ASSESSING THE POTENTIAL OF PENNSYLVANIAN PEDOGENIC PHYLLOSILICATE STABLE ISOTOPES AS A PALEOTEMPERATURE PROXY

The clay mineralogy, chemistry, hydrogen and oxygen-isotope composition of phyllosilicate samples from ten Pennsylvanian paleosols of the Illinois basin are presented in order to assess their utility as proxies of low-latitude terrestrial paleotemperatures. The majority of the samples are mineralogical mixtures of illite-smectite (I/S), and kaolinite. The chemistry and mineralogy of the mixtures provide unique oxygen and hydrogen isotope fractionation factors for each sample. The measured δ¹⁸O and δD values are used in conjunction with the calculated fractionation factors to estimate temperatures of phyllosilicate crystallization in equilibrium with paleo-meteoric water.

The range of phyllosilicate δ¹⁸O and δD values is 17.2‰ to 23.0‰ and -56‰ to -27‰, respectively. Assuming the phyllosilicates preserve a record of isotopic equilibrium with Pennsylvanian meteoric waters, these oxygen and hydrogen isotope values correspond to crystallization temperatures that range from 33 to 70ºC. These calculated phyllosilicate crystallization temperatures are inconsistent with a pedogenic origin, and instead suggest post-pedogenic, diagenetic alteration of the isotopic composition of the phyllosilicates. Accordingly these calculated crystallization temperatures are most likely erroneous, as it is unclear if the phyllosilicate isotope values reflect isotopic equilibrium with paleo-meteoric waters.

Comparison of phyllosilicate δ¹⁸O and δD values from laterally equivalent paleosols across the basin reveals distinct isotopic trends. Specifically, from the margin to the interior of the basin, δD values progressively decrease, while δ¹⁸O values progressively increase. These isotopic trends result in increasingly higher calculated crystallization temperatures from the margin to the interior of the basin. A similar spatial trend in clay mineralogy is observed across the basin, in which I/S becomes more illitic from the margin to the interior of the basin. These isotopic and mineralogic trends are considered to be a result of diagenetic crystallization of pedogenic minerals in response to burial by ~1.5 to 2 km of southward-thickening, post-Pennsylvanian strata, and perhaps a interval of middle Permian elevated heat flow associated with magmatic intrusions in the southern part of the basin.