Llanvirnian (Ord.) iron ooids in Baltoscandia: element mobility, REE distribution patterns, and origin of the REE

INCLUDING

ELSEVIER

ISOTOPE GEOSCIENCE

Chemical Geology 125 ( 1995) 45-60

Llanvirnian (Ord.) iron ooids in Baltoscandia: element mobility, RElE distribution patterns, and origin of the REE Ulf Sturesson l’pl,sala Universitet, Institute of Earth Sciences, Norbyvagen 22, S-75236 Uppsala, Sweden

Received 9 June 1995; accepted 13 June 1995 after revision

Abstract The chemical and mineralogical compositions of Llanvirnian (Ordovician) iron and associated phosphate ooids from Sweden, Estonia and Russia have been examined. The oolites are generally thin and lean (mud supported), and dominated by Feoxyhydroxide (goethite, limonite) ooids. One Swedish oolite contains a mixture of chamosite, goethite and hematite ooids, and it is suggested that the two latter types were derived from chamosite ooids by early diagenetic alteration. The REE distribution patterns were unaffected by these reactions. All ooids in the area show a typical negative Eu anomaly inherited from the source material. Major-and minor-element contents in limonite and chamosite ooids are similar, whereas their REE patterns are different. Limonite ooids have a LREE/HREE ratio from 1 to 2.4, and chamosite ooids from 3 to 5. The amounts of REE in the ooids are strongly dependent on the amount of phosphate.

1. Introduction

The greatest production of iron oolites in the geological history occurred during the Ordovician and Jurassic periods. Ordovician oolites can be found in N. Africa, Spain, Portugal, France, Germany, the U.K. (England, Wales), Poland, the Czech Republic (Bohemia) and other places (H.L. James, 1966). The ooid production shows thr,ee major peaks, Llanvirn, Caradoc and Ashgill. The ool.ites are usually thin but laterally persistent, and can locally be concentrated into oolites of commercial importance (Young, 1989). The iron oolites in Baltoscandia occur at two distinct stratigraphic levels in the Lower and Middle Ordovician (Lower and Upper Llanvirn) . The oolites are mainly thin ( < 1 m) and lean ( < 30% ooids) and have a geographical distribution from Norway, Sweden, along the northern Estonian coast, to the St. Petersburg [SBI 0009-2541/95/$09.50 0 1995 Ekevier Science B.V. All rights reserved SSDIOOO9-2541(95)00076-3

area of Russia (Fig. 1). Fe-oxyhydroxides (goethite, limonite) dominate, but chamosite, hematite and phosphate ooids also occur. At two localities in Sweden, Hlllekis and Gullhiigen (S22 and S23-S27, respectively, in Fig. l), volcanic ashes are closely associated with Middle Ordovician chamosite oolites. These ashes have been suggested to be the source of elements for ooid formation (Sturesson, 1992a, b). Similar close association of volcanic ash and iron oolites occur also in the Ordovician of Wales (Pulfrey, 1933; Trythall, 1989)) Portugal (Young, 1989) and Bohemia (Petranek, 1964; Petranek et al., 1988). Although there is no visible evidence of volcanic matter in oolites elsewhere in Baltoscandia, a volcanic origin cannot be excluded. In high-energy, ooid-forming environments there is little chance for volcanogenic material to be preserved in textural recognisable forms. It is possible, however, that some of the elements released during hydrolysis, particularly less mobile elements like the rare-earth

lJ. Stwesson /Chemical Geology 125 (1995) 45-60

46

Fig. 1. Locality map showing the sampling sites. Sample numbers identical with those in Tables 1 and 2.

elements (REE) , could be incorporated in the ooids and provide a volcanic signature. In such case, however, the variation in the ooid mineralogy might cause difficulties in the interpretation of chemical data. There is generally no single model accepted for iron oolite formation, and the variations in mineralogy and occurrence indicate possible formation by more than one process. It has been commonly suggested that chamosite ooids are primary, whereas goethite and hematite ooids might be derived from chamosite by early diagenetic alteration (Maynard, 1983; Odin et al., 1988, and references therein; Doering, 1990; Cotter, 1992), or on the other hand, ooids of different mineralogical composition may form contemporaneously under varying chemical conditions (van Houten and Purucker, 1984). If either scenario is true, these conditions should be reflected in the chemical composition of the ooids. However, there are few systematic studies of the abundance and distributions of major, and minor elements, and trace elements (including REE) in iron ooids. There are three goals to this study: ( 1) determine if elemental studies can help to reveal the mode of formation; (2) determine the degree of mobility of these elements during early diagenesis; and (3) evaluate the source material for the ooids. 2. Geologic setting Samples of Lower Ordovician ooids were taken from the island ijland (Sl to S8), the Siljan district (S12 to

