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Now showing items 1 - 16 of 99397

  • The cold and relatively dry nature of mantle forearcs in subduction zones

    Abers, G. A.   van Keken, P. E.   Hacker, B. R.  

    Some of Earth's coldest mantle is found in subduction zones at the tip of the mantle wedge that lies between the subducting and overriding plates. This forearc mantle is isolated from the flow of hot material beneath the volcanic arc, and so is inferred to reach temperatures no more than 600 to 800 degrees C - conditions at which hydrous mantle minerals should be stable. The forearc mantle could therefore constitute a significant reservoir for water if sufficient water is released from the subducting slab into the mantle wedge. Such a reservoir could hydrate the plate interface and has been invoked to aid the genesis of megathrust earthquakes and slow slip events. Our synthesis of results from thermal models that simulate the conditions for subduction zones globally, however, indicates that dehydration of subducting plates is too slow over the life span of a typical subduction zone to hydrate the forearc mantle. Hot subduction zones, where slabs dehydrate rapidly, are an exception. The hottest, most buoyant forearcs are most likely to survive plate collisions and be exhumed to the surface, so probably dominate the metamorphic rock record. Analysis of global seismic data confirms the generally dry nature of mantle forearcs. We conclude that many subduction zones probably liberate insufficient water to hydrate the shallower plate boundary where great earthquakes and slow slip events nucleate. Thus, we suggest that it is solid-state processes and not hydration that leads to weakening of the plate interface in cold subduction zones.
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  • Partially melted, mica-bearing crust in Central Tibet

    Hacker, B. R.   Ritzwoller, M. H.   Xie, J.  

    Surface wave tomography shows that the central Tibetan Plateau (the Qiangtang block) is characterized by S wave speeds as slow as 3.3 km/s at depths from 20-25 km to 45-50 km and S wave radial anisotropy of at least 4% (V-SH > V-SV) that is stronger in the west than the east. The depth of the Curie temperature for magnetite inferred from satellite magnetic measurements, the depth of the alpha-beta quartz transition inferred from V-P/V-S ratios, and the equilibration pressures and temperatures of xenoliths erupted from the middle to deep crust indicate that the Qiangtang crust is hot, reaching 1000 degrees C at the Moho. This inferred thermal gradient crosses the dehydration melting solidi for crustal rocks at 20-30 km depth, implying the presence or former presence of melt in the Tibetan middle to deep crust. These temperatures do not require the wholesale breakdown of mica at these depths, because F and Ti can stabilize mica to at least 1300 degrees C. Petrology suggests, then, that the Qiangtang middle to deep crust consists of a mica-bearing residue from which melt has been extracted or is being extracted. Wave speeds calculated for mica-bearing rocks with a subhorizontal to gently dipping foliation and 2% silicate melt are a good match to the wave speeds and anisotropy observed by seismology.
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  • Formation and Exhumation of Ultrahigh-Pressure Terranes

    Hacker, B. R.   Gerya, T. V.   Gilotti, J. A.  

    The reigning paradigm for the formation and exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction of crust to mantle depths and the return of crustal slices within the subduction channel all at plate tectonic rates. Additional processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study of UHP terranes for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism, and exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into the formation and exhumation of UHP terranes.
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  • Feedback between rifting and diapirism can exhume ultrahigh-pressure rocks

    Ellis, S. M.   Little, T. A.   Wallace, L. M.   Hacker, B. R.   Buiter, S. J. H.  

    The processes by which crustal rocks are buried to tremendous depths and subsequently exhumed to Earth's surface remain controversial. Rapid exhumation of Earth's youngest (ultra-) high-pressure (UHP) rocks in the Woodlark Basin, Papua New Guinea, is occurring within an active rift, in contrast to more common scenarios of UHP exhumation during plate convergence. We use 2D and 3D thermo-mechanical models to demonstrate that UHP exhumation can result from feedback between rifting and the diapiric rise of a previously subducted continental fragment through the lithosphere. We infer that this feedback is responsible for the exhumation of the UHP rocks in gneiss domes in the Woodlark Basin. Our models successfully reproduce UHP exhumation paths and rates, and geological structures within the gneiss domes. We show that UHP exhumation by diapirism is mechanically consistent in post-collisional rifts. Our models highlight the complex feedback between diapiric ascent and extension. (C) 2011 Elsevier B.V. All rights reserved.
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  • Controls on Trace Element Uptake in Metamorphic Titanite:Implications for Petrochronology

    Garber, J. M.   Hacker, B. R.   Kylander-Clark, A. R. C.   Stearns, M.   Seward, G.  

