2
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.
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: slpeyton@coalcreekresources.com)
Barbara Carrapa
Department of Geosciences, University of Arizona, 1040 E. 4th St., Tucson, Arizona, 85721, U.S.A.
(e-mail: bcarrapa@email.arizona.edu)
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 respectively, 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 vertical 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 basement 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
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16 Peyton and Carrapa
region influence the generation, migration, entrapment, and preservation of petroleum. Techniques 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 reflectance data.
INTRODUCTION
Geochronology and thermochronology use the radioactive 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 thermochronology, such decay is represented by spontaneous fissioning of 238U and the daughter product is represented by
damage tracks in the crystal structure produced by recoil of the fission products of 238U, called fission tracks.
For many crystalline minerals (e.g., apatite and zircon), 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 crystal lattice at high temperatures and is only retained
within the crystal below a temperature called the closure 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 thermochronometers, where daughter product is retained in
a crystal below the closure temperature but not above
it, facilitates explanations of thermochronologic techniques but is only valid for minerals that experience
steady, monotonic cooling (i.e., temperature always
decreases with time) (Dodson, 1973). Closure temperature 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)/
10711_ch02_ptg01_hr_015-036.indd 16
He dating, and the partial annealing zone (PAZ) for
­f ission-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 crystallization. For example, the (U-Th)/He technique involves the decay of U, Th, and to a lesser extent Sm,
to 4He (alpha particles). 4He is fully retained in apatite 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;
­K etcham 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, depending 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). Exhumation 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 exhumation to refer to rock removal. For a given sample,
thermochronometers with lower closure temperatures
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 exhumed 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
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 basins, low-temperature thermochronology can be used
to quantify the thermal history of a basin, evaluate hydrocarbon 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 geochronology of zircon and apatite, provides a powerful
tool for constraining the provenance and depositional
age of sedimentary rocks, as well as basin thermal 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
10711_ch02_ptg01_hr_015-036.indd 17
Figure 2. Closure temperature windows of thermochronometers 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).
deformation, uplift, or tilting, these techniques may illuminate the timing, rate, and amount of exhumation,
and if exhumation is a consequence of tectonic activity, 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 techniques, apatite fission-track (AFT) dating and apatite
(U-Th)/He (AHe) dating (Figure 2). These techniques
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18 Peyton and Carrapa
have closure isotherms that are located in the uppermost 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 techniques, 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 alternative 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 petroleum exploration in more detail.
TECHNIQUES
Fission-track Dating
Age Determination
Spontaneous fission of naturally occurring 238U produces 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 affect 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 relatively 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 fission tracks. The proxy for the daughter product for
fission-track dating is the density of spontaneous fission tracks formed by natural fission of 238U. Parent
nuclide concentration is measured using the externaldetector method, which is the most widely used and
10711_ch02_ptg01_hr_015-036.indd 18
best calibrated method at present (Hurford, 1990b,
1990a). U-free muscovite sheets (the external detectors) are placed adjacent to the polished grain mounts
and are irradiated with low-energy thermal neutrons
in a nuclear reactor. Standards of known U concentration are also included. The neutrons induce fission in
235
U, 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 inductively coupled plasma mass spectrometry (LA-ICPMS)
(Donelick et al., 2005). However, published calibration
studies are lacking, and this approach is still considered largely untested.
Fission-track ages are calculated using a decay
equation, similar to other geochronometers and thermochronometers. However, the densities of the induced and spontaneous fission tracks are entered
directly into the age equation instead of parent and
daughter nuclide concentrations (Equation 1). Fissiontrack 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,
l d 5 total decay constant of 238U, z 5 zeta calibration factor based on fission-track age standards,
g 5 ­g eometry factor for spontaneous fission track
registration, rd 5 induced fission track density for a
uranium standard, r s,i 5 spontaneous fission track
density for grain i, and r i,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).
Annealing
How fission tracks shorten or anneal is strongly dependent 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). Etching of fission tracks is discussed in more detail later.
