Mineral Reactions in the Sedimentary Deposits of the Lake Magadi Region, Kenya

Mineral reactions in the sedimentary deposits of the Lake Magadi region, Kenya

RONALD C. SURD AM Department of Geology, University of Wyoming, Laramie, Wyoming 82071 HANS P. EUGSTER Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, Maryland 21218

ABSTRACT hand, the initial alkalic zeolite that will form in a specific alkaline lake environment cannot yet be predicted with confidence. Among The authigenic minerals, principally zeolites, in the Pleistocene to the many parameters controlling zeolite distribution, the compo- Holocene consolidated and unconsolidated sediments of the sitions of the volcanic glasses and of the alkaline solutions are cer- Magadi basin in the Eastern Rift Valley of Kenya have been inves- tainly of primary importance. tigated. Samples were available from outcrops as well as drill cores. Most zeolite studies are concerned with fossil alkaline lakes. The following reactions can be documented: (1) trachytic glass + While it is generally easy to establish the composition of the vol-

H20 —» erionite, (2) trachytic glass + Na-rich brine —* Na-Al-Si gel, canic glass, the brines responsible for the authigenic reactions have + + (3) erionite + Na analcime + K + Si02 + H20, (4) Na-Al-Si long since disappeared. To overcome this handicap, it is desirable gelanalcime + H20, (5) calcite + F-rich brine—> fluorite + C03 , to investigate an alkaline lake environment for which glass as well (6) calcite + Na-rich brine —» gaylussite, and (7) magadiite —• as brine compositions can be determined — in other words, a + quartz + Na + H20. Erionite is the most common zeolite present, modern alkaline lake environment. Such an environment is Lake but minor amounts of chabazite, clinoptilolite, mordenite, and Magadi of Kenya. The Magadi basin is unique because the compo- phillipsite were also recognized. Erionite can form directly from sition and evolution of its waters have been very carefully

trachytic glass by the addition of H20 only. It is characteristic of the documented (Eugster, 1970; Jones and others, 1976). Magadi basin because of the low content of alkaline earths in the This paper describes the authigenic mineral suite that is now volcanic glasses and in the solutions interacting with them. forming in Lake Magadi and that has formed in its Pleistocene pre- Analcime is common in outcrops of the High Magadi and cursors and interprets the reactions responsible for these as- Oloronga Beds. It forms from erionite by a reaction probably ini- semblages in terms of glass chemistry and brine chemistry. We tiated by a lowering of the silica activity, which results from the hope that this approach will result in a better understanding of the transformation of magadiite to chert. Analcime in the drill-core interdependence between authigenic silicates and aqueous samples grew at the expense of a Na-Al-Si gel. This gel forms at the geochemistry. lake shore and is washed into the lake during flooding conditions. Fluorite is common in the core samples and can be explained by STRATIGRAPHY AND LITHOLOGIES reaction of the fluoride-rich brines with calcium in the sediments, principally detrital calcite. Authigenic albite and potassium The Pleistocene and Holocene history of the Magadi basin sedi- feldspar were not recognized, probably for reasons of reaction ments was described in detail by Baker (1958, 1963) and Eugster kinetics. (1969). Lake Magadi is located in the lowest part of the Eastern The presence of authigenic minerals can be accounted for by Rift Valley in Kenya, just north of the Tanzania border. It is mainly considering the chemical compositions of the starting materials, a trona-precipitating saline lake, which is fed by alkaline hot mainly volcanic glasses, and the brines they come in contact with. springs issuing at the perimeter of the basin. The sediments ac- Lake Magadi represents a unique opportunity for studies of cumulating now belong to the Evaporite Series (see Table 1), and diagenesis because authigenic minerals are forming there at the present time and because the evolution of its waters is well known. TABLE 1. STRAT1GRAPHIC UNITS, TECTONIC EVENTS, AND ABSOLUTE AGES OF INTRODUCTION FORMATIONS FOUND IN THE MAGADI BASIN

Zeolites are among the most common authigenic silicates of Units Age Lake sedimentary rocks (Hay, 1966). Among the most detailed studies (yr) are those of zeolites from saline alkaline lake environments (see Evaporite Series Sheppard and Gude, 1968, 1969a, 1973; Hay, 1970; Surdam and (slight tilting) 0 Magadi Parker, 1972). These zeolites generally formed by reaction of volcanic glass with saline alkaline solutions. The alkalic zeolites that High Magadi Beds (minor faulting?) High Magadi result from such reactions are commonly phillipsite, erionite, 9,100 clinoptilolite, and, to a lesser extent, mordenite and chabazite. Trachyte flows (minor) These zeolites may subsequently be transformed to analcime (as in caliche 780,000 the Big Sandy, Barstow, and Green River Formations) or to potas- Oloronga Beds sium feldspar (as in Pleistocene Lake Tecopa). Analcime may also (extensive grid faulting) >780,000 Oloronga react to form potassium feldspar (as in the Big Sandy, Barstow, and Plateau trachyte flows -1,000,000 Green River Formations). (extensive) to 1,700,000 The paragenetic sequence of volcanic glass —» alkalic zeolite —* Note: Data from Eugster (in prep.). analcime —» potassium feldspar is well established. On the other

Geological Society of America Bulletin, v. 87, p. 1739-1752, 11 figs., December 1976, Doc. no. 61208.

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the area of the active lake covers about 60 km2. There is evidence High Magadi and Oloronga Beds are discussed separately. The drill for the existence of two larger precursor lakes in the basin. The cores penetrated a very much thicker section of lake sediments than shoreline of the lake from which the High Magadi Beds were de- is available in the outcrops at the margin of the basin. Exact corre- posited is still visible, and the High Magadi Beds have been dated lation of the stratigraphic units from outcrop to drill core is as 9,100 yr. old (Butzer and others, 1972). The older lake is repre- difficult. Therefore, the drill-core mineralogy is tabulated sepa- sented by the Oloronga Beds, which have been dated by K-A meas- rately from the outcrop mineralogy. A tentative correlation will be urements on a glassy trachyte flow as at least 780,000 yr. old discussed later. (Fairhead and others, 1972; Eugster, in prep.). Below the Oloronga Beds are the extensive plateau trachyte flows that cover the Rift MINERALOGY AND CHEMISTRY OF Valley floor and are as old as 1.7 m.y. (Baker, 1958; Baker and HIGH MAGADI AND OLORONGA BEDS others, 1971). Outcrops of High Magadi and Oloronga Beds occur abundantly High Magadi Beds, Outcrop Samples in the alluvial flats and at the foot of the hills surrounding Lake Magadi (for sample locations, see Fig. 1). The detailed distribution The mineralogy of the High Magadi Beds is characterized by the of the two sets of lake beds is rather complex, because there is a following groups of minerals: (1) detrital silicates, (2) saline miner- considerable region of overlap. However, good Oloronga outcrops als, (3) calcite, (4) sodium silicates and quartz, and (5) authigenic are most common southwest, northwest, and northeast of Lake zeolites. The most abundant detrital silicate is anorthoclase, which Magadi, whereas the High Magadi Beds can best be studied east occurs as fragments of single crystals. These are generally fresh or and southeast of the lake. A detailed account of the stratigraphy, slightly altered to clay minerals and (or) calcite. These fragments lithologies, and sedimentary history will be published elsewhere are identical in mineralogy with the phenocrysts in the trachytic (Eugster, in prep.), and a brief summary will suffice here. lavas and were derived either from pyroclastic ash falls or directly The sedimentary rocks that constitute the High Magadi and from the lavas. Similar anorthoclase fragments are abundant in the Oloronga Beds are very similar; however, the Oloronga Beds are modern muds. Another common detrital mineral is amphibole. The commonly more indurated, and they are capped by a thick caliche amphibole also occurs as crystal fragments, many of which are al- (Kunkar) limestone (50 to 75 cm thick). This caliche is an important stratigraphic marker because it occurs throughout the basin, whereas the post-Oloronga lava flows are restricted to the northwest area (loc. 665 and 696). The sedimentary rocks are prin- LITTLE cipally bedded chert and tuffaceous material. The High Magadi MAGADI cherts have clearly been derived from magadiite horizons (Eugster, 1969), and the Oloronga cherts probably had a similar origin. The tuffaceous material varies greatly. Some tuffs must have formed from pyroclastic debris associated with individual volcanic events. Figure 1. Index and sample-locality map Such tuffs may contain pumice fragments as much as 1 cm in of Lake Magadi region, Kenya, East Africa. diameter. Others also contain detrital material and range from coarse, cross-bedded sandstone to finely laminated mudstone. However, even the clearly detrital components are of volcanic deri- vation. In fact, the sediments of the lower sequence of the High Magadi Beds (Eugster, 1969) may consist principally of eroded Oloronga material. There are no perennial rivers flowing into the Magadi Basin at present, and this condition probably also applied to High Magadi and, perhaps, even Oloronga time. Hence, the sediments in the basin were either derived from within the basin or were carried in by wind. Since the sediments are all underlain by thick sequences of trachytic flows, most of the sediments, except for the cherts, calcite, and organic fractions, are obviously of volcanic origin. EXPLANATION SAMPLING AND SAMPLE LOCATIONS OLORONGA BEDS Samples were obtained from outcrops, shallow pits, and drill HIGH MAGADI BEDS cores. The High Magadi Beds were examined at 27 locations (see MODERN MUDS Fig. 1), represented by a total of 179 samples. Most of the samples were collected from the High Magadi tuff (tuffaceous silt of the LAVA SAMPLES Upper Sequence of the High Magadi Beds of Eugster, 1969). The à BRINE SAMPLES Oloronga Beds were examined at 20 localities (see Fig. 1). A total O DRILL CORES of 142 samples were collected at these locations. In addition, core samples from four drill holes located in the lake itself were available (see Fig. 1). These are F6 (437-ft. depth), J10 (140-ft. depth), G16 (55-ft. depth), and H4 (141-ft. depth). Eighty-six core samples were examined in this study. Modern muds from Lakes Magadi and Natron were also examined. All bulk samples were examined by x-ray diffraction. Selected samples were examined by microprobe, scanning electron microscope, and bulk chemical analyses, as well as x-ray diffraction of mineral separates. Thin sections were cut from samples containing typical mineral assemblages. SCALE The authigenic mineral assemblages from outcrop samples of the + 0 I 2 3 4 5 km

