Lesser Antilles Arc Research Articles

A review of historical lahars, floods, and landslides in the Prêcheur river catchment (Montagne Pelée volcano, Martinique island, Lesser Antilles)

Abstract The Prêcheur river catchment belongs to the Montagne Pelée volcano massif located in the north of Martinique island, in the Lesser Antilles arc. In June 19–20, 2010 two high-discharge lahars overflowed and partially damaged the Prêcheur bridge, interrupting the road traffic for several days, and inundating part of the Abymes quarter. These non-eruptive, rain-triggered lahars were the consequence of the mobilization of loose volcanic material accumulated in the river bed by a massive landslide of the Samperre cliff that occurred on May 11, 2010. In order to improve lahar risk assessment for the Prêcheur river, we review the existing knowledge on lahars and floods for the period 1851–2011. We improved the database mainly on the basis of geophysical (seismic and acoustic) data for the most recent period (1980-present). We show that: (1) the largest event ever observed in Prêcheur is the eruption-related lahar of May 8, 1902 at 06:00 UTC, (2) high-discharge, non-eruptive lahars able to partially destroy or submerge the bridge occur every 10.3 years in average since 1950, (3) geophysical records show that there are more low magnitude lahars than previously recognized because witness accounts report the larger events and therefore filter out the smaller ones; by contrast geophysical instruments are able to detect both high and low magnitude events, (4) the recent paroxysms of 2009–2010 belong to a landslide-lahar crisis which started in 2004; the onset of the landslide-lahar crisis was unnoticed by the population due to the low magnitude of the events. The results of this compilation stress the importance of the presence of geophysical instrumentation sufficiently close to the river for monitoring small scale landslides and lahars. Indeed, these low magnitude events are likely to be unnoticed by the local population and therefore likely to be absent from the collective memory and local newspapers. However, they can inform about the imminence of more significant events, as observed in 1980 and 2010.

La Désirade island (Guadeloupe, French West Indies): a key target for deciphering the role of reactivated tectonic structures in Lesser Antilles arc building

Abstract In this paper we present and discuss new investigations performed on both the magmatic basement and the sedimentary formations of La Désirade. We report structural and sedimentary evidences for several episodes of deformation and displacement occurring prior to the present day tectonics. The main faults, respectively N130 ± 10°, N040 ± 10° and N090 ± 10°, previously considered as marker of the current tectonic regime corresponds to reactivated tectonic structures developed first during late Cretaceous compression and second during Pliocene to early Pleistocene extension. We demonstrate also the importance of late Pliocene-early Pleistocene and middle-late Pleistocene vertical movements in this part of the Lesser Antilles fore-arc as well as the role of compressive tectonics in the over thickened character of the arc basement in the Guadeloupe archipelago.

The recent Plinian explosive activity of Mt. Pelée volcano (Lesser Antilles): The P1 AD 1300 eruption

Plinian explosive eruptions represent a major volcanic hazard in the Lesser Antilles Arc that must be carefully assessed based on reconstructions of past activity. The present study focusses on a detailed time evolution of the P1 eruption (AD 1300) at Mt Pelee volcano (Martinique). After an initial dome-forming stage, a Plinian phase commenced. The P1 Plinian-style sequence is mostly a pumice fall deposit with an inversely graded base, interbedded with a surge deposit, and overlain by final flow/surge deposit. Field data on deposit dispersal, thickness, and grain-size distribution are used together with physical models to reconstruct the dynamical evolution of this eruption. We find that the mass eruption rate increased from 2×107 to 9×107 kg s − 1, producing a 19–22-km-high Plinian plume, initially stable but which ultimately collapsed to form a ~1.3-km-high fountain. Empirical models of deposit thinning suggest that the minimum volume of pyroclastic deposits is 0.15 km3, about 25 % that previously estimated. Published data on magmatic water contents in glass inclusions are used together with mass discharge rates to elucidate the mechanisms leading to column collapse. Conditions at the base of the column were close to the plume/fountain transition soon after the Pelean/Plinian-style transition due to the competing effects of increase in both gas content and mass discharge rate. After a short stage of partial collapse, the column underwent a total collapse due to an increasing discharge rate.

