- Email: [email protected]
Dolomite of possible mantle origin, southeast Sinai
Chemical Geology, 56 (1986) 281--288
281
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
[1]
DOLOMITE OF POSSIBLE MANTLE ORIGIN, SOUTHEAST SINAI R O N B O G O C H 1, M O R D E C K A I M A G A R I T Z 2 and A N N I E M I C H A R D 3 Geological Survey o f Israel, Jerusalem 95 501 (Israel) 2 Isotope Departmen t, Weizmann Institute o f Science, R e h o v o t 76 1 O0 (Israel) 3 Centre de Recherches, P~trographiques et G~ochimiques, N a n c y (France)
(Received July 25, 1985; accepted for publication March 10, 1986) Abstract Bogoch, R., Magaritz, M. and Michard, A., 1986. Dolomite of possible mantle origin, southeast Sinai. Chem. Geol., 56: 281--288. Dolomite and breunnerite occur in large amounts as veins and tabular bodies in association with the Tarr albitite within late Precambrian greenschist-facies metamorphics of southeast Sinai. Isotopic data for the dolomite (mean 6180 = +6.9%0; mean 513C =--8.1%0; mean 5 D =--65%0; mean end = +3.4;mean ~Sr/S~Sr = 0.70422), together with their low incompatible-element and high transition-metal contents, suggest that the dolomite derived from a mantle source. The isotopic composition of the breunnerite indicates contamination by metamorphic water during the replacement phase of the dolomite by the breunnerite.
1. I n t r o d u c t i o n E x p e r i m e n t a l studies (Wyllie, 1 9 7 8 ; Eggler e t al., 1 9 7 9 ) have established t h a t c a r b o n a t e minerals ( d o l o m i t e , magnesite) can be stable phases in p e r i d o t i t i c assemblages in the m a n t l e e n v i r o n m e n t . T h e i m p o r t a n c e o f CO2 in m a n t l e m e t a s o m a t i s m has b e e n s u m m a r i z e d b y Spera (1981), and c a r b o n a t i t e s in alkaline r o c k associations originated (pre-immiscibili t y ) d e e p in t h e u p p e r m a n t l e (Le Bas, 1981). D i r e c t e v i d e n c e f o r t h e p r e s e n c e o f CO2 in the m a n t l e is given in several r e c e n t papers (e.g., Pineau and J a v o y , 1 9 8 3 ; Byers et al., 1984). H o w e v e r , o n l y rarely have m a n t l e c a r b o n a t e minerals, as sampled f r o m kimberlitic x e n o liths, b e e n n o t e d , and t h e n in m i n u t e a m o u n t s ( M c G e t c h i n and B e s a n c o n , 1 9 7 3 ; Hervig and Smith, 1981). D o l o m i t e and b r e u n n e r i t e (Fe-magnesite) o c c u r in relatively large a m o u n t s in associa0009-2541/86/$03.50
t i o n with t h e albitite c o m p l e x at Wadi Tarr in s o u t h e a s t Sinai. An earlier s t u d y o f the tracee l e m e n t c h e m i s t r y o f these c a r b o n a t e s (enr i c h m e n t in t r a n s i t i o n metals and Sc, and dep l e t i o n in Sr and the L R E E ) ( B o g o c h et al., 1 9 8 4 ) led t o the c o n c l u s i o n t h a t t h e depositing fluids had b e e n in e q u i l i b r i u m with mafic/ u l t r a m a f i c material. H o w e v e r , this c o u l d imply t h a t t h e t r a c e - e l e m e n t assemblage derived f r o m ferromagnesian minerals a t t a c k e d b y CO2-bearing solutions o f an u n r e l a t e d source. This s t u d y presents stable (C, H and O) and radiogenic (Sr, Nd) i s o t o p e m e a s u r e m e n t s on these carbonates, which indicate a m a n t l e origin for the d o l o m i t e . 2. D e s c r i p t i o n o f the c a r b o n a t e s T h e T a r r albitite c o m p l e x consists o f metasomatic albitite bodies (totalling > 2 kin2; Bogoch, 1 9 8 3 ) with associated actinolite-rich
© 1986 Elsevier Science Publishers B.V.
282 rocks, and dolomite--breunnerite. The disposition of the complex is strongly controlled by two shear zones within Late Proterozoic, poly-metamorphosed meta-sedimentary and meta-pyroclastic rocks, mainly in the greenschist facies. Within the shear zones are large (up to 100 m across), rotated, allochthonous, tectonically emplaced blocks of feldsparfree hornblendite and gabbro, the former exhibiting ghost textures of an olivine--pyroxene ultramafite. The dolomite and breunnerite have been described in detail (Bogoch et al., 1982; Bogoch, 1983). Dolomite is the dominant (~70%) species. On fresh surface, the dolomite is light buff coloured and the breunnerite is medium to dark brown. Both minerals weather to a dark beige colour. These carbonates occur as veins (generally anastamosing), several centimeters to 1 m in width, as breccia cement in shear zone fault rocks, and as sheet-like bodies. The largest of the latter is 200 m long, 60 m wide and 10 m thick. The dolomite is very coarsely crystalline, from ~0.5 cm to rarely 20 cm on cleavage faces. Breunnerite varies from several millimeters to 5 cm in m a x i m u m diameter. Flow textures are absent, but at a few sites, there is a tendency for the dolomite to grow with its long axis perpendicular to the wall of the host rock. The breunnerite is seen to replace dolomite in several locations, and this paragenesis is assumed for most occurrences. The coarsegrain size of the dolomite and its local growth perpendicular from the walls indicate deposition from a (non-magma) fluid. 3. Methods Carbon dioxide for mass spectrometric analyses was obtained by reacting the dolomite and breunnerite with 95% phosphoric acid (specific gravity 1.80) at 50°C for at least 48 hr., with yields greater than 95%. The CO2 was analysed with a Varian ® M250 mass spectrometer. Water was extracted from fluid inclusions in carbonate by crushing the samples in an
evacuated stainless-steel tube. The water, after purification, was converted to hydrogen by passing it over uranium at 800°C (Godfrey, 1962). Results are expressed in the conventional " 5 " per mil notation. Hydrogen and oxygen isotopes are relative to SMOW and carbon isotopes relative to PDB (NBS 1 9 : 5 1 8 0 = -2.200/00 PDB; 513C = +1.96°/00 PDB). The phosphoric acid procedure is known to involve a kinetic isotope effect on oxygen (Sharma and Clayton, 1965). The c o m m o n l y used value (0.84o/00) was determined for 25°C and was used in this study since no correction for 50°C is available. Sr and Nd isotopic compositions and Rb, Sr, Sm and Nd concentrations were determined on ~100-mg of sample spiked before attack by HC1 using the technique described by Michard et al. (1985). For samples with low REE contents, a first enrichment was obtained by REE coprecipitation with ferric hydroxide followed by Fe removal on an anion-exchange column. Isotope standards analysed in the laboratory gave the following results: Eimer & Amend ® STSr/S6Sr = 0.707985 +- 30; BCR1 '43Nd/144Nd = 0.512639 + 15. 4. Results and discussion In terms of their trace-element geochemistry and istopes, the Tarr samples are apparently anomalous. Relative to other igneous, metamorphic and sedimentary carbonates, they are characterized by high Sc and transition-metal contents and by a depletion in Sr and the REE (Table I; see Fig. 3). Their unusually low Sr, Ba and REE contents would appear to eliminate their classification as carbonatites as characterized by Le Bas
(1981). 6'~O varies from +5.3 to +8.5°/00 in the dolomite and from +15.9 to +22.1°/00 in the breunnerite; 513C-values have a narrower range, from - 7 . 5 to -8.30/00 in dolomite and - 4 . 6 to -7.70/00 in the breunnerite. These data are represented in Table II and Fig. 1, to-
283 TABLE I
TABLE II
Selected trace-element data (in ppm) of Tarr dolomite and breunnerite (after Bogoch, 1983 ; Bogoch et at., 1984)
6~sO -, 6~3C- and 6D-values (in °/00) in Tarr area marbles, and dolomite and breunnerite
Dolomite Cr Ni Sc Sr Rb Ba Nb La Ce Nd Sm Eu Yb Lu
77 21 70 141 <5 <100 <3 6.8 15.9 7.7 1.7 0.8 1.4 0.2
215 293 75 76 <5 <100 <3 0.4 0.7 0.7 0.07 0.06 0.6 0.2
ii
\\ \
/ /
20
\\
,/
t
I'
/
©
•
• ///
/ t
u3
./
o
/
0 "~ o' • ~ \ .\ i c \
/I
\,
>15
J• i I 'L \
c ©
Rock/mineral
6 ~80
5 ~3C
W-393A W-393C W-318 W-335 W-348 W-262A W-195B W-319F W-405 W-324C W-324A W-98 W-349 W-133 W-372-1B W-341 W-342 W-262A
marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic
+13.8 +14.2 +12.7 +18.4 +16.7 +17.1 +18.0 +15.2 +14.5 +10.7 +15.0 +11.7 +13.7 +13.9 +17.6 +18.3 +17.8 +16.7
+0.9 1.7 -2.8 -5.1 -3.2 -3.2 -2.7 -4.8 -5.0 -2.3 -3.6 -4.5 -1.0 -3.0 -3.6 -3.6 -3.2 -2.7
W-IO5B W-IO2F W-IO2A
dolomite dolomite dolomite
+5.3 +5.5 +8.5
-8.1 7.5 8.4
W-327C W-355
dolomite dolomite
+7.8 +7.4
-8.3 8.3
W-105C W-IO2B W-265A W-347
breunnerite breunnerite breunnerite breunnerite
+15.9 +18.3 +19.1 +22.1
-7.7 -6.0 - 4.6 6.1
5D
c
•
•
/
\ (;
/
-59 -69, -66, -66
\
(;
.2 C~
Sample Breunnerite
-91
//
To o io L5 D o l o m i t e
'1
/
•
Breunnerile
()
Morble
•
Merble
colcih¢ -
dolornlhc
5 L
io
I
I
I
-5 0 813C%o (relative to PDB)
Fig. 1. 6 ~80 vs. a ~3C diagram of dolomite, breunnerite and country rock marbles.
