All ICDP Publications with Abstracts
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34.
Landsat infrared analysis of fumarole activity at Unzen Volcano: Time-series comparison with gas and magma fluxes
Journal of Volcanology and Geothermal Research,
89
(1-4)
57 – 64
1999
ISSN: 03770273Keywords:▾
Japan; fumarole; Landsat thematic mapper; magma; volcanic eruption; volcanic gas
Abstract: ▾ We analyse shortwave infrared thermal data of the phase 1 eruption of Unzen Volcano (Japan) extracted from eight nighttime Thematic Mapper (TM) images taken from the Landsat 5 satellite between October 1991 and November 1992. We identify two discrete regions of the dome that were heated to high temperature by the ongoing eruptive activity; a fumarolically heated region and an area associated with the effusion of new lava. We concentrate analysis on the fumarolically heated region and investigate the relationships between parameters derived from the infrared radiance data and the nature of the fumarolic gas and magma fluxes. Temporal variations in the parameters derived from the radiance data closely follow those observed in the measured rate of magma effusion. The positive correlation observed between the fumarolically driven shortwave infrared flux and the magma discharge rate (r2 = 0.64) indicates that degassing occurred efficiently and in proportion to the amount of magma supplied. Over our monitoring period, the data suggest that gas accumulation within the edifice did not occur, this conclusion agreeing with a previous finding obtained using correlation spectrometer (COSPEC) analysis of SO2 flux rates. A positive correlation (r2 = 0.56) was also found between the mean radiance of the pixels in the fumarolically heated region and the overall size of that region. This suggests a potential mechanism whereby, when gas pressure within the edifice increased, excess gas escaped through additional pathways to the surface as well through an increased flux at the main fumarolic vents.
33.
Geomagnetic field intensity at Hawaii for the last 420 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii
Journal of Geophysical Research: Solid Earth,
104
(B7)
15317-15338
1999
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
magnetic intensity; paleomagnetism; Quaternary, Hawaii; United States
Abstract: ▾ Four hundred twenty five new paleointensity (Thellier-Thellier) determinations (out of 545 analyzed samples) have been obtained from core HSDP, which penetrates about 1000 meters (208 flows) of the Mauna Loa and Mauna Kea volcanic series encompassing the last 420 kyr. Rock magnetic investigations identify pseudo-single-domain magnetite as the main magnetic mineral. Inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude. The inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events. The paleointensity record from the Mauna Loa sequence is not very detailed and does not allow precise comparison with other data in the 0-50 kyr interval. The record from the Mauna Kea sequence, on the contrary, is very detailed and documents relatively short-lived episodes of low and high field strength from 15 to 60 μT. The average virtual dipole moment (8.7 ± 3.0 1022 A.m2) is not significantly different from the value reported by Kono and Tanaka [1995] for the last 2.5 Myr. A comparison with other data from Hawaii and other geographical regions is described in detail. There are no drastic changes in paleointensity with the inclination anomaly, in agreement with previous results from Hawaii but in contrast with most published results which, however, consider data from polarity transition. Spectral analysis of a particularly detailed portion of the record, between 420 and 326 kyr, documents significant periodicities at 36, 8, 5, and 4 ka in the inclination record but not in the intensity record, suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity. Copyright 1999 by the American Geophysical Union.
32.
Juvenile volcanic glass erupted before the appearance of the 1991 lava dome, Unzen volcano, Kyushu, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
113 – 121
1999
ISSN: 03770273Keywords:▾
Japan; lava dome; magma chemistry; volcanic eruption; volcanic glass
Abstract: ▾ Fugen-dake, the main peak of Unzen volcano, began to erupt on 17 November 1990 after 198 years of dormancy. Following a period of steam-dominated tephra eruptions, a new lava dome appeared at the bottom of Jigokuato Crater in the summit region on 20 May 1991. Non-hydrated vesiculated glass was detected in the precursory tephra eruptions, beginning 12 February 1991. Determination of whether glass pyroclasts are hydrated or not appears to be an effective means of ascertaining whether the tephra contains juvenile material from ascending magma. The detection of juvenile pyroclasts at Fugen-dake more than 3 months before the appearance of the lava dome suggests that monitoring of early tephra emissions is an effective way to help forecast impending magmatic eruptions.
31.