Martinique: a Clear Case for Sediment Melting and Slab Dehydration as a Function of Distance to the Trench

In subduction zones, melting and dehydration of the subducted slab introduce material into the mantle wedge and modify its chemical and isotopic composition. As a consequence, island arc lavas differ significantly from mid-ocean ridge basalts and ocean island basalts. In some arcs, the composition of lavas is strongly influenced by the sedimentary material introduced with the slab; in others, magma composition is mainly affected by aqueous fluids released by the slab. The Lesser Antilles arc is known for its extreme continental-crust-like signature but for some Lesser Antilles lavas subducted sediments are barely involved and enrichment in fluid-mobile elements (Ba, U, Sr, Pb, etc.) is the dominant feature. Here we evaluate whether La/Sm is a quantitative proxy of sediment involvement in volcanic arcs, and we relate dehydration and melting processes to the temperature and pressure conditions of the slab. We use Martinique as a case study because in this island both dehydration and sediment melting fingerprints coexist. We measured major and trace elements for about 130 age-constrained samples, carefully chosen to cover all volcanic phases of Martinique (25 Ma to present). Using these results we demonstrate that: (1) weathering does not modify the La/Sm ratio; (2) fractional crystallization of amphibole and/or garnet does not increase La/Sm by more than 20%; (3) rare earth element transfer from wall-rock to magma during fractionation is not significant; (4) melting of the mantle source increases La/Sm by only about 20%. As a consequence, we show that the proportion of slab sediment incorporated in the mantle wedge controls the La/Sm ratio of the source. The observed correlations between La/Sm and Nd and Hf isotopic compositions indicate that the effect of sediment addition is the overwhelming factor: La/Sm is a good proxy for slab sediment proportion in Martinique. We observe a geographical gradient between slab dehydration and sediment melting on the island...

Heat flow in the Lesser Antilles island arc and adjacent back arc Grenada basin

Using temperature gradients measured in 10 holes at 6 sites, we generate the first high fidelity heat flow measurements from Integrated Ocean Drilling Program drill holes across the northern and central Lesser Antilles arc and back arc Grenada basin. The implied heat flow, after correcting for bathymetry and sedimentation effects, ranges from about 0.1 W/m2 on the crest of the arc, midway between the volcanic islands of Montserrat and Guadeloupe, to <0.07 W/m2 at distances >15 km from the crest in the back arc direction. Combined with previous measurements, we find that the magnitude and spatial pattern of heat flow are similar to those at continental arcs. The heat flow in the Grenada basin to the west of the active arc is 0.06 W/m2, a factor of 2 lower than that found in the previous and most recent study. There is no thermal evidence for significant shallow fluid advection at any of these sites. Present‐day volcanism is confined to the region with the highest heat flow.

Caribbean Plate margin evolution: constraints and current problems

Oceanic crust was generated at multiple spreading centres during the Jurassic and Early Cretaceous, forming a “proto-Caribbean” oceanic domain. During the Cretaceous, part of that crustal domain thickened into an oceanic plateau, of petrologic Mid-Ocean Ridge (MOR) to Ocean Island Basalt (OIB) affinity. Simultaneously, the South and North American continental plates developed rifting and tholeiitic magmatism in the Middle America region (Venezuela and Cuba). The rifting created space for the proto-Caribbean oceanic domain. Petrological and regional correlations suggest that, beginning in the Cretaceous, the proto-Caribbean domain was involved into two main stages of subduction, referred to as first and second “eo-Caribbean” phases. Each phase is characterized by oblique convergence. The older (mid-Cretaceous) stage, involved in subduction (probably eastward dipping) of thin proto-Caribbean lithosphere, with generation of Island Arc Tholeiitic (IAT) and Calc-Alkaline (CA) magmatism, accompanied by high pressure - low temperature (HP - LT) metamorphic effects, and formation of arc units and ophiolitic melanges (Guatemala, Cuba, Hispaniola and Puerto Rico, in the northern margin; Venezuela in the southern). The Late Cretaceous second stage consisted of westward dipping intra-oceanic subduction; it is recorded by tonalitic arc magmatism related to the onset of the Aves - Lesser Antilles arc system. Since the Late Cretaceous, the inner undeformed portions of the Caribbean oceanic plateau (i.e. the Colombian and Venezuelan Basins) were trapped east of the Pacific subduction of the Chortis, Chorotega and Choco blocks, ultimately building the Central American Isthmus. From Tertiary to Present, continuous eastward movement of the Caribbean Plate with respect to the Americas, gave rise to transpression along both the northern and southern margins, marked by scattered and dismembered ophiolitic terranes.

