Geochemistry of the Lesser Antilles volcanic island arc

New analyses of 1518 rocks for major and certain trace elements are used to examine chemical variations between the 15 larger volcanic islands of the Lesser Antilles island arc. The depth to the top of the subduction zone dipping westward at about 40° lies about 100km below all the volcanoes of the arc. Most of the sampled eruptions are post-Miocene (5-1 m.y.) although south of Martinique, the Oligocene-Miocene and the younger arc are superimposed.There is a chemical variation along the arc axis, from alkalic (southern) through calc-alkalic (central) to tholeiitic (northern) volcanic suites. Three islands are examined in detail as type examples of this variation, i.e. Grenada (south), Dominica (centre), and St. Kitts (north). The Grenada suite includes basanites, alkalic basalts, and subalkalic basalts, andesites and dacites. The subalkalic basalts, andesites and dacites each fall into three chemical groupings along the axis of the arc, distinguished especially by K, Zr, Ni and Cr abundances. The whole Lesser Antilles assemblage is characterised by low K abundances and low K/Rb ratios, compared with other island arcs.The magmas are believed to have evolved through processes of partial melting and crystal fractionation. Partial melting of garnet Iherzolite at about 100km depth in a relatively ‘fertile’ zone of upper mantle in the southern sector, above the subducted slab of basaltic ocean crust, could have produced the undersaturated alkalic magmas. In the central and northern sectors, where the crustal structures are more complex, partial melting may have occurred within more ‘barren’ upper mantle, to produce tholeiitic and calc-alkalic magmas depleted in certain trace elements. In either case, water was probably added to the melted zone from the subducted and hydrated oceanic crust, since the whole arc assemblage was erupted explosively and the rocks are rich in A1₂O₃, plagioclase is very calcic, and amphibole is an important phase. The second process was crystal fractionation at low pressure, as evidenced by the abundance of cumulate xenoliths. Separating phases for the southern volcanoes were olivine, calcic augite and Cr-spinel, followed by hornblende, anorthite and Ti-magnetite at lower temperatures. There is little evidence of the higher-temperature fractionation controls for the central and northern volcanoes.     

Peridotite xenoliths from Grenada,Lesser Antilles Island Arc

Ultramafic xenoliths comprising harzburgite, lherzolite (reacted harzburgite) and spinel-rich dunite, occur in alkali olivine basalts (M series) of Grenada in the Lesser Antilles island arc. Textures are protogranular, porphyroclastic and granular; the latter are restricted to dunites and areas of the harzburgites/lherzolites where interaction with host magma has occurred. Primary mineralogy comprises olivine, orthopyroxene, clinopyroxene, and spinel. Harzburgites are residual from a fractional partial melting event totaling ~22%. Infiltration of harzburgite by (and reaction with) basalt has produced: a wehrlite, with partial dissolution of primary spinel, an increase in the oxygen fugacity (ƒ_O2) from primary values 1–2 log ƒ_O2 units above the fayalite-magnetite-quartz (FMQ) buffer, to 2–2.5 log units above the buffer; reaction of orthopyroxene to form patches of intergrown olivine and clinopyroxene, and bronzite andesite glass (60 wt%, SiO₂ 18–20 wt% Al₂O₃ and 3–4 wt% Na₂O) with flat to light rare earth element-depleted, chondrite-normalized abundances. Refertilisation of the mantle by reacting melts, producing a clinopyroxene-rich lithology, may form a source of ankaramitic (high-Ca) arc basalts.Editorial responsibility: T.L. Grove     

The Geochemistry of the Igneous Rock Suite of St Martin, Northern Lesser Antilles

