V43E-01 INVITED
Ultrahigh Pressure Metamorphism (UHPM): Quo Vadimus?
UHPM is the petrologic record (min. P-T equivalent to Coe stability) of transport of continental lithosphere to asthenospheric depth, and return to crustal depth and incorporation into continents. The record of UHPM is scale- independent, but the issue of unit size (boudin/slice/terrane) and whether UHPM is recorded are important. Although the requirement for external hydration may limit equilibration in protoliths, cofacial compatible, indicative and diagnostic assemblages and lack of confidence in thermobarometry obfuscate the record of UHPM. Polymetamorphism and/or overprinting by multiple UHPM events may be possible through successive Wilson cycles and may be cryptic to add complexity. Our ability to image and study inclusion assemblages in zircons has been a major breakthrough, but are diamonds anybody's best friend? Is the max. depth from which continental crust may be retrieved equivalent to P = 10GPa or >10GPa? Will evidence remain to infer such depth? Commonly we tie dates to P-T, which yields rates that constrain mechanisms; advances in numerical modeling will enable progress in testing models of exhumation. UHPM is registered in the rock record since the Cryogenian-Ediacaran; it is inferred to record deep subduction. UHP rocks decorate sutures in Phanerozoic subduction-to-collision orogens. During the Mesoarchean-Tonian, subduction-to-collision orogenesis was marked by eclogite - high-pressure granulite metamorphism (E-HPGM). The implied change in geodynamics was a legacy of Rodinia, and related to the birth of the Pacific during the formation of Gondwana. Hoffman-type supercontinent breakup (turns inside out by subduction of complementary superocean) was the process by which segments of Rodinia were reassembled to form Gondwana by suturing of Braziliano - Pan-African belts, leaving orphaned Laurasian segments to combine with each other and Gondwana to form Pangea. In contrast, Wilson cycles (continental lithosphere rearranged by formation- destruction of internal oceans) dominated the Phanerozoic, when the continental lithosphere was restricted to one hemisphere. The formation of Laurasia, its amalgamation with Gondwana to form Pangea, and the fragmentation of Pangea involved successive major orogenic systems--the Appalachian/Caledonian-Variscide- Altaid and the Cimmerian-Himalayan-Alpine. The Iapetus, Rhenohercynian, Rheic, Paleo-Tethys and Neo-Tethys Oceans were closed by consumption of internally-generated ocean lithosphere by ribbon-continent terrane export from the Gondwanan side, and, for the Variscides, continent collision. Limited subduction of short-lived (warm) lithosphere before choking subduction by arc or ribbon-continent terrane collision may have limited transport of water to the mantle wedge, and suppressed development of small-scale convection, arc magmatism and backarc formation. In the complementary ocean hemisphere, circumferential subduction created accretionary orogens, commonly with paired metamorphic belts; whether UHPM is common in these orogens remains important to resolve. Why UHPM generally is associated with subduction of short-lived lithosphere is unclear? Large-scale mantle tomography for S-wave velocity structure of lower mantle supports a simple pattern of convection for modern Earth. High-velocity anomalies (colder) match Phanerozoic subduction zones and are interpreted as slabs. Low-velocity structures (LVS) display large positive geoid anomalies, shallow seafloor and elevated topography, and extensive hot spot volcanism, features indicating warmer mantle consistent with high- bulk-modulus domes composed of pyroxenite, linking them to subducted MORB. The breakup of Pangea was a legacy of Gondwana amalgamation (African LVS; a LVS is likely to develop under Asia in the future as a legacy of the formation of Asia.
