Isotopic effects in fracture-dominated reactive fluid-rocksystems
A mathematical model is presented that describes the effectsof pore fluid aqueous diffusion and reaction rate on the isotopicexchange between fluids and rocks in reactive geo-hydrological systemswhere flow is primarily through fractures. The model describes a simplesystem with parallel equidistant fractures, and chemical transport in thematrix slabs between fractures by aqueous diffusion through a stagnantpore fluid. The solid matrix exchanges isotopes with pore fluid bysolution-precipitation at a rate characterized by a time constant, R(yr-1), which is an adjustable parameter. The effects of reaction on theisotopes of a particular element in the fracture fluid are shown todepend on the ratio of the diffusive reaction length for that element (L)to the fracture spacing (b). The reaction length depends on thesolid-fluid exchange rate within the matrix, the partitioning of theelement between the matrix pore fluid and the matrix solid phase, theporosity and density of the matrix, and the aqueous diffusivity. For L/b<0.3, fluid-rock isotopic exchange is effectively reduced by a factorof 2L/b relative to a standard porous flow (single porosity) model. ForL/b>1, the parallel fracture model is no different from a porousflow model. If isotopic data are available for two or more elements withdifferent L values, it may be possible to use the model with appropriateisotopic measurements to estimate the spacing of the primaryfluid-carrying fractures in natural fluid-rock systems. Examples aregiven using Sr and O isotopic data from mid-ocean ridge (MOR)hydrothermal vent fluids and Sr isotopes in groundwater aquifers hostedby fractured basalt. The available data for MOR systems are consistentwith average fracture spacing of 1-4 m. The groundwater data suggestlarger effective fracture spacing, in the range 50-500 m. In general, forfractured rock systems, the effects of fracture-matrix diffusive exchangemust be considered when comparing isotopic exchange effects for differentelements, as well as for estimating water age using radioactive andcosmogenic isotopes.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director. Office of Science. Basic EnergySciences
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 920248
- Report Number(s):
- LBNL-60809; GCACAK; R&D Project: 468202; BnR: KC0303020; TRN: US200818%%1107
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 70, Issue 5; Related Information: Journal Publication Date: 2006; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
Similar Records
Sorption and porosity heterogeneity: Effects on radionuclide transport
TOUGHREACT User's Guide: A Simulation Program for Non-isothermal Multiphase Reactive Geochemical Transport in Variably Saturated Geologic Media, V1.2.1