Abstract Rock fracture enhances permeability and provides pathways through which fluids migrate. During contact metamorphism, fluids contained in isolated pores and fractures expand in response to temperature increases caused by the dissipation of heat from magmas. Heat transport calculations and thermomechanical properties of water‐rich fluids demonstrate (1) that thermal energy is a viable mechanism to produce and maintain pore fluid pressure ( P f ) in a contact metamorphic aureole; (2) that the magnitude of P f generated is sufficient to propagate fractures during the prograde thermal history (cause hydrofracture) and enhance permeability; and (3) that P f ‐driven fracture propagation is episodic with time‐scales ranging from years to thousands of years. Because P f dissipation is orders of magnitude faster than P , f buildup, P f oscillations and cyclical behaviour are generated as thermal heating continues. The P f cycle amplitude depends on the initial fracture length, geometry and the rock's resistance to failure whereas the frequency of fracture depends on the rate of heating. Consequently, oscillation frequency also varies spatially with distance from the heat source. Time series of fluid pressures caused by this process suggest that cyclical fracture events are restricted to an early time period of the prograde thermal event near the intrusive contact. In the far field, however, individual fracture events have a lower frequency but continue to occur over a longer time interval. Numerous fracture cycles are possible within a single thermal event. This provides a provisional explanation for multiple generations of veins observed in outcrop. P f cycling and oscillations may explain several petrological features. If pore fluids are trapped at various positions along a pressure cycle, the large amplitude of P f variations for small fractures may account for different pressures recorded by fluid inclusions analysed from a single sample. P f oscillations, during a single thermal episode, also drive chemical reactions which can produce complex mineral textures and assemblages for discontinuous reactions and/or zoning patterns for continuous reactions. These can mimic polymetamorphic or disequilibrium features. Temporal aspects of fracture propagation and permeability enhancement also constrain the likely timing of fluid flow and fluid‐mineral interactions. These data suggest that fluid flow and fluid‐mineral reactions are likely to be restricted to an early period in the prograde thermal history, characterized by high P f coincident with relatively high temperatures, fracture propagation and consequent increases in permeability. This early prograde hydration event is followed by diffusional peak metamorphic reactions. This relationship is evident in the complex mineralogical textures common in some metamorphosed rocks.
Journal Article Quantitative Simulation of the Hydrothermal Systems of Crystallizing Magmas on the Basis of Transport Theory and Oxygen Isotope Data: An analysis of the Skaergaard Intrusion Get access D. NORTON, D. NORTON Department of Geosciences, University of ArizonaTucson, Arizona 85721, U.S.A. Search for other works by this author on: Oxford Academic Google Scholar H. P. TAYLOR, JR. H. P. TAYLOR, JR. Division of Geological and Planetary Sciences, California Institute of TechnologyPasadena, California 91125, U.S.A. Search for other works by this author on: Oxford Academic Google Scholar Journal of Petrology, Volume 20, Issue 3, August 1979, Pages 421–486, https://doi.org/10.1093/petrology/20.3.421 Published: 01 August 1979 Article history Received: 01 November 1978 Revision received: 17 January 1979 Published: 01 August 1979
A numerical model of heat and mass transfer within porphyry copper environments and equilibrium phase relations in the system CaO-FeO-MgO-Al 2 O 3 -SiO 2 -Cu 2 O-H 2 S-H 2 SO 4 -H 2 O-CO 2 are combined into a theoretical analysis of hydrothermal and chemical conditions during skarn formation in siliceous limestone.Heat and mass transfer calculations indicate temperature-pressure-fluid flux evolution within host-rock contact zones can be subdivided into three events: (1) early conductive heating (0 to [asymp]5,000 yr) when fluid fluxes remain 5 x 10 (super -7) g cm (super -2) s (super -2) are realized as temperatures decline through the H 2 O critical region to [asymp]300 degrees C, and (3) late convective cooling ([asymp]30,000 to [asymp]400,000 yr) when fluid fluxes and temperatures gradually return to ambient values. Pressure changes during this history are several tens of bars or less.Space-time