Noble gases in Archaean metasediments from Isua (Greenland) and the Pongola supergroup (South Africa)

111

Chemical Geology (Isotope Geoscience Section), 52 (1985) 111-117 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

NOBLE GASES IN ARCHAEAN METASEDIMENTS FRQM WA (GREENLAND) AND THE PONGOLA SUPERGROUP (SOUTH AFRICA) G. ROSKAMP and L. SCHULTZ Max-Planck-Znstitut

fiir Chemie, D-6500 Mainz (Federal Republic (Accepted

for publication

October

of

Germany)

4, 1984)

Abstract Roskamp, G. and Schultz, L., 1985. Noble gases in Archaean metasediments Pongola Supergroup (South Africa). In: F.A. Podosek (Guest-Editor), Geol. (Isot. Geosci. Sect.), 52: 111-117.

from Isua (Greenland) and the Terrestrial Noble Gases. Chem.

Old sediments may contain trapped ancient atmosphere. The variation with time of the atmospheric 40Ar/‘6Ar ratio may yield clues to the degassing history of the Earth and the evolution of the atmosphere. We have measured Ar, Kr and Xe in samples from Isua (-3.7 Ga old) and the Pongola Supergroup (- 3 Ga). All gases analyzed have isotopic compositions indistinguishable from atmospheric values except for an excess of ‘OAr. Carbon separates from Isua contain more ‘OAr than can be explained by their K contents, possibly indicating acquisition of ‘OAr during metamorphic events. No ‘OArls6Ar ratio below today’s atmospheric value was found. Carbon separates contain gas concentrations up to 55 times higher than those measured in bulk samples. Thus, carbon, which accounts for N 1% of the bulk samples, appears to be a major noble-gas carrier in these samples. Isua and Pongola samples were different in their release pattern of noble gases during stepwlse heating experiments. In all samples the elemental fractionation, relative to atmospheric abundances, indicates an adsorptive trapping of rare gases from an atmospheric reservoir or incomplete degassing of heavy noble gases from the Earth. If the Xe concentration of these samples is representative of all sediments, the deficit of atmospheric Xe compared to planetary abundances cannot be explained by the noble-gas inventory of these rocks.

1. Introduction The observation that the Earth’s atmosphere contains radiogenic noble-gasnuclides has led to the suggestion that these isotopes were produced in the interior of the Earth by radioactive decay and were subsequently degassedinto the atmosphere. Several degassing models have been proposed between the ex-

tremes of a catastrophic degassingearly in the Earth’s history or a continuous loss of noble gases. The isotopic composition of ancient atmospheric Ar is closely related to all such degassing models and, hence. the thermal history of the Earth. Calculations of the evolution of atmospheric 40Ar/36Ar, for assumed K values of the whole Earth and different degassing mechanisms, have been per-

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112

formed by several authors (i.e. Ozima and Kudo, 1972; Schwarzman, 1973; Ozima, 1975; Bernatowicz and Podosek, 1978; Hamano and Ozima, 1978). Starting 4.5 Ga ago with the lowest known meteoritical ratio of 40Ar/36Ar of (2.9 + 1.7) . low4 (GBbel et al., 1978) different paths of an atmospheric evolution to the present ratio of 296 are possible. The most promising way to measure the atmospheric 40Ar/36Ar evolution with time is the analysis of ancient sedimentary samples which possibly incorporated palaeoatmospheric noble gases(Cherdyntsev and Kolesnikov, 1965; Phinney, 1972; Alexander, 1975; Cadogan, 1977; Podosek et al., 1980; Turner and Jones, 1982; Butterfield and Turner, 1983). It is possible that most of these gasesreside in special mineral phases. Good candidates for such potential noble-gas carriers are carbon phases (Frick and Chang, 1977). A separation and analyses of phases with low K concentrations would also reduce the in situ produced radiogenic 40Ar. Trapped ancient noble-gas components may be affected by temperature and pressure variations since the time of sediment formation. Therefore the measurement of isotopic composition and elemental noble-gas abundances may yield information about the noble-gas behaviour in geophysical processes(i.e. Kirsten, 1968; Fanale and Cannon, 1971; Kaneoka, 1980; Bernatowicz, 1981; Podosek et al., 1981; Jambon et al., 1982). We have analyzed the Ar, Kr and Xe concentrations and their isotopic compositions in bulk samples and carbon separates from Archaean sedimentary rocks. These measurements were made with the aim to observe palaeoatmospheric gasestrapped during sediment formation and to determine the degree of relative fractionations of noble gasesin our samples. 2. Samplesand experimental procedure The samples analyzed are from the 3.7-Ga old Isua metasediments, west Greenland, and