Colonizing the Caribbean: is the GAARlandia land‐bridge hypothesis gaining a foothold?

Colonizing the Caribbean: is the GAARlandia land‐bridge hypothesis gaining a foothold?

Tectonic context of moderate to large historical earthquakes in the Lesser Antilles and mechanical coupling with volcanoes

[1] The oblique convergence between North American and Caribbean plates is accommodated in a bookshelf faulting manner by active, oblique-normal faults in the northern part of the Lesser Antilles arc. In the last 20 years, two M > 6 earthquakes occurred along a large, arc parallel, en echelon fault system, the 16 March 1985 in Redonda and 21 November 2004 in Les Saintes. A better understanding of active faulting in this region permit us to review the location and magnitude of historical earthquakes by using a regional seismic attenuation law. Several others moderate earthquakes may have occurred along the en echelon fault system implying a strong seismic hazard along the arc. These faults control the effusion of volcanic products and some earthquakes seem to be correlated in time with volcanic unrest. Shallow earthquakes on intraplate faults induced normal stress and pressure changes around neighboring volcano and may have triggered volcanic activity. The Redonda earthquake could have initiated the 1995 eruption of Montserrat's Soufriere Hills by compressing its plumbing system. Conversely, pressure changes under the volcano increased Coulomb stress changes and brought some faults closer to failure, promoting seismicity. We also discuss the magnitude of the largest 11 January 1839 and 8 February 1843 megathrust interplate earthquakes. We calculate that they have increased the stress on some overriding intraplate faults and the extensional strain beneath several volcanoes. This may explain an increase of volcanic and seismic activity in the second half of the 19th century culminating with the devastating, 1902 Mount Pelee eruption.

The Mw = 6.3, November 21, 2004, Les Saintes earthquake (Guadeloupe): Tectonic setting, slip model and static stress changes

On November 21, 2004, a magnitude 6.3 earthquake occurred offshore, 10 km south of Les Saintes archipelago in Guadeloupe (French West Indies). There were more than 30000 aftershocks recorded in the following two years, most of them at shallow depth near the islands of the archipelago. The main shock and its main aftershock of February 14, 2005 (Mw = 5.8) ruptured a NE-dipping normal fault (Roseau fault), mapped and identified as active from high-resolution bathymetric data a few years before. This fault belongs to an arc-parallel en echelon fault system that follows the inner edge of the northern part of the Lesser Antilles arc, accommodating the sinistral component of oblique convergence between the North American and Caribbean plates. The distribution of aftershocks and damage (destruction and landslides) are consistent with the main fault plane location and attitude. The slip model of the main shock, obtained by inverting jointly global broadband and local strong motion records, is characterized by two main slip zones located 5 to 10 km to the SE and NW of the hypocenter. The main shock is shown to have increased the Coulomb stress at the tips of the ruptured plane by more than 4 bars where most of the aftershocks occurred, implying that failures on fault system were mainly promoted by static stress changes. The earthquake also had an effect on volcanic activity since the Boiling Lake in Dominica drained twice, probably as a result of the extensional strain induced by the earthquake and its main aftershock.

