(U-Th)/He Case Study
The Fresne-1 well is located
in the Southern Taranaki Graben, between the North and South
Islands of New Zealand. The well was drilled on one of
several prominent Late Miocene inversion structures in the region.
The stratigraphy of the section intersected in the well establishes
tight constraints on the timing of inversion to between 15.5
and 2 Ma. The seismic section through the structure (after
Knox, 1982) suggests between 2 and 3.5 km of section have been
eroded from the structure during inversion. This well
therefore provides an excellent test-bed for use of paleo-thermal
techniques in determining the timing of cooling and amount of
section eroded from the structure.
AFTA data from the Fresne-1
well were first reported by Kamp and Green (1990). Since
that time, methods for extraction of thermal history information
from AFTA data have improved considerably. In particular,
the mono-compositional kinetic model (Laslett et al., 1987)
used at the time of that study has been superseded by a multi-compositional
model which incorporates the effects of variation in kinetic
response between apatites of different composition (chlorine
content), and provides more accurate definition of both the
magnitude and timing of maximum paleotemperatures from AFTA
data. Therefore, for this study the samples from the Fresne-1
originally analysed by Kamp and Green (1990) have been reanalysed
using latest methods, in which chlorine contents are measured
in every apatite grain analysed. Cl contents were measured
using an automated electron microprobe, which takes the apatite
grain locations from the computer-controlled microscope system
and directly locates each grain for analysis.
Figure 2 shows fission track
ages plotted against depth (wrt kb) in the Fresne-1 well.
Values for each sample are generally very similar to those reported
by Kamp and Green (1990), showing a progressive reduction from
around 80 Ma from the shallowest part of the Late Cretaceous
to Early Tertiary coal measures section to values consistently
around 5 to 10 Ma at depths greater than 1500 metres.
This pattern of variation is characteristic of sections that
have been hotter in the past, with the point marking the transition
from rapidly decreasing ages to consistent values corresponding
to the paleotemperature at which all samples are totally annealed
prior to the onset of cooling. Mean track length and track
length distributions show complementary behaviour. Shallow
samples are dominated by shorter tracks, representing partially
annealed tracks formed prior to the onset of cooling, with only
a small proportion of longer tracks formed after cooling.
With increasing depth, as fission track ages are progressively
reduced, due to increasing maximum paleotemperatures, so is
the mean length of the shorter component in each sample, representing
an increasing degree of partial annealing of tracks formed prior
to the onset of cooling. In the three deepest samples,
all tracks were totally annealed prior to cooling, and only
longer tracks, formed after cooling, are present.
Figure 3 shows the variation
of fission track ages of individual apatite grains with chlorine
content within each sample. In samples 8694-7 and 8,
in which the central or pooled fission
track ages, characterising data from the entire collection of
grains from each sample, are higher than the stratigraphic age
of the samples, the individual grain ages show little or no
variation with Cl content. But in sample 8694-9, in which
the central fission track age is reduced to less than 50% of
the stratigraphic age, the individual grain ages show a clear
and consistent trend, with those grains with less than 0.1 wt%
Cl having very young ages, and with ages increasing with Cl
content such that in grains with greater than 0.3 wt% Cl, ages
are indistinguishable from the stratigraphic age. Results
from deeper samples show similar effects, but with the transition
from very young ages to much older ages occurring at progressively
higher Cl contents with increasing depth. In the deepest
sample, 8694-14, all grains except for that with the highest
Cl content between 0.6 and 0.7 wt% Cl give very young ages.
These trends represent the progressive
overprinting of tracks formed prior to the paleo-thermal maximum.
In the deeper samples, the lower Cl-content grains were totally
annealed prior to the onset of cooling (the fission track ages
of these grains provides the best constraint on the time of
cooling), with the range of Cl contents which were totally annealed
prior to the onset of cooling increasing with depth, corresponding
to progressively higher maximum paleotemperatures.
Results from a small number
of grains, circled in Figure 2, do not fit within this general
pattern. These results highlight the fact that secondary
controls on annealing rates exist, in addition to the first-order
control exerted by Cl content. Data from these grains
have been removed prior to detailed thermal history interpretation.
