- Email: [email protected]
Origins of the S-type Cape Granites (South Africa)
Goldschmidt Conference Abstracts 2006
Quantifying the isotopic fractionation of lithium during clay formation at various temperatures N. VIGIER1, A. DECARREAU2, R. MILLOT3, J. CARIGNAN1, S. PETIT2, C. FRANCE-LANORD1 1
CRPG, Vandoeuvre-le`s-Nancy, cnrs-nancy.fr) 2 HYDRASA, Poitiers, France 3 BRGM, Orle´ans, France
France
(nvigier@crpg.
Recent studies have shown that the surface geochemical cycle of lithium isotopes is mainly controlled by isotopic fractionations associated with clay formation. Weathering of continental crust by superficial waters can produce residual phases in soils that are depleted in 7Li, and high d7Li river waters. Also, the neoformation of smectite in the ocean could explain the seawater Li isotopic signature (d7Li = 31.2&), significantly heavier than its sources (d7Li = 9& for hydrothermal flux and 23& for dissolved continental flux), considering a minimum isotope fractionation of 19& during Li uptake by clays. We have constrained the lithium isotopic fractionation linked to Li–Mg substitution during smectites (hectorites) crystallization. For this, we have experimentally synthesized hectorites at various temperatures (from 25 to 250 °C), using solutions highly enriched in lithium. All the exchangeable lithium has then been removed with NH4Cl before clay isotopic analyses. Li isotopic compositions (d7Li) for reference materials, the synthesized clays and corresponding solutions have been measured using an Elan 6000 ICP-MS. Some of them have also been measured using a Neptune MC-ICP-MS, for intercalibration purpose. Both techniques are consistent within analytical uncertainties (1& and 0.5&, respectively, at the 2r level). d7Li obtained for the reference materials are consistent with published values. The measured isotopic bias (D7Li) between hectorite and solution is significant, and, decreases with increasing temperature (from 1.6& at 250 °C to 11& at 25 °C). The solution matrix has little effect on the isotopic fractionation but on the relative amount of lithium incorporated into the clay structure. The theoretical isotopic fractionations, as a function of temperature, have been estimated and are consistent with the experimental values, except for the highest temperatures for which some simplification used in the calculation may not be valid. Both, experimental and theoretical results, suggest a maximum Li isotope fractionation of 13& at seafloor temperature, significantly lower than the values used for ocean budget. A low isotope fractionation would imply either a ratio of hydrothermal/continental inputs for lithium that is significantly lower than estimated, or another Li sink in the ocean. doi:10.1016/j.gca.2006.06.1258
A673
Origins of the S-type Cape Granites (South Africa) A. VILLAROS1, G. STEVENS1, I.S. BUICK2 1
Department of Geology, University of Stellenbosch, South Africa ([email protected]; [email protected]) 2 School of Geosciences, Monash University, Melbourne, Australia ([email protected]) The Pan-African Cape Granite (CG) Suite, South Africa, consists of S- (560–540 Ma), I- (540–515 Ma) and A-type (515– 510 Ma) plutons and extrusive rocks. They intruded the low-grade (greenschist-facies) Malmesbury Supergroup (750–610 Ma) during and after the Saldanian orogeny (580–545 Ma). The syn- to late-tectonic S-type CG vary in composition from granodioritic to leucogranitic and contain biotite, cordierite and occasionally garnet. These granites host fine-grained granitic enclaves, metasedimentary xenoliths (predominantly amphibolite-facies) and rare metamafic xenoliths. The Sm–Nd and Rb–Sr geochemistry of the S-type granites indicates that all have a purely crustal origin. The narrow range of Nd-isotope compositions (eNd(550Ma) = 4.0 to 4.7) matches those of the Malmesbury Group and the metasedimentary xenoliths (eNd(550Ma) = 4.3 to 10.2; mostly 4.3 to 5.1) this suggests that the Malmesbury Group is the source of S-type CGs. The eNd values of the magmatic enclaves are typically very similar to those of the granites ( 4 to 5), although some with eNd as high as 2.3 at 550 Ma, possibly indicate a second source. Thermobarometry using the mineral assemblage (Cpx–Amp– Pl–Bt–Qtz) from a metamafic xenolith result in a peak P–T estimate of 10 ± 1 kb and 850 ± 50 °C. This is interpreted to reflect the metamorphic conditions in the magma source region. Similarly, the highest grade, but non-restitic, metasedimentary xenoliths (Grt–Bt–Pl–Qtz) result in P–T estimates of 750 °C and 7 kb, possibly representing conditions in the metamorphic terrain overlying the melting zone. Zoned garnet within the plutons varies in composition from Alm70Pyr25Grs2Sps3 in the interiors to rim overgrowths of Alm70Pyr10Grs2Sps18. Both differ from the Alm60Pyr15Grs15Sps10 garnet cores in the metasedimentary xenoliths. The two garnet generations in the granites are interpreted to record different stages of the P–T evolution of the magma. Modelling of the phase stabilities in these compositions suggests that the cores record pressures of 5–7 kb (at 750 °C), while the rims formed at 3–4 kb and a temperature close to the solidus (650 °C). Collectively, these results suggest that the S-type CG magmas resulted solely from biotite fluid-absent partial melting of tectonically thickened (P35 km) Malmesbury Group like metasediments along a convergent continental margin. doi:10.1016/j.gca.2006.06.1259
Quantifying the isotopic fractionation of lithium during clay formation at various temperatures N. VIGIER1, A. DECARREAU2, R. MILLOT3, J. CARIGNAN1, S. PETIT2, C. FRANCE-LANORD1 1
CRPG, Vandoeuvre-le`s-Nancy, cnrs-nancy.fr) 2 HYDRASA, Poitiers, France 3 BRGM, Orle´ans, France
France
(nvigier@crpg.