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tered. The source of the amphibole is similar to that of the anor- mary calcite rhombs are common in the magadiite layers. thoclase. Illite is an accessory mineral in many of the samples. Some Magadiite and quartz are ubiquitous in the samples; they are the illite is probably of detrital origin in conjunction with feldspar al- dominant minerals in the magadiite interval (Eugster, 1969). How- teration, but some may have formed by authigenic reactions. ever, magadiite and quartz are also found disseminated throughout The High Magadi Beds do not contain primary, bedded saline the section. minerals. However, the pore waters of these sediments are com- The authigenic zeolites are erionite, analcime, and chabazite. monly saline alkaline brines. Efflorescent crusts formed from these Erionite is found in nearly every sample and commonly is the pre- brines consist of trona, thermonatrite, and halite. All outcrops of dominant authigenic silicate. It occurs as felted masses and bundles the High Magadi Beds are coated with such crusts. of needles (Fig. 2). In some samples the erionite masses clearly re- Calcite is present in most of the samples examined. Most of it place glass shards. Analcime is also very common but is less abun- occurs as aggregates of anhedral crystals replacing the silicate ma- dant than erionite. The analcime occurs as irregular patches replac- trix and as veinlets. This calcite is clearly a late product and proba- ing erionite and as spherulites and euhedral single crystals (Fig. 3). bly was derived from the evaporation of dilute ground waters. Pri- Analcime is always associated with quartz or with quartz plus magadiite. Chabazite occurs as euhedral rhombs. Textural evidence indicates that it formed later than the erionite. The mineral assemblages of the authigenic silicates in the High Magadi Beds are summarized in Table 2. No unaltered glass shards, clinoptilolite, or fluorite were found in the outcrops of the High Magadi Beds. In the alluvial flats adjacent to Lake Magadi, a thin veneer of modern unconsolidated sedimentary deposits covers the High Magadi Beds. The thickness of this veneer varies from a few cen- timetres to 1 m. It consists of detrital and pyroclastic material that is generally slightly coarser grained than the typical High Magadi Beds. The latest pyroclastic addition was from the Oldoinyo Lengai volcanic eruption of 1966. The modern muds have not been studied in detail, but the detrital mineralogy of these sediments is very similar to that of the High Magadi Beds. The authigenic mineralogy, however, is markedly different in several respects, because these sedimentary deposits contain abundant unaltered glass shards, gaylussite, and erionite but no analcime (Fig. 4). They also contain gelatinous material of the type described by Eugster and Jones (1968). In zeolite separates there are traces of other zeolites such as clinoptilolite and chabazite. Calcite occurs in small amounts. The origin of the tuffaceous material in the High Magadi Beds and modern muds can best be elucidated by comparing the compo- Figure 2. SEM micrograph of sample of Oloronga Beds (633G). Note felted mass of erionite needles that average about 5 to 10 /¿m in length. sition of the Pleistocene trachytic lavas with that of the tuffaceous Erionite of High Magadi Beds is identical in appearance. TABLE 2. AUTHIGENIC SILICATE ASSEMBLAGES OF HIGH MAGADI BEDS

Mineral assemblage No. of occurrences

Erionite + analcime + quartz 78 Erionite + analcime + quartz + magadiite 48 Erionite + quartz + magadiite 24 Erionite + analcime + chabazite + quartz 11 Magadiite + quartz 8 Erionite + quartz 4 Analcime + quartz 3 Erionite + magadiite 2 Analcime + chabazite + quartz 1

Figure 4. Photomi- crograph of sample of modern mud (644) from Lake Magadi. Needlelike crystals are erionite; they are 10 to 15 ¡xm long. Darker equant grains are partially altered glass fragments. Unpolarized Figure 3. Photomicrograph of irregular patches of analcime (dark) that light. have replaced erionite. Note relict erionite needles (approximately 10 to 15 /urn in length). Unpolarized light. Sample 609D, from High Magadi Beds.

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rocks. Table 3 gives bulk chemical analyses of four pre-Oloronga the erionite with average trachyte based on a Barth standard cell of (14, 103A, 103D, 331) and two post- Oloronga lavas (698, 696). 160 oxygens. Assuming conservation of aluminum, erionite can Sample 225 is from a glassy selvage of a lava flow. Table 3 also obviously form from trachyte or trachytic glass without addition of gives six bulk analyses of erionite-rich tuffaceous rocks (four from any material except water. the High Magadi Beds [613C, 614C, 615C, 639D] and two from the Oloronga Beds [633A, 633G]) as well as six bulk analyses from Oloronga Beds, Outcrop Samples analcime-rich rocks, all of which come from the High Magadi Beds. The detrital and saline mineralogy and calcite occurrences in the The analyses of the trachytes in this study show the same compo- Oloronga Beds are similar to those of the High Magadi Beds. How- sitional range as those reported by previous workers (Baker, 1958). ever, there is a greater variety of authigenic silicates. In addition, The tuffaceous rocks are so similar in composition to the trachytes authigenic fluorite is present in some outcrops of the Oloronga that there is little doubt that the tuffaceous material was derived Beds. from the same volcanic source. There are some systematic dif- Erionite again is the most common authigenic silicate; it occurs ferences associated with the transition from trachytic material to in almost every sample (Table 5), typically as felted masses and zeolitized tuffs. The most obvious difference is the decrease in total bundles of needles. Analcime is also common and again is always iron. The average total Fe in the trachytes is 8.4 wt percent, associated with quartz. It is probably less abundant in the whereas the average total Fe in the tuffs is 6.4 wt percent. There is Oloronga Beds than in the High Magadi Beds. The analcime occurs little or no change in amounts of Si02, A1203, MgO, or CaO; how- as masses and spherules replacing alkalic zeolites. Chabazite also is ever, amounts of Na20 and K20 are slightly lower in the tuffaceous a common authigenic silicate and is much more abundant in the rocks. The average Na20 and K20 in the trachytes is 6.3 and 5.4 wt Oloronga Beds than in the High Magadi Beds. Typically it occurs percent, respectively, whereas the average Na20 and KaO in the in the tuffaceous rocks as rhombs. Another authigenic silicate that tuffs is 5.5 and 3.8 wt percent, respectively. is present in many samples is clinoptilolite (Fig. 5). Clinoptilolite In the transition from erionite-rich tuffaceous rocks to and chabazite are mutually exclusive. Phillipsite was found at one analcime-rich tuffaceous rocks, an obvious but slight decrease in locality (667); it typically occurs as spherules and prismatic crys-

K20 occurs. The erionite-rich rocks average 3.92 wt percent K20, tals. Mordenite was found at one locality (671) also. Quartz is but the analcime-rich rocks average 3.72 wt percent K20. ubiquitous in the Oloronga Beds, but magadiite is much less com- Table 4 presents a direct comparison of the composition of mon than in the High Magadi Beds. Fluorite, with the exception of trachytic glass with that of erionite from the same sample of a few minor scattered occurrences, is restricted to locality 633, but modern mud (644). These compositions are also compared with at that locality it is very abundant. Essentially unaltered glass was that of the average trachyte of Table 3. The composition of the found at one locality (1042) in Oloronga Beds. The glass occurs as erionite single crystals is very similar to those of the average pumice fragments (1 cm in diameter) in channel deposits at the ex- trachyte and tuffaceous rocks. Table 4 also shows a comparison of treme northeastern edge of the basin.