The volcanic evolution of Martinique Island: Insights from K–Ar dating into the Lesser Antilles arc migration since the Oligocene

The Lesser Antilles island arc bifurcates into two lines in its northern part, with an old branch to the east and a recent active branch to the west. Martinique is located at the southern tip of the separation. The two arcs diverge northward, and at maximum divergence are separated by a 50 km wide depression. Despite this separation, which suggests a jump in volcanism, activity has been almost continuous in Martinique Island with a slow displacement of the eruptive centers to the west. Considering timing of emplacement, previous authors defined three cycles of activity, the old, intermediate and recent arcs, of Late Oligocene–Early Miocene, Mid Miocene and Late Miocene to present ages, respectively. The present study investigates the timing of emplacement of the volcanic units in Martinique Island in order to constrain the activity of the old and intermediate Lesser Antilles arcs, as recorded on this island. Unspiked K–Ar age determinations on groundmass and plagioclase separates (Cassignol–Gillot technique) were conducted on 20 samples from the old and intermediate volcanic chains. Martinique has evolved as eight distinct volcanic centers: (1) Basal Complex and Sainte Anne Series (24.8 ± 0.4–20.8 ± 0.4 Ma) for the old arc; (2) Vauclin–Pitault Chain (16.1 ± 0.2–8.44 ± 0.12 Ma) and (3) South-western Volcanism (9.18 ± 0.16–7.10 ± 0.10 Ma) for the intermediate arc; and (4) Morne Jacob volcano (5.14 ± 0.07–1.54 ± 0.03 Ma), (5) Trois Ilets Volcanism (2.358 ± 0.034 Ma and 346 ± 27 ka), (6) Carbet Complex (998 ± 14 to 322 ± 6 ka), (7) Mount Conil (543 ± 8 to 127 ± 2 ka) and (8) Mount Pelée (126 ± 2 ka to present) for the recent arc ( Germa et al., 2010, 2011a).We propose migration rates of 1.1–1.4 km/Myr westward, toward the back arc region throughout the whole volcanic history of Martinique Island. These rates, together with geochemical evidence for a more enriched signature in the youngest magmas, are consistent with a geodynamic evolution involving the migration of the northern volcanic front away from the trench in response to the subduction of an aseismic ridge, possibly influenced by convergence between North-American and South-American plates.

Basin modelling across the emergent Lesser Antilles island arc, SE Caribbean and impact on petroleum systems

We performed forward basin modelling along a profile in the south-eastern Caribbean to evaluate the effect of an evolving island arc on the evolution of petroleum systems in adjacent sedimentary basins. The geological model used is a published interpretation of a NW-SE regional 2D seismic line across the Lesser Antilles island arc, in which the Grenada and Tobago basins are interpreted as a single large basin during the Eocene that was split by the developing Lesser Antilles arc, north of the Caribbean-South America collision zone, during the middle Miocene. South of the collision zone, the eastern offshore Trinidad region has developed as a foreland basin since the middle Miocene. Basin modelling shows that formation of the Lesser Antilles arc caused an increase in temperature that subsequently decayed in space and time. Heating had a major impact on the maturation of Palaeogene source rocks adjacent to the arc, which became mature and over-mature. The heating also affected reservoir rocks that were deposited before and during arc formation, but not after the arc was formed. No temperature effects are observed in the eastern offshore Trinidad–Columbus Basin region.

Discovery of Lower Cretaceous synmetamorphic thrust tectonics in French Lesser Antilles (La Désirade Island, Guadeloupe): Implications for Caribbean geodynamics

