Rare gases in a phlogopite nodule and a phlogopite-bearing peridotite in South African kimberlites

Earth and Planetary Science Letters, 36 (1977) 181-186 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

181

[2]

R A R E GASES IN A PHLOGOPITE NODULE AND A PHLOGOPITE-BEARING PERIDOTITE IN

SOUTH AFRICAN KIMBERLITES ICHIRO K A N E O K A

Geophysical Institute, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo (Japan) NOBUO T A K A O K A

Department of Physics, Faculty of Science, Osaka University, Toyonaka-shi, Osaka (Japan) and KEN-ICHIRO AOKI

Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, Sendal (Japan)

Received April 13, 1977

Rare gas isotopes in a phlogopite nodule and a phlogopite-bearingperidotite nodule in South African kimberlites were studied to examine the state of rare gases in the deep interior of the kimberlite region. Within the experimental error of 1 - 2%, rare gas isotopic compositions are atmospheric except for radiogenic 4He and 4OAr. No excess 129Xe was observed. In phlogopite, Ne is more depleted, whereas the heavier rare gases are more enriched than the atmospheric rare gases relative to 36Ar. Together with other data these results suggest that the state of rare gases in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by a process such as mixing of crustal materials.

1. Introduction In recent years, the existence o f terrestrial primordial rare gases has been suggested in seawater [ 1 ], submarine pillow basalts [2,3], CO2 well gases [4,5] and some m a n g e materials [6,7]. Especially the discovery o f excess 129Xe in CO 2 well gases [5] and in an Hawaiian xenolith [7] gives a very important constraint on the evolution o f the terrestrial atmosphere, provided excess 129Xe still exists ubiquitously in the earth's interior. This excess 129Xe seems to be accompanied with the mantle-derived CO2 [7], though their relationship is not clear. It is now well established that some phlogopites in kimbeflites and kirnbeflite nodules were formed in the earth's deep interior [8]. Such phlogopites are expected to gather volatile components in the earth's

deep interior during their formation. Hence they may be suitable materials to investigate the state o f rare gases in the earth's deep interior. Furthermore, kimberlite is known to contain relatively large amounts o f CO2 [8]. In this respect, if some isotopic anomalies in terrestrial rare gases associate with CO 2 o f deep origin, the nodules in kimberlites are a good candidate to test the possible existence o f such isotopic anomalies in terrestrial materials. For these reasons, we have analysed rare gases in a phlogopite nodule and a phlogopite-bearing peridotite in South African kimberlites. 2. Samples Sample DU-02 is a fresh phlogopite ( 4 0 - 6 0 mesh) separated from a phlogopite nodule in kimberlite, col-

182

lected at Du Toitspan, Kimberley, South Africa. 4°Ar/39Ar age analyses indicate that the phlogopite nodule was brought to the earth's surface about 84 m.y. ago [9]. Sample BF-03 is a phlogopite-bearing peridotite nodule in kimberlite, collected at Bultfontein, Kimberley, South Africa. The peridotite consists of forsterite, enstatite, phlogopite, potassic richterite and chromite in decreasing'order of abundance [10]. Rb/Sr analyses indicate that the peridotite was brought to the earth's surface about 90 m.y. ago [11]. This sample is relatively fresh. It has been estimated petrologically that sample BF-03 was brought to the earth's surface from a depth of about 100 km or less [10], while sample DU-02 was formed at a shallower region such as in the lower crust or in the uppermost mantle [12]. 3. Experimental The sample gases were extracted from each sample at 600, 1200 and 1700°C for 30 minutes, respec-

mx

tively. After purification with titanium-zirconium getters at about 800°C, the light rare gases (He, Ne), Ar and the heavy rare gases (Kr, Xe) were analysed separately with a Nier-type mass spectrometer (20 cm radius) with a 17-stage Cu-Be secondary electron multiplier at the Osaka University. Concentrations are determined by peak height comparison with the calibrated air standard;mass discrimination is also corrected with the same standard. The amount of 3He in the air standard is too small to be used as the standard. Hence the amount of 3He in the sample is estimated by assuming the same sensitivity as 4He, though this assumption increases an ambiguity in the 3He content by 20-30%. The present mass spectrometer has a resolving power of about 600 and the separation of 3He from HD and H 3 is complete. However, the relatively large concentrations of 4He in the present samples inhibit detection of 3He amounts less than 10 -12 cm 3 STP/g because of the increase in noise at the 3He mass range due to the scattering of 4He ions. Except for 3He, errors in the concentrations are estimated to be about 10%. More details on the experimental conditions have been reported elsewhere [13].