TABLE 3. CHEMICAL ANALYSIS OF LAVA FLOWS AND TUFFACEOUS ROCKS FROM LAKE MAGADI REGION

Si02 AI2O3 Fe203* MgO CaO NazO K20 MnO Total

Trachytes 14 64.4 14.7 6.7 0.5 1.1 6.2 5.7 0.3 99.6 103A 62.4 13.7 9.0 0.6 1.2 6.2 5.9 0.3 99.3 103D 61.8 13.5 9.7 0.6 1.8 6.0 5.1 0.3 98.8 331 62.3 13.7 8.7 0.4 1.2 6.3 5.4 0.2 98.2 698 63.8 13.4 7.4 0.5 1.4 6.2 5.4 0.2 98.3 696 62.5 14.2 8.4 0.6 1.0 6.6 5.4 0.3 99.0 225 61.8 12.8 9.0 0.4 1.2 6.5 4.6 0.3 96.6

Avg. 62.71 13.71 8.41 0.51 1.27 6.29 5.36 0.27 98.53

Iriontte-ricb tuffs 613C 62.0 13.2 6.6 0.5 1.0 5.5 4.3 93.1 614C 60.4 13.0 6.6 0.6 1.2 5.8 3.1 90.7 615C 61.2 13.1 6.4 0.5 1.3 5.8 3.8 92.1 639D 63.6 13.6 6.7 0.6 0.8 5.9 4.0 95.2 633A 66.8 11.9 4.4 0.5 0.3 5.2 4.0 93.1 633G 60.4 12.2 5.4 1.2 1.4 4.9 4.3 89.8

Avg. 62.40 12.83 6.01 0.65 1.00 5.52 3.92 92.33

1nalcime-rich tuffs 610C 61.2 13.1 7.1 0.6 1.0 5.9 3.5 92.4 632B 62.0 12.9 6.6 0.6 0.8 5.6 3.8 92.3 632C 62.4 12.8 6.7 0.6 0.9 5.6 3.8 92.8 638D 63.2 13.1 6.2 0.6 0.8 5.8 3.5 93.2 66 ID 63.2 13.2 7.0 0.6 0.8 5.1 3.9 93.8 660 64.0 13.2 6.9 0.6 0.6 5.5 3.8 94.6

Avg. 62.67 13.05 6.75 0.60 0.82 5.58 3.72 93.19 Note: Analyst: J. Murphy. Method: HF digestion and atomic absorption analysis (see Bernas, 1968). * Total iron.

Core Samples — Evaporite Series, High Magadi Beds, and chabazite, and analcime are much more restricted in occurrence. Oloronga Beds Clinoptilolite is usually associated with fluorite. In order to compare the mineralogy of the cores with that of the Mineralogical data for the four cores F6, J10, G16, and H4 are outcrops, it is necessary to identify the stratigraphic boundaries in presented in Table 6. The suite of detrital minerals is the same as the four cores. These assignments can be made only in a tentative that described from the surface outcrops, except for traces of manner (see Table 7), partly because of the very large differences in pyroxene. However, illite abundance is markedly decreased in the thickness and partly because the core samples are soaked in brine core samples. Among the saline minerals, trona is the most abun- and have a different appearance from the outcrop samples. In the dant phase. Halite and thermonatrite formed from interstitial F6 core, the top of the Oloronga Beds was located at 268 ft, be- brines during sample preparation. Some of the nahcolite occur- cause calcite becomes a very common mineral below this depth. rences listed could be primary. Calcite is essentially absent in the This decision was based on the fact that in outcrop the Oloronga upper part of the cores but is present in the lower parts. Calcite and Beds are always capped by a thick caliche horizon (see Table 1). fluorite seem to be mutually exclusive. The boundary between the Evaporite Series and the High Magadi Magadiite occurs scattered throughout the cores, whereas maka- Beds was located from the drill-core descriptions. The top of the tite (see also Sheppard and others, 1970) is restricted to the upper High Magadi Beds is characterized by a dense black plastic "clay" parts. Quartz is ubiquitous in the cores. Two very broad peaks unit, which can be correlated with the High Magadi Tuff (Eugster, were observed in the diffraction patterns of many core samples. 1969). These peaks are located at approximately 5.6 A and 4.0 A. The former is assigned to a hydrous sodium aluminosilicate gel and the Comparison of Core and Outcrop Data latter to amorphous silica. The aluminosilicate gel is presumably analogous to the gel described by Eugster and Jones (1968) from It should be pointed out that a number of differences exist be- the Magadi region, whereas the amorphous silica could have been tween the core and outcrop samples. They are that (1) calcite is derived from windblown grasses or diatoms (for a list of diatoms ubiquitous in the outcrops of the High Magadi Beds but is gener- found in the High Magadi Beds, see Eugster and Chou, 1973, ally absent in the High Magadi interval of the drill cores, (2) illite is p. 1147). The gel is shown in Figures 6 and 7. very much less abundant in the core samples for both the High Erionite and fluorite are present in many samples and are by far Magadi and Oloronga intervals, (3) the two amorphous phases the most common authigenic minerals in the cores. Clinoptilolite, have been encountered in the core only (except for modern muds), (4) makatite is essentially restricted to the Evaporite Series in the drill cores and has not been found in outcrop, (5) for the High TABLE 4. CHEMICAL ANALYSES OF TRACHYTIC GLASS Magadi Beds fluorite has been encountered only in the core sam- AND ERIONITE FROM A MODERN MUD (SAMPLE 644) ples, (6) analcime is equally abundant in the outcrop and core sam- OF LAKE MAGADI ples of the Oloronga Beds, whereas for the High Magadi Beds it is much more prevalent in the outcrop samples, and (7) except for Chemical analyses Barth standard cell one definite occurrence in the cores, chabazite is restricted to the Oxide Glass * Avg. Erionite"' Element Trachyte Erionite§ outcrop samples. (wt %) trachytef CHEMISTRY OF MODERN MAGADI WATERS Si02 59.2 62.7 60.0 Si 58.7 51.7 AI2O3 n.d. 13.7 14.8 AL 15.0 15.0 Before we attempt to interpret the authigenic silicate occurrences Fe203 n.d. 8.4 n.d. Fe 5.9 n.d. MGO <0.5 0.5 0.1 Mg 0.7 0.1 discussed in the preceding section, we must briefly summarize the CaO <1.0 1.3 0.6 Ca 1.3 0.6 compositional data of the Magadi basin waters and brines. Al- NazO 6.1 6.3 6.5 Na 11.5 10.9 though all of the waters analyzed are, of course, from modern sam- K2O 5.0 5.4 4.9 K 6.4 5.4 MnO n.d. 0.3 n.d. Mn 0.2 n.d. er oxides 98.6 er oxygen 160 Note: n.d. = not determined. * Microprobe analysis. | From Table 3. § Calculated relative to A1 content of trachyte Barth standard cell.

TABLE 5. AUTHIGENIC SILICATE ASSEMBLAGES OF THE OLORONGA BEDS

Mineral assemblage No. of occurrences

Erionite-quartz 43 Analcime-chabazite-erionite-quartz 26 Quartz 16 Erionite-clinoptilolite-quartz 15 Analcime-erionite-quartz 11 Phillipsite-erionite-quartz 9 Erionite-chabazite-quartz 7 Erionite-clinoptilolite-magadiite-quartz 5 Erionite-mordenite-quartz 3 Erionite-magadiite-quartz 3 Phillipsite-quartz 2 Figure 5. Photomicrograph of prismatic clinoptilolite crystals (20 ¿im Clinoptilolite-quartz 1 wide) growing from matrix of felted erionite: Unpolarized light. Sample Mordenite-magadiite-quartz 1 635B from Oloronga Beds.

