A preliminary study of 15N14N in octahedral growth form diamonds

CHEMICAL GEOLOGY 15;OTOPE GI','OS,'('IE.V('E

ELSEVIER

Chemical Geology 116 (1994) 43-59

A preliminary study of SN/14N in octahedral growth form diamonds S t u a r t R. Boyd '~, C.T. P i l l i n g e r Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK

(Received February 15, 1993; revision accepted December 31, 1993 )

Abstract The ~ ~3C, ~ 'SN and nitrogen concentrations have been determined in 43 octahedral diamonds from southern Africa, Australia and North America. The sample set included locations such as Finsch (South Africa), a mine which produces diamonds having fi~3C-values close to - 5%0, and Argyle (Australia), a mine which produces ~3Cdepleted diamonds. The nitrogen within "high-O ~3C" diamonds (d ~3C = - 6.4 to - 2.9%0) was generally depleted in J5N relative to atmospheric nitrogen ( ~ S N = - 12.3 to + 5.9%o: mode ~ -4%o), similar to the results which have been obtained previously from fibrous diamond. It is believed that the volatiles from which these diamonds grew are primitive, being derived from a source located beneath the continental lithosphere, which may have changed little (in terms of~J3C and ~'SN) since the mid-Archean. The "low-g J3C" diamonds (~ ~3C = - 19.4 to - 9.5%o) contained nitrogen generally enriched in J5N relative to air ( ~ S N = - 3.0 to + 16.4%0: mode ~ + 5%0). It is suggested that the major carbon isotope heterogeneity within the mantle, as represented by diamonds such as these, is located in deep, mechanically unstable regions of the continental lithosphere. The isotopic compositions of both C and N are consistent with this heterogeneity resulting from the subduction of crustal material; however, isotope fractionation related directly to diamond growth cannot be ruled out.

I. Introduction D i a m o n d s , originating f r o m depths in excess o f 150 km, carry valuable i n f o r m a t i o n regarding the nature and the isotopic c o m p o s i t i o n o f volatiles in the deep sub-continental mantle. Indeed, d i a m o n d is the only " c o m m o n " m a n t l e mineral to contain high concentrations o f nitrogen b o u n d

[MB]

within the crystal lattice (Kaiser a n d Bond, 1959 ). A knowledge o f the isotopic c o m p o s i t i o n o f nitrogen within the m a n t l e is necessary for the understanding of the origin of the nitrogen within the external reservoirs ( a t m o s p h e r e + crust) and, in addition, the isotopic c o m p o s i t i o n of the nitrogen present in d i a m o n d s can p r o v i d e additional constraints on the origin of the carbon, for which there is not a general consensus (see Kirkley et al., 1991 ).

~Present address: Laboratoire de G6ochimie des Isotopes Stables, Universit6 de Paris VII, 4 place Jussieu, F-75251 Paris Cedex 05, France.

0009-2541/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0009-2541 ( 94 )00008-V

44

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

I.I. Nitrogen within the mantle and external reservoirs: an isotopic imbalance? Atmospheric N2 is isotopically homogeneous (Mariotti, 1983) and has a ~SN-value of 0 % o since it is used as the standard for reporting isotopic measurements of nitrogen. Nitrogen within sediments (e.g., Peters et al., 1978; Sweeney et al., 1978 ), metasediments (Haendel et al., 1986; Bebout and Fogel, 1992 ) and igneous rocks with a sedimentary contribution (S.R. Boyd et al., 1993a) is normally enriched in 15N relative to air, having ~SN-values between + 1 and + 17%o: see S.R. Boyd et al. (1993a) for a simple model of the cycling of nitrogen within the continental crust. So the average ~SN-value of the external reservoirs must be positive. If the nitrogen present within the external reservoirs had been simply outgassed from the mantle then one would expect that nitrogen present within the mantle would also have a positive ~SN-value. The first determinations of ~SN in diamonds suggested that this was not the case (Javoy et al., 1984). Diamonds commonly contain nitrogen in concentrations between 500 and 1500 ppm (e.g., Javoy et al., 1984; S.R. Boyd et al., 1987, 1992) and most have fi~3C-values close to -5%0 (see Galimov, 1991; Kirkley et al., 1991 ) similar to other samples from the upper mantle (e.g., Javoy et al., 1986; Mattey, 1987). Thus one may expect that the ~SN-value of the nitrogen present within diamonds with ~3C-values close to -5%0 would be representative of the nitrogen within the upper mantle. In the first systematic study of the fi~SN-values of diamond, Javoy et al. (1984) found that nitrogen within large crystal aggregates from Zaire, which had ~3C-values close to - 5°/oo,was markedly depleted in ~SN relative to the external reservoirs, the average ~15N-value being close to - 5%0. S.R. Boyd et al. ( 1987 ) confirmed the results for Zaire and went on to show that ~SN depletion (~15N=-8.7 to - 1.7°/oo) was apparently a global phenomenon in diamonds with an upper-mantle carbon isotope signature (S.R. Boyd et al., 1992 ). In order to explain the apparent nitrogen isotope imbalance, Javoy et al. (1986) proposed a

heterogeneous accretion model for the Earth. They suggested that the bulk of the Earth was composed of material akin to the enstatite chondrites (depleted in ~SN) and that a large proportion of the nitrogen now present in the atmosphere and crust was added by carbonaceous chondrites (enriched in t 5N) during the latter stages of planetary accretion. These initial nitrogen isotope studies of diamond had concentrated on fibrous diamonds which are unusual in that they may be comparatively young ( < 350 Ma) and related to kimberlite magmatism (see S.R. Boyd et al., 1994 in this issue). Thus there was the possibility that they were not giving a representative ~SN-value of mantle nitrogen. The most common diamonds are octahedral. By this we refer to the mode of growth, addition of carbon to the { 111 } faces, and not the external shape which may be modified by resorption. One of the principal objectives of this study was to determine the ~15N-value of the nitrogen present within octahedral growth-form diamonds, having ~3C-values characteristic of the upper mantle, for comparison with the resuits obtained from fibrous diamonds.

