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Hf isotopes and zircon recrystallization: A case study
Goldschmidt Conference Abstracts 2006
Metamorphic evolution along the slab/ mantle interface within subduction zones ROBERT L. KING, GRAY E. BEBOUT Earth and Enviromental Sciences, Lehigh University, Bethlehem, PA 18015, USA ([email protected]) One view of the metamorphic subduction system supposes that coupled deformation and prograde/metasomatic reaction between the chemically enriched slab and the depleted mantle wedge promotes the formation of complexly hybridized lithologies along the slab/mantle interface. This type of model is based upon direct observations of the structure and petrology of high-P/T metamorphic complexes (e.g., Bebout and Barton, 2002; King et al., 2006) and detailed thermo-mechanical numerical models that also account for chemical mass transfer (e.g., Gerya and Yuen, 2003; Gorczyk et al., 2006). Direct analyses of metamorphic me´lange zones indicates hybridization via mechanical mixing dominates petrochemical evolution, while some elemental or isotopic systems are fractionated by metamorphic reactions or metasomatism. In such me´lange zones, geochemical signatures of particular input reservoirs are heterogeneously recorded by metamorphic products whose phase equilibria are poorly known, but will be clearly distinct compared to the record of the oceanic lithosphere. Furthermore, the enhanced permeability structure of these me´lange zones indicates they are the most ideal site for fluid flow (Ague, 2006), suggesting that progressive reaction between mobile fluids not in equilibrium with ambient mineralogy may control fluid chemistry (Zack and John, 2006; King et al., 2006). Integrating results from this body of research results in only one important conclusion: it appears unlikely that inputs to subduction zones are representative of the bulk petrology or geochemistry of the metamorphic system that actively promotes arc magmatism. While these results should be scrutinized due to the paucity of the metamorphic record as compared to the voluminous data for arcs and subduction inputs, this body of recent data suggests traditional models for magmatism and recycling within convergent margins may dangerously over-simplify the system. The metamorphic evolution of subducted material along the slab/mantle interface is largely unconstrainted, and this represents the most significant dilemma facing subduction-zone research.
References Ague, J.J., 2006. Chem. Geol. (submitted). Bebout, G., Barton, M., 2002. Chem. Geol. 187, 79–106. Gerya, T.V., Yuen, D.A., 2003. PEPI 140, 293–318. Gorczyk, W. et al., 2006. G-cubed (in press). King, R.L. et al., 2006. Earth Planet. Sci. Lett., (in press). Zack, T., John, T., 2006. Chem. Geol. (submitted). doi:10.1016/j.gca.2006.06.644
A319
Hf isotopes and zircon recrystallization: A case study P.D. KINNY1, G.J. LOVE1, N.J. PEARSON2 1
Applied Geology, Curtin University, Perth, Australia (P.Kinny @curtin.edu.au) 2 GEMOC, Macquarie University, Sydney, Australia (norm. [email protected]) ‘Metamorphic zircon’ is a broad term that encompasses precipitates from partial melts and fluids, solid-state diffusive growth from the breakdown of Zr- and Si-bearing phases, and in situ recrystallisation products of pre-existing zircon. Each of the above may result in complexly structured zircon grains in which metamorphically grown or modified layers partly or completely surround remnants of older grains. Features such as transgressive core–rim boundaries and poorly zoned rims with distinctive Th/U ratio are common but non-unique indicators of metamorphic zircon. In order to correctly interpret the equally complex U–Pb age data obtained from such zircon grains in terms of geological events and processes, it is useful to be able to distinguish between the different types of metamorphic zircon, whether representing newly grown or recrystallized older material. We have made in situ hafnium isotope determinations by LA-MC-ICPMS coupled with U–Pb age determinations by SHRIMP on zircon samples from high-grade metamorphic rocks of the Lewisian Gneiss Complex and Moine Supergroup of northern Scotland. In samples of granulite-facies tonalitic and trondhjemitic gneisses of the LGC, no detectable difference in 176 Hf/177Hf was found between zoned zircon cores and poorly zoned to unzoned metamorphic rims, despite variable resetting of the U–Pb systems. In contrast, samples from migmatitic rocks showed distinctly different 176Hf/177Hf ratios between cores and rims. The results of our study support the prediction that where there is a sufficient time difference between zircon formation and subsequent metamorphic modification, Hf isotopic composition can discriminate between recrystallization and new growth because newly added zircon would be expected to have a more radiogenic Hf composition with respect to original zircon, whereas recrystallized zircon should essentially retain its original Hf composition. doi:10.1016/j.gca.2006.06.645