S21), Gotland (S9 to Sl l), northern Estonia (El to E2) and Russia (Rl ) (Fig. 1). Stratigraphical and lithological descriptions of these localities have been given by Orviku ( 1940), Hessland ( 1949), Sturesson ( 1986,1988a, b), Nordlund ( 1989)) and Sturesson and Bauert ( 1994). The oolitic sequence on &and (S l-S6) comprises four Fe-rich beds ( I 10 cm thick) within the Asuphus expansus zone of the Kunda Stage. Three of these beds contain light-grey chamosite ooids and one contains a mixture of chamosite, goethite and hematite ooids. The limestone below the oolites is grey in colour and rich in glauconite. All ooids contain appreciable amounts of phosphate, and the phosphate ooids in neighbour outcrops (S7S8) are supposed to be coeval with the iron oolite (Nordlund, 1989). They always occur in small amounts, usually < 0.5 wt%. The Lower Ordovician ooids of the Siljan district (S12-S21) are either chamositic or limonitic (Hessland, 1949). They are particularly abundant at the northern outcrop, (S15-S21) (Fig. 1) where the oolite is _ 2.5 m thick, with a continuous series from superficial ooids at the base of the section to fully developed ooids at the top. The uppermost section also contains a minor amount of small, grey chamosite ooids. Ooids are also abundant at the southern locality ( S 12S 14), where the oolite is - 1 m thick, with limonite ooids in the lower section and chamosite ooids in the upper section, separated by a zone of mixed ooids. At

U. Sturesson /Chemical Geology 125 (1995) 45-40

both localities there is a hardground (glauconitic and impregnated with goethite) at the base of the oolites, marking the boundary between the Volkov and Kunda Stages. Hematite and phosphate ooids are absent at both these localities. The Lower Ordovician oolite of Gotland (S9-S 11) is - 0.60 m thick and contains grey chamosite ooids; in the lower section mixed with hematite ooids. The oolite rests on a glauconitic limestone terminated by a discontinuity surface. Glauconite grains form the nucleus in many ooids. The Estonian and Russian oolites (Lower Ordovician) are dominated by limonite ooids (U. Sturesson and T. Saadre, unpublished data, 1994). Minor amounts of phosphatic ooids occur, but there are no hematite ooids apparent. In the Middle Ordovician Estonian iron oolites, phosphate ooids always occur at the top of the oolites (Sturesson and Bauert, 1994). Middle Ordovician (Llanvirn-Llandeilo) ooids from south-central Sweden (S23-S26) consist of chamosite with minor a.mounts of hematite. The ash beds associated with the oolites are calcitized with a relict texture of the ash preserved as illitical rims on primary glass shards (Sturesson, 1992a). In a nearby quarry (S22) coeval chamosite ooids are associated with a thin, black phospho:rite (Sturesson, 1992b)

3. Materials and methods The ooids were cleaned from the limestone matrix and the internal calcite by etching in 10% acetic acid, washed in deionized water, and dried at 95°C. The samples from Gland ( S l-S6) were selected from the bed containing a mixture of red hematite, yellow goethite, and grey chamosite ooids. The phosphate ooids from Gland (S7SS) were taken from a bed just above a conspicuous phosphatic hardground. From the southern outcrop in the S iljan district (S 12-S 14)) two types of ooids were selected: brown limonite ooids and black chamosite ooids, and from the northern outcrop samples were taken at six levels (S15-S21) from + 1.00 m to the top of section at + 2.50 m (Sturesson, 1988b). The Estonian and Russian oolites (El, E2 and Rl, respectively) are all composed of limonite ooids. Samples were taken from horizons with the largest and best developed ooids. From the Gotland core (S9-Sl l ) samples of chamosite and hematite ooids were hand-

41

picked under the microscope. As these ooids often contain glauconite grains as nucleus, one sample of glauconite from the glauconitic limestone below the oolite was collected for REE analysis. Samples of Middle Ordovician chamosite ooids, volcanic ash from south-central Sweden (S23-S27), and phosphorite (S22) were treated as above. Chemical analyses of the material were made by plasma spectroscopy [inductively coupled plasmamass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) 1. 0.125 g of the samples were melted with 0.375 g of LiB02 and then dissolved in diluted HN03. Three measurments were made of each sample and the instrumental precision for major elements is better than +_1.5% (RSD) . The precision for REE and other trace elements vary with concentration. RSD for REEenriched samples (e.g., phosphate ooids) is better than f 3% and for depleted samples (e.g., limonite ooids) it is better than f 10%. Electron microprobe scans of Fe, Si, Ca, P, Mg, Al and Ti were made across the lamination of chamosite ooids from Gland. Three scans with three elements in each were made with the Fe distribution as reference in all three. The traverse length is 600 pm in which 200 spots were analysed in each run (Fig. 2). X-ray diffraction analysis (XRD) was made on powdered, air-dried material with Cu-K, radiation. The angular range is from 28= 4” to 65”, and the scanning speed was 2.4 min- ‘. Scaning electron microscopy (SEM) and energydispersive X-ray spectroscopy (EDS) (Philips@ and EDAX@,respectively) were used for the identification of minerals in the core and cortex of the ooids.