1.2. On the origin of carbon in diamonds Although most diamonds have 313C-values characteristic of the upper mantle (close to - 5%o), a significant number have "anomalous" values. The variability in 313C is not random but restricted to certain genetic classes of diamond (Galimov, 1984, 1991; KJrkley et al., 1991 ) and the so-called "eclogitic" paragenesis' diamonds have attracted the most attention. "Eclogitic" diamonds are of octahedral growth form, have 613C-values within the range - 34.4 to +3%0 (Kirkley et al., 1991 ) and contain silicate inclusions which arc: characteristic of eclogites (e.g., Meyer, 1987 ). In contrast to the eclogitic diamonds are the more common "peridotitic paragenesis" which are again of octahedral growth form but contain silicate inclusions characteristic of peridotites (e.g., Meyer, 1987). "Peridotitic" diamonds generally have less variable carbon isotope compositions with fil3C-values between - 10 and - 1o/oo,the mode

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (I 994) 43-59

in the distribution being close to - 5%o. It is believed that peridotitic diamonds have formed from primitive carbon (e.g., Kirkley et al., 1991 ). Perhaps the most popular model to explain eclogitic diamonds is that they have resulted from the recycling of organic carbon back into the mantle (e.g., Kesson and Ringwood, 1989; Kirkley et al., 1991 ). Although others have repeatedly argued that the observed variations in ~13C could be reflecting primordial heterogeneities within the mantle (e.g., Deines et al., 1991 ) or that the variations were the result of high-temperature isotope fractionation processes (Galimov, 1984, 1991; Javoy et al., 1986). Although previous studies concentrated on fibrous diamonds, preliminary results suggested that '3C depletion in octahedral diamonds may be accompanied by marked ~5N enrichment (Javoy et al., 1984; S.R. Boyd et al., 1987, 1992). Any coupling between the ~ ~3C- and ~' 5N-values of octahedral diamonds would bear directly on existing models for the origin of ~3C-depleted diamonds. A second aspect of this study was to determine the ~' 5N-value of the nitrogen present in '3C-depleted octahedral diamonds.

2. Samples studied and analytical techniques Brief descriptions of the diamonds studied are provided in Table 1. Generally, the samples were of small size ( < 10 mg) and only one sample (Fin#7) was sectioned by laser (S.R. Boyd et al., 1987 ) prior to analysis. The techniques used for the determination of ~3C, ~lSN and nitrogen concentration have been detailed previously (S.R. Boyd et al., 1987, 1988, 1993b; Wright et al., 1988 ). Typical errors were _+0.1%o for ~'3C, _+0.5%0 for ~lSN and _+15°/0 for nitrogen abundance. Where possible infrared (IR) absorption spectra were obtained from the diamonds prior to the other analyses although a number of the specimens were analysed for J 13C, ~ 15N and nitrogen abundance before the IR facility became available. The IR absorption spectra provide information regarding the thermal history of a diamond (cf. section 2.3 in S.R. Boyd et al., 1994 in this issue, and references therein). Since dia-

45

Table 1 Details of the samples studied Sample

Origin

Colour ~

Shape b

El-El2

Finsch mine, South Afria Finsch mine, South Africa Finsch mine, South Africa Finsch mine, South Africa Premier mine, South Africa Premier mine, South Africa Premier mine, South Africa Jagersfontein, South Africa Jagersfontein, South Africa Jagersfontein, South Africa Williamson mine, Mwadui, Tanzania Arkansas, U.S.A. Arkansas, U.S.A. Argyle, Western Australia

b

rd, i

gn

rd

c

rd

b

i

b, c

o

b

rd

y

m

g

i

b

rd

c

i

c

f

c

i

gn c

rd i, rd

Dll F#6 Fin#7 Pol-Po3 Pdl P#1

G I-G6 J2 J5 W1-WIO AK1, 3-5 AK2 A4-A24

For further information on each of the intrusions see Dawson (1980), Mitchell (1986) and Nixon (1987). aColours: c=colourless; b=brown; gn=green; y=yellow; g=grey. bShapes: i=irregular; o=octahedra; rd=rounded dodecahedra; m = macle; f= fragments of diamonds (original shape not known).

monds can be zoned strongly (e.g., S.R. Boyd et al., 1992), the ~13C_ and ~'SN-values and nitrogen concentration ( _+IR) were all measured on the same fragment (or block in the case of Fin#7) from a sample.

3. Results The results are given in Table 2. Fig. 1 compares the ~3C-values obtained in this study (Finsch, Premier, Jagersfontein and Argyle) to those which have been published previously for diamonds of known paragenesis. For Finsch and

S.R. Boyd, C T. Pillinger /Chemical Geology 116 (1994) 43-59

46

Table 2 (continued)

Table 2 Results

Sample Sample

~13C

6ZSN

(%ov s .

(%ov s .

P D B

air)

)

[N ] (ppm)

Type

613C (%0 vs. PDB)

6'5N (%o vs. air)

[N] (ppm)

Type

+5.8 + 8.0 +5.2 + 5.5 - 1.4 2.1 +3.1 + 2.6 + 16.4 + 16.6

660 600 1,570 1,630 630 800 450 360 470 640

IaAB+P IaAB+P IaAB+P IaAB+P IaAB+P IaAB + P IaA+P IaA+P IaAB + P IaAB+P

+2.1 -0.8 + 1.8 + 5.2 45.1 +5.7 +9.0 +9.4 +12.1 - 1.8 -0.2 + 8.7 + 7.3 + 8.0 - 1.9 +8.0

16 140 110 560 480 1,980 410 700 175 130 210 60 30 200 35 130

IaB IaB IaB+P IaB+P IaB+P laB laB laB IaA laB laB laB -

Arkansas, U.S.A.: Finsch mine, South Africa: El E2 E4 E6 E7 E8

EIO Ell E12 DII Dll F¢~6 F#6 F#6 Fin#7-A Fin#7-B Fin#7-C Fin#7-D Fin#7-E Fin#7-F Fin#7-G

-3.5 - 3.0 - 3.4 -4.1 -6.4 -4.7 -5.1 -4.0 -3.3 -6.2 -6.0 -5.3 -4.5 -4.6 -5.3 -5.3 -5.1 -5.3 -5.3 -5.2 -5.2

-7.4 - 3.7 - 3.4 -4.9 +5.9 -4.3 -6.4 -2.7 - 10.8 +4.6 +2.4 -4.6 -5.1 -2.8 -10.5 -8.5 -1.7 -2.0 -1.1 -5.1 -8.6

340 70 400 340 80 400 470 260 400 560 70 380 430 450 380 580 140 410 384 266 470

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. IaA(B) +P IaA(B)+P IaAB+P IaAB+P IaAB+P IaAB+P IaA(B)+P

260 250 5 140 460 440 350

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

140 3 410 20 640 130

n.d. n.d. n.d. n.d. n.d. n.d.

