All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
94.
The application of Raman spectroscopy to the study of gas hydrates
Annals of the New York Academy of Sciences,
912859-872
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; water, aqueous solution; conference paper; crystallization; gas; gas analysis; Raman spectrometry
Abstract: ▾ Raman spectroscopy is reviewed with particular emphasis placed on its application to gas hydrates. Experimental examples discussed include studies of the totally symmetric C-H stretching vibration v1 (A1) of methane in both synthetic and natural hydrate samples (from the JAPEX/JNOC/GSC Mallik 2L-38 research well); a comparison of the coupled O-H vibrations of water in the host lattice of CH4 hydrate and ice I(h) at low temperature; and local structural details of the relaxation of the hydrogen-bonded water on crystallization to structure II hydrate of amorphous tetrahydrofuran (THF) aqueous solutions. This paper is intended to be an introduction to Raman spectroscopy with specific examples from research at the National Research Council of Canada, and is aimed at those who wish to apply the technique as a tool to investigate gas hydrates.
93.
1 - Baikal drilling project
In Koji Minoura, Editor
,
Lake Baikal
Page 1-14
Publisher
Elsevier Science B.V.
,
Amsterdam
2000
1-1492.
Numerical experiments for seismic tomography using the Singular Value Decomposition (SVD) technique
Science Reports of the Kyushu University, Department of Earth and Planetary Sciences,
21
(1)
9-18
2000
ISSN: 13480545Abstract: ▾ The purpose of this study is to learn how to consider setting the borehole and arrangement of the measuring instruments for seismic tomography. For this purpose we made an underground structure model of Unzen Volcano based on the results of previous studies, and simulated the seismic tomography identifying this model with the true structure because one of some plans in "Scientific drilling project at Unzen Volcano" is seismic tomography by using a borehole approaching conduit" We located sources and receivers along the borehole and at the surface, and calculated synthetic seismograms at the receiver positions by a 2-D finite difference method. We measured the first P arrival time in the synthetic seismograms, and constructed the observational equation for seismic tomography. We investigated the solution of this equation by the Singular Value Decomposition (SVD) technique. We found we can consider borehole location and arrangement of sources and receivers through such a simulation by Singular values and eigenvalue that got in this way.
91.
UNAM Scientific shallow-drilling program of the Chicxulub impact crater
International Geology Review,
42
(10)
928-940
2000
Abstract: ▾ A scientific drilling program is being carried out by the National Autonomous University of Mexico (UNAM) at the southern sector of the Chicxulub impact crater in the Yucatan Peninsula, Mexico. Eight boreholes, ranging in depth from 60 m to 702 m, with a total of 2.62 km of continuos core, were recovered. A high recovery rate of up to 99% (overall average recovery rate for the eight boreholes is 87%) allows us to investigate in detail the stratigraphy of the impact lithologies and the Tertiary carbonate sequence. Three of the boreholes (UNAM-5, UNAM-6, and UNAM-7, with core recovery rates from 89 to 99%) sampled impact breccias that were classified in two units-an upper breccia sequence rich in basement clasts, impact glass, and fragments of melt (suevite-like breccia) and a lower breccia sequence rich in limestone, dolomite, and evaporite clasts (bunte-like breccia). Depths of contact between the Tertiary carbonate sequence and the impact breccias are 332.0 m in UNAM-5, 222.2 m in UNAM-7, and 282.8 m in UNAM-6, giving the depth to the K/T boundary. In UNAM-7, the contact between the upper and the lower breccias is at 348.4 m, which yields a thickness of 126.2 m for the suevite-like breccia. The rest of the boreholes sampled part of the Tertiary carbonate sequence (∼200 m thick), composed mainly of limestones, dolomitized carbonates, and calcarenite, with some fossiliferous horizons.
90.