4. Results 4.1. XRD

The mineralogical composition of the iron ooids fom Gland (S7-S8) shows that the red ooids are entirely hematitic (Fe,O,) , whereas yellow ooids contain francolite (Ca,( PO&O,) ,( OH,F) ), chamosite (Fe,,Mg0,8A13.0Si2.8010( OH) a according to Schoen, 1964)) and a minor amount of goethite (FeOOH) ; grey chamosite ooids have a significant amount of francolite and a higher content of calcite than hematite and goe-

(I. Sw-esson / Chemical Geology 125 (1995) 45-60

48

ica, particularly in the upper part of the section. At the outcrop (S12-S14), brown ooids are limonitic and have a chemical composition nearly identical to those above, whereas the black ooids consist of chamosite and a small amount of francolite (Sturesson, 1988b). Grey chamosite ooids from Gotland (S9-S 11) display a high degree of crystallinity with a well-defined peak at 14 A, and a low francolite content. Red ooids contain only hematite. The Estonian and Russian ooids (El, E2 and Rl ) show well-defined peaks for goethite, and they are brown in colour. The distinction between limonite and goethite ooids is here somewhat arbitrary. Limonite is not a definite mineral but represents a mixture of various Fe-oxyhydroxides, of which goethite is the chief constituent. Ooids with a low content of goethite are dark brown and here termed limonite ooids (group B in Fig. 3). Bright-yellow ooids have high amounts of goethite and consequently called goethite ooids (group A in Fig. 3)

b

4.2. Electron microprobe scan Stepwise electron microprobe scans across the ooid lamination from the nucleus and towards the margin showed that the lamination consists of alternating chamosite and phosphate (Fig. 2). The number of Fe peaks is N 15, which means that the average lamina thickness is _ 40 pm. 4.3. Chemical composition

/ 0

I

600

pm

Fig. 2. Electron microprobe scans perpendicular to the lamination of a chamosite ooid from bland, showing the relations between some major elements. Fe is included for all three scans. The chamosite elements Fe, Si, Al and Mg have generally similar trends, the apatite Ca and P have the opposite trend to these, and Ti is below the detection limit. The analysis shows that the lamination is due to variations in the ratio between chamosite and apatite. Scan lengthis 600mm. core is to the left.

thitic ooids (Sturesson, 1986). It is possible that the amount of goethite is underestimated by XRD due to its poor crystallinity. Brown ooids from the Siljan district (S U-S2 1) display a poor crystallinity, and only goethite is revealed from the XRD. A hump in the diffractogram indicates the presence of amorphous sil-

The chemical compositions of the ooids are listed in Table 1 and Table 2. The spider diagrams in Fig. 3 show the compositions of the hematite, goethite, chamosite and limonite ooids normalised to the North American Shale Composite (NASC) (Gromet et al., 1984). The goethite (limonite) ooids have a narrow concentration range and a small deviation from the shale standard, except for depletion of mobile elements such as Na, K, Sr, and an enrichment of Fe (Fig. 3, group B). The chamosite ooids are more variable in composition, containing higher amounts of PzOs and Ca from the fiancolite. Hematite and goethite ooids (Fig. 3, group A) which occur close together with chamosite ooids, have elemental distributions similar to these. This indicates that if they are derived from chamosite ooids this alteration had a minor impact on the relative abundance of many elements.

U. Sturesson /Chemical Geology 125 (1995) 45-60

49

100 -

Group A: goethite and hematite ooids

10

I

--

-G--

S 1 (hem)

-

SZ(hem)

--

0.1 --

100 --

Group B: limonite ooids

10 --

1 --

0.1 -100 --

Group C: chamosite ooids

r---l

10 --

-

s4

-

ss

-

S6

P

1 --

0.1 --

302

Fe0 A1203

Na20 CaO

K20

SC

MnO

MLiO TiO2

p206

zr

Ni Cr

Sr

W Ba

Fig. 3. Spider diagram illustrating the composition of major, minor and trace elements in limonite, chamosite and goethitekmatite ooids, normal&d to the NASC standard (Gromet et al., 1984). Group A represents the goethite and hematite ooids derived from chamosite ooids in group C according to a diagenetic model discussed. Group B am brown limonite ooids, with a low phosphate content.

0.42 1.02 0.27 0.14 8.21 4.83

0.15

0.46

1.15

0.25

0.14

7.49

4.76

NazO

K,G

MgG TiOl

MnO

P@s

Sc (ppm) Cr

99.2

83.3

19.6 4.84

22.0

15.6 2.97

Gd

Tb

C

1.67

15.5

2.85

27.0

11.1

66.8 12.2

93.6

90.9 19.7

385

477 78.0

209

12”

217 48.4 142

22.8

103

6.31

25.6

0.01

0.77

1.24

0.88

0.11

36.7

11.6

6.19

12.2

S5

C

1.40

12.0

2.30

21.6

8.82

51.7 9.58

72.1

71.8 18.2

300

372 61.9

169

12”

196 51.7 726

29.8

111

5.56

21.4

0.02

0.38

1.73

0.65

0.08

30.7

16.9

8.49

13.7

S6

3.05

1.30

3.31

P

1.62

15.5

2.79

26.4

11.4

67.5 13.1

101

103 21.3

456

581 94.4

246

n.d.

338 25.9 338

14.5

25.7

3.30

38.0

0.01

0.09

0.23

0.19

0.24

50.5

S7

2.90

1.42

3.70

P

1.72

14.2

2.81

26.9

11.8

69.9 13.6

105

106 20.8

469

597 97.8

254

n.d.

377 25.3 n.d.