700 600 560 620 900 720 30 520

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

Premier mine, South Africa: Pol Pol Po2 Po3 Pdl P~I P#I P~:I

-5.1 -5.3 -3.6 - 2.9 -5.2 -4.9 -5.0 -4.8

-1.3 -2.3 -5.5 - 1.0 -6.3 - 12.3 -11.7 - 10.7

Williamson mine, Mwadui, Tanzania: WI W2 W3 W7 W8

WIO

-3.4 -3.3 -4.6 -4.7 -4.7 -4.4

-7.7 - 1.6 + 1.3 -3.3 +0.5

Jagersfontein, South Africa: G1 G2 G3 G4 G5 G6 J2 J5

-18.2 -17.2 17.0 - 19.4 18.0 -16.6 -16.8 -4.8

+5.6 +4.0 + 3.0 +2.9 +2.5 +3.8 -3.0 -2.7

AK1 AK1 AK2 AK2 AK3 AK3 AK4 AK4 AK5 AK5

-5.7 -17.0 -5.6 -5.5 - 11.3

Argyle, Western Australia: A4 A8 A8

AI5 AI5 AI6 A16 AI6 AI8 AI8 A18 A20 A20 A24 A24 A24

-10.5 -11.0 - ll.4 -9.5 -9.8 -12.8 -12.0 - 12.1 -14.5 - 13.4 -12.8 - 10.8 - 10.8 - 12.0 - 10.3 -12.2

T h e A r k a n s a s samples were first analysed on J u n e 18, 1985 w h e n there was no facility on the N extraction line for the recovery o f CO2 hence the lack o f 6~3C data for one set o f analyses. T h e samples were subsequently re-analysed on M a y 24, 1987 a n d it can be seen that there is generally good agreem e n t between the 6 ~5N-values and N concentrations for both sets o f analyses, suggesting m i n i m a l zoning in these diam o n d s . All other samples were analysed between October 17, 1985 a n d July 13, 1987. n.d. = not determined; - = no data.

Premier (Deines et al., 1984) there is no difference in the 6~3C-values of the peridotitic and eclogitic parageneses whereas for Jagersfontein (Deines et al., 1991 ) and Argyle (Jaques et al., 1989 ), 13C depletion is commonest amongst the eclogitic paragenesis diamonds. Thus most of our inclusion-free samples cannot be placed into a paragenesis on the basis of 6~3C although it is probable that most of the diamonds from Argyle

47

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994)43-59

40-

Finsch

Jaeersfontein 5 Peridotitic

-~

g 40-

~a

Eclogitic E z

fl[q

[1

5

Eclogitic

Z

°

M 5 This study

, 40

Premier

25

Peridotitic

Argyle Peridotitic

rat%

40

~.o~ E Z

.ID

E z

!

Eclogitic

25

c L Eclogiti

5 This study

This study I I

t.. I

-20

-10

0

8~3C

-2~0

I

-1'0

0

8~3C

Fig. 1. Histograms comparing our c~3C-values to those published previously: Finsch and Premier (Deines et al., 1989); Argyle (Jaques et al., 1989); and Jagersfontein (Deines et al., 1991 ). For the current sample set, if multiple analyses revealed that the diamond was zoned in c~13Cthen this diamond will appear more than once on the histogram. For example, A24 appears twice.

belonged to the eclogite paragenesis. However, what is most important is that there was nothing unusual about the sample set. Similarly for these four locations, the IR absorption spectra (where measured) and nitrogen contents were similar to those that have been reported previously (Deines et al., 1984, 1989, 1991; Harris and Collins, 1985; Jaques et al., 1989). In particular, a significant number of Type laB diamonds were found to be present in the suite of Argyle diamonds (see Table 2 ). The 5~3C- and 5tSN-values are plotted in Fig. 2 and those obtained from the diamond that was laser-sectioned are depicted in Fig. 3. The diamonds with the higher t~13C-values (between - 6 . 4 and -2.9%o), hereafter referred to as

"high-513C,, are characterised by nitrogen which is generally depleted in 15N relative to atmospheric nitrogen. In contrast the diamonds with lower 513C-values (between - 1 9 . 4 and - 9.5%o), hereafter referred to as "1ow-513C", contain nitrogen generally enriched in 15N relative to atmospheric nitrogen. Although there is a distinct separation of these diamonds in terms of 513C (Fig. 2. ), there is a notable overlap in 5 ~SN. Since nitrogen is the trace element in diamond (and is unstable within the lattice) the chances of N-isotope fractionation occurring during growth are much higher (see S.R. Boyd et al., 1994 in this issue). Consider the samples from Finsch: the range in 513C is only 3.2%0; however, 515N varies by 16.4%o. Thus when using nitro-

48

S.R. Boyd, C.T. Pillinger /Chemical Geology 116 (I 994) 43-59

Finsch, Premier, Williamson,Jagersfontein(1 sample), Arkansas (3 samples) i -5 Fibrous ~ diamond

813C

~

Key O Finsch Premier Williamson O Jagersfontein [] Arkansas + Argyle

-15 -

-25

~-

-20

~

~++

/ ++++ (

~

++

+ -t:~

E'~ /

I Argyle,Jagersfontein(7 samples), I Arkansas(2 samples)

[

-10

_

I

0

+10

+20

815N Fig. 2. Summary plot of the isotope data. Included is the region defined by fibrous diamonds (37 samples; Boyd et al., 1987, 1992; S.R. Boyd, 1988 ). The dotted line is the N isotopic of the atmosphere (0%0). The high-dl3C samples are generally depleted in ~5N relative to air and are quite similar to fibrous diamonds, whereas the low-c~tSN samples are generally enriched in tSN relative to air.

