Manihiki Plateau Research Articles

Depleted mantle wedge and sediment fingerprint in unusual basalts from the Manihiki plateau, central Pacific Ocean: Comment and Reply: COMMENT

[Ingle et al. (2007)][1] reported 40Ar/39Ar ages for Manihiki plateau rocks and suggested some of them were contemporaneous (~120 Ma) with lavas from the Ontong Java and Hikurangi plateaus, each some 2500 km distant ([Ingle et al., 2007][1], their Fig. 1). Critical examination of the radiometric

A geomagnetic paleointensity stack between 0.8 and 3.0 Ma from equatorial Pacific sediment cores

We have conducted a paleomagnetic study of six sediment cores taken from the equatorial Pacific, three from the West Caroline Basin and three from the Manihiki Plateau, in order to make a relative paleointensity stack during the Matuyama and late Gauss Chrons. The age of the bottom of the cores ranges from 1.2 to 3.0 Ma. The sediments show little downcore changes in the proxies of magnetic grain size and mineralogy and are hence suitable for estimating relative paleointensity. The age of the cores is controlled by correlating variations in magnetic concentration (magnetic susceptibility and/or anhysteretic remanent magnetization (ARM)) to target oxygen‐isotope (δ18O) curves. All cores from the Manihiki Plateau show an upward decrease of the natural remanent magnetization (NRM) intensity normalized by ARM and isothermal remanent magnetization (IRM) with a decrease in sedimentation rate. Such a trend was removed before being converted to relative paleointensity. This observation implies that a sedimentation‐rate change can affect relative paleointensity estimation. Relative paleointensity records from the six cores coincide well with each other within uncertainty of age. We constructed a stacked curve between 0.8 and 3.0 Ma (the equatorial Pacific paleointensity stack EPAPIS‐3Ma) after adjusting age; the number of cores stacked is three to four throughout the record. Quasiperiodic paleointensity lows occur in the EPAPIS‐3Ma, and corresponding paleointensity lows can be found in previously published records with some shifts in age. At least, parts of the paleointensity minima seem to be accompanied by geomagnetic excursions. The “asymmetric sawtooth pattern” of paleointensity variations is not observed at the Matuyama‐Gauss boundary and thereafter in our record. A spectral analysis shows that ∼100 kyr orbital eccentricity frequency may exist in paleointensity variations during the Matuyama and late Gauss Chrons.

Tectonic evolution of the Pacific–Phoenix–Farallon triple junction in the South Pacific Ocean

Analysis of multibeam and gravity data reveals the tectonic history of the mid-Cretaceous (119–107 Ma) Penrhyn basin in the equatorial south Pacific Ocean. The basin formed during a period of heightened geodynamic activity and cessation of magnetic reversals. Similarities in the geometry of the Tongareva triple junction and the Rodriguez triple junction in the Indian Ocean make this study an interesting comparison between modern and ancient tectonics. Changes in abyssal hill trends during the formation of the basin suggest either a change in the location of the Euler pole describing the relative motion between the Pacific and Farallon plates, or a significant period of oblique spreading. Interaction between the local stress field associated with the break-up of the Manihiki plateau and the regional stress field controlling major plate motions complicated the tectonic evolution of the Penrhyn basin. Construction of velocity triangles from abyssal hill trends and measurements of the triple junction trace suggests that the triple junction oscillated between ridge–ridge–ridge and ridge–ridge–fault configurations. At least two reorganizations in the geometry of the triple junction occurred within 10 Ma of the initial rifting of the Manihiki plateau. Both changes in triple junction geometry coincide with discontinuities in the triple junction trace and result in right-lateral displacements of the triple junction trace. Changes in the bathymetric expression of the triple junction trace suggest a period of triple junction propagation controlled by rift propagation shortly after the change in Euler pole location.

