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Use of oxygen isotope ratios in correlation of tuffs, East Rudolf Basin, northern Kenya
Earth and Planetary Science Letters, 25 (1975) 291-296 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands
USE OF OXYGEN ISOTOPE RATIOS IN CORRELATION OF T U F F S , EAST RUDOLF BASIN, NORTHERN KENYA THURE E. CERLING*, DONALD L. BIGGS and CARL F. VONDRA Department of Earth Science, Iowa State University, Ames, Iowa (USA) and HARRY J. SVEC Department of Chemistry, Iowa State University, Ames, Iowa (USA) Received June 5, 1974 Revised version received January 14, 1975 Oxygen isotope determinations were made using CoF3 to extract oxygen from 27 volcanic glass samples from the East Rudolf Basin, northern Kenya. Results show that the older tufts are progressively enriched in 180 and that this index can be used in the correlation of volcanic ash units. This method could not distinguish individual samples from the youngest units studied because their ranges of 8180 overlap. The 8180 values for the shards in the Tulu Bor Tuff, the KBS Tuff, the Koobi Fora Tuff and the Chari Tuff range from 14.5 to 16.4, from 8.9 to 9.5, from 6.6 to 7.0 and from 7.0 to 7.2, respectively, in decreasing age. Determinations from pumice cobbles are consistently higher than the above values.
1. Introduction Volcanic ash beds are particularly useful as stratigraphic markers as they are deposited instantaneously (in the geologic time-sense) over a wide region and most o f them have some distinctive property that can be used to distinguish them from other ash units. Indices o f refraction and mineral assemblages have been generally used to characterize glasses. New techniques have recently led to the identification of individual ashes by studying elemental compositions as determined by neutron activation analysis, X-ray fluorescence and arc-emission spectroscopy. The purpose o f this paper is to present another instrumental technique for the identification o f volcanic ash units: that o f lso/160 analysis. This study resulted from the failure o f some of the above techniques to resolve correlation of the tuffs o f the East R u d o l f Basin, northern Kenya. The * Present address: Department of Geology and Geophysics, University of California, Berkeley, California 94720.
Plio-Pleistocene sequence in the northeastern portion o f the Lake Rudolf Basin is o f wide interest because of the abundance o f hominid fossils and associated stone artifacts in the sediments. However, because of the discontinuous nature o f the outcrops and the rapid lateral and vertical changes characteristic o f fluvial-lacustrine deposits, it is not possible to trace most strata from one locality to another. The area o f interest is shown in Fig. 1 and stratigraphic relationships are shown in Fig. 2. The tuffs studied were the Tulu Bor Tuff, the KBS Tuff, the Koobi Fora Tuff Complex, and the Chari Tuff which have recently been dated at 3.18 m.y., 2.61 m.y., 1.56 m.y., and 1.28 m.y., respectively [1]. The localities for the individual specimens are shown in Fig. 1. Four Tulu Bor Tuff samples were identified in the field; two others were from ruffs thought to be in similar stratigraphic positions (Table 1). All the KBS Tuff samples except one (sample 7) were from an ash bed that could be traced from Koobi Fora to the Karari Ridge; sample 7 is a tentative field correlation. All samples from the Koobi Fora Tuff Complex and from
292
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the Chaff Tuff could be identified in the field. No stratigraphic units can be traced from the Karari Ridge to the Ileret Area because of poor exposure. The Chari Tuff and the Koobi Fora Tuff are separated by about 40 km and both occur near the top of their respective sections (Fig. 2). A more detailed description of the geology of the region has been given by Vondra et al. [2] and by Bowen and Vondra [3].
related with increasing age. Although this is due in some cases to an original difference in the 180 content of the ash, usually the increasing ratios are caused by an enrichment in laD through exchange with meteoric or seawater. Stuckless and O'Neil [6] have used 180/160 ratios to show whether iostopic exchange by meteoric waters has taken place. Therefore, each tuff should give a unique 180/160 ratio corresponding to its particular history and thus provide a basis for correlation.
