A SeaMARC II survey of recent submarine volcanism near Easter Island

SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km² and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km². The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.     

Submarine Hydrothermal Mineralization on the Izu–Bonin Arc,South of Japan: An Overview

Abstract

Considerable effort has been expended in studying the Izu–Bonin Arc over the past 15 years. In particular, 43 dives of the Shinkai 2000 have been undertaken there to discover and evaluate the extent of submarine hydrothermal activity and mineralization. Most effort has been focused on Myojin Knoll (23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives).

The Izu–Bonin Arc is divided in two by the Sofugan Tectonic Line. Eight submarine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the arc is quite different from that of the southern sector. Volcanic rocks from the northern sector are more acidic than those from the southern sector.

Evidence for submarine hydrothermal mineralization has been observed at four seamounts along the Izu–Bonin Arc (Myojin Knoll, Myojinsho, Suiyo Seamount, and Kaikata Caldera), and submarine hydrothermal activity is evident at another three seamounts along the arc (Kurose Hole, Mokuyo Seamount, and Doyo Seamount).

The most extensive submarine hydrothermal mineral deposit so far located on the Izu–Bonin Arc is the Sunrise deposit at Myojin Knoll. This deposit, at least 400 m in diameter and 30 m high, is associated with black smoker venting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hydrothermal vents there was 278°C. Some of the sulfide chimneys contained as much as 49 μg/g Au and 3,400 μg/g Ag. The sunrise deposit is one of the largest submarine volcanic massive sulfide deposits so far discovered in midocean ridge, backarc, or arc settings and has an estimated mass of 9 × 10⁶ t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide deposits may be found in other caldera along the Izu–Bonin Arc.

The geological variability along the arc, the high seismicity, the occurrence of active volcanism and submarine hydrothermal venting, and a proven submarine hydrothermal mineral potential coupled with the proximity of the region to Japan suggest that the Izu–Bonin Arc could profitably serve as a natural laboratory for the long-term monitoring of the seafloor.     

Submarine Hydrothermal Mineralization on the Izu-Bonin Arc, South of Japan: An Overview

Considerable effort has been expended in studying the Izu-Bonin Arc over the past 15 years. In particular, 43 dives of the Shinkai 2000 have been undertaken there to discover and evaluate the extent of submarine hydrothermal activity and mineraliza tion. Most effort has been focused on Myojin Knoll (23 dives), Suiyo Seamount (6 dives), and Kaikata Caldera (10 dives). The Izu-Bonin Arc is divided in two by the Sofugan Tectonic Line. Eight submarine caldera are located north of this line but only one is south of it. The physiography of the northern sector of the arc is quite different from that of the southern sector. Volcanic rocks from the northern sector are more acidic than those from the southern sector. Evidence for submarine hydrothermal mineralization has been observed at four seamounts along the Izu-Bonin Arc (Myojin Knoll, Myojinsho, Suiyo Seamount, and Kaikata Caldera), and submarine hydrothermal activity is evident at another three seamounts along the arc (Kurose Hole, Mokuyo Seamount, and Doyo Seamount). The most extensive submarine hydrothermal mineral deposit so far located on the Izu-Bonin Arc is the Sunrise deposit at Myojin Knoll. This deposit, at least 400 m in diameter and 30 m high, is associated with black smoker venting, inactive sulfide chimneys, massive sulfides, hydrothermal Mn crusts, and a hydrothermal vent fauna. The maximum recorded temperature of the hydrothermal vents there was 278°C. Some of the sulfide chimneys contained as much as 49 μg / g Au and 3,400 μg / g Ag. The sunrise deposit is one of the largest submarine volcanic massive sulfide deposits so far discovered in midocean ridge, backarc, or arc settings and has an estimated mass of 9 x 10 6 t. This deposit may be of the Kuroko-type. The discovery of the Sunrise deposit in 1997 gives hope that other, similarly large, sulfide deposits may be found in other caldera along the Izu-Bonin Arc. The geological variability along the arc, the high seismicity, the occurrence of active volcanism and submarine hydrothermal venting, and a proven submarine hydrothermal mineral potential coupled with the proximity of the region to Japan suggest that the Izu-Bonin Arc could profitably serve as a natural laboratory for the long-term monitoring of the seafloor.     

