DOLOMITIZATION OF A TRIASSIC ATOLL IN A NOVEL SEAFLOOR FLUID FLOW SYSTEM BY REACTION OF LIMESTONE WITH DIFFUSE EFFLUENT, LATEMAR CARBONATE BUILDUP, DOLOMITES, NORTHERN ITALY

Massive dolomite (Dol) typically forms at depth and elevated T through replacement of limestone (Ls) by its reaction with flowing dolomitizing fluid.  Our study addresses questions about the flow geometry, dolomitizing fluid, and flow mechanism involved in one instance of dolomitization.  Dolomitization in the study area was arrested with both Dol and Ls well-exposed in 3D. A sharp interface between unreacted Ls and completely reacted Dol indicates that Dol formed by the disequilibrium flow mechanism.  The distribution of Dol was mapped on km- to m-scales and directly images an orthogonal lattice of interconnected vertical and bedding-parallel fluid flow channels.

The ⁸⁷Sr/⁸⁶Sr of Latemar Dol is 0.7076-0.7079, and fluid inclusions have salinities of 3.6-5.1 wt % NaCl equivalent [1], implying that seawater (SW) or SW-derived fluid was the dolomitizing fluid. Dol has δ¹⁸O = 21.5-27.4‰ (VSMOW), corresponding to T = 52-88°C (assuming equilibration with fluid of δ¹⁸O = 0).  Electron microprobe and LA-ICPMS data for the Dol show enrichment in Fe (1,626-19,000 ppm), Mn (66-431 ppm), and Zn (1.7-16 ppm) relative to unreacted Ls. The concentration of Fe in Dol displays a positive linear correlation with that of Mn and Zn but not Cu. On the basis of these data, we suggest a dolomitizing fluid, analogous to diffuse effluent (DE) at mid-ocean ridges, that was a mixture of SW and hydrothermal fluid produced by reaction between SW and the adjacent Predazzo igneous complex that was active during dolomitization. Up to 15 concentric oscillatory zones of trace element concentration are visible in single Dol grains with backscatter electron and CL imaging. The zoning, the distribution of Dol in the field, and δ¹⁸O_Dol variability at the outcrop scale (23-27.4‰) indicate that fluid flow occurred in multiple, spatially restricted pulses.

The amount of Dol is a function of the time-integrated fluid flux, q, which can be estimated using the spatial extent of dolomitization in the field and the Ca/Mg of the dolomitizing fluid.  Assuming DE as the dolomitizing fluid, a geothermal gradient of 100°C/km, an input fluid at T=150°C, and 1 km of Dol along the flow path, q = (3.8-5.4)·10⁷ cm³ fluid/cm² rock. Results justify the interpretation of δ¹⁸O_Dol in terms of T of formation.

[1] Wilson, E. N. et al., (1990). Am. J. Sci., 290, 741-796.