2. Proposed correlation technique 3. Procedure Studies by Taylor [4] and by Garlick and Dymond [5] on altered and unaltered glass show that each glass or ash has an tSo/160 ratio that could be roughly cor-
Ttie samples analyzed in this study were collected in the 1972 field season from tuff sections that could
293 KOOBI FORA AREA ILERET
AREA
KARARI RIDGE AREA
KUBI ALGI AREA o
TUFF COMPLEX
i o a
2222
Fig. 2. Stratigaphic column of East Rudolf Basin. Modified from Bowen and Vondra [31.
be traced lateraUy into well-documented stratigraphic sections. The samples were disaggregated by gentle grinding with mortar and pestle; they were then dry sieved and the 63-125-micron fraction collected. The glass was separated from the heavier mineral fraction by heavy liquids. Samples were then washed with acetone and allowed to air-dry. Each sample was also dried in a vacuum at 100°C for 24 hours prior to analysis. Oxygen was liberated from the glass separates by reaction with cobalt trifluoride. Flesch et al. [7] used this reagent for analysis of chromates; Sakai and Honma [8] applied it to silicates, using KHF2 as a flux. The reaction of CoF3 and glass is: SiO2 (glass) + 4CoFa = SiF4 + 02 + 4COF2 The glass sample was mixed with an excess of CoFa in a dry box (cobalt trifluoride reacts very quickly with water; therefore low humidity conditions must be maintained). The mixture was placed in a copper reaction tube with copper ribbon packed firmly over the mixture to prevent blow-over during evacuation. The tube was then connected to a large sam-
pie volume in front of the inlet system of the mass spectrometer and slowly evacuated. Sakai and Honma [8] found that complete evolution of oxygen from quartz took from 1 to 10 hours at 250-300°C; magnetite took 3 hours at 300°C; olivine required much higher temperatures. They also reported that 180 enrichment occurred when the evolution of oxygen was incomplete. Glass samples in this study were found to be very reactive at 300°C and evolution of oxygen took less than one hour; the amount of oxygen evolved was monitored by a manometer connected to the sample volume. HF and SiF4 were removed by a liquid nitrogen cold trap. It was found, however, that the SiF4 was incompletely removed by this method: therefore the ionizing voltage used in this study was 40 volts (below the appearance potential of SiF: 2÷, 47 eV). The sample was analyzed as 02 on a Nier-type mass spectrometer [9] that has been modified in this laboratory. The unknown was compared to tank oxygen, which was later compared to NBS No. 28 [ 10]. Replicate determinations were done
294 on several o f the samples and was found to be within 0.3 8 units. Results are given in the standard 8 notation. However, because of the standardization o f the tuffs to tank oxygen (8 = -+0.3), that o f tank oxygen to NBS No. 28 (8 = -+0.3), and that of NBS No. 28 to SMOW (8 = +0.12) [10], the overall standard deviation is 8 = -+0.45.
to tank oxygen. Therefore:
Rx RSMOW
-
R~ x X Rtank X RNBS Rtank
RNBS
RSMOW
and 8x/SMOW = 1.029078x/tank + 29.07 The range o f 8180 (relative to SMOW) in this study is from about +6 to about +16. Taylor [4] reports that the natural range o f unaltered volcanic glass is from about +6 to about +10. Garlick and Dymond [5] observed that 8180 values of glass shards in deepsea sediments increase with age, from +8 in the Pleistocene to +20 in the Eocene. They attribute this trend to isotopic exchange with seawater.
4. Results The results of 27 sample determinations are reported in Table 1. The values have been adjusted to SMOW (Standard Mean Ocean Water) by comparing tank oxygen to NBS No. 28 (+10.0 relative to SMOW [10] ). NBS No. 28 was found to be - 1 8 . 4 7 relative
TABLE 1 Oxygen isotope ratios of 27 samples
Sample No.