To further highlight the importance
of compositional effects in these data, Figure 4 shows the fission
track age of individual Cl content groups within each sample,
plotted against depth. While the behaviour of the data
in this plot are more erratic than those in Figure 2, due mainly
to the small numbers of grains involved, the trend is clear
with apatites with higher Cl contents achieving a particular
degree of age reduction at progressively deeper levels, corresponding
to higher maximum paleotemperatures. As a guide to visualising
these effects, also shown in Figure 4 are the trends of fission
track age with depth for separate compositional groups corresponding
to the best-fit thermal history solution presented in later
discussion. Predicted patterns of age vs Cl content within
each sample corresponding to these solutions are also shown
in Figure 3. Similar trends are also evident within the
track length data in these samples, but these are not illustrated
Thermal history information
is extracted from AFTA data by comparing measured parameters
(fission track age and track length distributions) with values
predicted from a range of likely thermal history scenarios,
systematically varying the magnitude and timing of the maximum
palaeotemperature and using rigorous statistical procedures
to define the range of conditions which are compatible with
the measured data within 95% confidence limits. VR data
are from Lowery (1988) and have been converted to maximum paleotemperatures
using the kinetics of Burnham and Sweeney (1989), using the
thermal history framework defined by AFTA.
Note that we do not attempt
to constrain the whole thermal history of each AFTA sample.
Instead we focus on those aspects of the thermal history that
control the development of the AFTA parameters specifically
the maximum paleotemperature of each sample, and the time at
which cooling from that paleotemperature began. Note also
that because AFTA and VR data preserve no information on the
approach to the paleo-thermal maximum, it is necessary to assume
a value of heating rate (10°C/Ma for this study). In addition,
because the main cooling phase began quite recently, the number
of tracks formed after cooling is small, and therefore the AFTA
data provide very little control on the cooling history, and
as a first pass we have assumed linear cooling at
Figure 5 shows paleotemperature
constraints from AFTA and VR in the Fresne-1 well, plotted against
depth (rkb). This plot illustrates the high degree of
consistency between the maximum paleotemperatures derived from
AFTA and VR. The combined paleotemperature constraints
define a linear profile, with a slope which is similar to that
of the present-day temperature profile, suggesting a most likely
explanation of heating due solely to deeper burial.
Combining results from all eight
samples, the best estimate from AFTA of the time at which cooling
began is between 9 and 8 Ma.
The slope of a line fitted to
the palaeotemperature profile in Figure 5 provides an estimate
of the palaeogeothermal gradient at the paleo-thermal maximum.
Extrapolating the fitted linear profile to an assumed palaeo-surface
temperature then provides an estimate of the amount of section
removed by erosion. Statistical techniques (using likelihood
theory, due to the ranges of paleotemperature allowed by the
AFTA data) allow definition of the range of each parameter allowed
by the palaeotemperature constraints within 95% confidence limits
(Bray et al., 1992).
Figure 6a shows the range of
allowed values of palaeogeothermal gradients and removed section
which are consistent with the palaeotemperature constraints
from AFTA and VR in the Fresne-1 well. The plot also highlights
the correlation between allowed values of the two parameters,
such that higher palaeogeothermal gradients require correspondingly
lower values of removed section, and vice versa. The maximum
likelihood palaeogeothermal gradient is 24.5°C with upper and
lower 95% confidence limits of 30 and 19°C/km, respectively.
The present-day thermal gradient of 28°C/km falls well within
this trend, and given the relatively late timing for the main
cooling phase, it seems most likely that the paleo-thermal gradient
at the time at which cooling began was close to this value.
A paleogeothermal gradient of 28°C/km corresponds to between
2550 and 2800 metres of removed section, as shown. Thus,
we adopt the mid-point value of 2675 metres as the best estimate
of the amount of section removed on the Late Miocene-Pliocene
unconformity in the Fresne-1 well. This is highly consistent
with independent estimates from reconstructing seismic sections
Taking values of 28°C/km for
the paleogeothermal gradient and 2675 metres of section removed,
reconstructed thermal histories for units intersected in the
Fresne-1 well are shown in Figure 6b.
Figure 7a (upper) shows the
equipment used to measure (U-Th)/He ages (courtesy of CSIRO
Division of Petroleum Resources, Sydney). The CSIRO He
extraction and analysis facility comprises an all-metal He extraction
and gas-handling line connected to a dedicated on-line Balzers
Prisma 200 quadrupole mass spectrometer. Gas extraction
is performed by using either of the 2 identical single vacuum
resistance furnaces, where samples are heated to ~900°C
for ~15 minutes. The line and furnace are evacuated to
~10-8 mbar via ion, turbo and backing pumps.
Active gases, particularly hydrogen, are removed using SAES
getters. The analysis procedure is operated by LabVIEW
automation software supplied by Prof. Ken Farley, Caltech.
Figure 7b (lower) shows (U-Th)/He
ages in the eight samples from Fresne-1, plotted against depth.
Fission track ages, and the variation of stratigraphic age with
depth, are also shown, for comparison. In the shallower
samples, the He ages are much younger than fission track ages
in the same sample, while the difference is much less pronounced
in the deeper samples. Helium ages in the shallower samples
are similar to the fission track ages in the deeper samples.