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TABLE 6. MINERAL ASSEMBLAGES OF CORE SAMPLES

Ap Px An I H Nh Th Tr Cc F K Mg Mk Q s AS G Ch CI E F6 11 '7" XX X X XX tr. 20'6" X XX X XX tr. tr. 30'4" X XX tr. XX tr. tr. tr. 35' X XX tr. X tr. tr. tr. 40' XX tr. XX tr. tr. tr. tr. tr. tr. 44'6" X X XX XX X tr. tr. 49'9" XX X X XX tr. tr. tr. 57'6" X XX tr. tr. tr. X X tr. 59'9" XX tr. tr. tr. XX tr. tr. 69'6" XX tr. XX tr. tr. X tr. 79'5" tr. XX XX tr. tr. X X X tr. X 80' tr. XX XX tr. tr. X 85'6" X XX tr. tr. XX tr. tr. tr. 89'8" tr. X XX tr. X X XX X tr. 94'6" X XX X X XX tr. 99'7" tr. XX X tr. tr. XX 109' X XX XX X 110' tr. XX tr. X XX X 113'9" tr. XX tr. X XX tr. X X 120' tr. XX tr. X XX X tr. 131'4" tr. X tr. X XX XX tr. 135' tr. X XX tr. X XX 140' tr. XX X XX 144' X X XX XX X 150' tr. XX X X XX tr. 161' tr. X tr. X XX XX X tr. 166'10" tr. tr. XX tr. XX tr. 170' tr. XX tr. tr. XX 186' tr. XX XX tr. 188' XX XX X X X tr. tr. 197' tr. XX XX tr. X X X X tr. 199' XX X X X XX tr. tr. 212' tr. XX X XX X X X X 225' X XX tr. tr. XX 237' tr. XX X tr. XX 245' tr. X X tr. XX XX 246'6" tr. XX X XX X 249' tr. X X X XX XX X 252' XX X tr. tr. XX 264' tr. XX X XX 268'6" XX XX 269'6" XX X X X X XX 286'6" XX XX X X 292' tr. XX tr. XX 300' tr. XX X X X XX X X 317' tr. XX X X XX tr. tr. 326' tr. XX tr. X XX tr. 342' tr. X tr. tr. tr. tr. X XX X 350' XX tr. X tr. X XX 355' tr. X X XX XX pies, there is good reason to believe that the water chemistry has The evolution of brines such as those listed in Table 8 has been not changed significantly since Oloronga time. The principal evi- discussed by Jones (1966), Garrels and Mackenzie (1967), Hardie dence can be found in the abundance of magadiite-derived cherts and Eugster (1970), and Eugster (1970). Typical dilute bicarbonate that occur at all levels of the Magadi basin deposits (Eugster, in waters that eventually lead to alkaline brines by evaporative con-

prep.). Baker (1958) published data from a detailed report on centration are formed by the weathering of feldspars by C02- Magadi hydrochemistry by J. A. Stevens (unpub., 1932). More re- charged ground waters. Such waters can form from a variety of cent data were presented by Eugster (1970). This work has been igneous and metamorphic rocks (see also Eugster and Hardie, updated further by Jones and others (1976). 1975). An example (M546) is shown in Table 8, representing the Table 8 gives the compositions of three typical hot springs and Ewaso Ngiro, a perched perennial stream that flows along the two lake brines. The hot springs are essentially sodium-bicarbonate western margin of the Eastern Rift Valley and eventually into Lake waters, but the lake brines are sodium-carbonate waters. Chloride Natron. This water, collected during an unusually high stage of is the next most important constituent, but even the lake brines do flow (July 1970), is representative of the dilute waters that enter the not normally reach saturation relative to halite. Calcium and mag- Eastern Rift Valley ground-water system from the adjacent nesium are virtually absent in the hot springs and completely ab- mountains. During evaporative concentration and subsequent un- sent in the lake brines, as would be expected for such alkaline derground flow, calcium and magnesium are precipitated as carbo- waters. Silica and fluoride values are unusually high, but this can nates. Most of the potassium and Si02 are also removed, the readily be acounted for (Jones and others, 1967; Eugster, 1970). former presumably by adsorption on clay minerals and the latter by

TABLE 6. (Continued)

Ap Px An I H Nh Hi Tr Cc K Mg Mk Q AS Ch CI Am

360' X tr. X tr. XX XX X X 395' tr. XX tr. tr. X XX tr. X 410' tr. XX tr. tr. XX tr. X 434' tr. XX tr. X X XX X X tr. X 437' XX

J10 38' tr. XX XX X X 48' tr. tr. XX tr. X XX tr. 50' tr. tr. XX tr. X X X tr. tr. 54'9" X XX X X XX tr. 63' X XX X XX X X 72'11" XX tr. tr. XX 73' XX tr. XX 74'8" tr. X tr. XX XX X tr. 84-4» XX tr. tr. XX 91' X XX X X X XX 95' XX X tr. tr. X XX X tr. 99' tr. tr. XX tr. XX tr. 104' X XX tr. tr. XX

G16 25'ii" tr. XX XX X tr. X tr. 39' X XX XX X tr. tr. tr. tr. 47'7" tr. tr. XX tr. tr. X XX X X 54'8" tr. XX X XX X X tr.

H4 11' X tr. XX tr. XX X 33' tr. tr. XX tr. XX tr. tr. tr. 50' XX XX X X X tr. tr. X 54' X tr. XX XX X tr. X tr. tr. 63' XX X XX tr. X tr. X tr. 70' X X XX XX tr. tr. tr. tr. 80' X XX XX X tr. X tr. 90' X tr. X XX XX X X tr. tr. 99' XX XX tr. tr. tr. 106'11" tr. XX tr. X XX 112' X X XX tr. X tr. tr. XX tr. 120' X X X 128' X XX tr. XX X X tr. 141' X XX XX X Note: XX = abundant, X = minor, tr. = trace. Ap = amphibole Tr = trona S = silica (cristobalite) Px = pyroxene Cc = calcite AS — amorphous silica An = anorthoclase F = fluorite G = Na-Al-Si gel I = illite K = kenyaite Ch = chabazite H = halite Mg = magadiite CI = clinoptilolite Nh = nahcolite Mk = makatite E = erionite Th = thermonatrite Q = quartz Am = analcime precipitation of silicates or silica cement. Meanwhile, values for waters still contain some calcium, and there is also evidence that sodium, chlorine, and fluorine increase continuously. locally derived surface waters may contain some alkaline earths The beginning of this trend is represented by M1020, a (Eugster, in prep.). This could have been derived either from rain- ground-water sample from the Magadi basin, locality 667, taken water dissolving the Oloronga caliche cap or from weathering of from a well dug into a large alluvial channel northeast of Little trachytic material. Such surface waters are now very local and Magadi. This well is used by Masai for watering their cattle. ephemeral, but they may have been more widespread during the M1020 is the most dilute ground-water inflow into the Magadi time of deposition of the High Magadi and, particularly, Oloronga basin encountered, and it still contains considerable amounts of Ca Beds. available for the formation of secondary calcite. Continued evaporative concentration and mineral precipitation follow the FORMATION OF AUTHIGENIC ZEOLITES trend described above and lead to the hot springs compositions and FROM TRACHYTIC GLASS eventually the lake brines. Detailed chemical balances for the Magadi systems have been presented by Eugster (1970) and Jones It is not surprising that the tuffaceous Quaternary sediments at and others (1976). Lake Magadi contain abundant zeolites, for many workers study- In summary, the hot springs and lake brines in the Magadi basin ing the authigenesis of tuffaceous rocks have documented the reac- are very rich in sodium, bicarbonate-carbonate, silica, and fluoride, tion glass —» zeolite in saline alkaline lakes (Deffeyes, 1959; Hay, but they virtually lack alkaline earths. The most dilute ground- 1964, 1970; Sheppard and Gude, 1968, 1969a, 1973). The combi-