1 102-0420 2 102-0431C 3 130-0409 4 130-0505 5 012-0211 Aa 6 125-0101 7 010-0315 8 102-0445 9 104-0432 10 105-0006 11 105-9203 12 105-9203A
Field correlation
Locality
Lithology
Tulu Tulu Tulu Tulu Tulu Tulu
Koobi Fora Ridge Koobi Fora Ridge Karari Ridge Karari Ridge Base Suregei Cuesta Kubi Algi
tuff tuff tuff tuff pumice tuff
+14.94 +14.55 +16.43 +15.14 +7.16 +6.97
lleret Koobi Fora Ridge Koobi Fora Ridge
+9.16 +9.23 +9.51 +9.18 +8.91 +11.81 +9.25 +8.98
Bor Tuff Bor Tuff Bor Tuff Bor Tuff Bor Tuff?. Bor Tuff?.
180/160 ratio relative to SMOW
14 130-0212
KBS Tuff?. KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff
Karari Ridge Karari Ridge
tuff tuff tuff tuff tuff pumice tuff tuff
15 001-9902CC 16 006-0561F 17 006-0561H 18 006A-056 lc 19 006A-056 le 20 007-9822
Chari Tuff Chari Tuff Chari Tuff Chari Tuff Chari Tuff Chari Tuff
Ileret Ileret Ileret Ileret Ileret Ileret
pumice tuff pumice tuff pumice tuff
+7.72 +7.00 +8.14 +7.24 +7.11 +7.04
21 22 23 24 25 26 27
Koobi Koobi Koobi Koobi Koobi Koobi Koobi
Koobi Koobi Koobi Koobi Koobi Koobi Koobi
tuff pumice tuff tuff tuff pumice tuff
+6.82 +7.53 +6.55 +7.01 +6.68 +6.92 +7.02
13 130-0203
101-0215 101-0221 101-0222 101-0223 101-0225 102-0461A 102-0464
Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff
Fora Fora Fora Fora Fora Fora Fora
Ridge Ridge Ridge Ridge Ridge Ridge Ridge
295 6 0 ~s per rail
÷6,
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.,,,
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.,.
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KOOBI FORA TUFF COMPLEX
w
o o.
Iz: I(/)
-;
lili lilili IIll --'OO\ ? ?
KBS
TUFF
[]
TULU BOR TUFF
• O0
•
Fig. 3. Oxygen isotope determinations of 27 volcanic ash samples. Open figures represent pumice samples;closed figures represent tuff samples. Question marks refer to those samples whose field correlation is not positive.
This technique effectively distinguishes the different tuffs of this study (Fig. 3). The 8180 values of the Tulu Bor Tuff are outside the range of unaltered volcanic glass; microscopic examination of the glass shards show them to have alteration rims. Previous field correlations of the Tulu Bor Tuff with samples 5 and 6 are not confirmed by this study. Although the ~ 180 values of these two samples are similar to the values of the Chari Tuff and the Koobi Fora Tuff Complex, these ruffs cannot be correlated with them because both tuffs occur substantially below the Chari and Koobi Fora Tuffs. The low 8180 values of these samples cannot be readily explained by the authors. Previous field correlations of the KBS Tuff are confirmed by this technique. Because all samples are in the upper range of unaltered glass [4] it is probable, but not certain, that isotopic alteration has occurred. Examination of the KBS Tuff in thin section reveals that some shards have slight alteration rims. It should be noted that the one ratio that is outside the grouping of the other KBS Tuff samples is from a pumice cobble associated with the tuff. The ranges of the Chari Tuff and the Koobi Fora Tuff Complex are not sufficiently different to distinguish these two units whose relative ages were unknown at the time of this study; nor do they allow a firm correlation of these two tuffs which occur near the top of the Koobi Fora Formation (Fig. 2).
