Martial Caroff

Origin of anomalous rare-earth element and yttrium enrichments in subaerially exposed basalts: Evidence from French Polynesia

Abstract Basalts from French Polynesian islands occasionally display extremely high abundances and anomalous distributions of rare-earth elements (REE) and yttrium, whereas other incompatible element concentrations and O, Sr, Nd and Pb isotopic ratios do not differ from those of “normal” basalts from the same area. The REE- and Y-enriched basalts contribute up to 15% of the sample set, suggesting that this feature is more widespread than previously thought. REE-Y enrichment is related to the presence of rhabdophane-type REE-Y-phosphate identified through electron microprobe analyses in the most enriched sample and inferred from leaching experiments in the others. This phenomenon is confined to subaerially exposed basaltic sequences, indicating a close relationship to supergene processes. This is supported by negative Ce anomalies in these basalts, since decoupling of Ce from the other REE is restricted to oxidizing, low-temperature, aqueous environments. Similar Nd isotopic ratios for enriched and normal basalts allow us to exclude the possibility that additional REE and Y are derived from marine sediments or guano, but rather suggest an origin from the local basalts. Moreover, light REE enrichment in the REE-Y-phosphates suggests short migration distances of the fluids, supporting the conclusion that additional REE and Y were mobilized from weathered basalts and transported by descending meteoric waters.

Trace element behavior in the alkali basalt-comenditic trachyte series from Mururoa Atoll, French Polynesia

Abstract Numerous drill holes have penetrated close to 1000 m of the volcanic pile of Mururoa atoll. The rocks are a typical example of mildly alkaline intraplate basaltic volcanics ranging from Mg-rich compositions to comenditic trachytes. Evolved basalts and hawaiites are the dominant rock types. Benmoreites and trachytes are relatively uncommon. The available KAr ages indicate a long period of activity between 11.8 and 10.7 Ma. Nevertheless, all the volcanic rocks studied are cogenetic in a broad sense (identical 143 Nd 144 Nd and initial 87 Sr 86 Sr ratios, constancy of ratios of highly incompatible elements). No indications of the occurrence of assimilation or magma mixing have been found. The Mururoa series is thus suitable for the study of fractionation-related processes. Three crystallization/fractionation models have been tested using trace elements, the mineral proportions taken from mass-balance calculations on major elements and individual distribution coefficients determined from trace element data on phenocrysts/host rock pairs. Closed-system fractional crystallization (CSF) is consistent with all of the trace element data. It satisfactorily reproduces the trends observed for compatible and incompatible elements, including the complex behavior of Y and rare-earth elements which are fractionated by kaersutite and apatite in mugearitic and benmoreitic magmas. In situ crystallization and equilibrium crystallization produce trends very close to Rayleighs law model for most incompatible elements. In contrast, in situ and equilibrium crystallization models produce significant underdepletions (compared to CSF) in compatible elements (Sc, Cr, Co, Ni). The corresponding patterns are not consistent with our analytical data in intermediate and evolved rocks. Cooling calculations indicate, however, that the length of volcanic activity in Mururoa ( ≥ 1 Ma) is hardly compatible with closed-system fractional crystallization of a single magma batch. Various models of open-system fractionation have thus been tested, but generally they do not fit the observed trace element patterns. However, one peculiar case of open-system fractionation in a periodically replenished magma chamber is consistent with the data. In this model, batches of mantle-derived parent liquids filling up the chamber at the beginning of each cycle evolve by fractional crystallization. All the corresponding residual liquids either crystallize or erupt at the end of each cycle, before the next replenishment. Thus, the apparently cogenetic Mururoa series is likely to result from fractional crystallization occurring under similar conditions through time, either in several magma chambers or in a single periodically refilled reservoir.

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

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.

Assimilation of ocean crust by hawaiitic and mugearitic magmas: an example from Eiao (Marquesas)

Abstract Three holes were drilled in the northern (Dominique), central (Sophie) and southern (Naore) parts of the volcanic shield of Eiao island in the Marquesas archipelago, French Polynesia. These were continuously cored with an excellent recovery and reached depths of 500 m (Sophie) and 800 m (Dominique and Naore). K–Ar dating indicates that the volcanic sequence penetrated was emplaced between 5.52±0.05 Ma and 4.95±0.04 Ma. Petrographic types include tholeiites, alkali basalts, picrobasalts, hawaiites, mugearites and trachytes. Intermediate lavas (hawaiites and mugearites) are fairly common. They occur within stratigraphic units denoted D1, D2, D3, S1, S2, S3, N1, and N2, respectively (D: Dominique; S: Sophie; N: Naore). The D3, S1 and N1 hawaiites and mugearites appear to be derived from fractional crystallization of parental alkali basalt magmas. The other groups are characterized by variable enrichments in Sr (D1, D2 and S3), P2O5 (D2), Th (S2 and N2) and light rare earth elements (D2) or by depletion in heavy rare earth elements (S2 and N2). We suggest that these trace element characteristics reflect open system differentiation processes which occurred during the building of Eiao volcanic shield. Two stages of assimilation coupled with fractional crystallization (AFC) are proposed. The calculated compositions of the two contaminants suggest that they were leucocratic materials located at different levels of the oceanic crust. Finally, a water-induced assimilation process is invoked to explain the origin of the S3 hawaiites: these lavas are enriched in Al2O3, Na2O and Sr, inferred to reflect plagioclase breakdown within the Eiao plumbing system.

