U-series disequilibria in early diagenetic minerals from Lake Magadi sediments, Kenya: Dating potential

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Geochimica PI Cosmochimica Acro Vol. 56, pp. 133I-I 34 I Copyright 0 1992 Pergamon Press plc. Printed in U.S.A.

+ .OO

U-series disequilibria in early diagenetic minerals from Lake Magadi sediments, Kenya: Dating potential and CLAUDE HILLAIRE-MARCEL Centre de Recherche en GCochimie isotopique et en Gtoehronologie (GEOTOP), Universite du Q&bec B Montreal, CHRISTIANGQETZ*

BP 8888, WCC.A, Montreal, H3C 3P8, Canada (Received November 1, 1990; accepted in revisedform December 13, 199 1)

the southern end of the Gregory Rift Valley, Lake Magadi occupies the bottom of a relatively large drainage basin. It is presently covered by a thick bona crust, which overlies two silty-clay units deposited during Late Pleistocene high lake stands. These units consist of a mixture of detrital grains (anorthoclase, amphiboles, quartz), clays (illite, authigenic zeolites), phosphates, and sedimentary sodium silicates and cherts. A late diagenetic calcite is occasionally observed. The authigenic and/or diagenetic mineralogical phases were extracted and analyzed for their uranium and thorium isotope contents. All vs. 234U/232Th)isochrons defining yielded highly correlated (23413/232Thvs. 238U/232Th)and (2~/~zTh two-component mixing systems (detrital and authigenic phases}. The detrital component is characterized by large excesses of 23@Th(over 234U) and by *3”Th/232Th ratios carrying an imprint of the source rocks (e.g., Precambrian basement vs. volcanics) and indicating efficient uranium-leaching processes during the previous pedologic cycle. The slope of the isochrons defines the 234U/238Uand z30Th/234U ratios of the authigenic component, i.e., the age of the uranium-uptake episode. Zeolites yield an age of 10.4 + 0.6 ka. This age is in agreement with the 14Cchronology already established for the most recent high lake level episode in the basin ( lO,~-12,~ yr BP). ~urn-~~~tes and cherts yield distinct ages of 98.5 + 20 and 6 + 3 ka, respectively, for the lower and upper lacustrine units. These ages allow the conclusion that ( 1) sedimentary silicates are of late diagenetic origin, and (2) that the lower lacustrine unit was deposited during a former high Lake Magadi level, possibly during the lacustrine episode dated at 135 2 10 ka from uranium-series measurements on littoral stromatolites. Finally, the late diagenetic calcite, which yields an age of about 5 ka, indicates a sign&ant change in the sediment interstitial water chemistry. This is also shown by the occurrence of calcitic oncolites in flat deposits assigned to a mid-Holocene minor oscillation of Lake Magadi level on the basis of their 14Cages. It is concluded that umnium-ales measurements in diagenetic minerals may give access to the age of their formation. Abstract-At

precursor to cherts; and (2) zeolites (erionite, analcime, chabazite) deriving from volcanic glasses, also thought to be “authigenic” ( SURDAMand EUGSTER, 1976 ) . A coring program of Lake Magadi deposits allowed recovery of sediments below the thick Late Holocene trona crust, spanning at least two Late Quaternary high lake level episodes (BAKER, 1958; HILLAIRE-MARCELand CASANOVA,1987). The coring was completed within the framework of a multidisciplinary research project aimed at the reconstruction of hy~olo~~ changes in eastern Africa during the last climatic cycle (TAIEB et al., 1989, 1991). The most recent two high lake standstills are well recorded in the topography, at about 60 m above the present lake level (Fig. 1 ), by two superimposed generations of littoral stromatolites which were dated by 14C and U-series methods at lO,OOO-12,000 yr BP and 135 +- 10 ka, respectively ( HILLAIRE-MARCELet al., 1986). The scarcity of the organic matter in the cored sequences did not permit a reliable 14Cchronology to be extended beyond the younger episode. A study of uranium-series disequilibria in sedimentary minerals was therefore undertaken in order to assess their dating potential. We report here on the encouraging results of this study.

INTRODUCMON

URANIUM-SERIESDISEQU~LIBRIA in calcium carbonates and gypsum from lacustrine environments are relatively well documented (KAUFMAN, 1964, 197 1; PENG et al., 1978; HENDY et al., 1979; SZABO and BUTZER, 1979; CAUSSE and HILLAIRE-MARCEL, 1986). Little attention has, however, been paid to these isotopes and their dating potential in sedimentary and early diagenetic silicates. In highly reactive geochemical environments, e.g., the Gregory Rift Valley lakes, such minerals may form during the early d&genetic period (EUGSTER, 1980). In the Lake Magadi basin (southern Kenya; Fig. I), a large variety of sedimentary minerals are observed both in hydrothermal pools ( EUGSTER, 1970; SURDAM and EUGSTER, 1976) and in recent Quaternary lacustrine sediments (BAKER, 1958). They include: ( 1) sodium silicates (ma~diite, kenyaite, makatite; SHEPPARD et al., 1970) considered by HAY (1968), EUGSTER (1970), and CYNEILand HAY ( 1972) to represent “authigenic” minerals * Dr. Christian Goetz passed away in May 1991,a few weeks alter we received comments on the original version of our manuscript from its handling editor, Henry Schwartz. Data and inte~re~tions put forth in this paper are principally from one chapter of Christian’s doctoral dissertation (Universid Aix-Marseille II). Formerly at the Laboratoire de G6ologie du Quatemaire of the CNRS, at Marseille (France), Christian Goetz was about to occupy a research position at the Institut de Physique du Globe, in Paris. He was 3 I years old.

GEOLOGICAL AND HYDROLOGICAL SEITING Lake Magadi and its Tanzanian counterpart Lake Natron (Fig. 1) occupy the deepest depression of the Gregory Rift 1331

C. Go&z and C. Hillaire-Marcel

1332 /,

36’E

FIG. I. The drainage basin of the modern and paleolakes, Natron and Magadi.

Valley, some 20 km wide and 100 km long, at an altitude of about 600 m. Along their western limit, the Nguruman fault escarpment shows successive layers of basaltic lava flows, occasionally interrupted by Pli~Plei~~ne lacustrine deposits. Further west, the Precambrian basement outcrops. Several volcanoes surround the basin. To the south, the frequently active Oldoinyo Lengai centre has a strong carbonatitic component ( HAY, 1983; JAVOYet al., 1989), whereas most other volcanoes (e.g., Gelai, Shompole, and Sambu) range from trachytic to phonolitic types ( DAWSON, 1962). As a consequence of the modem arid conditions ( VINCENS and CASANOVA,1987), both lakes are almost dry and are covered by an extensive trona crust as thick as 30 m in the central arm of Lake Magadi ( SURDAMand EUGSTER, 1976; JONESet al., 1977; PAGE and SIMON, 1987), where the trona has been exploited by the Magadi Soda Company since World War I. Small pools of water persist throughout the year, caused by hy~#e~~ springs, which have a very high concentration of sodium carbonate and chloride ( EUGSTER, 1980).

