Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

The Transition to Potassic Alkaline Volcanism in Island Arcs: The Ringgit--Beser Complex, East Java, Indonesia

The origin of potassic lavas with within-plate characteristicsin island are settings is unclear. The volcanic complex of Ringgit—Beser,situated in eastern Java, has erupted lavas of both normal islandare calc-alkaline type and atypical potassic lavas, includingsome highly magnesian lavas. The occurrence of these primitivelavas gives an unusual insight into the source characteristicsof the potassic lavas. The lavas from Ringgit—Beser have a wide range of K₂O(1.1–6.4 wt. %) and MgO contents (18.0–1.6 wt.%).The most magnesian lavas have high Ni and Cr contents. The calc-alkalinelavas have incompatible trace element patterns typical of islandare lavas with enrichments in large ion lithophile elements(LILE) and light rare earth elements (LREE) relative to highfield strength elements (HFSE) and heavy REE (HREE). The potassiclavas may be divided into two series on the basis of Ba andNb contents, with the enriched potassic (EK) series having higherBa and Nb contents for a given MgO content than the potassic(K) series. The EK and K series lavas have some incompatibletrace element ratios similar to within-plate lavas (e.g., highCe/Pb, low LILE/HFSE ratios, and low B/Be). However, both theEK series and K series lavas have negative Ti and Zr anomalies,and the EK series lavas have high Ba/La similar to are lavas.There is little distinction in Sr and Nd isotopes between theK and EK series, but the calc-alkaline lavas have lower ₈₇Sr/₈₆Srand higher ₁₄₃Nd/₁₄₄Nd ratios than the potassic lavas. The EKseries lavas have lower ₂₀₆Pb/₂₀₄Pb and higher ₂₀₈Pb/₂₀₄Pb thanthe K series lavas, but similar ₂₀₇Pb/₂₀₄Pb ratios. The K serieslavas define an almost horizontal trend in ₂₀₇Pb–₂₀₆Pbspace. The Pb isotopic ratios indicate that the EK series lavasare derived from a single mantle source, whereas the K seriesoriginate from a mixture of two mantle components. Calc-alkalinelavas have Pb isotope ratios similar to other calc-alkalineand tholeiitic lavas from Java, and plot on a mixing line betweenIndian Ocean mid-ocean ridge basalt (MORB) and Indian Oceansediment. Incompatible trace element and Pb isotope data for the calc-alkalinelavas indicate that these lavas have a similar source to othercalc-alkaline lavas erupted in Java, namely melts of the IndianOcean MORB mantle fluxed by fluids from the subducted slab.The potassic lavas originate from enriched mantle sources withinthe wedge which have not been affected by recent subductionprocesses. The EK series lavas are derived from a metasomatizedzone which has EMI-type characteristics. The K series lavasare derived from mixing of melts from Christmas Island-type(EMII) mantle and the metasomatized zone. The metasomatizedzone is probably situated at the base of the lithosphere andthe Indian Ocean MORB and Christmas Island-type mantle componentsare situated in the asthenosphere of the wedge. Isotopic datafor Ringgit—Beser lavas confirm that the mantle wedgeof the Sunda arc is extremely heterogeneous (Foden & Varne,1980; Varne, 1985; Wheller et al., 1987). The similarity in geochemistry between Indonesian potassic lavasand those erupted in continental settings indicates that themagma source is essentially the same, namely a metasomatizedphlogopite-rich layer generated by melts of recycled subductedlithosphere. The lack of negative Ti anomalies in the continentalpotassic lavas is ascribed to lower oxidation states in themantle in continental settings.     