gen isotopes to characterise a diamond population it will be the mode in the distribution which will be important. This is well known for carbon: there is overlap in the ~3C-values of Finsch peridotitic and Argyle eclogitic diamonds; however, the modes in the distribution show that these populations are clearly distinct (Fig. 1 ). Fin#7 (Fig. 3) is of particular interest since both the optical appearance (transparent plate) and the results for ~3C (-5.2___0.1%0) would suggest that the diamond was homogeneous. However, the zoning in both ~ ~5N and the nitrogen aggregation state showed that this was not the case. The central part of the diamond contained nitrogen that was quite highly aggregated (Type IaAB) whilst in the outer zone nitrogen was only mildly aggregated (predominantly Type IaA). Also the J ~5N-values of the two zones differed; in the centre the values were between - 2.0 and - 1.1 o~ whereas the values towards the edge were lower, being between - 10.5 and - 8.5%o. These results indicate that this diamond grew in at least two stages. However, unlike coated diamonds s.s. (see S.R. Boyd et al., 1994 in this issue), the presence of platelets (Fig. 3 ) and also

some limited development of the IaB aggregate in the outer zone demonstrates that the latter stage of diamond genesis occurred a significant period of time prior to the eruption of the kimberlite. Dating of mineral inclusions in Finsch diamonds have revealed at least two major periods of diamond genesis, both of which preceded the eruption of the kimberlite (120 Ma, in Nixon, 1987 ): peridotitic diamonds are of Archean age (3.3 Ga; Richardson et al., 1984) whereas the eclogitic diamonds formed during the Proterozoic ( 1.6 Ga; Richardson et al., 1990). Whether the interior of Fin# 7 formed during the earlier event, acting as a seed for renewed growth in the Proterozoic, or whether it is simply reflecting further periods of diamond growth in the source regions for Finsch diamonds is open to question, although the results show that, like coated diamonds (S.R. Boyd et al., 1994 in this issue), detailed studies of internal zoning can provide information which is otherwise lost. Similarly the results obtained from one of the samples from Argyle (A18) suggested multi-

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

49

discussion, we first compare and contrast the growth of these two varieties of diamond.

-2 -4 -6 ~13C

-5.0 ~ -5.5

C

~15 N

-8 -10

600

[N] ppm 2oo4°° t IR spectralvariations A

©

E©

<

B 1

2

3

4

5

6

7

4mm Fig. 3. The covariation of Ot3C, 615N, nitrogen abundance and nitrogen aggregation state across sample Fin#7. For an account of the significance of IR absorption spectra see Boyd et al. ( 1992 ), Evans and Qi (1992) and Boyd et al. ( 1994 in this issue). The data obtained by FTIR were reduced by plotting the nominal peak heights of the IaA (A), laB (B) and platelet (P) absorptions. The reduced data, a qualitative profile of the covariation of the A- and B-aggregate nitrogen and platelets are shown in the lowest part of the figure.

stage growth; one fragment from the edge of the diamond was Type IaA with a ¢jlSN-value of -0.2%0 whilst a second fragment from the interior was Type laB with a 3~SN-value of + 12.1%o.

4. Diamond genesis: differences between fibrous and octahedral

Since the largest data base for nitrogen in diamonds is for the fibrous variety, one of the principal aims of this study was to compare the ~SNvalues of nitrogen in octahedral diamonds, having ~13C-values close to -5%0, to that present within fibrous diamonds. In order to simplify the

4.1. Octahedral

Within a single kimbedite, there may be several sub-populations of octahedral diamonds which can be distinguished by isotopic composition, inclusion mineralogy, nitrogen content and nitrogen aggregation state or age (e.g., Deines et al., 1989). This suggests numerous, independent diamond growth events, within comparatively small regions of the mantle when compared to the total volume of the mantle sampled by the kimberlite. It is apparent that each event was unrelated to kimberlite magmatism since dating of mineral inclusions (e.g., Richardson et al., 1984, 1990) and the IR absorption spectra (Evans and Qi, 1982 ) both demonstrate that octahedral diamonds are xenocrysts in their kimberlite hosts. Available data suggest that these diamonds range in age from 3.3 to 0.9 Ga (e.g., Richardson et al., 1984, 1990; Richardson, 1986; Burgess et al., 1989, 1992) The nature of the medium from which octahedral diamonds grow is not as well known as that for fibrous diamonds although the internal zoning which may be present within octahedral diamonds (Harrison and Tolansky, 1964) is most easily explained by growth from volatilerich melts/fluids as opposed to growth in the solid state. This internal zoning also reveals that the growth of a single octahedral diamond may have been very complicated and protracted. Periods of growth may have been interspersed by periods of partial resorption, resulting in truncated growth zones (Harrison and Tolansky, 1964). A significant period of time separating two periods of growth can be detected by determining the nitrogen aggregation state in the different zones (see Fig. 3 ). 4.2. Fibrous

Within a given kimberlite there is only one obvious generation of fibrous diamond probably linked to kimberlite magmatism (see S.R. Boyd et al., 1994 in this issue). Thus the box depicted

50

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

in Fig. 2 will be for diamonds of Phanerozoic age, since those studied by S.R. Boyd et al. (1987, 1992) came from kimberlites which had eruption ages of between 350 and 71 Ma. Fibrous diamonds grew from COz-H20-rich (Chrenko et al., 1967), ultrapotassic (Navon et al., 1988) fluids believed to be derived from within the convecting mantle (S.R. Boyd et al., 1987, 1992). Compared to octahedral diamonds, this class of diamond grew rapidly hence the fibrous nature, high micro-inclusion content and comparatively uncomplicated internal structure.

