EVIDENCE FOR NORTHWEST-DIRECTED H2O-RICH SYNMETAMORPHIC FLUID FLOW, NW CASCADE CORE, WA

Fluid flowing through a marble body will change composition to become more CO₂-rich as it reacts with the protolith minerals that constitute the marble body. These reactions produce minerals that occur in a well documented sequence. Identification of these minerals can yield an estimate of the degree of fluid interaction, and from this the direction of fluid flow can be inferred.

Twenty-five samples from the Soda Springs marble quarry near the west end of Lake Wenatchee, WA were analyzed petrographically (and in some cases using SEM/EDS) to determine the mineral assemblage in each sample. A phase diagram for the CaO-Al₂O₃-SiO₂-CO₂-H₂O system was created using Perplex; by reference to this diagram, together with published estimates of the pressure and temperature of metamorphism, the composition of the fluid in equilibrium with each solid mineral assemblage was determined.

There is a large spatial variance in the composition of the fluid: some rocks located near each other show distinctly different fluid compositions. Based on this analysis, the fluid was found to be less than 15% CO₂ when it entered the marble body and greater than 15% as the fluid exited the body. It is estimated that the fluid flowed from southeast to northwest along preexisting bedding planes. The stress fields that drove this fluid flow are not well constrained by the samples in this study; however fluid flowing from southeast to northwest is consistent with a pressure gradient parallel to fluid flow. This stress field is most consistent with published transpressive tectonic models.