Lesser Antilles Island Arc Research Articles

Complex Martinique Intermediate‐Depth Earthquake Reactivates Early Atlantic Break‐Up Structures

AbstractEarthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth.

Magmatic and rare Eocene to Precambrian xenocrystic zircons in Dominica reveal discrete crystallization episodes and multiple sediment sources

The effects of sediment contribution to the Lesser Antilles island arc have been well explored with whole-rock trace-element chemistry and isotopic studies. To better understand the source of these sediments, we analyzed >400 zircons for U-Pb ages and trace-element chemistry in eight andesitic-dacitic ignimbrites and lavas younger than 100 ka from the island of Dominica in the central Lesser Antilles arc. The overwhelming majority of the zircons analyzed were magmatic in origin, with U-Pb ages younger than 10 Ma, but predominantly younger than 300 ka. Zircon trace-element chemistry is consistent with derivation from an oceanic-island arc, and positive εHf(t) values (+5 to +15) support a juvenile depleted mantle source. Rare Precambrian to Eocene xenocrystic zircons (36) were also found in the Dominican volcanics and record sediment dispersal from several different terranes as the Caribbean plate migrated eastward along the northern margin of South America. Although some previous detrital zircon studies in the region suggested zircons younger than 100 Ma were derived from the Great Arc of the Caribbean, the younger Dominican zircon xenocrysts (300−50 Ma) have elevated Th/Yb and U/Yb, as well as variable positive/negative εHf(t) values, inconsistent with an oceanic arc origin. These zircons in Dominica were most likely derived from the Eastern and Central Cordillera of the Andes, which experienced a flare-up in magmatism ca. 65−45 Ma. As the Great Arc of the Caribbean traversed along the South American margin, terrigenous sediments transported via river systems and turbidites accumulated in the forearc basin. Older zircons (1800−300 Ma) have a slightly different chemistry and equivocal source(s), including the Andes, northern Venezuelan coastal ranges, and/or the Guyana Shield.

Magma storage conditions over the past 4 Ma on Martinique Island, Lesser Antilles

We constrain, for the first time, the storage conditions beneath recent (<5 Ma) volcanic edifices in Martinique Island, combining petrography, geochemistry, and thermobarometry. Specifically, we investigate Morne Jacob volcano (5.2–1.5 Ma), Pitons du Carbet complex (998 ka – 322 ka), Trois Îlets Volcanic field (2.35 Ma – 346 ka), and Mt. Conil – Mt. Pelée system (<550 ka). Lava samples range in composition from basalt to dacite, and in age from 4.1 Ma (Morne Jacob) to 1929 CE (Mount Pelée). For the last ∼5 Ma, the volcanoes in Martinique were fed by andesitic to dacitic magma stored in the upper 6 km of the crust, at temperatures between 900 and 1100 °C. These shallow reservoirs were frequently replenished and remobilized by basaltic and basaltic-andesite magmas originating from the lower crust with temperatures as high as 1200 °C. Erupted products contain simple to more complex mineral assemblages, and plagioclase phenocrysts exhibit textures and compositional profiles typical of antecrysts, identified as (1) normal, (2) oscillatory, (3) sieved, (4) resorbed core, and (5) resorbed rim. All complexes also show a crystallization gap in the middle crust (10–18 km). Trois Îlets, a distributed volcanic field, is the only complex that shows continuous crystallization through the entire crust, distinct magma batches and storage regions for each individual volcano. More importantly, we see drastic changes in terms of rock composition, storage conditions, and crystal populations in pre - and post – collapse lavas. Large flank collapse (> 5 km3) can induce decompression as large as 5 MPa in the upper crust and as low as 0.1 MPa down to 10–12 km. Such decompression magnitudes might be sufficient to enhance ascent of basaltic magma from depth, initiate dike intrusions in the mid to upper crust, and remobilize shallow silicic reservoirs. Such events are likely to initiate magma mixing in shallow reservoirs. This noteworthy eruption-triggering mechanism may lead to the incorporation of distinct crystal populations and subsequently contribute to heightened rates of eruption. In summary, the magma storage conditions and plumbing system architecture for the recent volcanoes in Martinique closely resemble those observed in other volcanoes within the Lesser Antilles Island Arc, such as Soufriere Hills Volcano or La Soufriere de Guadeloupe. These newly established correlations offer valuable insights that can aid in the assessment of potential future volcanic eruptions.

