An Introduction to Low-temperature Thermochronologic ......coil of the fission products of 238U,...

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Copyright ©2013 by The American Association of Petroleum Geologists.

DOI:10.1306/13381688St653578

2An Introduction to Low-temperature Thermochronologic Techniques, Methodology, and ApplicationsS. Lynn Peyton Coal Creek Resources Inc., 1590 S. Arbutus Pl., Lakewood, Colorado, 80228, U.S.A.(e-mail: [email protected])

Barbara CarrapaDepartment 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 nor-mal 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 flu-ids. 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 tem-perature 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

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, p. 15–36.

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16 Peyton and Carrapa

He dating, and the partial annealing zone (PAZ) for fission-track dating (Figure 1). By measuring the amount of both parent nuclide and daughter product within a crystal, we can calculate the time when the crystal passed through this temperature window, called the cooling age. Minerals such as apatite and zircon can therefore be used as thermochronometers, with their ages recording cooling rather than crystal-lization. For example, the (U-Th)/He technique in-volves the decay of U, Th, and to a lesser extent Sm, to 4He (alpha particles). 4He is fully retained in apa-tite below ~40°C, partially retained between ~40°C and 70°C, and not retained above ~70°C (Farley, 2000; Farley, 2002). The closure temperature for He in zircon, in contrast, is ~170–190°C (Reiners et al., 2004), and the PRZ ~130–180°C (Reiners and Brandon, 2006). Note that the temperature ranges for PAZs and PRZs also vary with cooling rate (Reiners and Brandon, 2006). For the fission-track technique, all fission-tracks are annealed and their concentration, and thus age, is zero above ~120°C in apatite (Laslett et al., 1987; Ketcham et al., 1999), and ~240°C in zircon (Zaun and Wagner, 1985). Partial annealing of fission tracks occurs between ~60°C and 120°C in apatite, depend-ing on the chemistry of the apatite (Green et al., 1989b) and between ~180°C and 350°C in zircon (Tagami, 2005). Figure 2 shows the closure temperature ranges of many thermochronometers.

Cooling of rocks may occur due to exhumation, fluid flow, a decrease in geothermal gradient caused by the cessation of flow of hydrothermal fluids, or a decrease in basal heat flow (Ehlers, 2005). Exhumation is defined as the upward displacement of rock with respect to the surface (England and Molnar, 1990); this can result from erosion or tectonic exhumation (i.e., footwall exhumation due to normal faulting). Ex-humation typically results in cooling, as rocks move from greater depth (higher temperatures) to shallower depths (cooler temperatures) below the surface. The term denudation refers to downward movement of the surface with respect to a rock (e.g., Brown et al., 1994) and is often used interchangeably with exhu-mation to refer to rock removal. For a given sample, thermochronometers with lower closure temperatures

INTRODUCTION

Geochronology and thermochronology use the radio-active decay of a parent nuclide and the accumulation of a corresponding daughter product to date either the crystallization age or cooling age of a mineral. A daughter product may either be a daughter nuclide, such as 4He in (U-Th)/He dating, or the effects created by a daughter nuclide. In fission-track thermochronol-ogy, such decay is represented by spontaneous fission-ing of 238U and the daughter product is represented by damage tracks in the crystal structure produced by re-coil of the fission products of 238U, called fission tracks. For many crystalline minerals (e.g., apatite and zir-con), fission tracks gradually shorten and eventually disappear at high temperatures, as disturbed atoms or ions diffuse back into place and the crystal structure reforms (anneals). Fission tracks can only accumulate below the temperature where rapid annealing occurs, called the annealing or closure temperature. Similarly, for (U-Th)/He dating, 4He can diffuse out of a crys-tal lattice at high temperatures and is only retained within the crystal below a temperature called the clo-sure temperature.

Dodson (1973) defined closure temperature as the temperature of a mineral (e.g., apatite or zircon) at the time given by its radiometric age. It varies with both the dating technique used and the mineral being dated. The concept of closure temperature for thermo-chronometers, where daughter product is retained in a crystal below the closure temperature but not above it, facilitates explanations of thermochronologic tech-niques but is only valid for minerals that experience steady, monotonic cooling (i.e., temperature always decreases with time) (Dodson, 1973). Closure tem-perature will vary depending upon the cooling rate of the sample: Faster cooling results in higher closure temperatures, while slower cooling results in lower closure temperatures (Reiners and Brandon, 2006, and references therein).

In reality, thermochronometers have a temperature window over which the daughter product starts to be retained in the system. This temperature window is called the partial retention zone (PRZ) for (U-Th)/

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.

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An Introduction to Low-temperature 17

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 tempera-ture before the lower one.

As our understanding of low-temperature thermo-chronologic techniques has expanded in recent years, the number of applications for these techniques has also increased. For example, advances in understand-ing the diffusion of 4He in apatite and other miner-als over the last decade (e.g., Shuster et al., 2006; Flowers et al., 2009) have led to proliferation of the use of (U-Th)/He dating. Similarly, there have been advances in understanding fission-track annealing in apatite (e.g., Ketcham et al., 2007b). In sedimentary ba-sins, low-temperature thermochronology can be used to quantify the thermal history of a basin, evaluate hy-drocarbon maturation and fluid flow, and to study the provenance of sedimentary rocks (e.g., Burtner and Nigrini, 1994; Sobel and Dumitru, 1997; Osadetz et al., 2002; Armstrong, 2005). Combining multiple dating techniques, especially in conjunction with U/Pb geo-chronology of zircon and apatite, provides a powerful tool for constraining the provenance and depositional age of sedimentary rocks, as well as basin ther-mal history (Rahl et al., 2003; Campbell et al., 2005; Bernet et al., 2006; van der Beek et al., 2006; Carrapa et al., 2009). In areas that have experienced tectonic

deformation, uplift, or tilting, these techniques may il-luminate the timing, rate, and amount of exhumation, and if exhumation is a consequence of tectonic activ-ity, the timing of the tectonic event (e.g., Deeken et al., 2006; Carrapa et al., 2011).

This chapter provides an overview of the two most widely used low-temperature thermochronology tech-niques, apatite fission-track (AFT) dating and apatite (U-Th)/He (AHe) dating (Figure 2). These techniques

Figure 1. Schematic age-elevation profile showing relative positions of the PAZ, PRZ, and fossil PAZ and PRZ. Modified from Armstrong (2005).

Figure 2. Closure temperature windows of thermochronom-eters and geochronometers. Modified from Carrapa (2010). (1) Farley (2000); (2) Green et al. (1989b); (3) Reiners et al. (2004); (4) Zaun and Wagner (1985); (5) Purdy and Jäger (1976); (6) Chamberlain and Bowring (2001); (7) Dahl (1997); (8) Dahl (1997) and Mezger and Krogstad (1997).

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have closure isotherms that are located in the upper-most few kilometers of Earth’s crust and are ideally suited to study the timing and rates of upper crustal geologic processes that have a thermal signature, such as burial and exhumation (sensu England and Molnar, 1990). After reviewing the applications of these tech-niques, we also discuss potential pitfalls, problems, and limitations. Other papers summarizing these techniques are available, and the interested reader is referred to them for additional information and alter-native perspectives (Gallagher et al., 1998; Farley, 2002; Gleadow et al., 2002; Reiners, 2002; Ehlers and Farley, 2003; Donelick et al., 2005; Reiners and Brandon, 2006). Peyton and Carrapa 2013 reviews and summarizes published low- temperature thermochronology studies of the Laramide Rocky Mountain region to date, and discusses the application of these techniques to petro-leum exploration in more detail.