Pumice samples of these two tuffs are of particular interest; samples 15, 17 and 19 of the Chari Tuff and samples 22 and 26 of the Koobi Fora Tuff Complex are pumice cobbles. Three of these samples have ~i180 values considerably greater than the other samples of each respective tuff indicating that the pumice fragments have undergone greater amounts of exchange with percolating waters than have the associated glass shards. There are several possible interpretations of the 180/160 data with regard to the relationship of the Chari Tuff and the Koobi Fora Tuff Complex. The tuffs could be the same tuff that has undergone slightly different diagenetic histories in different localities; this is feasible because the East Rudolf Basin lies less than 50 km west of the Gregory Rift and the two areas where the ruffs outcrop are separated by about 30 km. A second possibility, favored by the authors, is that these two stuffs are of similar age although products of different eruptions. This conclusion is supported by the recent K - A r dating of Fitch et al. [ 1] which dates the Koobi Fora Tuff Complex at 1.56 m.y. and the Chari Tuff at 1.28 m.y. The rate of isotopic alteration in the East Rudolf region appears to be greater than that observed by Garlick and Dymond [5] for exchange with seawater. This could be due to minor hydrothermal events associated with rifting [ 1] or to exchange with the
296 alkaline waters o f the Lake Rudolf region. Lake Rudolf presently occupies a closed basin and has a pH o f 9.5. No 180/160 ratios were measured on etched specimens to study the effect o f the altered edges of the glass. This may have been a major factor governing the observed ratios although Garlick and Dymond report internal isotopic homogeneity [5]. No determinations o f the oxygen isotope ratios o f the associated crystalline matter in the tuffs were made. Although it is not expected that the exchange in sanidine or other minerals would be as great as in the glasses, it would be o f interest to determine these ratios to provide a clue to their respective diagenetic histories.
5. Conclusions (1) Study o f 180/160 ratios is a valid approach to correlation problems in volcanic ash. The 8180 value of a particular ash is due primarily to its individual alteration history and therefore is peculiar to that ash. (2) Previous field correlation of the KBS Tuff in the East Rudolf Basin is confirmed, although no definite conclusions regarding the relationship between the Koobi Fora Tuff Complex and the Chari Tuff could be drawn on the basis of 180/160 ratios. (3) The Tulu Bor Tuff and the KBS Tuff have undergone significant 180 enrichment in their respective glasses, whereas the younger Koobi Fora Tuff Complex and the Chari Tuff have undergone little isotope exchange.
Acknowledgements Field work was conducted under the financing o f National Science Foundation Grant GA-25684 and
the National Geographic Society. Special thanks are extended to Bruce Bowen, Richard Leakey, Glynn Isaac, Ian Findlater and all members o f the East Rudolf Expedition for help in the field, and to members of Physical Chemistry Group VII o f Iowa State University for help and encouragement in the laboratory. The manuscript was substantially improved by the comments o f Dr. Harmon Craig and two reviewers for ESPL.
References 1 F.J. Fitch, I.C. Findlater, R.T. Watkins and J.A. Miller, Dating of the rock succession containing fossil hominids at East Rudolf, Kenya, Nature 251 (1974) 213. 2 C.F. Vondra, G.D. Johnson, B.E. Bowen and A.K. Behrensmeyer, Preliminary stratigraphical studies of the East Rudolf Basin, Kenya, Nature 231 (1971) 245. 3 B.E. Bowen and C.F. Vondra, Stratigraphical relationships of the Plio-Pleistocene deposits, East Rudolf, Kenya, Nature 242 (1973) 391. 4 H.P. Taylor, The oxygen isotope geochemistry of igneous rocks, Contrib. Mineral. Petrol. 19 (1968) 1. 5 G.D. Garlick and D. Dymond, Oxygen isotope exchange between volcanic materials and ocean water, Geol. Soc. Am. Bull. 81 (1970) 2137. 6 J.S. Stuckless and J.R. O'Neil, Petrogenesis of the Superstition-Superior volcanic area as inferred from strontiumand oxygen-isotope studies, Geol. Soc. Am. Bull. 84 (1973) 1987. 7 G.D. Flesch, H.J. Svec and H.G. Staley, The absolute abundance of the chromium isotopes in chromite, Geochim. Cosmochim. Acta 20 (1960) 300. 8 H. Sakai and H. Honma, Isotope analysis of oxygen in rocks and minerals, II. Potassium hydrogen-fluoride-cobalt (III) fluoride method for the liberation of oxygen from silicate and oxide minerals, Shitsuryo Bunseki (Mass Spectroscopy) 15 (1967) 95. 9 A.O. Nier, A mass spectrometer for isotope and gas analysis, Rev. Sci. Inst. 18 (1947) 398. 10 I. Friedman and J.D. Gleason, A new silicate intercomparison for 180 analysis, Earth Planet. Sci Lett. 18 (1973) 124.
