Holocene carbonatite-nephelinite tephra deposits of Oldoinyo Lengai, Tanzania

Journal of Volcanology and Geothermal Research, 37 (1989) 77-91

77

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

HOLOCENE CARBONATITE-NEPHELINITE TEPHRA DEPOSITS OF OLDOINYO LENGAI, TANZANIA RICHARD L. HAY Department of Geology, University of IUinois, 1301 West Green Street, Urbana, Ill. 61801, U.S.A. (Received February 11, 1988; revised and accepted August 9, 1988 )

Abstract Hay, R.L., 1989. Holocene carbonatite-nephelinite tephra deposits of Oldoinyo Lengai, Tanzania. J. Volcanol. Geotherm. Res., 37: 77-91. Natrocarbonatite and nephelinite tephra have been erupted together from Oldoinyo Lengai over the past few thousand years. The oldest deposits with recognizable natrocarbonatite are thin tuff beds probably between 2000 and 5000 years in age. Tuffs and agglomerates with an age of about 1250-2000 years contain evidence strongly suggestive of natrocarbonatite ash. The volcano is mantled by neOhelinite-carbonatite ash deposits, termed the Footprint Tuff, that were erupted about 600 years ago. Evidence is contradictory as to whether natrocarbonatite was discharged during the cone-building phase of eruptions, which ended about 15,000 years ago. Nephelinite-carbonatite ash erupted in 1966 contains an unidentified mineral, designated NCS, with a chemical composition of Na4.ogCa2.76Si5014.81.It may be genetically related to natrocarbonatite magma as it has thus far been identified only in the younger tephra deposits of Oldoinyo Lengai, most of which contain evidence of natrocarbonatite. A tephra deposit termed the Footprint Tuff contains footprints thought to be preserved by the rapid recrystallization of primary natrocarbonatite. Calcite of natrocarbonatite origin forms an estimated 15-20% of the airfall tuffs, and natrocarbonatite probably equalled or exceeded the volume of nephelinite tephra at the time of eruption. Nyerereite ( [Nao.82Ko.ls] 2 Ca [ C03 ] 2) and gregoryite ( [ Nao.TsKo.o5] [ Cao.17C03 ] ) were primary minerals in the natrocarbonatite, as in modern lavas of Oldoinyo Lengai. Unlike modern lavas, the groundmass contained a substantial amount of silicate material. Noncarbonate minerals in the Footprint Tuff include nepheline, melilite, augite, wollastonite, melanite, fluorite, and NCS. The Footprint Tuff was cemented soon after deposition, very likely by trona. Gaylussite, pirssonite, or both, were probably later alteration products in the transformation of natrocarbonatite ash to form calcite. Oxygen and carbon isotopic re-equilibration were essentially decoupled in alteration of natrocarbonatite ash. J~sO values of this calcite suggest extensive to complete oxygen exchange, but J13C values are in and near the range for unaltered natrocarbonatite. Carbon exchange may have been retarded in ash alteration by the high pH of pore fluid.

Introduction Oldoinyo Lengai in Tanzania (Fig. 1 ) is the only volcano known to have erupted carbonatite tephra and lava in historic time. Eruptions have been recorded at intervals since 1880 (Dawson, 1962a). Most of these produced ash deposits with sublimates of soda salts, and na0377-0273/89/$03.50

trocarbonatite lava was erupted from 1960 until 1966, when nephelinite-carbonatite ash was discharged in highly explosive eruptions (Dawson et al., 1968). Eruptions in 1983-1987 produced minor tephra and numerous natrocarbonatite flows on the floor of the crater (C. Nyamweru, 1988, pers. commun., 1988). The present paper describes premodern

© 1989 Elsevier Science Publishers B.V.

78 I/ ~ ~I" -/

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eruptive natrocarbonatite, both modern and prehistoric. Samples were studied microscopically, and selected fractions were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), and by a JEOL 50-A electron microprobe. Examination of thin sections with reflected light proved especially useful in identifying relict igneous textures in natrocarbonatite clasts replaced by finely crystalline calcite, which is white in reflected light. Carbon- and oxygen-isotopic measurements were made to determine the degree of isotopic re-equilibration with atmospheric CO2 and meteoric H20 in transformation of natrocarbonatite to calcite.

i ~. \

s ",.-'IX."',. t

~\

i"--~,,. 2°45`

.._

00

~l \

~'/_/"

Loolmurwak "\ \1

0

~

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50'

Fig. 1. Map of Oldoinyo Lengai after Guest et al. (1961). Locality A is where large natrocarbonatite lava clasts were found in stream-laid deposits.

tephra deposits that document the eruption of natrocarbonatite magma over the past few thousand years. The principal natrocarbonatite-bearing deposit was brought to my attention by visitors to Laetoli, 90 km southwest of Oldoinyo Lengai. They commented on similarity of the Laetoli Footprint Tuff, of Pliocene age (Hay and Leakey, 1982 ), to footprint-bearing ash layers on Oldoinyo Lengai. The Laetoli Footprint Tuff originally contained natrocarbonatite ash (Hay, 1978), and the Footprint-bearing ash deposits of Oldoinyo Lengai, here termed the Footprint Tuff, were visited and sampled in 1979 for evidence of natrocarbonatite and its possible bearing on the making and preservation of footprints. Field studies in 1982 and 1983 were extended to older tephra deposits of Oldoinyo Lengai. The Footprint Tuff was studied mainly on the north and east slopes of the volcano and adjacent terrain. Samples from the Footprint Tuff were studied in the laboratory in order to determine the various forms of altered natrocarbonatite, the original nature of the natrocarbonatite, and the mode (s) of its alteration. Study was also made of the noncarbonate materials associated with