TECHNIQUES

Fission-track Dating

Age DeterminationSpontaneous fission of naturally occurring 238U pro-duces two large daughter nuclei. Once formed, these charged nuclei repel each other, moving in opposite directions through the crystal lattice. Because they are large particles, as they pass through a mineral they af-fect its crystal structure, forming damage zones called “fission tracks”. In general, the density of fission tracks within a crystal will increase with time, and with increasing concentration of 238U. However, if the crystal is at a temperature above or within the PAZ for a sufficiently long period of time, the crystal lattice will reorganize and the tracks will disappear (anneal). Apatite and zircon fission track PAZ temperatures are ~60°C to 120°C (Wagner, 1968; Green et al., 1989b) and 180°C to 350°C (Tagami, 2005), respectively. The rela-tively low PAZ temperatures for apatite fission tracks make this technique particularly useful for evaluating cooling histories of rocks in the upper ~4 km (~2.4 mi) of the crust.

Unlike other thermochronometers such as AHe dating, where both parent and daughter nuclides are measured directly using mass spectrometry, for fission-track dating both the parent and daughter products are measured indirectly by counting fis-sion tracks. The proxy for the daughter product for fission-track dating is the density of spontaneous fis-sion tracks formed by natural fission of 238U. Parent nuclide concentration is measured using the external-detector method, which is the most widely used and

best calibrated method at present (Hurford, 1990b, 1990a). U-free muscovite sheets (the external detec-tors) are placed adjacent to the polished grain mounts and are irradiated with low-energy thermal neutrons in a nuclear reactor. Standards of known U concentra-tion are also included. The neutrons induce fission in 235U, creating fission tracks in both the apatite and the adjacent mica sheet. The number of induced fission tracks is proportional to concentration of 235U, and since the ratio 235U/238U is constant in nature we can calculate the abundance of 238U provided we know the thermal neutron flux. The induced track density is counted from the detector. An alternative method is to measure 238U directly by laser-ablation induc-tively coupled plasma mass spectrometry (LA-ICPMS) (Donelick et al., 2005). However, published calibration studies are lacking, and this approach is still consid-ered largely untested.

Fission-track ages are calculated using a decay equation, similar to other geochronometers and ther-mochronometers. However, the densities of the in-duced and spontaneous fission tracks are entered directly into the age equation instead of parent and daughter nuclide concentrations (Equation 1). Fission-track age is given by

ti 5 1/ld ln[1 1 ld z g rd (rs,i/ri,i)] Equation 1

where t is the age, i refers to individual grain i, ld 5 total decay constant of 238U, z 5 zeta calibra-tion factor based on fission-track age standards, g 5 geometry factor for spontaneous fission track registration, rd 5 induced fission track density for a uranium standard, rs,i 5 spontaneous fission track density for grain i, and ri,i 5 induced fission track density for grain i. For more details on AFT dating, we refer the reader to Gallagher et al. (1998) and Donelick et al. (2005).

AnnealingHow fission tracks shorten or anneal is strongly de-pendent upon temperature and the orientation of the fission track with respect to the crystallographic c-axis. Annealing in apatite also correlates with other parameters, called annealing kinetic parameters, such as the concentration of Cl or OH (in atoms or ions per formula unit) in the crystal, or the diameter of fission-track etch pits parallel to the apatite crystal c-axis, defined as Dpar (Donelick, 1993; Carlson et al., 1999; Donelick et al., 1999; Ketcham et al., 1999). Etch-ing of fission tracks is discussed in more detail later. Modeling of a single sandstone sample using these three different kinetic proxies showed a single time-temperature history, thus documenting the validity

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of each proxy (Ketcham et al., 1999). Dpar is the most commonly used proxy for annealing kinetics due to its low cost and relative ease of measurement.

In general, apatites with smaller Dpar are typical of Fl-rich apatite compositions and are characterized by lower annealing temperatures, whereas apatites with larger Dpar are often Cl-rich and are characterized by higher annealing temperatures. Annealing studies showed that Fl-rich apatite from Durango, Mexico, which is used as a standard for both AFT and AHe dating, completely anneals at ~110°C, whereas Cl-rich apatite needs higher temperatures (in some cases >130°C) to fully anneal (Green et al., 1989b). Anneal-ing not only depends on the annealing kinetics (e.g., Green et al., 1986), but also on the duration of heating experienced by the sample (Green et al., 1989b). The degree of annealing can be determined by measuring confined-track lengths within a sample (Green et al., 1986; Green et al., 1989b).

Track LengthsThe initial length of a fission track is ~16 µm in apatite (Carlson et al., 1999). If the track forms in a crystal that is cooler (i.e., shallower) than the PAZ, its length will change only slightly over time. If the track forms in a crystal that is at a higher temperature than the PAZ, the track will anneal rapidly and will not be preserved over geologic time (Green et al., 1986). If a crystal ex-periences rapid cooling from temperatures above the PAZ to temperatures below the PAZ, nearly all fission tracks will form at temperatures below the PAZ and all will be close to their original length, regardless of the formation age of the rock. In this case the AFT age will represent the time of rapid cooling. If the rock is a volcanic ash that does not experience burial and reheating, the AFT age will represent the formation age of the ash. A wide distribution of track lengths will result when a sample resides at temperatures within the PAZ for a significant amount of time (rela-tive to its age); the fission tracks will start to anneal and shorten, with older tracks shortening more than younger tracks. If this sample is then cooled to tem-peratures below the PAZ, any new tracks formed will retain their original length, whereas older tracks were shortened or annealed during their time in the PAZ, resulting in a bimodal distribution of track lengths. The AFT age for this sample will be older than the youngest cooling event, and younger than any prior cooling events. Track-length distribution also depends on apatite annealing kinetics as discussed above. A study of borehole samples from the Otway Basin in Australia nicely illustrates the effect of temperature and annealing on length distribution (Gleadow and Duddy, 1981). Track lengths are therefore additional

data that can be used to constrain thermal history and are recorded along with the number of tracks when-ever possible.

For fission tracks to be visible under a microscope they must first be etched using acid. Different etching techniques may result in a different number of tracks ex-posed and thus result in different final ages (Ketcham et al., 1999; Murrell et al., 2009). After mineral separation, apatite crystals are mounted in epoxy and polished to expose internal grain surfaces. A standard etching rec-ipe (5.5 M HNO3 for 20 s at 21°C) is typically followed by most laboratories to reveal the spontaneous fission tracks, which allows for comparison of data from differ-ent sources. After irradiation, the external detector mica sheets are etched in 40% HF for 45 minutes to reveal the induced tracks. Both natural and induced fission tracks are counted manually using a microscope; however, because manual counting of tracks is partly subjec-tive, each analyst calculates their own correction factor based on standards, called the “zeta” (z) value (Equa-tion 1). When measuring track lengths, only tracks that are confined within the crystal and do not intersect the polished grain surface are counted. Confined tracks become etched via other tracks, fractures and cleav-age planes. Only confined tracks that intersect other tracks should be counted; tracks that intersect fractures or cleavage planes often exhibit anomalous annealing behavior and should be ignored (Donelick et al., 2005, and references therein). Because the annealing prop-erties of tracks vary with their orientation within the crystal (Donelick, 1991), a correction is applied to each measured track length based on the angle of the track with respect to the crystallographic c-axis ( Ketcham et al., 2007a).

Corrected AFT AgesCorrected AFT ages are often reported in published studies from the late 1980s and 1990s (e.g., Green et al., 1989a; Burtner and Nigrini, 1994). Ages were corrected for partial annealing by applying a correction factor, calculated from the relationship between mean track length and AFT age (Green et al., 1989a). Corrected ages were interpreted to be the AFT age if no anneal-ing had occurred. The ease and availability of forward and inverse modeling of AFT ages has, for the most part, eliminated the need for corrected ages.