Lake Natron is also fed by a few perennial streams, one of which (Enwase Ngiro; Fig. I ) drains a large basin and has reasonably “fresh” waters (dissolved solids > 200 ppm; JONES et al., 1977). The depression has been occupied by a larger body of water on several occasions during late Pliocene and Pleistocene times. Two well-developed, intensively faulted, volcano-sedimentary formations (Humbu and Moinik Formations; ISAAC, 1965) are observed on the west bank of Lake Natron. They span the Late Pliocene-Early Pleistocene as indicated by paleomagnetic records ( THOUVENYand TAIEB, 1986). In the Lake Magadi area, the Oloronga beds (BAKER, 1958), which essentially consist of cherts, are the likely correlative units, although their assignment to a precise lacustrine phase would be speculative. High lake levels also existed in more recent time. As mentioned above, the latest is well recorded by a belt of stromatolites around both lakes, some 60 m above the bottom of the depression (CASANOVAand HILLAIRE-MARCEL,1987). The maximum lake level is r4C-dated between 12,000 and 10,000 yr bp (HI~LAIRE-MARCELet al., 1986). The shoaling phase lasted at least until the early Holocene, as shown by the age of 9000 yr BP obtained on fish remains (T~la~~a; BUTZERet al., 1972) from silt deposits with tuff layers, largely spread at the margin of the modern Lake Manyara, which are known as the High Magadi Beds (BAKER, 1958). Indications for a mid-Holocene Lake Magadi level still slightly above its present elevation (a few meters at most) have been found at a few places around the present shoreline: Tilapia beds ( HILLAIRE-MARCELet al., 1986) and calcitic oncolites in “Bat” deposits (CASANOVA,1986) yielded r4Cages ranging from ca. 7000-5000 yr BP. This mid-Holocene episode may eventually correlate with a high stand at Lake Bogoria, farther to the north, dated at approximately 5000 yr BP (TIERCELIN and VINCJZNS,1987 1. The sediment accumulated below the recent trona crust in Lake Magadi contains detrital particles originating from sources that may have varied through time. During high lake level stands, the large drainage basin of the Enwase Ngiro fed directly into the Lake Magadi subbasin of the paleolake Ma~di-Natron (Fig. 1). Detrital inputs from the Precambrian basement into the Lake Magadi subbasin could be expected during such episodes. In contradistinction, during relatively low lake level phases, such as the mid-Holocene and modem situations, the Enwase Ngiro f&s Lake Natron, whereas each arm of Lake Magadi is fed by material from local sources, as it may be seen on Fig. 1. This will be important when looking at the geochemical characteristics for the detrital fraction in the sediment. MATERIALS AND METHODS As already mentioned, a few pools of shallow saline water persist all year long, in Lake Magadi, near the major hydrothermal springs. The brines usually contain rich populations of bacteria and cyanobacteria (BAKER, 1986). The sediment consists of low-Eh organic rich muds. They were sampled at four sites with a 60 cm long box-corer (Fig. 2). The Late Quaternary lacustnne units underlying the salt crust were cored in each of the Lake Magadi structural subbasins, in the western, central, and eastern arms (Fig. 2)) at places were the trona crust was thin or even missing. Two cores, respectively, from the western

1333

U-series radionuclide dating of lake sediment

migration; see HILLAIRE-MARCEL and CAUSSE,1989) ; (b) selective extraction of uranium and thorium on anion exchange resins; (c) Electrodeposition of thorium and uranium on steel discs; and (d) counting in B-counters ( 2s4Th) and a-spectrometers (all other isotopes). Piston

cores

Modern Trona High

pools crust

Magadi

be

Oncolites Bedrock

Results are reported on Tables 1,2 and 3, and in Figs. 4 and 6, in the form of isochron plots (23@Th/232Th vs. 2WU/232Th)and (234U/ *‘*Th vs. *38U/23*Th) (see Luo and Ku, 1990, or BISCHO!=F and FITZPATRICK,1990, for a detailed discussion of isochron dating with total sample dissolution). Isochron equations were calculated as in BROOKSet al. ( 1977) (see also YORK, 1969, 1966; MCINTYREet al. 1966). RESULTS

Lithological Changes in Core MAG-87-01 Map 87-62’ Man 87-os’r

5 -

km -

. 4

Coring at site MAG-87-01 ended at about 2.3 m on a highly resistant chert layer that may belong to the early (?) Pleistocene Oloronga Beds (BAKER, 1958), which outcrop on both sides of the structural depression in the eastern arm of Lake Magadi. Four facies are observed downcore (Fig. 3): from O-40 cm, the upper unit (Unit I) is represented by homogeneous greyish to brownish silts; between 40 and - 130 cm, the second unit (Unit II) shows silty-clay layers alternating

.’

.:i

‘. ..

Mao 6790

L

Mag

87-01 MINERALOGY

LITHOLOGY (cm1

Sample

(%I

FTC. 2. Location of coring and sampling sites in Lake Magadi.

(MAC-87-02 ) and eastern arms (MAC-87-0 1) were subsampled for uranium-series measurements. In this article, we will refer primarily to the second one (MAC-87-01; Fig. 3), collected at a site characized by low sedimentation rates. 411samples were treated as follows: ) Aliquots were used for grain size distribution and mineralogical analysis. X-ray diffraction analysis of clay minerals was done on the ~2 pm fraction. Alter crushing, the coarser fraction was also subjected to X-ray diffraction controls to verify optical identifications of coarse minerals. Occurrences of calcite as thin veins and aggregates of rhombohedral crystals are frequent: Total carbonate determinations by acid treatment and CO*-release measurements were assumed to represent calcite abundances, inasmuch as sodium carbonates were virtually absent in the X-ray diagrams. 1 For the purpose of uranium and thorium isotope measurements, mineralogical fractions were isolated as follows: (a) Sedimentary sodium silicates, largely amorphous, and cherts occur as small nodules or thin interstratified veneers often mixed with aggregates of calcite. They were hand-picked in the coarser fraction, and the associated aggregates of calcite were dissolved in HCl. The HClleachates were then considered to contain the radioisotopes linked to calcite. The residue, i.e., the sedimentary sodium silicates and cherts, was finally dissolved with a mixture of hydrofluoric and perchloric acids [ HF + HClO,]. (b) Zeolites and illites constitute the bulk of the 15 pm fraction. After a weak HCl (pH - 1.5) removal of carbonates, if any, this fraction was completely dissolved with a [ HF + HClO,] mixture. 3) Chemical extraction of uranium and thorium from each subsample was done according to traditional techniques (e.g., LALLY, mixture al1982): (a) Spiking with a known 234Th-228Th-232U lowing the determination of chemical yields, and counting kfficiencies (the double thorium-spike permits the control of natural disequilibria between 232Thand its daughter 228Th,due to 228Ra

60

160

Calcite

I%. 3. Core MAC-87-O 1 lithology and mineralogical assemblages; thorium/uranium sample depths.

