Sulfide inclusion chemistry and carbon isotopes of African diamonds

Geochimica et Cosmochimica Acta, Vol. 59, No. IS, pp. 3173-3 188. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/95 $950 + .I0

0016-7037(95)00205-7

Sulfide inclusion chemistry and carbon isotopes of African diamonds PETER DEINES’

and J. W.

HARRIS~

of Geosciences, The Pennsylvania State University, University Park, PA 16802, USA ‘Department of Geology and Applied Geology, University of Glasgow, Glasgow Cl2 8QQ, Scotland ‘Department

(Received

June 24, 1994; accepted

in revised form April 21, 1995 )

Abstract-Significant differences in the composition of sulfide mineral inclusions among diamond suites from Koffiefontein, Orapa, Premier, Roberts Victor, Jagersfontein, Sierra Leone, Star, and Mwadui have been found. The mode of the Ni content of the monosulfide (mss) inclusions lies between 8 and 10 wt%, i.e., between the means for mss from Siberian diamonds with inclusion of the eclogitic (3 wt% Ni) and peridotitic (23 wt% Ni) paragenesis. Considering the Ni/Fe ratios of the diamond mss inclusions and mantle olivines, together with experimental and naturally observed Ni/Fe distribution coefficients, we conclude that less than 20% of the mss inclusions of the African diamonds (mostly from Koffiefontein) could have been in chemical equilibrium with mantle olivine. This observation is in sharp contrast with the reported relative abundance of silicate inclusions in Koffiefontein diamonds (93% peridotitic, 7% eclogitic) and lends support to the proposal that a separate sulfide diamond paragenesis should be recognized. The 6 “C distributions of sulfide containing diamonds differs among kimberlites, however, for each kimberlite sulfide and silicate inclusion containing diamonds cover the same 6 “C range. Sulfides with high Ni concentrations can occur in diamonds of low as well as high “C content. The current observations, in conjunction with other chemical properties of diamonds suggest that fluid reactions rather than silica melt equilibria may be important in diamond formation. A dominance of fluid processes would have significant implications for the interpretation of the chemical and geochronological record of diamond inclusions. 1. INTRODUCTION

sequence pyrite (py ), pyrrhotite (PO), pentlandite (pn), chalcopyrite (cp) , cubanite (cb) , and heazlewoodite (hz), as temperature decreases (Craig and Kullerud, 1969). The effect of pressure on the stability fields of these minerals is believed to be small (Craig and Scott, 1976). Note that the mineral abbreviations given above will be used throughout the manuscript. The formation of sulfide-bearing diamonds brings together at least five elements (C, S, N, H, 0) capable of forming volatile compounds under mantle conditions. Because in diamonds C, S, N, and H are enriched locally over average mantle concentrations, these elements may be most significant for the elucidation of the origin of this mineral. Sellschop ( 1992) has reviewed existing, and provided new data, on the presence of these light volatiles in diamonds and, more recently, Mathez et al. ( 1993) also have reported on the oxygen content of diamonds. It has been proposed in a recent review (see Kirkley et al., 1991) that low “C contents are restricted to E-Type diamonds, and Yefimova et al. ( 1983) conclude that sulfides with high Ni content are restricted to P-Type diamonds. If both of these assertions are correct, one would expect that mss with Ni contents in excess of 8 wt% would not occur in diamonds with S “C values below - 10%0. This paper provides evidence to the contrary. We have investigated the composition of sulfide inclusions and the carbon isotopic composition of their host from five kimberlites (Koffiefontein, Premier, Roberts Victor, and Jagersfontein, in South Africa, and Orapa, in Botswana), for which we previously studied the relationship between 6”C, nitrogen content of the hosts, and the silicate inclusion chemistry. The present study reports, in addition, on sulphide inclusions and the carbon isotope signature in diamonds from

This paper presents the first detailed study of sulfide inclusion compositions in diamonds from south, west, and east Africa and examines their relationships to the “C content of their hosts. Sulfide inclusions are of particular interest because they are the commonest syngenetic inclusion observed in diamonds and are thus, seemingly overrepresented when compared to the petrographic makeup of the mantle (Gurney, 1989). Meyer ( 1987) has suggested diamonds with sulfide inclusions might represent a specific paragenesis. The mineralogy and chemical composition of silicate and oxide inclusions in diamonds worldwide, belong to two major parageneses, eclogitic (E-Type diamonds) and peridotitic (PType diamonds). Minor parageneses, known as websteritic (W-Type diamonds), transitional in chemical composition between E- and P-types and lherzolitic (L-type diamonds), have been recognized. Within both the eclogitic and peridotitic association, compositional differences within and between inclusions in diamonds from different kimberlite diatremes attest to the complex origins of diamond. For African diamonds, the relationship between silicate parageneses and sulfide inclusion chemistry remains unexplored, principally because very few samples with coexisting sulfides and silicates have become available for study. This situation is not the case for Siberian diamonds and the works of Yefimova et al. (1983) and Sobolev (1984) have been summarized by Gurney ( 1989). The average Ni content of sulfide inclusions from P-Type diamonds (mean = 22.8, range = 16.5-29.8 wt%) is significantly higher than that from E-Type diamonds (mean = 2.9, range 0.5-8.2 wt%). It is thought that the primary sulfide included during diamond growth is a monosulfide solid solution (mss), from which subsequent subsolidus exsolutions lead to a mineral 3173

P. Deines and J. W. Harris

3174

Sierra Leone, the Mwadui kimberlite, and associated alluvial deposit at Alamasi both in Tanzania and the Star mine in

m

South Africa.

9

E

0

-5.

8

2. EXPERIMENTAL

3. RESULTS 3.1. Nickel Content of Sulfide Inclusions and S’C of Host The overall relationship between carbon isotopic composition of the host and Ni content of sulfide inclusions which have not reequilibrated (i.e., the mss analyses) are shown in Fig. 1. Although these data are dominated by the sulfides of Koffiefontein, additional information from the other sources shows that there is no correlation between the two variables, but that there are two clusters, one extending from about 6 “C = -2 to -7%0 with Ni ranging from about 3 wt% up to 27 wt%, whilst the second has 6°C values between - 10 to -20%0 with Ni contents between 4 and 12 wt%. For the first cluster the silicate paragenesis of the diamonds is unknown. In the second cluster of eight diamonds, two belong to the WType and one to the E-Type paragenesis. The Ni content of five of the eight inclusions (including one E- and one WType) exceeds that of the highest value suggested by Yefimova et al. ( 1983) for E-Type diamonds.

3.2. Sulfide Inclusion Mineralogy and Chemistry From our previous work (Deines et al. 1984, 1987, 1991a,b, 1993), we know that significant differences exist amongst the carbon isotope frequency distribution of diamond suites from different southern African kimberlites. We have observed that while for some kimberlites no distinction in the carbon isotope frequency distribution between E- and P-Type diamonds could be made, in others, these distributions differed significantly. In addition, there are significant differences in the composition of garnet, clinopyroxene, olivine, and orthopyroxene when E- and/or P-Type inclusion suites from different kimberlites are compared.

.

. .*

. . .

(I’*&

.

.

?? = .

?? *

. .

2

-iTi The experimental procedures for the preparation of carbon dioxide from diamonds have been described by Deines et al. (1984) and carbon isotopic compositions are reported in the standard delta notation as deviations from the PDB reference. The sulfide inclusions, usually between 50-200 pm in largest dimension, were broken out of the diamond by crushing, mounted in epoxy and their surfaces mechanically polished to a 0.25 pm finish using diamond abrasives. Analyses were carried out on a Cambridge Instruments Microscan Mark 5 electron microprobe with an operating voltage of 20 kv and 30 nA. Count times were 40 s for peaks and 20 s for background. Unknowns were determined against a py standard for S and pure metals for all remaining elements, using PET and LIF crystal spectrometers. As there was a tendency, especially for minor phases, to nucleate along sulphide edges, considerable difficulty was sometimes experienced in obtaining analyses free of adjacent mineralogy. In Tables 2-8, such analyses are designated either by a percentage content of the minor phase, or if the mineralogy is uncertain, a question mark is present. The absence of a designation indicates that the chemical composition of the mineral could be determined accurately. In the cases of mss analyses, these were identified after a careful examination under high magnification revealed no exsolution textures and, in addition, if the mss analysis contained more than 2.5 wt% of Ni.