(U-Th)/He Dating

(U-Th)/He dating is based upon the decay of 238U, 235U, and 232Th to 206Pb, 207Pb, and 208Pb, respectively. 4He nuclei (alpha particles) are emitted at each step in this decay series and are the daughter nuclides for this

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dating system. Equation 2 shows the decay equation for (U-Th)/He dating:

4He 5 8238U[exp(l238t) 2 1] 1 7235U[exp(l235t) 2 1] 1 6232Th[exp(l232t) 2 1] Equation 2

where l238 is the decay constant of 238U, and so forth; U, Th, and He are the number of atoms of each iso-tope; and t is the (U-Th)/He age. Below ~70ºC, which is the closure temperature of He in crystalline apatite, He is largely retained in a crystal, and calculated AHe ages will record a cooling age (Wolf et al., 1996; Farley, 2000). Above this closure temperature, He escapes from apatite through diffusion and the calculated AHe age will be zero (Farley, 2000).

Similar to the fission-track technique, there is a PRZ of He between ~40 and 70ºC in apatite. If a crystal re-sides in the PRZ for a sufficient time, some He will dif-fuse out of the crystal (Wolf et al., 1998). Upon further cooling, calculated ages will be older than the most recent cooling episode, but younger than the previous cooling episode; that is, they will be partially reset. Partially reset ages cannot be interpreted as cooling ages. Thermal modeling, also referred to as thermal-kinetic and thermal history modeling, must be used to interpret thermochronometer ages that have been partially reset. AHe ages should typically be younger than AFT ages because the closure temperature and PAZ temperature range for AHe dating are lower (i.e., shallower in the upper crust) than for AFT dating. Assuming a geothermal gradient of 25ºC/km, rocks ~3 km below Earth’s surface will have a zero AHe age until they are exhumed to shallower depths. Thus, AHe dating should record uplift and exhumation in regions that have experienced little burial (<~3 km), such as the Laramide ranges and sedimentary basins of the western United States.

For AHe dating, a binocular microscope is used to select clear, inclusion-free apatite crystals with widths >60 µm. Inclusions of U-rich minerals, such as mon-azite and zircon, within apatite crystals can have a significant influence on the AHe age and should be avoided (House et al., 1997). However, if crystals con-tain inclusions that are not readily visible using the microscope, it is unlikely that they will have a large impact on the crystal age (Vermeesch et al., 2007). Crystals are photographed and measured before being wrapped in either a platinum or niobium tube. They are then degassed by laser-heating using Nd:YAG and CO2 lasers, and after cryogenic purification the 4He is measured by quadrupole mass spectrometry (House et al., 2000). Standard procedures are described in Rein-ers et al. (2004). The degassed crystals (and the tube) are then dissolved in nitric acid and the concentrations

of U, Th, and Sm measured using an inductively cou-pled plasma mass spectrometer (ICP-MS).

During the decay process, He nuclei have sufficient energy to travel ~20 µm through an apatite crystal lat-tice before stopping. When the decay occurs within 20 µm of the crystal edge, some He nuclei will be ejected from the apatite crystal (Farley et al., 1996). Therefore, for a given concentration of parent nuclides, there will be less He than expected within the crystal, and calculated AHe ages will be too young. A correc-tion to account for this loss of He, called the alpha ejec-tion correction, must be applied to the calculated AHe age, taking into account the dimensions of the crystal (Farley et al., 1996; Farley, 2002).

DiffusionThe understanding of diffusion of He in apatite has been evolving rapidly in recent years as the technique has become more widely used. Factors such as the size of a crystal may affect how much He is lost to diffu-sion, with larger crystals losing proportionally less He than smaller crystals. When cooling through the PRZ has been slow enough for He diffusion to occur, larger apatite crystals will have older AHe ages than smaller apatite crystals from the same sample (Reiners and Farley, 2001). The low concentration of He in the outer 20 μm of a crystal due to alpha-particle ejection results in decreased He diffusion and, therefore, older-than-expected AHe ages after applying the alpha-ejection correction (Meesters and Dunai, 2002b, 2002a). Recent work has shown that radiation damage of apatite may have a significant effect on He diffusion (Shuster et al., 2006; Flowers et al., 2009; Gautheron et al., 2009). Radi-ation damage is caused by recoil of a parent nuclide of U, Th, or Sm as it decays by ejecting an alpha particle. These damage sites may form traps for He and result in a range of AHe ages from the same sample that are proportional to the effective U content of the apatite crystals, defined as eU 5 [U] 1 0.235[Th] (Flowers et al., 2007; Flowers et al., 2009).

Diffusion kinetics determined from laboratory dif-fusion experiments on Durango apatite have become the standard model for diffusion of He in apatite (Wolf et al., 1996; Wolf et al., 1998; Farley, 2000). The Durango-diffusion model predicts that AHe ages will always be younger than AFT ages. In contrast, radia-tion damage diffusion models show that, under cer-tain circumstances, AHe ages may be older than AFT ages. These older-than-expected AHe ages occur when apatite crystals have a sufficiently high concentration of U, Th, and Sm and have resided in the PRZ for sig-nificant percentage of their age, conditions commonly met by exhumed cratonic rocks such as those exposed in the northern Rocky Mountains (Peyton et al., 2012)

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or the western Canadian shield (Flowers, 2009). Flow-ers et al. (2007) also documented this effect in a study of Permian and Triassic sedimentary rocks from the Grand Canyon region.

Sampling

Sample size is often beyond the control of the thermo-chronologist, especially when samples consist of core or cuttings from a wellbore. For field samples, the Uni-versity of Arizona Laserchron Center recommends collecting 5–10 kg (11–22 lb) of crystalline rock, and 10–15 kg (22–33 lb) of sedimentary rock (https://sites.google.com/a/ laserchron.org/laserchron/home/). AHe and AFT ages have been determined successfully from subsurface samples as small as ~100 g (~0.1 kg), although a small apatite yield may not provide enough fission-track length measurements (e.g., Naeser, 1989; Peyton et al., 2012). When possible, field samples should be collected away from ridges and lightning-prone areas, and the outer few centimeters of exposed rock should be removed while at the outcrop. It is im-portant to break up field samples into small, fist-sized pieces while on the outcrop to prevent later contami-nation. Intermediate or felsic crystalline rocks usually contain more apatite and zircon than an equivalent volume of mafic rocks, so priority should be given to these compositions. When dating detrital minerals from sedimentary rocks, very fine-grained or coarser clastic material (grain size >~60 µm) usually provides the best yield of apatite and zircon. Well sorted, mono-genic sandstones such as quartzarenites typically have a low apatite yield, whereas polygenic and lithic-rich sandstones are more likely to contain apatites. Shales and siltstones are too fine-grained to yield crystals large enough for analysis; limestones, dolomites, and evaporites do not contain crystalline apatite and zircon.

The number of samples taken from a vertical transect or wellbore may be constrained by the analysis budget or the availability of core/cuttings or outcrop. We rec-ommend sampling wellbores and vertical transects every 250 m to 500 m (820 to 1640 ft) if possible.

Apatite and zircon are separated from whole rock by crushing using a jaw crusher and roller mill. A Wilf-ley water table may be used to provide the first level of density-based liquid separation, followed by siev-ing, drying, and magnetic and heavy-liquid density separations (Donelick et al., 2005). If the mineral to be dated is from a crystalline rock, typically between two and 10 crystals (we recommend seven) are dated per sample using the (U-Th)/He technique, and ~20 crystals using the fission-track method. If the mineral being dated is from a sedimentary rock, ~100 grains

per sample should be analyzed using either technique in order to produce statistically sound results ( Vermeesch, 2004). However, the high cost of AHe dat-ing may preclude analysis of more than a few tens of grains, making this approach less robust than others for sedimentary samples.