Located east of Guadeloupe, the island of La Désirade exhibits the oldest rocks of the Lesser Antilles arc and the eastern Caribbean plate. An old magmatic basement is composed of (1) late Jurassic ophiolitic complex with meta‐basaltic pillow lavas and interbedded radiolarites, (2) acid igneous complex comprising meta‐quartz‐diorite and meta‐rhyolitic lavas flows, and (3) meta‐diabasic/microdioritic dyke swarm complex. We present and discuss the discovery of synmetamorphic thrust tectonics in this island. Based on detailed structural analysis we evidenced two main compressive events. The first event (D1) is characterized by pervasive folding associated with thrust development. In the northeastern part of the island, a major thrust fault, the Grand Abaque Thrust, has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit. The second deformation event (D2) is featured by a main pervasive cleavage S2 observed at the regional scale associated with upright folding (F2) and by the development of two sets of conjugated steeply dipping strike‐slip shear zones, dextral 130°N and sinistral 20°N striking, respectively. Microstructural observations indicate that Greenschist facies metamorphism is clearly contemporaneous with the development of D1 and D2 superimposed tectonic structures. Ar/Ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a D2 event. These analyses yield well‐defined and concordant plateau ages at 106.2 ± 1.7 Ma and 107.2 ± 1.8 Ma. Thus, shortening and thickening tectonics occurred during Lower Cretaceous (Albian). In the available framework of Caribbean geodynamics, the synmetamorphic thrust tectonics we discovered in the French Lesser Antilles is related to the collision between an Aptian‐Albian oceanic plateau and the Andean‐Cordilleran east‐dipping subduction zone. Our results support subduction polarity reversal during Lower Cretaceous.

DEM-based reconstruction of southern Basse-Terre volcanoes (Guadeloupe archipelago, FWI): Contribution to the Lesser Antilles Arc construction rates and magma production

We combine radiometric ages and geomorphologic investigations to quantify the relief creation of well preserved volcanic surfaces, applied to the volcanoes of southern Basse-Terre (Guadeloupe, Lesser Antilles Arc). The last 650 ka volcanic evolution of this island has been modeled using ten main stages constrained by K–Ar ages previously obtained on lava flows and domes from the volcanic massifs of the Axial Chain (1000–435 ka) and from the Grande Découverte Volcanic Complex (250 ka–present). Based on the construction of a 250,000-point database inferred from the analysis of the Guadeloupe Digital Elevation Model, 3D reconstructions of the successive volcanic stage landforms were calculated and the correlated geochronological maps drawn using ArcGIS software. Volumes and rates of construction were computed for each time span separating these ten stages. The average construction rates calculated here are 2.4 ± 0.3 × 10 −4 km 3/yr for the last million years, 0.9 ± 0.3 × 10 −4 km 3/yr for the last 100 ka, and 0.9 ± 0.2 × 10 −4 km 3/yr for the last 15 ka. Although Basse-Terre volcanism is characterized by a marked dominance of effusive products, our estimates should be considered as minimum values because the material that went into the sea during explosive events was not taken into account. However, we note that a relatively high construction rate of 4.5 ± 1.3 × 10 −4 km 3/yr has been obtained for the Icaques volcano, which was emplaced during the 630–600 ka time interval, within the depression formed by the first large-scale flank collapse having affected southern Basse-Terre. The sudden release of the lithostatic load induced by this mass-wasting event could explain this value, which is significantly higher than the average value of 0.8 ± 0.1 × 10 −4 km 3/yr obtained for the last 650 ka. Finally, the comparison with other analogous volcanic massifs from islands and continental arcs points to a relatively low magmatic production for southern Basse-Terre, which could be tentatively related to the relatively slow subduction rate of the Atlantic plate.

The K–Ar Cassignol–Gillot technique applied to western Martinique lavas: A record of Lesser Antilles arc activity from 2 Ma to Mount Pelée volcanism

