The application of C isotope measurements to the identification of the sources of C in diamonds: a review

Applied Geochemistry, Vol. 6, pp, 477494, 1991

0883-2927/91 $3.00 + .00 © 1991 Pergamon Press plc

Printed in Great Britain

The application of C isotope measurements to the identification of the sources of C in diamonds: a review MELISSA B. KIRKLEY, JOHN J. GURNEY, MARSHALL L. OTTER, STUART J. HILL* an d LEON R. DANIELSt Department of Geochemistry, University of Cape Town, Rondebosch 7700, South Africa (Received 27 November 1990; accepted in revised form 6 May 1991) Abstract--In the past decade, the isotopic compositions of C in >600 inclusion-bearing diamonds have been determined. Such analyses have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/or orthopyroxene inclusions with refractory compositions (high Mg, Cr), have 613C values predominantly between -10 and -1%o, with a sharp peak in the distribution near -5%0; (2) eclogitic diamonds, which commonly contain inclusions of omphacitic clinopyroxene, Cr-poor pyrope, and/or eclogitic accessory minerals such as rutile, kyanite, coesite or sanidine, have 613C values between -34 and +3%O, with a smaller peak near -5%o; (3) the isotopic compositions obtained for suites of diamonds from individual occurrences are, in general, unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities; (4) isotopic zoning patterns and heterogeneities are found in some diamonds; cores of coated diamonds tend to be depleted in 13C relative to the rims, and within single octahedral diamonds 613C variations of nearly 6%ohave been reported. Because expected C isotope fractionations at mantle temperatures are small, attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle C source have been unsuccessful. Nevertheless, fractionation is probably responsible for some of the observed variation in 613C values. Two other models have also been proposed to account for the diamond characteristics outlined above. The "primordial model" suggests that the range and distribution of C isotope compositions are inherited from primordial C in the mantle which has an inhomogeneous isotopic composition, such as that found in meteorites. The "subduction model" suggests that subducted, crustal C is the source of C in diamonds, as organic and inorganic C compounds in the crust exhibit a range of 613C values similar to that observed in diamonds. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics.

INTRODUCTION ISOTOPICANALYSESof C in diamonds began as soon as the value of stable isotope measurements as a means of characterizing and tracing the origins of Earth materials was recognized in the 1930s. In the past decade, however, C isotope analysis has become a standard procedure in diamond studies. The - 1 0 0 0 measurements of diamond 613C values now available (see the following section for definition of terms) serve as an important constraint on models of diamond genesis. The first published analysis of the C isotope composition was that of NIER and GULBRANSEN(1939). Their sample was from the Kimberley mines, South Africa, and gave a 613C value of -1.2%0. This early work was furthered by CRAIG (1953) who obtained 613C values of - 2 . 4 to -4.7%0 for six diamonds from the Kimberley mines. An improved calibration of the C (Stockholm) standard used in that work led to an

*Present address: AMPAL (Pty.) Ltd, P.O. Box 802, Gaborone, Botswana. tPresent address: Geocontracts, Botswana (Pty.) Ltd, Pvt Bag 140, Gaborone, Botswana. xo

6-5-a

adjustment of those values (WICKMAN, 1956) and a corrected range of - 1 . 9 to -4.2%0. WICK~IAN (1956) analysed diamonds of gem and industrial quality from central and western African kimberlite pipes and placers and reported 613C values of - 3 . 0 to -9.6%0, with one lower value of -13.8%0 considered to be due to organic C combination on the diamond surface. WICKMAN(1956) also r e p o r t e d t ~ l a C values for a few specimens from each of several diverse localities (i.e. African, Russian, North and South American, and Australian), all of which were between - 1 . 9 and -4.7%0, except for a single New South Wales (alluvial) diamond which was enriched significantly in 13C (813C = +2.7%o). Subsequent analyses of additional NSW diamonds have confirmed their 13C-enriched character (613C = - 3 . 3 to +2.4%0; SOBOLEV, 1984). VINOGRADOVet al. (1965) concluded that there was little variation in the isotopic composition of Yakutian d i a m o n d s ((~13C values from - 5 . 6 to -8.8%0) and that they were similar to diamonds from elsewhere in the world. The following year, VINOGRADOVet al. (1966) reported significantly 13C-depleted values of - 28.4 to - 27.8%0 for cryptocrystalline diamonds ("carbonado") from Brazil. It began to be suspected that such 13C-depleted diamonds might not be of kimberlitic

477

478

M. Kirkley et al.

200

TERMINOLOGY Carbon isotope compositions

150 ~13Csample = [ Rsample -- Rstandard] X 1000, Rstandard J

L

100 50 0

p.... ~'~,,~ -35

-30

-25 -20 -15 -10

', . . . . -5

, ....

0

5 i

61~CpD. (% o) FIG. 1. Histogram of diamond C isotope values. Data from WICKMAN(1956), VINOGe~,DOVet al. (1965, 1966), KeAVTSOV et al. (1975), SMIRNOVet al. (1979), SOBOLEVet al. (1979), DEINES (1980), GALIMOVet al. (1980), HALL and SMITH (1984), JAVOYet al. (1984), SOBOLEV(1984), DIENES et al. (1987, 1989), HILL (1989), JAQUESet al. (1989), MCCANDLESSet al. (1989), OTTER(1989) and DANIELS(1991).

origin, because m o r e were being found in Brazilian and Russian placers than at known kimberlite localities (SMIRr~ov et al., 1979). It was significant, therefore, when SMIRNOVet al. (1979) found (~13C values as low as -20.5%oo in diamonds from Lesothan kimberlites. The most 13C-depleted diamonds reported to date are polycrystalline diamond intergrowths from the Mir kimberlite, Yakutia, with 613C values of - 3 4 . 1 to -34.4%o (SOBOLEV et al., 1979). The significance of 13C-depleted diamonds relative to the rest of the world-wide population was clearly portrayed by DEINES (1980) who compiled available 613C data into a histogram, such that the full range and distribution of diamond isotopic compositions was apparent. Addition of new data in m o r e recent compilations (e.g. GALIMOV, 1984; HARRIS, 1987; Fig. 1 of this work) has not changed the distribution significantly.

where R = 13C/12C. Thus, 6 is the difference in isotopic ratio between a sample and a standard, expressed in parts per thousand or per mil (%0). For example, a sample with (~13C = -10%o is depleted in 13C by 10%0 (or 1%) relative to the standard, whereas a sample with 613C = 5%0 is enriched in 13C by 5%0 relative to the standard. All 613C values herein are reported relative to the PDB standard (belemnite from the Peedee Formation). Accuracies may vary among authors, but in general, C isotope analyses are considered to be accurate to within _+0.05%0. Ultramafic rocks

The group of ultramafic rocks relevant to this discussion is classified according to the quantities of the minerals olivine and pyroxene they contain, as illustrated in Fig. 2 (STRECKEISEN, 1976), and most of them contain accessory garnet or spinel (usually chromite). Fragments of such rock types are known as xenoliths when they are plucked from their source in the mantle (50-->200 km below the surface of the Earth's crust) and carried to the surface by igneous rocks, such as kimberlites and lamproites, which originate at such depths. Kimberlite and lamproite

Kimberlite and lamproite are the two igneous rock types which are mined for diamonds, and which, when eroded, release diamonds that subsequently may be concentrated into alluvial or placer deposits. The major mines where diamonds are currently extracted from kimberlite or lamproite, as well as other minor kimberlite occurrences mentioned in this study, are given in Table 1. Formal definitions of these rock types are given by CLEMENTet al. (1984) and JAQUESet al. (1984), respectively, but in general, both are volatile-rich (containing carbonates and/or hydrous minerals), K- and Mg-rich, and SiO2-poor

Olivine //~

Harzbur~/ 40/ / Olivine / / OrthopyroxenRe?/

lO.//

Orthopyro~enite~ , ~ Orthopyroxene" 10

Dunite. . . . . . . . . . . . . . . . . . . .

I hefzolte~~

hrnte

~,40 . . . . . . . . . . . . . . . \ \ Olivine OlivineWebsterite ~Clinopyrox enite W.bst.rite ...........

]

T -t

¢ i

\ \1o

y , ~ Clinopyroxenite 90 ' Clinopyroxene. . . . .

FIG. 2. International Union of Geological Sciences (IUGS) nomenclature for ultramafic rocks based on olivine, orthopyroxene and clinopyroxene proportions, assuming <5% spinel or garnet (SrRECKmSEN, 1976).

479

C isotopes in diamonds: a review Table 1. Major (non-alluvial) diamond mines of the world (with *), and other minor occurrences mentioned in this review Country Mine Tanzania Mwadui*

Surface area (ha)

Approx. grade (carats/100tons)

146

20-25

Zaire Mbuji Mayi*

15

Botswana Jwaneng* Letlhakane* Orapa*

51 12 113

South Africa Beilsbank Bultfontein* DeBeers* Dutoitspan * Finsch* Koffiefontein* Monastery Premier* Roberts Victor Star Wesselton*

(dikes) 10 5 11 18 10 1 32 0.5 (dikes) 9

% Gems

Kimberlite (K) Lamproite (L)

40

K

660

<5

K

140 25-30 68

50 40 15

K K K

>95 50 ? ? 25 50 ? 20 ? 95 ?

K K K K K K K K K K K

40 35-40 15-24 18-22 80-100 12 40 30-35 27 30 20-40

Swaziland Dokolwayo*

3

27

50

K

Australia Argyle* Ellendale 4*

45 84

680 14

10 60

L L

7

8

15

K

7 52

>50 ?