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K₂O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; ⁸⁷Sr/⁸⁶Sr ratios decrease and ɛ_Nd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7121 to 0.7105 and ɛ_Nd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, ²⁰⁶Pb/²⁰⁴Pb ∼17.2, ²⁰⁷Pb/²⁰⁴Pb ∼15.5, and ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛ_Nd values and unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios have relatively low ⁸⁷Sr/⁸⁶Sr ratios (the EM I array), whereas basalts with low ɛ_Nd values and high ⁸⁷Sr/⁸⁶Sr ratios have radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios and low ɛ_Nd values and ²⁰⁶Pb/²⁰⁴Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998     

Geochemical variations in Andean basaltic and silicic lavas from the Villarrica-Lanin volcanic chain (39.5° S): an evaluation of source heterogeneity,fractional crystallization and crustal assimilation

At 39.5° S in the southern volcanic zone of the Andes three Pleistocene-recent stratovolcanoes, Villarrica, Quetrupillan and Lanin, form a trend perpendicular to the strike of the Andes, 275 to 325 km from the Peru-Chile trench. Basalts from Villarrica and Lanin are geochemically distinct; the latter have higher incompatible element abundances and La/Sm but lower Ba/La and alkali metal/La ratios. These differences are consistent with our previously proposed models involving: a) a west to east decrease in an alkali metal-rich, high Ba/La slab-derived component which causes an across strike decrease in degree of melting; or b) a west to east increase in the contamination of subduction-related magma by enriched subcontinental lithospheric mantle. Silicic and mafic lavas from the stratovolcanoes have overlapping Sr, Nd and O isotopic ratios. Silicic lavas also have geochemical differences that parallel those of their associated basalts, e.g., rhyolite from Villarrica has lower La/Sm and incompatible element contents than high-SiO₂ andesite from Lanin. At each volcano the most silicic lavas can be modelled by closed system fractional crystallization while andesites are best explained by magma mixing. Apparently crustal contamination was not an important process in deriving the evolved lavas. Basaltic flows from small scoria cones, 20–35 km from Villarrica volcano have high incompatible element contents and low Ba/La, like Lanin basalts, but trend to higher K/Rb (356–855) and lower ⁸⁷Sr/ ⁸⁶Sr (0.70361–0.70400) than basalts from either stratovolcano. However all basalts have similar Nd, Pb and O isotope ratios. The best explanation for the unique features of the cones is that the sources of SVZ magmas, e.g., slab-derived fluids or melts of the subcontinental lithospheric mantle, have varying alkali metal and radiogenic Sr contents. These heterogeneities are not manifested in stratovolcano basalts because of extensive subcrustal pooling and mixing. This model is preferable to one involving crustal contamination because it can account for variable Sr isotope ratios and uniform Nd and Pb isotope ratios among the basalts, and the divergence of the cones from across-strike geochemical trends defined by the stratovolcanoes.     

New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec

O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ¹⁸O does not increase northwards as would be expected from the ~ 4 fold increase in subduction rate if slab-derived fluids had high ¹⁸O/¹⁶O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. ¹⁸⁷Os/¹⁸⁸Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust.     

Sources of primitive alkaline volcanic rocks from the Central European Volcanic Province (Rhön,Germany) inferred from Hf,Os and Pb isotopes

Basanites and nephelinites from the Tertiary Rhön area (Germany), which are part of the Central European Volcanic Province (CEVP), have high MgO, Ni and Cr contents and prominent garnet signatures indicating that they represent near-primary magmas formed by melting of a CO₂-bearing peridotitic mantle source at high pressure. The Pb and Hf isotope (and previously published Nd and Sr isotope) ratios of the Rhön lavas are rather uniform, whereas the Os isotope composition is highly variable. For the most primitive basanites, Pb, Os and Hf isotope compositions fall within the range of enriched MORB and some OIB. Other basanites and nephelinites with low Os concentrations have distinctly more radiogenic Os (¹⁸⁷Os/¹⁸⁸Os: 0.160–0.469) isotope compositions, which are inferred to originate from crustal contamination. The samples with the highest Os concentrations have the lowest Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os_(23 Ma): 0.132–0.135), and likely remain unaffected by crustal contamination. Together with their fairly depleted Sr, Nd and Hf isotope ratios, the isotopic composition of the Rhön lavas suggests derivation from an asthenospheric mantle source. Prominent negative K and Rb anomalies, however, argue for melting amphibole or phlogopite-bearing sources, which can only be stable in the cold lithosphere. We therefore propose that asthenospheric melts precipitated at the asthenosphere-lithosphere thermal boundary as veins in the lithospheric mantle and were remelted or incorporated after only short storage times (about 10–100 million years) by ascending asthenospheric melts. Due to the short residence time incorporation of the vein material imposes the prominent phlogopite/amphibole signature of the Rhön alkaline basalts but does not lead to a shift in the isotopic signatures. Melting of the lithospheric mantle cannot strictly be excluded, but has to be subordinate due to the lack of the respective isotope signatures, in good agreement with the fairly thin lithosphere observed in the Rhön area. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature portions incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhön or many other continental alkaline lavas with similar Pb isotope signatures.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