gether with stable isotope data obtained on samples of calcite and dolomite marbles found in the Precambrian country rocks.
T h e o x y g e n i s o t o p e s o f t h e d o l o m i t e are w i t h i n t h e r a n g e f o r p r i m a r y i g n e o u s carb o n a t i t e ( T a y l o r e t al., 1 9 6 7 ) . T w o o f t h e s a m ples h a v e 61SO-values w i t h i n t h e m o r e restricted range suggested by Boettcher and O'Neil (1980) for the primary mantle. The o t h e r t h r e e s a m p l e s are s l i g h t l y 1sO e n r i c h e d w h i c h are c o n s i s t e n t w i t h t h e l a t e - e n r i c h m e n t m o d e l in c a r b o n a t i t e s p r e s e n t e d by P i n e a u e t al. ( 1 9 7 3 ) . T h e c a r b o n i s o t o p e s are c l o s e t o the range characterizing carbon-bearing material d e r i v e d f r o m t h e m a n t l e . S u c h v a l u e s w e r e r e p o r t e d n o t o n l y i n c a r b o n a t i t e s , b u t also i n d i a m o n d s ( D e i n e s , 1 9 8 0 ; V i n o g r a d o v e t al., 1 9 8 0 ; J a v o y e t al., 1 9 8 4 ) , k i m b e r l i t e ( D e i n e s a n d G o l d , 1 9 7 3 ) , v o l c a n i c gases as i n A f a r
284 (Allard, 1979), CO2 trapped in mid-ocean ridge basalt (Pineau et al., 1976; Moore et al., 1977; Pineau and Javoy, 1983), or more generally, carbon in oceanic basalt (Des Marais and Moore, 1984). Certain primary dolomite samples from carbonatites yield 51SO-values similar to those at Tarr [e,g., Fen, Norway -- sample P-20 (Pineau et al., 1973); Carghill, Ontario, Canada (Dontsova et al., 1977)]. 5D-values found in fluid inclusions in dolomite are within the range ( - 5 8 to - 7 9 % 0 ) suggested for mantle H20, based on hydrogen phases in kimberlite (Boettcher and O'Neil, 1980). The oxygen isotope composition of the breunnerite (Fe-bearing magnesite) is much more enriched in 180 and slightly in 13C. Although the isotopic fractionation between magnesite and water is not known, it may be possible to estimate the values from published data on the oxygen isotope fractionation. Using the data of O'Neil et al. 4
OL
0.50
I
0.75
I
1.00 Ionic radii (A)
1.25
Fig. 2. Range o f 1000 ]n cx ( ~ 1 8 0 ) b e t w e e n MgCO8 and w a t e r at 500°C; calculations based on linear e x t r a p o l a t i o n of CaCO3, SrCO3, and BaCO 3 (data f r o m O'Neit et al., 1969) and non-linear e x t r a p o l a t i o n t h r o u g h " d o l o m i t e " points ( X ) calculated f r o m dolom i t e - c a l c i t e fractionation (data from O'Neil and Ep-
stein, 1967; Sheppard and Schwartz, 1970).
(1969) to trace the variation of 61sO with ionic radius, we can roughly estimate the breunnerite--water fractionation to be 2.5 to 3.5% 0 at 500°C (Fig. 2). In any case, the dolomite--magnesite fractionation cannot be very large, so that the large difference in 6180-values between the two minerals suggests that an additional source of water at lower temperature was involved in the breunneritization of the dolomite. The 8;sO in the breunnerite is similar to that found in the carbonate rocks of the metamorphic terrane (Fig. 1), suggesting that the fluid from which the breunnerite formed was contaminated by a late metamorphic fluid. Although only one ~D-value (-91°/00) was obtained in the breunnerite (very low amount of water in the samples), it is distinctly different from that of the dolomite (Table II). The 613C-values are n o t equal in the dolomite and breunnerite. Fig. 1 suggests that the 513C of breunnerite may have moved slightly from dolomite values towards values of meta-sedimentary carbonates. They are intermediate between dolomite and marbles. The isotope record of the breunnerite could result from complete exchange of oxygen isotopes of the replaced dolomite and partial exchange or replacement for carbon with the marbles (Pineau et al., 1973). The preservation of mantle carbon isotopic composition of the dolomite in the closely spatially associated breunnerite, is an indication of the preservation of the original carbonate (carbon isotope) values. The same relation between the later fluid and the isotope data is typical of most carbonate recrystallization processes at low and medium temperatures. It results from the difference in the water/ rock ratio in solutions for carbon and oxygen isotopes; the carbon is dominated by the rock, and oxygen by the fluid (Pineau et al., 1973; Magaritz, 1983). A similar model was suggested by Blattner and Cooper (1974) in their study of carbonatire veins in metamorphic terrain of the Haast Schist in New Zealand, where the emplace-
285
ment of the carbonate veins was associated with the end of the metamorphic phase. The carbon isotope mantle values were preserved while the oxygen values were shifted by several per mil due to the effects of metamorphism. The 61sO enrichment in the late fluids associated with formation of breunnerite from dolomite is similar to that in platform carbonatites (Pineau et al., 1973; Dontsova et al., 1977). In the Kordor intrusion of the Kolo province (N.W. Russian Republic, U.S.S.R.), the late Fe-rich phase is enriched by 4--50/00 relative to the earlier-stage calcite (7.3--8.7%o). In the Tanga intrusion in the East Sayan province (E. Russian Republic, U.S.S.R.), the enrichment in the Fe phase is 6--8%0 (from 6.4--7.30/00 to 13.2-- 14.4°/00). Radiogenic isotope data are presented in Table III. The measured *43Nd/144Nd and STSr/S6Sr were corrected for in situ decay (from the time of formation), assuming an age of 600 Ma based on the late Proterozoic age of the area (Bielski, 1982). The range of eNd (+2.5 to +4.7) and STSr/S6Sr (0.70348 to 0.70562) values o f the dolomite are commensurate with data of mantlederived silicate rocks (e.g., DePaolo, 1980; Richard and All~gre, 1980; White and Hofmann, 1982; Stern and Hedge, 1985), and for the Sr isotopes, with carbonatites (Faure and Powell, 1972; Samoylov et al., 1980). The eNd-Values in the breunnerite preserve the primary dolomite mantle values. The displace-
ment of Sr isotope values to higher ratios while preserving the original Nd isotope values is typical of crustal alteration (e.g., Hawkesworth et al., 1979). The involvement o f crustal fluid from the metamorphic terrain as suggested by the oxygen isotope data, is supported by the high STSr/S6Sr ratios in the breunnerite (Shimron et al., 1973; Table III). The low REE, Sr and Rb contents in the dolomite rule out a source of metasomatized mantle as suggested for most xenoliths in alkali basalt and kimberlite (Boettcher and O'Neil, 1980; Wass and Rogers, 1980; Irving, 1980). On the other hand, such values suggest an unmetasomatized source as shown b y Menzies (1983) for a group of nodules with low REE contents, and derived from depleted mantle of an oceanic characteristic. The depletion may indicate the presence of MORB-type mantle under continental crust (Menzies, 1983). Fig. 3 demonstrates the differences between the REE (normalized) curves for the Tarr carbonates and carbonatites, both in the total REE abundances and in their L R E E / H R E E ratios. Relative to limestones, dolostones and their marble derivatives and particularly to carbonatites, the Tarr carbonates have lower L R E E / H R E E ratios. The REE patterns of the dolomite and breunnerite differ due to crystallographic effects. The LREE content of the breunnerite is lower than that in the dolomite which is consistent with the lack of the
T A B L E III A n a l y t i c a l results o f radiogenic i s o t o p e s Sample No.