Metamorphic evolution along the slab/ mantle interface within subduction zones ROBERT L. KING, GRAY E. BEBOUT Earth and Enviromental Sciences, Lehigh University, Bethlehem, PA 18015, USA ([email protected]) One view of the metamorphic subduction system supposes that coupled deformation and prograde/metasomatic reaction between the chemically enriched slab and the depleted mantle wedge promotes the formation of complexly hybridized lithologies along the slab/mantle interface. This type of model is based upon direct observations of the structure and petrology of high-P/T metamorphic complexes (e.g., Bebout and Barton, 2002; King et al., 2006) and detailed thermo-mechanical numerical models that also account for chemical mass transfer (e.g., Gerya and Yuen, 2003; Gorczyk et al., 2006). Direct analyses of metamorphic me´lange zones indicates hybridization via mechanical mixing dominates petrochemical evolution, while some elemental or isotopic systems are fractionated by metamorphic reactions or metasomatism. In such me´lange zones, geochemical signatures of particular input reservoirs are heterogeneously recorded by metamorphic products whose phase equilibria are poorly known, but will be clearly distinct compared to the record of the oceanic lithosphere. Furthermore, the enhanced permeability structure of these me´lange zones indicates they are the most ideal site for fluid flow (Ague, 2006), suggesting that progressive reaction between mobile fluids not in equilibrium with ambient mineralogy may control fluid chemistry (Zack and John, 2006; King et al., 2006). Integrating results from this body of research results in only one important conclusion: it appears unlikely that inputs to subduction zones are representative of the bulk petrology or geochemistry of the metamorphic system that actively promotes arc magmatism. While these results should be scrutinized due to the paucity of the metamorphic record as compared to the voluminous data for arcs and subduction inputs, this body of recent data suggests traditional models for magmatism and recycling within convergent margins may dangerously over-simplify the system. The metamorphic evolution of subducted material along the slab/mantle interface is largely unconstrainted, and this represents the most significant dilemma facing subduction-zone research.
References Ague, J.J., 2006. Chem. Geol. (submitted). Bebout, G., Barton, M., 2002. Chem. Geol. 187, 79–106. Gerya, T.V., Yuen, D.A., 2003. PEPI 140, 293–318. Gorczyk, W. et al., 2006. G-cubed (in press). King, R.L. et al., 2006. Earth Planet. Sci. Lett., (in press). Zack, T., John, T., 2006. Chem. Geol. (submitted). doi:10.1016/j.gca.2006.06.644
A319
Hf isotopes and zircon recrystallization: A case study P.D. KINNY1, G.J. LOVE1, N.J. PEARSON2 1
Applied Geology, Curtin University, Perth, Australia (P.Kinny @curtin.edu.au) 2 GEMOC, Macquarie University, Sydney, Australia (norm. [email protected]) ‘Metamorphic zircon’ is a broad term that encompasses precipitates from partial melts and fluids, solid-state diffusive growth from the breakdown of Zr- and Si-bearing phases, and in situ recrystallisation products of pre-existing zircon. Each of the above may result in complexly structured zircon grains in which metamorphically grown or modified layers partly or completely surround remnants of older grains. Features such as transgressive core–rim boundaries and poorly zoned rims with distinctive Th/U ratio are common but non-unique indicators of metamorphic zircon. In order to correctly interpret the equally complex U–Pb age data obtained from such zircon grains in terms of geological events and processes, it is useful to be able to distinguish between the different types of metamorphic zircon, whether representing newly grown or recrystallized older material. We have made in situ hafnium isotope determinations by LA-MC-ICPMS coupled with U–Pb age determinations by SHRIMP on zircon samples from high-grade metamorphic rocks of the Lewisian Gneiss Complex and Moine Supergroup of northern Scotland. In samples of granulite-facies tonalitic and trondhjemitic gneisses of the LGC, no detectable difference in 176 Hf/177Hf was found between zoned zircon cores and poorly zoned to unzoned metamorphic rims, despite variable resetting of the U–Pb systems. In contrast, samples from migmatitic rocks showed distinctly different 176Hf/177Hf ratios between cores and rims. The results of our study support the prediction that where there is a sufficient time difference between zircon formation and subsequent metamorphic modification, Hf isotopic composition can discriminate between recrystallization and new growth because newly added zircon would be expected to have a more radiogenic Hf composition with respect to original zircon, whereas recrystallized zircon should essentially retain its original Hf composition. doi:10.1016/j.gca.2006.06.645