Plume Component Research Articles

Subpicosecond laser ablation of copper and fused silica: Initiation threshold and plasma expansion

We investigated the subpicosecond laser ablation of copper and fused silica under 100 fs laser irradiation at 800 nm in vacuum by means of fast plume imaging and time- and space-resolved optical emission spectroscopy. We found that, to the difference of copper ablation, the laser-generated plasma from a fused silica target exhibited one “main” component only. The “slow” plasma component, observed during copper ablation and usually assigned to optical emission from nanoparticles was not detected by either plasma fast imaging or optical emission spectroscopy even when fused silica targets were submitted to the highest incident fluences used in our experiments. The characteristic expansion velocity of this unique component was about three times larger than the velocity of the fast plume component observed during copper ablation. The dependence of laser fluence on both plasma expansion and ablation rate was investigated and discussed in terms of ablation efficiency and initiation mechanisms.

Rethinking geochemical feature of the Afar and Kenya mantle plumes and geodynamic implications

We discuss the spatial and temporal variation in the geochemistry of mantle sources which were sampled by the Eocene to Quaternary mafic magmas in the vicinity of the Afar and Kenya plume upwelling zones, East Africa. Despite the contributions of lithospheric and crustal sources, carefully screened Eocene to Quaternary mafic lavas display wide range of Sr‐Nd‐Pb isotopic and incompatible trace elemental compositions that can be attributed to significant intraplume heterogeneity. The geochemical variations reflect the involvement of at least four mantle plume components as sources for the northeastern Africa magmatism: (1) isotopically depleted but trace element‐enriched component; (2) component characterized by radiogenic Pb isotope signatures (HIMU?); (3) enriched mantle‐like component; and (4) high‐3He/4He‐type (as HT2‐type basalts) plume component. The first component disappears in the Miocene‐Quaternary magmatism, and the second component is hardly recognized after the eruption of Miocene basalt in southern Ethiopia. Plume‐unrelated depleted asthenosphere starts to involve at a nascent stage of seafloor spreading centers in the Red Sea and Gulf of Aden. The other two‐plume components have persisted from the late Eocene to present, but their proportions have changed through time and space. We propose a model of multiple impingements of plumelets within the broad upwelling zone connected to the African Superplume in the lower mantle beneath southern Africa. The plumelet contains a matrix of high‐3He/4He‐type component with blobs, streaks, or ribbons of other components.

Correlation between plasma expansion and damage threshold by femtosecond laser ablation of fused silica

We experimentally investigated the ultrafast ablation of fused silica under 100 fs laser pulses at 800 nm in vacuum. The ablation plume dynamics was monitored by plasma fast imaging and time- and space-resolved optical emission spectroscopy (OES). We used optical microscopy to determine the ablation threshold and rate in multi-shot operation at 100 Hz.We found that, unlike metal ablation, the plasma generated by femtosecond laser pulses from a fused silica target had only one ‘main’ component. This evidence was supported by the results of OES. The characteristic expansion velocity of this unique component was about one order of magnitude higher than the velocity of the fast plume component observed during metal ablation. We identified in optical emission spectra Si and O neutral atoms only, characterized by very different expansion velocities. In contrast to metal ablation, the ‘slow’ plasma component usually assigned in the literature to optical emission from nanoparticles was not detected by either plasma fast imaging or OES even for the highest laser fluence used in our experiments. The influence of laser fluence on both plasma expansion and ablation rate was investigated. We considered that the ablation mechanism in this case was dominated by thermal material removal processes.

A plume-triggered delamination origin for the Columbia River Basalt Group

The Columbia River Basalt Group reveals a complete and detailed stratigraphic succession to assess the interplay of lithospheric and asthenospheric processes. This record of chemical change through time is used to evaluate genetic models for Columbia River Basalt volcanism. We recognize four primary constraints on source melting: (1) a plume component appears to be the dominant source of Imnaha Basalt; (2) Grande Ronde Basalt is best interpreted as being derived from a mafic pyroxenite or eclogite source; (3) the sequence of source melting must correspond with the stratigraphic record; and (4) working models must explain a step-function chemical change at the Imnaha–Grande Ronde stratigraphic boundary. We can envision only three potential models to satisfy these primary constraints: (1) melting of a mantle plume entrained with eclogite, (2) plume interaction with the Juan de Fuca plate, and (3) delamination triggered by plume emplacement. The first two of these are inconsistent with the time-stratigraphic sequence of melting and cannot satisfy all four primary constraints. In contrast, a model of plume-triggered delamination accurately predicts a progressive sequence of melting that satisfies each of the primary constraints. Such a model is consistent with recent numerical experiments demonstrating that delamination is the expected result of plume emplacement beneath thin Mesozoic lithosphere lying adjacent to a thick cratonic boundary. We test this model by comparing the observed history of uplift and tectonism in eastern Oregon and adjacent Washington to that predicted by the numerical models to reveal consistent stress regimes and strikingly similar topographic and structural profiles.