1

I

-lO-

P 2 g 6

z y

. -15.

-20

:

.

.

.’

. P-Type

E-Type

-25: 0

10 Ni Concentration

30

20 of Mss Inclusions

(wt%)

FIG. 1. Comparison of the Ni content of mss inclusions of African diamonds and 6°C of the host. Filled circles, analyses of African diamonds; filled triangle and filled square, Siberian E- and P-Type diamonds, respectively. Since the average carbon isotopic composition of the diamond hosts of the Siberian diamonds is not known, their Ni content has been plotted at an arbitrary 6°C value; the bars indicates the range of Ni concentrations.

Such differences also exist for the diamond suites containing sulfide inclusions examined in this study. Table 1 lists the observed sulfide mineralogy. Significant differences among the inclusion mineralogies include a high abundance of mss at Koffiefontein in a frequent association with a Cu-phase, mostly cp, a high relative abundance of po at Orapa and Jagersfontein, a more frequent occurrence of pn sometimes associated with hz at Roberts Victor, and the only four-phase sulfide assemblage in diamonds from Sierra Leone. These differences in mineralogies may reflect differences in the mss composition included in the diamond during its formation. The chemical compositions of individual sulfide types also demonstrates that there are significant concentration differences of the minor elements among kimberlites. For example, the average Cu content of pentlandite inclusions from Koffiefontein (0.91 wt%) is significantly higher than that of pentlandite inclusions from the diamonds in the other kimberlites (Cu = 0.20-0.41 wt%) . The elevated Cu content is also ohserved in the Koffiefontein mss (mean 1% ) . It is interesting to note that there is no significant difference in Cu content of mss with Ni contents between 3.5 and 10.0 wt% Ni and between 10.0 and 27.0 wt% Ni (see Table 2). The sulfide data suggest that the sulfide component present during diamond formation may have had locally different compositions. For this reason we firstly examine the relationship between sulfide chemistries of Russian and African diamonds and then compare the sulfide and carbon isotope chemistry separately for each of the diamond sources in the present study. In Fig. 2, the composition of mss from African diamonds has been compared with the summary values of Siberian diamonds provided by Gurney ( 1989). Although larger datasets would be preferable, there is an indication of systematic differences in mss compositions between African and Siberian diamonds. Accordingly, one may question whether the Yefimova et al. ( 1983) proposal for differences in the Ni contents of mss between E- and P-Type Siberian diamonds can be

Sulfide inclusions and 6°C in diamonds

3175

TABLE1. The abundance of sulphide minerals (number of observations) in diamonds from different lcimberlites. Min. Assemblage

Koff.

orapa

Prem.

Jag.

R.V.

S.L.

0.

mss mss + cp

Ins.5

+

cp?

mss + p”

Ins

+cp +

p”

“P P” cb I

PY

2

PO + P”

3

I

PO + CP 1

pn +cp‘?

3

pn + hz 2

pa + p” + cp

2

I 2

po + pn + cp” PO + P” + PY PO + P” + CP + PY Total

33

22

8

15

17

Min. = Mineral, Koff. = Koftiefontem, Prem. = Premier, Orap. = Orapa. Jag. = Jagersfontein, R.V. = Roberts Victor, S.L. = Sierra Leone, mss = monosulfide solid solution, cp = chalcopyrite, pn = pentlandite, po = pyrrhotite, hz = heazlewoodite, py = pyrite, 0 = Others, ? = complete identification of all phases present uncertain.

applied directly to other diamond occurrences. In the following comparisons between south African and Siberian sulfide inclusions in diamonds, this reservation should be kept in mind. 3.3. Koffiefontein The geologic setting of this kimberlite, the relative abundance of different inclusions in diamonds, and the chemistry of the silicate inclusions was summarized by Rickard et al. ( 1989). Their data emphasize the high abundance of sulfide and P-Type inclusions. While 42% of the inclusions in diamonds represent sulfides, 30% are of P-Type, and only 7% of E-Type, the remainder are clouds and graphite. The carbon isotope record of diamonds and its relationship to the inclusion chemistry was investigated by Deines et al. (1991a). With three exceptions, one olivine containing (6°C = - 12.7%0) and two eclogitic diamonds containing garnets (6°C = -11.44 and -11.81%0), the carbon isotopic compositions for E- and P-Type diamonds fall in the range from - 1.80 to -8.86%0. The 6°C frequency distributions for Eand P-Type diamonds are indistinguishable and their mean

S’“C is -5.03 ? 1.3%0 (n = 59). The carbon isotopic composition of the sulfide containing diamonds and the composition of their inclusions have been compiled in Table 2. The 6 ‘?C distribution, shown in Fig. 3, is very similar to the earlier data. Of the 37 6’“C analyses, thirty-three fall in the range -3.2 to -6.85%0 with a mean of -5.06%0 + 0.92%0. Four diamonds have carbon isotopic compositions outside this range. Three of these contain inclusions of mss (6’C = -15.88, -16.00, -16.63%0) and one a pn (6’C = - 14.45%0). Figure 4 shows that the Ni concentration in mss has a wide range, a skewed distribution, a broad maximum between 4 and 10 wt% Ni, and appears to be continuous. Whether there is indeed a second mode between 22 and 26 wt% would have to be established through further analyses. We find that the Co and Ni content of the mss are highly correlated (r = 0.62, n = 28), so that their variation might be a consequence of a common process. The chemical composition of the sulfides and its relationship to the carbon isotopic composition can be examined in Fig. 5. The mean Ni content of the three mss inclusions ( K213B, K235B, K242) occurring in the three 6 ‘“C depleted

3176

P. Deines and J. W. Harris TABLE 2. Carbon isotope and Sample

6°C

Min.

o/o0

sulfide inclusion compositions for Koffiefontein diamonds.

CU

Fe

Ni

CO

Z”

s

wt%

wt%

wt%

Wt%

wt%

wt%

Total

n

K201

-3.77

mss

0.69

55.14

0.36

0.00

38.83

99.97

4

K202

-6.05

mss

1.37

56.16

3.88

0.33

0.00

38.23

99.97

4

~~(24)

9.31

50.82

2.84

0.34

0.43

37.17

loo.91

I

22.96

39.43

1.17

0.20

0.00

34.74

98.50

2

K202

4.95

K205

-3.21

cb

Iv.09

-6.85

mss

1.33

42.33

17.02

0.54

0.00

38.26

99.48

3

36.85

2.27

0.22

0.00

35.42

99.97

4

K209

CP(?)

25.21

K210

-4.18

mss

0.61

36.67

25.95

0.53

0.00

36.64

100.40

3

K211

-3.84

mss

1.10

56.75

3.52

0.43

0.00

38.66

100.46

4

K212A

-4.97

mss

1.04

50.19

9.32

0.5 1

0.00

38.04

99.09

2

K214

-4.24

mss

0.71

51.02

8.84

0.46

0.00

38.03

99.06

2

K215B

-5.52

mss

0.97

52.90

7.15

0.38

0.00

37.59

98.99

3

K216

-4.30

mss

1.64

52.80

8.15

0.49

0.00

36.96

100.04

3

CPWJ)

21.12

41.09

2.23

0.27

0.00

34.89

99.59

1

“us

0.91

53.72

7.14

0.42

0.00

38.09

100.28

4

cti57)

20.16

42.06

2.11

0.29

0.11

34.90

99.63

mss

0.72

50.08

9.78

0.44

0.00

38.01

99.03

3

mss

0.95

50.93

10.33

0.20

0.01

36.68

99.10

5

P”

1.20

29.32

34.82

0.76

0.00

32.71

98.81

1

mss

0.69

47.07

13.88

0.54

0.00

37.85

100.03

1

CP(?)

