Date of Award

12-2001

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Claudia I. Mora

Committee Members

Lee R. Riciputi, Steven G. Driese, Michael E. Essington

Abstract

A series of investigations were conducted to evaluate microscale evidence for basinal fluid migration in the Illinois basin during diagenesis of the Upper Cambrian Mount Simon Sandstone. Samples were examined using transmitted light and cathodoluminescence (CL) petrography, fluid inclusion analysis, and Secondary Ion Mass Spectrometry (SIMS) analysis of 18O/16O ratios and trace element compositions. Preliminary investigation of in situ laser ablation 40Ar-39Ar age dating techniques on authigenic K-feldspar over growths was also completed.

Two major generations of quartz overgrowths are observed, on the basis of transmitted light and CL petrography and fluid inclusion studies. High-resolution (100 μm x 100 μm), gray-scale images of CL in authigenic quartz overgrowths were acquired using the electron microprobe. Generation 1 quartz overgrowths have low intensity, medium- to dark- gray CL and are associated with fluid inclusion homogenization temperatures (Th) of 60 - 95°C. Generation 2 quartz overgrowths have light-gray CL and are related to fluid inclusion Th of 100 - 145°C. Th decreases from the southern part of the basin towards the north. Burial temperatures estimated by stratigraphic reconstruction and thermal history modeling did not exceed 60 - 90°C, thus, passage of at least two generations of basinal fluid flow, at temperatures similar to or slightly elevated relative to burial temperature (Th = 60 - 145°C) through the Mount Simon Sandstone are indicated. Salinities of these Na-Ca-K-Mg-Cl-H2O fluids vary from 18 wt% to 25 wt% NaCl equivalent, suggesting that both diagenetic fluids were highly saline basinal brines.

Ion-microprobe oxygen isotope analyses of authigenic K-feldspar and quartz reveal a south-to-north-increasing trend in δ18O values. Average K-feldspar δ18O (V-SMOW) values increase systematically from +14 ± 1‰ in the southernmost and deepest borehole samples from Illinois to +24 ± 2‰ in the northernmost outcrop samples in Wisconsin. A similar south-to-north trend was observed for quartz overgrowths (22 ± 2‰ to 28 ± 2‰). Within-sample δ18O variations of up to 9‰ are much greater than analytical precision of the ion probe. This may be, in part, the result of sampling different generations of cements, as supported by CL and fluid inclusion studies in quartz. CL zones within quartz overgrowths were analyzed to evaluate δ18O values in different generations of quartz cements. Generation 1 quartz cements have δ18O values that are equal to (within analytical precision) or greater than (up to 6.8‰ difference) generation 2 quartz cements. Overall, however, generation 1 cements are isotopically-enriched relative to generation 2 quartz cements. Constraints on fluid compositions using fluid-inclusion temperatures that were point-matched to δ18O values of quartz overgrowths containing these fluid inclusions reveal two distinct diagenetic fluids, corresponding to generation 1 (T < 95°C) and generation 2 (T > 95°C) fluids. the δ18O values of generation 1 fluid are about -5‰ in the southern portion of the basin and increase to ~ 0‰ in the northern part of the basin; δ18O values of generation 2 fluid increases from ~ +2‰ in the south to ~ +7‰ in the north. For the generation 1 diagenetic event (T < 95°C), fluid temperatures are consistent with burial temperatures in the deeper part of the southern basin, where the fluid may have originated. Fluid temperatures in the northern part of the basin are lower than in the southern basin, but still higher than burial temperatures.

Ion-microprobe studies of authigenic K-feldspar and quartz reveal that Rb, Sr, Ba, Pb, Fe, Mg, B, Ti, and Cl are present in trace amounts in authigenic K-feldspar. In quartz cements, K, Al, Fe, B, Ti, Mg, and Cl were detected. Strongly covariant relationships among Sr-Ba-Pb-Rb, Na-Mg-Ca-Cl, and K-Al in authigenic cements were attributed to their similar geochemical behaviors and incorporation modes, continuous chemical changes in the fluid during migration and cement precipitation, and the requirements of solid solution charge compensation. Concentrations of Ba, Sr, Rb, Pb, Fe, Ca, and Ce in diagenetic fluids were estimated on the basis of trace element contents in authigenic K-feldspar and distribution coefficients between sanidine and hydrothermal fluids. No correlation between fluid compositions and positions in the basin (south to north) is apparent. Total K+Na+Li+Mg+Ca concentrations are significantly higher in generation 2 quartz cement, compared to generation 1 cements; B and K are both notably enriched in generation 2. These differences suggest that later brines had a different composition. The lack of regional trends indicates more local control of the physiochemical environment on trace element concentrations in authigenic minerals, compared to their oxygen isotope of fluid inclusion compositions.

Preliminary trials of in situ UV-laser probe 40Ar/39Ar dating of authigenic K-feldspar cements were largely unsuccessful in releasing Ar gas out of the overgrowths; rather, the UV-laser mostly shattered the grains, without melting them. Ar-laser step heating tests on areas ~ 50 x 50 μm in size released gas from both detrital and authigenic K-feldspar. An overwhelming amount of atmospheric air was also released from the ceramic adhesive, resulting in very large analytical errors related to air-compound corrections. Procedures need to be taken to reduce air in the ceramic adhesive during thin section preparation, if this ceramic is used to substitute for epoxies.

This study demonstrates that in situ microscale ion probe analytical techniques have significant advantages over conventional analysis in obtaining oxygen isotope compositions and trace-element concentrations from very fine-grained authigenic minerals. High-resolution ion probe analysis, in concert with detailed CL petrography and fluid inclusion studies, may provide valuable geochemical constraints on low-temperature diagenesis and basinal fluid migration, revealing multiple events, regional flow directions, fluid compositions, and temperatures and possible fluid sources.

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