The Parkfield, California earthquake experiment: An update in 2000
Current Science,
79
(9)
1226-1236
2000
ISSN: 00113891Abstract: ▾ The US Geological Survey, in cooperation with other institutions, continues to monitor the San Andreas Fault (SAF) near Parkfield, California, hoping to capture high resolution records of continuous deformation before, during and after a magnitude 6 earthquake, as well as the details of its rupture initiation and strong ground motion. Despite the failure of the prediction that the next M 6 Parkfield earthquake would occur before 1993, Parkfield still has a higher known probability (1 to 10% per year) than anywhere else in the US of a M 6 or greater earthquake. Parkfield instrumentation is still largely in place, although there have been losses due to attrition as well as improvements made possible by new technology. Most Parkfield data sets are now available via the Internet, and all others may be obtained upon request from individual investigators. Detailed seismic monitoring has shown that events with identical seismograms, recurring in exactly the same locations, account for a high proportion of the background seismicity at Parkfield. Geophysical studies have revealed that fault zone seismic and electrical properties are consistent with high fluid content. The rate of interseismic slip on the SAF changed significantly in late 1992 or early 1993, during a period of relatively high seismic activity. The strain-rate change, measured by borehole tensor strainmeters and the two-colour electronic distance-measuring network, was also manifested as shortened recurrence intervals of repeating microearthquakes. Whether or not the accelerated deformation turns out to be an intermediate-term precursor to the next M 6 Parkfield earthquake, documenting the variation of interseismic strain rates with time has important implications for fault dynamics and seismic hazard estimation. Two possible instances of pre-earthquake signals have been recorded at Parkfield: water-level and strain changes over a period of three days prior to the nearby 1985 Mw 6.1 Kettleman Hills, California, earthquake and anomalous electromagnetic signals prior to the M 5 earthquake near Parkfield on 20 December 1994. Future work planned at Parkfield includes a National Science Foundation proposal to construct an SAF Observatory at Depth (SAFOD), as part of the Earthscope initiative. The Observatory will consist of a 4-km-deep borehole to penetrate the SAF and a shallow microearthquake cluster on Middle Mountain, directly above the hypocenter of the 1966 Parkfield earthquake.
89.
Sedimentation processes and new age constraints on rifting stages in Lake Baikal: Results of deep-water drilling
International Journal of Earth Sciences,
89
(2)
183 – 192
2000
ISSN: 14373254Keywords:▾
Russian Federation; depositional environment; paleoclimate; paleoenvironment; Pliocene; Quaternary; rifting; sedimentation rate
Abstract: ▾ With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5-1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60-70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07-1.31 Ma, a date which exceeds the age of previous geological models.
88.
Results from a TEM survey for gas hydrate delineation in permafrost
SEG Technical Program Expanded Abstracts,
19
(1)
1347-1350
2000
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Electric logging; Gases; Hydration; Permafrost; Petroleum prospecting; Surveys, Electrically conductive; Hydrate zones; Induced polarization; Resistive zones; TEM surveys; Time domain electromagnetics, Gas hydrates
Abstract: ▾ In conjunction with the 1998 Mallik 2L-38 gas-hydrate research well program (Dallimore et al, 1999), two time-domain electromagnetic sounding profiles were surveyed in an attempt to delineate a known gas-hydrate zone at depth. The research well is located on Richard's Island in the Mackenzie Delta, Northwest Territories. Electrical resistivity logging at the Mallik site in 1972 and 1998 defined a resistive zone from 900 m to 1100 m coincident with a gas-hydrate zone lying within unfrozen and electrically conductive sediments. A resistive layer, interpreted as the gas-hydrate zone was detected by the TEM survey at all soundings except for those overlain by thin permafrost and a thick unfrozen zone. This thin permafrost/thick unfrozen zone assemblage resulted in greater than expected conductivity-thickness product and limited the depth investigation of the survey. In addition, late-time negative transients interpreted as induced polarization effects, obscured the clear detection of gas hydrates. © 2000 Society of Exploration Geophysicists.
87.
Relation between gas hydrate and physical properties at the Mallik 2L-38 research well in the Mackenzie delta
Annals of the New York Academy of Sciences,
91294-100
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; natural gas, Arctic; Canada; conference paper; gas; gas analysis; porosity; river; salinity; sand; sediment; temperature; water content
Abstract: ▾ As part of an interdisciplinary field program, a 1150-m deep well was drilled in the Canadian Arctic to determine, among other goals, the location, characteristics, and properties of gas hydrate. Numerous physical properties of the host sediment were measured in the laboratory and are presented in relation to the lithology and quantity of in situ gas hydrate. Profiles of measured and derived properties presented from that investigation include: sediment wet bulk density, water content, porosity, grain density, salinity, gas hydrate content (percent occupancy of non-sediment grain void space), grain size, porosity, and post-recovery core temperature. The greatest concentration of gas hydrate is located within sand and gravel deposits between 897 and 922 m. Silty sediment between 926 and 952 m contained substantially less, or no, gas hydrate perhaps because of smaller pore size.
