Rare Earth Elements in the Early Archaean Isua Iron-Formation, West Greenland

135

RARE EARTH ELEMENTS IN THE EARLY ARCHAEAN ISUA IRONFORMATION, WEST GREENLAND PETER W. UITTERDIJK APPEL* Geological Institute, Oester Voldgade 10, D K 1350, Copenhagen (Denmark)

ABSTRACT Appel, P.W.U., 1983. Rare earth elements in the early Archaean Isua iron-formation, West Greenland. Precambrian Res., 20: 243-258. The rare earth element (REE) patterns in the 3.8 Ga-old Isua iron-formation are generally flat, resembling those of some primitive basalts. Samples with positive, negative or no europium anomaly were found. It is shown that diagenesis and metamorphism did not significantly change the REE patterns. The presence or absence of europium anomalies in iron-formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the Archaean and Precambrian. The REE contents cannot be used to distinguish Algoma-type from Superior-type iron-formations. There appears to be a striking similarity between the Archaean submarine exhalations and modern submarine hydrothermal systems. It is considered likely that Archaean and early Precambrian seawater had a chondritic REE pattern with a slight enrichment of light REE.

INTRODUCTION

The Isua supracrustal belt is situated in the central part of an ancient gneiss complex in West Greenland.'' Radiometric dating of the supracrustal belt has given an age of 3.8 Ga. (Moorbath et al., 1973;Michard-Vitrac et al., 1977). Thus, the Isua supracrustals are the oldest terrestrial rocks known. This old age, however, is the most unique feature of the Isua rocks. The supracrustals do not significantly differ from younger Precambrian supracrustals of similar metamorphic grade. At Isua the common supracrustal belt assemblage consists of basic and acid volcanics, intercalated horizons of chemically precipitated carbonates and iron-formation, ultrabasic intrusives, minor amounts of clastic sediments and strata-bound sulphide mineralizations. The striking similarity of the Isua supracrustals compared to younger Precambrian supracrustal belts shows that the broad scale sedimentary environments in which Archaean and younger Precambrian supracrustals were depo*Present address: Geological Survey of Greenland, Oester Voldgade 10, DK 1350, Copenhagen, Denmark.

136

sited did not change drastically with time. This is important in the evaluation of the geochemical secular trends for major and trace elements through the early Precambrian. An example of a supposed secular trend is the behaviour of rare earth elements (REE), especially europium (Eu). Fryer (1977 a, b) and Laajoki (1975) found that Eu anomalies in Precambrian iron-formations could be used as indicators for the absence of free oxygen in the atmosphere during the Archaean t o middle Precambrian. GEOLOGY OF THE ISUA IRON-FORMATION

A brief description of the geology of the Isua iron-formation is given here (for more information see Appel, 1979, 1980). The Isua supracrustals (Fig. l),which are enclosed in the younger Amasoq gneisses, can be divided into five main sequences. The quartzitic sequence in the northeastern part, consists mainly of quartzites with intercalated horizons of iron-formation, carbonates and amphibolites. The amphibolitic sequence is mainly composed of layered and massive amphibolites, presumably derived from volcanics. Horizons of iron-formation, carbonates and quartzites are interlayered in the amphibolites. Most of the quartzites in these two sequences are metacherts. After deposition, the supracmstals suffered several phases of deformation and were metamorphosed under low to medium grade amphibolite facies conditions. The Isua iron-formation is found throughout the supracrustal belt, within the quartzitic and amphibolitic sequences. The individual iron-formation horizons range from a few cm t o several tens of metres in thickness, and can be traced for up t o a few hundred metres along strike. The iron-formation can be divided into different facies according t o the concepts of James (1954). There is no clear distribution pattern of the different facies through the supracrustal belt and nowhere has a carbonate-oxide facies transition along strike been observed. There is, however, a tendency for oxide facies to be most abundant in the eastern part of the belt, whereas the more reduced facies tend t o be t o the west. A brief description of the different facies of iron-formation is given here (for further information see Appel, 1980).

Oxide facies A major iron ore body in the northeastern part of the belt consists of oxide facies. The horizons of oxide facies in the rest of the belt rarely exceed 10 m in width. This facies is characteristically banded, with quartzrich bands alternating with magnetite-rich bands. The individual bands range from a fraction of a millimetre t o tens of centimetres in thickness; the thicker bands usually show internal layering. The main minerals are quartz (metachert) and magnetite. Small amounts of actinolite and carbonate occur. Pyrite is found in large subhedral scattered grains up t o 1cm.