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nation of highly reactive volcanic ash and saline alkaline solutions diagram emphasizes the important parameters silica, alkali, and al- in sedimentary environments invariably results in the formation of kaline earth activities. Based on the selected analyses, the compo- zeolites (Hays, 1966). sitional ranges of the three zeolites are clearly separated, with phil- The Magadi region has been characterized by volcanism lipsite occupying the region of low silica and low alkaline earth, throughout Quaternary time; as recently as 1966, Oldoinyo Lengai erionite occupying the region of intermediate silica and very low contributed tuffaceous material to the sediments of Lake Magadi. alkaline earth, and clinoptilolite occupying the region of high silica In addition, saline alkaline lakes existed in the Magadi basin at and high alkaline earth. least as far back as 780,000 yrs ago (Eugster and Chou, 1973). Using the techniques of Korzhinskii (1959), an activity diagram Thus, in the Lake Magadi region, throughout Quaternary time, the can be constructed from the mole fraction plot (Fig. 8, insert). necessary conditions have been present for the formation of zeo- Judging from the relations shown in Figure 8, erionite can be ex- lites. Further evidence supporting the glass —> zeolite reaction in the pected to form in an environment characterized by high silica, high Lake Magadi region is that in the modern tuffaceous muds and in sodium, and extremely low calcium activities. This is exactly com- the Pleistocene High Magadi Beds there is a reciprocal relation be- patible with the observations summarized above and hence ac- tween glass and zeolite. counts for the predominance of erionite in the Magadi basin. The detailed reaction mechanism for the formation of erionite Erionite now becomes more apparent. As Table 4 illustrates, erionite can form from trachytic glass by the addition of water only. This water Erionite is the most prevalent alteration product of glass in the cannot be dilute surface or ground waters, because such waters Lake Magadi region. This distinguishes the Lake Magadi region would necessarily contain calcium obtained either from the weath- from other well-studied zeolite deposits such as the tuffaceous ering of feldspars or the dissolution of caliche. On the other hand, rocks of the Lake Tecopa, Barstow, and Wikieup areas (Sheppard the alkaline Lake Magadi brines are free of calcium and are an ideal and Gude, 1968, 1969a, 1973) and poses the basic question; Why agent for the hydration and solution of trachytic glass. is erionite so much more abundant at Lake Magadi? To answer this question, consider the following observations: (1) The trachytic Phillipsite glass, which is the starting material, is rich in alkalis and poor in alkaline earths. (2) There is an abundance of silica and silicate Phillipsite is restricted to the Oloronga Beds exposed at the Wells phases, such as magadiite and hydrous sodium aluminosilicate gels, in the Ndopa Hills (loc. 667) north of Lake Magadi (see Fig. 1).

indicating high activities of Si02 and Na20. (3) The inflow waters The rocks containing phillipsite are cross-bedded tuffaceous into the Magadi basin and the brines in the basin are rich in sodium sandstone and siltstone with a few thin laminated lake beds and and silica and are largely devoid of alkaline earths. These observa- tuffs that are characterized by mud cracks. This section of the tions suggest that erionite is the preferred initial zeolite in an envi- Oloronga Beds is unlike the rest of the Oloronga and High Magadi ronment rich in sodium and silica and poor in alkaline earths, espe- cially calcium. This conclusion can be checked further by considering the compositional range of the other common alkalic zeolites, such as phillipsite and clinoptilolite. Thirteen acceptable published analyses of erionite, phillipsite, and clinoptilolite are presented in Table 9. The sole criteria for choosing these 13 analyses and reject-

ing others was the A1203 + Fe203 to MgO + CaO + NazO + K20 balance. If the balance was not within 5 percent, the analyses were rejected. The compositional relationship between phillipsite, erion-

ite, and clinoptilolite can best be illustrated by a diagram of Si02/ (A1,03 + Fe2Oa) versus (CaO + Mg0)/(Na,0 + K20) (Fig. 8). This

Figure 7. Photomicrograph of gelatinous material under high magnifica- tion. Width of field in micrograph is approximately 50 fj.m. Unpolarized light.

TABLE 7. TENTATIVE STRATIGRAPHIC ASSIGNMENTS IN DRILL CORES

Depth (ft) F6 Jio G16 H4

Evaporite Series 0-60 0-34 0-25 0-91 Figure 6. Photomicrograph of gelatinous material from drill core (F6 — High Magadi Beds 60-268 34-104+ 25-35 91-141+ 161 ft). Width of field in micrograph is approximately 300 ¿im. Note so- Oloronga Beds 35-55+ called ropey texture of gel. Unpolarized light. 268-437+

Beds in that it contains a preponderance of fluviatile sandstones transpiration, resulting in a very saline and alkaline soil. It is in this and siltstones. This highly variable assemblage of sedimentary de- environment that the tuffaceous components of the soil are altered posits is probably a mixture of alluvial and lake-margin deposits. to phillipsite. The ground water characterizing these soils, although Hay (1964) described the formation of phillipsite in the saline at times very alkaline (pH = 9.5 to 10.6), differs from the Magadi soils of Olduvai Gorge. As he pointed out, sodium carbonate and Lake brines in one very important respect — it is not saturated with bicarbonate have been concentrated at the land surface by evapo- amorphous silica. Unlike a closed-basin lake, these soils are an

TABLE 8. CHEMICAL COMPOSITIONS OF GROUND WATERS, HOT SPRINGS, AND BRINES, LAKE MAGADI AREA, IN MILLIGRAMS PER KILOGRAM

M546* M1020f M48§ M62§ M14§ M87b# M91#

Li n.d. n.d. 1.0 1.0 1.2 n.d. n.d. Na 7.0 310 12,800 11,100 12,600 111,000 128,000 K 2.3 28 199 199 239 1,470 n.d. Ca 6.5 24 0.0 0.0 1.6 n.d. n.d. Mg 3.7 3.7 0.0 0.0 0.0 n.d. n.d. Al n.d. n.d. 0.79 0.56 0.71 n.d. n.d. Si02 20 64 92 90 91 802 1,150 HCOa 48 615 15,800 12,400 15,600 3,250 n.d. co3 0.0 0.0 3,220 3,710 3,540 93,700 94,400 so4 2.4 9.0 155 176 147 1,570 2,635 F 0.2 3.1 156 144 162 1,350 1,530 Cl 4.0 178 6,020 5,240 5,950 58,300 85,200 Br n.d. 0.67 115 158 70 216 314 PO4 0.03 1.7 9.2 10 11 51 88 B n.d. 0.13 8.8 8.3 8.8 74 102 Total dissolved solids 77 920 30,500 26,600 30,400 267,000 321,000 Temp. (°C) 16 24 70.5 82.5 81 26 39 pH 7.0 8.00 8.98 9.13 9.5 10.52 10.4 Note: n.d. = not determined. 4 From Ewaso Ngiro, a perennial river near western scarp of Eastern Rift Valley, collected 9/11/70. t Ground-water sample from locality 1020 (see Fig. 1) collected 6/5/73 (data from Jones and others, 1976). § Magadi hot springs (from Eugster and Jones, 1968). # Magadi lake brines (from Eugster, 1970).

TABLE 9. CHEMICAL COMPOSITION OF PHILLIPSITE, ERIONITE, AND CLINOPTILOLITE

1 2 3 4 5 Wt % Mol Wt % Mol Wt % Mol Wt % Mol Wt % Mol

Phillipsites * Si02 54.16 0.9015 56.34 0.9377 58.95 0.9812 59.69 0.9935 A1203 17.00 0.1660 15.22 0.1493 15.14 0.1485 14.70 0.1442 Fe203 1.35 0.0085 1.28 0.0080 0.92 0.0058 0.67 0.0042 MgO 0.58 0.0142 0.09 0.0022 0.62 0.0154 0.47 0.0116 CaO 0.70 0.0125 1.80 0.0321 1.37 0.0240 0.84 0.0150 Na20 4.92 0.0794 5.34 0.0861 3.72 0.0620 4.19 0.0676 K2O 6.01 0.0638 3.53 0.0375 5.18 0.0550 5.25 0.0557

