Society Hotspot Research Articles

Laser In Situ U–Pb Isotope Dating of Carbonate Rocks in Weijia Guyot in the Western Pacific Ocean and Its Geological Significance

Shallow-water carbonate rocks constitute a crucial component of large guyots, arising in distinct environments and harboring valuable insights into the evolutionary stages of seamount islands as well as the tectonic conditions of the underlying oceanic plate. Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) was used to conduct in situ U–Pb isotope dating of carbonate minerals with low uranium content collected from Weijia Guyot. This dating approach yielded crucial evidence for the vertical development of the seamount. Our study indicates that shallow-water carbonate rocks in Weijia Guyot had a temporal range between 91 My and 137 My. The carbonate rocks underwent two growth phases, Hauterivian to Barremian and Cenomanian to Turonian, with a hiatus of approximately 20 My. Since the Hauterivian age, the shield volcano of Weijia Guyot is essentially complete, with its seamount top exposed at or near sea level and receiving its first stage of shallow-water carbonate sedimentation. Based on the dating of both shallow-water carbonate rocks and hawaiite within the Weijia Guyot, it is inferred that approximately 10 My elapsed from shield-building volcanism to late alkalic volcanism. During the Turonian age, the main reason for the second phase of shallow-water carbonate rocks in the seamounts was the regional tectonic uplift triggered by the drift of the Weijia Guyot along with the Pacific Plate toward the Society hotspot.

MAGLAB: A computing platform connecting geophysical signatures to melting processes in Earth's mantle

Decompression melting of the upper mantle produces magmas and volcanism at the Earth's surface. Experimental petrology demonstrates that the presence of CO2 and H2O enhances peridotite melting anywhere within the upper mantle down to approximately 200–300 km depth. The presence of mantle melts with compositions ranging from carbonate-rich to silicate-rich unavoidably affects the geophysical signals retrieved from Earth's mantle. Geochemical investigations of erupted intraplate magmas along with geophysical surveys allow for constraining the nature and volume of primary melts, and a sound formalism is required to integrate these diverse datasets into a realistic model for the upper mantle including melting processes. Here, we introduce MAGLAB, a model developed to calculate the composition and volume fraction of melts in the upper mantle, together with the corresponding electrical conductivity of partially molten mantle peridotites at realistic pressure-temperature conditions and volatile contents. We use MAGLAB to show how the compositions of intraplate magmas relate to variations in lithosphere thickness. Progressive partial melting of a homogeneous peridotitic mantle source can in theory create the diversity of compositions observed among the spectrum of intraplate magma types, with kimberlite melts beneath thick continental shields, alkaline magmas such as melilitite, nephelinite and basanite beneath thinner continents and relatively old plus thick oceanic lithospheres, and ‘regular’ basalts beneath the youngest and thinnest oceanic lithospheres as well as beneath significantly thinned continental lithospheres. MAGLAB calculations support recent experimental findings about the role of H2O in the upper mantle on producing primary kimberlitic melts in addition to CO2. We demonstrate the robustness of MAGLAB calculations by reproducing the compositions of erupted melts as well as associated mantle electrical conductivities beneath the Society hotspot in the Pacific Ocean. A comparison of our simulations with magnetotelluric surveys at various oceanic settings shows that the heterogeneities in electrical conductivity of Earth's upper mantle are related to variations in volatile content via the presence of small (generally <<1 wt%) and heterogeneously distributed fractions of CO2-H2O-bearing melts.

The new pedagogy of a Human: the prolegomena

"A Human", as the meaning and center of gravity of Ukrainian science, education, society, still remains terra incognita for reformers of Ukrainian education. "А democratic society will only succeed when universal education will form a philosophical worldview in the public." On the basis of a "figurative" audit of the hot spots of society, the necessity and possibility of changing the foundation of educational policy has been proved. The reader’s attention is focused on the CODE of the nation. The scale of the problem itself and the tools for its solving have been depicted. In this article, the author shows only the directions and topics of reasoning, which can become the prolegomena for the new pedagogy of a Human.

