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Trapped fluids associated with Cr-diopside rich veins in spinel lherzolite xenoliths: Implications for mantle metasomatism
46 Nd-Sr-Pb ISOTOPIC CARBONATITES
SYSTEMATICS
OF
YOUNG
HEARD ISLAND: AN EXAMPLE OF LARGE ISOTOPIC VARIAnONS ON A SMALL OCEANIC ISLAND
D. ROUSSEAU, C J ALLEGRE & J.B. DAWSON (Lab. Geochimie & Cosmochimie IPG - 4 place Jussieu 75252 Paris Cedex 05 )
I.BARLING, S.L.GOLDSTEIN, G.E.WHELLER, and I. A. NICHOLLS (Max-Planck-Institut fUr Chemie, Postfach 3060, 6500 Mainz, F. R. Gennany)
Sr, Nd and Pb isotopic data are reported for 20 young carbonatites from both oceanic and continental settings. Samples come from Canary, Cape Verde, Kaiserstuhl (West Germany), Marocco, East Africa, Angola and from India and Brazil. As previously established, most carbonatites confirm their similar isotopic characteristics with ocean island basalts. But we notice some exceptions for India, and Brazil data (shifted towards the enriched pole on the Sr-Nd diagram) and Angola (high 208Pb/ 204 Pb). We tried to compare carbonatites with their corresponding basalts from the same region. The oceanic carbonatites, just as Kaiserstuhl, lie clearly within the OIB domain. The Maroccan, and East African data lie within or slightly below the mantle trend and we can consider they roughly overlap their regional corresponding basalts. On the other hand, the Angolan, Indian and Brazilian data plot out the ocean island array. For the Angola carbonatites we cannot infer any possible source to explain these data. The India and Brazil carbonatites, are located adjacent and contemporaneous to continental flood basalts, respectively the Dekkan and Parana ones. The data plot clearly within their corresponding CFB. In conclusion, first, there ·is no evidence to imply continental contamination, except for Angola. Then, the carbonatites do not seem to imply one single mantle source type. OIB mantle source as well as continental lithosphere can be inferred for the Carbonatite mantle source.
TRAPPED FLUIDS ASSOCIATED WITH Cr-DIOPSIDE RICH VEINS IN SPINEL LHERZOLITE XENOLITHS: IMPLICATIONS FOR MANTLE METASOMATISM
Heard Island lies ca.440 km southeast of the Kerguelen Islands on the Kerguelen plateau. Pleistocene to present day volcanism has centered on two vents: Big Ben, the main volcano, and Mt.Dixon on the Laurens Peninsula. Isotopic compositions of Sr, Nd and Pb fall within or near the Kerguelen field and define linear arrays. Sr and Nd isotopes range from 87Sr/86Sr =0.7048-0.7060, and ENd=+ 1.7 to -2.2. The Laurens Peninsula series (LPS) are petrographically simple and isotopically homogeneous, having the lowest Sr and the highest Nd and Pb isotope ratios. Big Ben basanites also have a small isotopic range, with 87S r/86S r =0.7052 and eNd = -0.4; they are distinct petrographically having Ti-augite rimmed diopsidic megacrysts and a striking ilmenite-rich ground mass. The Big Ben Basalts have a larger range of isotopic compositions, they include the highest Sr and the lowest Nd and Pb isotopic ratios and extend to values identical to the basanites. Big Ben Basalts are petrographically complex, with features such as reverse zoning and several distinct olivine populations indicating a hybrid history. Other Indian Ocean islands fall on extensions of Heard Island's linear isotopic arrays, implying that simple two component mixing between a distinct Heard source and a common Indian Ocean component can explain Heard trends; however on some trace element plots Heard data fonn triangular fields. Such trace element distributions, especially in conjunction with petrographic distinctions, require a third compositional end member corresponding to the Big Ben basanites. The incompatible element characteristics of these three end-members are distinct. Whilst the LPS and Big Ben basanites show similarities to other Indian Ocean basalts, the Big Ben Basalt series, unlike any reported Indian Ocean suite, shows relative :\b depletion and K enrichment indicating a possible sedimentary source component.Thus, whereas Heard Island as a whole fom1s one end of an Indian Ocean mantle array, on a local scale three distinct components are required to explain trace element trends.
