DOLOMITE IN THE CAVE ENVIRONMENT - CARLSBAD CAVERN, NEW MEXICO, USA
Under permit, we obtained previously broken samples from various settings in the cave. Two water sources for carbonates can be distinguished by their δ13C and δ18O values. (1) Carbonates deposited by meteoric recharge show increasing δ13C as CO2 degasses, and increasing δ18O with evaporation. Dolomite becomes less ordered near water bodies, as shown by decreasing 2θ  X-ray peaks, suggesting that dolomite crystallization is inhibited by the greater retention of water of hydration by Mg2+. Low δ18O in smectite-rich dolomite may be due to slow evaporation, whereas high δ18O and δ13C in botryoidal dolomite correlates with high rates of CO2 loss and evaporation. (2) Where evaporation-condensation cells are most active in the cave, speleothems produced by evaporation of dripping condensation water have low δ13C, because the moisture is at CO2 equilibrium with cave air and degassing is impossible. Low δ18O in these carbonate minerals suggests slow evaporation, which allows Mg2+ to concentrate while more stable carbonates precipitate. Dolomite with 2θ  < ~30.8 degrees and low δ18O has formed in low-Mg water together with calcite and aragonite, while dolomite with 2θ  ~31.0 degrees and moderate δ18O form alone in Mg-rich water.
The highest 2θ  for dolomite (~greatest ordering) is at the base of massive deposits beneath former condensation drip points. These consist of shingled rhombs (<50μm) indicating slow 2-dimensional nucleation. They also contain euhedral quartz from evaporative residue. Dolomite rhombs (<5μm) associated with authigenic clay have intermediate 2θ  values and appear to be nonreplacive precipitates. Petrographic analysis shows that dolomite in the interiors of large finely crystalline bodies has replaced aragonite and finely crystalline carbonate, while the outermost dolomite consists of fibrous layers and rounded rhombs after calcite.