_ Legend _ Ouortzitic sequence Amphibolitic sequencf

"._.I Carbonote - beoring sllceous schists

Gorben omphibolite Ultromofics Mylonite gneisses Amttsoq gneisses

I

. . . . .

Fig. 1. Simplified geological map of the Isua supracrustal belt. Area A: samples 1-4. Area B: samples 14-16 and 23-27. Area C: sample 17. Area D: samples 19-20 and 28-32. Area E: samples 8 and 13. Area F: samples 5-7 and 9-10. Area G:samples 11-12. Area H: samples 18 and 21-22.

@ Sample

area

138

Within the oxide facies, several 10-30 cm thick intraformational breccias occur, consisting of flat angular quartz-rich fragments embedded in a magnetite-rich matrix. Carbonate facies These occur in up to 4 m thick horizons, traceable for up to a few hundred metres along strike, mostly in the amphibolitic sequence. This facies is mostly layered with 2-3 cm thick carbonate layers alternating with up to 0.5 cm thick magnetite-rich layers. The carbonate is siderite with a small percentage of magnesium and manganese. Graphite in the form of scattered tiny flakes or spherical aggregates, also makes up a small percentage. Actinolite and grunerite occur locally, whereas quartz is absent or occurs in trace amounts only. A peculiar sub-facies is locally predominant. It is a massive rock, with carbonate forming the major mineral and magnetite occurring in subordinate amounts only. The most peculiar feature of this subfacies is the magnesium content which ranges from 10 to 32% MgO, in contrast to the normal carbonate facies which has an average MgO content of 5%.

-

-

Silicate facies This facies is found throughout the belt in horizons up to several tens of metres thick and can be sub-divided into several sub-facies. The most common consists of centimetre thick alternating magnetite and grunerite layers, with appreciable amounts of quartz present locally. Another type is finely laminated with magnetite and actinolite laminae. The third type is the finely laminated shaly iron-formation, consisting mainly of mag netite with 15% grunerite and 5% graphite. Massive horizons of silicate facies iron-formation occur locally, 8nd consist of grunerite with a few scattered subhedral magnetite grains. In silicate facies, pyrrhotite and chalcopyrite are found in small amounts only.

-

-

Sulphide facies

A transitional facies occurs between the sulphide and silicate facies, consisting of magnetite, chalcopyrite and pyrrhotite, together with varying amounts of actinolite and hornblende. Sulphide facies occur exclusively as thin horizons in the amphibolitic sequence. ANALYTICAL TECHNIQUES

The samples for mineral separates were crushed and sieved and fractions between 45 and 74 pm were separated using a hand magnet, Franz isodynamic magnetic separator and heavy liquids. This process provided very pure concentrates of magnetite, quartz, carbonates and iron silicates. The

139

mineral separates were analysed using neutron activation analysis (NAA) by R. Gwozdz, Ris$ National Laboratory (RISP). The samples for whole-rock analysis were crushed and analysed using NAA by B. Spettel, Max-PlanckInstitute (MPI). The NAA results for REE are listed in Table I, and their chondrite (Cl) normalised patterns are shown in Figs. 2-5. Chondrite REE contents are from Evenson et al. (1978). The total amounts of REE (ZREE) are calculated from the chondrite normalised patterns, and listed in Table I. La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb I

I

I

I

I

I

I

I

Lu I

2

c

0 c

0.2

-

(D

w w

a

2 1.5-

, , I La Ce Pr Nd I

1-2

'1-)----I

I

I

I

l-1.5

Srn Eu Gd Tb Oy Ho Er Tm Yb Lu Fig. 2. Rare earth element patterns in oxide facies iron-formation. Sample numbers as in Table I.

It might seem odd to normalise the REE contents in the Isua iron-formation to that of carbonaceous chbndrites when other authors have used other standards, e.g., North American Shale (NAS), in their studies of ironformations. It is not really logical to use either standard in the study of ironformations. Indeed, the only reason for using NAS is because it is a sedimentary rock, but NAS and iron-formations do not have more in common than present deep-sea ooze and coastal conglomerates. The main reason for using the chondrite (Cl) as a standard is because it contains the primordial REE abundance patterns, and is widely used and recognised by most geochemists. Samples of iron-formation were anlaysed by both RISQ and MPI in order to check the results. The chondrite normalised results are plotted in Fig. 5, where samples 7 and 18 were analysed by MPI, and samples 7A and 18A were analysed by RISP. The vertical bars on the RISQ results show the amount of error for each element. The results from the two laboratories agree reasonably well in their overall REE pattern. The RISQ results, however, tend to be slightly lower than those from MPI, especially in the heavy REE.

