Island Hotspot Research Articles

Seismic imaging of hotspot-related crustal underplating beneath the Marquesas Islands

VOLCANISM in active continental rift zones1,2or on rifted continental margins3is frequently associated with unusually high lower-crustal seismic velocities, indicating the presence of large igneous intrusions at the base of the crust. The only comprehensive investigation of crustal underplating at an oceanic hotspot, beneath Hawaii4–6, has yielded controversial results7. Here we report the results of seismic refraction experiments across the Marquesas Islands hotspot trace, which show that the island chain is underlain by crust 15–17 km thick, and that a large lower-crustal region (275 km wide and 2–8 km thick) has seismic velocities (7.3–7.75 km s−1) indicative of crustal underplating. Wide-angle reflections occur near the base of the normal lower-crustal velocities and at the base of the underplating complex; we interpret these reflectors as the relict pre-hotspot Moho and the current post-hotspot Moho, respectively. The high-velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks. The volume of the high-velocity body is impressive, amounting to nearly twice the volume of volcanics erupted at the surface.

Tahiti: Geochemical evolution of a French Polynesian Volcano

The island of Tahiti, the largest in French Polynesia, comprises two major volcanoes aligned NW‐SE, parallel with the general trend of the Society Islands hotspot track. Rocks from this volcanic system are basalts transitional to tholeiites, alkali basalts, basanites, picrites, and evolved lavas. Through K‐Ar radiometric dating we have established the age of volcanic activity. The oldest lavas ( ∼1.7 Ma) crop out in deeply eroded valleys in the center of the NW volcano (Tahiti Nui), while the main exposed shield phase erupted between 1.3 and 0.6 Ma, and a late‐stage, valley‐filling phase occurred between 0.7 and 0.3 Ma. The SW volcano (Tahiti Iti) was active between 0.9 and 0.3 Ma. There is a clear change in the composition of lavas through time. The earliest lavas are moderately high SiO2, evolved basalts (Mg number (Mg# = Mg/Mg+Fe2+) 42–49), probably derived from parental liquids of composition transitional between those of tholeiites and alkali basalts. The main shield lavas are predominantly more primitive olivine and clinopyroxene‐phyric alkali basalts (Mg# 60–64), while the later valley‐filling lavas are basanitic (Mg# 64–68) and commonly contain peridotitic xenoliths (olivine+orthopyroxene+clinopyroxene+spinel). Isotopic compositions also change systematically with time to more depleted signatures. Rare earth element patterns and incompatible element ratios, however, show no systematic variation with time. We focused on a particularly well exposed sequence of shield‐building lavas in the Punaruu Valley, on the western side of Tahiti Nui. Combined K‐Ar ages and magnetostratigraphic boundaries allow high‐resolution age assignments to this ∼0.7‐km‐thick flow section. We identified an early period of intense volcanic activity, from 1.3 to 0.9 Ma, followed by a period of more intermittent activity, from 0.9 to 0.6 Ma. Flow accumulation rates dropped by a factor of 4 at about 0.9 Ma. This change in rate of magma supply corresponds to a shift in activity to Tahiti Iti. We calculated the composition of the parent magma for the shield‐building stage of volcanism, assuming that it was in equilibrium with Fo89 olivine and that the most primitive aphyric lavas were derived from this parent by the crystallization of olivine alone. The majority of the shield lavas represent 25 to 50% crystallization of this parent magma, but the most evolved lavas represent about 70% crystallization. From over 50 analyzed flow units we recognize a quasi‐periodic evolution of lava compositions within the early, robust period of volcanic activity, which we interpret as regular recharge of the magma chamber (approximately every 25±10 kyr). Volcanic evolution on Tahiti is similar to the classic Hawaiian pattern. As the shield‐building stage waned, the lavas became more silica undersaturated and isotopic ratios of the lavas became more MORB‐like. We propose that the Society plume is radially zoned due to entrainment of a sheath of viscously coupled, depleted mantle surrounding a central core of deeper mantle material. All parts of the rising plume melt, but the thermal and compositional radial gradient ensures that greater proportions of melting occur over the plume center than its margins. The changing composition of Tahitian magmas results from lithospheric motion over this zoned plume. Magmas erupted during the main shield‐building stage are derived mainly from the hot, incompatible element‐enriched central zone of the plume; late‐stage magmas are derived from the cooler, incompatible element‐depleted, viscously coupled sheath. A correlation between Pb/Ce and isotope ratios suggests that the Society plume contains deeply recycled continental material.

