An Introduction to Low-Temperature Thermochronologic Techniques, Methodology, and Applications, in C

S. Lynn Peyton, Barbara Carrapa, 2013, An introduction to low-temperature thermochronologic techniques, methodology, and applications, in C. Knight and J. Cuzella, eds., Application of structural methods to Rocky Mountain hydrocarbon exploration and development: AAPG Studies in Geology 65, 2 p. 15–36. An Introduction to Low-temperature Thermochronologic Techniques, Methodology, and Applications S. Lynn Peyton Coal Creek Resources Inc., 1590 S. Arbutus Pl., Lakewood, Colorado, 80228, U.S.A. (e-mail: [email protected]) Barbara Carrapa Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, Arizona, 85721, U.S.A. (e-mail: [email protected]) ABSTRACT Low-temperature thermochronometers can be used to measure the timing and the rate at which rocks cool. Generally, rocks cool as they move towards Earth’s surface by erosion or normal faulting (tectonic exhumation of the footwall), or warm as they are buried by sediments and/or thrust sheets, or when they are intruded by magma and associated hydrothermal fluids. Changes in heat flow or fluid flow can also cause heating or cooling. Apatite fission-track and apatite (U-Th)/He dating have low closure temperatures of ~120°C and ~70°C respec- tively, and are used to date cooling in the upper ~3–4 km (~1.8–2.4 mi) of Earth’s crust. Age-elevation relationships from samples collected from different elevations along verti- cal transects or from wellbores are used to calculate exhumation rates and the time of onset of rapid exhumation. The spatial distribution of cooling ages can be used to map faults in base- ment or intrusive rocks where faults can be difficult to recognize. Cooling ages from detrital minerals in sedimentary rocks can be used to constrain provenance. If sedimentary samples reached temperatures high enough to reset the thermochronometers, then ages may provide information on the cooling history of the basin. Forward thermal modeling can be used to test proposed thermal history models and predict thermochronometer ages. Inverse thermal modeling finds a best-fit thermal history that provides a good statistical match to measured thermochronometer ages. Both types of thermal modeling may help constrain maximum temperature of the sample and time spent at that temperature. Thermochronometer ages can be used as constraints in basin modeling. Maturation of kerogen to petroleum in a sedimentary basin is controlled by the maximum temperature reached by the kerogen and the amount of time it spends at or near that temperature (i.e., the thermal history of the basin). The timing of tectonics and the formation of structures in a Copyright ©2013 by The American Association of Petroleum Geologists. DOI:10.1306/13381688St653578 15 10711_ch02_ptg01_hr_015-036.indd 15 6/5/13 7:59 AM 16 Peyton and Carrapa region influence the generation, migration, entrapment, and preservation of petroleum. Tech- niques such as low-temperature thermochronology that illuminate the relationship between time and temperature during basin evolution can be valuable in understanding petroleum systems. These techniques are especially powerful when multiple dating techniques (such as apatite fission-track, zircon fission-track, and apatite (U-Th)/He dating) are applied to the same sample and when they are combined with other thermal indicators such as vitrinite re- flectance data. INTRODUCTION He dating, and the partial annealing zone (PAZ) for fission-track dating (Figure 1). By measuring the Geochronology and thermochronology use the radio- amount of both parent nuclide and daughter product active decay of a parent nuclide and the accumulation within a crystal, we can calculate the time when the of a corresponding daughter product to date either crystal passed through this temperature window, the crystallization age or cooling age of a mineral. A called the cooling age. Minerals such as apatite and daughter product may either be a daughter nuclide, zircon can therefore be used as thermochronometers, such as 4He in (U-Th)/He dating, or the effects created with their ages recording cooling rather than crystal- by a daughter nuclide. In fission-track thermochronol- lization. For example, the (U-Th)/He technique in- ogy, such decay is represented by spontaneous fission- volves the decay of U, Th, and to a lesser extent Sm, ing of 238U and the daughter product is represented by to 4He (alpha particles). 