20.7

26.4

3.43

38.6

0.00

0.09

0.22

0.17

0.22

49.9

S8 6.09 3.68

H

0.33

2.66

0.38

3.35

1.34

7.42 1.50

10.5

8.90 2.26

33.5

40.6 7.31

17.9

24.9

20.0 54.2 42.2

31.5

365

2.3”

1.33

0.02

0.63

0.83

0.25

0

2.04

82.5

S9

C

0.81

6.10

1.Ol

8.14

3.36

18.7 3.88

24.8

22.2 6.18

82.6

98.2 18.8

43.1

24.1

26.8 81.9 20.5

88.2

642

2.3

3.96

0.11

1.05

4.58

0.32

0.04

7.19

34.9

16.6

21.9

SIO

GL

0.17

1.32

0.21

2.08

0.98

5.40 1.11

7.74

5.88 1.59

23.5

32.9 5.55

13.1

12’

14.0 28.9 40.0

51.9

131

8.41

1.45

0.01

0.05

3.00

7.68

0.20

2.19

21.8

10.1

48.1

Sll

L

0.58

4.37

0.58

3.79

1.13

0.79 5.49

4.50

2.95 0.79

10.9

15.7 2.22

8.06

20.0

54.1 107 80.8

48.5

115

10.9

0.81

0.05

0.78

1.15

0.50

0.07

0.99

68.3

6.37

8.89

s12

C

0.89

6.63

1.09

8.95

3.50

18.2 3.33

22.6

20.9 4.94

82.2

96.9 17.2

43.3

20.5

115 72.3 86.7

73.9

205

8.09

6.13

0.04

0.63

2.79

0.26

0.17

9.79

36.8

14.3

20.2

s13

L

0.48

3.66

0.51

3.04

0.91

0.64 4.60

4.01

2.63 0.68

9.00

13.9 1.87

7.06

15.9

55.3 105 79.4

45.1

114

10.7

0.82

0.05

0.75

1.18

0.55

0.12

1.22

66.9

6.33

8.80

s14

H = hematite; G = goethite; C = chamosite; P-phosphate; L - limonite; GL = glauconite.

C

1.18

9.46

1.40

16.3

7.55

39.5 1.99

58.2

54.0 13.6

221

291 46.1

129

12”

187 47.9 281

35.8

113

7.20

20.3

0.03

0.31

1.85

0.65

0.10

32.3

19.2

8.67

13.4

S4

‘Determinationsat, or near the detection limit.

n0 data.

Mineral abbreuiations:

G

1.17

o.d. -

H

0.69

8.93

1.30

14.3

6.43

34.1 6.80

49.0

43.2 10.3

183

230 37.7

105

30.0

131 65.9 160

22.7

150

6.62

14.1

0.03

0.29

1.16

0.47

0.12

22.1

H

0.56

Lo

5.28

0.82

8.53

3.68

19.1 3.68

27.3

23.9 5.42

8.27 5.29 41.8

S3

Mineral

0.93

4.20

Yb

Br

Tm

2.93

6.88

Dy Ho

EU

Sm

Nd

PI

130 21.2

105 16.8

ce

60.4

49.4

39.6

41.1

97.6 86.8 129

14.8

La

95.2 62.8 105

13.3

127

W

Ba

zr

Sr

Ni

0.12

19.2

cao

113

19.3

47.8

49.2

8.41 5.02

8.84

5.23

S2

Si02 (wt8)

Sl

Al& Fe203

Element

L

6.85

S16

8.54

s17

7.15

S18

60.7

s19

0.48

4.02

0.54

3.54

1.17

0.89 6.17

5.51

4.10 0.95

15.5

L

0.53

3.65

0.60

3.42

1.17

0.80 4.99

4.14

3.02 0.72

L

0.64

4.32

0.65

4.14

1.32

0.89 5.76

5.18

3.73 0.92

9.29 11.91

L

0.50

3.66

0.50

3.27

1.03

4.82 0.76

4.13

3.04 0.71

1.29

L

1.26

9.39

1.50

11.5

4.49

24.1 4.30

29.4

23.6 5.79

7.6

22.7 15.7 19.1 16.3 124 3.22 2.29 2.89 2.35 20.7

9.72

18.8

50.4 92.5 66.1

48.8

126

10.4

0.90

0.06

0.58

1.16

0.59

0.06

0.52

68.3

6.05

9.20

S15

The chemical composition of the Lower Ordovician ooids determinedby ICP-AES (major and trace elements) and ICP-MS (REE)

Table 1

L

1.59

11.8

1.93

14.8

5.71

30.5 5.38

38.7

31.4 7.29

128

162 27.6

81.9

s20

L

1.43

11.4

2.11

19.2

7.86

43.5 8.00

55.7

46.4 11.3

190

238 41.2

118

S21

5.50

7.91

L

E2

0.42

2.92

0.40

2.53

0.83

L

0.83

6.17

0.96

7.02

2.49

4.18 0.60 13.3 2.29

3.41 14.3

2.37 11.6 0.54 3.14

9.19 45.2

14.3 59 2.00 9.85

7.77 28.3

12”

