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Lake Olduvai was a saline, highly alkaline closed-basin lake in which lacustrine sediments of Bed I and lower Bed II of Olduvai Gorge were deposited between ca. 1.92 and 1.70 Ma. The lake consisted of a lacustrine area, the central basin (CB), surrounded by lake-margin terrain that was alternately flooded and dried. The lacustrine deposits consist largely of authigenic clay and carbonate minerals, zeolites, K- feldspar, chert, and pyrite. Claystones of the CB and most of the eastern lake-margin (ELM) terrain consist chiefly of Mg-rich authigenic illite, smectite, and interlayered illite-smectite (I/S). These clays were formed penecontemporaneously with detrital-clay deposition by interaction of the detrital clays with the saline, highly alkaline water. The Lake Olduvai clays average ∼65%–70% Mg 2+ , SiO 2 , and alkali ions from lake water, and the remaining 30%–35% is detrital clay. Lacustrine clays of the ELM indicate that this area was flooded by saline water for substantial periods of time. Hominids occupied the ELM at times of lower lake levels, when the clays were dried. Calcite is the most abundant carbonate mineral and occurs chiefly as sand-size euhedral crystals dispersed in claystone and less commonly as calcite-crystal limestones and nodules. The sand-size calcite crystals were formed diagenetically at shallow depths, possibly from a gaylussite and/or micritic calcium carbonate precursor. Dolomite occurs chiefly in the form of dolostones, some of which are probably of replacement origin. The oxygen isotope composition of lake water and pore fluid of the CB varied over ∼11‰, as determined from calculated \({\delta}^{18}O_{H_{2}O}\) values of authigenic minerals. Clay minerals were formed chiefly in the more saline waters. Sand-size calcite crystals were generated mostly from less saline pore fluids, and limestone nodules were formed chiefly from dilute pore fluid. Dolostones were formed from fluids of varying salinity. Hydrogeology of the central basin was complex, and postdepositional fluid movements are documented by differing \({\delta}^{18}O_{H_{2}O}\) values for different authigenic minerals of the same samples. Pore fluids moved within the lake deposits for a considerable period of time after deposition, as shown by δ 18 O values of quartz and K-feldspar formed at different times. Downward flow of lacustrine fluids is indicated by zeolitic alteration of the underlying tuffaceous alluvial deposits. Lake Olduvai became overall shallower during the deposition of Bed I, and the subaerial exposure increased, leading to occasional playa conditions. These changes very likely resulted from increasing aridity and tectonic activity. The lake that deposited lower Bed II was larger than that responsible for upper Bed I, and episodes of wetter climate alternated with drier periods in which salinity and probably temperatures were higher than in the Bed I lake.

In the lower Paleozoic of the Illinois Basin, three illite polytypes are found: 2M 1 of detrital origin, and 1M d and 1M of diagenetic origin. Illite polytype quantification of detrital 2M 1 illite and diagenetic 1M d and 1M illite, combined with K-Ar age dating, allows extrapolation to apparent detrital and diagenetic illite ages. Kinetic modeling of smectite illitization, combined with the calculated age of illitization, can evaluate different origins of illite. The diagenetic illite in the lower Paleozoic of the Illinois Basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events. The Upper Ordovician Maquoketa Group contains diagenetic illite (dominantly 1M d with minor 1M ) with an extrapolated age of ∼360 m.y. (356–377 m.y.) and formed from smectite at temperatures of 50–100 °C. This age falls within the span of dates for illite/smectite (I/S) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a K-rich brine from the Michigan Basin, Upper Mississippi Valley area, or Forest City Basin. In Ordovician and Cambrian shale partings and sandstone older than the Maquoketa Group, the diagenetic illite ( 1M d in shale and 1M in sandstone) has an age of ∼300 m.y. and formed at temperatures