Geological reconstruction of Fangataufa atoll, South Pacific

Several boreholes drilled by the Commissariat à l'Energie Atomique have reached and passed through the volcanic bedrock of Fangataufa atoll. The sampled volcanic rocks under the coral ring were produced during both aerial and submarine activity, whereas rocks drilled under the lagoon were erupted during submarine volcanism only. The bathymetric data show that the atoll has a “starfish” shape. The rift zones are elongated in N-S, N70–80 and N120 directions; these three main directions are also the directions of structural discontinuities in the lithosphere. Reconstruction of the atoll's topography before erosion using a slope angle of about 16° shows that the maximum height reached by the volcano was about 1300 m above sea level. For comparison, the maximum height of Méhetia island (southeast of Tahiti) is approximately 435 m. The successive construction stages are: (1) initiation of volcanism along the rift zones and construction of a central volcano; (2) production of brecciated lavas; (3) emergent volcanism; and (4) central and aerial activity. The present day position of the aerial volcanic rocks under the coral reef and the submarine products under the lagoon is discussed with reference to two hypotheses. The first is based on sea level changes and the second on a tectonic origin (collapse of the atoll's flanks). Using recent geochronological data, the submarine construction of the atoll related to the hot-spot activity lasted about 1.1 Ma. The accumulation rate was approximately 0.7 cm/yr (1.5 × 10⁻³ km³/yr) and the aerial volcanic activity lasted about 2 Ma (1.5 × 10⁻⁵ km³/yr).     

Geological and petrologic evolution of seamounts near the EPR based on submersible and camera study

Observations from 17 ALVIN dives and 14 ANGUS runs plus laboratory study of basalt samples collected with ALVIN help to constrain the morphologic, volcanic and petrologic evolution of four seamounts near the East Pacific Rise (EPR). Comparison among the four volcanoes provides evidence for a general pattern of near-EPR seamount evolution and shows the importance of sedimentation, mass wasting, hydrothermal activity and other geologic processes that occur on submerged oceanic volcanoes. Seamount 5, closest to the EPR (1.0 Ma) is the youngest seamount and may still be active. Its summit is covered by fresh lavas, recent faults and hydrothermal deposits. Seamount D is on crust 1.55 Ma and is inactive; like seamount 5, it has a breached caldera and is composed exclusively of N-MORB. Seamounts 5 and D represent the last stages of growth of typical N-MORB-only seamounts near the EPR axis. Seamounts 6 and 7 have bumpy, flattish summits composed of transitional and alkalic lavas. These lavas probably represent caldera fillings and caps overlying an edifice composed of N-MORB. Evolution from N-MORB-only cratered edifices to the alkalic stage does not occur on all near-EPR seamounts and may be favored by location on structures with relative-motion-parallel orientation.     

Oceanography and marine biology, 10: Harold Barnes (editor). George Allen and Unwin,London, 1972, 565 pp., £10.50

A bathymetric survey of the Gilliss Seamount, in the northwest Atlantic Basin, using a multi-beam sonar array system reveals an extremely complex morphologic character of this feature. A new chart provides the most detailed topographic presentation of an Atlantic seamount published to date and highlights the similarity of the Gilliss Seamount with terrestrial strata-volcanoes. Bottom photographs and samples reveal pillow-Iava formation. Seismic profiles show that the volcanic basement is irregularly covered by acoustically transparent deposits that are as much as 668 m thick. Volcanic debris and sediments locally are displaced down the flanks of the seamount. Bottom photographs and cores indicate that the transparent layer has accumulated slowly by deposition from suspensate-rich (mostly clay and planktonic foraminifera) water masses that flow around the mid to lower sectors of this submarine volcano. Bottom-current activity also modifies the abyssal plain turbidite-hemipelagic sequence surrounding the seamount.     