39.5 92.6 61.3

91.0

279

13.5

0.69

0.03

0.89

1.19

0.81

0.09

0.54

68.9

El

5.22

6.84

L

0.72

5.02

0.77

4.78

1.71

8.44 1.36

9.86

6.31 1.46

24.2

35.6 5.13

16.1

13.1

52.7 124 64.7

46.9

169

17.7

1.63

0.04

0.92

1.14

0.60

0.11

1.33

68.5

RI

&

$

z g 2

U 5

F $

g

;

s E 3. g

%

$

5

f=

CJ.Sturesson /Chemical

51

Geology 125 (1995) 45-60

Table 2 Rare-earth elements (RISE) in Middle Ordovician ooids, ash and phosphorite from south-central Sweden determined with ICP-MS, except for S22 which is analyzed with instrumental neutron activation analysis (INAA) Element

s22

La (ppm) Ce Pf Nd Sm EU Gd Tb DY Ho Er Tm Yb LU

190 640 1,200 126 21 13 78

17 2.2 phosphorhe, INAA

Sample

S23

S24

s25

S26

s27

6.46 21.6 2.28 9.17 2.03 0.625 2.57 0.462 2.43 0.615 1.81 0.260 1.98 0.240

6.67 18.6 1.92 7.55 1.86 0.548 2.36 0.452 2.79 0.682 2.24 0.386 3.07 0.392

15.7 46.9 5.40 18.7 4.02 1.25 4.85 0.884 4.85 1.12 2.97 0.470 3.64 0.437

18.1 47.3 5.66 24.2 4.90 1.05 5.44 0.891 6.71 1.61 5.45 0.859 5.92 0.811

41.3 96.3 11.2 46.9 9.39 2.16 9.90 1.53 10.6 2.46 8.21 1.28 8.89 1.27

chamosite ooids

chamosite ooids

hematitic ooids

ash, <63 pm

ash, 63-250 pm

The Al/Fe ratio reveals two distinct groups of ooids: a chamosite group with a ratio between 0.30 and 0.41, and a limonite, goethite and hematite group with a ratio from 0.06 to 0.10. These values are similar to those

2500

2000

I

reported by H.E. James and van Houten ( 1979) for Miocene chamosite and goethite ooids from Colombia. Phosphate ooids and glauconite grains belong to the chamosite group in this sense.

.

r+O.Ql

1500 k P 1 w

Chamosite

1000

Limonlte

500

0 0

5

10

15

20

25

30

35

40

Cont. P2O5 %

Fig. 4. The relation between the total amount of REl3 and the amount of phosphate. in the ooids. The horizontal bars show the range of the data for each type of ooid. 18 data pairs were used for the regression analysis.

U. Sturesson /Chemical Geology I25 (I 995) 45-60

52

T

I3

Sweden, S 1 to S 8

10

i

-+-

S 1 (hematite)

-

S 2 (hematite)

-

s 3 (goethite)

-

S 4 (chamosite)

-t-

‘....--..-“.- ‘....

/I

S 6 (chamosite)

-

S 7 (phosphate)

( 1-

S6(phos;h&j

1

10 1 .'.-. ....-.-..-. ....--.. .... ..-. -..

r --+-

s 5 (ChamoSlte)

-

Sweden, S 23 - S 27

Eu/Eu’=6.664 (6’0.054,

S 23 ooids

...-..-....- .....

1

N=6)

j

Fig. 5. The rare-earth element (REE) distribution in ooids and volcanic ash determined with an inductively coupled plasma-mass spectrometer

(ICP-MS) and normalised to chondrite standard. A. Iron and phosphate ooids from bland (S 1%). B. Middle Ordovician chamosite ooids and volcanic ash from south-central Sweden (S23S27).

In examined ooids the concentrations of the REE, are proportional to the phosphate content (Fig. 4). Limonite (group B) ooids are poor in REE, whereas the phosphate ooids are enriched in REE. Chamosite ooids (group C) and goethite-hematite ooids associated with them (group A) have a wide phosphate content range and thus a varying REE concentration. The distribution pattern of the REE is almost identical for the iron and phosphatic ooids in the samples from Gland having a considerable light REE (LREE) enrichment and a pronounced negative Eu anomaly of 0.68 (Fig. 5A). The Ce anomalies (shale-normahsed data) are very small: the phosphate ooids = - 0.05 and the ferriferous ooids = - 0.077 (s = 0.005). The average ratio of light REE/heavy REE (LREE/HREE) varies from 4.4 for hematite and goethite ooids to 5.9

for phosphate; average for all samples is 5.09 (s = 0.70, N=8). The ooids from the Siljan district display two different REE distribution patterns (Fig. 6A and B ) : in the lower parts of the sections, limonite ooids with a low phosphate content ( N 1.2%) have a low content of REE and a low LREE/HREE ratio ( 1.27, s=O.20, N = 4). In the uppermost parts the ooids become more chamositic and their P,O, content is N 3-596 (Sturesson, 1988b). The REE pattern shows an enrichment of the LREE and thus a higher LREE/HREE ratio (3.29, s=O.41, N-3). TheEu/Eu*anomalyof -0.6-0.7 is present in all the samples. Hematite and chamosite ooids from Gotland have similar patterns and display high LREE/HREE ratios (3.49, s = 0, N- 2) similar to thechamosite ooids from

53

II. Sturesson /Chemical Geology 125 (1995) 45-60 loo3

Sweden, S 15 to S 21

Sweden, S 12 to S 14 B

100 i

B f 1

Eu/Eu’=O.6S (s=O.OZI. N=3)

EulEu’=0.69

I

10 EulEu*=O.EZ (~90.014, N=4)

----SIS

-s,e

-.s19

1

--Sl,

-sso

-

S 13 chamorlte

-

S 14 go@thite

-alI)

-s21

7

104 Estonia and Russia

C

Sweden. S 9 to S 11

D

Grbttingbo co”

1

I I;

10

B

I

EulEu’n0.76, 0.68 and 0.57 (top

/-FM

-El

--

. bottom)

E2

-

s 11,

gIa”CO”lt.