variations in solution-mineral equilibria commensurate with calculated temperature-pressure evolution are described from activity diagrams that combine silicate-fluid and sulfide-fluid topologies. The diagrams incorporate explicit provision for silicate-solution compositions reported and oxidation states inferred from natural systems. Equilibrium constraints are responsible for many ore-gangue associations (e.g., chalcopyrite-andraditic garnet and bornite-wollastonite) and paragenetic features (e.g., decomposition of (garnet, clinopy-roxene)-sulfide-oxide to calcite-amphibole-sulfide-oxide at temperatures 2 concentration (in nonideal H 2 O-CO 2 fluid mixtures) during sub-400 degrees C garnet precipitation is
The porosity (P) of rock is described by: P-total = P-flow + P-diffusion + P-residual. Experimental results show that P-total of fractured rock in hydrothermal systems ranges from 0.2 to 0.01, and P-diffusion ranges from 0.001 to 0.00001. Data from the literature and from field studies of fractures indicate that P-flow is 0.001-0.00001. Consequently, the greater part of P-total in plutonic environments results from residual pores which are not interconnected to P-flow or P-diffusion. Permeability is a function of the abundance and geometry of continuous flow paths. Observations suggest that a planar fracture is a good first order approximation for fractures in plutonic environments. Analysis of aperture, abundance, and continuity suggests that permeabilities on the order of 1.0 nm/sup 2/ may be characteristic of large portions of the crust. The model proposed permits definition of the interface between circulating hydrothermal fluids and reactant minerals in a manner consistent with the physical phenomena and partial differential equations which describe the processes of advection-diffusion-reaction.
The Skaergaard magma chamber formed approximately 55 m.y. ago along the embryonic rift between North America and Europe as tholeiitic basalt magma flowed upward along fractures in basement gneiss and then infiltrated the stratigraphic unconformity at the base of a 7‐ to 9‐km‐thick section of continental basalts. The magma deflected and faulted the overburden as it formed a 4.5‐km‐thick, 189 km 3 , laccolithlike chamber with elliptical form ( a = 6 km, b = 4 km) in map view. As the chamber grew, its feeder pipes were eroded into conical depressions; blocks of gneiss were rafted to the chamber top, and some blocks were fused and entrained in the main magma as “immiscible” fluids. Crystallization and cooling produced at least four distinct fracture events: (1) At 1050°–1000°C, residual magma accumulated in fractures in the Layered Series, forming gabbro pegmatites, (2) at 1050°–700°C, near‐vertical fractures were formed, providing channels for the main pulse of meteoric‐hydrothermal activity; these fractures developed near the margin of the magma chamber, then expanded outward into the permeable basalts and inward, following the gabbro‐magma interface. Ground waters derived from joints in the surrounding basalts flowed into the gabbro, were heated, lowered the 18 O/ 16 O ratio of the intrusion, and filled its fractures with hornblende, clinopyroxene, biotite, and magnetite‐ilmenite, (3) at 800°–750°C, volatile‐rich granophyric melts derived from sloped blocks of basement gneiss expanded and crystallized as both sill‐like and dike‐like bodies in the gabbro, and (4) below 700°C, fractures continued to form and hydrothermal activity continued to cool the intrusion. The relative age, abundance, continuity, and mineralogy of the veins are consistent with parameters used in previous studies of this intrusion that predict the occurrence of a high‐temperature hydrothermal system and a time‐ and volume‐averaged permeability of 10 −13 cm 2 . Our new data indicate that the permeability of the layered gabbro decreased with time because the flow channels were sealed by high‐temperature mineral deposition. We thus conclude the following: (1) layered gabbros fracture in response to local stress at conditions just below their solidus temperature if the confining pressures are typical of the upper crust. This observation contravenes the conceptual viewpoint that the style of deformation at such elevated temperatures is only by plastic flow, and (2) because an extensive fracture network develops at these near‐solidus temperatures in layered gabbros, the bulk of the hydrothermal alteration of such bodies takes place at extremely high temperatures. This helps clarify the apparent paradox that extreme 18 O depletions are found in “fresh” layered gabbros.