These He ages are clearly responding to cooling at around the
same time as recorded in the fission track ages.
But because of the effects of
He loss at present-day temperatures, these (U-Th)/He ages cannot
be interpreted directly as dating specific cooling events.
Instead, the cumulative effects of the thermal histories of
the samples within the sedimentary basin context must be taken
Figure 8a shows the first
pass thermal history solutions derived from the AFTA and
VR data, from Figure 6b. Note that the heating and cooling
rates of 10°C/Ma assumed in interpreting the data appear to
be realistic, based on this reconstruction. The youngest
unit below the unconformity is dated at 15.5 Ma (Kamp and Green,
1990), the total amount of heating involved is ~75°C, and cooling
began at around 8.5 Ma. These combine to suggest a heating
rate very close to 10°C/Ma. Similarly, with a depositional
age of around 2 Ma for the youngest sediment above the unconformity,
cooling by 75°C in 6 Ma again suggests an overall cooling rate
close to 10°C/Ma. Thus, this reconstruction for units
intersected in the Fresne-1 well provides a self-consistent
description of the data discussed so far.
Figure 8b shows the (U-Th)/He
ages measured in apatites from the eight AFTA samples in the
Fresne-1 well (from Figure 7b). As a starting point to
more fully understanding these Helium age results, we have modelled
the values expected in each sample on the basis of the thermal
history framework obtained from the AFTA and VR data (Figure
8a). This has been carried out using software provided
by Prof. Ken Farley of Caltech, based on the systematics presented
in Farley (2000) and references therein. Predicted ages
are shown for four grain radii (since grain size affects diffusion
rates, larger grains retain more He than smaller grains, for
a given thermal history). The mean radii in the samples
analysed from Fresne-1 are generally around 50 µm, and
this trend in Figure 8b is the most appropriate for direct comparison
with the measured ages. In general, the predicted and
measured ages in Figure 8b show a fair degree of agreement,
particularly at the shallowest and deepest extremes of the depth
range, while predicted values from the middle of the sampled
interval are higher than measured values.
Given that the predictions are
based purely on diffusion systematics derived from extrapolation
of results from laboratory conditions, it is not, at present,
clear whether the slight mis-match between measured and predicted
ages arises because the real thermal histories are somewhat
different from those in Figure 8a, or because of slight errors
in the diffusion systematics. Similar comparisons of measured
(U-Th)/He ages with predicted values in samples from wells located
in the Otway Basin of S.E. Australia by House et al. (1999)
suggested that the diffusion systematics could be extrapolated
to geological conditions with confidence, and on this basis,
we proceed to consider how the thermal histories in Figure 8a
could be refined to give a better fit to the measured ages.
Figure 9a shows an alternative
thermal history style, characterised by rapid cooling from the
maximum at 8.5 Ma, with all cooling achieved within 1 Ma.
Figure 9b shows a comparison of (U-Th)/He ages predicted from
these histories with the measured values. In this case,
rapid cooling predicts much older ages than in Figure 8b, and
a much worse fit to the measured ages. Thus, this scenario
can be eliminated.
As a further alternative, Figure
10a illustrates a thermal history scenario involving protracted
cooling, where 50°C of the total 75° cooling occurs between
4 and 2 Ma. The predicted age trends in Figure 10b now
show an improved match to the measured ages, especially at depths
greater than ~1000 metres, although the predicted ages in the
two shallowest samples are younger than the measured values.
This suggests that these samples perhaps cooled earlier than
the deeper samples, suggesting a thermal history scenario involving
two discrete cooling episodes, as illustrated in Figure 11.
Figure 11a illustrates a thermal
history scenario which is based on that illustrated in Figure
10, but now involves two discrete cooling episodes, with an
initial phase at 8.5 Ma and a later cooling phase beginning
at 4 Ma (again with 50°C of cooling since 4 Ma). Figure
11b shows the age trends predicted from this scenario, and in
this case the agreement between measured and predicted ages
is extremely good across the whole depth range.
While it remains, to some extent,
uncertain whether this treatment represents over-reliance on
the extrapolation of laboratory diffusion systematics, and more
tests are required in controlled geological conditions, this
procedure certainly illustrates the potential of the (U-Th)/He
technique to complement AFTA and VR data in sedimentary basins
to provide further definition of thermal history styles, particularly
in terms of refining the most recent, low temperature, phase
of the history.
We are grateful to Ken Farley
of Caltech for providing the simulation software used to predict
(U-Th/He ages for this study. We also acknowledge the
efforts of our co-workers on this project, Dr. Peter Crowhurst
of CSIRO Division of Petroleum Resources, Sydney, and Prof.
Peter Kamp, The University of Waikato, Hamilton, New Zealand.
is the registered trademark of Geotrack International.
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