All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
219.
REE inverse modeling of HSDP2 basalts: Evidence for multiple sources in the Hawaiian plume
Geochemistry, Geophysics, Geosystems,
4
(2)
2003
ISSN: 15252027Abstract: ▾ The rare earth element (REE) concentrations of lavas from the Hawaiian Scientific Drilling Project (HSDP2) can be used to provide additional constraints on phase equilibria and the nature of the Hawaiian source. Major element analyses separate Mauna Kea lavas into two distinct populations, a high-silica and a low-silica suite. The low-silica samples can be separated stratigraphically into an upper low-silica alkalic series and a low-silica tholeiitic group that occurs deeper in the section. These contrasting groups could result from different extents of source partial fusion, or lithologically distinct source regions, or some combination of both factors. Petrologic modeling is performed to calculate that primary magma compositions contain about 20% MgO, and can be formed by 8-15% melting of a depleted mantle source for low-silica alkalic and high-silica lavas, respectively. The low-silica tholeiites could be generated by higher degrees of melting of a more fertile source. REE ratios and various isotopic systems reinforce the division of the low-SiO2 samples into the upper alkalic series, characterized by high Gd/Yb, and the deeper low-silica tholeiitic group, with low Gd/Yb. REE inverse modeling of fractionation-corrected basalts is consistent with lower degrees of melting to generate the late-stage alkalic lavas, with garnet present as a residual phase. The relatively constant Gd/Yb for low-silica tholeiites suggests that garnet is not an important residual phase during partial melting, implying higher extents of melting. The low-silica tholeiites are characterized by relatively enriched isotopic signatures that are consistent with contributions from a primitive source or from recycled subduction components. High 3He/4He associated with the lowsilica lavas could derive from primitive mantle, mass transfer from the core, or from a refractory lithospheric contribution to a recycled subduction package. However, the combination of major element, REE and isotopic data suggests that the deeper low-silica suite is sampling the relatively fertile, interior part of the Hawaiian plume, whereas the high-silica lavas are extracted from the more depleted periphery; later alkalic lavas are generated from a depleted source as the volcano moves off the hot spot. © 2003 by the American Geophysical Union.
218.
Quasi-continuous depth profiles of rock magnetization from magnetic logs in the HSDP-2 borehole, Island of Hawaii
Geochemistry, Geophysics, Geosystems,
4
(4)
2003
ISSN: 15252027Abstract: ▾ A quasi-continuous magnetic log has been obtained in the Hawaii Scientific Drilling Project 2 (HSDP-2) between 600 m and 1800 m, which corresponds to a time interval of approximately 350 ka to 480 ka. A tri-axial borehole magnetometer was employed to measure the horizontal and vertical magnetic fields. Measurements were taken in downhole and uphole runs, with a good correlation between the two. In a first step the logs were corrected for the transfer function of the employed low-pass filter and then for the logging depths. To calculate rock magnetizations from magnetic components, we used a multidisk cylindrical model for the penetrated rocks. The disk thickness corresponds with 0.1 m to the logging sampling rate. Magnetic borehole logging in the HSDP-2 hole has established the following: Massive lava flows can be distinguished from those with prevailing hyaloclastites and enables us to supplement the lithology, especially in depth intervals with poor core recovery. The inclinations of rock magnetization derived from the magnetic log agree well with those measured in core samples from HSDP-2 hole. The same applies to magnitudes of magnetizations from logging, as the sum of induced and remanent magnetizations, with laboratory determinations of the remanent magnetizations of core samples. We observe a distinct discrepancy between the local present-day geocentric axial dipole (GAD) inclination of 35.6° and the mean logging inclination of 22.7°. Furthermore, a systematic inclination decrease with depth is observed. Logging and core inclinations in HSDP-2 can be brought into agreement with core inclinations in the HSDP-1 pilot hole by shifting depths of HSDP-1 100 m downward. The correlation of inclination data between the two boreholes and the known age-depth relation of HSDP-1 is used to reexamine the "age versus depth" model curve for Mauna Kea of Sharp et al. (manuscript in preparation, 2003). We identify tentatively two logged inclination minima with excursion of the geomagnetic field in Brunhes, namely, "Levantine" at 360-370 ka and "Unknown" at 400-420 ka. © 2003 by the American Geophysical Union.
217.
The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP-2 drill core
Geochemistry, Geophysics, Geosystems,
4
(5)
2003
ISSN: 15252027Abstract: ▾ [1] We analyzed Pb isotopic compositions of 50 samples from the HSDP-2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple-spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/ 204Pb from 37.97 to 38.22. In 207Pb/204Pb- 206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/ 204Pb-206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as "Kea-lo8," "Kea-mid8," and "Kea-hi8." The 206Pb/204Pb isotope ratios exhibit rapid shifts by ̃0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ̃100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP-2 in terms of mixing of end-members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP-2 arrays converge to a common end-member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/ 204Pb ratios. This topology suggests that the HSDP-2 arrays were produced by mixing of at least four end-members. The origin of these end-members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end-member of the three Pb isotope arrays contains material with elevated m values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP-2 Kea-lo8, Keamid8, and Kea-hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea-lo8 array is present between ̃180 and 370 ka and samples more peripheral parts of the plume, while the Kea-mid8 and Kea-hi8 arrays occur in the deeper parts of the core (̃370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding "lengths" of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small-scale heterogeneities derived from the D″ layer in the lower mantle. © 2003 by the American Geophysical Union.