Electromagnetic evidence for volatile‐rich upwelling beneath the society hotspot, French Polynesia

AbstractWe have conducted a seafloor magnetotelluric survey that images, for the first time, three‐dimensional electrical conductivity structure in the upper mantle beneath the Society hotspot. A striking feature in our model is a high‐conductivity anomaly a few hundred kilometers in diameter, which is continuous from the lowest part of the upper mantle to a depth of approximately 50 km below sea level. Using theoretical and experimental results from mineral physics, we interpret the high‐conductivity anomaly as evidence of the melt fraction up to 2.2 vol.%, which is robust regardless of assumed temperature, and the existence of carbonated silicate melt beneath the hotspot. Our results suggest that the Society hotspot is a pathway for ascending volatiles from the deeper part of the upper mantle to the surface.

Mantle plumes beneath the South Pacific superswell revealed by finite frequency P tomography using regional seafloor and island data

AbstractWe present a new tomographic image beneath the South Pacific superswell, using finite frequency P wave travel time tomography with global and regional data. The regional stations include broadband ocean‐bottom seismograph stations. The tomographic image shows slow anomalies of 200‐300 km in diameter beneath most hot spots in the studied region, extending continuously from the shallow upper mantle to 400 km depth. Narrow and weak slow anomalies are detected at depths of 500–1000 km, connecting the upper mantle slow anomalies with large‐scale slow anomalies with lateral dimension of 1000–2000 km prevailing below 1000 km depth down to the core‐mantle boundary. There are two slow anomalies around the Society hot spot at depths shallower than 400 km, which both emerge from the same slow anomaly at 500 km depth. One of them is located beneath the Society hot spot and the other underlies 500 km east of the Society hot spot, where no volcanism is observed.

Unusual seismic activity in 2011 and 2013 at the submarine volcano Rocard, Society hot spot (French Polynesia)

AbstractWe analyze two seismic events that occurred on 27 May 2011 and 29 April 2013 at the Rocard submarine volcano which overlies the Society hot spot. The Polynesian Seismic Network recorded for the first time unusual associated short‐ and long‐period signals, with perfectly monochromatic (0.0589 Hz) Rayleigh wave trains of long period and duration. None of the numerous observations of long‐period (10–30 s) signals previously associated with volcanic activity in Japan, Italy, Mexico, Indonesia, Antarctica, and the Hawaiian Islands have the characteristics we observed at Rocard. We propose a tentative model for these unusual and rather enigmatic signals, in which the movement of lava excited the resonance of a shallow open conduit under a high hydrostatic pressure of ~400 bars.

Seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of OBS array records at 60 Ma seafloor

AbstractWe analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 Ma seafloor in the southeastward of Tahiti island, the South Pacific, by the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot project. We first obtained one‐dimensional shear wave velocity model beneath the array from average phase velocities of Rayleigh waves at a broadband period range of 5–200 s. The obtained model shows a large velocity reduction at depths between 40 and 80 km, where the lithosphere‐asthenosphere boundary might exist. We then estimated shear wave azimuthal anisotropy at depths of 20–100 km by measuring azimuthal dependence of phase velocities of Rayleigh waves. The obtained model shows peak‐to‐peak intensity of the azimuthal anisotropy of 2%–4% with the fastest azimuth of NW–SE direction both in the lithosphere and asthenosphere. This result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid‐ocean ridge but is roughly parallel to the ancient plate motion at depths of 20–60 km. The fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60–100 km. Additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190–210 km.

Upper mantle structure beneath the Society hotspot and surrounding region using broadband data from ocean floor and islands

We determined the three-dimensional shear wave velocity structure beneath the South Pacific superswell down to a depth of 200 km by analyzing fundamental Rayleigh wave records from permanent and temporary land-based and seafloor seismometers in the Pacific Ocean. Data from the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES) project yield excellent spatial resolution of velocity anomalies in the central part of the superswell, near the Society hotspot. Localized slow anomalies are found near hotspots in the upper mantle, but the vertical profiles of the anomalies vary from location to location: Slow anomalies near the Samoa, Macdonald, Pitcairn, and Society hotspots extend to at least 200 km depth, while a slow anomaly near the Marquesas hotspot extends only to ~150 km depth. Owing to the recently deployed seafloor array, horizontal resolutions of slow anomalies near the Society hotspot are substantially improved: The slow anomalies are about 300 km in lateral extent and have velocity anomalies as low as −6 %. The lithosphere thickness is estimated to be ~70 km in the vicinity of all hotspots, which may indicate thermal erosion by mantle plumes.