1334

C. Goetz and C. HiIl~re-MarceI

Table 1. Th and U contents and activity ratios in L.ake Table 2. Other relevant measurements. Magadi samples. _____._..__.._.__.___.._..._.._..._...-..-..._.._..._..~...~.._..._..-...-.._...__._ .,._.._.___.._..._.._._. 23qJ 234U 23m 234u 2qJ 232m 23m 23% Nud. t~@d tPdt9 .__..___._..__..__.__.._..._.._ ..._...__.__.._.._..._ ^__..._.. _..._.._..._ .._..._.. _ .._.._..._.. __.._.. _‘..

Field

U

Th

Modam sedkents Box I 253fo.07 Box2 0.94M.03

24.0i0.3

1.16kO.04 0.33iO.01

0.38iO.01 0.5OM.01

7.OitIl

1.3OAO.os 0.41f0.01

0.54fO.M

0.52to.01

Box3 5.35ztO.15

24.6zkO.4 1.38ztG.03 0.67M.02

0.93kO.03 0.5W.02

~0x4

31~0.5

1.30~0.03 0.24k0.01

0.32k0.01

14.0i0.3

1.41M.04

2.27ztO.08 0.9sM.02

24~1.05

asifo.02

CarboMles 7.5kO.2

Prti

5.8M.l

R3x

16.lM.4

6.4M.2

RC

4.0M.4

5.7M.2

RSx

9.O9zkO.04

Pr& R70

11.8ztO.5 1.3ktO.05

1.53foOS

7.5M.S

11.5M.6

t.53f0.03

7.4ii3.4

11.4iO.s 1.5ktO.05

5.5i0.2

1.56iO.04

5.1M.3

7.8iYO.3 1.2&0.03

5.QM.I

4.1fo.l

1.49fo.03

4.3kO.2

6.S0.2

5.3fo.2

3.7s. 1

1.46kO.04

4.4fo.2

6.4i0.3 1.18ti.03

125M.3 33.1fo.7

2~.08

4.3i0.2 22.6i0.7

l.SM.03

1.61fo.09 7AtO.4

R9a

1.45M.04

8.9kO.5

1.49iO.03

4.5i0.2

1.45fo.03

12.8k0.s 1.44M.04 6.7iO.3 1.31iO.03

5.8iO.2

l.KktcO.03 0.9lkO.03

1.07iO.06 1.19iO.03

R2B

1.64.k0.w o.ts&O.O4 1.47iO.04

6.3zkO.4

9.X0.5

R3B

3,07M.o9

224kO.09

1.33kO.04

4.2tIO.4

5.4t0.5

R4B

1.5lfO.02

257kO.06

1.33kO.02 1.8OM.04 2.4OkO.06 1.37M.03

1.59kO.05 1.4iO.l

R5B

1.2lkO.03 1.68M.05

1.32iO.04

2.9i0.1

1.14M.04

R6B

1.39M.03

22OS.09

1.39ztO.W 1.89kO.08 2.62iO.09

1.14M.02

R7B

1.19M.02

25M.l

1.34M.02

1.44io.09

1.15kO.02

1.34f0.02

1.7crtO.08

1.38H.03

2.06kO.06 2.84kO.09 1.71fo.W

l.WO.03

Pr8B 1.71iO.03 295kO.07 R9D

1.i39w.03

28M.2

22fo.l

1.93io.09 2.4M.l

l.Mio.06

ZeoLes RlC

2OOM.05 6.15fo.15

l.llM.03

R2C

2Olfo.03

1.45kO.02

RX

3.77M.06 12.7OkO.06 1.5OkO.02

1.4OH.03 5.0?.0.1

l.llfo.04

O.Wfo.03

4.4M.l

6.4PO.15

1.42kO.04

4.3kO.l

6.45kO.15 1.49iO.04

1.16iO.04 0.65fo.02

0.7sfO.03 0.9X0.03

Pr 4c

l.lW.03

RSC

282kO.07 4.45kO.15 1.44fo.04

R6C

1.83iO.06

R7C

261fo.07

3.96kO.09 1.37M.04 2.03kO.07 2.78fO.09 1.13fo.W

RSC

1.84M.05

272M.07

R9c

3.52M.08

6.X0.2

5.OiO.1 1.34fo.05

1.96kO.08 1.13io.04

1.46iO.05 2.0~0.~

2.&!&l

1.18iO.05

l.SliO.05

1.05kO.03

3.0M.l

1.22iO.04

1.49kO.04 1.61iO.06

2.39.kO.09 1.12kO.05

1.73M.03

1.39ztO.04 4.73i0.15

1.07iO.06 1.19kO.03

PNF2 0.3OkO.01 7.05M.11

l.lOkO.04 0.13f0.01

Phosphares PhFl 265M.07

1.43f0.03 1.23f0.04

0.61SIO2 0.39M.01

0.86kO.02 1.09kO.02 0.48iO.01 0.89M.02

Sodie silicaks wtd ckr& from core MAG 8742

upperUnit L.owexunit

0.46M.01 0.11 fo.Ol

1.44f0.05 1.32fO.09

12.5f1.7 1.3 M.1

lE.Oi2.5 1.8iO.1

2.0M.3 1.07 a.09

Enwase Ngiro (4.9 f0.2) 10.” 1.62 f0.08 Brims (West) (29 fs) lo-’ 1.46 f0.03 Brines (East) (4.6fl.6) 10-l 1.44 fO.O1

______.._ ..___._-------_--

__________ ____._____ --________ __________ -----___-_ -*________ .--

(data with 1 standasddeviation)

