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81.
Ultrahigh‐temperature (UHT) granulite facies rocks from the Achankovil Shear Zone area and the southern domain of the Madurai Granulite Block in South India contain monazite useful for in situ microprobe U–Pb dating. The UHT rocks examined consist of garnet + cordierite (retrograde) + quartz + mesoperthite + biotite + plagioclase + Fe‐Ti oxides ± orthopyroxene ± sillimanite and accessory zircon and monazite. Sillimanite occurs only as inclusions in garnet. Microstructural observations suggest garnet, orthopyroxene, spinel and mesoperthite are products of peak metamorphism. Post‐peak formation of cordierite ± orthopyroxene ± quartz and cordierite + spinel + Fe‐Ti oxides assemblages is also observed. Geothermobarometry on orthopyroxene and garnet‐orthopyroxene bearing assemblages suggest peak UHT conditions of T = 940–1040°C and P = 8.5–9.5 kbar. This was followed by a retrograde stage of 3.5–4.5 kbar and 720 ± 60°C, estimated from garnet‐cordierite assemblages. A small population of rounded, probably detrital, monazites in these rocks yield ages from Meso‐ to Neoproterozoic indicating a heterogeneous source. The youngest associated spot ages are 660–600 Ma suggesting protolith deposition up to ca 600 Ma. In contrast, the vast majority of monazites that crystallized during the latest metamorphic event show late Neoproterozoic to Cambrian ages. Probability‐density plots of monazite age data show a ‘peak’ between 533 and 565 Ma, but this peak need not reflect a particular thermal event. Collating ages from homogenous metamorphic monazites associated with minerals stable at peak P‐T conditions suggests peak metamorphism in these rocks occurred at 580–600 Ma. Together with a re‐evaluation of available data from adjacent granulite blocks in southern India, these data suggest the main metamorphic event coinciding with the suturing of India with the Gondwana amalgam probably occurred 580–600 Ma. The 500–550 Ma ages commonly reported in previous studies might represent post‐peak thermal events. 相似文献
82.
Takuya Watanuki Andrew S. Murray Sumiko Tsukamoto 《Earth and Planetary Science Letters》2005,240(3-4):774-789
There are only a few luminescence dating studies of loess sediments in Japan, but interleaved with these deposits are many well-described tephras of known age based on 14C and fission track analysis; these independent age controls provide an opportunity to test the reliability of loess luminescence ages. This study provides such a comparison at two sites in central Honshu, the largest island in the Japanese archipelago. Samples were collected from sequences of interleaved volcanic tephra and loess deposited on a Middle Pleistocene river terrace in the Niigata Prefecture and on an Early to Middle Pleistocene dissected fluvial surface at in the Tochigi Prefecture, Honshu. Equivalent doses (De) were estimated from fine grains (4–11 μm) using both polymineral IR-OSL and (post-IR) blue-OSL, and quartz blue-OSL. The blue-stimulated luminescence signals could be represented by up to three exponentially decaying components; only the most light sensitive of these components was used in the final De estimation. Almost all the estimates of De from polymineral IR-OSL are smaller than those from polymineral (post-IR) blue-OSL and quartz blue-OSL, whereas the latter two are in good agreement. The blue-light stimulated luminescence ages using the most light sensitive component are shown to be in good agreement with the independent control, up to 500 ka. Although the IR-OSL signals underestimate the known age, a simple laboratory fading test is found empirically to correct for this underestimation. We conclude that the most rapidly blue-stimulated luminescence signals from quartz extracted from our loess samples give reliable ages, and that future loess dating studies should concentrate on these signals. 相似文献
83.
Kshitij C. Shrestha Yoshikazu Araki Takuya Nagae Toshihiro Omori Yuji Sutou Ryosuke Kainuma Kiyohito Ishida 《地震工程与结构动力学》2013,42(5):725-741
This paper presents the results on shaking table tests of half‐scale brick walls performed to investigate the effectiveness of newly developed Cu–Al–Mn superelastic alloy (SEA) bars in retrofitting of historical masonry constructions. Problems associated with conventional steel reinforcing bars lie in degradation of stiffness and strength, or pinching phenomena, under cyclic loading, and presence of large residual cracks in structures during and after intense earthquakes. This paper attempts to resolve the problems by applying newly developed Cu–Al–Mn SEA bars, characterized by large recovery strain, low material cost, and high machinability, as partial replacements for steel bars. Sets of unreinforced, steel reinforced, and SEA‐reinforced specimens are subjected to scaled earthquake excitations in out‐of‐plane direction. Whereas steel‐reinforced specimens showed large residual inclinations, SEA‐reinforced specimens resulted in stable rocking response with slight residual inclinations. Corresponding nonlinear finite element (FE) models are developed to simulate the experimental observations. The FE models are further used to examine the sensitivity of the response with respect to the variations in experimental conditions. Both the experimental and numerical results demonstrate the superiority of Cu–Al–Mn SEA bars to conventional steel reinforcing bars in avoiding pinching phenomena. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
84.
Federica Schiavi Katsura Kobayashi Takuya Moriguti Eizo Nakamura Massimo Pompilio Massimo Tiepolo Riccardo Vannucci 《Contributions to Mineralogy and Petrology》2010,159(4):541-561
During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous
magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions
and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity,
Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards.
Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as
well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types
show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses,
along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower
δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined
with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP
magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous
degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early
volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma
erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre-
and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights
the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic
activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system. 相似文献