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Carbon and oxygen isotope composition of carbonates from the Qaqarssuk Carbonatite Complex, southern West Greenland
( hw~/~~u/ (;eolog~~ (Isotope 1 Isc\ li‘r Sc~cncc Publishers
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763
Carbon and oxygen isotope composition of carbonates from the Qaqarssuk Carbonatite Complex, southern West Greenland
tii~udscn. C‘. and Buchardt. .IIIIC ( omplex. southern
B.. 1091. Carbon and oxygen West Greenland. (‘hem. Geol..
isotope composition ofcarbonatcs (Isot. Cicosci. Sect. ). 86: 763-273.
from
the Qaqarssuk
(.-arhon-
‘>table isotope data from the Jurassic Qaqarssuk Carbonatite Complex show that carbonatltcs lntrudcd during the maln 1111I-usive event have cS”O-values ranging from + 7 to + 8.1 o/o0SMOW and ri’2C-valucs ranging from - 3.5 to - 3.1% PDB. I ate-stage so\ ite wns are enriched in light carbon relative to the main-stage carbonatitcs, with fi’7C-values ranyng lro1n - 5 to - 4% PDB. This is interpreted as loss of heavy carbon to a gas phase. I atc-srage REE-carbonatitcs have 0” ‘C-values m the same range as the main-stage carbonatites. but elevated 0“ ‘C-values rcl~ti\e to late-stage swites. The REE-carbonatites have elevated O’“O-values ( +7.4 to +4.?% SMOW) relative to both maln-stage carbonatites and late-stage wvitr. I ‘arbonates In mctasomatlcall) altered basement and contaminated carbonatltc have elc\ated rS”O-values ( +X.1 to + ~.8%a SMOW). probably caused b> exchange of oxygen with the basement. I )\)gen isotope geothermometrq give temperatures in the range 3 13-608 C. which IS low relative to the expected lgncous temperatures. These low temperatures are explamed as caused by subsolidus reactlons such as exsolution and recrbjtallization which can be observed In the carbonates. There is poor correspondence between oxygen and carbon isotope gcothermometq as well as with solvus geothermomctry. indicating that the calibration of the isotope geothermomctcr csrnhlishcd in metamorphic carbonate rocks cannot be applied directly to carbonatites.
Introduction Carbon and oxygen isotope distributions in carbonates from carbonatites have been studicd by, e.g., Taylor et al. ( 1967). Deines ( 1970, I 989 1, Deines and Gold ( 1973 ), Pineau et al. ( 1973). Suwa et al. (1975), Andersen (1984, 1987 ) and Nielsen and Buchardt ( 1985 ). The resulting data base covers a wide range in isotopic compositions (6’80-values from +6 to -3O%o SMOW and #‘C-values from -8 to c 3O/oo PDB). The “subvolcanic carbonatite complexes” generally have high #xO-values “~‘~cwt~t~~ldrcw:: ( opcnhagcn
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compared to the mantle (Fig. 1 ), and show less isotopic scatter and lower 6”O-values than carbonatites from “higher crustal levels” (Deines and Gold, 1973). In the subvolcanic complexes contamination caused by assimilation of wall rock as well as late-stage hydrothermal alteration has been used as explanation for the elevated #‘O-values and compositional variation (Andersen, 1984, 1987). Deines and Gold (1973) explain the higher O”O-values with decreasing depth as being due to either: ( 1) loss of isotopically light water during pressure reduction, (2) equilibration of carbonates with magmatic water at low temperature, or (3) influx of meteoric water (at a temperature of < 250’ C). Calcu-
B.V
264
C. KNUDSEN
a--L. 10
1‘1
/ ?h
t
i:
.-,-.I SIJO
&ND
B. BIIC’HAKDl
i-i
Fig. 1. Plot of carbon and oxygen isotope compositional fields for calclte In carbonatites (ri’ ‘C,,,,H and 0’ ‘KO~M, ,u i. Tht. compositional fields are from Deines and Gold ( 1973) and the trends arc from Pineau et al. ( 1973). calculated assuming Rayleigh fractionation between carbonate and a gas phase at 7OO’C. The,figurPs 2 and 0.5 indicate the mole ratio H,(C ’ COz in the gas phase.
lations by Pineau et al, ( 1973) suggest that the increase in 6180- and 6’3C-values with time or with decreasing depth may be due to Rayleigh fractionation of the oxygen and carbon isotopes between crystallizing carbonate and a coexisting vapour phase. The Qaqarssuk Carbonatite Complex was mapped and sampled as part of a resource evaluation programme concerning phosphor, niobium and lanthanide accumulation of possible economical interest (Knudsen, 1989). The degree of exposure in the complex is good (25%) and 248 cores have been drilled in the complex (average 17 m long). As a consequence, it has been possible to identify a wide range in carbonatite types with well-known field and textural relations (Knudsen, 1991). Therefore the Qaqarssuk Carbonatite Complex is considered well suited for a study of the effects of primary igneous fractionation and wall rock contamination on the oxygen and carbon isotope distribution in a carbonatite magma. The aim of this study is: ( 1) To investigate the evolution of oxygen and carbon isotope distribution during the magmatic evolution of the Qaqarssuk Carbonatite Complex. (2 ) To trace exchange of oxygen and carbon isotopes with external sources during:
(a) Metasomatic alteration of the pre-existing basement. (b) Emplacement of narrow carbonatite veins in fenite. (c) Autometamorphic events such as hydration of olivine. (d) Late- and post-magmatic alteration. (3 ) To calculate a minimum intrusion temperature from oxygen and carbon isotope fractionation between calcite and dolomite, compared with the calcite-dolomite solvus geothermometer. 2. Experimental
technique
Samples from the different types of carbonatite present in the complex were crushed and carbonates in the 75-150-pm fraction were analysed. The modal calcite/dolomite ratio was determined by means of X-ray diffractometry (XRD ). Measurements of oxygen and carbon isotopic ratios were carried out by mass-spectrometric analyses of CO? liberated by treating the carbonate samples with 98O/o H,P03 at 25°C (McCrea, 1950). Pure calcite samples were reacted for 12 hr after which all calcite was dissolved. Pure dolomite samples were reacted for 24 hr. This period of time is not enough to ensure total dissolution of dolomite
C 4’ 1) 0 :\OIOt’l
COMPOSITtONOk
( \RBONUES
FROMTHE
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(e\‘en after 72 hr., some dolomite remains), but our experiments have demonstrated that the analytical error originating from incomplcte reaction is insignificant (less than the reproducibility) for both carbon and oxygen. Mixtures of calcite and dolomite were treated by a fractionated reaction procedure based on the much slower reaction rate of dolomite than calcite (Espstein et al., 1963; Fritz, 1967 ). The CO? extracted after 2 hr. of reaction is thus considered to represent only the calcite phase. After additional 4 hr. of reaction, all calcite has dissolved, but at the same time a considerable amount of dolomite has contributed to the CO,. and the gas is therefore pumped away. Finally. the gas extracted after additional 18 hr. is considered to represent only the dolomite phase. Acid fractionation factors for calcite and dolomite are 1.01025 and 1.01109, respecti\ ely (Sharma and Clayton, 1965), and dolomite values have been recalculated to calcite values. The mass spectrometer used is a Finnigan Mat’“’ 250, at the laboratory for stable isotopes. Geological Institute, University of Copenhagen. Results are given relative to PDB and SMOW (carbon and oxygen. respectively 1. and the data have been corrected according to Craig ( 1957). Reproducibility measured as standard deviation for 10 standard yampies is better than t0.02%0 for carbon and ? 0.07%0 for oxygen. The microprobe analyses of the carbonates were carried out on the JEOL” 733 Superprobe at the Institute of Mineralogy. University ofcopenhagen, using four wavelength-dispersive X-ray spectrometers ( 15 kV, 10 nA and beam diameter 10 pm, 10-s lifetime). The results were calibrated relative to natural calcite. dolomite and magnesite standards. 3, General geology of the Qaqarssuk C‘arbonatite Complex The Qaqarssuk Carbonatite Complex covers an arca of 15 km’. The complex consists of carhonatite ring-dykes, intruded into the Archean
(‘OMPLk.\(
265
basement 173 Ma ago (Larsen et al., 19X3), surrounded by metasomatically altered basement rocks. The ring-dykes have elliptical to semi-rectangular shapes (Fig. 2). The dykes generally dip outwards, and the steepest dips are found at the margin of the complex (Knudsen, 199 1). The carbonatites near the margin of the complex are generally very fine grained and often contain highly deformed, partly digested fenite inclusions. The carbonatite ring-dykes consist mainly of calcite-dominated carbonatite (sovite), olivine sovite and dolomite carbonatite. The sovite and dolomite carbonatite ringdykes are cut by coarse-grained late-stage snvite veins, REE (rare-earth element) carbonatite veins, ferrocarbonatite and lamprophyre dykes. There is a general increase in the content of Sr and light lanthanides in the carbonate with a simultaneous decrease in the Y/La ratio during the evolution of the Qaqarssuk Carbonatite Complex (Knudsen, 199 1 ). This trend is found in the whole-rock geochemistry as well as in zoned apatite crystals (Knudsen, 199 1). Olivine sovite is considered the most “primitive”carbonatite. Then follow the major sovite and dolomite carbonatite ring-dykes, while the coarse-grained late-stage sovite veins and REEcarbonatite veins and ferrocarbonatite dykes are the latest and most evolved carbonatites. The sovites contain increasing amounts of silicate minerals and inclusions of metasomatical altered basement towards the contact to the wall rock. and there is a gradational contact from the sovite to the metasomatically altered rocks via silicate-rich sovites. The degree of metasomatical imprint on the basement rocks decreases away from the complex as well as from the individual carbonatite ring-dykes. In the core of the complex the basement rocks are often altered to ultramafic rocks (glimmerite and hornblendite ), grading into fenites essentially composed of albite, alkali amphibole and alkali pyroxene. In the distal altered basement rocks quartz is preserved, and
266
(‘. KNI’DSEN
1 km
~-
ANI)