W-IO2A W-327C W-IO2F W-347 W-IO2B
Mineral
dolomite dolomite dolomite breunnerite breunnerite
* C a l c u l a t e d for 6 0 0 Ma.
Contents (ppm) Rb
Sr
Sm
Nd
0.896 1.24 0.091 0.151 0.25
213 175.5 9.39 99.4 42.4
2.43 2.25 0.092 0.210 0.130
11.06 7.94 0.343 0.662 0.413
l*3Nd/t*4Nd
a~Sr/S6 Sr
end*
s 7Sr/86Sr*
0 . 5 1 2 5 2 3 (13) 0.512782(23) 0 . 5 1 2 6 6 1 (38) 0.512735(48) 0 . 5 1 2 7 2 2 (24)
0 . 7 0 3 5 9 0 (36) 0.703742(35) 0 . 7 0 6 0 2 1 (55) 0 . 7 0 8 5 3 3 (40) 0 . 7 0 7 7 5 2 (38)
+2.5 +4.7 +3.0 +2.4 +2.0
0.70348 0.70355 0.70562 0.70849 0.7076
286
5. Conclusions and genetic model
I0,000
5,000
\ I,OOC
I 50(
t~
g: c~ Z 0 37 t>
-\
0{.--
5C
Ld dL
09 I LLJ
!C
~"~
,,~..TOrr
SEDIMENT/ \ \ L§ ~
Tarr /" Breunnerite 0:[
La I prr ]
smTm~Tdl D~Y I E~r Y'b I Ce Nd Eu Tb Ho Tm Lu Fig. 3. Rare-earth elements (normalized to chondrite; Wakita et al., 1971). Carbonatite envelope after Cullers and Graf (1984). S e d i m e n t a r y carbonate envelope (limestones, dolostones and marbles) after Haskin et al. (1966), Jarvis et al. (1975), and Bellanca et al. (1981). larger Ca 2+ cation in the breunnerite. HREE are similar in both minerals, indicating replacement of the Mg> in the crystal lattices. Field evidence for the Tarr carbonates indicates that t h e y were deposited from (hydrous) fluids and not a magma. There is no evidence for the replacement of ferromagnesian silicate minerals (Bogoch, 1983), and therefore it appears that the solutions were not barren, but were themselves enriched in Mg, Ca, Fe and the associated trace elements. They became progressively enriched in Mg and Fe at later stages, mixed with metamorphic waters, and resulted in the formation of the breunnerite.
(1) The Tarr dolomite has mantle isotopic characteristics: 5 ~sO = +5.3 to +8.5%0; 81BC = --7.5 to --8.4%0; 5D = --59 to --69%o ; 87Sr/86Sr = 0.70348--0.70562; eNd = +2.5 to +4.7. (2) The dolomite differs from carbonatite in its trace-element geochemistry: low total REE, St; high Cr, Ni, Sc; and low LREE/ HREE ratios. (3) The trace-element compositions should reflect the type of mantle material in which the dolomite initially formed. Therefore, this mantle material is different from that in which carbonatites formed. This difference may be explained in the following ways. (a) The mantle source is geochemically depleted in LREE. A partial melt would also be relatively depleted in the incompatible elements so that the CO2-rich phase would concentrate only the a m o u n t of elements present. (b) In most cases, the a m o u n t of the CO2-rich phase is very small relative to the silicate phase. In the present case, the relative a m o u n t of carbonate could have been large, so that the total concentration of the incompatible elements would have been less enriched than in, for example, carbonatites. (c) The carbonate phase was in equilibrium with the early stage of magma crystallization in the mantle and hence is relatively enriched in the transition metals and depleted in the incompatible elements. The following genetic model is considered to best fit the geological and geochemical data for carbonate minerals in the Tarr albitite complex: (1) Uprise of carbonate-bearing peridotitic material during which the carbonate dissociates rapidly (Wyllie et al., 1983). (2) Emplacement o f ultramafic material, at last stage via the shear zones, within which is trapped the CO2 and cations in the form of a fluid. (3) Mobilization of the CO2-bearing fluid into even higher structural levels and deposi-
287
tion of carbonate as " h d y r o t h e r m a l " - t y p e vein and replacement deposits during late stages of metamorphism (the relatively large volume of carbonate would appear to preclude the entrapment of all the CO: within vesicles). (4) The fluid gradually became enriched in Mg and Fe, and metamorphic water penetrated the reservoir. The breunnerite, which then replaced the dolomite became enriched in 1sO and slightly in 13C. The strontium isotopes of the breunnerite are distinctly enriched in the radiogenic 87Sr reflecting the metamorphic water.
Acknowledgements The authors gratefully acknowledge assistance in the field by D. Vadal and in the laboratory by A. Barzilai and H. Eldad. Dr. Y. Nathan is thanked for his review of the manuscript, which was further improved by the comments of Dr. M. Javoy.
References Allard, P., 1979. 13C/12C and 34S/32S ratios in magmatic gases from the ridge volcanism in Afar. Nature (London), 282: 56--58. Bellanca, A., Di Salvo, P., MSller, P., Neri, R. and Schley, F., 1981. Rare earth and minor element distribution and petrographic features of fluorites and associated Mesozoic limestones of northwestern Sicily. Chem. Geol., 32: 255--269. Bielski, M., 1982. Stages in the evolution of the Arabian--Nubian massif ir Sinai. Ph.D. Thesis, Hebrew University, Jerusalem, 155 pp (in Hebrew, with English summary). Blattner, P. and Cooper, A.F., 1974. Carbon and oxygen isotopic composition of carbonatitic dikes and metamorphic country rock of the Haast Schist terrain, New Zealand, Contrib. Mineral. Petrol., 44: 19--27. Boettcher, A.L. and O'Neil, J.R., 1980. Stable isotope, chemical and petrographic studies of high pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites. Am. J. Sci., 280-A: 594-621. Bogoch, R., 1983. Petrology and geochemistry of the Tarr albitite complex, south-east Sinai. Ph.D. Thesis, Ben Gurion University, 134 pp.