102 4. Results and discussions

100

102

-...--..~.

164 t

t

20Ne36Ar

84Kr

132Xe

Fig. 1. Rare gas abundance patterns in phlogopite (e), in air (o) [15], in an Hawaiian xenolith (zx) [7], in water-dissolved air (+) [15] and in a Fig Tree shale (0) [14]. The relative abundances of 20 Ne and 132 Xe normalized to 36 Ar for Hawaiian xenolith and Fig Tree shale axe estimated from reported 22 Ne and 130 Xe concentrations, assuming atmospheric composition of Ne and Xe isotopes.

The results of rare gas concentrations are tabulated in Table 1 and abundance patterns normalized to 36Ar are plotted in Fig. 1 together with other terrestrial samples. In phlogopite DU-02, most 4He was degassed at 600°C, whereas Ne, Ar and probably Kr were mainly degassed in the 1200°C fraction. However, more than 95% of Xe was released only at 1700°C. This probably reflects the difference of the diffusion rate for each element. For peridotite BF-03, however, the relative degassing rates between the 1200 and 1700°C fractions are different from those of phlogopite. 4He was still retained at higher temperature, whereas a smaller fraction of Xe was released at 1700°C than at 1200°C. In phlogopite, the rare gases are enriched and their concentrations are larger than those of total peridotite by one to two orders of magnitude. Furthermore, the heavier rare gases are more enriched than the lighter ones in phlogopite, where 4He would be mostly radio-

183 genic since only 1 - 2 ppm U is sufficient to produce the observed 4He during 100 m.y. As shown in Fig. 1, the abundance pattern of the rare gases in phlogopite is clearly different from that in the Hawaiian xenolith in which Ne and Xe are enriched as compared to 36Ar [7]. Except for Kr, the abundance patterns of phlogopite are rather similar to those observed in the Fig Tree shale [14] or atmospheric rare gases dissolved in water [ 15]. Since the real amount of Kr should be larger than the ob.served one, the relative abundance of Kr may also be similar to those observed in these terrestrial materials. Since phlogopite contains relatively large amounts of water (e.g. 4.09% of H20(+) for DU-02 [12]), the rare gases might have been concentrated through hydrous phases within this mineral. However, the enrichment factor of Xe in phlogopite is larger than that expected from the dissolution of atmospheric rare gases in water. This suggests that the relative concentration of Xe may be larger in the earth's deep interior than that in the atmosphere. This suggestion is compatible with the explanation that the relative depletion of atmospheric Xe compared with chondrite Xe is due to adsorption of Xe on sediments [16]. For sample BF-03, the total amounts of Ne and Kr are only minimum values. The 132Xe/36Ar ratio (--- 3.2 × 10 -3) may be reliable and is larger than that of the atmosphere (---7.4 × 10-4). Since this sample contains phlogopite to some extent, it may reflect the high concentration of Xe in phlogopite and/or the relative enrichment of Xe in the earth's deep interior. In Table 2, isotopic compositions of the rare gases are shown for each sample at each temperature fraction. Except for 4He and 4°Ar, no anomalies are found from the atmospheric rare gas values. Within the experimental error of about 1-2%, excess 129Xe was not observed compared to atmospheric Xe isotopes. Since the amounts of Kr and Xe are relatively large in the present samples, the production of fissiogenic Kr and Xe during 100 m.y. would not be observable even if a U content as high as 50 ppm is assumed. In effect, 136Xe/132Xe and 134Xe/132Xe ratios are atmospheric within the experimental errors. This fact suggests that the usage of 132Xe as a normalizing component causes no serious problem for comparing each Xe isotope in the present case. In this respect, even if excess 129Xe exists in the samples, it should be less than 1-1.5% of the total 129Xe and

