All ICDP Publications with Abstracts
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753.
Rapid eclogitisation of the Dabie-Sulu UHP terrane: Constraints from Lu-Hf garnet geochronology
Earth and Planetary Science Letters,
273
(1-2)
203 – 213
2008
ISSN: 0012821XKeywords:▾
Asia; China; Dabie-Sulu Belt; Eurasia; Far East; Chemical elements; Garnets; Geochronology; Hafnium; History; Land use; Lead alloys; Lutetium; Metamorphic rocks; Mineralogy; Minerals; Mining; Neodymium; Silica; Trace elements; Zircon; Chemical compositions; Dry conditions; Eastern China; eclogite; Eclogites; eclogitisation; Genetic relationships; HF systems; High-precision; Lu-Hf geochronology; Major elements; Mineral growth; Orogenic belts; Porphyroblasts; Re-equilibration; Regional distribution; Regional scales; Short time; Sm-Nd geochronology; Subsequent cooling; Temperature conditions; U-Pb dating; U-Pb zircon; UHP metamorphism; ultrahigh-pressure; chemical composition; eclogite; garnet; geochronology; hafnium; lutetium; orogenic belt; P-T-t path; terrane; ultrahigh pressure metamorphism; Silicate minerals
Abstract: ▾ The Qinling-Dabie-Sulu orogenic belt in eastern China is one of the largest ultrahigh-pressure (UHP) terranes worldwide. Mineral Sm-Nd- and zircon U-Pb dating has been widely used to reveal the metamorphic history of this collisional orogen. However, the exact timing of the UHP metamorphic event(s) remains controversial and ages ranging from 245 Ma to 220 Ma have been suggested. We present high precision garnet-cpx Lu-Hf ages for six eclogites from the Dabie and Sulu areas. All ages fall in a narrow range between 219.6 and 224.4 Ma. Five samples define a mean age of 223.0 ± 0.9 Ma and one sample yields a slightly younger age of 219.6 ± 1.4 Ma. This very tight age range is particularly remarkable considering the large regional distribution of sample localities (on the order of 100 km at the time of UHP metamorphism) and the wide variety of garnet and eclogite chemical compositions represented. Two samples yield Sm-Nd ages that are indistinguishable from their Lu-Hf ages, albeit with larger uncertainties. The identical ages of eclogites from both the Dabie and the Sulu region emphasize their close genetic relationship and similar metamorphic histories. The Lu-Hf results appear to date a punctuated event of garnet growth. Alternatively, the Lu-Hf garnet ages may represent the onset of rapid, contemporaneous uplift and subsequent cooling. However, trace element zoning of Lu and Hf is still preserved in garnet porphyroblasts, even in those with a homogeneous major element distribution. Thus, complete re-equilibration of the Lu-Hf system during peak-temperature conditions probably did not occur. The garnet forming event can be placed toward the final stage of the UHP metamorphism, in agreement with some published U-Pb zircon ages. A possible trigger for this short-lived and widespread mineral growth episode may have been a fluid that became available at that stage of the metamorphic history. Although HREE-depleted patterns of older zircon grains may indicate the presence of an older generation of garnet, complete eclogitisation may have been inhibited during the major part of the prograde P-T path due to dry conditions during most of the UHP metamorphism. The uniform Lu-Hf (and Sm-Nd) ages of all investigated Dabie and Sulu eclogites suggest that garnet growth and thus possibly fluid availability were limited to a short time interval over a remarkably large regional scale. © 2008 Elsevier B.V. All rights reserved.
752.
PDF orientations in shocked quartz grains around the Chicxulub crater
Meteoritics and Planetary Science,
43
(4)
745-760
2008
Abstract: ▾ We measured 852 sets of planar deformation features (PDFs) in shocked quartz grains in impactite samples of the Yaxcopoil (YAX-1) core and from 4 Cretaceous/Tertiary (K/T) boundary deposits: the Monaca, the Cacarajícara, and the Peñalver formations in Cuba, and DSDP site 536, within 800 km of the Chicxulub crater, in order to investigate variations of PDF orientations in the proximity of the crater. Orientations of PDFs show a broad distribution with peaks at ω {1013}, π {1012}, and ξ {1112}, plus r, z {1011} orientations with minor c(0001), s{1121}, t{2241} plus x{5161}, and m{1010} plus a{1120} orientations. Planar deformation features with c(0001) orientation are relatively more abundant in the proximity of the Chicxulub crater than in distal sites such as North America, the Pacific Ocean, and Europe. This feature indicates that in the proximity of the crater, part of the shocked quartz grains in the K/T boundary deposits were derived from the low shock pressure zones. Moreover, the orientations of PDFs with ξ {1122} plus r, z {1011} are high in our studied sites, and frequencies of these orientations decrease with increasing distance from the crater. On the other hand, absence of c(0001) and the rare occurrence of PDFs with ξ {1122} plus r, z {1011} orientations in the sample from the YAX-1 core that was taken at the top of the impactite layer of the Chicxulub crater suggests that the sampling horizon that reflects a certain cratering stage is also an important factor for variations in shocked quartz. © The Meteoritical Society, 2008.
