Determining the history of fossil hydrologic systems that is recorded in vein minerals can be complicated by heterogeneity in the oxygen and carbon isotope compositions at millimeter and smaller scales. Several samples of diagenetic dolomite from two location in the Miocene Monterey Formation, California, have δ18O and δ13C values that vary over a few millimeters by ∼28‰ and 6.5‰, respectively (relative to the Pee Dee belemnite isotope standard). These variations reflect episodic flow and mixing of waters from different sources with distinct diagenetic histories. On the other hand, isotopic conpositions of dolomite in samples with gradual shifts in isotopic compositions and correlated δ18O and δ13C values may have been controlled by mass diffusion and thermal conduction between episodic flow events.

Primary fluid-inclusion assemblages from samples collected at one location yielded homogenization temperatures of 87.8 ± 3.6 °C (2σ). These temperatures were combined with measured δ18O values very nearly collocated in each dolomite sample to show that δ18Owater values ranged between −0.8‰ and +2.5‰ (relative to standard mean ocean water). The samples from this Monterey Formation dolomite thus indicate their precipitation from water more enriched in 18O than late Miocene and younger seawater and must reflect modification of the δ18O values of the involved waters by mineral-dehydration reactions prior to dolomite precipitation. Samples from the other location contain no primary fluid- inclusion assemblages and few abrupt shifts in isotopic compositions, perhaps reflecting recrystallization.

Small-scale variations in isotopic compositions of vein minerals could be interpreted as the result of a changing regional hydrologic pattern, but we think such an interpretation for the Monterey Formation samples would be incorrect. Instead, we infer that these variations provide important information about localized effects on the ancient hydrologic system and that these effects bear on the origins of the paleofluids responsible for the mineral precipitation.

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