Gravity study of the Pitcairn‐Easter Hotline

Shipboard free air gravity and bathymetric anomalies with an extension of 400 km were identified across the Pitcairn‐Easter hotline in the South Pacific. The anomalies are associated with one of the positive geoid undulations observed in the area from satellite data. Several smaller topographic features, volcano‐tectonic ridges oriented N65°E, are superimposed on the topographic high. Admittance computations and direct modeling show that the swell topography is compensated by a low density zone within the lithosphere, 4 to 8 km below the crust The volcano tectonic ridges are locally compensated in a classical Airy sense. The swell and the associated ridges were probably created by the action of a thermal anomaly resulting from the interaction of the Easter Island hotspot and of the Easter Microplate accretion centers.

Geochemistry of hydrothermal manganese deposits from the Pitcairn Island hotspot, southeastern Pacific

Hydrothermal Mn crusts from a submarine volcano in the Pitcairn Island region have been studied mineralogically, radiometrically, and by bulk and partition chemical techniques. The crusts contain layers that comprise three morphological types. A mechanism of formation by downward growth from a Mn-cemented surface sand hardpan is proposed. Age dating of the crusts suggests relatively recent, and rapid, formation. Mineralogically, the crusts exhibit variable amounts of 10Å and 7Å manganite. Chemical data, however, indicate that the crusts do not show element associations characteristic of marine birnessite, suggesting that the 7Å manganite phase may have transformed from structurally unstable 10Å manganite as a result of air drying. Trace element concentrations in massive, finely laminated types 2 and 3 crust layers are significantly lower than those in similar crusts from island arc and back-arc settings and, in general, are comparable with, or lower than, those of the most trace element poor crusts from mid-oceanic ridge environments. REE data suggest that these crusts are almost pure hydrothermal deposits with little or no hydrogenous component. In the types 2 and 3 crusts, only Mo shows a significant enrichment that is likely to be hydrothermal in origin, although it is enriched to lower levels than in hydrothermal crusts from most other geological settings. Friable, Mn-cemented sand type 1 crusts contain elevated trace element concentrations, in part as a result of incorporation of biogenic and volcaniclastic phases, but also as a result of hydrogenous inputs consistent with their greater age and closer proximity to seawater. In contrast to types 2 and 3 crusts which show no Li enrichment, type 1 crusts are enriched in Li to levels above those that can be accounted for by hydrogenous input and a hydrothermal source for it is proposed. The observed variability in the composition of hydrothermal Mn crusts between midplate hotspot, mid-oceanic ridge, island arc, and back-arc environments may be influenced by variability in basement composition and sea water/rock ratio in the hydrothermal systems. As both parameters are directly related to tectonic setting, this may play an important role in controlling the chemistry of hydrothermal deposits.

Oxygen thermobarometry of abyssal spinel peridotites; the redox state and C-O-H volatile composition of the Earth's sub-oceanic upper mantle

