HR: 13:55h
AN: H13I-02    [Abstracts]
TI: Pore Size Controls on the Temporal Evolution of Porosity in Reacting Basalts
AU: * Neuhoff, P S
EM: neuhoff@ufl.edu
AF: University of Florida, 241 WIlliamson Hall, Gainesville, FL 32611-2120 United States
AU: Spear, J E
EM: geojane@ufl.edu
AF: University of Florida, 241 WIlliamson Hall, Gainesville, FL 32611-2120 United States
AB: Basaltic lavas exhibit considerable, yet heterogeneous, porosity (typically 10-25 % in vesiculated zones, but sometimes up to 85 % in scoraceous flowtops) and concomitantly large permeabilities (0.001 to 10 darcies). The heterogeneous distribution of porosity occurs not only on the scale of a lava flow (e.g., between vesiculated flowtops and massive centers of thick aa flows) but also at smaller scales in terms of the abundance and size distribution of pores. Low temperature chemical alteration of basaltic lavas (i.e., weathering and zeolite facies metamorphism) results in dissolution of primary basalt phases (glass, anorthite-rich plagioclase, olivine, pyroxenes) and precipitation of a repeatable sequence of mineral parageneses. Secondary minerals formed during this process, such as zeolites and mafic phyllosilicates, have open crystal structures (low densities) and thus large molar volumes. The net effect is an increase in mineral volume. Reaction path modeling of typical zeolite facies mineral parageneses in basaltic lavas indicates that the net increase in mineral volume (the volume of new minerals formed minus that of the minerals dissolved) is up to 27 %. The new volume of mineral is accommodated by replacement (often pseudomorphic) of the primary phases and growth of minerals within pore spaces. Superposition relations within vesicles indicate that mafic phyllosilicates form first, followed by one or more generations of zeolites and other silicates. Observations of the distribution of various stages in this paragenesis between individual pores (which range in size from sub-mm to over one cm) indicate that pore size and shape largely control the extent of reaction progress recorded at the scale of an individual pore. For instance, within a given sample, mafic phyllosilicate linings on pores are observed to be of uniform thickness. If the thickness of the phyllosilicate lining exceeds the radius of the pore (or one half the minimum thickness of an elongate pore), the pore becomes completely filled during this stage and is not part of the storage capacity of the lava. Similarly, late stage mineral generations and residual porosity, if present, are typically only developed in the largest pores. These relations, along with recently discovered evidence of restricted fluid flow networks around and between vesicles in zeolite facies lavas, suggest that porosity modification in these systems is largely controlled by local (mm-scale) transport of material from the lava matrix into the pores and can be used to generate geometric predictions of the fate of pore space during reactive fluid flow. The fact that pore size and shape influences the preservation of porosity (and thus to some extent permeability) during reactive fluid flow in basaltic lavas suggests that semi-empirical models of reaction-porosity-permeability couplings that are typically used in other media do not apply well to vesicular materials.
DE: 8429 Lava rheology and morphology
DE: 1829 Groundwater hydrology
DE: 1832 Groundwater transport
DE: 1045 Low-temperature geochemistry
DE: 1010 Chemical evolution
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting