STABLE ISOTOPES OF STRAIN FRINGES IN APTIAN SLATES NEAR LOURDES, FRANCE

Metamorphic fluid flow and fluid temperatures are often poorly constrained in low-grade metamorphic systems. This study utilizes conventional microsampling techniques for stable isotope analysis of mm-scale, pyrite-centered, fibrous quartz and calcite strain fringes. These features document progressive growth of quartz and calcite during deformation. The samples used in this study are from a well-known strain fringe locality in the Pyrenees Mountains near Lourdes, France. Cretaceous (Aptian) limestones in the area are of marine origin with calcite oxygen isotope ratios (δ¹⁸O_cc) that range from 25.9 to 27.6‰ SMOW and calcite carbon isotope ratios ( δ¹³C_cc) that range from 1.7 to 5.4‰ PDB (n=16). Values of δ¹⁸O_cc throughout the slate unit from both strain fringes and matrix material are tightly constrained and fall between 21.5 and 22.5‰ (n=26). The lower δ¹⁸O_cc in the slate suggests the recrystallization of calcite at elevated temperatures. δ¹³C_cc values in the slate range from 1.6 to 4.1‰ and vary with stratigraphic height. δ¹⁸O_cc from four strain fringes are 22.8±0.5‰ (n=8) and D_qtz-cc values are 1.7±0.3‰. Calcite-quartz oxygen isotope thermometry yields apparent temperatures of 447±72°C. However, these calculations are not an accurate reflection of equilibrium temperatures based on metamorphic grade and calculated oxygen isotope ratios of water in equilibrium. These anomalously high apparent temperatures may reflect the scale at which these fringes were analyzed, and may be resolved through the use of ion microprobe analysis.