H. HI’< Ii.
51”4O‘W
Lzl Carbonatltc r;l Lj
P and Nb Late stage
LIL]
REE
rich sovite
carbonatltc
r~:]
Basement
Ultramaflte
Fig. 2. Geological
map of the Qaqarssuk
Carbonatite
Complex
these rocks are characterised by hematite staining and alkali pyroxenes and alkali amphiboles along joints and fractures. Exsolution textures are frequently found in carbonate minerals in the main-stage carbonatites where the carbonatite has only been subjected to little (syn- to late-magmatic) deformation, whereas the carbonatite is totally recrystallized with calcite and dolomite occurring as separate crystals in more deformed carbonatites. This recrystallization indicates that
subsolidus equilibration of the carbonates is a widespread phenomenon in the carbonatite. The olivine in the olivine sovite has often been replaced by hydrous phases such as phlogopite or amphibole and by dolomite. Increasing replacement of olivine has been observed in the samples GGU 252520+32050~ + 3204144 320506 (Table I ). This indicates that water may have been introduced at a late- 01 post-magmatic stage. Intense low-grade alteration (sericitization
C \&I:,)
I\Ol-OPt
( OP*1I’OSlT1ONOF~‘.~KROI\;.~‘TFS
FKOMTtiL
of feldspar and hematitization of magnetite) is mainly located in the marginal part of the complex (Kjzrgaard et al., 1987), but locally ;I heavily altered, hematite-bearing carbonatite similar to “r0dberg” from the Fen Carbonatitc Complex, south Norway (Andersen, I984 ) is found in the central parts of the complex (
: WI:’
1~x1 data on calclrc
and dolomltc
from
MlllWll
the Qaqarssuk Modal %I of GlrbO-
‘67
~~~~)4KSSIIK(‘-\RRON~T11L(‘O~IPLL\;
4. Results The distribution of oxygen and carbon isotopes is given in Table I and illustrated in Figs. 3 and 4. 4. 1 Chrhon is0toyw.s in dcite
The (S1?Z-values in calcite are in the range from - 4.69 to - 3.25%0 PDB (except for the highly altered 252.540) and in dolomite from - 3.35 to -2.52°/oo PDB. There is a trend from
C’arbonatite
(%I \‘5. PDB)
und dolomilt~
Complex.
#V)
Partial
mineral
(% vs. SMOW)
(‘a0
MgO
SrO
54.2 29.9 53.2 ‘Y.5
(1.7 ix.4
0 3 0 I
0.7
0 3
20. I
0 z
anal>33
(wt.% __ F;cO
1
Whole
Mn( 1
\I’:La
rock
1 ppm)
natec calcite dolomltc calute dolomite ‘(,?I
.,
XY
‘0.“
calcirc dolomite
‘il.?’ ‘I
calute dolomltc calcltc calcite calute dolomltc calcite dolomltc dolomltr calcite calcite calcite calcltc dolomltc calcite calclte nkerltc calcite calcite dolomite calcite calcltc calcite calclte
‘204
Ii
204 “.\
tine-gralned dolomite carbonatlte fine-gralned dolomite carbonatlte late stage s0vite late stage sevite tatc stage s0vite late stage s0wte REE-carbonatitc REE-carbonatite REE-carbonatite fcrrocarbonatlte lamprophyre hydrothcrmall) altered carbonatite calcite \cin calcite vein hornblcndile gllmmeritc
- 3.25 - 3.04 - 3.55 -3. I3
+ 7.06 f7.67 +7.11
- 3.55
+7.10
+7.c)3
0.2 3. I 0.1 I.6
0.I 0.2 0.3 0.4
lJ.20
I .OY6
0 IX
Y93 646
+7.24 f8.1’)
31.0 79.6
2.0 20 2
0 3 0 I
0.2 7.0
0.2 0.5
0 ‘0
+6.90 + 7.80 + 7.40 + 7.02 +1.s7 + 7.50 +7.93 +X.84 + IO.39
51.9 19.3 52.7 52.5 48.4 79.7 55.3 2Y.Y 19.7
1.1 18.1 0.7 I.0 4.7 17.7 0.8 I I. I 16.4
0.4 0 2 I .(I 0.6 0.3 0.3 0.8 0.3 0.3
0.6 4.2 0.X 0.8 I.4 5.0 0.6 5.6 6.4
0.4 0.3 0.4 0.3 0.3 0.4 0.3 0.8 0.6
0. I Y
1.15x
YY 92 31) 70 59 41 IO0
~ 3.46 ~ 3.0X -4.68 - 4.06 - 3.27 - 3.11 - S.YO -3.35 -2.7-l
0 I3 0.13 0.18
7 799 _,__ 2.4OS 7 I0
(I.17
1.760
0.20
I.111
97 9Y IO0 I 00 Y5 99 90 81 80 47 53 96 95 98 9Y
-3.49 -4.6X - 4.60 -4.68 -7.84 - 3.6 I ~ 3.74 - 2.52 -4.69 ~ I .04 - 2.73 -3.57 ~-4.40 -4.20 -3.51
+7.53 i7.74 +7.50 +7.1 I +9.x +7.54 1-8.26 +9.93 +7.72 + 16.3’) + IO.58 +9.1 I +8.75 + 8.10 +.X.46
50.9 49.9
0.4 I.8
I 3 0.8
0.5 1.1
OS 0.6
0. I7 0. IO 0. I I
7.537 ~,I09
78.0
I I.')
0.4
12.7
0.7
0.1) I 0. I3 0.1 I
I Y.S52 I.107 I.379
o.oi
5.591
71 2’)
1.271
268
0
0
OIlvIne
m
0
S0wte
A
fi
Dolomite
4
0
Fme-graned carbonatlte
F
s0vtte V carbonatlte
,-,
Late
V
Lanthanlde-rich
0’
UltramaflL
dolomite
A
4.2. Oxygen isotopes The 6 ‘*O-values
in calcite
is in the range
carbonatlte rock
CalcIte
vein
Strongly
Fig. 3. Plot of 6’% PDB vs. 6’80sMow for carbonates from the Qaqarssuk from the same sample. One sample (GGU 252540 calcite) is off-scale.
early, “primitive” carbonatite (low Sr and REE and high Y/La ratio), which is relatively 13Cenriched, to late-stage, “evolved” carbonatite (high Sr and REE and low Y/La ratio ) which is relatively 13C depleted (Figs. 3 and 5; Table I). An exception from this pattern are the REEcarbonatite veins which have high Sr, high REE, low Y/La and are relatively rich in “C. The trend is most clearly seen in the calcites. There is a similar trend in the early dolomites, but not seen in the sparse data on the late dolomites (Fig. 5 ). It should be noticed that the ferrocarbonatite is relatively rich in 13C.
SOVlte
Lamprophyre d.’
Ferrocarbonatlte
stage
dltered
carbonatlte
Carbonatite
Complex.
LJWS connect m~nc‘~-al~
from + 6.90 to + 9.1 l%o SMOW (except for 252540) and in dolomite from +7.50 to + 10.58°/oo SMOW. There is no relation between a’*O-values and the geochemical evolution in the carbonatites as described for the carbon isotopes above. Fresh and uncontaminated carbonatites have 6l*O-values in the range +6.90 to +7.53%/m SMOW. There are increased 6 “O-values in the samples which are formed by interaction between carbonatite and basement (Fig. 3 ), with 6”0values from + 8.20 to + 8.46%0 SMOW in calcite from ultramafic rocks (hornblendite and glimmerite ) formed by metasomatic alteration of wall rock. Fine-grained dolomite carbonatite, ferrocarbonatite and lamprophyre
L 5x10-h
l
t
.
c
.
.
t c
l
* A’k
1000°C
D _cc
-
. 05-
. .
. .
. .