<20 20

K K

U.S.A. Sloan U.S.S.R. Mir* Udachnaya*

relative to other, more common types of ultramafic rocks. Their major mineral components are olivine and phlogopite, and they commonly contain xenoliths (or single mineral grains derived from xenoliths, known as xenocrysts) of upper mantle origin. They may or may not contain diamonds; when present, diamond is only a rare accessory. The mineralogy of kimberlites and lamproites are compared in Table 2. ISOTOPIC COMPOSITION OF CARBONIN DIAMONDS The peak in the distribution of diamond 613C values between - 1 0 and -1%o, with the mean value of approximately -5%0 (Fig. 1), encompasses the range of 613C values exhibited by carbonatites (TAYLOR et al., 1967; CONWAYand TAYLOR, 1969; DEINES, 1970; DEINES and GOLD, 1973; SHEPPARDand DAWSON, 1973), kimberlite carbonates (SHEPPARD and DAWSON, 1975; KOBELSKIet al., 1979; KIRKLEYet al., 1989) and dissolved C and CO2 vesicles in MORB (mid-ocean ridge basalt) and OIB (ocean island basalt) glasses (PINEAUet al., 1976; DES MARAISand MOORE, 1984; MATrEY et al., 1984; EXLEY et al., 1986). Step-heating analyses of C in basalts (ibid.) indicate that 13C-depleted C with - 3 2 < 613C < -9-0%0 is evolved during the low temperature steps

(<500°C); this finding suggests to some that this 13C-depleted C is bound loosely and resides on sample surfaces or in cracks. There is currently active debate, however, as to whether such ~3C-depleted C represents an externally derived contaminant, or a precipitate from magmatic vapour (e.g. see discussion by TINGLE, 1989). In the latter case, the 13C-depleted component would represent mantlederived C, but having an isotopic composition that has been modified by fractionation during nearsurface magma degassing. Hence, the full range of 613C values found in diamonds (from - 3 4 . 4 to +2.7%o) is much larger than that exhibited by other hosts of (unmodified) mantle-derived C. The 613C distribution shown in Fig. 1 is not mirrored by any suite of diamonds from an individual kimberlite or lamproite occurrence. Rather, diamonds from individual localities have unique sets of isotopic compositions, examples of which are illustrated in Fig. 3. With few exceptions, diamonds from the Finsch, Premier (DEINES et al., 1984, 1989), Star (HILL, 1989) and Dokolwayo (DANIELS, 1991) kimberlites and from the Ellendale 4 and 9 lamproites (JAQUESe l al., 1989) occupy the relatively narrow range of 613C values between - 1 0 and -1%o.

480

M. Kirkley et al. Table 2. Comparison of the mineralogy of kimberlites and lamproites Mineral and generalized formula

Kimberlite

Major minerals Olivine (Mg,Fe)2SiO 4 Phlogopite KMg3SiaA1Olo(OH,F)2 Diopside CaMgSi20 6 Calcite CaCO 3 Serpentine Mg3Si2Os(OH)4 Monticellite CaMgSiO4 Leucite KAISi20 6 Amphibole (K,Na)ECa(Mg,Fe,AI)sSisO22(OH)2 Enstatite (Mg,Fe)SiO 3 Sanidine KAISi30 8

×

×

x × × x ×

× ×

× ×

× x

Minor minerals Apatite Cas(PO4)3(OH,F,C1 ) Perovskite CaTiO 3 Ilmenite FeTiD 3 Spinel (Mg,Fe)(AI,Fe,Cr)20 4 Priderite (K,Ba)(Ti,Fe)sO16 Nepheline NaAISiO 4 Wadeite K2ZrSi30 9

×

×

×

× × x × × ×

× x

Xenocrystic minerals Olivine (Mg,Fe)2SiO 4 Garnet Mg3AI2Si 3 ° 1 2 Diopside Ca(Mg,Fe)Si30 6 Enstatite (Mg,Fe)SiO3 Spinel (Mg,Fe)(A1,Fe,Cr)20 4

R o b e r t s Victor d i a m o n d s (DEINES et a l . , 1987) yield a b i m o d a l 613C distribution, with p e a k s n e a r - 5 as well as n e a r -15%O. T h e Sloan d i a m o n d s (OTI'ER, 1989; OTTER et a l . , 1990) exhibit a wide r a n g e of 6 ]3 C (o) FINSCH

Lamproite

×

×

×

×

×

×

×

×

×

×

values with a distribution displaying several peaks. T h e p e a k in the distribution of ~13C values, for d i a m o n d s f r o m t h e Argyle l a m p r o i t e , is in an u n u s u a l position n e a r -12%o (JAQUES et al., 1989).

(b) PREMIER

(d) STAR

(c) ROBERTS VICTOR

511

50

gl4O

4O

15

3O 10

5[

20

10

10

o_,15. . . . -;o ('!-" "

1

I

o

r-i

-15

-1

-10

~15 c

(,) DOKOLWAYO

...,

....

, ....

-15

-5

, ....

-10

-5

,

o

0

815C

~15c

(f) ARGYLE

(g) ELLENDALE 4 & 9

,-[-

, . . . . , .... -15 -10

, -5

, , 0

61SC

•

(h) SLOAN 15

0

o

i

10

0

1

, " ~

-25

" ,

" " " . . . .

-20

-15

5

" " , . . . . . . . . . .

-10

~1~1c

-5

O

0

1

1 ~11 c

5

0

.

15

.

.

.

.

.

.

.

.

.

10

.

.

.

.

0

0 ~1S c

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

25

20

15

10

613C

FIG. 3. Histograms of diamond C isotope data obtained from individual kimberlite and lamproite occurrences. Data from (a,b) DEINES et al. (1989), (c) DEINES et al. (1987), (d) HILL (1989), (e) DANIELS (1991), (f,g) JAQUESet al. (1989), (h) OTrER (1989).

5

0

C isotopes in diamonds: a review

100 o3 Ld G3

(a)

PERIDOTITIC DIAMONDS

80

< Z <

60

~

40

m

Z

20

i ....

-30

i , • . r"~ . . . .

-25

-20

Z

i • r"~ • I '

-15

-10

.r

....

L

i.,,

-5

o

-5

0

613CpDB

100 (b)

~

ECLOGITIC DIAMONDS

80

z <

60

2~ Z

2o

-30

-25

-20

-15

-10

~13CpD B

FI6.4. Histograms of diamond C isotope data subdivided according to (a) peridotitic and (b) eclogitic inclusion parageneses. Data sources as in Fig. 3, and SOBOLEV(1984). CORRELATION OF CARBON ISOTOPE COMPOSITION WITH INCLUSION PARAGENESIS Diamonds are classified into parageneses on the basis of the mineralogy and chemistry of their mineral inclusions. Because the inclusions are generally considered cogenetic with the diamonds, the parageneses are assumed to represent the host rocks in which diamonds crystallized. Most diamonds are classified readily into one of two parageneses (MEYER and BOYD, 1972); peridotitic diamonds commonly contain olivine, orthopyroxene, Ca-poor/Cr-rich garnet, and/or chromite inclusions; whereas, eclogitic diamonds typically contain Cr-poor garnet, omphacitic clinopyroxene, rutile, kyanite, coesite, and/or sanidine inclusions. The limited set of radiogenic isotope data available presently from diamond inclusions indicates that peridotitic diamonds are older than eclogitic diamonds, i.e. peridotitic inclusions are - 3 . 3 Ga (RICHARDSON et al., 1984), whereas eclogitic inclusions range in age from --2.4 to 0.9 Ga (RJcHARDSON et al., 1990; SMZTnet al., 1989). Carbon isotope compositions of diamonds correlate with their parageneses (SOBOLEVet al., 1979) in that most peridotitic diamonds have a restricted range of 613C values, predominantly between - 1 0 and -1%0 (Fig. 4a), whereas eclogitic diamonds are responsible for the wide range of 613C values (Fig.

481

4b), depleted and enriched relative to the peridotitic range, but with a smaller peak near the peridotitic mean. Note that the full range of 613C values shown in Fig. 4b is smaller than that in Fig. 1 because the paragenesis of many of the diamonds included in Fig. 1 is not known. When diamonds from individual kimberlite and lamproite localities are separated according to paragenesis, the same pattern is evident (Fig. 5a,b). There are ambiguities in the correlation of isotopic and inclusion compositions. That is, some diamonds do not have inclusion compositions and corresponding 613C values that conform readily to either the peridotitic or eclogitic paragenesis. For example, diamonds containing Cr-rich clinopyroxene inclusions (as opposed to Cr-poor eclogitic clinopyroxenes) are usually classified as "peridotitic". However, the sub-calcic garnets which characterize many peridotitic diamonds could not have crystallized in equilibrium with clinopyroxene. Hence, the sub-Ca garnet paragenesis would be more correctly termed "harzburgitic", whereas Cr-rich clinopyroxene inclusions probably represent a "lherzolitic" paragenesis. Two Yakutian diamonds containing the fourphase assemblage, olivine + pyrope + diopside + enstatite, led SOBOLEV (1972) to describe them as lherzolitic. OTI~ERand GURNEY (1989) considered the peridotitic inclusions in Sloan diamonds to be lherzolitic based not only on the occurrence of a Cr-rich clinopyroxene inclusion, but also on the more calcic compositions of olivine, orthopyroxene and Cr-rich garnet inclusions. DEINES et al. (1984) reported that clinopyroxene-bearing peridotitic (i.e lherzolitic) diamonds from Premier have relatively high 6t3C values of - 2 . 9 6 to -2.57%0. This is part of a general correlation, observed among diamonds of both peridotitic and eclogitic parageneses from Premier and Finsch, in which diamonds with relatively high 13C contents host inclusions with high CaO contents and low Mg/(Mg + Fe) ratios (DEINES et al., 1984). Clinopyroxenes included in the 13C-depleted group of eclogitic diamonds from Roberts Victor (Fig. 5a) are enriched significantly in A120 3, FeO and MnO compared with the clinopyroxene inclusions from the more X3C-enriched group (DEINES et al., 1987). GURNEY et al. (1984) and MOORE and GURNEY (1989) found "websteritic" diamonds at Orapa and Monastery, respectively, in which orthopyroxene inclusions coexist with phases of eclogitic affinity (garnet and/or clinopyroxene). SMITHet al. (1989) determined that a websteritic clinopyroxene inclusion (coexisting with orthopyroxene and garnet) in diamond (613C = -6.0%0) from the Kimberley mines has a low Sm/Nd ratio and unradiogenic Nd similar to the harzburgitic garnet inclusions in Kimberley diamonds, which were dated at 3.3 Ga by RICHARDSONet al. (1984). However, the younger age of the websteritic clinopyroxene (maximum Nd model age of 2.1 Ga) precludes a relation between these two parageneses. SOBOLEV (1972) considered a "wehrlitic" paragenesis

482

M. Kirkley et al.

(a)

FINSCH

PREMIER

ROBERTS VICTOR

STAR 20

40

PERtDOTITIC

PERIDOTrrfc

'] 10

2o/-

51-

101-

10

0 II ' . ~ 0

,

. . . .

j . . . .