Eruptive dynamics reflect crustal structure and mantle productivity beneath volcanoes

Abstract Volcanoes exhibit a wide range of eruptive and geochemical behavior, which has significant implications for their associated risk. The suggested first-order drivers of intervolcanic diversity invoke a combination of crustal and mantle processes. To better constrain mantle-crustal-volcanic coupling, we used the well-studied Lesser Antilles island arc. Here, we show that melt flux from the mantle, identified by proxy in the form of boron isotopes in melt inclusions, correlates with the long-term volcanic productivity, the volcanic edifice height, and the geophysically defined along-arc crustal structure. These features are the consequence of a variable melt flux modulating the pressure-temperature-composition structure of the crust, which we inverted from xenolith mineral chemistry. Mafic to intermediate melts reside at relatively constant temperature (981 ± 52 °C; 2σ) in the middle crust (3.5–7.1 kbar), whereas chemically evolved (rhyolitic) melts are stored predominantly in the upper crust (&amp;lt;3.5 kbar) at maximum depths that vary geophysically along the arc (6–15 km). Our findings are applicable worldwide, where we see similar correlations among average magma geochemistry, eruptive magnitude, and rate of magma input.

Variable element enrichment sources and contributions to volcanic rocks along the Lesser Antilles Island Arc

Understanding relationships between different components involved in subduction-related recycling of sediment at arc volcanic systems has long been a focus of study. It is generally understood that volcanic arc lavas incorporate materials from the down-going slab including fluids, fluid mobile elements (FME), and fluid modified large ion lithophile elements (LILE), largely derived from subducted sediments and serpentinized ultramafic rocks. Past studies have presumed bulk sediment contributions rather than addressing localized variations in sediment composition. We examine trace element variability along the Lesser Antilles Island Arc (LAIA) and show that sediment inputs via fluid transfer to the mantle wedge are necessary to explain high B (up to 50 ppm), high B/La ratios, and higher As, Cs, Rb in the central LAIA. In contrast, the northern portion of LAIA is enriched in Ba, Sr and U/Th, with low FME. Grenada has low FME abundances, low Ba/La, but higher Sm/La. Our results suggest that modification of LAIA mantle source by fluid addition is important for the central islands but find that sediment enrichment is more likely in the northern section of the LAIA. Li/Yb ratios suggest the Grenada signature is chemically more comparable to sediment from DSDP Site 144 (Demerara Rise), while Martinique magmas in the central LAIA are affiliated to sediments from Site 543 (offshore Dominica). Unexpectedly, the northern portion of LAIA is not directly relatable to Site 543 sediments but instead trends back toward a sediment source similar in composition to Site 144 but with lower B, lower La/Sm and higher Ba. Binary mixing models suggest the northern islands are likely influenced by small amounts of sediment melt similar to average Site 144 sediments, but with Ba concentrations ranging from 1,250 to 1,500 ppm. Trace element ratios then likely result from different levels of contribution from sediment packages both spatially and temporally along the arc, with some variation evident among samples from the same island. We suggest that the process of recycling subducted sediment along the arc is variable along the length of the LAIA, wherein the trace element relationships are likely strongly influenced by the types of sediment being subducted along strike.