USE OF OXYGEN ISOTOPE RATIOS IN CORRELATION OF T U F F S , EAST RUDOLF BASIN, NORTHERN KENYA THURE E. CERLING*, DONALD L. BIGGS and CARL F. VONDRA Department of Earth Science, Iowa State University, Ames, Iowa (USA) and HARRY J. SVEC Department of Chemistry, Iowa State University, Ames, Iowa (USA) Received June 5, 1974 Revised version received January 14, 1975 Oxygen isotope determinations were made using CoF3 to extract oxygen from 27 volcanic glass samples from the East Rudolf Basin, northern Kenya. Results show that the older tufts are progressively enriched in 180 and that this index can be used in the correlation of volcanic ash units. This method could not distinguish individual samples from the youngest units studied because their ranges of 8180 overlap. The 8180 values for the shards in the Tulu Bor Tuff, the KBS Tuff, the Koobi Fora Tuff and the Chari Tuff range from 14.5 to 16.4, from 8.9 to 9.5, from 6.6 to 7.0 and from 7.0 to 7.2, respectively, in decreasing age. Determinations from pumice cobbles are consistently higher than the above values.
1. Introduction Volcanic ash beds are particularly useful as stratigraphic markers as they are deposited instantaneously (in the geologic time-sense) over a wide region and most o f them have some distinctive property that can be used to distinguish them from other ash units. Indices o f refraction and mineral assemblages have been generally used to characterize glasses. New techniques have recently led to the identification of individual ashes by studying elemental compositions as determined by neutron activation analysis, X-ray fluorescence and arc-emission spectroscopy. The purpose o f this paper is to present another instrumental technique for the identification o f volcanic ash units: that o f lso/160 analysis. This study resulted from the failure o f some of the above techniques to resolve correlation of the tuffs o f the East R u d o l f Basin, northern Kenya. The * Present address: Department of Geology and Geophysics, University of California, Berkeley, California 94720.
Plio-Pleistocene sequence in the northeastern portion o f the Lake Rudolf Basin is o f wide interest because of the abundance o f hominid fossils and associated stone artifacts in the sediments. However, because of the discontinuous nature o f the outcrops and the rapid lateral and vertical changes characteristic o f fluvial-lacustrine deposits, it is not possible to trace most strata from one locality to another. The area o f interest is shown in Fig. 1 and stratigraphic relationships are shown in Fig. 2. The tuffs studied were the Tulu Bor Tuff, the KBS Tuff, the Koobi Fora Tuff Complex, and the Chari Tuff which have recently been dated at 3.18 m.y., 2.61 m.y., 1.56 m.y., and 1.28 m.y., respectively [1]. The localities for the individual specimens are shown in Fig. 1. Four Tulu Bor Tuff samples were identified in the field; two others were from ruffs thought to be in similar stratigraphic positions (Table 1). All the KBS Tuff samples except one (sample 7) were from an ash bed that could be traced from Koobi Fora to the Karari Ridge; sample 7 is a tentative field correlation. All samples from the Koobi Fora Tuff Complex and from
292
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Fig. 1. Geographic map of study area. Numbers show sample localities. Chevron pattern indicates areas of volcanic rocks.