APPLICATIONS AND INTERPRETATION

Age-elevation Profiles

A common sampling approach for low-temperature thermochronologic studies is to collect surface sam-ples at different elevations along a transect, usually in an area of high topographic relief (Fitzgerald et al., 1995). Results are displayed as thermochronometer age versus elevation. It is assumed that all samples in a transect passed through each isotherm at the same elevation (Figure 3A and B). Hence, samples at higher elevations should have older ages than those at lower elevations, because they were exhumed through the closure temperature earlier. Age-elevation profiles can therefore illustrate the exhumation history of an area; ages that form a steep slope on an age- elevation profile indicate the timing of rapid exhumation, and the exhumation rate is represented by the slope ( Figure 3C). If tectonics and exhumation are related, then we may be able to better understand the tectonic history of an area using age-elevation profiles.

After correcting for borehole deviation, samples from a borehole are more likely to form a true-vertical transect than samples from a surface transect. Ther-mochronometer ages decrease with depth in a bore-hole as temperature increases (Figure 1). Within the temperature range of the present-day PAZ or PRZ, fission tracks begin to anneal and He is only partially retained, resulting in a more-rapid decrease in age with depth. At depths and temperatures greater than the present-day PAZ and PRZ, AHe and AFT ages are zero. At depths above (i.e., temperatures below) the present-day PAZ or PRZ, ages will represent a pre-vious cooling event in crystalline rocks, and either an older cooling age or a detrital age in sedimentary rocks. Detrital ages represent the cooling ages of the source terranes of the sedimentary rock hosting the analyzed mineral and have not been reset after depo-sition. When sedimentary samples have been buried deeply enough for thermochronometer ages to be fully reset, and are then later exhumed, age-elevation profiles provide information on the thermal history of the sedimentary basin itself, especially when com-bined with other thermal indicators such as vitrinite reflectance.

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An important benefit to collecting and analyzing samples from an elevation transect or borehole is that the thermal histories of the samples must be related to each other. Thus, each sample will provide constraints on the viability of thermal models calculated for every other sample in the profile, and it may be possible to estimate paleogeothermal gradients.

Present-day PAZs and PRZs can be recognized on an age-elevation profile by their low slope and typical sigmoidal shape (Figure 1). An increase in exhumation rate can result in the preservation of this low slope

at higher elevations, where it is called a “fossil” PAZ or PRZ (Figure 1). Fossil PAZs or PRZs can be recog-nized on an age-elevation profile by a rapid increase in age with small increases in elevation, and hence exhumation rates calculated from the slope of the age- elevation profile are very slow. If an age- elevation profile includes the base of a fossil PAZ or PRZ, we can estimate the time of onset of rapid exhumation from the time when the slope changes. By assuming a paleogeothermal gradient, we can also estimate the amount of exhumation that has occurred. If the base

TClosure

TClosure

0

Slope = Exhumation rate (km/Ma)

Ele

vatio

n (k

m a

bsl)

Thermochronometer Age (Ma)

TClosure

depth all samples

Slope ≠ Exhumation rate (km/Ma)

TClosure

depth

Sample (1)Sample (2)Sample (3)

(1)

(2)

(3)

TClosure

(1)

(2)

(3)

Thermochronometer Age (Ma)

Ele

vatio

n (k

m a

bsl)

0

Denudation

Sedimentation and compaction

Advection of mass and heat

Advection of mass and heat

A B

C

D E

Figure 3. Effect of topog-raphy on age-elevation profiles. (A) Horizontal closure isotherm with sam-ples collected up a range front. (B) Closure isotherm deformed by topography, samples collected in a ver-tical wellbore or cliff face. (C) Age-elevation profile resulting from A or B. (D) Isotherms deformed by topography, denudation and sedimentation. (E) Age-elevation profile resulting from D. Slope of best-fit line through sample ages is not the cor-rect exhumation rate. From Ehlers (2005).

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of a fossil PAZ or PRZ is not preserved, the onset of rapid cooling is unknown and we can only estimate a minimum amount of exhumation.

Figure 4A shows an age-elevation profile from Peyton et al. (2012) for all published low-temperature thermochronology data from the Bighorn Range, in-cluding both surface and subsurface data. To correct for possible topographic effects, sample ages were plotted against sample depth below the Precambrian-Cambrian unconformity (Figure 4B), rather than against elevation (after Crowley et al., 2002). A fossil PAZ and fossil PRZ can be recognized in these data, and the inclusion of subsurface data allows the identi-fication of the present-day PRZ.

Forward and Inverse Modeling

Forward modeling involves calculating a thermo-chronometric age from a proposed time-temperature path, using diffusion or annealing kinetics derived from laboratory experiments. Forward modeling is used to check if a time-temperature path provides a plausible explanation of measured thermochrono-metric ages, and is also a useful way to predict and understand the effect of thermal history on ages and track-length distributions. Although forward mod-els can be constrained by independent geological data, they do not provide a unique time-temperature solution.

Inverse modeling involves calculating time- temperature paths that match the measured thermo-chronometer ages to within a specified amount of statistical error, assuming a starting time and temper-ature. The present-day sample temperature and any known geological controls are also used to constrain the inversion. Commonly, a best-fit time-temperature path and a range of good- and acceptable-fit paths are found using a Monte Carlo simulation (Ketcham, 2005).

Until recently, only AFT data were used for in-verse modeling. Track-length distribution, AFT age, and a kinetic parameter such as Dpar (Donelick, 1993) represent the entire thermal history of an apatite crystal from cooling through the PAZ to the present-day temperature and provide significant constraints on possible thermal histories. Recent advances in understanding the effect of radiation damage on He retention in apatite have shown that the AHe age-eU distribution of aliquots from a single sample is dependent upon the thermal history experienced by that sample (Shuster et al., 2006; Flowers et al., 2009). Therefore, inverse modeling of AHe age-eU pairs can be used to investigate the range of possible

thermal histories that could produce the observed AHe age-eU distribution (e.g., Flowers, 2009; Peyton et al., 2012). Just as a broad or bimodal distribution of fission-track lengths indicates that a sample has re-sided at temperatures within the AFT PAZ, so a cor-relation of AHe age with eU concentration indicates that a sample has resided at temperatures within the AHe PRZ. Such ages cannot simply be interpreted as the time elapsed since the sample passed through the closure temperature of the thermochronometer. Modeling is the only way to gain understanding of the thermal histories of slowly cooled samples, or of samples that have resided in the PAZ or PRZ for a significant time (relative to their age) and have a par-tially reset age.

Ages calculated from forward modeling and time-temperature paths determined from inverse modeling may both be useful for understanding real AHe and AFT ages but should be interpreted with caution. Modeling results are dependent on the ki-netic parameters that constrain annealing and diffu-sion. Although both forward and inverse modeling are based on a wealth of annealing and diffusion studies (e.g., Green et al., 1986; Laslett et al., 1987; Duddy et al., 1988; Green et al., 1989b; Wolf et al., 1998; Farley, 2000; Farley, 2002; Reiners et al., 2004; Shuster et al., 2006; Ketcham et al., 2007b; Flowers et al., 2009), we suggest that modeling should only be used to test hypotheses and not as a basis for a new hypothesis.

Several computer programs are available for for-ward and inverse modeling. HeFTy (Ketcham, 2005) can be used to forward and inverse model AFT, AHe, and vitrinite reflectance data. Others such as Monte Trax (Gallagher, 1995) and AFTSolve (Ketcham et al., 2000) can only model AFT data. Ketcham (2005) pro-vides more details on forward and inverse modeling and summarizes available software.