216.
Temperatures at the base of the Laurentide Ice Sheet inferred from borehole temperature data
Geophysical Research Letters,
30
(18)
CRY 3-1 - 3-4
2003
ISSN: 00948276
Publisher: American Geophysical Union
Keywords:▾
Boreholes; Glaciers; Ice; Melting; Atmospheric temperature; Glacial geology; Surface properties, Melting points; Borehole temperature; Geographic difference; Ground surface temperature; Last Glacial Maximum; Laurentide ice sheets; Surface temperatures; Temperature history; Temperature profiles, Geophysics; Ice, basal temperature; Last Glacial Maximum; Laurentide Ice Sheet; paleoclimate, North America
Abstract: ▾ We use temperature profiles from 4 deep (>1600 m) boreholes across Canada to determine ground surface temperature histories (GSTH's) through and after the Last Glacial Maximum (LGM). Inversion yields the temperature history at the base of the glacier and the surface temperature evolution after the glacial retreat. The results indicate geographic differences in basal temperature history across the Ice Sheet. During the Last Glacial Maximum, temperatures at the base of the Ice Sheet were lower in eastern Canada, at the southeastern edge of the glacier, than in central Canada, southwest of the glacier center. At all sites, basal temperatures were above the melting point of ice during and after the LGM, which may explain the highly unstable character of the Ice Sheet. The GSTH's are consistent with information on the history of the Laurentide ice sheet and provide quantitative constraints on glacier flow dynamics.
215.
Long continental records from Lake Baikal
2003
214.
The Gulf of Corinth: An active half graben?
Journal of Geodynamics,
36
(1-2)
323 – 340
2003
ISSN: 02643707
Publisher: Elsevier Ltd
Keywords:▾
Greece; Gulf of Corinth; active fault; deformation; faulting; graben; neotectonics; rift zone; tectonics
Abstract: ▾ The Gulf of Corinth is often considered as a typical example of a more or less simple half graben with major border faults to the south and a flexure of the northern shore. This paper reviews new data, especially subsurface data, from both onshore and offshore, compiled or acquired through the Corinth Rift Laboratory EEC project. This data indicate that (1) the Gulf of Corinth is bordered both north and south by active faults; (2) there is a lot of them, and not only the one bordering the Peloponnese coastline are still active; and (3) distinct opening phases may be recognized on the area. During the first phase, the depocenter was located near the northern shore, whereas during the most recent phase, and only in the centre and the eastern sector, the depocenter moved towards the southern shore. Furthermore, active tectonic, in this western sector, is characterised by a general uplift of the Peloponnese that leads to the formation of new faults, i.e. the Doumena, Aigion and Helike faults and to selective reactivation of older ones such as the Pirgaki fault. We suggest that this current tectonic phase started about 150-120 000 years ago in the Aigion area and about 350 000 years ago eastward (Corinth-city) and that it represents a third opening phase during the growth of the Gulf of Corinth. © 2003 Elsevier Ltd. All rights reserved.
213.
The heat transfer in the region of the Mauna Kea (Hawaii) - constraints from borehole temperature measurements and coupled thermo-hydraulic modeling
Tectonophysics,
371
(1-4)
23 – 40
2003
ISSN: 00401951
Publisher: Elsevier B.V.
Keywords:▾
Hawaii; Mauna Kea; United States; borehole; heat flow; heat transfer; hydraulic property; temperature gradient; thermal structure
Abstract: ▾ The objectives of this paper are an understanding of the thermal and hydraulic field because of a negative temperature gradient and cold temperatures in the 1-km-deep borehole of the Hawaiian Scientific Drilling Project (HSDP), located near the coast line. The temperature pattern is attributed to a superposition of thermal and hydraulic processes. In the deeper borehole (HSDP-2, depth 3.1 km) detailed temperature monitoring was performed. Temperature measurements reveal two different thermal regimes. The upper part is characterised by cold temperatures and a negative temperature gradient similar to those observed in the shallow pilot borehole. Below 1100 m, increasing temperatures are observed. Different processes, such as topographically driven groundwater flow, ingress of salt water and conductive heat flow are investigated by numerical modeling. A pure conductive scenario fails to match the temperature measurements, implying that both borehole sections are overprinted by advective conditions. Coupled fluid and heat flow modeling with solute transport yield results that agree with observed temperatures. The results of these simulations suggest that meteoric water flow from the mountain range controls the thermal conditions in the upper part of the borehole. Below this level, the thermal regime is additionally affected by circulation of salt water from the nearby ocean. Each of these flow systems has been observed at other locations: topographically driven fresh water at locations with pronounced topography and ingress of salt water is typical for islands or coastal areas. At Hawaii, they coincide and influence each other, resulting in a salt water interface occurring at greater depth than expected. © 2003 Elsevier B.V. All rights reserved.