TABLE I Rare earth elements and aluminium (%), and yttrium contents (ppm) in different facies of iron-formation. Samples 1 and 2, wholerock oxide facies. Samples 3 (quartz) and 4 (magnetite), mineral separates from oxide facies. Samples 5-8 and 11-13, whole-rock carbonate facies. Samples 9 (magnetite) and 10 (carbonate), mineral separates from carbonate facies. Samples 14 (cummingtonite), 15 (magnetite) and 1 6 (actinolite) mineral separates from sulphide-type facies. Samples 17 and 18 whole-rock sulphide facies. Samples 1 S 2 5 whole-rock silicate facies. Samples 26 (magnetite) and 27 (cummingtonite) mineral separates from silicate facies. Samples 28 (cummingtonite), 29 (magnetite) and 30 (quartz) mineral separates of silicate facies. Samples 3 1 (cummingtonite) and 32 (magnetite) mineral separates from silicate facies (n.a. = not analysed)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

La

Ce

2.58 2.88 0.15 1.23 4.19 2.88 7.70 6.61 3.89 1.96 0.68 0.57 9.26 0.05 1.54 0.38 2.68 19.3 1.15 0.38 0.28 10.1 1.08 0.99 4.25 0.61 0.51 0.194 0.49 1.03 0.82 1.47

4.20 3.40 0.212 3.18 6.65 4.90 14.0 12.5 13.30 3.88 1.22 1.40 16.8 3.44 1.94 6.60 36.1 1.43 0.90 2.0 24.6 2.04 1.75 8.70 1.39

Pr

Nd 1.50

4.22 11.0 8.54

2.0

0.73 1.06 9.1

20.0 0.72 1.05 17.0 0.26 1.17 1.00 5.80

0.79 2.21 2.05 4.44

0.68

Sm

Eu

0.34 0.43 0.03 0.18 1.09 1.11 2.40 2.39 0.76 0.50 0.20 0.34 2.47 0.20 0.25 2.60 1.08 2.93 0.24 0.24 0.35 4.65 0.34 0.25 1.23 0.10 0.11 0.05 0.05 0.29 0.10 0.11

0.20 0.18 0.02 0.14 0.64 0.48 1.25 1.98 0.34 0.23 0.13 0.16 1.65 0.11 0.12 1.67 0.32 0.30 0.21 0.22 0.02 0.29 0.38 0.28 0.91 0.11 0.12 0.04 0.08 0.16 0.09 0.16

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

XREE

0.29 0.56 0.05 0.21 0.69 1.60 3.01 3.06 0.32 0.80 0.20 0.43 2.55 0.50

0.050 0.094 0.007 0.07 0.102 0.26 0.46 0.451 0.04 0.14 0.032 0.065 0.39 0.09

11.0 11.8 0.8 7.3 22.0 23.2 55.2 51.0 25.5 13.1 4.2 6.2 57.0 3.3

2.99 0.71 0.84 1.57 0.37 0.22 0.42 0.28 0.75 0.13 0.70 1.69 3.96 0.15 0.44 0.18 0.46 0.30 0.81 0.31 0.26 0.15

0.42 0.11 0.19 0.061 0.064 0.106 0.52 0.067 0.068 0.12 0.04 0.05 0.03

28.0 19.8 96.6 6.1 5.3 8.3 87.2 7.7 7.1 27.1 4.1 3.6 2.3

0.08 0.46 0.10 0.10 0.65 1.12 0.18 0.43 0.64 3.70 0.70

0.25

0.11

0.96 1.09

0.47

0.04 0.25 0.06 0.57 0.10 0.64 0.15 0.66 0.95 0.69

1.00 6.7 0.50 0.55 1.20

0.58 0.08 0.09 0.19 1.34 0.10 0.10 0.21

0.05

1.1 3.4 0.54 0.75 1.16 7.6 0.65 0.72 1.32

0.03 0.07

0.30 0.05

6.7

Y

Al

0.0 n.a n.a 20.4 16.4 58.6

1429 212

n.a. n.a.