-15

,

-10

. , •

. . . .

0'...,

-5

-15

-10

6~SC

40

-5

....

-15

,, -10

61~ C

ECLOGmC

40 t-

PERIDOITflC

..,

-5

-15

ECLOGmC

-5

I0

6~SC

61:mC

20 i

ECLC~ITIC

£CLOGmC

3o/r

2o/-

1

5t-

lo/-

10

0 ~ , -15

(b)

-~0

-15

-5

-10

-5

-15

-10

-5

dis C

613C

~15 c

DOKOLWAYO

ARGYLE

ELLENDALE 4 & 9

-15

.

.

.

.

.

,

-10

-5

SLOAN 1ok

PERIDOTITIC

PERIDOTITIC

PERIDOTITIC

PERIDOTITIC el-

m

i,o 15

61"

10

4

5 5F

-25

-20

-15

-10

-5

0

2

0:..,

.... -15

~ -10

.

. -5

0

2t"

, .... -15

, . -10

.

. . -5

, 0

O~ , . . . . , . . . . , . . . . , . . . . , . . . -30 -25 - ~ -15 -10

- 5 "'(~

~,Sc

~ls C

615C

~15 C

.

1OF ECLOGITIC

2o

ECLOGmC

ECLOGITIC

ECLOGITIC

8F lO

15

| )

5

j

1o

o , . n . , . N .... m .......... -25

-20

-15

-10

81SC

-5

0

i.

-15

-10

-5

81SC

o._,s,. . . . . . . .-,o ....... ~13e

-~

° - ~ :'2s : ~ o %

:;o

-'5 ....

81~

FIG. 5. Histograms of diamond C isotope data obtained from (a) selected South African occurrences, and (b) occurrences outside South Africa. Data sources as in Fig. 3. to be established on the basis of the inclusion assemblage Ca-rich garnet + olivine + Cr-diopside. Coexisting inclusions of relatively Fe-rich chromite and ureyitic clinopyroxene in a diamond aggregate from Orapa (613C = - 3 . 8 % ) are also considered to be wehrlitic (McCANDLESS et al., 1989). The C isotope compositions of the websteritic and wehrlitic parageneses have not been yet established. In addition,

several, diamonds from the Argyle lamproite (HALL and SMITH, 1984) and the Monastery (MooRE and GURNEY, 1989) and Sloan (OTrER and GURNEY, 1989) kimberlites contain coexisting peridotitic and eclogitic inclusions, indicating that mixing of parageneses is possible. The occurrence of subgroups within the major parageneses shows that more precise definitions of

C isotopes in diamonds: a review diamond inclusion parageneses and their corresponding C isotope signatures are dearly needed, and the petrogenetic links among diamond parageneses have yet to be deciphered fully. In general, however, it may be stated that few peridotitic diamonds have 613C values outside the range - 1 0 to -1%o, and that diamonds with t~13C values outside the - 1 0 to -1%0 range, as well as some within this range, are typically eclogitic.

CORRELATION OF CARBON ISOTOPE COMPOSITION WITH OTHER DIAMOND CHARACTERISTICS

Several studies of diamond C isotope compositions have reported correlations with other diamond characteristics such as colour, crystal habit or shape, N content, N aggregation state, N isotopic composition, and occurrence (pipe vs placer). Explanations of the observed correlations are commonly lacking, however. GALIMOV(1984), summarizing the results of Russian studies, reported that the mean ~13C value of diamonds from placers is shifted toward more negative values by 1-2%o relative to diamonds from the source kimberlite pipe. In addition, GAUMOV(1984) stated that isotopically light diamonds are more common among coloured and, in particular, yellow stones. However, DEINES (1980) found no difference in t~13C between coloured and colourless diamonds from Premier, and only slightly higher 613C values (0.5 +_ 0.2%0) in colourless relative to coloured stones from the Dan Carl mine on the Bellsbank kimberlite dike. For diamonds from Premier, D~INES (1980) noted that aggregate diamonds are slightly enriched, and irregularly shaped diamonds are slightly depleted in 13C, relative to well-formed diamonds (octahedra, dodecahedra, and macles). Oa'rER et al. (1990) also found a higher proportion of aggregate diamonds among the 13C-enriched ( - 5 . 9 to -3.8%0) diamond subpopulation from the Sloan kimberlite. However, GALIMOV (1984) found diamond aggregates in Russian kimberlites to be more commonly depleted in 13C. JAQUESel al. (1989) found that Argyle peridotitic diamonds of planar octahedral habit have 613 C values between - 9.0 and - 3.2%0, with a mean near -6%0, whereas Argyle peridotitic diamonds with resorbed forms have 613C values within this range, but with a lower mean 613C value of -7.4%0. Nitrogen is the most abundant trace impurity in diamonds and several investigations have sought correlations between C and N isotope compositions, N content and N aggregation state (see DEINES et al., 1989, for a review of previous studies). As yet, the results have been interesting but inconclusive. In one of the earlier surveys of this type, MILLEDGE et al. (1983) found a wide spread in 613C values (from - 3 1 . 9 to -0.5%0) among the rare Type II diamonds (i.e. those with low to undetectable N contents).

483

Inasmuch as Type I stones (those with N present as single or multiple atom substitutions) have 613C values between - 1 1 . 0 and -6.4%0, MILLEDCE et al. (1983) suspected that Type II diamonds correspond to the eclogitic paragenesis. DEINES et al. (1989), however, found that most Type II diamonds from the Finsch and Premier kimberlites were peridotitic. Eclogitic diamonds from Roberts Victor Group A (613C = - 1 5 . 5 + 0.4%0) are Type If (with one exception containing only 69 ppm N), whereas Group B diamonds (613C = - 5 . 6 + 0.6%0) have higher N contents (205-632 ppm N; DEI~ES et al., 1987). DEINES et al. (1987) concluded, on the basis of the C and N data and inclusion chemistry, that Group A and Group B diamonds formed in two separate environments, and are not linked by a single fractionation process. DEINES et al. (1989) found significant differences in N aggregation state between diamonds from Finsch and Premier and suggested that this is a function of different thermal histories in the mantle regions beneath the two diatremes. A t both kimberlites, however, eclogitic diamonds contained more N (15-1639 ppm) than did peridotitic diamonds (7-1206 ppm), which suggests that the processes which determined the N contents of the diamonds were similar for both localities. DEINES et al. (1989) concluded that the relations between C and N isotope composition and N content of diamonds must be complex and will, therefore, be difficult to interpret.

ISOTOPIC HETEROGENEITIES IN DIAMONDS Because most of the early C isotope analyses were of single diamonds or diamond fragments, little or no information was obtained about isotopic heterogeneity within diamonds. However, more detailed investigations have revealed complex variations within individual specimens. Examination of polished diamond surfaces under cathodoluminescence often reveals an intricate pattern of "growth layers" in octahedral diamonds (Fig. 6), which may relate to variation in impurity content which may in turn relate to sampling of different C reservoirs, or to variable conditions within a single reservoir. Other diamonds have readily observable structural inhomogeneities, such as "coated diamonds", which consist typically of fibrous coats overgrown on octahedral cores. Polycrystalline diamonds range from coarse aggregates of two or more randomly oriented diamond crystals, to micro- or cryptocrystalline intergrowths, commonly known as "carbonado" or "bort". Carbon isotope compositions are correlative sometimes with these structural features. For example, SWART et al. (1983b) reported a maximum variation of nearly 4%0 between the core and coat of a coated diamond (613 C = -11.03 and -7.32%0, respectively) and, in five other specimens, found that cores were usually isotopically light relative to coats. GALIMOV (1984) reported that 613C values for the cores of coated

484

M. Kirkley et al.

Fl6.6. Optical cathodoluminescence image of a polished diamond slab (5 x 5 × 1 mm) showing octahedral growth layers revealed by variations in luminescent intensity (GERNEKEand OJ'rER, 1989).

Yakutian stones fluctuate between - 1 7 and -4%0, whereas the coats vary within a narrow 613C interval ( - 8 to -6%0). Similar results were obtained by B OYD et al. (1987) for coated diamonds from Mbuji Mayi, Zaire, with the coats having consistent OI3c values between - 5 . 9 and -7.5%0, and six out of eight cores being depleted in 13C by up to 2.7%0 relative to the coats. Cores of the other two coated stones were enriched in ~3C by up to 2%0 relative to the coats. BovDet al. (1987) proposed that the relatively homogeneous 6~3C values found in the coats on octahedral diamonds, as well as in cubic diamonds, represent the addition of a C component from a single homogeneous reservoir, believed to be related to the kimberlite eruption. Isotope variations are also significant in some visibly homogeneous diamonds. JAvoY et al. (1984) reported up to 5.8%o variation within a small octahedral diamond from Mbuji Mayi. OTTER (1989) found significant 613C variations in small octahedra from Sloan. Interestingly, the variation was found only in the group of 13C-depleted eclogitic diamonds (O13C < -13%o) and the largest within-diamond variation among Sloan diamonds of 4.2%0 was found in the diamond with the lowest average d13C value of -29.4%0 (OTTER, 1989; OTTER et al., 1990). Three fragments of an octahedral diamond from the Dokolwayo kimberlite, which had a colourless core grading into a yellow external surface, had dl3c values of - 19.7, - 17.3 and - 16.4%o, respectively (DANIELS, 1991). WILDING (1990) analysed diamond X3C con-

tents by ion microprobe and detected a range of bl3c values from --10.9 to --5.8%0 in an octahedral diamond from Bultfontein. He noted sharp changes in 613C occurring between major zones of growth detected by cathodoluminescence, but little or no variation within each zone, and no regular trends of 13C depletion or enrichment from core to rim.

MODELS FOR THE DERIVATION OF DIAMOND CARBON ISOTOPE COMPOSITIONS

Models formulated to explain the C isotope characteristics of diamonds should be able to account for the following observations: (1) correlations of isotopic composition with paragenesis (and corresponding age) of the inclusions; (2) the unique isotopic compositions observed at individual kimberlite/lamproites; and (3) isotopic zoning patterns and heterogeneities. Correlations of C isotope composition with other diamond characteristics such as colour, crystal shape, N content, N isotope composition and N aggregation state should also be compatible with the given model. Three models which have been proposed to explain the isotopic characteristics of diamonds are: (1) the fractionation model which evaluates the isotopic compositions that would result during diamond precipitation by a Rayleigh fractionation process; (2) the primordial model which suggests that the C

C isotopes in diamonds: a review

CO2

(a)

> C + 02

485

(b) CH4

( f 0 2 decreases) Avap_dm d

,

-IO

.

