John Hower

Mechanism of burial metamorphism of argillaceous sediment: 1. Mineralogical and chemical evidence

A detailed mineralogical and chemical investigation has been made of shale cuttings from a well (Case Western Reserve University Gulf Coast 6) in Oligocene-Miocene sediment of the Gulf Coast of the United States. The 10-µm fractions from the 1,250- to 5,500-m stratigraphic interval were analyzed by x-ray diffraction. Major mineralogic changes with depth take place over the interval 2,000 to 3,700 m, after which no significant changes are detectable. The most abundant mineral, illite/smectite, undergoes a conversion from less than 20 percent to about 80 percent illite layers over this interval, after which the proportion of illite layers remains constant. Over the same interval, calcite decreases from about 20 percent of the rock to almost zero, disappearing from progressively larger size fractions with increasing depth; potassium feldspar (but not albite) decreases to zero; and chlorite appears to increase in amount. Variations in the bulk chemical composition of the shale with depth show only minor changes, except for a marked decrease in CaO concomitant with the decrease in calcite. By contrast, the <0.1-µm fraction (virtually pure illite/smectite) shows a large increase in K2O and Al2O3 and a decrease in SiO2 The atomic proportions closely approximate the reaction smectite + Al+3 + K+ = illite + Si+4. The potassium and aluminum appear to be derived from the decomposition of potassium feldspar (and mica?), and the excess silicon probably forms quartz. We interpret all the major mineralogical and chemical changes as the response of the shale to burial metamorphism and conclude that the shale acted as a closed system for all components except H2O, CaO, Na2O, and CO2. Compositional changes in the shale as a function of metamorphic grade closely parallel compositional changes in shale as a function of geologic age.

Kinetics of illite formation

An activation energy of 19.6 +- 3.5 kcal/mole was found for conversion of synthetic beidellite with the composition Al/sub 2/Si/sub 3/./sub 66/Al/sub 0/./sub 34/O/sub 10/(OH)/sub 2/K/sub 0/./sub 34/ to mixed-layer illite/smectite. The size of this activation energy and the rate constants suggest that (1) the alteration of smectite to illite during diagenesis involves the breaking of chemical bonds in the 2:1 layers; (2) either an equilibrium or a kinetic interpretation for the range of mixed layering found in burial diagenetic sequences is compatible with the kinetic data; and (3) the formation of illite from smectite on the ocean floor will not be seen, even if the reaction is favored thermodynamically, because the reaction rate is too slow.

Late-Stage Dehydration in Deeply Buried Pelitic Sediments

Late-stage dehydration of shales containing montmorillonite or highly expandable illite-montmorillonite consists of the conversion of montmorillonite to illite layers, thus removing interlayer water. This conversion involves interlayer potassium fixation and probably is a result of ionic substitution within the montmorillonite alumino-silicate layer. Therefore, the rate at which dehydration takes place--and, indeed, whether it takes place at all--should be dependent on the chemical conditions of diagenesis, particularly pore-water composition. Detailed X-ray diffraction studies of cuttings from several Gulf Coast wells have resulted in a quantitative interpretation of late-stage dehydration of shales, caused by loss of interlayer water in montmorillonite. In the wells studied late-stage dehydration begins in the depth range of 6,000-8,000 ft and continues over a stratigraphic interval of 4,000-10,000 ft or more. Higher geothermal gradients result in shorter dehydration intervals. Montmorillonite dehydration rates reach maxima at the very inception of the diagenetic reaction and very near its close, when new illite layers are introduced into the illite-montmorillonite in an ordered fashion. At the close of the diagenetic reaction the illite-montmorillonite still contains about 20 percent hydrated layers which persist to tota depth in all wells studied. The temperature of inception of late-stage dehydration, as well as of its close, appears to be dependent on pressure, but other parameters such as pore-water chemistry may be equally important.

Mechanism of burial metamorphism of argillaceous sediment: 2. Radiogenic argon evidence