Modeling of a single sandstone sample using these
three different kinetic proxies showed a single timetemperature history, thus documenting the validity
6/5/13 7:59 AM
An Introduction to Low-temperature 19
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 Clrich apatite needs higher temperatures (in some cases
>130°C) to fully anneal (Green et al., 1989b). Annealing 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 Lengths
The 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 experiences 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 (relative 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 temperatures 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
10711_ch02_ptg01_hr_015-036.indd 19
data that can be used to constrain thermal history and
are recorded along with the number of tracks whenever 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 exposed 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 recipe (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 different 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 subjective, each analyst calculates their own correction factor
based on standards, called the “zeta” (z) value (Equation 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 cleavage 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 properties 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 Ages
Corrected 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 annealing 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,
U, and 232Th to 206Pb, 207Pb, and 208Pb, respectively.
4
He nuclei (alpha particles) are emitted at each step in
this decay series and are the daughter nuclides for this
235
6/5/13 7:59 AM
20 Peyton and Carrapa
dating system. Equation 2 shows the decay equation
for (U-Th)/He dating:
4
He 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 isotope; 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 resides in the PRZ for a sufficient time, some He will diffuse 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 thermalkinetic 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 monazite 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 contain 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 Reiners et al. (2004). The degassed crystals (and the tube)
are then dissolved in nitric acid and the concentrations
10711_ch02_ptg01_hr_015-036.indd 20
of U, Th, and Sm measured using an inductively coupled plasma mass spectrometer (ICP-MS).
During the decay process, He nuclei have sufficient
energy to travel ~20 µm through an apatite crystal lattice 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 correction to account for this loss of He, called the alpha ejection correction, must be applied to the calculated AHe
age, taking into account the dimensions of the crystal
(Farley et al., 1996; Farley, 2002).
Diffusion
The 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 diffusion, 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-thanexpected 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). Radiation 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 diffusion 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, radiation damage diffusion models show that, under certain 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 significant percentage of their age, conditions commonly
met by exhumed cratonic rocks such as those exposed
in the northern Rocky Mountains (Peyton et al., 2012)
6/5/13 7:59 AM
An Introduction to Low-temperature 21
or the western Canadian shield (Flowers, 2009). Flowers 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 thermochronologist, especially when samples consist of core
or cuttings from a wellbore. For field samples, the University 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/­l aserchron.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 lightningprone areas, and the outer few centimeters of exposed
rock should be removed while at the outcrop. It is important to break up field samples into small, fist-sized
pieces while on the outcrop to prevent later contamination. 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, monogenic 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 recommend 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 Wilfley water table may be used to provide the first level
of density-based liquid separation, followed by sieving, 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
10711_ch02_ptg01_hr_015-036.indd 21
per sample should be analyzed using either ­technique
in order to produce statistically sound results
(­Vermeesch, 2004). However, the high cost of AHe dating 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 samples 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. Thermochronometer ages decrease with depth in a borehole 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 previous 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 deposition. 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 combined with other thermal indicators such as vitrinite
reflectance.
6/5/13 7:59 AM
22 Peyton and Carrapa
A
B
TClosure
TClosure
Elevation (km absl)
C
Slope = Exhumation rate
(km/Ma)
0
TClosure depth all samples
Figure 3. Effect of topog-
Thermochronometer Age (Ma)
D
E
(2)
Advection of
mass and heat
(3)
TClosure
Elevation (km absl)
(1)
Denudation
Sedimentation
and compaction
Slope ≠ Exhumation rate
(km/Ma)
(1)
(2)
(3)
0
Advection of
mass and heat
TClosure depth
Sample (1)
Sample (2)
Sample (3)
Thermochronometer Age (Ma)
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
10711_ch02_ptg01_hr_015-036.indd 22
raphy on age-elevation
profiles. (A) Horizontal
closure isotherm with samples collected up a range
front. (B) Closure isotherm
deformed by topography,
samples collected in a vertical 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 correct exhumation rate. From
Ehlers (2005).
at higher elevations, where it is called a “fossil” PAZ
or PRZ (Figure 1). Fossil PAZs or PRZs can be recognized on an age-elevation profile by a rapid increase
in age with small increases in elevation, and hence
­e xhumation 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
6/5/13 7:59 AM
An Introduction to Low-temperature 23
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, including both surface and subsurface data. To correct
for possible topographic effects, sample ages were
plotted against sample depth below the PrecambrianCambrian 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 identification of the present-day PRZ.