,

-8

.

,

-6

"~

(f02 increases)

4~oo

.

,

-4

> C + 2H2

Avap_dm d

.

.

-2

.

.

0

61aCee"

.

.

.

.

-8

-7

,-v

- - 1 ~oo

.

-6

-5

-4

61sCene

FIG. 7. Approximate distributions of diamond C isotope values which would result from diamond precipitation from (a) CO 2 and (b) CH4 vapours under the conditions discussed in the text. (Modified from DEINES, 1980.)

isotope characteristics of diamonds are relict from the terrestrial accretion of solar nebular materials; and (3) the subduction model which proposes that the C isotope characteristics of diamonds are inherited from crustal C carried into the diamond stability zone via subduction. These models are described and compared below.

Frac6ona~on mode~

At the high temperatures (>1000°C) expected within the diamond stability field in the mantle, C isotope fractionations between diamond and most of the expected coexisting solid or liquid C-bearing phases (e.g. graphite, or dissolved C in a melt) are small, of the order of 1%o (BoTTINGA,1969b; DEINES, 1980). Hence, it is difficult to derive the full range of 613C values illustrated in Fig. 1 by solid-solid or solid-liquid isotope exchange reactions within a relatively homogeneous, mantle C reservoir. However, larger fractionations are expected if a vapour phase is involved in diamond formation. Diamond-carbonate fractionations are also relatively large, i.e. -3%O at mantle temperatures (BoxxtNGA, 1969a,b), and can be modelled in a manner similar to the d i a m o n d ~ O 2 fractionations discussed below. DEINES (1980) determined that CH 4 and/or CO 2 are the only plausible vapour sources from which diamond could precipitate, given the expected constraints on mantle pressures, temperatures, fO2, and H 2 0 contents. Carbon monoxide would be an insignificant vapour phase under mantle conditions. DroNES (1980) invoked Rayleigh fraetionation to model the range and distribution of diamond 613C values that would result during diamond precipitation for CO 2 vapour upon decreasing fOE, and from CH 4 vapour upon increasing fOE. The initial

conditions assumed were 1020°C, 45 kbar, and an initial isotopic composition and variability of the C source vapour of - 5 . 0 + 0.3?'oo (lo). Because diamond is depleted in 13C relative to CO 2 vapour by -4%° under these conditions (BoTrlNGA, 1969b), the first diamond to precipitate from the CO2 reservoir would have a 613C value of -9%0, i.e. 4%Olower than that of the source vapour. Removal of this relatively 13C-depleted C from the reservoir results in the vapour (and additional diamond precipitating from it) becoming increasingly enriched in 13C during diamond formation. The result is a distribution of diamond 613C values that is strongly skewed toward •13C values greater than the initial diamond 613C value (Fig. 7a). On the other hand, because diamond is enriched in 13C relative to CH4 vapour by -1%O (BoTTINGA, 1969a,b), diamond precipitation from CH 4 vapour will result in a slightly negatively skewed distribution (Fig. 7b). Neither the 613C distribution derived through diamond precipitation from CO2 nor from CH 4 under the assumed initial conditions (Fig. 7) resembles the strongly negatively skewed distribution exhibited by the diamond population (Fig. 1). In particular, the most ~3C-depleted diamond values are not accounted for by the precipitation of diamond from either vapour. JAVOYet al. (1986) pointed out that low 613C values near -30%o could be produced by Rayleigh fractionation of an isotopically homogeneous C source if cumulative effects were considered, i.e. if the fractionated residuum was isolated and again fractionated. However, Fig. 7b suggests that 98% of the C in the CH 4 vapour would be utilized before the 613C value of diamond precipitating from the reservoir reached -8%o. Hence, even if the residual CI-I4 was isolated and again fractionated, the amount of diamond with 613C values below -8%0 that could be produced would be miniscule. The curve in Fig. 7b would be shifted farther to the

486

M. Kirkley et al.

left, hence, larger quantities of 13C-depleted C could be obtained, if the fractionation factor involved were larger. JAVOYet al. (1986) called upon CO 2 outgassing from a C-bearing melt as a means of obtaining low 613C values in the melt, by assuming a fractionation factor (CO2-dissolved C in melt) of 4.9%o. However, MATrEY et al. (1990) have disputed the >4%o fractionation factor used by JAVOYet al. (1986), and propose that it is actually closer to 2%o. Further, it must be noted that magma outgassing occurs only at shallow crustal levels and not at mantle depths. FREUND et al. (1980) proposed that atomic C can dissolve in rocks and magmas and that diamond may form when such rocks and magmas at mantle depths become supersaturated with C. If this were the case, an additional opportunity to produce a wide range of diamond 613C values would be available, because isotopic fractionations between atomic C and other C phases would be large, probably of the order of 10%o greater than the fractionations expected between well-known C phases. However, the results of FREUND et al. (1980) have not been substantiated by independent studies (e.g. TINeLE et al., 1989). Thus, it appears that no presently recognized mantle fractionation mechanism is readily capable of producing the very low 613C values exhibited by some diamonds. DEINES (1980) also concluded: . . . it has become clear that no combination of variation in fractionation factors (resulting from the participation of a vapor and from pressure and temperature variations) and reservoir effects could lead to the observed 13C distribution of all diamonds. Hence the large range in 13C of diamonds may either reflect isotopic heterogeneities in the mantle, or the participation of carbon that has passed through the surface carbon cycle . . . . In spite of the above conclusion, it should be noted that the 613C distribution of all diamonds need not be modelled by a single fractionation process. As shown in Figs 4 and 5, suites of diamonds from individual kimberlite/lamproite localities exhibit unique 613C distributions, none of which is similar to the composite histograms of Figs 1 and 3. Each suite, as well as diamond subpopulations within a suite, may have formed from separate, distinct reservoirs, and/or by different precipitation mechanisms. Hence, particular cases of diamond precipitation (as well as diamond resorption) m a y indeed involve CO 2 and/or CH 4 vapour such that diamond-vapour reactions could be responsible for at least some of the variation within diamond 613C distributions. SWART et al. (1983b), for example, suggested that the trend from isotopically light cores to heavier coats on coated diamonds could be the result of Rayleigh fractionation from a CO2 source. A most important conclusion to be drawn from the extensive investigation of DEINES (1980) is that source inhomogeneity is an essential condition of any model which attempts to identify the sources of C in diamonds. Both the primordial C model and the

40

30

~, 20

0

,

V1 '

i

30

.

.

.

.

.

~ .

.

.

.

i

.

.

.

.

.

,,,[~ .

.

.

.

-1 r

10

-20

•

' 0

613Cpos FIe. 8. Histogram illustrating the distribution of isotopic compositions of C in iron meteorites (D~INES and WICgMAN,1973, 1975, and references therein). subducted crustal C model comply with this condition and invoke diamond derivation from isotopically inhomogeneous mantle C sources. Primordial carbon in the mantle

The distribution of 613C values for C-bearing phases in iron meteorites (DEINES and WICgMAN, 1973, 1975), has a peak near -5%o and a negatively skewed distribution (Fig. 8) which resembles the composite of diamond 6a3C values (Fig. 1). The similarity of Figs 1 and 8, in conjunction with the inability of Rayleigh fractionation processes to explain the range and distribution of diamond isotopic compositions, led DEINES (1980) to suggest that diamonds may represent C which has retained its primordial isotopic inhomogeneity. Beyond recognition of the similarity in the diamond and iron meteorite 613C distributions, however, the details of how diamonds might be derived from primordial C have not been yet addressed. A primary consideration must be the assumption of the primordial C model that C in iron meteorites is representative of C in the Earth's mantle. Iron meteorites are believed to represent the cores of their disaggregated parent bodies. Assuming that the Earth formed in a manner similar to that of these parent bodies, then C in taenite (Ni-Fe alloy) and cohenite ((Fe,Ni)3C) of iron meteorites (613C = - 18 to -22%o; Fig. 9a), could be assumed to be correlative with C in the Earth's core, but is not necessarily representative of C in the Earth's mantle. Carbon in the graphite nodules and silicate inclusions in iron meteorites might be better equated with mantle C. The relatively restricted range of 613 C values of these C phases (613C = - 3 to -14%o; Fig. 9b,c) is similar to that observed in peridotitic diamonds, but does not correspond with the >30%O spread of values exhibited by the eclogitic diamonds. Carbon in "residues" from acid digestion of meteoritic material (Fig. 9d) is problematic, because it may represent C in solution in taenite, kerogen-like organic compounds

C isotopes in diamonds: a review 30

20

(o) COHENITE, TAENITE

(b) GRAPHITE NODULES

Z

z ~0

487

10

2E

z

o

..,

. . . . . . . . .

-30

"1

, . . . . . . . . .

-20

,.

--,

. . . . . . . . .

-30

-10

i . . . . . . . . .

