 |
| Titel |
A new approach to deriving thermal history information from apatite (U-Th)/He analyses which exploits the natural dispersion of single crystal age determinations |
| VerfasserIn |
Roderick Brown, Romain Beucher, Kerry Gallagher, Cristina Persano, Finlay Stuart, Paul Fitzgerald |
| Konferenz |
EGU General Assembly 2011
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250052938
|
|
|
|
|
|
| Zusammenfassung |
| Near surface tectonic and geomorphic processes typically involve erosion that leads to
cooling of rocks through low temperatures characteristic of the upper few kilometers of the
Earth’s crust (circa 50-150 °
C). Over the last decade or so major progress has been
made in developing both the theoretical and practical aspects of apatite (U-Th)/He
thermochronometry (e.g. Farley, 2002) and the thermal sensitivity (40-70 °
C) of this
technique has made it one of the most widely used methods of quantifying the low
temperature thermal histories of rocks in geoscience investigations across an extremely wide
range of geological settings. It is now standard practice, and generally seen as best practice
(Farley et al., 2010), to analyse single grains. These individual prismatic crystals (particularly
of apatite and zircon) are usually broken, i.e. they are fragments of larger crystals that have
broken parallel to the weak cleavage direction at right angles to the c-axis (for apatite) during
mineral separation. This is clearly indicated by the common occurrence of only 1 or no clear
crystal terminations present on separated apatite grains (e.g. Farley, 2002, Farley et al.,
2010).
Using a numerical model with a finite cylinder geometry to approximate 4He diffusion in
hexagonal apatite crystals we show that much of the natural dispersion typically seen for
single crystal (U-Th)/He age measurements (i.e. the dispersion that exceeds that expected
statistically from the analytical precision of measurements alone) is explained when broken
grains are treated explicitly as fragments of larger grains. Our experiments indicate that
natural dispersion is often of the order of 40-50% (and sometimes greater) for samples with
well rounded diffusive profiles, and that the mean age of multiple fragment ages approaches
the true whole crystal age for a sufficient number of analyses (circa 20-30). Where the
majority of fragments analysed have no terminations then the sample age is always
substantially overestimated for samples with highly rounded 4He profiles. The results of our
numerical experiments accords very well with degree and pattern of natural dispersion
seen in several real data sets and reproduces the common observation that rapidly
cooled samples with ‘young’ ages show least dispersion while ‘older’ samples with
more complex T-t histories show large dispersion. This source of dispersion is a
natural consequence of analysing broken crystals and the shape of the combined
axial and radial diffusion profiles within prismatic crystals such as apatite and we
believe it is a primary cause of much of the dispersion observed in (U-Th)/He data
sets.
A key challenge to deriving T-t histories from these data, and therefore constraints on
rates of erosion or relief development, is that there is usually no unique cooling path
consistent with any given (U-Th)/He age determination. This is particularly demanding for
samples that have experienced slow to moderate cooling or prolonged residence within the
partial retention zone and therefore have strongly rounded 4He distributions. Major progress
has been made in this area with the development of the 4He/3He technique which uses a
constant background 3He distribution (induced by proton irradiation) coupled with a step
heating protocol to obtain information about the diffusive 4He profile within a single grain
(Shuster and Farley, 2005).
We demonstrate a new approach to deriving T-t paths consistent with single grain apatite
(U-Th)/He age measurements that explicitly uses the natural dispersion described above and
exploits the valuable information about the spatial distribution of 4He within the individual
crystal fragments. The strategy involves finding a single common (to all fragments) T-t
history that optimizes the fit to each fragment age (for which eU and radius are known) using
a finite cylinder model geometry and with the initial crystal lengths (which are unknown)
fitted as model parameters. The modelling approach yields similar information to the recently
developed 4He/3He technique (Shuster and Farley, 2005) with the advantage that it is
inherently a multi grain method capable of jointly fitting a common T-t history to
as many single grain analyses as are available from a single sample. Unlike the
4He/3He technique it is also clearly unaffected by analyses of broken fragments
but rather explicitly accommodates and exploits the T-t information within these
grains.
Our new modelling strategy explicitly exploits the information about the shape of the 4He
distributions inherent in the individual fragment ages, leading to improved constraints on
viable thermal history models, especially those for samples that have experienced significant
diffusive loss of 4He. Some changes to criteria for selecting grains for analysis are indicated
in order to maximize the effectiveness of this new approach. These include selecting a wide
range of grain sizes (specifically prism lengths), perhaps even manually breaking grains to
ensure that this is possible, and analyzing a larger number of single grains per sample (circa
15-20 grains per sample) than is usually the case where typically only 3-5 grains are
analysed.
The advantage of this new approach is that it is essentially a generalisation of the current
approach to modelling T-t histories from (U-Th)/He data with the assumption that single
grain measurements are made on whole crystals being relaxed. It can therefore explicitly
accommodate all the details of the current approach such as the effects of temporally variable
diffusivity (e.g. radiation damage models), zonation of U and Th and arbitrary grain size
variations and will work equally effectively for whole or broken crystals, or indeed the
more likely situation where there is a mixture of both. But, just like the current T-t
modelling approach, other causes of natural dispersion other than fragmentation
need to be assessed and accounted for. However, we believe that much of the so
called ‘enigmatic’ dispersion documented for single crystal apatite (U-Th)/He age
determinations, especially for those samples that have had protracted T-t histories within the
partial retention zone, is quantitatively explained by our explicit fragmentation
model.
Farley, K.A., 2002, (U-Th)/He dating: techniques, calibrations and applications, Reviews
in Mineralogy and Geochemistry, v. 47, p. 819-844.
Farley, K.A., Shuster, D.L., Watson, E.B., Wanser, K.H. and Balco, G., 2010, Numerical
investigations of apatite 4He/3He thermochronometry, Geochemistry Geophysics Geosystems,
v. 11, Q10001, 18 pp., doi:10.1029/2010GC003243.
Shuster, D.L. and Farley, K.A., 2005, 4He/3He thermochronometry: theory, practice and
potential applications, Reviews in Mineralogy and Geochemistry, v. 58, p. 181-203. |
|
|
|
|
|
|