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Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites: implications for petrogenesis and mantle metasomatism
Earth and Planetary Sctence Letters, 83 (1987) 80 -84 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
80
121
Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites" implications for petrogenesis and mantle metasomatis.m Jennifer S. Ongley ~, Asish R. Basu ~ and T. Kurtis K y s e r 2 t Department of (;eologtcal Sciences. Untt:erst(v of Roche~ter, Rochester, N Y 1462 7 { U S.A.) " Department of Geological Sciences. Untt'ersi(v of Saskatchewan, Saskatoon, Sask. (Canada) Received August 21. 1986; revised version received February 2, 1987 81~O values of coexisting garnet, clinopyroxene and phlogopite for twelve compositionally and texturally diverse Roberts Victor eclogite xenoliths range from 4 3.8 to + 7.1, 4-4.0 to + 7.4 and r 5.9 to + 7.4, respectively. Differences between the 31~O values of coexisting garnets and clinopyroxenes are normally zero: however, there is some variation in the 81~O values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity, possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite, amphibole and celsian. The 8~XO value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene, as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet. The large range in 81KO values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the Roberts Victor eclogites represent metamorphosed, altered basalts. The eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite, and the source of this fluid ".,,'as probably unrelated to the eclogitc.
!. Introduction:
The Roberts Victor kimberlite pipe in South Africa is well-known for its diamond production and for the conspicuous predominance of eclogites among its xenolith population. Two hypotheses have been advanced to explain the origin of these eclogites. MacGregor and Carter [1] subdivided the Roberts Victor eclogites into two major groups based on texture, mineralogy and major element chemistry and proposed an igneous origin involving fractionation and simultaneous crystallization of garnet and clinopyroxene from mafic magmas at high pressures. More recently, MacGregor [2,3], using a synthesis of the major and trace element and isotopic data of these eclogites suggested that the eclogites represent metamorphosed subducted oceanic crust. The results of the first oxygen isotopic measurements showed that the Roberts Victor eclogites had a wide range of lSO/1~'O ratios that were initially attributed to fractional crystallization of a mafic melt at high pressures [4,5]. In 0012-821X/87/$03.50
" 1987 Elsevier Science Publishers B.V
light of recent oxygen isotope analyses of oceanic lithosphere and ophiolites [6-8], the previous "anomalous" oxygen isotope data [4,5] of the Roberts Victor eclogites can now be interpreted to result from metamorphism of hydrothermally-altered oceanic crust [2,3,9,10]. Petrographic analyses of the eclogites indicate that secondary alteration is ubiquitous and often extensive. This alteration occurs as phlogopite-rich veins and as fine-scale alteration of the primary phases, garnet and clinopyroxene. The effect of this secondary alteration on the isotopic and chemical composition of the primary minerals is unknown, This study examines the oxygen isotopic composition of coexisting primary and secondary phases in the Roberts Victor eclogites to constrain the composition of metasomatic fluids. In addition, the small differences in the oxygen isotopic composition of primary garnet and pyroxene is discussed as further evidence that the eclogitcs represent subducted oceanic crust.
81 spectrometry. The isotope analyses are reported in the familiar delta notation in units of per mil relative to the V - S M O W standard. Duplicate analyses indicate that the 8180 values agree to better than + 0 . 2 and the 8180 value of NBS-28 quartz standard is + 9.6 in the Saskatoon laboratory.