1.36kO.04

So& siIkules and cheris R 1B 1.85M.04

243fo.05 1.02#.03

Walers

I418

R&l

UpperUnit Lower Unit

0.14io.01 0.6oB.02 PNF3 249M.07 10.8fo.2 1.4OiO.040.711to.020.99fO.030.71M.02 ----_I_---II_-~--(datawith 1standarddeviation)

with Ii~t-floury sedimentary silicate horizons, which are more abundant at the top and the bottom of the unit; between - 130 and 135 cm, the third unit (Unit III) contains indurated ferrugineous layers and nodules, which were slightly reduced at their surface when the core was opened, and the lower unit (Unit IV), not unlike Unit II above, shows alternating greenish silty-clays and sedimentary silicates. These lithological changes indicate two major lacustrine episodes, Units IV and Ii, separated by a gap represented by

the iron-rich layers, Unit III, very probably formed under aerial canditions. The top unit may correspond to the shoaling facies of the last lacustrine episode, which culminate some 12,000- 10,000 yr BP (see above). We may assume that Unit II was essentially deposited during this high lake level phase. Unfortunately, it was impossible to date this unit with 14C because the organic carbon content was beiow the analytical threshold of 1%. Inasmuch as the upper unit in core MAG87-02 from the western arm of Lake Magadi (see below), correlates with the present Unit II, this assignment seems highly probable: Measurements by accelerator mass spectrometry yielded 14Cages of 12,090 t 120 and 10,800 rl: 120 yr BP for organic-rich horizons interbedded with magadiite layers, respectively, at depths of about 230 and 118 cm downcore MAG-87-02 (TAIEB et al., 1989). The age of the older lacustrine episode represented by Unit IV in core MAC-87-01 is unknown. As a consequence, the duration of the aerial phase which allowed iron concentration to occur in Unit III cannot be interpolated. Semiquanti~tive dete~inations of mineralo~c~ abundance changes downcote am reported in Fig. 3. The “detrital” component in the lacustrine sediment, which constitutes the bulk of the 0.5 mm-5 pm fraction, essentially indudes anorthoclase, amphibole, and quartz grains, likely inherited from the Precambrian basement. The proportion of these minerals is around 10% (in weight) and varies only slightly throu~out the sequence, except for one horizon at about

Table 3. =U/W

and =Ti@%h disequilibria in modem Lake Uagadi clays

__------~-_~~l--__---_________~~_~_ Field ?J =l’h =I%-Excess Number =%J PLTtr ( dpm@* ______---^l--_-^_--__-__-____---_______-~_ Box I Box2 Box 3 Box 4

1.16SN4 1.30 ko.05 1.38kO.03 1.30 a.03

1.35 SW2 1.97 SW9 2.79 Lt.o.05 3.00 kO.02 2.6150.06 9.6 5~0.9 1.98 kO.05 7.4 LkO.9

___I~_~__-___-________I_____________LII_~~ * =Th-Excess

= qh

dpm/g tt228ntpgnt)

-11

U-series radionuclide dating of lake sediment 1.2 -

MODERN SEDIMENTS

2~u/=Th

0.8 -

0.6

u0U/u2Th 0

16

-

0.4

2y

U/2’2

0.8

0 1.2

CARBONATES

Th

1335

a~ TN232 Th

-

, 0

1

I

I 0.4

I 0.8

I

1

Th

1.2

I 1.6

AGE=5500*4OOy

u”Tb/232Th

"1

= U/“’

I

Slope = 1.54 f 0.05

231 W

8

-

4

2’*v/

8

12

16

ZEOLITES

2’?h

2y u/2’2 Th

232 Th 0

0

,

1

4

“‘Th

0 Slope = 1.56 f 0.03

y =

2 MAG-87-02 MAG-87-O

CORE CORE

I

I

I

8

AGE

2’?h

/

3

CORE CORE

I

I 4

0

I

MAG-87-01 MAG-87-02

(0.091 f 0.005) x + (0.88 f 0.02)

1

234U/ 2’%h 0

8

-2y

2

U/232

4

6

8

PHOSPHATES

l-h

1 16

= 104OOf6OOy

2’sU/2’?h 0

I

12

I

I 2

0

4

2w) TN

232 Th

1

I

0

I

4

AGE=

I

6

I 8

11 OOOf6OOy

3

6-

Slope = 1.422 f 0.003

4-

2

y

=

(0.096 f 0.005) + (0.60 f 0.01)

2-

0 0

I 2

I

I 4

1

“*

U/232

I 6

1

I-h I 8

0

2

4

6

8

FIG. 4. Isochrons of modem sediments and of various mineral phases from Lake Magadi cores.

90- 100 cm downcore, which shows much higher proportions ( - 30%). The major component of the sediment corresponds to the fine fraction ( ~0.5 pm). It contains illite and zeolites (erionite, analcime, and chabazite) originating from the in situ hydrolysis of volcanic glass (see SURDAMand EUGSTER, 1976). This fraction represents roughly 80% (in weight) or more in the bottom and top units of the core.

The sedimentary sodium silicates, intimately mixed with genetically related cherts, are particularly abundant at the base and top of Unit II, where they reach a maximum of about 60% in weight. They include magadiite, kenyaite, and makatite, considered by EUGSTER ( 1970) to represent authigenic minerals deriving from the dilution of sodium carbonate-rich brines by fresh water.

C.

1336

Goetz and C. Hillaire-Marcel

Finally, the thin calcite veneers and aggregates are fairly equally spread throughout the sequence and amount to a few percent by weight of the total sediment. They are very often intimately mixed with the sedimentary silicates and are considered to have formed during a late (?) diagenetic stage ( EUGSTER, 1970). Lithological Changes in Core MAG-87-02

A detailed description of core MAG-87-02 can be found in TAIEB et al. ( 199 1). Three units have been identified: ( 1) from 870 to 728 cm downcore, a lower laminated unit with zeolites, sodium silicates, and silts: (2) from 728 to 278 cm, a middle unit of homogeneous beige silty-clay with pyrite; and (3) from 278 to 6 cm, an upper laminated clay unit not unlike the lower one. Both the upper and lower units show a relatively high content in organic carbon (about 5% in weight) and are rich in pollen grains, indicative of a climate more humid than the modern one ( TAIEBet al., 199 1). They are tentatively correlated with Units II and IV from core MAG-87-01 assigned to high lake level episodes. Thorium and Uranium Concentrations and Isotope Activity Ratios in “Modern” Sediments

The organic-rich sediments box-cored in the persistent pools of water show very high concentrations of thorium (up to -3 1 pg/g; Table 1) compared to those of the bedrock, (-3 pg/g; GOETZ, 1990). This indicates significant mass losses during the previous pedologic evolution of the sedimented particles. Uranium-leaching certainly occurred at the same time: Uranium-series studies in East African soil profiles actually show strong excesses of 230Thover its parent isotope 234U (GOETZ, 1990). When particles are eroded, then deposited in low-Eh environments, post depositional uraniumuptake results in reduced 230Th excesses, occasionally in an inverse 230Th/234U disequilibrium (Table 1, sample 066). This “authigenic” uranium is characterized by relatively high 234U/238Uactivity ratios. The small pools are fed by strongly mineralized hydrothermal springs with high uranium concentrations (from about S-30 rig/g;; Table 2). The uranium, leached from the aquifer rocks during the hydrothermal circuit, is characterized by a 234U/238Uactivity ratio averaging 1.45 (Table 2; GOETZ, 1990). It Seems likely that large fluxes of 228Raalso occur, since a strong excess in 228Th over 232Th is observed in the sediments (Table 3 ) .