The island of St Martin lies inthe inactive part of the northernLesser Antilles island arc. The island consists of volcaniclasticsediments overlain and intruded by volcanic and plutonic rocks,which are in turn overlain by Miocene limestones. The extrusiveand intrusive rock suites are closely spaced in time (around27 Ma) although field relations suggest that the volcanic rockswere intruded by the plutons. Pluton emplacement gave rise tothermal metamorphism of the volcanic and volcaniclastic carapace,and to widespread hydrothermal alteration throughout the island. Geochemically, the igneous rocks of St Martin form a mildlytholeiitic to calc-alkaline typical subduction-related suite.The extrusive rocks are basalts to andesites, and the magmasappear to have differentiated largely through fractional crystallizationof plagioclase, clinopyroxene, and olivine. The REE displayflat chondrite-normalized patterns, with no significant Eu anomalydespite convincing evidence for plagioclase fractionation. Theplutonic rocks are more silica-rich diorites to granites, containingplagioclase, amphibole, and, less commonly, K-feldspar, sphene,zircon, and pyroxene. REE patterns are slightly LREE enrichedbut display distinct negative Eu anomalies. The fractionationof amphibole and accessory phases may have been important inthe evolution of the plutonic suite, as REE contents do notincrease overall with differentiation. Sr and Nd isotopic ratios of the St Martin suite form restrictedranges which vary little with differentiation, or between theextrusive and intrusive suiteSi ⁸⁷Sr/⁸⁶Sr ratios are slightlyhigher and ¹⁴³Nd/^l44Nd slightly lower than for volcanic rocksuites from the currently active northern Lesser Antilles volcanicarc. Some of the high ⁸⁷Sr/⁸⁶Sr ratios are explained in termsof hydrothermal alteration involving a high ⁸⁷Sr/⁸⁶Sr fluid,associated with pluton emplacement. Pb isotope ratios are similarto those of the currently active northern Lesser Antilles arc,and correlate with SiO². Such correlations, together with largeranges of incompatible (and immobile) trace element ratios suggestthat open-system differentiation occurred during the evolutionof the St Martin suite. The composition of magma sources in the northern Lesser Antillesarc apparently has not changed significantly over the last 30Ma, despite a westward shift in the locus of arc magmatism.Addition of a slab-derived fluid to the mantle wedge is responsiblefor the high relative abundances of large ion lithophile elements(LILE) and enrichment in radiogenic Pb and Sr relative to mid-oceanridge basalt (MORB). Subsequent differentiation may involveassimilation of the arc basement in St Martin, which is believedto consist of Cretaceous to early Tertiary arc material, similarto that encountered in the Greater Antilles. ^*Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721     

Mineralogy and petrology of Grenada,Lesser Antilles island arc

The mineralogy and petrology of volcanic and plutonic rocks from the island of Grenada are described. The volcanic rocks include basanitoids, alkalic and subalkalic basalts, andesites and dacites. Phenocryst phases in the basanitoids and basalts are olivine (Fo_90–71), zoned calcic augite, spinel ranging from ferrian pleonaste through chromite to titaniferous magnetite, and plagioclase. Some of the basalts contain pargasitic amphibole. Andesites and dacites generally contain hypersthene and augite, and one pigeonite-hypersthene-augite-bearing andesite was found. Apatite commonly occurs as a phenocryst in the andesites and dacites and quartz is present in some dacites as well as being a possible xenocryst in both alkalic and subalkalic basalts. Plutonic cumulates found as ejected fragments in tuffs and ashes are composed of variable proportions of olivine, magnetite, calcic augite, amphibole and plagioclase. One peridotitic (ol-cpx-opx) fragment was found but spinel or garnet peridotitis are absent. Despite the alkalic nature of the association, calcalkalic characteristics such as calcic plagioclase, restricted Feenrichment in coexisting pyroxenes and generally low TiO₂ content relative to oceanic suites are present in Grenada. Estimates of conditions of equilibration of the basanitoids with potential upper mantle materials using the results of high-pressure experiments are compared with estimates from thermodynamic data. Equating and basanitoid with hypothetical garnet peridotite assemblages gives a pressure and temperature of equilibration in the region of 35–38 kbar and 1550–1625 ° K. Experimental results are not supportive of these estimates.     