@Ii+ 0
7;,,,,.,, is the temperature FIN. 4. a. Plot of I/T“,,,,,, vs. A”C,_,.. and calcite using the Mg-calcite geothermometer.
b.motof‘I : 7-z5 ,,,,“,\‘s.L~fxo,,_cc. c. I’lotof I’Y‘,,( ( vs. 3”0,,_, C, The .solrd
rqfiwnc~t~
JLht I,).( .+0.35. d. Plot of 7,,,,,,, vs. ‘Z;,. .rcr,,and 7.c are temperatures ti\ cly. between dolomite-calcite pairs. e. I’lot of ‘f,. vs. -To.
5OO’C
TO
calculated on Mg fraction between co-existing
lrnc is from Sheppard
and Schwartz
( 1970) with z1’3CI, ,-,=0.37
( ‘C) calculated on oxygen and carbon isostope fractionation.
dykes rich in partly digested basement inclusions, calcite veins in the wall rock, and REEcarbonatite veins have 8’0-values of +7.54 to +9.93%0 SMOW. The olivine sovite in which the olivine has been hydrated, dolomite and calcite have elevated 6’80-values relative to dolomite and calcite in the olivine sevite with fresh olivine. In
dolomite
the “rodberg” /I”0-values SMOW are found.
respec-
up to + I 6.39°/oo
4.3. Geothermomutr~~ Dolomite is enriched in 13C and I80 as compared to calcite in samples where both phases have been analysed (Fig. 3 ) . This is in accord-
(‘. KNI’DSEN
I
I
AND
B ljll(
H-\KI)
I
tion between the dolomite-calcite pairs ( Fig. 3). LI’~O,,_~(.,=1000In CY”~,,_(i j’ =0.45*10hT ‘-0.40 1 12i AJC I>_(..= < 1000 In cy’“CI)_ct ! =0.18.10hT- ‘+().I7 :
and T
(Cal-tx1n
, = [0.18-10h/(~1’-~C,)~(c-0.17~]~ i3h)
-5I 0
I
05
Fig. 5. Hot of 6°C PI)B vs. wt.% SrO in carbonates from the Qaqarssuk Carbonatitc Complex. Lines connect calcite-dolomite pairs in the same sample. S.wnho/s as Fig. 2.
ante with the findings of Shieh and Taylor (1969), Deines (1970), and Sheppard and Schwarz (1970), whereas Andersen ( 1987) finds that 6’*0,,,<6’80,, in coexisting calcite and dolomite from the Fen Carbonatite Complex. One exception from the pattern in the Qaqarssuk carbonatites is the “rodberg”, which contains calcite veins likely to be deposited later than the ankerite and hencenot likely to have been in equilibrium with the dolomite. The change in isotopic fractionation with temperature between calcite and dolomite can be expressed as a general fractionation equation: 1000 In a=A/T*+B
(1)
where czis the fractionation factor; A and B are constants;and T is the temperature in K. Thus, decreasing Twill lead to increasing fractiona-
where d denotes the difference in %oobetween the &values for co-existing dolomite and calcite. This gives temperatures (Table II ) in the range from 608” to 304’C on the oxygen isotope distribution between calcite and dolomite, and from 1850” to 415’C on the carbon isotopes. (d”C,-,., in sample 32042I is too low to give a meaningful temperature). T,, and T,- are not identical, T,, generally yielding a lower temperature than T(:. but there seemsto be a relation between T,, and 7;. ( Fig. 4e) as there is a relation betweend’“C,, , j and d’80u_(.c (Fig. 4~). For comparison the temperature has been calculated on the basis of the Mg-calcite solvus thermometer using the relation (Gittins, 1979): (4) log,,, x&u,, = - 1690/T (K)+0.795 TABLE
2
Stable isotope and Mg fractlonatlon paws, and the calculated temperatures atite <‘omplex Sample GGU No.
-f”C,,-, ( (?60 vs. PDB)
data from calcite-dolomltc for the Qaqarssuk Cyarhon--.-_I_.~~
.~~
,l’80n_C.C (o/o0 YS. SMOW)
Molt% MgCO, in calcltc
(I; c) y
;c;,,
;;<;:<;.
1.68 10.47 I .9z 2.06 3.00 5.15
1848 822 415 653 S76
394 608 334 313 315 304
384 679 400 40X 4i6 53x
7i-‘s/o
0.2 I
0.61
_(.?041 I 3204l4 .?20430 j20460 320507
0.06 0.32 0.55 0.38 0.42
0.18 0.82 0.91 0.90 0.95
__~
-.~
( \YI)
t ISo7OI’L
U)MPOSIl’IOKOF
(~4KROUZTES
FKOMTHE()~O-\KSSII1((‘~KHON.~TlTF(
FeCO,<
f g. 6 f‘riangular plot of molecular proportions of Ca. Mg and Fc in carbonates. The compositional fields show the ccampositional variation observed in the Qaqarssuk Carbonatlte C‘omplex. L~nec- connect calcite-dolomite pairs i? the \amc sample. .~~vJ/w~ as in Fig. 2.
The calculated temperatures (Table II ) are generally low compared to igneous temperatures (600-900°C; Gittins, 1979) which can be expected in carbonatites. This is consistent with the observation of subsolidus re-equilibration (e.g., exsolution) observed in the carbonates. The compositions of the carbonates are illustrated in Fig. 6. The solvus temperatures do not show a systematic relation to the isotope temperatures (Fig. 4a and b), but they give temperatures in the same range as T,,. Solv us temperatures have been calculated for 14 other carbonatite samples and fall in the range 273-598’C. 5. Discussion
The general evolution towards enrichment in Sr in the carbonates coupled with depletion in ’ C indicates that there was a general ‘-‘C depletion during the evolution of the carbonatite, from the main-stage carbonatites to the late-stage s0vite veins. The REE-carbonatites have high Sr and high a’%-values relative to the late sravite veins.
OLlI’l.L:\
??I
There is no experimental result available about the fractionation of carbon isotopes between carbonate crystals, carbonatite melt and CO?. According to Rayleigh fractionation models presented by Pineau et al. ( 1973 ) and Deines ( 1989)) an enrichment in ‘jC is to be expected during evolution of carbonatites. This is the opposite of what is seen from main-stage carbonatites to late-stage s0vite in the Qaqarssuk Carbonatite Complex, and this enrichment in light carbon is not likely to be caused by fractionation by e.g., crystal separation. On the basis of the low S’C-values found in the carbonatite lavas of Oldoinyo Lengai, Tanganyika, Suwa et al. ( 1975) suggested that heavy carbon was preferentially fractionated into and lost with the gas phase. Theoretical computations by Bottinga ( 1968 ) indicate that “C-enriched carbon will be fractionated into a gas phase relative to calcite at temperatures of > 200°C. According to this, the depletion in heavy carbon from the main-stage carhonatites to the late-stage sarvite can be explained as loss of heavy carbon to a gas phase. Volatiles were lost: ( 1) by diffusion into the wall rock during the fenitisation; (2 ) through veins; and (3 ) through explosive events forming, e.g. the ferrocarbonatites. It can accordingly be expected that carbonates precipitated from volatites in the metasomatically altered rocks, in veins and in ferrocarbonatites are enriched in “C relative to the late-stage s0vite. The REE-carbonatites are rather porous ( miarolitic) and have been interpreted as deposited in equilibrium with a gas phase (Knudsen, 199 1). The REE-carbonatites as well as the ferrocarbonatite have elevated 6 ’ ‘C-values (and 6’80-values). This ‘jC (and “0) enrichment is similar to the evolution predicted by Pineau et al. ( 1973) and Deines ( 1989) using a Rayleigh fractionation model (Figs. 3 and 7), and similar to the evolution described by Nielsen and Buchardt ( 1985 ) . Another possibility for generation of the elevated G”C-values (and 6”O-values) in the very late veins and dykes is that the porous
C‘.KNUDSEN
Fig. 7. Plot of compositional fields and evolutionary trends for the 6’YI PDR vs. S’*OsMow in carbonates from the Qaqarssuk Carbonatite Complex, compared with other “subvolcanic” carbonatite complexes. The compositional fields ( 1-4) are from Deines and Gold ( 1973).
REE-carbonatite veins and ferrocarbonatite dykes may have been affected by a very late stage of hydrothermal alteration. This is seen in the marginal parts of the carbonatite (Kjzrgaard et al., 1987) where the ferrocarbonatite dykes are located. As the REE-carbonatite samples are collected in the central part of the complex, this explanation is less likely for their part.