Zeolites have been known since the mid-1750s, but prior to the early 1950s, most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks, particularly basaltic lava flows. Indeed, most of the large attractive zeolite specimens in museum collections were obtained from lavas. In recent years, zeolites have been recognized as important rock-forming constituents in low-grade metamorphic rocks and in a variety of sedimentary rocks. Most zeolites in sedimentary rocks are finely crystalline, that is they occur as microscopic or submicroscopic crystals, and they are therefore of little appeal to mineral collectors; however, deposits of this type are voluminous and have great geologic significance and economic potential. Zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks, and they form in sedimentary rocks of diverse lithology, age, and depositional environment. About twenty species of zeolites have been reported from sedimentary rocks, but only eight zeolites commonly make up the major part of zeolitic rocks. These are analcime, chabazite, clinoptilolite, erionite, heulandite, laumontite, mordenite, and phillipsite. This chapter will consider chiefly the zeolites in sedimentary rocks, with emphasis on volcaniclastic deposits, which contain the largest concentrations of zeolites. The occurrence of zeolites in lava flows is mentioned only briefly. Journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the Mineralogy and Geology of Natural Zeolites was first published in 1977 as Volume 4 of the Mineralogical Society of America’s Reviews in Mineralogy . The present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals. A wide variety of zeolites has been identified in sedimentary deposits, with the most common being clinoptilolite, analcime, heulandite, laumontite, and phillipsite. Less abundant zeolites include chabazite, erionite, mordenite, natrolite …

Much controversy has arisen over both the depositional and diagenetic environments of the lacustrine sediments and the chemical evolution of waters within those sediments that make up the upper Oligocene Creede Formation. Previous studies of the neighboring Creede Ag-Pb-Zn district proposed that saline and isotopically heavy fluids from the Creede Formation were involved in ore deposition. Examination of rhythmically laminated limestones from two Continental Scientific Drilling Program drill cores (CCM-1 and CCM-2) provided a unique opportunity to study carbonate sediments of a caldera lake. The upper parts of CCM-2 (50–148 m depth) are characterized by laminations of low-Mg calcite containing brine shrimp fecal pellets (with an aragonite precursor), organic matter (including bacterial mat–like laminae), calcite pseudomorphs after gypsum, and cements of bladed low-Mg calcite and tabular clinoptilolite. The degree of recrystallization and crystal coarsening is greater in the lower part of CCM-2 (172–309.4 m depth) and in CCM-1 (8.8–155.2 m depth). These observations suggest that ancient Lake Creede was an evaporative, partially meromictic, saline to hypersaline, near-neutral, Na-Mg-SO 4 -Cl or Cl-SO 4 lake. SO 2− 4 and H + were supplied to the lake by the oxidation of volcanic SO 2(gas) , which buffered the alkalinity of the lake. Brine shrimp in the epilimnion are consistent with salinities greater than seawater and with warm surface temperatures. The lack of bioturbation in rhythmically laminated carbonate-siliciclastic couplets indicates either meromictic conditions or saline conditions or both. Bacterial mat-like laminae suggest that lake waters were either relatively shallow or clear. Displacively grown gypsum crystals indicate times when evaporative ground-water conditions dominated (e.g., during low lake levels), and their absence may indicate changes in water chemistry and lake level. Sulfate reduction during burial resulted in the conversion of gypsum and iron oxides to calcite and pyrite. Maximum estimates for the duration of the lake range from 132 323 to 76 340 yr and are based on rhythmites in CCM-2. Results from this study support the hypothesis of saline and isotopically heavy waters evolving within the Creede Formation prior to ore deposition.