2. Samples and methods
Twelve samples representing all the diverse textural and mineralogical varieties of eciogites found in the Roberts Victor kimberlite pipe were selected from the collections of J.J, G u r n e y and from those initially studied by I.D. MacGregor. The petrography and chemical compositions of the minerals of the eciogites were determined. Some samples contained pyroxene and garnet with reaction rims, exsolution features and chemical zoning. The twelve samples selected for the present study encompass a variety of textures, mineralogies and a wide range of temperatures of equilibration estimated on the basis of M g - F e partitioning between the garnets and clinopyroxenes [11]. The eclogites were crushed to 8 0 - 1 2 0 mesh size and the minerals were separated by a combination of Franz Magnetic separation, density separations with heavy liquids, and, finally, hand-picking under a binocular microscope. The final hand-picking under the binocular microscope was particularly important for the clinopyroxenes as it was necessary to inspect each grain to select only the clear, glassy clinopyroxenes for isotopic analysis. Clean mineral separates were essential to obtain credible oxygen isotope analysis. All our final separates consisted of 100% of the pure, unaltered minerals. The oxygen isotopic compositions were measured according to the BrF s procedure at the University of Saskatchewan by isotope ratio mass
3. Results and discussion
6180 values of the mineral separates are listed in Table 1. The 8aSO values of the whole rocks are estimated from the modal abundances. Although the 8180 values of most of the minerals were reproducible to +0.1, some of the minerals, such as those in samples HRV-187, HRV-15 and RV(W) shown in Table l, did not produce reproducible values. These inconsistent results are possibly due to isotopic zoning within the minerals, either within grains or a m o n g mineral grains, as a result of the introduction of fluids that also produced seco n d a r y phases such as phlogopite, amphibole, celsian and albite which rim the garnets, and to a lesser extent the clinopyroxenes of these samples. In general, however, the values for the mineral separates are consistent and reproducible, with AlsO (clinopyroxene-garnet) values ranging from 0.0 to 0.3%. Compilation of the data from this study with those of the earlier studies of Garlick et al. [5] and Jagoutz et al. [12] indicate that coexisting garnet and clinopyroxene generally have similar oxygen
TABLE 1 3180 values (%~)of coexisting garnets, clinopyroxenes and phlogopites of Robers Victor eclogites measured in this study Sample
Garnet
Clinopyroxene
Phlogopite
(cpx-gnt)
Whole rock ~
HRV-272 HRV-20! HRV-187 RV-102 HRV-93 R-52 R-19 HRVol5 R-13
+6.1 +3.8 +6.4 +5.4 +7.0 +6.7 +6.8 +5.9 +6.1
+6.3 +4.0 +5.7 b +5.5 +7.0 +6.9 +6.9 +6.1 b +6.4
+5.9 +6.4 +7.1 +6.7 +7.4
0.2 0.2 -0.7 0.1 0.0 0.2 0.1 0.2 0.3
+6.2 +3.9 +6.1 +5.5 +7.0 +6.9 +6.8 +6.0 +6.3
R-II
+7.1
+7.4
0.3
+7.2
R-7 RV(W)
+6.4 +6.9 b
+6.4 +7.2
0.0 0.3
+6.4 +7.0
" Whole rock 8180 estimates are based on modal mineralogy. Reproducibility greater than 0.2.
-
82
isotopic compositions, so that most samples have A180 (clinopyroxene-garnet)= 0 regardless of the whole-rock 6180 values (Fig. 1). Such small differences between the ~ 8 0 values of garnet and clinopyroxene are expected for isotopic equilibration at high temperatures implying that the garnet and clinopyroxene formed at the same time during the transition from basalt to eclogite. Variations in whole-rock 8~sO values are substantial, from + 2.2 to +8.3 for these eclogites, and since these eclogites are clearly of mantle-derivation (e.g. [11]), there must be substantial variations in the oxygen isotopic composition of the mantle• Kyser et al. [13] and Gregory and Taylor [10] have implied that the extremes in the 8tSO values of these eclogites are outside the range of 5-7, expected for "normal" mantle materials. Gregory and Taylor [10] recently have suggested that the large range in the 8~sO values of olivine from the mantle may result from melting these eclogites and then by reacting these melts with the peridotitic hosts of the mantle, although no silicate liquids erupted from the mantle have such extreme values. MacGregor and Manton [3] have recently extended the chemical and petrographic classification of MacGregor and Carter [1] by using the available 8xSO data as a discriminant, with the type I and type lI eclogites having 8t~O values greater than +5.8 and less than + 5.3, respectively. The compiled analyses, however, suggest
that there may be a gap in the 8xsO values, between the 4.5 and 5.3 continuum of values (Fig. 2). It has been pointed out by MacGregor [2], among others [8], that the range of 6tSO values observed in the Roberts Victor eclogites compare favorably with those found in seawater-altered lithospheric crust, implying derivation of the Roberts Victor eclogites from subducted oceanic crust [14]. If this comparison is valid, a continuum of 8~sO values is expected from the data of altered pillow basalts and sheeted dikes of gabbros of ophiolitic complexes [8]. Textural relations of the phlogopites with the garnets and pyroxenes of the eclogites point to their secondary, metasomatic origin. This is evidenced by the veins and rims of phlogopites around the primary clinopyroxenes and garnets of these eclogites. Therefore, the oxygen isotopic composition of the phlogopites may provide im-
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8 Fig. 1. Correlation between the 81~'O values of coexisting garnet and clinopyroxene or phlogopite in the two different groups of Roberts Victor eclogites. Data from this study, (iarlick et al. [5] and from Jagoutz et al. [12]. This diagram illustrates that the garnets and the clinopyroxenes are usually in isotopic equilibrium, while phlogopitc is often not in equilibrium with coexisting minerals.
Fig. 2. Compilation of all available ~lSO values measured in Roberts Victor eclogite minerals from three sources: this study. (.iarlick el al. [5], and Jagoutz et al. [12]. "l"his compilation indicates the possibility of a gap in the 8tsO values between • 4.5 and -5.3.