We present 23 new ages from three volcanic complexes of the Lesser Antilles arc in Martinique Island (French West Indies). These ages obtained with the K–Ar Cassignol–Gillot technique are distributed within the whole Quaternary. They allowed us to reconstruct a detailed history of successive volcanic growth and flank collapse stages. Trois Ilets Volcanism has been active during at least 2 Ma, between 2.35 ± 0.03 Ma and 346 ± 27 ka, with monogenetic volcanoes of basaltic-andesite to andesitic compositions. We here propose that magma mixing, which characterizes this volcanism, could have been initiated between 617 and 346 ka by the activation of arc-parallel and arc-transverse fault systems. Meanwhile, the Carbet complex was active 25 km to the north from 998 ± 14 to 322 ± 6 ka, and was partially destroyed by a flank collapse after 602 ± 10 ka. Together with geochemical data, our ages show that Mount Conil and Mount Pelée volcanoes are parts of the same edifice sharing a single magmatic reservoir. Mount Conil started to emerge before 543 ± 8 ka, and andesites erupted until 127 ± 2 ka, when a flank collapse destroyed the western flank of the edifice, probably triggering the emplacement of Piton Marcel, the last eruption of this first stage. We note that this collapse occurred during the transition from oxygen stages 6 to 5, i.e. during glacial to interglacial change, when eustatic level rapidly increased. After that, and until present, Mount Pelée volcano was built with periods of cone growth intercalated by flank collapse events. We here show that a peak of activity occurred between 550 and 330 ka in western Martinique within the three complexes, which are spaced of 15–25 km. Since 330 ka volcanic activity is limited to the northernmost Mount Conil–Mount Pelée complex. Our data are in agreement with the regional scale observations that the whole recent Lesser Antilles arc was subject to a high volcanic activity since 600 ka, probably linked to an increase in magma production. This permanent establishment of rising magma in regularly spaced batches and tectonically controlled, could explain the individual chemical evolution of each edifice and the different eruptive dynamisms occurring at the same time along the recent arc.

Differentiation and Source Processes at Mt Pelée and the Quill; Active Volcanoes in the Lesser Antilles Arc

Volcanic rocks erupted at Mt Pelee (Martinique; central Lesser Antilles) and the Quill volcano (Statia; northern Lesser Antilles) define distinct differentiation trends, each of which can be accounted for largely by fractional crystallization of plagioclase, amphibole and Fe–Ti oxides. This assemblage is seen commonly in associated cumulate nodules, although the petrography of the lavas is pyroxene + plagioclase + Fe–Ti oxides. Thus differentiation is controlled, in part, by cryptic amphibole fractionation. At a given degree of differentiation incompatible trace element abundances tend to be higher at Mt Pelee, and REE patterns are more fractionated than is the case at the Quill. Isotopic ratios of Sr and Pb correlate with indices of differentiation (e.g. wt % SiO2) at both volcanoes (more convincingly at Mt Pelee), indicating that differentiation is also an open-system process. When the differentiation trends for the two volcanoes are compared they do not converge towards a single parental magma composition, suggesting that the primary magmas for the Quill and Mt Pelee are different. This difference is most probably due to mantle source variations. The source of the Mt Pelee magmas appears to be more enriched in incompatible elements, consistent with a greater proportion of admixed subducted sediment. This observation is in agreement with previous studies that have documented an increasing sediment contribution southwards along the arc. However, comparison with available data for other volcanoes along the arc does not reveal a consistent along-arc trend, suggesting that a model of sediment–source mixing is oversimplified and that additional factors such as variable fluid contributions from the subducted slab may be important.

The Transoceanic 1755 Lisbon Tsunami in Martinique

On 1 November 1755, a major earthquake of estimated M w=8.5/9.0 destroyed Lisbon (Portugal) and was felt in the whole of western Europe. It generated a huge transoceanic tsunami that ravaged the coasts of Morocco, Portugal and Spain. Local extreme run-up heights were reported in some places such as Cape St Vincent (Portugal). Great waves were reported in the Madeira Islands, the Azores and as far as the Antilles (Caribbean Islands). An accurate search for historical data allowed us to find new (unpublished) information concerning the tsunami arrival and its consequences in several islands of the Lesser Antilles Arc. In some places, especially Martinique and the Guadeloupe islands, 3 m wave heights, inundation of low lands, and destruction of buildings and boats were reported (in some specific locations probably more enclined to wave amplification). In this study, we present the results of tsunami modeling for the 1755 event on the French island of Martinique, located in the Lesser Antilles Arc. High resolution bathymetric grids were prepared, including topographic data for the first tens of meters from the coastline, in order to model inundations on several sites of Martinique Island. In order to reproduce as well as possible the wave coastal propagation and amplification, the final grid was prepared taking into account the main coastal features and harbour structures. Model results are checked against historical data in terms of wave arrival, polarity, amplitude and period and they correlate well for Martinique. This study is a contribution to the evaluation of the tele-tsunami impact in the Caribbean Islands due to a source located offshore of Iberia and shows that an 8.5 magnitude earthquake located in the northeastern Atlantic is able to generate a tsunami that could impact the Caribbean Islands. This fact must be taken into account in hazard and risk studies for this area.