Variations in the 40* Ar of the whole rock and the 2 O in the inferred burial metamorphic reactions of argillaceous sediment from Texas Gulf Coast well CWRU (Case Western Reserve University) 6. These data strongly support the occurrence of the reactions involving the illite/smectite mixed-layer clay inferred from depth-dependent mineralogical and chemical changes reported in a companion paper by Hower and others (this issue). The 40Ar released from all samples was generally greater than 40 percent radiogenic ( 40* Ar), allowing precise measurement of differences of 40* Ar with depth. The whole-rock K-Ar apparent age shows a decrease from about 150 m.y. to 75 m.y. over just the same depth interval (1,850 to 3,700 m) that the illite/smectite mixed-layer clay progressively changes from 20 percent illite layers to 80 percent. This decrease in apparent age of the total shale is due to loss of 40* Ar from the rock. The 40* Ar loss is not caused by outgassing from increasing temperature, because the finest ( 40* Ar with depth. The coarser fraction, from which the 40* Ar loss is occurring, concentrates the older 40* Ar—rich phases of the rock such as K-feldspar and mica. These results strongly imply that the K 2 O for the new illite layers of the illite/smectite is derived by chemical decomposition of the K-feldspar and mica. The gain in 40* Ar of the finest fraction also just corresponds with depth to the change in mineralogy of the illite/smectite and a large gain of K 2 O in this fraction from 2 to 5 percent K 2 O. The 40* Ar gain indicates that the mean time of K 2 O gain, which is the mean time of burial metamorphism, was about 18 m.y. ago.

THE MINERALOGY OF GLAUCONITE

The mineral in monomineralic glauconite pellets is an iron-rich mixed-layer illite-smectite (here called glauconite), often composed almost entirely of illite layers. The nature of the interlayering is closely analagous to that of aluminous illite-smectite and varies with the proportions of the layer types: >30 per cent smectite, randomly interstratified; 15–30 per cent smectite, allevardite-like ordering; < 15 per cent smectite, ‘IMII’ ordering.Glauconite is analagous to aluminous illite-smectite chemically as well as structurally. A good correlation has been found between the number of potassium atoms per O10(OH)2 in structural formulas calculated from the chemical analyses and the proportion of illite layers as determined by X-ray powder diffraction methods. This relationship indicates a remarkably systematic increase in the potassium content of the illite layers with an increasing proportion of illite layers. This feature and the existence of ordered interlayering at high proportions of illite layers can be explained by crystal-chemical effects of illite layers on neighboring smectite layers. Glauconite differs from aluminous illite-smectite in that glauconite contains significantly less potassium per illite layer than does aluminous illite-smectite with the same proportion of illite layers except near the pure illite composition. The strength with which the interlayer potassium is held and the ease of conversion of smectite to illite layers in glauconite may be attributed to its IM structure and, perhaps, to its high octahedral iron content, which lead to stronger bonding of potassium by allowing a higher tilt angle of the O-H axis of hydroxyls adjacent to the potassium ion.The apparent octahedral cation occupancy in excess of two-thirds of the octahedral positions in many glauconites appears largely attributable to the presence of significant amounts of interlayer hyd-roxy-iron, aluminum and magnesium complexes in the smectite layers.

THE HYDROTHERMAL TRANSFORMATION OF SODIUM AND POTASSIUM SMECTITE INTO MIXED-LAYER CLAY

The transformation of sodium and potassium smectite into mixed-layer clay was followed in hydrothermal kinetic experiments. Glasses of beidellite composition and the Wyoming bentonite were used as starting materials. Temperatures ranged between 260 and 490°C at 2 kbar pressure, and run times ranged between 6 hr and 266 days.The course of the reactions was found to be strongly affected by interlayer chemistry. When potassium was the interlayer cation, increasing reaction produced the series: randomly interstratified illite/smectite-ordered interstratifled illite/smectite-illite. This sequence is equivalent to that formed in shales during burial diagenesis. With interlayer sodium and temperatures above 300°C, an aluminous beidellite (Black Jack analog)-rectorite-paragonite series was realized. The difference between these two diagenetic families is discussed. Below 300°C, sodium beidellite formed randomly interstratifled mixed-layer clay much like potassium beidellite, except that a higher layer charge was required to produce sodium mica-like layers. The higher charge resulted from sodium’s higher hydration energy. The difference in hydration energy between potassium and sodium may account for the fixation of potassium rather than sodium in illite during burial diagenesis.The appearance of ordered interlayering in mixed-layer phases is also related to interlayer chemistry. Ordering formed in sodium clays at high expandabilities, whereas it never appeared in the potassium clays above approximately 35% expandable. The appearance of ordering may be partly related to the polarizing power of the mica-like layers.Phase diagrams, constructed from the kinetic experiments and from the composition and occurrence of natural clays, are presented for the systems paragonite and muscovite-2 quartz-kaolinite-excess water. This study also reports the first synthesis of a Kalkberg-type mixed-layer clay.