Geologic evolution of the Canarian Islands of Lanzarote, Fuerteventura, Gran Canaria and La Gomera and comparison of landslides at these islands with those at Tenerife, La Palma and El Hierro

In this paper we discuss the results of a swath bathymetric investigation of the Canary archipelago offshore area. These new data indicate that volcanism is pervasive throughout the seafloor in the region, much more that would be suggested by the islands. We have mapped tens of volcanic edifices between Fuerteventura and Gran Canaria and offshore Tenerife, La Gomera, El Hierro and La Palma. Volcanic flows are present between Tenerife and La Gomera and salic necks dominate the eastern insular slope of La Gomera. This bathymetry also supports land geologic studies that indicate that the oceanic archipelago has acquired its present morphology in part by mass wasting, a consequence of the collapse of the volcanic edifices. In the younger islands, Tenerife, La Palma and El Hierro, the Quaternary (1.2 to 0.15 Ma) debris avalanches are readily recognizable and can be traced offshore for distances measured in tens of km. Off the older islands, Lanzarote, Fuerteventura, Gran Canaria and La Gomera (<20 to 3.5 Ma), the avalanches have been obscured by subsequent turbidity current deposition and erosion as well as hemipelagic processes. The failure offshore western Lanzarote is in the form of a ramp at the base of the insular slope bound on the seaward side by a scarp. Its size and the lack of evidence of rotation along its landwards side precludes the possibility that it is a slump. It probably represents a slide whose outer scarp is caused by break-up of the slide. Mounds on the ramp’s surface may represent post-displacement volcanic structures or exotic blocks transported to their present locations by the slide. The failures offshore Fuerteventura are so large that, although they occurred in the Miocene-Pliocene, exotic blocks displaced from upslope are still recognizable in the insular margin morphology. The Canary Island insular margin appears to be a creation of Miocene-Pliocene mass wasting and more recent turbidity current deposition and erosion, and hemilepagic deposition. Failures offshore La Gomera are due to debris flows and/or turbidity currents. These events have obscured earlier mass wasting events. An erratum to this article is available at .     

Magmatic evolution of the Easter microplate-Crough Seamount region (South East Pacific)

The Easter microplate-Crough Seamount region located between 25° S–116° W and 25° S–122° W consists of a chain of seamounts forming isolated volcanoes and elongated (100–200 km in length) en echelon volcanic ridges oriented obliquely NE (N 065°), to the present day general spreading direction (N 100°) of the Pacific-Nazca plates. The extension of this seamount chain into the southwestern edge of the Easter microplate near 26°30 S–115° W was surveyed and sampled. The southern boundary including the Orongo fracture zone and other shallow ridges (< 2000 m high) bounding the Southwest Rift of the microplate consists of fault scarps where pillow lava, dolerite, and metabasalts are exposed. The degree of rock alternation inferred from palagonitization of glassy margins suggests that the volcanic ridges are as old as the shallow ridges bounding the Southwest Rift of the microplate. The volcanics found on the various structures west of the microplate consist of depleted (K/Ti < 0.1), transitional (K/Ti = 0.11–0.25) and enriched (K/Ti > 0.25) MORBs which are similar in composition to other more recent basalts from the Southwest and East Rifts spreading axes of the Easter microplate. Incompatible element ratios normalized to chondrite values [(Ce/Yb)_N = 1–2.5}, {(La/Sm)_N = 0.4–1.2} and {(Zr/Y)_N = 0.7–2.5} of the basalts are also similar to present day volcanism found in the Easter microplate. The volcanics from the Easter microplate-Crough region are unrelated to other known South Pacific intraplate magmatism (i.e. Society, Pitcairn, and Salas y Gomez Islands). Instead their range in incompatible element ratios is comparable to the submarine basalts from the recently investigated Ahu and Umu volcanic field (Easter hotspot) (Scientific Party SO80, 1993) and centered at about 80 km west of Easter Island. The oblique ridges and their associated seamounts are likely to represent ancient leaky transform faults created during the initial stage of the Easter microplate formation ( 5 Ma). It appears that volcanic activity on seamounts overlying the oblique volcanic ridges has continued during their westward drift from the microplate as shown by the presence of relatively fresh lava observed on one of these structures, namely the first Oblique Volcanic Ridge near 25° S–118° W at about 160 km west of the Easter microplate West Rift. Based on a reconstruction of the Easter microplate, it is suggested that the Crough seamount (< 800 m depth) was formed by earlier (7–10 Ma) hotspot magmatic activity which also created Easter Island.     