-20

613Cpom

,--

-10

813CFem

20

(d) RESIDUES

(C) GRAPHITE IN SIUCATE INCLUSIONS (n

(n 15

15

z

z

~o m X ~ z

Is

5

z

.rl .... rl

o -30

-20

-10

6i3Cpem

-30

.....

mn

-20

..... -10

1-1 .... (

OiSC~m

FIG. 9. Histograms of C isotope data from various C phases in iron meteorites (DEINESand WICKMAN, 1973, 1975, and references therein).

similar to those found in carbonaceous chondrites, and/or terrestrial contaminants (DEINES and WICKMAN, 1973). More recent studies of meteorites employing step-heating techniques have found that C with 613C ~ - 3 0 to -25%0 is liberated typically at temperatures <500°C. Such C could represent terrestrial contamination and cannot unequivocally be attributed to indigenous C in the meteorite (e.g. SWART et al., 1983a). Whether or not the residues in iron meteorites fall into this category is not known. However, the wide range of C isotope compositions appropriate for the formation of diamonds is present in iron meteorites only if the residues represent indigenous C. Preservation of primordial isotopic inhomogeneity since the formation of the Earth - 4 . 6 Ga ago is an inherent condition of the primordial C model. Because little is known about the first billion years following Earth's accretion, one can only speculate as to whether or not this condition would be met. It has been proposed that enough heat was retained in the early Earth for the upper mantle to have partially or completely melted (e.g. TURCOTTE, 1984; CONDIE, 1989). Complete melting would result in a magma ocean; less-than-complete melting would produce variable quantities of magma. In either case, excess heat was probably dissipated by relatively rapid and vigorous convection (e.g. BURKE and 142100, 1978). In turn, such melting and convective processes would be

expected to homogenize the C isotope compositions of the primordial mantle. Convective homogenization could explain the relatively narrow range of peridotitic diamond 613C values between - 1 0 and -1%o. That C in M O R and OI basalts, carbonatites, and kimberlite carbonates (references cited above) also have 6]3C values within this range is consistent with isotopic homogenization of mantle C, and suggests that the mean of this range is an accurate representation of the isotopic composition of C in the convecting asthenosphere (GuRr~EY, 1989). Whether or not extensive mantle melting and convection did occur in the early Earth, the disparity between the relatively homogeneous peridotitic diamond and other mantle-derived 613C values, and the wide 613C range exhibited by eclogitic diamonds, must be explained. The different C isotope values in peridotitic and eclogitic diamonds might be reconciled with the primordial model by calling upon accretion of two different primordial components. For example, it has been proposed that the accretionary development of the Earth included a late-stage veneer of carbonaceous chondrite-like meteoritic material (e.g. JAVOY and PINEAU,1983; DREIBUS and W.~NKE,1984). Bulk analyses of carbonaceous chondrites (KERRIDGE, 1985) yield the distribution of C isotope compositions illustrated in Fig. 10, where the 613C range can be seen to be similar to that of eclogitic diamonds.

488

M. Kirkley et al. monds (or capable of producing that composition through fractionation) preserved that isotopic composition through the igneous and metamorphic processes that characterized the first few billion years between the Earth's accretion and diamond crystallization.

5

Subducted carbon model 0

.

r

r

-30

.

.

.

.

.

n .

.

.

.

r

.

.

.

.

.

.

.

.

.

-20

~

.

-10

.

.

.

.

.

.

.

.

i

0

613CpDa

FIG. 10. Histogram of the isotopic composition of bulk C in carbonaceous chondrites (data from KERRID6E, 1985, and references therein). Carbon and other volatile phases in carbonaceous chondrites have not experienced temperatures > -600°C, thus, their C isotope compositions cannot be equated directly to those of diamonds. GIBSON (1976) investigated the effects of thermal metamorphism on carbonaceous chondritic material (Murchison C2 chondrite) using vacuum pyrolysis, and GIBSON and CHAN6 (1980) determined the C isotope compositions of the sample residues from the earlier heating experiments. The latter authors found that heating to temperatures of 800°C changed the initial 613C value of -6.6%0 to a value of -16.5%o. How closely such experiments represent natural, primitive mantle conditions is unknown. However, the results suggest that it may be possible to produce low 613C values in diamonds through heating of carbonaceous chondrite material, e.g. through metamorphic recrystallization in the mantle. In order for the primordial model to be compatible with the observed correlations between isotopic composition and inclusion paragenesis, the low 13C isotopic signature would then have to be imparted to host rocks with eclogitic mineralogy. Finally, the significantly younger age of eclogitic diamonds would suggest, according to the primordial C model, that such late-accreting carbonaceous material remained as a separate reservoir, distinct from the material which served as the source of peridotitic diamonds, for approximately 2 Ga prior to its recrystallization as diamondiferous eclogite. In summary, the isotopic composition of peridotitic diamonds may well be explained by a primordial C model, that is, it may represent an originally wider range of 613C values which was homogenized during melting and convection following accretion of the Earth. Fractionation mechanisms such as those discussed in the previous section may have produced small variations about this mean to yield the observed range of - 10 to - 1%o in peridotitic diamonds. On the other hand, the isotopic composition of eclogitic diamonds can be explained by the primordial model only if a primordial component with C isotope compositions characteristic of eclogitic dia-

The range of 613C values of eclogitic diamonds spans that of present-day crustal carbonaceous materials, i.e 613C values of organic matter range from - - 3 5 to -20%o, and inorganic marine carbonates have 613C values of + 2%0. This comparison has led to the suggestion that eclogitic diamonds may represent crustal C which was carried into the mantle in a subducted oceanic slab comprised of basaltic oceanic crust (which recrystallized to eclogite under mantle conditions), + organic matter in sediments, _+ inorganic carbonates (e.g. FRANK,1969; SOBOLEVand SOBOLEV, 1980). It should be noted that the subducted C model is concerned mainly with the origin of the wide range of eclogitic diamond 613C values, and assumes that the more homogeneous peridotitic diamond 613C values were derived in a different manner, e.g. through homogenization of primordial C, as discussed in the previous section (e.g. JAQUESet al., 1989). Several assumptions are inherent in the subduction model. Firstly, crustal C with the appropriate isotopic composition, i.e. similar to that of modern organic matter and carbonates, had to (a) exist and (b) be subducted, prior to diamond crystallization. The ages of eclogitic diamonds indicate that crustal C had to be introduced into the mantle prior to - 2 Ga ago. Secondly, reactions that occurred during subduction and conversion of crustal C into diamond cannot have fractionated C isotopes substantially, such that the isotopic signatures of crustal C sources are preserved. The organic matter in ancient sediments had a range of 613C values similar to, and even wider than, present-day organic matter. This is illustrated in Fig. 11. Marine carbonates have maintained relatively constant isotopic compositions (0 + 5%o) through time (e.g. SCHIDLOWSKIet al., 1983). Hence, there appears to be no problem with the availability prior to 2 Ga ago of C having a range of 613C values similar to that of diamonds. A discussion as to whether or not the sedimentary veneer of a descending oceanic slab would be subducted or scraped off is beyond the scope of this paper. JAvov et al. (1982), and later DES MARAIS (1985), however, calculated that the amount of C which has been outgassed from the mantle at midocean ridges exceeds the amount stored in crustal and atmospheric reservoirs, hence, reinjection of C into the mantle via subduction of sediments must occur.

C isotopes in diamonds: a review

489

(e) ORAPHITIEIN ~ IETAIIE]I)IIENI~ (>looo uQ; >~UPHmm.rrE o 1 ~ AUSTRALIA

SOUTH AFRICA

[.ARAH[[DY GP 2

"o x

TRANSVAAL SUPGP GRIQUALAND OP HAM[RSIrY OP & TUR[[ CR[EK GP

VEHTERSDORP SUPGP

,.=,

FORT[$CUE GP

PONGOLA SUPGP [ CR[[K GP WAG~R[WOONA GP

i

SWAZILAND SEO

, ....

-60

, ....

-50

2

J ....

-40

L ....

-30

i ....

-20

L ....

-10

,

o- 3 2 m

0

-28

-24.

615CP0e

-20

-16

[-1-1. -12

6~3Cpoe

Fl6. 11. The isotopic composition of organic C in ancient sediments (modified from SCmDLOWSKIet al., 1983, with data from HAVESet al., 1983). (b) ECLOglTICDIAMOND61=C DATA 16

Suffice it to say that such a subduction process is considered feasible. Isotopic fractionations that occur during subduction may be evaluated by considering the behaviour of carbonaceous materials during regional and contact metamorphism (KIRKLEYand GURNEY, 1989; KIRKLEY et al., 1990). Loss of liquid and gaseous hydrocarbons occurs predominantly during catagenesis and diagenesis (<200°C, <7 km depth) of sedimentary organic matter (e.g. HUNT, 1979). This usually causes c~13C values of the residual C to increase, as hydrocarbons are depleted in ]3C relative to the original material. However, the magnitude of the isotopic shift may vary from only a few per rail, to a maximum of -20%0 (McKIRDY and POWELL, 1974). The 613C value inherited by the graphitic endproduct, therefore, ranges from the virtually unchanged initial organic signature, to a value enriched significantly in 13C. A c~I3Cof -10%o appears to be the maximum attained by graphite during metamorphism of organic C (McKIRDY and POWELL, 1974; HOEFS and FREY, 1976). Because the C isotope fractionation between graphite and diamond is small (<0.5%0 at 700°C; BOTTINGA, 1969b), the isotopic composition of graphite is unlikely to be changed significantly through recrystallization to diamond. A compilation of published 613C data for organically-derived graphite in ancient metasediments (>1000 Ma, >amphibolite grade) yields the distribution shown in Fig. 12a. This distribution is virtually identical to that of one of the subpopulations of eclogitic diamonds from the Sloan kimberlite ("Group III" of OTrER, 1989). The laC-depleted diamonds from Roberts Victor ("Group A" of DEINES et al., 1987) also have 613C values within this range (Fig. 12b). As the maximum 613C value attained during metamorphism of organic matter ~ - 10%o (McKIRDY and POWELL, 1974; HOEFS and FREY, 1976), eclogitic diamonds with 613C values > -10%o must be derived by means other than simple metamorphism of or-

f

1~

I~

I

12

1

~ OROUP nl | ROIIE'STS VICTOR gROUP A

1

\ \

z

4

"

\

o -32"

-2a

-24

i

-20

-16

.

12

61a0poe Fro. 12. Histograms comparing (a) the isotopic composition of graphite in ancient metasediments (> 1000 Ma, >amphibolite grade; BARKERand FRIEDMAN,1969; ANDREAE,1974; W]ES et al., 1981) and (b) the isotopic composition of diamonds from Sloan (Group III of O'rrER, 1989) and Roberts Victor (Group A of DEINESetal., 1987). ganic C. As Fig. 13a shows, graphite becomes enriched in 13C through C isotope exchange with carbonate during metamorphism of calc-silicate assemblages, such that it commonly attains 613C values of - 2 -+ 4%o (e.g. VALLEYand O'NEIL, 1981). Eclogitic diamonds from the Star mine (HILL, 1989) and from New South Wales placers (SOBOLEV,1984) have relatively high (513C values of - 4 to +2%o (Fig. 13b), similar to the values derived through graphitecarbonate exchange. In addition, diamonds from these localities contain Ca-enriched inclusions such as grossular garnet (SoBOLEV, 1984; HILL, 1989), and in one case, a dolomite inclusion (HILL, 1989), which is consistent with crystallization within carbonaterich host rock. The value of C, which is considered to be indigenous to mid-ocean ridge and ocean island basalt glasses (i.e. directly mantle-derived, rather than due to C compounds derived during or following eruption), is within the relatively restricted range - 8 to -3%o (DES MARAISand MOORE, 1984; MATrEY et al., 1984; EXLEYet al., 1986). Upon subduction, these ~ 1 3 C

490

M. Kirkley et al. 2000

1500 (...;, lO 1000 z i.d -.j

a.