5. Discussion

5. I. High ~13C.Finsch, Premier and Williamson, Jagersfontein (1 sample) and Arkansas (3 samples) The O15N-values obtained from these diamonds were predominantly negative, similar to fibrous diamonds (Fig. 2 ). The greater spread in the isotope data (both C and N) for octahedral diamonds (see also Fig. 1 ), compared to fibrous diamonds, is probably due to the more complicated conditions of growth outlined above: isotopic fractionation will be favoured during slow growth from a limited supply of volatiles (see S.R. Boyd et al., 1994 in this issue ). The simplest conclusion is that the volatiles for both classes of diamond were derived from a similar mantle source (in terms of dl3C and 3tSN), believed to be the convecting mantle (S.R. Boyd et al., 1992). The similarity in d~3C between oceanic basalts (e.g., Javoy et al., 1986; Mattey, 1987), hypabyssal facies Group I kimberlite carbonates (Sheppard and Dawson, 1975; Kirkley et al., 1989 ), all fibrous diamonds (S.R. Boyd et al., 1992) and most octahedral diamonds (Galimov, 1991; Kirkley et al., 1991) suggests that carbon in the convecting mantle is globally quite homogeneous (fil3C ,-,-5O/oo). Given that this value is also characteristic of the oldest diamonds (e.g., Finsch peridotitic paragenesis; Fig. 1 ) has led to suggestions that homogenisation was complete very early in Earth

history (S.R. Boyd, 1988; Kirkley et al., 1991 ). The similarity in ~15N between fibrous diamonds, which are believed to be of Phanerozoic age (S.R. Boyd et al., 1992), and the current results for Finsch and Premier (Fig. 2 ) leads to the possibility that the same is true for nitrogen. However, unlike carbon, there is not a simple relationship between the nitrogen isotope results obtained from diamonds and those which have been obtained from oceanic basalts. Table 3 lists the data currently available for oceanic basalts together with a harzburgite for which 613C was also available ( - 4 . 9 to -4.0%o). The data for MORB are dominated by the results of Exley et al. (1987) and, unlike diamonds, the ~15N-values were generally positive: + 7.5 + 1% (for Atlantic and Pacific) although two samples from the Indian Ocean were negative. For a very gasrich mid-ocean ridge basalt (MORB) (Atlantic), Javoy and Pineau also found negative values with an average of - 3.6%o and the harzburgite (Nadeau et al., 1990) is in the range of fibrous diamond (see Fig. 2). At present we are unsure as to why both the homogeneity and negative values suggested by the diamond data are not more generally reflected in the data for MORB. There is a suggestion in the data that the samples with the higher concentrations have the more negative values (Table 3 ) and it cannot be ruled out that there has been some fractionation during degassing and eruption onto the sea-floor (Javoy et al., 1986). This can only remain as speculation until work similar to that which has been performed on the partitioning of carbon isotopes between gaseous and dissolved species in basalts (cf. Javoy et al., 1978; Mattey et al., 1990; Mattey, 1991; Javoy and Pineau, 1991; Blank et al., 1993; Pineau and Javoy, 1994) has been repeated for nitrogen. The new data are thus compatible with the idea that nitrogen within the deep mantle is indeed depleted in 15N relative to the external reservoirs, lending support to the heterogeneous accretion model of Javoy et al. (1986). However, it must be noted that the oldest diamonds are only 3.3 Ga (Richardson et al., 1984), hence the nitrogen in diamonds will only be representative of the mantle after the formation of the core.

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

51

Table 3 Compilation o f nitrogen isotope data that have been obtained from oceanic basalts and a harzburgite Sample

[N ] (ppm)

31 sN ( % vs. air)

Reference

OIB (Hawaii) OIB (Hawaii) MORB (n = 2) OIB (Hawaii) MORB (A, P ) ( n = 6 ) MORB (I.O.) ( n = 2 ) OIB (Hawaii) MORB (A) Harzburgite

~ 1 0.4 0.9, 1.4 0.2-1.2 0.2-2.1 0.3, 0.4 6 12 8

+ 17 - 0.4 - 0.1, 0.8 + 12.8 to + 15.5 +7.5 _+ 1.0 - 4 . 5 , - 1.9 - 1.1 -3.5 - 5.0

Becker and Clayton (1977) Sakai et al. (1984) Sakai et al. (1984) Exley et al. (1987) Exley et al. (1987) Exley et al. (1987) Javoy et al. ( 1986 ) Javoy and Pineau ( 1991 ) Nadeau el al. (1990)

OIB = ocean island basalt; MORB = mid-ocean ridge basalt; n = number of samples analysed; A = Atlantic; P = Pacific; I.O. -- Indian Ocean.

Studies of iron meteorites have shown that they can contain significant amounts of nitrogen (up to 80 ppm) of highly variable isotopic composition ( 6 1 5 N = - 9 6 to +156°oo; Franchi et al., 1993; Prombo and Clayton, 1993) although Prombo and Clayton (1993) relate the variations in ~ ~5N to cosmochemical processes rather than processes occurring during chemical fractionation on the meteorite parent bodies. It is thus quite possible that a significant proportion of the total nitrogen originally accreted is presently within the core, whether the 815N-value of the residual nitrogen within the mantle was affected remains open to question. At present the best we can say is that a nitrogen isotope imbalance between the "post-core" mantle and external reservoirs is either indicating heterogeneous accretion or only minor outgassing of mantle nitrogen after core formation. Both imply that the bulk of nitrogen in the atmosphere was acquired at a very early stage. 5.2. Zow-t~13C diamonds: Argyle, Arkansas (2 samples) and Jagersfontein (7 samples)

The "low-613C" diamonds from all three locations show a marked spread in 8~5N ( - 3 . 0 to +16.6%o) and unlike the "high-813C'' diamonds, the majority of the samples were enriched in ~5N relative to air, similar to results obtained for octahedral diamonds (cores of coated stones) from Zaire (S.R. Boyd et al., 1987),

Botswana (S.R. Boyd et al., 1992) and Angola (S.R. Boyd et al., 1992). Similarly the only 13Cdepleted diamond studied by Javoy et al. (1984) was also enriched in ~5N. We first consider where in the mantle these diamonds may occur.