212.
The hydromechanical behaviour of a fracture: An in situ experimental case study
International Journal of Rock Mechanics and Mining Sciences,
40
(7-8)
1257 – 1270
2003
ISSN: 13651609
Publisher: Elsevier BV
Keywords:▾
Elasticity; Fracture; Granite; Quarrying; Water; Hydromechanical behavior; fracture flow; hydromechanics; in situ stress; rock mechanics; stress measurement; Rock mechanics
Abstract: ▾ Relationships between flow distributions and mechanical opening in a single natural fracture are investigated in situ through field experiments, at a scale of about 1 m, in a granitic quarry. Experiments have been conducted at various injection flow-rates while the normal stress applied to the fracture was controlled by hydraulic flat jacks. Variations of the collected flow-rate (monitored through contiguous flow collectors distributed along the fracture periphery) with the injection pressure are fully reproducible. They show that the fracture opens only above a threshold pressure which increases with the externally applied stress. This threshold is non-zero with no applied flat jacks pressure which raises questions on the reliability of hydraulic jacking techniques for the measurement of the normal stress on preexisting fractures. It is shown that equivalent hydraulic aperture and mechanical opening are comparable only above a critical mean fracture opening estimated to be around 15-20 μm for the tested granite. For mean fracture openings smaller than this value, the standard time scale used for stress measurements distorts the results. It is also shown that channeling effects may control flow away from the injection hole so that the hydraulic jacking stress measurement technique may overestimate the mean normal stress acting on the fracture plane by as much as 4 MPa. It is concluded that hydraulic testing techniques for normal stress measurements should not include results from the fracture opening phase. Moreover, criteria should be established for validating results from the closing phase in order to demonstrate the absence of channelling effects. Finally, it is shown that, because of the elastic response of the rock, water injection in a fracture system decreases the interstitial pressure ahead of the increasing pressure front associated with the water flow. © 2003 Elsevier Ltd. All rights reserved.
211.
The mechanics of unrest at Long Valley caldera, California. 2. Constraining the nature of the source using geodetic and micro-gravity data
Journal of Volcanology and Geothermal Research,
127
(3-4)
219-245
2003
ISSN: 03770273
Publisher: Elsevier
Keywords:▾
Deformation; Gravitational effects; Microgravity processing, Ellipsoidal sources, Geology, caldera; geodetic datum; geometry; geostatistics; GPS; gravity; modeling; uplift, California; Long Valley Caldera; United States
Abstract: ▾ We model the source of inflation of Long Valley caldera by combining geodetic and micro-gravity data. Uplift from GPS and leveling, two-color EDM measurements, and residual gravity change determinations are used to estimate the intrusion geometry, assuming a vertical prolate ellipsoidal source. The U.S. Geological Survey occupied the Long Valley gravity network six times from 1980 to 1985. We reoccupied this network twice, in the summer of 1998 (33 stations), and the summer of 1999 (37 stations). Before gravity data can be used to estimate the density of the intrusion, they must be corrected for the effect of vertical deformation (the free-air effect) and changes in the water table. We use geostatistical techniques to interpolate uplift and water table changes at the gravity stations. The inflation source (a vertical prolate ellipsoid) is located 5.9 km beneath the resurgent dome with an aspect ratio equal to 0.475, a volume change from 1982 to 1999 of 0.136 km3 and a density of around 1700 kg/m3. A bootstrap method was employed to estimate 95% confidence bounds for the parameters of the inflation model. We obtained a range of 0.105-0.187 km3 for the volume change, and 1180-2330 kg/m3 for the density. Our results do not support hydrothermal fluid intrusion as the primary cause of unrest, and confirm the intrusion of silicic magma beneath Long Valley caldera. Failure to account for the ellipsoidal nature of the source biases the estimated source depth by 2.9 km (a 33% increase), the volume change by 0.019 km3 (a 14% increase) and the density by about 1200 kg/m3 (a 40% increase). © 2003 Elsevier B.V. All rights reserved.
210.
The suevite of drill hole Yucatàn 6 in the Chicxulub impact crater
Meteoritics and Planetary Science,
38
(9)
1299-1317
2003
Abstract: ▾ The suevite breccia of the Chicxulub impact crater, Yucatàn, Mexico, is more variable and complex in terms of composition and stratigraphy than suevites observed at other craters. Detailed studies (microscope, electron microprobe, SEM, XRF) have been carried out on a noncontinuous set of samples from the drill hole Yucatàn 6 (Y6) located 50 km SW from the center of the impact structure. Three subunits can be distinguished in the suevite: the upper unit is a fine-grained carbonate-rich suevite breccia with few shocked basement clasts, mostly altered melt fragments, and formerly melted carbonate material; the middle suevite is a coarse-grained suevite with shocked basement clasts and altered silicate melt fragments; the lower suevite unit is composed of shocked basement and melt fragments and large evaporite clasts. The matrix of the suevite is not clastic but recrystallized and composed mainly of feldspar and pyroxene. The composition of the upper members of the suevite is dominated by the sedimentary cover of the Yucatàn target rock. With depth in well Y6, the amount of carbonate decreases and the proportion of evaporite and silicate basement rocks increases significantly. Even at the thin section scale, melt phases of different chemistry can be identified, showing that no widespread homogenization of the melt took place. The melt compositions also reflect the heterogeneity of the deep Yucatàn basement. Calcite with characteristic feathery texture indicates the existence of formerly pure carbonate melt. The proportion of carbonate to evaporite clasts is less than 5:1, except in the lower suevite where large evaporite clasts are present. This proportion constrains the amount of CO2 and SOx released by the impact event.