CARBON AND NITROGEN ISOTOPES IN THE MANTLE S.R.Boyd and C.T.Pillinger
H.E.F. AMUNDSEN, T. ANDERSEN and E.A.J. BURKE (Mineralogical-Geological Museum, Sars gt. 1, 0562 Oslo 5, Norway) The nature of fluids in the upper mantle has
implications for element migration during metasomatic processes, the mantle solidus and oxidation state. The focus of this study are fluid inclusions associated with Cr-diopside rich veins in spinel lherzolite xenoliths from northwestern Spitsbergen. Two main types of primary textured inclusions occur: (1) high-density CO 2 fluid; and (2) glass + amphibole + C02 fluid. Microthermometric measurements and laser Raman microprobe analyses suggest that the CO 2 is pure or nearly pure. The recalculated bulk composition of type (2) inclusions resembles a volatile rich high-AI basaltic liquid. This liquid is in equilibrium with the coexisting minerals at ca 11S0°C, using experimental data on pyroxene- and olivine-liquid equilibria. The microtextures and fluid inclusion data suggest that the Cr-diopside rich veins formed by crystallization of pyroxenes during infiltration of immiscibly coexisting high-AI basaltic liquid and C02 fluid. Vein and lherzolite minerals have near identical compositions, indicating that the compositions of the infiltrating fluids were buffered by a spinel lherzolite (FOgO) mineralogy. Thus, the infiltration process resulted mainly in an increase in the pyroxene/olivine ratio in the bulk lherzolites, leaving mineral compositions largely unchanged. Type (2) inclusions have also been found in spinel lherzolites showing high Ga/Al ratios, but otherwise no textural evidence of metasomatic enrichment. This suggests that spinel lherzolites with anomalously high pyroxene contents may reflect metasomatic processes similar to those represented by the pyroxene rich veins.
(Dept. Earth Sciences, Opcn University, Walton Hall, Milton Keynes, U.K.)
(i 13 e and (i15N measurements have been performed on coated and octahedral (+ modified forms) diamonds from Australia, Africa, Siberia and N. America. Amongst the octahedral diamonds two broad groups can be distinguished. Group 01 diamonds (e.g. Finsch, South Africa) have (i13e values between·3 and -7%. (similar to most other upper mantle samples) and contain nitrogen generally depleted in 15N relative to AIR: S15N ca.·5%. (range +5 to ·16%.). Group 02 diamonds (e.g. Argyle, W.Australia) have variable s13e values (·2 to -21Y~) and contain N generally enriched in 15N relative to AIR: ca. +5%. (range ·5 to + 16"/~). The coatings on diamonds from Sierra Leone, Siberia, Angola, Botswana and Zaire have a restricted range in isotopic composition ((i13e -5 to ·7.5Y~, S15N ·2 to -8Y~). Both group 01 and 02 diamonds were present as cores within these samples. The coats of coated diamonds have been interpreted as being phenocrysts (1). Their isotopiC uniformity on a regional scale suggests the existence of a fairly homogeneous e and N reservoir underlying the continental lithosphere which supplies the volatiles associated with kimberlite eruptions. The Siberian pipe (Udachnaya) is of mid· Palaeozoic age which implies that, by this time, the extreme carbon Isotope heterogeneity within the mantle (+3 to ·35%.) was restricted to the continental lithosphere. The isotopic characteristics of the coats are similar to the 01 diamonds. Since diamonds from Finsch and Premier (both 01) have been dated at 3.3 Ga and 1.2 Ga respectively (2,3) there appears to have been little change in the 013C and S15N value of the asthenospheric reservoir since the mid-Archean. The origin of the isotopic characteristics of the group 02 diamonds is uncertain. The observation that they tend to occur in diatremes located close to craton margins (or off the craton in the case of Argyle) indicates that they are associated with younger rather than older lithosphere which argues against the primordial heterogeneity model of Deines et al. (4).
