Adjacent Seamounts Research Articles

Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.

Geodynamic evolution of the Galápagos hot spot system (Central East Pacific) over the past 20 m.y.: Constraints from morphology, geochemistry, and magnetic anomalies

We report results of magnetic data from the Nazca Plate and of geochemical (major element and Sr‐Nd‐Pb‐isotope) analyses of rocks dredged from the Galápagos hot spot tracks (Cocos, Carnegie, Malpelo and Coiba Ridges and adjacent seamounts) in the Central East Pacific. Magnetic anomalies indicate that the Malpelo and Carnegie Ridges were once attached and that seafloor spreading separated the two ridges between 14.5 Ma and 9.5 Ma. The variations in Sr‐Nd‐Pb isotopic composition show that three of the mantle components currently observed at the Galápagos (Central, Southern, and Eastern) existed in the hot spot for at least 20 m.y., whereas the Northern Galápagos mantle component has been present for at least ∼15 Ma. Our data are consistent with the existence of a compositionally zoned/striped Galápagos plume since ∼20 Ma. Combined constraints from the morphology of the hot spot tracks, the magnetic record, and the isotope geochemistry of the rock samples provide new insights into the hot spot‐ridge geometry and interaction of the Galápagos hot spot with the Cocos‐Nazca spreading center (CNS) over the past 20 m.y. At 19.5 Ma a ridge jump moved the spreading axis to the northern edge of the hot spot. Between 19.5 and 14.5 Ma, the spreading axis was located above the center of the hot spot. At 14.5 Ma, a new ridge jump moved the spreading axis to the south, splitting the paleo‐Carnegie Ridge into the present Carnegie and Malpelo Ridges. The repeated ridge jumps reflect capture of the northwardly drifting spreading center by the Galápagos hot spot. At 11–12 Ma an offset of the spreading axis lay above the plume center. Spreading between the Carnegie and Malpelo Ridges continued until 9.5 Ma.

New volcanological and volatile data provide strong support for the continuous existence of Gal�pagos Islands over the past 17�million�years

The first systematic rock sampling of volcanoes along the Galapagos hotspot tracks (the aseismic Cocos, Carnegie, Malpelo and Coiba ridges and adjacent seamounts) in the area between the Galapagos Islands and Central and South America was carried out on R/V Sonne cruise 144-3. Guyot-shaped seamounts, paleo-beach or intertidal wave-cut platform deposits, the structure and texture of volcanic rocks, and low sulfur contents of fresh glasses dredged at these volcanoes imply that ocean islands existed continuously above the Galapagos hotspot for at least the past 17 million years. These new data significantly extend the time period over which the unique endemic Galapagos fauna could have evolved, providing a complete solution to the long-standing enigma of the evolution of Galapagos land and marine iguanas.

10.1016/0967-0653(95)94328-n

10.1016/0967-0653(95)94328-n

Geochemistry and significance of basaltic rocks dredged from the South Tasman Rise and adjacent seamounts

Basalts dredged from two seamounts separated by some 200 km and built on ca 50–35 Ma oceanic crust west of Tasmania have slightly LREE‐depleted N‐MORB affinities and were probably generated in a spreading ridge or near‐ridge setting. They are compositionally very similar to 60 Ma basalts drilled at DSDP Site 282, midway between these seamounts. Taken together, these basalts provide a useful dataset for the compositional features of MORB erupted early in the rifting history of Tasmania separating from Antarctica. Well‐preserved intraplate alkaline basalts with HIMU geochemical affinities have been sampled from Cascade Seamount, which forms the peak of the East Tasman Plateau. Far more altered alkaline intraplate basalts dredged at several other locations within the South Tasman Rise south of Tasmania, and on seamounts built on oceanic crust and thinned continental crust south of the East Tasman Plateau, also show typically HIMU affinities for immobile trace‐element ratios (e.g. low Zr/Nb values, 3–5), but have undergone strong rare‐earth element mobility associated with sea‐floor weathering and replacement of glass by chemisorbed biogenic phosphate. Dolerites with some strong affinities to the Jurassic dolerites of Tasmania were dredged in the central section of the South Tasman Rise.

Trade-off in production between adjacent seamount chains near the East Pacific Rise

BATHYMETRIC data and side-scan sonar images collected near the southern East Pacific Rise between 15° and 19° S1, 2 have revealed many seamounts on the west flank of the rise, most of which are organized into chains running perpendicular to the rise axis. The number of seamounts of >5 km diameter is at least twice that expected for average East Pacific sea floor3. The formation of numerous, small ( 5 Myr), linear seamount chains and the tendency for fresh lava flows to be found primarily at the near-ridge ends of the chains suggest that there are plume-like sources in the upper mantle, which are relatively stationary, active for extended periods and affected by adjacent sources.