TIARES Project—Tomographic investigation by seafloor array experiment for the Society hotspot

We conducted geophysical observations on the French Polynesian seafloor in the Pacific Ocean from 2009 to 2010 to determine the mantle structure beneath the Society hotspot, which is a region of underlying volcanic activity responsible for forming the Society Islands. The network for Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES, named after the most common flower in Tahiti) is composed of multi-sensor stations that include broadband ocean-bottom seismometers, ocean-bottom electro-magnetometers, and differential pressure gauges. The network is designed to obtain seismic and electrical conductivity structures of the mantle beneath the Society hotspot. In addition to providing data to study the mantle structure, the TIARES network recorded unprecedented data of pressure and electromagnetic (EM) signals by tsunamis associated with large earthquakes in the Pacific Ocean, including the 2010 Chilean earthquake (Mw 8.8).

Evidence of a partitioned dynamo reversal process from paleomagnetic recordings in Tahitian lavas

Lavas erupted at the Society hotspot during the Matuyama‐Brunhes (M‐B) polarity reversal record transitional field behavior containing two tight paleodirectional groups that when averaged are antipodal at the 95% confidence level, and thus correlate to antipodal clustered virtual geomagnetic poles (VGPs). The occurrence of these observations––data obtained from two published records of the M‐B transition from distinct sections of a succession of flows on Tahiti––is associated with a time when the strength of the axial dipole was significantly reduced. One of the clusters was recorded by lavas that were not erupted in succession, suggesting that significant time had passed during this volcanic activity. Time spent during the formation of the antipodal cluster is unknown, although it resides in the same location as VGP clusters associated with four other transitional events obtained from Society hotspot lavas. Calculated VGPs at the Society hotspot for both “polarities” of the averaged historic field––less the axial dipole term––are found in the cluster locations. These findings offer strong evidence for a two‐tiered dynamo process in which nearly the entire axial dipole component undergoes both demise and regeneration quasi‐independently from that of the remainder of the field, the pattern of which is tied to long‐held physical conditions of the lower‐most mantle.

Mapping upper mantle flow beneath French Polynesia from broadband ocean bottom seismic observations

Upper mantle structures and flow related to the South Pacific Superswell and to the associated short‐lived hotspots are investigated using seismic stations deployed in French Polynesia. Beside island stations, broadband ocean bottom seismometers (BBOBS) have been deployed between the various Polynesian archipelagos to investigate the oceanic upper mantle “unperturbed” by thickened crust or hotspot activity. We investigate seismic anisotropy measured at BBOBS stations and found it rather similar to that from island stations. This confirms the ability of OBS to be used for anisotropy purposes and suggests some homogeneity in the upper mantle pervasive deformation. The two OBS installed on the head of the Society hotspot provide different anisotropy signatures than most of the other French Polynesian stations: one is “isotropic” to SKS waves and the other provides a fast split direction normal to the average value observed in this area, suggesting a more complex mantle flow induced by the plume upwelling.

New K-Ar ages of the Society Islands, French Polynesia, and implications for the Society hotspot feature

Abstract We report 52 new K-Ar age determinations of volcanic rocks from the Society Islands in French Polynesia. The newly obtained ages range from 0.51 to 4.61 Ma, which are consistent with previously reported ones. The magnetostratigraphy based on 45 data sets combining K-Ar ages and paleomagnetic polarity agrees with the geomagnetic polarity time scale at the 2s level, except for a few age data. Assuming the absolute motion of the Pacific plate, paleopositions of the dated rocks are estimated from the K-Ar ages, showing a narrow distribution in the western part of the easternmost volcano of the active hotspot region. This suggests that a hotspot of a few tens of kilometers in diameter might have been fixed nearly at the eastern margin of the presently active region in the Society archipelago and that the volcanism of each island might have persisted for about 1 myr.

Topography of the mantle discontinuities beneath the South Pacific superswell as inferred from broadband waveforms on seafloor

We analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the South Pacific superswell to determine the depths of the mantle discontinuities using a receiver function method. We estimated the thickness of the mantle transition zone (MTZ) to be 245 km on average beneath the superswell region, which is close to the global average. The MTZ is found to be thinned locally beneath the Society hot spot by 30 km. Temperature anomalies computed from the thinned transition zone and the Clapeyron slope of the olivine phase transformations are 150–200 K beneath the Society hot spot. Previous studies, using land-based data, suggested the presence of a hot MTZ beneath the Pitcairn hot spot. The locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle. The normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present.