南海火山玻璃的分布特征、化学成分及源区探讨

研究了南海火山玻璃的分布特征、化学成分及源区,结果表明,表层沉积物中的火山玻璃含量基本呈南北向分布,由北部、西部、南部大陆架分别向中部、东部深海盆逐渐增加;柱样沉积物中的火山玻璃在深海盆出现的层数多、含量大.火山玻璃化学组分之间的相关性不显著,表层沉积物中的火山玻璃属于安山岩,柱样沉积物中的火山玻璃的岩石类型广泛,主要属于安山岩和流纹岩.菲律宾岛弧火山带、南海深海盆火山喷发以及印尼岛弧火山带是南海火山玻璃主要源区,火山玻璃搬运和沉积主要受台风、越赤道气流和环流的影响和控制.     

New seismo-stratigraphic and marine magnetic data of the Gulf of Pozzuoli (Naples Bay, Tyrrhenian Sea, Italy): inferences for the tectonic and magmatic events of the Phlegrean Fields volcanic complex (Campania)

A detailed reconstruction of the stratigraphic and tectonic setting of the Gulf of Pozzuoli (Naples Bay) is provided on the basis of newly acquired single channel seismic profiles coupled with already recorded marine magnetics gathering the volcanic nature of some seismic units. Inferences for the tectonic and magmatic setting of the Phlegrean Fields volcanic complex, a volcanic district surrounding the western part of the Gulf of Naples, where volcanism has been active since at least 50 ka, are also discussed. The Gulf of Pozzuoli represents the submerged border of the Phlegrean caldera, resulting from the volcano-tectonic collapse induced from the pyroclastic flow deposits of the Campanian Ignimbrite (35 ka). Several morpho-depositional units have been identified, i.e., the inner continental shelf, the central basin, the submerged volcanic banks and the outer continental shelf. The stratigraphic relationships between the Quaternary volcanic units related to the offshore caldera border and the overlying deposits of the Late Quaternary depositional sequence in the Gulf of Pozzuoli have been highlighted. Fourteen main seismic units, both volcanic and sedimentary, tectonically controlled due to contemporaneous folding and normal faulting have been revealed by geological interpretation. Volcanic dykes, characterized by acoustically transparent sub-vertical bodies, locally bounded by normal faults, testify to the magma uprising in correspondence with extensional structures. A large field of tuff cones interlayered with marine deposits off the island of Nisida, on the western rim of the gulf, is related to the emplacement of the Neapolitan Yellow Tuff deposits. A thick volcanic unit, exposed over a large area off the Capo Miseno volcanic edifice is connected with the Bacoli-Isola Pennata-Capo Miseno yellow tuffs, cropping out in the northern Phlegrean Fields.     

Ocean-bottom seismometer observations of seismic activity at Loihi Seamount,Hawaii

This study reports the result of deep ocean-bottom seismometer recording of an undersea volcanic event in progress. An array of five three-component, isolated sensor ocean-bottom seismometers (ISOBS) was deployed for 28 days on the summit and flanks of Loihi Seamount, Hawaii, to monitor seismicity. The deployment was prompted by reports from the Hawaiian Volcano Observatory (HVO) of a swarm of small-magnitude events located beneath the active submarine volcano in late September, 1986. Monitoring of this earthquake swarm by the University of Hawaii commenced 1 October 1986. Although seismicity tapered off rapidly after 11 October, more than 200 events were located. Systematic changes in spatial clustering during the initial swarm activity suggest changing patterns of stress within this crustal volume, possibly due to induced stress resulting from magma movement in the underlying crust or deep portions of the edifice.Most of the very shallow (< 10 km) events were located beneath the summit and southwest flank of the elongate edifice. No shallow tremor was observed despite a search through the data, although such tremor may have ceased prior to deployment of the ocean-bottom seismometers (OBS). Constraints on the association between seismicity and observed topographic and tectonic elements of Loihi are also of primary importance. Many of the earthquakes located near the steep flanks generated rock falls that were recorded on the OBSs. This is consistent with the results of dredge and bottom photography data indicating that the flanks are covered with fragments of shattered lava pillows and flows. Dike intrusion and mass wasting are major influences on the morphology of Loihi. Intact flows have been observed near the deep portion of the south rift zone; however, few events were located in that region during this swarm.     