21.18

39.99

3.16

0.27

0.00

35.98

100.58

1

-6.61

P”

0.80

31.94

33.56

0.40

0.00

32.60

99.30

7

K224

-4.88

mss

1.11

53.27

7.20

0.41

0.00

37.27

99.25

4

K225

-14.45

P”

I .03

30.32

34.58

0.43

0.00

32.49

98.84

4

K226

-6.44

mss

1.26

34.21

27.19

0.60

0.30

36.12

99.69

5

K22lB

-4.99

mss

0.98

54.96

4.25

0.38

0.00

38.17

98.73

7

-4.80

mss

0.60

38.96

22.13

0.50

0.00

36.79

98.98

4

-16.63

mss

1.14

56.41

4.50

0.42

0.20

37.27

99.94

5

K216 K217

-5.46

K217 K218A K219 K220

-4.64 -4.05 -4.92

K720 K221A

-5.49

K221A K223

K230 K231B K232

-4.04

K232 K233

-4.81

s

mss

I .89

53.41

5.98

0.37

0.00

37.33

98.98

8

~~(48)

17.50

42.61

2.43

0.20

0.00

36.06

98.79

1

s

K234A

-6.39

mss

2.15

50.23

8.96

0.42

0.00

37.69

99.46

10

K235B

- 16.00

mss

0.61

53.18

8.04

0.44

0.00

36.49

98.76

5

mss

1.13

45.56

15.87

0.54

0.02

36.84

99.96

7

CPQ)

8.67

41.78

13.52

0.44

0.11

35.23

99.74

2

K236

-6.25

K236 K237

-4.87

mss

I .25

49.04

10.16

0.44

0.01

37.72

98.61

6

K238

-5.82

mss

0.48

38.12

22.41

0.50

0.02

36.86

98.39

2

CP(15)

5.51

38.19

19.33

0.45

0.00

36.16

99.64

I

37.16

24.39

0.50

0.04

36.77

99.54

8

K238 K239

-6.17

mss

0.69

K240

-3.59

CP(?)

25.47

35.16

3.65

0.22

0.00

34.65

99.15

2

K241

-5.53

P”

0.48

30.87

34.81

0.42

0.01

32.86

99.45

3

K242

-15.88

mss

0.82

55.08

5.96

0.51

0.00

36.84

99.22

2

K243

-4.87

“us

1.03

52.88

7.07

0.35

0.01

38.07

99.40

7

K244B

-4.97

mss

1.39

52.17

6.79

0.38

0.04

38.61

99.38

5

K247

-4.73

s

K248

-5.81

s

cp = chalcopynte, mss = monosulfide solid solution, p” = pentlandite. n = number of sulfide inclusion analyses, () estimated percent of phase present in mixture of mss and cp, (?) a third phase in addition to cp and mss must be present to acccunt for the analysis, s sulfide inclusion lost in preparation

Sulfide inclusions and 6°C in diamonds

L

6

2

5

5 =

s 2

%

70 jm

3177

1 0

5

10

15

20

25

;

Ni Content wt%

‘r

~

-I

0.5

0.6

0.7

(

Co Content wt%

Cu Content wt%

FIG. 2. Comparison of mss compositions from south African diamonds with the range and mean composition of mss from Siberian diamonds as summarized by Gurney (1989). Filled square: mean composition of mss from Siberian PType diamonds, filled triangle: mean of mss from Siberian E-Type diamonds.

diamonds is 6.2 wt% and that of the pn from the S “C depleted diamond (K225) is 30.3 wt%. One might propose that samples K213B, K235B, K242, and K225 came from one mantle environment of low S ‘“C and that the original mss had a Ni content of 6.2 wt%. If the pn with a Ni content of 30.3 wt% formed through exsolution from such a mss, one would ex-

6

161412. lo-

:

a-

$ e

pect, on the basis of mass balance, that other low Ni sulfides, such as po, cp, and py. would occur and these would need to be more abundant than the pn. Such phases were not observed. The occurrence of pn, therefore, may indicate that the mineral formed from a high Ni mss. If this is the case, then a high Ni mss and a low “C content of the diamond host are not necessarily mutually exclusive. 3.4. Orapa The geology of the kimberlite was summarized and detailed carbon isotope studies of diamonds with silicate inclusions

Koffiefontain

6:

IL

4207 . . . -30

10~

. -25

. .

.

. r-b

-20

. . . . . . . .. . .

-15

-10

-5

9-

.

Koffiefontein

0

16-

7.

Orapa

1412$10. E s.

6. 6-

f!

4-

IL

20, -30

2r! -25

!".d -20

613C

r.rt! -15

a/o0

-10

-5

0

vs. PDB

FIG. 3. Carbon isotope frequency distribution of Koffiefontein and Orapa sulfide containing diamonds.

lo_( 0

I 2

I,, 4 6

8

10

, 12

, 14

16

, 18

, 20

, 22

, 24

26

, 28

Wt% Ni in Mss FIG. 4. Ni content of mss from Koffiefontein diamonds.

30

3178

P. Deines and J. W. Harris

Koffiefontein

0 mss

A cp

??

pn

+---ocoexisting

sulfides

Fe Premier

Orapa I

O-O

\

coexisting

sulfides

/

0 mss

\

o---o

coexisting

sulfides

FIG. 5. The sulfide composition in terms of the ternary components Fe-Ni-S and the carbon isotope composition of their hosts. In the complete ternary, the extent of monosulfide solid solution (mss) at 1 atm and 1000°C is shown as well as the composition of pyrite (py), pyrrhotite (PO), pentlandite (pn), heazlewoodite (hz). The composition field which has been enlarged for the individual locations is indicated as a bold outline. Also shown are the ranges of mss estimated for Siberian P-Type (squares) and E-Type diamonds (triangles). Koffiefontein: The isotopic composition of the ‘C-depleted diamonds has been indicated, the mean isotopic composition of all other diamonds is -5.14 2 0.92%0: mss = monosulfide solid solution, cp = chalcopyrite, pn = pentlandite. Orapa: The carbon isotopic composition of all samples has been indicated. premier: The isotopic composition of one ‘“C-depleted diamond has been shown, the carbon isotopic composition of all other host diamonds is -4.78 2 0.4%0.

and from diamondiferous eclogites were previously reported by Deines et al. (1991b, 1993). The data for sulfide containing diamonds are compiled in Table 3. The carbon isotope data for OR38B (W-Type), OR42F (W-Type), and OR57C (E-Type) have been reported earlier. The S “C distribution of sulfide containing diamonds (Fig. 3) resembles the summary distribution of Orapa diamonds with silicate inclusions (see Deines et al., 1993). In the earlier dataset (Deines et al., 1993), two major modes were recognized, mode Ml around - 1%0 and M2 around -6%0. It was also suggested that there might be a third mode at about - 14%0. The data presented here would enhance the mode at - 14%00and thus provide additional evidence that the 6 ‘?C distribution of Orapa diamonds is trimodal. The sulfide composition and 6°C content of the host can be inspected in Fig. 5. In general, the Ni content in the sulfide inclusions from Orapa appears to be lower compared to Koffiefontein. Three diamonds with isotopic compositions between - 11.4 and - 15.2%0 include mss with Ni contents which exceed the upper limit of the range for E-Type diamonds proposed by Yefimova et al. ( 1983). The data dem-

onstrate that high Ni contents in sulfides can be observed both in diamonds of low and high “C contents. 3.5. Premier Mine In earlier work, Deines et al. (1984) outlined the general geology of this kimberlite and reported in detail on the relationships between the carbon isotopic composition of diamonds and their inclusion chemistry. The new carbon isotope and sulfide composition data are summarized in Table 4 and Fig. 5. 6°C values fall in the range of -4.03 to -5.37%0, except for one sample with an isotopic composition of - 14.1%~ The S “C distribution is indistinguishable from that found in our earlier studies of Premier diamonds. The “C depleted diamond (P210A) contains coexisting po, cp, and pn. Since the relative proportions of these minerals are not known, the composition of the mss from which they formed, through exsolution, cannot be reconstructed. However, if the original mss was similar to that of P216 (A, B ) this would indicate also that at Premier, isotopically light carbon could occur together with mss of 10 wt% Ni.