Bogoch, R., Halicz, L. and Nathan, Y., 1982. Breunnerite from the Tarr albitite complex, Sinai. Am. Mineral., 67: 822--825. Bogoch, R., Eldad, H. and Nathan, Y., 1984. Scandium-bearing carbonates of the Tarr albitite complex, southeast Sinai. Geochim. Cosmochim. Acta, 48: 885--887. Buyers, C.D., Christie, D.M., Muenow, D.W. and Sinton, J.M., 1984. Volatile contents and ferric-ferrous ratios of basalt, ferrobasalt and rhyodacite glasses from the Gal~pagos 95.5°W propagating rift. Geochim. Cosmochim. Acta, 48: 2239--2246. Cullers, R.L. and Graf, J.L., 1984. Rare earth elements in igneous rocks of the continental crust: predominantly basic and ultrabasic rocks. In: P. Henderson (Editor), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 237--274. Deines, P., 1980. The carbon isotopic compositions of diamonds: relationship to diamond shape, color, occurrence and vapor composition. Geochim. Cosmochim. Acta, 44: 943--996. Deines, P. and Gold, D.P., 1973. The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon. Geochim. Cosmochim. Acta, 37: 1709--1733. DePaolo, D.L., 1980. Sources of continental crust: neodymium isotope evidence from the Sierra Nevada and Peninsular ranges. Science, 209: 684-687. Des Marais, D.J. and Moore, J.G., 1984. Carbon and its isotopes in mid-oceanic glasses. Earth Planet. Sci. Lett., 69: 43--57. Dontsova, Ye.I., Kononova, V.A. and Kuzentsova, L.D., 1977. The oxygen-isotope composition of carbonatites and similar rocks in relation to their source and mineralization. Geochem. Int., 14(4): 1--11. Eggler, D.H., Kushiro, I. and Holloway, J.R., 1979. Free energies of decarbonation reactions at mantle pressures, I. Stability of the assemblage forsterite-enstatite--magnesite in the system MgO--SiO~-CO~--H~O to 60 kbar. Am. Mineral., 64: 288--293. Faure, G. and Powell, J.L., 1972. Strontium Isotope Geology. Springer, Berlin, 188 pp. (see especially pp. 56--59). Godfrey, J.D., 1962. The deuterium content of hydrous minerals from the East-Central Sierra Nevada, and Yosemite National Park. Geochim. Cosmochim. Acta, 26: 1215--1245. Haskin, L.A., Wildeman, T.R., Frey, F.A., Collins, K.A., Keedy, C.R. and Haskin, M.A., 1966. Rare earths in sediments. J. Geophys. Res., 71: 6091-6105. Hawkesworth, C.J., O'Nions, R.F. and Arcurus, R.J., 1979. Nd and Sr isotope geochemistry of island arc volcanics, Grenada, Lesser Antilles. Earth Planet. Sci. Lett., 45: 237--248.
288
Hervig, R.L. and Smith, J.V., 1981. Dolomite-apatite inclusion in chrome-diopside crystal, Bellsbank kimberlite, South Africa. Am. Mineral., 66: 346--349. Irving, A.J., 1980. Petrology and geochemistry of composite ultramafic xenoliths in alkalic basalts and implications for magmatic processes within the mantle. Am. J. Sci., 280-A: 389--426. Jarvis, J.C., Wildeman, T.R. and Banks, N.G., 1975. Rare earths in the Leadville limestone and its marble derivatives. Chem. Geol., 16: 27--37. Javoy, M., Pineau, F. and Demaiffe, D., 1984. Nitrogen and carbon isotopic composition in the diamonds of Mbuji Mayi (Zaire). Earth Planet. Sci. Lett., 68: 399--412. Le Bas, M.J., 1981. Carbonatite magmas. Mineral. Mag., 44: 56--65. Magaritz, M., 1983. Carbon and oxygen isotope composition of recent and ancient coated grains. In: T.M. Peryt (Editor), Coated Grains. Springer, Berlin, pp. 27--37. McGetchin, T.R. and Besancon, J.R., 1973. Carbonate inclusions in mantle-derived pyrope. Earth Planet. Sci. Lett., 18: 408--410. Menzies, M., 1983. Mantle ultramafic xenoliths in alkaline magmas: evidence for mantle heterogeneity modified by magmatic activity. In" C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths. Shiva, Orpington, pp. 92--110. Michard, A., Gurriet, P., Soudant, M. and Albar~de, F., 1985. Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta, 49: 601--610. Moore, J.G., Batchelder, J.N. and Cunningham, C.G., 1977. CO~-filled vesicles in mid-ocean basalt. J. Volcanol. Geotherm. Res., 2: 309--927. O'Neil, J.R. and Epstein, S., 1967. The oxygen isotope fractionation in the system dolomite--calcite --carbon dioxide. Science, 152: 198--201. O'Neil, J.R., Clayton, R.N. and Magada, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547--5558. Pineau, F. and Javoy, M., 1983. Carbon isotopes and concentrations in mid-ocean ridge basalts. Earth Planet. Sci. Lett., 62: 239--257. Pineau, F., Javoy, M. and All4gre, C.J., 1973. l~tude syst~matique des isotopes de l'oxyg~ne, du carbone et du strontium dans la carbonatites. Geochim. Cosmochim. Acta, 37: 2363--2377. Pineau, F., Javoy, M. and Bottinga, Y., 1976. ~3C/ ~2C ratios of rocks and inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing on the problem of the isotopic composition of deepseated carbon. Earth. Planet. Sci. Lett., 29: 413-421. Richard, P. and All~gre, C.J., 1980. Neodymium and
strontium isotope study of ophiolite and orogenic lherzolite petrogenesis. Earth Planet. Sci. Lett., 47: 65--74. Samoylov, V.S., Plyusnin, G.S., Sandimirova, G.P. and Chernysheva, E.A., 1980. Isotopic composition of strontium from carbonatite of the USSR. Dokl. Acad. Sci. U.S.S.R., Earth Sci. Sect., 238: 125--127. Sharma, T. and Clayton, R.N., 1965. Measurements of 180/160 ratios of total oxygen from carbonate. Geochim. Cosmochim. Acta, 29- 1347--1353. Sheppard, S.M.F. and Schwarcz, H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between metamorphic calcite and dolomite. Contrib. Mineral. Petrol., 26: 161--198. Shimron, A.E., Brookins, D.G., Magaritz, M. and Bartov, Y., 1973. Origin of the intrusive carbonate rocks between the Gulf of Elat and Gulf of Suez Rifts. Isr. J. Earth Sci., 22: 243--254. Spera, F.J., 1981. Carbon dioxide in igneous petrogenesis, II. Fluid dynamics of mantle metasomatism. Contrib. Mineral. Petrol., 77: 56--65. Stern, R.J. and Hedge, C.E., 1985. Geochronologic and isotopic constraints on Late Precambrian crustal evolution in the Eastern Desert of Egypt. Am. J. Sci., 285: 97--127. Taylor, Jr., H.P., Frechen, J. and Degens, E.T., 1967. Oxygen and carbon isotope studies of carbonatites from the Laacher See district, West Germany, and the Aln5 District, Sweden. Geochim. Cosmochim. Acta, 31: 407--430. Vinogradov, V.I., Kranov, A.A., Kuleshov, V.N. and Sulerzhitskiy, L.D., 1980.13C/12C and ~sO/l~O ratios and ~4C concentration in carbonatites of the Kaliango volcano (East Africa). Int. Geol. Rev., 69: 43--57. Wakita, H., Rey, P. and Schmitt, R.A., 1971. Abundances of the 14 rare-earth elements and 12 other trace elements in Apollo 12 samples: five igneous and one breccia rocks and four soils. Proc. 2nd Lunar Sci. Conf., pp. 1319--1329. Wass, S.Y. and Rogers, N.W., 1980. Mantle metasomatism -- precursor to continental alkaline volcanism. Geochim. Cosmochim. Acta, 44: 1811-1823. White, W.M. and Hofmann, A.W., 1982. Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution. Nature (London), 296: 821-825. Wyllie, P.J., 1978. Mantle fluid compositions buffered in peridotite--CO2--H20 carbonates, amphibole and phlogopite. J. Geol., 86: 687--713. Wyllie, P.J., Huang, W.-L., Otto, J. and Byrnes, A.P., 1983. Carbonation of peridotites and decarbonation of siliceous dolomites in the system CaO-MgO--SiO2--CO2 to 30 kbar. Tectonophysics, 100: 359--388.
