Mantle Hotspot Research Articles

Tracing hotspot traces in the Andes

Two segments of subduction of the Nazca plate beneath the South American plate occur at low angles based on seismic hypocenter locations, approaching nearly horizontal below ~100 km in depth. In contrast with most of the rest of the subduction zone, the two segments, beneath central Chile, and central and northern Peru, lack active volcanoes along the crest of the Andes and have more subdued topography to the east of the Andean crest. Each low-angle subduction segment occurs to the east of the intersection of inferred mantle hotspot traces on the Nazca plate with the Peru-Chile Trench: the Nazca ridge (at the southern part of the Peruvian segment), and the Juan Fernández island-seamount chain (offshore the Chilean segment). A third inferred trace, the Galápagos-Carnegie ridge, may be correlated with a zone on incipient low-angle subduction beneath Colombia. 
 The importance of such hotspot traces in contributing to low-angle subduction beneath the Andes is strengthened by updated South American-Nazca plate reconstructions, including three oceanic hotspot traces, in comparison with a new isotopic date compilation of igneous rocks from the mountain range. The Juan Fernández hotspot trace, reconstructed from Pacific-hotspot models to the Nazca-Farallon plate, encountered the subduction zone offshore southern Peru ~65 Ma, broadening arc volcanism to the east; the trace-trench intersection migrated gradually and then rapidly southward, widening the arc east to Bolivia and northern Argentina; it then stabilized about 13 Ma offshore central Chile, producing the contemporary low-angle Pampean segment. The Juan Fernández hotspot may also have been responsible for formation of the Manihiki Plateau on the Pacific plate much earlier, ~125 Ma. The Easter-Nazca hotspot trace intersected the subduction zone beneath Colombia before ~50 Ma and migrated southward beneath Ecuador beginning ~15 Ma, with progressive low-angle subduction implied by migrating volcanic cessation along the Andean crest to southern Peru. The Galápagos-Carnegie hotspot trace only recently encountered the subduction zone, apparently inducing a new low-angle segment and cessation of magmatism in Colombia. The reconstructions and magmatic history provided here strongly support a previously proposed genetic relationship of hotspot traces and low-angle subduction. Additionally, the reconstructions suggest remnants of older subducted traces in the asthenosphere may have sourced post-rift magmatism in eastern Brazil and Paraguay, which cannot be explained otherwise by simple hotspot mechanisms.

Temporal and spatial distribution and genesis of primary indium deposits around the world

Because rare dispersed elements including indium play an increasingly extensive and indispensable role in the field of modern high technology, the exploration, development and research of its resources are in full swing. The abundance of indium in the earth's crust is extremely low, and its original independent deposits have not been found so far. Indium is mainly parasitic in bulk non-ferrous metal deposits in the form of associated by-products. Because of its origin, the genetic types of indium deposits are as diverse as those non-ferrous metal deposits it parasitizes. The distribution of indium in geological time and earth space is uneven. The formation of indium ore deposit is closely related to tectonic-magmatic activity. It is the product of plate tectonic movement, continental rifts and mantle plumes or mantle hotspots in the interior of ancient continents. The emergence of indium ore deposit is inseparable from magmatic activity, especially granite magmatic activity. Ore-forming materials mainly come from the upper mantle and/or deep crust. Indium deposits formed under high temperature conditions have enrichment mechanisms. In addition to the enrichment factors of traditional metal deposits, the enrichment of nano-scale indium minerals through the nano effect is also a very important enrichment method. The indium deposits formed at the end of the differentiation and evolution of granite magma may not be enriched in one go, but is the result of multiple enrichments.