Stratigraphy and age The stratigraphy given here (Table 1) is based on Dawson (1962a) supplemented by the present study. The oldest exposed deposits of Oldoinyo Lengai are yellow tuffs and agglomerates, abbreviated YTA, with a small amount of lava. These form the bulk of the volcano, over which the younger deposits form a veneer. The YTA contains nephelinite and phonolite ejecta. It is widely overlain by black tuffs and agglomerates, or BTA, of nephelinite composition that are generally 1-3 m thick on the east and northeast slopes and at least 15 m thick on the west and northwest flanks. The BTA commonly overlies a deeply eroded surface of the YTA. Low on the northeast slopes the BTA is locally separated from the YTA by 1-2 m of clayey weathered rubble with two airfall tuffs, each 1-2 cm thick, that contain the oldest clear evidence of carbonatite volcanism. Melanephelinite lava and a few parasitic cones followed deposition of the B TA. The Footprint Tuff covers most of the volcano, including the parasitic cones, and some adjacent terrain (Fig. 2). It is as much as 3 m thick on the upper slopes. Ejected blocks of nephelinite lava commonly as much as 15 cm in diameter widely overlie the Footprint Tuff on the northeast side of the volcano, and blocks of lava 1-2 m in diameter are common in one area.

79 TABLE 1 Stratigraphic subdivisions of Oldoinyo Lengai and correlatives at Olduvai Gorge Olduvai Gorge

Oldoinyo Lengai Strat. subdivision

Age

Strat. subdivision

Deposits of historic eruptions

1880 to present

None None

Ejected blocks 1 Footprint Tuff1

~ 600 a

None None

Melanephelinite lavas and cones BTA (Black tufts and agglomerates)

Age

1250-200 a

Namorod Ash Older BTA ash

Localized rubble with thin airfall tuffs 1

None

YTA (Yellow tuff and agglomerates)

Naisiusiu Beds Ndutu Beds

1250 a

15 ka 30-400 ka

1Stratigrapic units established here; other units of Oldoinyo Lengai are from Dawson (1962a). Data on Olduvai Gorge are from Hay (1976).

Fig. 2. View of Oldoinyo Lengai from the northeast. Dark deposits in lower one-third of photo are tufts and lapilli tufts of the BTA. Above are lighter-colored, commonly grass-covered airfall deposits of the Footprint Tuff, which mantle the lower and middle slopes of the volcano. The YTA is widely exposed in the upper slopes of the volcano. T h e ejected blocks r e p r e s e n t a v i o l e n t explosive episode t h a t p r e c e d e d t h e historic e r u p t i o n s . T h e Y T A h a s b e e n c o r r e l a t e d w i t h t u f t s of the N d u t u a n d Naisiusiu Beds of Olduvai Gorge, 60 k m to t h e s o u t h w e s t a n d d o w n w i n d f r o m O1doinyo L e n g a i ( H a y , 1976). T h e N d u t u B e d s

h a v e a n age s p a n of a b o u t 30-400 ka, a n d t u f t s of t h e N a i s i u s i u B e d s were d e p o s i t e d a b o u t 15,000 y e a r s ago. T h e B T A is late H o l o c e n e . A t h i n c a l c r e t e n e a r t h e t o p of t h e B T A gave a C14 d a t e o f 1300 a, a n d a d a t e of 2050 a was obt a i n e d f r o m calcrete of a w e a k l y d e v e l o p e d p a -

80 leosol near the middle of the BTA (Hay, 1976). A major eruption of the BTA is represented at Olduvai Gorge by the Namorod Ash, dated at about 1250 a. Older wind-worked ash from BTA eruptions is also found at Olduvai Gorge, and pedogenic calcretes developed beneath the Namorod and older ashes give C-14 dates of 21909130 a. The older dates seem excessive as the wind-worked ash is far less altered than eolian tuffs of the Naisiusiu Beds, dated at about 15,000 a. Hence an age of about 1250-2000 a seems reasonable for the BTA. Differing colors of the YTA and BTA are chiefly a result of their differing age and degree of alteration. Glass and nepheline are extensively altered to zeolites in the YTA, whereas nepheline is unaltered and glass only slightly altered in the BTA. Tuffs between the YTA and BTA are little altered and very likely closer in age to the BTA than to the YTA. The Footprint Tuff is estimated to have an age of about 600 a on the basis of its position above the Namorod Ash in the floor of Loolmurwak Crater, near Oldoinyo Lengai (Fig. 1 ). Here unconsolidated sediments are exposed to

a depth of about 5 m in a narrow fissure. The top of the Namorod Ash lies at a depth of 1.2 m, and the Footprint Tuff, 20 cm thick, lies in the middle of the overlying sequence of silts. The fissure is probably of recent origin, very likely over the past 50-100 years, in view of its vertical walls in unconsolidated sediments. The age of the Footprint Tuff is about 600 a assuming a recent age for the fissure and a uniform sedimentation rate for the silts above the Namorod Ash.

