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Metasomatism in intraplate and suprasubduction lithospheric mantle
Goldschmidt Conference Abstract 2006
Metasomatism in intraplate and suprasubduction lithospheric mantle M. COLTORTI, C. BONADIMAN Department of Earth Sciences, University of Ferrara, 44100 Ferrara, Italy ([email protected]) A comparison between petrographic and geochemical features of minerals and glasses from mantle xenoliths hosted in intraplate alkaline and orogenic calc-alkaline s.l. basalts are presented. Clinopyroxene and amphibole are the main repositories for incompatible trace elements in the mantle, thus providing invaluable clues on the nature of the metasomatizing agents affecting the lithospheric mantle. Their use is however limited by their presence and by their crystallographic constrains. In this respect, glasses produced by reactions with metasomatic fluids represent a very useful tool, but they are less commonly found, especially in suprasubduction environments. The nature of the metasomatizing agents in intraplate settings are by far much better understood than their orogenic counterpart. Several diagrams using minerals and glasses were put forward aiming at defining the nature of the metasomatizing agents, i.e. Na-alkaline silicate, K-alkaline silicate or carbonatite in intraplate settings. Fewer data are however available on the geochemical features of the metasomatic phases in suprasubduction settings, and, by consequence, on the nature of the metasomatizing agents involved. This can vary from volatile-bearing fluids to pure melts, which, in its turn, may involve oceanic crust plus variable amount of sediments, resulting in an extremely complex range of fluid compositions, percolating upward and reacting with the overlying mantle wedge. Few cases where a ‘‘clear’’ subduction signature can be recognized in cpx and amph are presented and their geochemical characteristics compared with those from intraplate metasomatism. In these cases intraplate metasomatism is overprinted on the previous subduction enrichment. This fact bear some analogies with the magmatic events which can be observed on the surface, where intraplate alkaline magmatism follows, in a time span of about 10–15 Ma, the calc-alkaline s.l. magmatism. Amphibole represents the best tool for investigating the different metasomatic styles and may also be used for recording the transition between the two different events. The role that accessory phase, such as rutile, is playing during melting or dehydratation of the slab and the recycling of this phase into the mantle as possible Nb- and Ti-rich reservoirs are addressed. If this is the case the physical and chemical relationships between subducted slab and intraplate magma generations can be also tentatively put forward. doi:10.1016/j.gca.2006.06.131
A109
High resolution trace metal analysis of benthic foraminifera reveal nutrient excursions in Antarctic Intermediate Water M.R. COOK1, H. ELDERFIELD1, R. ZAHN2, K. PAHNKE3 1
Department of Earth Sciences, University of Cambridge, Cambridge, UK ([email protected]) 2 Institucio´ Catalana de Recerca i Estudis Avanc¸ats, ICREA i Universitat Auto`noma de Barcelona, Spain (rainer.zahn@ uab.es) 3 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA ([email protected]) The use of d13C within foraminifera as a nutrient proxy is complicated by air-sea exchange processes that cause isotopic deviations due to equilibrium fractionation between the ocean and atmosphere. These effects can be resolved by the utilisation of proxies such as cadmium, which co-varies with phosphate in the modern ocean. We present a high resolution record of trace metal variability from core MD97-2120, located on Chatham Rise, near New Zealand at a depth of 1210 m. MD97-2120 is located close to the core layer of Antarctic Intermediate Water (AAIW) and is thus uniquely placed to study changes in AAIW and the redistribution of heat and freshwater within the upper ocean. A previous study of d13C and d18O in benthic foraminifera from MD97-2120 demonstrated increased production of AAIW and Southern hemisphere warming coincident with cooling and decreased deepwater convection in the North Atlantic (Pahnke and Zahn, 2005). The interdependence of water mass conversion has implications for the forcing of climatic instability and thermohaline circulation. In this study we present Cd/Ca and Mg/Ca analysis for the Benthic foraminifera Hoeglundina elegans, Trifarina angulosa, and Uvigerina sp. from MD97-2120 for the past 150 ka, at a mean temporal resolution of 161 years. Cd/Ca and Mg/Ca may be used for the determination of intermediate water nutrient content and temperature, respectively. This allows separation of air-sea exchange and nutrient signals within the d13C record, which has significant implications for the movement of the SubAntarctic Front and the rate of formation of AAIW in the South Pacific.