Multiple Thermochronology Techniques per Sample

When samples cannot be collected over an elevation profile, thermal histories can be constrained by apply-ing multiple thermochronometers such as AHe, AFT, zircon He (ZHe), and zircon fission-track (ZFT) dating (Figure 2) to a single sample (e.g., Guenthner et al., 2010). If the sample cooled quickly from temperatures above the highest closure temperature to temperatures below the lowest closure temperature, then each ther-mochronometric age can be interpreted as the time since the sample passed through each thermochro-nometer’s closure temperature. Age is plotted against temperature rather than elevation, and the slope of

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Figure 4. Age-elevation profile of thermochronologic results from the Bighorn Range. (A) Ages plotted against elevation. (B) Ages plotted against depth below the Precambrian-Cambrian unconformity. Inset shows same data but with an expanded time scale for more detail. All error bars are 2s. From Peyton et al. (2012). Reprinted by permission of the American Journal of Science. 1000 m (3281 ft).

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the plot represents the cooling rate. The amount of ex-humation experienced by a sample can be estimated by assuming a paleogeothermal gradient and by for-ward and inverse modeling. Interpretation becomes more complicated if, at some point during its history, the sample resided in the PAZ or PRZ for one or more of the techniques. Forward or inverse modeling may then be required to understand the thermal history of the sample.

Multidating of single crystals, where multiple tech-niques are applied to the same grain or crystal, is an emerging technique with applications in provenance and tectonic studies. Examples of double dating in-clude ZHe and zircon U/Pb dating (e.g., Rahl et al., 2003; Campbell et al., 2005; Reiners et al., 2005) and ZFT and U/Pb dating (e.g., Bernet et al., 2006). Carrapa et al. (2009) triple dated apatite grains from the An-des using AHe, AFT, and U/Pb techniques together with 40Ar/39Ar dating of detrital white micas from the same synorogenic strata. The apatite U/Pb ages provided the sediment source crystallization history and matched zircon U/Pb ages for the same samples ( DeCelles et al., 2007), thus helping resolve prove-nance. The 40Ar/39Ar white mica ages and AFT ages recorded sediment source exhumation histories in the Paleozoic and Cenozoic respectively, and AHe ages recorded Cenozoic basin incision. The combination of these different techniques allowed for the resolution of multiple tectono-thermal events related to different phases of mountain building.

Structural Mapping

In areas where faults are difficult to identify, such as crystalline basement or igneous intrusives, the base of a fossil PAZ or PRZ, if preserved, can be used as a structural marker. This approach assumes that at one time the base of the PAZ or PRZ was at a single elevation across the study area (i.e., it was flat) and was deformed during or after cooling. The sense of fault displacement can be determined from thermo-chronometric age changes across the fault. For ex-ample, the hanging wall of a normal fault will have older thermochronometric ages than the footwall, and conversely, the hanging wall of a reverse fault or thrust will have younger ages than the footwall. If we can estimate the exhumation rate, perhaps us-ing an age-elevation profile, then we can calculate the approximate throw across a fault using the thermo-chronometric ages. Several authors have documented basement structure in Laramide ranges by mapping the base of the AFT PAZ (Strecker, 1996; Kelley and Chapin, 1997; Kelley, 2005).

Detrital Thermochronology

Applying low-temperature thermochronologic tech-niques to sedimentary rocks can help constrain sedi-ment source exhumation ages and orogenic patterns, maximum depositional age of the sedimentary rock, and paleodrainage and paleotopography (Bernet et al., 2001; Spiegel et al., 2004; Bernet et al., 2006; Carrapa et al., 2006; van der Beek et al., 2006; Carrapa and DeCelles, 2008). The main assumptions of detrital thermochronology are (1) different sediment source regions have different cooling ages and impart dif-ferent, distinguishable ages to the sedimentary basin fill and (2) the sedimentary rocks being studied have not reached temperatures high enough to totally reset thermochronometer ages (i.e., the grains being dated record cooling ages of the original sediment source areas). When ages of detrital samples are partially re-set, they only provide an estimate of maximum bur-ial temperature. Thus, the cooling age of the detritus within a sedimentary rock, called the detrital cooling age, cannot be younger than its depositional age, and the youngest cooling age found in a sedimentary rock provides a constraint on the maximum age of deposi-tion. When cooling ages within sedimentary strata are younger than the depositional age of the hosting strata (i.e., fully reset after deposition), they provide infor-mation on the timing of basin exhumation and defor-mation (e.g., Carrapa et al., 2011).

Lag TimesIf the depositional age of a sedimentary rock can be independently determined, perhaps using U/Pb dat-ing of zircon from intercalated tuff layers, then the concept of lag time can be used to investigate orogenic patterns and source exhumation. Lag time is defined as the difference between the cooling age of a detri-tal grain and the depositional age of the sedimentary rock that contains the grain (Garver et al., 1999). It provides a measure of cooling and exhumation rates. The higher the exhumation rate, the shorter the time a sample will take to be exhumed from the closure-temperature depth to the surface and then transported and deposited. With this approach it is assumed that no temporary storage of material has occurred be-tween source and basin.

When an orogen is experiencing steady-state exhu-mation (i.e., uniform exhumation through time), the lag time remains constant up section in a sedimen-tary sequence (Figure 5). If the exhumation rate of the source is increasing through time, (e.g., the orogen is in a constructional phase and topography is growing), then lag time will decrease up section (Bernet et al., 2001) (Figure 5). If the exhumation rate is decreasing

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through time (reducing topography) or is episodic (Carrapa et al., 2003), then lag times will increase up-ward in a sedimentary sequence (Figure 5).

Lag times can also be used to estimate the cooling and exhumation rate of the source orogen:

Cooling rate 5 (Tc2Ts)/Δt (Equation 3)

Exhumation rate 5 ((Tc2Ts)/G)/Δt (Equation 4)

where Δt 5 lag time, Tc 5 closure temperature, Ts 5 surface temperature, and G 5 geothermal gradient (Bernet et al., 2001). Care must be exercised when using these equations, because (1) when exhumation is very fast, isotherms are perturbed towards the surface and the geothermal gradient is not constant and (2) the clo-sure temperature of a thermochronometer is affected by the cooling rate (e.g., Garver et al., 1999). For example, apatite that has experienced rapid cooling will have a higher closure temperature and older AHe or AFT age than apatite that has experienced slower cooling (Far-ley, 2000). Sedimentary rocks that have been reworked and redeposited should be avoided because the lag time for these rocks will be too large and ages will not be representative of the most recent orogenic processes.

Unroofing Sequences and ProvenanceAFT and AHe ages from a sedimentary sequence will reflect the cooling ages of the original source terranes if the apatite has not reached high-enough temperatures

to reset ages after deposition. Bedrock from higher elevations in the source terrane will have older cool-ing ages than bedrock from lower elevations, and will generally be eroded and redeposited earlier than rock from lower elevations. Hence, detrital thermochrono-metric ages from synorogenic sedimentary rocks should show a general inverse correlation to the cool-ing ages of the source terrane, with older cooling ages deeper in the basin and younger cooling ages shal-lower in the basin. Dating of both the source terrane and the basin fill can help a researcher integrate the cooling, erosional, and tectonic histories of an area and provide both age and rate of cooling, along with changes in the exhumation rate (Spiegel et al., 2001; Coutand et al., 2006; Kuhlemann et al., 2006; Carrapa and DeCelles, 2008).

