Society Hotspot Research Articles

Shear wave splitting and upper mantle deformation in French Polynesia: Evidence for small‐scale heterogeneity related to the Society hotspot

We determined shear wave splitting parameters at four island sites in French Polynesia: Tiputa (TPT) on Rangiroa in the Tuamotu archipelago; Papeete (PPT) on Tahiti in the Society Islands; Tubuai (TBI) in the Cook‐Austral island chain; and Rikitea (RKT) on Mangareva in the Gambier Islands. We also examined splitting at Pitcairn (PTCN) on Pitcairn Island; because of the short time of operation of PTCN, our results there are preliminary. We find substantial differences in splitting, most likely caused by variable upper mantle deformation beneath the five stations. At TPT the fast split shear wave (ϕ) direction is N66°W±4°, parallel to the current Pacific‐hotspots relative motion (APM) vector; the delay time between fast and slow waves is 1.3±0.2 s. At PPT, on Tahiti, we could detect no splitting despite many clear SKS observations. At TBI, on Tubuai we detected splitting with a delay time of 1.1±0.1 s and a ϕ direction midway between the local APM direction and the fossil spreading direction (N86°W±2°), as locally indicated by the nearby Austral Fracture Zone. At RKT in the Gambier Islands, ϕ trends N53°W±6°, 16° clockwise of the local APM azimuth, and delay time at RKT is 1.1±0.1 s. Results at PTCN include ϕ near N38°W±9° and a delay time of 1.1±0.3 s. These different results imply variable upper mantle deformation beneath the five sites. We interpret splitting at TPT and, possibly, RKT as indicative of asthenospheric flow or shear in the APM direction beneath the stations. At PPT, azimuthal isotropy indicates deformed upper mantle with a vertical symmetry axis, or absence of strong or consistently oriented mantle deformation fabric beneath Tahiti. Either effect could be related to recent hotspot magmatism on Tahiti. At TBI, splitting may be complicated by juxtaposition of different lithospheric thicknesses along the nearby Austral Fracture Zone, resulting in perturbation of asthenospheric flow. The absence of splitting related to fossil spreading in French Polynesia indicates that upper mantle deformation processes postdating lithosphere formation are important at all four sites within that region. The ϕ azimuth at PTCN does not align with either the fossil direction or the APM direction, but our best individual determination of splitting parameters at this station lies within 10° of the local APM at Pitcairn Island.

Mehetia Island, South Pacific: geology and petrology of the emerged part of the Society hot spot

Mehetia is the youngest island of the Society hot-spot chain, representing the summit of a large submarine volcanic edifice (4035 m in height). Its conical shape results from strombolian-type eruptions. Mehetia comprises two major volcanic formations. The “old edifice” made up of interbedded pyroclastic rocks and lava flows, forms the main part of the island. The base of its stratigraphic pile is dated at 70,000–75,000 yr B.P. by K/Ar. Younger events (⩽ 31,000 yr B.P.) have built up a volcanic cone, with a summit hydromagmatic crater, and emplaced lava flows extending south of the island where they overlie coral reef limestones. Between the two eruptive periods, an erosional phase caused the partial destruction of the old edifice. Six successive stages of construction of the island are described from stratigraphic relations. Mehetia does not show any indication of present-day volcanic or fumarolic activity but should be considered as an active volcano as evidenced by the 1981 volcanoseismic crisis. Mehetia lavas include oceanites, primitive and evolved alkali basalts and basanites together with rare hawaiites and mugearites. Two main petrogenetic processes (i.e. accumulation and crystal fractionation) are inferred from bulk-rock composition and microprobe data on phenocrysts. Oceanites are derived from basalts through accumulation of olivine and clinopyroxene. Major and trace elements trends suggest evolution through crystal fractionation processes involving olivine, clinopyroxene, and Fe-Ti oxides.

Submarine intraplate volcanism in the South Pacific: Geological setting and petrology of the society and the austral regions