Forward and Inverse Modeling
Forward modeling involves calculating a thermochronometric 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 thermochronometric ages, and is also a useful way to predict and
understand the effect of thermal history on ages and
track-length distributions. Although forward models 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 thermochronometer ages to within a specified amount of
statistical error, assuming a starting time and temperature. 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 inverse 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 presentday 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
10711_ch02_ptg01_hr_015-036.indd 23
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 resided at temperatures within the AFT PAZ, so a correlation 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 partially 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 kinetic parameters that constrain annealing and diffusion. 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 forward 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) provides 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 applying 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 thermochronometric age can be interpreted as the time
since the sample passed through each thermochronometer’s closure temperature. Age is plotted against
temperature rather than elevation, and the slope of
6/5/13 7:59 AM
24 Peyton and Carrapa
BIGHORN RANGE
A
4000
Surface samples
3000
10
Subsurface samples
1000
20
30
0
40
-1000
50
AHe Peyton et al. (2012)
AFT Peyton et al. (2012)
AHe Crowley et al. (2002)
AFT Cerveny (1990)
-2000
-3000
0
100
200
300
Temperature °C
Elevation (m)
2000
60
70
400
Age (Ma)
B
Fossil PAZ
-250
250
-250
Fossil PRZ
1250
1750
2250
2750
3250
Approximate
present-day
PRZ
3750
4250
1250
1750
2250
2750
3250
3750
4250
AHe Peyton et al. (2012)
AFT Peyton et al. (2012)
AHe Crowley et al. (2002)
AFT Cerveny (1990)
4750
5250
4750
5250
250
750
Depth below pC unconformity (m)
Depth below pC unconformity (m)
750
0
20
40
60
80
100
120
140
Age (Ma)
0
100
200
300
400
Age (Ma)
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).
10711_ch02_ptg01_hr_015-036.indd 24
6/5/13 7:59 AM
An Introduction to Low-temperature 25
the plot represents the cooling rate. The amount of exhumation experienced by a sample can be estimated
by assuming a paleogeothermal gradient and by forward 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 techniques are applied to the same grain or crystal, is an
emerging technique with applications in provenance
and tectonic studies. Examples of double ­dating include 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 Andes using AHe, AFT, and U/Pb techniques ­together
with 40Ar/ 39Ar dating of detrital white ­m icas 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
(­D eCelles et al., 2007), thus helping resolve provenance. 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 thermochronometric age changes across the fault. For example, 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 using an age-elevation profile, then we can calculate the
approximate throw across a fault using the thermochronometric 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).
10711_ch02_ptg01_hr_015-036.indd 25
Detrital Thermochronology
Applying low-temperature thermochronologic techniques to sedimentary rocks can help constrain sediment 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 different, 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 reset, they only provide an estimate of maximum burial 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 deposition. When cooling ages within sedimentary strata are
younger than the depositional age of the hosting strata
(i.e., fully reset after deposition), they provide information on the timing of basin exhumation and deformation (e.g., Carrapa et al., 2011).
Lag Times
If the depositional age of a sedimentary rock can be
independently determined, perhaps using U/Pb dating 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 detrital 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 between source and basin.
When an orogen is experiencing steady-state exhumation (i.e., uniform exhumation through time), the
lag time remains constant up section in a sedimentary 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
6/5/13 7:59 AM
26 Peyton and Carrapa
10
5
0
15
20
20
0
5
10
C
on
st
a
nt
la
15
25
e
g
20
tim
25
30
increas
ea
dy
-s
t
30
umatio
ing exh
at
e
35
35
ex
hu
m
at
io
n
40
n
on
humati
sing ex
decrea
st
Detrital cooling ages (Ma)
45
25
30
35
Depositional ages (Ma)
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).