5.2 The oxygen isotopes In the unaltered and uncontaminated carbonatites there is fairly little variation in the oxygen isotopic composition ranging from +6.9 to + 7.5% SMOW in calcite. This is within the 6180 range of primary, high-temperature carbonatites ( + 6 to + 8%0) of Deines and Gold ( 1973 ). There is no major increase in IgO from early carbonatites to the late-stage sovite. This is compatible with the small fractionation of oxygen isotopes between minerals and fluid at high temperatures (O’Neil et al., 1969). Fractionation of oxygen between the carbonates, silicates and oxides has not been investigated in the present work. 180-enriched oxygen will be preferentially fractionated into the carbonates relative to silicates and oxides
4NDB.BI'C'HAKL)I
olivine, phlogopite and magnetite ) onway and Taylor, 1969 ). If the fractionaI E”‘? tion of oxygen between carbonates and silicates was of importance this would give a relative enrichment in 6”O in carbonates in samples rich in silicates (e.g.. GGU 320507). This is not seen, and fractionation of oxygen between carbonates and silicates, at least in the early magmatic stage, is regarded as being of minor importance. The calcite in metasomatically altered rocks, in calcite veins in the basement and in lamprophyre (rich in partly digested basement inclusions) all have elevated 6”O-values in the range from + 7.5 to + 16%0 SMOW. The local Archaean, mainly granodioritic basement is likely to have a d”O-value in the range + 7 to + 13%/m SMOW (Taylor, 1974), and the carbonates are considered to have been contaminated by exchange of oxygen with these “Oenriched basement rocks. Moreover, the ‘“Oenriched oxygen in the basement would have been preferentially fractionated into the carbonates if there had been an exchange of oxygen carbonate and silicates during the metasomatic alteration. The high 6”O-values in REE-carbonatites and ferrocarbonatites relative to the main-stage carbonatites and late-stage sovite veins may be explained as: ( 1) loss of isotopically light water during pressure release; (2 ) influx of meteoric water, and (3) equilibration with magmatic water at low temperatures or a vapour phase. These alternatives can explain the observations with regard to the oxygen isotopes. However, loss of the gas phase would cause a depleof > 200” C tion in “C at temperatures (Bottinga, 1968) and, as 613C is elevated in these rocks, the first alternative is regarded as less likely. The second alternative may explain the high 6180-values found in the marginally located ferrocarbonatites, but is regarded as less likely in the REE-carbonatites in the core of the complex. The REE-carbonatites and ferrocarbona-
tites have isotopic compositions (Fig. 3) as could be expected by Rayleigh fractionation between carbonates and a vapour phase (Pincau et al., 1973) (Fig. 1) with a starting compositron similar to the late-stage sovites (Fig. 3 ). and this is considered the most likely explanation. The carbonatites with hydrated olivines have htgher 6’xO-values than the unaltered olivine sovite. This is consistent with the interpretat ron of the alteration of olivine as being due to equilibration with magmatic water at lower temperatures. It is not likely to be due to influx of meteoric water, because the equilibration stopped at a fairly high temperature. at which equilibration with meteoric water would lead to depletion in 180.
There is a relation between S’80L)-c‘c and l’3c,)-.(.I (Fig. 4c) as well as between the car!Jon and oxygen isotopic temperatures (Fig. -te ) . but with a slightly different slope (Fig. 4c ) and a very different axial intercept compared to what was found by Sheppard and Schwartz ( 1970). The fairly poor correspondence between the carbon and oxygen iosotopic temperatures indicates that the calibration used (based on metamorphic limestone: Sheppard and Schwartz, 1970) cannot be directly applied to the carbonatite material. The oxygen isotopic temperatures (To) fall in the same range as the solvus temperature (Fig. 4d ) , whereas the carbon isotopic temperatures ( To) generally are higher than the solvus temperatures. However, the present material is too small, and the correspondence bet ween isotope temperatures and solvus temperatures is too poor to propose a new calibration of the isotopic temperatures. 6. Conclusions 1 ) Geochemically
tive”
olivine
and isotopically “primisovite, sovite and dolomite car-
bonatite indicate that the 6’“O and 6’C ( isoprimitive topic composition ) of the carbonatite magma was in the range + 7 to +7.5% SMOW and -3.5 to -3.1°/oo PDB, respectively. (2 ) There was a depletion in heavy carbon isotopes from deposition of main-stage carbonatite to late-stage sovite (Fig. 7). This is likely to be caused by loss of heavy carbon isotopes to a gas phase. ( 3 ) Carbonates in metasomatically altered basement and contaminated carbonatite have elevated 6’80-values, likely to be caused by exchange of oxygen with the basement. (4) Isotope geothermometry gives temperatures below expected igneous temperatures, explained as caused by subsolidus reactions in the carbonates. The results show that the calibration of the isotope geothermometer established in metamorphic carbonate rocks cannot be directly applied in carbonatites. Acknowledgements Thanks are due to J. Ronsbo for guiding the microprobe analysis, to L.M. Larsen and A.K. Pedersen for valuable comments on the manuscript. The field work was supported by the European Economic Community through contract No. MSM 119DK. C.K. is funded by the Danish Natural Science Research Council. contract No. 1 l-5835. The mass spectrometer facility is funded by the Danish Natural Science Research Council, grant No. 1 l-9372. Publication is authorised by the Director of the Geological Survey of Greenland. References .Andersen. T.. 1984. Secondary processes in carbonatitcs: petrology of “rodberg” (hematite-calcite-dolomitecarbonatite) in the Fen Central Complex. Tclcmark (South Norway ). Lithos, 7: 127-145. Andersen. T.. 1987. Mantle and crustal components in a carbonatite complex, and the evolution of carbonatite magma: KEE and isotopic evidence from the Fen com-
274 plex. southeast Norway. Chem. Geol. (Isot. Geosci. Sect.), 65: 147-I 66. Bottinga, Y.. 1968. Calculation of fractionation factors for carbon and oxygen isotopic exchange in the system calcite carbon dioxide water. J. Phys. Chem., 72: 800808. Conway, C‘.M. and Taylor. H.P.. 1969. “Q/“‘O and “C/ 12C’ratios of coexisting minerals in the Oka and Magnet Cove carbonatite bodies. J. Geol.. 77: 618-626. Craig, H.. 1957. Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analyses of carbon dioxid. Geochim. Cosmochim. Acta. 12: 133-149. Deines. P.. 1970. The carbon and oxygen isotopic composition of carbonates from the Oka carbonatite complex, Quebec, Canada. Geochim. Cosmochim. .4cta. 34: 1199-1225. Deines. P.. 1989. Stable isotope variations in carbonatites. In: K. Bell (Editor), Carbonatites: Genesis and Evolution. Unwin Hyman, London, pp. 301-359. Deines, P. and Gold, D.P., 1973. The isotoptc composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deapseated carbon. Geochim. Cosmochim. Acta. 37: 1709-l 733. Epstein, S., Graf, D.L. and Degens. E.T., 1963. Oxygen isotope studies on hte origin of dolomite. In: H. Craig, S.L. Miller and G.J. Wasserburg (Editors), Isotope and Cosmic Chemistry. North-Holland Publishing Co., Amsterdam, pp. 169- 180. Fritz. I’., 1967. Oxygen and carbon isotope composition of carbonates from the Jura of Southern Germany. Can. J. Earth Sci., 4: 1247-l 267. Gittins. J., 1979. Problems inherent in the application of calcite-dolomite geothermometry to carbonates. Contrib. Mineral. Petrol.. 69: l-4. Kjargaard, M.. Knudsen, C. and Abrahamsen, N., 1987. Geophysical investigations of the Qaqarssuk Carbonatite Complex, southern West Greenland. Rapp. Gronl. Geol. Unders., 125: 34-40. Knudsen, C.. 1989. Pyrochlore group minerals from the Qaqarssuk Carbonatite Complex. In: P. Moller (Editor), Lanthanides, Tantalum and Niobium. Springer. Berlin, pp. 80-99.
(‘ KNl:IXjtN,ANI)
B. HlI(
HL\KL)I
Knudsen, c‘., 1991. Petrology, geochemistry and cconomic geology of the Qaqarssuk Carbonatite C‘omplex. southern West Greenland. Miner. Deposna. Monogr. Ser. 2V. I. Larsen, L.M., Rex. D.C. and Secher. K., 1Y83. The age ol carbonatites, kimberlitcs and lamprophyres from southern West Greenland: recurrent alkaline magmatism during 2500 million years. Lithos. 16: 2 15-X I McCrea. J.M., 1950. On the isotope chemistry ofcarbonales and a paleotemperature scale. J. Chcm. Phys., IX. 849-857. Nielsen. T.F.D. and Buchardt, B.. 1985. Sr-C -0 isotopes in nephelinitc rocks and carbonatites. Gardincr complex, Tertiary of East Greenland. Chcm. Geol.. 53: 707217. O’Neil. J.R., Clayton. R.N. and Mayeda. T.K.. 19hY. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys.. 5 I: 5547-5558. Pineau, F.. Javoy. M. and .4llegre. C.J., 1973. Etude systematique des isotopes de I’oxygdne, du carbonc et du strontium dans les carbonatites. Geochim. C’OS~Ichim. Xcta. 37: 2363-2377. Sharma. T. and Clayton, R.. 1965. Measurements of ‘“( )I IhO-ratios of total oxygen from carbonatites. Cicochim. Cosmochim. .4cta. 29: 1347- 1353. Sheppard, S.4.F. and Schwartz. H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contrib. Mineral. Petrol.. 26: 16 I- 198. Shich. Y.N. and Taylor, H.P.. 1969. Oxygen and carbon isotope studies of contact metamorphism of carbonate rocks. J. Petrol.. IO: 307-33 I. Suwa, K., Oana, S., Wada. H. and Osaka. S.. 1975. Isotope geochemistry and petrology of .African carbonatitcs Phys. Chem. Earth, 9: 735-745. Taylor. H.P., 1974. The application of oxygen and hydrogen isotopy studies to problems of hydrothermal aiteration and ore deposition. Econ. Gcol.. 69: 843-883. Taylor, H.P., Frechen, J. and Degens. E.T.. 1967. Oxygen and carbon isotope studies of carbonatites from Laacher See district, West Germany, and the r\lno district, Sweden. Geochim. Cosmochim. Acta. 3 1: 407430.