If sediment source terranes have different cool-ing histories, then different detrital-age populations will represent the different tectono-thermal events of the source regions, as long as partial resetting of ages due to burial is insignificant. At least 100 grains should be dated from a detrital sample and the age distribution of the grains plotted. If Gaussian distri-butions are fitted to the peaks on the age-distribution plot, the best-fit peak ages are inferred to represent the age of different populations in the source area (Brandon, 1992, 1996). When AFT dating is used, only track lengths pertinent to each population of grain ages should be analyzed for thermal modeling (e.g., Carrapa et al., 2006).

Basin Modeling

In petroleum exploration the primary goals of basin modeling are to better understand the timing of pe-troleum generation, migration, trap formation, and the degree of source rock maturity. These processes depend on the burial, exhumation, and thermal his-tory of a sedimentary basin. Various source-rock thermal maturity data, such as vitrinite reflectance (%Ro), Rock-Eval pyrolysis data, Tmax, Hydrogen Index, and so forth, along with thermochronometric data, are used to validate and refine basin models. Input parameters include estimates of stratigraphic thickness and age, porosity-depth relationships (to calculate decompacted thicknesses), surface tempera-tures, basal heat flow, and thermal properties of the sediments.

Thermochronometric ages generated from a ba-sin model are compared to actual measured ages and the basin model adjusted to provide a good fit to the measured data. Because source-rock maturity data contain no timing information, the inclusion of

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Figure 5. Lag time plot showing age trends (red arrows) going up sequence for different exhumation scenarios. If exhumation is constant (steady-state), lag time remains constant up sequence. If exhumation is increasing, lag time decreases up sequence. If exhumation is decreasing, lag time increases up section. Modified from Carrapa (2009).

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thermochronologic data in basin modeling may help refine basin models by providing specific informa-tion on timing, such as the onset, rate, and duration of rapid cooling or exhumation, as well as informa-tion on maximum paleotemperature. The application of low-temperature thermochronology to basin modeling is discussed in more detail in Chapter 3 (Peyton and Carrapa 2013). Many other reviews of the application of low-temperature thermochronology, and in particular AFT dating, to sedimentary-basin analysis have also been published (e.g., Green et al., 1989a; Naeser et al., 1989; Armstrong, 2005).

PITFALLS AND COMPLICATIONS

Many problems and limitations of thermochronom-eters are better understood today than in recent years, and are now addressed routinely during analysis. Is-sues that affect both AFT and AHe dating include the effects of near-surface processes, slow cooling, wildfires, lightning strikes, and complications from using wellbore core and cuttings (such as sample con-solidation, nonvertical wells, and contamination from caving and drilling mud additives). AFT ages are af-fected by variations in annealing kinetics (Green et al., 1986; Ketcham et al., 2007b), track-length reproduc-ibility (Barbarand et al., 2003; Ketcham et al., 2009), and etching protocol (Murrell et al., 2009). AHe ages are affected by alpha-particle ejection (Farley et al., 1996), radiation damage (Shuster et al., 2006; Flowers et al., 2009; Gautheron et al., 2009), grain size (Rein-ers and Farley, 2001), He implantation (Kohn et al., 2008; Reiners et al., 2008), and zonation of U and Th (Hourigan et al., 2005). All of these issues are dis-cussed here so that readers can evaluate published thermochronologic results and identify potential prob-lems. Some issues with AHe dating, such as age scat-ter within a sample and anomalously old AHe ages, are not yet fully understood and are still being stud-ied. Many recent examples show anomalously old AHe results compared with corresponding AFT data and other geological constraints (e.g., Crowley et al., 2002; Belton et al., 2004; Hendriks and Redfield, 2005; Fitzgerald et al., 2006; Green et al., 2006; Danisík et al., 2008; Spiegel et al., 2009; Peyton et al., 2012). These anomalously old AHe ages, which are likely caused by He implantation (Kohn et al., 2008; Reiners et al., 2008) and/or radiation damage (Shuster et al., 2006; Flowers et al., 2009; Gautheron et al., 2009), often occur in con-tinental areas that have been subjected to long (hun-dreds of millions of years), complex thermal histories involving reburial, slow cooling/exhumation rates, and/or long residence time in the He PRZ.

Issues Affecting Both AFT and AHe Dating

Effects of Near-surface ProcessesA one-dimensional, age-versus-elevation approach to interpreting thermochronometric ages assumes that all samples passed through the closure isotherm at the same elevation, and that samples followed a vertical trajectory to the surface. This is an oversimplified ap-proach because surface topography distorts the geo-thermal field and the shape of the isotherms, causing an upwarping of isotherms beneath mountain ranges relative to beneath adjacent plains (Figure 3D) (Stüwe et al., 1994). Migration of drainage divides further complicates the shape of the isotherms (Stüwe and Hintermüller, 2000). The effect of topography on iso-therms diminishes with depth, and therefore impacts AHe dating more than AFT dating. Similarly, it affects AFT dating more than higher temperature thermo-chronometers, such as ZFT and Ar-Ar dating.

It is unlikely that samples follow a vertical path to the surface, especially in faulted areas; both compres-sional and extensional faulting normally involve a component of horizontal displacement as well as ver-tical displacement, resulting in nonvertical exhumation pathways (Ehlers, 2005). Thus, in areas of complex tec-tonics and significant topographic relief, caution should be used when interpreting age-elevation profiles.

For age-elevation profiles to be valid, the horizontal length of the sampling transect must be small compared to the wavelength of the topography, or the wavelength of the topography must be small (<~10 km); otherwise samples may have passed through the same isotherm at different elevations relative to sea level. This is likely to result in age errors of ~10% for short-wavelength, high-elevation landscapes with average erosion rates of >1 mm/yr; for landscapes with erosion rates on the order of 0.5 mm/yr the effect is only significant for wavelengths >~20 km (>~12.4 mi) (Stüwe et al., 1994; Ehlers and Farley, 2003; Braun, 2005; Ehlers, 2005). A more recent paper by Valla et al. (2010) explored the effects of tran-sient topography and lateral offset of sample locations on thermochronometric ages and stressed the need for ap-plying more than one thermochronometer when at-tempting to reconstruct paleorelief and exhumation. Thermal-kinetic modeling of samples collected from vertical profiles can help to resolve spatial differences in temperature-time paths and exhumation (Ketcham, 2005).

Wellbore Cuttings and CoresMany thermochronology studies use cuttings from petroleum wellbores as samples (e.g., Omar et al., 1994; Beland, 2002; Peyton et al., 2012). Cuttings are usually collected by exploration companies at either

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~3 m (10 ft) or ~10 m (30 ft) intervals. Due to the limited amount of material available for dating at each depth interval, cuttings must be consolidated over a depth interval which may vary based on availability of mate-rial. The possible effects of creating a composite sample over a depth range on the resultant thermochronomet-ric ages must therefore be considered. In addition, cut-tings are not instantaneously transported away from the drill bit, resulting in an unknown amount of mix-ing. Peyton et al. (2012) inspected cuttings from a well in the Bighorn Range that crossed a thrust with Pre-cambrian crystalline basement over Phanerozoic sedi-mentary rock, and estimated that contamination with crystalline basement decreased to 5% of the cuttings 160 m (524 ft) below the thrust. The amount of mixing or contamination from caved material will likely vary between wells, but the AHe or AFT age of contami-nants will typically be older than the actual cooling age for a particular depth, because the contaminants are from shallower depths that cooled earlier. Excep-tions occur when rocks from the shallow section of a well have not been thermally reset and ages are detrital cooling ages. If the sedimentary rocks represent an un-roofing sequence of the source terrane, older ages will occur deeper in the section than younger ages. Core samples are preferable to cuttings because a larger vol-ume of rock may be available for sampling, and core samples are not affected by contamination and mixing.