209.
Three-dimensional crustal structure of Long Valley caldera, California, and evidence for the migration of CO2 under Mammoth Mountain
Journal of Geophysical Research: Solid Earth,
108
(3)
ESE 6-1 - 6-16
2003
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Abstract: ▾ A temporary network of 69 three-component seismic stations captured a major seismic sequence in Long Valley caldera in 1997. We performed a tomographic inversion for crustal structure beneath a 28 km × 16 km area encompassing part of the resurgent dome, the south moat, and Mammoth Mountain. Resolution of crustal structure beneath the center of the study volume was good down to ∼3 km below sea level (∼5 km below the surface). Relatively high wave speeds are associated with the Bishop Tuff and lower wave speeds characterize debris in the surrounding moat. A low-Vp/Vs anomaly extending from near the surface to ∼1 km below sea level beneath Mammoth Mountain may represent a CO2 reservoir that is supplying CO2-rich springs, venting at the surface, and killing trees. We investigated temporal variations in structure beneath Mammoth Mountain by differencing our results with tomographic images obtained using data from 1989/1990. Significant changes in both Vp and Vs were consistent with the migration of CO2 into the upper 2 km or so beneath Mammoth Mountain and its depletion in peripheral volumes that correlate with surface venting areas. Repeat tomography is capable of detecting the migration of gas beneath active silicic volcanoes and may thus provide a useful volcano monitoring tool.
208.
Trace element abundances of Mauna Kea basalt from phase 2 of the Hawaii Scientific Drilling Project: Petrogenetic implications of correlations with major element content and isotopic ratios
Geochemistry, Geophysics, Geosystems,
4
(6)
2003
ISSN: 15252027Abstract: ▾ [1] The temporal geochemical variations defined by lavas erupted throughout the growth of a single volcano provide important information for understanding how the Hawaiian plume works. The Hawaii Scientific Drilling Project (HSDP) sampled the shield of Mauna Kea volcano to a depth of 3100 meters below sea level during Phase 2 of the HSDP. Incompatible element abundance ratios, such as La/Yb, Sm/ Yb, Nb/Zr, and Ti/Zr, in conjunction with SiO2 abundance and radiogenic isotopic ratios, especially He and Pb, in the reference sample suites of the Mauna Kea portion of cores from Phases 1 and 2 of the HSDP define three distinct geochemical groups. The upper 550 m of Mauna Kea lavas in the Phase 2 core include the Postshield Group with eruption ages of ̃200 ka to <370 ka. These lavas have relatively low SiO2 content, 3He/4He and 206Pb/204Pb, and they define a trend to relatively high La/Yb, Sm/Yb, and Nb/Zr. The eruption of these lavas coincides with migration of the Mauna Kea shield off the hot spot. As a result, extent of melting decreased, melt segregation occurred at greater depth, within the garnet stability field, and a geochemically distinct component associated with the periphery of the plume was sampled. Deeper in the Phase 2 core two other geochemical groups of lava are intercalated. One group has relatively low SiO2 abundance and high Nb/Zr Ti/Zr, 3He/4He and high 208Pb/204Pb at a given 206Pb/204Pb. These are distinctive geochemical characteristics of lavas erupted at Loihi seamount. Variations in incompatible element abundance ratios (e.g., Sm/Yb versus Nb/Zr and La/Yb versus Ti/Zr) define mixing trends between these low SiO2 lavas (Loihi-type) and lavas belonging to a high SiO2 group that are the dominant lava type in the shield part of the core (Kea-type). These two groups are presumed to reflect components intrinsic to the plume. Correlations of incompatible element abundance ratios, such as La/Nb, with radiogenic isotope ratios show that Hawaiian shields contain different proportions of geochemically distinctive components. The Koolau shield contains a recycled sedimentary component that is not present in the Mauna Kea shield. The anomalously high Ba/Th in Hawaiian lavas is inferred to be a source characteristic. Ba/Th is correlated with some radiogenic isotope ratios in Kilauea and Mauna Loa lavas, but there is no correlation in Mauna Kea lavas which range in Ba/Th by a factor of 2.6. Copyright 2003 by the American Geophysical Union.
207.