The Sinya Beds of the Amboseli Basin in Tanzania and Kenya consist largely of carbonate rocks and Mg-rich clays that are intensely deformed where exposed in and near former meerschaum mines. The carbonate rocks consist of limestone and dolomite in Tanzania, but only dolomite has been identified in Kenya. Sepiolite and mixed-layered kerolite/stevensite (Ke/St) are subordinate constituents of the carbonate rocks. The carbonate rocks and overlying bedded sepiolite were deposited in a semiarid lake basin at the foot of the large volcano Kilimanjaro. Calcite and dolomite of the carbonate rocks have ~tsO values 4-6%0 lower than calcite and dolomite of the late Pleistocene Amboseli Clays, suggesting that the Sinya Beds were deposited in the middle or early Pleistocene under a different climatic regime when meteoric water had lower 6~sO values than at present. Mg-rich clay minerals form veins and fill cavities in the Sinya Beds. The principal clay minerals are sepiolite and Ke/St, some of which contains substantial AI and Fe (A1-Ke/S0. NEWMOD | modeling and other X-ray diffraction (XRD) data suggest that most of the Ke/St contains 25-50 percent kerolite layers, but minor amounts ofkerolite-rich Ke/St are present in some samples. Illite with an inferred high content of Fe or Mg is a minor constituent of the samples with A1-Ke/St. The cavity-filling clays were chemically precipitated, as shown by field relationships and SEM study. The early-deposited clays of veins and cavities are principally Ke/St with minor sepiolite, and the latest clay is sepiolite (meerschaum), generally with minor Ke/St. The ~tsO values of cavity-filling Ke/St range from 22.5-25.6%0 and correlate with mineral composition, with the highest values associated with the highest content of stevensite and the lowest values with the highest content of kerolite. This relation suggests that high salinities favored stevensite and low salinities favored kerolite. ~sO values of sepiolite (meerschaum) fall in the middle of the range for Ke/St, suggesting that salinity was not the main control on sepiolite precipitation. High values of amo2/a~2§ may have been a major factor in sepiolite precipitation. Different mixtures of dilute ground water and saline, alkaline lake water in pore fluids may largely account for the differences in clay mineralogy of cavity-filling clays. Sepiolite is the dominant clay mineral in lacustrine sediments of the Amboseli Basin, and the cavity-filling sepiolite may reflect a high proportion of lake water. The low-A1 Ke/St may have formed from fluids with a higher proportion of ground water. Detrital clay was very likely a factor in forming the A1-Ke/St, for which ~mO values suggest a saline environment.

Core KM-3 at Searles Lake, California, comprises 693.4 m of lacustrine sediments deposited over the past 3.2 m.y. The lake water evolved from moderately saline, slightly alkaline, and dominated by Na, Ca, C1, SO4, and HCO3 + CO3 to a highly alkaline brine dominated by Na and CO3 ions. Sediments are chiefly muds and evaporites. Montmorillonite and illite are the principal detrital clay minerals supplied to Searles Lake at present and probably during the late Pliocene and Pleistocene. The drill core is divided into three diagenetic zones on the basis of clay-mineral reactions. The upper zone (0-291.1 m) contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, which average 60-70% of the

The mineralogy, chemistry, and mineral textures of tuffs and tuffaceous sediments from two drill cores, CCM-1 and CCM-2, were studied to better understand depositional and diagenetic conditions of the Creede Formation of Colorado. Similarities between phenocrystic crystal assemblages and relict shard shapes and sizes in correlated tuffs indicate minimal offset between CCM-1 and CCM-2. Vertically zoned sequences of authigenic silicates in tuffs reflect alteration of silicic-intermediate vitric tuffs within an elevated geothermal gradient (100-110 °C/km). The abundance of clinoptilolite and Al-smectite in drill core CCM-2 is consistent with diagenetic waters having low to moderate salinity and a pH of 7-9. The higher-grade mineral assemblage, including analcime, quartz, minor heulandite, and albite, in CCM-1 reflects hotter fluids migrating through more permeable strata. The presence of kaolinite and mixed-layer clays in nontuff lithologies in CCM-1 indicates intermittently low cation/H+ ratios and SiO 2 ( a q ) activities. Alteration occurred in a relatively open chemical system based on calculated gains and losses from silicic to intermediate end-member compositions of Fisher Quartz Latite compared to Creede Formation tuffs. On the basis of comparisons with studies of zeolite formation in other burial diagenetic systems, the observed authigenic silicate assemblage (opal-CT, quartz, clinoptilolite, and analcime) in tuffs of the Creede Formation probably formed within ∼3-4 m.y. of deposition. This assemblage was later overprinted by a hydrothermal event (K-feldspar, I/S, C/S, chlorite, and quartz) in CCM-1 at 17.5 Ma. Authigenic silicate assemblages in tuffs poorly constrain the salinity of diagenetic pore waters because they could form in waters ranging from fresh to moderately saline with a pH ≤ 9. The inferred low to moderate salinities for silicate alteration and relatively open chemical system suggest that salinities were progressively lowered from moderate to high during deposition of ancient Lake Creede to lower salinities during silicate reactions associated with later burial diagenesis and hydrothermal alteration.