Magnetic and geochemical scanning reveals growth history of marine ferromanganese crust on Detroit Seamount, northwest Pacific since the early Miocene

The Hawaiian Chain-Emperor Seamounts is known to be one of the most active regions influenced by mantle hotspot. However, previous studies have provided limited information on the growth process and metal enrichment of marine ferromanganese crusts as indicators of post-eruption history. In this study, we conducted a geochronological and major elemental study of a hydrogenetic ferromanganese crust, from the Detroit Seamount using high-resolution magnetic and geochemical scanning. Our findings are as follows: (1) A total of 39 magnetozones are recognized in the studied section of the crust, which can be preliminarily correlated to the geomagnetic polarity timescale from chrons C1n to C5Dn. This correlation yields growth rates of 1.48–2.18 mm/Myr over the past ∼18 Myr. (2) Columnar and lamellar growth structures are identified with distinct metal enrichment patterns. A significant transition from lamellar to columnar growth occurred at ∼11.6 Ma. (3) The quantified elements can be clustered into three groups, (Mn, Cu–Co–Ni, and Al–Si), (Fe and Ti), and Ca. A strong correlation between metal enrichment in the crust and the global benthic foraminifera δ13C record is then observed. Based on these results, we suggest a predominant role of deep-sea redox conditions in the growth history of ferromanganese deposits on the Detroit Seamount since the Early Miocene.

Quantifying Permanent Uplift Due To Lithosphere‐Hotspot Interaction

AbstractVertical motions that accompany the passage of the lithosphere over a mantle hotspot can shed light on the nature of the hotspot and its effect on the lithosphere. However, quantifying the temporal vertical and spatial extent, is challenging due to the paucity of evidence in the geological record. Here, we utilize dense seismic and well data covering the intersection of the Great Meteor Hotspot (GMH) track with the U.S. Atlantic continental margin to constrain the surface expression of the hotspot passage under the lithosphere. The continuous sedimentary record of the eastern North American margin during its passage over the hotspot allows determination of the timing, magnitude, width and rate of denudation. We find that a ∼300 km wide region was denuded by up to 850 m between ∼97 and 86 Ma, ∼10 m.y. after the passage of the GMH. Stratigraphic relationships suggest a decaying rock uplift rate with time and no subsequent sagging. The broad, long‐lasting, and delayed uplift was modeled as a surface manifestation of either sub‐lithospheric mantle depletion, permanently eroded base of the continental lithosphere, or intrusions of depleted magma. We consider sub‐lithospheric depletion to be the most likely cause, based on seismic imaging results.

Provenance of the Late Cretaceous Pab Formation, Sulaiman fold‐thrust belt, Pakistan: Insight from the detrital zircon U–Pb geochronology and sandstone petrography

This study provides an insight to the provenance of the Late Cretaceous Pab Formation exposed in the Sulaiman fold‐thrust belt, Pakistan. The methodology adopted consisted of sandstone petrography and U–Pb detrital zircon geochronology. The sandstone petrography indicates craton interior and recycled orogenic provenances that points to an Indian provenance. The observed basaltic lithics were possibly derived from Rajmahal traps instead of Deccan traps, supported by younger detrital age. The detrital zircon ages exhibit a resemblance to the central and southern Indian provenances, while some detritus also matches the age spectrum of Tethyan Himalayas (~400–800 Ma). Combining sandstone petrography and U–Pb geochronology results, it is suggested that in the Late Cretaceous time, the sediments were mainly fed by the southern and central sources of the Indian Craton. This provenace is possible, as during the Cretaceous time, thermal doming of Indian Plate in the southern part occurred when it was passing over a mantle hotspot. This movement over the hotspot uplifted the southeastern region and increased sediment supply from southern and central Indian provenances and deposited along the western margin as the Pab Formation.

