American Research Laboratories, Crown Mines, Republic of South Africa ... Keywords: agq diamond, diamond inclusions, eclogite, Sr and Nd isotopes. :ks. R.
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Sr and Nd isotopic systematics of diamondbearing eclogite xenoliths and eclogitic inclusions in diamond from southern Africa C. B. Smrrn,r J. J. GunNEy,2 J. \J7. HeRRts,3 D. N. RontNsoN,a S. R. Sssea and E. Jecourzr tMax Planck Institut
filr Chemie, Abteilung Kosmochemie, Mainz, Federal Republic of Germany; zDepartment of Geochemistry, (Jniaersity of Cape Town, Rondebosch, Republic of South Africa; 3Department of Applied Geology, (Jniaersity of Strathclyde, Glasgow, United Kingdom; and aAnglo American Research Laboratories, Crown Mines, Republic of South Africa
ier ,dJ
:al ch R.
C. sh
-A
ABSTRACT Diamond, diamond-graphite and graphite eclogite xenoliths from four southern African kimberlites have varying Sr and Nd isotopic compositions, ranging from 'depleted' to highly 'enriched' on an Sr-Nd isotope correlation diagram. In most samples, clinopyroxene and garnet were in Nd isotopic equilibrium at the time of kimberlite emplacement, precluding internal mineral-mineral age estimates. However, even if the samples date from before the time of pipe emplacement the isotopic characteristics cannot simply reflect ageing, and isotopic character is probably inherited from isotopically varying source rocks. By implication, a single source for carbon is unlikely. Garnet and clinopyroxene in one sample with a relatively low equilibration temperature preserve an apparent age of about 440 My, and the parent must have had a history of LREE enrichment despite the degree of LREE depletion actually measured in the eclogite. The sample cannot, therefore, represent a tiquid; it must either be a cumulate or residuum from melt extraction. The Nd isotopic composition of eclogitic garnet inclusions from a single Finsch diamond is very radiogenic, coupled with high Sm/Nd. The model age is 1670 My, considerably younger than the peridotitic garnets in diamonds from the same pipe dated by Richardson et al (1984). Keywords: agq diamond, diamond inclusions, eclogite, Sr and Nd isotopes.
I7.I INTRODUCTION The association of diamond and eclogite is well established in the xenolith and diamond populations of a number of kimberlites. Diamond, diamond-graphite and graphite eclogites are known from a number of localities, and are apparently more common than diamond-bearing peridotite nodules. Minerals of the eclogite paragenesis are presumably as common as syngenetic
inclusions in diamonds, though in contrast to diamond-bearing xenoliths are apparently less abundant than peridotite paragenesis inclusions. In some kimberlites, such as the Orapa pipe, where eclogite dominates both the xenolith
population and the diamond inclusions, disaggregation of diamond eclogite during emplacement can account for much or most of the production. In addition to the economic significance, the isotopic character of eclogite xenoliths and diamond inclusions may provide information on the compositicn, degree of homogeneity and evolution of the subcontinental lithosphere if it is assumed that most diamond-bearing rocks are derived from old stabilized lithosphere as opposed to convecting asthenosphere.
A number of
isotopic studies
of
eclogite
xenoliths have been conducted, though only the studies by Kramers (1979) and Jagoutz et al (1984) on Roberts Victor samples are reasonably
--l'
C. B. Smith et al.
854
modal compositions. Samples are generally somewhat altered, with garnets rimmed by varying
Composite samples of hundreds of diamonds are
generally required, and Richardson (1986) has carried out the first studies of well-characterized eclogite paragenesis inclusions. Analyses of single or composite inclusions from individual diamonds
are desirable in that uncertainties inherent in composite diamond samples can be eliminated. Here we present the first Sr and Nd isotopic analyses of clinopyroxene and garnet separated from diamond- and graphite-bearing eclogite xenoliths, and the preliminary results for eclogite garnet inclusions . extracted from individual diamonds.
