Experimental investigation of the breakdown of dolomite in rock cores at 100 MPa, 650-750  C

Abstract

The kinetics of the breakdown reaction dolomite = periclase + calcite + CO 2 were investigated using cores of dolomitic marble. Two samples of Reed Dolomite from southwestern Nevada were cut into cylinders approximately 4 × 6 mm in size. The cores were sealed in gold capsules with isotopically enriched water (H 2 18 O or HD 18 O 0.5 16 O 0.5 ). The samples were heated in a cold-seal hydrothermal apparatus to 650-750 °C at 100 MPa for durations ranging from 2-59 days. The cores were then sectioned and examined by EPMA, XRD, and SIMS techniques. All experiments showed some amount of reaction regardless of duration or temperature. Reaction products occurred mainly along grain boundaries, fractures within grains, and along sample edges. Ion images and isotope-ratio analysis indicated that reaction products exchanged with infi ltrating fl uids. Reaction rates were calculated from measured extents of reaction, which were determined from automated EPMA modes. At 700 °C, we measured reaction rates ranging from 3.8 × 10 -14 to 2.3 × 10 -12 mol/mm 2 ·s. The extent of reaction is proportional to the square root of time, suggesting a diffusion-controlled process. A shrinking-core model for the dolomite breakdown reaction fi ts the grain-size data, suggesting that diffusion of H 2 O and CO 2 through the mantle of reaction products controlled the rate. Apparent activation energies for that diffusion are ~283 ± 32 kJ/mol for coarse-grained samples and ~333 ± 36 kJ/mol for fi ne-grained samples. Initial reaction occurred relatively fast near the surface of dolomite grains, but continued diffusion through the reaction products ultimately controlled the rate of dolomite breakdown.

Application of calcite, Mg‐calcite, and dolomite as Raman pressure sensors for high‐pressure, high‐temperature studies

“…Because of the calcite ↔ calcite II phase transition (Figure 6), the calcite Raman pressure sensor is applicable only under relatively low‐pressure conditions (e.g., below 1.4 GPa). The dolomite Raman pressure sensor can be applied over a wider pressure range, but is not suitable for high‐temperature experiments (e.g., >600°C) as it may break down into other phases 20–23 . Another limitation of the calcite and dolomite Raman pressure sensors is that under hydrous conditions, both minerals are reactive at temperatures greater than 400°C.…”

“…The dolomite Raman pressure sensor can be applied over a wider pressure range, but is not suitable for high-temperature experiments (e.g., >600 C) as it may break down into other phases. [20][21][22][23] Another limitation of the calcite and dolomite Raman pressure sensors is that under hydrous conditions, both minerals are reactive at temperatures greater than 400 C. Therefore, pressure determination results should be treated with caution and additional checks should be made. For instance, comparison measurements made under ambient conditions both before and after high P-T experiments are recommended to check for potential shifts in the calcite or dolomite Raman peak positions as caused by changes in their chemical compositions under high P-T conditions.…”

“…In contrast, dolomite is stable up to 17 GPa before it transforms to a high‐pressure dolomite II structural phase under ambient temperature conditions 18,19 . At temperature beyond 600°C, dolomite breaks down into periclase, calcite, and CO 2 at low‐pressure conditions (e.g., 100 MPa) or into magnesite and aragonite at high‐pressure (e.g., >5 GPa) conditions 20–24 . As for the role of Mg in carbonates (e.g., Mg‐calcite and dolomite), results from recent studies show that the phase transition pressures among CaCO 3 polymorphs and the P‐T condition for the decarbonation reaction with silicate magmas are highly dependent on the MgCO 3 mole fraction in the samples 24–26 .…”

“…18,19 At temperature beyond 600 C, dolomite breaks down into periclase, calcite, and CO 2 at low-pressure conditions (e.g., 100 MPa) or into magnesite and aragonite at highpressure (e.g., >5 GPa) conditions. [20][21][22][23][24] As for the role of Mg in carbonates (e.g., Mg-calcite and dolomite), results from recent studies show that the phase transition pressures among CaCO 3 polymorphs and the P-T condition for the decarbonation reaction with silicate magmas are highly dependent on the MgCO 3 mole fraction in the samples. [24][25][26] Nevertheless, our knowledge about the geochemical behavior of carbonates in the deep earth, especially for the role of MgCO 3 mole fraction on the phase transitions among CaCO 3 polymorphs and the chemical reactions with silicate minerals, needs to be expanded, as few experiments have been carried out on the reactions of calcite, Mg-calcite, and dolomite with silicate minerals (e.g., feldspar and pyroxene) under hydrous or anhydrous conditions.…”

Supplemental Material: Seismic fault weakening via CO₂ pressurization enhanced by mechanical deformation of dolomite fault gouges

“…Previous laboratory experiments studies have suggested a greatly varied activation energy for dolomite decomposition from 97 to 333 kJ/mol (Olszak-Humienik and Jablonski, 2015, Gunasekaran and Anbalagan 2007, DeAngelis et al, 2007. To obtain the suitable activation energies for numerical modeling, we performed a parametric simulation with varying activation energy to find the best-fit activation energies for explaining thermal behavior of dolomite up on grinding in our laboratory experiments.…”