Erionite f Si02 61.40 1.0220 57.24 0.9527 60.39 1.0052 59.51 0.9903 AI2O3 13.26 0.1301 13.93 0.1366 13.32 0.1307 14.20 0.1393 Fe203 1.48 0.0093 1.95 0.0122 1.31 0.0082 0.73 0.0046 MgO 0.23 0.0057 0.15 0.0037 0.49 0.0122 0.14 0.0035 CaO 0.57 0.0102 0.00 0.0000 1.30 0.0232 0.01 0.0002 Na20 4.21 0.0679 6.24 0.1007 3.48 0.0561 5.92 0.0955 K2O 5.42 0.0575 4.10 0.0435 4.33 0.0460 3.64 0.0386

Clinoptilolite§ Si02 66.40 1.1050 65.30 1.0867 65.21 1.0852 64.70 1.0763 63.69 1.0599 AI2O3 11.64 0.1141 11.47 0.1125 12.61 0.1236 12.43 0.1219 12.47 0.1223 Fe203 0.47 0.0029 1.36 0.0085 0.40 0.0025 0.44 0.0028 0.62 0.0039 MgO 0.86 0.0213 1.22 0.0303 1.09 0.0270 0.34 0.0084 1.42 0.0352 CaO 2.55 0.0455 1.80 0.0321 1.84 0.0328 1.26 0.0223 2.25 0.0401 Na20 1.85 0.0298 1.76 0.0284 1.76 0.0284 4.32 0.0697 2.46 0.0397 K2O 1.93 0.0205 2.49 0.0264 3.79 0.0402 2.28 0.0242 1.80 0.0191 * Samples 1 to 4 from Hay (1964). t Sample 1 from Mariner (1971); samples 2 to 4 from Sheppard and Gude (1969b). § Samples 1 to 3 from Mariner (1971); samples 4 and 5 from Sheppard and Gude (1969a).

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open system, and the efflorescences of sodium carbonate and published two analyses of chabazites. These chabazites have Si/Al bicarbonate, and with them the silica, are flushed out and away ratios similar to the Magadi erionite, but the cations are grossly dif- every wet season. As a result, the formation of phillipsite from vol- ferent; in particular, the calcium content is considerably higher in canic glass in these alluvial deposits is virtually identical to the the chabazite. The presence of chabazite in the sediments of the process described by Mariner and Surdam (1970). Thus, the phil- Lake Magadi region probably can be explained in one of two ways: lipsite in the Oloronga Beds is the consequence of alkaline solutions (1) It is formed during authigenesis as a minor phase as a result of low in alkaline earths, and undersaturated with respect to amor- the release of the small amounts of Ca++ in the glass. (2) It could be phous silica, reacting with volcanic glass and resulting in the for- formed as a result of the influx of fresh water, as would be the case mation of an authigenic phase with a relatively low Si/Al ratio (see at a delta of a surface tributary to the lake or in outcrops under the Mariner and Surdam, 1970). influence of dilute ground waters. These processes may not have The relatively unaltered glass at locality 1042 (see Fig. 1) is even been important during the deposition of the High Magadi Beds, but farther removed from the lake deposits and the influence of alkaline during the deposition of the Oloronga Beds the lake was larger and ground waters and lake brines. This glass has been preserved in al- more dilute, and there may have been more fresh surface water en- luvial channels located considerably above the lake level and tering it. ground-water table and below the soil profile. Clinoptilolite and Mordenite Chabazite Clinoptilolite and mordenite are the most siliceous zeolites in the Chabazite is nowhere the most abundant zeolite in either the Lake Magadi basin. Clinoptilolite occurs in some outcrops of the High Magadi or Oloronga Beds, but it does occur in small amounts Oloronga Beds as well as in the High Magadi interval of the drill in a few samples of the High Magadi Beds, and it is even more cores, whereas mordenite is restricted to a single outcrop of the abundant in the Oloronga Beds. It is less abundant in the core. Oloronga Beds. According to Figure 8, the formation of clinoptilo- Chabazite is generally associated with erionite and locally with lite requires very high activities of silica and alkalis, combined with analcime. There is some textural evidence observed in thin sections a slightly higher activity of alkaline earths than that necessary for that suggests that chabazite forms after erionite. The compositional erionite growth. The same holds true for the formation of data for chabazites formed in saline alkaline lakes is very scanty. mordenite. The necessary conditions for the growth of clinoptilo- Gude and Sheppard (1966) and Sheppard and Gude (1973) have lite and mordenite are very difficult to reach in a highly alkaline environment, because any Ca and Mg added should immediately precipitate as carbonate. Some petrographic evidence (see Fig. 5) suggests that clinoptilolite and mordenite formed later than erionite, indicating that the alkaline earths may have been supplied by residual glass and other reaction products. The alkaline earths also may have been provided by percolating dilute ground waters. However, we do not fully understand the mechanisms by which clinoptilolite and mordenite have formed in the Magadi basin.

FORMATION OF ANALCIME FROM A ZEOLITE PRECURSOR

Analcime is a very common constituent of saline alkaline lake deposits (see, for example, Hay, 1966; Sheppard and Gude, 1969a; Hay, 1970; Surdam and Parker, 1972; among others.). Data gathered from tuffaceous sediments strongly support the conclusion that analcime does not form directly from the alteration of glass, for the association of analcime with glass has not been reported. Analcime is, however, commonly associated with a variety of alkalic zeolites. Sheppard and Gude (1969a) described petrographic evidence showing both clinoptilolite and phillipsite altering to analcime. The analcime in the tuffaceous Magadi sediments is not associated with glass; it is either associated with erionite or is in a reciprocal relationship with erionite (Tables 2, 5, 6). We suggest that analcime is the reaction product of a zeolite precursor, generally erionite, as illustrated by the following reaction:

+ Na0.5K0 5AlSi3.5O9 • 3H20 + 0.5Na Magadi erionite

->NaAlSi2Os • H20 + 0.5K+ + 1.5Si02 + 2HaO . analcime Si 0;

AI20j • Fez 03 Sheppard and Gude (1969a), in studying the diagenesis of the tuffs in the Miocene Barstow Formation, suggested a correlation Figure 8. Alkalic zeolites from Table 9 plotted on mole fraction diagram. Circles are phillipsite (Ph), triangles are erionite (E), and crosses are between the composition of the zeolite precursor and the Si/Al ratio clinoptilolite (CI). In erionite field, triangle with point down is typical com- of analcime. They observed that analcime associated with and deposition of erionite from High Magadi Beds (see Table 4). Insert shows rived from clinoptilolite has a relatively high number of silicon schematic activity diagram constructed from mole fraction plot. atoms per unit cell (34.5 to 35.1), whereas analcime associated