I7.2
SAMPLES
17.2.1 Diamond eclogite xenoliths The ten diamond- and/or graphite-bearing eclogite xenoliths analysed to date are from four kimberlites. Three diamond eclogites (samples
amounts of kelyphite and clinopyroxene commonly having turbid alteration zones generally attributed to decompression melting followed by preferential alteration of those zones' The samples are essentially bimineralic, with accessory rutile occurring in AKl/10 and JJG144 in addition to diamond. The small sizes and shapes of the Orapa and Newlands samples suggest size reduction in the plant prior to collection. The
rounded shapes and polished surface features of the Excelsior samples, however, are probably the results of abrasion in the kimberlite during emplacement. The Excelsior samples are distinctive in comprising dominantly coarse grained garnet, with some of the specimens lacking clinopyroxene and resembling garnet megacrysts with accessory diamond. A photograph of a similar sample from a Barkly V/est kimberlite was published by Bosch (1971). Microprobe analyses show that these are eclogitic, however. Microprobe analyses of constituent minerals in all but one of the Orapa and Roberts Victor samples are given by Hatton and Gurney (1979),
Shee and Gurney (1979) and Robinson et al (19S4). Analyses of sample XM23 from Orapa and the Excelsior and Newlands samples are given in Table 17.1. Garnet chemistry best reflects the compositional variability of eclogite, and in
Fig. 17.1 samples are approximate Type 1 single specimen with diamond and graphite from Roberts Victor (HRV247) has been described by
Hatton and Gurney (1979). Three diamond-
bearing samples from the Excelsior kimberlite (8x2, Ex8 and Ex10) are from a newly collected
suite of fifteen xenoliths and have not
been
described previously. A single specimen from the Newlands kimberlite (JJG114) has also not been described before, but is similar to other Newlands diamond eclogites described by Bonney (1899, 1901). According to the petrographic and chemi-
cal criteria of MacGregor and Carter (1970) and McCandless and Gurney (1986) all but one of the rocks are coarse grained Type I eclogites with grey-green clinopyroxene and orange-red garnet. XMIl (Orapa) is a Type 2 eclogite, having apple green clinopyroxene and pinkish red garnet. Vith the exception of HRV247, all of the rocks are small, being 1 to a few centimetres in maximum dimension, precluding reliable estimates of the
s
in the Orapa specim somewhat arbitrary in that there is probably
a
continuum of compositions between subdivisions, and between Type I and Type 2 eclogites.
Nevertheless, it provides a convenient framework
within which to illustrate chemical variability among the samples since, according to Hatton (1978), increase in Ca content of the garnet corresponds to breakdown of an orthopyroxene-
clinopyroxene buffer reaction in the crystallizing melt. The samples analysed for isotopic character do not represent the full compositional range of southern African diamond eclogites (Reid et al I976), though garnet compositions include Main
Group, Fe*Ca-rich and Ca-rich types (Fig. 17.1). The Type 2 graphite eclogite from Orapa (XMl1) is of the websterite association defined by
Shee (1978), though there is no orthopyroxene in
the rock. The Orapa diamond eclogites have a large compositional range and in this regard are similar to Roberts Victor samples (Hatton 1978;
Sr and Nd isotopic ststematics of diamond-bearing eclogite xenoliths TABLE
l7.I
855
Microprobe analyses of clinopyroxene and garnet in diamond eclogite xenoliths and garnet inclusions in Finsch diamond.
l0 sio2
MnO
55.26 0.44 9.70 0.06 4.22 0.05
Mgo
10.1 5
CaO
14.r4
Na20
Kzo
5.50 0.10
Total
99.62
Tio, Al2o3 CrrO3 FeO
40.57 0.45 23.07 0.04 15.05 o.29 12.19 9.23 0.14
I1.84
9.79
9.16 0.14 na
13.41
6.21 0.10
10.32 0.18 na
5.72 0.17
na
100.14
100.98
100.82
99.70
100.95
100.46
Lt.24 0.03
4.t5
ndt 9.05
ll.r5
13.r7
na* 101.03
l4
55.58 0.38 10.40 0.06 4.65 0.04 9.55 13.87 5.90 0.03
40.29 0.48 22.88 0.07 15.68 0.28
39.66 0.48 22.89 0.06 15.97 0.27
55.74 0.45
55.
0.39
t0.32 0.07 4.69
nd
39.78 0.29 23.05 0.04 18.84 0.36 I 1.30 7.
l8
0.1
I
39.07 0.33 23.05 0.04 17 44 0.36 10.92 8.93 0.13 na
to0.2l
)9.r4 0.34 22.22 0.05 22.05 0.81
8.32 7.68
0.14 na 100.75
r Not analysed. f Not detected. Sample key
Excelsior diamond eclogites:
lcpx,ExZ,n:26 Exl0, z : l0 5gat,Ex8,n:2
3 cpx,
Newlands diamond eclogite JJG144:
6
cpx,n: 2
2gar,Ex2,n:2 4gar,Exl0,n:2 7
gar,n:
2
9
gar,n:
4
Orapa diamond eclogite XM23:
8
cpx,n: 6
Finsch diamond 11798 l18 l0 gar inclusions, n : 6 Analyses were performed at U.C.T., and values given are averages of z grains.