The Role of Crustal Accretion Variations in Determining Slab Hydration at an Atlantic Subduction Zone

AbstractWe present a 2D P‐wave velocity model from the outer rise region of the Lesser Antilles island arc, the first wide‐angle seismic study of outer rise processes at an Atlantic subduction zone. The survey consists of 46 OBS receivers over a 174 km profile with velocities resolved to 15 km below top basement. The final velocity model, produced through tomographic inversion, shows a clear decrease in the velocity of the lower crust and upper mantle of the incoming plate as it approaches the trench. We attribute this drop to outer rise bend‐related hydration, similar to Pacific cases, but superimposed on spatial variations in hydration generated at the slow‐spreading ridge axis. In thin, tectonically controlled crust formed under magma‐poor spreading conditions the superposition of these sources of hydration results in compressional velocities as low as 6.5 km s−1 beneath the PmP reflector. In contrast, segments of crust interpreted as having formed under magma‐rich conditions show velocity reductions and inferred hydrous alteration more like that observed in the Pacific. Hence, variations in the style of crustal accretion, which is observed on 50–100 km length scales both along and across isochrons, is a primary control over the distribution of water within the slab at Atlantic subduction systems. This heterogeneous pattern of water storage within the slab is likely further complicated by along strike variations in outer rise bending, subducting fracture zones and deformation at segment ends and may have important implications for our understanding of long‐term patterns of hazard at Atlantic subduction systems.

Ongoing tectonic subsidence in the Lesser Antilles subduction zone

SUMMARYGeological estimates of vertical motions in the central part of the Lesser Antilles show subsidence on timescales ranging from 125.000 to 100 yr, which has been interpreted to be caused by interseismic locking along the subduction megathrust. However, horizontal GNSS velocities show that the Lesser Antilles subduction interface is currently building up little to no elastic strain. Here, we present new present-day vertical velocities for the Lesser Antilles islands and explore the link between short- and long-term vertical motions and their underlying processes. We find a geodetic subsidence of the Lesser Antilles island arc at 1–2 mm yr−1, consistent with the ∼100-yr trend derived from coral micro-atolls. Using elastic dislocation models, we show that a locked or partially locked subduction interface would produce uplift of the island arc, opposite to the observations, hence supporting a poorly coupled subduction. We propose that this long-term, margin-wide subsidence is controlled by slab dynamic processes, such as slab rollback. Such processes could also be responsible for the aseismic character of the subduction megathrust.

Zirconium isotopic composition of the upper continental crust through time

The stable isotopic composition of insoluble, refractory elements such as titanium (Ti) or zirconium (Zr), which are modified by magmatic differentiation but, a priori, are poorly affected by weathering or diagenesis, serve as powerful potential proxies to reconstruct the compositional evolution of the continental crust. Here we present the evolution of the Zr stable isotopic compositions (δ94/90ZrIPGP-Zr, per mille deviation of 94Zr/90Zr from IPGP-Zr standard) of the continental crust through time, using 38 sedimentary samples from the upper continental crust (UCC), including 12 Holocene loesses from the Chinese Loess Plateau and Xinjiang, three oceanic sediments from the sea floor outboard of the Lesser Antilles island arc and 23 glacial diamictite composites with depositional ages ranging from ∼ 2.9 Ga to 0.3 Ga from South Africa, South America, Canada, USA and China. The samples show limited Zr isotopic variations with δ94/90ZrIPGP-Zr values of 0.043‰ - 0.109‰ for loess; 0.069‰ - 0.083‰ for oceanic sediments and 0.031‰ - 0.118‰ for glacial diamictites; their Zr-weighted average values are, 0.081 ± 0.044‰ (2SD, n = 12), 0.073 ± 0.015‰ (2SD, n = 3) and 0.078 ± 0.047‰ (2SD, n = 23), respectively. The isotopic similarity among loess, oceanic sediments and glacial diamictites, suggests that zircon enrichment effects previously documented in some sedimentary samples have not biased the Zr isotope compositions of these sedimentary rocks from their source rocks. Two groups with or without Zr enrichment have similar average δ94/90ZrIPGP-Zr values (0.075 ± 0.040‰ and 0.080 ± 0.046‰). There is no correlation between Zr isotope compositions and any proxy of chemical weathering (e.g., Al2O3/SiO2, Fe2O3/SiO2, CIA, K2O/Al2O3 and δ7Li). The δ94/90ZrIPGP-Zr values are quite constant for these sedimentary samples regardless of their depositional ages and locations. Therefore, the UCC appears to have had a constant Zr isotopic composition between 3 Ga and present, and is homogeneous at a large scale. Combining data for sedimentary reference materials from the literature and the sedimentary rocks in this study, we suggest a Zr-weighted δ94/90ZrIPGP-Zr value of 0.077 ± 0.058‰ (2SD, n = 44) for the UCC, which is statistically distinct (t test, p value = 2.88 × 10−10) and higher than that of the mantle (0.040 ± 0.044‰, n = 72). Combining the δ94/90ZrIPGP-Zr values of different terrestrial reservoirs, the δ94/90ZrIPGP-Zr of the BSE and bulk Earth is constrained to be 0.041 ± 0.041‰.