through time (reducing topography) or is episodic
(Carrapa et al., 2003), then lag times will increase upward 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 closure 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 (Farley, 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 Provenance
AFT 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
10711_ch02_ptg01_hr_015-036.indd 26
to reset ages after deposition. Bedrock from higher
elevations in the source terrane will have older cooling ages than bedrock from lower elevations, and will
generally be eroded and redeposited earlier than rock
from lower elevations. Hence, detrital thermochronometric ages from synorogenic sedimentary rocks
should show a general inverse correlation to the cooling ages of the source terrane, with older cooling ages
deeper in the basin and younger cooling ages shallower 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 cooling 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 distributions 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 petroleum generation, migration, trap formation, and
the degree of source rock maturity. These processes
depend on the burial, exhumation, and thermal history of a sedimentary basin. Various source - rock
thermal maturity data, such as vitrinite reflectance
(%R o), 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 temperatures, basal heat flow, and thermal properties of the
sediments.
Thermochronometric ages generated from a basin 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
6/5/13 8:00 AM
An Introduction to Low-temperature 27
thermochronologic data in basin modeling may help
refine basin models by providing specific information on timing, such as the onset, rate, and duration
of rapid cooling or exhumation, as well as information 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 thermochronometers are better understood today than in recent years,
and are now addressed routinely during analysis. Issues 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 consolidation, nonvertical wells, and contamination from
caving and drilling mud additives). AFT ages are affected by variations in annealing kinetics (Green et al.,
1986; Ketcham et al., 2007b), track-length reproducibility (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 (Reiners 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 discussed here so that readers can evaluate published
thermochronologic results and identify potential problems. Some issues with AHe dating, such as age scatter within a sample and anomalously old AHe ages,
are not yet fully understood and are still being studied. 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 continental areas that have been subjected to long (hundreds of millions of years), complex thermal histories
involving reburial, slow cooling/exhumation rates,
and/or long residence time in the He PRZ.
10711_ch02_ptg01_hr_015-036.indd 27
Issues Affecting Both AFT and AHe Dating
Effects of Near-surface Processes
A 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 approach because surface topography distorts the geothermal 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 isotherms diminishes with depth, and therefore impacts
AHe dating more than AFT dating. Similarly, it affects
AFT dating more than higher temperature thermochronometers, such as ZFT and Ar-Ar dating.
It is unlikely that samples follow a vertical path to
the surface, especially in faulted areas; both compressional and extensional faulting normally involve a
component of horizontal displacement as well as vertical displacement, resulting in nonvertical exhumation
pathways (Ehlers, 2005). Thus, in areas of complex tectonics 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 transient topography and lateral offset of sample locations on
thermochronometric ages and stressed the need for applying more than one thermochronometer when attempting 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 Cores
Many 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
6/5/13 8:00 AM
28 Peyton and Carrapa
~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 material. The possible effects of creating a composite sample
over a depth range on the resultant thermochronometric ages must therefore be considered. In addition, cuttings are not instantaneously transported away from
the drill bit, resulting in an unknown amount of mixing. Peyton et al. (2012) inspected cuttings from a well
in the Bighorn Range that crossed a thrust with Precambrian crystalline basement over Phanerozoic sedimentary 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 contaminants will typically be older than the actual cooling
age for a particular depth, because the contaminants
are from shallower depths that cooled earlier. Exceptions 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 unroofing 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 volume 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 azimuth 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 communication). 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 using 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 sampling 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.
10711_ch02_ptg01_hr_015-036.indd 28
Slow Cooling
When samples have cooled through the PRZ or PAZ
slowly, or resided within the PRZ or PAZ for a significant 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 interpreted 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 resetting and identify possible temperature-time paths
is crucial to understanding partially reset ages.
Wildfires and Lightning
Wildfires may reach high enough temperatures that
some resetting of AFT and AHe ages may occur
(Reiners, 2009). To prevent wildfires from influencing 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 Kinetics
Apatite 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 annealing behavior of each sample analyzed (Ketcham et al.,
1999; Ketcham, 2005). Early fission-track studies did
not measure any kinetic parameters and must be interpreted with caution (e.g., Bryant and Naeser, 1980).