Reid er al 1976). The Excelsior samples differ from those of the other kimberlites in that 12 of the 15 eclogites have compositionally restricted garnets, suggesting that the samples analysed for isotopes are genetically related (Fig. 17.1). Three of the Excelsior samples have garnet with somewhat lower Ca levels and higher and more varying Mg levels; these rocks have not yet been analysed Mg
Fig. 17.1 Compositions of garnets in diamond eclogite and Finsch diamond. Fields 1-4 are approximate Type I eclogite subdivisions for Orapa by Shee (1978): L
main group; 2. Ca*Fe enrichment trend; 3. Ca enrichment trend; 4. kyanite-bearing. Sample XMll is a Type 2 eclogite; all others are Type l. Sample XMll contains graphite only; samples JJG889 and HRV247 contain graphite and diamond. O Orapa; I Newlands; O Roberts Victor; I Excelsior; O Eicelsior samples not analysed for rsotopes.
for isotopic character and will not be dealt with further in this paper. Itrflith the exception of the Type 2 graphite eclogite XMll, clinopyroxene and garnet are characterized by elevated K2O levels (up to 0.10wt%) and Na2O levels (up to 0.I5wt%) respectively, a feature typical of diamond eclogite in general (McCandless & Gurney 1986) and consistent with high pressure origins (Reid et al 1976). Equilibration temperatures calculated for an assumed pressure of 50 kb using the method of Ellis and Green (1979) vary, ranging from about
856
C. B. Smith et al.
1000 to 1280'C. Kp values and remperature estimates are given in Table 17.2. Garnet and clinopyroxene are chemically homogenous in all samples except HRY247. tU7ith regard to that sample, Hatton and Gurney (1979) documented considerable compositional variation in MgO across one slab of the nodule, though equilibration temperatures for different sections of the rock do not vary markedly. Diamonds in these samples have morphological features generally similar to those described by Robinson (1979), octahedral or twinned forms predominating. In one of the Excelsior samples containing a number of small I mm diameter
is not uniform, with colourless and yellow-white crystals, and one green crystal, present. Preliminary
TAtsLE
17.2 Ko
values and equilibration temperatures in diamond eclogite samples. Temperatures calculated by the method of Ellis and Green
(re7e)
Sample KD TCC
at P
:
50
kb)
Ex2
1173
3.51
t046
HRV247
3.49 4.00
l0l5
Newlands Roberts Victor
XMl l
3.20
990
Orapa
AKl/10 xM23
xM3l
JJG88e JJG144
2
Excelsior Excelsior
3.03 3.12 2.90 3.28 3.06
Exl0
tt77 1281
tl2l
t094 1048
octahedral diamonds, diamond colour
Type
carbon isotopic analyses of these stones yield 5r3C of -5.4 to - 5.5 in th"e colourless to yellow-white formsl carbon in the green crystal is somewhat
I7.3 ANALYTICALMETHODS
lighter with 6t3C of about -5.8. A second Excelsior sample has diamonds with 6'3C of -5.76. Carbon isotopic composition values are
Locality
Type I
Orapa Orapa Orapa Orapa
Analytical methods were essentially the same as given by Jagoutz and rtrTenke (1986); aspects ofthe techniques deemed important to these particular samples are discussed below.
broken. Several of the smaller garnets were analysed by microprobe (Table l7,l), and the presumption that these inclusions are similar to those analysed for isotopic compositions is supported by the homogeneous maior element compositions of several grains. The garnets are similar in composition to five other eclogitic garnets from
The eclogite xenoliths are small and not in pristine condition. Consequently, reliability of results hinges upon obtaining the cleanest possible mineral separates, and this is probably the most important aspect of the analysis procedures. Mineral separates of both clinopyroxene and garnet are necessarily small due to minimal nodule size and to the difficulty of obtaining pure separates from partially altered starting material. Samples were carefully crushed by hand in a clean agate mortar and pestle, and sieved into size fractions ranging from 200 to 500 pm diameter. Sized separates were then cleaned in acetone using ultrasonic agitation, dried and passed through a high sensitivity magnetic separator to separate garnet and clinopyroxene and to check that both minerals were homogeneous with respect to Mg/Fe ratios. All samples were found to be homogeneousl subpopulations of garnet or clinopyroxene were not present and this was verified by microprobe in two cases. Hand cleaning under a microscope was performed in several stages, the final cleaning being done in alcohol facilitating identification of internal cracks or inclusions. During crushing the grains tend to fracture along alteration filled cracks and
Finsch diamonds analysed by Gurney et al (1979), and have notably high Fe and Mn contents compared with garnets in eclogite xenoliths. The diamond contained no co-existing diopside; estimates of equilibration temperature are not possible.