Sulfide

inclusions and 6°C in diamonds

TABLE 3. Carbon isotope and sulfide inclusion Sample

UC

Min.

o/o0

compositions

CU

Fe

Ni

CO

Z”

wt%

wt%

wt%

wt%

wt%

3179

for Orapa diamonds. S

Total

n

wt%

OR38B

-15.75

mss

0.10

54.61

4.50

0.00

0.00

39.63

98.84

I

OR42F

-15.22

mss

I .95

51.73

8.50

0.17

0.00

37.14

99.49

2

OR57C

-12.38

mss*

I .30

52.21

7.97

0.16

0.02

37.85

99.5 1

4

OR20lA

-9.46

PO*

0.66

58.55

0.87

0.00

0.00

39.78

99.84

8

OR202A

-8.97

PO*

0.17

59.37

0.69

0.00

0.00

39.59

99.81

6

ORZOZA

cti?)

27.02

34.44

1.15

0.04

0.09

35.38

98.12

I

OR202B

PO*

0.06

59.71

0.31

0.00

0.00

40.0 I

100.09

3

OR203A

-10.53

PO

0.30

57.54

I .27

0.00

0.00

38.92

98.03

4

ORZOA

-6.05

POP”CP

0.04

52.41

9.38

0.31

0.00

38.27

100.41

I

POP”CP

0.00

59.30

1.26

0.00

0.00

39.19

99.75

I

PO’

I .20

56.73

1.89

0.05

0.00

39.76

99.62

3

OR204A OR205A

-13.14

OR206A

-11.43

Po

I.15

57.60

1.15

0.M)

0.00

39.55

99.45

3

OR207A

-16.41

PO*

0.15

58.63

1.24

0.00

0.02

39.41

99.44

4

OR208C

-19.18

Pop”*

0.36

57.98

1.83

0.00

0.00

39.42

99.59

2

OR209A

-18.25

Pop”*

1.27

59.16

1.23

0.00

0.00

38.26

99.31

14

OR210A

-25.63

OR21 IA OR211A OR212A

-12.13

ORZIZB

-17.50

OR213A

-6.21

OR213A OR214A

0.49

58.86

1.02

0.13

0.00

39.74

100.24

2

0.57

57.11

2.61

0.00

0.00

39.63

99.92

2

PO

0.93

56.69

2.55

0.00

0.00

39.66

99.83

2

PO*

0.91

58.29

I .34

0.35

0.00

39.12

100.01

2

PO*

0.88

58.27

I .43

0.36

0.00

38.99

99.93

6

Po

0.05

58.52

1.59

0.68

0.00

39.05

99.55

4

44.99

18.76

0.78

0.00

36.82

101.38

I

51.08

9.22

0.43

0.00

38.87

99.81

IO

POP”

0.03

-I 1.37

mss*

0.23

cp’?

19.58

35.85

9.61

0.25

0.00

36.20

101.49

I

-5.59

mss

1.51

38.96

21.53

0.41

0.00

37.16

99.57

3

OR214A OR21SA

PO POP”’

mss*

1.40

39.41

21.39

0.37

0.00

36.79

99.36

OR217

-12.2

Po

0.91

56.97

2.20

0.33

0.00

38.84

99.25

OR218A

-5.45

PO*

1.70

56.77

1.51

0.29

0.00

39.50

99.77

OR219A.B

-11.1

Po

0.08

59.60

0.26

0.00

0.00

39.87

99.87

POP”

0.11

57.72

2.40

0.09

0.00

39.16

99.48

POP”CP

11.52

36.30

13.24

0.46

0.03

34.52

96.07

Po

0.50

57.34

2.20

0.12

0.00

39.46

99.61

OR215A

OR219A OR219B ORZZOA

-12.5

Mineral abbreviations are the same as in Table I and 2. Addition of ‘*’to a mineral abbreviation indicates an average of spot and broad beam analyses. Generally, no signiticant differences between the two types of analyses were observed.

The two sulfide inclusions in diamond P216 both exhibit variations in their chemistries across the surfaces analyzed; Ni contents, for example, vary between 6.8 and 13.6 wt%. These differences indicate chemical disequilibrium on a micron scale. We find a positive correlation between Co and Ni (r = 0.99, n = 6) and a negative correlation between Ni and Fe (r = 0.99, II = 6)) as well as a positive correlation between the Ni/Fe ratio and S content (r = 0.92, n = 6). If the measured variations in Ni, Fe, and S were attributable to the incipient formation of po and pn a negative correlation between Ni/Fe and S would be expected. Hence, we suggest that the observed chemical differences reflect variations in the original mss. 3.6. Roberts Victor We have discussed in an earlier publication (Deines et al., 1987) the geology of this kimberlite and the relationships be-

tween silicate inclusion chemistry and 6°C of the diamond host. The carbon isotopic compositions of our present (Table 5) and earlier work are indistinguishable. The relationship between diamond 6 “C and sulfide composition can be inspected in Fig. 6. The three low 6 “C diamonds (RV45, RV70, RV72), studied by us previously (Deines et al., 1987), all include po. These diamonds are E-Type because they also contain, respectively, cpx, gnt, and cpx plus gnt. The chemistry of these minerals is distinct from all other E-Type inclusions and all garnets and clinopyroxenes from Roberts Victor eclogite xenoliths (Hatton, 1978)) because of their exceptionally high Mn and Fe contents. They are also differ significantly from the compositions of eclogites for which MacGregor and Manton ( 1986) postulated a subduction origin (Deines et al., 1987, Fig. 6). There are several observations which lead us to suggest that the original mss phase at Roberts Victor might have been

P. Deines and J. W. Harris

3180

TABLE

Sample

4.

Carbon isotope and sulfide inclusion

61,C

Mill.

CU

Fe

PO

0.80

Po

0.26

F20lA P205A

-5.37

P207A

-4.81

compositions

for Premier diamonds.

Ni

CO

Zn

S

Total

58.73

2.20

0.26

0.00

36.17

98.16

3

59.45

0.37

0.11

0.03

38.89

99.10

4

n

Po

0.15

59.75

0 50

0.14

0.00

38.87

99.41

4

P207A

P”

0.14

29.96

34.10

2 06

0.00

32.78

99.04

I

P207A

CP

33.42

30.51

0.03

0.08

0.00

34.26

98.30

1

PO

0.26

59.85

0.33

0.17

0.02

38.64

99.28

3 1

PZIOA

-14.10

P2lOA

P”

0.29

31.41

34.98

0.45

0.04

32.82

100.00

P210A

CP

32.20

31.83

0.09

0.16

0.14

36.72

101.14

1

59.35

0.16

0.13

0.00

38.98

98.82

3 1

P215A

-4.65

Po

0.20

P216A

-4.91

mss

0.71

54.02

6.83

0.18

0.00

37.93

99.66

P216A

mss

0.75

49.56

10.83

0.41

0.00

37.77

99.32

1

P216A

mss

0.78

52.53

8.08

0.25

0.00

37.92

99.55

8

F216B

mss

0.63

50.51

9.28

0.28

0.00

37.94

98.63

1

P216B

mss

0.55

46.68

I3 56

0.51

0.01

37.72

99.03

I

mss

0.75

48.15

II.73

0.44

0.00

37.78

98.89

7

PO

0.78

58.64

I .08

0.13

0.02

38.78

99.41

3

P”

0.21

28.28

36.63

2.04

0.05

32.78

99.97

3

PO

0.11

59.75

0.21

0.12

0.08

38.90

99.17

3

P216B P217A

-4.03

P217A P218A

-4.93

Abbreviations

are the same

TABLE

5.

as in Tables

Carbon

1,2 and 3

isotope and sulfide inclusion

for Roberts Victor diamonds.

Sample

6W

Min.

RV20lA

-5.66

P”

0.14

30.43

35.81

0.03

0.00

0.08

33.73

loo.22

RV202A

-4.47

Pn

0.11

32.02

34.19

0.30

0.01

0.00

33.68

100.28

RV204A

-5.09

PoPn

0.09

56.70

3.21

0.00

0.00

0.00

39.80

99.80

P”

0.07

27.05

38.54

0.90

0.08

0.00

33.67

100.31 99.86

RV204A RV205A

-3.07

CU

compositions

Fe

Ni

CO

Zn

CI

s

Total

PoPn

0.07

55.75

4.86

0.02

0.00

0.M)

39.16

RV205A

POP”

0.11

56.52

3.11

0.12

0.00

0.00

39.62

99.48

RV205A

CP(?)