281
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
[1]
DOLOMITE OF POSSIBLE MANTLE ORIGIN, SOUTHEAST SINAI R O N B O G O C H 1, M O R D E C K A I M A G A R I T Z 2 and A N N I E M I C H A R D 3 Geological Survey o f Israel, Jerusalem 95 501 (Israel) 2 Isotope Departmen t, Weizmann Institute o f Science, R e h o v o t 76 1 O0 (Israel) 3 Centre de Recherches, P~trographiques et G~ochimiques, N a n c y (France)
(Received July 25, 1985; accepted for publication March 10, 1986) Abstract Bogoch, R., Magaritz, M. and Michard, A., 1986. Dolomite of possible mantle origin, southeast Sinai. Chem. Geol., 56: 281--288. Dolomite and breunnerite occur in large amounts as veins and tabular bodies in association with the Tarr albitite within late Precambrian greenschist-facies metamorphics of southeast Sinai. Isotopic data for the dolomite (mean 6180 = +6.9%0; mean 513C =--8.1%0; mean 5 D =--65%0; mean end = +3.4;mean ~Sr/S~Sr = 0.70422), together with their low incompatible-element and high transition-metal contents, suggest that the dolomite derived from a mantle source. The isotopic composition of the breunnerite indicates contamination by metamorphic water during the replacement phase of the dolomite by the breunnerite.
1. I n t r o d u c t i o n E x p e r i m e n t a l studies (Wyllie, 1 9 7 8 ; Eggler e t al., 1 9 7 9 ) have established t h a t c a r b o n a t e minerals ( d o l o m i t e , magnesite) can be stable phases in p e r i d o t i t i c assemblages in the m a n t l e e n v i r o n m e n t . T h e i m p o r t a n c e o f CO2 in m a n t l e m e t a s o m a t i s m has b e e n s u m m a r i z e d b y Spera (1981), and c a r b o n a t i t e s in alkaline r o c k associations originated (pre-immiscibili t y ) d e e p in t h e u p p e r m a n t l e (Le Bas, 1981). D i r e c t e v i d e n c e f o r t h e p r e s e n c e o f CO2 in the m a n t l e is given in several r e c e n t papers (e.g., Pineau and J a v o y , 1 9 8 3 ; Byers et al., 1984). H o w e v e r , o n l y rarely have m a n t l e c a r b o n a t e minerals, as sampled f r o m kimberlitic x e n o liths, b e e n n o t e d , and t h e n in m i n u t e a m o u n t s ( M c G e t c h i n and B e s a n c o n , 1 9 7 3 ; Hervig and Smith, 1981). D o l o m i t e and b r e u n n e r i t e (Fe-magnesite) o c c u r in relatively large a m o u n t s in associa0009-2541/86/$03.50
t i o n with t h e albitite c o m p l e x at Wadi Tarr in s o u t h e a s t Sinai. An earlier s t u d y o f the tracee l e m e n t c h e m i s t r y o f these c a r b o n a t e s (enr i c h m e n t in t r a n s i t i o n metals and Sc, and dep l e t i o n in Sr and the L R E E ) ( B o g o c h et al., 1 9 8 4 ) led t o the c o n c l u s i o n t h a t t h e depositing fluids had b e e n in e q u i l i b r i u m with mafic/ u l t r a m a f i c material. H o w e v e r , this c o u l d imply t h a t t h e t r a c e - e l e m e n t assemblage derived f r o m ferromagnesian minerals a t t a c k e d b y CO2-bearing solutions o f an u n r e l a t e d source. This s t u d y presents stable (C, H and O) and radiogenic (Sr, Nd) i s o t o p e m e a s u r e m e n t s on these carbonates, which indicate a m a n t l e origin for the d o l o m i t e . 2. D e s c r i p t i o n o f the c a r b o n a t e s T h e T a r r albitite c o m p l e x consists o f metasomatic albitite bodies (totalling > 2 kin2; Bogoch, 1 9 8 3 ) with associated actinolite-rich
© 1986 Elsevier Science Publishers B.V.
282 rocks, and dolomite--breunnerite. The disposition of the complex is strongly controlled by two shear zones within Late Proterozoic, poly-metamorphosed meta-sedimentary and meta-pyroclastic rocks, mainly in the greenschist facies. Within the shear zones are large (up to 100 m across), rotated, allochthonous, tectonically emplaced blocks of feldsparfree hornblendite and gabbro, the former exhibiting ghost textures of an olivine--pyroxene ultramafite. The dolomite and breunnerite have been described in detail (Bogoch et al., 1982; Bogoch, 1983). Dolomite is the dominant (~70%) species. On fresh surface, the dolomite is light buff coloured and the breunnerite is medium to dark brown. Both minerals weather to a dark beige colour. These carbonates occur as veins (generally anastamosing), several centimeters to 1 m in width, as breccia cement in shear zone fault rocks, and as sheet-like bodies. The largest of the latter is 200 m long, 60 m wide and 10 m thick. The dolomite is very coarsely crystalline, from ~0.5 cm to rarely 20 cm on cleavage faces. Breunnerite varies from several millimeters to 5 cm in m a x i m u m diameter. Flow textures are absent, but at a few sites, there is a tendency for the dolomite to grow with its long axis perpendicular to the wall of the host rock. The breunnerite is seen to replace dolomite in several locations, and this paragenesis is assumed for most occurrences. The coarsegrain size of the dolomite and its local growth perpendicular from the walls indicate deposition from a (non-magma) fluid. 3. Methods Carbon dioxide for mass spectrometric analyses was obtained by reacting the dolomite and breunnerite with 95% phosphoric acid (specific gravity 1.80) at 50°C for at least 48 hr., with yields greater than 95%. The CO2 was analysed with a Varian ® M250 mass spectrometer. Water was extracted from fluid inclusions in carbonate by crushing the samples in an
evacuated stainless-steel tube. The water, after purification, was converted to hydrogen by passing it over uranium at 800°C (Godfrey, 1962). Results are expressed in the conventional " 5 " per mil notation. Hydrogen and oxygen isotopes are relative to SMOW and carbon isotopes relative to PDB (NBS 1 9 : 5 1 8 0 = -2.200/00 PDB; 513C = +1.96°/00 PDB). The phosphoric acid procedure is known to involve a kinetic isotope effect on oxygen (Sharma and Clayton, 1965). The c o m m o n l y used value (0.84o/00) was determined for 25°C and was used in this study since no correction for 50°C is available. Sr and Nd isotopic compositions and Rb, Sr, Sm and Nd concentrations were determined on ~100-mg of sample spiked before attack by HC1 using the technique described by Michard et al. (1985). For samples with low REE contents, a first enrichment was obtained by REE coprecipitation with ferric hydroxide followed by Fe removal on an anion-exchange column. Isotope standards analysed in the laboratory gave the following results: Eimer & Amend ® STSr/S6Sr = 0.707985 +- 30; BCR1 '43Nd/144Nd = 0.512639 + 15. 4. Results and discussion In terms of their trace-element geochemistry and istopes, the Tarr samples are apparently anomalous. Relative to other igneous, metamorphic and sedimentary carbonates, they are characterized by high Sc and transition-metal contents and by a depletion in Sr and the REE (Table I; see Fig. 3). Their unusually low Sr, Ba and REE contents would appear to eliminate their classification as carbonatites as characterized by Le Bas
(1981). 6'~O varies from +5.3 to +8.5°/00 in the dolomite and from +15.9 to +22.1°/00 in the breunnerite; 513C-values have a narrower range, from - 7 . 5 to -8.30/00 in dolomite and - 4 . 6 to -7.70/00 in the breunnerite. These data are represented in Table II and Fig. 1, to-
283 TABLE I
TABLE II
Selected trace-element data (in ppm) of Tarr dolomite and breunnerite (after Bogoch, 1983 ; Bogoch et at., 1984)
6~sO -, 6~3C- and 6D-values (in °/00) in Tarr area marbles, and dolomite and breunnerite
Dolomite Cr Ni Sc Sr Rb Ba Nb La Ce Nd Sm Eu Yb Lu
77 21 70 141 <5 <100 <3 6.8 15.9 7.7 1.7 0.8 1.4 0.2
215 293 75 76 <5 <100 <3 0.4 0.7 0.7 0.07 0.06 0.6 0.2
ii
\\ \
/ /
20
\\
,/
t
I'
/
©
•
• ///
/ t
u3
./
o
/
0 "~ o' • ~ \ .\ i c \
/I
\,
>15
J• i I 'L \
c ©
Rock/mineral
6 ~80
5 ~3C
W-393A W-393C W-318 W-335 W-348 W-262A W-195B W-319F W-405 W-324C W-324A W-98 W-349 W-133 W-372-1B W-341 W-342 W-262A
marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-dolomitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic marble-calcitic
+13.8 +14.2 +12.7 +18.4 +16.7 +17.1 +18.0 +15.2 +14.5 +10.7 +15.0 +11.7 +13.7 +13.9 +17.6 +18.3 +17.8 +16.7
+0.9 1.7 -2.8 -5.1 -3.2 -3.2 -2.7 -4.8 -5.0 -2.3 -3.6 -4.5 -1.0 -3.0 -3.6 -3.6 -3.2 -2.7
W-IO5B W-IO2F W-IO2A
dolomite dolomite dolomite
+5.3 +5.5 +8.5
-8.1 7.5 8.4
W-327C W-355
dolomite dolomite
+7.8 +7.4
-8.3 8.3
W-105C W-IO2B W-265A W-347
breunnerite breunnerite breunnerite breunnerite
+15.9 +18.3 +19.1 +22.1
-7.7 -6.0 - 4.6 6.1
5D
c
•
•
/
\ (;
/
-59 -69, -66, -66
\
(;
.2 C~
Sample Breunnerite
-91
//
To o io L5 D o l o m i t e
'1
/
•
Breunnerile
()
Morble
•
Merble
colcih¢ -
dolornlhc
5 L
io
I
I
I
-5 0 813C%o (relative to PDB)
Fig. 1. 6 ~80 vs. a ~3C diagram of dolomite, breunnerite and country rock marbles.