    Petrochronology-the interpretation of isotopic dates with complementary elemental data-requires understanding the relationship between trace elements in chronometers and the petrological evolution of their host rocks. Titanite is a useful petrochronometer for crustal processes, but how titanite records host rock evolution is uncertain. We present an extensive titanite U-Pb and chemical dataset from felsic gneisses and leucosomes in the Western Gneiss Region (WGR) of Norway. Mineral textures, U-Pb dates, and major, minor, and trace element chemistry reveal three titanite populations: (1) Precambrian igneous titanite [high light rare earth elements (LREE), Th, Pb, Zr; low Al, F]; (2) Caledonian recrystallized titanite (low LREE, Th, Pb) that formed from dissolution-reprecipitation of the Precambrian titanite and co-crystallized with allanite; (3) Caledonian neocrystallized titanite (high Al, F and variable REE). Although titanite records multiple igneous and metamorphic events in the WGR, we use a principal components analysis to identify distinct petrological and thermal effects on trace element uptake that hold across all titanite populations. Coupled with textural observations, these data show that different trace element patterns between populations predominantly represent the activity of different rock reactions during continental subduction and exhumation; using correlations between principal component scores and trace element abundances or ratios, we discriminate which phases co-crystallized with titanite. Our results further demonstrate that thermal and fluid partitioning effects can complicate interpretations of rock petrology from titanite trace elements, but these factors can be assessed by measuring specific trace elements (e.g. Al, Zr).
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  • Subduction, collision and exhumation in the ultrahigh-pressure Qinling-Dabie orogen

    Hacker, B. R.   Ratschbacher, L.   Liou, J. G.  

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  • Presentation of the Dana Medal of the Mineralogical Society of America for 2013 to Max W. Schmidt

    Hacker, B. R.  

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  • P-T-t-D record of crustal-scale horizontal flow and magma-assisted doming in the SW Mongolian Altai

    Broussolle, A.   Stipska, P.   Lehmann, J.   Schulmann, K.   Hacker, B. R.   Holder, R.   Kylander-Clark, A. R. C.   Hanzl, P.   Racek, M.   Hasalova, P.   Lexa, O.   Hrdlickova, K.   Burianek, D.  

    The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite-magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite-bearing schists. Theoldest structure in the migmatite-magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn-tectonic, with up to several tens of metres thick granitoid sills. Sillimanite-ilmenite-magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6-7kbar at 710-780 degrees C. Reorientation of sillimanite into the S2 fabric indicates that the S1-S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3-4kbar and 680-750 degrees C during development of the S2 steep fabric, and post-tectonic andalusite indicates further decompression to 2-3kbar and 600-650 degrees C. Widespread crystallization of post-tectonic muscovite is explained by the release of H2O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3kbar and 500-530 degrees C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5-3 kbar and 530-570 degrees C; replacement of cordierite by fine-grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3-4kbar and 540-620 degrees C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best-developed oscillatory zoning is 356 +/- 1.0 [+/- 7]Ma (Pb-207-corrected U-238/Pb-206), considered to date the crystallization from melt in the cordierite stability similar to 680 degrees C and 3.5kbar, whereas the patchy BSE-dark domains give a date of 347 +/- 4.2 [+/- 7]Ma interpreted as recrystallization at subsolidus conditions. The earliest sub-horizontal fabric is associated with the onset of magmatism and peak of P-T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite-migmatite core with supracrustal rocks. This study provides information about tectono-thermal history and time-scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn-orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.
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  • Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U-Th/He, Sm-Nd, and Lu-Hf dating

    Hacker, B. R.   Kelemen, Peter B.   Rioux, Matthew   McWilliams, Michael O.   Gans, Philip B.   Reiners, Peter W.   Layer, Paul W.   S?derlund, Ulf   Vervoort, Jeffrey D.  

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  • Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints

    Hacker, B. R.   Mosenfelder, J. L.   Gnos, E.  

    Understanding ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 40Ar/ 39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to ~825 K in 1-2 m.y. Hornblende ages from metamorphic rocks and from cross-cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below ~825 K in 1-2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100-200 mm/yr. On the basis of age relationships and thermal considerations, the authors favor a two-stage model for the initial stages of Samail ophiolite emplacement: intraoceanic thrusting over <2-m.y.-old lithosphere at 150 km/m.y, parallel to a spreading ridge for 1-2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and the authors propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting
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  • Time and Metamorphic Petrology: Calcite to Aragonite Experiments

    Hacker, B. R.   Kirby, S. H.   Bohlen, S. R.  

