Abstract
We have used clumped isotope thermometry to study a fault-hosted hydrothermal calcite vein associated with the Mississippi Valley Type (MVT) mineralization on the Derbyshire Platform in the southern Pennines, UK. This is the first published dataset obtained using a new mass spectrometer, MIRA, optimized for clumped isotope analysis and an associated clumped isotope–temperature calibration. We analysed multiple generations of vein growth at high spatial resolution in two transects across the vein. The vein grew episodically at temperatures between 40 and 100°C. We interpret each episode of growth as being associated with an increasing flux of formation waters from deep sedimentary basins next to the mineralized platform and an accompanying increase in the precipitation temperatures. Heat is conserved in the fluid as it ascends along the fault surface and, thus, flow must have been fast and restricted to short-lived pulses. The flux could have been driven by high pore pressures associated with rapid sedimentation, hydrocarbon generation and diagenesis in the basinal facies of the Visean Bowland–Hodder group. Natural hydraulic fracturing of shale units and failure of capillary seals, possibly triggered by uplift, allowed the release of fluids into aquifers within the sediment pile. The transmission of high pore fluid pressures from the shales to the fault zone, aided by the compressibility of the gas phase in two-phase pore fluids, may have resulted in fault rupture, accompanied by enhanced fracture permeability and rapid fluid flow. Vein growth ceased as pore pressures dissipated. Such behaviour is closely related to a seismic valve type model for mineralization. Supplementary material: Details of the methods and equipment are available at https://dx.doi.org/10.6084/m9.figshare.3808329.v9
Highlights
D flow must have been fast and restricted to short-lived pulses
The data for bulk and clumped isotope analysis of all samples and standards are reported in Table 1 and plotted in Figures 4 and 5
Each area is characterized by a rising temperature from a minimum of 40∘ to a maximum of 90°C. This pattern is mirrored by antithetic changes in δ18O, falling from a maximum value of -8‰ to a minimum of -10‰VPDB
Summary
D flow must have been fast and restricted to short-lived pulses. The flux could have been driven by E high pore pressures associated with rapid sedimentation, hydrocarbon generation and diagenesis in the basinal facies of the Visean Bowland-Hodder group. Increases in pore fluid pressure, for example due to fluid injection, can lead to rupture and an increase in seismic activity This is readily explained by the Navier-Coulomb criteria for brittle failure and the decrease in effective stress that results from the elevated pore fluid pressure (Price 1966; Sibson 1981). The flow is interpreted to result from either seismic pumping due to a dilatancy-diffusion type process or a seismic valve mechanism in which fault rupture leads to leakage of an over-pressured aquifer or reservoir of fluid (Nur 1972; Sibson 1981).
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