83
portant constraints on the composition of metasomatic fluids in the mantle. The five phlogopites analyzed in this study show a 81~O range of + 5.9 to +7.4, which is well within the range reported by other workers [15,16] in mantle-derived amphiboles and phlogopites. However, the 8180 values of the phlogopites correlate positively with the whole-rock values of their host (Fig. 1). In addition, substantial differences between the 8180 values of the phlogopites and the primary minerals indicate that isotopic equilibration between the metasomatic fluid and the rock was not attained, although the K 2 0 contents of the coexisting clinopyroxenes increase with increasing modal amounts of the phlogopites (Fig. 3). These observations, collectively, can be interpreted in the following manner: the phlogopites are results of secondary metasomatic fluids which also carried other large ion lithophile elements like potassium and which affected the chemical composition of the eclogites. The oxygen isotopic compositions of the metasomatic fluids may have been buffered by the host rock eclogites rather than the metasomatic fluid buffering the country rock. The metasomatic fluids were derived mostly from sources unrelated to the eclogites. Various radiogenic isotopic systematics of the Roberts Victor eclogites point to a possible Archean age [12,17,18] of eclogite facies metamorphism. The oxygen isotopic compositions of the Roberts Victor eclogites compare favorably (Fig. 4) to those of a recent study [19] from a suite of mafic and ultramafic samples from the Onverwacht Group, the basal unit of the Archean greenstone sequence of the Barberton Mountains of 0 25 o 20 -. ~.0
c a~
_
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o
o
~
~
015
o~o
o 05
o oo
4) 0
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I
[
I
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MCCal o,~, Ph'ogopl~e
Fig. 3. Plot of KzO contents of clinopyroxenes(determined by electron microprobe) against the modal phlogopite in five phlogopite-bearing eclogitesof this study.
MACQ~IAJ~IE iSlAND
II
LLI I""
-r
~A&
R O B E r t S VICTOR m
I 4
I 6
I 8
lo 1
I 12
I '4
S'60/o Fig. 4. Comparison of the ranges of ~180 values of three Phanerozoic ophiolites [8,20,21], the Onverwacht Group of marie and uhramafic rocks [19] and the Roberts Victor cclogitcs.
South Africa and to Phanerozoic ophiolites as represented by the Oman ophiolite [8], the Macquarie Island ophiolite [20] and the Chilean ophiolites [21]. It is clear from the comparisons in Fig. 4 that the Roberts Victor eclogite data show a widely variable range, unlike the narrow range of oxygen isotopic ratios commonly found in the mantle-derived volcanic rocks and ultramafic xenoliths of alkalic basalts and kimberlites, but similar to the range displayed by these ophiolites which are widely believed to be hydrothermally altered oceanic crust. 4. Conclusion
8~80 values of coexisting garnets, clinopyroxenes and phlogopites in the Roberts Victor eclogites, in the light of recent oxygen isotopic studies of hydrothermaily altered oceanic lithospheric crust, can now be interpreted to indicate that these eclogites represent metamorphosed, subducted, ancient oceanic crust that has been subjected to secondary metasomatism. 8t80 values of the phlogopites, reported here for the first time, sometimes differ substantially from those of coexisting garnets and clinopyroxenes of the bulk rock, although there is a positive correlation between the 8180 of the phlogopites and those of the bimineralic eclogites. These data are interpreted to indicate that the eclogites were subjected to infiltration metasomatism in the mantle and that the
84 o x y g e n i s o t o p i c c o m p o s i t i o n o f t h e fluids w e r e a f f e c t e d b y t h e c o u n t r y rocks.
Acknowledgements This
research was
supported
by
NSF
grant
E A R 8307613 to A . R . B . a n d b y N a t i o n a l S c i e n c e and
Engineering
Research
Council
of
Canada
g r a n t s to T . K . K . Drs. J.J. G u r n e y a n d I.D. M a c G r e g o r g e n e r o u s l y p r o v i d e d the e c l o g i t i c x e n o l i t h s a n a l y z e d in t h e p r e s e n t s t u d y . D i s c u s s i o n s w i t h I.D. M a c G r e g o r at v a r i o u s s t a g e s o f this w o r k arc gratefully acknowledged. We thank Karlis Muehl e n b a c h s a n d a n a n o n y m o u s r e v i e w e r for s u g g e s tions which i m p r o v e d this manuscript.