not as large as observed in the Hawaiian xenolith. Furthermore, Ne isotopes are of atmospheric values within the experimental errors. As discussed before, all 4He can be explained as radiogenic components produced after sample extrusion. Hence, even if 3He had existed in the samples when they were extruded, the 3He/4He ratios have been reduced by both generation of radiogenic 4He and possible loss of He due to diffusion. The K content for phlogopite DU-02 and peridotite BF-03 is determined to be 8.68 and 0.847 wt.%, respectively [ 12,10]. Assuming that all 36Ar represents an atmospheric component, we can estimate the content of radiogenic 4°Ar for these samples. Adopting this assumption, we obtain K-Ar ages of 198 m.y. for DU-02 and 368 m.y. for BF-03. Extrusion ages have been estimated to be about 84 m.y. for DU-02 [9] and about 90 m.y. for BF-03 [11]. Hence these samples clearly contain excess or inherited 4°Ar, which suggests that they were formed at the relatively deep interior of the earth [17]. This conjecture is compatible with that inferred petrologically [10,12]. It is inferred from these observations that the isotopic composition of the rare gases in the upper mantle of the South African kimberlite region is nearly atmospheric. This does not apply to the 3He/4He ratio, since no definite 3He/aHe ratio was obtained in this work. It is concluded that excess 129Xe is not always associated with CO2 of deep origin. A possibility is that the rare gas compositions in the upper mantle of this region might have been changed by some processes like admixture of crustal materials. A somewhat higher initial (875r/86Sr) ratio of kimberlite and kimberlite nodules (0.703-0.708) [18] relative to oceanic ridge tholeiites (0.702-0.704) [19] may support this suggestion. Furthermore, rare gas abundance patterns in the present samples are similar to those of type 1 defined by Ozima and Alexander [20]. The rare gases incorporated via a hydrous phase might have caused such a tendency. Although selective Ne loss from the samples may be another explanation, the relative freshness of the samples precludes such a possibility. In this respect, a peridotite nodule of upper mantle origin from the Oki-D~go Island also shows a similar rare gas abundance pattern like the present samples [21]. This peridotite nodule does not contain phlogopite and extruded only about 3.6 m.y.

<0.67

<11.3

total

1700

(<4.9) <9.0 <2.3

<18.9

total

600 ** 1200 1700

<14.4 <1.2 <3.3

(X 10 -12 cm 3 STP/g)

(°C)

600 1200 1700

3He

Temperature

0.081

207.7

(0.02) 153 54.7

2755.3

2270 485 0.3

4He

36Ar

0.0036

0.0406

(0.0084) 0.0256 0.0150

0.532

0.159 0.322 0.041

0.015

0.337

(0.006) 0.260 0.077

2.36

0.149 2.02 0.191

(X 10 -8 cm 3 STP/g)

20Ne

4.34

1447

(1.89) 1300 147

7742.4

99.4 7450 193

4OAr

* A part of Kr was lost due to insufficient collecting procedures. ** The fraction probably lost the sample gases. Hence the total represents the addition of 1200°C and 1700°C fractions for BF-03.

Blanks 1700°C, 30 minutes

BF-03 (0.717 g) peridotite Bultfontein, S. Africa

DU-02 (0.720 g) phlogopite, Du Toitspan, S. Africa

Sample

Rare gas concentrations in nodules in South African kimberlites

TABLE 1

132Me"

0.014

0.467<

(0.038<) 0.404< 0.063<

6.58<

0.97< 4.27< 1.34

0.002

0.107

(0.002) 0.094 0.013

12.91

0.093 0.518 12.3

(X 10 -10 cm 3 STP/g)