V43E-02
Cathodoluminescence (CL) Petrography of UHP-metamorphic Rocks: New Avenues for Petrological Applications
Since the first application of CL microscopy on rock samples (Sippel, 1965; Marfunin, 1979), this technique has become a standard tool in mineralogy. First CL-studies, however, have mainly focused on sedimentary rocks and on zircon-zonations connected with ion probe dating. Here, we would like demonstrate the great advantages of a hot cathode CL microscope applied to UHP metamorphic rocks, in particular in conjunction with electronmicroprobe (EMP)-studies. The compilation of data presented include samples from Dora Maira/Italy, Kokchetav/Kazakhstan, Sulu/China, Western Gneiss region/Norway and Pohorje/Slovenia. Colored images of important but otherwise invisible growth features and internal structures of minerals, like small-scale growth zoning, inhomogeneities, exsolution lamellae, dissolution and deformational effects etc. can be easily achieved within seconds. Examples shown are oscillatory zoning patterns of garnet and kyanite, change of morphology during growth, regular and irregular zoning patterns of garnet, jadeite, omphacite, carbonates, clinopyroxene (partly with cracking/annealing structures), exsolution textures (dolomite/Mg-calcite; K-feldspar/pyroxene), different CL-colours of SiO2-phases (coesite: bluish- green, quartz: dark red to violet, chalcedony: yellow), and special features of accessory minerals like zircon, diamond, apatite, and bearthite. Although being mainly descriptive, the conclusions drawn open new avenues for petrological applications. The knowledge of detailed structural characteristics of rock-forming minerals has an invaluable impact on the more precise and detailed derivation of PT-paths and thus on the evolution of geodynamic processes with any metamorphic rocks involved. Thus we propose the CL-technique to be used more intensively for routine investigations of thin sections as a pathfinder prior to chemical characterization by EMP.
V43E-03 INVITED
Ultra-High Pressure Metamorphism: Where do we go From Here?
Ultra-High Pressure Metamorphism (UHPM) has its roots in the early 1960s discovery that glaucophane is stable only at high pressure and low temperature – implying that blueschists were carried to 10s of km rapidly and returned to the surface equally rapidly. Controversy erupted. Only with the advent of plate tectonics was the conceptual problem overcome and blueschists recognized as identifying subduction zones. Similar controversies arose upon discovery of coesite (1984) and diamond in metamorphosed crustal lithologies, and were repeated when microstructural indicators identified exhumation from greater depths (1996). Such findings are now commonplace from collision terranes and depths of subduction have been pushed to >350 km and exhumation from ~400 km. In the debates about the reliability of each discovery, the potential information content of UHPM rocks has often been lost in arguments about depth. It is time for us to develop ways to tease out of these rocks information about the exhumation trip up the subduction zone and, perhaps, also the voyage downwards. Solid inclusions in zircon have been used somewhat for this purpose but there must be ways to use isotopes and fluid inclusions to detect fluid flow through these rocks that must have occurred, probably during both downward and upward transport. We have probably only scratched the surface of information contained in the felsic carriers of the mafic and ultramafic rocks that record the greatest depths. Most of this felsic component has partially melted on the way back up or at least reverted to low-pressure assemblages. How do we interrogate them beyond examining their zircons for inclusions of high-pressure minerals? There is also now emerging a dimension of UHPM beyond subduction zones. Microdiamonds have now been identified in tiny melt pools of xenoliths in Ocean Island Basalts (OIBs). Silicates and oxides from the lower mantle, including phase egg (a clear identifier of continental material), have been discovered in diamonds. Most recently, coesite and diamond have been found and stishovite implied in chromitite from unmetamorphosed and unshocked ophiolites. Thus, UHPM of subducted material has struggled for 40+ years against the very old idea that continents and mantle have been very efficiently segregated by isostasy. Now, we are witnessing the dawn of a similar revolution showing that there are also traces of the very deep earth that can be carried to the surface by volcanic processes that can carry messages from much deeper than the depth of generation of the magmas.