--=--

S 9. hematite

-?-

S 10. chamositr

1

Fig. 6. Chondrite-nomalised

and(D) Gotland (S9-Sll).

REE distributions for ooids from: (A, B) the Siljan district (S.12321);

(C) Estonia and Russia (El, E2, Rl);

54

Fig. 7. NASC-normalised REE distributions for limonite and chamosite ooids. The inset illustrate the hat shape of the REE pattern defined as Gd/ (La * Lu) 1/Z(normalised) and the HREE/LREE ratio represented by the Lu/La as a function of La concentration. La is taken as a proxy for the total amount of REE.

aland and the chamosite ooids from the Siljan district, and the Eu/Eu* ratio is in the same range (Fig. 6D). The P205 content is 1.3% in the hematite ooids and 4.0% in the chamosite ooids. The REE pattern for the glauconite grains from the bed below the oolite is

almost identical to that for the hematite ooids with a LREE/HREE = 4.07. The Estonian and Russian limonite ooids have REE patterns similar to the limonite ooids in Sweden, as well as to the chamosite ooids (Fig. 6C). The LREE/

CJ.Sturesson /Chemical

HREE ratio of 1.83 (s = 0.49, N= 3) is -50% higher than in the Swedish limonite ooids but still not within the range of chamosite ooids (Fig. 6A, B and D) . The Eu anomaly in Estonian ooids is less pronounced than in the Swedish ooids (Fig. 6C, note the different scale). REE concentrations normalised to NASC (Gromet et al., 1984) for 13 ooid samples from various localities in Baltoscandia are shown in Fig. 7. There are two obvious trends in the REE distributions: a decrease of the HREE (Dy to Lu) /LREE (La to Sm) ratio with increasing amounts of REE, and a related change of the patterns towards a more convex shape. The former feature is demonstrated by the Lu/La ratio in the inset in Fig. 7 where CLais used as a proxy for the BEE. The later feature is illustr,ated by the [ Gd/ (La * Lu) “2] / La ratio (NASC-normalised values). The enrichment of the REE has a small1influence on the LREE (La to Sm) distributions, whereas the HREE distribution becomes progressive1.y more negative with increasing ZREE. The Ce anomalies for these samples are small (mean - 0.044, s = 01.025,N= 13). This is within the range for a normal marine environment (Wright et al., 1987). The volcanic ashes and chamosite ooids from southcentral Sweden have the general features in common, i.e. a steep LREE dislribution and an almost horizontal HREE distribution ( Fig. 5B). This pattern is similar

Geology 125 (1995) 45-60

55

to some limonite ooids like those in Estonia and Russia (Fig. 6C). The Eu anomaly is present in all but is more pronounced in the ashes (0.84 vs. 0.65).

5. Discussion

Most studies of iron oolites have focused on rich (60-80% ooids, sensu Bhattacharyya, 1989) oolites of economic importance. These oolites are usually rather thick due to redeposition and accumulation by currents, and might have been reworked and transported far from the site of their formation (Maynard, 1983, chapter 2). Conversely, lean oolites ( < 30% ooids) with mud supported ooids better represent the site of formation. The lean oolites in Baltoscandia exhibit several stages in their evolution and are thus well suited for a study of the chemical changes during their formation and diagenetic evolution. Differences in chemical and mineralogical compositions of the ooids can be attributed to formation in different environments, and/or to diagenetic alterations of the original material. For the interpretation of the l$EE patterns and other chemical and mineralogical properties of the ooids, it is important to elucidate the processes of ooid formation based on petrographic features as the composition of the nucleus, ooid shape and size, and thickness of the lamination.

Fig. 8. The suggested mckbl for the conversion of chamosite ooids to hematite ooids via the. intermediate goethite ooids, displaying the element mobility. The transformation of ooid mineralogy is also accompanied by a loss of secondary carbonate (Sturesson, 1988a). It is suggested that these reactions are early diagenetic taking place at the sediment-seawater interface.

56

U. Sturesson /Chemical

Geology 125 (1995) 4540

Fig. 9. A. Detail of a phosphate ooid from &and with echinoderm nucleus and the lamination partly preserved by concentric Reflected light, picture length -0.5 mm. B. Back-scattered electron image of a phosphate ooid with pyrite in the cortex and nucleus. Scale bar= 100 mm.