20.30

41.60

I .oo

0.00

0.00

0.00

37.12

100.02

RV207A

-4.38

P”

0.39

31.24

34.06

0.25

0.00

0.00

33 68

99.62

RV208A

-4.82

POP”

0.13

55.03

5.06

0.00

0.02

0.00

39.21

99.45

RV209A

-5.97

CP

30.40

29.68

3.76

0.1 I

0.00

0.40

34.55

98.90

PoPn

0.09

51.67

7.55

0.00

0.02

0.74

39.45

99.52

CP

32.60

30.78

0.68

0.00

0.00

0.06

35.46

99.58

P”

2.44

31.34

31.27

0.16

0.M)

0.00

33.67

98.88

0.05

46.28

0.13

0.00

0.00

0.00

53.26

99.72

RV209A RV2WB RV213A

-5.98

RV217A

-3 74

PY

RV33C

-6.36

P”

0.12

31 45

34.02

0.32

0.00

0.00

33.29

99.20

hz

0.04

3.41

69.43

0.01

0.02

0.00

27.79

100.70

RV33C RV52C

-4.27

RV45E

-15.37

RV54C

-4.73

pnhz

0.53

25 56

41.63

0.23

0.00

0.04

33.12

101.11

PO

0.09

59.21

0.50

0.00

0.00

0.00

38.96

98.76

PO

0.11

5751

I .70

0.00

0.00

0.63

39.70

99.65

28.65

37.56

0.27

0.M)

0.M)

33.63

100.19

Pn

0.08

RV70K

-15.14

PO

0.18

59.04

0.29

0.00

0.00

0.00

39.36

98.87

RV72C

-15.73

PO

0.02

6001

0.37

0.00

0.00

0.00

39.05

99.45

-5.87

hzpn

0.08

21.95

44.15

0.23

0.00

0.05

33.31

99.11

P”

0.12

31.33

34.29

0.24

0.00

0.03

33.50

99.51

P”

0.55

26.69

37.86

0.62

0.00

0.22

33.34

99.28

RV54C

RV215A RV215A RV218A

Abbreviations

-5.45

are the same

as m Tables

1, 2, and 3.

n

Sulfide inclusions and 6°C in diamonds

Roberts

Jagersfontein

Victor

O--O

coexisting sulfides

Sierra

O--O

3181

coexisting sulfides

Leone

coexisting sulfides

0-0

Star Mine

v----o Alamasi

o--o

Mwadui

FIG. 6. The sulfide composition in terms of the ternary components Fe-Ni-S and the carbon isotope composition of their hosts, the range of the components Fe-Ni-S shown is identical to that of Fig. 5. Roberts Victor: The isotopic compositions of three “C-depleted diamonds are shown, the rest has a mean isotopic composition of -4.99 -C0.95%0. Jagersfontein, Sierra Leone, Star Mine, Alamasi, Mwadui: The carbon isotopic composition of all diamond hosts has been shown.

significantly higher in Ni than at Orapa. These include the frequent occurrence of pn, the high Ni content of that mineral, and the occurrence of py and hz as a coexisting phases. The coexistence of high Ni pn and py and high Ni pn with hz is indicated by the low temperature equilibria in the Fe-Ni-S system, (Craig and Scott, 1976).

3.7. Jagersfontein A summary of the geology of the kimberlite, the carbon isotope record of its diamonds and the chemistry of their silicate inclusions was given by Deines et al. (1991a). The new 6 ‘?C data (Table 6) are consistent with the earlier work. The relationships between the sulfide chemistries and carbon isotopic composition of the host can be inspected in Fig. 6. This figure demonstrates that sulfides of very similar compositions can occur in diamonds differing significantly in 6 “C. For example, pns with nearly the same Ni content occur in diamonds which differ in 6°C by more than 10%0; pos occur in diamonds which vary in 6’“C between -4 and -20.8%0 and cp is observed in diamonds of -5.4 and - 17.7%0. In our earlier study (Deines et al., 1991a), we made similar observations for the relationship between silicate inclusion chemistry and the 6 “C of the host.

Sample 5218, the only diamond for which a mss inclusion was found, has a carbon isotopic composition of -19.4%0. The mss has a Ni content of 9.15 wt% which falls between those considered to be characteristic for Siberian E- and PType diamonds. It is interesting to note that this mss has the highest Cr content measured for any of the sulfide phases from Jagersfontein. Gurney ( 1989) indicates that the occurrence of chromite and high Cr garnet inclusions are characteristics of the P-Type association. We have also found in this kimberlite a diamond (S’“C = -19.07%0) containing a chromite and peridotitic diamonds with S”C values as low as -19.1%0 (Deines et al., 1991). There can be little doubt that in this kimberlite, ‘“C depleted carbon occurs together with mineral chemistries which are thought to be characteristic for the peridotitic paragenesis.

3.8. Sierra Leone The geological setting, petrology, and mineral chemistry of the kimberlites of Sierra Leone were reviewed by Tompkins and Haggerty ( 1984). The kimberlites center on Koidu on the southwest margin of the west African craton and consist of a dike swarm along which three small kimberlite pipes and a so-called dyke enlargement have developed. The diamonds of

P. Deines and .I. W. Harris

3182

TABLE6. Carbon isotom and sulfide inclusion comDositions for Jaeersfontein diamonds. 61X

Min.

CU

Fe

Ni

CO

Zn

CI

S

Total

n

J201

-19.23

PO

0.29

60.38

0.21

0.00

0.00

0.09

38.33

99.30

3

1202

-19.79

PO

0.06

60.62

0.25

0.00

0.00

0.02

38.38

99.33

1

3203

-17.05

PO

0.12

60.01

0.26

0.00

0.00

0.00

38.69

99.08

J206

-3.47

Pn

0.73

29.48

32.49

0.42

0.00

0.39

36.38

99.89

I I

J208

-20.15

PO

0.04

60.56

0.13

0.00

0.00

0.00

38.50

99.24

5

1209

-15.94

PO

0.17

60.50

0.19

0.00

0.00

0.00

38.49

99.35

5

Sample

P”

0.28

31.02

31.30

3.30

0.00

0.00

33.26

99.16

I

1209

PO

0.12

60.33

0.21

0.00

0.00

0.00

38.34

99.00

3

J209

Pn

0.08

34.10

28.34

3.53

0.00

0.00

33.89

99.94

1

PO

0.08

56.99

3.24

0.00

0.00

0.00

39.69

100.00

4

P*

0.03

26.69

37.57

1.34

0.05

0.00

33.12

98.80

I

cPi90)

31.20

29.33

3.44

0.42

0.02

0.00

33.96

98.37

1

PO

0.04

60.65

0.26

0.00

0.00

0.00

38.37

99.32

3

POP”

0.04

48.84

14.04

2.21

0.M)

0.00

34.63

99.76

1

J213

-18.62

PO

0.15

60.56

0.18

0.00

0.00

0.00

38.37

99.26

5

J215

-5.38

PO

0.09

57.81

2.00

0.01

0.00

0.00

39.55

99.24

5

CP

33.09

31.02

0.07

0.00

0.00

0.00

34.96 99.51

3

P”

0.05

27.64

38.10

0.56

OSUI

0.37

33.57

100.29

3

POP”