A massive white sepiolite deposit at Amboseli precipitated from magnesium and silica released during the ground water dolomitization of an earlier lacustrine sepiolite. Kerolite has since formed in proximity to the massive sepiolite as an alteration product of sepiolite and as a ground water precipitate when the pH is below 8. Authigenic sepiolite also occurs in the overlying younger sediments. Kerolite is likely to occur but has not yet been positively identified. Presently, ground waters within the Amboseli Basin are supersaturated with respect to sepiolite and kerolite. This supersaturation results from the weathering of alkaline olivine basalts on the edge of the basin. The precipitation of sepiolite and/or kerolite does not control ground water compositions in the basin. These reactions are slow compared to other aqueous-mineral reactions such as those maintaining carbonate mineral equilibria. Equilibrium between disordered-dolomite and calcite buffers the log a Mg2+/(a H+)2 as a function of log f CO 2 in ground waters in the proximity of the massive sepiolite. This reaction can help explain the presence of sepiolite associated with dolomites in other near-surface deposits besides Amboseli.

Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ~7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ~9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and meriinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or meriinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/ or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

The island of St. Vincent consists of an active volcano and the remains of two or more that are extinct. Late Pleistocene subaerial ash deposits forming the uppermost geologic unit over much of the island are the object of the present study. Most of the deposits are between 20 and 40 feet thick, but they reach a maximum thickness of at least 120 feet. They consist of andesitic lithic, crystal, and vitric material or their weathered equivalents. Plagioclase ($An_{50-100}$), hypersthene, augite, and olivine are the most abundant minerals. The ash was erupted from the Soufriere. The pattern of grain size and thickness suggests that it was distributed by northwesterly Pleistocene winds quite different from those of the present. Four lithologic fades have been separated on the basis of sorting, grain size, and degree of weathering-namely, the Soufriereslope, eastern-coastal, western-coastal, and upland facies. Differences between them can be explained by primary distribution of the ash and subsequent removal o...

SUMMARY Monoclinic K-feldspar, analcime, searlesite and phillipsite of diagenetic origin are present in samples of clay and tuff from U. S. Geological Survey cores of Pleistocene and Recent sediments of Searles Lake, California. Authigenic K-feldspar was identified in 37 of the 72 samples studied; analcime was identified in 21, searlesite in 8, and phillipsite in 3. Most of the K-feldspar, analcime, and searlesite occur in non-tuffaceous claystones, which form about 46% of the 875-ft. thickness of sediments that was cored. Phillipsite was found only in three rhyolitic tuff laminae within the upper 110 ft. of beds. Most of the K-feldspar and analcime crystals are less than 2 μ in diameter, and maximum diameters are 0.10 mm for K-feldspar and 0.02 mm for analcime. Phillipsite occurs as lath-shaped crystals 0.01–0.02 mm long, and searlesite principally forms spherulites 0.02–0.2 mm in diameter. Microcline, orthoclase, albite, andesine, and quartz predominate in the sand and silt fraction of the clays; illite, montmorillonite, chlorite, and kaolinite predominate in the detrital clay fraction. Grains of quartz and andesine are extensively etched at various levels below a depth of 220 ft., and only skeletal crystals of andesine remain in some clays. Volcanic glass has been completely dissolved or replaced by authigenic silicate minerals in the few tuffs. Distribution of clay minerals in the core suggests that montmorillonite and chlorite have been destroyed in many of the clays. Alkaline brine saturates the surface and subsurface sediments, and the high activity ratio of alkali ions to hydrogen ions in this brine probably accounts for the widespread formation of K-feldspar, zeolites, and searlesite. Na, K, and B for the authigenic silicates were probably supplied by the brine and by saline minerals in contact with the brine. Montmorillonite may have provided additional K. Solution of glass provided the Si and Al required to form authigenic silicate minerals in the tuffs, and solution of quartz, plagioclase, and montmorillonite most likely supplied the Si and Al for authigenic silicates in the clays.

