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Illite/Smectite Formation and Potassium Mass Transfer During Burial Diagenesis of Mudrocks: A Study from the Texas Gulf Coast Paleocene-Eocene
D. N. Awwiller (1)
Mudrock samples from a well penetrating Eocene and Paleocene strata (1235-4455 m) of the Texas Gulf Coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis, and the mechanism by which detrital smectitic illite/smectite (I/S) is transformed into diagenetic illitic I/S.
Idealized smectite and illite end-member compositions for interstratified I/S are estimated to be [K0.00X+10.56Mg0.39Fe0.57Al1.13Si3.90O10(OH)2] and [K0.53X+10.18Mg0.17Fe0.16Al2.28 Si3.40O10(OH)2], respectively. There is no obvious difference in the amount of K addition to I/S in random- and ordered-interstratified I/S. Aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite-to-illite reaction is a complete dissolution-precipitation reaction rather than a solid-state [K+1 + Al+3] for Si+4 substitution reaction. Depth-relat d mineral trends and mass-balance calculations suggest that illitic I/S may form from kaolinite, and possibly illite or mica, as well as from smectitic I/S.
Although most whole-rock element abundances are invariant over the sampled depth interval, K2O content increases from ca. 2.0 weight percent at 1500 m of burial to ca. 3.8 weight percent at 4000 m. Some of this increase is probably caused by import of K into the shale via fluids derived from interbedded sandstones, implying that Gulf Coast mudrocks behave as open chemical systems during burial diagenesis. If this is the case, a minimum of 103 pore volumes of fluid must have passed through the most K-enriched shales to have introduced the added potassium, provided that the K content of present-day formation water is representative of that of ancient formation water.
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