Nonvertical wellbores are another potential source of uncertainty in sample depth. Sometimes the azi-muth and inclination of a wellbore are measured and recorded as a deviation survey, allowing for measured depths to be corrected to true vertical depth. Without a deviation survey there is no choice but to assume that a borehole is vertical, but if that assumption is incorrect, depth will be overestimated and elevation underestimated.

Samples from cuttings may also be contaminated by additives to the drilling mud. Recent studies have shown that drilling mud used in the Piceance Basin of Colorado contained zircons which could skew zircon dating results (A. J. Vernon, 2009, personal commu-nication). We assume that contamination of apatite is also possible with subsurface samples, although this has not been studied.

Although there are several potential pitfalls to us-ing well cuttings, meaningful ages can still be expected from the analysis of cuttings because the depth range within samples is small compared to the entire sam-pling depth range of the well (typically ~3 km [~1.6 mi]). Some small scatter in ages should be expected, and it should be recognized that older-than-expected AHe and AFT ages may reflect caving from shallower in the borehole.

Slow CoolingWhen samples have cooled through the PRZ or PAZ slowly, or resided within the PRZ or PAZ for a signifi-cant amount of time, the (U-Th)/He or fission track ages no longer represent cooling ages, and do not have direct geological significance. A correlation of AHe age with eU concentration for multiple grains or aliquots from a sample, or a broad or bimodal distribution of fission tracks, indicates that ages should not be inter-preted as the time since the sample passed through a closure temperature, but rather as partially reset ages. Thermal modeling to evaluate the degree of partial re-setting and identify possible temperature-time paths is crucial to understanding partially reset ages.

Wildfires and LightningWildfires may reach high enough temperatures that some resetting of AFT and AHe ages may occur (Reiners, 2009). To prevent wildfires from influenc-ing results, the outer few centimeters of a field sample should be removed before processing (Mitchell and Reiners, 2003). Because lightning strikes on peaks and ridges can also reset thermochronometric data, surface samples should be collected in protected locations.

Issues Affecting AFT Dating

Variation in Annealing KineticsApatite composition affects the annealing temperature of fission tracks in apatite (Green et al., 1986), but does not seem to have an effect on AHe age (Warnock et al., 1997). Fission tracks in chlorapatite anneal at higher temperatures than those in fluorapatite (Green et al., 1986). Modern fission-track analyses measure a kinetic parameter, typically either chlorine content (Cl wt%) using an electron microprobe, or more commonly Dpar, the mean fission-track etch diameter (Donelick, 1993). Both Cl wt% and Dpar have been shown to be reliable proxies for annealing kinetics (e.g., Ketcham et al., 1999). These kinetic parameters are reported along with AFT ages and fission-track lengths, and are entered into thermal-modeling software to determine the anneal-ing behavior of each sample analyzed (Ketcham et al., 1999; Ketcham, 2005). Early fission-track studies did not measure any kinetic parameters and must be inter-preted with caution (e.g., Bryant and Naeser, 1980).

Track Length Reproducibility and Thermal ModelingOne of the main issues in AFT thermochronology and inverse thermal-kinetic modeling is the reproducibility of track length and Dpar measurements between dif-ferent analysts. Ketcham et al. (2009) found significant

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variation amongst analysts asked to measure induced initial track lengths (L0), and also in their sampling of lightly annealed (long) and highly annealed (short) track populations. Ketcham et al. (2007a) documented that normalizing track lengths to a common crystal-lographic c-axis projection improved reproducibility of AFT inversion results by reducing analyst bias, be-cause tracks with different orientations to the crystal c-axis have different lengths and annealing properties (Donelick, 1991). Using a fixed L0 of 16.30 µm (Green et al., 1986) for all analysts resulted in a large variation in inversion results and was not recommended (Ketcham et al., 2009). Ketcham et al. (2009) suggested that bias in track-length measurements between analysts can be cor-rected by having analysts perform a blind L0 calibration.

Confined fission tracks (i.e., where a complete track is preserved in the apatite being analyzed) are rare in natu-ral samples and most studies only report a few tens of track-length measurements. Ideally, 100 confined track lengths per sample should be measured, corrected for their orientation to the crystallographic c-axis, and L0 cal-ibrated (Ketcham et al., 2009). For detrital samples, track lengths should be measured for each age population. For samples composed of a single age population (e.g., from basement rocks), the number of confined tracks available for track-length measurement can be increased by irra-diating the sample with 252Cf-derived fission fragments (Donelick and Miller, 1991). These fragments produce additional damage to the apatite crystal lattice, increas-ing the number of etching pathways and therefore the number of etched confined tracks.

Other important issues that should be considered when interpreting AFT modeling results are that (1) at temperatures <60oC annealing is very slow, and small variations in length measurement (a few tenths of a micron) can alter predicted annealing temperatures by tens of degrees (Ketcham et al., 2009) and (2) only sam-ples that are characterized by a single age population (i.e., pass the x2 statistical test, Galbraith, 1981) should be modeled. When studying detrital samples with multiple age populations, each population should be modeled using only lengths pertinent to that popula-tion (e.g., Carrapa et al., 2006); modeling a mix of ages and populations (van der Beek et al., 2006), or mod-eling detrital samples that do not pass x2 or have in-sufficient grain measurements (Barnes et al., 2008), produces equivocal results. In general, modeling de-trital populations is less robust than modeling in-situ samples, and results should be interpreted carefully.

Etching ProtocolIn a series of laboratory experiments, Murrell et al. (2009) showed that different etching methods had a significant effect on AFT ages and modeling results.

They recommended etching with 5.5 M nitric acid for 20 s at 21oC. This etching protocol was originally pro-posed by Donelick and used by Carlson et al. (1999) in their AFT annealing experiments. Results of these an-nealing experiments were used to calibrate the anneal-ing models of Ketcham et al. (1999) and Ketcham et al. (2007b), which are frequently used in thermal-history modeling.

Issues Affecting AHe Dating

Alpha-particle EjectionAs discussed earlier, alpha particles can travel up to ~20 μm in apatite when they are ejected from the par-ent nuclide during radioactive decay. If that decay occurs within ~20 μm of the crystal edge, some per-centage of the alpha particles will be ejected from the crystal and not measured during analysis. A correction for this effect, called the alpha-ejection correction, is routinely applied during data processing and is based on the crystal dimensions and geometry (Farley et al., 1996; Farley, 2002).

When apatite is at temperatures where He can partially diffuse out of the crystal (i.e., when it re-sides within the PRZ), He depletion from the outer ~20 μm of the crystal due to alpha ejection will result in decreased diffusive He loss. Applying a standard alpha-ejection correction to the measured age will re-sult in an overcorrection of the AHe age by as much as ~20%, depending on the thermal history (Farley, 2000; Meesters and Dunai, 2002b). Thermal-modeling programs such as HeFTy (Ketcham, 2005) include this effect so that modeled ages can be compared directly to measured ages.

Grain SizeThe effect of slow cooling through the PRZ on AHe ages will vary depending on the size of the grain be-ing dated. Smaller apatite grains that cool slowly will lose a larger fraction of their He through diffusion than larger crystals. Larger crystals therefore often have older ages than smaller crystals from the same sample (Reiners and Farley, 2001). Reiners and Far-ley (2001) presented AHe ages for multiple aliquots from two samples from the Bighorn Range of north-central Wyoming. AHe ages for each sample ranged between 98 and 348 Ma, and 107 and 232 Ma, with grain radii between 32 and 99 µm, and 42 and 103 µm, respectively. Forward modeling showed that the maxi-mum temperature during pre-Cenozoic burial was the most influential parameter on age-grain size distribu-tion. Grain-size effects are most significant when sam-ples have resided with the AHe PRZ for a long time,

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and least significant for samples that have experi-enced rapid, recent cooling (Reiners and Farley, 2001). Reiners and Farley (2001) also illustrated how forward modeling of samples showing an age-grain size dis-tribution can be used to investigate paleogeothermal gradients or to constrain local paleogeography. Both forward and inverse thermal modeling are important methods for understanding these age distributions because they incorporate grain-size effects (Ketcham, 2005).