from the -3-Ga old sediments of the Pongola Supergroup, South Africa. Isua metasediments occur together with volcanic rocks in a supracrustal sequence about 150 km NE of Godthab. The area has been mapped by the Geological Survey of Greenland (Bridgwater and McGregor, 1974). All samples belong to the amphibolithic sequence of the western belt of the supracrustals. They mainly consist of banded iron formation rocks and show a variation in their carbon content up to 10% (Monster et al., 1979). The sedimentary rocks of the Pongola Supergroup are also associated with volcanic rocks. All samplesare from the White Mfolozi river valley in northern Natal, mapped and described by Matthews (1964). The Pongola Supergroup is divided into an older Insuzi Group and a younger Mozaan Group. The sandstone PO 37 is from the Upper Insuzi Group, while the shales are from the Upper Mozaan Group (PO56 and PO57). The sample preparation comprises physical and chemical procedures to get homogeneous bulk samples and different mineral fractions: Sample Isua PA 3 was separatedinto a magnetite- and a silicate-rich fraction (PA 3 M and PA 3 S, respectively) by use of a hand magnet. A carbon-rich sample (Isua 2795 SH) was prepared by density separation from the whole-rock sample Isua 2795. Other carbon separateswere obtained as insoluble residues of a hydrofloric acid treatment. All acidtreated residues as well as single mineral phaseswere investigated by X-ray diffraction. Graphite is common in Isua samples but not detectable in Pongola samples.Since the latter contain considerable carbon as well it appears that they must have a high kerogen content. The K concentrations of aliquots of several bulk samples and carbon residues were determined by neutron activation analysis (NAA) techniques (W&&e et al., 1977). The noble gaseswere analyzed in an allmetal 60” mass spectrometer with 25-cm radius. Samples were degassedat - 1600°C or in stepwise heating experiments. Blank

113

3. Results and discussion

corrections were made for the resistanceheated (R.H.) double-walled vacuum extraction oven for different temperatures. Typical blank values for an extraction temperature of 1600°C are 4 lo-’ cm3 STP, 1 lo-‘* cm3 SIP, and 8 lo-i3 cm3 STP for 40Ar, S4Kr and 13*Xe,respectively. To minimize memory effects, Ar, Kr and Xe were separatedusing a temperature-controlled charcoal trap and analyzed individually. The sensitivities of the mass spectrometer for these gaseswere determined by peak height comparison with gas mixtures of a known isotopic and elemental composition. l

Table I contains the concentrations of K, 36Ar, 84Kr and “*Xe and the 40Ar/36Arratio. The data for the individual steps of stepwise heating extractions are only given in graphic form. Within the limits of error the isotopic composition of Kr and Xe is indistinguishable from atmospheric values. Ar also has an atmospheric 38Ar/36Ar ratio but the 40Ar/36Ar ratios in bulk samples and carbon separates are both well above the atmospheric value (Fig. 1). The high. 40Ar/36Ar ratios of -10,000 or more of the Isua samples reflect

l

l

TABLE

I

Rare-gas and K concentrations Sample* *

Sample weight (mg)

in bulk samples and separates

K

( 10e8 cm3 STP g-l) “Ar

*‘Kr

13aXe

4oAr/S6Ar

0.0089 * 0.0005

zt 0.82

0.0205 * 0.0011 0.357 f 0.017 0.0450 + 0.0027 0.0244* 0.0022 0.788 * 0.039

i 0.18 * 0.06

0.0908 f 0.0046

Zsua :