4He is fully retained in apa- damage tracks in the crystal structure produced by re- tite below ~40°C, partially retained between ~40°C coil of the fission products of238 U, called fission tracks. and 70°C, and not retained above ~70°C (Farley, For many crystalline minerals (e.g., apatite and zir- 2000; Farley, 2002). The closure temperature for He in con), fission tracks gradually shorten and eventually zircon, in contrast, is ~170–190°C (Reiners et al., 2004), disappear at high temperatures, as disturbed atoms or and the PRZ ~130–180°C (Reiners and Brandon, 2006). ions diffuse back into place and the crystal structure Note that the temperature ranges for PAZs and PRZs reforms (anneals). Fission tracks can only accumulate also vary with cooling rate (Reiners and Brandon, below the temperature where rapid annealing occurs, 2006). For the fission-track technique, all fission-tracks called the annealing or closure temperature. Similarly, are annealed and their concentration, and thus age, for (U-Th)/He dating, 4He can diffuse out of a crys- is zero above ~120°C in apatite (Laslett et al., 1987; tal lattice at high temperatures and is only retained Ketcham et al., 1999), and ~240°C in zircon (Zaun within the crystal below a temperature called the clo- and Wagner, 1985). Partial annealing of fission tracks sure temperature. occurs between ~60°C and 120°C in apatite, depend- Dodson (1973) defined closure temperature as the ing on the chemistry of the apatite (Green et al., 1989b) temperature of a mineral (e.g., apatite or zircon) at and between ~180°C and 350°C in zircon (Tagami, the time given by its radiometric age. It varies with 2005). Figure 2 shows the closure temperature ranges both the dating technique used and the mineral being of many thermochronometers. dated. The concept of closure temperature for thermo- Cooling of rocks may occur due to exhumation, chronometers, where daughter product is retained in fluid flow, a decrease in geothermal gradient caused a crystal below the closure temperature but not above by the cessation of flow of hydrothermal fluids, or a it, facilitates explanations of thermochronologic tech- decrease in basal heat flow (Ehlers, 2005). Exhumation niques but is only valid for minerals that experience is defined as the upward displacement of rock with steady, monotonic cooling (i.e., temperature always respect to the surface (England and Molnar, 1990); decreases with time) (Dodson, 1973). Closure tem- this can result from erosion or tectonic exhumation perature will vary depending upon the cooling rate (i.e., footwall exhumation due to normal faulting). Ex- of the sample: Faster cooling results in higher closure humation typically results in cooling, as rocks move temperatures, while slower cooling results in lower from greater depth (higher temperatures) to shallower closure temperatures (Reiners and Brandon, 2006, and depths (cooler temperatures) below the surface. The references therein). term denudation refers to downward movement of In reality, thermochronometers have a temperature the surface with respect to a rock (e.g., Brown et al., window over which the daughter product starts to 1994) and is often used interchangeably with exhu- be retained in the system. This temperature window mation to refer to rock removal. For a given sample, is called the partial retention zone (PRZ) for (U-Th)/ thermochronometers with lower closure temperatures 10711_ch02_ptg01_hr_015-036.indd 16 6/5/13 7:59 AM An Introduction to Low-temperature 17 Figure 1. Schematic age-elevation profile showing relative positions of the PAZ, PRZ, and fossil PAZ and PRZ. Modified from Armstrong (2005). are expected to record younger ages than those with higher closure temperatures because as a rock is ex- humed it passes through the higher closure temperature before the lower one. As our understanding of low-temperature thermochronologic techniques has expanded in recent years, the number of applications for these techniques has also increased. For example, advances in understanding the diffusion of 4He in apatite and other minerals over the last decade (e.g., Shuster et al., 2006; Flowers et al., 2009) have led to proliferation of the Figure 2. Closure temperature windows of thermochronom- use of (U-Th)/He dating. Similarly, there have been eters and geochronometers. Modified from Carrapa (2010). advances in understanding fission-track annealing in (1) Farley (2000); (2) Green et al. (1989b); (3) Reiners apatite (e.g., Ketcham et al., 2007b). In sedimentary ba- et al. (2004); (4) Zaun and Wagner (1985); (5) Purdy and sins, low-temperature thermochronology can be used Jäger (1976); (6) Chamberlain and Bowring (2001); to quantify the thermal history of a basin, evaluate hy- (7) Dahl (1997); (8) Dahl (1997) and Mezger and Krogstad drocarbon maturation and fluid flow, and to study the (1997). provenance of sedimentary rocks (e.g., Burtner and Nigrini, 1994; Sobel and Dumitru, 1997; Osadetz et al., deformation, uplift, or tilting, these techniques may il- 2002; Armstrong, 2005). Combining multiple dating luminate the timing, rate, and amount of exhumation, techniques, especially in conjunction with U/Pb geo- and if exhumation is a consequence of tectonic activ- chronology of zircon and apatite, provides a powerful ity, the timing of the tectonic event (e.g., Deeken et al., tool for constraining the provenance and depositional 2006; Carrapa et al., 2011). age of sedimentary rocks, as well as basin ther- This chapter provides an overview of the two most mal history (Rahl et al., 2003; Campbell et al., 2005; widely used low-temperature thermochronology tech- Bernet et al., 2006; van der Beek et al., 2006; Carrapa niques, apatite fission-track (AFT) dating and apatite et al., 2009).

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