,84Kr,

4~

---1.00 0.191 ± 0.004 667.5 -+ 0.4

<0.009 0.040 0.201 0.199 -=1.00 0.308

<0.01 <0.01 0.067 0.981 0.153 0.779 ~-1.00 0.380 0.325

36Ar 3BAr 40At

78Kr 80Kr 82Kr 83Kr 84Kr 86Kr

124Xe 126Xe

+ 0.005 ± 0.010

± 0.007 -+ 0.014 ± 0.005 ± 0.021

± 0.003 <0.01 <0.01 0.072 0.988 0.150 0.793 -1.00 0.384 0.329

<0.012 0.034 0.204 0.203 ~1.00 0.300

± 0.007 ± 0.004

+- 0.002 ± 0.009 + 0.004 ± 0.009

± 0.009

± 0.007 ± 0.004 -+ 0.004

-=1.00 0.184 + 0.002 3694 ± 37

9.81 +- 0.08 0.029 +- 0.002 =-1.00

<3.2 × 10 - 7 -=1.00

<0.01 <0.01 0.071 0.983 0.153 0.790 -=1.00 0.388 0.327

0.0065 0.040 0.204 0.203 -~1.00 0.303

---1.00 0.188 1011

9.94 <0.033 =--1.00

0.0009 0.002 0.004 0.003

0.002 0.008 0.004 0.012

± 0.004 ± 0.004

+ ± ± ±

± 0.007

± ± ± ±

± 0.002 ± 10

± 0.15

<1.1 × 10 - 3 =-1.00

<0.01 <0.01 0.069 0.990 0.152 0.788 ~-1.00 0.384 0.318

<0.009 0.040 0.204 0.203 w-l.00 0.301

0.005 0.014 0.002 0.011 ± 0.007 ± 0.008

± ± ± +

± 0.003

± 0.003 ± 0.005 -+ 0.004

---1.00 0.189 ± 0.003 4981 + 59

9.74 ± 0.15 <0.14 ---1.00

<5.9 x 10 - 6 ~-1.00

* The isotopic compositions of atmospheric rare gases are from references listed below. He: B.A. Mamyrin, G.S. Anufriyev, I.L. Kamenskiy and I.N. Tolstikhin, Geochem. Int. 7 (1970) 498. Ne: P. Eberhardt, O. Eugster and K. Marti, Z. Naturforseh. 20a 0 9 6 5 ) 623. At: A.O. Nier, Phys. Rev. 77 (1950) 789. Kr, Xe: A.O. Nier, ibid. 79 (1950) 450.

129Xe 130Xe 131Xe 132Xe 134Xe 136Xe

128Xe

9.73 -+0.12 0.034 ± 0.003 ~1.00

2°Ne 21Ne 22Ne

± 0.008 ± 0.005 ± 0.004

<5.4 × 10 - 7 ~1.00

31-Ie 4He

1200 °C

1700 °C

600 °C

1200 °C

Sample BF-03

Sample DU-02

Isotopic compositions of rare gases in nodules in South African kimberlites (± represents one standard deviation)

TABLE 2

<0.01 <0.01 0.068 0.984 0.149 0.792 ---1.00 0.384 0.338

+ 0.010 -+ 0.016

± 0.008 ± 0.016 ± 0.005 ± 0.011

± 0.007

0.0036 0.0034 0.0713 0.983 0.152 0.788 ~-I.00 0.389 0.329

0.0062 0.0396 0.202 0.202 ~l.00 0.306

~I.00 0.187 295.5

---1.00 0.188 ± 0.001 1898 ± 8

± 0.005 ± 0.004 ± 0.008

9.81 0.029 ~1.00

9.87 ± 0.07 <0.083 =--1.00

<0.017 0.044 0.203 0.200 =-1.00 0.299

1.4 x 10 - 6 ~-1.00

<4.3 X 10 - 6 ---1.00

1700 °C

Atmosphere *

Let

g

186 ago [22]. Hence such rare gas abundance patterns may not be rare even for mantle-derived rocks. The abundance patterns are surely affected by both the original abundance patterns and the fractionation processes of each element: the present samples would be such examples. These points should be checked by analysing mantle-derived rocks from other regions.