Influences of grain size and salinity on pressure-temperature thresholds for methane hydrate stability in JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research-well sediments
Bulletin of the Geological Survey of Canada
(544)
229-240
1999
ISSN: 00687626Abstract: ▾ This paper summarizes laboratory determinations of the pressure-temperature (P-T) phase equilibrium conditions for methane hydrate stability in sediments recovered from the gas-hydrate-bearing interval at JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well. Three test samples consisted of quartz-rich sand with in situ pore-water salinities of 4 ppt (parts per thousand), 20 ppt, and 40 ppt. A fourth sample was dominated by silt, with a salinity of 31 ppt. Initially, methane hydrate was regrown in the sediments, followed by the determination of P-T stability thresholds between 0°C and 12°C. Comparisons with published data for methane hydrate stability in pure gas-water systems indicate no appreciable shift in P-T stability conditions in sand with salinity of 4 ppt, but suggest a progressively increasing shift towards the higher pressure, lower temperature region for sand samples with elevated salinity. Test results for the saline silt sample indicate an additional shift in the stability threshold attributed to the effect of the porous medium in fine-grained sediments.
30.
How did the foreland react? Yangtze foreland fold-and-thrust belt deformation related to exhumation of the Dabie Shan ultrahigh-pressure continental crust (eastern China)
Terra Nova,
11
(6)
266 – 272
1999
ISSN: 09544879
Publisher: Blackwell Publishing Ltd.
Keywords:▾
China; Dabie Shan; collision zone; crustal deformation; exhumation; fold and thrust belt; foreland basin; subduction; Triassic
Abstract: ▾ During the Triassic collision of the Yangtze and Sino-Korean cratons, the leading edge of the Yangtze crust subducted to mantle depths and was subsequently exhumed as a penetratively deformed, coherent slab capped by a normla shear zone. This geometry requires a reverse shear zone at the base of the slab, and we suggest that the Yangtze foreland fold-and-thrust belt constitutes this zone. Lower Triassic rocks of the eastern foreland record NW-SE compression as the oldest compressional stress field; onset of related deformation is indicated by Middle Triassic clastic sedimentation. Subsequent Jurassic stress fields show a clockwise change of compression directions. Based on nearly coeval onset and termination of deformation, and on a common clockwise change in the principal strain/stress directions, we propose that the foreland deformation was controlled by the extrusion of the ultra high-pressure slab. Widespread Cretaceous-Cenozoic reactivation occurred under regional extension to transtension, which characteristically shows a large-scale clockwise change of the principal extension directions during the Lower Cretaceous.
29.
Heat flow and geothermal field of the Baikal region
Geologiya i Geofizika,
40
(3)
287 – 303
1999
ISSN: 00167886Abstract: ▾ Data on the geotemperature field of the central part of the Baikal Rift Zone (BRZ) and adjacent regions of the Siberian Platform and Transbaikalia are summarized. About 800 measurements of heat flow were made here, 700 of them were carried out in the Baikal basin (shallow-depth measurements in the upper layer of sediments). The average heat flow through the Baikal bottom is 71±21 mW/m2. At some sites of the bottom extremely high flows (up to 6000-8000 mW/m2) confined to the local centers of discharge of underwater sources have been revealed. In the montane framing of the rift troughs heat flow is much less intense - no more than 15-40 mW/m2. In the south of the Siberian Platform and in Transbaikalia average heat flow is 40±9 and 52±11 mW/m2, respectively. This differentiated heat flow distribution in the BRZ is, probably, the result of active water exchange in intermount artesian basins, which leads to cooling of feeding zones (mountain ridges) by meteogenic waters and further warming-up of the sedimentary cover of these basins (discharge areas). Special studies in the North-Muya tunnel supported this hypothesis. The validity of numerous shallow-depth measurements of heat flow in Lake Baikal is corroborated by the temperatures measured in the first underwater boreholes BDP-93 and BDP-96. By the location of the lower boundary of the gas hydrate layer of the Baikal sediments, 500 estimates of heat flow in the Southern and Central basins were made. In accordance roith these estimates, average heat flow is 76±10 mW/m2, which also corroborates the above average heat flow value. Heat flow data were used to forecast deep temperatures by the geothermal method. The paper analyzes the forecast temperature distributions at depths of 10, 20, 30, and 40 km. The validity of a geothermal forecast for the rift troughs is corroborated by the results of thermobarometric study of mineral inclusions from deep-seated volcanites of East Sayan and the Vitim Plateau. These data also evidence that heat flow and deep temperatures within the BRZ are underestimated.