Plume-related stable isotope compositions and fluid–rock interaction processes in the Basal Complex of La Palma, Canary Islands, Spain

Abstract Chemical and isotopic compositions of amphiboles, biotites, pyroxenes and feldspars from gabbros and basalts of La Palma, Canary Islands, were studied to determine primary, plume-related compositions and effects of late-stage water–rock interactions. All the studied amphiboles have Sr isotope ratios close to those typical for the mantle, excluding the possibility of significant seawater influence. The pyroxenes and amphiboles also have stable isotope compositions that are typical for mantle-derived phases, whereas biotites and feldspars show signs of interaction with meteoric water. On the basis of the oxygen isotopic compositions, the infiltrating meteoric water derived from precipitation at an approximate elevation of 3500 m above sea level, indicating that La Palma reached this height when the gabbro complexes were formed. The unaltered hydrogen and oxygen isotope compositions of amphiboles show a trend from normal mantle ranges to −90‰ and 5.1‰, respectively; these values are very close to compositions found in other Canary Island complexes by earlier studies, and support the theory that these compositions reflect a plume component originating from depth, rather than local phenomena.

Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F 0.5 J/cm2), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences.

Geochemistry and petrogenesis of Cretaceous oceanic plateau lavas in eastern Jamaica

Basaltic lavas of Turonian to Coniacian age belonging to the Bath–Dunrobin Formation occur with intercalated island arc tuffs in the south of the Blue Mountain inlier, have been interpreted as being derived from the Caribbean oceanic plateau. This study presents new major and trace element and Sr–Pb–Nd–Hf isotopic data for these igneous rocks. The Jamaican rocks are altered by tropical weathering, hydrothermal and metamorphic processes, which have mobilised many of their elements (e.g. K and Ba). Consequently, the basalts and dacitic tuffs have been classified by using immobile trace elements. The trace element and ɛ Hf(i)– ɛ Nd(i) geochemistry suggests that the basaltic lavas are derived from a chemically similar source region by variable degrees of partial melting. The Caribbean plateau basalts lie on a mixing line between a depleted plume component and HIMU in Nd–Hf isotopic space. The Pb isotope data also demonstrate that the Jamaican plateau lavas are composed of a larger HIMU component than the other plateau lavas within the Caribbean region. The intercalated island arc tuffs are the first to be found in any oceanic plateau succession in the Caribbean and imply that the Caribbean oceanic plateau at ∼ 90 Ma was relatively close to the subduction zone along South America and the Great Arc of the Antilles.

Magma Genesis and Mantle Dynamics at the Harrat Ash Shamah Volcanic Field (Southern Syria)

A geochemical and petrological study of Miocene to recent alkali basalts, basanites, hawaiites, mugearites, trachytes, and phonolites erupted within the Harrat Ash Shamah volcanic field was performed to reconstruct the magmatic evolution of southern Syria. The major element composition of the investigated lavas is mainly controlled by fractional crystallization of olivine, clinopyroxene, +/- Fe-Ti oxides and +/- apatite; feldspar fractionation is restricted to the most evolved lavas. Na2O and SiO2 variations within uncontaminated, primitive lavas as well as variably fractionated heavy rare earth element ratios suggest a formation by variable degrees of partial melting of different garnet peridotite sources triggered, probably, by changes in mantle temperature. The isotopic range as well as the variable trace element enrichment observed in the lavas imply derivation from both a volatile- and incompatible element-enriched asthenosphere and from a plume component. In addition, some lavas have been affected by crustal contamination. This effect is most prominent in evolved lavas older than 3.5 Ma, which assimilated 30-40% of crustal material. In general, the periodicity of volcanism in conjunction with temporal changes in lava composition and melting regime suggest that the Syrian volcanism was triggered by a pulsing mantle plume located underneath northwestern Arabia.