grain boundaries. Hence, contaminants tend to be concentrated on grain surfaces in the mineral separates. To ensure removal of such material that may have been missed under the microscope, mineral separates were rigorously leached, with
similar to the median value of eclogite association diamonds from Siberian kimberlites (Galimov 1984), but the Excelsior diamonds are signifi-
cantly lighter isotopically than production diamonds from the Dan Carl Mine, Bellsbank (6"C : -3.34 + 0.95), and Premier (6t,C : -4.75 + 1.37) analysed by Deines (1980). Carbon isotopic analyses of diamonds from other samples are as yet unavailable.
17.2.2 Diamond inclusion
A
2.3 c yellow octahedral diamond from the Finsch kimberlite contained 35 orange-red eclogitic garnet inclusions weighing l.2mg in total. One large inclusion, about 1 mm3 in volume constituted the greatest part of the sample weight and was extracted intact on the diamond being
-
Sr and Nd isotopic systematics of diamond-bearing 3-5 min of ultrasonic agitation at each leaching step. Garnets were leached in warm 6N HCI overnight, followed by warm 5olo HF overnight. Clinopyroxenes were leached in warm 6N HCI overnight followed by cold 5% HF for up to 30 min. rtrTeight losses during leaching were of the
of 5-l0o/o. The pure mineral separates ranged from 5 to l0 mg in weight in most cases, and dissolution was performed in sealed Savillex beakers at 100 to 12.0"C following several l0 min bakes in a
eclogite xenoliths
857
(+
2 ng Nd for the isotopic composition) neces_ sarily meant that fewer blocks oi data could be collected, and even then ion beams were not of ideal intensity. Standards of similar size were
order
17.4 RESULTS 17.4.1 Eclogite xenolirhs Analytical results for clinopyroxene and garnet separares are given in Table 17.3. Sm and Nd
samples were neglegible except
in the case of the diamond inclusion. Even in ihat instance blank corrections were less than the mass spectrometer and weighing errors. Reported diamond inclusion data are nevertheless blank corrected.
analysed as rhe meral species; Nd was analysed as oxide on Re filaments. Pr and Ce were monitored and corrected for (small amounts of pr were commonly present), and the ,otNd7,o.*O rario was measured,to ensure data quality. Ba and/or BaF was checked for in all analyses but was generally present in trace amounts too low to interfere with
usin resu Nd.
raa11
"ffi:"::::Til.,1 itored by t46Nd7
blank level, The small amount of sample analysed
C., B.
858 TABLE I7.3
Sr and Nd isotopic compositions and Rb, Sr, Sm and Nd concentradons in clinopyroxene and garnet in diamond eclogites. .
Sample
Nd
Sm
Excelsior
Ex2 Ex2 ExIO Ex10 ExS
cpx gar
cpx gar gar
0.443
0.949 0.366 0.984 0.961
raTsm/raaNd r43Nd/r4Ndo rNd
2.172 t.207
0.123 0.475
1.851
0.120 0.408
1.458 1.165
0.499
Orapa
AKl/10 AKI/I0 XM31 JJG889 XM23 XMI I
cpx
0.134
t 1 +3 o.5t27o t 2 0.51300 t 2 0.51308 t 3 0.51305
0.70669
l2
0.70681
+
6
0.0006
0.70255
+
3
o.7o6t5
t
6
0.51166 0.51366
+ 3 -19.1 t 5 +rs9
0.51095 0.01 173.1 0.51095 0.005 0.93
0.0002 0.019
0.70426 0.70492
+ +
2
0.518r7
t 5
2.020
0.245
0.691
0.937
0.697
Finsch diamond inclusion 2.19
0.00r5 0.024 0.01I
0.70593
0.51291 0.51306
Newlands
718 gar 2.52
72.7 0.24 63.9 0.92 0.89 -
0.007 0.83
0.0001 0.007
JIG144 cpx 0.818 JJGI44 gar 1,07
0.1 12
0.038
0.5t27 | 0.51262 0.51270 0.51271
0.51190 0.01 280.4 0.51 193 0.002 0.78
0.51196 0.51225
0.195
0.5t266
87Rb786sr 8'sr78usr,
Sr
-r3.3 -7.6
0.085 0.390
0.261 0.126
Rb
0.0001 0.0001 0.0002 0.0008
7.80 1.687
Roberts Victor HRV247C cpx HRV247C gar
0.082
+7.0 +8.6
l4rNd/reNdi
137.) 2.01 0.007 142.7 0.01 282.3 0.009 r21.3 0.110 391.3
l.ll 1.089
cpx cpx cpx cpx
0.32 t.25t
+r.2
3 +5.3 t 3 +8.2 0.5t275 l2 +2.r 0.51319 12 +10.7 0.51273t3 +1.8 o.5t25o I 2 -2.7
0.107
gar
+2.0 +8.2
0.5t274
0.995 0.742 6.000 1.705 5.278 0.591 3.195 5.338 30.30
798
Snith et al.
