Cation diffusion in calcite: determining closure temperatures and the thermal history for the Allan Hills 84001 meteorite

The Calcite-Dolomite Solvus Temperature and T-X(CO2) Evolution in High-Grade Impure Marble from Thongmön Area, Central Himalaya: Implications for Carbon Cycling in Orogenic Belts

Impure dolomitic marble from the Great Himalayan Sequences (GHS) in Thongmön area, central Himalaya, is first systematically reported here concerning its petrographic features, textural relations, and fluid evolution. The Thongmön impure marble is characterized by the assemblage of calcite + dolomite + forsterite + spinel + phlogopite + clinohumite ± diopside ± retrograde serpentine. Three groups of calcite and dolomite occurring both as inclusions and in the matrix were identified: group I is represented by relatively magnesium-rich calcite (Cal) (CalI:XMg = 0.10–0.15) and almost pure dolomite (Dol) (DolI:XMg = 0.47–0.48), corresponding to the Cal-Dol solvus temperatures of 707–781 °C; group II is characterized by vermicular dolomite exsolutions (DolII:XMg = 0.45–0.46) in Mg-rich calcite and Mg-poor calcite (CalII:XMg = 0.05–0.08) adjacent to DolII, and the recorded solvus temperatures are 548–625 °C; group III is represented by nearly pure calcite (CalIII:XMg = 0.003–0.02) and Ca-rich dolomite in the matrix (DolIII:XMg = 0.33–0.44). Isobaric T-X(CO2) pseudosection at a peak pressure of 15 kbar in the system K2O-CaO-MgO-Al2O3-FeO-SiO2-H2O-CO2 suggests that the peak fluid composition of the Thongmön forsterite marble is restricted to X(CO2) < 0.04 at T > 780 °C due to a potential infiltration event of H2O-rich fluid. Alternatively, the forsterite marble is a retrograde product subordinated to the GHS exhumation process, and its fluid composition is relatively CO2-rich (0.6 < X(CO2) < 0.8 at 5 kbar, 750 °C) at a nearly isothermal decompression stage. In either case, we suggest that the carbon flux contributed by metacarbonate rocks in an orogen setting to the global carbon cycling must be considered.

Micro-CT X-ray imaging exposes structured diffusion barriers within biofilms

In nature, bacteria predominantly exist as highly structured biofilms, which are held together by extracellular polymeric substance and protect their residents from environmental insults, such as antibiotics. The mechanisms supporting this phenotypic resistance are poorly understood. Recently, we identified a new mechanism maintaining biofilms - an active production of calcite minerals. In this work, a high-resolution and robust µCT technique is used to study the mineralized areas within intact bacterial biofilms. µCT is a vital tool for visualizing bacterial communities that can provide insights into the relationship between bacterial biofilm structure and function. Our results imply that dense and structured calcium carbonate lamina forms a diffusion barrier sheltering the inner cell mass of the biofilm colony. Therefore, µCT can be employed in clinical settings to predict the permeability of the biofilms. It is demonstrated that chemical interference with urease, a key enzyme in biomineralization, inhibits the assembly of complex bacterial structures, prevents the formation of mineral diffusion barriers and increases biofilm permeability. Therefore, biomineralization enzymes emerge as novel therapeutic targets for highly resistant infections.