the Chaff Tuff could be identified in the field. No stratigraphic units can be traced from the Karari Ridge to the Ileret Area because of poor exposure. The Chari Tuff and the Koobi Fora Tuff are separated by about 40 km and both occur near the top of their respective sections (Fig. 2). A more detailed description of the geology of the region has been given by Vondra et al. [2] and by Bowen and Vondra [3].
related with increasing age. Although this is due in some cases to an original difference in the 180 content of the ash, usually the increasing ratios are caused by an enrichment in laD through exchange with meteoric or seawater. Stuckless and O'Neil [6] have used 180/160 ratios to show whether iostopic exchange by meteoric waters has taken place. Therefore, each tuff should give a unique 180/160 ratio corresponding to its particular history and thus provide a basis for correlation.
2. Proposed correlation technique 3. Procedure Studies by Taylor [4] and by Garlick and Dymond [5] on altered and unaltered glass show that each glass or ash has an tSo/160 ratio that could be roughly cor-
Ttie samples analyzed in this study were collected in the 1972 field season from tuff sections that could
293 KOOBI FORA AREA ILERET
AREA
KARARI RIDGE AREA
KUBI ALGI AREA o
TUFF COMPLEX
i o a
2222
Fig. 2. Stratigaphic column of East Rudolf Basin. Modified from Bowen and Vondra [31.
be traced lateraUy into well-documented stratigraphic sections. The samples were disaggregated by gentle grinding with mortar and pestle; they were then dry sieved and the 63-125-micron fraction collected. The glass was separated from the heavier mineral fraction by heavy liquids. Samples were then washed with acetone and allowed to air-dry. Each sample was also dried in a vacuum at 100°C for 24 hours prior to analysis. Oxygen was liberated from the glass separates by reaction with cobalt trifluoride. Flesch et al. [7] used this reagent for analysis of chromates; Sakai and Honma [8] applied it to silicates, using KHF2 as a flux. The reaction of CoF3 and glass is: SiO2 (glass) + 4CoFa = SiF4 + 02 + 4COF2 The glass sample was mixed with an excess of CoFa in a dry box (cobalt trifluoride reacts very quickly with water; therefore low humidity conditions must be maintained). The mixture was placed in a copper reaction tube with copper ribbon packed firmly over the mixture to prevent blow-over during evacuation. The tube was then connected to a large sam-
pie volume in front of the inlet system of the mass spectrometer and slowly evacuated. Sakai and Honma [8] found that complete evolution of oxygen from quartz took from 1 to 10 hours at 250-300°C; magnetite took 3 hours at 300°C; olivine required much higher temperatures. They also reported that 180 enrichment occurred when the evolution of oxygen was incomplete. Glass samples in this study were found to be very reactive at 300°C and evolution of oxygen took less than one hour; the amount of oxygen evolved was monitored by a manometer connected to the sample volume. HF and SiF4 were removed by a liquid nitrogen cold trap. It was found, however, that the SiF4 was incompletely removed by this method: therefore the ionizing voltage used in this study was 40 volts (below the appearance potential of SiF: 2÷, 47 eV). The sample was analyzed as 02 on a Nier-type mass spectrometer [9] that has been modified in this laboratory. The unknown was compared to tank oxygen, which was later compared to NBS No. 28 [ 10]. Replicate determinations were done
294 on several o f the samples and was found to be within 0.3 8 units. Results are given in the standard 8 notation. However, because of the standardization o f the tuffs to tank oxygen (8 = -+0.3), that o f tank oxygen to NBS No. 28 (8 = -+0.3), and that of NBS No. 28 to SMOW (8 = +0.12) [10], the overall standard deviation is 8 = -+0.45.
to tank oxygen. Therefore:
Rx RSMOW
-
R~ x X Rtank X RNBS Rtank
RNBS
RSMOW
and 8x/SMOW = 1.029078x/tank + 29.07 The range o f 8180 (relative to SMOW) in this study is from about +6 to about +16. Taylor [4] reports that the natural range o f unaltered volcanic glass is from about +6 to about +10. Garlick and Dymond [5] observed that 8180 values of glass shards in deepsea sediments increase with age, from +8 in the Pleistocene to +20 in the Eocene. They attribute this trend to isotopic exchange with seawater.