86.
Recycled oceanic crust observed in 'ghost plagioclase' within the source of Mauna Loa lavas
Nature,
404
(6781)
986-990
2000
ISSN: 00280836DOI:10.1038/35010098
Keywords:▾
basalt; mantle plume; melt inclusion; oceanic crust; petrogenesis; petrology; recycling; strontium, article; geography; geology; natural science; oceanic regions; priority journal; United States; volcano, United States
Abstract: ▾ The hypothesis that mantle plumes contain recycled oceanic crust is now widely accepted. Some specific source components of the Hawaiian plume have been inferred to represent recycled oceanic basalts, pelagic sediments or oceanic gabbros. Bulk lava compositions, however, retain the specific trace- element fingerprint of the original crustal component in only a highly attenuated form. Here we report the discovery of exotic, strontium-enriched melt inclusions in Mauna Loa olivines. Their complete trace-element patterns strongly resemble those of layered gabbros found in ophiolites, which are characterized by cumulus plagioclase with very high strontium abundances. The major-element compositions of these melts indicate that their composition cannot be the result of the assimilation of present-day oceanic crust through which the melts have travelled. Instead, the gabbro has been transformed into a (high-pressure) eclogite by subduction and recycling, and this eclogite has then been incorporated into the Hawaiian mantle plume. The trace-element signature of the original plagioclase is present only as a 'ghost' signature, which permits specific identification of the recycled rock type. The 'ghost plagioclase' trace-element signature demonstrates that the former gabbro can retain much of its original chemical identity through the convective cycle without completely mixing with other portions of the former oceanic crust.
85.
Overlapping volcanoes: The origin of Hilo Ridge, Hawaii
Geology,
28
(6)
547 – 550
2000
ISSN: 19432682
Publisher: Geological Society of America
Keywords:▾
Hawaii [(ISL) Hawaiian Islands]; Hawaii [United States]; Hawaiian Islands; Hilo; Kohala; Mauna Kea; United States; Isotopes; Hilo Ridge; Isotope ratios; Kohala; Mauna Kea; Scientific drilling; isotopic ratio; shoreline; submarine; terrace; volcanism; volcano; Volcanoes
Abstract: ▾ The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano, but is crossed at ~1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the Hawaii Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism.
84.
Occurrences of natural gas hydrates beneath the permafrost zone in Mackenzie Delta. Visual and X-ray CT imagery
Annals of the New York Academy of Sciences,
9121021-1033
2000
ISSN: 00778923
Publisher: New York Academy of Sciences
Keywords:▾
methane; natural gas, Canada; computer assisted tomography; conference paper; gas; gas analysis; oil industry; porosity; sand; sediment; X ray analysis
Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 research well was drilled to a depth of 1150 m beneath the permafrost zone in the Mackenzie Delta, N.W.T., Canada, early in 1998. A large amount of natural gas hydrates were successfully retrieved from a variety of sandy and gravel sediments. Over 110 m of gas hydrate-bearing sediments were found to be distributed between 897 m and 1100 m deep. Approximately 37 meters of core were recovered in this interval with most of the recovered gas hydrates being less than 2 mm in size occurring mainly in intergranular porosity of silty to clean massive sand and conglomerate (granule to pebble). Typically, hydrate-bearing strata were between 10 cm and more than one meter thick with an estimated porosity of 25 to 35%. The largest form of hydrate was about 2 cm in diameter, occurring as clasts and intergranular porosity within granular sands. Occurrences of natural gas hydrate have been observed visually at the drill site and in core samples preserved in pressurized storage vessels utilizing an X-ray CT scanner technique. Quantitative assessments of gas hydrate concentrations in core samples have been made based on pressure response of dissociation vessels and direct volumetric measurements. Six types of gas hydrate have been recognized: (1) pore-space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The X-ray CT images proved useful for characterizing macroscopic forms of gas hydrate. Finer grained occurrences were more difficult to study, however the distribution of gas hydrates and granular grains ran be recognized. The occurrences of natural gas hydrates in the Mallik well are compared to the previous natural gas hydrate core samples obtained from ODP/DSDP programs and other field studies.
83.
Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust
Tectonophysics,
301
(3)
191-215
1999
ISSN: 0040-1951Keywords:▾
UHP rocks, seismic properties (V and V), densities, crustal lithology, delamination
Abstract: ▾ The Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then rapidly exhumed from the mantle back into the crust. About 30 UHP rock samples representing major lithologies were collected from surface exposures and their physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure). The experimentally determined data include compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of Vp and Vs. Using a regional geotherm, velocity–depth profiles were calculated for the different lithologies spanning compositions from felsic through intermediate to mafic, and metamorphic grades up to granulite and eclogite facies. The velocity data, in particular the Poisson ratio (Vp/Vs ratio), were used to interpret lithologically the seismic models evaluated from seismic refraction data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper-lower and lowermost crust) with an average thickness of 34 km. From our results, we infer that a mixture of about 90% felsic gneiss with variable amounts of high Vp amphibolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both the Vp and Poisson ratio of the upper-lower and lowermost crust, respectively. From the combined Vp and Poisson ratio (Vp/Vs) data, we conclude that eclogite is not a major constituent of the present Dabie deep crust. Combining the laboratory-derived in-situ seismic data (Vp and Poisson ratio) with refraction seismic data, along with evidence from geological and geochemical investigations, suggests that lower crustal delamination played an important role in the modification of the East China crust.
82.
Production potential of the Mallik field reservoir
Bulletin of the Geological Survey of Canada
(544)
377-390
1999
ISSN: 00687626Abstract: ▾ Immense volumes of naturally occurring gas hydrate in different parts of the world, onshore and offshore, have encouraged the belief that gas hydrate in the next century may become a viable energy resource. Various issues need to be resolved to convert gas hydrate from an energy resource to an energy reserve of real commercial value. The production capability of a gas hydrate reservoir and the gas production technique that could be utilized should be addressed through geological and petrophysical studies, well-production tests and reservoir simulation. To make the simulation of practical value, the controlling mechanisms of fluid flow, kinetics, and heat transfer should be incorporated in the model. The Mallik gas hydrate accumulation in the Mackenzie Delta has exhibited promising potential to be considered a gas reserve through the assessments made of the Mallik L-38 and 2L-38 wells. The data available from both wells and the results of production tests in JAPEX/JNOC/GSC Mallik L-38 gas hydrate research well accommodate basic requirements for comprehensive modelling of the reservoir and production of gas from the in situ gas hydrate through various methods.
81.
Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan)
Journal of Volcanology and Geothermal Research,
89
(1-4)
255 – 273
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; gravity anomaly; lava dome; volcanic eruption
Abstract: ▾ Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943-1945: Showa-Shinzan and 1977-1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60 ± 10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).
80.
Pore-water geochemistry, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
165-175
1999
ISSN: 00687626Abstract: ▾ A pore-water research program was designed to measure dissolved components in interstitial water from sediment core samples collected during the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research-well project. Pore waters from the gas-hydrate-bearing samples had an average salinity of 8 ppt compared to 34 ppt for non-gas-hydrate-bearing samples. The difference in salinities suggests that 80-90% of the pore space in the gas-hydrate-bearing sediment was filled with gas hydrate, which dissociated during recovery. Potassium concentration was also measured in pore water, to estimate the amount of drill-mud contamination in pore-water samples, since the drill mud contained brine solution made from potassium chloride. On average, pore-water salinities were estimated to be enhanced by 2 ppt due to drill-mud contamination.
79.
Physical properties of sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
95-100
1999
ISSN: 00687626Abstract: ▾ A 1150 m deep gas hydrate research well was drilled in the Canadian Arctic in February and March 1998 to investigate the interaction between the presence of gas hydrate and the natural conditions presented by the host sediments. Profiles of the following measured and derived properties are presented from that investigation: water content, sediment wet bulk density, grain size, porosity, gas hydrate quantity, and salinity. These data indicate that the greatest concentration of gas hydrate is located within sand and gravel deposits between 897 m and 922 m. American Society for Testing and Materials 1997: Standard test method for specific gravity of soil solids by gas pycnometer D 5550-94; in American Society for Testing and Materials, Annual Book of ASTM Standards, v. 04.09, Soil and Rock, West Conshohocken, Pennsylvania, p. 380-383.
78.