Late Devonian rifting in the central North Sea: Evidence from altered felsic volcanic rocks in the Embla oil field

In the Embla oil field on the northern flank of the Mid North Sea High, the central North Sea, multiple quartz porphyric volcanic beds at ca. 4600 m depth form part of a volcano-sedimentary interval above the Caledonian basement as interpreted from seismic data. Zircon U–Pb laser ablation ICPMS date one bed to 374 ± 3 Ma, indicating that the volcanic rocks and interbedded sediments are early Famennian and correlate to the Buchan Formation. The volcanic rocks have been extensively clay and carbonate altered in a near-surface environment, but high field strength element data show that the protoliths were alkali rhyolites, yielding intra-plate signatures in tectonic discrimination diagrams. Famennian quartz porphyric volcanic rocks have also been reported from well A17-1 on the southern flank of the Mid North Sea High. The Famennian volcanism on the northern and southern flanks testify to an active magmatic environment in the central North Sea in the early Famennian, supporting the existence of a late Devonian proto-Central Graben rift extending northwards into the central North Sea. The rift is likely an early example of strain localisation to a zone of reduced crustal strength along the Caledonian suture between Avalonia and Baltica.     

Variations in axial processes on the Mid-Atlantic Ridge: The median valley of the MARK area

Submersible observations and photogeology document dramatic variations in the distribution of young volcanic rocks, faulting, fissuring, and hydrothermal activity along an 80 km-long segment of the Mid-Atlantic Ridge south of the Kane Transform (MARK Area). These variations define two spreading cells separated by a cell boundary zone or a small-offset transform zone. The northern spreading cell is characterized by a median ‘neovolcanic’ ridge which runs down the axis of the median valley floor for 40 km. This edifice is as much as 4 km wide and 600 m high and is composed of very lightly sedimented basalts inferred to be < 5000 years old. It is the largest single volcanic constructional feature discovered to date on the Mid-Atlantic Ridge. The active Snake Pit hydrothermal vent field is on the crest of this ridge and implies the presence of a magma chamber in the northern spreading cell. In contrast, the southern cell is characterized by small, individual volcanos similar in size to the central volcanos in the FAMOUS area. Two of the volcanos that were sampled appear to be composed of dominantly glassy basaltic rocks with very light sediment cover; whereas, other volcanos in this region appear to be older features. The boundary zone between the two spreading cells is intensely faulted and lacks young volcanic rocks. This area may also contain a small-offset ( < 8 km) transform zone. Magmatism in the northern cell has been episodic and tens of thousands of years have lapsed since the last major magmatic event there. In the southern cell, a more continuous style of volcanic accretion appears to be operative. The style of spreading in the southern cell may be much more typical for the Mid-Atlantic Ridge than that of the northern cell because the latter is adjacent to the 150 km-offset Kane Transform that may act as a thermal sink along the MAR. Such large transforms are not common on the MAR, therefore, lithosphere produced in a spreading cell influenced by a large transform may also be somewhat atypical.     

Vesteris Seamount: An enigma in the Greenland Basin

Vesteris Seamount is a solitary submarine volcano located at 73°30 N, 9°10W in the Greenland Basin. Steeply rising from a base depth of 3100 m to a minimum depth of ~ 130 m and striking 030°/210°, the feature lies ~ 300 km east of the east Greenland margin on an otherwise nearly flat and featureless seafloor. The main body of the seamount appears to have been formed episodically, the last of which culminated about 110 000 years ago. Subsequent, lower intensity volcanic activity continued sporadically until about 25 000 years ago, as evidenced by ash layers found in cores near the base of the feature. The smoothed surfaces at the summit make it likely that the seamount actually broached the surface during the Weichselian glacial period, between 8000 and 13 000 years ago. Two multibeam bathymetric investigations aboardPFS Polarstern during ARKTIS II/4 (1984) and ARKTIS VII/1 (1990), combined with geologic sampling, single-channel seismic profiling and underwater television coverage, have resulted in a new interpretation of both the morphology and origins of the seamount. Data collected aboardPolarstern from ARKTIS II/4 (1984) have been previously reported by Hempelet al. (1991), however, when combined with the ARKTIS VII/1 (1990) data set, a more detailed interpretation of the morphology and structure was feasible. This included the elongated shape of the feature and showed the existence of several small volcanic cones on the seamount flanks.The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Petrology, Geochemistry and Nd-Sr-Pb Isotopic Properties of Volcanic Rocks in Daheishan Island, Penglai, Shandong Province