751.
Preliminary achievement of the Chinese Cretaceous continental scientific drilling project-SK-I
Acta Geologica Sinica (English Edition),
82
(1)
9-20
2008
ISSN: 10009515
Publisher: Science Press
Abstract: ▾ To obtain the global environmental and climatic records is going to be the aim of the international scientific drilling projects. As a paradigm of greenhouse climate in the geological history, the Cretaceous provides significant records of the global climate changes under the condition of greenhouse climate. But currently, the international scientific drilling programs on the Cretaceous have been focusing on marine sediments; in contrast, no terrestrial records have been recovered by any scientific program. Newly completed SK I and SK II drilling located at the northern part of the Songliao Basin are supposed to be an important breakthrough in this domain. After more than one year effort, the drilling project has been finished, and 2485.89 in of the cores have been obtained, with a high recovery ratio of 96.46%, and that is the longest and continuous Cretaceous terrestrial core all over the world. A number of important investigations and achievement on the cores from SK I and SK II have been carried out. One of the most significant contributions of this program is to get records with a resolution of ten thousand year by the centimeter-grade sampling and analysis, instead of the traditional records on million years of timescale, and hopefully, its research can provide an important reference to solve currently global warming. Combined with raw data and upcoming results for the core, an integrated accomplishment of the scientific can be made in ten profiles such as biostratigraphy, chemostratigraphy, magnetostratigraphy and geomicrobiology and so on. The preliminary progress has been achieved on sedimentology, organic geochemistry, cyclostratigraphy, paleolimnology, geomicrobiology and reconstruction of Pco'2. All the cores will be preserved in professional storeroom after being cut for our researches, and scientists all over the world are highly welcome. The follow-up the SK II drilling program has also been selected as one of the candidate programs of ICDP (International Continental Drilling Project).
750.
Post-impact carbonate deposition in the Chicxulub impact crater region, Yucatan Platform, Mexico
Current Science,
95
(2)
248-252
2008
Abstract: ▾ The Chicxulub crater has attracted considerable attention as one of the largest terrestrial impact structures and its association with the Cretaceous/Palaeogene boundary. Analyses of stable isotopes and magnetostratigraphic results for the Paleocene carbonate sequence in the Santa Elena borehole are used to investigate the post-impact sequence and estimate the age of basal sediments in the southern crater sector. Studies of impact ejecta and cover sediments and modelling of post-impact processes suggest erosion effects due to seawater back surge, block slumping and partial rim collapse of post-impact crater modification. Correlation of stable isotope patterns with the global pattern for marine carbonate sediments provides a stratigraphic framework for the basal Paleocene carbonates. Magnetic polarity constrains correlation of stable isotope variations with the reference Cenozoic isotopic data suggest that the first 17 m above the breccia-carbonate contact represents about 2.5 Ma. The stable isotope data suggest a gap of less than 0.1 Ma, whereas the magnetic polarity data (absence of reverse-polarity samples above impact breccia contact) suggest a gap up to 0.25 Ma.
749.
Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis
Studia Geophysica et Geodaetica,
52
(4)
493 – 512
2008
ISSN: 00393169
Publisher: Springer New York
Keywords:▾
earthquake hypocenter; earthquake swarm; microearthquake; seismic migration; seismicity; strike-slip fault; waveform analysis
Abstract: ▾ We present the pattern of seismic activity in the period between 2001 and 2007 for the Novä Kostel focal zone, which is recently the most active zone of the West-Bohemia/Vogtland earthquake swarm region. While the year 2001 was characterized by dying out of the 2000-swarm activity in the form of a few microswarms, almost no seismicity occurred in the period between 2002 and 2003. Since 2004 an elevated seismic activity occurs in the form of repeating microearthquake swarms. We used a relative location method to relate the hypocenter positions of the post-swarm activity to the geometry of the 2000-swarm cluster. We found that the activity has concentrated in several clusters, which have been repeatedly activated. Some clusters coincide with the position of the previous activity; the others have activated so far inactive deep segments at the southern edge of the Novä Kostel fault. Besides the shift of the hypocenters to the edges of the previously active area we observe a southward migration of the activity and an increase of maximum depths of earthquakes from 10 to 13 km. The waveform similarity analysis disclosed that some fault patches consist of only a single, repeatedly activated fault plane, while the others consist of multiple, differently oriented fault planes activated almost simultaneously. Most of the focal mechanisms are consistent with the geometry of hypocenters showing NNW-SSE trending steep fault planes with left-lateral strike-slip mechanisms and varying dip-slip component. © Institute of Geophysics of the ASCR, v.v.i 2008.
