Kinematics of fibrous vein growth: insights from stable isotopes and trace element data
Abstract
Veins are important recorders of thermal, hydrological, structural, and geochemical conditions during deformation. Fibrous or bladed veins are particularly useful, because the mineral fibers are believed to grow continuously or episodically over what may be a significant geologic time period. Thus, individual mineral fibers document the complex, dynamic changes occurring coeval with vein growth and mineral precipitation. Geochemical and structural analyses have been used to constrain the kinematic history of a bed-parallel fibrous calcite vein from the Upper Jurassic La Casita Formation, Sierra Madre Oriental, Mexico. The La Casita Formation, correlative with the Smackover Formation of the northern US Gulf Coast, consists of shale and is part of a sedimentary succession that unconformably overlies evaporites of the Minas Viejas Formation. The fibrous vein examined in this study was taken from the backlimb of the frontal fold of the Monterrey salient near Saltillo. Optical petrographic observation reveals that the calcite fibers have blade or tapering lath shapes with widths from 0.1-1.0 mm. Other minerals present in the vein are pyrite, gypsum, bitumen, and iron oxides. High-resolution, closely spaced stable isotope and elemental analyses were carried out along five traverses across the vein width. The d18O values vary in a narrow range, with an average value of +20.8 permil (VSMOW). The d13C values increase systematically along the fibers, from the walls of the vein toward the suture plane. In all five traverses, the d13C increase is relatively constant, with lower values next to the vein wall (+1.2 to +1.6 permil PDB) and higher values along the suture line (+3.6 to +4.1 permil PDB). The vein minerals, fibrous calcite and accessory pyrite, are interpreted to be the products of high temperature reactions between light hydrocarbons and dissolved sulfate, known as thermochemical sulfate reduction (TSR). Reactants such as light hydrocarbons and products such as CO2, H2S, and H2O contributed to what are interpreted to have been high fluid pressures during vein growth. The origin of vein minerals is probably related to: (1) methanogenesis and decarbonation reactions within the shale, and (2) migration of high-temperature brine during late stages of folding.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2005
- Bibcode:
- 2005AGUFM.T53C1456F
- Keywords:
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- 0454 Isotopic composition and chemistry (1041;
- 4870);
- 8005 Folds and folding;
- 8010 Fractures and faults