Field description of Footprint Tuff On the lower slopes of the volcano the Footprint Tuff is generally 15-30 cm thick and is divisible into two units of subequal thickness. The lower unit is laminated and mostly medium-grained; the upper unit is massive to crudely stratified, mostly coarse-grained, and contains lithic ejecta commonly as large as 5 cm in diameter. To the northwest, the upper unit contains coarse biotite, not found elsewhere. A short distance upslope the entire Footprint Tuff is laminated, and the upper unit is recognizable

Fig. 3. Hand sample of the Footprint Tuffwith light-colored calcite-richlaminae. Calcite exceeds the volume of noncarbonate ash particles in some laminae of the lower 2 cm. Sample is from the lower part of the Footprint Tuff on the east side of the volcano.

81

only by coarse lithic ejecta. Thickness of the Footprint Tuff is 60-90 cm at the highest level studied, an estimated 500 m above the base of the volcano. Laminated tuffs mantle irregularities on the underlying surface and are clearly airfall deposits; the massive to crudely stratified tuffs were at least locally redeposited, possibly by the sheetwash of heavy rains. The fol-

lowing description of airfall tufts will refer to the laminated deposits unless otherwise noted. Ash laminae of the airfall tufts are 0.1-1 cm thick and range from moderate gray with minimal calcite to light gray and white where calcite forms a matrix for the gray ash particles (Fig. 3 ). White layers with abundant calcite occur principally in the lower 5-10 cm. Ash layers

Fig. 4. A. Surface of the Footprint Tuff with a polygonal cementation pattern. B. Cemented surface of the Footprint Tuff with a polygonal fracture pattern.

82 are concordant, suggesting that they accumulated over a short period, perhaps several weeks or months during which rainfall was insufficient to cause sheetwash or gullying. Calcified clumps of sturdy, thick-leaved grass as much as 15 cm high are buried in upright position by the airfall tuffs, further pointing to their rapid accumulation. Tuffs are weakly cemented except for the upper 2-3 cm, which may be well cemented and have polygonal joint and erosion patterns that range from centimeters to decimeters in scale (Fig. 4A, B). Similar polygonal patterns have been noted in other natrocarbonatite-bearing airfall tuffs (Hay, 1978; Deans and Roberts, 1984). Air bubbles, or vesicles, are found in many of the finer-grained laminae. The vesicles resemble those of vesicular A horizons of desert soils and resulted from incomplete saturation of the ash layers by rainfall. Footprints of hare, bovids, birds, and indeterminate small mammals were noted on bedding surfaces within the tuff. The Footprint Tuff is chiefly stream-laid and as much as 70 cm thick in an area of braided streams bordering Oldoinyo Lengai on the northeast. These deposits are chiefly massive to crudely stratified beds 5-20 cm thick that may have been transported and deposited as slurry-like masses by overloaded streams. Vesicles are common and can be as much as 1 cm long. Angular clasts as much as 10 cm in diameter can be dispersed in the massive deposits or form thin discontinuous breccias. The clasts comprise natrocarbonatite, silicate lava, and ijolite. Some tuffs are well sorted, cross-bedded, and have been reworked by running water. Plant fragments, both woody and calcified, are common in the stream-laid deposits.

Petrography Modern natrocarbonatite The nature of modern eruptive natrocarbo-

natite of Oldoinyo Lengai is essential to understanding the pre-modern tephra deposits in which the natrocarbonatite is altered. Lavas erupted from 1960 to 1966 were composed almost wholly of nyerereite ( [Nao.s2Ko.1s]2Ca [C03 ] 2) and gregoryite ([Nao.TsKo.os]2[Cao.17C03]), which occur as phenocrysts and groundmass constituents. Analyzed lavas contain no more than 0.10% Si02 and 0.3% total Fe (Dawson, 1962b; Dawson et al., 1987). A partly altered pre-1960 lava sample differs in the presence of rare fluorite and calcite and slightly higher contents of SiO2 and Fe203. Four samples of lava erupted in 1985 and provided by Celia Nyamweru consist largely of gregoryite and nyerereite, which are chemically similar to those of the 1960-1966 lavas (Nyamweru, 1988). One sample contains 21% of acid-insoluble material, which is about three quarters fluorite and one quarter apatite. The fluorite occurs principally as fibroradial aggregates of hairlike crystals, and the apatite, identified by XRD, forms small rounded crystals intergrown with fluorite. Ash discharged in the explosive eruptions of 1966 was a mixture of natrocarbonatite and nephelinite tephra, and the following description is based on Dawson et al. (1968) supplemented by study of sample BD 882 from the crater rim, provided by J.B. Dawson. Carbonate materials formed about 25% of the chemically analyzed sample of Dawson et al. (1968). The non-carbonate fraction is bimodal in size distribution, with concentrations of ash between 0.2 and 2 mm and finer than 0.02 mm. The coarse fraction includes mineral grains, lava clasts, and spherical aggregates of fine ash, many of the latter containing crystals as nuclei. The dominant mineral is nepheline, and others include melanite, melilite, wollastonite, nyerereite, partly altered gregoryite, trona, and an unidentified mineral (Table 2). The trona is presumably an alteration product of natrocarbonatite ash.