Offshore evidence for a huge landslide of the northern flank of Tahiti‐Nui (French Polynesia)

A bathymetric analysis of the flanks of Tahiti‐Nui (French Polynesia) reveals the existence of a major catastrophic north‐directed landslide during its eruptive history. The related debris‐avalanche (DA) deposits extend north 70 km from the island shore and have a total volume of about 800 km3. Two contrasted facies of the debris are recognized: (1) relatively small‐sized material, including a few blocks with well‐developed flow structures (elongate downslope‐directed ridges and transversal pressure ridges), and (2) a scattering of large kilometer‐sized individual blocks that form, in part, a prominent bathymetric hummock in the proximal central part of the landslide deposits. Bathymetric analyses suggest that the Tahitian failure evolved through two main successive shots. The DA debris were rapidly overlain by subsequent volcanic material, producing a smooth surface down to 1500 m below sea level. The on‐land and offshore structures indicate a listric fault geometry for the landslide scar. The volume removed by the landslide is thus estimated between 400 km3 and 450 km3. Discrepancy in the volume of the scar, and the debris derived from it, is due in part to expansion of the debris, to fragmentation of the avalanche, and to incorporation of sediments. Comparison with a recently recognized southern Tahitian landslide (Clouard et al., 2001) and with landslides recognized around the Hawaiian Islands and the Canary Islands shows that the huge northern Tahitian landslide described here has a reduced mobility in its central part. Perhaps the older Tetiaroa eruptive complex to the north acted as a buttress to inhibit movement of the DA products. The northern and southern Tahiti‐Nui landslides moved in opposite directions away from the main central E‐W directed rift zone that served as the main magma feeding structure. Then, the huge northern landslide strongly controlled the later volcano‐structural evolution by capturing subsequent volcanic activity to the north, despite the northwestward displacement of the Pacific plate over the Society hot spot.

Degassing of CO 2 and H 2O in submarine lavas from the Society hotspot

We analyzed CO 2– δ 13C–H 2O– δD in vesicles of a suite of 23 vesicular submarine volcanic glasses (4 alkalic basalts and 19 trachytes) from Teahitia, Volcano 4 and Rocard seamounts and Mehetia Island in the active area of the Society hotspot (French Polynesia, South Pacific). Vesicle C and H 2O concentrations ([C vesicle] and [H 2O vesicle]) vary in the range 1–1487 ppm C and ∼0–5431 ppm H 2O, respectively. The δ 13C of CO 2 in vesicles ( δ 13C vesicle) vary from − 3.8‰ to − 9.7‰ and the δD of H 2O in vesicles ( δD vesicle) vary from − 32‰ to − 71‰. The δ 13C vesicles values are correlated with the vesicularity, Log([C vesicle]) and the δD vesicle values. The Log([H 2O vesicle]) is also correlated to the δD vesicle values and Log([C vesicle]). These observations show that late-stage open-system degassing, following a first stage of closed-system degassing, is the chief process controlling the concentration and isotopic ratio variations observed in the vesicles. The highest measured δ 13C vesicle values fall in a typical mantle range (− 4.0 ± 0.5‰). Taking into account the effect of vesicle bursting and crystal fractionation, the reconstructed pre-eruptive carbon and water concentrations in the alkalic basalt parental magma are ∼3810 ppm C and 14,360 ppm H 2O (1.43 wt.%). These concentrations correspond to carbon and water fluxes for the Society hotspot during the last 900 kyr of 2.57 × 10 4 tons C yr − 1 and 97 × 10 3 tons H 2O yr − 1 and volatile concentrations of the source of 190 ppm C and 720 ppm H 2O. These values are similar in magnitude to previous estimates for other hotspots. Our study emphasizes that H 2O degassing must be taken into account before discussing H 2O content and δD value variations in the lavas in terms of source variation in the case of the Society hotspot.