+
2 t 2 +
0.5 1286
0.51291 0.51267 0.51306
0.51266 0.51243
0.03
-
t 4 t 4 0.70643 x 5 0.7847 + 2 0.7080e t 3 0.7038 0]0?89
+
2
t5
+r07.9
Errors in isotopic analyses are 2o std errors of running means. Neglecdng weighing errors, uncertainties in concentration determinations (in parts/106) are generally better than 0.6% for Sr and Sm, and 0.2% for Nd. Relative errors in Rb concentrations are much larger (up to 50olo) due to large blank corrections required for very small samples. Sr, Sm and Nd concentration errors are also larger for the diamond inclusion, the in-run errors being 0.8% and 0.5olo for Sm and Nd respectively. Subscripts, and i on isotopic ratios indicate^present day and emplacement age corrected values, respectively. eNd values are calculated for present day ratios (bulk rrig-7r+r1i6 earth r43Nd/r44Nd :
,.. .o..."r.i foi
:
age in Fig. 17.2 earth 0.1936)'lr47sm:6.54X 10 r2y-r.Emplacementagestakenas:Excilsiorll4MxOrapagbMy;RobertsVictorl25My. Initial ratios for JJGI44 are calculated for 440 My, the apparenr two point gar-cpx age. r43Nd/r44Nd :0.51264 in BCR -l;87Sr/ susr 0.51264);.-eNd values
:
(bulk
emplacement
0.70802 in the Eimer and Ahmend SrC.o, standa.d. The diamond incluiion data are correcred for small blank contributions.
enough to require a significant correction for the time of kimberlite emplacement. As with the Sr
isotopic compositions, initial r43Nd/t44Nd rarios of
clinopyroxene and garnet define an apparent age
of about
440 My, r43Nd/r44Nd ratio
with an apparent initial
of 0.51095. This is strikingly
the clinopyroxenes vary greatly, ranging from 0.51095 to 0.51306. In the Sr-Nd correlation diagram (Fig. 17.2), the variation is even more striking in that samples do not define a single trend. Compared to most mantle-derived mat-
unradiogenic, particularly for a sample with such a degree of REE depletion (note the high Sm/Nd in both clinopyroxene and garnet, Table 17.3), and is rather similar to the case of peridotitic
erials the Excelsior samples, rhe graphite eclogite
Richardson et al (1984).
I and JJG889 all have clinopyroxenes with anomalously radiogenic Sr relative to their Nd isotopic composirions. Co-existing clinopyroxene and garnet in the XMI
Excelsior samples, AKl/10 and HRV247 define
times of pipe emplacement, within the limits of analytical error. The rocks are either young, or
more likely mineral phases were maintained in isotopic equilibrium until sampled by the kimberlite. This is not so in the case of JJGl44, in which
garnet inclusions
in
diamonds measured by
In light of the chemical variation within HRV247 (Hatton & Gurney 1979) it might have been expected that this nodule should exhibit isotopic disequilibrium. However, the separates analysed for isotopic compositions were from one small portion of the rock only, and disequilibrium
may occur on the scale of the entire sample. Further tests on different portions of the rock are in progress. Garnet has not yet been analysed in samples
Sr and Nd isotopic systematics of diamond-bearing eclogite xenoliths
859
17.5 DISCUSSION 17.5.1 Eclogite xenoliths a
llc 889
Ex
! z-^.