Boron isotope ratios of surface waters in Guadeloupe,Lesser Antilles

Large variations are reported in the B concentrations and isotopic ratios of river and thermal spring waters in Guadeloupe, Lesser Antilles. Rivers have δ¹¹B values around 40‰ and B concentrations lower than 30 μg/L, while thermal springs have δ¹¹B of 8–15‰ and B concentrations of 250–1000 μg/L. River samples strongly impacted by hydrothermal inputs have intermediate δ¹¹B and B contents. None of these surface water samples have δ¹¹B comparable to the local unweathered volcanic rocks (around 0‰), implying that a huge isotopic fractionation of 40‰ takes place during rock weathering, which could be explained by preferential incorporation of ¹⁰B during secondary mineral formation and adsorption on clays, during rock weathering or in the soils. The soil-vegetation B cycle could also be a cause for such a fractionation. Atmospheric B with δ¹¹B of 45‰ represents 25–95% of the river B content. The variety of the thermal spring chemical composition renders the understanding of B behavior in Guadeloupe hydrothermal system quite difficult. Complementary geochemical tracers would be helpful.     

Thin section petrography of northern Lesser Antilles ceramics

Thin sections from 44 sherds representing eight prehistoric sites on four islands (Barbuda, Montserrat, Anguilla, St. Martin) in the northern Lesser Antilles (West Indies) were examined using a petrographic microscope. Point counting distinguished three temper associations: (1) exclusively volcanic,(2) volcanic and carbonate, and (3) volcanic, carbonate, and grog. Exclusively volcanic or dominantly volcanic (with low carbonate) temper associations occur in all Saladoid and many post-Saladoid sherds, with plagioclase feld-spar and volcanic rock fragments being most abundant. Thin sections with significant carbonate content or with grog were restricted to post-Saladoid sherds from Barbuda and to a lesser extent Anguilla. the presence of volcanic grains in sherds from the limestone islands of Barbuda and Anguilla indicates that volcanic islands in the region served as sources of pottery (or volcanic temper) for these two islands.     

Geochemical Evolution of the Soufriere Hills Volcano, Montserrat, Lesser Antilles Volcanic Arc

The geochemical evolution of Montserrat provides an importantbackground to understanding the current activity of this islandarc volcano. Here we present major and trace element, and U-,Th- and O-isotope data for rocks generated in the last 300 kyrthat provide constraints on the magmatic processes occurringbeneath the volcano. Samples range from low- to medium-K calc-alkalinebasalts to dacites. Three suites can be distinguished on thebasis of major and trace element compositions: the South SoufrièreHills suite; the Soufrière Hills suite, including thelava from the current eruption; and the mafic inclusions. Magmaticdifferentiation of the magma that crystallized to form the maficinclusions appears to have been governed by closed-system processes,modelled by fractional crystallization (F     

Oxidised phase relations of a primitive basalt from Grenada,Lesser Antilles

A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO₂) from Grenada, Lesser Antilles, at anhydrous, H₂O-undersaturated and H₂O-saturated conditions in the pressure range 1 atm to 1.7 GPa. \(\hbox{Fe}^{3+}/\Upsigma\hbox{Fe}\) of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO₂) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO ? 2.5 to + 3.8 show that increasing fO₂ mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H₂O, respectively. H₂O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo_91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of \({{K}_{D}}_{\rm ol-melt}^{\rm Fe-Mg}\) when all Fe is assumed to be in the Fe²⁺ state (\({K}_{D}^{{\rm Fe}_T}\)). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO₂ to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.     

The Petrology of Plutonic Blocks and Inclusions from the Lesser Antilles Island Arc

