The geology and geophysics of the ambrym caldera,New Hebrides

Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are.     

中国内蒙古白云鄂博Fe-Nb-REE矿床地质特征与陨石撞击成因

中国内蒙古白云鄂博Ｆｅ－Ｎｂ－ＲＥＥ矿床由于其独特的矿床地质与矿物地球化学特征而明显有别于世界上其它绝大多数的Ｆｅ、Ｎｂ、ＲＥＥ矿床。本文提出该矿床是约１５００Ｍａ前一块富含ＲＥＥ、Ｎｂ的铁陨石撞击在华北地台北缘碳酸盐沉积陆架上形成的。现在矿床矿体中心的块状铁矿石更多地代表了陨石的成分,矿体外侧富含ＲＥＥ、Ｎｂ的环状白云岩是陨石撞击浅海碳酸盐沉积陆架释放的巨大能量使海水（含Ｍｇ）迅速受热溶解陨石中的ＲＥＥ、Ｎｂ等形成的含金属热液蚀变沉积成因方解石质碳酸盐沉积物的结果。块状铁矿石和环状白云岩之间矿物成分异常复杂、结晶颗粒十分细小的条带状、侵染状Ｆｅ－Ｎｂ－ＲＥＥ矿石组成的矿体则是撞击瞬间导致的由陨石（Ｆｅ、Ｎｂ、ＲＥＥ）、海水（Ｍｇ、Ｎａ、Ｋ等）和碳酸盐（含少量硅质夹层）（Ｃａ、Ｓｉ、Ａｌ等）组成的超高温、多组分、富水体系在快速冷却条件下反应生成的产物。撞击释放的巨大热能,在撞击体（陨石）下部相对不易释放,导致部分碳酸盐沉积物在富含ＲＥＥ、Ｎｂ、Ｍｇ等的热液参与下融熔,然后侵入到周围岩石裂隙形成矿区内众多的富含ＲＥＥ、Ｎｂ的白云岩脉。撞击的瞬间导致大量岩石、矿物碎屑、海水、蒸汽弥漫天空,同时引发巨大的海啸,然     

A seamless system for the collection and cultivation of extremophiles from deep-ocean hydrothermal vents

Pele's Pit is a 300-m-deep pit crater whose summit rises to a depth of 1000 m on the Loihi submarine volcano 34 km south of the island of Hawaii. Hydrothermal vents with water temperatures up to 198/spl deg/C have been observed at the basal edge of the pit. We fabricated a "seamless" system comprising submersible-mounted samplers, a helium-activated sample transfer system, and a series of 1-l Teflon-lined stainless-steel bioreactors that maintain ambient temperature and pressure in hydrothermal vent fluids collected from hydrothermal vents in the pit crater. The bioreactors can sustain pressures to 150 atm. and temperatures over 165/spl deg/C. The system was deployed during October 2001 Pisces V submersible operations on Pele's Pit during which the maximum vent water temperature measured was 90/spl deg/C.     

Petrology and geochemistry of back-arc basin basalts from Lau Basin spreading ridges at 15°, 18° and 19°S

Summary Fresh back-arc basin lavas were recovered during five dives of the submersible Mir during the 1990 cruise of the research vessel Akademik Mstislav Keldysh to the Lau Basin. Three dives were conducted on the central spreading center of the King's Triple Junction (KTJ) in the northeastern part of the Lau Basin east of Niuafo'ou Island at approximately 15°S. The lavas from the KTJ can be divided into types I and II based on their similarities to N-MORB and the BABB magma type ofSinton andFryer (1987) respectively. One dive each was made on the Central Lau Spreading Center (CLSC) at 18°S and the Eastern Lau Spreading Center (ELSC) at 19°S. Lavas sampled on the CLSC were associated with active hydrothermal sulphide chimneys occurring at the base of a collapsed caldera structure on the central volcanic axial high. Sampled lavas from both the CLSC and ELSC are all of type I geochemistry.The results of the Keldysh 90 cruise are integrated with previous work to evaluate the geochemical characteristics of the Lau Basin crust as a whole and geochemical zonation models for back-arc basin development. An important part of this review of Lau Basin basalt geochemistry is the recognition of boninites and rocks of boninitic affinity which occur at off-ridge locations throughout the Lau Basin. These are suggested to erupt during the initial stages of development of new spreading ridges associated with episodes of ridge jump or the propagation of back-arc spreading ridges into arc crust.Hf-Th-Ta systematics of the Lau Basin lavas are used to identify unmodified mantle source compositions and possible subduction-related enriched components. Unmodified mantle source compositions range from D-, N-, and E-MORB to OIB. All Lau Basin lavas show some evidence of enrichment by a H₂O ± LILE ± LREE-enriched component, a slab derived hydrous fluid. The slab-derived fluid is not homogeneous in composition perhaps reflecting the presence or absence of a subducted sediment component.The sampled lavas from the KTJ confirm the uniqueness of the BABB magma type ofSinton andFryer (1987).