Reference Pahnke, K., Zahn, R., 2005. Science 307, 1741–1746. doi:10.1016/j.gca.2006.06.132
Metasomatism in intraplate and suprasubduction lithospheric mantle M. COLTORTI, C. BONADIMAN Department of Earth Sciences, University of Ferrara, 44100 Ferrara, Italy ([email protected]) A comparison between petrographic and geochemical features of minerals and glasses from mantle xenoliths hosted in intraplate alkaline and orogenic calc-alkaline s.l. basalts are presented. Clinopyroxene and amphibole are the main repositories for incompatible trace elements in the mantle, thus providing invaluable clues on the nature of the metasomatizing agents affecting the lithospheric mantle. Their use is however limited by their presence and by their crystallographic constrains. In this respect, glasses produced by reactions with metasomatic fluids represent a very useful tool, but they are less commonly found, especially in suprasubduction environments. The nature of the metasomatizing agents in intraplate settings are by far much better understood than their orogenic counterpart. Several diagrams using minerals and glasses were put forward aiming at defining the nature of the metasomatizing agents, i.e. Na-alkaline silicate, K-alkaline silicate or carbonatite in intraplate settings. Fewer data are however available on the geochemical features of the metasomatic phases in suprasubduction settings, and, by consequence, on the nature of the metasomatizing agents involved. This can vary from volatile-bearing fluids to pure melts, which, in its turn, may involve oceanic crust plus variable amount of sediments, resulting in an extremely complex range of fluid compositions, percolating upward and reacting with the overlying mantle wedge. Few cases where a ‘‘clear’’ subduction signature can be recognized in cpx and amph are presented and their geochemical characteristics compared with those from intraplate metasomatism. In these cases intraplate metasomatism is overprinted on the previous subduction enrichment. This fact bear some analogies with the magmatic events which can be observed on the surface, where intraplate alkaline magmatism follows, in a time span of about 10–15 Ma, the calc-alkaline s.l. magmatism. Amphibole represents the best tool for investigating the different metasomatic styles and may also be used for recording the transition between the two different events. The role that accessory phase, such as rutile, is playing during melting or dehydratation of the slab and the recycling of this phase into the mantle as possible Nb- and Ti-rich reservoirs are addressed. If this is the case the physical and chemical relationships between subducted slab and intraplate magma generations can be also tentatively put forward. doi:10.1016/j.gca.2006.06.131
A109
High resolution trace metal analysis of benthic foraminifera reveal nutrient excursions in Antarctic Intermediate Water M.R. COOK1, H. ELDERFIELD1, R. ZAHN2, K. PAHNKE3 1
Department of Earth Sciences, University of Cambridge, Cambridge, UK ([email protected]) 2 Institucio´ Catalana de Recerca i Estudis Avanc¸ats, ICREA i Universitat Auto`noma de Barcelona, Spain (rainer.zahn@ uab.es) 3 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA ([email protected]) The use of d13C within foraminifera as a nutrient proxy is complicated by air-sea exchange processes that cause isotopic deviations due to equilibrium fractionation between the ocean and atmosphere. These effects can be resolved by the utilisation of proxies such as cadmium, which co-varies with phosphate in the modern ocean. We present a high resolution record of trace metal variability from core MD97-2120, located on Chatham Rise, near New Zealand at a depth of 1210 m. MD97-2120 is located close to the core layer of Antarctic Intermediate Water (AAIW) and is thus uniquely placed to study changes in AAIW and the redistribution of heat and freshwater within the upper ocean. A previous study of d13C and d18O in benthic foraminifera from MD97-2120 demonstrated increased production of AAIW and Southern hemisphere warming coincident with cooling and decreased deepwater convection in the North Atlantic (Pahnke and Zahn, 2005). The interdependence of water mass conversion has implications for the forcing of climatic instability and thermohaline circulation. In this study we present Cd/Ca and Mg/Ca analysis for the Benthic foraminifera Hoeglundina elegans, Trifarina angulosa, and Uvigerina sp. from MD97-2120 for the past 150 ka, at a mean temporal resolution of 161 years. Cd/Ca and Mg/Ca may be used for the determination of intermediate water nutrient content and temperature, respectively. This allows separation of air-sea exchange and nutrient signals within the d13C record, which has significant implications for the movement of the SubAntarctic Front and the rate of formation of AAIW in the South Pacific.
Reference Pahnke, K., Zahn, R., 2005. Science 307, 1741–1746. doi:10.1016/j.gca.2006.06.132