83 TABLE 2 Mineral composition of tephra deposits of Oldoinyo Lengai Cement

Primary minerals 2

Stratigraphic unit 1

Calcite

Zeolite

Nph

Mel

Aug

Wo

NCS 3

Mln

Sph

Per

Nyr

Grg

1966 ash Footprint t u f f

0 XX-XXX

0 0

XXX XXX

X X-XX

X XX

X X-XX

XX X-XX

XX X×

0 0-+

0 0-×

X XX

+ XX

Melenephelinite cone 0

0

XXX 0

XX

XX

XX

X×

0

0

0

0

BTA Thinairfalltuffs YTA

X-XX 0 XXX

XXX XXX XXX

XX X-XX XX

0-X 0 0

0-X X-XX 0

X-XX X 0-X

+ 0 X-XX

X + 0-X

07 0?-X 0

0? 0? 0

XX-XXX XX 0-X

X X-XX 0-X

IStratigraphic units are listed in order of increasing age. 2Abbreviations are: Nph = nepheline; Mel = melilite; Aug = augite; Wo = wollastonite; Mln = melanite; Sph = sphene; Per = Perovskite; Nyr = nyerereite; and Grg = gregoryite. Not listed are apatite and magnetite, which are minor constituents of all deposits, and alkali feldspar, found only in the YTA, and sodalite, found only in the BTA. Symbols are: X X X =abundant; X X = c o m m o n ; X = i n minor amounts; + = rare; 0 = absent; and 0? -- not identified but presumed possible because of evidence for natrocarbonatite. 3Crystals of NCS are wholly altered in the BTA and thin airfall tufts and were identified on the basis of shape and inclusion-rich zones.

Mineral NCS The unidentified mineral forms an estimated 5-10% of the 1966 ash and ranges in crystal size from about 0.01 to 0.1 mm. The larger crystals are cubic, have low birefringence, and a mean refractive index of 1.640. Zones parallel to crystal faces differ in refractive index and contain locally high concentrations of mineral inclusions 2-5/zm in diameter. The smaller crystals are platy and approximately square with rounded corners. The major cations are Na, Ca, and Si, and minor cations are A1, Mg, Mn, and Fe. The lack of detectable A1, Mg, and Mn in one or more analyses suggests that they represent contamination by the small inclusions. Oxygen is the most likely anion as S and C1 were not detected. The analysis with the smallest amounts of elements other than Ca, Na, and Si has a formula arbitrarily based on 32 oxygen ions as follows: Nas.7~K0.o4Ca~.9oMgo.14Fe3+o.o7Silo.7oO32. Neglecting minor constituents, the formula is Na4.ogCa2.76SisO14.s1 as based on 5 Si atoms. This is close to the artificial compound Na4Ca3Si5015 formed in crystallization of Na-rich calcic glasses (Segnit, 1953). The artificial compound has a refractive index of 1.620, some-

what lower than the mineral in BD 882. The Na-Ca silicate of Oldoinyo Lengai will be designated NCS.

Footprint Tuff

The tufts now consist largely of noncarbonate ash particles similar in many respects to those of the 1966 eruption. The ash is bimodal with most in the ranges of 5-20 #m and 0.1-1.0 ram. Fine ash in some layers forms spherical aggregates 0.50-0.75 mm in diameter and accretionary coats around larger ash particles. Most laminae are well sorted, but many are poorly sorted. Textures are chiefly grain supported but can be fenestral or matrix supported in the lower 5-10 cm (Fig. 3). White, finely crystalline calcite fills fenestrae and forms the matrix for noncarbonate ash. Massive streamlaid deposits are poorly sorted and contain much fine-grained ash. The coarser noncarbonate ash is composed of crystals and less commonly brown glass and microcrystalline nephelinite lava. Nepheline is the most common mineral, and others include sodic augite, melanite, melilite, and wollaston-

84

ire (Table 2 ). Chemical analyses of melilite and wollastonite are given in Table 3. Crystals are commonly coated by microcrystalline lava or glass molded to the shapes of the crystals. Similar coated crystals are common in melilite-carbonatite tuffs of the Laetolil Beds in Tanzania (Hay, 1978). Brown glass and lava of the Footprint Tuff contain microphenocrysts of the same minerals found as individual crystals. Fine ash has the same minerals and also includes fluorite and translucent isotropic material with a refractive index of about 1.50. The fluorite and isotropic material are from natrocarbonatite tephra such as described below. Crystals of NCS are in all samples and are generally 0.03-0.1 mm in diameter. The NCS is partly to wholly altered to opal (?), which also commonly replaces the marginal 30-60/~m of wollastonite. Clasts of calcified natrocarbonatite are recognizable in about a quarter of the airfall tuff samples and are found chiefly in the poorly sorted layers. Clasts are about 0.02-1.0 mm in size and form at most a few percent of the laminae. By contrast, clasts of calcified natrocarbonatite form an estimated 5-10% of the massive stream-laid tufts. Most are between 0.02 and 1.0 mm in size, but clasts 1-10 cm in diameter are found in a few beds. The larger clasts TABLE 3 Microprobe analyses of silicate minerals in tephra deposits of Oldoinyo Lengai (1)

(2)

(3)

(4)

(5)

Si02 A120~ FeO MnO MgO CaO Na20 K20

50.79 0 0.41 0 0.44 26.15 21.45 0.14

50.88 0.61 1.05 0.54 0.46 26.54 21.87 n.d.

50.20 1.00 0.80 0 0 25.70 20.57 0.27

44.40 9.27 6.94 0 3.76 29.50 6.04 n.d.

51.45 0 1.04 0.43 0 47.00 0 n.d.