The major elements, trace elements, K-Ar age and Sr-Nd-Pb isotopic systems of the Cenozoic volcanic rocks in Daheishan Island and Cishan, Penglai, Shandong Province are measured. The volcanic rocks ( olivine-nephelinite and nepheline-basanite ) in DaheishanIsland erupted periodically in an interval of 0.32 Ma, from 8.72 Mm 8.39 Ma, 8.08 Ma to 7.73Ma. The volcanic rocks are all rich in light REEs. They are similar to the OIB-type alkali basalt in the trace elements normalized model by primordial mantle: rich in high field elements such as Nb and Ta, and imcompatible elements such as Cs, Rb, Ba, Th, U. The volcanic rocks show a depletion of K and Rb elements. It is suggested by the trace elements that the olivine-nephelinite in Daheishan Island is originated fi‘om deep resources under the continental mantle, ε Nd (0) values of the volcanic rocks in Daheishan Island and Cishan are 5.31 - 8.51 and 7.33 respectively, suggesting that the volcanic rocks are from the depleted mantle resources, which have higher Sm/Nd ratios than the CHUR. ^143Nd /^144Nd ratios of Daheishan Island olivine-nephelinite and Cishan alkali basalts are 0.512 910 - 0.513 074 and 0.513 014 resoeetivelv. The ^87Sr /^86Sr of Daheishan lsland volcanic rocks are lower than that of Cishan, 0.703 427 - 0.703 482 and 0.703 895 respectively. The Daheishan Island olivinenephelinite has the Pb isotopic values as follows: ^206Pb /^204pb= 18.028 9 ～17.972 8,^207Pb /^204Pb= 15,435 8 - 15.402 2 and ^208Pb /^204Pb=38.087 6 - 37.997 5, lower than those of Cishan basanite. The Cishan basanite has ^206Pb / ^204Pb = 18.240 1, ^208Pb /^204Pb = 15.564 5 and ^208Pb /^204Pb=38.535. The authors suggest that the olivine-nephelinite in Daheishan Island is similar to the E-type MORB or Hawaii OIB, and the alkali basalts in Cishan similar to the Kerguelen OIB. The dominant mantle components of DM PREMA and perhaps DM( Dupal type ) are the dominant mantle components for volcanic rocks in Daheishan Island and Cishan. The PREMA component plays an important role.     

Co–rich Mn crusts from the Magellan Seamount cluster: the long journey through time

The Magellan seamounts began forming as large submarine shield volcanoes south of the equator during the Cretaceous. These volcanoes formed as a cluster on the small Pacific plate in a period when tectonic stress was absent. Thermal subsidence of the seafloor led to sinking of these volcanoes and the formation of guyots as the seamounts crossed the equatorial South Pacific (10–0°S) sequentially and ocean surface temperatures became too high for calcareous organisms to survive. Guyot formation was completed between about 59 and 45 Ma and the guyots became phosphatized at about 39–34 and 27–21 Ma. Ferromanganese crusts began formation as proto-crusts on the seamounts and guyots of the Magellan Seamount cluster towards the end of the Cretaceous up to 55 Ma after the formation of the seamounts themselves. The chemical composition of these crusts evolved over time in a series of steps in response to changes in global climate and ocean circulation. The great thickness of these crusts (up to 15–20 cm) reflects their very long period of growth. The high Co contents of the outer parts of the crusts are a consequence of the increasing deep circulation of the ocean and the resulting deepening of the oxygen minimum zone with time. Growth of the Co-rich Mn crusts in the Magellan Seamount cluster can be considered to be the culmination of a long journey through time.     

Geochemical features of the Okhotsk Sea Cenozoic volcanism

Cenozoic volcanic rocks were discovered on major rises of the inner Okhotsk Sea. In this paper, these rocks are geochemically typified, and the geodynamic conditions of their formation reconstructed. For this purpose, mineralogical and geochemical analyses as well as radioisotope age determinations were carried out. The radioisotope age determinations show that the Cenozoic volcanic rocks were formed during the Paleogene–Pleistocene. Within this period, Eocene and Plio-Pleistocene volcanic complexes are particularly prominent. Mineralogical and geochemical analyses show that the rocks belong to the calc-alkaline volcanic series. The Eocene volcanic rocks were formed under subaerial conditions, whereas the Plio-Pleistocene volcanics were formed under submarine conditions. The results of the study suggest that the Okhotsk Sea Basin was formed during the destruction of the Asian continental margin, the Eocene volcanism reflecting subaerial, the Plio-Pleistocene volcanism submarine stages of the Okhotsk Sea evolution.     