Uranium-series Concentrations and Activity Ratios in PaIeoIake Sediments

All mineral phases that were analyzed for their thorium and uranium isotope contents show evidence of a dilution by “authigenic” uranium of the radioisotopes inherited with the detrital fraction. The latter are “labelled” by the 232Thcontents, and the authigenic input is shown by variable 238U/ 232Th ratios (from about 1-9) and also by 234U/238U ratios much greater than the secular equilibrium value of 1.O(Tables I, 2, and 3). The only distinctive feature between all these minerals is the much higher uranium content observed in calcite. DISCUSSION

The Uranium “Diluting” the “Modem” Sediments

The first point we will address concerns the “authigenic” uranium diluting radioisotopes inherited with the detrital particles box-cored in the shallow pools of hydrothermal waters. The corresponding isochron plot in Fig. 4 depicts a mixing system between a detrital component, labelled by the yintercept [ ( 23”Th/232Th)0 - 0.49 + 0.021, and an authigenic component characterized by 234U/238U and 23qh/234U activity ratios of - 1.49 + 0.07 and -0.04 t 0.03, respectively (isochron slopes). The “authigenic” uranium shows an isotopic composition indistinguishable within standard deviations from that measured in modem hydrothermal waters ( 1.44 f ~0.0 1; GOETZ, 1990). Due to the large relative error on the isochron slope, the “age” of the authigenic uranium lacks in precision (4.9 + 3.3 ka; Table 4). The Uranium of the Diienetic MAG-87-01

Calcite in Core

Here again, thorium and uranium isotopes define an almost perfect two-component mixing system as shown on Fig. 4. The data points fit on highly correlated (I > 0.99) isochrons, which implies that a single phase of uranium-uptake occurred during a relatively short period of time (i.e., that the diagenetic calcite was formed during a single and short event). From the 234U/238U (-1.54 rt 0.05) and 23’?h/234U (0.050 + 0.003) ratios of the “authigenic” component (i.e., the isochron slopes), a mean age of 5.5 + 0.4 ka can be derived for calcite precipitation. This is much younger than the age of the last high Lake Magadi level (ca. 12,000-10,000 yr BP).

Table 4. Isochron equations for modem sediments and diagenetic minerals in core MAC-87-01

A: (=“ThPTh) *

= a (=‘W-h) b

+b a1

B: (=‘IJpqh) b

= a’ (=Uf=Th)

+ b’

AGE (W

(=‘U/=U),

0.044 fo.029

0.487 fo.016

1.490 f0.068

-0.064 M.022

CL&OlWttX

0.050 fo.003

0.880 M.024

1.537 HI.045

-0.204 Ml.170

PhosphaIer

0.096 iO.005

0.601 fo.013

1.422 fo.003

-0.040 M.007

ll.0M.6

1.44io.04

Zcolites

0.091 fo.005

0.880 fo.019

1.559 fo.028

-0.299 iQ.038

10.4 i0.6

1.58 M.03

No-silicolcs (Unit II) 8 Chcra (Unit IV)

0.056 fo.024 0.622 SO86

1.030 fo.023 -0.098 iO.210

1.553 M.068 1.504 i0.243

-0.47 M.10 -0.282 fo.443

6.3 XI.8 98.5120

1.57 fo.07 1.70 fo.34

Modern

sedimenls

4.9 0.3

1xl H.07

5.5 fo.4

1.55 M.05

U-series radionuclide dating of lake sediment Mag

85-01

LITHOLOGY

GRANULOMETRY

‘4C

(cm)

(%I 50

0

100

100 BP

7202 a5 BP 050 * 100 BP _----

40 -_-_-_ -_---50 ---.-~..... 60 ---E--x--_ 70-

---I_ -80 -- ------I_ 90 -_ ----100 ----

110 --

-

-

_--

-E_ _---_--

-

120 50 a

Silts and sands

p-71

Clays

1250

CASANOVA,

160

1986

656 m ( HILLAIRE-MARCELand CASANOVA, 1987) by waters collected in a drainage basin extending as far north as the Mau escarpment (some 100 km northwards). Diatom assemblages in core MAG-88-02 (BARKER, 1989) indicate that the paleolake waters were diluted compared to modem brines. Percolation of these waters through the deposits which were roughly 2 m thick in the eastern arm apparently resulted in a complete diagenetic transformation of the fine sediment fraction into zeolites, including the lower sedimentological Unit IV assigned to a former lacustrine episode. Simultaneously, some “authigenic” uranium was incorporated into the zeolites. A rapid closure of the thorium/ uranium system in zeolites prevailed as indicated by the rather good correlation of data points on the isochron lines (Fig. 4). This mechanism of “synsedimentary” zeolite formation by percolation of diluted waters had previously been proposed by SURDAMand EUGSTER ( 1976). It should be emphasized, however, that this mechanism involved simultaneously the two lithological units (II and IV) assigned to distinct lacustrine episodes. With respect to thorium and uranium isotopes, either the zeolites from the lower unit were reset back to zero by the recent phase of uranium uptake, or they formed largely during this late episode. The same conclusion can be drawn for zeolites from core MAG-87-02, in the western arm: Uranium-series data from the lower and upper lacustrine units fit on the same isochrons (Fig. 4).

(pm) 2000

FIG. 5. Lithology of flat deposits and 14Cages obtained on calcitic oncolites from core MAG-85-0 1,in the western arm of Lake Magadi (from

1337

).