748.
Physical rock properties in and around a conduit zone by well-logging in the Unzen Scientific Drilling Project, Japan
Journal of Volcanology and Geothermal Research,
175
(1-2)
13-19
2008
ISSN: 03770273Keywords:▾
Acoustic logging; Acoustic wave velocity; Degassing; Drilling; Forestry; Gamma rays; Harvesting; Human computer interaction; Hydraulic structures; Levees; Lithology; Logging (forestry); Neutron logging; Oil well logging; Porosity; Rock drilling; Rocks; Scale (deposits); Spontaneous potential logging; Structural geology, Alternating layers; Conduit structure; Data support; High porosity; Log data; Low resistivity; Magma ascent; Neutron porosity; P-wave velocities; Physical Properties; Pyroclastic rocks; Rock properties; Scale formation; Scientific drilling; Spontaneous potential; Unzen volcano; Wave velocities; Well logging, Well drilling, degassing; drilling; lava; lithology; magma; permeability; rock property; volcanic eruption; wave velocity; well logging, Corrosion; Degassing; Deposits; Drilling; Hydraulic Structures; Logging; Porosity; Velocity, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ The objective of the Unzen Scientific Drilling Project (USDP) is not only to reveal the structure and eruption history of the Unzen volcano but also to clarify the ascent and degassing mechanisms of the magma conduit. Conduit drilling (USDP-4) was conducted in 2004, which targeted the magma conduit for the 1990-95 eruption. The total drilled length of USDP-4 was 1995.75 m. Geophysical well logging, including resistivity, gamma-ray, spontaneous potential, sonic-wave velocity, density, neutron porosity, and Fullbore Formation MicroImager (FMI), was conducted at each drilling stage. Variations in the physical properties of the rocks were revealed by the well-log data, which correlated with not only large-scale formation boundaries but also small-scale changes in lithology. Such variations were evident in the lava dike, pyroclastic rocks, and breccias over depth intervals ranging from 1 to 40 m. These data support previous models for structure of the lava conduit, in that they indicate the existence of alternating layers of high-resistivity and high P-wave velocity rocks corresponding to the lava dikes, in proximity to narrower zones exhibiting high porosity, low resistivity, and low P-wave velocity. These narrow, low-porosity zones are presumably higher in permeability than the adjacent rocks and may form preferential conduits for degassing during magma ascent. © 2008 Elsevier B.V.
747.
Phase relations and liquid lines of descent in hydrous ferrobasalt - Implications for the skaergaard intrusion and Columbia river flood basalts
Journal of Petrology,
49
(9)
1687-1727
2008
ISSN: 00223530Keywords:▾
crystallization; differentiation; experimental study; flood basalt; hydrothermal system; magma; modeling; precipitation (chemistry); redox conditions; silicate mineral, Arctic; Columbia River; Greenland; North America; Skaergaard Intrusion
Abstract: ▾ Crystallization experiments using a hydrous ferrobasalt as starting material, conducted at 200 MPa, 940-1200°C, at a wide range of water activities (0.1-1) and redox conditions (QFM - 3 to QFM + 4, where QFM is the quartz-fayalite-magnetite oxygen buffer), show that H2O influences significantly the differentiation history of ferrobasaltic magmas. A combination of our data with published experiments on dry ferrobasalt at 1 atm provides an extensive experimental database for modeling and quantifying crystallization and differentiation processes within a typical Fe-rich tholeiitic system under both dry and hydrous conditions. The addition of H2O decreases liquidus temperatures and changes significantly the proportions, temperature range and sequence of crystallizing mineral phases. The dry liquidus is at about 1170°C whereas the liquidus for H2O-saturated melts is at ∼1060°C. The main phases crystallizing from H2O-bearing ferrobasalt at the investigated conditions are olivine (OL), clinopyroxene (CPX), plagioclase (PL), magnetite (MT), hematite (HM), ilmenite (ILM) and amphibole (AM). The phase assemblage is similar to that of the dry system except for the presence of HM at extremely oxidizing conditions and AM at low temperatures (< 950°C) and H2O-saturated conditions. The important observation made in this study is that the stability of Fe-Ti-oxides, and in particular MT, as well as the simultaneous coprecipitation of MT and ILM, are almost independent of the activity of H2O (a H2O) in the system, whereas the liquidus temperatures of the silicate minerals are dramatically depressed by increasing a H2O. The stabilities of oxides are controlled mainly by the redox conditions prevailing in the system. The most pronounced effect of a H2O on the liquidus temperatures of silicates is observed for PL, which shows a considerable delay in crystallization with progressive magma differentiation. Early crystallization of Fe-Ti-oxides in H2O-bearing ferrobasaltic compositions precludes any significant Fe enrichment during differentiation. As Fe enrichment is a characteristic feature of the Skaergaard intrusion, it implies that the Skaergaard parental magma did not contain considerable amounts of water. On the other hand, our experiments indicate that the differentiation of some ferrobasaltic series from the Columbia River flood basalt province might have occurred in magmatic systems containing significant amounts of volatiles (∼0.5-3 wt % H2O). © The Author 2008. Published by Oxford University Press. All rights reserved.