---

Zusammenfassung Dieser Artikel berichtet über erste Ergebnisse einer Forschungsexpedition in das Lau Basin, ausgeführt mit dem Forschungsschiff Akademiker Mstislav Keldysh und dem Mini-U-Boot Mir. Drei Tauchfahrten führten zur zentralen Achse der King's triple junction (KTJ) im nordöstlichen Teil des Lau Basins östlich der Insel Niuafo'ou (15°S). Die dort beprobten Laven werden auf der Basis ihrer chemischen Zusammensetzung eingeteilt in die Typen I und II, entsprechend der Klassifikation vonSinton andFryer (1987) in N-MORB und BABB. Eine vierte Tauchfahrt zum zentralen Lau Spreading Center (CLSC, 18°N) erbrachte Fragmente von Sulphidschloten eines aktiven Black Smoker innerhalb einer kollabierten Calderastructur. Eine fünfte Tauchfahrt zum östlichen Lau Spreading Center (ELSC, 19°N) erbrachte Laven vom Typus I (entsprechend N-MORB).Unter Berücksichtigung früherer Arbeiten gestatten die Ergebnisse der Keldysh 90 Expedition eine geochemische Charakterisierung des Lau Basins, sowie eine Bewertung bestehender Zonierungsmodelle fur die Entstehung und Evolution von Backarc Basins. Entlang der gesamten Achse im Lau Basin, aber abseits des eigentlichen aktiven Spreading Centers, werden Boninite und Laven boninitischer Affinität beobachtet. Diese Boninite können als integraler Teil des Frühstadiums eines Backarc Spreading Centers gedeutet werden, und entstehen möglicherweise dort, wo Spreading Centers in die Kruste eines Inselbogens vordringen oder dann, wenn es zu einer abrupten Positionsänderung der Achse kommt.Die Hf-Th-Ta Konzentrationen und Verhältnisse in Lau Basin Laven gestatten die Identifikation mehrerer Mantel-Endglieder (entsprechend D-MORB, N-MORB, E-MORB, und OIB), sowie einer an H₂O und LILE-LREE angereicherten Komponente, die mit Subduktionstätigkeit und anschließender Mantelmetasomatose in Zusammenhang gebracht wird. Die angereicherte Komponente war vermutlich inhomogen aufgrund variabler Anteile subduzierter Sedimente. Die Laven des Lau Basins bestätigen die vonSinton andFryer (1987) vorgeschlagene geochemische Sonderstellung von BABB Magmentypen.

---

---

With 8 Figures     

Noble gas systematics of deep rift zone glasses from Loihi Seamount, Hawaii

We report new noble gas fusion and crushing data for six pillow rim glasses, recovered between 3 and 5 km water depth on the south rift zone of Loihi Seamount, Hawaii. Helium abundances of the glasses vary from 0.3 to 2.3 μcc/g, with ⁴He/³He ratios between 30000 and 27000 (24–27 R_A), similar to previously reported values. The neon data form a correlation line which is similar to the Loihi-Kilauea line reported by Honda et al. [1], but extends to much higher ratios, up to 12.9 and 0.0382 for the ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios, respectively. This provides conclusive evidence for the suggestion that the Hawaiian plume, thought to originate in the lower mantle, has a solar-like ²⁰Ne/²²Ne composition [1], but a slightly higher ²¹Ne/²²Ne ratio. ⁴⁰Ar/³⁶Ar ratios of the deep rift-zone glasses are as high as 2600, and show a positive correlation with neon isotopic ratios. In contrast to neon and argon, all xenon isotopic compositions are isotopically indistinguishable from air, which either suggests preferential atmospheric contamination of xenon, or could indicate an atmospheric xenon isotopic composition for the lower mantle.     

Magnetic measurements over the Hawaiian ridge and their vulcanological implications

The results of an aeromagnetic survey of the Hawaiian Islands are studied in terms of the local magnetic anomalies defined and their relation to known centers of vulcanism. A quantitative analysis as to the depth of origin of the anomalies indicates that in nearly all cases they can be attributed to sub-surface intrusive rock masses at depths of the order of 2 to 6 kilometers. That the intrusive rocks defined are probably related to intrusions of magma derived from the underlying mantle is indicated by both quantitative studies of associated gravity anomalies and seismic refraction crustal measurements. On the basis of these integrated geophysical studies, it appears that there has been extensive intrusion of mantle-derived magma into the crust along the East-West trending submarine Molokai Fracture Zone and that where this fracture system intersected a Northwest-Southeast zone of tectonic weakness or fracture there was vulcanism that led to the formation of the Hawaiian Islands. Although magnetic data are lacking as yet over the entire Hawaiian Ridge from Hawaii to Midway Island, it is not an unreasonable hypothesis that the Ridge as a whole developed in a similar manner over its entire length as a progressive feature with time, in response to a secular shift in crustal stress pattern at the intersection of two major translational fault systems.