Clumped isotope evidence for the formation of nonplanar dolomite textures at near-surface temperatures

Abstract

Dolomite textures are widely interpreted to reflect physical, mineralogical, and geochemical conditions of crystal growth. In particular, nonplanar dolomites, which display non-faceted crystal boundaries and a low percentage of crystals with compromise boundaries with preserved crystal-face junctions, have long been cited as evidence of crystal growth in fluids warmer than a theoretical dolomite critical roughening temperature (CRT) of ~50-100°C. No direct empirical evidence exists, however, to support the claim that nonplanar dolomites form exclusively above the CRT. The present study offers new Δ47 clumped isotope data from nonplanar dolomites from the Paleocene-Eocene Umm er Radhuma Formation (Qatar) that show nonplanar dolomite can form below the proposed CRT. These dolomites are interpreted to have experienced only near-surface to shallow burial conditions since deposition, and lack common burial features such as two-phase liquid-vapor inclusions, stylolites, compaction reduced porosity, and buria l cements. Scanning electron microscope images reveal that relatively large dolomite crystals (typically > 100 µm) comprise non-faceted mosaics with indistinct crystal boundaries, indicating a nonplanar texture. Thin-section petrographic measurements confirm the nonplanar texture, as the proportion of dolomite crystals with compromise boundaries with preserved crystal-face junctions ranges from 9% to 20% with an average of 14%, defining these dolomites as nonplanar sensu stricto (≤ 30%). The new Δ47 clumped isotope data from these nonplanar dolomites reveals average crystallization temperatures ranging from 38.8 to 54.2°C and overall averaging 44.1°C. Calculated uncertainties, however, indicate the nonplanar dolomites could have formed at temperatures as low as 29.1°C or as high as 65.3°C. More than three quarters (~78%) of the samples have mean temperatures that fall below 50°C, and all samples have calculated uncertainties indicating possible temperatures below 50°C but not all indicate possible tempera tures above 50°C. Furthermore, these calculated uncertainties overlap with the crystallization temperatures of planar and mimetic dolomites higher in the section, suggesting that all dolomites formed under similar temperature conditions, and therefore texture is unlikely driven solely by crystallization temperature. Cumulatively, these results indicate that the nonplanar dolomite formed in a shallow burial setting at temperatures near or below the proposed dolomite CRT. The new Δ47 data, in conjunction with textural observations from natural dolomites and hundreds of published high-temperature experiments, suggest that nonplanar dolomite cannot be reliably used as an indicator of high-temperature environments of dolomitization.