The Kinetics of the Smectite to Illite Transformation in Cretaceous Bentonites, Cerro Negro, New Mexico

Abstract

Abstract--The thermal effects, as well as the survivability and origins of microorganisms in Cretaceous rocks, are evaluated from the timing and extent of the smectite to illite transformation in Cretaceous bentonites collected from cores outside the thermal aureole of the Pliocene Cerro Negro volcanic neck. Overall, randomly ordered mixed-layered illite-smectite (I-S) is the predominant clay mineral in these bentonites, and the K-Ar ages of I-S range from 36 to 48 Ma (21 analyses, two additional analyses were outside this range). Increased temperature from burial is thought to be the primary factor forming I-S in these bentonites. Kinetic model calculations of the smectite to illite transformation are also consistent with I-S formed by burial without any appreciable thermal effects due to the emplacement of Cerro Negro. In a core angled toward Cerro Negro, the percentages of illite layers in I-S from the bentonite closest to Cerro Negro are slightly higher (32-37%) than in most other bentonites in this study. The KAr ages of the closest I-S are slightly younger as a group (38-43 Ma; Average 41 Ma; N = 4) than those of I-S further from Cerro Negro in the same core (41-48 Ma; Average = 44 Ma; N 6). A small amount of illite in this I-S may have formed by heat from the emplacement of Cerro Negro, but most illite formed from burial. Vitrinite reflectance, however, appears to record the effects of heating from Cerro Negro better than 1-S. Tentatively, the temperature of this heat pulse, based on vitrinite data alone, ranged from 100 to 125~ and this is most evident in the CNAR core. The upper temperature, 125~ approximates the sterilization temperatures for most microorganisms, and these temperatures probably reduced a significant portion of the microbial population. Thermophiles may have survived the increased temperatures from the combined effects of burial and the intrusion of Cerro Negro.

Acid-sulphate hydrothermal alteration of andesitic tuffs and genesis of halloysite and alunite deposits in the Biga Peninsula, Turkey

“…This ensured equivalence of the respective portions taken for Ar and K measurements. The measurements were done using procedures such as those of Elliott et al (1999). The amount and isotopic composition of the Ar were determined by mass spectrometry after isotope dilution with 38 Ar.…”

Modeling of smectite illitization in burial diagenesis environments

“…Their best match produced the values n = 5 (in S n ) and m = 1 (in [K/Na] m ). Elliott et al (1999) also found that the equation of Pytte and Reynolds (1989) with a high order was the best in modeling illitization of bentonite affected by burial, although they expressed their doubts that such equation is the best model for all types of illitization sequences. Altaner (1989) coupled diffusional rates of K in a bentonite bed with illitization rates, in order to model the illite-layer proportion distribution in I-S within the bentonite bed.…”

“…The reaction has been modeled by several authors using field samples and laboratory experiments to produce kinetic equations. Some of these equations have been tested against the progress of the reaction in sedimentary basins where the several variables could be reasonably constrained (Pytte and Reynolds, 1989;Elliott et al, 1991;Velde and Vasseur, 1992;Huang et al, 1993;Hillier et al, 1995;Elliott and Matisoff, 1996;Elliott et al, 1999), with the result that none of the kinetic equations has been found to be consistently superior to the others. Hence, the search for the equation governing the rate of smectite illitization is still open.…”

Eogenetic diagenesis of Chinle sandstones, Petrified Forest National Park (Arizona, USA): A record of Late Triassic climate change

“…This mixed-layer clay has two possible sources: (i) it could be recycled from old mudrocks within the Chinle Basin (i.e. the Moenkopi Formation); or (ii) it could be the result of low temperature diagenetic weathering of volcanic ash (Srodon, 1984;Inoue et al, 1990;Cuadros & Altaner, 1998;Elliott et al, 1999).…”