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listelement.badge.dso-typeЭлемент, EVIDENCE FOR MULTI-STAGE METASOMATISM OF CHLORITE-AMPHIBOLE PERIDOTITES (ULTEN ZONE, ITALY): CONSTRAINTS FROM TRACE ELEMENT COMPOSITIONS OF HYDROUS PHASES(2007) Marocchi M.; Morten L.; Hermann J.Peridotites from the Ulten Zone (Upper Austroalpine Domain, Central-Eastern Alps, Italy) derive from a mantle wedge environment and record a complex metamorphic history. This study focuses on amphibole–spinel peridotites and chlorite–tremolite peridotites. Chlorite is generally closely intergrown with Cr–spinel, indicating that it grew at the expense of former Al–spinel. No garnet relics or chlorite pseudomorphs after garnet have been found. Moreover, amphibole and chlorite trace element patterns display no fractionation in HREE, suggesting that these chlorite peridotites never equilibrated in the garnet stability field.On the basis of textures, bulk rock and mineral major and trace element compositions three stages of metasomatism are documented in the mantle rocks. We propose that the earliest stage represents melt impregnation at plagioclase peridotite conditions and is unrelated to subduction. The metasomatism leading to spinel– and chlorite–amphibole peridotites is related to the progressive influx of a fluid with crustal signature, derived from neighbouring subducted continental crust as the mantle wedge peridotites approach the slab. The observation that garnet peridotites and chlorite peridotites, which never equilibrated in the garnet stability field, are hosted within the same gneisses, suggests that slices of mantle wedge peridotite with different P-T trajectories can be sampled by subducted and exhumed crust.listelement.badge.dso-typeЭлемент, ORIGINS OF XENOLITHIC ECLOGITES AND PYROXENITES FROM THE CENTRAL SLAVE CRATON, CANADA(2007) Aulbach S.; Pearson N.J.; O'reilly S.Y.; Doyle B.J.Major- and trace-element and Sr^Nd^Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxe- nite xenoliths were used to establish their origins and evolution in the subcontinental lithospheric mantle beneath the central Slave Craton, Canada. The majority of eclogites can be assigned to three groups (high-Mg, high-Ca or low-Mg eclogites) that have distinct trace-element patterns. Although post-formation metasomatism involving high field strength element (HFSE) and light rare earth element (LREE) addition has partially obscured the primary compositional features of the high-Mg and high-Ca eclo- gites, trace-element features, such as unfractionated middle REE (MREE) to heavy REE (HREE) patterns suggestive of garnet-free residues and low Zr/Sm consistent with plagioclase accu- mulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low P REE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagi- oclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyrox- ene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher P REE and absence of Eu anomalies. In con- trast, the subchondritic Zr/Hf and positive slope in the HREE of the low-Mg eclogites are similar to Archaean second-stage melts and point to a previously depleted source for their precursors. Low ratios of fluid-mobile to less fluid-mobile elements and of LREE to HREE are consistent with dehydration and partial melt loss for some eclogites. The trace-element characteristics of the different eclogite types translate into lower eNd for high-Mg eclogites than for low-Mg eclogites.listelement.badge.dso-typeЭлемент, INTERMEDIATE ALKALI - ALUMINO-SILICATE AQUEOUS AOLUTIONS RELEASED BY DEEPLY SUBDUCTED CONTINENTAL CRUST: FLUID EVOLUTION IN UHP OH-RICH TOPAZ - KYANITE QUARTZITES FROM DONGHAI (SULU, CHINA)(2007) Frezzotti M.L.; Ferrando S.; Compagnoni R.; Dallai L.Minerals, fluid inclusions and stable isotopes have been studied in ultrahigh-pressure (UHP) OH-rich topaz–kyanite quartzites from Hushan (west of Dongai), in southern Sulu (China). The quartzites underwent a metamorphic evolution characterized by a peak stage (3·5 GPa and 730–820°C) with the anhydrous assemblage coesite + kyanite I, followed by an early near-isothermal decompression stage (2·9 GPa and 705–780°C) with growth of kyanite II, muscovite, and OH-rich topaz, and by decompression-cooling stages, represented by paragonite (1·9 GPa and 700–780°C) and pyrophyllite (0·3 GPa and 400°C) on kyanite (I and II) and OH-rich topaz, respectively. These rocks may exhibit unusually low δ18O and δD