DOLOMITE IN THE CAVE ENVIRONMENT - CARLSBAD CAVERN, NEW MEXICO, USA

Since its last phase of formation by sulfuric acid nearly 4 My ago, Carlsbad Cavern, New Mexico, has been the site of speleothem deposition that includes a variety of evaporative Ca-Mg carbonate minerals and some silicates. Dolomite is common and shows a broad range of ordering. The stable cave environment has preserved an unusually good record of dolomite growth.

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 [104] 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 [104] < ~30.8 degrees and low δ¹⁸O has formed in low-Mg water together with calcite and aragonite, while dolomite with 2θ [104] ~31.0 degrees and moderate δ¹⁸O form alone in Mg-rich water.

The highest 2θ [104] 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 [104] 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.