Geochronology and evolution of quaternary volcanism at the Keli Highland,Greater Caucasus

The paper reports newly obtained stratigraphic, petrographic, and isotope-geochronological data on modern moderately acid lavas from the Keli Highland at the Greater Caucasus and presents a geological map of the territory, in which 35 volcanoes active in Late Quaternary time were documented by the authors. The total duration of volcanic activity at the highland was estimated at 250 ka. The volcanic activity was discrete and occurred in three phases: Middle Neopleistocene (245−170 ka), Late Neopleistocene (135−70 ka), and Late Neopleistocene-Holocene (<30 ka). Newly obtained lines of evidence indicate that certain volcanoes erupted in the latest Neopleistocene-Holocene. The first phase of volcanic activity was connected mainly with lava volcanoes, and eruptions during the later phases of volcanic activity in this part of the Greater Caucasus produced mainly lavas. The most significant eruptions are demonstrated to occur in the territory during the second phase. The major evolutionary trends of volcanic processes during the final phase in the Keli Highland are determined. It was also determined that the overwhelming majority of volcanoes that were active less than 30 ka B.P. are spatially restricted to long-liven local magmatic zones, which were active during either all three or only the final two phases of activity. These parts of the territory are, perhaps, the most hazardous in terms of volcanic activity.     

Lead Isotope composition and origin of the quaternary lavas of Elbrus Volcano, the Greater Caucasus: High-precision MC-ICP-MS data

A systematic study of Pb isotope composition was carried out for Elbrus Volcano, one of the Europe’s largest volcanoes, using high-precision method of multi-collector inductively coupled plasma mass spectrometry. The measurement error of Pb isotope ratios was estimated from the results of replicate analyses of international BCR-1 and AGV-1 standards as ±0.03% (±2SD). The study of a representative collection showed that dacites of all three phases of the Elbrus eruptive activity are characterized by relatively small-scale variations of Pb isotope composition: ²⁰⁶Pb/²⁰⁴Pb 18.621–18.670, ²⁰⁷Pb/²⁰⁴Pb 15.636–15.659, and ²⁰⁸Pb/²⁰⁴Pb 38.762–38.845. New Pb isotope geochemical characteristics in combination with existing Sr-Nd data indicate that the parental magmas of Elbrus are of mixed mantle-crust origin. They were formed by interaction of mantle-derived melts with continental crust of the Greater Caucasus during continental collision between the Eurasian, Arabian, Turkish, and Iranian plates.     

Isotopic evidence for interaction between Öræfajökull mantle and the Eastern Rift Zone,Iceland

The off-rift central volcano of Öræfajökull has very distinctive EM1-like isotopic compositions compared with other Icelandic lavas. New Pb–Nd–Sr isotopic data from Öræfajökull show strong correlations interpreted as a result of mixing. End-members are a depleted mantle source incorporating 0.5 % subduction-processed sediment and a mantle source with an isotopic signature similar to lavas of the Reykjanes Peninsula. Sr–Nd–Pb isotopic correlations of Icelandic Eastern Rift Zone (ERZ) lavas are almost completely distinct from those of the Reykjanes Peninsula and the Western Rift Zone (WRZ) and require a high-²⁰⁷Pb/²⁰⁴Pb, low-¹⁴³Nd/¹⁴⁴Nd end-member that resembles Öræfajökull compositions, which is very distinct from the enriched end-members suggested for the Reykjanes Peninsula and the WRZ. Given the similar depth and degree of melting at rift zones, variation in the observed enriched end-members between rift zones must indicate spatial variations in enriched mantle sources within the shallow mantle under Iceland rather than purely mixing of melts from a bi-lithological mantle. This is consistent with observations that the ERZ lavas erupted closest to Öræfajökull exhibit the most Öræfajökull-like isotopic compositions, implying that a homogenised Öræfajökull source with positive ?²⁰⁷Pb is focused under the Öræfajökull centre and its associated flank zone. This then mixes laterally with the dominant negative-?²⁰⁷Pb ERZ mantle source. Like Reykjanes Peninsula and WRZ lavas, the ERZ mantle source has strongly negative Δ²⁰⁷Pb and low K/Nb (<170), and these provide evidence for a recycled oceanic crust contribution. The range in ²⁰⁶Pb/²⁰⁴Pb in mantle sources with negative Δ²⁰⁷Pb was probably generated by heterogeneity in ²⁰⁶Pb/²⁰⁴Pb and μ in the recycled oceanic crust, which is the dominant source of incompatible elements in Icelandic lavas.     