From alkali basalt to phonolite in hand-size samples: vapor-differentiation effects in the Bouzentès lava flow (Cantal, France)

Abstract The Bouzentes lava flow is a 20-m-thick alkali basalt flow emplaced during the last stage of formation of the Cantal stratovolcano at 4.2 Ma. Its upper part has 1- to 20-cm-thick vesicle-rich segregation sheets which recur every 0.1–2 m. These horizontal veins are hawaiitic in composition. They are characterized by hypertrophic development of their minerals (‘pegmatoids’) and by glassy phonolitic segregation vesicles. Internal differentiation within the Bouzentes lava flow was triggered by an unusually high water content, as suggested by pre-emptive iddingsite alteration of olivine phenocrysts. The proposed model of formation of the segregation sheets includes the upward motion of diapirs of residual melt plus addition of vapor from the bottom of the central liquid lens to the base of the upper solidified crust of the cooling lava flow. Olivine settling appears to have been inhibited or at least retarded by upward migration of melt plus vesicles. Most of the features observed in Bouzentes recall the internal differentiation processes usually described within thick Hawaiian lava lakes. The segregation vesicles are believed to result from an increase of gas solubility in residual melt during the crystallization process.

Epiclastic deposits and ‘horseshoe-shaped’ calderas in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago), French Polynesia

Occurrences of debris avalanche deposits newly identified in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago) are described and interpreted here. In both islands,the breccias are located within horseshoe-shaped residual calderas. In Tahiti,the epiclastic formations,up to 500 m thick,lie on the floor of the central depression and in the valley of the northwards running Papenoo River. In Ua Huka,the breccias crop out within a depression limited by a semicircular crest in four bays along the southern coast. Their thickness is ca. 100 m. A few clasts collected in the Tahitian breccias and some rocks forming their substratum have been dated (K^Ar datings) and analysed (major and trace elements,Sr^Nd isotopes) for this study. Using these data,we show that the debris avalanche(s) occurred in Tahiti Nui at the end of the growth of the shield volcano (between 570 000 and 390 000 years ago),maybe in consequence of the emplacement of the plutonic body which occupies the central part of the caldera. In Ua Huka,the collapse took place nearly 3 Ma ago,between the construction of the shield volcano and that of the inner one. The southwards orientation of the caldera,like that of the neighbouring island Nuku Hiva,might reflect a preferential direction of weakness in the substratum of the central Marquesas. > 2002 Elsevier Science B.V. All rights reserved.

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

Geology, geochemistry and geochronology of Bora Bora island (Society islands, French Polynesia). 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 diam- eter) 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 .

Géochimie des basaltes de l'île de Ua Huka (archipel des Marquises): variation du taux de fusion partielle et hétérogénéité de la source mantellique

Abstract The main shield volcano of Ua Huka Island (Marquesas Archipelago) was emplaced between 2.2 and 2.4 Ma, and then affected by two caldera collapse events. After a 0.9 Ma-long gap, volcanic activity resumed with the emplacement of two smaller volcanoes in the southwest part of the island, between 1.5 and 0. 75 Ma. The geochemical characteristics of Ua Huka mafic lavas, which range from olivine tholeiites to alkali basalts and basanites, are consistent with a temporal decrease in partial melting degrees of a heterogeneous mantle source. The associated temporal variation of the isotopic signatures of Ua Huka basalts implies a more important contribution of a Depleted MORB Mantle (DMM) end-member during the genesis of the youngest basanitic lavas. Such a variation was not previously documented in the Marquesas Archipelago.

Timescale of open-reservoir evolution beneath the south Cleft segment, Juan de Fuca ridge

Lavas erupted at the southern end of the intermediate Juan de Fuca ridge (Cleft segment) are mostly cogenetic and their chemical diversity results from melt evolution in an open magma system. In the present study, we apply a theoretical model allowing the time evolution of this periodically recharged and tapped magma chamber to be estimated. In our mathematical procedure, the melt quantity supplied to the reservoir varies through time following a sinusoidal function. The rare earth element concentrations in the refilling melt were calculated on the basis of the REE distribution in lavas. This theoretical composition is akin to that previously estimated for a Mg#70 MORB from mineralogical and chemical data. Then, we approached the temporal evolution of the reservoir using a set of suitable parameters deduced from the geometry of the crust and magma system beneath the Cleft segment. Particularly, we considered two end-members scenarios for the melt repartition through the magma reservoir beneath the Cleft segment: the “gabbro glacier” model (crystal nucleation and growth occur within one single melt lens and crystals subside vertically and laterally) and the “sheeted sill” model (crystallization takes place within a network of connected sills located at various depths within the crust). We estimated that the magma chamber is refilled every thousand years and that the melt resides approximately one hundred years within the reservoir.

Geological and petrologic evolution of Huahine island (Society archipelago, French Polynesia) : an unusual intraoceanic shield volcano

Huahine (Leeward Islands, Society Archipelago) is composed of two islands, Huahine Nui and Huahine Iti, separated by the shallow Port Bourayne and Maroe bays and surrounded by a common lagoon. The two islands, however, belong to a single basaltic and trachybasaltic shield volcano, the emerged part of which was constructed during a very short period, between 2.65 and 2.52 Ma. The volcano is made of composite basaltic flows belonging to three distinct petrogenetic types, which derive from low degrees of partial melting of heterogeneous mantle sources. This building stage lead to the formation of a central caldeira. Then, a WSW-ENE trending graben formed separating Huahine Nui from Huahine Iti. As a consequence, Huahine differs from most of the other Polynesian islands which display large collapse structures opened toward the sea. After a period of inactivity of at least 0.25 m.y., magmatic activity resumed, leading to the emplacement of five trachyphonolitic intrusions along N-S trending deep regional fractures. These lavas, which do not result from the fractional crystallization of the shield basalts, are considered as derived from the melting of a deep intrusive network of dykes.