Radiation DamageThe term “radiation damage” refers to damage of an apatite crystal lattice caused by recoil of a parent nuclide of U, Th, or Sm as it decays by ejecting an alpha particle (He nucleus). Recent work has led to a better understanding of the influence of radiation damage on the diffusion of He in apatite, and has led to the development of models that predict the relationship between AHe age and radiation dam-age (Green et al., 2006; Shuster et al., 2006; Flow-ers et al., 2009; Gautheron et al., 2009; Shuster and Farley, 2009). All of these models assume that He can be trapped in radiation damage sites at tem-peratures higher than the PRZ. The Shuster et al. (2006) and Flowers et al. (2009) models predict that apatite crystals from the same bedrock sample that have experienced the same thermal history will have AHe ages proportional to the amount of ra-diation damage, which is proportional to eU (see earlier definition). However, the influence of grain size may make this age-eU correlation appear random (Peyton et al., 2012). Variation in parent nuclide concentration is therefore a possible expla-nation for age scatter observed in multiple grains from the same sample. Depending upon the thermal history of the sample, this scatter may vary from zero Ma when the sample has been cooled rapidly through PRZ to hundreds of Ma when the sample cooled slowly through the PRZ (e.g., Flowers et al., 2007; Flowers, 2009; Flowers et al., 2009; Peyton et al., 2012). The trapped He results in older-than-expected ages and essentially changes the diffusion properties of the apatite.

Radiation damage can also explain, under certain circumstances, some anomalously old AHe ages. An AHe age may be older than the corresponding AFT age for the same sample if the sample presently resides within a fossil PRZ and has experienced sufficient radiation damage (Flowers et al., 2009; Peyton et al., 2012). Samples that have experienced rapid cooling through the PRZ should always have AHe ages that are younger than the corresponding AFT age.

Bad Neighbors/He ImplantationIf neighboring crystals or mineral phases containing high levels of U and Th (e.g., zircon, monazite, etc.) were within ~20 µm of the apatite crystal being dated, it is possible that He (alpha particles) may have been injected into the dated crystal from these neighbors. This additional He would result in anomalously old AHe ages. These U- and Th-rich phases may be in the form of weathering products (Reiners et al., 2008) or “bad [mineralogic] neighbors” (Spencer et al., 2004; Kohn et al., 2008). The influence of He implantation on AHe age is not restricted to slowly cooled cratonic rocks; the effect has also been documented in rapidly cooled volcanogenic sedimentary rocks (Spiegel et al., 2009). To date, workers have investigated the use of mechanical grain abrasion to remove the outer 20 µm of apatite crystal to eliminate any implanted He with some success (e.g., Kohn et al., 2008; Spiegel et al., 2009). However, grain abrasion has yet to become a routine and tractable part of AHe dating. Orme (2011) investigated chemical washing to remove the outer part of the apatite crystal and found that AHe age scatter was reduced but not eliminated.

Zonation of Uranium and ThoriumZonation of U and Th can affect how much He is lost due to alpha ejection. More He will be lost by alpha ejection if there is a zone with high U and Th concen-tration at the edge of the crystal compared to a high-concentration zone at the center. It is unlikely that zonation causes errors in (U-Th)/He age greater than about 30% (Wolf et al., 1996; Hourigan et al., 2005). At present there is no way to tell if U and Th zonation is affecting AHe or ZHe ages, unless the crystal has been double dated using both AFT and AHe dating.

CONCLUSIONS

Low-temperature thermochronometers such as AFT and AHe dating have many potential uses in un-derstanding the structural and thermal evolution of petroleum-producing regions, especially when ap-plied together. Age-elevation profiles from ranges adjacent to sedimentary basins can provide the tim-ing and rate of exhumation of the source area of the sediments. If the cooling and exhumation are directly related to tectonics, they also constrain the timing of deformation, which in turn may have implications for hydrocarbon migration and trapping. Age-elevation profiles from wells within a sedimentary basin pro-vide information on the timing and amount of burial and exhumation of the basin. If shallow sedimentary

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rocks have not been thermally reset, their thermo-chronometer ages are detrital cooling ages and may help constrain the provenance and maximum possi-ble depositional age of the rock, as well as the unroof-ing histories of the source areas. Thermochronometer ages can provide specific information on timing and amount of cooling, and can be used to constrain basin models of sediment accumulation, leading to a better understanding of the level and timing of maturation of petroleum source rocks.

While AFT dating is a well established low- temperature thermochronometer, the manual process of identifying and counting fission tracks and measur-ing track lengths still has the potential of introducing analyst bias into the results (Ketcham et al., 2009). Au-tomated counting techniques are being developed but are not yet fully tested or widely available (Gleadow et al., 2006; Gleadow et al., 2009). Track-length meas-urements and a kinetic parameter such as Cl wt% or Dpar , along with track counts, are required for the most complete interpretation of thermal history from AFT data.

Our understanding of the diffusion of He in apatite is also still advancing. Rocks that have spent a long time (relative to their age) in the PRZ, such as cratonic basement rocks, may have AHe ages that have been significantly impacted by radiation damage, result-ing in age scatter and possibly AHe ages older than corresponding AFT ages. For these kinds of thermal histories, grain-size variations can also result in AHe age variation for a single sample. Thermal modeling is critical for understanding the possible thermal his-tories of these samples, and indeed these variations of age with grain size and radiation damage provide ad-ditional constraints on possible thermal histories. He implantation can cause anomalously old AHe ages, re-gardless of the thermal history of the rock.

It is important to analyze an appropriate number of samples from a wellbore or elevation transect because the thermal histories of the samples are related to each other, further constraining possible modeling solu-tions and reducing uncertainty. Similarly, a sufficient number of grains should be analyzed to allow for the identification of multiple age populations in a detrital sample and to recognize the effects of radiation dam-age or He implantation on AHe ages. Partially reset ages should be interpreted with caution and should be modeled when possible.

Despite the complications associated with fission track and (U-Th)/He thermochronology highlighted in this paper, these systems are much better under-stood today than just a decade ago. Greater knowl-edge of annealing and diffusion kinetics expands the uses of these techniques because more detailed

information can be extracted from the data than previ-ously. Advancements in AFT and AHe dating can be used to model and interpret older ages if all the nec-essary parameters we have discussed are available. However, older studies did not typically measure all the parameters used today, such as track lengths, track orientations, Dpar, and so forth. In these cases, forward modeling can be used to place limits on possible inter-pretations or to test the validity of conclusions drawn from the data.

The real power of low-temperature thermochronol-ogy resides in the application of multiple techniques to the same crystal (e.g., triple dating of apatites), dat-ing of different minerals from the same sample (e.g., AFT, ZFT, AHe, ZHe), and thermal-kinetic modeling of multiple thermochronometers. Combining these techniques with other thermal indicators, such as vit-rinite reflectance, and applying them to multiple sam-ples with related thermal histories from a borehole or elevation transect, further reduces uncertainties.

ACKNOWLEDGEMENTS

We thank Shari Kelley, Jim Steidtmann, Doug Waples, and Rich Bottjer for their constructive reviews. Connie Knight offered encouragement and helpful sugges-tions for improving the manuscript. We are grateful to Cirque Resources for providing a quiet place to write.

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