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Fission-Track Dating

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Fission-Track Dating 1. Introduction and Application 3. Sampling Methods Fission-track (FT) dating is utilized in a number of geologic studies to obtain time-temperature information on bedrock and sediment deposit. Tracks are produced by fission decay events of 238U and thus the mineral lattice damage (track) is equivalent to daughter isotopes measured in other radiometric dating methods. In typical FT dating several hundred of apatite or zircon grains are mounted, and polished to expose internal mineral surfaces. The spontaneous tracks that intersect this internal surface are etched chemically and become visible with an optical microscope (Fig. 1). Track densities are commonly determined by counting the number of tracks in a certain area. Determination of the amount of parent isotope is traditionally accomplished by neutron irradiation that induces Figure 1. Fission tracks in a detrital apatite from the Cretaceous fission of 235U. The inducted tracks are etched and counted Jackass Mountain Group in southern BC. Fission tracks are in an external detector over the same area of the chemically enlarged before counting with high magnification on spontaneous track count. an optical microscope. Grain is about 200 µm across. Fission tracks form continuously at all temperatures and Sampling for fission-track dating involves collection of several depths, but anneal completely at high temperatures of kilograms of candidate rock from lithologies in the field that are >120°C in apatite and >250–300°C in zircon. There is a thought to have the highest potential to yield apatite and/or temperature window where track annealing is slowed down zircon. In typical field sampling, we generally use new large significant enough that it causes track accumulation and canvas sample bags, and we fill these slightly over half way. length shortening. This partial annealing zone for tracks in Generally the rock is coarsely crushed in the field so that sample apatite and zircon is at temperatures ranging from 60–120°C pieces (≤10 cm) can go directly into a jaw crusher in the lab. and 210–300°C, respectively. Particularly for apatite, the track Collecting appropriate samples, which will have a high yield of lengths are measured and used for thermal history modeling. suitable grains, is the most challenging aspect of field collection. FT analysis is commonly used to study geologically young mountain belts, where rock cooling is a function of uplift and erosion, tectonic denudation, and differential exhumation due to faulting or varying climate driven surface processes. Detrital FT dating is used to investigate changes in source rock exhumation through time. FT analysis on modern sediment is used to gain an integrated cooling signal over an entire catchment, which may otherwise not be accessible for bedrock sampling. Due to its thermal sensitivity FT analysis is a powerful tool to study the thermal history of basin strata such as determining maximum burial/heating and timing of cooling and basin inversion. Figure 2: Collecting samples from schistose turbidites in southern Alaska. 2. Age Range and Suitable Geologic Material Datable range depends on the uranium concentration of the In the sedimentary environment, hydrodynamic sorting of heavy mineral. Typical FT ages range from 1–500 Ma. For zircon with minerals (apatite, zircon) results in concentration in coarser- typical uranium concentrations, dates in the Paleozoic grained beds. For example, in turbidites we spend a considerable become complicated due to high track densities. Very high- amount of time looking for the coarsest beds, and then we try to density fission tracks can be counted using a Scanning sample the bottom of graded beds to get the coarsest available fraction. Sampling is less complicated in plutonic rocks, especially Electron Microscope (SEM), which extends the datable range if granitic in composition. Note that zircon is relatively robust and to ~1 Ga. Apatite and zircon are the most commonly used resistant during diagenesis and alteration, but apatite is not. Thus minerals in FT dating, which are found in acidic to if your study is targeting apatite, it is important to collect rocks intermediate magmatic rocks, clastic sediment, and their that are as fresh and unweathered as possible. metamorphic derivatives. Eva Enkelmann, UofC Thermochronology lab, Calgary, Canada, [email protected] John I. Garver, Union College Fission-track lab, Schenectady, NY, [email protected] • In exhumation studies, a common problem is the 4. Sample Integrity and Considerations thermal effects of young igneous activity (e.g. dikes). Key Assumptions for FT Dating In these settings it should be obvious to avoid sampling rocks near dikes and sills, but it is not uncommon for Most fission-track studies are aimed at documenting either: a) the thermal veil to be larger and more substantial. exhumation and rock cooling; or b) provenance and source terrain cooling. In all applications there are a few key assumptions in the • dating methodology. One of the most important is that the dating In provenance studies, the lithology of the source approach has full quantified track density and that that track rocks matter, and some high-yield rock types can density represents all fission tracks on an internal plane in the dominate the signal. Detrital FT ages may reflect crystal (dated surface is a polished internal surface of the crystal, cooling due to source rock exhumation or volcanic and that surface needs to have 4π geometry). Another is that the input uranium determination is accurate and representative of the area • with counted fission tracks. In most FT dating the external detector In provenance studies, stratigraphy and facies matter. method is used. In this approach, thermal neutron irradiation When strata are used to understand exhumation, the causes induced fission in 235U, and the uranium determination is deposition age of basin strata is required to evaluate calculated using the isotopic ratio between 235U and 238U. partial or full annealing of tracks due to post- For exhumation studies the relationship between closure depositional heating. temperature and depth. These complicated and interrelated components include determination of the effective closure 5. Laboratories in North America temperature, which is dependent on the rate of cooling, and the disposition of the geothermal gradient in space and time. In • Appalachian State University, Gabriel Casale, provenance studies there are a number of simplifying assumptions [email protected] that need to be made, including some knowledge of changes of the • source terrain and drainage basin through time, the transfer of Dalhousie University, Isabelle Coutand, [email protected] datable minerals to the site of deposition, and the effects of post- • https://www.dal.ca/faculty/science/earth- depositional thermal annealing sciences/faculty_staff/faculty/coutand_i.html Primary Considerations at the Outcrop • Stanford University, Trevor Dumitru, Grain size and composition matter • https://pangea.stanford.edu/research/groups/thermoc • Target rocks of felsic to intermediate composition hronology/index.php?page=1 • Medium to coarse-grain metamorphic and sedimentary rocks tend to have better yields. • Union College, John Garver, [email protected] • Medium to coarse sand deposits on river banks • http://minerva.union.edu/garverj/FT/FThome.html Sample size • • Typical magmatic and metamorphic rocks 3-4 kg University of Arizona, Stuart Thomson, [email protected] • Most clastic sedimentary rocks, 2–6 kg • https://sites.google.com/site/arizonaftlab/home • Unconsolidated sand from modern environments, collect a 1 gallon bag, but 100 grams if heavies are • University of Calgary, Eva Enkelmann concentrated with a gold pan. [email protected] • http://enkelmann.weebly.com/facilities.html Considerations for taking samples • • Elevation of samples are important in most typical Occidental College, Ann Blythe, [email protected] exhumation studies. So, if a positive age vs. elevation relationship is expected, document sample elevation. Commercial FT Labs: • GeoSep Services, Paul O’Sullivan, • Fault zones, and other environments that may have [email protected] experienced hot fluids to sufficient temperatures to • http://geoseps.com/ cause track annealing. Eva Enkelmann, UofC Thermochronology lab, Calgary, Canada, [email protected] John I. Garver, Union College Fission-track lab, Schenectady, NY, [email protected] .
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