Poisson's Ratio Structure Beneath the Nazca Ridge

AbstractThe Nazca Ridge (NR) was formed near the interaction of a hotspot mantle plume and an active spreading center. We use active‐source wide‐angle seismic data to obtain 2‐D Vp and Vs tomographic models, and hence the Poisson's ratio (ν) structure beneath the NR. Results show a ∼2 km thick seismic layer 2A with ν values of 0.25–0.32 in the uppermost crust interpreted as pillow basalts with a low degree of fracturing and/or hydrothermal alteration. The 2A/B boundary layer presents ν values of 0.27–0.29 consistent with pillow basalts/sheeted dykes units. A ∼3 km layer 2B overlies a ∼10 km layer 3 with ν values of 0.24–0.3 at the 2/3 boundary layer. The lowermost layer 3 presents ν values of 0.28 ± 0.02 suggesting an increase in Mg content (≥10% wt). The NR crust (∼15 km thick) requires an increment of the asthenospheric mantle potential temperature in ∼100°C formed by passive adiabatic decompression melting.

Crustal Structure across the West Antarctic Rift System from Multicomponent Ambient Noise Surface Wave Tomography

Abstract Approximately 2 yr (2010–2011) of continuous seismic records from a subset of the Antarctic component of the Polar Earth Observing Network (POLENET-ANET) seismic network deployed in West Antarctica are used to compute the nine components of the correlation tensor between each pair of stations in the network. Rayleigh wave velocity information from the vertical and radial components was extracted in the form of group and phase velocity dispersion curves, whereas the transverse component provided complimentary Love wave velocity information. The multicomponent Rayleigh wave measurements (ZZ, RR, ZR, and RZ) were averaged and used to infer the measurement uncertainties. The Rayleigh and Love wave group and phase velocities were then regionalized in space using a 2D deterministic tomography. A transect that spans the West Antarctic rift system was extracted from the tomography at individual periods between 7 and 60 s for the four types of surface wave velocities (i.e., Rayleigh and Love phase and group velocities). A transdimensional Bayesian joint inversion algorithm was used to invert these four datasets for a 1D model of isotropic shear-wave velocity versus depth at each point along the transect. In this way, surface wave dispersion curves from multicomponent noise correlations were used to build a 2D isotropic shear-wave velocity model down to ∼55 km depth. In this model, the top of the large low-velocity zone beneath Marie Byrd Land was imaged (up to a 5% decrease in velocity at ∼50 km depth), which provides further evidence for a mantle hot spot beneath the crust that supports the high topography in this region. We also observed a large velocity contrast in the lower crust beneath Marie Byrd Land at a depth where previous long-period seismicity has been observed. This strong contrast occurs more shallow than in previous crustal models, which compared to our model identify a deeper Moho (∼5–10 km deeper) beneath Marie Byrd Land. This new model has implications for interpreting earthquake locations in this region and perhaps necessitates that we revisit past hypocenter estimation studies using updated velocity models for the region.

ВУЛКАНЫ ГОРЯЧИХ ТОЧЕК (НОВАЯ ВЫСТАВКА В МУЗЕЕ ЗЕМЛЕВЕДЕНИЯ МГУ)

The new exhibition is located in Hall 5 «Geotectonics» in the Earth Science Museum at Moscow State University and devoted to an urgent problem of volcanic manifestations in areas of hot spots. Among the large number of active volcanoes on our planet, a significant place is occupied by volcanoes of hot spots and mantle plumes, which are characterized by the eruption of a huge amount of pyroclastic material (Hawaii, Yellowstone) and often form large igneous provinces (Eastern Siberia, Kerguelen, Deccan, Iceland). The exhibition presents geological samples from the funds and collections of the museum staff. All samples are characteristic of hot spot volcanoes and mantle plumes from different regions of the world.