Gcoscicncc .Srctmj. B.V.. .Amstcrdam
86 ( I90 I ) 263-374
763
Carbon and oxygen isotope composition of carbonates from the Qaqarssuk Carbonatite Complex, southern West Greenland
tii~udscn. C‘. and Buchardt. .IIIIC ( omplex. southern
B.. 1091. Carbon and oxygen West Greenland. (‘hem. Geol..
isotope composition ofcarbonatcs (Isot. Cicosci. Sect. ). 86: 763-273.
from
the Qaqarssuk
(.-arhon-
‘>table isotope data from the Jurassic Qaqarssuk Carbonatite Complex show that carbonatltcs lntrudcd during the maln 1111I-usive event have cS”O-values ranging from + 7 to + 8.1 o/o0SMOW and ri’2C-valucs ranging from - 3.5 to - 3.1% PDB. I ate-stage so\ ite wns are enriched in light carbon relative to the main-stage carbonatitcs, with fi’7C-values ranyng lro1n - 5 to - 4% PDB. This is interpreted as loss of heavy carbon to a gas phase. I atc-srage REE-carbonatitcs have 0” ‘C-values m the same range as the main-stage carbonatites. but elevated 0“ ‘C-values rcl~ti\e to late-stage swites. The REE-carbonatites have elevated O’“O-values ( +7.4 to +4.?% SMOW) relative to both maln-stage carbonatites and late-stage wvitr. I ‘arbonates In mctasomatlcall) altered basement and contaminated carbonatltc have elc\ated rS”O-values ( +X.1 to + ~.8%a SMOW). probably caused b> exchange of oxygen with the basement. I )\)gen isotope geothermometrq give temperatures in the range 3 13-608 C. which IS low relative to the expected lgncous temperatures. These low temperatures are explamed as caused by subsolidus reactlons such as exsolution and recrbjtallization which can be observed In the carbonates. There is poor correspondence between oxygen and carbon isotope gcothermometq as well as with solvus geothermomctry. indicating that the calibration of the isotope geothermomctcr csrnhlishcd in metamorphic carbonate rocks cannot be applied directly to carbonatites.
Introduction Carbon and oxygen isotope distributions in carbonates from carbonatites have been studicd by, e.g., Taylor et al. ( 1967). Deines ( 1970, I 989 1, Deines and Gold ( 1973 ), Pineau et al. ( 1973). Suwa et al. (1975), Andersen (1984, 1987 ) and Nielsen and Buchardt ( 1985 ). The resulting data base covers a wide range in isotopic compositions (6’80-values from +6 to -3O%o SMOW and #‘C-values from -8 to c 3O/oo PDB). The “subvolcanic carbonatite complexes” generally have high #xO-values “~‘~cwt~t~~ldrcw:: ( opcnhagcn
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compared to the mantle (Fig. 1 ), and show less isotopic scatter and lower 6”O-values than carbonatites from “higher crustal levels” (Deines and Gold, 1973). In the subvolcanic complexes contamination caused by assimilation of wall rock as well as late-stage hydrothermal alteration has been used as explanation for the elevated #‘O-values and compositional variation (Andersen, 1984, 1987). Deines and Gold (1973) explain the higher O”O-values with decreasing depth as being due to either: ( 1) loss of isotopically light water during pressure reduction, (2) equilibration of carbonates with magmatic water at low temperature, or (3) influx of meteoric water (at a temperature of < 250’ C). Calcu-
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264
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Fig. 1. Plot of carbon and oxygen isotope compositional fields for calclte In carbonatites (ri’ ‘C,,,,H and 0’ ‘KO~M, ,u i. Tht. compositional fields are from Deines and Gold ( 1973) and the trends arc from Pineau et al. ( 1973). calculated assuming Rayleigh fractionation between carbonate and a gas phase at 7OO’C. The,figurPs 2 and 0.5 indicate the mole ratio H,(C ’ COz in the gas phase.
lations by Pineau et al, ( 1973) suggest that the increase in 6180- and 6’3C-values with time or with decreasing depth may be due to Rayleigh fractionation of the oxygen and carbon isotopes between crystallizing carbonate and a coexisting vapour phase. The Qaqarssuk Carbonatite Complex was mapped and sampled as part of a resource evaluation programme concerning phosphor, niobium and lanthanide accumulation of possible economical interest (Knudsen, 1989). The degree of exposure in the complex is good (25%) and 248 cores have been drilled in the complex (average 17 m long). As a consequence, it has been possible to identify a wide range in carbonatite types with well-known field and textural relations (Knudsen, 1991). Therefore the Qaqarssuk Carbonatite Complex is considered well suited for a study of the effects of primary igneous fractionation and wall rock contamination on the oxygen and carbon isotope distribution in a carbonatite magma. The aim of this study is: ( 1) To investigate the evolution of oxygen and carbon isotope distribution during the magmatic evolution of the Qaqarssuk Carbonatite Complex. (2 ) To trace exchange of oxygen and carbon isotopes with external sources during:
(a) Metasomatic alteration of the pre-existing basement. (b) Emplacement of narrow carbonatite veins in fenite. (c) Autometamorphic events such as hydration of olivine. (d) Late- and post-magmatic alteration. (3 ) To calculate a minimum intrusion temperature from oxygen and carbon isotope fractionation between calcite and dolomite, compared with the calcite-dolomite solvus geothermometer. 2. Experimental
technique
Samples from the different types of carbonatite present in the complex were crushed and carbonates in the 75-150-pm fraction were analysed. The modal calcite/dolomite ratio was determined by means of X-ray diffractometry (XRD ). Measurements of oxygen and carbon isotopic ratios were carried out by mass-spectrometric analyses of CO? liberated by treating the carbonate samples with 98O/o H,P03 at 25°C (McCrea, 1950). Pure calcite samples were reacted for 12 hr after which all calcite was dissolved. Pure dolomite samples were reacted for 24 hr. This period of time is not enough to ensure total dissolution of dolomite
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(e\‘en after 72 hr., some dolomite remains), but our experiments have demonstrated that the analytical error originating from incomplcte reaction is insignificant (less than the reproducibility) for both carbon and oxygen. Mixtures of calcite and dolomite were treated by a fractionated reaction procedure based on the much slower reaction rate of dolomite than calcite (Espstein et al., 1963; Fritz, 1967 ). The CO? extracted after 2 hr. of reaction is thus considered to represent only the calcite phase. After additional 4 hr. of reaction, all calcite has dissolved, but at the same time a considerable amount of dolomite has contributed to the CO,. and the gas is therefore pumped away. Finally. the gas extracted after additional 18 hr. is considered to represent only the dolomite phase. Acid fractionation factors for calcite and dolomite are 1.01025 and 1.01109, respecti\ ely (Sharma and Clayton, 1965), and dolomite values have been recalculated to calcite values. The mass spectrometer used is a Finnigan Mat’“’ 250, at the laboratory for stable isotopes. Geological Institute, University of Copenhagen. Results are given relative to PDB and SMOW (carbon and oxygen. respectively 1. and the data have been corrected according to Craig ( 1957). Reproducibility measured as standard deviation for 10 standard yampies is better than t0.02%0 for carbon and ? 0.07%0 for oxygen. The microprobe analyses of the carbonates were carried out on the JEOL” 733 Superprobe at the Institute of Mineralogy. University ofcopenhagen, using four wavelength-dispersive X-ray spectrometers ( 15 kV, 10 nA and beam diameter 10 pm, 10-s lifetime). The results were calibrated relative to natural calcite. dolomite and magnesite standards. 3, General geology of the Qaqarssuk C‘arbonatite Complex The Qaqarssuk Carbonatite Complex covers an arca of 15 km’. The complex consists of carhonatite ring-dykes, intruded into the Archean
(‘OMPLk.\(
265
