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18O-tracing of the hydrothermal alteration of pyrochlore
Goldschmidt Conference Abstracts 2006
Nd isotope and trace element characteristics of the Mesoarchean (3075 Ma) Ivisaartoq greenstone belt, SW Greenland: Evidence for two distinct subarc mantle sources 1
2
A. POLAT , R. FREI , Y. DILEK
3
1
Department of Earth Sciences, University of Windsor, Canada ([email protected]) 2 Geological Institute, University of Copenhagen, Denmark ([email protected]) 3 Department of Geology, Miami University, USA (dileky@ muohio.edu) The Mesoarchean (3075 Ma) Ivisaartoq greenstone belt in SW, Greenland contains well-preserved primary magmatic features, including pillow lavas, cpx-bearing ultramafic cumulates, volcanic breccias, and immiscible liquid textures. In addition there are variably deformed actinolite schists and serpentinites in the belt. Field relationships and petrographic observations indicate that all volcanic and intrusive rock types in the Ivisaartoq belt are part of the same lithotectonic assemblage, sharing a common history deformation and metamorphism. Field, petrographic and geochemical data suggest that the Ivisaartoq rocks underwent at least two stages of post-magmatic calc–silicate metasomatic alteration between 3075 and 2961 Ma, resulting in mobilization of most major and trace elements. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents a relic of a dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and dunitic to wehrlitic sills, and a lower layer of leucogabbros and anorthosites. The Sm–Nd isotope systems were disturbed in strongly altered actinolite schists, likely during 2800–2900 Ma tectonothermal events, whereas they appear to have remained relatively undisturbed in the pillow lavas, cumulates, and gabbros. Gabbros and pillow lavas have similar initial Nd isotope compositions (eNd = 0.30–+3.04), consistent with a variably depleted, heterogeneous mantle source. Cumulates, on the other hand, were derived from a more depleted (eNd = +4.23–+4.97) mantle source. These isotopic characteristics indicate that two distinct Mesoarchean mantle sources existed beneath the Ivisaartoq belt (sensu lato). The lower eNd (<+3.0) values in gabbros and pillow basalts may indicate an Nd-enriched component in the source region. We suggest that the enriched component had been added to the mantle wedge and into the mantle source of gabbros and pillow lavas at variable proportions by recycling of older continental material (sediments), with super-chondritic Nd/Sm ratios. doi:10.1016/j.gca.2006.06.1460
A499
18
O-tracing of the hydrothermal alteration of pyrochlore
P. PO¨ML1, M. MENNEKEN1, T. STEPHAN2, D. NIEDERMEIER1, T. GEISLER1, A. PUTNIS1 1
Institut fu¨r Mineralogie, Westfa¨lische Wilhelms-Universita¨t, Mu¨nster, Germany ([email protected]) 2 Institut fu¨r Planetologie, Westfa¨lische Wilhelms-Universita¨t, Mu¨nster, Germany The disposition and long-term storage of high-level nuclear waste and ‘‘surplus’’ plutonium is a topic of particular concern for the modern society. Minerals of the pyrochlore group have been suggested as a host matrix for plutonium. In this study we compare the hydrothermal alteration of crystalline and metamict, i.e. self-irradiation-damaged pyrochlore, using 18O-enriched solutions. The 18O-tracing experiments were conducted on (1) a natural crystalline, Ta-based pyrochlore from Congo, (2) a natural heavily radiation-damaged Ti-based pyrochlore-(Y) from Norway, and (3) a synthetic crystalline pyrochlore, (Ca0.76Ce0.75Gd0.23 Hf0.21)Ti2O7. From each sample a polished cuboid with edge sizes around 3 mm was prepared for the experiments. The letters ‘‘WWU’’ were engraved into the surfaces of the Ti-based pyrochlore samples using an Excimer laser. Each cuboid was treated in a cold-sealed silver reactor in 1 ml of a 1 M HCl solution containing 43.5% 18O at 250 °C for 72 h (Tibased pyrochlores) or at 200 °C for 14 days (Ta-based pyrochlore). After the experiments the colours of the samples turned from brown or black to almost white. All three cuboids were either intact or parts of it (e.g., an edge) could be recovered. This observation indicates that the reaction is pseudomorphic. Backscattered electron imaging on cut sections revealed both altered and unaltered areas. Whereas the Ta-based pyrochlore was partly transformed into a defect-pyrochlore characterized by a deficit in the A-site cations (i.e., Ca and Na), the Ti-based pyrochlore samples were transformed into a mixture of rutile/anatase (TiO2). In the case of the metamict Ti-based pyrochlore aeschynite-(Y) was found at the interface between the rutile/anatase assemblage and the unreacted pyrochlore. Imaging the 18O distribution in the reacted material by time-of-flight mass spectrometry reveals in all cases a strong enrichment in 18O with a sharp gradient (on a micrometer scale). Our observations suggest that the alteration mechanism is a dissolution–reprecipitation mechanism that operates at a moving front rather than a diffusion-controlled solid-state process. We emphasize here that our results produced under relatively extreme conditions agree well with those derived from experiments conducted under moderate conditions rather expected in a nuclear repository (Zhang et al., 2004).