Total

99.38

101.96

98.54

99.90

99.92

(1)-(3) are of a crystal of NCS from ash of the 1966 eruption. (4) Melilite from the Footprint Tuff. (5) Wollastonite from the Footprint Tuff.

were probably transported in mudflows from an area with coarse ejecta, possibly high on the volcano. The higher content of recognizable natrocarbonatite ash particles in the massive stream-laid deposits may be largely due to their high content of fine-grained noncarbonate ash, which preserves outlines of the natrocarbonatite particles. Calcite-rich layers of airfall tuff probably contained much natrocarbonatite, and it may have been the major constituent in laminae with fenestral and matrix-supported textures, where calcite fills space formerly occupied by some other material, presumably natrocarbonatite. Natrocarbonatite clasts larger than 1 cm consist of altered lava that is yellowish-brown, nonvesicular, and porphyritic. Nyerereite phenocrysts 1-2 mm long constitute 10-20% of the natrocarbonatite and are represented by rectangular cavities partly filled by calcite crystals 10-20/lm in diameter. Rare rounded pseudomorphic cavities may represent gregoryite phenocrysts. Most of the groundmass has been transformed to porous iron-stained sparry calcite lacking relict igneous texture. However, patches of isotropic noncarbonate material, most common around altered phenocrysts, retain the texture of groundmass nyerereite and gregoryite (Fig. 5A). Nyerereite is represented by tabular to acicular cavities 100-300/~m long and the gregoryite by rounded cavities 20-40 zm across, about the same size and shape as in modern lavas. The isotropic matrix for altered nyerereite and gregoryite has a refractive index of 1.50-1.54 and includes fluorite and minor appatite. The isotropic material is colorless to yellowish-brown and forms 20-50% of the groundmass in which relict texture is preserved. Fluorite occurs as small globular crystals and as irregular, commonly vermicular crystals as much as 30 pm long. Wisps and clots of yellowish-brown to brown isotropic material occur in the groundmass areas replaced by sparry calcite. The noncarbonate fraction forms 25.2% of a sample digested in 5% acetic acid. The isotropic material is noncrystalline and

85

chemically complex. It gives neither an XRD pattern nor a diffraction shoulder. Silicon and calcium are the major cations, and next in abundance is a group comprising P, Mn, Mg, Sr, and probably Ba. Several other elements are present in smaller amounts (Table 4, nos. 1 and 2 ). Analyzed amounts of principal elements are given as oxides except for Ca, of which 12.8% is assigned to the estimated 25% fluorite. The low total in the XRF analysis, even when corrected for fluorine, shows that about 40% of the sample remains unidentified. Weight loss was subsequently determined on heating to 115°C (2.3% loss), then to 520°C (8.0% loss), and finally to 1020 ° C ( 13.3% loss). The sample fused at 1020°C to a glass with abundant globular crystals of fluorite. The weight loss on heating may represent chiefly HeO, and gives a weight percent total of 80-82%. TABLE4 Chemical composition of noncarbonate materials in natrocarbonatite ejecta Oxide/compound

(1)

(2)

(3)

SiO2 TiO2 A1203 Fe~O:~ MnO MgO CaO CaF2 SrO BaO Na20 K20 P205 Cl

10 <0.2 < 0.2 1 2.3 0.6 13.2 ~ 25 0.6 < 0.9 0.3 < 0.2 4.2 0

20 <0.4 < 0.4 2 4.6 1.2 26.6 1.2 < 1.8 0.6 0.4 8.5 0

24.1 0 0 1.3 5.7 13.4 12.0 0.5 0.4 1.2 1.8 6.8 0.2

Total

< 58.7

< 69.7

67.4

(1) XRF analysis of acid-insoluble fraction of natrocarbonatite flast from the Footprint Tuff. Also present are La (<0.2%), Ce (<0.2%), Zn (<0.3%), and Pb (<0.3%). The sample contains an estimated 25% fluorite and 5% apatite. (2) Analysis (1) after deleting CaF2 and recalculating to about 70 wt.% for comparison with (3). (3) Altered crystals in coarse natrocarbonatite block of Kerimasi. Analysis is an average of 4 microprobe analyses.

The isotropic material may well be altered glass from which substantial amounts of silica and other ions have been leached, as in palagonitization of basaltic glass. However, when recalculated to exclude 25% fluorite, its chemical composition is similar in some respects to brown isotropic pseudomorphs after small hexagonal platy crystals of an unidentified mineral (Table 4, no. 3) in an ejected block of coarse natrocarbonatite from Kerimasi volcano, to the south of Oldoinyo Lengai (Hay, 1983). Thus, the isotropic material may be an alteration product of crystalline material. Natrocarbonatite clasts less than about 1 cm in diameter are partly or wholly white because of finely crystalline calcite. Fragments 1-10 mm in diameter commonly have white mottles and/ or alteration rinds in which nyerereite is replaced by finely crystalline calcite (Fig. 5B). Fragments smaller than 1 mm are chiefly finely crystalline calcite with little or no relict igneous texture. Some of the smaller clasts do, however, have a tabular shape suggestive of nyerereite crystals. Secondary calcite is found in all samples of the Footprint Tuff. It occurs as a cement or matrix, as a filling of pores and fenestrae, and as a replacement of plant materials and natrocarbonatite ejecta. Calcite crystals are chiefly 21 0 / t m in diameter except in the larger natrocarbonatite clasts, where they are generally 1040 ttm in size. Calcite probably averages 15-20% of the airfall tuffs, judging from measured amounts of 13% and 29% in samples representing extremes in calcite content. A typical sample of massive stream-laid tuff contains 27% calcite. If, as seems likely, most of the secondary calcite is from natrocarbonatite tephra of the Footprint Tuff, then the original content of natrocarbonatite tephra may well have exceeded that of silicate tephra. For example, 1520% CaCO3 is equivalent to 65-87% of natrocarbonatite, using the ratio of alkali carbonates to calcium carbonate in the 1960 lavas (Dawson, 1962b).