gether with stable isotope data obtained on samples of calcite and dolomite marbles found in the Precambrian country rocks.
T h e o x y g e n i s o t o p e s o f t h e d o l o m i t e are w i t h i n t h e r a n g e f o r p r i m a r y i g n e o u s carb o n a t i t e ( T a y l o r e t al., 1 9 6 7 ) . T w o o f t h e s a m ples h a v e 61SO-values w i t h i n t h e m o r e restricted range suggested by Boettcher and O'Neil (1980) for the primary mantle. The o t h e r t h r e e s a m p l e s are s l i g h t l y 1sO e n r i c h e d w h i c h are c o n s i s t e n t w i t h t h e l a t e - e n r i c h m e n t m o d e l in c a r b o n a t i t e s p r e s e n t e d by P i n e a u e t al. ( 1 9 7 3 ) . T h e c a r b o n i s o t o p e s are c l o s e t o the range characterizing carbon-bearing material d e r i v e d f r o m t h e m a n t l e . S u c h v a l u e s w e r e r e p o r t e d n o t o n l y i n c a r b o n a t i t e s , b u t also i n d i a m o n d s ( D e i n e s , 1 9 8 0 ; V i n o g r a d o v e t al., 1 9 8 0 ; J a v o y e t al., 1 9 8 4 ) , k i m b e r l i t e ( D e i n e s a n d G o l d , 1 9 7 3 ) , v o l c a n i c gases as i n A f a r
284 (Allard, 1979), CO2 trapped in mid-ocean ridge basalt (Pineau et al., 1976; Moore et al., 1977; Pineau and Javoy, 1983), or more generally, carbon in oceanic basalt (Des Marais and Moore, 1984). Certain primary dolomite samples from carbonatites yield 51SO-values similar to those at Tarr [e,g., Fen, Norway -- sample P-20 (Pineau et al., 1973); Carghill, Ontario, Canada (Dontsova et al., 1977)]. 5D-values found in fluid inclusions in dolomite are within the range ( - 5 8 to - 7 9 % 0 ) suggested for mantle H20, based on hydrogen phases in kimberlite (Boettcher and O'Neil, 1980). The oxygen isotope composition of the breunnerite (Fe-bearing magnesite) is much more enriched in 180 and slightly in 13C. Although the isotopic fractionation between magnesite and water is not known, it may be possible to estimate the values from published data on the oxygen isotope fractionation. Using the data of O'Neil et al. 4
OL
0.50
I
0.75
I
1.00 Ionic radii (A)
1.25
Fig. 2. Range o f 1000 ]n cx ( ~ 1 8 0 ) b e t w e e n MgCO8 and w a t e r at 500°C; calculations based on linear e x t r a p o l a t i o n of CaCO3, SrCO3, and BaCO 3 (data f r o m O'Neit et al., 1969) and non-linear e x t r a p o l a t i o n t h r o u g h " d o l o m i t e " points ( X ) calculated f r o m dolom i t e - c a l c i t e fractionation (data from O'Neil and Ep-
stein, 1967; Sheppard and Schwartz, 1970).
(1969) to trace the variation of 61sO with ionic radius, we can roughly estimate the breunnerite--water fractionation to be 2.5 to 3.5% 0 at 500°C (Fig. 2). In any case, the dolomite--magnesite fractionation cannot be very large, so that the large difference in 6180-values between the two minerals suggests that an additional source of water at lower temperature was involved in the breunneritization of the dolomite. The 8;sO in the breunnerite is similar to that found in the carbonate rocks of the metamorphic terrane (Fig. 1), suggesting that the fluid from which the breunnerite formed was contaminated by a late metamorphic fluid. Although only one ~D-value (-91°/00) was obtained in the breunnerite (very low amount of water in the samples), it is distinctly different from that of the dolomite (Table II). The 613C-values are n o t equal in the dolomite and breunnerite. Fig. 1 suggests that the 513C of breunnerite may have moved slightly from dolomite values towards values of meta-sedimentary carbonates. They are intermediate between dolomite and marbles. The isotope record of the breunnerite could result from complete exchange of oxygen isotopes of the replaced dolomite and partial exchange or replacement for carbon with the marbles (Pineau et al., 1973). The preservation of mantle carbon isotopic composition of the dolomite in the closely spatially associated breunnerite, is an indication of the preservation of the original carbonate (carbon isotope) values. The same relation between the later fluid and the isotope data is typical of most carbonate recrystallization processes at low and medium temperatures. It results from the difference in the water/ rock ratio in solutions for carbon and oxygen isotopes; the carbon is dominated by the rock, and oxygen by the fluid (Pineau et al., 1973; Magaritz, 1983). A similar model was suggested by Blattner and Cooper (1974) in their study of carbonatire veins in metamorphic terrain of the Haast Schist in New Zealand, where the emplace-
285
ment of the carbonate veins was associated with the end of the metamorphic phase. The carbon isotope mantle values were preserved while the oxygen values were shifted by several per mil due to the effects of metamorphism. The 61sO enrichment in the late fluids associated with formation of breunnerite from dolomite is similar to that in platform carbonatites (Pineau et al., 1973; Dontsova et al., 1977). In the Kordor intrusion of the Kolo province (N.W. Russian Republic, U.S.S.R.), the late Fe-rich phase is enriched by 4--50/00 relative to the earlier-stage calcite (7.3--8.7%o). In the Tanga intrusion in the East Sayan province (E. Russian Republic, U.S.S.R.), the enrichment in the Fe phase is 6--8%0 (from 6.4--7.30/00 to 13.2-- 14.4°/00). Radiogenic isotope data are presented in Table III. The measured *43Nd/144Nd and STSr/S6Sr were corrected for in situ decay (from the time of formation), assuming an age of 600 Ma based on the late Proterozoic age of the area (Bielski, 1982). The range of eNd (+2.5 to +4.7) and STSr/S6Sr (0.70348 to 0.70562) values o f the dolomite are commensurate with data of mantlederived silicate rocks (e.g., DePaolo, 1980; Richard and All~gre, 1980; White and Hofmann, 1982; Stern and Hedge, 1985), and for the Sr isotopes, with carbonatites (Faure and Powell, 1972; Samoylov et al., 1980). The eNd-Values in the breunnerite preserve the primary dolomite mantle values. The displace-
ment of Sr isotope values to higher ratios while preserving the original Nd isotope values is typical of crustal alteration (e.g., Hawkesworth et al., 1979). The involvement o f crustal fluid from the metamorphic terrain as suggested by the oxygen isotope data, is supported by the high STSr/S6Sr ratios in the breunnerite (Shimron et al., 1973; Table III). The low REE, Sr and Rb contents in the dolomite rule out a source of metasomatized mantle as suggested for most xenoliths in alkali basalt and kimberlite (Boettcher and O'Neil, 1980; Wass and Rogers, 1980; Irving, 1980). On the other hand, such values suggest an unmetasomatized source as shown b y Menzies (1983) for a group of nodules with low REE contents, and derived from depleted mantle of an oceanic characteristic. The depletion may indicate the presence of MORB-type mantle under continental crust (Menzies, 1983). Fig. 3 demonstrates the differences between the REE (normalized) curves for the Tarr carbonates and carbonatites, both in the total REE abundances and in their L R E E / H R E E ratios. Relative to limestones, dolostones and their marble derivatives and particularly to carbonatites, the Tarr carbonates have lower L R E E / H R E E ratios. The REE patterns of the dolomite and breunnerite differ due to crystallographic effects. The LREE content of the breunnerite is lower than that in the dolomite which is consistent with the lack of the
T A B L E III A n a l y t i c a l results o f radiogenic i s o t o p e s Sample No.