Mineralogy and Early Diagenesis of Carbonate Sediments

ABSTRACT The mineralogy and important trace elements of Recent carbonate sediments and environmentally comparable Pleistocene carbonate rocks have been studied. All of the rocks and most of the sediments employed were obtained in southern Florida. Additional sediment samples came from the Bahama Banks, the South and East China Seas, and the Persian Gulf. Most sediments were found to consist of aragonite, high-magnesium calcite, and low-magnesium calcite. Neither dolomite nor vaterite was encountered. Shallow water carbonate sediments contain an average of about 70 percent of unstable forms of CaCo3 with aragonite predominating and high-magnesium calcite dominant over low-magnesium calcite. Deep water carbonate sediments are composed predominantly of low-magnesium calcite, and high-m gnesium calcite dominates aragonite. Compositional differences between shallow and deep water carbonates are believed mainly to result from differences in the importance of contributions of skeletal material by certain groups of organisms in the two environments. The mineralogy of the Pleistocene carbonate rocks studied showed them to consist mainly of low-magnesium calcite and indicated that under near-surface conditions in nature a stability sequence runs as follows: low-magnesium calcite>aragonite>high-magnesium calcite. Diagenesis of carbonate sediments is accompanied by an important loss in the level of abundance of the elements, magnesium, strontium, barium, and manganese. These elements, including magnesium, are lost from the chemical system and do not appear to form new min rals in place, although they may later do so at some other place. Since the chemical system is not closed, the significance of important volume changes which accompany diagenesis can not yet be assessed.

Radiometric dating of time of thrusting in the disturbed belt of Montana

Mesozoic sedimentary rocks overridden by thrust plates in the disturbed belt of northwestern Montana have been metamorphosed by burial beneath these plates. Bentonite in the Cretaceous section has been converted to potash bentonite by this metamorphism, allowing radiometric dating of the emplacement of the thrust plates by the K-Ar method. Ages determined thus far range from 72 to 56 m.y. B.P. The oldest ages coincide with estimates for the beginning of thrusting from stratigraphic and structural evidence. Field evidence does not allow a reliable estimate for the end of thrusting. Thus, the younger ages, which indicate completion of thrusting at the end of Paleocene time, represent additional data of interest to the structural history of the disturbed belt.

An explanation for low radiometric ages from glauconite

Abstract K-Ar and Rb-Sr ages from glauconites are about ten to twenty per cent lower than the age of sedimentation. Previous studies have indicated that these low ages are not attributable to normal diffusion loss of Ar from glauconite crystallites. The possibility of argon loss from ‘open’ potassium sites, such as on crystal surfaces and from expanded layers, was investigated by acid dissolution techniques. These studies show that potassium is removed from glauconites with low expandabilities at three different rates. The highest dissolution rate corresponds to cation exchange and comprises five to ten per cent of the total potassium. About five per cent of the total potassium is removed at a much slower rate than that of cation exchange, but at an order of magnitude faster than the bulk of the potassium. Activation energies calculated from rate constants determined at 50° and 80°C, for one sample gave values of 19 kcal/mole for the lowest dissolution rate and 14 kcal/mole for the intermediate rate. It appears that low radiometric ages from glauconites can be largely explained by the presence of potassium in sites where argon is readily lost, although such factors as late epigenetic gain of potassium by glauconite may also contribute to their low radiometric ages. A method is described for making quantitative corrections for such daughter product loss in radiometric age determinations.

Kaolinite synthesis; the role of the SiAl and (alkali)/(H (super +) ) ratio in hydrothermal systems

The Si/Al ratio of an hydrothermal system plays an important role in kaolinite synthesis. If the atomic Si/Al ratio of a system is greater than 2-0, kaolinite will disappear at 345 + 5°C and 2 kbars water pressure according to the reaction kaolinite + 2 quartz → pyrophyllite + H2O. If the atomic Si/Al ratio is less than 2-0, however, kaolinite will persist until 405°C where it will react according to the equation 2 kaolinite → pyrophyllite + 2 boehmite + 2 H2O. The Si/Al ratio of the system and temperature are also factors in determining whether b-axis ordered or disordered kaolinite will crystallize. The ordered variety is favored by a lower Si/Al ratio and a higher temperature than is the disordered form.Hydrothermal experiments also show that kaolinite can be synthesized at 150°C and 5 bars pressure in distilled water from amorphous starting materials. Previous investigators were unsuccessful in forming kaolinite under these conditions because their systems were contaminated with alkalis.Attempts to synthesize halloysite and dickite failed, but halloysite was converted to kaolinite at 150°C, suggesting that halloysite can be synthesized only at low temperatures.