Three main groups of plutonic nodules are present in the LesserAntilles arc and are interpreted as (a) phenocryst clusters,(b) metamorphosed wallrock xenoliths and (c) cumulate texturedxenoliths. Large cumulate blocks display felsic-mafic layering,slump structures and auto-intrusive features. The majority ofspecimens are ad- and heteradcumulates with fewer ortho- andcrescumulates. Interstitial scoria and glass are present ina variety of samples. Plagioclase, amphibole, cino- and orthopyroxene,olivine, magnetite, biotite, ilmenite, quartz and apatite arepresent in various proportions in individual blocks. Plagioclaseand amphibole are modally predominant. Significant variationalong the arc is displayed in the rarity of orthopyroxene andabundance of amphibole in the southern islands compared withthe common presence of two pyroxenes in the northern islands. Plagioclase varies from An_100–36 with very low orthoclasecomponent, and usually precedes amphibole in a given crystallizationsequence. Only on Grenada are plagioclase-free blocks present.Olivine is restricted to assemblages where coexisting plagioclaseis more calcic than An₈₉ and its composition range is Fo_90–59.Clinopyroxene is predominantly calcic augite and cinopyroxene,olivine and plagioclase all coexist stably with amphibole. Ageneral trend of decreasing Ca content in clinopyroxene fromsouth to north in the arc is present. Orthopyroxene ranges from En_73–49 and is most common inassemblages where the coexisting plagioclase is more sodic thanAn₈₃ Coexisting pyroxenes define temperatures in the range 800–1050?C. Amphibole compositions include pargasite, magnesiohastingsite,magnesio-hornblende and tschermakitic hornblende. The K contentof the amphiboles increases from north to south in the samesense as the general tholeiitic-calcalkalic-alkalic variationof parental magmas. Magnetite is the dominant spinel phase but ferrian chromiteand chromian magnetite are present in some Grenada cumulatesand pleonaste is found in rare St. Kitts samples. Ilmenite ispresent in blocks from several islands; coexisting Fe-Ti oxidesdefine a temperature range of 710?-950 ?C at of 15.5 to 10.0 bars. Biotite, quartz and apatiteare restricted to evolve cumulate types. Some modification of interstitial scoria/glass compositionsfrom equilibrium melts has occurred in the majority of samples,but general similarity with erupted lava types is importantevidence for the cognate relationship of the cumulate assemblages.The role of H₂O is crucial in determining the calcic natureof island arc plutonic plagioclase when compared with relativelydry, layered tholeiitic plutons. Some modal and chemical featuresof cumulate-lava comparisons suggest plagioclase flotation maybe significant. A variety of thermodynamic calculations indicate temperaturesand pressures of crystallization in the range 850–1050?C, 4–10 kb. No evidence exists for systematic along-arcvariations in these parameters. Standard amphibole crystallinesolution models give unsatisfactory results for calculations. Some distinctive contrasts between cumulate and phenocryst modesare present. The abundance of amphibole in equilibrium withbasaltic melts in the plutonic situation compared with its rarityin lavas is striking. Plagioclase coexisting with a given meltis more anorthitic in the plutonic than the phenocryst mode. Least squares fractionation tests demonstrate the possibilityof relating basalt-andesite-dacite suites by fractional crystallizationof the cumulus phases. Trace element systematics of cumulate-lavasuites for individual islands also generally support this hypothesis.The suggestion of sole amphibole fractionation for the generationof andesite from basalt is discarded. ^* Present address: CRAE, Box 39598, Darwin, N.T. 5798, Australia.     

Relations between fracture patterns, seismicity and plate motions in the Lesser Antilles

Measurements of orientations of joints that have been made on the Leeward and Windward Islands (Lesser Antilles) from St. Martin to St. Vincent are reported. The preferred orientations of the joints have been determined by a statistical analysis. Assuming the joints to be shearing phenomena, the principal directions of the stress field that caused them, can be calculated. The latter are in very good agreement with the principal stress directions obtained by a statistical analysis of fault plane solutions of earthquakes in the area; the inferred maximum compression direction being NW-SE (ca. N 130°E). This corresponds to a model which assumes that the American plate is subducted in that direction beneath the Caribbean plate.     