W-IO2A W-327C W-IO2F W-347 W-IO2B
Mineral
dolomite dolomite dolomite breunnerite breunnerite
* C a l c u l a t e d for 6 0 0 Ma.
Contents (ppm) Rb
Sr
Sm
Nd
0.896 1.24 0.091 0.151 0.25
213 175.5 9.39 99.4 42.4
2.43 2.25 0.092 0.210 0.130
11.06 7.94 0.343 0.662 0.413
l*3Nd/t*4Nd
a~Sr/S6 Sr
end*
s 7Sr/86Sr*
0 . 5 1 2 5 2 3 (13) 0.512782(23) 0 . 5 1 2 6 6 1 (38) 0.512735(48) 0 . 5 1 2 7 2 2 (24)
0 . 7 0 3 5 9 0 (36) 0.703742(35) 0 . 7 0 6 0 2 1 (55) 0 . 7 0 8 5 3 3 (40) 0 . 7 0 7 7 5 2 (38)
+2.5 +4.7 +3.0 +2.4 +2.0
0.70348 0.70355 0.70562 0.70849 0.7076
286
5. Conclusions and genetic model
I0,000
5,000
\ I,OOC
I 50(
t~
g: c~ Z 0 37 t>
-\
0{.--
5C
Ld dL
09 I LLJ
!C
~"~
,,~..TOrr
SEDIMENT/ \ \ L§ ~
Tarr /" Breunnerite 0:[
La I prr ]
smTm~Tdl D~Y I E~r Y'b I Ce Nd Eu Tb Ho Tm Lu Fig. 3. Rare-earth elements (normalized to chondrite; Wakita et al., 1971). Carbonatite envelope after Cullers and Graf (1984). S e d i m e n t a r y carbonate envelope (limestones, dolostones and marbles) after Haskin et al. (1966), Jarvis et al. (1975), and Bellanca et al. (1981). larger Ca 2+ cation in the breunnerite. HREE are similar in both minerals, indicating replacement of the Mg> in the crystal lattices. Field evidence for the Tarr carbonates indicates that t h e y were deposited from (hydrous) fluids and not a magma. There is no evidence for the replacement of ferromagnesian silicate minerals (Bogoch, 1983), and therefore it appears that the solutions were not barren, but were themselves enriched in Mg, Ca, Fe and the associated trace elements. They became progressively enriched in Mg and Fe at later stages, mixed with metamorphic waters, and resulted in the formation of the breunnerite.
(1) The Tarr dolomite has mantle isotopic characteristics: 5 ~sO = +5.3 to +8.5%0; 81BC = --7.5 to --8.4%0; 5D = --59 to --69%o ; 87Sr/86Sr = 0.70348--0.70562; eNd = +2.5 to +4.7. (2) The dolomite differs from carbonatite in its trace-element geochemistry: low total REE, St; high Cr, Ni, Sc; and low LREE/ HREE ratios. (3) The trace-element compositions should reflect the type of mantle material in which the dolomite initially formed. Therefore, this mantle material is different from that in which carbonatites formed. This difference may be explained in the following ways. (a) The mantle source is geochemically depleted in LREE. A partial melt would also be relatively depleted in the incompatible elements so that the CO2-rich phase would concentrate only the a m o u n t of elements present. (b) In most cases, the a m o u n t of the CO2-rich phase is very small relative to the silicate phase. In the present case, the relative a m o u n t of carbonate could have been large, so that the total concentration of the incompatible elements would have been less enriched than in, for example, carbonatites. (c) The carbonate phase was in equilibrium with the early stage of magma crystallization in the mantle and hence is relatively enriched in the transition metals and depleted in the incompatible elements. The following genetic model is considered to best fit the geological and geochemical data for carbonate minerals in the Tarr albitite complex: (1) Uprise of carbonate-bearing peridotitic material during which the carbonate dissociates rapidly (Wyllie et al., 1983). (2) Emplacement o f ultramafic material, at last stage via the shear zones, within which is trapped the CO2 and cations in the form of a fluid. (3) Mobilization of the CO2-bearing fluid into even higher structural levels and deposi-
287
tion of carbonate as " h d y r o t h e r m a l " - t y p e vein and replacement deposits during late stages of metamorphism (the relatively large volume of carbonate would appear to preclude the entrapment of all the CO: within vesicles). (4) The fluid gradually became enriched in Mg and Fe, and metamorphic water penetrated the reservoir. The breunnerite, which then replaced the dolomite became enriched in 1sO and slightly in 13C. The strontium isotopes of the breunnerite are distinctly enriched in the radiogenic 87Sr reflecting the metamorphic water.
Acknowledgements The authors gratefully acknowledge assistance in the field by D. Vadal and in the laboratory by A. Barzilai and H. Eldad. Dr. Y. Nathan is thanked for his review of the manuscript, which was further improved by the comments of Dr. M. Javoy.
References Allard, P., 1979. 13C/12C and 34S/32S ratios in magmatic gases from the ridge volcanism in Afar. Nature (London), 282: 56--58. Bellanca, A., Di Salvo, P., MSller, P., Neri, R. and Schley, F., 1981. Rare earth and minor element distribution and petrographic features of fluorites and associated Mesozoic limestones of northwestern Sicily. Chem. Geol., 32: 255--269. Bielski, M., 1982. Stages in the evolution of the Arabian--Nubian massif ir Sinai. Ph.D. Thesis, Hebrew University, Jerusalem, 155 pp (in Hebrew, with English summary). Blattner, P. and Cooper, A.F., 1974. Carbon and oxygen isotopic composition of carbonatitic dikes and metamorphic country rock of the Haast Schist terrain, New Zealand, Contrib. Mineral. Petrol., 44: 19--27. Boettcher, A.L. and O'Neil, J.R., 1980. Stable isotope, chemical and petrographic studies of high pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basalts and kimberlites. Am. J. Sci., 280-A: 594-621. Bogoch, R., 1983. Petrology and geochemistry of the Tarr albitite complex, south-east Sinai. Ph.D. Thesis, Ben Gurion University, 134 pp.