28.
Groundmass pargasite in the 1991-1995 dacite of Unzen volcano: Phase stability experiments and volcanological implications
Journal of Volcanology and Geothermal Research,
89
(1-4)
197 – 212
1999
ISSN: 03770273Keywords:▾
Japan; crystallization; dacite; pargasite; volcanic eruption
Abstract: ▾ Pargasite commonly occurs in the dacitic groundmass of the 1991-1995 eruption products of Unzen volcano. We described the occurrence and chemical compositions of amphibole in the dacite, and also carried out melting experiments to determine the low-pressure stability limit of amphibole in the dacite. The 1991-1995 ejecta of the Unzen volcano show petrographic evidence of magma mixing, such as reverse compositional zoning of plagioclase and amphibole phenocrysts, and we used a groundmass separate as a starting material for the experiments. Reversed experiments show that the maximum temperature for the crystallization of amphibole is 930°C at 196 MPa, 900°C at 98 MPa, and 820°C at 49 MPa. Compared with the experimental results on the Mount St. Helens dacite, present experiments on the Unzen dacitic groundmass show that amphibole is stable to pressures ca. 50 MPa lower at 850°C. Available Fe-Ti oxide thermometry indicates the crystallization temperature of the groundmass of the Unzen dacite to be 880 ± 30°C, suggesting that the groundmass pargasite crystallized at > 70 MPa, corresponding to a depth of more than 3 km in the conduit. The chlorine content of the groundmass pargasite is much lower than that of phenocrystic magnesiohornblende in the 1991-1995 dacite of Unzen volcano, indicating that vesiculation/degassing of magma took place before the crystallization of the groundmass pargasite. The present study shows that the magma was water oversaturated and that the degassing of magma along with magma mixing caused crystallization of the groundmass amphibole at depths of more than 3 km in the conduit.
27.
Ground temperature change observed at Unzen Volcano associated with the 1990-1995 eruption
Journal of Volcanology and Geothermal Research,
89
(1-4)
65 – 71
1999
ISSN: 03770273Keywords:▾
Japan; prediction; surface temperature; volcanic eruption
Abstract: ▾ Ground temperature associated with eruptive activity of Unzen Volcano, southwestern Japan, was observed in a cave about 680 m west of the central vent. Based on the data during 1991-1996, eliminating the effects of seasonal change, the temperature rose to the highest level probably around the first half of 1992. Hereafter the temperature decreased gradually year by year at least up to the autumn of 1996, beyond the level recovered at the beginning of the lava extrusion in May 1991. Although it is not necessarily conclusive, the present observation suggests a possible rise in ground temperature prior to the commencement of the first phreatic eruption in November 1990. If so, a careful observation may help to predict future eruptions by detecting a precursory rise of the ground temperature in this volcano.
26.
Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: The 1990-1995 eruption at Unzendake volcano, Kyushu, Japan
Journal of Volcanology and Geothermal Research,
89
(1-4)
23 – 34
1999
ISSN: 03770273Keywords:▾
Japan; crustal deformation; GPS; ground motion; lava flow; magmatism; volcanic earthquake; volcanic eruption
Abstract: ▾ Global positioning system (GPS) measurements made around Unzendake volcano, Kyushu, Japan, since January 1991 have detected ground deformation caused by magma intrusion and lava discharge. In the intermittent phreatic and phreatomagmatic eruption stage, the ground was inflating. After growth of the lava dome and following frequent pyroclastic flows at Unzendake volcano, the ground began deflating. These ground deformations are explained by the inflation and deflation of a Mogi's source model (a point source model) located about 6 km west of the active crater at a depth of 11 km, at an aseismic region. The observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation. The horizontal displacements at the reference GPS station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other GPS baseline systems. These observations validate our estimates for the pressure source. Based on the relation between the deformation volume of the ground surface and the discharged volume of the lava, it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater. Magma is estimated to be supplied to the reservoir at an average rate of 1.1 x 105 m3/day; magma intrusion began in December 1989 at the latest and continued for 1.9 x 103 days.
25.