with and derived from phillipsite (a silica-deficient zeolite) has a low released is not flushed from the system during the reaction. There number of silicon atoms per unit cell (33.1 to 34.4). Boles (1972) are at least two explanations for this observation: (1) As noted studied the reaction clinoptilolite or heulandite —» analcime ex- above, illite is much more common in the outcrops than in the core perimentally and has shown that the Si/Al ratio of the analcime samples, suggesting that at least some of the illite may be of au- product is largely a function of the Si/Al ratio of the zeolite reac- togenic origin. (2) On the other hand, tuffs also contain organic tant. From both field observation and laboratory experiments, material, probably as colloids, which can readily take up potas- there is strong support for the reaction zeolite —> analcime; the sium. analcime inherits its Si/Al ratio from a zeolite precursor. The method outlined by Saha (1961) was used to determine the FORMATION OF ANALCIME FROM compositions of analcime from the High Magadi and Oloronga GEL PRECURSOR Beds. Six samples of analcime associated with erionite are compared to one sample of analcime associated with phillipsite (from In addition to forming from a zeolite precursor, analcime also the High Natron Beds, which are probably equivalent to the High can form from a hydrous sodium aluminosilicate gel, as suggested Magadi Beds.) Results of the present study are shown in Figure 9, by Eugster and Jones (1968). Their data on the chemical composi- along with the data of Sheppard and Gude (1969a). The silica con- tion of some typical Lake Magadi gels are given in Table 10. Recent tent of analcime associated with erionite is significantly higher than field work has demonstrated that these gels are more widespread the silica content of analcime associated with phillipsite, as would than had been previously suggested and that they occur throughout be expected if the analcime inherited its Si/Al ratio from a zeolite the Magadi basin. They are found at or near the orifices of most of precursor such as erionite. These data strongly support the sugges- the hot springs, as well as at the shores of Lake Magadi and Little tion that some of the analcime in the tuffaceous rocks in the Lake Magadi, where alkaline brines are in contact with trachytic debris. Magadi region is the result of the reaction of a zeolite precursor. At the north end of Little Magadi, gel accumulations have been en- Analcime is common in outcrops of the High Magadi Beds but is countered that are as much as 30 cm thick. The reaction responsi- scarce in the High Magadi interval of the drill cores. The transition ble for these gels has been elucidated by Eugster and Jones (1968). erionite to analcime involves the addition of sodium and the loss of Trachytic material reacts with alkaline solutions according to + silica and potassium. Lowering the activity of silica and increasing trachyte + Na + H20 —> Na-Al-Si gel + Si02, assuming that the activity of sodium is also associated with the reaction of aluminum is conserved during the reaction. Observations of the gel magadiite to quartz (Bricker, 1969). In fact, there is evidence that accumulations north of Little Magadi during four field seasons the transition erionite to analcime is coupled to the magadiite to suggest that the gelatinous material is washed into the lake yearly, chert conversion: samples containing analcime as a major phase particularly during the rainy seasons. Therefore, it is not surprising rarely contain even a trace of magadiite. Two mechanisms have that similar gels are commonly present in the drill core samples been suggested for the magadiite to chert conversion. O'Neil and (Figs. 6, 7). In these samples the gels are generally associated with Hay (1973) used isotopic evidence to show that quartz may crystal- amorphous silica. lize spontaneously from magadiite in the presence of sodium-rich On the other hand, there seems to be an antithetical relation be- brines. Eugster (1969) observed the in situ conversion of magadiite tween the gel and analcime. In samples of core F6, an evolution of to chert in outcrops and shallow pits and attributed the reaction to analcime from gel can be demonstrated (see Fig. 10), clearly indi- the removal of sodium by more dilute ground water. Because of the cating that analcime grows at the expense of the gel. This may ac- prevalence of analcime in the outcrops, we believe that the lower- count for the fact that analcime is more abundant in the deeper ing of silica activity associated with the magadiite to chert transi- parts of the core; however, the analcime distribution is not strictly tion, and hence the erionite to analcime transition, is probably ac- controlled by depth. Some erionite may also have formed from gels complished by percolation of ground waters through the outcrops. in the core samples; however, we have no direct evidence for or The erionite to analcime reaction releases 0.5 mol of K+ for every against such a reaction. 1 mol of analcime formed. As the data of Table 3 show, there is in Crystallization of Magadi gels under hydrothermal (350°C and fact a slight difference in potassium content between erionite-rich 2 kb pressure) conditions yielded analcime (Eugster and Jones, and analcime-rich tuffs, but it is obvious that most of the potassium 1968). In order to study the crystallization products at lower tem-

TABLE 10. CHEMICAL COMPOSITIONS OF Figure 9. Plot showing ANHYDROUS GELS FROM THE LAKE MAGADI AREA correlation between silicon content of analcime and precursor zeolite. Compo- Sample no. sition of analcime deter- M47H* M48J M48H M62H M14J ANALCIME ASSOCIATED WITH : mined from x-ray diffrac- Si02 53.75 40.14 39.21 46.73 51.50 tometer data by measure- O ERIONITE, LAKE MAGADI ment of displacement of Ti02 1.03 1.29 0.81 0.75 1.26 PHILLIPSITE, LAKE NATRON AI O 1 (639) peak of analcime. A 2 3 11.30 18.00 21.22 19.03 10.65 Data from Barstow Forma- P2O3 J PHILLIPSITE, BARSTOW FM. Fe 0 1 tion is after Sheppard and 2 3 9.61 11.50 6.38 6.72 9.76 FeO J Gude (1969a). CLINOPTILOLITE, BARSTOW FM. MgO 1.62 2.92 1.40 1.23 2.66 CaO 1.96 4.47 4.75 3.07 4.20 OO Na20 15.67 18.24 22.54 18.50 13.98 A ooo o K2O 5.06 3.44 3.69 3.96 5.99 H2O- 90-93 52-63 52-63 64-72 72-77 • •• •• Loss of ignition • A • AAA« to 900°C —,—I—I—I—I—I—I—I—I—I—- (purified samples) 61.63 45.94 2.34 2.28 n.d. 33.0 34.0 34.5 Note: Data from Eugster and Jones (1968); n.d. = not determined. Si ATOMS PER UNIT CELL * Localities are listed in Figure 1 as 14, 47, 48, and 62. (96 oxygen)

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peratures and pressures, purified Magadi gels were placed in suggested by Sheppard and Gude (1969c), by reaction of Na2C03 solutions of varying pH and heated at 1-atm pressure at villiaumite. This accounts for the paucity of fluorite in the outcrops 60°C for five weeks; these experiments yielded magadiite + gel and its abundance in the core samples. (Table 11). Similar experiments were run with gels in Na2C03 so- Gaylussite is abundant at certain horizons in many of the lutions heated to 80°C for two weeks, and they yielded magadiite + modern mud samples. It forms by the interaction of alkaline brines erionite. In the latter experiments, erionite and magadiite formed in with calcium. The calcium source is either detrital calcite derived all solutions that originally had a pH of 9.0 or greater. When from the High Magadi and Oloronga Beds or dilute percolating K2C03 solutions were substituted for the Na-rich solutions with all ground water or surface runoff. The reaction is of the type calcite +

other conditions remaining constant, the gels still yielded erionite Na2C03 + H20 —» gaylussite. + magadiite. No silicate phases formed in Ca-rich solutions. DETRITAL VERSUS AUTHIGENIC FELDSPARS AUTHIGENIC FORMATION OF FLUORITE AND GAYLUSSITE The most common detrital mineral in the Magadi sedimentary rocks is anorthoclase (Table 6). It was suspected at first that the Fluorite was reported in a Magadi drill core (Sheppard and primary anorthoclase might alter in the highly concentrated others, 1970). As mentioned above, fluorite is absent in outcrops of sodium-carbonate brines of Lake Magadi to albite or potassium the High Magadi Beds, present in some outcrops of the Oloronga feldspar. Neither the petrographic nor x-ray diffraction studies re- Beds, and ubiquitously present in the core samples (see Table 6). vealed the presence of authigenic albite or potassium feldspar. The Also, there is a marked antithetical relation between calcite and anorthoclase is surprisingly fresh, and in the few samples where its fluorite in the drill core samples. This relation suggests a reaction of edges are altered, the reaction products are calcite and white mica. the type The absence of albitization of detrital feldspars in the Magadi sedimentary rocks is not surprising, for the youngest albite found in CaC03 + 2F~ -> CaF2 + C03=. calcite fluorite altered tuffaceous material in alkaline lake environments is Eocene in age (Green River Formation; see Iijama and Hay, 1968; Surdam Calcium necessary for the fluorite reaction can be derived from and Parker, 1972). The absence of albitization in younger tuffa- other sources, such as detrital minerals or gaylussite, whereas ceous rocks must be due either to a kinetic effect or to a lack of the fluorine is always supplied by the brine. Magadi brines are ex- slightly higher temperatures required for the conversion of anal- ceptionally high in fluorine, containing as much as 1,600 ppm cime to albite (Campbell and Fyfe, 1965). fluoride (see Eugster, 1970). The inflow waters into the Eastern Rift More surprising is the lack of authigenic potassium feldspar in Valley system contain fluorine concentrations that are normal for the Magadi sedimentary rocks. Potassium feldspar is a common igneous and metamorphic waters. Because bicarbonate is the dom- alteration product in the Green River Formation as well as in many inant anion, evaporative concentration leads to precipitation of Pleistocene and older saline alkaline lake deposits (Hay, 1966; carbonate minerals, removing calcium before saturation relative to Sheppard and Gude, 1968, 1969a; Hay, 1970). In these deposits, fluorite can occur. Fluorine remains in the brine and continues to the reactions zeolite —* K-feldspar and analcime —K-feldspar have be concentrated, until finally villiaumite (NaF) precipitates. both been documented (Sheppard and Gude, 1969a; Iijama and Villiaumite has been found only in the trona deposits at the surface Hay, 1968). Some insight into this problem can be gained by com- (see Baker, 1958, p. 45). Fluorite forms either where fluoride-rich paring the solution compositions in equilibrium with analcime and brines come in contact with calcium-bearing minerals or, as potassium feldspar with the compositions of the Magadi Lake brines. Figure 11 shows the stabilities of analcime and potassium feldspar relative to the activities of Na+, K+, and H+ (at 25°C and 1-atm pressure) as well as the activities of those lake brines for F6 161' which field pH and potassium analyses are available. The concen- TABLE 11. CRYSTALLIZATION OF PURIFIED MAGADI GELS AT 1-ATM PRESSURE