The authors have applied the spinel peridotite oxygen barometer to abyssal spinel peridotites from the mid-Atlantic, central Indian, southwest Indian and American-Antarctic ocean ridge systems. The results indicate that the oxygen fugacity (F{sub O{sub 2}}) of the suboceanic mantle is on average 0.9 {plus minus}0.7 (n = 33; {plus minus} 1sd) log units below the Fayalite-Magnetite-Quartz (FMQ) f{sub O{sub 2}} buffer, in excellent agreement with f{sub O{sub 2}} estimates of MORB glasses (FMQ {minus}1.20 {plus minus} 0.63, n = 87; {plus minus} 1 sd). The agreement between MORBs and their mantle source region suggests that the rapidly quenched liquids have not undergone significant oxidation (by hydrogen degassing, for example) during their ascent and eruption. Their results also show that the suboceanic mantle is more reduced than the subcontinental mantle, for which the average value of log f{sub O{sub 2}} lies approximately at FMQ (log f{sub O{sub 2}} = FMQ + 0.24 {plus minus} 0.5, n = 54; {plus minus} 1 sd). Abyssal spinel peridotites from the Islas Orcadas fracture Zone (FZ), near the Bouvet Island hotspot, are the most reduced samples in our suite (log f{sub O{sub 2}} = FMQ {minus}1.67 to {minus}2.32), and they are compatible with a graphite-saturatedmore » fluid being present in this source region. In general, however, upper mantle f{sub O{sub 2}}'s are too high for graphite to be stable, and they estimate an activity of carbon relative to graphite of about 0.05 under the approximate conditions of MORB generation (P {approximately} 10 kb, T {approximately} 1,325{degree}C, log f{sub O{sub 2}} {approximately} FMQ {minus}0.9). Under these conditions an average CO{sub 2} content in the mantle of {approximately} 215 to 545 ppm would be consistent with the fluid compositions of MORB glasses.« less

Active submarine volcanism on the society hotspot swell (west Pacific): A geochemical study

The present work deals with the petrography and geochemistry of lavas dredged from five active submarine volcanoes (named Mehetia, Moua Pihaa, Rocard, Teahitia, and Cyana) from the southeast end of the Society Islands hotspot trace. Most samples are basic and alkaline, ranging from 16 to 5 wt % MgO, with about 5% normative nepheline. Fractionation modelling based on major and minor compatible element variations suggests that olivine and minor clinopyroxene were the major fractionating phases and implies a maximum range of fractionation of 30–35%. Rocard and Cyana have yielded more evolved, trachy‐phonolitic, glassy samples. These evolved samples are thought to be derived by removal of 70% cumulate from the basalts. Both basaltic and phonolitic samples are incompatible‐element enriched, with La/YbN ≈ 15 in most of the basalts. The trachy‐phonolite patterns show middle rare earth element (REE) depletion and negative Eu anomalies. The Moua Pihaa basalts have flatter patterns than the other basalts (La/YbN = 7.5–12.4). All samples, with the exception of a sample from Moua Pihaa which has elevated 206Pb/204Pb, fall on linear Sr‐Nd‐Pb isotopic arrays, suggesting two end‐member mixing. The most depleted end‐member is shown to be a pristine ocean island basalt magma with no detectable contribution from a depleted, mid‐ocean ridge basalt (MORB) upper mantle. The flatter REE patterns and higher 206Pb/204Pb of the Moua Pihaa sample are taken to indicate a more depleted, U‐enriched (high μ) component in its source. This component may be altered oceanic crust. The Sr isotopic variations in the samples excluding Moua Pihaa correlate positively with Rb/Nb, Pb/Ce, and SiO2 variations, indicating a component of mantle enriched by injection of material from a subducted oceanic slab. Correlation of 207Pb/204Pb with 87Sr/86Sr suggests that the subducted material is geochemically old. Mapping the geochemical variations shows that the contribution to the lavas from the subduction component is greater over the north of the hotspot than in the south. The absence of a MORB component in the Society magmatism, the small volumes of the Polynesian hotspot volcanoes, and the lack of more intense volcanic activity near the center of the Pacific Supers well, all lead us to conclude that the latter is unlikely to be caused by a large convective plume. The Superswell is more probably located above a region in the asthenospheric mantle which, due to its higher content of recycled continental debris, is anomalously hot.