Geochemistry of Lau Basin volcanic rocks: influence of ridge segmentation and arc proximity

Abstract Side-scan sonar imaging of the Central Lau Basin (SW Pacific) has revealed a Central Lau Spreading Centre (CLSC) propagating southwards at the expense of an Eastern Lau Spreading Centre (ELSC) with a small Intermediate Lau Spreading Centre (ILSC) forming a ‘relay’ between the two. Volcanic rocks sampled along these spreading centres, and from two adjacent seamounts, are glassy to fine-grained pillow lavas and sheet flows of basalts, ferrobasalts and andesites. The evolved rocks are mostly confined to the propagating tip of the CLSC and can be explained by a high rate of cooling relative to magma supply, as invoked for magma genesis at propagating ridges elsewhere. Compared with equivalent rocks from the eastern Pacific, the most evolved members of the CLSC suite require similarly high degrees (>90%) of fractional crystallization from their basaltic parents. Fractional crystallisation cannot, however, account for the compositional differences between CLSC and ELSC lavas. Whereas the composition of the CLSC lavas lies just within the compositional spectrum of typical N-MORB, the ELSC lavas are distinctly enriched in alkali and alkaline earth elements, reach oxide and apatite saturation at lower Fe, Ti and P concentrations and generally show greater vesicularity despite slightly greater depths of eruption, all indicative of a water-rich subduction component. They also have lower contents of Ni, Sc, Na and Fe and higher contents of Si at a given MgO concentration that indicate a more depleted and more hydrous mantle beneath the ELSC compared with the CLSC. These results provide further evidence that, beneath the Central Lau Basin, the source composition changes progressively from MORB-type to island arc tholeiite type as the subduction zone is approached, both eastwards from the CLSC to ELSC and southwards along the ELSC to the Valu Fa Ridge. They also indicate that the composition of the subduction component may vary systematically away from the arc, with Th, LREE, Ba, Rb and H (as H 2 O) all present close to the arc, only Ba, Rb and H 2 O present at intermediate distances and just H 2 O perceptible at the furthest distances.

Axial and off‐axial heterogeneity of basaltic rocks from the East Pacific Rise at 12°35′N–12°51′N and 11°26′N–11°30′N

Surface ship operations and 40 submersible dives have provided a large amount of field observations and rock samples from two segments of the EPR at 12°35′N–12°51′N and at 11°26′N–11°30′N and from four adjacent seamounts centered at less than 18 km from the rise axis. The basaltic samples show a great variety of morphological features and a diversity in the proportions of early formed mineral phases and chemical composition. There is no correlation between lava flow morphology, geological settings, and compositions. Based on mineral and compatible element composition, these basalts were classified into three types. The least evolved olivine‐basalts and highly phyric plagioclase‐basalts (type A) have high Fo87–89, high Mg # (66–70), and high Ni (>100) contents. The most evolved plagioclase‐olivine basalts with or without pyroxene (type B and C) have relatively low Fo78–86, low Mg # (55–65), and low Ni (60–100 ppm) contents. Simple crystal fractionation (3–18%) accounts for some of the observed compositional range. However, based on their incompatible element contents, the above basalts are divided into three groups (the various types A to C are found in each group): depleted basalts with low K/Ti (0.04–0.15) ratios, low Zr (<100 ppm) and low Nb (<0.4 ppm) contents, undepleted basalts with the highest K/Ti (0.25–0.46), Zr (150–170 ppm), and Nb (8–16 ppm) contents and the transitional basalts having compositions between these two extremes. These three basaltic groups occur within limited portions of the axial graben and its limbs in areas of less than 10 km in length and less than 3–4 km in width near 12°43′N–12°51′N on the EPR. Similar volcanic diversity in a more limited area (<3 km2) is observed on off‐axial seamounts and other constructional features centered up to 6 and 18 km from the rise axis. Depleted and undepleted melts are believed to have been formed during sequential batch melting of a composite mantle. The transitional basalts probably result from a mixing of depleted and undepleted liquids which have undergone variable degrees of crystal fractionation. Two distinct mantle sources are distinguishable by their incompatible element contents and their susceptibility to melting; we suggest that a lherzolitic mantle, with minerals having a variable behavior under partial melting, constitutes this type of composite mantle. Indeed, clinopyroxene, enriched in incompatible elements, will be consumed at an earlier stage of melting than the olivine plus orthopyroxene. Eruption of compositionally distinct magma batches in close proximity on axial and off‐axial structures implies periods of quiescence between successive magmatic stages and cycles.

Determination of Magnetic Polarities of Dredged Basalts: A Limitation of the Method

Magnetic polarity determinations were attempted on 17 unoriented specimens of basalt dredged from 11 locations on Juan de Fuca Ridge and adjacent Heck and Heckle Seamount Chains (Northeast Pacific Ocean). Irving's method involving comparison of NRM and ARM demagnetization curves appears to work consistently only when the NRM has been demagnetized to about 2% or less of its initial intensity and when the ratio of intensity of initial ARM to intensity of residual NRM is high (in the order of 50). For 11 samples meeting these requirements, the polarity determined matches that predicted from the magnetic anomalies in the area of the dredge station. For the other specimens, it is shown that the position of the ARM demagnetization curve was not stabilized under the conditions of the experiments and normally magnetized samples cannot be reliably distinguished from reversed rocks.

Preliminary report on magnetic anomalies between Adak, Alaska, and Kwajalein, Marshall Islands

A continuous record of variations of the total magnetic field intensity was taken along a single flight line from Adak, Alaska to Kwajalein, Marshall Islands, on Project Volcano during the summer of 1947. Anomalies as large as 750 gamma extending over distances of 20 to 30 mi were found over open ocean areas between Adak and Midway, where ocean depths range from 2000 to 3000 fathoms. A rather complete survey, at an altitude of 1500 ft above sea level, was made over Bikini Atoll and the adjacent seamount. A broad negative anomaly of 750 gammas was recorded over Bikini, with several superimposed more localized anomalies; those associated with the adjacent seamount were larger than those associated with Bikini.