Nannofossil carbonate fluxes during the Early Cretaceous: Phytoplankton response to nutrification episodes, atmospheric CO2, and anoxia

Greenhouse episodes during the Valanginian and Aptian correlate with major perturbations in the C cycle and in marine ecosystems, carbonate crises, and widespread deposition of Corg‐rich black shales. Quantitative analyses of nannofossil micrite were conducted on continuous pelagic sections from the Southern Alps (northern Italy), where high‐resolution integrated stratigraphy allows precise dating of Early Cretaceous geological events. Rock‐forming calcareous nannofloras were quantified in smear slides and thin sections to obtain relative and absolute abundances and paleofluxes that are interpreted as the response of calcareous phytoplankton to global changes in the ocean‐atmosphere system. Increased rates of volcanism during the formation of Ontong Java and Manihiki Plateaus and the Paranà‐Etendeka large igneous province (LIP) are proposed to have caused the geological responses associated with early Aptian oceanic anoxic event (OAE) 1a and the Valanginian event, respectively. Calcareous nannofloras reacted to the new conditions of higher pCO2 and fertility by drastically reducing calcification. The Valanginian event is marked by a 65% reduction in nannofossil paleofluxes that would correspond to a 2–3 times increase in pCO2 during formation of the Paranà‐Endenteka LIP. A 90% reduction in nannofossil paleofluxes, which occurred in a 1.5 myr‐long interval leading into OAE1a, is interpreted as the result of a 3–6 times increase in pCO2 produced by emplacement of the giant Ontong Java and Manihiki Plateaus. High pCO2 was balanced back by an accelerated biological pump during the Valanginian episode, but not during OAE1a, suggesting persisting high levels of pCO2 in the late Aptian and/or the inability of calcareous phytoplankton to absorb excess pCO2 above threshold values.

Petrology of the greenstones of the Lower Sorachi Group in the Sorachi–Yezo Belt, central Hokkaido, Japan, with special reference to discrimination between oceanic plateau basalts and mid‐oceanic ridge basalts

Abstract The Lower Sorachi Group of the Sorachi–Yezo Belt in central Hokkaido, Japan is a peculiar accretionary complex characterized by numerous occurrences of greenstones (metabasalts and diabases), which are mostly composed of aphyric basalts. Clinopyroxene‐rich phenocryst assemblage in phyric basalts is different from olivine–plagioclase assemblage in mid‐oceanic ridge basalts (MORB). The greenstones are geochemically uniform, and show a lower‐Ti trend than MORB in an FeO*/MgO‐TiO2 diagram, mostly plotting on the island arc tholeiite (IAT) field in a TiO2−10MnO−10P2O5 diagram. In a MORB‐normalized spider diagram, the greenstones show a flat pattern from P to Y, which are lower than those of normal mid‐oceanic ridge basalt (N‐MORB). These indicate that the greenstones were derived by a higher degree of partial melting from a depleted mantle similar to a N‐MORB source, and experienced olivine–clinopyroxene fractional crystallization. However, a positive spike of Nb in the spider diagram cannot be explained, and may be attributed to mantle heterogeneity. These characteristics are analogous to those of oceanic plateau basalts (OPB) such as in Ontong Java Plateau, Manihiki Plateau and Nauru Basin, suggesting that the greenstones in the Lower Sorachi Group are of oceanic plateau origin. The present study proposes new field divisions to distinguish OPB from MORB in the conventional FeO*/MgO–TiO2 and TiO2−10MnO−10P2O5 diagrams.

Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the southwestern Pacific Basin

The trace of the ridge-ridge-ridge triple junction that connected the Pacific, Farallon, and Phoenix plates during mid-Cretaceous time originates at the northeast corner of the Manihiki Plateau near the Tongareva atoll, for which the structure is named. The triple junction trace extends >3250 km south-southeast, to and beyond a magnetic anomaly 34 bight. It is identified by the intersection of nearly orthogonal abyssal hill fabrics, which mark the former intersections of the Pacific-Phoenix and Pacific-Farallon Ridges. A distinct trough is commonly present at the intersection. A volcanic episode from 125 to 120 Ma created the Manihiki Plateau with at least twice its present volume, and displaced the triple junction southeast from the Nova-Canton Trough to the newly formed Manihiki Plateau. Almost simultaneously, the plateau was rifted by the new triple junction system, and large fragments of the plateau were rafted away to the south and east. The Tongareva triple junction originated ca. 119 Ma, when carbonate sedimentation began atop the Manihiki Plateau. Subsequent spreading rates on the Pacific-Phoenix and Pacific-Farallon Ridges averaged 18–20 cm/yr until 84 Ma.