5.2.1, A deeper mantle origin for 13C-depleted diamonds The inclusion study of Deines et al. (1991) suggested that many of the ~3C-depleted diamonds from Jagersfontein originated from depths greater than those usually associated with diamond genesis: ~ 150 km (e.g., Meyer, 1985 ). Indeed Deines et al. ( 1991 ) suggested that some of the samples originated from deep within the asthenosphere (400 km). A deep origin for '3Cdepleted diamonds is also consistent with the nitrogen aggregation state of Argyle diamonds in relation to their "mantle residence time". Harris and Collins (1985 ) were the first to report that the nitrogen present in Argyle diamonds is severely aggregated with there being an unusual amount of Type IaB diamonds (see also Table 2). The extent of nitrogen aggregation in a diamond depends mainly upon the temperature of the mantle source region and the length of time that it was in the mantle (Evans and Qi, 1982). The degree of aggregation of some of the diamonds from Argyle suggested storage at 1400 °C for a length of time approximately equal to the age of the Earth (Harris and Collins, 1985). However, equilibration temperatures for Argyle

52

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

diamonds range between 1085 ° and 1575 °C with an average of only ~ 1250°C (Griffin et al., 1988). Furthermore, the diamonds were in the mantle for only ~400 Ma, the difference between the age of the diamonds and the eruption of the Argyle lamproite (Richardson, 1986). Harris and Collins (1985) suggested that some process had occurred within the source region of Argyle diamonds to "speed up" the nitrogen aggregation process. It is known that vacancies produced by radiation damage can lead to enhanced rates of nitrogen aggregation (Collins, 1978, 1980). A second way of producing vacancies within the lattice is by the plastic deformation or shearing of diamonds. If the diamonds in the Argyle source regions had suffered plastic deformation, during storage within the mantle, then the rate of nitrogen aggregation may have been enhanced and a possible explanation is that the Argyle diamonds formed in deep, mechanically unstable regions of the mantle.

5.2.2. Asthenosphere or continental lithosphere? Deines et al. ( 1991 ) went on to suggest that the 13C-depleted diamonds from Jagersfontein characterised the carbon in the asthenosphere beneath Jagersfontein ( ~ - 20%o). If so, then the similarity in the results obtained from ~3Cdepleted octahedral diamonds from Jagersfontein, Zaire, Botswana, Arkansas and Argyle would lead to the conclusion that ~3C depletion is widespread in the asthenosphere beneath Africa and is also present beneath America and Australia. This view seems difficult to reconcile with the carbon isotope results that have been obtained from other upper-mantle samples, which suggests that carbon within the convecting mantle has a restricted range in ~laC (see above). It seems difficult to envisage how large volumes of the convecting mantle could be characterised by ~3C-values between - 10 and - 20%0 and never be seen in upper-mantle samples other than diamonds. For example, why - - when an ocean such as the Atlantic opened - - was there not upwelling of this material to the mid-ocean ridges, which give t~13C-values close to -5°oo, and not between - 10 and - 20%o?

Thus we would prefer to locate ~3C-depleted diamonds within deep, mechanically unstable continental lithosphere in which they may suffer plastic deformation during storage. For Argyle, a source within continental lithosphere is also consistent with models regarding the origins of the Western Australian lamproites (McCulloch et al., 1983 ) and the 875r/86Srinit of eclogitic inclusions from Argyle diamonds (Richardson, 1986). However, we acknowledge that at Jagersfontein the inclusion data do suggest an asthenospheric origin for many of the ~3C-depleted diamonds (Deines et al., 1991 ).

5.2.3. On the origin ofl3C-depleted diamonds There are essentially three models, that have been proposed to account for the existence of the large range in the t~13C-values of diamonds, which have been reviewed recently by Kirkley et al. ( 1991 ). It has been suggested ( 1 ) that the variations may be reflecting primordial heterogeneities that have survived since the Earth's accretion; (2) that they are due to the recycling of crustal material; and (3) that the carbon isotopes were fractionated within the mantle. Because the subject has been reviewed recently we only provide brief outlines of each model and then indicate whether our data are consistent with the model or not.

5.2.3.1. Primordial heterogeneity. In this model the variations in ~ ~3C are regarded as being relict heterogeneities which have survived since the accretion of the Earth. The basis for the model is that the range of ~ 13C for bulk meteorites is similar to that observed for diamonds. S.R. Boyd (1988) and Kirkley et al. (1991) point out the difficulty in preserving primordial isotope heterogeneities. The majority of the samples from the upper mantle, including the oldest known diamonds (Richardson et al., 1984) have ~3Cvalues close to -5%0 suggesting that carbon within the convecting mantle is homogeneous and that homogenisation was complete at an early stage, the current results suggesting that the same may be true for nitrogen (see Fig. 2 ). To avoid homogenisation, any primordial heterogeneities would have been trapped at an early

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

stage, presumably in the oldest continental lithosphere. However, the locations producing 13Cdepleted diamonds (e.g., Jagersfontein, Arkansas) are often found near craton margins, suggesting that the source regions for the diamonds are in relatively young lithosphere (S.R. Boyd, 1988 ). Furthermore, Argyle diamonds have been dated at only 1.6 Ga (Richardson, 1986). Deines et al. ( 1987 ) suggest that there are "no compelling reasons" for carbon isotope homogenisation following accretion. S.R. Boyd ( 1988 ) noted that, since most meteorite groups with low fi~3C have oxygen isotope compositions distinct from terrestrial oxygen (Clayton et al., 1976), then, if there had not been homogenisation following accretion, oxygen isotope anomalies (~170-~80) should be expected. The study of Robert et al. ( 1992 ) suggests that the oxygen isotopes of the Earth are homogeneous and, due to the similarity between the Earth and the Moon, the authors suggested that homogenisation of the oxygen isotopes may even have occurred before accretion.