There are several indications of changing physical and chemical conditions during the formation of the northern oolite in the Siljan district (SE-S21) : the

layers of pyrite.

ratio cortex thicknesslooid length increases up’wards from 0 to -0.7, the nucleus material changes from exclusively echinoderm fragments at the base to an

U. Sturesson /Chemical Geology 125 (1995) 45-60

echinoderm-trilobite mixture in the middle, to mostly unrecognisable nuclei at the top of the section, and there is also a decrease in ooid length from - 2 to 1.5 mm. The chamositic ooids at the top of the section often have alternating limonite laminae, indicating varying chemical conditions during the formation. There are also variations in the chemical composition of the ooids in the section: in the upper part there is a pronounced increase of the phosphate content, and a decrease of Ti (Sturesson, 1988b). In the southern oolite (S12-S14) most identified nuclei consist of trilobite fragments, all algal bored and stained brown by Fe-oxyhydroxides. The amount of ooids with nuclei of unknown origin and with recrystallized cortex increases upwards. The amount of phosphate in the ooids is higher in the upper part of the section which contain chamosite ooids (Sturesson, 1988b). A ch,ange in the ooid formation environment is therefore likely also in this case. Based on red/grey changes of the limestone colour, formation of phosphatized hardgrounds and variations in ooid mineralogy, Hessland ( 1949) suggested that sea-level changes had occurred, causing local stagnancy followed by periods of ventilation of the seawater. He also argued for volcanic ash as the most plausible source of the ooid iron. The nuclei in the Gland iron ooids are dominantly carbonate skeletal grains but algal boring activity and recrystallization have rendered them unrecognisable. In the phosphate ooi’ds from Gland glauconite grains and phosphatized skeletal material are common as nuclei, as well as ooid fragments and the nucleus is always small compared to the cortex thickness (Sturesson, 1988a). The Ibland ooids vary in size; chamosite ooids are significantly smaller than goethite and hematite ooids. The amount of secondary calcite is highest in the chamosite ooids (57.2-59.4%) and lowest in the hematite ooids (4.0-4.4%) with intermediate goethite ooids ( 14.6~-16.1%). The phosphate ooids are significantly smaller than the iron ooids, and have a low calcite content (5.4-5.6%). Sturesson ( 1988a) suggested that the iron ooids originated as chamosite ooids, and that some of them were altered to goethite and hematite ooids, by oxidation and dehydration during early diagenesis. Chauvel and Guerrak (1989) arrived at the same conclusion for the formation of Ordovician iron oolites in France. The proposed model in Fig. 8 summarises the mobility of the most important elements during the

51

diagenesis of the ooids from Gland ( S l-S8), based on analyses of calcite-free material. During the alteration of the chamosite ooids there is a continuous loss of elements towards hematite, with a leaching of many elements such as Si, Al, Ca, Mg and P. The amount of secondary calcite is also reduced (Sturesson, 1988a) and replaced by Fe-oxyhydroxides and -oxides. Beside Fe, only a few elements are enriched in the process. However, the differences in chemical composition are rather small (Table 1) , and there are remnants of chamosite also in the goethite. The phosphate ooids from Gland (S7-S8) have not been included into the model as they always occur in very low concentrations and never in the same beds as the iron ooids. They are probably formed under later diagenetic reaction of chamosite ooids in a reducing environment (Bhattacharyya, 1989). The iron in these ooids is often bound to pyrite around and within the ooid cortex (Fig. 9 A,B) . The proposed model is supported by the discovery of modern analogues of chamosite ooids at Cape Mala Pascua, Venezuela, growing directly on the sea floor and exposed to oxidizing seawater (Kimberley, 1994). These ooids are composed of almost pure authigenic Fe-rich silicate and have a pm-thick oxidized rim of ferric hydroxide. Nucleus composition and lamination are similar to the ooids discussed here, and the ooids are associated with phosphatic sediments (Kimberley, 1989). Limonite ooids from Sweden, Estonia and Russia (Fig. 3, group B) also contain Fe, Si, Al and Mg in proportions similar to chamosite ooids (Fig. 3, group C).The only significant difference is the higher content of phosphate and Ba in the chamosite group. The amount of immobile TiOz and SC is nearly the same in the ooid types. Limonite ooids from the Siljan district (S15-S21) show a concentric lamination of amorphous SiOZ (Sturesson, 1988b) after treatment with HCl. The chemical elements of chamosite are present, but no chamosite can be seen in XRD, except for the uppermost samples, and there only as traces. These facts indicate that there is a transition from limonite ooids in the lower part of the section to more chamositic in the upper part, but environmental conditions for true chamosite ooid formation were not at hand. REE distribution patterns can be useful for the interpretation of genetic and diagenetic environments of the ooids (Piper, 1974; Graf, 1978; Elderfieldet al., 1981) but comparisons with data in the literature is severely

