47191-AC8
Annealing of Fission Tracks in Wide-range, Radiation-damaged Detrital Zircon
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. With a deeper understanding of radiation damage and track stability we are in the process of developing a 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. 200°C and higher) under geological conditions. We are testing this hypothesis on clastic rocks of the Beekmantown Group in eastern NY State, one of the major targets for gas exploration in the Appalachian Basin. We initiated three experiments that address the internal structure, track stability, and radiation damage in zircon. The overall goal is to understand how radiation damage in zircon affects annealing of fission tracks, which will allow use of damaged zircons as sensitive low-temperature thermochronometer. We have three main experiments aimed at revealing the complexity between radiation damage and annealing: [1] ZFT Ages of detrital zircon from Cambrian strata (NY). We separated zircon from seven samples of the Cambrian Potsdam Fm. Using the SEM HDFT dating, a new technique that we are developing (see Montario & Garver, 2008) the ZFT ages of 466 detrital grains were determined from samples around the Adirondack massif. ZFT ages from the Potsdam Fm. range from ~300 Ma to over 2000 Ma. To the west, where thermal maturity is low (CAI ~ 2), ZFT grain ages are older than the age of deposition, and most grains cooled at ~550 Ma and ~740 Ma, which reflects cooling of the basement source. Samples from the eastern Adirondacks, from rocks with greater thermal maturity (CAI values ~4.5-5), are mainly old with pre-depositional ages, but they also have a young ZFT population at ~280-290 Ma, which coincides with Alleghanian burial. Our next goal is to understand this overdispersion through an analysis of radiation damage and track retention. [2] Annealing of Radiation Damage in highly damaged zircon. To investigate annealing of radiation damage in zircons, large pegmatitic zircons from the Grenville basement were analyzed These crystals have a ZFT age of 513±30 Ma with uranium between 150-350 ppm. For thermal annealing and Raman studies, 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 these higher incremental temperatures as indicated by the size of etch pits of later-induced fission tracks, and by Raman scattering. Raman scattering indicates that annealing at these incremental temperatures causes a significant change in internal ordering. A key implication of our data is that zircon grains likely have inherited radiation damage between geologic events, and therefore this damage likely affects the systematics of geochronology using this mineral. [3] Measurement of internal radiation damage by Raman Spectroscopy. A key question is whether the grain-to-grain variation in radiation damage can be used to explain overdispersion of grain ages in reheated samples. Our best prospect for measuring radiation damage in zircon is through Raman spectroscopy. FT dating of zircon from 34 samples from quartzites of the Phyllite-Quartzite (PQ) Unit 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. 12 Ma, but many grains unreset and as old as 100 Ma). After the ZFT age and uranium concentration of 334 zircons was determined, micro-Raman laser spectroscopy measurements were conducted with a high-resolution Raman system using a solid-state laser (632.817 nm) with 20 mW power at the sample surface. In this suite there is a clear relationship between uranium and Raman laser scattering. To relate overdispersion of ZFT ages and radiation damage, a suite of nine zircons of a substantial range of uranium concentrations were annealed at 1000°C. These zircon grains were examined and measured using Raman spectroscopy both prior to and after annealing. Finally, a suite of 97 synthetic zircons containing no uranium and therefore no radiation damage were measured. Thirty-five out of those 97 synthetic zircons were doped with 0.8-3.6%wt Hf. These measurements allowed us to understand the relationship between Raman scattering and uranium plus radiation damage. Our analysis indicates that uranium concentration has major direct effect on the Raman spectral data of the v3 and v1 [SiO4] bands from the zircons, and the amount of radiation damage can probably be quantified from these measurements. Ongoing studies will relate the Raman wavenumber to annealing characteristics on the annealing of heated detritial samples that are characteristic of the Cambrian of eastern NY State. The reset grains in these strata record the last thermal maximum that was likely related to gas generation that is the current target of significant exploration in the State