0.02

42.19

19.19

0.15

0.00

0.35

37.16

99.06

PO*

1.14

59.71

0.24

0.00

0.00

0.10

38.19

99.38

3

0.00

0.59

39.13

99.90

1 1

1209

J211

-17.70

1211 1211 1212

-4.04

J212

1215 J215 1216

-6.00

J216 J217 J218

-4.26 -19.42

mss* mss*

J218

0.06 0.04

48.25 50.73

II.87 9.15

0.00 0.00

0.00

0.50

39.08

99.50

38.04

99.84

4

38.56

99.41

2

J219

-20.43

PO

0.10

61.55

0.16

0.00

0.00

0.00

J22 1

-17.47

PO

0.09

60.51

0.25

0.00

0.00

0.00

Abbreviations are the same as Tables 1,2 and 3

the present study come from the 100 m levels of number 1 and 2 pipes and from the near surface mining of a dyke enlargement. Chemical and isotopic composition data for diamond samples from Sierra Leone are given in Table 7. The relationship between the carbon isotopic composition and the sulfide chemistry can be examined in Fig. 6. All diamonds with sulfide inclusions have carbon isotopic compositions which are particularly low in “C. Diamond (SL18) with the lowest “C content (-33.0%0) includes the assemblage po-pn-py and the pn has the highest Ni content of the inclusions in Sierra Leone diamonds. Three other diamonds with inclusions (SL3, SL21, SL23), are also low in 6°C and SL3 (-26.08%0) contains the only four phase sulfide assemblage (po-pn-py-cp) found in this study. From the polished sulfide sections, the dominant phase appears to be po in all cases, however, because the modal abundance of these sulfide phases is unknown, the composition of the original mss cannot be reconstructed. Diamonds with silicate inclusions from the Sierra Leone kimberlite have not yet been investigated in any detail for their relationships between silicate chemistry and carbon isotopic composition. As nine chromite-bearing diamonds from this source were available, we determined their 6”C, which ranges from -1.31 to -9.41%0. The cbromite- and sulfidebearing diamonds, therefore, have very distinct isotopic com-

positions and show the largest range in 6 “C of any of the African diamond suites. It would thus be of particular interest to investigate silicate bearing diamond suites from Sierra Leone in greater detail. 3.9. Star Mine, Mwadui, and Alamasi The Star Mine kimberlite is located about 15 km northeast of Theunissen, Orange Free State, South Africa and has been described by Mitchell and Meyer ( 1989) and dated at 124 Ma by MacIntyre and Dawson ( 1976). The Mwadui kimberlite, the largest economic diamond pipe in the world, lies in the Shinyanda District of Tanzania. The geology is outlined by Gobba (1991) and the pipe has been dated at between 40-53 Ma by Davis (1977) and Raber (1978). The diatreme has been eroded only very slightly and the hypabyssal facies has not been exposed. Alamasi diamonds are considered to be alluvial accumulations from the adjacent Mwadui pipe. Only a few samples were analyzed from these occurrences and the results are given in Table 8. The relationship between sulfide chemistry and carbon isotopic composition can be viewed in Fig. 6. For Tanzanian diamonds, the range of S “C is large ( 12.5%0), but, unfortunately the dataset does not allow any meaningful comparisons to be made between the sulfide inclusion compositions of low and high 6°C diamonds.

3183

Sulfide inclusions and 6°C in diamonds TABLE 7. Sample

Carbon isotope and sulfide inclusion compositions for Sierra Leone kimberlite diamonds.

8’3C

SLI SLI SL3

-26.08

SL3 SL3 SL3 SL9

-22.50

SLIO

16.28

SL18

-33.00

SL18

Min.

-21.07

s

Total

n

59.88

0.35

0.20

0.00

38.84

99.40

P”

20.25

2.01

0.03

35.54

99.14

1

PO

0.09

60.06

0.36

0.15

0.03

38.61

99.30

3

PY

2.88

43.62

0.03

I .72

0.00

52.18

100.42

P”

0.15

37.41

25.89

0.73

0.03

34.26

98.46

~~(84)

29.35

33.21

0.08

0.27

0.15

37.44

loo.49

PO

3

0.16

59.82

0.44

0.13

0.00

38.68

99.18

0.12

60.01

0.34

0.1 I

0.01

38.56

99.15

29.56

32.51

4.88

0.02

33.65

loo.94

I

51.99

98.77

I

5

s PO

0.32

4

0.39

44.20

0.10

2.10

0.00

PO

0.09

59.30

I.05

0.16

0.00

38.89

99.48

P”

0.15

35.83

26.16

2.87

0.02

35.86

100.88

I 3

PO

SL23

Zll

40.86

PY

SL23

CO

0.14

SLIB

SL2 I

Ni

0.45

P”

-23.97

Fe

PO

SL18

SL2 1

CU

PO

0.10

60.44

0.47

0.17

0.00

38.67

99.84

P”

0.17

36.26

25.30

3.40

0.00

33.15

99.17

I

0.12

58.35

1.55

0.16

0.05

38.94

99.17

2

SL26

-26.88

PO

SL30

-2.52

chr

SL3 I

-4.85

chr

SL32

-1.31

chr

SL33

-9.41

ctu

SL35

-3.19

CtK

SL35A

-3.43

chr

SL36

-2.1 I

chr

SL37

-1.61

chr

SL38A

-4.84

chr

Abbrewations are the same as in Tables I, 2. and 3. chr = chromite

3.10.The Relationship Between Carbon and Sulfur

Isotopic Compositions For some of the samples we investigated, Eldridge et al. ( 1991) have reported the sulfur isotopic composition of the sulfide inclusions. The 6”% results and our 6°C measurements are shown in Fig. 7. The data indicate that diamonds from the various kimberlites are probably characterized by different SX4Sand 6 “C mean values as well as ranges of 6 “C and 6?S variability. A similar observation was made for the 6°C values of carbonatite carbonates and the 6’4S values of their associated sulfides (Deines, 1989). Koffiefontein and Premier diamonds show the most restricted 6 “C and 6’S ranges and the mean SX4Svalues are close Cl?&.The data for Orapa, Roberts Victor, Jagersfontein, and Sierra Leone show both wider 634S and 6°C ranges and possibly a positive (Orapa), negative (Sierra Leone), or no (Jagersfontein, Roberts Victor) correlation between the two variables. Additional data are required to establish whether indeed correlations between 6°C of diamond P4S of their inclusions exist. Such relationships might indicate correlated fractionation processes of C and S, whose identification would aid in the evaluation of C precipitation processes or mixing of sources of distinct S’4S and 6°C.

4. DISCUSSION

There is a significant difference between the Ni content of the mss in Siberian diamonds and those from Africa examined in this study. Whether this is the result of sampling or indicates true chemical difference between diamonds from the two continents is not clear at this time. The diamonds from Siberia were generally large (N. V. Sobolev, pers. commun.) and contained silicates coexisting with sulfides. The mss chemistry was computed as the weighted average of the measured composition of individual sulphide phases and their modal abundance. The sizes of the African diamonds were small, between 1 and 3 mm, and in only three samples from Orapa (OR38,OR42, OR57) and three from Roberts Victor (RV45, RV70, RV72) did silicates coexist with mss. Hence, a meaningful comparison of sulfide chemistry and coexisting silicate parageneses is not possible. In order to provide some insight into how the Ni content of mss inclusions in African diamonds is related to the Ni content of olivine associated with diamond we have examined the dependence of the Ni/Fe ratio of olivines on their Mg number. In Fig. 8 these variables are shown and comprise olivines included in diamonds (Premier, Venezuela, Ghana, Williamson, and Finsch; Hervig et al., 1980), and in lherzolite xenoliths (Bultfontein, De Beers, Frank Smith, Kilboume

P. Deines and J. W. Harris

3184

TABLE 8. Carbon isotope and sulfide inclusion diamonds. S?.IIlPk

compositions

for the Star Mine, Alamasi and Mwadui

6°C

Min.

CU

Fe

Ni

CO

Ztl

s

Total

n

S201A

-4.84

mss

0.13

56.78

3.45

0.44

0.02

38.83

99.65

4

S201A

-4.84

P”

0.3 1

32.14

31.14

1.00

0.04

34.89

99.51

I

S201A

-4.84

CP

32.75

31.28

0.73

0.11

0.11

34.57

99.56

3

A201A

-2.96

P”

0.62

27.60

37.71

0.36

0.00

32.62

98.91

3

A201A

-2.96

P”

1.39

38.21

23.46

0.36

0.00

35.49

98.91

2

-14.91

s

-2.26

mss

1.63

38.70

21.62

0.49

0.03

36.55

99.01

1

mss

1.36

38.85

22.15

0.49

0.00

36.51

99.36

1

STAR MINE

ALAMASI

A202

MWADUI MWlA MWlB MW206

-4.55

s

MW207

-3.02

s

MW8

-2.84

PO

0.10

60.13

0.42

0.14

0.03

38.66

99.48

3

MW8

-2.84

P”

0.14

36.01

29.19

0.49

0.06

33.58

99.46

1

Abbreviations are the same as in Table 1, 2, and 3.