Track Length Reproducibility and Thermal Modeling
One of the main issues in AFT thermochronology and
inverse thermal-kinetic modeling is the reproducibility
of track length and Dpar measurements between different analysts. Ketcham et al. (2009) found significant
6/5/13 8:00 AM
An Introduction to Low-temperature 29
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 crystallographic c-axis projection improved reproducibility
of AFT inversion results by reducing analyst bias, because 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 corrected 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 natural 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 calibrated (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 irradiating the sample with 252Cf-derived fission fragments
(Donelick and Miller, 1991). These fragments produce
additional damage to the apatite crystal lattice, increasing 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 samples 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 population (e.g., Carrapa et al., 2006); modeling a mix of ages
and populations (van der Beek et al., 2006), or modeling detrital samples that do not pass x2 or have insufficient grain measurements (Barnes et al., 2008),
produces equivocal results. In general, modeling detrital populations is less robust than modeling in-situ
samples, and results should be interpreted carefully.
Etching Protocol
In a series of laboratory experiments, Murrell et al.
(2009) showed that different etching methods had a
significant effect on AFT ages and modeling results.
10711_ch02_ptg01_hr_015-036.indd 29
They recommended etching with 5.5 M nitric acid for
20 s at 21oC. This etching protocol was originally proposed by Donelick and used by Carlson et al. (1999) in
their AFT annealing experiments. Results of these annealing experiments were used to calibrate the annealing 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 Ejection
As discussed earlier, alpha particles can travel up to
~20 μm in apatite when they are ejected from the parent nuclide during radioactive decay. If that decay
occurs within ~20 μm of the crystal edge, some percentage 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 resides 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 result 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 Size
The effect of slow cooling through the PRZ on AHe
ages will vary depending on the size of the grain being 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 Farley (2001) presented AHe ages for multiple aliquots
from two samples from the Bighorn Range of northcentral 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 maximum temperature during pre-Cenozoic burial was the
most influential parameter on age-grain size distribution. Grain-size effects are most significant when samples have resided with the AHe PRZ for a long time,
6/5/13 8:00 AM
30 Peyton and Carrapa
and least significant for samples that have experienced rapid, recent cooling (Reiners and Farley, 2001).
Reiners and Farley (2001) also illustrated how forward
modeling of samples showing an age-grain size distribution 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 Damage
The 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 damage (Green et al., 2006; Shuster et al., 2006; Flowers 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 temperatures 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 ­e xperienced the same thermal history will
have AHe ages proportional to the amount of radiation 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 explanation 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-thanexpected 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
­s ufficient 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.
10711_ch02_ptg01_hr_015-036.indd 30
Bad Neighbors/He Implantation
If 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 Thorium
Zonation 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 concentration at the edge of the crystal compared to a highconcentration 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 understanding the structural and thermal evolution of
­p etroleum-producing regions, especially when applied together. Age-elevation profiles from ranges
adjacent to sedimentary basins can provide the timing 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 provide information on the timing and amount of burial
and exhumation of the basin. If shallow sedimentary
6/5/13 8:00 AM
An Introduction to Low-temperature 31
rocks have not been thermally reset, their thermochronometer ages are detrital cooling ages and may
help constrain the provenance and maximum possible depositional age of the rock, as well as the unroofing 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 measuring track lengths still has the potential of introducing
analyst bias into the results (Ketcham et al., 2009). Automated counting techniques are being developed but
are not yet fully tested or widely available (Gleadow
et al., 2006; Gleadow et al., 2009). Track-length measurements 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, resulting 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 histories of these samples, and indeed these variations of
age with grain size and radiation damage provide additional constraints on possible thermal histories. He
implantation can cause anomalously old AHe ages, regardless 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 solutions 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 damage 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 understood today than just a decade ago. Greater knowledge of annealing and diffusion kinetics expands
the uses of these techniques because more detailed
10711_ch02_ptg01_hr_015-036.indd 31
information can be extracted from the data than previously. Advancements in AFT and AHe dating can be
used to model and interpret older ages if all the necessary 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 interpretations or to test the validity of conclusions drawn
from the data.
The real power of low-temperature thermochronology resides in the application of multiple techniques
to the same crystal (e.g., triple dating of apatites), dating 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 vitrinite reflectance, and applying them to multiple samples 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 suggestions for improving the manuscript. We are grateful to
Cirque Resources for providing a quiet place to write.
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