V43E-04 INVITED
Osbornite (TiN) and boron nitride nanoinclusions in coesite from Tibet: a first record of nitrogen in a terrestrial ultrahigh pressure environment
We report here discovery of nitrides in podiform chromitite of the Luobusa ophiolite, Tibet, representing the mantle section of a ¡°fossil" fragment of oceanic crust which marks the Early Tertiary (~65Ma) suture zone between Asia and India. Nanometric crystals of TiN and BN are included in coesite which, together with kyanite, TiO2-II and several other still unidentified phases rim a FeTi-alloy pellet; this sample, together with an OsIr alloy pellet containing microdiamond were extracted from the massive chromitite (Yang et al., Geology, 2007). Both TiN and BN form bright-grey-contrast particles in secondary-electron scanning electron microscope imaging. Several focused ion beam foils were prepared for a transmission electron microscope (TEM) and a nano secondary ion mass spectrometry (nanoSIMS) studies. Because energy dispersive X-ray spectra (EDS) of boron and nitrogen K-lines have overlaps with each other and with Ti L-lines, we have used electron energy loss spectroscopy (EELS) in TEM to determine the presence of boron and nitrogen K-edges and to separate them from Ti L-lines. Electron diffraction patterns identify the cubic boron nitride (c-BN) structure. TiN is stoichiometric (Ti=77.20wt%; N=22.80wt%) and is also cubic. Both phases contain trace carbon. Osbornite, is usually found only in meteorites, although it also has been reported as inclusions in carbonado diamonds and as inclusion in a corundum grain from lamproitic breccia in the Asov block, Eastern-European platform, Ukraine. Until now, boron nitride was known only as an industrial compound, with both hexagonal and cubic structures. With the nanoSIMS Cameca-50 using the Cs beam we have measured nitrogen and carbon isotopes in both the TiN and c-BN inclusions: δ 15N= +(6-10) ‰ and δ 13C= (+1) ¨C (-10) ‰. The results show that both minerals are characterized by δ 15N similar to crustal rocks involved in the subduction process and their δ 13C values suggest mixture of heavy (mantle) and lighter (crustal) carbon. These observations, coupled with the occurrence of the TiN and c-BN in coesite, record a mantle history of nitrogen and indicate that natural osbornite formation is not restricted to high-temperature gas-to-solid condensation reported from studies of meteorites. These nitrides may have been formed during ultrahigh pressure metamorphism of nitrogen- and boron-bearing sediments subducted to mantle depths or, given that this material may have come to the surface from > 300 km (Yang et al., 2007), they could possibly reflect a role of nitrogen in Earth's core formation and cast light on the geochemical budget of nitrogen ¡°escaped from" and ¡°retained within" the Earth during its accretion and atmosphere formation.
V43E-05 INVITED
Diamond and Unusual Minerals Discovered from the Chromitite in Polar Ural: A First Report
Ultrahigh pressure (UHP) minerals, such as diamond, coesite, and pseudomorphs of octahedral olivine, and as well as about 80 other mineral species have been recovered from podiform chromitites of the Luobusa ophiolite, southern Tibet, and a new mineral, Luobusaite (Fe0.82Si2), has been approved recently by CNMMN. The UHP minerals from Luobusa are controversial because they have not found in situ and because ophiolites are currently believed to form at shallow levels above oceanic spreading centers. More detailed study and experimental work are needed to understand the origin and significance of these unusual minerals and investigations of other ophiolites are needed to determine if such minerals occur elsewhere. For this purpose, we collected about 1500 kg of chromitite from two orebodies in an ultramafic body in the Polar Urals. Thus far, more than 60 different mineral species have been separated from these ores. The most exciting discovery is the common occurrence of diamond, a typical UHP mineral in the Luobusa chromitites. Diamonds from Ural chromitite are clear, colorless, well-developed crystals with octahedral morphology, generally 0.2-0.3 mm in size. Attached with the diamonds and perhaps also occurring as inclusions within them are many minerals as chromite, MnNiCrFe alloy, native Si and Ta, corundum, zircon, feldspar, garnet, moissanite, confirming their natural origin and suggesting a long residence time in the mantle. Other mineral group include: (1) native elements: Cr, W, Ni, Co, Si, Al and Ta; (2) carbides: SiC and WC; (3) alloys: Cr-Fe, Si-Al-Fe, Ni-Cu, Ag-Au, Ag-Sn, Fe-Si, Fe-P, and Ag-Zn-Sn; (4) oxides: NiCrFe, PbSn, REE, rutile and Si- bearing rutile, ilmenite, corundum, chromite, MgO, and SnO2; (5) silicates: kyanite, pseudomorphs of octahedral olivine, zircon, garnet, feldspar, and quartz,; (6) sulfides of Fe, Ni, Cu, Mo, Pb, Ab, AsFe, FeNi, CuZn, and CoFeNi; and (7) iron groups: native Fe, FeO, and Fe2O3. These minerals are very similar in composition and structure to those reported from the Luobusa chromitites. For examples, some spherules of native iron contain spherical inclusions of FeO, exactly like comparable grains in the Luobusa sample.