~efs. 1 Boyd et aI., 1987, Earth Planet Sci. Lett., 86, 341-353 Richardson et aI., 1984, Nature, 310, t98-202,: 3Richardson, 1986, Nature, 322, 623-625, 4 Deines et aI., 1986, 4th Int. Kimberlite eonf., Extended Abstr., 383·385)
SYSTEMATICS
OF
YOUNG
HEARD ISLAND: AN EXAMPLE OF LARGE ISOTOPIC VARIAnONS ON A SMALL OCEANIC ISLAND
D. ROUSSEAU, C J ALLEGRE & J.B. DAWSON (Lab. Geochimie & Cosmochimie IPG - 4 place Jussieu 75252 Paris Cedex 05 )
I.BARLING, S.L.GOLDSTEIN, G.E.WHELLER, and I. A. NICHOLLS (Max-Planck-Institut fUr Chemie, Postfach 3060, 6500 Mainz, F. R. Gennany)
Sr, Nd and Pb isotopic data are reported for 20 young carbonatites from both oceanic and continental settings. Samples come from Canary, Cape Verde, Kaiserstuhl (West Germany), Marocco, East Africa, Angola and from India and Brazil. As previously established, most carbonatites confirm their similar isotopic characteristics with ocean island basalts. But we notice some exceptions for India, and Brazil data (shifted towards the enriched pole on the Sr-Nd diagram) and Angola (high 208Pb/ 204 Pb). We tried to compare carbonatites with their corresponding basalts from the same region. The oceanic carbonatites, just as Kaiserstuhl, lie clearly within the OIB domain. The Maroccan, and East African data lie within or slightly below the mantle trend and we can consider they roughly overlap their regional corresponding basalts. On the other hand, the Angolan, Indian and Brazilian data plot out the ocean island array. For the Angola carbonatites we cannot infer any possible source to explain these data. The India and Brazil carbonatites, are located adjacent and contemporaneous to continental flood basalts, respectively the Dekkan and Parana ones. The data plot clearly within their corresponding CFB. In conclusion, first, there ·is no evidence to imply continental contamination, except for Angola. Then, the carbonatites do not seem to imply one single mantle source type. OIB mantle source as well as continental lithosphere can be inferred for the Carbonatite mantle source.
TRAPPED FLUIDS ASSOCIATED WITH Cr-DIOPSIDE RICH VEINS IN SPINEL LHERZOLITE XENOLITHS: IMPLICATIONS FOR MANTLE METASOMATISM
Heard Island lies ca.440 km southeast of the Kerguelen Islands on the Kerguelen plateau. Pleistocene to present day volcanism has centered on two vents: Big Ben, the main volcano, and Mt.Dixon on the Laurens Peninsula. Isotopic compositions of Sr, Nd and Pb fall within or near the Kerguelen field and define linear arrays. Sr and Nd isotopes range from 87Sr/86Sr =0.7048-0.7060, and ENd=+ 1.7 to -2.2. The Laurens Peninsula series (LPS) are petrographically simple and isotopically homogeneous, having the lowest Sr and the highest Nd and Pb isotope ratios. Big Ben basanites also have a small isotopic range, with 87S r/86S r =0.7052 and eNd = -0.4; they are distinct petrographically having Ti-augite rimmed diopsidic megacrysts and a striking ilmenite-rich ground mass. The Big Ben Basalts have a larger range of isotopic compositions, they include the highest Sr and the lowest Nd and Pb isotopic ratios and extend to values identical to the basanites. Big Ben Basalts are petrographically complex, with features such as reverse zoning and several distinct olivine populations indicating a hybrid history. Other Indian Ocean islands fall on extensions of Heard Island's linear isotopic arrays, implying that simple two component mixing between a distinct Heard source and a common Indian Ocean component can explain Heard trends; however on some trace element plots Heard data fonn triangular fields. Such trace element distributions, especially in conjunction with petrographic distinctions, require a third compositional end member corresponding to the Big Ben basanites. The incompatible element characteristics of these three end-members are distinct. Whilst the LPS and Big Ben basanites show similarities to other Indian Ocean basalts, the Big Ben Basalt series, unlike any reported Indian Ocean suite, shows relative :\b depletion and K enrichment indicating a possible sedimentary source component.Thus, whereas Heard Island as a whole fom1s one end of an Indian Ocean mantle array, on a local scale three distinct components are required to explain trace element trends.