Mineral systems and their putative link with mantle plumes

Abstract In this contribution, I discuss the putative link of mantle plumes with selected categories of mineral systems. Continental rifting and break-ups can be induced by the upwelling of mantle plumes, also resulting in the generation of a wide range of mineral deposits. These include magma-associated ores, anorogenic igneous events responsible for iron oxide–copper–gold (IOCG) deposits, carbonatites and hydrothermal-induced mineralization, as well as hydrocarbons, salt domes, petroleum and gas, and several mineral systems in continental passive margins. Amongst the magma-associated mineral systems, the Ni–Cu–platinum group element (PGE), Fe–Ti–V and Cr deposits are the economically most important, such as those of the Bushveld Igneous Complex in South Africa. Anorogenic magmas are generally alkaline and associated with IOCG mineral systems, as exemplified by the giant Olympic Dam and similar deposits in South America. Carbonatites are considered as a distal effect of hotspot mantle plumes, as shown by Mount Weld in Australia, which may be related to the Bushveld Superplume. Plume-related thermal anomalies are the principal factor for the inception of hydrothermal circulation and the genesis of a wide range of hydrothermal mineral systems in rift-related tectonic settings. These include large-scale sedimentary-rock-hosted metalliferous ores, such as sedimentary exhalative (SEDEX) deposits. A modern example of is provided by the Red Sea brine pools. Some key examples are presented in this paper.

The Qutn granite, a hotspot-related A-type granite from the northeastern Arabian Shield?

The Qutn granite is a 579 Ma alkaline ring complex consisting of an incomplete biotite granite outer rim and a porphyritic core. It belongs to the late Ediacaran Abanat suite which encompasses the youngest posttectonic granitic intrusives in the NE Arabian Shield. The Qutn rocks are geochemically classified as A-type granites and belong to A2-subtype of anorogenic granites. The Qutn granite is clearly postcollisional and probably represents the exhumed root of a collapsed caldera. Magma genesis is thought to have been induced by mantle delamination through the detachment of a part of the subcrustal lithospheric mantle following continental collision. The Qutn complex displays an exceptionally well-developed arcuate trail of satellite plutons diminishing in size away from the main Qutn mass. This type of distribution would indicate a migration of the center of magmatic activity in response to counterclockwise rotation around a pole situated about 10 km northeast of Jabal Qutn. In the past, some authors have commented on the possibility of such migrating complexes being the manifestation of plate movement above a mantle hot spot. Invoking a rising mantle plume to explain the situation at Qutn and similar small intrusions is untenable; a more realistic approach involving magma generation in the lower crust following mantle delamination combined with block rotation in the upper crust in response to sinistral strike-slip movement along the regional Najd transcurrent system is more plausible.

Satellite Gravity Constraints on the Antarctic Moho and Its Potential Isostatic Adjustments

AbstractWe investigate the impact of combining Gravity Field and Steady‐state Ocean Circulation Explorer (GOCE) satellite gravity anomaly and Bedmap2 terrain relief data to enhance Antarctic seismic Moho estimates over the satellite's coverage region south of latitude 60°S. The study considers improving the seismic surface wave‐inferred AN1 Moho (An et al., 2015a, https://doi.org/10.1002/2014JB011332) from misfits of the gravitational effects of the terrain and the seismic Moho computed at 250 km altitude. The updates also relate the GOCE gravity anomalies that correlate directly and inversely with the terrain's gravity effects as isostatic anomalies of the uncompensated mantle relief. These terrain‐correlated effects infer potential isostatic adjustments of the Moho that may help constrain the crust's stress field, track the Gamburtsev Subglacial Mountains to the Kerguelen mantle hotspot, and further test the putative Wilkes Land impact basin for its crustal attributes. Analysis of the gravity‐updated seismic AN1 Moho (sAMoho) estimates suggests that most are within the seismic errors of several kilometers or less. However, the gravity‐updated estimates that are deeper than the sAMoho estimates tend to characterize anomalously hot upper mantle where the assumed mantle‐to‐crust density contrast may be too low. These Moho difference estimates discern elevated heat flow for the Maud Rise, Kerguelen Plateau, Pacific‐Antarctic Ridge, and most of western Antarctica extending from the Pacific‐Antarctic Ridge along the Transantarctic Mountains and across the Ross Sea through Marie Byrd Land and the western margin of the Antarctic Peninsula Microplate to the Scotia Ridge. The methodology of this study also is effective in updating any Moho model for improved gravity and terrain data.