Trace element constraints on mantle sources during mid-Proterozoic magmatism: evidence for a link between the Gardar (South Greenland) and Abitibi (Canadian Shield) mafic rocks

Trace and major element compositions of mid-Proterozoic (1.20–1.16 Ga) basaltic lava flows and dikes from the Gardar Province (South Greenland) provide evidence for two geochemically distinct magma sources. Based on distinct features of incompatible trace element ratios, such as Th/Ta, Th/Tb, or Th/Hf, they differ by the composition of their mantle source and by their partial melting trends. One mantle source is compositionally transitional between mid-ocean ridge basalt (MORB)-type and ocean-island basalt (OIB)-type sources with relatively low Ta/Hf ratios (~0.2), moderate enrichment in light rare-earth elements (LREE), and slightly positive initial εNdvalues (+2). It can be attributed to either a lithospheric mantle source or a depleted astenospheric mantle plume component that has been enriched shortly prior to eruption. The other mantle source is characterized by high Ta/Hf ratios (~0.6), a more pronounced LREE enrichment, and initial εNdvalues around 0. Elevated CeN/YbN(7.0–9.8) and TbN/YbNratios (1.6–1.8) of the rocks derived from this source indicate the presence of garnet during melting, suggesting melt generation at depths > 70 km. This mantle source has the geochemical characteristics of an OIB-type source and is interpreted as originating from a mantle plume. Samples from the slightly younger (1.14 Ga) Abitibi dike swarm (Superior Province, Canada), spatially connected to the Gardar Province, show very similar trace element characteristics and the same two distinct magma sources can be identified. The geochemical similarities between the magma sources in South Greenland and Canada support the idea of a genetic link between the two magmatic provinces. This link strengthens the idea that the system was a long-lived major intracontinental rift zone.

Investigation of nanoparticle generation during femtosecond laser ablation of metals

The production of nanoparticles via femtosecond laser ablation of gold and copper is investigated experimentally involving measurements of the ablated mass, plasma diagnostics, and analysis of the nanoparticle size distribution. The targets were irradiated under vacuum with a spot of uniform energy distribution. Only a few laser pulses were applied to each irradiation site to make sure that the plume expansion dynamics were not altered by the depth of the laser-produced crater. Under these conditions, the size distribution of nanoparticles does not exhibit a maximum and the particle abundance monotonously decreases with size. Furthermore, the results indicate that two populations of nanoparticles exist within the plume: small clusters that are more abundant in the fast frontal plume component and larger particles that are located mostly at the back. It is shown that the ablation efficiency is strongly related to the presence of nanoparticles in the plume.

Crustal contamination and diversity of magma sources in the northwestern Ethiopian volcanic province

Geochemical and isotopic data are presented for Bure volcanic rocks from the northwestern Ethiopian plateau to investigate the processes involved in the genesis of the erupted magma and the nature of mantle source compositions during plume-induced continental rifting. The Bure area is mainly composed of flood basalts (30-25 Ma), ranging from transitional tholeiite to alkaline basalts, with a probable Quaternary alkaline basalt single lava flow, scoria cones and trachyte plugs. The transitional tholeiites have variable incompatible elements and isotopic ratios (87Sr/86Sr = 0.70329-0.70429, 143Nd/144Nd = 0.51281-0.51304 and 206Pb/204Pb = 18.36-19.08). In contrast, the alkaline basalts display a homogeneous incompatible element and Sr-Nd isotopic ratios (87Sr/86Sr = 0.70311-0.70363 and 143Nd/144Nd = 0.51279-0.51288) with a wide range of Pb isotopes (206Pb/204Pb = 18.91-19.83). Isotope and trace element variations within the Bure lavas reflect the involvement of: 1) an intrinsic isotopically depleted plume component, 2) a plume component having radiogenic Pb isotope and 3) an enriched crustal component. The first and third components played a significant contribution in the initial stage of plume-induced continental flood basalt magmatism.

Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.

Shrimp U–Pb zircon geochronology, geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China

The Cida A-type granitic stock (∼ 4 km 2) and Ailanghe I-type granite batholith (∼ 100 km 2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO ⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites. The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios ( ε Nd( t) = − 0.25 to + 1.35 and ( 87Sr/ 86Sr) i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε Nd( t) ratios (− 6.34 to − 6.26) and high ( 87Sr/ 86Sr) i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.

