Abstract

Carbonate minerals filling fractures in the Monterey Formation at four locations along the Santa Barbara Channel coast display a delta 18 O(SMOW) range from 20.7 to 35.7 per thousand and delta 13 C range from - 18.6 to + 16.6 per thousand . Many fracture fills display a covariation of delta 18 O and delta 13 C, and successive generations of carbonate have lower delta 18 O values. These isotope systematics can be interpreted in terms of delta 18 O values that were controlled by crystallization temperatures and the delta 18 O of connate pore water, and delta 13 C values that were controlled by depth-sequential organic matter degradation reactions. Each successive generation of carbonate precipitated at progressively higher burial temperatures from isotopically evolving connate pore water. These carbonate-filled fractures developed during burial and not during uplift. Using crystallization temperature estimates from fluid inclusion data, the change in delta 18 O of conate pore water with increasing temperature in the burial environment is modeled. From this model, the fracture fill delta 18 O values indicate precipitation over the following temperature intervals: 25-70 degrees C (Carpinteria Beach), 7-15 degrees C (Goleta Slough Beach), 45-100 degrees C (Refugio Beach), and 53-103 degrees C (Jalama Beach).

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