U-Pb dating of hydrothermal zircon: Fracturing and fluid flow in mantle peridotite at the MAR
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Research Article| April 01, 2006 Dating the mantle roots of young continental crust Nadine Wittig; Nadine Wittig 1Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen, Denmark, and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Search for other works by this author on: GSW Google Scholar Joel A. Baker; Joel A. Baker 1Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen, Denmark, and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Search for other works by this author on: GSW Google Scholar Hilary Downes Hilary Downes 2School of Earth Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK Search for other works by this author on: GSW Google Scholar Author and Article Information Nadine Wittig 1Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen, Denmark, and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Joel A. Baker 1Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen, Denmark, and School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand Hilary Downes 2School of Earth Sciences, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK Publisher: Geological Society of America Received: 14 Aug 2005 Revision Received: 14 Nov 2005 Accepted: 17 Nov 2005 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2006) 34 (4): 237–240. https://doi.org/10.1130/G22135.1 Article history Received: 14 Aug 2005 Revision Received: 14 Nov 2005 Accepted: 17 Nov 2005 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Nadine Wittig, Joel A. Baker, Hilary Downes; Dating the mantle roots of young continental crust. Geology 2006;; 34 (4): 237–240. doi: https://doi.org/10.1130/G22135.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Subcontinental lithospheric mantle xenoliths from beneath the French Massif Central contain clinopyroxene with high Lu/Hf and Hf isotope ratios (176Lu/177Hf = 0.1–10.8; εHf = +40 to +2600). These Lu-Hf isotope systematics yield model ages of 313–377 Ma and apparently date melt extraction in a mantle wedge during Variscan subduction, with the residual mantle subsequently forming the stabilizing mantle keel of young European continental crust. The extremely depleted Lu-Hf mantle systematics contrast with other isotopic signatures (Sr, Pb, Nd) that have been overprinted by mantle metasomatism. The high Lu/Hf values developed in mantle clinopyroxene by melt extraction, coupled with the robustness of this system to some types of metasomatism, provide a new chronometer for directly dating crust-mantle differentiation in both young and ancient continental regions. The process of metasomatic decoupling of Nd and Hf isotopes, which has been previously recognized in Hawaiian oceanic mantle and in French Massif Central subcontinental mantle, may be responsible for the prominent displacement of terrestrial rocks to lower εNd at a given εHf as compared to Bulk Earth reference values obtained from analysis of chondritic meteorites. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Abstract A combined secondary ion mass spectrometer and laser ablation‐(multicollector)‐inductively coupled plasma mass spectrometer study of zircon U‐Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element‐HREE patterns, negative Eu anomalies, and negative to low δ 18 O values of −11.3 to 0.9‰ and U‐Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U‐Pb ages and trace element and Hf‐O isotope compositions are similar to those for protolith zircons from ultrahigh‐pressure metamorphic rocks in the Dabie‐Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab‐mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust‐mantle interaction at the deep continental subduction zone.
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Before discussing the formation of the major geological units of the solid Earth, we should review the internal structure of our planet as described by seismic wave studies (Fig. 8.1). The most important discontinuities observed by seismologists down to the base of the mantle are:
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The Pb isotope compositions of amphiboles and clinopyroxenes in spinel peridotite and pyroxenite mantle xenoliths from the intra-plate Quaternary volcanic fields of the Eifel province (Germany) are strongly correlated with their Sr–Nd isotope and trace element compositions. High-temperature anhydrous xenoliths from a depth of around 60 km have trace element and Sr–Nd–Pb isotope compositions similar to the depleted source of mid-ocean ridge basalts (Depleted MORB Mantle, DMM). Amphibole-bearing xenoliths from shallower depths (<45 km) provide evidence for three temporally distinct episodes of mantle metasomatism in the subcontinental lithosphere: (1) aqueous fluids from an isotopically enriched (EM-like) mantle reservoir caused amphibole formation during deformation in the shallow continental lithospheric mantle and may be subduction related, probably associated with the last major tectonic event that influenced the area (Hercynian orogeny). (2) During a second phase of mantle metasomatism the EM-like lithospheric mantle was affected by melts from an ancient, HIMU-like (high time-integrated μ = 238U/204Pb) mantle source. The HIMU-like component introduced by these fluids had a much more radiogenic Pb isotope composition than the asthenospheric source of the widespread Cenozoic magmatism in Europe and may be linked to reactivation of ancient subducted crustal domains during the Hercynian orogeny or to early Cretaceous deep-sourced mantle plumes. (3) During a brief final stage the heterogeneously enriched EM–HIMU subcontinental lithosphere was locally modified by basaltic melts migrating along fractures and veins through the upper mantle as a consequence of the Cenozoic Eifel volcanism. Although a DMM component is completely lacking in the metasomatic fluids of the metasomatic episodes 1 and 2, the vein melts of episode 3 and the Cenozoic Eifel lavas require mantle sources containing three end-member components (DMM–HIMU–EM). Thus, mobilization of the more depleted mantle material occurred at the earliest in the Tertiary, contemporaneously with the development of the extensive rift system and main melt generation in Europe. Alternatively, the variety of Sr–Nd–Pb isotope signatures of the metasomatic agents may have been produced by melting of isotopically distinct mantle domains in a heterogeneous uprising mantle plume.
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Abstract Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.
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