Download TIME PASSES – ARGON ISOTOPES AND FLUIDS IN THE EARTH`S

TIME PASSES – ARGON ISOTOPES AND FLUIDS IN THE EARTH’S CRUST.
S. P. Kelley. Dept of Environment, Earth and Ecosystems ([email protected]).
Introduction: Recent improvements in quantifying the
fundamental parameters for noble gas solubility and diffusion mean that we can begin to quantify their distribution and
transport in the Earth’s crust, and question some of the long
standing assumptions about their residence and potential for
recycling into other terrestrial reservoirs. The starting point
for this exploration is a review of noble gases and halogens
in crustal fluids by Kendrick and Burnard [1], who noted that
time is an important parameter in the use of noble gas tracers
to understand crustal processes; whether it indicates the
residence time of water in a reservoir based on 4He acquired
from aquifer rocks, the 40Ar/39Ar age of fluid movements
based on trapped fluid inclusions, or 40Ar/36Ar ratios in deep
mine waters.
The combination of several noble gas tracers is a powerful
tool but argon isotopes offer a particular insight into the
system as a whole because they are easily measured and new
solubility measurements can be combined with the extensive
literature of geochronology to gain a wholistic view of noble
gas reservoirs and transport in the crust.
Laboratory experiments have now reliably quantified solubility in a range of minerals (e.g. [2. 3]) notably in minerals that
have the capacity to recycle noble gases into the mantle,
which means that we can bring together the understanding
from measuring the rates and timescales of crustal processes,
and studies of the noble gas geochemistry of terrestrial fluids.
Earlier work on excess argon e.g. [4]) is key to quantifying
noble gases in the deep crust since the many studies of excess argon (e.g. [4, 5, 6]) clearly identify crustal environments where radiogenic noble gases reach concentrations
that are significant relative to in-situ radiogenic 40Ar in potassium bearing minerals. Moreover, measurements of argon
isotopes in fluid inclusions from a range of crustual environments ranging from hydrothermal ore deposits to eclogites (e.g. [7, 8]) also provide clear indications of the fluid
compositions at depth.
Rather than a simple picture of variations in radiogenic contents with crustal age, or gradual depletion of atmospheric
argon in deeper fluids, what emerges is a dynamic and heterogeneous system, dependent on variations in solubility and
kinetics of diffusion as well as fluid transport and availability. The controls exerted by fluids are illustrated by deep
crustal rocks such as eclogites which has been the subject of
study for both noble gas geochemistry and geochronology,
and also high level systems such as authigenic K-feldspar
growth in sandstone reservoirs.
The comparison of natural fluid and mineral measurements
with laboratory solubility measurements allows us to predict
which minerals and environments are likely to have high
radiogenic contents and whether the mineral 40Ar/39Ar ages
will be significantly affected. This allows us to test the geochronology system works and improve the interpretation of
40
Ar/39Ar ages in metamorphic rocks.
Comparison of natural measurements also helps to assess the
noble gas contents of the crust and in particular its fluids
which are clearly dependent upon time and the potassium
content of both solid and fluild reservoirs. The assessment
does not reveal a simple picture of loss to the atmosphere,
but suggests a strong role for crustal processes in the distribution of argon and the 40Ar/36Ar ratio of the crust.
References:
[1] Kendrick, M.A. and Burnard, P. (2013) In: Burnard, P.
(ed.), The Noble Gases as Geochemical Tracers, Advances
in Isotope Geochemistry.
[2] Jackson C. R. M., Parman S. W., Kelley S. P. and
Cooper R. F. (2013) Earth Planet. Sci. Lett. 384, 178–187.
[3] Jackson C. R. M., Parman S. W., Kelley S. P. and
Cooper R. F. (2015). Geochim. et Cosmochim. Acta 159
1–15.
[4] Kelley, S. P. (2002) Chemical Geology 188:1-22.
[5] Warren, CJ; Hanke, F; Kelley, SP (2012) Chemical
Geology, 291, 79-86.
[6] Smye, A., Warren, C.J., Bickle, M.J., Holland, T.H.,
(2013). Geochimica et Cosmochimica Acta, 113, 94-112.
[7] Qiu, H.-N. & Wijbrans, J.R. (2006). Geochem. Cos.
Act. 70, 2354–2370.
[8] Hu, R., Wijbrans, J., Brouwer, F., Zhao, L., Wang, M.,
Qiu, H. (2015) Geoscience Frontiers, 6, 759-770.