Long Valley Coring Project
Bulletin. Geothermal Resources Council,
27
(2)
43-46
1998
ISSN: 01607782
Publisher: Geothermal Resources Council, Davis, CA, United States
Abstract: ▾ In December 1997, the California Energy Commission (CEC) agreed to provide funding for Phase III continued drilling of the Long Valley Exploratory Well (LVEW) near Mammoth Lakes, CA, from its present depth. The CEC contribution of $1 million completes a funding package of $2 million from a variety of sources, which will allow the well to be cored continuously to a depth of between 11,500 and 12,500 feet. The core recovered from Phase III will be crucial to understanding the origin and history of the hydrothermal systems responsible for the filling of fractures in the basement rock. The borehole may penetrate the metamorphic roof of the large magmatic complex that has fed the volcanism responsible for the caldera and subsequent activity.
24.
Osmium-isotope variations in Hawaiian lavas: Evidence for recycled oceanic lithosphere in the Hawaiian plume
Earth and Planetary Science Letters,
164
(3-4)
483-496
1998
ISSN: 0012821XKeywords:▾
hot spot; isotopic composition; oceanic crust; osmium, Pacific Ocean
Abstract: ▾ Isotopic heterogeneity in Hawaiian shield lavas reflects the presence of two distinct recycled components in the Hawaiian plume, both from the same packet of recycled oceanic lithosphere. Radiogenic Os-isotopes and anomalously heavy oxygen-isotopes in Koolau lavas reflect melt generation from recycled oceanic crust plus pelagic sediment. In contrast, Kea lavas have unradiogenic Os-isotopes but anomalously light oxygen-isotopes. Oxygen-osmium-lead isotope correlations preclude generation of the Kea isotopic signature from asthenospheric upper mantle or the in situ lithospheric mantle or crust. Instead, melting of recycled, hydrothermally altered ultramafic lower crust or lithospheric mantle in the Hawaiian plume can produce Kea-type lavas. The preservation of both upper- and lower-crustal oxygen isotope signatures in plume-derived Hawaiian lavas indicates that chemical heterogeneities with length scales of only a few kilometers can be preserved in the convecting mantle for long periods of time, probably on the order of 1 Ga or more.
23.
Scientific drilling continues in Long Valley Caldera, California
Eos, Transactions American Geophysical Union,
79
(36)
429-432
1998
Abstract: ▾ The Long Valley caldera region, located between the Sierra Nevada and the Basin and Range Province in eastern California, encompasses a large volcanic complex whose eruptive history began nearly 4 m.y.a. and continues to the present, with eruptions occurring, on average, every few hundred years. Eruptive activity in the area occurred as recently as 250 years ago with small eruptions from vents on Paoha Island in the middle of Mono Lake and 550–600 years ago from three vents at the southern end of the Inyo volcanic chain in the west moat of Long Valley caldera. The current unrest in the caldera began in 1980 and has included recurring earthquake swarms and uplift of the resurgent dome in the center of the caldera by over 70 cm [Bailey and Hill, 1990]. Long Valley caldera is one of several large calderas around the world that have shown similar signs of magmatic unrest in the last few decades.
22.
U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China
Earth and Planetary Science Letters,
161
(1-4)
215 – 230
1998
ISSN: 0012821X
Publisher: Elsevier
Keywords:▾
China; geochronology; Triassic; ultrahigh pressure metamorphism
Abstract: ▾ New SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accomodated ~100% N-S- stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ~219 to ~245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.
21.
Unconformity-like Reflection Pattern under the Moho in the Sulu Area
1997
20.
Crustal structure and development of Sulu UHPM Terrane in east-central China
Episodes,
20
(2)
100 – 103
1997
ISSN: 07053797
Publisher: International Union of Geological Sciences
Abstract: ▾ This article provides a brief view of development of the Sulu ultrahigh pressure metamorphic (UHPM) terrane involved in a conceivable intracontinental collision between the Sion-Korean Plate and the Yangtze Plate during the Mesozoic Era. Integrated study of geological and geophysical data reveals crustal structures in the Sulu terrane, and provides evidence for an evolution model of the UHPM terrane called "the dual intracontinental subduction model".
19.