5.2.3.2. Subduction of crustal volatiles? This model is perhaps the most popular, the basis being the close agreement in the range of fi13Cvalues for sedimentary carbon and diamonds (see Kirkley et al., 1991 ). The appeal of this model is that it places diamond genesis within the broad framework of plate tectonics and accounts not only for the fi~3C-values but also for the chemistry of the mineral inclusions, i.e. the eclogitic minerals are considered to be subducted oceanic crust or melts which were derived from it. Supporting evidence for this idea comes from the isotopic composition of sulphur (Chaussidon et al., 1987; Eldridge et al., 1991 ) and lead (Eldridge et al., 1991 ) in diamond inclusions which have characteristics akin to sediments. In addition, although the ~80-values of diamond inclusions have yet to be studied in detail, eclogite nodules in kimberlites can have ~laO-values similar to hydrothermally altered oceanic crust (see Kirkley et al., 1991 ). In Fig. 4 are shown the ~SN-values of fibrous and octahedral diamonds together with various crustal rocks. It can be seen that the ~5N-values

53

of the 13C-depleted diamonds match quite closely the values of the crustal rocks so, at "face value", the nitrogen isotope data are consistent with formation from recycled crustal volatiles. What needs to be considered is whether nitrogen can be subducted to depths in excess of 150 km. During the diagenesis of carbonaceous sediments, although the content of "organic nitrogen" decreases markedly, much may be converted to ammonium ions and, once formed, they become sorbed and subsequently fixed by clay minerals (e.g., Itihara and Honma, 1979; Williams et al., 1992 ). Having an ionic radius similar to rubidium, ammonium substitutes readily for potassium in minerals such as micas and feldspars (e.g., Itihara and Honma, 1979 ), lowgrade metapelites containing up to ~ 700 ppm N as ammonium. Haendel et al. (1986) have shown that with increasing metamorphic grade there is loss of ammonium from the sediments (perhaps linked to dehydration reactions) and that there is an increase in the ~15N-value of the residual nitrogen. Ammonium survives up to the point of melting being concentrated in migmatites produced from the anatexis of crustal sediments (Itihara and Honma, 1979). In the continental crust, the stability of ammonium is typified by granites ("S"-type) which have had sediment involved in their genesis (Itihara and Honma, 1979; Hall, 1988; S.R. Boyd et al., 1993b). In a study of the behaviour of nitrogen in subduction zone metamorphism Bebout and Fogel (1992) have obtained similar results to those of Haendel et al. ( 1986 ): with increasing metamorphic grade, the ammonium content of the rocks decreased and the ~15N-values of the residual ammonium increased. The maximum depth represented by the rocks studied was only 45 km and deeper subduction of nitrogen may be controlled largely by the geochemistry of potassium. However, one important consequence of subduction is that the cold slab disrupts the geothermal gradient, such that the stability fields of potassic minerals (potential host for NH2-) extend to much greater depths, e.g. in excess of 150 km for phlogopite (Wyllie and Sekine, 1982). Since this is approximately the upper limit for diamond stability then it is possible that the crustal nitro-

S.R. Boyd, C. T Pillinger / Chemical Geology 116 (1994) 43-59

54

Area equivalent to one analysis U Organic' nitrogen in recent marine sediments (Peters et al., 1978)

Ammoniacal nitrogen in metasediments 1: regional (Haendel et al., 1986). Ammoniacal nitrogen in an'S' type granite (Boyd et al., 1993a) Ammoniacal nitrogen in metasediments 2: subduction zone (Bebout and Fogel, 1992).

Low 813C diamonds

_mni.

.i,. .hi-=

--INN

.

m

High 813C diamonds Ii111 Fibrous diamond (Boyd eta|., 1987, 1992)

N! -15

-10

-5

I

815N

+5

I

+10

I

+15

+2(I

Fig. 4. Histogram comparing the ~SN-values of octahedral diamonds to those of fibrous diamonds and crustal rocks. The atmosphere (0%0) is also shown for reference. The "high-~3C" octahedral diamonds have ~SN-values similar to fibrous diamonds whereas the "low-t~13C" diamonds have values similar to crustal rocks. The highest d~SN-values in the metasediments correspond to those of highest metamorphic grade.

gen may be present in some diamonds. However, to generate these diamonds via recycling of organic matter would require a complicated sequence of events. To start with the C and N would be intimately associated in organic compounds. During metamorphism they would become disassociated: carbon would eventually be present as graphite, which can survive subduction, and nitrogen would be present as ammonium within potassic minerals. At present, it

is uncertain as to whether nitrogen can be recycled into the diamond stability field and the problem is made worse if ~3C-depleted diamonds form at depths of > 150 km. When the slab was within the diamond stability field a melt would need to be produced into which the graphite and ammonium both "'dissolve" (become associated again) to produce a volatile-rich component which later crystallises diamonds (low 8~3C, high 8~5N) with igneous like zoning.

S.R. Boyd, C. T. Pillinger / Chemical Geology 116 (1994) 43-59

5.2.3.3. Isotope characteristics intrinsic to diamond growth. Although the subduction model is quite popular, others have argued that atypical O~3C-values may be produced by high-temperature fractionation processes (e.g., Galimov, 1984, 1991: Javoy et al., 1986 ) occurring deep within the mantle. There have been several approaches, which generally invoke Rayleigh distillation, which are either at variance with new data or difficult to envisage (see Kirkley et al., 1991 ). In a recent paper, Galimov ( 1991 ) relates diamond genesis and carbon isotope fractionation to the influx of reduced CH4-bearing fluids from the asthenosphere into oxidised lithosphere. The author goes on to say that this situation is a "necessary prerequisite for significant carbon isotope fractionation".

S.R. Boyd et al. (1994 in this issue), favouring the model of Haggerty ( 1986 ) for the oxidation state of the upper mantle, propose that fibrous diamonds grew in reduced continental lithosphere from oxidised, asthenosphere-derived fluids. However, the model of S.R. Boyd et al. ( 1994 in this issue) is based on the direct observation of CO2 and H20 within inclusions in fibrous diamond. A problem with any distillation model for the origin of the isotopic characteristics of octahedral growth-form diamonds is that, for a given mine, a trend from "high-O~3C '' to "low-O~3C" diamonds is never observed (note the bimodal distribution in the ~ ~3C-values for Jagersfontein: Fig. 1 ). Also, with the new data, a trend from "high ~ 13C_1ow ~ 15N,, to "low ~ 13C_hig h ~ ~5N,, would be required, which is not apparent in Fig. 2. These anomalous regions appear to be isolated and quite similar worldwide, suggesting that they formed by similar processes. The major difference between the conditions of growth of highfi~3C and low-fi ~3C diamonds would appears to be chemical environment (peridotitic/eclogitic) and depth. Attempts to model isotope fractionation related directly to diamond growth have assumed isotopic equilibrium between the diamond and the volatile phase from which it is growing (e.g., Deines, 1980). S.R. Boyd et al. (1994 in this is-