746.
Permeability measurements on rock samples from Unzen Scientific Drilling Project Drill Hole 4 (USDP-4)
Journal of Volcanology and Geothermal Research,
175
(1-2)
82-90
2008
ISSN: 03770273Keywords:▾
Capillarity; Liquids; Rocks; Solids, Compressional wave velocities; Confining pressures; crack; Drill holes; drilled core; permeability; Permeability measurements; pore structure; Rock samples; Transient pulse; Unzen volcano, Rock drilling, atmospheric pressure; country rock; crack; degassing; dike; drilling; gas flow; granite; permeability; solidification; volcanic eruption; wave velocity, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ Permeability measurement was made on five rock samples from USDP-4 cores. Rock samples were collected from the conduit zone and its country rock. One sample (C14-1-1) is considered as a part of the feeder dyke for the 1991-1995 eruption. The transient pulse method was employed under confining pressure up to 50 MPa. Compressional wave velocity was measured along with permeability. The measured permeability ranges from 10- 19 to 10- 17 m2 at the atmospheric pressure, and is as low as that reported for tight rocks such as granite. The permeability decreases with increasing confining pressure, while the compressional wave velocity increases. Assuming that pores are parallel elliptical tubes, the pressure dependence of permeability requires aspect ratio of 10- 4-10- 2 at the atmospheric pressure. The pore aperture is estimated to be less than 1 μm. The estimated aspect ratio and pore aperture suggest that connectivity of pores is maintained by narrow cracks. The existence of cracks is supported by the pressure dependence of compressional wave velocity. Narrow cracks (< 1 μm) are observed in dyke samples, and they must have been created after solidification. Dyke samples do not provide us information of pore structures during degassing, since exsolved gas has mostly escaped and pores governing the gas permeable flow should have been lost. Both dyke and country rock samples provide us information of materials around ascending magma. Although the measured small-scale permeability cannot be directly applied to geological-scale processes, it gives constrains on studies of large-scale permeability. © 2008 Elsevier B.V. All rights reserved.
745.
Shock metamorphism of Bosumtwi impact crater rocks, shock attenuation, and uplift formation
Science,
322
(5908)
1678 – 1681
2008
ISSN: 10959203Keywords:▾
crater; deformation; numerical model; pressure; quartz; shock metamorphism; shock wave; uplift; article; calculation; Craterostigma; crystallography; priority journal; rock; scientific literature; shock wave
Abstract: ▾ Shock wave attenuation rate and formation of central uplifts are not precisely constrained for moderately sized complex impact structures. The distribution of shock metamorphism in drilled basement rocks from the 10.5-kilometer-diameter Bosumtwi crater, and results of numerical modeling of inelastic rock deformation and modification processes during uplift, constrained with petrographic data, allowed reconstruction of the pre-impact position of the drilled rocks and revealed a shock attenuation by ∼5 gigapascals in the uppermost 200 meters of the central uplift. The proportion of shocked quartz grains and the average number of planar deformation feature sets per grain provide a sensitive indication of minor changes in shock pressure. The results further imply that for moderately sized craters the rise of the central uplift is dominated by brittle failure.
744.
Session 15. The Evolution of the Biogeochemical Cycling of Phosphorus and Other Bioessential Elements
Astrobiology,
8
(2)
356--361
2008
743.
Water content and dry bulk density of piston core CON01-603-2 from CONTINENT Ridge
742.