Jadeitite and Related Rocks in Serpentinite Mélanges from the Rio San Juan Complex, Dominican Republic: Evidence for Both Isochemical Replacement and Metasomatic Desilication of Igneous Protoliths with Fluid-Assisted Jadeite Growth

Abstract —This study presents an overview of the systematic petrography, mineralogy, and geochemistry of jadeitite and jadeite-rich rocks found as blocks in the serpentinite mélanges of the Rio San Juan Complex (RSJC) of the northern Dominican Republic. The RSJC is one of the remnants of the subduction/accretionary complex of the Great Caribbean Arc that once spanned the gap between North and South America, moved relatively eastward to its present position as the Lesser Antilles island arc, and left collisional fragments along the two continental margins. Our systematic collection of heterogeneous samples ranges from jadeitite s.str. (sensu stricto) with ≥90 vol.% jadeite to quartz-rich rocks with jadeite and lawsonite. Two suites of rock types can be recognized. In the matrix-quartz-free rock suite, albite is the principal vein-filling or interstitial phase. Quartz is present only as inclusions in the cores of some jadeite crystals. In the matrix-quartz-bearing rock suite, quartz is abundant and albite is relatively rare. The first-order question concerning jadeite-rich rocks is whether jadeite precipitated from a high-pressure aqueous fluid (“vein precipitation” or “P-type”) or whether the jadeite-rich rock formed through comprehensive metasomatic replacement of an igneous protolith (“R-type”). Some examples occur as discordant veins and are clearly P-type. For most, however, classification has been equivocal. The systematic data on the petrography and whole-rock chemistry of jadeite rocks from the RSJC presented in this paper leads to significant clarification. A major argument against R-type genesis is that the metasomatic mass transfer required to produce jadeitite and jadeite-rich rocks from any normal igneous protolith is prohibitively complex. Using whole-rock, major-element compositions, we show that many members of the matrix-quartz-bearing rock suite from the RSJC can be derived by isochemical HP/LT metamorphism of normal oceanic plagiogranites subducted together with oceanic crust. Isocon analysis shows, furthermore, that more jadeite-rich rock types and also members of the matrix-quartz-free suite can be derived from such plagiogranites primarily by straightforward desilication, a realistic scenario in a serpentine-rich environment. The quartz inclusions found in jadeite crystals of the matrix-quartz-free suite corroborate a genetic path in which the plagioclase in a plagiogranite protolith reacts to jadeite + quartz. Later desilication and the formation of albite in the Si-undersaturated rock matrix leave tell-tale quartz inclusions as relics in jadeite crystals.

Eocene intra-plate shortening responsible for the rise of a faunal pathway in the northeastern Caribbean realm.

Intriguing latest Eocene land-faunal dispersals between South America and the Greater Antilles (northern Caribbean) has inspired the hypothesis of the GAARlandia (Greater Antilles Aves Ridge) land bridge. This landbridge, however, should have crossed the Caribbean oceanic plate, and the geological evolution of its rise and demise, or its geodynamic forcing, remain unknown. Here we present the results of a land-sea survey from the northeast Caribbean plate, combined with chronostratigraphic data, revealing a regional episode of mid to late Eocene, trench-normal, E-W shortening and crustal thickening by ∼25%. This shortening led to a regional late Eocene-early Oligocene hiatus in the sedimentary record revealing the location of an emerged land (the Greater Antilles-Northern Lesser Antilles, or GrANoLA, landmass), consistent with the GAARlandia hypothesis. Subsequent submergence is explained by combined trench-parallel extension and thermal relaxation following a shift of arc magmatism, expressed by a regional early Miocene transgression. We tentatively link the NE Caribbean intra-plate shortening to a well-known absolute and relative North American and Caribbean plate motion change, which may provide focus for the search of the remaining connection between 'GrANoLA' land and South America, through the Aves Ridge or Lesser Antilles island arc. Our study highlights the how regional geodynamic evolution may have driven paleogeographic change that is still reflected in current biology.