86

Fig. 5. A. Photomicrograph showing relict igneous texture in part of a large altered natrocarbonatite clast from the Footprint Tuff. Nyerereite microphenocrysts are represented by a calcite pseudomorph (CC) and a pseudomorphic cavity partly filled by calcite (CV). Groundmass consists of isotropic silicate material (S) with cavities pseudomorphous after nyerereite (N) and gregoryite (G). Remainder of clast is sparry calcite (black) and pore space (white). Photographed with plane light. Bar scale is 0.2 mm. B. Photomicrograph showing lath-shaped nyerereite crystals replaced by calcite in a 2-mm natrocarbonatite clast from the Footprint Tuff. Calcite is finely crystalline and appears white. Silicate material forms the groundmass for calcified nyerereite. Photographed with reflected light. Bar scale is 1.0 mm.

87 A l t e r a t i o n of n a t r o c a r b o n a t i t e Airfall tufts of the Footprint Tuff are sufficiently continuous over Oldoinyo Lengai to suggest that they were cemented to some extent soon after deposition and have remained cemented for the past 600 years. Meteoric water leaches the alkali ions from natrocarbonatite, and the initial cement may have been trona (Na2CO3" NaHC03- 2H20), which was identified in the 1966 ash by Dawson et al. (1968). Gaylussite (Na2Ca [ CO3 ] 2"5H20 ) and pirssonite (Na2Ca [ CO3 ] 2" 2H20 ) can be intermediate phases in calcification of natrocarbonatite lava. Dawson et al. (1987) identified pirssonite in altered pre-modern lava, and in the present study both gaylussite and pirssonite were identified in alteration rinds of lava samples from the eruptive episode that began in 1983. As an experiment in ash alteration, a 1-g sample of powdered natrocarbonatite lava was added to 50 cm 3 of distilled water at room temperature. After 4 weeks the sample was a mixture of gaylussite crystals and residual nyerereite. The water: rock ratio was much lower than this in the Footprint Tuff, but the result of this experiment taken together with observations on altered lavas suggests that gaylussite, pirssonite, or both, were intermediate phases in alteration of ash, and they may well have acted as a cement in the Footprint Tuff until altered to form calcite. Upward movement of Ca within the Footprint Tuff is indicated by the cemented crust, which was probably precipitated by surface evaporation of pore fluids. Ca was very likely relocated in forming the ubiquitous calcite cement.

Isotopic data The isotopic composition of calcite can be used to estimate the degree of isotopic re-equilibration with ambient solutions in the transformation of natrocarbonatite to calcite. Fresh natrocarbonatite lava has much lower J ~sO and J13C values than pedogenic calcite (Table 5). Analyzed natrocarbonatite lava has J~sO val-

ues of about + 7%o. Published (~13Cvalues range from - 4 . 5 to -12.6%o on unaltered lava erupted from Oldoinyo Lengai in 1960 (Suwa et al., 1975 ). The published values average about 8%o, which is slightly higher than the values obtained from lavas erupted in 1985 (Table 5, nos. 2,3 ). By comparison, the J'sO values ofpedogenic calcite in the vicinity of Oldoinyo Lengai are close to + 30%0, and with one exception the J'3C values are 0%0. The exception, with a J13C value of -2.2%0, is of a sample from the basal part of a laminar calcrete, which contains a considerable amount of BTA tephra and may contain nonpedogenic calcite. Oxygen and carbon isotope re-equilibration appear to have been essentially decoupled in alteration of natrocarbonatite ash. Ash collected within a day after eruption in August, 1966 exhibits the early stages of alteration. It has a JlSO value of + 16.8%o (Table 5, no. 4), significantly higher than fresh natrocarbonatite and indicating considerable oxygen exchange with meteoric water. The J13C value is -8.9%o, within the lower range of natrocarbonatite values. Altered ash in the Footprint Tuff is represented chiefly by calcite cement and pore fillings (Table 5, nos. 7-9) which have JlsO values from +25.8 to + 33.7%o, indicating extensive to complete exchange of magmatic by meteoric oxygen; J13C values range from - 5.9 to - 8.6%0, in the range for modern lavas. Extensive oxygen exchange is not unexpected for cements and pore fillings because of the opportunity for equilibration with meteoric water during transportation of CO~- in solution. Carbon equilibration may have been retarded by the slow rate of exchange between atmospheric CO2 and dissolved carbonate species at the high pH, about 9.0-9.5, of a solution in equilibrium with trona, gaylussite, pirssonite, or natrocarbonatite. As an example of the slow exchange in saline alkaline solutions, trona newly crystallized at the surface of the alkaline brine of Lake Magadi, Kenya, gave a C-14 date of 7100 a (Hay, 1968). The slow rate of equilibration is at least partly attributable to the low

