1) Clumped isotope thermometry was used to determine the growth temperatures of calcite crystals from a fault zone in the Peak District, UK, which ranged from 45°C to 92°C. The technique also allowed determination of the oxygen isotope composition of the mineralizing fluids. 2) The calcite precipitated from fluids that plotted on a mixing line between two end-members: a warm, evolved end-member with temperatures over 80°C and oxygen isotope values over 3.5‰, and a cooler, depleted end-member with temperatures under 45°C and oxygen isotope values under -2‰. 3) Sub-sampling of the calcite crystals revealed a complex, episodic pattern

1. The Geology of Geomechanics Conference, London, October 2015
Clumped isotope thermometry – a tool to further detail fluid processes in
fault zones, a view from the South Pennine Orefield, Peak District, UK
Daniel Myhill1
, Paul Dennis1
, and Alina Marca1
1
Stable Isotope Laboratory, School of Environmental Sciences, University of East Anglia,
Daniel.myhill@uea.ac.uk and P.dennis@uea.ac.uk
Clumped isotope thermometry of vein calcite from Lower Carboniferous limestone (Peak
District, UK) allows for mineralising fluid composition (δ18
OVSMOW) and temperature (T(Δ47)
[°C]) to be independently determined. The degree of ordering of heavy isotopes 13
C and 18
O
in the carbonate lattice is inversely proportional to temperature. Measuring the deviation of
ordering from a stochastic distribution allows us to estimate the mineral growth temperature.
With the bulk oxygen isotope composition of the carbonate, δ18
Ocarb, we can deduce the
δ18
Ofluid value. In this study we used clumped isotopes to determine the growth temperature
of single calcite crystals of > 80 mm length from Dirtlow Rake (rake: local term for a wrench
fault), part of the Castleton fault system, Peak District, UK. Sub-sampling of the crystals
along their growth axis shows complex mineralising fluids with an episodic saw-tooth pattern
to both temperature and isotopic composition.
Vein calcite precipitated between 45°C and 92°C, with parent fluids plotting on a well-
defined two end-member mixing line in T-δ18
Ofluid space. A warm, isotopically evolved end-
member (T > 80°C, δ18
Ofluid > 3.5 ‰, VSMOW) mixes with a cooler, isotopically depleted
end-member (T < 45°C, δ18
Ofluid < -2 ‰, VSMOW) characteristic of meteoric groundwaters.
This episodic saw-tooth signal of both the temperature and isotope distributions demonstrates
fluid evolution in the fault zone is complex, involving mixing of two or more end-members.
Sharp temperature transitions imply rapid advection of heat due to fast fluid transport rates
and rapid changes in mixing ratios. The time dependent rates of fluid flow within the fault
zone depend on the development of pore fluid pressure, the external stress field and the
overall effective stress. Contributing processes may include seismic pumping, seismic valve
and the generation of gas pulsars. Using simple numerical models we explored the effects of
both development of overpressure in adjacent basinal shales as a source of deep, hot water
and the possible impacts of rapid influx of near-surface waters as a result of brittle failure
into the fault zone. We find the thermal and isotopic evolution of fluids is consistent with
typical values for fluid pressures and fault zone permeabilities.
Clumped isotope thermometry is therefore a useful tool in further aiding our understanding of
fluid processes in fault zones by independently constraining paleo-fluid compositions and
temperature. This, coupled with other techniques, such as petrography, trace element and
noble gas analysis, can build a better understanding of fluid processes in fault zones.