Lithospheric architecture of a Phanerozoic orogen from magnetotellurics: AusLAMP in the Tasmanides, southeast Australia

We present a resistivity model of the southern Tasmanides of southeastern Australia using Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) data. Modelled lower crustal conductivity anomalies resemble concentric geometries revealed in the upper crust by potential field and passive seismic data. These geometries are a key part of the crustal architecture predicted by the Lachlan Orocline model for the evolution of the southern Tasmanides, in which the Proterozoic Selwyn Block drives oroclinal rotation against the eastern Gondwana margin during the Silurian period. For the first time, we image these structures in three dimensions (3D) and show they persist below the Moho. These include a lower crustal conductor largely following the northern Selwyn Block margin.Spatial association between lower crustal conductors and both Paleozoic to Cenozoic mafic to intermediate alkaline volcanism and gold deposits suggests a genetic association i.e. fluid flow into the lower crust resulting in the deposition of conductive phases such as hydrogen, iron, sulphides and/or graphite.The 3D model resolves a different pattern of conductors in the lithospheric mantle, including northeast trending anomalies in the northern part of the model. Three of these conductors correspond to Cenozoic leucitite volcanoes along the Cosgrove mantle hotspot track which likely map the metasomatised mantle source region of these volcanoes. The northeasterly alignment of the conductors correlates with variations in the lithosphere-asthenosphere boundary (LAB) and the direction of Australian plate movement, and may be related to movement of an irregular LAB topography over the asthenosphere.By revealing the tectonic architecture of a Phanerozoic orogen and the overprint of more recent tectono-magmatic events, our resistivity model enhances our understanding of the lithospheric architecture and geodynamic processes in southeast Australia, demonstrating the ability of magnetotelluric data to image geological processes over time.

Plate-driven micro-hotspots and the evolution of the Dragon Flag melting anomaly, Southwest Indian Ridge

Micro hotspots represent excess volcanism at the ultraslow spreading Southwest Indian Ridge (SWIR) unrelated to mantle hotspots, but to focused melt flow in the mantle, wide volcano spacing, and/or increased mantle fertility. Individual micro hotspot can reflect variations in regional mantle fertility, simultaneously affecting 100's of km of ridge, or extreme melt focusing to isolated segments in regions with unusually thick lithosphere. The Dragon Flag melting anomaly, erupting isotopically moderately depleted low-K tholeiite, is the best example of the former: an enormous ridge-centered volcano at 50.5°E. It is far from Crozet, and lies at the apex of a V-shaped trace consisting of ∼1 km anomalously elevated seafloor that extends ∼400 km to the northwest and southeast, indicating a sudden onset of excess volcanism at ∼11-8 Ma. This trend is opposite to that predicted by the hotspot framework, with inconsistent geochemistry. Narrowgate at 14.7°E is one of many micro hotspots that represent isolated large volcanic centers bounded by long amagmatic ridge sections. It also lies at the apex of an eastward V-shaped bathymetric trend, but erupts large volumes of alkali basalt.Both micro hotspot varieties are an order of magnitude smaller than Wilson's classic hotspots, and are not fixed in the hotspot referenced frame. They can grow and remain stationary for extended periods, or migrate freely with respect to each other; driven by the evolving plate tectonic stress field. Dragon Flag, represents a plate reorganization during a prolong period of enhanced melt supply. Narrowgate, on the other hand represents excess volcanism due to extreme deep melt focusing of low degree melt from an unusually wide region in the mantle to an isolated volcanic segment where thick lithosphere caps melting at great depth. To constrain the origins of the micro hotspots six examples are evaluated in terms of their major and isotopic composition, prior plate history, depth and extent of melting and tectonic context. This includes new major, trace element, and heavy isotope data for the Dragon Flag Supersegment, and unpublished data for the Joseph Mayes and Narrowgate segments.