The southeastern prolongations of the Society and Austral islands volcanic chains are terminated by several recent submarine volcanoes (300–3800 m in height) built on irregularly shaped crustal swells or bulges (3600–3950 m in depth). The crustal swells (about 100 km in width) is bounded by deeper abyssal hill regions (>4000 m in depth) where old volcanoes with thick Fe‐Mn coatings are encountered. The rocks sampled on this ancient oceanic crust are depleted mid‐ocean ridge basalts (MORBs) similar to modem volcanics encountered on the East Pacific Rise. The volcanics of the Society hot spot consist of ankaramites, picrites, alkali‐basalts, basanites, tephrites, and trachytes. Highly vesiculated pillow lavas show a wide compositional range in their large ion lithophile elements (LILE) (K2O=1–4%, Nb=25–80 ppm, Zr=200–400 ppm, Ba=300–840 ppm) and compatible element variations (Mg #=40–70, Ni=80–500 ppm), and low Zr/Nb ratios (5–8). Low‐K tholeiites (K2O<0.15%, Nb=3–6 ppm, Zr=56–74 ppm, Zr/Nb=10–25), also found associated with alkali‐basalts and basanites from recent volcanoes, are believed to have been exposed during rejuvenation of ancient edifices. The Macdonald volcano (Austral island chain), with a base diameter of 45 km at a depth of 3850 m, was sampled from its summital area (<100 m in depth) down to 2000 m in depth. Highly vesiculated pillow lavas, volcanic ejecta, and accidental rock debris recovered differ from those of the Society hot spot volcanoes by their lower Zr/Nb (3–5), Rb/Sr (<0.045), and (Ce/Yb)N (5–10) ratios and their lower Ba (350–400 ppm) and light rare eath elements (LREE) content. The accidental rock debris (metabasalt, metadolerite, isotropic gabbros, and metagabbros) were scattered during phreatic explosive events on the flanks of the Macdonald volcano. Volcanic ejecta of basanite lapilli partially covering the accidental rock debris were formed during more recent phreatomagmatic explosions. The most undersaturated glassy pillow lava encountered in the Austral hot spot is a normative nepheline‐rich rock with incipient amphibole recovered from an ancient seamount (Ra seamount) located at about 100 km northwest of the Macdonald volcano. Based on their high LREE content and low Zr/Nb (3–8) and on their high (Ce/Yb)N (5–20) (5–20), (La/Sm)N (1–5), and (La/Lu)N (7–40) ratios with respect to the MORBs, it is speculated that the intraplate volcanics (Austral and Society volcanoes) have originated from the partial melting of mixed mantle sources made up of lherzolite (MORB mantle source) and another complementary source enriched in LREEs and Nb. A recycling of continental crust or pelagic sediments in subduction zones represents a possible supply of LREEs but not of Nb. Continental metasomatized peridotites and oceanic amphibole‐peridotites such as Zabargad and St. Paul's Rocks samples were proposed as potential mantle sources for intraplate magmas. Using modal and chemical (Zr, Nb, Ce, and Yb) data available in the literature, a model of partial melting of a composite mantle, obtained by mixing homblendite (Queensland's xenoliths) and spinel‐lherzolites, was tested as being a possible origin for the studied rocks. This model provides adequate Zr/Nb ratios and suggests that the Society mantle source contained smaller amount of homblendite (<5 wt %) and underwent lower degree of partial melting (about 5 %) than that of the Austral hot spot (10 wt % and 10–20% respectively). However, the complementary mantle source in the Society hot spot requires larger amount of LREEs than it is necessary for the Austral volcanoes. While the Zabargad amphibole‐peridotites are reasonable candidates, the metasomatized peridotite xenoliths from continental alkali‐basalts and the amphibole‐peridotite mylonites from St. Paul's Rocks are more suitable as a potential source for the Society parental melts enriched in Ce and La. Hence the intraplate lava compositions could be the result of recycling and remelting of an ancient oceanic upper mantle which is partially metasomatized during its lateral transfer off the ridge axis. Local mantle heterogeneities and variable amounts of melting account for the occurrence of several different magmatic lineages through crystal fractionation processes in shallow magmatic reservoirs underlying individual volcanic centers of the Society and Austral hot spots.

Morphological and structural studies of the Society and Austral hotspot regions in the South Pacific

The southeastern extension of the Society (Teahitia hotspot) and Austral (Macdonald hotspot) volcanic chains were extensively studied (1986–1989) using multi-channel echo-sounding (Seabeam), bottom imaging systems and submersible dives. Sea floor within these hotspot regions consists of recently constructed submarine highs built on crust that shows morphological and structural discontinuities with the surrounding, ancient abyssal hill regions (> 4200 m in depth). Recent volcanic and hydrothermal activity is present on several volcanoes within the Society hotspot and on the Macdonald volcano. Previous studies carried out in the Society hotspot region showed several geological provinces: 1. (1) Eight major highs (> 1000 m in height) which extend from above sea level down to a depth of 3750 m. These features have been formed by lateral eruptive events along rift zones which are aligned along four main structural directions—N150°–180°, N80°, N30°–40° and N110°—which correspond to the orientations of ancient ridge axes, transform faults and Riedel-type shear zones, respectively. 2. (2) The sea floor, which slopes away from the base of each major volcano (3750–4200 m depth), has an average slope of 0.5°, and is characterized by smooth topography and volcanic cones less than 500 m high. 3. (3) The surrounding abyssal hill region, which has an average slope of 0.1° near the Society hotspot, consists of faulted terrane with a N170° orientation, and ancient elongated structural highs. From the structural lineations observed in the Society hotspot volcanoes, a reconstruction of volcanic features has been made. It is proposed that the formation of the seamounts occurs in an organized fashion along pre-existing preferential structural orientations that are associated with zones of crustal weakness.

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.

Mapping present-day geochemical variations across the society hotspot

The presence of variable amounts of amphibole ± phlogopite in a garnet websterite and a garnet clinopyroxenite from the Beni Bousera peridotite massif provides evidence for post-formation metasomatism. Textural observations associated with major- and trace-element mineral compositions allowed us to distinguish two metasomatic episodes, which occurred at different stages of the Beni Bousera massif evolution. The garnet websterite has recorded interaction with LREE-rich silicate melts before the uplift of the massif. Amphibole/clinopyroxene and amphibole/garnet trace-element ratios closely approach partition coefficient values, indicating that chemical equilibrium was attained between amphibole and pyroxenite matrix minerals. The geochemical signatures of the putative alkaline interacting melts are similar to those of recent basaltic magmas erupted in Morocco, suggesting a common peridotite mantle source. In contrast, amphibole from the garnet clinopyroxenite is in chemical disequilibrium with the pyroxenite matrix minerals. In this clinopyroxenite the crystallization of amphibole and plagioclase occurred at lower T (and P) conditions, most probably during the ascent of the Beni Bousera massif and its emplacement into the crust. The melt responsible for this later metasomatic episode was LREE-depleted and HREE-enriched, suggesting that it resulted from decompression melting of a garnet-bearing source (with garnet as a melting phase), similar to the garnet-bearing pyroxenites outcropping in the Beni Bousera massif.