Prolonged Magmatic and Tectonic Development of the Caribbean Igneous Province Revealed by a Diving Submersible Survey

Using the diving submersible survey NAUTICA we investigated the central part of the Caribbean large igneous province (CLIP) to observe and sample internal portions of this proposed oceanic plateau. Most of the samples are gabbroic and doleritic rocks; basalts are scarce. Radiometric dating by ⁴⁰Ar/³⁹Ar incremental heating experiments indicate that the intrusive rocks are Campanian in age (81–75 Ma). In some places these intrusive rocks underlie older Santonian (85–83 Ma) extrusive basaltic rocks, suggesting that the Campanian rocks represent a sill injection and an underplating episode. Results of the diving program supplemented by information from ODP and DSDP drilling sites document a 20 m.y. period (94–75 Ma) of igneous activity in the submerged portion of the Caribbean large igneous province (CLIP). In the northern part of the Beata Ridge late Campanian and/or post Campanian uplift is documented by prominent Maastrichtian (71–65 Ma) erosion and the establishment of a Paleocene-middle Eocene (65–49 Ma) carbonate platform. During and after the uplift an extensional period is indicated by seismic images and the subsidence (3 km depth) of the carbonate platform. Paleocene ages (55–56 Ma) determined on some volcanic samples are attributed to localised decompression mantle melting that accompanied the extension. We document a prolonged period of magmatic and tectonic events that do not fit with the current models of short-lived plateau formation during mantle plume initiation but shares many similarities with the constructional histories of other oceanic large igneous provinces.     

晚中新世南海珍贝-黄岩海山岩浆活动及其演化:岩石地球化学和年代学证据

南海海盆15°N附近呈东西向展布的珍贝-黄岩海山被认为是32~17Ma前南海海盆的残留扩张中心.对采自黄岩海山的两个火山岩样品(9DG,9DG-2)进行了岩石学、地球化学和年代学研究.两个样品的SiO₂含量分别为60.3%和63.6%,Al₂O₃含量分别为17.56%和17.55%,TiO₂含量分别为0.48%和0.31%,碱度率分别为3.88和3.62.根据岩石学和岩石化学分类,样品属碱性系列的粗面岩.对稀土元素和微量元素分析表明岩石具有洋岛玄武岩(OIB)型配分型式,轻重稀土总量比(∑c(LREE)/∑c(HREE))和球粒陨石标准化镧镱比((La/Yb)N)分别高达17.22和27.23,并具有铕负异常和锶、磷、钛亏损的特点.样品9DG的锶-钕-铅同位素分析结果为锶-87的含量与锶-86的含量之比值为0.704183,钕-143的含量与钕-144的含量之比值为0.512827,铅-206的含量与铅-207的含量之比值为18.68668,铅-207的含量与铅-204的含量之值为15.67962,铅-208的含量与铅-204的含量之比值为39.00261,表明初始岩浆来自软流圈地幔,具有与珍贝海山玄武岩相似的同位素组成.经钾-氩法测年,粗面岩的年龄为(7.77±0.49)Ma,略晚于珍贝海山玄武岩的年龄[(9.1±1.29)~10.0±1.80Ma],属于南海扩张期后晚中新世火山活动的产物.对比珍贝海山玄武岩的地球化学和同位素特征,认为两者有相同的岩浆源区,但是它们经历了不同程度的结晶分异过程,在晚中新世期间珍贝-黄岩海山可能有地幔柱活动.     

总磁异常△T与油气田火山岩展布研究

东海某油气田研究区分布有火山岩。鉴于火山岩具有磁性这一特征,基于总磁异常△T成果,采用总磁异常△T变倾角化极技术、小波多尺度分解技术,研究了火山岩在该区的展布与厚度。研究结果表明,具备必需的物性、磁性体分布规模等条件,在油气田开发中采用磁测成果研究磁性体展布是可行的。