References I I.D. MacGregor and J.L. Carter, The chemistry of clinopyroxenes and garnets of eclogite and peridotite xenoliths from the Roberts Victor Mine, South Africa. Phys. Earth Planet. Inter. 3, 391-397, 1970. 2 I.D. MacGregor, The Roberts Victor eclogites: ancient oceanic crust, Abstr. Geol. Soc. Am. Annu. Meei. 650, 1985. 3 i.D. MacGregor, and W.I. Manton, The Roberts Victor eclogites: ancient oceanic crust, J. Geophys. Res., in press, 1986. 4 D.E. Vogel and G.D. Garlick. Oxygen isotope ratios in metamorphic eclogites, Contrib. Mineral. Petrol. 28, 183-191, 1970. 5 G.D. Garlick, I.D. MacGregor, and D.E. Vogel, Oxygen isotope ratios in eclogites from kimberlites, Science 172, 1025-1027, 1971. 6 M. Magaritz and H.P. Taylor, Jr., Oxygen, hydrogen and carbon isotope studies of the Franciscan formation, Coast Ranges, California, Geochim. Cosmochim. Acta 40, 215-234, 1976. 7 E.T.C. Spooner, R.D. Beckinsale, W.S. Fyfe and J.D. Sinewing. 1SO-enriched ophiolitic metabasic r~xzks from E. Liguria (Italy), Pindos (Greece), and Trtx'~los (Cyprus). Contrib. Mineral. Petrol. 47, 41, 1974. 8 R.T. Gregory and H.P. Taylor, Jr., An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail ophiolite, Oman: evidence for 81sO-buffering of the oceans by deep ( > 5 kin) seawater-hydrothermal circulation at mid-ocean ridges, J. Geophys. Res. 86, 2737-2755, 1981.
9 J.S. Ongley, A.R. Basu and T.K. Kyscr, Oxygen isotopic study of coexisting garnets, clinopyroxenes and phlogopites from Roberts Victor eclogite xenoliths, Abstr. Trans. Am. Geophys. Union 67, 16. 397, 1986. 10 R.T. Gregory and H.P. Taylor, Jr.. Non-equilibrium, mctasomatic tSO/160 effects in upper mantle mincr',d assemblages, Contrib. Mineral. Petrol. 93. 124-135, 1986. l l A.R. Basu, J.S. Ongley and I.D. MacGregor. Eclogites. pyroxene geotherm and layered mantle convection, Science 233, 1303-13()5. 1986. 12 E. Jagoutz, J.B. Dawson, S. Horncs, B. Spettael and II. Wanke, Anorthositic oceanic crust in the Archean, Lunar Planet. Sci. XV, 395-396, 1984. 13 T.K. Kyser, J.R. O'Ncil and I.S.E. Carmichael, Oxygen isotope thermometry of basic lavas and mantle nodules, Contrib. Mineral. Petrol. 77, 11-23, 1981. 14 H. Halmstaedt and R. Doig, Eclogitc nodules from kimbcrlite pipes of the Colorado Plateau-samples of subducted Franciscan-type oceanic lithosphere, in: Physics and Chemistry' of the Earth 9, L. Ahrens, J.B. Dawson, A.R. Duncan and A.J. Erlank, eds., pp. 95-112, Pergamon Press, Oxford, 1975. 15 S.M.F. Shepard and J.B. Dawson, Hydrogen. carbon, and oxygen isotope studies of megacwst and matrix minerals from Lcsothan and South African kimberlites, in: Physics and Chemistry of the Earth 9, L. Ahrens, J.B. Dawson, A.R. Duncan and A.J. Erlank, eds., pp. 747-763, Pergamon Press, Oxford, 1975. 16 A.L. Boettcher and J.R. O'Neil, Stable isotopes, chemical, and petrographic studies of high-pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basahs and kimberlites, Am. J. Sci. 280A. 594-621, 1980. 17 W.I. Manton and M. Tatsumoto, Some Pb and Sr isotopic measurements on eclogites from Roberts Victor Mine, South Africa, Earth Planet. Sci. Lett. 10, 217-226, 1971. 18 J.D. Kramers, Lead, uranium, strontium, potassium and rubidium in inclusion-bearing diamonds and mantle-derived xenoliths from southern Africa, Earth Planet. Sci. Lett. 42, 58-70, 1979. 19 S.E. Hoffman, M. Wilson and D.S. Stakes. Inferred oxygen isotope profile of Archean oceanic crust, Onverwacht Group, South Africa, Nature 321, 55-58, 1986. 20 J.D. Cocker, B.J. (.}tiffin and K. Muehlenbachs, Oxygen and carbon isotope evidence for sea water-hydrothermal alteration of the Macquarie Island Ophiolite, Earth Planet. Sci. Lett. 61, 112-122, 1976. 21 C.R. Stern, M.J. DeWitt and J.R. Lawrence, Igneous and metamorphic processes associated with the formation of Chilean ophiolites and their implication for ocean floor metamorphism, seismic layering, and magnetism, J. Geophys. Res. 81. 4370-4380, 1976.