5. Summary (1) Rare gas isotopes in phlogopite separated from a phlogopite nodule and in a phlogopite-bearing peridotite nodule show atmospheric values except for radiogenic 4He and 4°Ar. No excess 129Xe was observed. (2) Normalized to 36Ar, Ne is more depleted, Xe and probably Kr are more enriched in phlogopite than in the atmosphere. (3) As an explanation for the present results, the rare gas state in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by some processes like admixture of crustal materials which contain atmospheric rare gases.

Acknowledgements Prof. M. Ozima of University of Tokyo reviewed the manuscript and Mr. K. Nagao of Osaka University assisted us in performing the present experiments. Mrs. T. Osaki has typewritten the manuscript. We express our sincere thanks to them. This study is supported by the Matsunaga Science Foundation.

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4 W.A. Buffer, P.M. Jeffey, J.H. Reynolds and G.J. Wasserburg, Isotopic variations in terrestrial xenon, J. Geophys. Res. 68 (1963) 3283. 5 M.S. Boulos and O.K. Manuel, The xenon record of extinct radioactivities in the earth, Science 174 (1971) 1334. 6 I.N. Tolstikhin, B.A. Mamyrin, L.B. Khabarin and E.N. Erlikh, Isotope composition of helium in ultrabasic xenoliths from volcanic rocks of Kamchatka, Earth Planet. Sci. Lett. 22 (1974) 75. 7 E.W. Hennecke and O.K. Manuel, Noble gases in an Hawaiian xenolith, Nature 257 (1975) 778. 8 J.B. Dawson, Kimberlites and their relation to the mantle, Philos. Trans. R. Soe. Lond. Ser. A. 271 (1972) 297. 9 I. Kaneoka and K. Aoki, 40Ar/39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites (in preparation). 10 K. Aoki, Origin of phlogopite and potassic richteritebearing peridotite xenoliths from South Africa, Contrib. Mineral. Petrol. 53 (1975) 145. 11 H.L. Allsopp and D.R. Barrett, Rubidium-strontiumage determinations on South African kimberlite pipes, Phys. Chem. Earth 9 (1975) 605. : 12 K. Aoki, Phlogopites and potassic richterites from mica nodules in South African kimberlites, Contrib. Mineral. Petrol. 48 (1974) 1. 13 N. Takaoka, A low-blank, metal system for rare-gas analysis, Mass Spectrom. 24 (1976) 73. 14 R.A. Canalas, E.C. Alexander, Jr. and O.K. Manuel, The terrestrial abundance of noble gases, J. Geophys. Res. 73 (1968) 3331. 15 E. Mazor and R.O. Fournier, More on noble gases in Yellowstone National Park hot waters, Geoehim. Cosmoehim. Acta 37 (1973) 515. 16 F.P. Fanale and W.A. Cannon, Physical adsorption of rare gas on terrigenous sediments, Earth Planet. Sci. Lett. 11 (1971) 362. 17 I. Kaneoka, Investigation of excess argon in ultramafie rocks from the Kola Peninsula by the 40Ar/39Ar method, Earth Planet. Sci. Lett. 22 (1974) 145. 18 D.R. Brett and G.W. Berg, Complementary petrographic and strontium-isotope ratio studies of South African kimberlite, Phys. Chem. Earth 9 (1975) 619. 19 Z.E. Peterman and C.E. Hedge, Related strontium isotopic and chemical variations in oceanic basalts, Geol. Soc. Am. Bull. 82 (1972) 493. 20 M. Ozima and E.C. Alexander, Jr., Rare gas fractionation patterns in terrestrial samples and the earth-atmosphere evolution model, Rev. Geophys. Space Phys. 14 (1976) 385. 21 I. Kaneoka and N. Takaoka, unpublished data. 22 1. Kaneoka, E. Takahashi and S. Zashu, K-Ar ages of alkali basalts from the Oki-D~go Island, J. Geol. Soc. Jpn. 83 (1977) 187.