Details

Autor(en) / Beteiligte

• Penniston-Dorland, Sarah C

Titel

Sources, mechanisms, and pathways of chemically reactive fluid transport during metamorphism from analysis of geochemical tracers

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2005

Link zum Volltext

• ProQuest

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Geochemical fluid tracers ([special characters omitted] δ18O) and reaction progress constrain sources, mechanisms, and pathways of chemically reactive fluid flow during metamorphism. In the first of two parts, a dichotomy in apatite compositions in metacarbonate rocks is documented in five contact aureoles in Italy, Scotland, and U.S.A. Rocks that record low [special characters omitted] contain apatite that is an F-OH solid solution; rocks that record high [special characters omitted] contain apatite that is relatively Cl-rich. The halogen content of fluid coexisting with analyzed apatites was characterized to determine the origin and significance of the dichotomy in apatite composition. The Cl-rich apatites are best explained by crystallization at relatively low [special characters omitted] and aHF rather than at high a HCl or [special characters omitted]. In contrast, the F-OH apatite formed by infiltration of rock by and equilibration with relatively H2O-rich, high F/Cl fluid. Calculated halogen contents indicate that the non-CO2 fraction of fluid in equilibrium with both rock groups had seawater-like salinity and that the reactive H2O- and F-rich fluid that infiltrated the low-[special characters omitted] group had a plutonic source. The second project investigated the significance of small-scale variations in the progress (ξ) of infiltration-driven reaction in marls during Barrovian regional metamorphism in east-central Vermont. Reaction progress, [special characters omitted], and δ18OCal (proxy for δ 18OFluid) were measured along traverses perpendicular to layering at a variety of scales. Along millimeter- and centimeter-scale traverses, there are significant differences in ξ but statistically uniform [special characters omitted] and δ18OCal within error of measurement. Along meter- to decameter-scale traverses some samples record statistically significant differences in [special characters omitted] and δ18OCal over distances as small as 0.4 m, while other samples record uniform fluid composition over distances up to 4.2 m. Simple models demonstrate that cm-scale variations in ξ result not from channelized fluid flow but from layer-by-layer variations in the initial composition and amount of mineral reactants and cross-layer homogenization of fluid composition by diffusion/dispersion during reaction. Significant m-scale cross-layer variations in [special characters omitted] and δ18OCal rule out fluid flow across layers and point to layer-parallel flow in channels on the order of 10 m wide. Because there are no local sources of H2O, reactive fluid must have been external.