Geochemistry of basalts from the Dumisseau Formation, southern Haiti: implications for the origin of the Caribbean Sea crust

Basalt and diabase from the Cretaceous Dumisseau Formation, southern Haiti have Mg-numbers of 43–63, TiO 2 contents of 1.6–3.9% and La abundances of 3.6–15.3 ppm. La/Ta ratios average 10, and indicate that the basalts are oceanic in character, distinct from the arc associations forming the northern part of Haiti. Oldest lavas have low TiO 2 (1.6%) and are LREE-depleted, similar to N-MORBs, whereas overlying lavas have higher TiO 2 (2–3.9%) and are LREE-enriched, similar to E-MORBs or hotspot basalts. 87Sr 86Sr ratios vary from 0.70280 to 0.70316, 143Nd 144Nd from 0.512929 to 0.513121, and 206Pb 204Pb from 19.00 to 19.27. LREE-depleted lavas have high 143Nd 144Nd (0.51309–0.51310) typical of MORBs, whereas 143Nd 144Nd in the LREE-enriched lavas varies widely (0.512929–0.513121). Chemical features of the Dumisseau basalts are equivalent to those of Caribbean seafloor basalts recovered on DSDP Leg 15, and support the contention that the Dumisseau is an uplifted section of Caribbean Sea crust. Oldest lavas are analogous to MORB-like basalts cored at Leg 15 Sites 146, 150, 152 and 153, and the overlying lavas are analogous to incompatible-element-enriched basalts cored at Site 151 on the Beata Ridge. Isotopic compositions of the Dumisseau basalts overlap with those of the eastern Pacific Galapagos and Easter Island hotspots. However, the presence of N-MORB basalts in the lower part of the Dumisseau and at the majority of Leg 15 Sites indicates that the anomalously thick Caribbean crust probably did not originate as a hotspot-related basaltic plateau, but may have been generated by on-ridge or near-ridge hotspot magmatism.

Variations in subsidence rates along intermediate and fast spreading mid-ocean ridges

The nature of subsidence near the ridge crest of the intermediate and fast spreading mid-ocean ridges of the Indian and Pacific Oceans is investigated using surface—ship bathymetry and magnetics profiles. The ridge can be divided into discrete sections, apparently bounded by distinct structural features such as major fracture zones, in which bathymetry plotted against crustal age forms a well-defined envelope with a width roughly the amplitude of the local bathymetry. The averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient, C1, in the range of 340–390 m Myr−1/2. The best fitting subsidence curve, however, never reproduces the amplitude of the axial topographic high. The most notable region displaying unusual behaviour is the East Pacific Rise between 9°S and 22°S. In this region, the western flank of the ridge is subsiding at 200–225 m Myr−1/2 while the eastern flank is subsiding at ‘normal’ rates of 350–400 m Myr−1/2. Other anomalous areas include the region between the Easter Island hot spot and the Chile Rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m Myr−1/2, and the region east of the Australia-Antarctic Discordance in which the northern flank is subsiding at 440 m Myr−1/2. This area may also be subsiding asymmetrically although there is not much data from the southern flank. The asymmetric subsidence in the 9°S-22°S region of the East Pacific Rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 Myr old crust. Oligocene-aged crust on the western flank is subsiding at more normal rates, but is 500 m shallow with respect both to the world-wide average and to the conjugate crust on the eastern flank. The simplest model to explain these observations is that the western flank is underlain by a hotter mantle, perhaps as the result of upwelling resulting from the large-scale return circulation from the trenches. Depending on the depth of compensation, the observed asymmetry could result from a lateral temperature gradient of 0.05–0.10°C km−1 and a total lateral temperature variation of under 100°C.

The volcanoseismic swarms of 1981–1983 in the Tahiti‐Mehetia Area, French Polynesia

During the years 1981–1983, three intense seismic swarms took place in the Tahiti‐Mehetia area of French Polynesia at the presumed location of the Society Island hot spot. The 1981 swarm featured 4000 earthquakes, with a maximum magnitude ML = 4.3, in the immediate vicinity of the island of Mehetia; the 1982 swarm occurred along the flank of the major Teahitia seamount, and involved more than 9000 events; a second swarm occurred at Teahitia in 1983 and involved 3000 events. Although no precise constraint can be placed on the depth of individual events from their travel times to Polynesian stations, features in the evolution of the Mehetia swarm are generally consistent with the probable ascent of a magma body toward the surface. In the case of Teahitia, the recording of abundant tremors of both high and low frequency, particularly intense during the 1983 swarm, is directly similar to cases of documented volcanic eruptions. The swarms are interpreted as episodes of active volcanism, part of the process of building the next island in the chain.Appendix tables are available with the entire article on micro‐fiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B84‐005; $2.50. Payment must accompany order.