basement 173 Ma ago (Larsen et al., 19X3), surrounded by metasomatically altered basement rocks. The ring-dykes have elliptical to semi-rectangular shapes (Fig. 2). The dykes generally dip outwards, and the steepest dips are found at the margin of the complex (Knudsen, 199 1). The carbonatites near the margin of the complex are generally very fine grained and often contain highly deformed, partly digested fenite inclusions. The carbonatite ring-dykes consist mainly of calcite-dominated carbonatite (sovite), olivine sovite and dolomite carbonatite. The sovite and dolomite carbonatite ringdykes are cut by coarse-grained late-stage snvite veins, REE (rare-earth element) carbonatite veins, ferrocarbonatite and lamprophyre dykes. There is a general increase in the content of Sr and light lanthanides in the carbonate with a simultaneous decrease in the Y/La ratio during the evolution of the Qaqarssuk Carbonatite Complex (Knudsen, 199 1 ). This trend is found in the whole-rock geochemistry as well as in zoned apatite crystals (Knudsen, 199 1). Olivine sovite is considered the most “primitive”carbonatite. Then follow the major sovite and dolomite carbonatite ring-dykes, while the coarse-grained late-stage sovite veins and REEcarbonatite veins and ferrocarbonatite dykes are the latest and most evolved carbonatites. The sovites contain increasing amounts of silicate minerals and inclusions of metasomatical altered basement towards the contact to the wall rock. and there is a gradational contact from the sovite to the metasomatically altered rocks via silicate-rich sovites. The degree of metasomatical imprint on the basement rocks decreases away from the complex as well as from the individual carbonatite ring-dykes. In the core of the complex the basement rocks are often altered to ultramafic rocks (glimmerite and hornblendite ), grading into fenites essentially composed of albite, alkali amphibole and alkali pyroxene. In the distal altered basement rocks quartz is preserved, and
266
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P and Nb Late stage
LIL]
REE
rich sovite
carbonatltc
r~:]
Basement
Ultramaflte
Fig. 2. Geological
map of the Qaqarssuk
Carbonatite
Complex
these rocks are characterised by hematite staining and alkali pyroxenes and alkali amphiboles along joints and fractures. Exsolution textures are frequently found in carbonate minerals in the main-stage carbonatites where the carbonatite has only been subjected to little (syn- to late-magmatic) deformation, whereas the carbonatite is totally recrystallized with calcite and dolomite occurring as separate crystals in more deformed carbonatites. This recrystallization indicates that
subsolidus equilibration of the carbonates is a widespread phenomenon in the carbonatite. The olivine in the olivine sovite has often been replaced by hydrous phases such as phlogopite or amphibole and by dolomite. Increasing replacement of olivine has been observed in the samples GGU 252520+32050~ + 3204144 320506 (Table I ). This indicates that water may have been introduced at a late- 01 post-magmatic stage. Intense low-grade alteration (sericitization
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of feldspar and hematitization of magnetite) is mainly located in the marginal part of the complex (Kjzrgaard et al., 1987), but locally ;I heavily altered, hematite-bearing carbonatite similar to “r0dberg” from the Fen Carbonatitc Complex, south Norway (Andersen, I984 ) is found in the central parts of the complex (
: WI:’
1~x1 data on calclrc
and dolomltc
from
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the Qaqarssuk Modal %I of GlrbO-
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4. Results The distribution of oxygen and carbon isotopes is given in Table I and illustrated in Figs. 3 and 4. 4. 1 Chrhon is0toyw.s in dcite
The (S1?Z-values in calcite are in the range from - 4.69 to - 3.25%0 PDB (except for the highly altered 252.540) and in dolomite from - 3.35 to -2.52°/oo PDB. There is a trend from
C’arbonatite
(%I \‘5. PDB)
und dolomilt~
Complex.
#V)
Partial
mineral
(% vs. SMOW)
(‘a0
MgO
SrO
54.2 29.9 53.2 ‘Y.5
(1.7 ix.4
0 3 0 I
0.7
0 3
20. I
0 z
anal>33
(wt.% __ F;cO
1
Whole
Mn( 1
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rock
1 ppm)
natec calcite dolomltc calute dolomite ‘(,?I
.,
XY
‘0.“
calcirc dolomite
‘il.?’ ‘I
calute dolomltc calcltc calcite calute dolomltc calcite dolomltc dolomltr calcite calcite calcite calcltc dolomltc calcite calclte nkerltc calcite calcite dolomite calcite calcltc calcite calclte
‘204
Ii
204 “.\
tine-gralned dolomite carbonatlte fine-gralned dolomite carbonatlte late stage s0vite late stage sevite tatc stage s0vite late stage s0wte REE-carbonatitc REE-carbonatite REE-carbonatite fcrrocarbonatlte lamprophyre hydrothcrmall) altered carbonatite calcite \cin calcite vein hornblcndile gllmmeritc
- 3.25 - 3.04 - 3.55 -3. I3
+ 7.06 f7.67 +7.11
- 3.55
+7.10
+7.c)3
0.2 3. I 0.1 I.6
0.I 0.2 0.3 0.4
lJ.20
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0 IX
Y93 646
+7.24 f8.1’)
31.0 79.6
2.0 20 2
0 3 0 I
0.2 7.0
0.2 0.5
0 ‘0
+6.90 + 7.80 + 7.40 + 7.02 +1.s7 + 7.50 +7.93 +X.84 + IO.39
51.9 19.3 52.7 52.5 48.4 79.7 55.3 2Y.Y 19.7
1.1 18.1 0.7 I.0 4.7 17.7 0.8 I I. I 16.4
0.4 0 2 I .(I 0.6 0.3 0.3 0.8 0.3 0.3
0.6 4.2 0.X 0.8 I.4 5.0 0.6 5.6 6.4
0.4 0.3 0.4 0.3 0.3 0.4 0.3 0.8 0.6
0. I Y
1.15x
YY 92 31) 70 59 41 IO0
~ 3.46 ~ 3.0X -4.68 - 4.06 - 3.27 - 3.11 - S.YO -3.35 -2.7-l
0 I3 0.13 0.18
7 799 _,__ 2.4OS 7 I0
(I.17
1.760
0.20
I.111
97 9Y IO0 I 00 Y5 99 90 81 80 47 53 96 95 98 9Y
-3.49 -4.6X - 4.60 -4.68 -7.84 - 3.6 I ~ 3.74 - 2.52 -4.69 ~ I .04 - 2.73 -3.57 ~-4.40 -4.20 -3.51
+7.53 i7.74 +7.50 +7.1 I +9.x +7.54 1-8.26 +9.93 +7.72 + 16.3’) + IO.58 +9.1 I +8.75 + 8.10 +.X.46
50.9 49.9
0.4 I.8
I 3 0.8
0.5 1.1
OS 0.6
0. I7 0. IO 0. I I
7.537 ~,I09
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0.4
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0.7
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0
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0
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V
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0’
UltramaflL
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A
4.2. Oxygen isotopes The 6 ‘*O-values
in calcite
is in the range
carbonatlte rock
CalcIte
vein
Strongly
Fig. 3. Plot of 6’% PDB vs. 6’80sMow for carbonates from the Qaqarssuk from the same sample. One sample (GGU 252540 calcite) is off-scale.
early, “primitive” carbonatite (low Sr and REE and high Y/La ratio), which is relatively 13Cenriched, to late-stage, “evolved” carbonatite (high Sr and REE and low Y/La ratio ) which is relatively 13C depleted (Figs. 3 and 5; Table I). An exception from this pattern are the REEcarbonatite veins which have high Sr, high REE, low Y/La and are relatively rich in “C. The trend is most clearly seen in the calcites. There is a similar trend in the early dolomites, but not seen in the sparse data on the late dolomites (Fig. 5 ). It should be noticed that the ferrocarbonatite is relatively rich in 13C.
SOVlte
Lamprophyre d.’
Ferrocarbonatlte
stage
dltered
carbonatlte
Carbonatite
Complex.
LJWS connect m~nc‘~-al~
from + 6.90 to + 9.1 l%o SMOW (except for 252540) and in dolomite from +7.50 to + 10.58°/oo SMOW. There is no relation between a’*O-values and the geochemical evolution in the carbonatites as described for the carbon isotopes above. Fresh and uncontaminated carbonatites have 6l*O-values in the range +6.90 to +7.53%/m SMOW. There are increased 6 “O-values in the samples which are formed by interaction between carbonatite and basement (Fig. 3 ), with 6”0values from + 8.20 to + 8.46%0 SMOW in calcite from ultramafic rocks (hornblendite and glimmerite ) formed by metasomatic alteration of wall rock. Fine-grained dolomite carbonatite, ferrocarbonatite and lamprophyre
L 5x10-h
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.
.
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. .
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b.motof‘I : 7-z5 ,,,,“,\‘s.L~fxo,,_cc. c. I’lotof I’Y‘,,( ( vs. 3”0,,_, C, The .solrd
rqfiwnc~t~
JLht I,).( .+0.35. d. Plot of 7,,,,,,, vs. ‘Z;,. .rcr,,and 7.c are temperatures ti\ cly. between dolomite-calcite pairs. e. I’lot of ‘f,. vs. -To.