The polygenetic Kahouanne Seamounts in the northern Lesser Antilles island arc: Evidence for large-scale volcanic island subsidence

Remotely operated vehicle (ROV) explorations of the Kahouanne Seamounts, located 25 km SE of Montserrat in the northern Lesser Antilles island arc, have discovered the occurrence of honeycomb-scalloped erosional features on volcanic and limestone outcrops at depths of up to 600 m below sea level (mbsl). These features, combined with the flat-topped morphology of the seamounts, the occurrence of shallow-water carbonates (rhodoliths, benthic foraminifera), and the presence of oxidized, highly-vesicular volcanic fragments, suggest that the seamounts were once subaerial islands and have subsequently subsided to their present depth. The subsidence is likely to have been in response to a combination of 1) graben development southeast of the island of Montserrat where faulting and extension are driven by the accommodation of slip convergence associated with oblique subduction along the Lesser Antilles volcanic arc and 2) regional subsidence inferred from studies of nearby carbonate platforms. 40Ar/39Ar dating of basalt/andesite lavas from the central seamount indicates active volcanism around 5.4 Ma. Shallow-water carbonates recovered from an apparent wave-cut terrace at ~600 mbsl in the same area, were deposited approximately 3 Ma, based on Sr-isotope stratigraphy, indicating significant subsidence of the complex since that time. The total subsidence is in line with displacements in the Kahouanne valley graben and regional subsidence rates inferred from carbonate platform depths around the islands of Guadeloupe and Martinique, implying that intra-arc subsidence has been a significant process shaping the present bathymetry and topography of this island arc.

Lateral variation in crustal structure along the Lesser Antilles arc from petrology of crustal xenoliths and seismic receiver functions

We reconstruct crustal structure along the Lesser Antilles island arc using an inversion approach combining constraints from petrology of magmatic crustal xenoliths and seismic receiver functions. Xenoliths show considerable island-to-island variation in xenolith petrology from plagioclase-free ultramafic lithologies to gabbros and gabbronorites with variable proportions of amphibole, indicative of changing magma differentiation depths. Xenoliths represent predominantly cumulate compositions with equilibration depths in the range 5 to 40 km. We use xenolith mineral modes and compositions to calculate seismic velocities (vP,vS) and density at the estimated equilibration depths. We create a five-layer model of crustal structure for testing against receiver functions (RF) from island seismic stations along the arc. Lowermost layer (5) comprises peridotite with physical characteristics of mantle xenoliths from Grenada. Uppermost layer (1) consists of 5 km of volcaniclastics and sediments, whose physical properties are determined via a grid inversion routine. The three middle layers (2) to (4) comprise igneous arc crust with compositions corresponding to the xenoliths sampled at each island. By inversion we obtain a petrological best-fit for the RF on each island to establish the nature and thicknesses of layers (2) to (4).Along the arc we see variations in the depth and strength of both Moho and mid-crustal discontinuity (MCD) on length-scales of tens of km. Moho depths vary from 25 to 37 km; MCD from 11 and 32 km. The Moho is the dominant discontinuity beneath some islands (St. Kitts, Guadeloupe, Martinique, Grenada), whereas the MCD dominates beneath others (Saba, St. Eustatius). Along-arc variability in MCD depth and strength is consistent with variation in estimated magmatic H2O contents and differentiations depths that, in turn, influence xenolith lithologies. A striking feature is steep, along-arc gradients in vP similar to those observed at other oceanic arcs. These gradients reflect abrupt changes in rates and processes of magma generation in the underlying crust and mantle. We find no evidence for large, interconnected bodies of partial melt beneath the Lesser Antilles. Instead, the crustal velocity structure is consistent with magma differentiation in vertically-extensive, crystal mush-dominated reservoirs. Along-arc variation in crustal structure may reflect heterogeneous upwelling within the mantle wedge, itself driven by variation in slab-derived H2O fluxes.

