMS to determine U/Pb ages on representative samples. To the west, where thermal maturity is low (CAI ~ 2), ZFT grain ages are older than the age of deposition but samples from the eastern Adirondacks where the rocks have greater thermal maturity (CAI values ~4½ -5), have a ZFT population younger than depositional age that is inferred to reflect post-depositional heating. 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 most important observations from the FT data are that there is not widespread resetting around the Adirondacks.

Radiation damage facilitates the annealing process in grains heated after strata have been deposited. In geologic conditions when high-damage grains are brought to temperatures of c. 180-200°C heterogeneous annealing results. To understand the annealing process, we have investigated annealing in the lab and in the field.

2.0 Laboratory annealing. Annealing experiments on large pegmatitic zircons (5-10 mm) from the Grenville basement of NY State show that these crystals have a ZFT age of 513 ± 30 Ma with uranium between 150-350 ppm and a U/Pb crystallization age of 999.7 +/- 9.1 Ma. Zircons were heated to determine the point of FT annealing and then progressive annealing of different aliquots was accomplished by heating to incremental temperatures of 750°, 850°, 950° and 1050°C for 20 hr. Radiation damage is progressively removed at higher incremental temperatures as indicated by: 1) a decrease in the size of etch pits of later-induced fission tracks; and 2) an increase in crystallinity as measure by Raman scattering. A key implication of these is that a significant fraction of alpha damage is more resistant to thermal annealing as compared to fission damage.

3.0 Field studies of annealing. 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 critical thermal zone for fission track annealing. The critical thermal zone corresponds to a temperature between about 180 and 250°C, where some grains show full annealing and some grains do not.

We aim to use mineralogical properties to isolate the low-retentive zircon that get reset in the critical thermal window. Traditionally the only approach used for this discrimination has been statistical. We are developing methods that are based on different attributes of the crystal properties. One method is to estimate the alpha dose (Dα) at the time of reheating and another is to measure the alpha dose using Raman Spectroscopy.

3.1 The Cambrian Postdam Formation was buried and subjected to heating in the Paleozoic. Unreset ZFT ages in this sandstone have populations primarily between about 550 and 780 Ma, but several key samples experienced significant reheating. 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 Taconic and to the Alleghenian Orogeny.

3.2 The Phyllite-Quartzite unit (PQU) rocks in Greece (Kythera) were derived from Paleozoic-cooled rocks in the Cretaceous, subducted, heated and exhumed in the Tertiary. FT dating of zircon from 34 samples from quartzites of the PQU in the Hellenic fore-arc reveal the systematics of heated suites of zircon that show over-dispersion of the age distribution (some reset at c. 10 Ma, but many grains unreset and as old as 100-400 Ma). As in the Potsdam Formation case discussed above, we separate reset grains from all others. In this case we use mineral properties revealed by Raman spectroscopy, to isolate grains of with high damage that have had all fission tracks annealed during burial. 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.