5OO’C
TO
calculated on Mg fraction between co-existing
lrnc is from Sheppard
and Schwartz
( 1970) with z1’3CI, ,-,=0.37
( ‘C) calculated on oxygen and carbon isostope fractionation.
dykes rich in partly digested basement inclusions, calcite veins in the wall rock, and REEcarbonatite veins have 8’0-values of +7.54 to +9.93%0 SMOW. The olivine sovite in which the olivine has been hydrated, dolomite and calcite have elevated 6’80-values relative to dolomite and calcite in the olivine sevite with fresh olivine. In
dolomite
the “rodberg” /I”0-values SMOW are found.
respec-
up to + I 6.39°/oo
4.3. Geothermomutr~~ Dolomite is enriched in 13C and I80 as compared to calcite in samples where both phases have been analysed (Fig. 3 ) . This is in accord-
(‘. KNI’DSEN
I
I
AND
B ljll(
H-\KI)
I
tion between the dolomite-calcite pairs ( Fig. 3). LI’~O,,_~(.,=1000In CY”~,,_(i j’ =0.45*10hT ‘-0.40 1 12i AJC I>_(..= < 1000 In cy’“CI)_ct ! =0.18.10hT- ‘+().I7 :
and T
(Cal-tx1n
, = [0.18-10h/(~1’-~C,)~(c-0.17~]~ i3h)
-5I 0
I
05
Fig. 5. Hot of 6°C PI)B vs. wt.% SrO in carbonates from the Qaqarssuk Carbonatitc Complex. Lines connect calcite-dolomite pairs in the same sample. S.wnho/s as Fig. 2.
ante with the findings of Shieh and Taylor (1969), Deines (1970), and Sheppard and Schwarz (1970), whereas Andersen ( 1987) finds that 6’*0,,,<6’80,, in coexisting calcite and dolomite from the Fen Carbonatite Complex. One exception from the pattern in the Qaqarssuk carbonatites is the “rodberg”, which contains calcite veins likely to be deposited later than the ankerite and hencenot likely to have been in equilibrium with the dolomite. The change in isotopic fractionation with temperature between calcite and dolomite can be expressed as a general fractionation equation: 1000 In a=A/T*+B
(1)
where czis the fractionation factor; A and B are constants;and T is the temperature in K. Thus, decreasing Twill lead to increasing fractiona-
where d denotes the difference in %oobetween the &values for co-existing dolomite and calcite. This gives temperatures (Table II ) in the range from 608” to 304’C on the oxygen isotope distribution between calcite and dolomite, and from 1850” to 415’C on the carbon isotopes. (d”C,-,., in sample 32042I is too low to give a meaningful temperature). T,, and T,- are not identical, T,, generally yielding a lower temperature than T(:. but there seemsto be a relation between T,, and 7;. ( Fig. 4e) as there is a relation betweend’“C,, , j and d’80u_(.c (Fig. 4~). For comparison the temperature has been calculated on the basis of the Mg-calcite solvus thermometer using the relation (Gittins, 1979): (4) log,,, x&u,, = - 1690/T (K)+0.795 TABLE
2
Stable isotope and Mg fractlonatlon paws, and the calculated temperatures atite <‘omplex Sample GGU No.
-f”C,,-, ( (?60 vs. PDB)
data from calcite-dolomltc for the Qaqarssuk Cyarhon--.-_I_.~~
.~~
,l’80n_C.C (o/o0 YS. SMOW)
Molt% MgCO, in calcltc
(I; c) y
;c;,,
;;<;:<;.
1.68 10.47 I .9z 2.06 3.00 5.15
1848 822 415 653 S76
394 608 334 313 315 304
384 679 400 40X 4i6 53x
7i-‘s/o
0.2 I
0.61
_(.?041 I 3204l4 .?20430 j20460 320507
0.06 0.32 0.55 0.38 0.42
0.18 0.82 0.91 0.90 0.95
__~
-.~
( \YI)
t ISo7OI’L
U)MPOSIl’IOKOF
(~4KROUZTES
FKOMTHE()~O-\KSSII1((‘~KHON.~TlTF(
FeCO,<
f g. 6 f‘riangular plot of molecular proportions of Ca. Mg and Fc in carbonates. The compositional fields show the ccampositional variation observed in the Qaqarssuk Carbonatlte C‘omplex. L~nec- connect calcite-dolomite pairs i? the \amc sample. .~~vJ/w~ as in Fig. 2.
The calculated temperatures (Table II ) are generally low compared to igneous temperatures (600-900°C; Gittins, 1979) which can be expected in carbonatites. This is consistent with the observation of subsolidus re-equilibration (e.g., exsolution) observed in the carbonates. The compositions of the carbonates are illustrated in Fig. 6. The solvus temperatures do not show a systematic relation to the isotope temperatures (Fig. 4a and b), but they give temperatures in the same range as T,,. Solv us temperatures have been calculated for 14 other carbonatite samples and fall in the range 273-598’C. 5. Discussion
The general evolution towards enrichment in Sr in the carbonates coupled with depletion in ’ C indicates that there was a general ‘-‘C depletion during the evolution of the carbonatite, from the main-stage carbonatites to the late-stage s0vite veins. The REE-carbonatites have high Sr and high a’%-values relative to the late sravite veins.
OLlI’l.L:\
??I
There is no experimental result available about the fractionation of carbon isotopes between carbonate crystals, carbonatite melt and CO?. According to Rayleigh fractionation models presented by Pineau et al. ( 1973 ) and Deines ( 1989)) an enrichment in ‘jC is to be expected during evolution of carbonatites. This is the opposite of what is seen from main-stage carbonatites to late-stage s0vite in the Qaqarssuk Carbonatite Complex, and this enrichment in light carbon is not likely to be caused by fractionation by e.g., crystal separation. On the basis of the low S’C-values found in the carbonatite lavas of Oldoinyo Lengai, Tanganyika, Suwa et al. ( 1975) suggested that heavy carbon was preferentially fractionated into and lost with the gas phase. Theoretical computations by Bottinga ( 1968 ) indicate that “C-enriched carbon will be fractionated into a gas phase relative to calcite at temperatures of > 200°C. According to this, the depletion in heavy carbon from the main-stage carhonatites to the late-stage sarvite can be explained as loss of heavy carbon to a gas phase. Volatiles were lost: ( 1) by diffusion into the wall rock during the fenitisation; (2 ) through veins; and (3 ) through explosive events forming, e.g. the ferrocarbonatites. It can accordingly be expected that carbonates precipitated from volatites in the metasomatically altered rocks, in veins and in ferrocarbonatites are enriched in “C relative to the late-stage s0vite. The REE-carbonatites are rather porous ( miarolitic) and have been interpreted as deposited in equilibrium with a gas phase (Knudsen, 199 1). The REE-carbonatites as well as the ferrocarbonatite have elevated 6 ’ ‘C-values (and 6’80-values). This ‘jC (and “0) enrichment is similar to the evolution predicted by Pineau et al. ( 1973) and Deines ( 1989) using a Rayleigh fractionation model (Figs. 3 and 7), and similar to the evolution described by Nielsen and Buchardt ( 1985 ) . Another possibility for generation of the elevated G”C-values (and 6”O-values) in the very late veins and dykes is that the porous
C‘.KNUDSEN
Fig. 7. Plot of compositional fields and evolutionary trends for the 6’YI PDR vs. S’*OsMow in carbonates from the Qaqarssuk Carbonatite Complex, compared with other “subvolcanic” carbonatite complexes. The compositional fields ( 1-4) are from Deines and Gold ( 1973).
REE-carbonatite veins and ferrocarbonatite dykes may have been affected by a very late stage of hydrothermal alteration. This is seen in the marginal parts of the carbonatite (Kjzrgaard et al., 1987) where the ferrocarbonatite dykes are located. As the REE-carbonatite samples are collected in the central part of the complex, this explanation is less likely for their part.