Shipboard Characterization of Tuvalu, Samoa, and Lau Dredge Samples Using Laser-Induced Breakdown Spectroscopy (LIBS)

Chemical analysis using laser-induced breakdown spectroscopy (LIBS) is well suited for field applications and was applied here for shipboard characterization of a large sample set during the RR1310 rock dredging expedition to the Tuvalu Seamounts. Although recently the most common data treatment for LIBS has consisted of a partial least squares approach to define sample groupings, we show that quantitative data of useful quality can be obtained with a univariate approach. Here, our analysis goal was a quantitative comparison of the total alkali (Na2O + K2O) versus silica (SiO2) contents of 586 representative dredge samples with known ranges in common rock types. Out of those samples, >400 form a single large group of alkalic basalts with minor basanites/tephrites (SiO2: 43-48 wt%, Na2O + K2O: 3-5 wt%), similar to known shield-stage compositions of the Rurutu and Samoa hotspots in the sampling area. In contrast, several dredge hauls contain samples with compositions that do not overlap with the majority of samples. This includes three dredges performed on the northern boundary of the Lau Basin that contain similar SiO2 compositions, but slightly higher total alkali (Na2O+K2O) content. Despite this difference, they classify as basanite/tephrite, similar to a subset of the main group. More importantly, similar compositions were previously reported from the same tectonic boundary, ascribed to hotspot mantle source material mixed into the Lau Basin back-arc. Although the quality of the compositional data suffices to enable sample selection for time-intensive analyses, higher precision is required for more in-depth petrogenetic interpretation. Error analysis based on repeat standard measurements suggests averaging 100 spectra per sample is optimal here, while use of a higher resolution spectrometer, together with better laser control, would improve results and interpretations.

Venus Interior Structure and Dynamics

No two rocky bodies offer a better laboratory for exploring the conditions controlling interior dynamics than Venus and Earth. Their similarities in size, density, distance from the sun, and young surfaces would suggest comparable interior dynamics. Although the two planets exhibit some of the same processes, Venus lacks Earth’s dominant process for losing heat and cycling volatiles between the interior and the surface and atmosphere: plate tectonics. One commonality is the size and number of mantle plume features which are inferred to be active today and arise at the core mantle boundary. Such mantle plumes require heat loss from the core, yet Venus lacks a measurable interior dynamo. There is evidence for plume-induced subduction on Venus, but no apparent mosaic of moving plates. Absent plate tectonics, one essential question for interior dynamics is how did Venus obtain its young resurfacing age? Via catastrophic or equilibrium processes? Related questions are how does it lose heat via past periods of plate tectonics, has it always had a stagnant lid, or might it have an entirely different mode of heat loss? Although there has been no mission dedicated to surface and interior processes since the Magellan mission in 1990, near infrared surface emissivity data that provides information on the iron content of the surface mineralogy was obtained fortuitously from Venus Express. These data imply both the presence of continental-like crust, and thus formation in the presence of water, and recent volcanism at mantle hotspots. In addition, the study of interior dynamics for both Earth and exoplanets has led to new insights on the conditions required to initiate subduction and develop plate tectonics, including the possible role of high temperature lithosphere, and a renewed drive to reveal why Venus and Earth differ. Here we review current data that constrains the interior dynamics of Venus, new insights into its interior dynamics, and the data needed to resolve key questions.

Crustal Formation on a Spreading Ridge Above a Mantle Plume: Receiver Function Imaging of the Icelandic Crust