Petrogenesis of Volcanic Rocks from Saipan and Rota, Mariana Islands, and Implications for the Evolution of Nascent Island Arcs

An ⁴⁰Ar/³⁹Ar age of 45·1 Ma determined for lavas fromnorthern Saipan confirms that these high-silica rhyolites eruptedduring the ‘proto-arc’ stage of volcanism in theIzu–Bonin–Mariana system, which is characterizedelsewhere by eruption of boninitic lavas. Incompatible traceelement concentrations and Sr, Hf, Nd, and Pb isotope ratiosfor these rhyolites are transitional between those of c. 48Ma boninitic lavas and post-38 Ma ‘first-arc’ andesitesand dacites from Saipan and Rota that have typical subduction-relatedcompositions. These transitional compositions are modeled bycrystal fractionation of parental tholeiitic basalt combinedwith assimilation of young boninitic crust. A second stage ofRayleigh fractionation in the upper crust is required by SiO₂concentrations that exceed 77 wt % and near-zero compatibleelement concentrations. First-arc magma compositions are consistentwith fractionation of basalt and assimilation of crust similarin composition to the first-arc magmas themselves. The mantlesources of the proto-arc and first-arc lavas from Saipan andRota are similar to those of Philippine back-arc basin basaltsbased on Nd and Hf isotopic compositions. The Pb isotope compositionsof these lavas are between those of Pacific sea-floor basaltsand Jurassic and younger cherty and clay-rich sediments. Thiscontrasts with the boninitic proto-arc volcanic rocks from Guamand Deep Sea Drilling Project Sites 458 and 459 that have Pbisotope compositions similar to Pacific basin basalts and volcaniclasticsediments. The preferred explanation for the difference in thenature of proto-arc volcanism between Saipan and other fore-arclocations is that the crust ceased extending 3–4 Myr earlierbeneath Saipan. This was caused by a change from mantle upwelling,fore-arc extension, and shallow melting to an environment dominatedby more normal mantle wedge convection, stable crust, and deepermelting. KEY WORDS: rhyolite; andesite; Mariana arc; isotope ratios; trace elements     

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.     

Pillow lavas of the Sierra Leone test site,Mid-Atlantic Ridge, 5°–7°N: Sr-Nd isotope systematics,geochemistry, and petrology

Based on detailed petrological, geochemical, and isotope-geochemical study, fragments of fresh pillow lavas with chilled glass margins dredged at the Sierra-Leone test site in the axial MAR rift zone between 5° and 7°N correspond to MORB tholeiites, which are not primitive mantle melts but were differentiated in intermediate magmatic (intrusive) chambers. Small-scale geochemical and Sr-Nd isotope heterogeneities were established for the first time in the basalts and their glasses. It was shown that some samples show significant nonsystematic differences in the ⁸⁷Sr/⁸⁶Sr ratio between the basalts and their chilled glasses and less significant difference in ?_Nd; higher Sr ratios can be observed both in the glasses and basalts of the same lava fragments. No significant correlation is observed between the isotope characteristics of the samples and their geochemistry; it was also shown that seawater did not affect the Sr and Nd isotope composition of the chilled glasses of the studied pillow lavas. It is suggested that such differences in isotope ratios are related to a small-scale heterogeneity of the melts owing to incomplete homogenization during their rapid ascent to the surface. The heterogeneity of the basaltic melts is explained by their partial contamination by the older plutonic rocks (especially gabbroids) of the lower oceanic crust, through which they ascended to the ocean floor surface. The wider scatter of the Sr isotopic ratios relative to Nd is related to the presence of xenocrysts of calcic plagioclase; correspondingly, the absence of a Nd mineral carrier in the rocks results in less distinct Nd isotope variations. It was shown that all of the studied basalts define a single trend along the mantle correlation array in the Sr-Nd isotope diagram. The causes of this phenomenon remain unclear.     

