Austral Chain Research Articles

Survival of lithium isotopic heterogeneities in the mantle supported by HIMU-lavas from Rurutu Island, Austral Chain

Thirty years ago, Hofmann and Hart [Hofmann, A.W., Hart, S.R., 1978. An assessment of local and regional isotopic equilibrium in the mantle, Earth Planet Sci. Lett. 38, 44–62] showed that local disequilibria of slowly diffusing radiogenic tracers (e.g. Sr) are preserved in the mantle over 1–2 Ga time scales. Recently, it was suggested that this is not the case for fast diffusing elements such as lithium [Halama, R., McDonough, W.F., Rudnick, R.L., Bell, K., 2008. Tracking the lithium isotopic evolution of the mantle using carbonatites, Earth Planet Sci. Lett. 265, 726–742], thus questioning the ability of lithium isotopes to constrain long-term effects of recycling of material with crustal signatures. A key issue in this debate is the identification in hotspot volcanism of Li isotopic fingerprint consistent with recycling. Previous studies proposed that HIMU type volcanism, which is thought to sample mantle domains that include subducted altered oceanic crust, has 7Li/ 6Li distinctively higher than the fresh mid-ocean ridge basalts. This work focuses on Rurutu island, where both HIMU (20.88 < 206Pb/ 204Pb < 21.42) and non-HIMU (19.11 < 206Pb/ 204Pb < 20.45) lavas occur. When considering only the freshest and most primitive lavas, the lithium isotopic signatures of HIMU (+ 5.4 < δ 7Li < + 7.9‰) and non-HIMU (+ 2.9 < δ 7Li < + 4.8‰) lavas do not overlap, thus supporting the idea that HIMU mantle has distinctly elevated δ 7Li. This result suggests that Li isotopic heterogeneities could survive diffusion over 1–2 billion years, the amount of time required to develop the highly radiogenic Pb signature of HIMU-lavas. Modeling lithium diffusion out of a lithium-rich, isotopically heavy altered oceanic crust reveals that isotopic disequilibrium persists over long time periods in comparison with the rapid decrease of chemical disequilibrium. For instance, a kilometer-thick altered oceanic crust loses most of its Li excess in a matter of a few tens of millions of years but could preserve 7Li/ 6Li distinctively higher than the ambient mantle over a time period in excess of 1.5 Ga. After 1.5 Ga, a kilometer-thick altered oceanic crust has heavy but uniform 7Li/ 6Li, while large isotopic variations persist in the nearby mantle (< 20 km). Thus, decoupling between Li and slowly diffusing radiogenic isotopes can be predicted in the mantle nearby subducted crust.

No thinning of the lithosphere beneath northern part of the Cook‐Austral volcanic chains

The Cook‐Austral volcanic chains are situated in the South Pacific. They are composed of several volcanic chains and isolated volcanoes corresponding to distinct events from 55 Ma to present time. The tectonic history of the volcanoes emplacement is quite complex. It involves the activity of at least three hot spots, and sometimes several volcanic stages coexist on the same volcano. The purpose of this study is to bring new information on the loading history of the northern part of the Austral chain by studying the variation of the elastic thickness of the lithosphere. We have used the high‐quality data acquired during the ZEPOLYF2 cruise which conducted in July 1999, a multibeam survey of this area, and we got several new ages on seamounts. In this region where several volcanic stages overlap, the simple elastic model applied on the present‐day bathymetry is not appropriate. We then reconstructed the loading history of each volcano or group of volcanoes using information provided by the radiometric ages and chemical signature of the volcanic stages or the volcanoes morphology. We show that the lithosphere is characterized by normal elastic thickness if one correctly isolates the different volcanic stages occurring on this area. The abnormally low values of the effective elastic thickness obtained previously over this area may be explained either by not taking into account the overlapping volcanism or by poor quality of the geophysical data used for the determination.

Pb isotopic variability in melt inclusions from the EMI–EMII–HIMU mantle end-members and the role of the oceanic lithosphere

Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.