[English]
Crosshole waveform tomography velocity and attenuation images of arctic gas hydrates
Volume 22
,
Page 2255-2258
Publisher
Society of Exploration Geophysicists
2003
Queen's University, Kingston, ON, Canada; GeoForschungsZentrum, Potsdam, GermanyKeywords:▾
Hydration; Petroleum prospecting; Tomography; Velocity, Attenuation factors; High resolution; Hydrate bearing sediments; Large-scale research; Quantitative images; Wave forms, Gas hydrates
206.
[English]
Operation overview of the 2002 mallik gas hydrate production research well program at the Mackenzie delta in the Canadian arctic
Volume 2003-May
,
Page 563-572
Publisher
Offshore Technology Conference
2003
Japan Petroleum Exploration Co., Ltd, Brazil; Japan National Oil Corporation, Brazil; Canadian Petroleum Engineering Inc., BrazilDOI:10.4043/15124-ms
Keywords:▾
Gas hydrates; Gas industry; Gasoline; Hydration; Offshore oil well production; Offshore oil wells; Offshore technology; Oil well logging; Well testing, Gas-hydrate production; Geological Survey of Canada; Japan national oil corporations; Operational management; Petroleum exploration; Reservoir simulation model; Scientific experiments; Thermal stimulation tests, Software testing
ISBN:
9781555632502
205.
Open fissure mineralization at 2600 m depth in Long Valley Exploratory Well (California) – insight into the history of the hydrothermal system
Journal of Volcanology and Geothermal Research,
127
(3)
347-363
2003
ISSN: 0377-0273Keywords:▾
Long Valley Caldera, hydrothermal system, thermal history, fluid inclusions, quartz crystallization, open fissures
Abstract: ▾ Long Valley Exploratory Well, drilled into the Resurgent Dome at Long Valley Caldera (California) to explore the potential of geothermal power in an active magmatic system, achieved temperatures of only ca. 100°C at 2500–3000 m depth, well below the range expected atop an active magma chamber. Open fissures encountered at 2600 m depth are coated by mm-sized idiomorphic quartz crystals with first- and second-order growth discontinuities. Specific growth defects indicating rapid crystallization reflect sudden changes in SiO2 supersaturation. Fluid inclusions contain low salinity (0–5 wt% NaCl) and low CO2 (<3 mole%) aqueous fluids, with V–L homogenization temperatures of 300–350°C, indicating trapping at more than 200°C above the ambient temperatures measured within the borehole today. Fluid composition and inclusion density varies between and within the growth zones, reflecting progressive changes in the hydrothermal system during crystallization. Episodic crystallization from supersaturated fluids is interpreted to reflect sudden changes in the convection pattern, presumably induced by seismic activity, with a more recent and dramatic reorganization resulting in convective cooling. The quartz crystals are sensitive recorders of the earlier higher temperature history, unaffected by the present-day situation.
204.
Oxygen isotope geochemistry of the second HSDP core
Geochemistry, Geophysics, Geosystems,
4
(8)
2003
ISSN: 15252027Abstract: ▾ Oxygen isotope ratios were measured in olivine phenocrysts (̃1 mm diameter), olivine microphenocrysts (generally ̃100-200 mm diameter), glass, and/or matrix from 89 samples collected from depths down to 3079.7 m in the second, and main, HSDP core (HSDP-2). Olivine phenocrysts from 11 samples from Mauna Loa and 34 samples from the submarine section of Mauna Kea volcano have δ18O values that are similar to one another (5.11 ± 0.10%, 1s, for Mauna Loa; 5.01 ± 0.07%, for submarine Mauna Kea) and within the range of values typical of olivines from oceanic basalts (δ18O of ̃5.0 to 5.2%). In contrast, δ18O values of olivine phenocrysts from 20 samples taken from the subaerial section of Mauna Kea volcano (278 to 1037 mbsl) average 4.79 ± 0.13%. Microphenocrysts in both the subaerial (n = 2) and submarine (n = 24) sections of Mauna Kea are on averagẽ0.2% lower in δ18O than phenocrysts within the same stratigraphic interval; those in submarine Mauna Kea lavas have an average δ18O of 4.83 ± 0.11%. Microphenocrysts in submarine Mauna Kea lavas and phencrysts in Mauna Loa lavas are the only population of olivines considered in this study that are typically in oxygen isotope exchange equilibrium with coexisting glass or groundmass. These data confirm the previous observation that the stratigraphic boundary between Mauna Loa and Mauna Kea lavas defines a shift from "normal" to unusually low δ18O values. Significantly, they also document that the distinctive 18O-depleted character of subaerial Mauna Kea lavas is absent in phenocrysts of submarine Mauna Kea lavas. Several lines of evidence suggest that little if any of the observed variations in δ18O can be attributed to subsolidus alteration or equilibrium fractionations accompanying partial melting or crystallization. Instead, they reflect variable proportions of an 18O-depleted source component or contaminant from the lithosphere and/or volcanic edifice that is absent in or only a trace constituent of subaerial Mauna Loa lavas, a minor component of submarine Mauna Kea lavas, and a major component of subaerial Mauna Kea lavas. Relationships between the δ18O of phenocrysts, microphenocrysts, and glass or groundmass indicate that this component (when present) was added over the course of crystallization-differentiation. This process must have taken place in the lithosphere and most likely at depths of between ̃5 and 15 km. We conclude that the low-δ18O component is either a contaminant from the volcanic edifice that was sampled in increasingly greater proportions as the volcano drifted off the center of the Hawaiian plume or a partial melt of low-δ18O, hydrothermally altered perdotites in the shallow Pacific lithosphere that increasingly contributed to Mauna Kea lavas near end of the volcano's shield building stage. The first of these alternatives is favored by the difference in δ18O between subaerial and submarine Mauna Kea lavas, whereas the second is favored by systematic differences in radiogenic and trace element composition between higher and lower δ18O lavas. Copyright 2003 by the American Geophysical Union.