We are tempted to assign this phase of CaC03 precipitation to the shoaling period following the mid-Holocene minor rise in Lake Magadi level. As already mentioned, this episode is recorded, in the western arm of Lake Magadi, by “flat” deposits containing calcitic oncolites, the 14Cages of which suggest shoaling at ca. 5000 yr BP (Fig. 5). Nevertheless, the age of the U-CaCOs coprecipitation phase in the sediments of the eastern arm confirms the hypothesis of EUGSTER ( 1970) regarding late diagenetic calcite deposition. The Diagenetic Uptake of Uranium by Zeolites in Core MAG-87-01

The uranium and thorium isotopic compositions of all zeolite fractions sampled throughout the sequence also fit on well-correlated isochrons (Fig. 4). This means that one single phase of uranium uptake is recorded by the zeolites. Isochron slopes yield 2?J/23*U and 23”Th/234Uratios of - 1.56 f 0.03 and 0.09 1 rt: 0.005, respectively (i.e., an age of 10,4 + 0,6 ka for the “authigenic” component). It may be concluded that uranium uptake in the zeolites occurred during a much earlier diagenetic stage than that allowing calcite to precipitate, i.e., at a stage with an interstitial water chemistry differing from that which allowed calcite to form later on. Moreover, zeolite formation appears to correspond to the last high Lake Magadi stand of the Pleistocene-Holocene transition. Indeed, during this episode, the basin was filled up to an altitude of about

The Age of the “Authigenic” Uranium in Sodium Silicate Minerals and Cherts

The horizons and nodules rich in magadiite, kenyaite, and makatite (SHEPPARD et al., 1970), thought to be “authigenic” minerals (EUGSTER, 1969, 1970; O’NEIL and HAY, 1972), are mixed with late diagenetic cherts that formed through Na-leaching process. As opposed to zeolites, these minerals do not show one single phase of uranium uptake throughout the sedimentary sequence: uranium-series data (Fig. 6), rather, indicate two phases. All samples from the upper lacustrine Unit II fit on an isochron with a 23?h/234U age of 6.3 f 2.8 ka, which would indicate uranium fixation during a late diagenetic phase. Compared with other minerals, the Na-silicates and cherts from Unit II yield a poorly correlated ( 23@Th/232Thvs. 234U/232Th) isochron; this is essentially due to one remote datum, sample PR 4B (Fig. 6), that may well be a chert nodule reworked from the lower Unit IV and incorporated into Unit II. This sample was collected in the horizon showing a peak in detrital inputs (95 cm downcore; Fig. 3) and, indeed, its thorium and uranium isotope composition fit on the isochron defined by the sedimentary silicate minerals from the underlying lacustrine Unit IV (Fig. 6). As a matter of fact, the three samples from Unit IV seem much older. Their isochron slopes (Fig. 6) yield an age of 98.5 f 20 ka. The Na-silicates and cherts from the lower lacustrine Unit IV possibly represent diagenetic minerals formed in sediments deposited during the high Lake Magadi episode, which culminated at 135 + 10 ka as shown by uranium-series measurements on littoral stromatolites ( HILLAIRE-MARCEL et al., 1986), and which preceded the 12,000-10,000 yr bp high stand. Here again, Na-silicates and cherts from the lower and upper lacustrine units in core MAG-87-02 (western arm) fit

1338

C. Goetz and C. Hiitaire-Marcel “‘U?=?‘h

/AGE

23?h?32Th -

- - t

COffE

87-01

{UNIT

- - CORE

87-02

(UPPER

UNIT)

6 -

I 15

/

/

IO

/t

I

10

5

= 1.55 f

y

4-

0.07

coaE

87-01 8752

(UNIT

II)

(LOWER

UNIT)

15

234U?32Th 3

0

*=kJ , ih=‘Th . I 20 25

r

,

=

(0.056

J/ o-Q?--

0.027)

x

+

(1.03

f

0.02)

___.+-_----4-

,

1 5

0

23? h?32Th 2

Q

f

SLIMPLE PR 48

2-

PC) 43

Of?6L---, . f 0

0 -

- - Ilt - - CURE

6-

Slope 3AhfPLE

ka 1

/

P’

i

: 6.3 f 2.8

lo-

It)

234U P3’Th I I 15 20

, 10

AGE: Y = (0.62

f 0.09)

X

98.5 f 20 ka

+

(-0.1

k 0.2)

9

SAMPLE

PF? 48

1

-o-

1

_ - t

CORE - - CORE

(UN17

874f 87-02

IV)

(LOWER

- 0 -

UN/T)

t

236U P=*Th 0,

I.

I

$.

1.I

I

1

0

*

0

1,

2

3

0

-

CORE

- CORE

1

87-01 8742

[UNIT

IV)

(LOWER

2

UNIT)

3

4

EfG. 6. Isochrons of sodium silicates and cherts from lacustrine Units Ii and IV (data for core MAG-87~2 are from 1990).

G~ETZ,

on the isochrons of 98.5 + 20 and 6.3 t 2.8 ka, respectively, already defined by core MAG-87-O I samples (Fig. 6). The Uranium anteing

the Basin

As a first-order approximation, a very simple mixing system prevails with respect to 234U/238Uactivity ratios in diagenetic minerals from sediments deposited during the last high stand of Lake Magadi (Unit II in core MAG-87-O 1; upper unit in core GAG-87-02). When plotted as a single isochron (Fig. 7a), their 234Uf 232Thvs. 23sU/232Th activity ratios point toward a 2?-J/238U ratio of - 1.43 for the “amhigenic” uranium (the isochron slope). This isochron has a negative “y-intercept,” indicating that the uranium inherited with the detrital

15

fraction had a 234U/2.‘8U ratio lower than the equilibrium value of one, i.e., that the detrital particles carried into the paleolake had previously lost some uranium during their pedologic cycle (with preferential leaching of 234U). More detailed examination shows small differences between the initial 234U/238Uratios calculated for each mineral (Fig. 7b). Compared to the modern Lake Magadi brines, all diagenetic minerals except phosphates show higher excesses of 234U(vs. its parent isotope 238U) . We thus hypothesize that at least two sources of u~nium are involved. One with a high *34Uexcess would have dominated during high lake stands; a second, with a lower excess, would characterize the modern low lake stand. Today, most of the uranium coming into Lake Magadi originates from highly mineralized hydrothermal springs.

ALLSAMPLES

10

0

/] 1

0

5

10 (a)

15

1.5

2

(W

FIG. 7. (a) 234U/*32Thand *3*U/232Thratios in all diagenetic minerals and modern sediments from Lake Magadi cores (lef?); (b) Initial zwU/2’8U ratios in diagenetic minerals (calculated from isochron equations) compared to those of modem brines, clays, and surface waters. (Data for travertine pipe carbonates are from ~ILLAIRE-MARKET et al., 1986.)