Preliminary results of the first scientific drilling on Lake Baikal, Buguldeika site, southeastern Siberia
Quaternary International,
373 – 17
1997
ISSN: 10406182
Publisher: Elsevier Ltd
Keywords:▾
Russian Federation, Siberia, Lake Baikal, Buguldeika Saddle; borehole logging; palaeoclimate; sedimentation; tectonics
Abstract: ▾ The Baikal Drilling Project (BDP) is a multinational effort to investigate the paleoclimatic history and tectonic evolution of the Baikal sedimentary basin during the Late Neogene. In March 1993 the Baikal drilling system was successfuly deployed from a barge frozen into position over a topographic high, termed the Buguldeika saddle, in the southern basin of Lake Baikal. The BDP-93 scientific team, made up of Russian, American and Japanese scientists, successfully recovered the first long (>100 m) hydraulic piston cores from two holes in 354 m of water. High quality cores of 98 m (Hole 1) and 102 m (Hole 2), representing sedimentation over the last 500,000 years, were collected in 78 mm diameter plastic liners with an average recovery of 72% and 90%, respectively. Magnetic susceptibility logging reveals an excellent hole-to-hole correlation. In this report the scientific team describes the preliminary analytical results from BDP-93 hole 1 cores. Radiocarbon dating by accelerator mass spectrometry provides an accurate chronology for the upper portion of Hole 1. Detailed lithologic characteristics, rock magnetic properties and inorganic element distributions show a significant change to the depositional environment occuring at 50 m subbottom depth, approximately 250,000 BP. This change may be due to uplift and rotation of the horst block in the Buguldeika saddle. The sedimentary section above 50 m is pelitic with varve-like laminae, whereas the section below 50 m contains a high proportion of sand and gravel horizons often organized into turbidite sequences. Accordingly, high resolution seismic records reveal a change in sonic velocity at this depth. It is inferred that sedimentation prior to 250 ka BP was from the west via the Buguldeika river system. After 250 ka BP the Buguldeika saddle reflects an increase in hemipelagic sediments admixed with fine-grained material from the Selenga River drainage basin, east of Lake Baikal. Variations in the spore-pollen assemblage, diatoms, biogenic silica content, rock magnetic properties, clay mineralogy and organic carbon in the upper 50 m of BDP-93-1 reveal a detailed record of climate change over approximately the last 250,000 years. These variables alternate in a pattern characteristic of glacial/interglacial climatic fluctuations. The present age model suggests that the climate signal recorded in Lake Baikal sediments is similar to Late Quaternary signals recorded in Chinese loess sections and in marine sediments. Copyright © 1996 INQUA/ Elsevier Science Ltd.
18.
Continuous paleoclimate record recovered for last 5 million years
Eos, Transactions American Geophysical Union,
78
(51)
597-601
1997
Abstract: ▾ From January to March 1996, an ice-based drilling platform (Figure 1) was positioned in 321 m of water above a submerged topographic high known as the Academician Ridge of Lake Baikal, south-central Siberia (Figure 2). The Russian Nedra drilling team expertly used this system to recover the first sediment cores from this region to span the last 5 m.y. before present (Ma). This operation was complicated and dangerous: there were constantly shifting blocks of ice nearly 1 m thick, the location was surrounded by wilderness areas nearly 400 km from the nearest population center (Irkutsk), and supplies and personnel had to travel 90 km over the ice pressure ridges in 4-wheel motorcycles and small vehicles.
17.
Hydrogeology of the Hawaii Scientific Drilling Project borehole KP-1 1. Hydraulic conditions adjacent to the well bore
Journal of Geophysical Research: Solid Earth,
101
(5)
11675-11682
1996
ISSN: 21699313
Publisher: American Geophysical Union
Abstract: ▾ Temperature and formation resistivity logs obtained in borehole KP-1 of the Hawaii Scientific Drilling Project indicate that the adjacent formation is characterized by several zones of distinctly different average temperature and water salinity. A series of hydraulic analyses and water sampling programs were conducted to rule out the possibility of local hydraulic effects associated with the presence of the borehole in the generation of these apparent groundwater zones. Hydraulic tests and sampling with the borehole cased to a depth of 710 m and open below that depth indicate that the deep aquifer contains seawater at a temperature nearly identical to that of the open ocean at the same depth. Various analyses give estimates of aquifer transmissivity of about 10-3 m2/s in the vicinity of the borehole. Isolation of this deeper aquifer from the overlying groundwater zones was investigated by perforating the casing at six locations and then measuring the changes in water level in the borehole, in the salinity of the fluid column, in the temperature profile of the fluid column, and in the rate of flow in the fluid column induced by the perforations. These results positively confirm that the zones of distinctly different formation properties indicated on the temperature and resistivity logs are not caused by flow in or around casing. Flow and fluid column salinity induced by the perforations also confirm significant differences between the hydraulic heads and geochemistry of the different groundwater zones inferred from the well logs.