Onset of the Mid‐Cretaceous greenhouse in the Barremian‐Aptian: Igneous events and the biological, sedimentary, and geochemical responses

Basalts and biostratigraphy dated at 125–120 Ma from the Ontong Java and Manihiki Plateaus in the western Pacific evidence the largest volcanic event in Earth history in at least the past 160 m.y. The intervening Nova‐Canton Trough rifted at about 121–118 Ma, and a number of guyots and seamounts formed concurrently or slightly later. Geological events that probably were responses to these volcanic/tectonic events occurred in the following chronostratigraphic order. Biotic fluctuations began at about 122.5 Ma. At about 122.0 Ma, 87Sr/86Sr began to decline slowly. Metal concentrations of Co, Mn, Pb, Yb, and Cu in sediments peaked at about 121.5–121.2 Ma. Changes in planktonic communities and sedimentation culminated in a nannoconid “crisis” just prior to 120.5 Ma and in the Selli black shale (OAE la) at about 120.5–119.5 Ma. A sharp drop in δ13C occurred at the beginning of the Selli event and rebounded into a longer positive excursion that reached a peak after the Selli event at about 119.5–118.5 Ma. At 120.5 Ma, 87Sr/86Sr declined rapidly and reached a minimum at about 116–113 Ma. We speculate that the intensity of these latter responses suggests a corresponding peak in volcanic/tectonic activity at about 121–119 Ma.

Northward abyssal transport through the Samoan passage and adjacent regions

A conductivity‐temperature‐depth/hydrographic survey in January–February 1994 and a 17‐month deployment of current meter moorings from September 1992 to March 1994 were carried out to determine the volume transport, water mass characteristics, and diathermal fluxes of northward flowing abyssal waters in the Samoan Passage and adjacent regions of the South Pacific Ocean. Geostrophic calculations relative to 1.2°C potential temperature indicated northward transport of 7.8 Sv in the Samoan Passage, 1.1 Sv through a gap in Robbie Ridge, and 2.8 Sv along the eastern flank of the Manihiki Plateau. All of the total of 11.7 Sv of northward geostrophic transport was in waters colder than 1.1°C. The northward transport distribution was bimodal in temperature, with a cold mode of 3.6 Sv in the range 0.65°–0.70°C occurring entirely in the Samoan Passage and a warm mode of 3.0 Sv in the range of 0.80°–0.85°C occurring mainly along the Manihiki Plateau. Within the Samoan Passage, 7.1 Sv of the northward transport was below 4000 m where the geostrophic calculation was confirmed by an equal estimate of transport from current meters during the simultaneous 3‐day period. The 17‐month mean transport from the moored array was 6.0 Sv ± 0.5. By using the observed temporally varying flow within the Samoan Passage together with the hydrographic snapshot across the region, an estimate of the total mean northward transport of 10.6 Sv ± 1.7 was obtained. Estimates of the flow across near‐bottom potential temperature surfaces indicate extraordinarily high rates of mixing, with heating of the abyssal layer up to 20 W m−2, corresponding to diffusivities up to 10−1 m2 s−1.

Biostratigraphy and assemblage composition of benthic foraminifera from the Manihiki Plateau, southwestern tropical Pacific

Abstract. Pleistocene and late Pliocene benthic foraminifera from the Manihiki Plateau (southwestern tropical Pacific) were studied at piston-core 34KL. A new benthic foraminiferal biozonation is proposed. It comprises the Nodogenerina sagriensis Partial Range Zone from core base to 566.5 cm and the Fissurina seminiformis Partial Range Zone from this level to core top. The boundary is defined by the last occurence of Nodogenerina sagriensis which is time equivalent to the ‘Stilostomella extinction’ in the Eastern Atlantic. High abundances of Cibicidoides wuellerstorfi indicate a strong influence of near-bottom currents. The absence of high-productivity sensitive species reveals a low flux of organic matter to the sea floor from which a considerable amount is adduced by lateral advection.