203.
{Diatom turnover in the early Paleogene diatomite of the Sengiley section, middle Povolzhie, Russia: A response to the initial Eocene thermal maximum?}
{Causes and consequences of globally warm climates in the early Paleogene}
Publisher
Geological Society of America
012003
202.
Noble gases in olivine phenocrysts from drill core samples of the Hawaii Scientific Drilling Project (HSDP) pilot and main holes (Mauna Loa and Mauna Kea, Hawaii)
Geochemistry, Geophysics, Geosystems,
4
(1)
2003
ISSN: 15252027
Publisher: Blackwell Publishing Ltd
Abstract: ▾ [1] We have determined concentrations and isotopic compositions of all noble gases in olivine phenocrysts from the Hawaii Scientific Drilling Project (HSDP) drill core, comprising Mauna Loa lavas in the top 247 m and Mauna Kea lavas down to the preliminary depth of 3109 m. Our aim was to describe the long-term isotopic evolution of noble gases over a significant time fraction of the active life of a major Hawaiian volcano. The He isotopic signature displays a clear temporal trend: 3He/4He ratios increase from MORBlike 9 RA in the youngest lavas to 15 RA in the Mauna Loa section and from ̃7 RA to ̃12 RA in the subaerial Mauna Kea deposits. They remain close to 12 RA in most of the submarine Mauna Kea samples, except for a few excursions with 3He/4He ratios of up to 21 RA in borehole depths between 2000 and 2600 m. The average 3He/4He ratio of 12 RA is lower than that observed in recent eruptions of Kilauea and Loihi seamount and supports the idea of a concentrically zoned Hawaiian plume [Kurz et al., 1996]. The Ne isotopic signature does not show a temporal evolution. It remains plume-like (plotting close to the Loihi-Kilauea correlation line in a Ne three-isotope diagram) over the whole Mauna Kea section in those samples which are not dominated by air-like Ne. Maximum 20Ne/22Ne and 21Ne/22Ne ratios reach 12.10 ± 0.36 and 0.0360 ±0.0042, respectively. 40Ar/36Ar ratios vary widely between 360 and ̃3300 in the ≥1000°C release steps due to variable atmospheric contributions. In at least one sample, a 40Ar/ 36Ar ratio of 14,300 ±910 demonstrates the presence of a MORB-like Ar component. Kr and Xe isotopic compositions are atmospheric throughout. We discuss several possibilities on how to explain the isotopic trends of the noble gases and their correlation to other geochemical parameters. Simple admixture of MORB-like noble gases to the plume component cannot account for the observations. We favor a model involving early melt extraction from the outer plume sections, followed by radiogenic ingrowth and, possibly, some interaction with ambient mantle material. © 2003 by the American Geophysical Union.
201.
Fluid flow in the resurgent dome of Long Valley Caldera: Implications from thermal data and deep electrical sounding
Journal of Volcanology and Geothermal Research,
127
(3-4)
329-345
2003
ISSN: 03770273
Publisher: Elsevier
Keywords:▾
Cooling; High temperature effects; Sediment transport; Water, Magma chambers, Geothermal fields, caldera; electrical resistivity; fluid flow; geochemistry; hydrothermal fluid; thermal regime, California; Long Valley Caldera; United States
Abstract: ▾ Temperatures of 100°C are measured at 3 km depth in a well located on the resurgent dome in the center of Long Valley Caldera, California, despite an assumed >800°C magma chamber at 6-8 km depth. Local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a Peclét-number analysis of temperature logs. These analyses reveal zones with fluid circulation at the upper and lower boundaries of the Bishop Tuff, and an upflow zone in the metasedimentary rocks. Vertical Darcy velocities range from 10 to 70 cm a-1. A 21-km-long geoelectrical profile across the caldera provides resistivity values to the order of 100 to >103 Ωm down to a depth of 6 km, as well as variations of self-potential. Interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome. To explain the unexpectedly low temperatures in the resurgent dome, we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly. Instead, we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past, and that a present-day hot water flow system is responsible for local hot anomalies, such as Hot Creek and the area of the Casa Diablo geothermal power plant. The source of hot water has been associated with recent shallow intrusions into the West Moat. The focus of planning for future power plants should be to locate this present-day flow system instead of relying on heat from the old magma chamber. © 2003 Elsevier B.V. All rights reserved.
200.