Geothermometry and geobarometry of a cummingtonite-bearing dacite from Martinique,Lesser Antilles

A dacite sample from a nuée ardente deposit at Pitons du Carbet, Martinique, contained cummingtonite, orthopyroxene, quartz, magnetite, and ilmenite as phenocrysts, and thus was suitable for evaluation of the conditions of crystallization of nuée ardente materials according to the method of Ewart et al. (1971). The estimations were obtained with the aid of a Fortran IV program, which permitted the operation of all the arrangements between the available microprobe analyses. The results indicated that the phenocrystic association crystallized at moderate temperatures (ca. 750° C), under high oxygen fugacities (> NNO) and total pressures (2.3–4.5 Kb, with a maximal density of points at about 3.5 Kb), and water pressures approximating total pressures. The groundmass Fe-Ti oxides equilibrated at lower temperatures and under relatively increasing oxygen fugacities, the variation of which is tentatively ascribed to the aerial emplacement of the nuée ardente materials. The phenocryst data imply that in the case studied the high fluid pressures characterizing the nuée ardente eruptions were not acquired at a shallow level because of the deep intratelluric stage of crystallization of these differentiated calcalkalic magmas.     

Petrology of the 1979 eruption of Soufriere volcano,St. Vincent,Lesser Antilles

Major element, trace element, and Sr isotope data are used to study the temporal variation in the chemistry of the ejecta from the 1979 eruption of Soufriere volcano, St. Vincent, and to compare the compositions of the 1979 and 1971/2 magmas. Both the 1971/2 and 1979 products were basaltic andesites almost identical in petrography. A small temporal variation in chemistry is apparent in the 1979 samples but these cannot be related to the 1971/2 lava by fractional crystallisation of phenocryst phases, and the two eruptions may therefore have sampled different batches of magma. ⁸⁷Sr/⁸⁶Sr ratios of the two magmas were identical within analytical error.Microprobe analyses of phenocryst phases and glasses from the 1979 ejecta are presented. Clinopyroxene phenocryst cores with very high Mg/Fe ratios indicate that the basaltic andesites are products of fractionation of magnesian parents. Such magmas are represented by lavas on St. Vincent similar to the microphyric alkali picrites found to the south in Grenada. A common origin for the basaltic andesites of both islands by fractional crystallisation of picritic magmas is suggested. Dacitic glass is abundant in the groundmass of scoria blocks from the eruption. It does not represent the liquid originally in equilibrium with the phenocryst phases, but rather this liquid modified by quench crystallisation. Published interpretations suggesting that dacitic glass compositions in tephra from eruptions of the Soufriere are evidence of mixed-magma eruptions are therefore rejected.     

A Petrogenetic Model for the Origin of the Calc-alkaline suite of Grenada, Lesser Antilles

Basanitoid, alkal basalt, andesite and dacite compositions fromGrenada, Lesser Antilles, show a differentation trend from nepheline-normative,through hypersthene-normative to quartz-normative. This unusualsequence, in which the liquids migrate through the low pressurethermal divide of olivine-clinopyroxene-plagioclase from criticallyundersaturated compositions, is interpreted from experimentaland chemical data as the result of crystallization of amphiboleunder a hydrous pressure of several kilobars.     

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles

St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO₂ NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd and SiO₂ rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and ²⁰⁸Pb/²⁰⁴Pb and between ²⁰⁸Pb/²⁰⁴Pband SiO₂ are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by ²³⁸U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of ²²⁶Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis     

Echinoids as hard substrates: varied examples from the Oligocene of Antigua,Lesser Antilles

A collection of unremarkably preserved fossil irregular echinoids from the Upper Oligocene (Chattian) Antigua Formation of Antigua, Lesser Antilles, nonetheless provides evidence of a range of palaeoecological interactions. A dead test of the heart urchin Eupatagus sp. formed a hard substrate for the attachment of gregarious Thecidellina? sp., a thecidoid brachiopod. Although obligate encrusters, these brachiopods more commonly occur as disarticulated valves free of the substrate in the Antillean fossil record. Elongate pits in test fragments were formed, variously, before and after the death of the host echinoids. These depressions on the external surface were formed either by invertebrates excavating domiciles or by claws or teeth; the echinoid later reclaimed the pits and grew new tubercles in the base. Post-mortem pits lack such new tuberculation. A test of Eupatagus sp. bears the boring Oichnus isp., formed either by a predator (gastropod?) or after the death of the echinoid (domicile), and a serpulid worm tube which grew on the test subsequent to the echinoid’s death. The echinoid fauna of the Antigua Formation has been easy to collect and specimens are to be found in many museums; they now await re-examination to reveal palaeosynecological data analogous to that determined from the fragments discussed herein.     