88 TABLE 5 Isotopic composition of carbonate materials from Oldoinyo Lengai and vicinity Material analyzed

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Natrocarbonatite lava (1960) Natrocarbonatite lava (1985) Natrocarbonatite lava (1985) BD 882, partly altered ash from eruption of August, 1966 2-cm natrocarbonatite clast from stream-laid Footprint Tuff 2-cm natrocarbonatite clast from stream-laid Footprint Tuff Calcite of massive stream-laid tuff of Footprint Tuff Calcite of fenestral airfall tuff of Footprint Tuff Calcite cement of airfall tuff of Footprint Tuff Surface layers of laminar calcrete within the BTA at the north foot of Oldoinyo Lengai Middle layers of same calcrete as no. 9 Basal part of same calcrete as nos. 9 and 10 Calcrete southeast of Oldoinyo Lengai with C-14 age of 3500 yr Late Holocene calcrete from Olduvai Gorge {478-236) Late Pleistocene calcrete from Olduvai Gorge (478-254)

jl80

j13C

(SMOW)

(PDB)

(%°)

(%0)

+7.1 +6.8 +7.3 + 16.8 +26.0 -26.0 +25.8 + 3O.9 + 33.7 +32.1 + 29.4 +29.3 +29.8 +31.0 +31.4

-7.6 -7.1 -6.8 - 8.9 -3.5 -4.1 -6.7 - 8.6 - 5.9 0.0 0.0 -2.2 +O.7 +0.6 +0.8

Data sources: 1 - O'Neil and Hay, 1973; 2-13 - D. Haymes; and 14, 15 - Cerling and Hay, 1986.

concentration of H2CO3 at high pH. This would constitute a very small "door" for exchange of atmospheric carbon with dissolved carbon species. A high total amount of inorganic carbon in solution, as HCO~- and CO~-, can be another factor causing a slow rate of equilibration with atmospheric CO2 (Lynn Walter, pers. commun., 1988). Altered natrocarbonatite lapilli have J~sO values of +26.0%0 (Table 5, nos. 5,6), within the range of values for altered ash. However, J13C values are - 3 . 5 and -4.1%o, distinctly higher than in altered ash and indicating a greater degree of carbon isotopic re-equilibration. The greater degree of carbon exchange in the lapilli is perhaps attributable to their larger size and slower alteration, thus allowing greater opportunity for equilibration with atmospheric CO2. The larger size of calcite crystals in the lapilli as compared to the tufts fits with slower crystallization of calcite in the lapilli. Blocks of altered natrocarbonatite from Kerimasi volcano have isotopic patterns comparable to those of the larger clasts in the Footprint Tuff. Unaltered sovite clasts and primary cal-

cite phenocrysts of a natrocarbonatite clast have j180 values of + 7.2 to + 7.5%0 and J13C values of - 4 . 2 to -4.3%0 (Hay, 1987). Calcified nyerereite in natrocarbonatite blocks gives JlsO values of + 22.2 to + 26.4%0, which represents about 60 to 80% oxygen exchange. The j13C values are mostly - 3 . 0 to -3.3%0, which represents about 25%0 carbon exchange. A thin pedogenic calcrete cementing and coating natrocarbonatite breccia on Kerimasi has a normal pedogenic J~sO value of +29.4%0 and a j13C value of -3.3%o, which is considerably lower than in the other analyzed calcretes (Table 5). It seems likely that the natrocarbonatite substrate contributed a significant fraction of the carbon in the pedogenic calcite.

Carbonatite eruptive history The stratigraphy of Oldoinyo Lengai demonstrates that natrocarbonatite and nephelinite magmas have been erupted together at intervals over the past few thousand years. The two thin nephelinite tufts between the YTA and BTA, probably between 2000 and 5000 years in

89 age, contain the oldest clear evidence of eruptive natrocarbonatite. One of the two tuffs contains calcified clasts 0.2-0.5 mm in diameter that are interpreted as altered natrocarbonatite. A few of the clasts have shapes suggestive of nyerereite. Both tuffs contain altered crystals of NCS and exhibit polygonal joint patterns similar to those found elsewhere in natrocarbonatite-bearing airfall tuffs. The BTA, with an age of about 1250-2000 a, contains evidence suggestive of natrocarbonatite. NCS is present in about half of the tuff samples examined, and an ejected block of carbonatite was noted by Dawson (1962a). Natrocarbonatite ash is the most likely source of the abundant calcite cement. Natrocarbonatite ash seems required to account for the rapid accumulation of calcite in the Stage IV calcrete profile beneath wind-worked ash of BTA eruptions at Olduvai Gorge (Hay and Reeder, 1978). The amount of calcite in the profile compares with that of calcretes developed over 100 ka or more in the southwestern U.S.A., where eolian dust and rainfall are the source of calcite. Silt-sized particles of calcite in eolian sediment near O1duvai Gorge gave a C-14 date of 1200+100 a (Hay, 1976), which is compatible with an origin from natrocarbonatite erupted 1250 to 2000 years ago. Melanephelinite cones that followed deposition of the BTA on the north side of the volcano are composed of spherical lava lapilli and small bombs. These are of interest here because of the common occurrence of NCS (table 2), which has been found elsewhere only in natrocarbonate-bearing tephra deposits. The NCS in the lapilli and bombs may reflect the interaction of natrocarbonatite with nephelinite magma. The Footprint Tuff is the product of explosive eruptions about 600 years ago in which the volume of natrocarbonatite very likely equalled or exceeded that of nephelinite. This natrocarbonatite was similar to that of the modern lavas except for a substantial content of silicate material. As noted earlier, ash deposits erupted be-