values acquired before undergoing UHP metamorphism. Five distinct fluid generations are recognized. Type I: concentrated peak solutions rich in Si, Al, and alkalis, present within multiphase inclusions in kyanite I. Type II: CaCl 2 -rich brines present during the growth of early retrograde OH-rich topaz. Type III, IV, and V: late aqueous fluids of variable salinity, and rare CO 2 present during amphibolite- and late greenschist-facies conditions. A number of conclusions may be drawn from these relationships that have an effect on fluid evolution in deeply subducted continental rocks. (1) At a pressure of about 3·5 GPa alkali–alumino-silicate aqueous solutions, with compositions intermediate between H 2 O fluid and melt (H 2 O > 25 and ≤ 50 wt %) evolved from quartzites, probably generated by dehydration reactions. (2) During early decompression stages, at the transition from UHP to high-pressure (2·9 GPa) conditions, brines of external origin with higher water contents (82 wt % H 2 O) initiated the growth of OH-rich topaz and muscovite. (3) The subsequent decompression, at P <2 GPa, was defined by a limited circulation of NaCl aqueous fluids, and CO 2 infiltration. Overall, fluid inclusions and stable isotopes highlight a metamorphic fluid–rock interaction characterized by internally derived intermediate aqueous solutions at UHP, followed by infiltration of Cl-rich brines with higher water activities.listelement.badge.dso-typeЭлемент, GEOCHEMISTRY OF ECLOGITES FROM THE DABIE-SULU TERRANE, EASTERN CHINA: NEW INSIGHTS INTO PROTOLITHS AND TRACE ELEMENT BEHAVIOUR DURING UHP METAMORPHISM(2007) Tang H.F.; Liu C.Q.; Nakai S.; Orihashi Y.The major and trace element compositions of nine eclogites from the Dabie–Sulu ultrahigh pressure (UHP) metamorphic terrane in eastern China were determined for both whole rock and the main constituent minerals, garnet and clinopyroxene. The results indicate that the eclogite protoliths originated from a basaltic magma, which formed in a continental setting as shown by isotopic and immobile element data. Based on the garnet REE characteristics, the eclogites can be roughly divided into two groups. Group 1 has LREE enrichment with no Eu anomaly for whole rock, and smooth LREE depletion but HREE enrichment pattern for garnet, whereas group 2 shows a depletion of LREE with a pronounced positive Eu anomaly and flat HREE pattern for both whole rock and garnet. From these features, we suggest that the protoliths for group 2 are Fe–Ti–gabbros with relatively high cumulus plagioclase and Fe–Ti oxide, whereas the group 1 eclogites are probably from basalts. Therefore, the unusual garnet REE pattern observed in group 2 can be considered as an important signature for identifying gabbro protoliths for eclogites. The identification of gabbro protoliths from the eclogites in the Dabie–Sulu terrane provides evidence for Neoproterozoic rift magmatism in the northern margin of the Yangtze craton. During ultrahigh pressure metamorphism in the Dabie–Sulu terrane, LILEs (including Ba, Rb, Th, U, K) had high mobility, but REEs and HFSEs were immobile, and trace element distribution equilibrium was approached between garnet and clinopyroxene. An estimate of mass balance indicates that garnet and clinopyroxene host the majority of HREEs and Y, and clinopyroxene is a significant host for Sr, but minor and accessory minerals predominantly account for LREEs, Th, U, and Zr.listelement.badge.dso-typeЭлемент, RECHARGE/SEEPAGE FROM AN ARRAY OF RECTANGULAR CHANNELS(2007) Choudhary M.; Chahar B.R.An exact analytical solution for the quantity of Recharge/Seepage from an array of rectangular channels underlain by a drainage layer and with pressure at a shallow/large depth has been obtained using an inverse hodograph and Schwarz–Christoffel transformation. The symmetry about the vertical axis has been utilized in obtaining the solution for half of the seepage domain only. The solution also includes relations for variation in seepage velocity along the channel perimeter and a set of parametric equations for the location of phreatic line. From this generalized case, particular solutions have also been deduced for an array of rectangular channels without pressure, without drainage layer or a single rectangular channel with drainage layer at finite/infinite depth. The solution is useful in quantifying artificial recharge and/or seepage loss from an array of rectangular channels.