The weathering and element fluxes from active volcanoes to the oceans: a Montserrat case study

The eruptions of the Soufrière Hills volcano on Montserrat (Lesser Antilles) from 1995 to present have draped parts of the island in fresh volcaniclastic deposits. Volcanic islands such as Montserrat are an important component of global weathering fluxes, due to high relief and runoff and high chemical and physical weathering rates of fresh volcaniclastic material. We examine the impact of the recent volcanism on the geochemistry of pre-existing hydrological systems and demonstrate that the initial chemical weathering yield of fresh volcanic material is higher than that from older deposits within the Lesser Antilles arc. The silicate weathering may have consumed 1.3% of the early CO2 emissions from the Soufrière Hills volcano. In contrast, extinct volcanic edifices such as the Centre Hills in central Montserrat are a net sink for atmospheric CO2 due to continued elevated weathering rates relative to continental silicate rock weathering. The role of an arc volcano as a source or sink for atmospheric CO2 is therefore critically dependent on the stage it occupies in its life cycle, changing from a net source to a net sink as the eruptive activity wanes. While the onset of the eruption has had a profound effect on the groundwater around the Soufrière Hills center, the geochemistry of springs in the Centre Hills 5 km to the north appear unaffected by the recent volcanism. This has implications for the potential risk, or lack thereof, of contamination of potable water supplies for the island’s inhabitants.

The eruptive history of Morne Jacob volcano (Martinique Island, French West Indies): Geochronology, geomorphology and geochemistry of the earliest volcanism in the recent Lesser Antilles arc

Martinique is the Lesser Antilles Island where the most complete volcanic history of the arc can be found from the Oligocene to the present time. In this study, we focused on the construction of Morne Jacob shield volcano in Martinique, which is the largest volcano of the Lesser Antilles. We have dated twenty representative samples from the Morne Jacob, by K–Ar based on the Cassignol–Gillot technique, used geochemical data and obtained morphological constraints that have helped us to reconstruct better the volcanic history of this shield volcano. Our results and the lack of reliable ages on other Plio-Pleistocene islands show that the Morne Jacob is the oldest volcano of the aerial recent arc that has been dated so far. It has a longer history than previously inferred, with different stages ranging between 5.2 and 1.5 Ma. A large basaltic to andesitic shield volcano was first built between 5.2 and 4.0 Ma (J1) with tholeiitic lavas characterized by an increase of SiO 2 content through time. After 800 kyr of repose, calc–alkaline andesites erupted over the first shield from 3.2 to 2.2 Ma (J2a). The accumulation of lavas over a hyaloclastitic basement provoked spreading and northeast creeping of the northern flank of the volcano. The mass movement induced regressive erosion, particularly at the centre of the edifice. Then, between 2.1 and 1.5 Ma (J2b), calc–alkaline lavas, which show a decrease of the SiO 2 content through time, erupted at the central vent and from peripheral fissures, over the previous shield and down to the Caribbean coast. Eruptive volumes were reconstructed, permitting estimates of minimum output rates for the volcano's history. Minimum volume erupted during the first stage is 112 ± 20 km 3 and 33 ± 25 km 3 during the whole second stage, yielding rates of about 0.107 km 3/kyr and 0.019 km 3/kyr, respectively. Considering the entire history of Morne Jacob shield volcano, between 5.14 ± 0.07 and 1.54 ± 0.03 Ma, we obtain a total volume of 145 ± 32 km 3 above the sea level, and a time-averaged construction rate of 0.040 ± 0.008 km 3/kyr. With comparison between the reconstructed paleotopographies and the DEM of present topography, we have calculated an eroded volume of 18 km 3 over the outcropping 210 km 2 that have occurred during the last 1.5 Myr. Finally, our set of twenty K–Ar ages, obtained on groundmass separates, allow us to revise the eruptive chronology of Morne Jacob volcano and to date the earliest subaerial volcanic activity so far identified for the recent Lesser Antilles arc.