Update on the fluid geochemistry monitoring time series for geothermal systems in Dominica, Lesser Antilles island arc: 2009–2017

With updated geochemical and isotopic compositions obtained over the period 2009 to 2017, this study presents revised fluid characterizations for the active volcanic-hydrothermal systems on the island of Dominica in the Lesser Antilles, which were first reported in 2011. Hydrothermal waters of Dominica cover a wide spectrum of pH, temperatures and chemical composition. The pH of the thermal waters ranges from acidic to neutral (pH values of 1–7.8) and the waters are predominantly Na-SO4 in character (Na = 14–2127 mg/L; SO4 = 1–1725 mg/L), likely formed as a result of dilution of acidic H2S-rich gases in near surface oxygenated groundwater, and have experienced limited water-rock interaction. The geochemical composition of the waters for most of the hydrothermal systems studied indicate no significant changes, with the exception of the Boiling Lake, which experienced a short (~6 week) episode of instability in November 2016 which appeared to be associated with a small mud-rich explosion. Unlike the last such event in December 2004, which was reported to be earthquake-triggered, this event is possibly the consequence of a moderate-sized landslide into the lake. The lake draining episodes have been accompanied by changes in composition between Na-SO4 and Na-Cl, which is attributed to hydrothermal fluid contributions from two different aquifers: a shallower acid-sulphate hydrothermal aquifer and a deeply-sourced brine aquifer.Reservoir temperatures determined by quartz geothermometers have not changed significantly over the monitoring period, suggesting steady-state degassing of the magma chambers. In two areas, temperatures have increased: Watten Waven (from 83–90 °C to 89–139 °C) and Sulphur Springs (from 145–152 °C to 93–243 °C). The elevated reservoir temperatures have affected the isotopic composition of the waters (δ18O = −5.7 to 9.1‰ and δD = −8 to 20.5‰), that reflect a dominantly meteoric source, with boiling/degassing and evaporation also playing an important role. The time series data suggests that some hydrothermal areas are experiencing increased steam evaporation over time whereas other waters are becoming more meteoric. The δ13CDIC is decoupled from the deuterium and oxygen-18 isotopes and shows very little variation over time, but a broad range in values from −11 to +5‰. The dominant process contributing to δ13CDIC is degassing of primarily magmatic CO2, as exhibited by the bubbling pools. Hydrothermal streams have experienced mixing with biogenic CO2 sources, including plant respiration and methanogenesis. The slight variations observed from site to site are likely a consequence of fractional degassing of the magma chamber during exsolution of CO2. Over the sampling period 2014–2017, the temperatures and δ13C values do not change, which suggests a current steady-state of degassing.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

New light on the geology of the Roundstone intrusion, its inversion and that of the Grampian metagabbro-gneiss complex, Connemara, western Ireland

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

Ancient xenocrystic zircon in young volcanic rocks of the southern Lesser Antilles island arc