High-resolution stratigraphy and the response of biota to Late Cenozoic environmental changes in the central equatorial Pacific Ocean (Manihiki Plateau)

An undisturbed 16 m late Pliocene-Pleistocene sediment core spanning 2.6 Myr of deposition was recovered from the Manihiki Plateau by the German research vessel Sonne in 1990. This core 34KL complements the heavily disturbed late Pliocene-Pleistocene core sections of DSDP Site 317, and is well suited for stratigraphic correlation. The sediments consist of calcareous microfossils (93–97% CaCO 3), minor portions of siliceous microfossils and detrital minerals. All important calcareous microfossil zones could be identified and correlated with the magnetostratigraphic and isotope stratigraphic events, stages and periods. Due to the high degree of silica dissolution, the late Quaternary radiolarian Buccinosphaera invaginata Zone is the only siliceous fossil zone which could be identified. An interval with Globigerinoides gomitulus/pink was found within the middle Pleistocene which is clearly distinguished from the Globigerinoides ruber/pink interval of the late Pleistocene. The magnetostratigraphic Gauss-Matuyama and Matuyama-Brunhes boundaries as well as the Olduvai and Jaramillo events were clearly identified. The δ 180 curve displays the 100 kyr (Milankovitch) and the 41 kyr (Laplace) cyclicity periods. The beginning of the Laplace Period and the last occurrence of the calcareous nannofossil Discoaster brouweri, which marks the end of the Pliocene, fall within the base of the Olduvai Event. There is also a marked drop in sedimentation rates around this time, which seems to be a regional phenomenon. Prominent 3.5 kHz subbottom reflectors at 8 ms and 15 ms reflection time are related to lithologic changes near the Mid-Pleistocene Revolution (MPR) and the beginning of the Laplace Period or the Pleistocene, respectively. It is possible to trace these reflectors laterally to the eroded eastern edge of the Manihiki Plateau where they can be tied to older strata.

Mineralogy and composition of manganese crusts and nodules and sediments from the Manihiki plateau and adjacent areas: Results of HMNZS Tui Cruises

Forty‐four sediment samples and 23 Mn crusts and Mn nodules have been analyzed for several elements in order to characterize their geochemical nature. Most of the Mn crusts and Mn nodules are of hydrogenous origin, although one crust (U375) from the flanks of the Rarotonga volcano is thought to be of hydrothermal origin. δ‐MnO2 is the principal Mn mineral in the crusts, whereas todorokite and birnessite occur in addition to δ‐MnO2 in the nodules. The sediments can be divided into carbonates, with calcite as the dominant phase, and deep‐sea clay, consisting mainly of illite and smectite.

New evidence for the production of EM-type ocean island basalts and large volumes of volcaniclastites during the early history of the Manihiki Plateau

Alkaline volcanic rocks including nephelinites, basanites and trachybasalts dredged from the volcanic pedestal of Rakahanga Atoll and from a volcanic edifice with 100 satellite volcanoes at the eastern edge of the Manihiki Plateau, ca. 40 km southwest of the atoll, fall well within the category of EM-type ocean island basalts. They indicate a hotspot involvement during the formation of the plateau basement. The rocks are thought to be products of explosive eruptions which took place subaerially or in shallow water in the Aptian. The volcanoes, together with other volcanic eruption centers, most likely were responsible for the formation of the 230 m thick volcaniclastite layer which rests on the basement for at least 5000 km 2 of the eastern part of the Manihiki Plateau. Erosion has prevented any substantial sediment cover on the volcanic cone field and most of the slope of Rakahanga and thin pelagic limestones were deposited instead at least since the Maastrichtian.

East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume?