Helium isotopes on the Macdonald seamount (Austral chain): constraints on the origin of the superswell

Abstract We present new helium isotope data from the Macdonald seamount (Austral chain). The helium isotopic ratio varies from 4 He/ 3 He=45 000 (R/Ra=16.0) to 200 170 (R/Ra=3.6). The helium content is between 1.5×10 −8 and 1.1×10 −5 ccSTP/g. These helium results show clearly the presence of primitive mantle material in the source of the Austral chain. Macdonald has the lowest 4 He/ 3 He ratio among the Polynesian submarine volcanoes, except Hawaii (Loihi). The simplest explanation for the primitive helium signature is the presence under Macdonald of a mantle plume that derives either from the 670 km or 2900 km boundary layers, or, eventually, from the top of a large mantle dome resulting from a stratified two-layer convection. This plume contains less-degassed material with low 4 He/ 3 He ratio. To cite this article: M. Moreira, C. Allegre, C. R. Geoscience 336 (2004).

Isotope and trace element variations in lavas from Raivavae and Rapa, Cook–Austral islands: constraints on the nature of HIMU- and EM-mantle and the origin of mid-plate volcanism in French Polynesia

We have examined the major and trace element and Sr–Nd–Hf–Pb–Os isotope variations in two suites of lavas from the islands of Raivavae and Rapa from the Cook–Austral chain to better constrain the nature and origin of the source(s) of these lavas. Combined with data from other Cook–Austral islands and seamounts, the Raivavae and Rapa samples reveal the presence of three distinct components involved in the genesis of these lavas, a HIMU component, a depleted (or DM) component, and an “enriched” (EM) component. Northern Cook–Australs lavas (those north of the Austral Fracture Zone (AFZ)) falling along the “Macdonald” pseudo-age progression reflect mixing of the HIMU and DM components, whereas lavas south of the AFZ reflect mixing of the DM and EM components.Strong depletions in potassium and other fluid soluble elements in HIMU lavas suggest that the HIMU source contains ancient recycled (dehydrated) oceanic crust. The EM lavas have systematically less radiogenic Os-isotopes than the HIMU lavas. Therefore, the enriched or EM component cannot be generated through the addition of pelagic or terrigenous sediment to the HIMU source. Instead, the EM component in the southern Cook–Australs may represent recycled subduction-modified sub-arc mantle. High 208Pb/204Pb values in the DM component relative to East Pacific Rise mid-ocean ridge basalts (MORB) preclude derivation of this component from assimilated lithospheric mantle. The Sr–Nd–Pb isotope characteristics of the DM component resemble Pacific alkalic seamount lavas, and may derive from easily fusible components (e.g., pyroxenite veins) dispersed in the Pacific upper mantle.The Raivavae and Rapa lavas define a Hf–Nd isotope correlation that is considerably steeper than the mantle Hf–Nd isotope array. The unusually low εHf values (for a given εNd value) characteristic of the HIMU source are consistent with the presence of recycled oceanic crust. However, the presence of low εHf values in the Rapa EM lavas reveals that anomalously low εHf values are not restricted to the HIMU source. Regardless of the ultimate origin of the Raivavae and Rapa lavas, the comparatively low εHf values in these lavas indicate that multiple low-εHf components are present in the mantle. The addition of these components may help resolve the apparent terrestrial Hf–Nd mass-imbalance with respect to estimates of the bulk earth Nd- and Hf-isotope composition.Islands and seamounts from the Cook–Austral chain do not follow a clear age progression as predicted by the standard plume model for the origin of ocean island basalts (OIB). Previous models for the origin of the Cook–Austral chain have appealed to the existence of multiple small plumes. However, these models are implausible because they do not explain how several deep-seated plumes become aligned with each other in the direction of plate motion. We propose that volcanism along the Cook–Austral chain (and perhaps in much of French Polynesia) is generated through self-perpetuating melting anomalies that preferentially sample enriched components in a marble-cake mantle. These melting anomalies preferentially form or strengthen where lithospheric boundary conditions are favorable, such as at pre-existing transform faults or where the lithosphere has been thinned by a previous period of volcanism.