Historic magmatism on the Reykjanes Peninsula,Iceland: a snap-shot of melt generation at a ridge segment

We present new compositional data on a suite of historic lava flows from the Reykjanes Peninsula, Iceland. They were erupted over a short time period between c. 940 and c. 1340 ad and provide a snap-shot view of melt generation and evolution processes beneath this onshore, 65 km long, ridge segment. The lavas are tholeiitic basalts (MgO 6.5–9.2 wt%) and sparsely (≪5%) olivine and/or plagioclase phyric (±trace clinopyroxene). Individual eruptive events show remarkable compositional homogeneity. Despite a limited variation in Sr–Nd isotope compositions, high-precision double-spike Pb isotope data show tight coherent arrays that, together with correlations with incompatible trace element ratios, indicate control by binary mixing processes. Poor correlations with elemental abundances require that this mixing took place prior to extensive fractional crystallisation. Olivines in the historic lavas have light δ¹⁸O values (+4.2 to +4.3‰), which is likely to be a feature of the enriched mantle source to Reykjanes Peninsula lavas. High precision Pb isotope analyses of other post-glacial Reykjanes Peninsula lavas show significant variability in ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb at lower ²⁰⁶Pb/²⁰⁴Pb values than in the historic lavas. This variation demonstrates that at least three compositionally distinct components within the mantle are required to explain the Pb isotope variations within the Reykjanes Peninsula as a whole. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Oxygen isotope composition of phenocrysts from Tristan da Cunha and Gough Island lavas: variation with fractional crystallization and evidence for assimilation

We have measured the δ¹⁸O values of the major phenocrysts (olivine, clinopyroxene and plagioclase) present in lavas from Tristan da Cunha and Gough Island. These islands, which result from the same mantle plume, have enriched radiogenic isotope ratios and are, therefore, prime candidates for an oxygen isotope signature that is distinct from that of MORB. Consistent differences between the δ¹⁸O values of olivine, pyroxene and feldspar in the Gough lavas show that the phenocrysts in the mafic Gough Island lavas are in oxygen isotope equilibrium. The olivines in lavas with SiO₂ <50 wt% have a mean δ¹⁸O value of 5.19‰, consistent with crystallization from a magma having the same oxygen isotope composition as MORB. Phenocrysts in all the Gough lavas show a systematic increase in δ¹⁸O value as silica content increases, which is consistent with closed-system fractional crystallization. The lack of enrichment in δ¹⁸O of the Gough magmas suggests that the mantle source contained <2% recycled sediment. In contrast, the Tristan lavas with SiO₂ >48 wt% contain phenocrysts which have δ¹⁸O values that are systematically ∼0.3‰ lower than their counterparts from Gough. We suggest that the parental mafic Tristan magmas were contaminated by material from the volcanic edifice that acquired low δ¹⁸O values by interaction with water at high temperatures. The highly porphyritic SiO₂-poor lavas show a negative correlation between olivine δ¹⁸O value and whole-rock silica content rather than the expected positive correlation. The minimum δ¹⁸O value occurs at an SiO₂ content of about 45 wt%. Below 45 wt% SiO₂, magmas evolved via a combination of assimilation, fractionational crystallization and crystal accumulation; above 45 wt% SiO₂, magmas appeared to have evolved via closed-system fractional crystallization. Received: 23 November 1998 / Accepted: 27 September 1999     

The geochemical variations of mid-Cretaceous lavas across western Shandong Province, China and their tectonic implications