8

5OO 4. 2 0

0 i

-15

-30

.

-lo

-5

o

613C1~11

(b)

10

EGLOelI'IC

DIAMOND

4taO

DATA

[==---]_

8

6

2 0

' . . . . -15

L ' , -10

,

'

'

I -5

0

61~C~,, FIG. 13. Histograms comparing (a) the isotopic composition of graphite in ancient calc-silicate rocks (>1000 Ma, >amphibolite grade; VALLEYand O'NEIL, 1981; WEXSet al., 1981) and (b) the isotopic composition of diamonds from New South Wales (SoBOLEV,1984) and Star (HILL, 1989).

basalt types would carry this C back into the mantle. Hence, eclogitic diamonds with isotopic compositions within the same range as basaltic C and CO2, such as those from Finsch, Roberts Victor Group B and Ellendale 4 and 9 (Fig. 5a,b), may reflect a recycled, but otherwise unaltered, mantle C isotope signature. Alternatively, the subducted slab might encounter sufficiently high temperatures and pressures such that mantle C present in the surrounding peridotitic material could mix and exchange with C in the slab, resulting in diamonds with a "typical mantle" isotopic signature. The ]3C-depleted component in basalts, which may be due to surficial organic contamination or to precipitation from volcanic gases during eruption, nevertheless, would be subducted with the descending crustal slab. A compilation of (~13Cvalues of the total C in basalts is illustrated in Fig. 14. In view of the wide range of 613C values found in basalts (Fig. 14), it might be possible to derive most eclogitic diamonds from the metamorphism of basalt such that there would be no need to call upon the subduction of sedimentary carbonaceous materials. However, we have in our collection diamondiferous eclogite xeno-

-20

-10

0

6tSCea. FIG. 14. Histogram of the isotopic composition of C in basalts. Data from PINEAUet al. (1976), PINEAUand JAvoY (1983), DES MARA]Sand MOORE (1984), MATrEYet al. (1984), EXLEYet al. (1986).

liths containing - 5 % diamond by volume. Also, graphite pseudomorphs after eclogitic diamonds comprise up to 15 % of certain garnet clinopyroxenite layers in the Beni Bousera peridotite massif of Morocco (PEARSON et al., 1989). Hence, a C source in addition to that in basalts is required for mass balance. Finally, eclogitic diamonds with intermediate 613C values between - 1 2 and -8%o, such as those from Argyle (JAQUES et al., 1989) and Sloan Group II (OTTER, 1989; OTTER et al., 1990) (Fig. 5b), could represent mixing of various quantities of C from the sources described above and represented in Figs 12, 13 and 14. If the slab experienced some degree of melting, diffusion would promote the homogenization of a range of crustal 613C values to some intermediate value, depending on the quantities of light and heavy C present originally. The subduction model, therefore, is able to explain the derivation of the full range of diamond 613C values, through the metamorphism of organically derived C and inorganic carbonate, plus homogenized asthenospheric C, and mixtures of these C sources. Other independent studies support the subduction model. Oxygen isotope compositions o f eclogite xenoliths.

Whereas mantle peridotite xenoliths typically have 61So values within a narrow range of -5.0-6.5%0 (e.g. KYSERet al., 1981), eclogite xenoliths exhibit a wider range of 6180 ~ 2-8%o (GARLICKet al., 1971; JAGOUTZ et al., 1984; MACGREGOR and MANTON, 1986; ONGLEY et al., 1987). JAGOUTZ et al. (1984) suggested that the O isotope variation in the eclogites is caused by relatively low temperature fluid-rock interaction, analogous to the hydrothermal alteration observed in modern-day oceanic crust (e.g. MUEHLENBACHS and CLAYTON, 1976). Due to their small size, 6lSo values of eclogitic inclusions in diamonds have not yet been determined. Hence, it can only be assumed at this time that the inclusions

C isotopes in diamonds: a review exhibit a range of 61So values similar to those of the xenoliths. The fact remains, however, that eclogite 6180 values are consistent with a subducted crustal source, and no mantle fractionation process has yet been recognized which can explain the wide range of 6180 values.

Sulphur isotope compositions of sulphide inclusions in diamonds. The 634S values for sulphide inclusions in eclogitic diamonds from Orapa exhibit a wider spread than is characteristic of S in unaltered mantle peridotite and in iron meteorites (CHAUSSIDON et al., 1987). Sulphur isotope compositions have become more variable with time as organic processes have exerted greater influence on the isotopic ratios (SCHIDLOWSK! et al., 1983). Hence, the relatively wide spread in 634S ratios for diamond inclusions is inconsistent with a meteoritic or direct mantle origin for the S, and instead suggests that organic processes could have been involved in producing the spread. Carbon isotope variations in back-arc basalts. Carbon in back-arc basin basalts has quite low 613C values of - 13.2 to - 9 . 8 % relative to C in MORB and OIB (MA~EY et al., 1984). These authors suggested that the light isotopic signature is the result of the addition of organic C into the back-arc basin magma source via subduction. That an organic C isotope signature is reflected in subduction-related basalts, suggests that it could also be reflected in diamonds, provided subduction can carry crustal materials to the depths of diamond formation.

CONCLUSIONS

The wide range of diamond 613C values is not readily explained by fractionation of a homogeneous mantle C reservoir by any presently known fractionation mechanisms. The isotopic data appear to require a mantle C source(s) with an inhomogeneous C isotope composition. Because so little is known about conditions prevailing during Earth's formation, models of diamond derivation from primordial C are difficult to assess. However, currently popular models of the accretion and early development of the Earth, as well as the consistent and relatively limited range of 613C values found in mantle-derived carbonates and in basaltic CO2 suggest that the isotopic composition of C in the mantle has been homogenized. It appears reasonable, therefore, to conclude that peridotitic diamonds which have 613C values encompassing this "typical mantle" range also represent homogenized primordial C. If eclogitic diamonds represent primordial C, then this places stringent restrictions on the diamond derivation model, i.e. accretion of a primordial component with a wide range in C isotope composition, that is not homogenized with the component represented by peridotitic diamonds, but

491

which is preserved during diamond crystallization within an eclogitic lithology at a later date. The subducted C model is consistent with the observation that eclogitic diamonds, as opposed to peridotitic diamonds, exhibit a wide range of C isotope signatures. The observed range of 613C values in eclogitic diamonds, as well as the mineralogy of the eclogitic inclusions, could both be derived through metamorphism or melting of basaltic crust plus a sedimentary component. The subduction model is also consistent with the relatively young ages of eclogitic diamonds, because organic C is known to have existed prior to 3 Ga ago, which allows 1 Ga for this material to be subducted and converted to diamond within eclogitic host rock. Therefore, until C (and O and S) isotope fractionation mechansims are discovered which can explain the anomalously wide range of isotopic compositions found in eclogitic diamonds, xenoliths and sulphide inclusions, or until the crystallization of diamond with the observed isotopic and paragenetic characteristics can be shown to be consistent with early stages of the Earth's development, the subducted crustal C model is most compatible with the available diamond data.

SUGGESTIONS FOR FUTURE RESEARCH

Much of the progress that has been made in diamond research in the past decade is the result of integrated studies in which both diamonds and their mineral inclusions were examined. It is important that this trend continues, with future diamond isotopic studies being made on inclusion-bearing diamonds, such that the findings may be extrapolated to diamondiferous host rocks in the mantle. The study of diamonds with two or more coexisting inclusions are particularly valuable. Subtle chemical distinctions in inclusion compositions should be noted, such that subparageneses, e.g. lherzolitic, wehrlitic, and websteritic, can be identified and characterized. Compilations of existing data, plus additional, detailed geochemical measurements (i.e. including those of major and trace elements and radiogenic isotopes) of diamondiferous peridotite and eclogite xenoliths will also be valuable in deciphering the origins of diamondiferous lithologies. Mixed parageneses and variable isotopic compositions within single diamonds suggest that diamonds partially crystallize within one host rock lithology, then are transported (via mantle convection?) to another lithology where crystallization proceeds. A better understanding of the phenomenon of mixed inclusion paragenesis is needed, as is an understanding of whether diamonds crystallize from fluids, melts or represent subsolidus recrystallization events. In addition, the question remains as to what degree of diamond crystallization, if any, can be related to the kimberlite magmatic event. Explanations for correlations between C isotope

492

M. Kirkley et El.

composition with diamond colour and crystal shape, and how these correlations relate to paragenesis are needed. Further studies of how C isotope compositions vary with N content, N aggregation state, and N isotope composition are required. In particular, how such characteristics correspond to diamond growth layering revealed by cathodoluminescence should be investigated. A t present, crustal (+organic) fractionations appear to be the only realistic means of deriving a wide range of 613C, 6180 and 634S values in mantle materials. A r e there fractionations that are being o v e r l o o k e d in the search for explanations of the large range of stable isotope compositions in the mantle? According to the subducted C model, hydrothermally altered oceanic basaltic crust + sediments should produce eclogite + diamond when subjected to mantle-like pressures, temperatures and O fugacities. Experimental confirmation of the model, as well as experimental investigation into the behaviour of C during partial melting of eclogite, and of eclogite/ peridotite assemblages, should be attempted. If successful, comparison of the range of eclogite compositions produced experimentally with that found in xenolith suites from kimberlites and lamproites would be very interesting and informative. Acknowledgements--Discussions with colleagues during the Diamond Workshop of the 28th International Geological Congress, Washington D.C., 1989 (convenors F. R. Boyd, H. O. A. Meyer and N. V. Sobolev) and during the Carbon Cycle and Mantle Eclogite Special Sessions of the American Geophysical Union Spring Meeting, Baltimore, 1990 (convenors T. N. Tingle and J. Smyth, respectively) inspired the compilation of this review. Special thanks to A. A. Levinson for constructive comments and encouragement. Editorial handling: Brian Hitchon.