2780 278OC 2781 2796 2795C 2795 SH 2796 2796C 2797 2797C 3022 PA3 PA3M PA3S

366.2 3.6 374.3 227.0 25.9 41.4 180.6

36.8 215.4 31.8 120.6 249.0

5 10

f 4ppm f 5ppm

5 5

* 4rvm * 4rvm

11

* 4wm

0.250 9.79 0.54 0.337 8.24 3.72 1.19

13

* 1 wm

72

* 7wm

14

* 2wm

205.3

t 0.013

f 0.50 f 0.05 * 0.018

29.7 * 3.0 0.394 f 0.020 11.73 0.05

f 0.57 f 0.01

0.263 * 0.015 0.237 it 0.012

0.1116 1.51

* 0.0066 f 0.15

0.1000

f 0.0050 0.0220t 0.0011 0.0120 f 0.0010 0.336 i 0.017

0.0393 f 0.0022 0.0030* 0.0002 0.691 0.0144

* 0.070 f 0.0010 f 0.017

0.0338 * 0.0017 0.705 f 0.035 0.0073 + 0.0008

0.349 0.0042 * 0.0005

0.0189 0.0108

0.0063 * 0.0016 0.0026* 0.0005

f 0.0012 f 0.0007

4,630*

200

10,620 * 70 2,110 i 240 12,300 i 250 10,600 * 1,060

3,920* 5,030i

40 52

12,200 f 1,220 4,710 f 30 14,800 i 540 188,000 f 2,400

2,960 f 5,260 *

30 50

Pongola: PO 37 Po37C PO 56 Po56C PO 67 Po57C

201.8

5.53 f 0.06%*1

5.5 170.4

42.2 194.0 9.1

4.29 f 0.02% 1.38 * 0.06% 5.13 f 0.06%**

0.95 * 0.05 40.6 f 3.5 0.86 f 0.05 3.07 * 0.16 0.55 15.1

i 0.03 f 0.7

0.0937 * 3.11 f 0.0784 f 0.206 f 0.0524 f 2.86 t

0.0049 0.22 0.0040 0.010

0.0033 0.14

0.0205i 0.0013 0.529 * 0.031 0.0259 * 0.0015 0.0628 * 0.0036

28,600 f 1,180

0.0185 1.005

50,500 f 3,740 2.250 * 55

i 0.0024 f: 0.052

1,550 f

11

35,400*

692

813*

7

*‘Kind of separate is given after the sample number: C = carbon separate obtained after H.F. treatment; bon separate received by den&y separation; M = magnetic fraction; S = silicate fraction. *‘Obtained from Laskowski (1982).

SH = car-

114 MECHANIC ‘MAGNETIC

)BULK I CARBON

PONGOLC

I

ot’ SE!+tRATI

l

PO 57

0

20,

PO 56

,” ::

PO 37

I SUA

0

l

0

i

PA3

S

;;i

PA3

M

-i.iL : B I

3022 2797 2796 2795 2761

4

2760

296

lo2

m l 0 0 l 0 l mm 0 0*

lo3

lo4

*

2!e 36A,

Fig. 1. 40Ar/36Ar ratios in bulk samples and separates of Pongola and Isua sediments. For comparison, the modern atmospheric value of 296 is indicated.

the presence of radiogenic Ar in such quantities that the detection of palaeoatmospheric Ar is impossible. In contradistinction, Pongola carbon residues show 40Ar/36Ar ratios different from bulk ratios and near the atmospheric value. A complete exchange of 4oAr from Kbearing minerals has not taken place in these scarcely metamorphozed Pongola rocks. Isua samples yield a K-Ar model age of about 10 Ga for bulk samples and 17 Ga for carbon residues. These unrealistic ages define excess 40Ar in all samples and are probably the result of incorporated Ar which is released from K-rich gneiss surrounding the Isua supracrustal belts. A number of thermal events are known to have occurred in the history of these rocks (compiled by Bridgwater et al., 1976) that may have led to a redistribution of 40Ar in these samples. In particular the carbon phases are able to adsorb these liberated gases. By these thermal events with temperatures up to 600°C (Schidlowski et al., 1979) the Isua rocks metamorphozed to the amphibolithic state. A palaeoatmospheric 40Ar/36Ar ratio in such metamorphozed samples with much excess 40Ar is not detectable. The -3 Ga old Pongola samples yield a mean K-Ar model age of -1 Ga for bulk

samples but only 0.3 Ga for the carbon residue PO 56 C. This is indicative of 40Ar loss from these samples. A complete Ar exchange within the Pongola samples is excluded by differences of the 40Ar/36Ar ratios of bulk samples and carbon residues. These differences reflect the low-grade metamorphism of Pongola samples with thermal events below 250°C. However, the 40Ar/36Ar ratios are still above the atmospheric value. This could be due to radiogenic 40Ar from the in situ decay of 40K in the residues. It was expected that the K concentrations of carbon separates would be low, e.g. below 20 ppm, as observed in the Isua samples. However, the carbon residues show rather high K concentrations. It is not yet clear if a K-rich phase survived the chemical treatment. Without such a possible phase the samples from Pongola would be good candidates for tracers of palaeoatmospheric Ar. 3.1. Degassing patterns A common technique for separating noblegas components of different origin is the stepwise increase of degassing temperature. This technique was used to separate possible atmoPONGOLA

600

1wo

uoo

‘C

em

loo0

woo ‘C

Fig. 2. Typical degassing pattern of Ar, Kr and Xe of Isua and Pongola samples. The fractional release of temperature steps between 100’ and 1600°C are presented. The total concentrations for 36Ar, 04Kr and IsaXe is in units of lo-* cm’ STP g-’ and in 10e4 cm3 STP g-l for 40Ar.

