Masters Theses

Date of Award

5-1995

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Claudia I. Mora

Committee Members

Steven Driese, Otto Kopp

Abstract

Paleoclimate interpretations based on paleosol stable isotope proxies require preservation of original pedogenic signatures and thorough understanding of mineral paragenesis (i.e., pedogenic or diagenetic). This study examined the oxygen isotope compositions of clays and pedogenic carbonate in three approximately time-equivalent vertic paleosols to establish the extent to which these minerals approached isotopic equilibrium at pedogenic and burial diagenetic temperatures. The paleosols are preserved in the late Mississippian Mauch Chunk (PA), Hinton (WV), and Pennington (TN) Formations, which were buried to 7-8 km, 3-4 km, and <3 km, respectively, within the Appalachian Basin. Preserved macro- and micromorphological features indicate that these paleosols were composed of > 30 %, chiefly smectitic, clay which experienced repeated cycles of wetting and drying as a result of strongly seasonal precipitation during pedogenesis. Presently, the paleosols are composed predominantly of discrete illite, which is inferred to have been authigenically produced through the smectite-to-illite transition. Size separations were made to isolate the smallest naturally occurring size fraction of clays, as determined by laser particle scattering. This size fraction was then analyzed to evaluate its mineralogy (using X-ray diffraction) and its oxygen isotope composition. Previous studies have documented the strong control of the smectite-to-illite transition on the isotopic evolution of pore fluids. Therefore, three possible environments of illite formation in the paleosols, defined by different temperatures and pore fluid oxygen compositions, were considered: 1) illitization occurs in the pedogenic environment and is kinetically driven by repeated cycles of wetting and drying smectitic clays, 2) illitization is thermally driven as result of increased depths of burial, and 3) illitization occurs in response to a pulse of hot, saline, orogenic fluids migrating through the paleosols. Illite-calcite equilibration temperatures, calculated equilibrium pore-fluid compositions estimated from appropriate O-isotope fractionation factors, and radiogenic (K/Ar) age dating were used to evaluate which environment best explains illitization within each paleosol. Illite and pedogenic calcite in the more-deeply buried Mauch Chunk and Hinton paleosols no longer preserve pedogenic signatures. Instead, calculated equilibration temperatures for the Mauch Chunk (180 °C) and Hinton (110 °C) indicate that these mineral phases most likely acquired their oxygen signatures during burial-driven illitization. Compaction of the paleosols subsequent to clay dewatering may have effectively prevented later orogenic fluids from migrating through them. That vertic paleosols behave as isotopically closed systems is supported by the preservation of isotopic equilibration temperatures that are below, and, in the case of the Mauch Chunk, well below, maximum burial temperatures. Interpretation of the Pennington data is more difficult. The method of illite formation must account for three important parameters defined by the data: 1) illitization of the paleosol is extensive despite very shallow depths of burial, 2) calcite retained a pedogenic oxygen isotope signature which indicates it did not undergo isotopic exchange during smectite dewatering and illite formation, and 3) the Pennington "parent" and paleosol materials have identical oxygen isotope compositions and have gone through the same degree of illitization. individually, none of the defined environments of illite formation can account for all of these parameters. Therefore, the pathway of Pennington illitization is best explained by a combination of different methods of illite formation. In this case, illitization most likely began in the pedogenic environment and continued to be driven during burial diagenesis. Illitization initiated in the pedogenic environment may have allowed coarsening and ordering of illite to proceed at the low temperature and low water/rock conditions necessary for the preservation of pedogenic oxygen isotope compositions in calcite. Extended periods of time in the shallow burial environment would account for extensive illitization and masking of mineralogical and isotopic differences between "parent" and paleosol illites. The results of this study suggest that oxygen isotopes in vertic paleosols are not well suited for paleoclimate interpretations, but are best suited for evaluating the physical and chemical conditions of diagenesis.

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