Modelling the thermal regime of permafrost and gas hydrate deposits to determine the impact of climate warming, mallik field area
Bulletin of the Geological Survey of Canada
(544)
391-401
2003
ISSN: 00687626Abstract: ▾ We apply a two-dimensional geothermal model to predict the permafrost and natural gas hydrate structure in theMallik field area, based on two paleoenvironmental scenarios deduced at otherwells in theMackenzieDelta area. ScenarioAindicated a subaerial history throughout theHolocene, and scenario B documented a several thousand year, subaqueous episode during theHolocene followed by recent subaerial exposure. The effects of these histories is limited largely to the 600mthick permafrost zone, with scenarioB predicting a substantial talik. The most defensible scenario can be resolved with ground temperatures or independent paleoenvironmental indicators. The effect of climatewarmingwill be apparent in awarming of the permafrost and, with marine transgression, creation of an underlying talik. Terrestrial methane hydrate deposits remain stable with increasing surface temperatures over several centuries, but the base of gas hydrate stability rises about 2 m after 300 years.
199.
A late quaternary diatom record of tropical climatic history from Lake Titicaca (Peru and Bolivia)
Palaeogeography, Palaeoclimatology, Palaeoecology,
194
(1-3)
139 – 164
2003
Keywords:▾
Bolivia; Lake Titicaca; Peru; Bacillariophyta; Chaetoceros; Chaetoceros muelleri; Chaetoceros muelleri; Cyclotella; Cyclotella andina; Cyclotella meneghiniana; Cyclotella stelligera; Cyclotella stelligera; biostratigraphy; diatom; lake level; Last Glacial Maximum; paleoclimate; Quaternary
Abstract: ▾ A composite high-resolution diatom stratigraphy from three piston cores and one box-core in the deep sub-basin of Lake Titicaca reveals large moisture variations during the past 30 kyr in the Altiplano region. Diatom sequences indicate orbital and millennial-scale variability in water level and salinity. The pelagic freshwater diatom species Cyclotella andina and Cyclotella stelligera dominate Glacial-age sediments, suggesting that the lake was above its present outlet, Generally, wet conditions continued until 11 000 cal yr BP, as indicated by high percentages of freshwater planktonic diatoms. Large pulses of benthic diatom species between about 11 000 and 10 000 cal yr BP suggest brief intervals of large-amplitude declines in lake level. During the early Holocene (10 000-8500 cal yr BP), a freshwater diatom assemblage suggests overflowing conditions. Pelagic freshwater diatoms are replaced ca, 8500 cal yr BP by the salinity-indifferent species Cyclotella meneghiniana and by benthic taxa, indicating the beginning of lake regression. During the mid-Holocene (6000-3500 cal yr BP), the abundance of the saline taxon Chaetoceros muelleri, coupled with high abundances of epiphytic and epipelic diatoms, indicates maximum salinity and lowest lake levels in the entire 30 000 year record. Lake transgression began ca. 4000 cal yr BP, and the lake achieved modern levels by about 1500 cal yr BP. These water-level changes imply changes in effective moisture, most likely resulting from large precipitation changes. Precipitation was high throughout the Last Glacial Maximum (21 000-18 000 cal yr BP), likely due to an enhanced South American Summer Monsoon during peak summer insolation in the Southern Hemisphere. In contrast, the mid-Holocene transition was dryer than today in association with an austral summer insolation minimum and the subsequent weakening of the summer monsoon. © 2003 Elsevier Science B.V. All rights reserved.
198.
A Look Inside the San Andreas fault at Parkfield Through Vertical Seismic Profiling
Science,
302
(5651)
1746-1748
2003
ISSN: 00368075Keywords:▾
Cracks; Earthquakes; Fluids; Scattering, Vertical seismic profiling (VSP), Seismology, geological structure; microearthquake; P-wave; S-wave; San Andreas Fault; vertical seismic profile, article; earthquake; electric resistance; geology; material state; priority journal; rock; sediment; United States, California; North America; Parkfield; United States
Abstract: ▾ The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby micro-earthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids.
197.
