Investigating the Fate of Mg-bearing Calcium Carbonates During Early Diagenesis

Abstract

The formation of dolomite at low temperature (<80ºC) is uncommon in modern environments, however dolomite is abundant in the geologic record. The paradox between the scarcity of modern dolomite and abundance in ancient dolomite has been of long-term interest to geologists. Further, because dolomite comprises many prolific carbonate reservoirs, there are practical needs for understanding dolomite formation as it benefits petroleum exploration and development. Dolomite abundance in geologic history also reflects the secular variation of seawater through time and is a key mineral in probing past change in climate.This study investigates whether dolomite can form from seawater in the presence of carboxylated organic matter (COM), and if the presence of COM further facilitates Mg incorporation into calcium carbonates under conditions that approximate early diagenesis and burial. The study compares Mg-incorporation in carbonate precipitated from a solution with the composition of an idealized Silurian seawater in the presence of functionalized (-COOH) organic matter followed by moderate increases in temperature (T) and pressure (P), typical of diagenesis. Three series of experiments were conducted to explore carbonate precipitation at surface conditions (T=40°C, and P=15 psi), followed by rapid sedimentation and burial during early diagenesis (T=40°C, and P=160 psi, 200 psi, 550 psi and 900 psi), and simulated diagenesis (calculated based on thermobaric gradient 1°C/122 psi; T=40°C, 40.5°C and 47°C; P=100 psi, 160 psi and 900 psi). High Mg-calcite (mol% MgCO3 ~ 11.5%) and aragonite were identified by XRD in all the vessels after incubation at surface conditions (T=40°C, and P=15 psi). As pressure increased apparent dissolution of calcite and spherulitic aragonite was observed based on analysis by SEM. During the simulation of diagenesis (additional increases in pressure and temperature), saturation indices for carbonate minerals increased, but mol%MgCO3 of calcite slightly decreased. Regrowth of calcite and new fabric of aragonite were observed under SEM, suggesting possible reprecipitation. Generally, there were few differences in bulk fluid geochemistry and mol%MgCO3 of calcite precipitates between experimental and control vessels, which is probably due to the slow incorporation rate and the short incubation time (4 months). However, amorphous calcium carbonates nucleated on the surface of COM exhibited solid Mg:Ca ratios slightly larger than calcite that precipitated homogeneously from solution. This suggests that COM may facilitate Mg incorporation into calcite but under experimental conditions in this study, does not facilitate detectable dolomite nucleation nor formation. Aside from COM, the effect of temperature and pressure on carbonate precipitation are also investigated here. Based on Phreeqc modeling of Silurian seawater composition, the saturation indices (SIs) of calcite, aragonite and dolomite increase with increasing temperature (< 65°C), and slightly decrease with increasing pressure. While detectable dolomite was not formed in experiments that simulated precipitation under sea surface conditions nor during subsequent simulated diagenesis, changes in mol%MgCO3 of calcite was observed as a function of pressure and COM. While pressure may play a role in the incorporation of Mg into the calcite structure, it is not clear from this study that its effect is large. Given adequate time for nucleation and precipitation, however, which may not have occurred due to the brevity of the experimental protocol, it is hypothesized that high Mg-calcite or dolomite is most likely formed through COM-mediated processes.