He,Ne, Ar,and C isotope systematics of geothermal emanations in the Lesser Antilles Islands Arc

We present He, Ne, Ar, and C isotope analyses of hydrothermal brines and gases from fumaroles, hot springs, mofettes and hydrothermal exploration drillings on the major islands of the Lesser Antilles Arc. The origin of hydrothermal brines, which have been analyzed also for O and H isotopes, is essentially meteoric-hydrothermal. Air-corrected isotope compositions of helium (2.2 Rc/Ra < ³He/⁴He < 8.6 Rc/Ra) and carbon (−20 < δ¹³C_PDB < +0.5) are variable and require a variety of crustal and magmatic sources. The diversity of δ¹³C_PDB and ³He/CO₂ ratios within individual volcanic centres suggests that crustal sources (e.g., limestone) contaminate magmatic CO₂ en route from high-level magma reservoirs (depth < 15 km) to the surface. A similar contamination may be found for magmatic helium on distal springs. The ³He/⁴He signature of summit fumaroles, thought to reflect the ³He/⁴He signature of high-level magmas, shows a remarkable systematic variation along the arc. In addition, there is a correlation throughout the arc between published Sr, Pb, and Nd isotope signatures of lavas and the ³He/⁴He signatures of summit fumaroles. On the northern islands (Nevis, Montserrat, Guadeloupe, and Dominica) summit fumaroles have the N-MORB signature (³He/⁴He = 8 ± 1 R/Ra), and the isotope signature of lavas is not dissimilar from comparable intra-oceanic arc tholeiites elsewhere. Variable enrichments in radiogenic Sr and Pb have been reported for lavas of individual volcanic centres of the Southern Islands (Martinique, St.Lucia, and Grenada), and summit fumaroles on these centres match these variations by variable radiogenic He-enrichments, i.e., lower ³He/⁴He ratios. This correlation suggests that radiogenic Sr and Pb enrichments of lavas and low ³He/⁴He signatures on summit fumaroles have a common origin, i.e., a terrigenous contaminant derived from the Orinoco depositionary fan. Crustal assimilation is thought to decouple the He isotope system from any other radiogenic isotope system and, therefore, we argue that the observed correlation of He, Sr, Pb, and Nd isotope systems is related to a terrigenous contaminant derived from subducted sediments. Support for this scenario also comes from the matching of low ³He/⁴He ratios and tectonic features of the forearc thought to favor the subduction of forearc sediments.The present study offers a first clue that, under suitable conditions, crustal helium from oceanic sediments might be subducted to the depth of arc magma sources and, possibly, even recycled into the deeper mantle.     

Magma evolution beneath Bequia,Lesser Antilles,deduced from petrology of lavas and plutonic xenoliths

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An?≤?98) and low forsterite (Fo?≤?84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H₂O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H₂O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H₂O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.     

Isotopic composition of strontium in three basalt-andesite centers along the Lesser Antilles arc

Si⁸⁷/Sr⁸⁶ ratios have been determined for lavas and py lastic rocks from three basalt-andesite centers along the Lesser Antilles arc—Mt. Misery on the island of St. Kitts, Soufriere on the island of St. Vincent, and Carriacou, an island of The Grenadines. The average Si⁸⁷/Sr⁸⁶ content of these rocks is 0.7038 for Mt. Misery, 0.7041 for Soufriere, and 0.7053 for Carriacou. All the Sr⁸⁷/Sr⁸⁶ values from each center are the same within analytical uncertainty (±0.0002). The constancy of strontium isotopic data within each center supports the hypothesis that basalts and andesites for each specific center investigated are generated from the same source — in agreement with petrographic and major- and minor-element data. Strontium isotopic compositions and elemental concentrations, particularly of strontium and nickel, indicate that this source was mantle peridotite and that the relationship between the respective basalts and andesites is probably fractional crystallization.Publication authorized by the Director, U.S. Geological Survey.