4605 4129 3292 18102

n.a. n.a.

n.a. n.a.

n.a. n.a. n.a. n.a.

n.a. n.a. n.a. n.a.

4.9 8.7 32.3

27.0 3.3 1.7 7.5

n.a.

6.0

n.a. n.a. n.a. n.a. n.a. n.a. n.a.

582 159 7410

9422 688 793 8733 9580 318 476 3917

n.a. n.a. n.a. n.a. n.a. n.a. n.a.

141

10

2

30

10 4

r'x---Y-x5

4

La Ce Pr Nd Sm EuGd Tb Dy Ho Er TmYb Lu Fig. 3. Rare earth element patterns in carbonate and sulphide facies iron-formation. Sample numbers as in Table I.

The following list gives the degree of error for the MPI analysis in percent for each element, where the first figure is the maximum error and the last figure the most common error. La (5,3), Ce (25, lo), Pr (20, 20), Nd (25, 15), Sm (5, 3), Eu (5, 3), Gd (10, 7), Tb (10, 7), Dy (20, lo), Ho (15, lo), Er (20,20), Tm (25, lo), Yb (5, 3), Lu (5, 3). Yttrium was analysed by J. Bailey (Geological Institute Copenhagen, Denmark). The detection limit is 0.5 ppm with an error of -10%. Aluminium has been analysed by XRF and AA, with an average error of -5%. A description of each of the analysed samples is given in the appendix.

142 La Ce Pr Nd 7134 ' '

Sm Eu Gd Tb Dy Ho Er TmYb Lu '

7

W

W

a

- 10

10-

5-

>ba -_--_

.

1-

0.4

-1

La Ce Pr Nd

SmEuGd Tb Dy Ho Er Tm Yb Lu

Fig. 4. Rare earth element patterns in silicate facies iron-formation. Sample numbers as in Table I.

CHEMISTRY

The REE are a group of 14 elements characterized by very similar chemical behaviour and they are usually trivalent. Cerium (Ce) and europium (Eu), however, can also occur in tetravalent and divalent states, respectively. Whether Ce is trivalent or tetravalent depends on the redox potential (Eh) of the system. Under highly oxidising conditions, tetravalent Ce will be fractionated from the trivalent REE, and precipitates from oxidising aqueous solutions will thus have strong Ce anomalies. Divalent Eu can likewise be fractionated from the trivalent REE. Hence Eu anomalies in iron-formations

143

L a Ce

Sm Eu

Yb Lu

80

h

--30

E30-a,

+ .-

--lo

_ _ _ - - - -- '''~718 _---_----

+j10.c 0

5

ki""-5

5--

0

+ Q,

> .-

+

E

-1 -30

1-

--;I/

- - - - - - - - _ _ _ _ _ _-0--07-.*o

I7A1l5

Fig. 5. Rare earth element patterns in duplicate samples of iron-formation. Samples 7 and 18 analysed by MPI. Samples 7A and 18A analysed by RISB.

have been used as indicators for oxidising v. reducing conditions in the Precambrian atmosphere (Laajoki, 1975; Fryer, 1977 a, b). However, it requires very special conditions for divalent Eu to survive in aqueous solutions. The Eh for the reaction Eu2+ =+ Eu3+ + e' is -0.43 volts, and decreases with increasing pH. It will, therefore, require highly reducing conditions to keep appreciable amounts of EuZ+in solution. In the depositional environment of the iron-formations, abundant Fez+ and Fe3+ was present, with a redox 'potential for the transition Fez+ +- Fe3+ + e- of + 0.77 volts. The Eh for this reaction decreases with increasing pH, but will always be well above the redox potential for the stability of divalent Eu. Thus, E d + cannot exist in the depositional environment where the iron-formations were deposited, Hence Eu in iron-formations cannot have been fractionated on the grounds of redox conditions from the other REE. Therefore, Eu anomalies in iron-formations do not give any indication of the composition of the atmosphere during the Precambrian. The total amount of REE (XREE) in the different facies of the Isua ironformation varies from -4 ppm to almost 100 ppm. There is no clear-cut relationship between X REE and the facies of iron-formation. There is, however, a tendency for carbonate facies to be high in XREE compared to oxide facies, whereas silicate facies cover the whole range from very low CREE contents to high X REE contents. Within the different mineral phases there is the same absence of clear relationships. In oxide facies quartz is low in REE compared to the coexisting magnetite, whereas the opposite relation-

