2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 9
Presentation Time: 1:30 PM-5:30 PM


PRICE, Jonathan D., WARK, David A. and WATSON, E. Bruce, Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 110 8th St, JSC 1W19, Troy, NY 12180, pricej@rpi.edu

Fluid flow within the lower crust is likely controlled by grain-scale pores, and the equilibrium shapes of these pores is best defined by the intersection angle of pore walls at the boundary between two crystalline grains (the right section of an edge pore).   The minimum energy configuration of this angle (known as the dihedral angle) has been well explored for pores with curved walls.  New results from synthetic materials illuminate the nature of this intersection with flat walls (facets), features common to many fluid-bearing rocks.

Quartz and clinopyroxene powder and distilled water were subjected to 900 oC and 1.4 GPa for 5 days to produce microcrystalline quartzite with an equilibrium pore structure. Oxide and fluoride powders of fluorotremolite stoichiometery were combined with dilute HF and subjected to 950 oC and 1.4 GPa for 5 to 25 days to produce microcrystalline amphibolite with an equilibrium pore structure. Sections of these products revealed pore-wall intersections composed of two flat faces (FF), two curved faces (CC), or one of each (FC). Quartzite samples contained 30% CC and 30% FF over fluid volumes (f ) ranging from 1.5 to 3%.   The amphibolite contained 9% CC, 75% FF in samples with f  < 4% and 93% FF,1% CC angles with f  from 4 to 12%.

A population of apparent dihedral angles on randomly oriented sections should yield a median within 1o of the actual angle and a cumulative frequency narrowly distributed around the actual dihedral angle (30% should fall within 2o of an actual angle of 40o).  The CC angles in the quartzite sections yielded a median of 30o and exhibited this narrow distribution.  However, FC and FF apparent angles yielded higher medians and showed an increasingly wider distribution (6% of apparent FF are within 2o of their median).  The angles in low-f amphibolite did not show an increase in the median (39o), but did exhibit a widening distribution with faceting. The apparent FF were also widely distributed (8% are within 2o) at higher f, but the small fraction of CC angles prohibited evaluating changes in the median and distribution with faceting.

The data show that intersection angles of faceted pores probably fall within a limited range, but are much more variable than measured CC and predicted dihedral angles. This suggests that faceted pores have somewhat variable geometries.