80
121
Oxygen isotopes in coexisting garnets, clinopyroxenes and phlogopites of Roberts Victor eclogites" implications for petrogenesis and mantle metasomatis.m Jennifer S. Ongley ~, Asish R. Basu ~ and T. Kurtis K y s e r 2 t Department of (;eologtcal Sciences. Untt:erst(v of Roche~ter, Rochester, N Y 1462 7 { U S.A.) " Department of Geological Sciences. Untt'ersi(v of Saskatchewan, Saskatoon, Sask. (Canada) Received August 21. 1986; revised version received February 2, 1987 81~O values of coexisting garnet, clinopyroxene and phlogopite for twelve compositionally and texturally diverse Roberts Victor eclogite xenoliths range from 4 3.8 to + 7.1, 4-4.0 to + 7.4 and r 5.9 to + 7.4, respectively. Differences between the 31~O values of coexisting garnets and clinopyroxenes are normally zero: however, there is some variation in the 81~O values of different fractions of the same mineral in four samples which suggests the presence of isotopic zonation and inhomogeneity, possibly resulting from the introduction of a secondary fluid which metasomatized the eclogites and resulted in the formation of phlogopite, amphibole and celsian. The 8~XO value of the metasomatic fluid is generally buffered by the isotopic composition of the primary garnet and clinopyroxene, as indicated by a correlation between the isotopic composition of phlogopite and the primary pyroxene and garnet. The large range in 81KO values of the eclogites and the similarity in the isotopic composition of coexisting pyroxene and garnet support the interpretation that the Roberts Victor eclogites represent metamorphosed, altered basalts. The eclogites were subjected to infiltration metasomatism in the mantle prior to their incorporation in the kimberlite, and the source of this fluid ".,,'as probably unrelated to the eclogitc.
!. Introduction:
The Roberts Victor kimberlite pipe in South Africa is well-known for its diamond production and for the conspicuous predominance of eclogites among its xenolith population. Two hypotheses have been advanced to explain the origin of these eclogites. MacGregor and Carter [1] subdivided the Roberts Victor eclogites into two major groups based on texture, mineralogy and major element chemistry and proposed an igneous origin involving fractionation and simultaneous crystallization of garnet and clinopyroxene from mafic magmas at high pressures. More recently, MacGregor [2,3], using a synthesis of the major and trace element and isotopic data of these eclogites suggested that the eclogites represent metamorphosed subducted oceanic crust. The results of the first oxygen isotopic measurements showed that the Roberts Victor eclogites had a wide range of lSO/1~'O ratios that were initially attributed to fractional crystallization of a mafic melt at high pressures [4,5]. In 0012-821X/87/$03.50
" 1987 Elsevier Science Publishers B.V
light of recent oxygen isotope analyses of oceanic lithosphere and ophiolites [6-8], the previous "anomalous" oxygen isotope data [4,5] of the Roberts Victor eclogites can now be interpreted to result from metamorphism of hydrothermally-altered oceanic crust [2,3,9,10]. Petrographic analyses of the eclogites indicate that secondary alteration is ubiquitous and often extensive. This alteration occurs as phlogopite-rich veins and as fine-scale alteration of the primary phases, garnet and clinopyroxene. The effect of this secondary alteration on the isotopic and chemical composition of the primary minerals is unknown, This study examines the oxygen isotopic composition of coexisting primary and secondary phases in the Roberts Victor eclogites to constrain the composition of metasomatic fluids. In addition, the small differences in the oxygen isotopic composition of primary garnet and pyroxene is discussed as further evidence that the eclogitcs represent subducted oceanic crust.
81 spectrometry. The isotope analyses are reported in the familiar delta notation in units of per mil relative to the V - S M O W standard. Duplicate analyses indicate that the 8180 values agree to better than + 0 . 2 and the 8180 value of NBS-28 quartz standard is + 9.6 in the Saskatoon laboratory.