4. Results The results of 27 sample determinations are reported in Table 1. The values have been adjusted to SMOW (Standard Mean Ocean Water) by comparing tank oxygen to NBS No. 28 (+10.0 relative to SMOW [10] ). NBS No. 28 was found to be - 1 8 . 4 7 relative
TABLE 1 Oxygen isotope ratios of 27 samples
Sample No.
1 102-0420 2 102-0431C 3 130-0409 4 130-0505 5 012-0211 Aa 6 125-0101 7 010-0315 8 102-0445 9 104-0432 10 105-0006 11 105-9203 12 105-9203A
Field correlation
Locality
Lithology
Tulu Tulu Tulu Tulu Tulu Tulu
Koobi Fora Ridge Koobi Fora Ridge Karari Ridge Karari Ridge Base Suregei Cuesta Kubi Algi
tuff tuff tuff tuff pumice tuff
+14.94 +14.55 +16.43 +15.14 +7.16 +6.97
lleret Koobi Fora Ridge Koobi Fora Ridge
+9.16 +9.23 +9.51 +9.18 +8.91 +11.81 +9.25 +8.98
Bor Tuff Bor Tuff Bor Tuff Bor Tuff Bor Tuff?. Bor Tuff?.
180/160 ratio relative to SMOW
14 130-0212
KBS Tuff?. KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff KBS Tuff
Karari Ridge Karari Ridge
tuff tuff tuff tuff tuff pumice tuff tuff
15 001-9902CC 16 006-0561F 17 006-0561H 18 006A-056 lc 19 006A-056 le 20 007-9822
Chari Tuff Chari Tuff Chari Tuff Chari Tuff Chari Tuff Chari Tuff
Ileret Ileret Ileret Ileret Ileret Ileret
pumice tuff pumice tuff pumice tuff
+7.72 +7.00 +8.14 +7.24 +7.11 +7.04
21 22 23 24 25 26 27
Koobi Koobi Koobi Koobi Koobi Koobi Koobi
Koobi Koobi Koobi Koobi Koobi Koobi Koobi
tuff pumice tuff tuff tuff pumice tuff
+6.82 +7.53 +6.55 +7.01 +6.68 +6.92 +7.02
13 130-0203
101-0215 101-0221 101-0222 101-0223 101-0225 102-0461A 102-0464
Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff Fora Tuff
Fora Fora Fora Fora Fora Fora Fora
Ridge Ridge Ridge Ridge Ridge Ridge Ridge
295 6 0 ~s per rail
÷6,
tel o Z LIJ
•;
~ZXZX Z~
-?
CHARI
'p
(relative
.,,,
to SMOW)
.,.
,.
TUFF
KOOBI FORA TUFF COMPLEX
w
o o.
Iz: I(/)
-;
lili lilili IIll --'OO\ ? ?
KBS
TUFF
[]
TULU BOR TUFF
• O0
•
Fig. 3. Oxygen isotope determinations of 27 volcanic ash samples. Open figures represent pumice samples;closed figures represent tuff samples. Question marks refer to those samples whose field correlation is not positive.
