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Notes: [Auszug] Vesicles and included CO2are enriched in deep-sea basalts that are also enriched in light rare earth and incompatible elements. This enrichment probably results from a unique deep mantle origin of such melts but may have been modified by CO2 bubbles rising in shallow magma ...

Notes: Abstract  High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea), Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep (∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights 〈300 m. The flat tops commonly have either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m. The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most of the rejuvenated-stage flat-topped cones formed on slopes 〈10° and formed adjacent semicircular steps down the flank of Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine flat-topped cones but formed under different cooling conditions.

Notes: Abstract The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass. Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 2∶1 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene (≈ Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite (≈ Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine. Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more “equilibrium” trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling. Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected. Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or〉 Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl. The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate. Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.

Notes: Abstract On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa14 composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000° C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800° C was followed by a 40 year period before the temperature reached 700° C. The olivine phenocrysts settled at an average rate of about 4 × 10−6 cm Sec−1 to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa55) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa20 Fa48) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000° C to at least as low as 800° C, the olivine phenocrysts were converted to Fa30–40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant ground mass olivine. Poikilitic hypersthene grew at the expense of pre-existing ferromagnesian minerals in the cumulate zone.

Notes: Abstract Sulfur analyses by X-ray fluorescence give an average content of 107 ppm for 9 samples of fresh subaerially-erupted oceanic basalt and 680 ppm for 38 samples of submarine erupted basalt. This difference is the result of retention of sulfur in basalt quenched on the sea floor and loss of sulfur in basalt by degassing at the surface. The outer glassy part of submarine erupted basalt contains 800±150 ppm sulfur, and this amount is regarded as an estimate of the juvenile sulfur content of the basalt melt from the mantle. The slower cooled interiors of basalt pillows are depleted relative to the rims owing to degassing and escape through surface fractures. Available samples of deep-sea basalts do not indicate a difference in original sulfur content between low-K tholeiite, Hawaiian tholeiite, and alkali basalt. The H2O/S ratio of analyzed volcanic gases is generally lower than the H2O/S ratio of gases presumed lost from surface lavas as determined by chemical differences between pillow rims and surface lavas. This enrichment of volcanic gases in sulfur relative to water may result from a greater degassing of sulfur relative to water from shallow intrusive bodies beneath the volcano.

Notes: Abstract The submarine Mahukona Volcano, west of the island of Hawaii, is located on the Loa loci line between Kahoolawe and Hualalai Volcanoes. The west rift zone ridge of the volcano extends across a drowned coral reef at about-1150 m and a major slope break at about-1340 m, both of which represent former shoreines. The summit of the volcano apparently reached to about 250 m above sea level (now at-1100 m depth) did was surmounted by a roughly circular caldera. A econd rift zone probably extended toward the east or sutheast, but is completely covered by younger lavas from the adjacent subaerial volcanoes. Samples were vecovered from nine dredges and four submersible lives. Using subsidence rates and the compositions of flows which drape the dated shoreline terraces, we infer that the voluminous phase of tholeiitic shield growth ended about 470 ka, but tholeiitic eruptions continued until at least 435 ka. Basalt, transitional between tholeiitic and alkalic basalt, erupted at the end of tholeiitic volcanism, but no postshield-alkalic stage volcanism occurred. The summit of the volcano apparently subcided below sea level between 435 and 365 ka. The tholeiitic lavas recovered are compositionally diverse.

Notes: Abstract A major carbonate reef which drowned 13 ka is now submerged 150 m below sea level on the west coast of the island of Hawaii. A 25-km span of this reef was investigated using the submersibleMakali'i. The reef occurs on the flanks of two active volcanoes, Mauna Loa and Hualalai, and the lavas from both volcanoes both underlie and overlie the submerged reef. Most of the basaltic lava flows that crossed the reef did so when the water was much shallower, and when they had to flow a shorter distance from shoreline to reef face. Lava flows on top of the reef have protected it from erosion and solution and now occur at seaward-projecting salients on the reef face. These relations suggest that the reef has retreated shoreward as much as 50 m since it formed. A 7-km-wide “shadow zone” occurs where no Hualalai lava flows cross the reef south of Kailua. These lava flows were probably diverted around a large summit cone complex. A similar “shadow zone” on the flank of Mauna Loa volcano in the Kealakekua Bay region is downslope from the present Mauna Loa caldera, which ponds Mauna Loa lava and prevents it from reaching the coastline. South of the Mauna Loa “shadow zone” the - 150 m reef has been totally covered and obscured by Mauna Loa lava. The boundary between Hualalai and Mauna Loa lava on land occurs over a 6-km-wide zone, whereas flows crossing the - 150 m reef show a sharper boundary offshore from the north side of the subaerial transition zone. This indicates that since the formation of the reef, Hualalai lava has migrated south, mantling Mauna Loa lava. More recently, Mauna Loa lava is again encroaching north on Hualalai lava.

Notes: Abstract A marine sampling program, utilizing the PISCES-5 submersible operated by the Hawaii Undersea Research Laboratory (NOAA), has confirmed the presence of a major submerged coral reef offshore from Ka Lae (South Point), Hawaii. The top of the reef is now 150–160 m below sea level. Radiocarbon and Useries dating indicates that it drowned about 13.9 ka by the combined effects of island subsidence (2.5 mm/year) and the rapid rise of sea level at the end of the last glaciation so that the relative submergence rate of more than 10 mm/year exceeded the upward growth rate of the reef. The submerged reef caps the offshore part of the southwest rift-zone ridge of Mauna Loa, which has apparently undergone little volcanic activity offshore since 170 ka, and possibly since 270 ka. This fact suggests that rift zone activity is becoming increasingly restricted toward the upper part of the volcano, a condition possibly heralding the end of the shield-building stage.

Notes: Abstract The Surtsey marine volcano was built on the southern insular shelf of Iceland, along the seaward extension of the east volcanic zone, during episodic explosive and effusive activity from 1963 to 1967. A 1600-m-long, east-west line of 42 bench marks was established across the island shortly after volcanic activity stopped. From 1967 to 1991 a series of leveling surveys measured the relative elevation of the original bench marks, as well as additional bench marks installed in 1979, 1982 and 1985. Concurrent measurements were made of water levels in a pit dug on the north coast, in a drill hole, and along the coastline exposed to the open ocean. These surveys indicate that the dominant vertical movement of Surtsey is a general subsidence of about 1.1±0.3 m during the 24-year period of observations. The rate of subsidence decreased from 15–20 cm/year for 1967–1968 to 1–2 cm/year in 1991. Greatest subsidence is centered about the eastern vent area. Through 1970, subsidence was locally greatest where the lava plain is thinnest, adjacent to the flanks of the eastern tephra cone. From 1982 onward, the region closest to the hydrothermal zone, which is best developed in the vicinity of the eastern vent, began showing less subsidence relative to the rest of the surveyed bench marks. The general subsidence of the island probably results from compaction of the volcanic material comprising Surtsey, compaction of the sea-floor sediments underlying the island, and possibly downwarping of the lithosphere due to the laod of Surtsey. The more localized early downwarping near the eastern tephra cone is apparently due to greater compaction of tephra relative to lava. The later diminished local subsidence near the hydrothermal zone is probably due to a minor volume increase caused by hydrous alteration of glassy tephra. However, this volume increase is concentrated at depth beneath the bottom of the 176-m-deep cased drillhole.