Hole, Lashaine, Letseng, Malaita, Matsoku, Mothae, Potrillo Maar, San Carlos, Serra Guadelupe, and Thaba Putsoa; Hervig and Smith, 1982). The observed trend between olivines included in diamond and those in lherzolites can be readily modeled by a batch melting process using the least fractionated lherzolite composition as a starting material. Estimates of the mss compositions which could be in equilibrium with these olivines was obtained as follows. Thompson and Barnes ( 1984) have reviewed existing, and provided new data, for the distribution of Ni and Fe between cogenetic sulfides and olivine. Their conclusion is that, for a variety of magmatic sulfide deposits, the mean distribution coefficient for Ni and Fe between the two phases:

has a range from 5 to 20 and a mean value of 9.8 (X denotes mole fractions). For a recent review see Naldrett ( 1989). Values for KD determined experimentally are significantly larger. Fleet and MacRae (1988) present a detailed study for the partitioning of Ni between olivine and monosulfide-oxide liquids in the temperature range 1300- 1395°C a range of oxygen and sulfur fugacities, as well as of olivine and sulfide melt compositions. They observe that the distribution coefficient exhibits only a small decrease from 35 to 29 with an increase in oxygen fugacity from the iron-wtistite to the quartz-fayalite-magnetite buffer. The authors suggest that a

value of KD = 30 would be applicable for upper mantle conditions. The expected relationship between sulfide Ni/Fe ratio and the Mg number of olivine present at various stages of partial melting are indicated in Fig. 8, for the smaller KD value, determined from natural mineral associations, and the larger one, based on the experimental studies. It is apparent that if the experimental Ku of 30 was applicable, none of the mss inclusions in diamonds would have high enough Ni/Fe ratios to be in equilibrium with the olivine inclusions in diamonds studied by Hervig et al. ( 1980). Even if the much smaller KD value of 10 (see Fig. 8), deduced from magmatic sulfides were applicable, less than 20% of the mss inclusions could be in equilibrium with olivine inclusions in diamonds. This observation is consistent with the identification of P-Type diamonds on the basis of their Ni content of the mss using the criteria of Yefimova et al. (1983). Therefore, only 20% of the mss found as inclusions in African diamonds belong to the P-Type association and, by inference, the remainder ought to be assigned to the E-Type paragenesis. This conclusion, however, is challenged by the evidence in Fig. 2, which demonstrates that the Ni content of most of the mss in African diamonds is significantly higher than that of mss coexisting with eclogitic minerals in Siberian diamonds. Hence, a classification as E-Type, on the basis of the criteria of Yefimova et al. ( 1983), would be inappropriate for many mss analyses. Furthermore, on the basis of the diamond

Sulfide inclusions

15

and 6°C in diamonds

3185

I

?? ??

lo-

6

??

5-

< s k m z (0

O-

AV

00 A A

v v

-5-

??

v

v

0

-25

-20

FIG. 7. Carbon and oxygen isotopic composition from Eldridge et al. (1991).

in Diamond in Lherzolite Diamond

-5:’ _ - ;

I;-I :

a, 2 =

:

_-_=:_-==--_-__==z-

--_---, *_ A/ -

// _z:-- --,

,+

I

0.10:

j i 1 0.01

:

___--__-_-

-

..

-- -

7 0.85

0.90

Olivine

-15

-10

Batch melting KD = 10 KD = 30 0.95

M9/(M9+Fe)

FIG. 8. Relationship between Ni/Fe and Mg/(Mg + Fe) of olivines from lherzolite (filled circles) (Hervig and Smith, 1982), diamonds (filled diamonds) (Hervig et al., 1980) and the trend expected during a batch melting process of undepleted lherzolite (solid lines). The dashed lines indicate the mss NilFe ratio expected in equilibrium with olivine at any stage of the melting process. Filled triangles, plotted at an arbitrary Mg/(Mg + Fe) ratio, indicate the mss composition found for south African diamonds.

-5

0

o/oovs.PDB

of diamonds and their sulfide inclusions.

inclusion abundance studies for Koffiefontein, (Rickard et al., 1989), for which most of the mss analyses were carried out, one would expect a P-Type/E-Type ratio of 12.6 (P-Type 93%). Hence, the mss data present a paradox. On the basis of the silicate inclusion abundance, one would expect that 93% of them would belong to the P-Type paragenesis, yet only 20% of them could be in equilibrium with olivine. On the other hand, for many of the mss inclusions the Ni content is significantly higher than that of E-Type Siberian diamonds. The present data, therefore, provide support for suggestion of

1.00.

z.

0

613C

+ Olivine Olivine

0

.“r o”:

0 0

-30

1 Mssin

80

v

-lO-

??

??

v

0 Ora a A Ko f! tefontein 0 Premier ??Roberts Victor v Jagersfontein ??Sierra Leone

Sulfur isotope data are

Meyer ( 1987) that in addition to the E- and P-Type a separate sulfide inclusion paragenesis should be recognized. There are several notable and as yet unexplained features of the association of sulfides with diamond, if one considers them to be formed in processes involving silicate melts. One, pointed out by Gurney ( 1989), is the seeming overabundance of sulfide inclusions compared to the mantle petrology. A second constitutes the fact that some sulfides are associated with refractory silicate compositions. If the refractory nature of the silicates reflects the composition of a residuum of partial melting, one might expect that sulfides would enter early forming melts and not be associated with the residuum. If one considers the refractory nature of the silicates to be the result of crystallization from a highly depleted magma one could anticipate that the sulfide should be removed in the fractionation process which caused the depleted nature of the magma. In addition, if there were to be any sulfide present it should be associated with the late, rather than the early forming silicates. Finally, there is the requirement that solids containing two volatile elements, C and S, become simultaneously oversaturated in several petrologically distinct environments in the presence of N and H of sufficiently high concentrations to account for their abundance in diamonds. In the case of PType diamonds, one has to consider also that some of the equilibration temperatures deduced for their inclusions are very close to the wet peridotite solidus and for a large number of them, slightly below it (Gurney, 1989). The existence of chemical disequilibria between silicate inclusions has also been commented on by Gurney ( 1989). In this study, we observed that a mss inclusion (P216) ranges in Ni content from 6.8 to 13.6 wt%. If the relatively small diamond grew in a melt within the mantle, such a disequilibrium is unexpected. Also, the trace element enrichment in eclogitic

3186

P. Deines and J. W. Hamis

garnet and clinopyroxene inclusions in Orapa diamonds (Richardson, 1989) and the trace element geochemistry of garnet inclusions in diamonds from Finsch and Koffiefontein (Shimizu et al., 1989) are very difficult to reconcile with a simple equilibrium relationship between the inclusions and a silicate melt. Consideration of the carbon isotope distribution adds additional complexity. No distinction can be made between the S’% distribution of P- and E-Type diamonds from Finsch, Premier, and Koffiefontein diamonds. In the case of the latter two, we find that the similarity extends to sulfide-containing diamonds. In the Orapa diamond suite E- and P-Type have about the same 6 ‘“C range, however, “C-depleted diamonds carry more often eclogitic mineral inclusion or inclusions of compositions transitional between E- and P-Type. The 6°C distribution for sulfide-containing diamonds from Orapa is consistent with that of diamonds with silicate inclusions. Jagersfontein diamonds also show a wide range of 6 “C in Eand P-Type diamonds, with depleted E-Type diamonds being more abundant than P-Type diamonds. Here also, diamonds with sulfide and silicate inclusions have the same 6 ‘“C range. In addition, in this kimberlite, highly ‘C-depleted asthenospheric diamonds occur. There are also a number of instances where subgroups of E- and P-Type diamonds have been identified, based on the combined consideration of their inclusion chemistry and carbon isotopic composition (e.g., Premier, Finsch, Roberts Victor, Jagersfontein). If one assumes that silicate melts played a role in the formation of diamonds and their inclusions, the variations in inclusion mineralogy and chemistry would be understood to be the result of different petrogenetic processes. The carbon isotope record would then imply that in some locations very different petrogenetic processes would lead to the same carbon isotope distribution, while at other locations this would not be the case. Furthermore, the record would indicate that a particular process could result in very different 6°C distributions at different locations. In addition, explanations for the 6 “C variability within diamonds (Swat? et al., 1983; Milledge et al., 1989) need to be provided. While it is not impossible to rationalize some of these observations, they add to the difficulty to explain a range of diamond features through processes involving melts. Many of these observations would be considered differently if the chemical system in which diamond formed was dominated by C with, not only minor contributions from S and other volatile constituents, but also silicate components. From this viewpoint, processes involving fluids rather than silicate melts would be the point of focus. Sellschop ( 1992) showed that in addition to nitrogen (up to 5000 ppma) and hydrogen (500-3600 ppma), oxygen (<20-3600 ppma) can occur in diamond. The author also points out that part of the oxygen can be accounted for by trace elements (over fifty elements have been detected in mineral inclusion free diamonds, and have been interpreted to represent microinclusion of silica melt), but that a residual remains which can be attributed to the presence of water. Diamond growth, from a fluid phase, capable of dissolving silicates, rather than a melt, is also indicated by the occurrence of microinclusions in coated diamonds (Navon et al., 1988; Navon, 1991; Gutbrie et al., 1991).