Migration rate of volcanism along the Foundation Chain, SE Pacific

Volcanism has migrated along the NW–SE-trending Foundation Chain, SE Pacific, at a rate of 91±2 mm/y for the past 21 million years (My). The Foundation Chain is at least 1900 km long, and was erupted mostly in the interior of the Pacific plate. The linear migration of intraplate, often geochemically enriched, volcanism along the Foundation Chain can be explained by the theory of tectonic plate drift over a narrow, stationary plume of hot mantle material upwelling from depth. The present hotspot (or at least its eastern extent) generated by this Foundation plume is now located on, or close to, the Pacific–Antarctic spreading-axis. Local spreading/microplate boundaries have migrated systematically towards the Foundation plume for at least the past 21 My, a process that was, however, interrupted by the creation of the Selkirk microplate. The Foundation Chain has erupted, therefore, onto ocean crust ranging in age from ∼17 Ma to ≥0 Ma, with distances between the Foundation plume and local spreading/microplate boundaries varying from ∼900 km to ≥0 km. Such interplay between the Foundation plume and local seafloor, spreading and microplate boundaries — probably in combination with changing stresses in the lithosphere — apparently induced major changes in the fabric (and composition) of the Foundation Chain, e.g., the switch from individual seamounts to elongated, en echelon, ridges at ≥5 Ma. It can also explain why ages for a few Foundation seamounts located at the edges of the Selkirk microplate do not lie along the overall linear trend of migrating volcanism. The linear velocity of the Pacific plate over the Foundation plume, calculated on the basis of the migration rate of volcanism along the Foundation Chain, is predicted by a number of rotation poles for the motion of the Pacific plate in a fixed hotspot reference frame. The fact that the measured migration rates of volcanism along the Foundation and Hawaiian chains are predicted by these rotation poles (1) confirms that the distribution of Foundation Chain ages records the absolute motion of the Pacific plate, and (2) supports the fundamental assumption of stationary mantle plumes in the case of Hawaii versus Foundation for at least the past 21 My. The Foundation Chain was created by a weak plume, primarily as relatively small, probably rapidly erupted (≤1 My) individual seamounts or seamount clusters at a rate of approximately one every My. Linear migration rates of volcanism are most likely defined unequivocally along such types of hotspot chain, on the basis of age–distance data. It should also be easier to identify and characterize volcanism created as a result of plume interactions with local spreading/microplate boundaries. Rotation poles for Pacific plate drift show that the initial stages of volcanism along the southern Austral Chain (≥34 Ma) could have been created by the Foundation plume, suggesting that the path of Pacific plate drift over the stationary Foundation plume could be at least 3000 km long.

Failure of plume theory to explain midplate volcanism in the southern Austral islands

It has long been recognized that the properties of the Cook–Austral chain (Fig. 1) of volcanoes in the South Pacific are difficult to reconcile with the theory that volcanic activity in plate interiors is produced by the drift of tectonic plates over narrow, stationary plumes1 of hot mantle material upwelling from depth. Radiometric dates2,3 from many island samples are younger or older than would be predicted if a single plume currently located at volcanically active Macdonald seamount4 was responsible for all of the volcanoes. Indeed, only the southernmost part of the Austral volcanic line has hitherto appeared to be consistent with plume activity, and then only within the past 6 million years (Myr)5,6. Here we report radiometric dates that demonstrate that these southern Austral volcanoes are actually composed of three distinct volcanic chains with a range of ages spanning 34 Myr and with inconsistent age progressions. Gravity anomalies and seafloor fabric suggest that the volume and location of volcanism in this region is controlled by stress in the lithosphere rather than the locus of narrow plumes rising from the deep Earth.

Contrasting old and young volcanism in Rurutu Island, Austral chain

We present new geochemical data (major and trace element, and Sr, Nd and Pb isotopic compositions) for volcanic rocks from Rurutu Island in Polynesia. The rocks are divided into two age groups: 13–10.8 Ma old basalts derived from the plume that is now active beneath the Macdonald seamount, and 1.8–1.1 Ma old volcanics that were produced during a younger volcanic event. Marked chemical differences exist between the two generations of lavas. Typical HIMU isotopic compositions are found in the old lavas but not in the young lavas, and unusual trace element fractionations (Ce/Pb, Zr/Hf and Ti/Eu) are restricted to the young lavas. These results are interpreted in a model in which the composition of the Quaternary lavas results from the interaction between magmas from an ascending young plume and the oceanic lithosphere under the island. The contaminating component in the lithosphere is thought to be a residual carbonatite liquid from the first volcanic episode. This contaminant has isotopic compositions similar to those of the first volcanic event but very high trace-element concentrations characterized by significant negative Ti and Pb anomalies. This model raises questions about the use of isotopic and trace element data from islands located downstream from another plume to understand the general composition and origin of the source of plume volcanism. However, it does not put into question the existence of extreme isotopic compositions (HIMU, EM I and EM II) in the mantle seurces of oceanic island basalts.