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40 Ar 39 Ar ages determined in this study for two Leg 129 basalts average 114.6 ± 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites ( 87 Sr 86 Sr init = 0.70360–0.70374 ; 143 Nd 144 Nd init = 0.512769–0.512790 ; 206 Pb 204 Pb meas = 18.355–18.386 ) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 ± 1.0 Ma 40 Ar 39 Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Biostratigraphy of Tertiary and Quaternary benthic bathyal foraminifers of DSDP Site 317 (Tropical Pacific)

A study is made of the benthic foraminifers (size fraction > 63 μm) recovered from 59 upper Eocene through Quaternary sediment samples at DSDP Site 317 (Leg 33), located at a depth of 2598 m in the central part of the Manihiki Plateau (South Pacific). The sediments cored are disturbed in only two samples. The stratigraphic assignements used are based on previous studies of planktic foraminifers and other microfossils. In total, 216 taxa are identified. A cluster analysis based on the 77 species which comprised 5% or more of the entire foraminiferal assemblage in at least one sample suggests the presence of 3 major biostratigraphic zones corresponding approximately to the following ages, zone A: middle Miocene-Quaternary; zones B-C: early Miocene-Oligocene; and zone D: Eocene. The most important faunal turnover occurred between the Eocene and the Oligocene; a less pronounced break took place between the early and the middle Miocene, and an additional minor turnover between the Oligocene and the early Miocene. Eighteen taxa are long-ranging, being recorded from the middle Eocene through the Pliocene-Quaternary. It is concluded that, in general, benthic foraminifers of the bathyal zone are poor worldwide stratigraphic guide fossils; the following taxa are conditionally considered as the most suitable in the Eocene-Quaternary sequence: Aragonia aragonensis, Quadrimorphina profunda, Nuttallides truempyi, Abyssamina poagi, Buliminella grata, Bulimina jarvisi, B. macilenta, Turrilina alsatica, Cibicides notocenicus, C. wuellerstorfi, Pyrgo murrhina. However, most of these species are relatively rare.

Minor and rare elements in manganese crusts and nodules and sediments from the Manihiki plateau and adjacent areas: Results of HMNZS Tui cruises

Six manganese crusts, 13 manganese nodules, and 16 sediments were analyzed by instrumental neutron activation analysis. Data were generated on selected major and minor elements but geochemical evaluations are based only on Fe, Sc, U, Th, and the rare earth elements (REE). Manganese crusts and manganese nodules have comparable trivalent REE contents and show a shale‐like distribution pattern. Both crusts and nodules are characterized by a positive Ce anomaly but the anomaly is higher in nodules. REE contents in manganese nodules show a linear dependence on the Fe content, and it is concluded that these elements are incorporated in the Fe‐rich (δ‐MnO2) phase. In the crusts, the REE correlate with Sc and are therefore assumed to be associated with the clay minerals. Uranium contents are significantly higher in the crusts than in nodules whereas Th is slightly higher in the nodules. There is a clear positive correlation between U and Th in nodules but there are too few data to make a similar conclusion for crusts. Compositional data suggest a division of the sediments into two groups. The carbonate sediments have much lower REE contents and a more pronounced negative Ce anomaly than the clays, while both show a lithogenous component as indicated by a slight negative Eu anomaly.