The Lesser Antilles arc is one of the best global examples in which to examine the effects of the involvement of subducted sediment and crustal assimilation in the generation of arc crust. Most of the zircon recovered in our study of igneous and volcaniclastic rocks from Grenada and Carriacou (part of the Grenadines chain) is younger than 2Ma. Within some late Paleogene to Neogene (~34–0.2Ma) lavas and volcaniclastic sediments however, there are Paleozoic to Paleoarchean (~250–3469Ma) xenocrysts, and Late Jurassic to Precambrian zircon (~158–2667Ma) are found in beach and river sands. The trace element characteristics of zircon clearly differentiate between different types of magmas generated in the southern Lesser Antilles through time. The zircon population from the younger arc (Miocene, ~22–19Ma, to Present) has minor negative Eu anomalies, well-defined positive Ce anomalies, and a marked enrichment in heavy rare earth elements (HREE), consistent with crystallization from very oxidized magmas in which Eu2+ was in low abundance. In contrast, zircon from the older arc (Eocene to mid-Oligocene, ~30–28Ma) has two different REE patterns: 1) slight enrichment in the light (L)REE, small to absent Ce anomalies, and negative Eu anomalies and 2) enriched High (H)REE, positive Ce anomalies and negative Eu anomalies (a similar pattern is observed in the xenocrystic zircon population). The combination of positive Ce and negative Eu anomalies in the zircon population of the older arc indicates crystallization from magmas that were variably, but considerably less oxidized than those of the younger arc. All the igneous zircon has positive εHf(t), reflecting derivation from a predominantly juvenile mantle source. However, the εHf(t) values vary significantly within samples, reflecting considerable Hf isotopic heterogeneity in the source.The presence of xenocrystic zircon in the southern Lesser Antilles is evidence for the assimilation of intra-arc crustal sediments and/or the recycling and incorporation of sediments into the magma sources in the mantle wedge. Most likely however, primitive magmas stalling and fractionating during their ascent through the Antilles crust entrained ancient zircon. This is evidence by the geochemistry of the study samples, which is inconsistent with any involvement of partially melted subducted sediment. Paleogeographic reconstructions show that the old zircon could derive from distant regions such as the Eastern Andean Cordillera of Colombia, the Merida Andes, and the northern Venezuela coastal ranges, transported for example by the Proto-Maracaibo River precursor of the Orinoco River.

Helium isotope systematics of volcanic gases and thermal waters of Guadeloupe Island, Lesser Antilles

The island of Guadeloupe is located in the middle of the 850km long Lesser Antilles island arc. Present-day volcanic and geothermal activity is concentrated in two systems both located in the southwestern part of the island (Basse Terre): the La Soufrière volcanic complex and the Bouillante hydrothermal system, some 20km to the northwest of the volcano. We report here the largest isotopic data set for helium isotopes in hydrothermal gases and waters from both systems, acquired between 1980 and 2012. 3He/4He ratios in the fumarolic gases of La Soufrière volcano have been quite homogeneous and stable over the last thirty years. The average ratio of 8.2±0.2 Ra confirms that the volcano is tapping a MORB-like mantle source. In contrast, the nearby Bouillante geothermal system displays a much lower 3He/4He ratio (4.5±0.1 Ra). He–C elemental and isotopic relationships show that both systems are actually fed by the same magmatic source, and that their marked difference in 3He/4He results from the 4He contamination of the Bouillante deep aquifer by the surrounding wallrock. This conclusion is strengthened by the spatial distribution of 3He/4He ratios which shows that La Soufrière fumaroles and the Bouillante geothermal system are the two end-members of a spatial trend of decreasing 3He/4He ratio with distance from La Soufrière summit dome, implying an increasing addition of radiogenic 4He from the host rocks away from the present-day active volcanic edifice.

Timing of geothermal activity in an active island-arc volcanic setting: First 40 Ar/ 39 Ar dating from Bouillante geothermal field (Guadeloupe, French West Indies)

Abstract Mineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias, newly discovered in the active Bouillante geothermal field (Guadeloupe archipelago), and investigated by 40 Ar/ 39 Ar geochronology in order to constrain the timing of geothermal activity in this part of the active Lesser Antilles island arc. The inverse isochron diagram indicates an age of 248±50 ka (2σ) for all adularia from one breccia sample ( n =8), with an initial 40 Ar/ 36 Ar ratio of atmospheric composition (309±12 (2σ)) attesting that this age is valid. This age is concordant with the weighted mean age of 290±40 ka for the same sample. Adularia from other samples yields concordant ages. The obtained 40 Ar/ 39 Ar ages can be related either to the magmatic activity of the Bouillante Volcanic Chain ( c. 850–250 ka ago) or to the initiation of the volcanic activity of the active Grande Découverte–Soufrière system (200 ka ago–present day). Our results demonstrate that the Bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area. The possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed.