144

ship is seen in silicate facies (see samples 3, 4 and 29, 30, respectively). The REE contents in coexisting magnetite and iron silicates are of,the same order of magnitude in silicate facies iron-formation (cf. samples 26, 27 and 28, 29 and 31, 32). None of the individual mineral phases have REE contents or REE patterns significantly different from the whole-rock composition. REE patterns from the different facies of the Isua iron-formation show the same general trend. The REE pattern is rather flat, resembling that of Isua basalt (Appel, 1979). The most common deviation from the flat pattern is a slight enrichment in light REE in the iron-formation. A few samples are depleted in lanthanum, e.g. sample 21. Europium is clearly anomalous in many of the samples. In some samples it is depleted by a factor up to 10; in others it is enriched by a factor up to 3. In several samples, however, there is no europium anomaly at all. There is no obvious relationship between the behaviour of europium and the facies of iron-formation. In oxide and carbonate facies about half the samples have positive Eu anomalies and the rest have no anomalies. In silicate facies two samples have strong negative Eu anomalies, whereas all others have positive Eu anomalies. It is noticeable that the silicate facies with negative Eu anomalies consist of magnetite and actinolite, whereas the silicate facies samples with positive anomalies mainly consist of magnetite and grunerite. ROLE OF METAMORPHISM

Before evaluating the REE results, the role of metamorphism has t o be considered. The REE are a group of elements in the third group of the periodic system, together with elements such as aluminium and yttrium. The group three elements have many chemical similarities and tend t o covariate in aqueous solutions. Figure 6 shows the relationship of aluminium and yttrium v. 2 REE in the Isua iron-formation. Both elements show a fairly good correlation with 2 REE. The relationship of aluminium and yttrium v. the individual REE (except Eu) are very similar t o the trends in Fig. 6. This was t o be expected since the elements were precipitated contemporaneously in an aqueous environment. During diagenesis many elements are highly mobile, e.g., Rb and Sr. However, aluminium and yttrium are generally immobile during metamorphism. The behaviour of REE during diagenesis and metamorphism is debated, however, the results shown in Fig. 6 strongly indicate that the REE were immobile, like aluminium and yttrium, during diagenesis and metamorphism of the Isua iron-formation. Rare earth elements all behave very similarly, except for europium which is dealt with below. This chemical similarity indicates that even if some metamorphic addition or removal of REE took place, it would not affect the distribution pattern of the REE, but merely push the general level slightly up or down. Is it possible that the Eu anomalies in some of the iron-formation samples are of diagenetic or metamorphic

145 I

1.000

0.500

0.015

5

h

Q

Y

f

.-

z

c

>

30 20 lo

50

C

100

REE(ppm)

Fig. 6. Aluminium (%) and ytttrium (ppm) v. ZREE (ppm) in different facies of ironformation. Sample 8 is not included. The correlation coefficients are A1 - ZREE = 0.79 and Y - ZREE = 0.81. Note that the scale for aluminium is logarithmic.

origin? This would require very reducing alkaline conditions with no Fe3+ or other oxidising ions present. If these conditions were met, which is highly unlikely, then Eu3+would be'reduced to the more soluble Eu2+with the result that the soluble Eu2+ compounds could be removed from the system, leaving a rock with a negative europium anomaly. The carbonate facies with a high graphite content would be the best candidate for strongly reducing conditions during diagenesis or metamorphism, but none of the carbonate facies samples have negative europium anomalies. Thus, the Eu anomalies are unlikely t o be of metamorphic or diagenetic origin. The REE patterns depicted in Figs. 2-4 are, therefore, considered to be pre-diagenetic and pre-metamorphic. DISCUSSION

Previous investigations of the REE distribution patterns in iron-formations revealed an apparent secular trend of europium anomalies (Laajoki, 1975; Fryer, 1977 a, b). These authors found that the early Precambrian ironformations had negative Eu anomalies, whereas younger iron-formations

