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42246-AC8
Application of Transport Theory to Quantitative Evaluation of Competing Models for Formation of Replacement Dolomite in the Latemar Carbonate Buildup, Dolomites, Northern Italy
Replacement dolomite in the Middle Triassic Latemar carbonate buildup formed when limestone was infiltrated by and reacted with Mg-rich fluid. Dolomite occurs in discrete bodies in sharp contact with unreacted limestone. The dolomite developed in a nearly orthogonal, structurally controlled lattice of vertical columns (replacement of limestone breccia pipes), vertical sheets (replacement along fractures and limestone-dike contacts), and nearly horizontal bedding-parallel sheets and tubes. The lattice directly images portions of the plumbing system in which the amount of fluid flow was sufficient to form dolomite. Decreases in the proportion of dolomite relative to limestone and in the proportion of vertical relative to horizontal dolomite-limestone contacts with increasing elevation indicate that the overall direction of fluid flow was upward in breccia pipes and along fractures and then outward along more permeable bedding horizons.
Dolomite is significantly enriched in Fe, Mn, and Zn, as well as in Mg, relative to calcite in precursor limestone but not in Cu, Ni, Co, Cr, Ba, or Pb. The Fe, Mn, and Zn content of dolomite varies spatially at the outcrop and larger scales and in time at a given position, as recorded by oscillatory zoning in individual dolomite crystals defined by Fe-rich and Fe-poor growth zones. Dolomite composition is interpreted in terms of a dolomitizing fluid that itself contained significant amounts of Fe, Mn, and Zn, as well as of Mg, and whose composition varied in both space and time. There is nearly complete overlap in the δ13C of dolomite (2.0-4.6‰, VPDB) and calcite (1.1-4.0‰). Average measured δ18O of dolomite is ≈1‰ less than the average measured δ18O of calcite. When measured δ18O of calcite is corrected for dolomite-calcite oxygen isotope fractionation, measured values of δ18O of dolomite and corrected values of δ18O of calcite define two nearly perfectly separated groups, 21.8-27.7‰ and 27.2-33.0‰ (VSMOW), respectively. Values of δ18O indicate that the dolomitizing fluid was variable in δ18O or temperature and confirm that most limestone was either infiltrated by a relatively small amount of dolomitizing fluid or escaped infiltration entirely.
The dolomitizing fluid is identified as one similar to modern diffuse effluent on the basis of salinity (similar to seawater); temperature (50°-90°C); 87Sr/86Sr (0.7076-0.7079); Ca/Mg (<1.4); and Fe, Mn, and Zn content (sufficient to cause enrichment of these elements in dolomite compared to limestone). Spatial and temporal variations in temperature and in the Fe, Mn, and Zn contents measured in modern diffuse effluent can account for equivalent variations in the δ18O and in the Fe, Mn, and Zn contents of replacement dolomite from the Latemar buildup.
Volumetric time-integrated fluid flux was in the range (2-4)·106 mol fluid/cm2 rock = (4-7)·107 cm3 fluid/cm2 rock. Estimation of time-integrated flux leads to an internally consistent framework for the appropriate interpretation of the oxygen, strontium, and carbon isotope composition of replacement dolomite. The oxygen and strontium isotope compositions reflect equilibration with dolomitizing fluid and provide a chemical and thermal fingerprint of the fluid. In almost all cases, the carbon isotope composition of dolomite, however, was simply inherited directly from the precursor limestone. A quantitative evaluation of the minor- and trace-element budget of dolomitization verifies that a fluid like modern diffuse effluent, but not seawater, is capable of supplying sufficient Fe, Mn, and Zn to enrich dolomite in these elements.
Analysis of temperature profiles adjacent to vertical tabular bodies of replacement dolomite indicate (a) that fluid flow occurred in spatially restricted pulses of short duration (<1 y), (b) that fluid fluxes were ≈0.02-2 cm3/cm2·s, similar to the flux of modern diffuse effluent, and (c) that dolomitzation in the Latemar buildup was accomplished by 100s or more pulses over a surprisingly short total duration of mineral-fluid reaction (≈100 y or less). The total duration of dolomitization was likely much longer depending on the time elapsed between pulses of fluid flow.
Conversion of limestone to dolomite occurred by a mechanism intermediate between the end-member cases of replacement at constant oxygen and carbon and replacement at constant volume.
The research has led to new collaborative projects, including (a) development and application of the clumped isotope thermometer for dolomite with scientists at Caltech and ETH-Zurich, (b) investigation of possible involvement of microbes in formation of dolomite in the Latemar buildup with scientists at ETH-Zurich, and (c) development of reactive transport models for systems consisting of both fluid and solid solutions with graduate student Nathan Winslow. The project supported the graduate training at Johns Hopkins University of Sarah Carmichael, who has finished her Ph.D. degree and is now on the faculty of Appalachian State University, and of Nathan Winslow who will complete his Ph.D. degree within the next 9 months.