2 and
a
The salient features of the diamond- and graphitebearing eclogite xenoliths analysed in this study
10
o
xM11
t Nd
suite alone, and second, the lack of definite indications as to their age except in the case of one xenolith. These two points are addressed below,
z group ll
!
kimberlrte
s ro
o
i !
'1o2
Despite the lack of age indications, the isotopic
xnvzaz
variation cannot be explained by assuming the
J)C144
tpresent
day)
'oo"rrryrurr'',on"ro,
7oa
'?1o
Fig.l7.2 Sr-Nd isotopic correlation diagram. Present day "517865r in clinopyroxene is plotted against emplacement age corrected l43Nd/l44Nd
in clinopyr-
oxene. Clinopyroxene and garnet are not in isotopic equilibrium in JJGI44; the measured
r43Nd7t*Nd ratio is plotted. The same may be true of JJG889, but garnet has ndt yet been analysed. Age-corrected kimberlite and present day oceanic basalt fields are shown for comparison. eNd values are for 100 My. O Orapa; I Newlands; (E Roberts Victor; I Excelsior.
XMll
and JJG889. In the last of these samples Sr and Nd are both radiogenic (Fig. 17.2), clearly an anomaly in light of the anticorrelarion typical of most terrestrial rocks. By analogy with JlGl44, garnet and clinopyroxene may not have been in isotopic equilibrium ar the time of pipe emplacement, and a true initial la3Nd/raaNd ratio could be much lower. Likewise in the case of XMl1 and XM3l, though in these instances the initial r43Nd/'onNd ratios of clinopyroxene
XM3l,
in
relation to Sr are more plausible mantle values.
17.4.2 Diamond inclusions Blank-corrected Sm and Nd concentrations in the Finsch diamond garnet inclusions are 2.53 and
2.24 partsl106 respectively, considerably higher than in the xenolith garners, with a much higher raTSmTraaNd rario of 0.697 (Sm/Nd : l.t5). The r43Nd/t44Nd
ratio of 0.51817 is extremely radio-
genic, corresponding to an Epsilon value of about
*108. The garnet is not similar to the Finsch peridotitic inclusions (Richardson et al 1984), which have higher Nd contents and anomalously
Iil
are, first, the extreme isotopic variation of the group of samples as a whole and within the Orapa
unradiogenic Nd isotopic compositions.
xenoliths are ancient and have undergone isotopic evolution which has caused the observed variations. Rb contents of eclogites are very low (excluding the secondary intergranular material), and Sr is therefore unsupported in the sense that 87517865r ratios of the clinopyroxenes could not have evolved from a common value to observed ranges even over protracted periods of time. The same logic applies to the Sm/Nd systemarics, although the argument is more complex because bulk rock Sm and Nd concentrations and Nd isotopic compositions must be reconstructed from estimates of modal mineral abundances. The
latter is difficult to derive from small, coarse grained rocks in which clinopyroxene may have been preferentially eroded while entrained in kimberlite (e.g. in the case of the Excelsior samples). Nevertheless, the variation in la3Nd/ laaNd
cannot be the result of isotopic ageing from
a single
initial value.
Throughout this paper
it
has been tacitly as-
sumed that diamond-bearing eclogites in southern
African kimberlites are recrystallized cumulates from melts of approximate basaltic composition (e.g. Hatton 1978; MacGregor & Carter 1970), the melts in turn being derived from chemically diverse sources. This is consistent with the isotopic data, is the simplest explanation for the varied isotopic character, and supports the model of eclogitic diamond genesis proposed by Haggerty (1986). By implication, the carbon required for diamond formation may also have diverse origins though ultimately the carbon must either be a primitive mantle component or recycled
through subduction
processes.
However, other models of eclogite formation are plausible. If the eclogites are regarded as recrystallized cumulates, simultaneous fractional crystallization-assimilation processes, such as advocated by Rudnick et al (1986) as responsible for
860
C. B. Smith et al.
the origin of Australian lower crustal xenoliths, could be relevant. Mixtures of isotopically primitive or depleted melt and isotopically enriched peridotite country rock could yield the observed diverse isotopic character. Eclogite could also represent fragments of subducted oceanic lithosphere (e.g. Ater et al 1984). Jagoutz et al (1984) presented evidence that some demonstrably ancient non-carbonaceous samples
they analysed were best explained by such
a
model. However, their samples were dissimilar to those of this study in being of a highly depleted isotopic character, some samples yielding ancient internal mineral ages according to Sm-Nd systematics, and having a more complex internal chemical and isotopic character, and petrographic dis-
equilibrium features.