AbstractIceland sits astride a mid‐ocean ridge underlain by a mantle hot spot. The interplay of these two geological processes has the potential to generate a complex and laterally variable crustal structure. The thickness of the Icelandic crust is a long running and controversial debate, with estimates ranging from a thin 20‐km crust to a thick 40‐km crust. We present new images of the first‐order seismic discontinuity structure of the Icelandic crust based on a joint inversion of receiver function and ambient noise‐derived surface wave dispersion data. Inversion results are validated through comparison to receiver functions multiphase common conversion point stacks across the densely instrumented Northern Volcanic Zone. We find a multilayered crustal structure consisting of a 6‐ to 10‐km‐thick upper crust underlain by either one or two discontinuities. The shallower discontinuity is found at depths of ≈20 km throughout Iceland. The deeper discontinuity is only present in some regions, defining the base of a lens‐like lower layer with maximum depths of 44 km above the center of the mantle plume. Either of these two discontinuities could be interpreted as the seismic Moho, providing an explanation why previous estimates of crustal thickness have diverged. Such structure may form via underplating of a preexisting oceanic crust as has been hypothesized in other ocean island plume settings. However, we demonstrate with a simple petrological model that variability in seismic discontinuity structure can also be understood as a consequence of compositional variation in melts generated with distance from the plume center.

Noble gas composition of Indian carbonatites (Amba Dongar, Siriwasan): Implications on mantle source compositions and late-stage hydrothermal processes

Within a stepwise crushing study we determined the noble gas composition of several calcite separates, one aegirine and one pyrochlore-aegirine separate of the carbonatite ring dyke complex of Amba Dongar and carbonatite sill complex of Siriwasan, India. Both carbonatites are related to the waning stages of volcanic activity of the Deccan Igneous Province ca. 65 Ma ago. Major observations are a clear radiogenic 4He⁎ and nucleogenic 21Ne⁎ imprint related to in situ production from U and Th in mineral impurities, most likely minute apatite grains, or late incorporation of crustal fluids. However, in first crushing steps of most calcites from Amba Dongar a well-resolvable mantle neon signal is observed, with lowest air-corrected mantle 21Ne/22Ne-compositions equivalent to the Réunion hotspot mantle source. In case of the aegirine separate from Siriwasan we found a neon composition similar to the Loihi hotspot mantle source. This transition from a mantle plume signal in first crushing step to a more nucleogenic signature with progressive crushing indicates the presence of an external (crustal) or in situ nucleogenic component unrelated and superposed to the initial mantle neon component whose composition is best approximated by results of first crushing step(s). This contradicts previous models of a lithospheric mantle source of the carbonatitic magmas from Amba Dongar containing recycled crustal components which base on nucleogenic neon compositions. Instead, the mantle source of both investigated carbonatite complexes is related to a primitive mantle plume source that we tentatively ascribe to the postulated Deccan mantle plume. If, as is commonly suggested, the present location of the Deccan mantle plume source is below Réunion Island, the currently observed more nucleogenic neon isotopic composition of the Réunion hotspot might be obliterated by significant upper mantle contributions. In addition, compared with other carbonatite complexes worldwide a rather significant contribution of atmospheric noble gases is observed. This is documented in cut-off 20Ne/22Ne-ratios of ca. 10.2 (Amba Dongar) and 10.45 (Siriwasan) and cut-off 40Ar/36Ar-ratios of about 1500. This atmospheric component had been added at shallow levels during the emplacement process or later during hydrothermal alteration. However, understanding the late-stage interaction between atmospheric gases and magmatic mantle fluids still requires further investigation.

Spatial and temporal variability in Marquesas Islands volcanism revealed by 3He/4He and the composition of olivine-hosted melt inclusions