V43E-06
Infrared and Raman spectroscopic observations on Central African carbonado and the implication to its origin
Carbonado diamonds from the Central African Republic (CAR) were investigated by means of spectroscopic observations. Raman spectra for the polished section of carbonado revealed that the surface of the sample gave the average Raman frequency at 1333.0 cm-1, while 10 micrometer below the sample surface gave the bimodal distribution of Raman frequency with the average at 1333.5 cm-1. These contrastive results indicate that the inside of carbonado retain the considerable residual pressure. The maximum residual pressure detected in this study from the 10 micrometer subsurface was 0.49 GPa. From infrared (IR) absorption spectra for crushed carbonado samples, no absorption due to diamond was observed because absorption bands due to mineral and fluid inclusions were too strongly observed. In contrast, the acid leaching treatment on the crushed grains gave rise to the observations of IR bands assignable to intrinsic diamond vibration and nitrogen impurity in diamonds. Nitrogen atoms in the CAR carbonado were not much aggregated and the extent of aggregation was intermediate between type Ib and type IaA. IR absorption bands from liquid water and carbonate were ubiquitously observed for the chemically treated samples. This suggests that the CAR carbonado samples contain fluid inclusion and are similar to the diamonds containing mantle-derived fluids. Overall, the experimental results obtained in the present study suggests that the CAR carbonado originated from some rapid heating event with a presence of fluid in the mantle and subsequent rapid cooling before aggregating impurity nitrogen in the diamond lattice.
V43E-07
Norwegian "External" Orthopyroxene Eclogites
Orthopyroxene (opx) eclogites have been recognised in the Norwegian Western Gneiss Complex (WGC) since the work of Eskola. They have played a key role in pressure-temperature evaluation, but their complexity has fueled controversy. "External" eclogites are those directly enclosed in gneiss and are distinguished from "internal" eclogites enclosed within mantle-derived peridotites. They are restricted to the UHP domains. Their petrography is complex, with abundant secondary amphibole. Garnets are idioblastic and prograde-zoned with inclusions of amphibole and cpx, but also platy and interlaminated with opx, with inclusions of graphite. Some bodies are pegmatitic. Opx eclogites were important in early thermobarometry in the WGC because they were the only parageneses capable of giving absolute P-T values. They yielded very high pressures (3-4GPa), and the platy garnets as exsolution lamellae suggested a prior high-T history, possibly at very high P. This supported a model of early mantle evolution and subsequent tectonic introduction into the crust. This was dependant on pairing low-Al opx cores with garnet, and on the origin of lamellar garnets. Opx rims adjacent to garnet tend to have higher Al, and these gave much lower pressuress (2GPa), and the precursor for grains showing garnet exsolution may, instead, have been low-P, high Al opx. This was consistent with a model of metamorphism in-situ in overthickened continental crust. Discovery of coesite and microdiamond confirmed the original high pressure values, and was further corroborated by thermobarometers using phengite and kyanite. Recent pressure estimates are in excess of 4GPa. Evidence for very high pressures in gneissic country-rocks led to models with wholesale insertion of continental crust into the mantle, but allowed for imbrication of low-P, HP and UHP nappes. Here again opx eclogites proved useful, as they were used to test the imbrication hypothesis, but there was found to be no PT contrast across