Fault gouge diagenesis at shallow burial depth: Solution–precipitation reactions in well-sorted and poorly sorted powders of crushed sandstone

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Using a new oedometric-type uniaxial cell, we determine rate constants for closed-system diagenesis of crushed quartz-rich aeolian sandstone powders in distilled water, heated for weeks at T ∼120°, at pore and confining pressures of ∼7 and 21 MPa. Results are presented for well-sorted distributions of fine and coarse size fractions, and for a poorly sorted mixture more representative of a fault gouge. The two well-sorted samples compact at nearly the same rate, but the poorly sorted gouge compacts much more rapidly at slow loading. The fine-size fraction reacts more quickly and fluids have a higher steady-state solute concentration than the coarse one. These results, combined with simple first order reaction rate theory, predict net dissolution followed by net precipitation for a simple two-phase mixture of particle sizes that is confirmed independently by the poorly sorted synthetic gouge. Observed super-saturation at relatively low effective pressures is consistent with Ostwald ripening. After the test the poorly sorted material is cemented sufficiently to be brought out as an intact, if friable, ‘rock’, indicating that fault sealing may be relatively rapid even under relatively closed conditions at depths of a few km.