55

sue) suggest that because diamond is so unreactive, it will grow in isotopic disequilibrium and be able to preserve any kinetic isotope effects associated with the surface reactions leading to diamond growth. The kinetics of the surface reactions will be an interplay between the reactivity of the diamond surface (controlled by pressure and temperature, i.e. depth), the rate of supply of volatiles to the diamond surface and the extent to which the volatile-rich fluid is saturated with carbon (see S.R. Boyd et al., 1994 in this issue). An isotope fractionation mechanism, capable of producing marked anomalies, which is linked to the chemical composition of the lithosphere together with pressure and temperature would result in diamonds, which grew in a similar environment, having similar characteristics worldwide. They would occur as isolated anomalies and be unrelated to high-~3C diamonds. Also, extreme ~3C depletion would only ever be seen in diamonds and not in other mantle-derived materials such as kimberlite carbonates or MORB. Until such time that sufficient information is available so that the growth of octahedral diamonds can be modelled in detail, the possibility that the isotopic heterogeneities now observed have resulted from purely physical/ chemical processes occurring within the mantle cannot be ruled out.

6. A working model Although this study can only be regarded as preliminary, in this section we provide a simple working model which we hope will lead to future research.

6. I. The mid-Archean (Fig. 5.1) By this time the major differentiations coremantle-atmosphere had occurred. Carbon and nitrogen within the convecting mantle were both globally quite homogeneous: ~ 3 C ~ - 5 % o , ~SN ~ - 5%o. Nitrogen in the atmosphere was isotopically heavier being close to 0%o. It is possible that the nitrogen content of the atmosphere

56

S.R. Boyd, C T. Pillinger / Chemical Geology 116 (I 994) 43-59

5.1: Archean

I

ammonium in the crust (sediments, metasediments and related crustal melts: S.R. Boyd et al., 1993a), reducing the total amount of nitrogen in the atmosphere and pushing the ~SN-value to lower values. Ammonium in deep-sea sediments and altered oceanic crust would have been available for recycling. Beneath the continents, thick lithosphere extended down to within the diamond stability field (Richardson et al., 1984; F.R. Boyd et al., 1985 ). We assume the model of Haggerty (1986) for the oxidation state: a reduced continental lithosphere overlying an oxidised asthenosphere. CO2-H20-rich melts from the asthenosphere intruded the lithosphere where they crystallised, releasing volatiles which resulted in the formation of the first generations of diamond (A and B in Fig. 5.1 ). These had isotopic compositions which reflected the deep-seated source (~3C and filSN both close to -5°/oo) although small-scale isotope fractionation led to dispersion around these values. Nitrogen aggregation commenced as soon as the diamonds had formed.

1

Asthenosphere 15N =-5~ 613C =-5%0

5.2: P r o t e r o z o i c

6.2. The Proterozoic (Fig. 5.2)

5.3: P h a n e r o z o i c

15N =-5%o 513 C =-5ff,~

1

2

3

Lamproite Group 1 Kimberlite

Fig. 5. A simple model for the origin ofoctahedral diamonds: for explanation see text.

and the also the ~ l S N - v a l u e w e r e higher than at present since biological fixation of nitrogen followed by diagenesis, metamorphism, etc., would have trapped heavy nitrogen (~ 15N ~ + 60/oo) as

During the Proterozoic there was further growth ofhigh-813C diamonds. In Fig. 5.2 this is represented as an overgrowth on one of the diamonds in population A which is equivalent to sample Fin# 7 (Fig. 3 ). Most importantly was the appearance of 13C-depleted, ~SN-enriched diamonds in deep mechanically unstable lithosphere. Plastic deformation during storage within the lithosphere would eventually lead to a decoupiing of nitrogen aggregation state in relation to mantle residence time. 13C-depleted diamonds may have resulted from the recycling of crustal material or the isotopic characteristics are the result of fractionation linked to growth. 6.3. The Phanerozoic (Fig. 5.3)

By the Phanerozoic, the Archean diamond populations A and B were Type IaAB with platelets. In the case of A, the Proterozoic overgrowth was mainly Type IaA with only limited development of platelets. For A and B the degree of ni-

S.R. Boyd, C.T. Pillinger / Chemical Geology 116 (1994) 43-59

trogen aggregation for all diamonds was directly coupled to the nitrogen concentration, time and temperature of storage within the lithosphere. Despite being amongst the younger diamonds, the Proterozoic population C were predominantly Type laB due to the enhanced rate of N aggregation which accompanied plastic deformation during storage in the lithosphere. Group I kimberlite (Smith, 1983 ), Group II kimberlite (Smith, 1983) and lamproite magmas erupted through the various source regions transporting the diamonds to the surface. Kimberlites which followed path 3 (Fig. 5.3) would only contain high-t~13C diamonds (e.g., Finsch), for path 2 there would be a bimodal distribution in 6~3C (e.g., Jagersfontein ) and for path 1, only 13C.de" pleted diamonds would be present. The closest approximation to the latter is Argyle although this is of late Proterozoic age (Pidgeon et al., 1986). In the case of Group I kimberlites (path 2: Fig. 5.3 ), the 6~3C-values of carbonates in the hypabyssal facies (Sheppard and Dawson, 1975; Kirkley et al., 1989) and any fibrous diamond coats which formed around the pre-existing diamonds (S.R. Boyd et al., 1994 in this issue) would be close to - 5%o, irrespective of the ~ 3Cvalues of the xenocrystal octahedral diamonds, reflecting the asthenospheric source for the magmas.

Acknowledgements

JeffW. Harris is thanked for providing sample Fin# 7 and Steve Bergman for the diamonds from Arkansas. We thank Michael Seal for laser-sectioning of Fin#7 and H.J. Milledge and M.J. Mendelssohn for the IR spectra. S.R.B. thanks NERC for a research studentship. S.R.B. is grateful to the following Australian ventures for continued financial support: Argyle Diamond Mines, Ashton Mining Ltd. and C.R.A. Exploration. Grey Bebout, Marc Chaussidon, John Gurney and JeffHarris are thanked for constructive criticism of the manuscript.

57

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