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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Carbon/Oxygen Isotopic Ratios as Evidence for Rapid CO2 Degassing and Carbonate Crystallization

James R. Boles and Thomas Carpenter
Department of Geological Sciences, Univ of California, Santa Barbara, CA 93106, [email protected]

Carbonate is the most common cement in fractures and fault zones and its presence is often attributed to rapid CO2 degassing as a result of a pressure drop within a propagating fracture or fault. An analogy to degassing during rock deformation is the rapid formation of carbonate speleothems in tunnels and carbonate scales in well bore tubing. In the man made settings, carbonate sprays can grow rapidly at a rate of millimeters per year. A positive correlation of carbon/oxygen isotopic ratios in carbonate has been cited as evidence of rapid crystallization, assuming the input fluid composition is relatively constant (Hendy, 1971). We have been investigating the stable isotopic geochemistry of carbonates from faults, fractures, speleothems in a tunnel, and scales from oil and geothermal well bore production tubing in order to determine if calcite crystallization is rapid enough to result in a co-varying carbon/oxygen (C/O) isotopic ratios. In the case of the tunnel speleothems, modern calcite crystallization is out of oxygen isotopic equilibrium with the incoming pore water, whereas in at least a few of the hydrocarbon wells where data is available, the carbonate is close to oxygen isotopic equilibrium with the pore water. The tunnel speleothems, as well as a number of well scales (but not all), show evidence of rapid carbonate crystallization based on the C/O isotopic criteria. The C/O isotopic ratios (PDB) have positive slopes and vary between 1.2 and 5.5 for calcite scales and speleothems but between .6 and 1.0 for aragonite scales. Carbon isotopic values (PDB) frequently are very positive (+16 to +30 per mil). In general, calcites and dolomites from fault/fracture zones do not show these variations, suggesting that these carbonates have formed during a period unrelated to rapid pressure drop induced by the rupturing event. We conclude that rapid degassing may not be responsible for carbonate precipitation in these faults and fractures and instead the carbonate grows relatively slowly during periods of high fluid flux.

Posted with permission of The Geological Society of America; abstract also online ( © Copyright 2005 The Geological Society of America (GSA).