SHRIMP U–Pb dating, trace elements and the Lu–Hf isotope system of coesite-bearing zircon from amphibolite in the SW Sulu UHP terrane, eastern China
Geochimica et Cosmochimica Acta,
72
(12)
2973-3000
2008
ISSN: 0016-7037Abstract: ▾ In this study, we link mineral inclusion data, trace element analyses, U–Pb age and Hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi-stage metamorphic evolution of amphibolites from the Sulu ultrahigh-pressure (UHP) terrane, eastern China. Zircon grains separated from amphibolites from the CCSD-MH drill hole (G12) and Niushan outcrop (G13) were subdivided into two main types based on cathodoluminescence (CL) and Laser Raman spectroscopy: big dusty zircons with inherited cores and UHP metamorphic rims and small clear zircons. Weakly zoned, grey-white luminescent inherited cores preserve mineral inclusions of Cpx+Pl+Ap±Qtz indicative of a mafic igneous protolith. Dark grey luminescent overgrowth rims contain the coesite eclogite-facies mineral inclusion assemblage Coe+Grt+Omp+Phe+Ap, and formed at T=732–839°C and P=3.0–4.0GPa. In contrast, white luminescent small clear zircons preserve mineral inclusions formed during retrograde HP quartz eclogite to LP amphibolite-facies metamorphism (T=612–698°C and P=0.70–1.05GPa). Inherited zircons from both samples yield SHRIMP 206Pb/238U ages of 695–520Ma with an upper intercept age of 800±31Ma. The UHP rims yield consistent Triassic ages around 236–225 and 239–225Ma for G12 and G13 with weighted means of 229±3 and 231±3Ma, respectively. Small clear zircons from both samples give 206Pb/238U ages around 219–210Ma with a weighted mean of 214±3Ma, interpreted as the age of retrograde quartz eclogite-facies metamorphism. Matrix amphibole from both samples indicate Ar–Ar ages of 209±0.7 and 207±0.7Ma, respectively, probably dating late amphibolite-facies retrogression. The data suggest subduction of Neoproterozoic mafic igneous rocks to UHP conditions in Middle Triassic (∼230Ma) times and subsequent exhumation to an early HP (∼214Ma) and a late LP stage (∼208Ma) over a period of ∼16 and 6Myr, respectively. Thus, early exhumation from a mantle depth of 120–100km to about 60km occurred at an average rate of 0.3cm/y, while subsequent exhumation to a middle crustal level took place at approximately 0.54cm/y. These exhumation rates are considerably slower than those obtained for UHP rocks in the Dora Maira and Kokchetav massifs (2–3cm/y). Based on similar P–T estimates and trace element and Hf isotope compositions, Sulu amphibolites can be identified as retrograde UHP eclogites. The εHf(800) of +8 implies a significant input from the depleted mantle to the Sulu–Dabie terrane during the middle Neoproterozoic. Overgrown rims are characterized by a distinct trace element composition with low Lu/Hf and Th/U and significantly higher 176Hf/177Hf ratios than inherited cores, consistent with formation during/after garnet (re-)crystallization and fractionation of the Lu–Hf system during UHP metamorphism. The combined dataset suggests homogenization of the 176Hf/177Hf ratio within the metamorphic mineral assemblage and during protolith formation. Observed variations are explained by mixing of material from both domains during laser ablation, e.g., due to partial recrystallization of inherited cores.
741.
The potential of Lake Ohrid for long-term palaeoenvironmental reconstructions
Palaeogeography, Palaeoclimatology, Palaeoecology,
259
(2-3)
341 – 356
2008
ISSN: 00310182
Publisher: Elsevier B.V.
Keywords:▾
Albania; Eurasia; Europe; Lake Ohrid; Macedonia [Southern Europe]; Southern Europe; Bacillariophyta; biogeochemistry; bioturbation; chronology; comparative study; core analysis; grain size; lacustrine deposit; mass movement; paleoenvironment; physical property; reconstruction; sedimentation rate; seismic survey; tectonics; Tertiary
Abstract: ▾ Lake Ohrid, at the Macedonian/Albanian border, was likely tectonically formed during the Tertiary and therefore is one of the oldest lakes in Europe. However, only a few studies exist concerning the potential of Lake Ohrid sediments for long-term palaeoenvironmental reconstructions within the scope of future potential deep-drilling campaigns. Therefore, as a first step, a transect of short surface sediment cores was investigated for chronology, physical properties, grain size, and biogeochemistry. The results were compared with information derived from a shallow hydro-acoustic seismic survey. The investigations indicate a rather uniform and bioturbated sedimentation in the central part of the lake basin with mean sedimentation rates of ca. 0.5-1 mm/year. The sediment composition is dominated by authigenetic carbonates. Diatom frustules or fragments form the major part of biogenic matter deposits, as indicated by the relatively high contents of biogenic opal and low contents of total organic carbon and total nitrogen. The shallow hydro-acoustic seismic survey indicates that horizons of sediment redeposition occur sporadically. Towards the shore of the lake, the sedimentation rate increases and sedimentation is increasingly influenced by local inflows or mass-movement processes triggered by tectonic activities. Thus Lake Ohrid has a high potential for palaeoenvironmental reconstructions on a multi-decadal scale and provides additional information concerning tectonic activity in the region. © 2007 Elsevier B.V. All rights reserved.
740.
[English]
Formation evaluation of gas hydrate bearing sediments
Publisher
Society of Petrophysicists and Well-Log Analysts (SPWLA)
2008
Department of Geology, University of Leicester, Leicester, LE1 7RH, United Kingdom; British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG, United Kingdom; RWE Dea AG, Hamburg, 22297, Germany739.