This technique effectively distinguishes the different tuffs of this study (Fig. 3). The 8180 values of the Tulu Bor Tuff are outside the range of unaltered volcanic glass; microscopic examination of the glass shards show them to have alteration rims. Previous field correlations of the Tulu Bor Tuff with samples 5 and 6 are not confirmed by this study. Although the ~ 180 values of these two samples are similar to the values of the Chari Tuff and the Koobi Fora Tuff Complex, these ruffs cannot be correlated with them because both tuffs occur substantially below the Chari and Koobi Fora Tuffs. The low 8180 values of these samples cannot be readily explained by the authors. Previous field correlations of the KBS Tuff are confirmed by this technique. Because all samples are in the upper range of unaltered glass [4] it is probable, but not certain, that isotopic alteration has occurred. Examination of the KBS Tuff in thin section reveals that some shards have slight alteration rims. It should be noted that the one ratio that is outside the grouping of the other KBS Tuff samples is from a pumice cobble associated with the tuff. The ranges of the Chari Tuff and the Koobi Fora Tuff Complex are not sufficiently different to distinguish these two units whose relative ages were unknown at the time of this study; nor do they allow a firm correlation of these two tuffs which occur near the top of the Koobi Fora Formation (Fig. 2).
Pumice samples of these two tuffs are of particular interest; samples 15, 17 and 19 of the Chari Tuff and samples 22 and 26 of the Koobi Fora Tuff Complex are pumice cobbles. Three of these samples have ~i180 values considerably greater than the other samples of each respective tuff indicating that the pumice fragments have undergone greater amounts of exchange with percolating waters than have the associated glass shards. There are several possible interpretations of the 180/160 data with regard to the relationship of the Chari Tuff and the Koobi Fora Tuff Complex. The tuffs could be the same tuff that has undergone slightly different diagenetic histories in different localities; this is feasible because the East Rudolf Basin lies less than 50 km west of the Gregory Rift and the two areas where the ruffs outcrop are separated by about 30 km. A second possibility, favored by the authors, is that these two stuffs are of similar age although products of different eruptions. This conclusion is supported by the recent K - A r dating of Fitch et al. [ 1] which dates the Koobi Fora Tuff Complex at 1.56 m.y. and the Chari Tuff at 1.28 m.y. The rate of isotopic alteration in the East Rudolf region appears to be greater than that observed by Garlick and Dymond [5] for exchange with seawater. This could be due to minor hydrothermal events associated with rifting [ 1] or to exchange with the
296 alkaline waters o f the Lake Rudolf region. Lake Rudolf presently occupies a closed basin and has a pH o f 9.5. No 180/160 ratios were measured on etched specimens to study the effect o f the altered edges of the glass. This may have been a major factor governing the observed ratios although Garlick and Dymond report internal isotopic homogeneity [5]. No determinations o f the oxygen isotope ratios o f the associated crystalline matter in the tuffs were made. Although it is not expected that the exchange in sanidine or other minerals would be as great as in the glasses, it would be o f interest to determine these ratios to provide a clue to their respective diagenetic histories.
5. Conclusions (1) Study o f 180/160 ratios is a valid approach to correlation problems in volcanic ash. The 8180 value of a particular ash is due primarily to its individual alteration history and therefore is peculiar to that ash. (2) Previous field correlation of the KBS Tuff in the East Rudolf Basin is confirmed, although no definite conclusions regarding the relationship between the Koobi Fora Tuff Complex and the Chari Tuff could be drawn on the basis of 180/160 ratios. (3) The Tulu Bor Tuff and the KBS Tuff have undergone significant 180 enrichment in their respective glasses, whereas the younger Koobi Fora Tuff Complex and the Chari Tuff have undergone little isotope exchange.
Acknowledgements Field work was conducted under the financing o f National Science Foundation Grant GA-25684 and
the National Geographic Society. Special thanks are extended to Bruce Bowen, Richard Leakey, Glynn Isaac, Ian Findlater and all members o f the East Rudolf Expedition for help in the field, and to members of Physical Chemistry Group VII o f Iowa State University for help and encouragement in the laboratory. The manuscript was substantially improved by the comments o f Dr. Harmon Craig and two reviewers for ESPL.
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