2. Samples and methods
Twelve samples representing all the diverse textural and mineralogical varieties of eciogites found in the Roberts Victor kimberlite pipe were selected from the collections of J.J, G u r n e y and from those initially studied by I.D. MacGregor. The petrography and chemical compositions of the minerals of the eciogites were determined. Some samples contained pyroxene and garnet with reaction rims, exsolution features and chemical zoning. The twelve samples selected for the present study encompass a variety of textures, mineralogies and a wide range of temperatures of equilibration estimated on the basis of M g - F e partitioning between the garnets and clinopyroxenes [11]. The eclogites were crushed to 8 0 - 1 2 0 mesh size and the minerals were separated by a combination of Franz Magnetic separation, density separations with heavy liquids, and, finally, hand-picking under a binocular microscope. The final hand-picking under the binocular microscope was particularly important for the clinopyroxenes as it was necessary to inspect each grain to select only the clear, glassy clinopyroxenes for isotopic analysis. Clean mineral separates were essential to obtain credible oxygen isotope analysis. All our final separates consisted of 100% of the pure, unaltered minerals. The oxygen isotopic compositions were measured according to the BrF s procedure at the University of Saskatchewan by isotope ratio mass
3. Results and discussion
6180 values of the mineral separates are listed in Table 1. The 8aSO values of the whole rocks are estimated from the modal abundances. Although the 8180 values of most of the minerals were reproducible to +0.1, some of the minerals, such as those in samples HRV-187, HRV-15 and RV(W) shown in Table l, did not produce reproducible values. These inconsistent results are possibly due to isotopic zoning within the minerals, either within grains or a m o n g mineral grains, as a result of the introduction of fluids that also produced seco n d a r y phases such as phlogopite, amphibole, celsian and albite which rim the garnets, and to a lesser extent the clinopyroxenes of these samples. In general, however, the values for the mineral separates are consistent and reproducible, with AlsO (clinopyroxene-garnet) values ranging from 0.0 to 0.3%. Compilation of the data from this study with those of the earlier studies of Garlick et al. [5] and Jagoutz et al. [12] indicate that coexisting garnet and clinopyroxene generally have similar oxygen
TABLE 1 3180 values (%~)of coexisting garnets, clinopyroxenes and phlogopites of Robers Victor eclogites measured in this study Sample
Garnet
Clinopyroxene
Phlogopite
(cpx-gnt)
Whole rock ~
HRV-272 HRV-20! HRV-187 RV-102 HRV-93 R-52 R-19 HRVol5 R-13
+6.1 +3.8 +6.4 +5.4 +7.0 +6.7 +6.8 +5.9 +6.1
+6.3 +4.0 +5.7 b +5.5 +7.0 +6.9 +6.9 +6.1 b +6.4
+5.9 +6.4 +7.1 +6.7 +7.4
0.2 0.2 -0.7 0.1 0.0 0.2 0.1 0.2 0.3
+6.2 +3.9 +6.1 +5.5 +7.0 +6.9 +6.8 +6.0 +6.3
R-II
+7.1
+7.4
0.3
+7.2
R-7 RV(W)
+6.4 +6.9 b
+6.4 +7.2
0.0 0.3
+6.4 +7.0
" Whole rock 8180 estimates are based on modal mineralogy. Reproducibility greater than 0.2.
-
82
isotopic compositions, so that most samples have A180 (clinopyroxene-garnet)= 0 regardless of the whole-rock 6180 values (Fig. 1). Such small differences between the ~ 8 0 values of garnet and clinopyroxene are expected for isotopic equilibration at high temperatures implying that the garnet and clinopyroxene formed at the same time during the transition from basalt to eclogite. Variations in whole-rock 8~sO values are substantial, from + 2.2 to +8.3 for these eclogites, and since these eclogites are clearly of mantle-derivation (e.g. [11]), there must be substantial variations in the oxygen isotopic composition of the mantle• Kyser et al. [13] and Gregory and Taylor [10] have implied that the extremes in the 8tSO values of these eclogites are outside the range of 5-7, expected for "normal" mantle materials. Gregory and Taylor [10] recently have suggested that the large range in the 8~sO values of olivine from the mantle may result from melting these eclogites and then by reacting these melts with the peridotitic hosts of the mantle, although no silicate liquids erupted from the mantle have such extreme values. MacGregor and Manton [3] have recently extended the chemical and petrographic classification of MacGregor and Carter [1] by using the available 8xSO data as a discriminant, with the type I and type lI eclogites having 8t~O values greater than +5.8 and less than + 5.3, respectively. The compiled analyses, however, suggest
that there may be a gap in the 8xsO values, between the 4.5 and 5.3 continuum of values (Fig. 2). It has been pointed out by MacGregor [2], among others [8], that the range of 6tSO values observed in the Roberts Victor eclogites compare favorably with those found in seawater-altered lithospheric crust, implying derivation of the Roberts Victor eclogites from subducted oceanic crust [14]. If this comparison is valid, a continuum of 8~sO values is expected from the data of altered pillow basalts and sheeted dikes of gabbros of ophiolitic complexes [8]. Textural relations of the phlogopites with the garnets and pyroxenes of the eclogites point to their secondary, metasomatic origin. This is evidenced by the veins and rims of phlogopites around the primary clinopyroxenes and garnets of these eclogites. Therefore, the oxygen isotopic composition of the phlogopites may provide im-
00--
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8 Fig. 1. Correlation between the 81~'O values of coexisting garnet and clinopyroxene or phlogopite in the two different groups of Roberts Victor eclogites. Data from this study, (iarlick et al. [5] and from Jagoutz et al. [12]. This diagram illustrates that the garnets and the clinopyroxenes are usually in isotopic equilibrium, while phlogopitc is often not in equilibrium with coexisting minerals.