[English]
Constraining the far-field in situ stress state near a deep south african gold mine
Volume 46
,
Page 555 – 567
2008
Shell International Exploration and Production, Inc., Houston, TX, United States; Stanford University, Stanford, CA, United States; University of Oklahoma, Norman, OK, United StatesKeywords:▾
African gold mine; Borehole images; Boundary elements; Far-field; Far-field stress; Fault slip; Forward modeling; Induced stress; Insitu stress; Mining excavation; Natural earthquake; Principal stress; Seismic event; State of stress; Stress field; Stress state; Virgin state, Earthquakes; Gold; Gold mines; Mines; Rock mechanics; Rocks; Stresses, Mining
738.
737.
Yellowstone plume-continental lithosphere interaction beneath the Snake River Plain
Geology,
36
(1)
51-54
2008
ISSN: 00917613Keywords:▾
Lithosphere; Mantle plume; Volcanic activity, Basalt; Geochemistry; Isotopes; Tectonics; Volcanoes, Volcanic rocks, basalt; continental lithosphere; isotopic composition; lead isotope; mantle plume; mantle source; neodymium isotope; strontium isotope, Idaho; North America; Snake River Plain; United States
Abstract: ▾ The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. The identification of a clear seismic image of a plume beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic-Paleozoic accreted terrains suggest a plume source while basalts erupted across the Proterozoic-Archean craton margin indicate an ancient subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that of the Wyoming province superimposes its inherent isotopic composition on sublithospheric plume and/or asthenospheric melts. The results show that Yellowstone plume could have a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group and that the plume source composition has persisted to the present day. © 2008 The Geological Society of America.
736.
Wellbore-stability study for the SAFOD borehole through the San Andreas Fault
SPE Drilling and Completion,
23
(4)
394-408
2008
ISSN: 10646671
Publisher: Society of Petroleum Engineers
Keywords:▾
Acoustic logging; Boring; Finite element method; Logging while drilling; Strike-slip faults, A-stable; California; Fault creep; Fluid penetration; Logging while drilling; Microearthquakes; Mud weights; San Andreas fault; Wellbore; Wellbore stability studies, Boreholes, borehole geophysics; borehole stability; creep; drilling; drilling fluid; finite element method; microearthquake; numerical model; San Andreas Fault, California; North America; Parkfield; United States
Abstract: ▾ This paper presents a wellbore-stability study of the San Andreas Fault Observatory at Depth (SAFOD) research borehole located near Parkfield, California, USA. In the summer of 2005, the SAFOD borehole was drilled successfully through the active trace of the San Andreas Fault (SAF) in an area characterized by fault creep and frequent microearthquakes. In this study, we report how the analysis of wellbore failures in the upper part of the hole, geophysical logs, and a model for stress gradients in the vicinity of the fault were used to estimate the mud weights required to drill through the fault successfully. Because logging-while-drilling (LWD) acoustic caliper data and real-time hole-volume calculations both showed that relatively little failure occurred while drilling through the SAF, the predicted mud weight was successful in drilling a stable borehole. However, a six-arm caliper log, run after drilling was completed, indicates that there was deterioration of the borehole with time, which appears to be caused by fluid penetration around the borehole. The LWD-resistivity measurements show that essentially no fluid penetration occurred as the hole was being drilled. Because of this, the mud weight used was capable of maintaining a stable wellbore. However, the resistivity data obtained after drilling show appreciable fluid penetration with time, thus negating the effectiveness of the mud weight and leading to time-dependent wellbore failure. Using finite-element modeling (FEM), we show that mud penetration into the fractured medium around the borehole causes failure with time. Copyright © 2008 Society of Petroleum Engineers.
735.
Well site selecting, core profile characteristics and distribution of the special lithology in CCSD-SK II
Acta Geologica Sinica (English Edition),
82
(5)
669 – 675
2008
ISSN: 10009515
Publisher: Science Press
Abstract: ▾ CCSD-SK II is one of the two drilling wells of the Cretaceous Scientific Drilling Project in Songliao Basin. It is located at limb section of the nose-like structure in the Gulong sag of the central depression, north of Songliao Basin. Based on the detailed description on the cores from the second member bottom of Nenjiang Formation to the third member top of Quantou Formation in CCSD-SK II, the core profile was established, and five common lithology (mudstone, silty mudstone, mudy siltstone, siltstone and sandstone) and eight special lithology (dolomite, volcanic ash, oil shale, lime mudstone, marlite, recrystalline limestone, ostracode clastic limestone and ostracode limestone) were identified. 172 layers of special lithology in CCSD-SK II were recognized with a total thickness of 14. 516m. They are distributed mainly in the Qingshankou Formation, the second and third member of Yaojia Formation and Nenjiang Formation. The detailed description of the cores and identification on special lithology are the base for the next studies.
734.
Water content and dry bulk density of pilot core to CON01-603-2, PANGAEA
733.