Magnetization of the Pacific Ocean lithosphere deduced from Magsat data

Magsat anomalies over the Pacific Ocean are related to volcanic plateaus and the edge effect of the Pacific Cretaceous Quiet Zone (PCQZ). Four prominent plateaus, Shatsky Rise, the Mid‐Pacific Mountains, Hess Rise, and Manihiki Plateau, are considered in this study. The major lateral magnetization contrasts associated with these plateaus are considered to arise from the topographic swells and the crustal roots, with root thicknesses estimated from topography using an Airy isostatic model. Shatsky Rise and the Mid‐Pacific Mountains are older than the Cretaceous Quiet Period and their natural remanent magnetization (NRM) is not a significant component of the magnetization contrasts that cause the Magsat anomalies associated with these plateaus. Their magnetic properties are characterized by effective magnetic susceptibility contrasts of 0.07 (±0.01, S.I., per unit volume) averaged over their topographic swells and crustal roots, equivalent to 0.04 if averaged over the entire crustal thicknesses (swell plus root plus intervening layer). This value agrees well with the estimates of previous investigators for these plateaus and for similar plateaus in other oceans. Two plateaus of Cretaceous Quiet Period age within the PCQZ, Hess Rise and Manihiki Plateau, have origins similar to those of Shatsky Rise and the Mid‐Pacific Mountains, and they probably have similar effective susceptibility contrasts but they may also carry Cretaceous Quiet Period NRM. Magnetic anomalies due to Hess Rise and Manihiki Plateau, calculated from the effective susceptibility contrast deduced for Shatsky Rise and the Mid‐Pacific Mountains, are substantially larger than the observed Magsat anomalies of these plateaus. There are also Magsat anomalies over parts of the Pacific where no plateaus exist Some of these anomalies are demonstrated as probably due to a contrast in NRM between the PCQZ and its surroundings. The total remanent magnetization contrast (magnetization contrast in amperes per meter times thickness in meters, equivalent to the dipole moment per unit area) of the PCQZ is 14,000–20,000 amperes (±30%), which cannot be simply explained by the absolute values of remanent magnetization predicted by conventional models of oceanic crustal magnetic properties. Modelling of the Cretaceous quiet zones in the northern Atlantic indicates a similar total remanent magnetization contrast. These results may be explained by including uppermost mantle remanent magnetization, with an intensity of about 1–2 A m−1 if magnetization persists to a depth of ≈7 below the seismic Moho, i.e, down to the depth of the crustal roots of the plateaus. Including NRM in the uppermost mantle of the PCQZ also serves to reduce the calculated amplitude of the Hess Rise and Manihiki Plateau Magsat anomalies if the NRM of the crustal roots is less than the NRM of the surrounding uppermost mantle. On this basis the estimated intensity of NRM in the uppermost 7 km of the PCQZ mantle is ≈1–3 A m−1, in very good agreement with the intensity estimated from the PCQZ edge effect anomalies. These estimates of uppermost mantle NRM are probably a maximum limit, recognizing that other sources may also contribute to the total remanent magnetization contrasts of the Cretaceous quiet zones.

New sidescan sonar and gravity evidence that the Nova-Canton Trough is a fracture zone

A 1990 sidescan sonar survey in the eastern region of the Nova-Canton Trough mapped 138°-striking abyssal-hill fabric trending into 70°-striking trough structures. The location and angle of intersection of the abyssal hills with the eastern Nova-Canton Trough effectively disprove a spreading-center origin of this feature. Free-air gravity anomalies derived from satellite altimetry data show continuity, across the Line Islands, of the Nova-Canton Trough with the Clipperton Fracture Zone. The Canton-Clipperton trend is copolar, about a pole at 30°S, 152°W, with other coeval Pacific-Farallon fracture-zone segments, from the Pau to Marquesas fracture zones. This copolarity leads us to postulate a Pacific-Farallon spreading pattern for the magnetic quiet zone region north and east of the Manihiki Plateau, with the Nova-Canton Trough originating as a transform fault in this system.

Consequences of plume-lithosphere interactions

Abstract Splitting or thinning of lithosphere above a mantle plume can result in voluminous melt generation, leading to the formation of large igneous provinces, or LIPs. Examples of LIPs include continental flood basalt provinces and oceanic plateaus. Basaltic samples from the Ontong Java Plateau, Nauru Basin and Manihiki Plateau, which are among the largest of the LIPs, have isotopic compositions within the range of ocean island basalts. The majority of continental basalts, however, record a trace element and isotopic contribution from the lithosphere through which they have erupted. We are thus unable to reconcile the available compositional data with models which derive the isotopic and large-ion lithophile element-enriched character of continental flood basalts solely from sub-lithospheric mantle plume sources. A combination of mantle sources is indicated, with the thermal energy being supplied by voluminous melts from a plume, and the lithospheric components in continental flood basalts being inherited by contamination of plume-derived melts by low melting point hydrous and carbonated fractions in the lithosphere. Successive injection of plume-derived melts serves to heat the lithosphere, reducing its viscosity and making it susceptible to rupture if allowed by regional plate forces. Furthermore, the lithosphere, including the mechanical boundary layer, may be thinned by thermal stripping from below, allowing the plume mantle to ascend and decompress further. Such a system has the potential for positive feedback leading to rapid melt generation. While we do not exclude recent models of LIP formation which require the sudden impact of a new mantle plume, we favour a model whereby the thermal anomaly builds gradually, incubating beneath a steady-state lithospheric cap.

