Column transport experiments were conducted at 225 degrees C and 300 degrees C using a crushed amphibolite schist from Fenton Hill, NM to evaluate lithium ion as a cation-exchanging tracer to interrogate fracture surface area in enhanced geothermal systems. Lithium exchange proceeded to equilibrium rapidly, and Li+ selectivity doubled from 225 degrees C to 300 degrees C, with the selectivity at both temperatures being much greater than at room temperature. Also, cation exchange was deduced to be occurring primarily in a thin "rim zone" (< 0.05 mm) on the rock surfaces. These results are all encouraging for using lithium ion to interrogate fracture surface area in enhanced geothermal systems.