Authorea

Paul Dennis edited untitled.tex over 8 years ago

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\textit{Oh, 1. Introduction There is abundant evidence for a close physical and chemical coupling between pore fluids and fracture in the upper crust ({Frank:1965wi, Hubbert:1959he, Nur:1973ju, Sibson:1981kt}). Increases in pore fluid pressure, for example due to fluid injection, have been observed to lead to rupture and an empty article!} increase in seismic activity. This is readily explained by the Navier-Coulomb criteria for brittle failure and the decrease in effective stress as a result of the elevated pore fluid pressure ({Price:1966uh, Sibson:1981kt}). Conversely, the changes in groundwater levels and the surface effusions of warm water that sometimes occur along fault traces following earthquakes show that failure can also have a profound effect on fluid flow, heat and mass transport ({Nur:1974ht} Sibson et al., 1975, Sibson, 1981,). The flow is interpreted to result from either seismic pumping as a result of dilatancy diffusion type processes or a seismic valve mechanism in which fault rupture leads to leakage of an overpressured aquifer or reservoir of fluid (Nur, 1972, Nur et al., 1973, Sibson, 1981). Evidence for episodic and rapid flow of fluid associated with fracture and faulting in the geological record is more equivocal. The presence in exhumed fault systems of banded hydrothermal mineralisation of varying degrees of complexity is often taken as evidence of pulsed fluid flow. Rock failure may result in brecciation of earlier generations of veins and the opening of large dilation voids that are then cemented by mineral precipitation from upwelling hydrothermal fluids ({Wright:2009ej}). The extent, however, to which mineral precipitation in the veins is contemporaneous with, and directly coupled to failure is open to question. Observations of epithermal mineralisation associated with, for example dilation jogs between en-echelon fault segments suggests that fluid flow accompanied by rapid pressure fluctuations may trigger high level boiling, or effervescence and rapid degassing of hydrothermal fluids. This results in precipitation of common gangue quartz and calcite as well as economic metalliferous deposits ({Sibson:1975cn, Sibson:1981kt, Sibson:1987dq, Henley:2000wk}). A defining feature of systems associated with episodic pulses of warm to hot fluid is the development of a thermal anomaly along the high permeability paths in which flow is focused. A perturbation of the temperature field has been observed in Mississippi Valley Type (MVT) mineralisation districts that lie around the upper Mississippi Valley and major Palaeozoic sediment basins in the continental USA (basal aquifer of the Arkoma basin in Cathles and co-workers show that in the Upper Mississippi Valley district there is a difference in mineral precipitation temperatures reported for Mississippi Valley Type (MVT) deposits and that typical of the host rock for the depth of burial at the time of mineralisation. Similar temperature differences have been reported between lead-zinc mineralisation temperatures and the host rock for deposits located in sedimentary basins to the south east of the Massif Central in France (Charef and Sheppard, 1986). In both cases a cogent argument can be made that the difference is due to heat advection associated with episodic pulsing of hot fluids from deeper regions of the sedimentary basins. In these example the fluids are thought to originate from overpressured brines in compacting sedimentary basins. Importantly, the fluid velocities required to produce the thermal anomaly are more than 1000 times greater than could be produced by the steady subsidence, compaction, and dewatering of the basins (Cathles and Smith, 1983). This suggests to us that the pulses result from a coupling between the pore fluid pressure and rock failure. We envisage that when pore fluid pressures approach lithostatic pressure either hydraulic fracturing or shear failure and development of dilation jogs along fault surfaces allows rapid dewatering of the sediment pile with channel flow of fluids into the basal aquifers. A corollary of the model proposed by Cathles and others is that the fluids involved in the mineralisation are trapped formation and not meteoric waters. Additionally the mineralisation involves restricted water volumes at consequent low water:rock ratios. Supporting evidence for formation waters is found in the fact that in the modern upper Mississippi Valley basins the deep waters are saline (Cathles, 1993). Were the waters to have been meteoric one expects the salinity to have been flushed out as recharge in areas of elevated terrain drives cross-basin flow along path lengths of several hundred kilometres. Thus the evidence of a thermal anomaly, coupled with remnant salinity in the sedimentary basin runs counter to the current paradigm for MVT formation which involves cross-basin flow of large volumes of gravity driven meteoric water (Garven plus other references). Rather a more dynamic regime involving fluid overpressure, fracture and fluid flow is implicated. To help us understand these processes we have used clumped isotope thermometry to determine the temperature at which hydrothermal vein calcite precipitated in dilation voids of a Variscan strike-slip fault in the southern Pennines of the United Kingdom. The veins are associated with Pb-Zn and fluorite mineralisation and the fault was extensively worked as an economic deposit. Located at the margin of a lower Carboniferous platform on which shelf carbonates were deposited, the fault separates the platform from a deep water basin infilled with deep water facies limestones and shales of lower to upper Carboniferous age. Clumped isotope geothermometry relies on the fact that the heavy isotopes 13C and 18O are ordered in the carbonate lattice with the degree of ordering being an inverse function of temperature. This is a result of the greater stability of the 13C-18O bond compared to bonds involving either no, or a single isotopic substitution. As temperature increases the isotopes tend towards a more random or stochastic distribution. Measurement of the degree of ordering allows us to estimate the temperature at which the distribution of isotopes in the calcite structure are locked in. This is analogous to the concept of a closure temperature for radiogenic isotopes or cation ordering in minerals. In this study this temperature is taken as the precipitation temperature of the calcite. A key advantage of the method is that the temperature estimate is based on the distribution of carbon and oxygen isotopes within a single phase and not on the partitioning of oxygen isotopes between calcite and it’s parent fluid as in the conventional oxygen isotope geothermometer. Thus combining the clumped isotope temperature (T(Δ47)) with the bulk oxygen isotope composition of the carbonate we can use published fractionation factor calibrations to calculate the isotopic composition of the parent fluid. Using these techniques we find: (i) the calcite precipitated at temperatures between 40° and 100°C. (ii) the parent fluids range in isotopic composition from -4‰VSMOW to +5‰VSMOW and represent mixtures of a cool, meteoric water and a more evolved formation water. (ii) the temperature at which the calcite precipitated is a conservative tracer for the fluid mixing. This implies that heat is rapidly advected as the hydrothermal fluid flow is focussed along the fault plane. (iii) The calcites exhibit zoned development characterised by cyclic and pulsed evolution of precipitation temperatures and fluid compositions as a result of varying mixing ratios of the two fluid end-members. (iv) The fluids have evolved under low fluid:rock ratios. You can get started by \textbf{double clicking} this text block and begin editing. You can also click the \textbf{Text} button below to add new block elements. Or you can \textbf{drag and drop an image} right onto this text. Happy writing!