Phanerozoic addition rates to the continental crust and crustal growth

Phanerozoic addition rates to the continental crust are calculated by using seismic profiles through magmatic arcs to measure the crustal volumes added during the active lifespans of the arcs. Data for 17 arcs give addition rates per kilometer of arc in the range 20 to 40 km³ km−1 Ma−1. From these data we deduce a world‐wide addition rate of 1.65 km³ a−1 after adding other contributions to the formation of the continental crust, e.g., from hot spot volcanism. We infer a subtraction rate, mainly by subducting sediments, of 0.6 km³ a−1 and arrive at a net crustal growth rate of about 1 km³ a−1. Growth of the continental crust is necessary to maintain approximately constant freeboard, because the secular decline in the heat production of the mantle causes the ocean basins to deepen. An equation for the growth of the continents as a function of the decline in terrestrial heat flow yields approximately constant growth rate since the Archean of 0.9 km³ a−1, in good agreement with the above estimate. On the average, Archean growth rates must have been 3 to 4 times the present rate. Island arc growth rates are inadequate to explain the formation of the Arabian‐Nubian Shield and the Archean granite‐greenstone terrain of the Superior Province, and a captured island chain in Oregon. We confirm the oceanic island origin of the Oregon terrain on the basis of the large growth rates of hotspot islands.

Tectonic history of aseismic ridges in the eastern Indian Ocean

We have devised a model for the origin of the Ninetyeast Ridge, the Broken Ridge–Naturaliste Plateau, and the Kerguelen Plateau in the eastern Indian Ocean which demonstrates that they were created by fixed hot spots now beneath the Amsterdam–St. Paul and Kerguelen Islands. The model employs reconstructions constrained by relative motions between plates; the model was deduced from sea-floor data and by paleolatitudes from Australian paleomagnetic pole data. Paleolongitudes were constrained by holding the Ninetyeast Ridge over the Amsterdam–St. Paul spot. Predicted age gradients and trajectories result, and these agree very well with geologic observations on the aseismic ridges. The Ninetyeast Ridge was laid down mainly from the Amsterdam–St. Paul hot spot, but also from the spot now under Kerguelen. The ridge is predicted to be from about 90 to 20 m.y. old. Broken Ridge–Naturaliste Plateau and the Kerguelen Plateau were both formed from the Kerguelen Islands hot spot and were joined prior to the Australia-Antarctic rift about 55 m.y. B.P. Broken Ridge–Naturaliste Plateau is predicted to be from more than 100 to about 80 m.y. old. Kerguelen Plateau should be from more than 100 to 0 m.y. old, but several volcanic hiatuses are predicted.

Classification, Characteristics, and Origin of Ophiolites

Tholeiitic (TH) series rocks occur in almost all types of tectonic settings, whereas calc-alkalic (CA) series rocks occur characteristically in island arcs and continental regions. Abyssal tholeiites and associated gabbros in mid-oceanic ridges show much narrower ranges of $$SiO_{2}$$ contents and FeO* (total iron as FeO)/MgO ratios than volcanic rocks in oceanic islands and island arcs. Abyssal tholeiites and oceanic island volcanics tend to show higher $$TiO_{2}$$ contents than island arc volcanics. These are among the diagnostic features of volcanic rock series. In terms of volcanic rock series, the ophiolitic complexes may be classified into a number of distinct classes, three of which are discussed here. The ophiolitic complexes of Class I are characterized by the presence of volcanic rocks of both CA and TH series. A typical example is the Troodos massif in Cyprus, which was formed probably in an island arc. The ophiolitic complexes of Class II are characterized by the presence of TH series volcanics. Some of them were formed probably in island arcs and others in mid-oceanic ridges. The ophiolitic complexes of Class III are characterized by the presence of TH and alkalic series volcanics. Examples are ophiolites in high-pressure (glaucophanitic) metamorphic terranes. The volcanic rocks in such ophiolites show a marked chemical resemblance to the volcanics in some hot spot islands as well as to some volcanic rocks on stable continents. A few more classes may exist.