Protoliths of most of the diamond eclogites cannot have been uhmodified MORB, presumably the dominant component of subducted lithosphere. If subducted basalts did suffer seawater alteration possibly complicated by subsequent metasomatism (e.9. Ongley et al 1986),
514
Ir
NEWLANDS DIAMOND ECLOGITE
ltG
\
11.4
tT l---b-:
!
z
: !
Z
['" Nx
512
l-
to CHUR ct - 3500 51001
0
500
1000
Age (Ny)
Fig. 17.3 Nd isotopic evolution diagram for JJGI44 (f ) with an apparent clinopyroxene-garnet age of 440 My. Present dayIa3Nd/1aaNd ratios of co-existing garnet and clinopyroxene are connected by evolution lines (-) to the initial ra3Nd/raaNd ratio at 440 My. Dashed evolution lines (---) from the 440 My composition represent the extremes of possible evolution paths of the precursor. Evolution lines
(-)
however, a range of isotopic compositions similar to that of the eclogites could possibly have been
for bulk earth (CHUR) and
peridotitic
garnet inclusions in Finsch diamonds are shown for comparison. O, O diamond-bearing and diamond-free eclogites from Orapa, Roberts Victor and Excelsior.
generated with time. Melting of such material, possibly admixed with pelagic sediment, could
also conceivably have produced the range of
GAR
observed isotopic compositions. Except in sample IIGl44, co-existing clinopyr-
lysed) and a very similar equilibration temperature. Isotopic disequilibrium must be suspected. The significance of the apparent internal age of
rium at the time of kimberlite
and/or clinopyroxene may not be closed systems relative to surrounding mantle rocks. Assuming that the eclogite body was isotopically isolated from surrounding mantle, 440 My must be regarded as a minimum age because garnet and clinopyroxene may not be completely closed
in samples in which both minerals were analysed were in isotopic equiliboxene and garnet
emplacement.
Hence the rocks are either young (within error of pipe emplacement ages) or have resided in the upper mantle at temperatures high enough to prevent closure of mineral isotopic subsystems on
the scale of a hand specimen.
Metamorphic textures in conjunction with lack of petrographic disequilibrium features, as well as isotopic evidence of ancient origins in the case of other eclogites (Allsopp et al 1969; Kramers 1979) and eclogitic diamond inclusions (this work; Richard-
son 1986) are supportive of the latter. The
Newlands sample JJGl44, however, has an apparent internal clinopyroxene-garnet Sm-Nd age of about 440My (Fig. 17.3), despite petrographic features similar to those of the other samples. In comparison to other Type I samples, the equilibration temperature of 1050"C (Table I 7.2) is low, though higher than for HRV247 or XMI l. Clinopyroxene from sample JJGS89 has an anomalous isotopic character (Fig. 17.2; garnet not yet ana-
JJGL44
is difficult to evaluate because garnet
systems relative to each other.
The sample and its original parent must have had a complex history. As shown in Fig. 17.3, Sm/Nd ratios of clinopyroxene, garner and the estimated bulk rock are high, indicative of LREE depletion. Yet the apparenr initial ratio of about 0.51095 is exceptionally low and rather similar to
that of peridotitic garnet inclusions in diamonds (Richardson et al 1984). This similarity extends to the unradiogenic Sr isotopic composition and implies a relatively primitive time-averaged Rb/Sr ratio. The eclogite must be either a cumulate or a residuum from melt extraction, and the parent prior to melting must have had an extreme degree of LREE enrichment for a long period of time.
--Sr and Nd isotopic systematics of diamond-bearing eclogite xenoliths
The time of genesis of the parent is not well defined, and the parent could itself have had a complex history. Direct separation from a chondritic reservoir could not have occurred less than 1350 My ago, the limiting case for an Sm/Nd ratio of 0 (Fig. 17.3). If the precursor is not a direct derivative of a chondritic composition but rather evolved in an environment similar to that of the peridotitic diamond inclusions, this could have happened as recently as 500 to 600 My ago (Fig. I7.3). Alternatively, this time could represent the 'true' age of the eclogite itself, 440 My possibly being a cooling age. Further work on this sample is in progress.
861
!
z !