Isotopic hyperbolas constrain sources and processes under the Lesser Antilles arc

The chemical and isotopic characteristics of island arc lavas are usually attributed to the combined effects of subducted materials and processes occurring in the mantle wedge. Most island arcs have compositions displaced towards continental crust values relative to MORB, none more so than the Lesser Antilles arc. Not only do lavas from the Lesser Antilles arc contain a high proportion of crustal material but they also display a very large range of chemical and isotopic compositions. Lavas on Martinique Island cover most of the variability known for the entire arc, and volcanism occurred almost continuously for the last 25 Ma. It is therefore an ideal target to establish the causes of the chemical and isotopic variability and to evaluate whether a secular relationship exists. We measured Pb, Sr, Nd and Hf isotopic ratios on 30 well-dated Martinique samples whose ages range between 25 Ma and the present. Samples were chosen to cover all volcanic phases, from island formation to the most recent eruptions. Our results cover most of the isotopic range defined by the entire arc, from fairly unradiogenic to highly radiogenic values; but more importantly, they define two distinct hyperbolas in isotopic space. One array is defined by old lavas (25 to 7.1 Ma), and the other by recent lavas (5.1 Ma to present). The hyperbolas are compatible with simple two-component mixtures but are not reproduced by AFC models. Using these hyperbolas, we can constrain the composition of the mixing end-members. Both depleted sources correspond to depleted mantle contaminated by a radiogenic Sr component, and the two enriched components resemble local sediments with little to no relative fractionation of Sr, Nd, Hf and Pb. We conclude on this basis that sediments were added to the mantle wedge by melting and not dehydration. Our interpretation for Martinique can be extrapolated to the entire Lesser Antilles arc because the isotopic arrays of Martinique and other islands are indistinguishable. More generally, if the same occurs under most other island arcs, the chemical and isotopic characteristics of arc volcanism might be directly linked to the amount and nature of subducted sediments.

Fluid Inputs to Magma Sources of St. Vincent and Grenada (Lesser Antilles): New Insights from Trace Elements in Olivine-hosted Melt Inclusions

St.Vincent and Grenada, known for the presence of high Mg-basalts, are two of the southernmost islands of the Lesser Antilles arc, located at the boundary between the Atlantic and Caribbean Plates. We report here secondary ionization mass spectrometry measurements of trace elements (V, Sr, Y, Zr, Nb, Ba, La to Eu) in basaltic melt inclusions trapped in olivine (Fo90^86) within high-MgO basalts. Combined with previous data on light elements and stable isotopes obtained on the same sample set, these results allow us to decipher the nature of the slab-derived fluids that modified the mantle wedge beneath these two islands. The melt inclusions exhibit trace element patterns that encompass those of the whole-rocks, but with a larger range of variability at Grenada. On the whole, such patterns are trace element enriched but not very fractionated compared with mid-ocean ridge basalt and display typical Ba and Sr enrichment. The variations of mobile/immobile element ratios (e.g. K/La, Sr/ Y) are poorly reconciled with crustal contamination processes but rather track the involvement of early aqueous fluids, which are also responsible for significant increases in Cl/F, B and Cl recorded by the melt inclusions. The variations of immobile element ratios such as La/Sm and Nb/Y cannot be accounted for by variable extents of melting alone, but also imply the influence of a hydrous sediment-derived melt for St. Vincent, whereas the by far larger enrichments of La and Nb in the Grenada samples require contributions of solute-rich fluids extracted at high temperature and pressure from both subducted oceanic crust and sediments. Moreover, a few Grenada melt inclusions display unusual Zr enrichment (up to 1274 ppm), a rare feature in basaltic melts that possibly involves the localized dissolution of zircon. The comparison of trace element distribution patterns in the melt inclusions (MI) and whole-rocks indicates the primary composition of the MI and the interaction of the melt with mantle paragenesis after their entrapment in olivine.