1339

U-series radionuclide dating of lake sediment Through the hydrothermal circuit, efficient uranium-leaching of the aquifer rocks occurs (as shown by the high uranium concentrations observed in hydrothermal springs; Table 2) with preferential removal of 234U (see KRONFELD, 1974; SZABO, 1982). Unfortunately, we did not collect hydrothermal spring water for uranium-isotope determinations. Modern Lake Magadi brines, which derive largely from hydrothermal inputs, indicate a maximum value ( - 1.45 ) for the isotopic composition of the uranium from deep origin (Table 3). A better indication may be found, however, in the 234U/ 238Uratio ( 1.24 f 0.0 1) measured in a travertine pipe built at the very outlet of a hydrothermal spring (see HILLAIREMARCEL et al., 1986). In contrast, during high stands, the lake is predominantly fed by surface or shallow aquifer waters (HILLAIRE-MARCELet al., 1986) drained by major rivers, notably the Enwase Ngiro (Fig. 1), which carries uranium originating from soil leaching processes. This uranium shows a much higher excess in 2MU(2?J/238U = 1.62 + 0.08; Table 3), which also characterizes minerals formed during episodes of diluted lake waters. It is concluded that uranium isotope ratios in Lake Magadi sediments and minerals likely constitute an indicator of the relative dilution of deep hydrothermal inputs by surface waters (Fig. 7b). ?h/“2Th

Ratios in Detrital Particles

Up to now, we have paid little attention to the “detrital” fraction, which is always the first term of the mixing systems we examined. As may be expected, this fraction is characterized by deficits in 234Ucompared to 238U,and by excesses of 2qh compared to 234U,resulting from uranium leaching during its previous pedologic cycle. Fine detrital fractions in core MAG-87-O 1 (eastern arm of Lake Magadi) point toward a (23@Th/232Th)ratio of about 0.88 -+ 0.02 for the “unsupported” 23@Th(the y-intercept of the isochron line of zeolites or from that of calcite; see Fig. 4). It may be interesting to note that the detrital fractions from littoral stromatolites formed during the last high Lake Magadi episode, i.e., between 12,000 and 10,000 BP, yielded a comparable value of 0.87

MODERN LOW LAKE MAGADI SOURCE ROCKS

f 0.02 ( HILLAIRE-MARCEL et al., 1986). A similar calculation for the detrital component of the modem sediments (boxcores) from the western arm of Lake Magadi yields a lower value of 0.49 + 0.02. The difference may have several causes: 1) grain size differences between detrital fractions (uraniumablation rates in soils-and therefore 23@Th-excesses-are much higher for fine than for coarse fractions, due to their larger reactive surface (e.g., HILLAIRE-MARCELet al., 1990)); 2) uranium-leaching rates in soils varying through time as a consequence of climatic changes (e.g., GHALEB et al., 1990); and 3) distinct source rocks. The latter cause is no doubt primarily responsible for the low 23@Th/232Thratio observed in modem sediments. These are weathering products of local basalts outcropping in the small catchment area of the western arm of Lake Magadi (Fig. 1b) ; these source rocks show a low 2qh / 232Thratios (0.66 + 0.02; GOETZ, 1990). By comparison, the crystalline rocks of the regional Precambrian basement, which supplied most of the detrital material during the high lake level episodes (Units II and IV in core MAG-87-01), yielded a 23@Th/232Thratio of 0.89 f 0.02; GOETZ, 1990) accounting for the values calculated above for the y-intercepts of the zeolite and calcite isochrons. The “unsupported” 2)orh seems, therefore, to label the source rocks of detrital supplies (Fig. 8 ) . This is confirmed by the initial 2qh/232Th ratio indicated by the y-intercept of the isochron plot for the phosphates in core MAG-87-02 (0.60 + 0.01). As seen on Fig. 1, core MAG-87-02 allowed the recovery of sediments deposited at the foot of a fault plane. The uplifted block of trachytes and basalts constituted a paleopeninsula penetrating deeply into the last high paleolake Magadi. Abundant illites ( TAIEB et al., 1989) are a distinctive feature of the local sediments, which carry in their 23@Th/232Thratio the signature of the mixed sources of terrigeneous material: the regional Precambrian basement and the local volcanic rocks.

“0,

/ ‘=Th

'1

sOlLPR0FR.J ON OMNfTE

SOIL PROFEE ONBASALT

-

?J/

Sccvhr Equili&iam 0.5

ZEOLJTES

0

STROAUTOLITES

=I-h 0

0

o

1

,,,,,,,,, 0

I

I

1 1 I

2’4U/“bTh

I

2

2?h/232Th and 234U/232Th ratios in source.rocks and sediments from the Lake Magadi basin: modem low GOETZ, 1990. For stromatolites: HILLAIREMARCELet al., 1986.) FIG. 8.

lake level situation vs. high paleolake situation. (Data for source rocks:

I

I

I

4

1340

C. Goetz and C. H~Ilai~-Marcel CONCLUSIONS

This study confirms previous interpretations concerning the s~n~men~/~rly diagenetic origin of zeolites and silicate minerals: ali samples from the ~~~ol~~cai unit assigned to the last high Lake Magadi level indicate ages of ca. 11,000 a for the coprecipitated uranium, which agrees with the 14Cchronology aiready established for this episode. Similarly, calcite nodufes and veins are demonstmted to belong to a late diagenetic evolution, at ca. 5,ooO BP, when the Lake Magadi water chemistry probably differed from the modern situation: calcium carbonate precipitation was apparently a dominant process, as shown by oncolites formation during this period. Our results indicate that early diagenetic minerals other than calcite can be dated by the tho~nm/uranium disequilibrium method, provided that a closure of the radioactive system prevails. This is ascertained with respect to the last high lake level episode but remains to be demonstrated for former lacustrine phases. Tho~um~umnium data may therefore provide minjmum ages for lacustrine oddments deposited during recent humid episodes in presently arid basins. A~knuw~edgme~s-~~~~ version of our manuscript benefited from discussions with Dr. A. Kaufman (Weizmann Institute) and Drs. J. Bourne and J.-C. Mareschai (UQAM). Critical readings by J. L. Bishoff. S. Szabo (USGS). and B. Blackwell (McMaster Univ.) helmed to clarify several‘aspects’of this paper. Thanks are also due to Ms. M, Laithier ~~QAM) for the drawings. financial support from the CNRS, France ( Programme PIRAT), and NSERC, Canada, is also acknowledged. Edit&xi