58

U. Sturesson /Chemical

hampered by the paucity of published data about REE in oolitic ironstones. It is clear, however, that there are two distinct groups of REE patterns in the studied material: the chamosite group with a LREE/HREE = 3-5 and the limonite group with LREE/HREE = l-2.4. It is also clear that the REE pattern from the chamosite ooids is retrieved almost unchanged into the goethite and hematite ooids derived from them, according to the model suggested in Fig. 8. It is well known that phosphate can incorporate considerable amounts of REE, and we have observed that the phosphate ooids from Gland (S7S8) display REE patterns identical to the coeval iron ooids (Sl-S6) (Fig. 5A). The role of the phosphate phase for the enrichment of REE in the ooids is illustrated in Fig. 4. The considerable difference in the phosphate content between limonite and chamosite ooids explains the difference between their NASC-normalised patterns in Fig. 7. The limonite ooids (CLa< 1) have a rather straight pattern with light REE-medium REE (LREEMREE) depletion, whereas the chamosite ooids have a typical enrichment of MREE with a maximum at Gd. This convex shape is typical for ancient phosphates and seems to be produced when REE, adsorbed on Feoxyhydroxides, are transferred to the phosphate phase during the earliest stages of diagenesis (GrandjeanLCcuyer et al., 1993). The Eu anomalies observed in all examined ooids have a very small range, (0.57-0.80, mean = 0.67, s = 0.06, N= 22). Such a narrow range cannot easily be explained by a discrimination of Eu in the sedimentary environment. The anomaly is probably inherited from the source material as Eh-values, low enough to reduce Eu3+, are rare under near-surface conditions (Bau and Miiller, 199 1) . The glauconitic limestones below the Gotland chamosite oolite, and the glauconite grains common as nuclei in many ooids, point to a close association between glauconite-and chamosite-forming environments. These are mutually exclusive, however, and glauconite never forms ooids. Glauconite is associated with deeper waters, whereas chamosite occurs in nearshore environments (Odin, 1988; Odin et al., 1988). The change from glauconite to chamosite could therefore indicate a sea-level drop. The glauconite has a REE distribution which shows the same trend as that for the ooids (Fig. 6D). The result can be interpreted so that eroded glauconite grains delivered the material to the

Geology 125 (I 995) 4560

ooids and the REE pattern was inherited from the glauconite, or glauconite grains and ooids have inherited their elements from a common source, such as volcanic ash. The similarity of the REE patterns between Middle Ordovician chamosite ooids in south-central Sweden (S23-S26, Fig. 1 ), and associated volcanic ash beds is striking (Fig. 5B). The Eu anomalies are of the same magnitude, and the LREE/HREE ratios are almost identical (2.13 vs. 2.16). The ash was suggested as the source material for the ooids (Sturesson, 1992a, b). Devitrifying volcanic glass at the sea bottom should also influence the amount and distribution of the REE in pore waters, and such diagenetically mobilised REE incorporated in apatite would retain some features of the volcanic REE pattern. The phosphorite at S22 (Fig. 1) , associated with chamosite ooids and volcanic ash, has a REE pattern nearly identical to the chamosite ooids in the Siljan district (S 12 to S21 in Fig. 6A and B) and on Gland, whereas the ash and ooid patterns resemble those for limonite ooids due to the low phosphate content (0.3%) (Fig. 5B). The Middle Ordovician Estonian limonite and phosphate ooids have REE distribution patterns essentially identical to the ooids from Dalarna and Gland, and the Eu anomaly (0.69) is the same (Sturesson and Bauert, 1994). If the REE patterns in the Middle Ordovician ooids are derived from the volcanic ash, then there is probably also a volcanogenic origin for the Lower Ordovician ooids in Baltoscandia. Hessland (1949) arrived at a similar conclusion using other arguments. He suggested that volcanic ashes rich in Fe were transported to the sea after having been deposited on land. Possible Lower Ordovician volcanic sources occur in the Scandinavian Caledonides, western Norway (Vogt, 1945; Stillman, 1986; Bruton, 1988), in the Wales basin (Kokelaar, 1992) and in the English Lake District (Hughes and Kokelaar, 1993).

6. Conclusions ( 1) The REE distribution patterns for the Lower and Middle Ordovician ooids in Baltoscandia display two important features: a significant negative Eu anomaly of N 0.6-0.7 with small variations between the localities and two distinct groups separated by their LREE/ HREE ratio -a limonite type with a ratio between 1

II. Sturesson / Chemical Geology 125 (1995) 45-60

and 2.4, and a chamosite type with a ratio between 3 and 5. (2) The assumed early diagenetic alteration of chamosite ooids to goethite and hematite ooids does not change their REE distribution patterns. (3) The major carrier of the REE in the ooids is the phosphate phase. (4) Similarities between the REE patterns of Middle Ordovician volcanic ashes and associated chamosite ooids suggest a volcanogenic source for these ooids. (5) Similarities between the REE patterns of Middle Ordovician ooids and coeval ashes, and between Middle Ordovician and Lower Ordovician ooids suggest that also the latter have a volcanogenic origin although no textural evidence of volcanic matter has been observed. (6) Major-and minor-element contents (except REE) are similar for chamosite and limonite ooids, which indicates that the conditions for their formation also must have been similar. (7) The REE patterns appear to be well suited as geochemical indicators for provenance studies of iron oolites. More data from other areas and ages will be necessary to validate the suggested hypothesis of a volcanic origin to iron oolites.

Acknowledgements

I would like to express my thanks to S. Morad, V. Ragnarsdottir and an anonymous reviewer for careful critical comments on the manuscript, and to J.S. Peel who revised the manuscript linguistically. The study was financially supported by a grant from the Swedish Natural Science Research Board (NFR), grant No. GGU 3424-306.

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