Plume-in-grid modeling for particulate matter

Three-dimensional grid models are now being commonly used to simulate particulate matter (PM) concentrations, especially fine PM (PM 2.5), which includes a significant fraction of secondary species formed in the atmosphere. These models usually do not address the subgrid-scale effects associated with emissions from large elevated point sources. This can lead to errors in the calculation of PM 2.5 concentrations downwind of these sources because of unrealistic representations of the dispersion and transformation processes that govern PM 2.5 formation. Here, we describe the development, application and evaluation of a plume-in-grid (PiG) model for PM. The model, based on an existing PiG model for ozone, is extended to include in the plume component state-of-the-science treatments of aerosol chemistry and dynamics as well as aqueous chemistry that are consistent with the treatments used in the host grid model. Application of this model to several SO 2 and NO x emitting power plants in the southeastern United States shows that the PiG treatment leads to significant differences in sulfate and total inorganic nitrate concentrations. Comparisons of model results, with and without PiG treatment, against measurements characterizing specific plume events, show that the PiG treatment captures the plume events more often and generally better than the standard grid-based approach.

Noble gases in carbonatites of Sung Valley and Ambadongar: Implications for trapped components

Carbonates, apatites and magnetites from Sung Valley (107 Ma) and carbonates from Ambadongar (65 Ma), two carbonatitic complexes of India, have been analysed for noble gases by vacuum crushing and pyrolysis (apatites only). During vacuum crushing 20Ne/ 22Ne and 129Xe/ 132Xe of up to 12.1 and 1.06 respectively in Sung Valley and up to 10.3 and 1.06 respectively in Ambadongar have been measured. This clearly indicates presence of mantle noble gases. The 3He/ 4He are up to ≈ 9.01 ± 0.85 R A for Sung Valley magnetites (where R A is the atmospheric 3He/ 4He of 1.4 × 10 − 6 ). The 21Ne/ 22Ne lie in the range of 0.031 to 0.069 for Sung Valley and 0.030 to 0.054 for Ambadongar. This is consistent with the presence of a MORB component admixed with crustal noble gases introduced into the lithospheric mantle during subduction. However, the crustal signatures are differently manifested in the carbonates and the late crystallising magnetites from Sung Valley owing to the relative variation of He/Ne ratios between the residual degassed magma (derived from the MORB source) and the vapour phase. As the relative He/Ne ratio of the magma increases progressively, assimilation of the crustal component from the lithosphere strongly affects the Ne, but not the He isotopic composition of the late crystallising magnetites. Ar and Xe compositions are also consistent with MORB contribution. Some dilution of magmatic signatures is clearly observed in the late stage ferrocarbonatites from Ambadongar and magnetites from Sung Valley, probably the effect of hydrothermal fluids bearing atmospheric gases. Unlike the Kola carbonatites which was dominated by plume component, both Sung Valley and Ambadongar have been generated during the waning stages of plume magmatism leading to dilution and replacement of plume signatures by increasing input from MORB and lithospheric components as the uprising plume induces their melting and subsequent entrainment.

Sr–Nd–Pb isotopic compositions of the Kovdor phoscorite–carbonatite complex, Kola Peninsula, NW Russia

The Sr, Nd and Pb isotopic compositions for the Kovdor phoscorite–carbonatite complex (PCC), Kola Peninsula, NW Russia, have been determined to characterize the mantle sources involved and to evaluate the relative contributions of a plume and subcontinental lithospheric mantle in the formation of the complex. The Kovdor PCC is a part of the Kovdor ultramafic–alkaline–carbonatite massif, and consists of six intrusions. The initial isotopic ratios of the analyzed samples, calculated at 380 Ma, display limited variations: ε Nd, + 2.0 to + 4.7; 87Sr/ 86Sr, 0.70319 to 0.70361 ( ε Sr, − 12.2 to − 6.2); 206Pb/ 204Pb, 18.38 to 18.74; 207Pb/ 204Pb, 15.45 to 15.50; 208Pb/ 204Pb, 37.98 to 39.28. The Nd and Sr isotope data of the Kovdor PCC generally fit the patterns of the other phoscorites and carbonatites from the Kola Alkaline Province (KAP), but some data are slightly shifted from the mixing line defined as the Kola Carbonatite Line, having more radiogenic 87Sr/ 86Sr ratios. However, the less radiogenic Nd isotopic compositions and negative Δ7/4 values of Pb isotopes of the analyzed samples exclude crustal contamination, but imply the involvement of a metasomatized lithospheric mantle source. Isotopic variations indicate mixing of at least three distinct mantle components: FOZO-like primitive plume component, EMI-like enriched component and DMM-like depleted component. The isotopic nature of the EMI- and DMM-like mantle component observed in the Kovdor samples is considered to be inherited from metasomatized subcontinental lithospheric mantle. This supports the previous models invoking plume–lithosphere interaction to explain the origin of the Devonian alkaline carbonatite magmatism in the KAP.

Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1

The enriched mantle 1 (EM-1) component in ocean–island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and < 3 Ma basanites and basalts from Tristan da Cunha, Inaccessible and Gough. Additionally, we have analysed Cretaceous mafic–potassic magmas from south-west Africa and eastern South America in order to establish the compositional variation of metasomatised lithospheric mantle that may have been delaminated during Gondwana break-up. The results of our rare-earth-element inversion and Sr-, Nd- and Pb-isotopic mixing models suggest that the ‘depleted’ mantle plume component resembles FOZO and that the composition of the ‘enriched’ mantle component in central South Atlantic OIB has varied both spatially and temporally. At least three different enriched mantle end-members are required to explain the compositional range of 100 to 30 Ma OIB magmas. These resemble the source regions of mafic–potassic magmas from: (i) the Congo craton and Damara belt of south-west Africa; (ii) the São Francisco craton and Brasilia belt of south-east Brazil; and (iii) the Rio Apa–Luis Alves craton of southern Brazil and Paraguay. The most isotopically enriched EM-1 basalts ( ɛNd = − 0.8 to − 4.5), generated on the Walvis Ridge and Rio Grande Rise between 89 and 78 Ma, appear to contain a 10% to 15% contribution from a melt source region with low ɛNd, 206Pb / 204Pb and high [La / Nb] n , similar in composition to metasomatised subcratonic lithospheric mantle beneath southern Brazil and Paraguay. Reconstructions of plate motions indicate that, at the time of continental break-up, this would have been located above the sites of subsequent EM-1 melt generation. Shallow-mantle recycling of metasomatised lithosphere delaminated from south-west Africa and south-east Brazil may explain the further variations in trace-element and isotopic ratios of 100 and 80 to 30 Ma basalts from the Walvis Ridge, respectively. Recent magmas from Tristan, Inaccessible and Gough contain a melt contribution from an enriched mantle source with high 206Pb / 204Pb, 208Pb / 204Pb and 87Sr / 86Sr ratios and low ɛNd, relative to bulk-Earth. This does not resemble the isotopic composition of mafic–potassic magmas from continents adjacent to the South Atlantic and we therefore propose that it is a deep-recycled mantle component. The high Pb-isotopic ratios of recent Tristan plume magmas are inconsistent with a melt contribution from recycled oceanic crust plus a few percent pelagic sediments. We suggest that the deep mantle component is recycled metasomatised lithospheric mantle. Recycled metasomatised lithospheric mantle, of variable composition, only appears to have been an intrinsic part of the sub-oceanic Tristan mantle plume and the bulk-rock composition of present-day Tristan plume related melts are therefore inappropriate for modelling the contribution of plume-derived melts to the Paraná–Etendeka continental flood-basalts. This relationship may also apply to other large igneous provinces and ocean islands along their associated hot spot tracks.

Morphological and geochemical variations along the eastern Galápagos Spreading Center