A vegetation and fire history of Lake Titicaca since the last glacial maximum
Palaeogeography, Palaeoclimatology, Palaeoecology,
194
(1-3)
259 – 279
2003
Keywords:▾
Bolivia; Lake Titicaca; Peru; Apiaceae; Apiales; Cyperaceae; Plantago; Plantago; Poaceae; Polylepis; Polylepis; deglaciation; fire history; Last Glacial Maximum; paleoclimate; pollen; Quaternary; vegetation history
Abstract: ▾ Fine-resolution fossil pollen and charcoal analyses reconstruct a vegetation and fire history in the area surrounding Lake Titicaca (3810 m, Peru/Bolivia) since ca, 27 500 cal yr BP (hereafter BP). Time control was based on 26 accelerator mass spectrometer (AMS) radiocarbon dates. Seventeen AMS dates and 155 pollen and charcoal samples between ca. 17 500 BP and ca. 3100 BP allow a centennial-scale reconstruction of deglacial and early- to mid-Holocene events. Local and regional fire signals were based on the separation of two charcoal size fractions, ≥ 180 μm and 179-65 μm. Charcoal abundance correlated closely with the proportion of woody taxa present in the pollen spectra. Little or no pollen was detected in the sedimentary record prior to ca. 21 000 BP. Very cold climatic conditions prevailed, with temperatures suggested to be at least 5-8°C cooler than present. Increases in pollen concentration suggest initial warming at ca. 21 000 BP with a more significant transition toward deglaciation ca. 17 700 BP. Between 17 700 BP and 13 700 BP, puna brava is progressively replaced by puna and sub-puna elements. The most significant changes between the Pleistocene and the Holocene floras were largely complete by 13 700 BP, providing an effective onset of near-modern conditions markedly earlier than in other Andean records. Fire first occurs in the catchment at ca. 17 700 BP and becomes progressively more important as fuel loads increase. No evidence is found of a rapid cooling and warming coincident with the Younger Dryas chron. A dry event between ca. 9000 BP and 3100 BP, with a peak between 6000 and 4000 BP, is inferred from changes in the composition of aquatics, and the marsh community as pollen of Cyperaceae is replaced by Poaceae, Apiaceae, Plantago and the shrub Polylepis. Human disturbance of the landscape is evident in the pollen spectra after ca. 3100 BP with the appearance of weed species. © 2003 Elsevier Science B.V. All rights reserved.
196.
Alteration of hyaloclastites in the HSDP 2 phase 1 drill core1. Description and paragenesis
Geochemistry, Geophysics, Geosystems,
4
(5)
2003
ISSN: 15252027Abstract: ▾ [1] The core from the Hawaii Scientific Drilling Project 2 Phase 1 provides a unique opportunity for studying the low-temperature alteration processes affecting basalt in suboceanic-island environments. In hyaloclastites, which make up about one half of the lower 2 km of this core (the portion that accumulated below sea level), these processes have resulted in zones of incipient, smectitic, and palagonitic alteration. The alteration of sideromelane in these hyaloclastites has four distinct outcomes: dissolution, replacement by two different textural varieties of smectite (i.e., reddened and green grain-replacive), and conversion to palagonite. All samples show evidence of the incipient stage of alteration, suggesting that every sample passed through that zone. However, most samples that show palagonitic alteration do not also show evidence of smectitic alteration and vice versa, suggesting these two outcomes represent divergent paths of alteration. Incipient alteration (1080 to 1335 m depth) includes fracturing and mechanical reduction of porosity from 40-45% to about 20-30%; growth of one form of pore-lining smectite; dissolution of sideromelane; and formation of sideromelane-grain replacements consisting of Fe-hydroxide-strained smectite, titaniferous nodules, and tubules. DNA-specific stains and morphological features indicate that tubules are the result of microbial activity. Smectitic alteration (1405 to 1573 m) includes growth of a second variety of pore-lining smectite, pore-filling and grain-replacing smectite, and cements of phillipsite and Ca-silicate minerals. Palagonitic alteration (1573 m to the deepest samples) includes replacement of margins of shards with palagonite and growth of pore-filling chabazite. The porosity is reduced by cementation to less than 4% at 1573 m. Porosity does not decrease further down hole, nor does the thickness of palagonite rims on shards increase through the zone of palagonitic alteration. In these samples, palagonite is not an intermediate alteration product in the development of smectite. Rather, in hyaloclastites from the HSDP core, palagonite has formed after all observed smectites. Current downhole temperatures at the boundaries between the three alteration zones are in the range from 12° to 15°C, suggesting that geochemical thresholds or vital effects, not temperature conditions, control different outcomes of alteration. © 2003 by the American Geophysical Union.
195.
Brief climate events in the sedimentary record of Lake Baikal between 130 and 70 kyr BP
Geologiya i Geofizika,
44
(7)
623 – 637
2003
ISSN: 00167886Abstract: ▾ The paper presents new biogenic silica (BiSi) and total organic carbon (TOC) data for the past 130 kyr from BDP-98 and BDP-96-2 cores, respectively, showing a distinct climate periodicity correlated with Late Pleistocene rhythms. The high-resolution sedimentary records from Lake Baikal allow approaching the problem of the length of the last interglacial and stability of its climate in the Northern Hemisphere. The Siberian archives for the interval of 70 to 130 kyr contain brief sub-Milankovitch millenial-scale excursions correlated with events in the high-resolution records of North Atlantic ice cores and European continental pollen sequences. The correlation of the mid-Eemian cooling about 122 kyr BP and the cold Montaigu event about 103 kyr BP with low production signals in the Baikal cores evidence for a climatic connection between the geographically distant North Atlantic, continental Europe, and continental Siberia regions. This connection was well pronounced during interglacials and interstadials and weak during the later glaciation. Rapid warm/cold transitions in the Baikal record, especially the Kazantsevo/Early Zyryanka transition, indicate that glaciation in Siberia began at 115-116 kyr BP, or 5-8 kyr earlier than in Europe and in the North Atlantic, as inferred from independent age models. The continuous climate record from Lake Baikal provides a solid stratigraphic background for detailed correlation of Siberian continental sections.