Fig. 2. Compilation of all available ~lSO values measured in Roberts Victor eclogite minerals from three sources: this study. (.iarlick el al. [5], and Jagoutz et al. [12]. "l"his compilation indicates the possibility of a gap in the 8tsO values between • 4.5 and -5.3.
83
portant constraints on the composition of metasomatic fluids in the mantle. The five phlogopites analyzed in this study show a 81~O range of + 5.9 to +7.4, which is well within the range reported by other workers [15,16] in mantle-derived amphiboles and phlogopites. However, the 8180 values of the phlogopites correlate positively with the whole-rock values of their host (Fig. 1). In addition, substantial differences between the 8180 values of the phlogopites and the primary minerals indicate that isotopic equilibration between the metasomatic fluid and the rock was not attained, although the K 2 0 contents of the coexisting clinopyroxenes increase with increasing modal amounts of the phlogopites (Fig. 3). These observations, collectively, can be interpreted in the following manner: the phlogopites are results of secondary metasomatic fluids which also carried other large ion lithophile elements like potassium and which affected the chemical composition of the eclogites. The oxygen isotopic compositions of the metasomatic fluids may have been buffered by the host rock eclogites rather than the metasomatic fluid buffering the country rock. The metasomatic fluids were derived mostly from sources unrelated to the eclogites. Various radiogenic isotopic systematics of the Roberts Victor eclogites point to a possible Archean age [12,17,18] of eclogite facies metamorphism. The oxygen isotopic compositions of the Roberts Victor eclogites compare favorably (Fig. 4) to those of a recent study [19] from a suite of mafic and ultramafic samples from the Onverwacht Group, the basal unit of the Archean greenstone sequence of the Barberton Mountains of 0 25 o 20 -. ~.0
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MACQ~IAJ~IE iSlAND
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S'60/o Fig. 4. Comparison of the ranges of ~180 values of three Phanerozoic ophiolites [8,20,21], the Onverwacht Group of marie and uhramafic rocks [19] and the Roberts Victor cclogitcs.
South Africa and to Phanerozoic ophiolites as represented by the Oman ophiolite [8], the Macquarie Island ophiolite [20] and the Chilean ophiolites [21]. It is clear from the comparisons in Fig. 4 that the Roberts Victor eclogite data show a widely variable range, unlike the narrow range of oxygen isotopic ratios commonly found in the mantle-derived volcanic rocks and ultramafic xenoliths of alkalic basalts and kimberlites, but similar to the range displayed by these ophiolites which are widely believed to be hydrothermally altered oceanic crust. 4. Conclusion
8~80 values of coexisting garnets, clinopyroxenes and phlogopites in the Roberts Victor eclogites, in the light of recent oxygen isotopic studies of hydrothermaily altered oceanic lithospheric crust, can now be interpreted to indicate that these eclogites represent metamorphosed, subducted, ancient oceanic crust that has been subjected to secondary metasomatism. 8t80 values of the phlogopites, reported here for the first time, sometimes differ substantially from those of coexisting garnets and clinopyroxenes of the bulk rock, although there is a positive correlation between the 8180 of the phlogopites and those of the bimineralic eclogites. These data are interpreted to indicate that the eclogites were subjected to infiltration metasomatism in the mantle and that the
84 o x y g e n i s o t o p i c c o m p o s i t i o n o f t h e fluids w e r e a f f e c t e d b y t h e c o u n t r y rocks.
Acknowledgements This
research was
supported
by
NSF
grant
E A R 8307613 to A . R . B . a n d b y N a t i o n a l S c i e n c e and
Engineering
Research
Council
of
Canada
g r a n t s to T . K . K . Drs. J.J. G u r n e y a n d I.D. M a c G r e g o r g e n e r o u s l y p r o v i d e d the e c l o g i t i c x e n o l i t h s a n a l y z e d in t h e p r e s e n t s t u d y . D i s c u s s i o n s w i t h I.D. M a c G r e g o r at v a r i o u s s t a g e s o f this w o r k arc gratefully acknowledged. We thank Karlis Muehl e n b a c h s a n d a n a n o n y m o u s r e v i e w e r for s u g g e s tions which i m p r o v e d this manuscript.