tween 1880 and 1960 yielded soluble Na salts that were almost certainly derived from natrocarbonatite ash. Natrocarbonatite is well documented in the ash deposits of the 1966 eruption. Evidence is contradictory as to whether natrocarbonatite was erupted during the conebuilding eruptions represented by the YTA. Neither NCS nor altered clasts of carbonatite have been identified in samples of the YTA. The small amount of calcite cement was deposited following cementation by zeolites, suggesting that it may be related to weathering of calciumbearing silicates rather than to natrocarbonatite. However, well-developed Stage IV pedogenic calcretes are associated with correlative tuffs of the Ndutu and Naisiusiu Beds at O1duvai Gorge, which originated in YTA eruptions of Oldoinyo Lengai (Hay and Reeder, 1978). Those associated with tuffs of the upper unit of the Ndutu Beds ( ~ 30-60 ka) and the Naisiusiu Beds ( ~ 15 ka) were developed much more rapidly than usual for calcretes of noncarbonatite origin, and natrocarbonatite ash should be considered a possible source of their calcium carbonate. The 87Sr/S6Sr value is 0.70462 + 0.00003 for a sample of calcrete from the base of the Naisiusiu Beds at Olduvai Gorge (Keith Bell, pers. commun., 1987). This implies a source of the calcium from volcanic rocks rather than from the Precambrian basement. Natrocarbonatite lava of Oldoinyo Lengai has a STSr/S6Sr value of 0.70443 + 0.00003, for example, whereas basement rocks of the Tanzanian Shield have values of about 0.747 (Keith Bell, pers. commun., 1987).

Conclusions

(1) Natrocarbonatite and nephelinite tephra have been erupted together from Oldoinyo Lengai at intervals over the past few thousand years. The Footprint Tuff, erupted about 600 years

90

ago, probably had the highest percentage of natrocarbonatite tephra. Evidence is contradictory as to whether natrocarbonatite was erupted during the cone-building phase of eruptions that ended about 15,000 years ago. (2) Nephelinite-carbonatite ash erupted in 1966 contains a previously unrecognized silicate mineral, here termed NCS, which has a chemical composition close to that of the artificial compound Na4Ca3SisO~5, formed in crystallization of Na-rich calcic glasses. It may be genetically related to natrocarbonatite magma as it has been identified only in the younger tephra deposits of Oldoinyo Lengai, most of which contain evidence of natrocarbonatite ash. (3) Natrocarbonatite ash probably equalled or exceeded the volume of nephelinite in ash of the Footprint Tuff. Calcite of natrocarbonatite origin forms 15-20% of the airfall tuffs and occurs as a cement and pore filling, as a matrix for noncarbonate particles, and as a replacement of plant materials and natrocarbonatite clasts. Nyerereite and gregoryite were the primary carbonate minerals, as in the modern lavas, and the groundmass for the carbonate minerals was composed of fluorite and silicate material containing Ca, P, Mn, Sr, Ba, and several other elements. (4) The Footprint Tuff was cemented soon after deposition. Trona was probably the first cement, and gaylussite may have been a later cement in transformation of natrocarbonatite ash to calcite. Calcium has been relocated within the Footprint Tuff to form a ubiquitous cement and locally a well-cemented crust. (5) Early cementation of ash layers resulted in preservation of footprints within the Footprint Tuff. (6) Oxygen and carbon re-equilibration were decoupled in transformation of natrocarbonatite ash to calcite cements and pore fillings within the Footprint Tuff. They have JlsO values indicating complete or nearly complete oxygen exchange and J13C values in the range of magmatic carbon. Carbon exchange may have been retarded by a high pH of pore fluids, which

reduced the rate of equilibration between atmospheric CO2 and carbonate species in solution.

Acknowledgements Field work was supported by grants from the National Geographic Society and the University of Illinois at Champaign-Urbana. Peter Jones and M.D. Leakey supplied transportation and camp facilities. I am grateful to J.B. Dawson for a sample of ash from the 1966 eruption of Oldoinyo Lengai and to Celia Nyamweru for lava samples from the 1985 eruptions. I am indebted to many persons for laboratory data. Most of the isotopic measurements were made at the University of Illinois by David Haymes, who assisted in other phases of the laboratory work. Keith Bell of Carleton University measured the S7Sr/S6Sr value of a calcrete sample from Olduvai Gorge. The XRF analysis was made by Robert Frost of the Illinois State Geological Survey, and microprobe analyses are by Cameron Begg of the University of Illinois. Discussions with R.J. Kirkpatrick, T.F. Anderson, and Lynn Walter were helpful in various aspects of the study. The efforts of three anonymous reviewers are greatly appreciated.

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Hay, R.L. and Leakey, M.D., 1982. The fossil footprints of Laetoli. Sci. Am., 246: 50-57. Hay, R.L. and Reeder, R.J., 1978. Calcretes of Olduvai Gorge and the Ndolanya Beds of northern Tanzania. Sedimentology, 25: 649-673. Nyamweru, C., 1988. Activity of Ol Doinyo Lengai Volcano, Tanzania, 1983-1987. J. Afr. Earth Sci., 7: 603610. O'Neil, J.R. and Hay, R.L., 1973. ~80/1~O ratios in cherts associated with the saline lake deposits of East Africa. Earth. Planet. Sci. Lett., 19: 257-266. Segnit, E.R., 1953. Further data on the system Na20-CaOSiO2. Am. J. Sci., 251: 586-601. Suwa, K., Oana, S., Wada, H. and Osaki, S., 1975. Isotope geochemistry and petrology of African carbonatites. In: L.H. Ahrens, J.B. Dawson, A.R. Duncan and A.J. Erlank (Editors), Physics and Chemistry of the Earth, 9: 735-745.