    Demonstrates the importance of kinetic studies on fully dense polycrystalline aggregates for understanding mineralogic phase changes in nature. Extrapolation of these results to geological time scales suggests that transformation of calcite to aragonite does not occur in the absence of volatiles at temperatures below 200degC. Kinetic hindrance is likely to extend to higher temperatures in more complex transformations
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  • Extreme enriched and heterogeneous Sr-87/Sr-86 ratios recorded in magmatic plagioclase from the Samoan hotspot

    Edwards, M. A.   Jackson, M. G.   Kylander-Clark, A. R. C.   Harvey, J.   Hagen-Peter, G. A.   Seward, G. G. E.   Till, C. B.   Adams, J. V.   Cottle, J. M.   Hacker, B. R.   Spera, F. J.  

    We report the major-element, trace-element, and Sr-87/Sr-86 compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock Sr-87/Sr-86 of 0.718592, and ALIA-115-21 with whole-rock Sr-87/Sr-86 of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure Sr-87/Sr-86 ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme Sr-87/Sr-86 heterogeneity in excess of 5000 ppm (where ppm of Sr-87/Sr-86 variability =3D 10(6) . [Sr-87/Sr-86(max) - Sr-87/Sr-86(min)]/Sr-87/Sr-86(avg)). In two of the plagioclase crystals, we identify the highest Sr-87/Sr-86 ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase. We find that in Sr-87/Sr-86-versus-Sr concentration space, the six plagioclase crystals overlap in a "common component" region with higher Sr-87/Sr-86 than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivine mineral inclusions (Fo =3D 74.5 +/- 0.8, 2 SD) in the high-Sr-87/Sr-86 zone of one plagioclase crystal to infer the bulk composition (Mg# =3D 46.8 +/- 0.8, 2 SD) of the extreme EM2 magma from which the olivine and high-Sr-87/Sr-86 plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower-Sr-87/Sr-86 melt to generate the observed intra-crystal plagioclase isotopic heterogeneity. By inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated Sr-87/Sr-86 ratios observed in the Samoan plagioclase is similar to 7% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment domain that constitutes 0.02% of the mass of the mantle, a much lower proportion than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute up to 7.7% of the mantle's mass), then only 0.25% of the LLSVPs are composed of sediment. This requires that the distribution of subducted sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation. (C) 2019 Elsevier B.V. All rights reserved.
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  • Observational Evidence for a Possible New Diffusion Path

    HACKER, B. R.   CHRISTIE, J. M.  

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  • Experimental deformation of a glassy basalt : Hacker, B R; Christie, J M TectonophysicsV200, N1/3, Dec 1991, P79–86

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  • Slow exhumation of UHP terranes: Titanite and rutile ages of the Western Gneiss Region, Norway RID E-7750-2011

    Kylander-Clark, A. R. C.   Hacker, B. R.   Mattinson, J. M.  

    U-Pb ages of titanite and rutile were obtained from the central Western Gneiss Region, Norway, to assess the style and timing of exhumation and cooling of the Western Gneiss UHP terrane. Approximately half of the titanite ages are concordant, the majority of which yield a limited age range from 393 to 390 Ma. The other titanite data are discordant, and define discordia arrays with upper intercept ages of either similar to 938 Ma or similar to 1.6 Ga, and a lower interceptof similar to 389 Ma. Concordant rutile analyses range from 385 to 392 Ma. Both titanite and rutile ages young WNW toward the core of the orogen and are similar to 4 Ma older than Ar-40/Ar-39 muscovite ages, corresponding to a cooling rate of similar to 90 degrees C/Ma. A well-defined boundary between concordant and discordant titanite ages, in combination with the WNW-increasing P-T gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west, suggests that the WGR remained coherent throughout its exhumation history, and was progressively unroofed from east to west.A390.2 +/- 0.8 Ma titanite in the Soroyane UHP domain indicates that exhumation occurred at a vertical rate of similar to 7 mm/yr for similar to 12 Ma. These rates are slower than estimates from smaller UHP terranes, but similar to other large UHP terranes, suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large UHP terranes that undergo protracted subduction and exhumation, and smaller UHP terranes that undergo rapid subduction and exhumation. (c) 2008 Elsevier B.V. All rights reserved.
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  • [Phys. Rev. B\r 96\r , 121107(R) (2017)]

    Wang, Aifeng   Graf, D.   Liu, Yu   Du, Qianheng   Zheng, Jiabao   Lei, Hechang   Petrovic, C.  

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