References I I.D. MacGregor and J.L. Carter, The chemistry of clinopyroxenes and garnets of eclogite and peridotite xenoliths from the Roberts Victor Mine, South Africa. Phys. Earth Planet. Inter. 3, 391-397, 1970. 2 I.D. MacGregor, The Roberts Victor eclogites: ancient oceanic crust, Abstr. Geol. Soc. Am. Annu. Meei. 650, 1985. 3 i.D. MacGregor, and W.I. Manton, The Roberts Victor eclogites: ancient oceanic crust, J. Geophys. Res., in press, 1986. 4 D.E. Vogel and G.D. Garlick. Oxygen isotope ratios in metamorphic eclogites, Contrib. Mineral. Petrol. 28, 183-191, 1970. 5 G.D. Garlick, I.D. MacGregor, and D.E. Vogel, Oxygen isotope ratios in eclogites from kimberlites, Science 172, 1025-1027, 1971. 6 M. Magaritz and H.P. Taylor, Jr., Oxygen, hydrogen and carbon isotope studies of the Franciscan formation, Coast Ranges, California, Geochim. Cosmochim. Acta 40, 215-234, 1976. 7 E.T.C. Spooner, R.D. Beckinsale, W.S. Fyfe and J.D. Sinewing. 1SO-enriched ophiolitic metabasic r~xzks from E. Liguria (Italy), Pindos (Greece), and Trtx'~los (Cyprus). Contrib. Mineral. Petrol. 47, 41, 1974. 8 R.T. Gregory and H.P. Taylor, Jr., An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail ophiolite, Oman: evidence for 81sO-buffering of the oceans by deep ( > 5 kin) seawater-hydrothermal circulation at mid-ocean ridges, J. Geophys. Res. 86, 2737-2755, 1981.
9 J.S. Ongley, A.R. Basu and T.K. Kyscr, Oxygen isotopic study of coexisting garnets, clinopyroxenes and phlogopites from Roberts Victor eclogite xenoliths, Abstr. Trans. Am. Geophys. Union 67, 16. 397, 1986. 10 R.T. Gregory and H.P. Taylor, Jr.. Non-equilibrium, mctasomatic tSO/160 effects in upper mantle mincr',d assemblages, Contrib. Mineral. Petrol. 93. 124-135, 1986. l l A.R. Basu, J.S. Ongley and I.D. MacGregor. Eclogites. pyroxene geotherm and layered mantle convection, Science 233, 1303-13()5. 1986. 12 E. Jagoutz, J.B. Dawson, S. Horncs, B. Spettael and II. Wanke, Anorthositic oceanic crust in the Archean, Lunar Planet. Sci. XV, 395-396, 1984. 13 T.K. Kyser, J.R. O'Ncil and I.S.E. Carmichael, Oxygen isotope thermometry of basic lavas and mantle nodules, Contrib. Mineral. Petrol. 77, 11-23, 1981. 14 H. Halmstaedt and R. Doig, Eclogitc nodules from kimbcrlite pipes of the Colorado Plateau-samples of subducted Franciscan-type oceanic lithosphere, in: Physics and Chemistry' of the Earth 9, L. Ahrens, J.B. Dawson, A.R. Duncan and A.J. Erlank, eds., pp. 95-112, Pergamon Press, Oxford, 1975. 15 S.M.F. Shepard and J.B. Dawson, Hydrogen. carbon, and oxygen isotope studies of megacwst and matrix minerals from Lcsothan and South African kimberlites, in: Physics and Chemistry of the Earth 9, L. Ahrens, J.B. Dawson, A.R. Duncan and A.J. Erlank, eds., pp. 747-763, Pergamon Press, Oxford, 1975. 16 A.L. Boettcher and J.R. O'Neil, Stable isotopes, chemical, and petrographic studies of high-pressure amphiboles and micas: evidence for metasomatism in the mantle source regions of alkali basahs and kimberlites, Am. J. Sci. 280A. 594-621, 1980. 17 W.I. Manton and M. Tatsumoto, Some Pb and Sr isotopic measurements on eclogites from Roberts Victor Mine, South Africa, Earth Planet. Sci. Lett. 10, 217-226, 1971. 18 J.D. Kramers, Lead, uranium, strontium, potassium and rubidium in inclusion-bearing diamonds and mantle-derived xenoliths from southern Africa, Earth Planet. Sci. Lett. 42, 58-70, 1979. 19 S.E. Hoffman, M. Wilson and D.S. Stakes. Inferred oxygen isotope profile of Archean oceanic crust, Onverwacht Group, South Africa, Nature 321, 55-58, 1986. 20 J.D. Cocker, B.J. (.}tiffin and K. Muehlenbachs, Oxygen and carbon isotope evidence for sea water-hydrothermal alteration of the Macquarie Island Ophiolite, Earth Planet. Sci. Lett. 61, 112-122, 1976. 21 C.R. Stern, M.J. DeWitt and J.R. Lawrence, Igneous and metamorphic processes associated with the formation of Chilean ophiolites and their implication for ocean floor metamorphism, seismic layering, and magnetism, J. Geophys. Res. 81. 4370-4380, 1976.