Formation of replacement dolomite in the Latemar carbonate buildup, Dolomites, northern Italy: Part 2. Origin of the dolomitizing fluid and the amount and duration of fluid flow

Abstract

Replacement dolomite in the Latemar carbonate buildup developed when limestone was infiltrated by reactive fluid. Minor-element, trace-element, and oxygen and carbon isotope compositions of dolomite and precursor limestone constrain the origin of the fluid and fundamental aspects of the flow. Inferred salinity (similar to seawater); temperature (45°–85°C); ⁸⁷Sr/⁸⁶Sr (0.7076–0.7079); Ca/Mg (<1.4); and Fe, Mn, and Zn content of the dolomitizing fluid are consistent with a fluid similar to modern diffuse effluent. Modern diffuse effluent itself is approximately a mixture of seawater with up to ∼25 percent high-temperature mid-ocean ridge hydrothermal vent fluid. Time-integrated fluid flux was in the range (2–4) · 10⁶ mol fluid/cm² rock or (4–7) · 10⁷ cm³ fluid/cm²rock. Estimation of time-integrated flux leads to an internally consistent framework for the appropriate interpretation of the oxygen, strontium, and carbon isotope compositions of replacement dolomite. The oxygen and strontium isotope compositions reflect equilibration with dolomitizing fluid and provide a chemical fingerprint of the fluid. The carbon isotope composition of dolomite, however, was simply inherited directly from the precursor limestone in nearly all cases. A quantitative evaluation of the minor- and trace-element budget of dolomitization verifies that a fluid like modern diffuse effluent, but not unmodified seawater, could supply sufficient Fe, Mn, and Zn to enrich dolomite in these elements compared to limestone. If the flux of dolomitizing fluid was similar to that of modern diffuse effluent, ∼0.02 cm³/cm² · s, the duration of fluid flow and mineral-fluid reaction was short, ∼100 years. The total duration of dolomitization, however, could have been much longer if fluid flow was episodic, as in modern seafloor hydrothermal systems, depending on the time elapsed between episodes of flow. Conversion of limestone to dolomite likely occurred by a mechanism intermediate between the end-member cases of replacement at constant oxygen and carbon and replacement at constant volume.