Overview of science program, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
11-17
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in February and March, 1998, in the Mackenzie Delta, Northwest Territories, Canada, to a depth of 1150 m. The scientific program was conducted through a collaborative agreement between the Japan National Oil Corporation and the Geological Survey of Canada with key participation by the Japan Petroleum Exploration Company and the United States Geological Survey. A primary objective of the well was to undertake a comprehensive scientific research program to study an arctic gas hydrate accumulation. Field research conducted as part of the Mallik 2L-38 program included collection of permafrost and gas-hydrate-bearing core samples, downhole geophysical logging, and a vertical seismic profile survey. Laboratory and modelling studies undertaken during the field program, and subsequently as part of a post-field research program, documented the sedimentology, biostratigraphy, physical/petrophysical properties, pore-water and gas geochemistry, geophysics, and reservoir characteristics of the Mallik field gas hydrate accumulation.
77.
Petrophysical environment of sediments hosting gas hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
109-124
1999
ISSN: 00687626Abstract: ▾ A petrophysical study has been performed on mudstone and sandstone samples from depths of 880-950 m to determine the petrophysical controls on gas hydrate distribution in the sedimentary sequence at the J APEX/JNOC/GSC Mallik 2L-38 well site, Northwest Territories, Canada. Within the cored interval of the Mallik 2L-38 well gas hydrate is hosted in two sandstone horizons with overlying and underlying mudstone horizons, with minor gas hydrate concentrations within some mudstone formations. Results indicate that, although the interbedded mudstone units have relatively high porosities (24-30%) and are at relatively shallow depths, they have a well developed framework-supported texture, probably due to high silt and sand content (56-78 weight per cent), and a maximum burial depth greater than present. Regardless of this, the minor matrix content (13-25 weight per cent) controls the fluid transport characteristics, resulting in extremely low mudstone permeability sections (2×10-21 m to 2×10-19 m2). There are indications that these low permeabilities and the storage pore sizes contribute to the gas hydrate distribution.
76.
Petrology of the 1991-1995 eruption at Unzen: Effusion pulsation and groundmass crystallization
Journal of Volcanology and Geothermal Research,
89
(1-4)
173 – 196
1999
ISSN: 03770273Keywords:▾
Japan; dacite; petrology; volcanic eruption
Abstract: ▾ Effusive eruption of dacite magma (2.1 X 108 m3) during 1991-1995 formed a lava dome at the summit of Unzen Volcano, Japan. The effusion rate was highest at the beginning, 4.0 X 105 m3/day (4.6 m3/s), and decreased roughly with time, to almost zero before this pattern was repeated with a second pulse of magma supply. The whole-rock chemistry of lavas shows significant variation attributable to variations in phenocryst abundance; the more mafic, the more abundant the phenocrysts. The pattern of chemical variation with time shows some difference from that of the effusion rate. All phenocrysts in dacite (plagioclase, hornblende, biotite, quartz and magnetite) show evidence of disequilibrium with melt. Although a glomerophyric aggregation of phenocrysts suggests coexistence with each other, phenocrysts are isotopically heterogeneous from species to species. The calculated initial melt composition was rhyodacite, and was nearly constant throughout the activity. In contrast, the bulk phenocryst population is andesite. A model explaining the textures and the isotopic heterogeneity is the capture of diorite fragments (or xenocrysts) by parental rhyodacite magma. It is suggested that, when effusion rate was high, less viscous crystal-poor magma exited from the reservoir. Groundmass glass and plagioclase microlite rims show temporal chemical variations correlating with the effusion rate; the higher the effusion rate, the more evolved the compositions. Groundmass crystallinity increased with decreasing effusion rate; from 33% to 50%. Textures in dome lavas suggest that groundmass crystallization had been mostly completed when magma reached the conduit top. The Fe-Ti oxide temperature (880-780°C) was low when the crystallinity was high. Micropumice erupted before dome growth provided a sample recording magmatic foam in the conduit. Porosity of dome lavas was lower at lower effusion rates. Collapse of foam magma and simultaneous escape of volatiles through the conduit top were probably responsible for the accompanying low-frequency earthquakes. Phenocrysts were broken and the breakdown rims on hornblende phenocrysts were torn off during collapse and successive compaction. When effusion waned, degassing and the resultant crystallization proceeded more completely, so that the magma became too viscous to flow in the conduit top and behaved as a plug, resulting in a temporary halt of effusion. In turn, groundmass crystallization in magma below the plug increased excess pressure in the upper parts of conduit due to slow cooling. The plug was scavenged when rising excess pressure overcame its effective strength. Then, the second pulse of magma supply began. Strong endogenous growth and extrusion of a lava spine in the later stage probably occurred for the same reason.
75.