16.
The 40Ar/39Ar and K/Ar dating of lavas from the Hilo 1-km core hole, Hawaii Scientific Drilling Project
Journal of Geophysical Research: Solid Earth,
101
(5)
11607-11616
1996
ISSN: 21699313
Publisher: American Geophysical Union
Keywords:▾
accumulation rate; alkali basalt; argon-40/argon-39 dating; lava flow; petrology; potassium/argon dating; recurrence interval; tholeiite, USA, Hawaii, Hawaii
Abstract: ▾ Mauna Kea lava flows cored in the HilIo hole range in age from <200 ka to about 400 ka based on 40Ar/39Ar incremental heating and K-Ar analyses of 16 groundmass samples and one coexisting plagioclase. The lavas, all subaerially deposited, include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic, transitional tholeiitic, and tholeiitic basalts. The lower section has yielded predominantly complex, discordant 40Ar/39Ar age spectra that result from mobility of 40Ar and perhaps K, the presence of excess 40Ar, and redistribution of 39Ar by recoil. Comparison of K-Ar ages with 40Ar/39Ar integrated ages indicates that some of these samples have also lost 39Ar. Nevertheless, two plateau ages of 391 ± 40 and 400 ± 26 ka from deep in the hole, combined with data from the upper section, show that the tholeiitic section accumulated at an average rate of about 7 to 8 m/kyr and has an mean recurrence interval of 0.5 kyr/flow unit. Samples from the upper section yield relatively precise 40Ar/39Ar plateau and isotope correlation ages of 326 ± 23, 241 ± 5, 232 ± 4, and 199 ± 9 ka for depths of -415.7 m to -299.2 m. Within their uncertainty, these ages define a linear relationship with depth, with an average accumulation rate of 0.9 m/kyr and an average recurrence interval of 4.8 kyr/flow unit. The top of the Mauna Kea sequence at -280 m must be older than the plateau age of 132 ± 32 ka, obtained for the basal Mauna Loa flow in the corehole. The upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to Mauna Kea as it rode the Pacific plate away from its magma source, the Hawaiian mantle plume. The age-depth relation in the core hole may be used to test and refine models that relate the growth of Mauna Kea to the thermal and compositional structure of the mantle plume.
15.
Mauna Loa lava accumulation rates at the Hilo drill site: Formation of lava deltas during a period of declining overall volcanic growth
Journal of Geophysical Research: Solid Earth,
101
(5)
11631-11641
1996
ISSN: 21699313
Publisher: American Geophysical Union
Keywords:▾
accumulation rate; coastal environment; lava delta; temporal variation; volcanic eruption, USA, Hawaii, Hawaii
Abstract: ▾ Accumulation rates for lava flows erupted from Mauna Loa, as sampled in the uppermost 280 m of the Hilo drill hole, vary widely for short time intervals (several thousand years), but overall are broadly similar to those documented elsewhere on this volcano since 100 ka. Thickness variations and accumulation rates for Mauna Loa lavas at the Hilo drill site have been strongly affected by local paleotopography, including funneling and ponding between Mauna Kea and Kilauea. In addition, gentle submerged slopes of Mauna Kea in Hilo Bay have permitted large shoreline displacements by Mauna Loa flows. Ages of eruptive intervals have been determined from published isotopic data and from eustatic sea level curves modified to include the isostatic subsidence of the island of Hawaii at 2.2-2.6 mm/yr. Prior to 10 ka, rates of Mauna Loa lava accumulation at the drill site varied from 0.6 to 4.3 mm/yr for dateable intervals, with an overall rate of 1.8 mm/yr. Major eruptive pulses at about 1.3 and 10 ka, each probably representing a single long-lived eruption based on lack of weathering between flow units, increase the overall accumulation rate to 2.4 mm/yr. The higher rate since 10 ka reflects construction of thick near-shoreline lava deltas as postglacial sea levels rose rapidly. Large lava deltas form only along coastal segments where initially subaerial slopes have been submerged by the combined effects of eustatic sea level rise, isostatic subsidence, or spreading of volcano flanks. Overall accumulation of 239 m of lava at the drill site since 100-120 ka closely balances submergence of the Hilo area, suggesting that processes of coastal lava deposition have been modulated by rise in sea level. The Hilo accumulation rate is slightly higher than average rates of 1-2 mm/yr determined elsewhere along the Mauna Loa coast, based on rates of shoreline coverage and dated sea cliff and fault scarp exposures. Low rates of coastal lava accumulation since 100 ka, near or below the rate of island-wide isostatic subsidence, indicate that Mauna Loa is no longer growing vigorously or even maintaining its size above sea level.