Original pH pH at completion Cation solution8. Resultsf

Figure 10. Diffrac- Experiments at 60°C for 5 weeks F6 360' tometer patterns over 7.63 7.48 Plain gel None interval 12° to 19° 29 8.45 9.16 Na Magadiite + gel (CuKa radiation). Se- 9.00 9.34 Na Magadiite + gel lected samples from drill 9.52 9.34 Na Magadiite + gel core F6 (Table 6). 9.99 9.81 Na Magadiite + gel Upper sample (161 ft) is 10.49 10.36 Na Magadiite + gel gel rich, whereas lower 10.13 7.19 Ca None sample (410 ft) is gel 8.49 8.67 K Magadiite + gel F6 342' free. Intermediate sam- 9.51 9.40 K Magadiite + gel ples contain gelatinous 10.04 9.77 K Magadiite + gel material reacting to analcime in varying pro- Experiments at 80°C for 2 weeks portions. 8.51 9.03 Na Magadiite + gel 9.02 9.15 Na Magadiite + erionite 9.53 9.61 Na Magadiite + erionite 10.45 10.20 Na Magadiite + erionite J\ F6 410' 9.02 9.13 K Magadiite + erionite 10.02 10.10 K Magadiite + erionite Note: Sample collected near locality 14 (see Fig. 1). * Na2C03, K2C03, and Ca(OH)2 solutions were added, • I....I....I. I f For chemical composition of gels, see Table 10. 12

tration values were recalculated approximately to activities by tion can be accomplished by more dilute percolating waters and

using activity coefficients of the NaCl-KCl-H20 system. Activities for (or) by the spontaneous crystallization of quartz. the lake brines lie very close to the analcime—potassium feldspar Some of the analcime present in the core samples can be ac- boundary, indicating that authigenic potassium feldspar could counted for by direct crystallization from a Na-Al-Si gel. This gel is form either by an addition of potassium or (perhaps) by a rise in encountered throughout the core; it now forms at the lake shore temperature. The analyses of the analcime-rich tuffs listed in Table and is washed into the lake during flooding conditions. Fluorite

3 show that abundant K20 is stored in these rocks. Therefore, we and gaylussite are also common authigenic minerals in the Magadi assume that the release of the K20 necessary for the formation of sedimentary rocks; the former occurs mainly in the core samples potassium feldspar is governed by kinetic factors. Much of the K20 and the latter in the modern muds. The presence of fluorite is not is contained in the unaltered anorthoclase and will not be released surprising, because the lake brines are exceptionally rich in fluorite. until the feldspars are altered. Both minerals form where local sources of calcium are available. Authigenic albite and potassium feldspars are absent at Magadi. SUMMARY AND CONCLUSIONS Albite is not to be expected because it has not been recognized in tuffaceous rocks younger • than Eocene. Authigenic potassium Authigenic minerals encountered in the sedimentary rocks of the feldspar, on the other hand, forms readily in saline, alkaline lake Magadi basin consist principally of zeolites, mainly erionite and environments. Magadi brines plot close to the analcime —» analcime, with minor amounts of chabazite, clinoptilolite, K-feldspar transition. The potassium necessary to accomplish the mordenite, and phillipsite. Except for analcime, the zeolites form transition is present in the analcime-rich tuffaceous rocks, but it is by reaction of trachytic glass with solutions. Erionite is the princi- stored principally in unaltered detrital feldspar fragments. Au- pal product, because it can form from trachytic glass simply by the thigenic potassium feldspar probably cannot form until these

addition of H20. It is characteristic of environments with high ac- feldspars react and release their potassium. tivities of sodium and silica combined with exceptionally low activ- There exist some systematic differences between the suite of au- ities of the alkaline earths. Clinoptilolite forms whenever some cal- thigenic minerals present in outcrops and those in core samples: cium and magnesium are available, either from within the sedi- calcite, illite, analcime, and chabazite are common in outcrops and ments or from percolating ground waters, while chabazite is to be much less abundant in the cores, whereas fluorite and the Na-Al-Si expected when the activities of alkaline earths are still higher and gels are essentially restricted to the core samples. These differences the silica activity is somewhat lower. Phillipsite is associated with can be accounted for by differences in the hydrologic conditions alluvial delta and lake-margin deposits and is characteristic of envi- within the lake and at the lake shore. Within the lake, the sediments ronments with low silica activity, such as soil horizons. Calcium is are in contact only with concentrated sodium-carbonate brines rich available either from secondary calcite in the outcrops of the High in fluorine and devoid of alkaline earths, whereas the outcrops, be- Magadi and Oloronga Beds, including the caliche horizons, or cause they are situated at the margins of the basin, are exposed to a from dilute ground waters and runoff. much wider range of solution compositions. These include fairly di- Analcime is common in the outcrop samples, and it is also lute waters derived from percolating ground waters and surface present in the core. In the outcrops it is clearly a reaction product runoff, as well as concentrated brines formed by dissolution of after erionite. Analcime is always associated with quartz, and we efflorescent crust or local evaporative concentration. assume that it forms when the silica activity is lowered in conjunc- No lateral variation in the distribution of authigenic minerals has tion with the magadiite —* chert transformation. This transforma- been observed in the High Magadi Beds, but the Oloronga Beds show an incipient variation from erionite —» phillipsite —» unaltered glass. However, the mineral zones are very restricted and much less developed than in most other saline alkaline lake deposits, because both precursor lakes were situated in a narrow fault trough, as is the present Lake Magadi. The wide playa flats that are necessary to produce the lateral zonation cannot develop under these conditions. Both the High Magadi and Oloronga lakes contained alkaline brines. This is evident from the abundant bedded cherts found in the deposits of these lakes. Nevertheless, it is probable that Lake Oloronga was generally larger and somewhat less saline than the High Magadi lake. Lake Oloronga could at times have merged with Lake Natron to the south to produce a very large body of water. The authigenic minerals found in the Magadi basin sedimentary rocks can be understood and interpreted successfully by considering the following aspects: (1) composition of the starting materials (trachytic glass, tuffaceous rocks, carbonates), (2) composition of the solutions interacting with them (runoff, ground water, hot springs, lake brines), (3) hydrologic conditions, and (4) kinetic factors. Because most of these aspects are well documented, Lake Magadi and its precursors have provided us a unique opportunity to study authigenic minerals in a saline alkaline lake environment as well as the processes responsible for their formation.

14 16 is 20 22 ACKNOWLEDGMENTS 2 2 Log (a K'/a H ) This work was supported in part by the donors of the Petroleum Figure 11. Stability of analcime and potassium feldspar in system Research Fund (Surdam, PRF #6678-AC2), administered by the

K20-Na20-Al203- Si02-H20 at 25°C and 1-atm total pressure (after Gar- American Chemical Society, and the National Science Foundation rels and Mackenzie, 1971). It is assumed that quartz and water are ubiquit- (Eugster, GA-1507). We thank the Magadi Soda Company for ous. Plot of Magadi brines is from Eugster (1970). permission to work at Magadi and for their generous support.

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R. C. Herrick did much of the x-ray diffraction work, and we Hardie, L. A., and Eugster, H. P., 1970, The evolution of closed-basin thank him for his careful efforts. Recrystallization experiments of brines, in Morgan, B. A., ed., Mineralogy and geochemistry of non- the gels were done by Jennie Ridgley. We are grateful to A. J. marine evaporites: Mineralog. Soc. America Spec, paper 3, p. 273- Gude, 3d, R. L. Hay, and R. A. Sheppard for very careful reviews 290. Hay, R. L., 1964, Phillipsite of saline lakes and soils: Am. Mineralogist, v. of an earlier version of this paper. 49, p. 1366-1387. 1966, Zeolites and zeolite reactions in sedimentary rocks: Geol. Soc. America Spec. Paper 85, 130 p. REFERENCES CITED 1970, Silicate reactions in three lithofacies of semi-arid basin, Olduvai Gorge, Tanzania, in Morgan, B. A., ed., Mineralogy and geochemistry Baker, B. H., 1958, Geology of the Magadi area: Geol. Survey Kenya Rept. of non-marine evaporites: Mineralog. Soc. America Spec. 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