Reference Zhang, Z., Li, H., Vance, E.R., McLeod, T., Scales, N., 2004. Mat. Res. Soc. Symp. Proc. 824. doi:10.1016/j.gca.2006.06.1461
Nd isotope and trace element characteristics of the Mesoarchean (3075 Ma) Ivisaartoq greenstone belt, SW Greenland: Evidence for two distinct subarc mantle sources 1
2
A. POLAT , R. FREI , Y. DILEK
3
1
Department of Earth Sciences, University of Windsor, Canada ([email protected]) 2 Geological Institute, University of Copenhagen, Denmark ([email protected]) 3 Department of Geology, Miami University, USA (dileky@ muohio.edu) The Mesoarchean (3075 Ma) Ivisaartoq greenstone belt in SW, Greenland contains well-preserved primary magmatic features, including pillow lavas, cpx-bearing ultramafic cumulates, volcanic breccias, and immiscible liquid textures. In addition there are variably deformed actinolite schists and serpentinites in the belt. Field relationships and petrographic observations indicate that all volcanic and intrusive rock types in the Ivisaartoq belt are part of the same lithotectonic assemblage, sharing a common history deformation and metamorphism. Field, petrographic and geochemical data suggest that the Ivisaartoq rocks underwent at least two stages of post-magmatic calc–silicate metasomatic alteration between 3075 and 2961 Ma, resulting in mobilization of most major and trace elements. Given the geological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc ophiolites, we suggest that the Ivisaartoq greenstone belt represents a relic of a dismembered Mesoarchean supra-subduction zone oceanic crust. This crust might originally have been composed of an upper layer of pillow lavas, picritic flows, gabbroic to dioritic dykes and dunitic to wehrlitic sills, and a lower layer of leucogabbros and anorthosites. The Sm–Nd isotope systems were disturbed in strongly altered actinolite schists, likely during 2800–2900 Ma tectonothermal events, whereas they appear to have remained relatively undisturbed in the pillow lavas, cumulates, and gabbros. Gabbros and pillow lavas have similar initial Nd isotope compositions (eNd = 0.30–+3.04), consistent with a variably depleted, heterogeneous mantle source. Cumulates, on the other hand, were derived from a more depleted (eNd = +4.23–+4.97) mantle source. These isotopic characteristics indicate that two distinct Mesoarchean mantle sources existed beneath the Ivisaartoq belt (sensu lato). The lower eNd (<+3.0) values in gabbros and pillow basalts may indicate an Nd-enriched component in the source region. We suggest that the enriched component had been added to the mantle wedge and into the mantle source of gabbros and pillow lavas at variable proportions by recycling of older continental material (sediments), with super-chondritic Nd/Sm ratios. doi:10.1016/j.gca.2006.06.1460
A499
18
O-tracing of the hydrothermal alteration of pyrochlore
P. PO¨ML1, M. MENNEKEN1, T. STEPHAN2, D. NIEDERMEIER1, T. GEISLER1, A. PUTNIS1 1
Institut fu¨r Mineralogie, Westfa¨lische Wilhelms-Universita¨t, Mu¨nster, Germany ([email protected]) 2 Institut fu¨r Planetologie, Westfa¨lische Wilhelms-Universita¨t, Mu¨nster, Germany The disposition and long-term storage of high-level nuclear waste and ‘‘surplus’’ plutonium is a topic of particular concern for the modern society. Minerals of the pyrochlore group have been suggested as a host matrix for plutonium. In this study we compare the hydrothermal alteration of crystalline and metamict, i.e. self-irradiation-damaged pyrochlore, using 18O-enriched solutions. The 18O-tracing experiments were conducted on (1) a natural crystalline, Ta-based pyrochlore from Congo, (2) a natural heavily radiation-damaged Ti-based pyrochlore-(Y) from Norway, and (3) a synthetic crystalline pyrochlore, (Ca0.76Ce0.75Gd0.23 Hf0.21)Ti2O7. From each sample a polished cuboid with edge sizes around 3 mm was prepared for the experiments. The letters ‘‘WWU’’ were engraved into the surfaces of the Ti-based pyrochlore samples using an Excimer laser. Each cuboid was treated in a cold-sealed silver reactor in 1 ml of a 1 M HCl solution containing 43.5% 18O at 250 °C for 72 h (Tibased pyrochlores) or at 200 °C for 14 days (Ta-based pyrochlore). After the experiments the colours of the samples turned from brown or black to almost white. All three cuboids were either intact or parts of it (e.g., an edge) could be recovered. This observation indicates that the reaction is pseudomorphic. Backscattered electron imaging on cut sections revealed both altered and unaltered areas. Whereas the Ta-based pyrochlore was partly transformed into a defect-pyrochlore characterized by a deficit in the A-site cations (i.e., Ca and Na), the Ti-based pyrochlore samples were transformed into a mixture of rutile/anatase (TiO2). In the case of the metamict Ti-based pyrochlore aeschynite-(Y) was found at the interface between the rutile/anatase assemblage and the unreacted pyrochlore. Imaging the 18O distribution in the reacted material by time-of-flight mass spectrometry reveals in all cases a strong enrichment in 18O with a sharp gradient (on a micrometer scale). Our observations suggest that the alteration mechanism is a dissolution–reprecipitation mechanism that operates at a moving front rather than a diffusion-controlled solid-state process. We emphasize here that our results produced under relatively extreme conditions agree well with those derived from experiments conducted under moderate conditions rather expected in a nuclear repository (Zhang et al., 2004).
Reference Zhang, Z., Li, H., Vance, E.R., McLeod, T., Scales, N., 2004. Mat. Res. Soc. Symp. Proc. 824. doi:10.1016/j.gca.2006.06.1461