CARBON AND NITROGEN ISOTOPES IN THE MANTLE S.R.Boyd and C.T.Pillinger
H.E.F. AMUNDSEN, T. ANDERSEN and E.A.J. BURKE (Mineralogical-Geological Museum, Sars gt. 1, 0562 Oslo 5, Norway) The nature of fluids in the upper mantle has
implications for element migration during metasomatic processes, the mantle solidus and oxidation state. The focus of this study are fluid inclusions associated with Cr-diopside rich veins in spinel lherzolite xenoliths from northwestern Spitsbergen. Two main types of primary textured inclusions occur: (1) high-density CO 2 fluid; and (2) glass + amphibole + C02 fluid. Microthermometric measurements and laser Raman microprobe analyses suggest that the CO 2 is pure or nearly pure. The recalculated bulk composition of type (2) inclusions resembles a volatile rich high-AI basaltic liquid. This liquid is in equilibrium with the coexisting minerals at ca 11S0°C, using experimental data on pyroxene- and olivine-liquid equilibria. The microtextures and fluid inclusion data suggest that the Cr-diopside rich veins formed by crystallization of pyroxenes during infiltration of immiscibly coexisting high-AI basaltic liquid and C02 fluid. Vein and lherzolite minerals have near identical compositions, indicating that the compositions of the infiltrating fluids were buffered by a spinel lherzolite (FOgO) mineralogy. Thus, the infiltration process resulted mainly in an increase in the pyroxene/olivine ratio in the bulk lherzolites, leaving mineral compositions largely unchanged. Type (2) inclusions have also been found in spinel lherzolites showing high Ga/Al ratios, but otherwise no textural evidence of metasomatic enrichment. This suggests that spinel lherzolites with anomalously high pyroxene contents may reflect metasomatic processes similar to those represented by the pyroxene rich veins.
(Dept. Earth Sciences, Opcn University, Walton Hall, Milton Keynes, U.K.)
(i 13 e and (i15N measurements have been performed on coated and octahedral (+ modified forms) diamonds from Australia, Africa, Siberia and N. America. Amongst the octahedral diamonds two broad groups can be distinguished. Group 01 diamonds (e.g. Finsch, South Africa) have (i13e values between·3 and -7%. (similar to most other upper mantle samples) and contain nitrogen generally depleted in 15N relative to AIR: S15N ca.·5%. (range +5 to ·16%.). Group 02 diamonds (e.g. Argyle, W.Australia) have variable s13e values (·2 to -21Y~) and contain N generally enriched in 15N relative to AIR: ca. +5%. (range ·5 to + 16"/~). The coatings on diamonds from Sierra Leone, Siberia, Angola, Botswana and Zaire have a restricted range in isotopic composition ((i13e -5 to ·7.5Y~, S15N ·2 to -8Y~). Both group 01 and 02 diamonds were present as cores within these samples. The coats of coated diamonds have been interpreted as being phenocrysts (1). Their isotopiC uniformity on a regional scale suggests the existence of a fairly homogeneous e and N reservoir underlying the continental lithosphere which supplies the volatiles associated with kimberlite eruptions. The Siberian pipe (Udachnaya) is of mid· Palaeozoic age which implies that, by this time, the extreme carbon Isotope heterogeneity within the mantle (+3 to ·35%.) was restricted to the continental lithosphere. The isotopic characteristics of the coats are similar to the 01 diamonds. Since diamonds from Finsch and Premier (both 01) have been dated at 3.3 Ga and 1.2 Ga respectively (2,3) there appears to have been little change in the 013C and S15N value of the asthenospheric reservoir since the mid-Archean. The origin of the isotopic characteristics of the group 02 diamonds is uncertain. The observation that they tend to occur in diatremes located close to craton margins (or off the craton in the case of Argyle) indicates that they are associated with younger rather than older lithosphere which argues against the primordial heterogeneity model of Deines et al. (4).
~efs. 1 Boyd et aI., 1987, Earth Planet Sci. Lett., 86, 341-353 Richardson et aI., 1984, Nature, 310, t98-202,: 3Richardson, 1986, Nature, 322, 623-625, 4 Deines et aI., 1986, 4th Int. Kimberlite eonf., Extended Abstr., 383·385)