14.
Introduction to special section: Hawaii Scientific Drilling Project
Journal of Geophysical Research: Solid Earth,
101
(B5)
11593-11598
1996
ISSN: 21699313
Publisher: American Geophysical Union
13.
Hydrogeology of the Hawaii Scientific Drilling Project borehole KP-1 2. Groundwater geochemistry and regional flow patterns
Journal of Geophysical Research: Solid Earth,
101
(5)
11683-11694
1996
ISSN: 21699313
Publisher: American Geophysical Union
Keywords:▾
fluid flow; freshwater; geochemistry; groundwater; hydrogeology; salinity, USA, Hawaii, Hawaii
Abstract: ▾ A series of downhole and surface water samples were taken from the 1-km-deep KP-1 borehole located on the eastern flank of the island of Hawaii. Early samples from depths of more than 700 m showed salinities nearly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and residual drilling mud clays. Later deep samples found only minor variations from seawater cation chemistry that are consistent with low-temperature weathering of basalts; δ18O values are equivalent to seawater values and are consistent with this interpretation. Carbon 14 activities of dissolved inorganic carbonate indicate a water age ranging from 5890 to 7170 years B.P. and fluid transport rates of 1.8 to 2.2 m/yr. Fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the Mauna Kea/Mauna Loa interface; borehole resistivity logs indicate that it is ∼200 m thick. Although it has not yet been possible to obtain samples of the freshwater zone without contamination from the deep saline fluids, the chloride concentrations of the low-salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg/L. Light stable isotope data indicate that the fresh water at 320 m is derived from recharge entering the island at an average elevation of 2000 m. Inferred 14C activities of the dissolved bicarbonate in the freshwater zone indicate an average calibrated age of 2200 years B.P. and an average fluid velocity of at least 14 m/yr. A regional water flow model is proposed that suggests that the fresh water found at the 320-m depth is derived from rainfall recharge from the middle elevations of Mauna Kea volcano. This rainfall is channeled beneath the Mauna Loa lavas by the thick soil layer separating the two volcanoes. A second shallow fresh-to-brackish water zone, derived from Mauna Loa recharge, is also inferred to exist below the carbonate formation that underlies the shallow basal lens. The results of our preliminary study of the groundwater system below the KP-1 drill site demonstrate that intervolcano and interflow aquicludes can have a substantial impact on water circulation and discharge from young island volcanoes.
12.
Hawaii scientific drilling protect: Summary of preliminary results
GSA Today,
6
(8)
x1-8
1996
ISSN: 10525173
Publisher: Geological Society of America
Abstract: ▾ Petrological, geochemical, geomagnetic, and volcanological characterization of the recovered core from a 1056-m-deep well into the flank of the Mauna Kea volcano in Hilo, Hawaii, and downhole logging and fluid sampling have provided a unique view of the evolution and internal structure of a major oceanic volcano unavailable from surface exposures. Core recovery was ∼90%, yielding a time series of fresh, subaerial lavas extending back to ∼400 ka. Results of this 1993 project provide a basis for a more ambitious project to core drill a well 4.5 km deep in a nearby location with the goal of recovering an extended, high-density stratigraphic sequence of lavas.
11.
