Abstract
Magnesium and sulfur are one of the major elements that control dolomitization in marine, low-temperature environments. However, conditions and reaction mechanisms, which are related to dolomitization are poorly understood. We investigated partly dolomitized limestone (Oker, Langenberg, Germany) of Upper Jurassic age ( approximately 153 Ma) that was deposited in a shallow marine, sabkha environment. X-ray diffraction (XRD), delta (super 18) O and delta (super 13) C isotope measurements, and electron microprobe (EMP) analyses were completed on porous dolomite, sandwiched by layers of limestone. The lower limestone layer consists of micritic calcite (-1.7 to -2.9 ppm of delta (super 18) O and 1.3 to -0.7 ppm of delta (super 13) C, VPDB), which formed under marine conditions. EMP analyses revealed low-Mg calcite (LMC) of (Ca (sub 0.96-0.99) Mg (sub 0.004-0.03) Sr (sub 0-0.01) Na (sub 0-0.01) Fe (sub 0-0.002) ) (sub 0.99-1.0) CO (sub 3) . Samples from the contacts between dolomite and limestone layers contain LMC, high-Mg calcite (HMC), and dolomite, which are associated in single grains. The dolomite core, (Ca (sub 0.97-1.14) Na (sub 0-0.01) ) (sub 0.97-1.14) (Mg (sub 0.75-0.97) Fe (sub 0-0.02) Mn (sub 0-0.01) ) (sub 0.76-0.99) [(C (sub 0.998-1.0) S (sub 0-0.002) )O (sub 3) ] (sub 2) , is approximately 10 mu m in diameter, and is surrounded by LMC, (Ca (sub 0.86-0.99) Mg (sub 0.006-0.05) Fe (sub 0-0.004) Na (sub 0-0.002) Mn (sub 0-0.001) ) (sub 0.96-0.97) CO (sub 3) , and subsequently deposited HMC, (Ca (sub 0.64-0.78) Mg (sub 0.19-0.32) Fe (sub 0-0.004) Na (sub 0-0.003) Mn (sub 0-0.002) ) (sub 0.9 1-0.99) CO (sub 3) . The "pure" dolomite (2.2 to 1.7ppm of delta (super 18) O and 1.7 to -0.1ppm of delta (super 13) C, VPDB) comprises of 2-50 mu m sized euhedral crystals of (Ca (sub 1.02-1.12) Na (sub 0-0.02) ) (sub 1.02-1.13) (Mg (sub 0.82-0.94) Fe (sub 0-0.02) Mn (sub 0 -0.002) ) (sub 0.83-0.97) [(C (sub 0.994-1.0) S (sub 0-0.006) )O (sub 3) ] (sub 2) , and shows alternate growth zones of S and Fe. XRD data confirm non-stoichiometric dolomite with 51-54 mol% of CaCO (sub 3) . The degree of order in dolomite, in respect to dolomite super structure, decreases from 83% to 42% with increasing S contents from 0.02 to 0.06 S [a.p.f.u.], respectively. The upper limestone layer contains no dolomite or HMC, and displays marine conditions; i.e., micritic LMC (-1.8 to -3.4ppm of delta (super 18) O and -1.6 to -4.0ppm of delta (super 13) C, VPDB). The euhedral shape and the (Fe, S)-growth zoning of the dolomite crystals, and the alternate zones of dolomite, LMC, and HMC suggest dolomitization via dissolution of primary carbonates and subsequent cyclic and abrupt changes in the chemical (Mg, Fe, and S), and isotopic composition of the interstitial solutions.