Tuff deposits of the Koko Crater group consist largely of alkali basalt glass, either fresh or palagonitized. Most of the deposits are progressively palagonitized at depth, and topographic relations of palagonite on Koko Crater indicate that the palagonite was formed after the cone had been deeply eroded. The principal authigenic minerals in the palagonite tuffs were deposited in following sequence: phillipsite, chabazite, analcime, montmorillonite together with opal, and calcite. The amount of authigenic minerals in a given sample is generally proportional to the amount of palagonite, indicating that the authigenic minerals are produced in palagonitization of glass. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a quarter of the SiO2, half of the Al2O3 and MgO, and three quarters or more of the CaO, Na2O, and K2O are lost in converting sideromelane to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby in zeolites, montmorillonite, opal, and calcite. Reaction of sideromelane with percolating ground water at low temperatures accounts for the vertical zoning from relatively fresh tuffs down into palagonite tuffs. The pH and ionic strength of percolating water probably increased with depth by solution and hydrolysis of glass, and where the pH and ionic strength became sufficiently high, the glass reacted to form palagonite and zeolites. Palagonite was formed by a microsolution-precipitation mechanism rather than by hydration or devitrification.

Alteration of primary igneous minerals was studied in sedimentary volcanic deposits of the middle Eocene Aycross formation, Pitchfork formation, and early basic breccia of the Absaroka Range, Wyoming. Olivine, hypersthene, augite, hornblende, and plagioclase are altered to varying extents in these beds. Chlorite, clay, barite, calcite, and chalcedony are present in pseudomorphs after the primary igneous minerals. The sequence of deposition, shown above from earliest to latest, is indicated by replacement relations in pseudomorphs and by successive coatings and replacements in amygdules. A groundmass of recrystallized clay minerals and authigenic chlorite characterizes the beds in which alteration is most complete. Zeolite and koninckite occur as authigenic minerals in some of the sedi entary deposits. Alteration of igneous minerals to clay was accomplished by weathering in a warm, humid climate after the sediment was deposited; later replacements by barite, calcite, and chalcedony took place after burial. Different degrees of alteration are explained by differences in rate of burial and depth of the water table. This alteration of primary igneous minerals greatly affects the reliability of stratigraphic correlations based upon mineral analyses, and may also lead to misidentification of the original composition of the volcanic material.

Sand-size spheroids of K-feldspar in the Cretaceous-Tertiary (C-T) boundary clay at Caravaca, southern Spain, were interpreted by Smit and Klaver as having solidified from a melt resulting from the impact of a large extraterrestrial body. Sand-size spheroids of K-feldspar, glauconite, and magnetite-quartz have been found in the C-T boundary clay in northern Italy, and spheroids of K-feldspar and pyrite were found in the boundary clay at Deep Sea Drilling Project Site 465A, in the central Pacific. These spheroids have textures similar to those of rapidly crystallized feldspar and mafic silicates. They are interpreted as diagenetically altered microcrystalline spherules of basaltic composition produced by the impact of a large asteroid in an ocean basin at the end of the Cretaceous. They are analogous to the glassy microtektites produced by impacts on more siliceous target rocks. 21 references, 4 figures.