146

had positive or no Eu anomalies. The suggested explanation was that europium under reducing conditions occurred in the soluble divalent state, thus being fractionated from the trivalent REE (Laajoki, 1975; Fryer, 1977 a, b). The divalent Eu was supposedly accumulated in the oceans as long as the reducing conditions prevailed. When oxygen appeared in the atmosphere, Eu ceased to be anomalous and was precipitated in its trivalent state (Laajoki, 1975; Fryer, 1977 a, b). The first iron-formations to be deposited had therefore positive Eu anomalies, whereas iron-formations deposited later had no Eu anomalies (Laajoki, 1975; Fryer, 1977 a, b). This theory is clearly inconsistent, not only with the physical chemistry of europium, but also with the REE patterns of the 3.8 Ga-old Isua ironformation. In the Isua area, the iron-formation shows positive, negative or no Eu anomalies. Fryer (1977 a, b) and Laajoki's (1975) model was clearly inspired by the behaviour of cerium in sediments. Cerium covariates with the other REE in Precambrian deposits. Present day seawater has a pronounced negative Ce anomaly, corresponding t o a positive Ce anomaly in deep sea Mn nodules. The explanation for this behaviour is quite simple. During present day oxidising conditions Ce is oxidised to its tetravalent state and Ce4" compounds are less soluble than trivalent Ce compounds. Therefore, Ce is fractionated in seawater from the trivalent REE. This feature seems to provide a means for investigating the oxygen content in the atmosphere via the presence or absence of Ce anomalies in sea floor deposits. However, in a recent paper, Elderfield and Greaves (1981) report ferromanganese nodules from the Pacific Ocean with negative Ce anomalies. Thus, it can be concluded that neither cerium nor europium anomalies can be used as indicators for the presence or absence of oxygen in the atmosphere. It has been suggested that the REE patterns could be used to distinguish different types of iron-formations. Graf (1978) suggested that the absolute contents of REE and the type of Eu anomaly could distinguish an Algomatype iron-formation from a Superior-type iron-formation. This is clearly inconsistent with the REE patterns in the Isua iron-formation, where the general level of REE varies by a factor of 20, and positive, negative or no europium anomalies occur. The REE content and distribution patterns in coexisting mineral phases in the Isua iron-formation do not show any systematic trends. A likely explanation is that precipitation of the REE was governed by solution chemistry rather than by scavenging. Is it possible to obtain any information from the REE regarding the genesis of iron-formations? The genesis of iron-formations has been debated during most of this century, but little progress has been made. What is agreed upon, is that iron-formations are chemical sediments, precipitated in a very quiet sedimentary environment, with a general absence of detrital material. It is still an open question where the material, mainly iron and silicon, came from. The most likely origin is hot brines pouring on to the

147

sea floor. The most plausible source for these brines is either direct magmatic exhalations, or brines which have been circulating in and leaching the oceanic crust. Both processes are observed today, e.g., in the Pacific Ocean. When hot brines of either origin are expelled on the sea floor, they mix with seawater. Most of the material carried in solution by the brines will subsequently precipitate. The chemistry of the precipitates will depend on the degree of mixing, temperature, redox conditions and pH, as well as on the original composition of the brines and the composition of the seawater. The degree of seawater brine mixing depends on the temperature, velocity and density of the brines, as well as the bottom topography. Thus, there are a considerable number of unknowns in this process. Is it possible to elucidate any of these problems by the study of REE patterns of the precipitated sediments? Modern submarine hydrothermal systems observed at the East Pacific rise have apparently much in common with Archaean submarine exhalations (Corliss et al., 1981). Corliss (personal communication, 1982) has found strong positive Eu anomalies in the hot metal-rich brines exhaled on the sea floor at the East Pacific Rise and he attributes the positive Eu anomalies to highly reducing conditions under which hot circulating water leached sea floor basalts at depth. The presence of methane and carbonaceous matter in the brines indicates such reducing conditions that all the iron must be in divalent form. During the leaching process Eu is reduced to Eu2+ which is more soluble than trivalent REE. When the brines ascend and are exhaled at the sea floor, the material carried in solution will precipitate under conditions with higher redox potential than at depth. The Eu carried in solution will be oxidized to Eu3+ and precipitated together with the other REE. If only a limited mixing of brine and seawater occur, the precipitates will have strong positive Eu anomalies. With extensive mixing, the REE pattern of precipitate will be modified by the seawater REE pattern. In close proximity to the modern exhalative sea floor vents, metalliferous deposits can be expected to be found with positive Eu anomalies. Further away the deposits will have REE patterns resembling that of seawater. This model is applicable to the Isua iron-formation. The horizons with positive Eu anomalies were thus precipitated close to the submarine exhalative vents and those horizons with no Eu anomalies were deposited further away from the vents, where extensive brine seawater mixing had taken place and the REE pattern of the brines were modified by the REE pattern of seawater. This leads to the suggestion that the Archaean seawater had a chondritic REE pattern, possibly with a slight enrichment of light REE. The negative Eu anomalies found in two silicate facies (samples 21 and 22) and one sulphide facies (sample 18) are puzzling. Possible explanations are, that the material in these horizons were leached from rocks which were previously depleted in europium or preferential leaching of mafic minerals in the sea floor basalts occurred.