Viscosity of andesite melts and its implication for magma mixing prior to Unzen 1991-1995 eruption
Journal of Volcanology and Geothermal Research,
175
(1-2)
208-217
2008
ISSN: 03770273Keywords:▾
Igneous rocks; Iron; Mixing; Viscometers; Viscosity, andesite melt; dissolved water; magma mixing; redox state of iron; Unzen, Hydrodynamics, andesite; magma chamber; redox conditions; rhyolite; viscosity; volcanic eruption; water content, Asia; Eurasia; Far East; Japan; Kyushu; Nagasaki [Kyushu]; Unzen Volcano
Abstract: ▾ The viscosity of an iron-bearing melt with composition similar to Unzen andesite was determined experimentally in the high (109-1010.5 Pa s) and low (5-1000 Pa s) viscosity range using a parallel plate viscometer and the falling sphere method, respectively. Falling sphere experiments were carried out in an internally heated argon pressure vessel and in a piston cylinder apparatus at 1323 to 1573 K and 200 to 2000 MPa. Creep experiments were performed in the temperature range of 747-845 K at 300 MPa. The water content of the melt varies from nominally dry to 6.2 wt.% H2O. The Fe2+/Fetot ratio was determined for each sample in the quenched glass using a colorimetric method. Pressure has minor influence on the viscosity compared with the effect of temperature, water content (main compositional parameter controlling the viscosity) or with the Fe2+/Fetot ratio (especially important at low water content of the melt). Based on our new viscosity data and literature data with measured Fe2+/Fetot ratio we propose a new empirical equation to estimate the viscosity η (in Pa s) of andesitic melts as a function of temperature T (in K), water content w (in wt.%) and Fe2+/Fetot ratio. The derived relationship reproduces the experimental data (87 in total) in the viscosity range from 100.5 to 1013 Pa s with a 1σ standard deviation of 0.17 log units. However, application of this calculation model is limited to Fe2+/Fetot Fetot > 0.3 and to temperatures above Tg. Moreover, in the high viscosity range the variation of viscosity with water content is constrained only by few experimental data and needs verification by additional measurements. The viscosity data are used to interpret mixing processes in the Unzen magma chamber prior to 1991-1995 eruption. We demonstrate that the viscosities of the rhyolite and andesite melts from the two end-member magmas are nearly identical prior and during mixing, enabling efficient magma mixing. © 2008 Elsevier B.V. All rights reserved.
732.
Using open hole and cased-hole resistivity logs to monitor gas hydrate dissociation during a thermal test in the mallik 5L-38 research well, Mackenzie Delta, Canada
Petrophysics,
49
(3)
285-294
2008
ISSN: 15299074Abstract: ▾ Gas hydrates, which are naturally occurring ice-like combinations of gas and water, have the potential to provide vast amounts of natural gas from the world's oceans and polar regions. However, producing gas economically from hydrates entails major technical challenges. Proposed recovery methods such as dissociating or melting gas hydrates by heating or depressurization are currently being tested. One such test was conducted in northern Canada by the partners in the Mallik 2002 Gas Hydrate Production Research Well Program. This paper describes how resistivity logs were used to determine the size of the annular region of gas hydrate dissociation that occurred around the wellbore during the thermal test in the Mallik 5L-38 well. An open-hole logging suite, run prior to the thermal test, included array induction, array laterolog, nuclear magnetic resonance and 1.1-GHz electromagnetic propagation logs. The reservoir saturation tool was run both before and after the thermal test to monitor formation changes. A cased-hole formation resistivity log was run after the test.Baseline resistivity values in each formation layer (Rt) were established from the deep laterolog data. The resistivity in the region of gas hydrate dissociation near the wellbore (Rxo) was determined from electromagnetic propagation and reservoir saturation tool measurements. The radius of hydrate dissociation as a function of depth was then determined by means of iterative forward modeling of cased-hole formation resistivity tool response. The solution was obtained by varying the modeled dissociation radius until the modeled log overlaid the field log. Pretest gas hydrate production computer simulations had predicted that dissociation would take place at a uniform radius over the 13-ft test interval. However, the post-test resistivity modeling showed that this was not the case. The resistivity-derived dissociation radius was greatest near the outlet of the pipe that circulated hot water in the wellbore, where the highest temperatures were recorded. The radius was smallest near the center of the test interval, where a conglomerate section with low values of porosity and permeability inhibited dissociation. The free gas volume calculated from the resistivity-derived dissociation radii yielded a value within 20 per cent of surface gauge measurements. These results show that the inversion of resistivity measurements holds promise for use in future gas hydrate monitoring. © 2008 Society of Petrophysicists and Well Log Analysts. All rights reserved.
731.