A search for seamounts in the Southern Cook and Austral Region

The existence of uncharted seamounts in the Cook‐Austral region of the South Pacific has been investigated using GEOS 3 and SEASAT radar altimeter observations. Three previously uncharted submarine seamounts, provisionally named GEOS A to GEOS C, have been located between Mauke and Rimatara and a fourth, GEOS D, has been located east of Rurutu. This confirms that the Aitutaki‐Mauke islands of the Southern Cooks are a continuation of the Austral chain. A second group of uncharted seamounts has been provisionally located some 200 km south of Rimatara and Maria and this is suggestive of a second seamount chain, south of the first, that includes Raratonga and Mangaia. Fabert Bank, to the south of Mangaia, appears to be mislocated by about 2° in longitude.

Tertiary Pacific Plate Motion Deduced from the Hawaiian-Emperor Chain

Nearly all linear island and seamount chains on the Pacific plate are parallel to small circles generated about either a Hawaiian pole at 72° N., 83° W. or an Emperor pole at 17° N., 107° W. The rates of rotation of the Pacific plate relative to the Hawaiian melting (hot) spot are calculated from age data from the Hawaiian-Emperor chain. Extrapolation of the known age progression along the Hawaiian chain yields a 27-m.y. estimate of the age of the Hawaiian-Emperor bend; however, recent radiometric ages from Koko seamount in the southern Emperor chain indicate that the Hawaiian-Emperor bend is 42 to 44 m.y. old. The Pacific plate apparently moved slowly, if at all, relative to the Hawaiian melting spot from about 20 to 42–44 m.y. ago. The rates of rotation calculated are 1.3° per m.y. about the Hawaiian pole (0 to 20–25 m.y.), The proposed rotational motion of the Pacific plate relative to the Hawaiian melting spot can be used to predict ages of seamounts and islands in other chains if the various melting spots are fixed with respect to one another. Almost all ages from other chains are consistent with the rotational model except for two K-Ar ages from the Austral chain. The proposed rotational motion of the Pacific plate can be used to reconstruct a paleomagnetic polar path of the Pacific plate if the melting spots are fixed with respect to the spin axis. The melting-spot polar path agrees well with the Late Cretaceous and limited Neogene paleomagnetic data. However, an extension of this polar path through the Late Cretaceous based on the Line Islands appears to be inconsistent with existing paleomagnetic data. The assumption that melting spots are fixed relative to the spin axis (and therefore the equator) can be tested by comparing the equatorial belt of high sedimentation with the sediment distribution predicted by the rotational-plate motion model. Almost every Deep Sea Drilling Project (DSDP) site presently located just north of the equator has its highest sedimentation rate at the time when the melting-spot motion model predicts that it was located at the equator. An equatorial sedimentation model adapted after Winterer (1972) is combined with the melting-spot motion model to generate a predicted isopach map of the post-middle Eocene equatorial sediments. The resulting isopach map is remarkably similar to the Ewing and others (1968) map of actual isopachs determined by seismic profiling. No data at present require motion of the Hawaiian melting spot relative to the spin axis. The tectonic histories of Japan and the Aleutians appear to reflect the proposed discontinuities of Pacific plate motion.

Relation of Macdonald Volcano to migration of volcanism along the Austral Chain

The growth of island chains by the linear migration of volcanic activity is manifest, to varying degrees, in the surface geology of a number of island groups. In Hawaii, the classic example, this pattern is attested by present-day volcanism, geomorphology, and geochronology. The recent discovery of the currently active Macdonald volcano at the southeastern extension of the Austral chain supports the validity of a linear volcanic migration pattern for that group as well.