148

Leaching of a basaltic pile will remove most of the incompatible elements, e.g., REE from the basalts, and thus the circulating hot water will attain a REE pattern resembling that of basalts (Piper et al., 1975). One exception is Eu, which during crystallization of a basaltic magma is built into plagioclase. A mild leaching, not affecting plagioclase, will produce a brine with a basaltic REE pattern, but with a negative Eu anomaly (Piper et al., 1975). Exhalations from a magma contain high proportions of incompatible elements. Submarine exhalations from a basaltic magma will have a basaltic REE pattern. An exception is if plagioclase in the magma has started t o crystallise, thus depleting the magma in Eu. Exhalations from such a magma will have a REE pattern resembling that of the basalt, but with a negative Eu anomaly. Disregarding europium for the moment, most REE patterns in the Isua iron-formation have the same general trend of a siight enrichment of light REE. This trend resembles that of some primitive basaltic rocks, and the average levels of REE in the Isua iron-formation and in Isua basalts are of the same order of magnitude (Appel, 1979). Therefore, it is considered likely that the brines or exhalations which supplied the material for the iron-formation were of basaltic pedigree, and obtained their metal content through leaching of sea floor basalt and/or directly from basaltic magmas. The REE patterns in the Isua iron-formation show that local small scale features, such as proximity t o hot vents and sea floor topography determined the site and facies type of iron-formation precipitated. This explains why none of the individual horizons of iron-formation can be traced more than a few hundred metres along strike. It also explains the irregular distribution pattern of the different facies of iron-formation, and the lack of transition facies between oxide and carbonate facies. CONCLUSIONS

The results of this investigation, compared with similar investigations of younger iron-formations and recent submarine hydrothermal systems, have led t o the following conclusions: (1) the REE content and distribution patterns in the Isua iron-formation have not been significantly changed during diagenesis and metamorphism; (2) the presence or absence of europium anomalies in iron-formations cannot be used as an indicator for the presence or absence of oxygen in the atmosphere during the Archaean and Precambrian; (3) REE distribution patterns cannot be used to distinguish Algomatype and Superior-type iron-formations; (4)the ultimate source of the material in the iron-formations are submarine exhalations from basaltic volcanism and/or leaching of sea floor basalts; (5) the Archaean and early Precambrian seawater had a chondritic REE pattern, with a slight enrichment of light REE.

149 ACKNOWLEDGEMENTS

Part of the work was done while the author held a research stipend at the Max-Planck-Institute for Chemistry (MPI), Mainz, W. Germany. The Isua programme at MPI was initiated by M. Schidlowski and C.E. Junge, and performed as part of the Sonderforschungsbereich 73 “Atmospheric Trace Components”, partially funded by the Deutsche Forschungsgemeinschaft. Whole rock analysis were made at MPI, Department of Cosmochemistry. The mineral separates were analysed at Risqi National Laboratory on a grant provided by the Danish Natural Science Research Council. The XRF analyses were done by John Bailey, Institute of Petrology, University of Copenhagen, Denmark. APPENDIX