REFERENCES

ANDREAEM. O. (1974) Chemical and stable isotope composition of the high grade metamorphic rocks from the Arendal area, southern Norway. Contr. Miner. Petrol. 47,299-316. BARKER F. and FRIEDMAN. I. (1969) Carbon isotopes in pelites of the Precambrian Uncompahgre formation, Needle Mountains, Colorado. Geol. Soc. Am. Bull. 80, 1403-1408. BOTIIN.GAY. (1969a) Calculated fractionation factors for carbon and hydrogen isotope exchange in the system calcite-carbon dioxide-graphite-methane-hydrogen-water vapor. Geochim. cosmochim. Acta 33, 49--64. BOaTINGAY. (1969b) Carbon isotope fractionation between graphite, diamond and carbon dioxide. Earth planet. $ci. Lett. $, 301-307. BoYo S. R., MATrEYD. P., PILLINGERC. T., MILLEDGEH. J., MENDELSSOnr~ M. and SEAL M. (1987) Multiple growth events during diamond genesis: an integrated study of carbon and nitrogen isotopes and nitrogen aggregation state in coated stones. Earth planet. Sci. Lett. 86, 341-353. BURKEK. and KIDD W. S. F. (1978) Were Archean conti-

nental geothermal gradients much steeper than those of today? Nature 272,240-241. CHAUSSIDON.M., ALBAREDEF. and SHEPPARD S. M. F. (1987) Sulphur isotope heterogeneity in the mantle from ion microprobe measurements of sulphide inclusions in diamonds. Nature 330, 242-244. CLEMENTC. R., SK~N.NERE. M. W. and SCOTTSMrrn B. H. (1984) Kimberlite redefined. J. Geol. 92, 223-228. CON'DIEK. C. (1989) Plate Tectonics and Crustal Evolution. Pergamon Press. CON.WAYC. M. and TAYLORn. P. JR (1969) o l a / o 16 and C13/C 12 ratios of coexisting minerals in the Oka and Magnet Cove carbonatite bodies. J. Geol. 77,618-626. CRAIG H. (1953) The geochemistry of the stable carbon isotopes. Geochim. cosmochim. Acta 3, 53-92. DANIELSL. R. (1991) Diamonds and related minerals from the Dokolwayo kimberlite, Swaziland. Ph.D. thesis, University of Cape Town. DEIN.ES P. (1970) The carbon and oxygen isotopic composition of carbonates from the Oka Carbonatite complex, Quebec, Canada. Geochim. cosmochim. Acta 34, 11991225. DEINES P. (1980) The carbon isotopic composition of diamonds: relationship to diamond shape, color, occurrence and vapor composition. Geochim. cosmochim. Acta 44, 943-961. DEINESP. and GOLD D. P. (1973) The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon. Geochim. cosmochim. Acta 37, 1709-1733. DEINES P., GURN.EYJ. J. and HARRISJ. W. (1984) Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite, South Africa. Geochim. cosmoschim. Acta 48, 325-342. DEINES P., GURN.EYJ. J. and HARRISJ. W. (1987) Carbon isotopic composition, nitrogen content and inclusion composition of diamonds from the Roberts Victor kimberlite, South Africa: evidence for 13C depletion in the mantle. Geochim. cosmochim. Acta 51, 1227-1243. DEIN.ES P., HARRISJ. W., SPEARP. M. and GURN.EYJ. J. (1989) Nitrogen and 13C content of Finsch and Premier diamonds and their implications. Geochim. cosmochim. Acta 53, 1367-1378. DEINES P. and WICKMAN.F. E. (1973) The isotopic composition of 'graphitic' carbon from iron meteorites and some remarks on the troilitic sulfur of iron meteorites. Geochim. cosmochim. Acta 37, 1295-1319. DEINES P. and WICKMANe. E. (1975) A contribution to the stable carbon isotope geochemistry of iron meteorites. Geochim. cosmochim. Acta 39,547-557. DES MARAISD. J. (1985) Carbon exchange between the mantle and the crust, and its effect upon the atmosphere: today compared to Archean time. Geophys. Mono. 32, 602-611. DES MARAiSD. J. and MOOREJ. G. (1984) Carbon and its isotopes in mid-ocean basaltic glasses. Earth planet. Sci. Lett. 69, 43-57. DREmUS G. and W)~NKEH. (1984) Accretion of the earth and the inner planets. Proc. 27th International Geological Congress 11, 1-20. EXLEYR. A., MATrEYD. P., CLAGUED. A. and PILLINGER C. T. (1986) Carbon isotope systematics of a mantle "hotspot": a comparison of Loihi Seamount and MORB glasses. Earth planet. Sci. Lett. 78, 189-199. FRANKF. C. (1969) Diamonds and deep fluids in the upper mantle. In The Application o f Modern Physics to the Earth and Planetary Interiors (ed. S. K. RUN.CORN.),Chap. 2, pp. 247-250. Wiley. FREUNO F., KATnREIN.H., WEN.OELERH. and KNOEEL R. (1980) Carbon in solid solution in forsterite--a key to the untractable nature of reduced carbon in terrestrial and cosmogenic rocks. Geochim. cosmochim. Acta 44, 13191333.

C isotopes in diamonds: a review GALIMOVE. M. (1984) 13C/12C of diamonds: vertical zonality of diamond formation in the lithosphere. Proc. 27th Int. Geol. Cong. 11,279-307. GALIMOV E. M., IVANOVSKAYAI. N., KLYUYEV YU. A., NEPSHA V. I., EFISHINA N. I., PLOTNIKOVAS. P., GALIULINAO. I., POBEREZHSKYV. A., GRITSIKV. V. and KoenL V. A. (1980) Isotopic composition and peculiarities of crystalline structure of natural polycrystals of diamond with Ionsdaleite. Translated from Geockhimya 4, 533539. GARLICKG. D., MACGREGORI. D. and VOGELD. E. (1971) Oxygen isotope ratios in eclogites from kimberlites. Science 172, 1025-1027. GERNEKE D. A. and OTrER M. L. (1989) Optical and SEM cathodoluminescence of diamonds. Proe. Electron Microsc. Soc. Southrn Afr. 19, 1%20. GIBSON E. K. JR (1976) Volatilization studies on the Murchison carbonaceous chondrite. Meteoritics 11,286-287. GIBSON E. K. JR and CHANG S. (1980) Carbon isotopic changes produced by thermal volatilization of the Murchison C2 chondrite. Lunar planet. Inst. Contr. 412, Paper 91. GURNEY J. J. (1989) Diamonds. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 2, pp. 935-965. Blackwell Scientific (Geol. Soc. Austr. Spec. Pub. 14). GURNEY J. J., HARRIS J. W. and RICKARD R. S. (1984) Silicate and oxide inclusions in diamonds from the Orapa Mine, Botswana. In Kimberlites H: The Mantle and Crust-Mantle Relationships (ed. J. KORNPROBST),pp. 3--9, Elsevier. HALL A. E. and SMrrH C. B. (1984) Lamproite diamonds--are they different? In Kimberlite Occurrence and Origin: A Basis for Conceptual Models in Exploration (eds J. E. GLOVER and P. G. HARRIS), Publs Geol. Dep. and University Extension, University of W. Aust., No. 8, pp. 167-212. HARRISJ. W. (1987) Recent physical, chemical, and isotopic research of diamond. In Mantle Xenoliths (ed. P. NlXON), Chap. 33, pp. 478-500. Wiley. HAYES J. M., KAPLAN I. R. and WEDEKING K. W. (1983) Precambrian organic geochemistry, preservation of the record. In Earth's Earliest Biosphere (ed. J. W. SCHOPF), Chap. 5, pp. 93-134. Princeton University Press. HILL S. J., (1989) A study of the diamonds and xenoliths from the Star kimberlite, Orange Free State, South Africa. M.S. thesis, University of Cape Town. HOEFS J. and FREY M. (1976) The isotopic composition of carbonaceous matter in a metamorphic profile from the Swiss Alps. Geochim. cosmochim. Acta 40,945-951. HUNT J. M. (1979) Petroleum Geochemistry and Geology. W. H. Freeman. JAGOUTZ E., DAWSON, J. B., HOERNES S., SPETTELB. and W~,NKE H. (1984) Anorthositic oceanic crust in the Archean earth. Abstract: 15th Lunar Planetary Science Conference, Houston (unpaginated). JAQUES A. L., HALL A. E., SHERATONJ. W., SMITHC. B., SUN S.-S., DREW R. M., FOUDOULISC. and ELLINGSENK. (1989) Composition of crystalline inclusions and Cisotopic composition of Argyle and Ellendale diamonds. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 2, pp. 1029-1041. Biackwell Scientific (Geol. Soc. Austr. Spec. Pub. No. 14). JAQUES A. L., LEWIS J. D., SMITH C. B., GREGORYG. P., FERGUSON J., CHAPPELL B. W. and McCULLOCH M. T. (1984) The diamond-bearing ultrapotassic (lamproitic) rocks of the West Kimberly region, Western Australia. In Kimberlites I: Kimberlites and Related Rocks (ed. J. KORNPROBST), pp. 225--254. Elsevier. JAvov M. and PINEAU F. (1983) Stable isotope constraints on a model earth from a study of mantle nitrogen. Meteoritics 18,320-321. JAVOV M., PISEAU F. and ALLEGRE C. J. (1982) Carbon geodynamic cycle. Nature 300, 171-173.

