FORMATION OF REPLACEMENT DOLOMITE BY INFILTRATION OF DIFFUSE EFFLUENT: LATEMAR CARBONATE BUILDUP, DOLOMITES, NORTHERN ITALY
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Massive dolomite typically forms at depth and elevated temperature through replacement of limestone by its reaction with flowing dolomitizing fluid. Analysis of the spatial distribution of elements, isotopes, and heat with transport theory leads to insights into the flow system that produced dolomite in the Latemar carbonate buildup. Dolomitization was arrested, and both dolomite and unreacted limestone well-exposed in three dimensions. Boundaries between the dolomitized and undolomitized regions were mapped on meter to kilometer-scales. The distribution of dolomite directly images an orthogonal lattice of interconnected vertical tube-like and bedding-parallel sheet-like fluid flow channels. The 87Sr/86Sr of Latemar dolomite and the salinity of fluid inclusions in dolomite, previously measured by others, imply that a seawater-derived fluid was the dolomitizing fluid. Dolomite has δ18O = 21.5-27.4‰ (VSMOW), corresponding to temperatures of 50-90°C (assuming equilibration with fluid of δ18O = 0). Electron microprobe and LA-ICPMS data for the dolomite show enrichment in Fe (1,600-19,000 ppm), Mn (66-430 ppm), and Zn (1.7-16 ppm) relative to unreacted limestone. The concentrations of Fe and Zn in dolomite display a positive linear correlation with that of Mn; concentrations of other transition metals show no correlation with Mn. These data suggest that the dolomitizing fluid is analogous to modern diffuse effluent at mid-ocean ridges, and was a mixture of seawater and hydrothermal fluid produced by reaction between seawater and rocks of the adjacent Predazzo igneous complex that was the driving mechanism for ii dolomitization. The distribution of dolomite and its variability in δ18O at the outcrop scale (23-28‰) indicate that fluid flow occurred in multiple, spatially restricted pulses. The time-integrated fluid flux, q, can be estimated from mass balance using the spatial extent of dolomitization in the field and the Ca/Mg of the dolomitizing fluid. Assuming diffuse effluent as the dolomitizing fluid, and temperatures of 50-90°C, and ≈ 1 km of dolomite along the flow path, q = 1.9·107 - 1.6·108 cm3 fluid/cm2 rock. Quantitative analysis of outcrop-scale temperature gradients (T ≈10-25°C/m) with heat transport theory suggests that dolomitization occurred in a minimum of ~400 fluid pulses over a total duration of flow and reaction possibly as short as ~30 years.