Sample description Samples 1 and 2: oxide facies iron-formation consisting of mm-thick alternating quartz and magnetite layers, with subordinate amounts of actinolite and trace amounts of carbonate. Average grain size of quartz 0.2 mm and of magnetite 0.002 mm. Samples 3 and 4: quartz and magnetite mineral separates of oxide facies iron-formation. Samples 5-8: carbonate facies iron-formation consisting of mm-thick alternating siderite and magnetite rich layers, with up to 10% grunerite and up to 5% graphite. Quartz is characteristically absent or occurs in trace amounts only. Graphite occurs mostly as scattered small flakes or spherical aggregates. In some very thin layers, however, graphite can amount to 75%. Average grain size of carbonate facies is 0 . 1 4 . 2 mm. Samples 9 and 10: magnetite and siderite mineral separates of sample 7. Samples 11 and 12: massive carbon&? facies iron-formation consisting of more than 80% Mg-Fe carbonate and -15% magnetite as scattered subhedral grains, together with minor amounts of grunerite. This rock type characteristically contains roughly equal amounts of iron and magnesium. Average grain size 0.5 mm. Sample 13: unusual type of iron-formation. Finely laminated with -90% magnetite, together with graphite, grunerite and garnet. Average grain size 0.5-1 mm. Samples 14, 1 5 and 16: cummingtonite, magnetite and actinolitic hornblende mineral separates of a sulphide-type iron-formation (see below). Samples 1 7 and 18: sulphide-type iron-formation. Can be regarded as a transition facies between sulphide facies and silicate facies iron-formation. This facies type consists of chalcopyrite, magnetite and actinolitic hornblende with small amounts of pyrrhotite and normal hornblende. Samples 1 9 and 20: silicate facies iron-formation, consisting of mm-thick alternating grunerite and magnetite rich layers, with small amounts of quartz locally. Average grain size 0.5 mm.

150

Samples 21 and 22: silicate facies iron-formation. Thinly laminated with alternating actinolite and magnetite rich laminae. Average grain size 0.050.1 mm. Samples 23-25: massive silicate facies iron-formation consisting of grunerite with scattered subhedral magnetite grains. Grain size up t o a few mm. Samples 26 and 27: magnetite and grunerite mineral separates from sample 25. Samples 28, 29 and 30: grunerite, magnetite and quartz mineral separates from sample 19. Samples 31 and 32: grunerite and magnetite mineral separates from silicate facies iron-formation. REFERENCES Appel, P.W.U., 1979. Stratabound copper sulfides in a banded iron-formation and in basaltic tuffs in the early Precambrian Isua supracrustal belt, West Greenland. Econ. Geol., 74: 45-52. Appel, P.W.U., 1980. On the Early Archaean Isua iron-formation, West Greenland. Precambrian Res., 11: 73-87. Corliss, J.B., Baross, J.A. and Hoffman, S.E., 1981. An hypothesis concerning the relationship between submarine hot springs and the origin of life on Earth. Proc. 26 Int. Geol. Congr., Geology of Oceans Symposium, Paris, July 7-17, 1980, Oceanologica Acta, pp. 59-69. Elderfield, H. and Greaves, M.I., 1981. Negative cerium anomalies in the rare earth element patterns of oceanic ferromanganese nodules. Earth Planet. Sci. Lett., 55 :

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Evensen, N.M., Hamilton, P.J. and O’Nions, R.K., 1978. Rare earth abundances in chondritic meteorites. Geochim. Cosmochim. Acta, 42: 1199-1212. Fryer, B. J., 1977a. Rare earth evidence in iron-formations for changing Precambrian oxidation states. Geochim. Cosmochim. Acta, 41: 361-367. Fryer, B.J., 197713. Trace element geochemistry of the Sokoman iron-formation. Can. J. Earth Sci,, 14: 1598-1610. Graf, I.L., 1978. Rare earth elements, iron-formations and seawater. Geochim. Cosmochim. Acta, 42: 1845-1850. Holland, H.D., 1973. The oceans: a possible source of iron in iron-formations. Econ. Geol., 68: 1169-1172. James, H.L., 1954. Sedimentary facies of iron-formation. Econ. Geol., 49: 235-293. Laajoki, K., 1975. Rare earth elements in Precambrian iron formations in Vayrylankyla, South Puolanka area, Finland. Bull. Geol. SOC. Finl., 47: 93-107. Michard-Vitrac, A., Lancelot, J., Allegre, C.J. and Moorbath, S., 1977. U-Pb ages on single zircons from the early Precambrian rocks of West Greenland and the Minnesota river valley. Earth Planet. Sci. Lett., 35: 449-453. Moorbath, S., O’Nions, R.K. and Pankhurst, R.J., 1973.Early Archaean age for the Isua iron-formation, West Greenland. Nature, 270: 43-45. Piper, D.Z., Veh, H.H., Bertrand, W.G. and Chase, R.L., 1975. An iron-rich deposit from the northeast Pacific. Earth Planet. Sci. Lett., 26: 114-120.