5.2 The oxygen isotopes In the unaltered and uncontaminated carbonatites there is fairly little variation in the oxygen isotopic composition ranging from +6.9 to + 7.5% SMOW in calcite. This is within the 6180 range of primary, high-temperature carbonatites ( + 6 to + 8%0) of Deines and Gold ( 1973 ). There is no major increase in IgO from early carbonatites to the late-stage sovite. This is compatible with the small fractionation of oxygen isotopes between minerals and fluid at high temperatures (O’Neil et al., 1969). Fractionation of oxygen between the carbonates, silicates and oxides has not been investigated in the present work. 180-enriched oxygen will be preferentially fractionated into the carbonates relative to silicates and oxides
4NDB.BI'C'HAKL)I
olivine, phlogopite and magnetite ) onway and Taylor, 1969 ). If the fractionaI E”‘? tion of oxygen between carbonates and silicates was of importance this would give a relative enrichment in 6”O in carbonates in samples rich in silicates (e.g.. GGU 320507). This is not seen, and fractionation of oxygen between carbonates and silicates, at least in the early magmatic stage, is regarded as being of minor importance. The calcite in metasomatically altered rocks, in calcite veins in the basement and in lamprophyre (rich in partly digested basement inclusions) all have elevated 6”O-values in the range from + 7.5 to + 16%0 SMOW. The local Archaean, mainly granodioritic basement is likely to have a d”O-value in the range + 7 to + 13%/m SMOW (Taylor, 1974), and the carbonates are considered to have been contaminated by exchange of oxygen with these “Oenriched basement rocks. Moreover, the ‘“Oenriched oxygen in the basement would have been preferentially fractionated into the carbonates if there had been an exchange of oxygen carbonate and silicates during the metasomatic alteration. The high 6”O-values in REE-carbonatites and ferrocarbonatites relative to the main-stage carbonatites and late-stage sovite veins may be explained as: ( 1) loss of isotopically light water during pressure release; (2 ) influx of meteoric water, and (3) equilibration with magmatic water at low temperatures or a vapour phase. These alternatives can explain the observations with regard to the oxygen isotopes. However, loss of the gas phase would cause a depleof > 200” C tion in “C at temperatures (Bottinga, 1968) and, as 613C is elevated in these rocks, the first alternative is regarded as less likely. The second alternative may explain the high 6180-values found in the marginally located ferrocarbonatites, but is regarded as less likely in the REE-carbonatites in the core of the complex. The REE-carbonatites and ferrocarbona-
tites have isotopic compositions (Fig. 3) as could be expected by Rayleigh fractionation between carbonates and a vapour phase (Pincau et al., 1973) (Fig. 1) with a starting compositron similar to the late-stage sovites (Fig. 3 ). and this is considered the most likely explanation. The carbonatites with hydrated olivines have htgher 6’xO-values than the unaltered olivine sovite. This is consistent with the interpretat ron of the alteration of olivine as being due to equilibration with magmatic water at lower temperatures. It is not likely to be due to influx of meteoric water, because the equilibration stopped at a fairly high temperature. at which equilibration with meteoric water would lead to depletion in 180.
There is a relation between S’80L)-c‘c and l’3c,)-.(.I (Fig. 4c) as well as between the car!Jon and oxygen isotopic temperatures (Fig. -te ) . but with a slightly different slope (Fig. 4c ) and a very different axial intercept compared to what was found by Sheppard and Schwartz ( 1970). The fairly poor correspondence between the carbon and oxygen iosotopic temperatures indicates that the calibration used (based on metamorphic limestone: Sheppard and Schwartz, 1970) cannot be directly applied to the carbonatite material. The oxygen isotopic temperatures (To) fall in the same range as the solvus temperature (Fig. 4d ) , whereas the carbon isotopic temperatures ( To) generally are higher than the solvus temperatures. However, the present material is too small, and the correspondence bet ween isotope temperatures and solvus temperatures is too poor to propose a new calibration of the isotopic temperatures. 6. Conclusions 1 ) Geochemically
tive”
olivine
and isotopically “primisovite, sovite and dolomite car-
bonatite indicate that the 6’“O and 6’C ( isoprimitive topic composition ) of the carbonatite magma was in the range + 7 to +7.5% SMOW and -3.5 to -3.1°/oo PDB, respectively. (2 ) There was a depletion in heavy carbon isotopes from deposition of main-stage carbonatite to late-stage sovite (Fig. 7). This is likely to be caused by loss of heavy carbon isotopes to a gas phase. ( 3 ) Carbonates in metasomatically altered basement and contaminated carbonatite have elevated 6’80-values, likely to be caused by exchange of oxygen with the basement. (4) Isotope geothermometry gives temperatures below expected igneous temperatures, explained as caused by subsolidus reactions in the carbonates. The results show that the calibration of the isotope geothermometer established in metamorphic carbonate rocks cannot be directly applied in carbonatites. Acknowledgements Thanks are due to J. Ronsbo for guiding the microprobe analysis, to L.M. Larsen and A.K. Pedersen for valuable comments on the manuscript. The field work was supported by the European Economic Community through contract No. MSM 119DK. C.K. is funded by the Danish Natural Science Research Council. contract No. 1 l-5835. The mass spectrometer facility is funded by the Danish Natural Science Research Council, grant No. 1 l-9372. Publication is authorised by the Director of the Geological Survey of Greenland. References .Andersen. T.. 1984. Secondary processes in carbonatitcs: petrology of “rodberg” (hematite-calcite-dolomitecarbonatite) in the Fen Central Complex. Tclcmark (South Norway ). Lithos, 7: 127-145. Andersen. T.. 1987. Mantle and crustal components in a carbonatite complex, and the evolution of carbonatite magma: KEE and isotopic evidence from the Fen com-
274 plex. southeast Norway. Chem. Geol. (Isot. Geosci. Sect.), 65: 147-I 66. Bottinga, Y.. 1968. Calculation of fractionation factors for carbon and oxygen isotopic exchange in the system calcite carbon dioxide water. J. Phys. Chem., 72: 800808. Conway, C‘.M. and Taylor. H.P.. 1969. “Q/“‘O and “C/ 12C’ratios of coexisting minerals in the Oka and Magnet Cove carbonatite bodies. J. Geol.. 77: 618-626. Craig, H.. 1957. Isotopic standards for carbon and oxygen and correction factors for mass spectrometric analyses of carbon dioxid. Geochim. Cosmochim. Acta. 12: 133-149. Deines. P.. 1970. The carbon and oxygen isotopic composition of carbonates from the Oka carbonatite complex, Quebec, Canada. Geochim. Cosmochim. .4cta. 34: 1199-1225. Deines. P.. 1989. Stable isotope variations in carbonatites. In: K. Bell (Editor), Carbonatites: Genesis and Evolution. Unwin Hyman, London, pp. 301-359. Deines, P. and Gold, D.P., 1973. The isotoptc composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deapseated carbon. Geochim. Cosmochim. Acta. 37: 1709-l 733. Epstein, S., Graf, D.L. and Degens. E.T., 1963. Oxygen isotope studies on hte origin of dolomite. In: H. Craig, S.L. Miller and G.J. Wasserburg (Editors), Isotope and Cosmic Chemistry. North-Holland Publishing Co., Amsterdam, pp. 169- 180. Fritz. I’., 1967. Oxygen and carbon isotope composition of carbonates from the Jura of Southern Germany. Can. J. Earth Sci., 4: 1247-l 267. Gittins. J., 1979. Problems inherent in the application of calcite-dolomite geothermometry to carbonates. Contrib. Mineral. Petrol.. 69: l-4. Kjargaard, M.. Knudsen, C. and Abrahamsen, N., 1987. Geophysical investigations of the Qaqarssuk Carbonatite Complex, southern West Greenland. Rapp. Gronl. Geol. Unders., 125: 34-40. Knudsen, C.. 1989. Pyrochlore group minerals from the Qaqarssuk Carbonatite Complex. In: P. Moller (Editor), Lanthanides, Tantalum and Niobium. Springer. Berlin, pp. 80-99.
(‘ KNl:IXjtN,ANI)
B. HlI(
HL\KL)I
Knudsen, c‘., 1991. Petrology, geochemistry and cconomic geology of the Qaqarssuk Carbonatite C‘omplex. southern West Greenland. Miner. Deposna. Monogr. Ser. 2V. I. Larsen, L.M., Rex. D.C. and Secher. K., 1Y83. The age ol carbonatites, kimberlitcs and lamprophyres from southern West Greenland: recurrent alkaline magmatism during 2500 million years. Lithos. 16: 2 15-X I McCrea. J.M., 1950. On the isotope chemistry ofcarbonales and a paleotemperature scale. J. Chcm. Phys., IX. 849-857. Nielsen. T.F.D. and Buchardt, B.. 1985. Sr-C -0 isotopes in nephelinitc rocks and carbonatites. Gardincr complex, Tertiary of East Greenland. Chcm. Geol.. 53: 707217. O’Neil. J.R., Clayton. R.N. and Mayeda. T.K.. 19hY. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys.. 5 I: 5547-5558. Pineau, F.. Javoy. M. and .4llegre. C.J., 1973. Etude systematique des isotopes de I’oxygdne, du carbonc et du strontium dans les carbonatites. Geochim. C’OS~Ichim. Xcta. 37: 2363-2377. Sharma. T. and Clayton, R.. 1965. Measurements of ‘“( )I IhO-ratios of total oxygen from carbonatites. Cicochim. Cosmochim. .4cta. 29: 1347- 1353. Sheppard, S.4.F. and Schwartz. H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between coexisting metamorphic calcite and dolomite. Contrib. Mineral. Petrol.. 26: 16 I- 198. Shich. Y.N. and Taylor, H.P.. 1969. Oxygen and carbon isotope studies of contact metamorphism of carbonate rocks. J. Petrol.. IO: 307-33 I. Suwa, K., Oana, S., Wada. H. and Osaka. S.. 1975. Isotope geochemistry and petrology of .African carbonatitcs Phys. Chem. Earth, 9: 735-745. Taylor. H.P., 1974. The application of oxygen and hydrogen isotopy studies to problems of hydrothermal aiteration and ore deposition. Econ. Gcol.. 69: 843-883. Taylor, H.P., Frechen, J. and Degens. E.T.. 1967. Oxygen and carbon isotope studies of carbonatites from Laacher See district, West Germany, and the r\lno district, Sweden. Geochim. Cosmochim. Acta. 3 1: 407430.