handling H. P. Schwartz

BAKERB. H. ( 1986) Tectonics and volcanism ofthe southern Kenya RiA Valley and its influence on sedimentation. In Sedjmentation in the Afn’can Rifts (ed. L. E. FROSTICKet al.); Geol. Sot. London Spec. Pub. 25, pp. 45-57. BARKER P. ( 1989) Diatom Analysis in the East &iicun Ri_fi f’a’uliey. ~e~~~~i~aryres~ts~rom L.&e ~agad~~ Kenya and Lake ~~an.yara, Tanzania. Univ. Loughborough (UK) W%rking Paper 3, pp. l30. BF,CHO~?F J. L. and FITZPATRICKJ. A. ( 1990) /I-series dating of impure carbonates: An isochron technique using total-sample dissolution. Gent&m. ~o~~o~~~~, .&a S&543-554. BROOKSC. HART S. R., and WENDTL. f 1977) Realistic use of twoerror regression treatment as applied to ~bid~um-strontium data. Rev. Geopkys. Space Phys. X0,551-577. BUZZERK. W., &AC G. L., RICHARDSON J. L., and W~S~BO~R~KAMAu C. f 1972) Radiocarbon dating of East African lake levels. Science 175. lO69-1076. GGANOVA J. ( 1986) Les stromatolites continentaux: pal&o&cologie, paliahydrologie, pal&climatologie. Application au rift Gregory. Thesis Doctorat d’Etat Univ. Aix-Marseille II, France. CASANOVAJ. and HILLAIR~-MARVEL C. ( 1987) ~hronologie et paI&ohydrologie des hauts niveaux quatemaires du bassin NatronMagadi ( Tanzanie-Kenya} d’aprb la composition isotopique ( “0, 13C 14C, U/Th) des stromatolites littoraux. Sci. G&/. Bull. 40, 121’-134. CAUSSE C. and HILLAIRE-MARCELC. ( 1986) Radi~bronol~~ fondies sur le d~~uilib~ (par d&ut/par exe&s) du 230Th appiiq&es B I’&ude de Iacs afticains: Wadi Shati ( Libye) et NammMagadi ( Kenya-Tan~n~e~ . Travatax er ~~~rn~nf~ 197, 5 1.

DAWSONJ. B. ( 1962) The geology of Oldoinyo Lengai. &Id. Yolcanoi. 24,349-387. DowsoN R. G. f 1963) Geology of the South Han Area. Ceol. Sure. Kenya, Apt. 60. EUGSZR H. P. ( 1969) Inorganic bedded cherts from the Magadi area, Kenya. Contrib. Mineral. Petrol. 22, 1-3 I. EUGSTERH. P. ( 1970) Chemistrv and origin of the brines of Lake Magadi, Kenya. M& Spec. Piper 3, ppy2 15-235. EUCSTER H. P. ( 1980) Lake Magadi, Kenya, and its precursors. In ~~~e~~ai~neBrines and ~va~or~t~&Env~~n~nts fed. A. NISSEHBAUM); ~~~~~~rne~s in Sed~mento~og.~2& pp. 195-230. Elsevier. EUGSTERH. P. ( 1986) Lake Magadi, Kenya: A model for rift valley hyd~h~mist~ and ~imen~tion? In Sed~mentatjun in the~~jcan R$is fed. L. E. FROSTICKet al.); Geol. Sot. London Spec. fib.. 2.X pp. t 77- 189. GHALE~ B., HILLAIRE-MARCELC., CAUSSEC., GAKIEPV C., and VAL.LIERES S. ( 1990) Fractionation and recycling of U and Th isotopes in a semi-arid endoreic depression of central Syria. Geochins. C~~srn~~bj~~. Acta 54, 1025- 1035. GoETZ C. f 19901 Potentiel chronoloa~aue des d~s~uil~b~s Th /U dam les ~ofits & altin-ation et les s&mints carott& bes lacs Ma&d; (Kenya) et Manyara (Tanzanie). Thesis Doctorat Univ. Aix-Marseille II, France. HAY R. I...( 1968f Chert and its sodium-silicate precursors in sodiumcarbonate lakes of East Africa. Co&%. Minerai. Petrol. i?, 22% 274. HAY R. L. ( 1983) Natr~ar~nat~te tephra of Kerimasi volcano, Tanzania. G&ogy 1I, 599-602. HENDY C. H., HEAL T. R., RAYNERE. M., SHAW,J., and WILSON A. T. ( 1979) Late Pleistocene Glacial Chronology of the Taylor Valley, Antarctica, and the Global Climate. &al. Res. II, 17% 184. HILLAIRE-MARCELC. and CASANOVAJ. ( 1987) Isotopic hydrology and ~aleohydrol~y of the Magadi ( Kenya )-Natro~ (Tanzania) basin during the Late Quatema~. Paiaeogr. ~alaeoc~~rna~[~i. Palaeoecuf. 58, 15S- 181. HILLAIRE-MARCEI. C. and CAUSS~:C. ( 1989) Chronoloie ThjU des concr&ions calcaires de varves du lac glaciaire Deschaillons ( Wisconsinien in&ieur). Canadian J. Earth Sci. 26, 1041-1052. H~~LA~RE-M~~~~~C., CAR%) O., and CA~ANOYA J. f 1986) ‘v and Th/ U dating of Pleistocene and Holocene Stroma~olites from East African Paleolakes. &at. &s. 25,312-329. H~LLAI~~MARCE~C., VALL~ERE~ S., GHALEBB., and MARESCHAI, J.-C. (1990) ~uilib~ Tk/U dans les sols carbonat& en climat subaride: estimation des llux #uranium et vitesse d’&osion. Le cas du bassin de Palmyre (Saie). C. R. Acad. Sci. Paris 31f,233238. ISAAC G. L. ( 196.5) The stratigraphy of the Peninj Beds and the provenance afthe Natron Aust~opith~ine mandible. Qu~ernur~a 7,101-130. JA~OY M., P~NEAUF., STAUDACHERT., CHEMINEEJ. L., and K%FT M. ( 1989). Mantle volatiles sampled from a continental rift: The 1988 eruption of Oldoinyo Len@ (Tanzania). Terra Abstr. 1, 324. JONESB, F., ELGSTERH. P.. and RETTZGS. L. ( 1977 ) Hydrochemistry of the Lake Magadi basin, Kenya. Geo&i!?~. Cu,~mo~h~rn~ ActLl41, 53-72. KAUFMAN A. (1964) Th230-Uz14dating of carbonates from lakes Lahontan and Bonneville. PhD dissertation, Columbia Univ. KAUFMAN A. ( 197I ) U-series dating of Dead Sea basin carbonates. &o&m. Cosmochim. Acta 35, 1269-l 28 i, KRONFELDJ. ( 1974) Uranium deposition and Th-234 alpha recoil: An explanation for extreme U-234/U-238 fractionation within the Trinity aquifer. Eurth Planet. Sci. Let&.21, 327-330. LAI.LYA. E. ( 1982) Chemical procedures. In ~~~~j~~~S&?.s Diseyuilihrium-Apt~on to environmental Problems fed. M. IVANOVICHand R. S. HARMON), pp. 79-106. Clarendon Press. Luo S. and Ku T.-L. ( 1990). U-series isochron dating: A generalized method employing total-sample dissolution. Geochim. Cosmachim. Acta 5% 555-564. MCINTYRE6. A., BRNXG C., COMPSFONW., and TUBEK.A. ( 1966) The Statistical Assessment of Rb-Sr isochrons. f. Get&r.~~ Res. ?I, 5459-5468.

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