Geochemical stratigraphy of lava flows sampled by the Hawaii Scientific Drilling Project
Journal of Geophysical Research: Solid Earth,
101
(5)
11729-11746
1996
ISSN: 21699313
Publisher: American Geophysical Union
Keywords:▾
basalt; geochemistry; lava flow; petrogenesis; stratigraphy, USA, Hawaii, Hawaii
Abstract: ▾ Geochemical discriminants are used to place the boundary between Mauna Loa flows and underlying Mauna Kea flows at a depth of about 280 m. At a given MgO content the Mauna Kea flows are lower in SiO2 and total iron and higher in total alkali, TiO2, and incompatible elements than the Mauna Loa lavas. The uppermost Mauna Kea lavas (280 to 340 m) contain alkali basalts interlayered with tholeiites and correlate with the postshield Hamakua Volcanics. In addition to total alkalis, the alkali basalts have higher TiO2, P2O5, Sr, Ba, Ce, La, Zr, Nb, Y, and V relative to the tholeiites and lower Zr/Nb and Sr/Nb ratios. Some of the alkali basalts are extensively differentiated. Below 340 m all the flows are tholeiitic, with compositions broadly similar to the few "fresh" subaerial shield-building Mauna Kea tholeiites studied to date. High-MgO lavas are unusually abundant, although there is a wide range (7-28%) in MgO content reflecting olivine control. FeO/MgO relationships are used to infer parental picritic magmas with about 15 wt % MgO. Lavas with more MgO than this have accumulated olivine. The Mauna Loa lavas have compositional trends that are controlled by olivine crystallization and accumulation. They compare closely with trends for historical (1843-1984) flows, tending toward the depleted end of the spectrum. They are, though, much more MgO-rich (9-30%) than is typical for most historical and young (<30 ka) prehistoric lavas. The unusual abundance of high-MgO and picritic lavas is attributed to the likelihood that only large-volume, hot, mobile flows will reach Hilo Bay from the northeast rift zone. FeO/MgO relationships are used to infer parental picritic magmas with about 17 wt % MgO. Again, lavas with more MgO than this have accumulated olivine. Systematic changes in incompatible element ratios are used to argue that the magma supply rate has diminished over time. On the other hand, the relatively constant Zr/Nb and Sr/Nb ratios that compare closely with historical and young (<30 kyr) prehistoric flows are used to argue that the source components for these lavas in the Hawaiian plume have remained relatively uniform over the last 100 kyr.
10.
Evolution of Mauna Kea volcano: Inferences from lava compositions recovered in the Hawaii Scientific Drilling Project
Journal of Geophysical Research: Solid Earth,
101
(5)
11747-11767
1996
ISSN: 21699313
Publisher: American Geophysical Union
Keywords:▾
basalt; geochemistry; lava flow; petrogenesis; petrology; temporal evolution; volcano, USA, Hawaii, Hawaii, Mauna Kea
Abstract: ▾ The lower 776 m of core recovered during the initial phase of the Hawaii Scientific Drilling Project (HSDP) contains lavas erupted from Mauna Kea volcano. Tholeiitic and alkalic basalts, including an Fe-Ti rich flow, are intercalated in the upper 58 m of Mauna Kea lavas. Similar basaltic sections are subaerially exposed on the lower east flank of Mauna Kea. The Fe-Ti rich lavas reflect large amounts of clinopyroxene, plagioclase, and olivine fractionation within the crust and upper mantle, but the range from tholeiitic to alkalic compositions reflects variable extents of melting of a garnet-bearing source. Based on abundances of incompatible elements, the extent of melting for a basanitoid was a factor of 2 less than that for nearly coeval tholeiitic lavas. All flow units in the lower 718 m of the HSDP core are tholeiitic lavas. Their variability in major element compositions reflect variable accumulation of olivine. Incompatible element abundance ratios in these lavas reflect a complex temporal variation in extent of melting. Within the tholeiitic part of the core, lavas from 800 m to 950 m formed by the largest extent of melting, whereas tholeiitic lavas from the bottom of the core and from just below the tholeiitic to alkalic transition formed by lower degrees of melting. Inferred melt compositions at 16% MgO show that the ∼200 to 400 ka Mauna Kea lavas from the HSDP core and the <250 ka subaerial exposures define an inverse correlation between SiO2 and FeO contents. Based on experimental studies, this correlation is caused by differing pressures of melt segregation. Furthermore, abundances of Nb and SiO2 are also inversely correlated in these calculated melts. In general, the younger lavas are relatively enriched in FeO and incompatible elements but are depleted in SiO2. These trends are interpreted to reflect an overall trend of increasing pressure of melt segregation and decreasing extent of melting with decreasing eruption age. There are, however, geochemical variations which indicate short-term reversals in this long-term trend. Previously, the geochemical trends accompanying the transition from tholeiitic to alkalic volcanism at Hawaiian volcanoes have been interpreted as reflecting the effects of increasing distance from the plume axis. The long-term geochemical trends of tholeiitic lavas in the HSDP core also reflect migration of Mauna Kea away from the Hawaiian plume.