Petrology and Fe-Ti oxide reequilibration of the 1991 Mount Unzen mixed magma
Journal of Volcanology and Geothermal Research,
89
(1-4)
213 – 230
1999
ISSN: 03770273Keywords:▾
Japan; dacite; geobarometry; magma chemistry; petrology; volcanic eruption
Abstract: ▾ A dacitic magma (64.5 wt.% SiO2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991-1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790 ± 20°C according to Fe-Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H2O-saturated) and possibly as high as 1130°C (2 wt.% H2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850°to 870°C. The groundmass Fe-Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890 ± 20°C temperatures; the oxide-oxide contacts give temperatures of 910 ± 20°C. The 900 ± 30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe-Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.
74.
Palynology, age, and paleoenvironmental interpretations from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well
Bulletin of the Geological Survey of Canada
(544)
81-93
1999
ISSN: 00687626Abstract: ▾ The JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well was drilled in 1998 to investigate the geological, geochemical, geophysical, and engineering properties of a gas hydrate accumulation previously identified in the Mallik L-38 well. Palynological analysis of core and cuttings from 670 m to 1150 m (TD) are reported here. Detailed quantitative analysis has been done on the 886-952 m cored interval that hosts the main gas hydrate accumulation. The pollen and spore evidence suggests the following biostratigraphic subdivisions for the 670-1150 m succession: 670-785 m, Late Miocene or older; 775-897 m, within the range of Early Miocene to Late Eocene; 897-930 m, probably Late Eocene; 930-995 m, Late Eocene; and 995-1151m within the range Early to Middle Eocene. Below 930 m the rocks are best assigned to the Richards and upper Taglu sequences. A dominantly continental succession is indicated, with a marginal marine and/or estuarine episode between about 945 m and 948 m, in the Late Eocene. The dinoflagellates in this interval are considered to be indigenous to the sampled rock. There is evidence of two episodes of edaphic-climatic dryness in the Late Eocene and probable Late Eocene.
73.
Overview of the 1990-1995 eruption at Unzen Volcano
Journal of Volcanology and Geothermal Research,
89
(1-4)
1 – 22
1999
ISSN: 03770273Keywords:▾
Japan; earthquake; lava dome; pyroclastic flow; volcanic eruption
Abstract: ▾ Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1 x 108 m3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230-540 m high). The eruption finished with extrusion of a spin endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.
72.
Regional gas hydrate occurrences, permafrost conditions, and Cenozoic geology, Mackenzie Delta area
Bulletin of the Geological Survey of Canada
(544)
31-43
1999
ISSN: 00687626Abstract: ▾ The occurrence of natural gas hydrate within Cenozoic sediments of the Mackenzie Delta-Beaufort Sea region has been well documented. In preparation for the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, a detailed evaluation of terrestrial gas hydrate occurrences was undertaken to assess the geological setting, sediment associations, pressure and temperature conditions, and the presence of free gas in the Mackenzie Delta-Beaufort Sea region. After an exhaustive review, it was determined that the Mallik L-38 site, drilled by Imperial Oil in 1972, offered the highest probability of encountering a thick gas hydrate occurrence with high gas hydrate concentrations. On the basis of openhole well-log evaluation, it was estimated that about Him of gas-hydrate-bearing strata occur at this location from 810.1 to 1102.3 m, within the zone of predicted methane hydrate stability and below the base of icebearing permafrost, estimated to be at 640 m.
71.
Properties of samples containing natural gas hydrate from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, determined using Gas Hydrate And Sediment Test Laboratory Instrument (GHASTLI)
Bulletin of the Geological Survey of Canada
(544)
241-250
1999
ISSN: 00687626Abstract: ▾ As part of an ongoing laboratory study, preliminary acoustic, strength, and hydraulic conductivity results are presented from a suite of tests conducted on four natural-gas-hydrate-containing samples from the Mackenzie Delta JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. The gas hydrate samples were preserved in pressure vessels during transport from the Northwest Territories to Woods Hole, Massachusetts, where multistep tests were performed using GHASTLI (Gas Hydrate And Sediment Test Laboratory Instrument), which recreates pressure and temperature conditions that are stable for gas hydrate. Properties and changes in sediment behaviour were measured before, during, and after controlled gas hydrate dissociation. Significant amounts of gas hydrate occupied the sample pores and substantially increased acoustic velocity and shear strength.
70.
Scientific results from JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well.
In Dallimore S.R., Uchida T., Collett T.S., Editor
Publisher
Geological Survey of Canada
1999
ISBN:
0660177773