Recent studies have shown that the surface geochemical cycle of lithium isotopes is mainly controlled by isotopic fractionations associated with clay formation. Weathering of continental crust by superficial waters can produce residual phases in soils that are depleted in 7Li, and high d7Li river waters. Also, the neoformation of smectite in the ocean could explain the seawater Li isotopic signature (d7Li = 31.2&), significantly heavier than its sources (d7Li = 9& for hydrothermal flux and 23& for dissolved continental flux), considering a minimum isotope fractionation of 19& during Li uptake by clays. We have constrained the lithium isotopic fractionation linked to Li–Mg substitution during smectites (hectorites) crystallization. For this, we have experimentally synthesized hectorites at various temperatures (from 25 to 250 °C), using solutions highly enriched in lithium. All the exchangeable lithium has then been removed with NH4Cl before clay isotopic analyses. Li isotopic compositions (d7Li) for reference materials, the synthesized clays and corresponding solutions have been measured using an Elan 6000 ICP-MS. Some of them have also been measured using a Neptune MC-ICP-MS, for intercalibration purpose. Both techniques are consistent within analytical uncertainties (1& and 0.5&, respectively, at the 2r level). d7Li obtained for the reference materials are consistent with published values. The measured isotopic bias (D7Li) between hectorite and solution is significant, and, decreases with increasing temperature (from 1.6& at 250 °C to 11& at 25 °C). The solution matrix has little effect on the isotopic fractionation but on the relative amount of lithium incorporated into the clay structure. The theoretical isotopic fractionations, as a function of temperature, have been estimated and are consistent with the experimental values, except for the highest temperatures for which some simplification used in the calculation may not be valid. Both, experimental and theoretical results, suggest a maximum Li isotope fractionation of 13& at seafloor temperature, significantly lower than the values used for ocean budget. A low isotope fractionation would imply either a ratio of hydrothermal/continental inputs for lithium that is significantly lower than estimated, or another Li sink in the ocean. doi:10.1016/j.gca.2006.06.1258
A673
Origins of the S-type Cape Granites (South Africa) A. VILLAROS1, G. STEVENS1, I.S. BUICK2 1
Department of Geology, University of Stellenbosch, South Africa ([email protected]; [email protected]) 2 School of Geosciences, Monash University, Melbourne, Australia ([email protected]) The Pan-African Cape Granite (CG) Suite, South Africa, consists of S- (560–540 Ma), I- (540–515 Ma) and A-type (515– 510 Ma) plutons and extrusive rocks. They intruded the low-grade (greenschist-facies) Malmesbury Supergroup (750–610 Ma) during and after the Saldanian orogeny (580–545 Ma). The syn- to late-tectonic S-type CG vary in composition from granodioritic to leucogranitic and contain biotite, cordierite and occasionally garnet. These granites host fine-grained granitic enclaves, metasedimentary xenoliths (predominantly amphibolite-facies) and rare metamafic xenoliths. The Sm–Nd and Rb–Sr geochemistry of the S-type granites indicates that all have a purely crustal origin. The narrow range of Nd-isotope compositions (eNd(550Ma) = 4.0 to 4.7) matches those of the Malmesbury Group and the metasedimentary xenoliths (eNd(550Ma) = 4.3 to 10.2; mostly 4.3 to 5.1) this suggests that the Malmesbury Group is the source of S-type CGs. The eNd values of the magmatic enclaves are typically very similar to those of the granites ( 4 to 5), although some with eNd as high as 2.3 at 550 Ma, possibly indicate a second source. Thermobarometry using the mineral assemblage (Cpx–Amp– Pl–Bt–Qtz) from a metamafic xenolith result in a peak P–T estimate of 10 ± 1 kb and 850 ± 50 °C. This is interpreted to reflect the metamorphic conditions in the magma source region. Similarly, the highest grade, but non-restitic, metasedimentary xenoliths (Grt–Bt–Pl–Qtz) result in P–T estimates of 750 °C and 7 kb, possibly representing conditions in the metamorphic terrain overlying the melting zone. Zoned garnet within the plutons varies in composition from Alm70Pyr25Grs2Sps3 in the interiors to rim overgrowths of Alm70Pyr10Grs2Sps18. Both differ from the Alm60Pyr15Grs15Sps10 garnet cores in the metasedimentary xenoliths. The two garnet generations in the granites are interpreted to record different stages of the P–T evolution of the magma. Modelling of the phase stabilities in these compositions suggests that the cores record pressures of 5–7 kb (at 750 °C), while the rims formed at 3–4 kb and a temperature close to the solidus (650 °C). Collectively, these results suggest that the S-type CG magmas resulted solely from biotite fluid-absent partial melting of tectonically thickened (P35 km) Malmesbury Group like metasediments along a convergent continental margin. doi:10.1016/j.gca.2006.06.1259