Major and trace element as well as Sr–Nd isotopic compositions of mid-Cretaceous lavas across western Shandong Province, China have been studied. These lavas can be generally divided into southern Shandong group (including Pingyi and Mengyin) and northern Shandong group (including Laiwu and Zouping) based on their geochemistry. The southern group lavas are characterized by extreme enrichment in LREE, large ion lithophile elements (LILE), and depletion in HFSE along with EMII-like Sr–Nd isotopic compositions, suggesting that the crustal involvements play a significant role in their petrogenesis. Comparing studies with Fangcheng basalts reveal that the Triassic continent–continent collision between the Yangtze craton (YC) and the North China craton (NCC), and subsequent extensive modification of the sub-continental lithospheric mantle (SCLM) beneath the south part of the NCC by silicic melts released from the subducted Yangtze lower crust, formed an enriched lithospheric mantle which was the source of the southern Shandong group lavas. In contrast, the northern Shandong group lavas are mildly enriched in LREE and LILE relative to those of the southern group lavas. The isotope compositions are also distinctive in that the Sr isotopic ratios are very low. Available geochemical evidence and comparing studies with spatially closed related mafic intrusions suggest that the SCLM feeding the northern group lavas seems to be linked to carbonatitic metasomatism and changed modal proportion of phlogopite and clinopyroxene in the mantle rather than subduction-related modifications. The contrasting geochemical characters of the mid-Cretaceous lavas across western Shangdong suggest that the SCLM of the NCC is spatially heterogeneous in Mesozoic.     

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.     

Distinguishing lower and upper crustal processes in magmas erupted during the buildup to the 7.7 ka climactic eruption of Mount Mazama, Crater Lake, Oregon, using 238U–230Th disequilibria

Uranium-series isotope ratios determined for 35 volcanic rocks and 4 glass separates erupted from ~36 to 4.8 ka at Mt. Mazama, Crater Lake, Oregon, identify both ²³⁰Th-excess and ²³⁸U-excess components. U–Th isotope compositions cover a wide range, exceeding those previously measured for the Cascade arc. Age-corrected (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) activity ratios range from 1.113 to 1.464 and from 0.878 to 1.572 (44.4 % ²³⁰Th-excess to 8.8 % ²³⁸U-excess), respectively. The most distinctive aspect of the data set is the contrast in U–Th isotope ratios between low and high Sr (LSr, HSr) components that have been previously identified in products of the 7.7 ka caldera-forming climactic eruption and preclimactic rhyodacite lavas. The LSr component exclusively contains ²³⁸U-excess, but the HSr component, as well as more primitive lavas, are marked by ²³⁰Th-excess. ²³⁰Th-excesses such as those recorded at Mt. Mazama are commonly observed in the Cascades. Melting models suggest that high ²³⁰Th-excesses observed in the more primitive lavas evolved through mixing of a mantle melt with a partial melt of a mafic lower crustal composition that contained garnet in the residuum that was produced through dehydration melting of amphibolite that was initially garnet free. Dehydration melting in the lower crust offers a solution to the “hot-slab paradox” of the Cascades, where low volatile contents are predicted due to high slab temperatures, yet higher water contents than expected have been documented in erupted lavas. The ²³⁸U-excess observed at Mt. Mazama is rare in Cascade lavas, but occurs in more than half of the samples analyzed in this study. Traditionally, ²³⁸U-excess in arc magmas is interpreted to reflect slab fluid fluxing. Indeed, ²³⁸U-excess in arcs is common and likely masks ²³⁰Th-excess resulting from lower crustal interaction. Isotopic and trace element data, however, suggest a relatively minor role for slab fluid fluxing in the Cascades. We propose that ²³⁸U-excess reflects melting and assimilation of young, hydrothermally altered upper crust. The processes related to generating ²³⁸U-excess are likely important features at Mt. Mazama that accompanied development of a large-scale silicic magma chamber that led to the caldera-forming eruption.     

