Oxygen and hydrogen isotope compositions of Permian pedogenic phyllosilicates:: Development of modem surface domain arrays and implications for paleotemperature reconstructions

Mineralogic, chemical, and oxygen and hydrogen isotope compositions of 15 different phyllosilicate samples from Permo-Pennsylvanian-age paleosols of the eastern shelf of the Midland basin of Texas and the southern Anadarko basin are presented. Mixtures of 2:1 phyllosilicates and kaolinite dominate most samples, although some samples consist of relatively pure 2:1 phyllosilicates. Chemical and mineralogic data are used in conjunction with published thermodynamic data to calculate hydrogen and oxygen isotope fractionation factors for each sample. In turn, application of measured oxygen and hydrogen isotope compositions of the phyllosilicates to temperature-dependent fractionation equations are used to calculate paleotemperatures of crystallization. The delta D values of the phyllosilicates range from -69 parts per thousand to -55 parts per thousand. The delta O-18 values range from 19.5 parts per thousand to 22.7 parts per thousand. If these samples preserve a record of equilibrium with paleo-meteoric waters, the isotopic compositions of the phyllosilicates correspond to paleotemperatures of phyllosilicate crystallization ranging from 22 +/- 3 degrees C to 35 +/- 3 degrees C. In particular, the stratigraphic trend of calculated temperatures from Midland basin samples suggests that Early Permian surface temperatures may have been up to 10 degrees C warmer than those of the latest Pennsylvanian. (c) 2005 Elsevier B.V. All rights reserved.