Age progression along the Society hotspot chain (French Polynesia) based on new unspiked K-Ar ages

Abstract New K-Ar dates of volcanic rocks from five of the nine islands of the Society Archipelago (Moorea, Huahine, Raiatea, Bora Bora and Maupiti), confirm a Pacific plate velocity of around 11 cm/a during the last 4.3 m.y. These new data allow us to analyse the age-distance relationship along the chain and to evaluate possible temporal variations in the activity of the Society hotspot. A clear increase of ages is observed along the linear chain away from the present Society hotspot location. The time-space relationship between Taiarapu, Tahiti-Nui and Moorea can be explained by a simple hotspot model. Nevertheless, the simple fixed hotspot model assuming constant Pacific plate velocity may need adjustments to fully explain the age progression along the Archipelago. The slight departures from a linear age distribution can be explained by changes in Pacific plate motion which occurred at 5 and 3 Ma. In addition, the contemporaneous magmatic activities in the pairs Bora-Bora/Tahaa, Raiatea/Huahine, Maiao/Moorea require additional lithospheric control on magma transport. Combined with the hotspot activity, lithospheric loading may have produced extension and triggered volcanism along already existing fractures linking paired islands. The most likely model for the Society chain, proposed by McNutt [1998], involves a plume originating from a wide deep thermally anomalous zone (the Pacific Superswell) as a rising diapir (hotspot of secondary type according to the classification of Courtillot et al. [2003]). It melted during ascent and ponded beneath the Pacific plate to form short linear island chains showing rather good age vs. distance correlations.

Thickness of the mantle transition zone beneath the South Pacific as inferred from analyses of ScS reverberated and Ps converted waves

We study the thickness of the mantle transition zone beneath the South Pacific in order to determine temperature anomalies associated with presumed hot plumes in the South Pacific Superswell. The ScS reverberation method is used to determine the average thickness between an event-station pair and the Velocity Spectrum Stacking (VSS) method is used to determine the thickness near each station. The thickness obtained by the ScS reverberation method shows that the average thickness beneath the Superswell is less than the globally averaged thickness by 6 km, while the average thickness beneath the Tonga–Fiji subduction is greater than the global average by 10 km. The VSS method shows nearly normal thickness beneath stations used in the Superswell region. We conclude that substantial thinning of the transition zone associated with presumed hot plumes occurs locally beneath the Society hot spot as revealed by our previous SS-precursor study, and is not widespread beneath the whole area of the Superswell.

Mantle transition-zone structure beneath the South Pacific Superswell and evidence for a mantle plume underlying the Society hotspot

Underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the South Pacific Superswell and surrounding regions. Differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal, and thus hotter than normal, over an area 500 km or less in diameter beneath the Society hotspot. There is no general difference, however, in transition-zone thickness between the Superswell area and its surroundings. Our observations support the inference that the thermal flux from lower to upper mantle beneath the South Pacific Superswell occur on the lateral scale of a mantle plume (several hundred kilometers) rather than that of a superplume (several thousand kilometers).

The island of Maupiti : the oldest emergent volcano in the Society hot spot chain (French Polynesia)

Abstract The island of Maupiti is the top of an eroded volcanic shield surrounded by a large lagoon and coral reef. The most important part of this shield is made up of alkali basaltic lavas grading upward into hawaiitic flows. The shield-building event is dated at ca. 4.51 ± 0.04 Ma. A few mugearitic and benmoreitic late dykes were emplaced at 4.21 ± 0.04 Ma. These chronological data are consistent with a Pacific plate motion of 11 cm/year with respect to a fixed Society plume. The Maupiti lava series results from the effects of fractional crystallisation of a parental alkali basalt magma displaying a prominent EM II isotopic and trace element signature, in many ways similar to that of the youngest Society hot spot lavas. This feature suggests that the geochemical signature of the Society plume did not change through time.

Géologie, géochimie et géochronologie de l'île de Bora Bora (Société, Polynésie française)

Bora Bora island, located at the northwestern edge of the Society archipelago represents the upper part of a shield volcano made up of a pile of meter-thick alkali basalt flows and rare hawaiites. The summit caldera (4.4 km in diameter) is still recognisable and a southwestern tilting of the volcanic edifice led to a relative uplift of the northern and eastern parts of the volcano. Bora Bora lavas derive from low partial melting degrees of an EMII mantle source the composition of which was influenced by continental materials. The subaerial volcanic activity corresponding to the presently emerged portion of the edifice took place between 3.45 and 3.10 Myr. Such ages are consistent with the formation of the island by being above the Society hot spot considered as fixed and a Pacific plate motion of 11 cm·yr –1.