The concept of subsolidus growth of diamonds with peridotitic inclusions was first discussed by Boyd and Finnerty ( 1980), and the idea of diamond growth in an open system from a fluid was invoked by Griffin et al. (1988) to explain some of the observations on inclusions from West Australian diamonds. Pearson et al. ( 1994) suggested that graphite in the cratonic lithospheric mantle could have a subsolidus metasomatic origin, and result from cooling of a C-H-O fluid permeating the lithosphere along fractures. Haggerty ( 1986, 1994) has suggested that mantle fluids are important for diamond growth. If diamonds are formed from deep mantle fluids, their occurrence in mantle peridotites, eclogites, and websterites may not be a consequence of the crystallization of the rocks, but a result of a subsequent process. The chemical nature of such a fluid is unknown but it could include the elements carbon, sulfur, nitrogen, and hydrogen. The oxidation state of the mantle is a topic of active discussion. Wood et al. ( 1990) indicate a range of close to four orders of magnitude in oxygen fugacity. The lowest oxidation states are observed in abyssal peridotites and are such that CH4 would be the major carbon bearing constituent of a C-H-O fluid. If diamond was formed through oxidation of methane, water can be produced. Deines ( 1980) has pointed out that the pressuretemperature dependence of the equilibrium CO, + CH, = 2H20 + 2C

(1)

is such that C precipitation can occur from a C&/CO2 mixture by lowering simultaneously pressure and temperature. Alternatively, C precipitation can occur if CH, is introduced into an environment in which a CO1 fluid is present. The points important to note are that a reagent, external to the C-O-H system, is not required for C precipitation to occur, and that at the site of diamond precipitation, high water fugacities may exist although the fluid introducing the carbon can be completely dry. One gram of water would be formed in the precipitation of a 6 mm diamond cube by reaction 1. The amount is large enough to play a significant role in the dissolution of silicates in the vicinity of the precipitation site (see Eggler, 1987 for a summary of element solubility data). Water formed during diamond precipitation could hence be an important factor for the formation of inclusions in diamond. Thus, the silicate components which formed the inclusions, might be locally derived or equilibrated. It should be noted that water would be generated as diamond precipitation proceeds and, since the environment in which diamond growth occurred would have to be open to some extent, water would be lost. From this viewpoint, the relative abundance of sulfide and silicate inclusions would be governed by the fluid chemistry rather than mantle petrology and the more frequent occurrence of sulfide inclusions would not necessarily be unexpected if the fluid contained sufficient sulfur. The coexistence of refractory silicate compositions and the Ni content of the sulfides could be understood, if the fluid interacted with the rocks into which it was introduced. The local concentration of several volatile elements in an environment, which is characterized by a high degree of silicate fractionation, would find an explanation as well. While the composition of silicate inclusions of P-Type diamonds might be established through local exchange between country rock and fluid, their equilibration

Sulfide inclusions

temperature would be that of the precipitation of the silicates from a fluid, rather than a peridotitic melt. Thus, the subsolidus equilibration of many P-Type diamonds might find an explanation. Since one pulse of fluid of a given S”C could lead to diamond formation in a variety of mantle rock types, the similarity in the 6°C distribution of diamonds with distinct mineral inclusion parageneses would be understandable. Likewise, the occurrence of chemically and isotopically distinct subgroups of E- and P-Type diamonds could be accounted for. Continued introduction of fluid of variable 6°C during diamond growth could also account for some of the isotopic composition zoning observed in diamonds (Swart et al., 1983), as well as the very large abrupt changes in 6 ‘“C on the order of IO?& over small distances which were reported by Milledge et al. (1989). Ciamond growth from a fluid (CO*-H20) phase of varying composition has been evoked to interpret carbon and nitrogen isotope variations in diamonds (Boyd et al., 1987, 1992). If local sources of sulfides existed in the rock into which the fluid intruded, isotope exchange with them might determine or significantly influence the 6j4S of the sulfide inclusions and thus explain some of the 6’4S variations observed among different sulfide inclusions within one diamond (Eldridge et al., 1991). The growth zonation (Harris, 1987) could be readily explained if the composition of the fluid varied as a function of time. A dominance of the fluid reactions and potential chemical and isotopic exchange with the rocks into which the fluid is introduced would have significant implications for the proper interpretation of diamond inclusion chemistry. The inclusion record would have to be understood as reflecting very local conditions, and extrapolation to larger mantle volumes would be difficult to justify. There would also be implications for the deduction of age information from diamond silicate inclusions. While isochron ages established on inclusions from one diamond might carry significant information on the time of diamond precipitation (e.g., Richardson et al., 1993; Smith et al., 1991), it would be arguable if this would be necessarily true for model ages derived from single mineral inclusions (e.g., Richardson et al., 1984). Thus, the reality of some of the very old ages reported for some diamonds might then be questioned. 5. CONCLUSIONS We conclude that:

1) The abundance of different sulfide inclusions as well as the composition of particular sulfide minerals varies among kimberlites. 2) The carbon isotope distribution of silicate and sulfide containing diamonds from a particular kimberlite are indistinguishable, while diamonds from different kimberlites can have different 6 “C distributions. 3) The mode of the Ni content of mss from African diamonds falls between the ranges for mss from E-Type and P-Type Siberian diamonds. 4) Mss with Ni contents significantly higher than those observed for mss from Siberian E-type diamonds are found in three “C depleted African diamonds. On the basis of sulfide type abundance and mass balance consideration,

and 6°C in diamonds

5)

6) 7)

8)

3187

we can reason that among diamonds from Koffiefontein, Orapa, Premier, and Jagersfontein, mss with Ni contents higher than 8 wt% Ni occur in diamonds with S’C values below - 1O%O. At most, 20% of the mss inclusions of African diamonds could have been in equilibrium with mantle olivines. Also, most of the remaining 80% could not be classified as EType based on the criteria developed for Siberian diamonds. The present work is consistent with the proposal that a separate sulfide inclusion paragenesis should be recognized. Fluid reactions rather silicate melt equilibria may be important for diamond precipitation. The fluid introducing the C could be essentially dry. Sufficient water can be created in the precipitation of diamond to play a significant role in the formation of the silicate inclusion mineralogy. If chemical and isotopic exchange between inclusion minerals and the enclosing rocks occurred, this interaction would have significant implications for the proper interpretation of the chemical and isotopic record of diamond inclusions and any physicochemical or chronological information deduced from it.

Acknowledgmenrs-The authors wish to thank De Beers Consolidated Mines Limited for making samples available. In particular, they are grateful for the support and encouragement for this study by J. B. Hawthorne, V. G. Anderson, J. Parker, and E. van Blerk are thanked for assisting in the collection of the sulfide-bearing diamonds. Drs. P. Hill and S. Keams of the Microprobe Laboratory in the Geology and Geophysics Department of Edinburgh University are also thanked for help with the sulfide analyses. Financial support for the work was provided by the National Science Foundation through grants EAR 90 117378 and EAR85 11549 to P. Deines. Editorial handling: T. K. Kyser

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