Cambrian-Ordovician strata of the Mississippi Valley have been extensively modified by potassic diagenesis. Ordovician vitric tuffs are altered to K-feldspar and illite-rich, mixed-layer illite-smectite (I/S), both of which have been dated by K/Ar. Rb/Sr dates for K-feldspar support its reliability as a K/Ar clock, despite a dislocation density of 10/sup 9/-10/sup 10/ cm/sup -2/. Age data document three Paleozoic episodes of potassic diagenesis, which have mean ages of 396 Ma (Early Devonian) for K-feldspar of the upper Mississippi Valley (UMV), 362 Ma (Late Devonian) for I/S of the UMV, and 265 Ma (Permian) for I/S of Missouri. Devonian dates from the UMV correspond to times of wide-spread epeirogeny in the US midcontinent, and diagenetic episodes are attributed to regional flow of basinal brines caused by ground-water recharge on uplifted arches. Potassium for K-feldspar may have come from the Michigan basin, and K for illite may have come from the east-central Iowa basin. K-feldspar and I/S of the UMV are in oxygen-isotopic disequilibrium, indicating that the K-feldspar crystallized from fluid that was either warmer or had lower ..delta../sup 18/O values than that of the I/S. Permian illitization apparently records northward movement of warm saline fluid as a result of the Ouachitamore » orogeny.« less

Three basaltic tephra deposits of Kilauea were studied in order to determine the nature and amount of weathering as a function of age and climate. One of these deposits, the Pahala Ash, is a regional tephra blanket 10,000 to 17,000 yrs old that is weathering under present rainfall ranging from about 25 to 635 cm/yr. The Keanakakoi Formation and Uwekahuna Ash, which contain the other deposits investigated, are less than 10,000 yrs old, and are present only in the vicinity of Kilauea caldera, where rainfall is from 115 to 255 cm/yr. Basaltic glass in the Pahala Ash is extensively weathered, whereas that in the Keanakakoi Formation generally is little altered. The principal weathering products of glass are palagonite and clay. Montmorillonite and calcite are weathering products of the Pahala Ash in the drier areas; gibbsite has formed at rainier sites. Plagioclase, pyroxene, and olivine are weathered only in the Pahala Ash. Chemical changes on weathering of the tephra deposits vary as a function of climate. From comparative analysis of unaltered and altered solids, SiO2 and Na + K + Ca + Mg seem to be lost under heaviest rainfall in average proportions as high as 2:1; in the driest areas they have been leached in the ratio of about 1:3. In an area of intermediate rainfall, SiO2 and Na + K + Ca + Mg are lost in an average ratio of 3:2, as shown by chemical composition of both solids and interstitial waters. However, an unresolved discrepancy occurs between the rate of leaching, as calculated from solids and that calculated from solution data, when an age of 10,000 yrs is taken for the upper part of the Pahala Ash. Ion-activity products calculated from the solution analyses provide a framework for elucidation of the weathering processes and products.

Zeolites and dawsonite have formed rapidly during the past 20,000 years in the sides of Olduvai Gorge and over the adjacent Serengeti Plain of northeast Tanganyika. Nephelinite tuffs were altered to form a surface calcrete and the minerals phillipsite, natrolite, chabazite, analcime, and dawsonite; sodic trachyte tuffs were altered to form phillipsite, erionite, and chabazite. Zeolites and calcrete were also formed at the land surface in the Olduvai region during two dry episodes of the Pleistocene before the gorge was eroded. Zeolites and dawsonite were formed by reaction of volcanic glass and nepheline with solutions of sodium carbonate and bicarbonate that were concentrated by evaporation in the soil and surface layers of rock. Hydration and carbonation appear to be the principal chemical changes in zeolitic alteration.