Bogoch, R., Halicz, L. and Nathan, Y., 1982. Breunnerite from the Tarr albitite complex, Sinai. Am. Mineral., 67: 822--825. Bogoch, R., Eldad, H. and Nathan, Y., 1984. Scandium-bearing carbonates of the Tarr albitite complex, southeast Sinai. Geochim. Cosmochim. Acta, 48: 885--887. Buyers, C.D., Christie, D.M., Muenow, D.W. and Sinton, J.M., 1984. Volatile contents and ferric-ferrous ratios of basalt, ferrobasalt and rhyodacite glasses from the Gal~pagos 95.5°W propagating rift. Geochim. Cosmochim. Acta, 48: 2239--2246. Cullers, R.L. and Graf, J.L., 1984. Rare earth elements in igneous rocks of the continental crust: predominantly basic and ultrabasic rocks. In: P. Henderson (Editor), Rare Earth Element Geochemistry. Elsevier, Amsterdam, pp. 237--274. Deines, P., 1980. The carbon isotopic compositions of diamonds: relationship to diamond shape, color, occurrence and vapor composition. Geochim. Cosmochim. Acta, 44: 943--996. Deines, P. and Gold, D.P., 1973. The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon. Geochim. Cosmochim. Acta, 37: 1709--1733. DePaolo, D.L., 1980. Sources of continental crust: neodymium isotope evidence from the Sierra Nevada and Peninsular ranges. Science, 209: 684-687. Des Marais, D.J. and Moore, J.G., 1984. Carbon and its isotopes in mid-oceanic glasses. Earth Planet. Sci. Lett., 69: 43--57. Dontsova, Ye.I., Kononova, V.A. and Kuzentsova, L.D., 1977. The oxygen-isotope composition of carbonatites and similar rocks in relation to their source and mineralization. Geochem. Int., 14(4): 1--11. Eggler, D.H., Kushiro, I. and Holloway, J.R., 1979. Free energies of decarbonation reactions at mantle pressures, I. Stability of the assemblage forsterite-enstatite--magnesite in the system MgO--SiO~-CO~--H~O to 60 kbar. Am. Mineral., 64: 288--293. Faure, G. and Powell, J.L., 1972. Strontium Isotope Geology. Springer, Berlin, 188 pp. (see especially pp. 56--59). Godfrey, J.D., 1962. The deuterium content of hydrous minerals from the East-Central Sierra Nevada, and Yosemite National Park. Geochim. Cosmochim. Acta, 26: 1215--1245. Haskin, L.A., Wildeman, T.R., Frey, F.A., Collins, K.A., Keedy, C.R. and Haskin, M.A., 1966. Rare earths in sediments. J. Geophys. Res., 71: 6091-6105. Hawkesworth, C.J., O'Nions, R.F. and Arcurus, R.J., 1979. Nd and Sr isotope geochemistry of island arc volcanics, Grenada, Lesser Antilles. Earth Planet. Sci. Lett., 45: 237--248.
288
Hervig, R.L. and Smith, J.V., 1981. Dolomite-apatite inclusion in chrome-diopside crystal, Bellsbank kimberlite, South Africa. Am. Mineral., 66: 346--349. Irving, A.J., 1980. Petrology and geochemistry of composite ultramafic xenoliths in alkalic basalts and implications for magmatic processes within the mantle. Am. J. Sci., 280-A: 389--426. Jarvis, J.C., Wildeman, T.R. and Banks, N.G., 1975. Rare earths in the Leadville limestone and its marble derivatives. Chem. Geol., 16: 27--37. Javoy, M., Pineau, F. and Demaiffe, D., 1984. Nitrogen and carbon isotopic composition in the diamonds of Mbuji Mayi (Zaire). Earth Planet. Sci. Lett., 68: 399--412. Le Bas, M.J., 1981. Carbonatite magmas. Mineral. Mag., 44: 56--65. Magaritz, M., 1983. Carbon and oxygen isotope composition of recent and ancient coated grains. In: T.M. Peryt (Editor), Coated Grains. Springer, Berlin, pp. 27--37. McGetchin, T.R. and Besancon, J.R., 1973. Carbonate inclusions in mantle-derived pyrope. Earth Planet. Sci. Lett., 18: 408--410. Menzies, M., 1983. Mantle ultramafic xenoliths in alkaline magmas: evidence for mantle heterogeneity modified by magmatic activity. In" C.J. Hawkesworth and M.J. Norry (Editors), Continental Basalts and Mantle Xenoliths. Shiva, Orpington, pp. 92--110. Michard, A., Gurriet, P., Soudant, M. and Albar~de, F., 1985. Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta, 49: 601--610. Moore, J.G., Batchelder, J.N. and Cunningham, C.G., 1977. CO~-filled vesicles in mid-ocean basalt. J. Volcanol. Geotherm. Res., 2: 309--927. O'Neil, J.R. and Epstein, S., 1967. The oxygen isotope fractionation in the system dolomite--calcite --carbon dioxide. Science, 152: 198--201. O'Neil, J.R., Clayton, R.N. and Magada, T.K., 1969. Oxygen isotope fractionation in divalent metal carbonates. J. Chem. Phys., 51: 5547--5558. Pineau, F. and Javoy, M., 1983. Carbon isotopes and concentrations in mid-ocean ridge basalts. Earth Planet. Sci. Lett., 62: 239--257. Pineau, F., Javoy, M. and All4gre, C.J., 1973. l~tude syst~matique des isotopes de l'oxyg~ne, du carbone et du strontium dans la carbonatites. Geochim. Cosmochim. Acta, 37: 2363--2377. Pineau, F., Javoy, M. and Bottinga, Y., 1976. ~3C/ ~2C ratios of rocks and inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing on the problem of the isotopic composition of deepseated carbon. Earth. Planet. Sci. Lett., 29: 413-421. Richard, P. and All~gre, C.J., 1980. Neodymium and
strontium isotope study of ophiolite and orogenic lherzolite petrogenesis. Earth Planet. Sci. Lett., 47: 65--74. Samoylov, V.S., Plyusnin, G.S., Sandimirova, G.P. and Chernysheva, E.A., 1980. Isotopic composition of strontium from carbonatite of the USSR. Dokl. Acad. Sci. U.S.S.R., Earth Sci. Sect., 238: 125--127. Sharma, T. and Clayton, R.N., 1965. Measurements of 180/160 ratios of total oxygen from carbonate. Geochim. Cosmochim. Acta, 29- 1347--1353. Sheppard, S.M.F. and Schwarcz, H.P., 1970. Fractionation of carbon and oxygen isotopes and magnesium between metamorphic calcite and dolomite. Contrib. Mineral. Petrol., 26: 161--198. Shimron, A.E., Brookins, D.G., Magaritz, M. and Bartov, Y., 1973. Origin of the intrusive carbonate rocks between the Gulf of Elat and Gulf of Suez Rifts. Isr. J. Earth Sci., 22: 243--254. Spera, F.J., 1981. Carbon dioxide in igneous petrogenesis, II. Fluid dynamics of mantle metasomatism. Contrib. Mineral. Petrol., 77: 56--65. Stern, R.J. and Hedge, C.E., 1985. Geochronologic and isotopic constraints on Late Precambrian crustal evolution in the Eastern Desert of Egypt. Am. J. Sci., 285: 97--127. Taylor, Jr., H.P., Frechen, J. and Degens, E.T., 1967. Oxygen and carbon isotope studies of carbonatites from the Laacher See district, West Germany, and the Aln5 District, Sweden. Geochim. Cosmochim. Acta, 31: 407--430. Vinogradov, V.I., Kranov, A.A., Kuleshov, V.N. and Sulerzhitskiy, L.D., 1980.13C/12C and ~sO/l~O ratios and ~4C concentration in carbonatites of the Kaliango volcano (East Africa). Int. Geol. Rev., 69: 43--57. Wakita, H., Rey, P. and Schmitt, R.A., 1971. Abundances of the 14 rare-earth elements and 12 other trace elements in Apollo 12 samples: five igneous and one breccia rocks and four soils. Proc. 2nd Lunar Sci. Conf., pp. 1319--1329. Wass, S.Y. and Rogers, N.W., 1980. Mantle metasomatism -- precursor to continental alkaline volcanism. Geochim. Cosmochim. Acta, 44: 1811-1823. White, W.M. and Hofmann, A.W., 1982. Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution. Nature (London), 296: 821-825. Wyllie, P.J., 1978. Mantle fluid compositions buffered in peridotite--CO2--H20 carbonates, amphibole and phlogopite. J. Geol., 86: 687--713. Wyllie, P.J., Huang, W.-L., Otto, J. and Byrnes, A.P., 1983. Carbonation of peridotites and decarbonation of siliceous dolomites in the system CaO-MgO--SiO2--CO2 to 30 kbar. Tectonophysics, 100: 359--388.