Three-dimensional seismic attenuation structure around the SAFOD site, Parkfield, California
Bulletin of the Seismological Society of America,
98
(6)
2934-2947
2008
ISSN: 00371106Keywords:▾
Alternating methods; California; Cenozoic; Earthquake datum; Event locations; Fault zones; Fluid pathways; Low velocities; Portable arrays; Q-values; Salinian basements; San andreas faults; Seismic attenuations; Seismic networks; Seismogenic zones; Site response; Source parameters; Three-dimensional (3D), Buildings; Crack closure; Earthquakes; Fluids; Sedimentology; Tectonics; Three dimensional, Sedimentary rocks, data inversion; fault zone; P-wave; S-wave; San Andreas Fault; seismic attenuation; seismic data; seismic source; three-dimensional modeling, California; North America; Parkfield; United States
Abstract: ▾ We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P- and S-wave t*-values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t* and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t*-values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low-Q area (Q p and Q s about 50-75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high-Q feature (Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low-Q feature (Q p and Q s about 50-80) attributed to a wedge of sedimentary rocks; uniformly low Q p- and Q s-values suggest that the wedge is fluid rich. A low-Q basin feature (Q p and Q's about 50-75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high-Q feature (Q p and Q s about 220-300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q-values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.
730.
The neglected side of speciation in ancient lakes: Phylogeography of an inconspicuous mollusc taxon in lakes Ohrid and Prespa
Hydrobiologia,
615
(1)
141 – 156
2008
ISSN: 15735117Keywords:▾
Eurasia; Europe; Lake Mikri Prespa; Lake Ohrid; Southern Europe; Bivalvia; Mollusca; Pisidium; Pisum sativum; Sphaeriidae (bivalves); Biodiversity; Electric network analysis; Graph theory; Network protocols; Sensor networks; endemic species; fauna; lake; mollusc; morphometry; phylogenetics; phylogeography; speciation (biology); taxonomy; Ancient lakes; Cryptic speciation; Morphometrics; Phylogenetics; Pisidium; Pisidium edlaueri; Pisidium maasseni; Lakes
Abstract: ▾ The morphologically remarkable endemic fauna within ancient lakes has received much attention in the literature. More inconspicuous taxa, however, often lack detailed molecular and morphometrical examination, although their proportion of the endemic fauna of an ancient lake must not be underestimated. Consequently, a better understanding of evolutionary patterns and processes within these lakes requires more knowledge about the often-neglected inconspicuous taxa. In the present study, we focus on the notoriously cryptic pea clam genus Pisidium (Bivalvia: Sphaeriidae). Though the genus is widely distributed, most endemic species are reported only from ancient lakes, including the European ancient sister lake system of Ohrid and Prespa on the Balkan Peninsula. Here we test for the first time hypotheses on the evolution of the endemic pea clams in this European biodiversity hotspot by molecular means. Combining a broad 16S phylogeny (comprising most European pea clam species), network analyses and morphometrical analyses, we found interesting biogeographical patterns and provide evidence for cryptic species in both lakes. Furthermore, we confirmed the proposed sister-species relationship of the endemics P. edlaueri in Lake Ohrid and P. maasseni in Lake Prespa, and we suggest scenarios of the endemic pea clam evolution within both lakes. The patterns of speciation found in the genus Pisidium are compared to patterns in morphologically distinct molluscan groups in lakes Ohrid und Prespa. © 2008 Springer Science+Business Media B.V.
729.
Signature of long supercycles in the Pleistocene history of Asian limnic systems
Journal of Paleolimnology,
40
(1)
445 – 452
2008
ISSN: 09212728Keywords:▾
Asia; Eurasia; Far East; Hvsgol Aymag; Lake Baikal; Lake Khubsugul; Mongolia; Russian Federation; biodiversity; biota; carbon isotope; eccentricity; glacial-interglacial cycle; paleolimnology; periodicity; Pleistocene; sediment chemistry; water budget
Abstract: ▾ The analysis of sediment chemistry and biota in drill cores from Lake Khubsugul in Mongolia (KDP-01) and Lake Baikal in Siberia (BDP-96/1), two great Eurasian freshwater lakes, detected prominent climate and biological events at 460-420 and 670 kyrs BP in addition to the orbital cycles of precession, tilt and eccentricity. The revealed long-term events were associated with notable changes in biodiversity and geography/landscapes, mainly in water budgets and weathering patterns. The span between 460-420 and 670 kyrs BP was the time when the climate and geographic conditions differed from those before and after these events. The corresponding 33-24 m (670-460 kyr) interval of the Khubsugul core lacked the usual signature of the Milankovitch glacial/interglacial cycles. Events of approximately these ages were found in some other continental ecosystems and in oceanic δ13C records. The two events may mark the phases of a 300-500-kyr long supercycle (or megastadial) in the evolution of continental ecosystems. Among other causes (e.g., regional tectonic events), this periodicity, being globally correlated, may be associated with the 400-kyr cycle of the Earth's orbital eccentricity. © 2007 Springer Science+Business Media B.V.