The Marquesas Islands are a volcanic chain having an oblique orientation to Pacific plate motion. They represent the surface manifestation of a mantle hotspot having a low melt flux and a large intrinsic chemical and isotopic heterogeneity. Here we provide new helium isotope data for olivine separated from 23 basalts and one gabbroic xenolith, plus whole rock major and trace element data for the same lavas. We also carried out trace element analyses of 41 olivine-hosted melt inclusions (OHMIs) from 15 of the basalts, and 40Ar/39Ar age determinations for basalt groundmass from the islands of Fatu Hiva and Hiva Oa. 3He/4He shows a large range of values from <5 RA up to 14.5 RA. Elevated 3He/4He ratios >9 RA are found at Nuku Hiva and Hiva Oa near the center of the archipelago, in agreement with previous findings (Castillo et al., 2007). The new ages for the Hiva Oa lavas indicate that 3He/4He increases from ~8 RA to 14.5 RA during the mid-late shield stage of volcanism (2.6–2.0Ma), followed by a decrease during the post-shield stage (<1.8Ma) after collapse of the Temetiu caldera. Lavas from Fatu Hiva, Ua Huka, Ua Pou, Tahuata and Motane have 3He/4He<8.3 RA, with values typically between 5 and 7 RA. Incompatible element ratios observed in the OHMIs show more variability than what is observed in the lavas because they represent a less extensive aggregation of melts derived from small scale chemical heterogeneities within the Marquesas mantle plume. The Cl/K ratio of OHMIs ranges from 0.02 to 0.4, potentially due to both mantle source heterogeneity and assimilation of brine or seawater-altered rocks at crustal levels. The geographic distribution of 3He/4He suggests the mantle plume is concentrically zoned in He isotope composition, with high 3He/4He material concentrated near its central axis that is sourced from hot, primitive material entrained near the core-mantle boundary region.

Deformation of oceanic crust in the eastern Gulf of Guinea: role in the evolution of the Cameroon Volcanic Line and influence on the petroleum endowment of the Douala-Rio Muni Basin

Abstract Evidence is presented from the interpretation of seismic data for the compressional deformation of intra-plate oceanic crust in the eastern Gulf of Guinea, a region occupied by the Douala-Rio Muni Basin and the Cameroon Volcanic Line. The deformation has taken the form of the reactivation of syn-kinematic structures in the fabric of the oceanic crust. The origin of the Cameroon Volcanic Line is usually attributed to a mantle hotspot, but interpretation of the seismic data indicates that strike-slip and/or compressional tectonics and reactivation of the fabric of the oceanic crust have all played an important role in its evolution. A regional tectonic model is presented that associates deformation in the eastern Gulf of Guinea with regional alpine-related far-field compressive stress. In this way, a causal link is invoked between deformation of the oceanic crust and strike-slip movement on the Central African Shear Zone, establishing a continent–ocean tectonic link. The evolution of the Cameroon Volcanic Line and the continent–ocean tectonic link has imparted a unique tectonostratigraphic history to the Douala-Rio Muni Basin and has influenced its petroleum endowment. The structural and combination trapping potential has been greatly enhanced in this environment and the structural architecture of the basin has influenced the depositional patterns of the deep water reservoir sands.

Seismic evidence for crustal underplating beneath a large igneous province: The Sierra Leone Rise, equatorial Atlantic

Wide-angle seismic profiles reveal anomalously thick crust with a high-velocity (> 7.3 km s− 1) zone under the Sierra Leone Rise, a major mid-plate elevation in the Atlantic lying between the Cape Verde platform and the Cameroon Volcanic Line. A profile recorded over the crest using an ocean-bottom seismometer and surface sonobuoys shows that beneath a 3 km water layer and 1 km of sediments, the basement extends to 16–20 km below sea level. Most velocity-depth values fall outside the expected range for Mesozoic–early Cenozoic ocean floor and stretched continental crust. The detection of 7.3–7.5 km s− 1 material beneath thick, lower-velocity volcanics suggests that magmatic underplating of the crust has occurred. A prominent change in velocity gradient 10–12 km below sea level may mark the transition to underplated material emplaced during the late Cretaceous–early Cenozoic. A pronounced change in Moho depth lies on the line of a long offset fracture zone extending from the African margin, implying underplating was influenced by a pre-existing discontinuity in the lithosphere. Other seismic lines show 7.0–7.2 km s− 1 basement above the underplated zone extending into water depths of almost 5 km. This is probably the intrusive foundation of early-formed crust over a mantle hot-spot. It is suggested that the development of the Sierra Leone Rise is distinct from other Atlantic hot-spot features to which it has been linked because of its setting in a region of intense lithospheric shear.