493

JAVOYM., PINEAU F. and DELORMEH. (1986) Carbon and nitrogen isotopes in the mantle. Chem. Geol. 57, 41-62. JAVOYM., PINEAUF. and DEMAIFFED. (1984) Nitrogen and carbon isotopic composition in the diamonds of Mbuji Mayi (Zaire). Earth planet. Sci. Lett. 68, 399-412. KERRIDGE J. F (1985) Carbon hydrogen and nitrogen in carbonaceous chondrites: abundances and isotopic compositions in bulk samples. Geochim. cosmochim. Aeta 49, 170%1714. KIRKLEY M. B. and GURNEY J. J. (1989) Carbon isotope modelling of biogenic origins for carbon in eclogitic diamonds. Extended Abstract 28th International Geological Congress Workshop on Diamonds, pp. 40-43. KIRKLEY M. B., GURNEYJ. J., OTTER M. L. and HILL S. J. (1990) Modelling subducted crustal carbon sources of eclogitic diamonds (Abst.). LOS, Trans. Am. Geophys. Union 71(17), 644. KIRKLEY M. B., SMITH H. S. and GURNEY J. J. (1989) Kimberlite carbonates--a carbon and oxygen stable isotope study. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 1, pp. 264-281. Blackwell Scientific (Geol. Soc. Austr. Spec. Pub. No. 14). KOBELSKI B. J., GOLD D. P. and DEINES P. (1979) Variations in stable isotope compositions for carbon and oxygen in some South African and Lesothan kimberlites. In Kimberlites, Diatremes and Diamonds: Their Geology, Petrology and Geochemistry (eds F. R. BOYD and H. O. A. MEYER), Vol. 1, pp. 252-271. American Geophysical Union. KRAVTSOVA. I., KROPOTOVAO. I., VOYTOVG. I. and IVANOV V. A. (1975) Isotopic composition of carbon of diamonds and carbon compounds in pipes of the east Siberian diamond province. Translated from Dokl. Akad, Nauk SSSR, 223(1), 206-208. KYSER T. K., O'NEIL J. R. and CARMICHAELI. S. E. (1981) Oxygen isotope thermometry of basic lavas and mantle nodules. Contr. Miner. Petrol. 77, 11-23. MCCANDLESS T. E., KIRKLEY M. B., ROBINSON D. N., GURNEY J. J., GRIFFINW. L., COUSENSD. R. and BOYDF. R. (1989) Some initial observations on polycrystalline diamonds mainly from Orapa. Extended Abstract 28th International Geological Congress Workshop on Diamonds, pp. 47-51. MACGREGORI. D. and MANTONW. I. (1986) Roberts Victor eclogites: ancient oceanic crust. J. geophys. Res. 91(B14), 14063-14079. McKIRDY D. M. and POWELL T. G. (1974) Metamorphic alteration of carbon isotopic composition in ancient sedimentary organic matter: new evidence from Australia and South Africa. Geology 2, 591-595. MATTEY D. P., CARR R. H., WRIGHT I. P. and P1LLINGERC. T. (1984) Carbon isotopes in submarine basalts. Earth planet. Sci. Lett. 70, 196--206. I~f_ATI'EYD. P., TAYLORW. R., GREEN D. H. and PILL1NGER C. T. (1990) Carbon isotopic fractionation between CO 2 vapour, silicate and carbonate melts: an experimental study to 30 kbar. Contr. Miner. Petrol. 104, 492-505. MEYER H. O. A. and BOYD F. R. (1972) Composition and origin of crystalline inclusions in natural diamonds. Geochim. cosmochim. Acta 36, 1255-1273. MILLEDGE H. J., MENDELSSOHNM. J., SEAL M., ROUSE J. E., SWART P. K. and Pillinger C. T. (1983) Carbon isotopic variation in spectral type II diamonds. Nature 303,791-792. MOORE R. O. and GURNEYJ. J. (1989) Mineral inclusions in diamond from the Monastery kimberlite, South Africa. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 2, pp. 102%1041. Blackweli Scientific (Geol. Soc. Austr. Spec. Pub. No. 14). MUEHLENBACHS K. and CLArrON R. N. (1976) Oxygen isotope composition of the oceanic crust and its bearing on seawater. J. geophys. Res. 81, 4365-4369. NIER A. O. and GULBgANSENE. A. (1939) Variations in the

494

M. Kirkley et al.

relative abundance of the carbon isotopes. Am. Chem. Soc. J. 61,697-698. NISBETE. G., BICKL~M. J. and MARTINA. (1977) The mafic and ultramafic lavas of the Belingwe Greenstone Belt, Rhodesia. J. Petrol. 18,521-566. ONGLEY J. S., BASUA. R. and KYSERT. K. (1987) Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites; implications for petrogenesis and mantle metasomatism. Earth planet. Sci. Lett. 83, 80-84. OTTER M. L. (1989) Diamonds and their mineral inclusions from the Sloan diatremes of the Colorado-Wyoming State Line kimberlite district, North America. Ph.D thesis, University of Cape Town. OTTER M. L. and GORNEVJ. J. (1989) Mineral inclusions in diamonds from the Sloan diatremes, Colorado-Wyoming State Line kimberlite district, North America. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 2, pp. 10421053. Blackwell Scientific (Geol. Soc. Austr. Spec. Pub. No. 14). OTTER M. L., GURNEYJ. J. and MCCANDLESS T. E. (1990) The carbon isotope composition of Sloan diamonds. EOS, Trans. Am. Geophys. Union 71(17), 644. PEARSON D. G., DAVIESG. R., NIXON, P. H. and MILLEDGE H. J. (1989) Graphitized diamonds from a peridotite massif in Morocco and implications for anomalous diamond occurrences. Nature 338, 60-62. PINEAU F. and JAvoY M. (1983) Carbon isotopes and concentrations in mid-oceanic ridge basalts. Earth planet. Sci. Lett. 62, 239-257. P~NEAOF., JAvoY M. and BOa'nNGAY. (1976) 13C/12Cratios of rocks and inclusions in popping rocks of the MidAtlantic Ridge and their bearing on the problem of isotopic composition of deep-seated carbon. Earth planet. Sci. Lett. 29, 413-421. RICHARDSONS. H., ERLANKA. J., HARRISJ. W. and HARTS. R. (1990) Eclogitic diamonds of Proterozoic age from Cretaceous kimberlites. Nature 346, 54-56. RICHARDSON S. H., GURNEYJ. J., ERLANKA. J. and HARMS J. W. (1984) Origin of diamonds in old enriched mantle. Nature 310, 198-202. SCHIDLOWSKI M., HAYES J. M. and KAPLAN I. R. (1983) Isotopic inferences of ancient biochemistries: carbon, sulfur, hydrogen and nitrogen. In Earth's Earliest Biosphere (ed. J. W. SCHOPF), Chap. 7, pp. 149-186. Princeton University Press. SHEPPARD S. M. F. and DAWSON J. B. (1973) 13C/12C and D/H isotope variations in "Primary Igneous Carbonatites". Fortsch. Mineral. 50, 128-129. SHEPPARD S. M. F. and DAWSON J. B. (1975) Hydrogen, carbon and oxygen isotope studies of megacryst and matrix minerals from Lesothan and South African kimberlites. In Physics and Chemistry o f the Earth (eds L. H. AHRENS, J. B. DAWSON, A. R. DUNCAN and A. J. ERLANK), Vol. 9, Chap. 48. pp. 747-763. Pergamon Press. SMIRNOVG. I., MOFOLOM. M., LEROTHOLIP. M., KAMINSKY F. V., GAUMOV E. M. and IVANOVSr~VAI. N. (1979) Isotopically light carbon in diamonds from some kimberlite pipes in Lesotho. Nature 278, 630. SMITH C. B., GURNEV J. J., H A m s , J. W., OTTER M. L., KmKLEY M. B. and JAGOUTZE. (1989) Nd and Sr isotope

systematics of large eclogite and lherzolite paragenesis single diamonds, Finsch and Kimberley Pool, R.S.A, Extended Abstract 28th International Geological Congress Workshop on Diamonds, pp. 102-104. SOBOLEVN. V. (1972) Petrology of xenoliths in kimberlitic pipes and indications of their abyssal origin. Proceedings 24th International Geological Congress, pp. 297-302. SOaOLEV N. V. (1984) Crystalline inclusions in diamonds from New South Wales, Australia. In Kimberlite occurrence and Origin: A Basis for Conceptual Models in Exploration (eds J. E. GLOVESand P. G. HARRIS), Pubis Geol. Dep. and Unversity, University ofW. Aust., No. 8, pp. 213-226. SOBOLEV N. V., GALIMOVE. M., IVANOVSKAYAI. N. and YEFIMOVAE. S. (1979) Isotope composition of carbon of diamonds containing crystalline inclusions. Translated from Dokl. Akad. Nauk SSSR 249(5), 1217-1220. SOBOLEV V. S. and SOBOLEV N. V. (1980) New proof of submersion to great depths of eclogitized rocks of the earth's crust. Translated from Dokl. Akad. Nauk SSSR 250(3), 683--685. STRECKEISEN A. (1976) To each plutonic rock its proper name. Earth Sci. Rev. 12, 1-33. SWARTP. K., GRADYM. M. and PILLINGERC. T. (1983a) A method for the identification and elimination of contamination during carbon isotopic analyses of extraterrestrial samples. Meteoritics 18, 137-154. SWARTP. K., PILLINGERC. T., MILLEDGEH. J and SEAL M. (1983b) Carbon isotopic variation within individual diamonds. Nature 303,793-795. TAYLOR H. P. JR, FR~CHEN J. and DEGENS E. T. (1967) Oxygen and carbon isotope studies of carbonatites from the Laacher Sea District, West Germany and the AID6 District, Sweden. Geochim. cosmochim. Acta 31, 407430. TINGLE T. N. (1989) More on the mantle carbon flux. EOS, Trans. Am. Geophys. Union 70(48), 1513-1514. TINGLE T. N., GREEN H. W. and FINNERTY A. A. (1989) Experiments and observations bearing on the solubility and diffusivity of carbon in olivine. In Kimberlites and Related Rocks (ed. J. Ross), Vol. 2, pp. 922-931. Blackwell Scientific (Geol. Soc. Aust. Spec. Pub. 14). TURCOTrE D. L. (1984) Mantle convection and geochemical cycles. Proceedings o f the 27th International Geological Congress, 11,449-466. VALLEY J. W. and O'NEIL J. R. (1981) 13C/12C exchange between calcite and graphite: a possible thermometer in Grenville marbles. Geochim. cosmochim. Acta 45,411419. VINOGRADOV A. P., KROPOTOVAO. I. and USTINOV V. I. (1965) Possible sources of carbon in diamonds as indicated by the C12/C 13 ratios. Geochem. Int. 2, 495-503. VINOGRADOV A. P., KROPOTOVAO. I., ORLOV Yu. L. and GmNENKO V. A. (1966) Isotopic composition of diamond and carbonado crystals. Geochem. Int. 3, 1123-1125. WEIS e. L., FRIEDMAN I. and GLEASON J. P. (1981) The origin of epigenetic graphite: evidence from isotopes. Geochim. cosmochim. Acta 45, 2325-2332. WlCKMAN F. E. (1956) The cycle of carbon and the stable carbon isotopes. Geochim. cosmochim. Acta 9, 136-153. WILDINO M. C. (1990) A study of diamonds with syngenetic inclusions. Ph.D. thesis, University of Edinburgh.