115

spheric contaminations, excess 40Ar, and in situ produced 40Ar. The release patterns obtained are given in Fig. 2. From the Isua samples most of the noble gases are released between 800” and 1200°C whereas Pongola samples degas at lower tem,peratures.Bulk samples and carbon residues show similar release patterns within one geological group, i.e. for Isua or Pongola samples, respectively. The characteristic difference:3 between Isua and Pongola degassingpatterns possibly reflect the low-grade metamorphism of Pongola samples and the medium-grade metamorphism of Isua rocks. Thus the lower amount of noble gasreleased at lower temperature from Isua samples is due to their high-temperature history. In spite of the fact that the 40Ar/36Ar ratio varies in different temperature steps of one sample up to a factor of 800 (PO 57), no value below the atmospheric ratio is observed. 3.2. Elemental pattern8

abundances

and fractionation

The noble-gascontent of mineral separates and carbon residues compared to the bulk content gives information about noble-gas carriers in these samples. In Isua and Pongola samples a considerable part of the nonradioISUA 2780

39 PONGO

genie gasesreside in the carbon phases(Fig. 3). Therefore, carbon is a significant carrier of trapped noble gases. For Isua samples radiogenic 4oAris also concentrated in the carbon phase. This indicates that the other rare gases are not necessarily primary trapped ones but may have been redistributed to carbon. The 40Ar concentration of the carbon sample PO 56 C (Fig. 3) is only 8% of that of the bulk value, which implies that the lowtemperature metamorphism is ineffective in redistributing noble gases.Its 40Ar/36Arratio of 315 + 3 in one temperature step (150600°C) is the lowest ratio measuredin this set of Archaean samples. Noble-gas abundance patterns of the Isua and Pongola samples are given in Fig. 4. The concentrations are presentedrelative to atmospheric concentrations, which are defined as the ratio of the atmospheric noble-gascontent divided by the mass of the Earth. All samples are enriched in Xe and partly in Kr, compared to the atmospheric concentrations. This is a common feature for sedimentary rocks (Canalas et al., 1968; Fanale and Cannon, 1971; Phinney, 1972; Ozima and Alexander, 1976; Podosek et al., 1980,198l). It indicates that a considerable part of the total terrestrial Xe might be locked in sedi-

LA

C

2%k =!!IFig. 3. Rare-gas concentrations in carbon separates relative to gas concentrations in the bulk of the same sample.

--I -OL* J

PO58C

I *4Kr

““xe

he

?4r

“Kr

‘“ke

Fig. 4. Noble-gas concentrations obtained from Isua and Pongola sediments relative to an atmospheric concentration C,, = atmospheric inventory/m= of the Earth. Relative seawater concentrations are based on data by Bieri et al. (1966).

116

mentary rocks. However, taking Xe concentrations of sedimentary Isua and Pongola rocks to be 20X higher than the atmospheric concentration (Fig. 4) and the abundancesof all sedimentary rocks as l/2000 of the Earth’s total mass (Fanale and Cannon, 1971), only N 1% of the atmospheric Xe is presently located in the sedimentary rocks. This is less than has been estimated from the Xe content of the Martinez, California, U.S.A., shale (Fanale and Cannon, 1971) or the tuffaceous black shale AS2 (11) (Phinney, 1972) with the assumption that they present l/2000 of the Earth’s total mass. However, Xe concentrations of other sedimentary rocks (Podosek et al., 1980) are in the rangeof Isua or Pongola bulk samples. Thus, these measured Xe concentrations suggestthat the whole Xe content of sedimentary rocks is smaller than the atmospheric Xe inventory. A comparison of elemental noble-gas ratios in the terrestrial atmosphere and certain meteorites show a deficit of Xe in the atmosphere by a factor of 20 (Signer and Suess,1963). If the Earth originally acquired noble gaseswith a planetary elemental pattern, then Xe reservoirs other than sedimentary rocks are required to explain the Xe deficit of the atmosphere.The missing Xe may be still in the interior of the Earth.

Acknowledgements The authors are most grateful to M. Freunde1 for mass spectrometric and computer assistance and to B. Spettel for the performance of the neutron activation analyses. We thank Professor F. Begemann for his critical advise in discussionsand ProfessorsA. Kr6ner and M. Schidlowski for making available the analyzed samples.The suggestionsof two unknown reviewers have improved the manuscript. Financial support by the Deutsche Forsch~sgemeinschaft is acknowled@d.

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