A field of volcanoes on the Manihiki Plateau: Mud or lava?

A field of about 100 cones surrounding a 1900 m high seamount was discovered on the northeastern edge of Manihiki Plateau (10° 18.5′S, 161°27.5′W) during a February 1987 cruise of the R.V. Moana Wave in the territorial waters of the Cook Islands. The cones appear as acoustically transparent piercements on single-channel seismic reflection profiles collected during this and earlier expeditions to the area. SeaMARC II sidescan images typically reveal nearly conical, steep-sided peaks of about 1 km diameter rising several hundred meters above the smooth and featureless plateau surface. Many of the cones appear to be formed by multiple flow units. A few have summit depressions, and moats partially surround some of the cones. Distributed like holes on a dart board, individual cones become more densely spaced about a bulls eye located near the eastern edge of the Manihiki Plateau, 50 km southwest of Rakahanga Island. At that location, about 40 cones coalesce to form the seamount, which is about 25 km in diameter and rises from a plateau depth of 3200 m to a summit at a depth of 1310 m. Sidescan images show that incised channels trend away from the seamount toward the east, where the irregular seafloor in the area truncates tilted, E-dipping sub-bottom reflectors. To the south and west, the seafloor and sub-bottom reflectors are smooth and dip gently to the southwest. Recent foraminiferal ooze was cored from a satellite cone. Dredging of the seamount summit recovered middle Eocene planktonic foraminifera embedded in burrowed limestone. Some of the limestone cobbles had manganese crusts; coralline fragments and Recent foraminifera were also recovered in the same dredge. Despite the apparent absence of igneous rocks, geopotential modelling indicates a basement high at this location. If fluid escape rather than magma flow is involved in the genesis of these cones, it is localized by faulting associated with basement uplift or the presence of an older igneous intrusion. At this time we cannot say with certainty whether the cones are igneous or sedimentary features.

Flow of Abyssal Water into the Samoa Passage

On the TEW Expedition a hydrographic section was made in June 1987 across the Samoa Passage; in addition, south of the Passage, a section was made through the Penrhyn and Samoa Basins and the Tonga Trench. In this paper the flow of the deep water with North Atlantic Deep Water and Antarctic characteristics below 3500 m is discussed. Temperature-salinity-oxygen relationships show a similarity of properties between the Tonga Trench and western boundary of the Samoa Basin. The salinity and silicate data show distinct signatures of North Atlantic Deep Water and Antarctic Bottom Water flowing into the Samoa Passage. Salinity and silicate data in the Samoa Basin and Passage are similar to those measured on the STYX Expedition (1968). Oxygen data show that the water along the Samoa Basin's western boundary is oxygen-rich relative to water in the eastern Basin, but with no oxygen maximum in deep water, as was observed on STYX. Geostrophic flow, based on a reference surface determined from T-S. O2 curves, shows strong northward components (> 10 cm s−1) in the Samoa Passage. On the west side of the Passage the flow was southward (<1 cm s−1) below 4600 m. This flow reversal was associated with a reversal of the sign of the horizontal potential temperature gradient and also with lower values of oxygen, salinity and silicate next to the boundary. The northward transport of Lower Circumpolar Water between the Manihiki Plateau and the Tonga Trench was 12.3 ± 3.6 × 106 m3 s−1; the flow into the Samoa Passage was 6.0 ± 1.1 × 106 m3 s−1. Assuming that only flow above 4800 m can exit the northern Samoa Basin because of topographic restrictions, the total northward transport into the north Tokelau Basin was 9.6 ± 1.8 × 106 m3 s−1.