Tectonic evolution and mantle structure of the Caribbean

AbstractWe investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic P wave velocity structure under the Caribbean region. In the upper mantle, positive seismic anomalies are imaged under the Lesser Antilles and Puerto Rico. These anomalies are interpreted as remnants of Atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the Lesser Antilles island arc during the past ~45 Myr. The imaged Lesser Antilles slab consists of a northern and southern anomaly, separated by a low‐velocity anomaly across most of the upper mantle, which we interpret as the subducted North America‐South America plate boundary. The southern edge of the imaged Lesser Antilles slab agrees with vertical tearing of South America lithosphere. The northern Lesser Antilles slab is continuous with the Puerto Rico slab along the northeastern plate boundary. This results in an amphitheater‐shaped slab, and it is interpreted as westward subducting North America lithosphere that remained attached to the surface along the northeastern boundary of the Caribbean plate. At the Muertos Trough, however, material is imaged until a depth of only 100 km, suggesting a small amount of subduction. The location and length of the imaged South Caribbean slab agrees with proposed subduction of Caribbean lithosphere under the northern South America plate. An anomaly related to proposed Oligocene subduction at the Nicaragua rise is absent in the tomographic model. Beneath Panama, a subduction window exists across the upper mantle, which is related to the cessation of subduction of the Nazca plate under Panama since 9.5 Ma and possibly the preceding subduction of the extinct Cocos‐Nazca spreading center. In the lower mantle, two large anomaly patterns are imaged. The westernmost anomaly agrees with the subduction of Farallon lithosphere. The second lower mantle anomaly is found east of the Farallon anomaly and is interpreted as a remnant of the late Mesozoic subduction of North and South America oceanic lithosphere at the Greater Antilles, Aves ridge, and Leeward Antilles. The imaged mantle structure does not allow us to discriminate between an “Intra‐Americas origin” and a “Pacific origin” of the Caribbean plate.

Chemical and isotopic characteristics of geothermal fluids from Sulphur Springs, Saint Lucia

Sulphur Springs is a vigorous, geothermal field associated with the active Soufrière Volcanic Centre in southern Saint Lucia, Lesser Antilles island arc. The ‘Sulphur Springs Park’ is an important tourist attraction (touted as the ‘world's only drive-through volcano’) with some of the hot pools being developed into recreational pools. Some 200,000 people visit the park each year. Since 2001, the hydrothermal fluids of Sulphur Springs have been sampled as part of an integrated volcanic monitoring programme for the island. Gas and water samples were analysed to characterise the geochemistry of the hydrothermal system, and to assess the equilibrium state and subsurface temperatures of the reservoir. This has also enabled us, for the first time, to establish baseline data for future geochemical monitoring.The gases are of typical arc-type composition, with N2 excess and low He and Ar content. The dry gas composition is dominated by CO2 (ranging from 601–993mmol/mol), with deeper magmatic sourced H2S-rich vapour undergoing boiling and redox changes in the geothermal reservoir to emerge with a hydrothermal signature in the fumarolic gases. Fluid contributions from magmatic degassing are also evident, mainly from the moderate to high contents of HCl and deeply-sourced H2S gas, respectively. Sulphur Springs hydrothermal waters have acid-sulphate type compositions (SO4=78–4008mg/L; pH=3–7), and are of primarily meteoric origin which have been affected by evaporation processes based on the enrichment in both δ18O and δD (δ18O=−1 to 15‰ and δD=−9 to 14‰ respectively) in relation to the global meteoric water line (GMWL). These waters are steam-heated water typically formed by absorption of H2S-rich gases in the near surface oxygenated groundwaters. Reservoir temperatures calculated from the evaluation of gas equilibria in the CO2–CH4–H2 system reveal higher temperatures (190 to 300°C) than those derived from quartz geothermometry (95 to 169°C), which appeared to be affected by dilution with meteoric waters. Generally, no significant variations in fluid geochemistry of the hydrothermal system were observed between 2001 and 2006, and we propose that there were no changes in the state of volcanic activity during this period.