Z
m
70
Ase (ca
!0
40
)
Fig.17.4 Nd
17.5.2 Diamond inclusion Assuming a single stage history arid derivation from a chondritic reservoir, the model age for the eclogitic garnet inclusions in the Finsch diamond is 1670 + 40 My (Fig.17.4; error from maximum analytical uncertainties). If the parent reservoir is not chondritic, but still within the compositional limits of normal depleted or enriched upper
mantle rocks, the age calculation varies little highly radiogenic Nd. The assumption of a more or less normal parent reservoir is supported by initial ratios of eclogitic inclusions in Premier and Argyle diamonds measured by Richardson (1986). On the other hand, anomalously depleted samples of upper mantle rocks are now known (Jagoutz et al 1984; Jagoutz 1986; McCulloch 1986; Shervais et al 1986), though the extent of such potential reservoirs is unknown. In this context it must be noted that Finsch diamonds are dominantly peridotitic, with something less than 3% being of the eclogitic paragenesis because of the
al 1979). Hence it is possible that this diamond may have formed in a localized event from a localized and compositionally anomalous (Gurney et
parent.
The model age, though predating considerably
the time of pipe emplacement-L2} My ago (Smith et al 1985) is substantially younger than the ages of sulphide inclusions (greater than 2000 My (Kramers 1979)) or peridotitic garnet inclusions (3300 My (Richardson et al 1984)) in Finsch diamonds. It is thus not possible to relate in a single event formation of eclogitic and peridotitic inclusion suites represented by silicate samples analysed to date, at least in the Finsch diamond population. The sulphide inclusions
isotopic evolution diagram for eclogitic garnet in Finsch diamond J1798 718. The model age is 1670 t 40 My (error from maximum analytical uncertainties), Field for 'normal' mantle comprises oceanic and continental basalts.
inclusions
analysed by Kramers (1979) may represent an additional diamond forming event. Richardson (1986) has now documented Proterozoic ages for eclogitic inclusions in two other kimberlites, and eclogitic and peridotitic inclusion suites could represent Proterozoic and Archaean processes respectively. The Proterozoic model age of the Finsch eclogitic garnet is similar to some of the older ages obtained in crustal rocks of the off-
craton Namaqua and Bushmanland provinces (e.g. Reid et al 1986), and it is tempting to speculate that the Finsch eclogitic diamonds could represent an upper mantle event, or events, related to those crustal processes.
17.6 CONCLUSIONS Eclogite xenoliths with accessory diamond, or both from four southern African kimberlites have greatly varying Sr and Nd graphite,
isotopic characters, suggestive of derivation from isotopically diverse sources. Clinopyroxene and garnet in most samples were in isotopic equilibrium at the time of emplacement, precluding
internal mineral pair age determinations. One sample from Newlands, however, has an apparent Sm-Nd clinopyroxene-garnet age of 440 My, but whether this corresponds to a'real' geologic event is uncertain. The Sm/Nd ratio of mineral phases and calculated bulk rock values of that sample indicate extreme depletion and are decoupled from the low apparent initial 'a3NdTrnoNd ratio
uC. B. Smith et al.
862
indicative of time-averaged LREE enrichment of the eclogite precursor. The largest silicate inclusions known in diamond tend to be of the order of I mm3 in volume, and isotopic analysis of individual inclusions are thus possible. Single rather than composite diamond samples can therefore be dated provided the
sample material is available. Eclogite garnet inclusions from a Finsch diamond yield an Nd model age of 1670 My. This result, in conjunction with recent work by Richardson (1986), suggests that eclogite paragenesis diamonds are of Proterozoic age in contrast to older peridotite paragenesis diamonds.
ACKNOWLEDGMENTS \We thank the De Teers Corporation and in particular J.B. Hawthorne of Anglo-American for
support and interest during this and related studies. J. Scott, A. van Niekerk and V. Anderson of De Beers helped in the selection of diamond inclusions. The Ardo Mining Company and mine managers J. Dutoit and A. Dodgen are gratefully acknowledged for donating valuable samples from the Excelsior kimberlite. R.S. Rickard helped with probe analyses, and A. Boos and H. Gorzawski provided carbon isotopic analyses. J. Burkholz and D. Schier ably maintained the chemistry laboratory during this work. M. Zadnik and D. Schier reviewed the first version of the paper. D.N.R. and S.R.S. acknowledge the AngloAmerican Corporation for permission to publish, Gabriele Schier typed the manuscript.
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