Rapid magmatic processes accompany arc–continent collision: the Western Bismarck arc, Papua New Guinea

New U–Th–Ra, major and trace element, and Sr–Nd–Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc–continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc–continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated ²⁰⁸Pb/²⁰⁴Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. ²³⁸U and ²²⁶Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia.     

The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curaçao,Netherlands Antilles: a remnant of an oceanic plateau

The island of Curaçao in the southern Caribbean Sea is composed mainly of a thick sequence (>5?km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ?_Nd=+6.1 to +6.6, ⁸⁷Sr/⁸⁶Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ?_Nd=+6.6 to +7.6, ⁸⁷Sr/⁸⁶Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curaçao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ?_Nd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curaçao lava succession. The Curaçao volcanic sequence is part of an oceanic plateau formed at about 88–90?Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (>200^°?C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.     

A Platinum Group Element and Re-Os Isotope Investigation of Siderophile Element Recycling in Subduction Zones: Comparison of Grenada, Lesser Antilles Arc, and the Izu-Bonin Arc

The picritic (MgO >13·5%) lavas of Grenada providea unique opportunity to evaluate the platinum group elements(PGE) and Os isotope compositions of primitive subduction-generatedmelts. Compared with other arc lavas they have undergone verylimited crustal contamination (     

Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios

High precision Sr-Nd isotope ratios together with Pb isotope ratios corrected for mass fractionation using a double spike are reported for an extensive suite of late Quaternary to Recent lavas of Iceland, the Kolbeinsey and Reykjanes Ridges, and a small number of basalts from further south on the Mid-Atlantic Ridge. Compared with global MORB, the Icelandic region is distinguished by having low ²⁰⁷Pb/²⁰⁴Pb for any given ²⁰⁶Pb/²⁰⁴Pb, expressed by negative Δ²⁰⁷Pb (−0.8 to −3.5) in all but four Icelandic samples. Most samples also have elevated ²⁰⁸Pb/²⁰⁴Pb (strongly positive Δ²⁰⁸Pb), which combined with their negative Δ²⁰⁷Pb is very unusual in MORB worldwide. The negative Δ²⁰⁷Pb is interpreted as a consequence of evolution in high-μ mantle sources for the last few hundred Ma. The region of negative Δ²⁰⁷Pb appears to correspond with the region of elevated ³He/⁴He, suggesting that both lithophile and volatile elements in melts from the whole region between 56 and 70°N are dominantly sourced in a plume that has incorporated recycled Palaeozoic ocean crust and unradiogenic He, probably from the deep mantle. At least four mantle components are recognized on Iceland, two with an enriched character, one depleted and one that shows some isotopic affinities to EM1 but is only sampled by highly incompatible-element-depleted lavas in this study. Within restricted areas of Iceland, these components contribute to local intermediate enriched and depleted components that display near binary mixing systematics. The major depleted Icelandic component is clearly distinct in Pb isotopes from worldwide MORB, but resembles the depleted mantle source supplying the bulk of the melt to the Kolbeinsey and southern Reykjanes Ridges. However, an additional depleted mantle source is tapped by the northern Reykjanes Ridge, which with very negative Δ²⁰⁷Pb and less positive Δ²⁰⁸Pb is distinct from all Icelandic compositions. These components must mostly mix at mantle depths because a uniform mixture of three Icelandic components is advected southward along the Reykjanes Ridge.Despite strong covariation with isotope ratios, incompatible trace element ratios of Icelandic magmas cannot be representative of old mantle sources. The observed parent-daughter ratios in depleted and enriched Icelandic lavas would yield homogeneous Sr, Nd, Hf and ²⁰⁶Pb isotope signatures ∼170 Ma ago if present in their sources. The heterogeneity in ²⁰⁷Pb/²⁰⁴Pb is not however significantly reduced at 170 Ma, and the negative present day Δ²⁰⁷Pb cannot be supported by the low μ observed in depleted lavas from Iceland or the adjacent ridges. Since μ is higher in melts than in their sources, it follows that all the depleted sources must be residues from <170 Ma partial melting events. These are thought to have strongly affected most incompatible trace element ratios.