a nappe contact, emphasising coherence of lithotectonic units after peak pressures were attained. Evidence for an early high T history in these opx eclogites has not been confirmed. Higher-Al opx rims appear to be a result of late amphibole growth and the low-Al opx cores do seem to be a primary UHP feature. WGC opx-eclogites have been recognised as a "lineage" distinct from a kyanite-bearing group. Opx and biotite are stabilised by high Mg, low Ca and low Al compared to normal basaltic compositions. Bulk compositions are picritic. Some may have originated from layers in mafic intrusions, but field and isotopic evidence suggest metasomatism involving interaction with continental crust. Pegmatitic character indicates crystallisation in the presence of fluids. Veins of glimmerite and garnetite with abundant zircon and apatite, and concentrations of magnesite all suggest interaction with K-rich, silicate and C-O-H enriched melts or fluids. Similar lithologies have been found in xenoliths from ultrapotassic volcanics in the Pamir above a subducted continental slab. Opx eclogites appear to be peculiar to the WGC and less well-known from other UHP terrains. Early workers noted similarities with opx eclogites from kimberlites; future work should again focus on parallels between opx eclogites and mantle xenoliths, and the insights to be gained on metasomatism, crust-mantle interactions and the fate of deeply subducted continental crust.
V43E-08
Sm-Nd Mineral Isochron Age Patterns from Garnet-bearing Peridotite of the Western Gneiss Region, Norwegian Caledonides: Discrete Mantle Events or Continuous Re- equilibration?
The garnet peridotites (and pyroxenites) of the UHP Western Gneiss Region of Norway give Sm-Nd garnet, clinopyroxene, whole rock, orthopyroxene, amphibole ages that range from ca. 1.7 Ga to 424 Ma. Most of these twenty seven ages are much older than the continent-continent collision that transferred these peridoitites from the mantle into the crust (i.e. the 400 Ma Scandian Orogeny) suggesting the garnet peridotites of the WGR are unique relative to those in other UHP terranes, which invariably give ages that overlap the time of UHP metamorphism of the enclosing country rocks. All but the youngest ages given by WGR peridotites reflect processes that occurred deep in the mantle beneath the Baltic Shield, but it is unclear if they date a series of discrete events related to the tectonic evolution of the Baltic Shield or if the ages reflect continuous, but variable, re-equilibration of the Sm-Nd system between phases during the residence of the peridotites in the mantle. Three ages overlap the 1.75 to 1.55 Ga Gothian Orogeny while twelve ages are within error of the 1.2 to 0.9 Ga Sveconorwegian Orogeny. The three youngest ages (438 to 424 Ma) are associated with a younger generation of garnets and may mark the beginning of eclogite-facies metamorphism of Baltica as it was subducted beneath Laurentia during the Scandian Orogeny. However, the remaining nine ages spread more or less continuously between these three major events. The overall pattern on a histogram is a range of ages with a pronounced peak at and near the Sveconorwegian Orogeny. The ages therefore appear to date continuous diffusion between minerals from garnet-bearing assemblages that formed originally during or, less likely, before the Gothian Orogeny interrupted by a pronounced thermal event during the Svconorwegian Orogeny and a recrystallization event during the early stages of the Scandian orogeny. The degree of re-equilibration was probably controlled by the ambient temperature of the peridotite body in the mantle, which was controlled, in turn, by their depth in the mantle and their proximity to hot mantle upwelling during the Sveconorwegian Orogeny.