As the eastern Galápagos Spreading Center (GSC) shallows westward toward the Galápagos Archipelago, axial morphology evolves from a low‐relief, valley‐and‐ridge terrain to an increasingly prominent axial ridge, closely mirroring the western GSC. Between the Inca Transform (∼85.5°W) and its western termination near 91°W, the eastern GSC comprises seven morphological segments, separated by five morphological discontinuities and the eastward propagating 87°W overlapping spreading center. Combined morphologic and geochemical data divide the eastern GSC into two domains independent of the fine‐scale morphologic segmentation. The western domain is defined by its axial ridge morphology and highly variable lava population. Elemental data define steep along‐axis gradients, reflecting a complex source that includes one or more hot spot–related components in addition to a highly depleted component. The eastern domain is defined by transitional, valley‐and‐ridge morphologies and a surprisingly invariant lava population. This population is dominated by shallow crystal fractionation processes and displays significantly less variability attributable to multiple source components. The Galápagos hot spot has long been known to have a symmetrical, long‐wavelength influence on crustal accretion along the GSC. Existing isotopic and new elemental data define twin “geochemical peaks” that we interpret as loci for transfer of distinct source components from the Galápagos plume to the GSC. Although Na8 and Fe8 values lie within the negatively correlated global array, Na8 increases with decreasing axial depth, contrary to global trends and consistent with emerging deep, hydrous melting models that predict decreasing overall extent of melting despite increasing melt production. Support for hydrous melting comes from decreasing heavy REE, increasing La/Sm and La/Yb, and the systematics of decreasing FeO and increasing CaO and Al2O3 with decreasing distance to the hot spot. Overall, an enriched, deep melt component appears to coexist in the shallow mantle with a ubiquitous, depleted primitive melt component, consistent with new models for channelized melt flow connecting a deep hydrous melt regime with the dry shallow mantle. Nevertheless, an absence of low‐Fe lavas suggests that hydrous melting is strictly limited beneath the eastern GSC, becoming dominant only near the western geochemical peak where input from a hydrous “Northern” or “Wolf‐Darwin” plume component is inferred.

Lead isotope composition of Central American volcanoes: Influence of the Galapagos plume

Lead isotopic analyses of lavas from Central America, both along and behind the volcanic arc, help to clarify source components in the mantle wedge. Analysis of previous Pb isotopic data had implied that little or no marine sediment lead was added to the Central American source region, as all samples fell within the MORB field, in contrast to other information (e.g., Ba/La, 10Be, 87Sr/86Sr) that indicated a high subduction component. The data presented here include several analyses of local marine sediment, showing it to be exceptionally unradiogenic in Pb and thus permitting high sediment contributions to the mantle source region without significant changes in Pb isotopes. Combined Pb‐Nd and Pb‐Sr isotopic diagrams clearly illustrate the influence of crustal contamination for samples from Guatemala and Honduras, and of subducted sediment for all lavas of the volcanic front. Samples collected behind the volcanic front are derived from mixing between enriched and depleted mantle sources, and in central Costa Rica (extending to the back arc) overlap Pb, Sr and Nd isotope values for both Cocos Island and some components of the Galapagos hot spot. The restricted geographical occurrence of the enriched mantle signature in Central America, coupled with the persistence of the signal well into the back arc region, imply that these lavas are sampling Galapagos plume‐influenced mantle. The presence of this plume component beneath southern Central America and extending to the northeast beneath the Caribbean confirms a Galapagos hot spot origin for this part of Caribbean Plate.

The CO 2-He-Ar-H 2O systematics of the manus back-arc basin: resolving source composition from degassing and contamination effects

We report new stepped heating He, Ar, CO 2 and water data on a petrogenetically diverse suite of lavas from the Manus back-arc basin, where a plume component has previously been identified. The aim of this study is to evaluate the superimposed effects of degassing and contamination in order to identify mantle source characteristics. CO 2 abundances and carbon isotopes in both the vesicle ([CO 2] up to 180ppm; δ 13C as low as -33.6 ‰) and glass ([CO 2] up to 270ppm; δ 13C as low as -34.3 ‰) phases reveal that samples have been modified by varying degrees of degassing. High water concentration samples (back-arc basin basalts (BABB) and arc type samples) show the highest degrees of degassing (i.e. lower δ 13C values and lower CO 2 contents). The results are modelled for both the glass and vesicle phases using batch and fractional degassing models. Parental melt compositions can be constrained to show the following CO 2 concentration trend: arc-type > BABB s.r. (southern rift) > MORB-2, E-MORB, X-BABB (extreme BABB), BABB > MORB-1 and MORB-smt. 4He/ 40Ar∗ ratios of samples (14.6–1100) are consistent with residual volatiles from a degassed source. Variations in CO 2/ 3He values are likely due to degassing, followed by contamination from a crustal source (either the subducting Solomon Sea Plate or the pre-existing crust through which the lavas erupt), as evidenced by high K 2O/TiO 2 ratios and low δ 13C. The CO 2/ 3He of the Manus plume is best estimated by the MORB-smt and MORB-1 samples at 3.1 ± 0.6 x 10 9. This value is similar to previous estimates of plume CO 2/ 3He values, which are either equal to or slightly greater than the upper mantle average of 2 x 10 9.