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     

Carbonatite Magmatism and Plume Activity: Implications from the Nd, Pb and Sr Isotope Systematics of Oldoinyo Lengai

New Nd (0.51261–0.51268), Pb (²⁰⁶Pb/²⁰⁴Pb: 19.24–19.26),and Sr (0.70437–0.70446) isotopic compositions from tennatrocarbonatite lavas, collected in June 1993 from OldoinyoLengai, the only known active carbonatite volcano, are relativelyuniform, and are similar to data from the 1960 and 1988 flows.Three of the samples contain silicate spheroids, one of whichhas Nd and Sr isotopic ratios similar to host natrocarbonatite,consistent with an origin by liquid immiscibility or the mixingof melts with similar isotopic compositions. Pb isotope datafor two samples of trona are inconsistent with its involvementin the genesis of natrocarbonatite. New Pb isotope data fromsilicate volcanic and plutonic blocks (ijolite, nephelinite,phonolite, syenite) from Oldoinyo Lengai are highly variable(²⁰⁶Pb/²⁰⁴Pb, 17.75–19.34; ²⁰⁷Pb/²⁰⁴Pb, 15.41–15.67;²⁰⁸Pb/²⁰⁴Pb, 37.79–39.67), and define near-linear arraysin Pb-Pb diagrams. The isotopic data for the silicate rocksfrom Oldoinyo Lengai are best explained by invoking discretepartial melting events which generate undersaturated alkalinesilicate magmas with distinct isotopic ratios. Pb isotope ratiosfrom most ijolites and phonolites are predominantly lower andmore variable than from the natrocarbonatites, and are attributedto interaction between silicate melts involving HIMU and EMIsource components and an additional component, such as lower-crustalgranulites, DMM or PREMA (prevalent mantle). Variations in Nd,Pb and Sr isotope ratios from Oldoinyo Lengai, among the largestyet documented from a single volcano, are attributed to mantlesource heterogeneity involving mainly the mixing of HIMU andEMI mantle components. Based on the new isotopic data from OldoinyoLengai and data from other East African carbonatites, and mantlexenoliths, we propose a two-stage model in an attempt to explainthe isotope variations shown by carbonatites in this area. Themodel involves (I) the release of metasomatizing agents withHIMU-like signatures from upwelling mantle (‘plume’)source, which in turn metasomatize the sub-continental (old,isotopically enriched, EMI-like) lithosphere, and (2) variabledegrees and discrete partial melting of the resulting heterogeneous,metasomatized lithosphere. KEY WORDS: carbonatite; isotopes; Oldoinyo Lengai; mantle plumes ^*Telephone: (613) 788–2660, ext. 4419. Fax: (613) 788–4490. e-mail: kbell{at}ccs.carleton.ca     

A lead isotopic study of the Stillwater Complex,Montana: constraints on crustal contamination and source regions

Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater Complex indicate a narrow range of apparent initial isotopic compositions (²⁰⁶Pb/ ²⁰⁴Pb=13.95; ²⁰⁷Pb/²⁰⁴Pb=14.95–15.01; ²⁰⁸Pb/²⁰⁴Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher ²⁰⁷Pb/²⁰⁴Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas. Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values, but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73–2.79 Ga), Late Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about 0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2–26) but often high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were generated at different levels within this crust/mantle system, before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs. This situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the Stillwater magmas. The late Archean crustal complex and the Stillwater Complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the Middle to Early Archean crust present in the region.     

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.     

The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust

ABSTRACT

The nature, magmatic evolution, and geodynamic setting of both inner and outer Makran ophiolites, in SE Iran, are enigmatic. Here, we report mineral chemistry, whole-rock geochemistry, and Sr–Nd–Pb isotope composition of mantle peridotites and igneous rocks from the Eastern Makran Ophiolite (EMO) to assess the origin and tectono-magmatic evolution of the Makran oceanic realm. The EMO includes mantle peridotites (both harzburgites and impregnated lherzolites), isotropic gabbros, diabase dikes, and basaltic to andesitic pillow and massive lava flows. The Late Cretaceous pelagic limestones are found as covers of lava flows and/or interlayers between them. All ophiolite components are somehow sheared and fragmented, probably in Cenozoic time, during the emplacement of ophiolite. This event has produced a considerable extent of tectonic melange. Tectonic slices of trachy-basaltic lavas with oceanic island basalt (OIB)-like signature seal the tectonic melange. Our new geochemical data indicate a magmatic evolution from fore-arc basalt (FAB) to island-arc tholeiite (IAT)-like signatures for the Late Cretaceous EMO lavas. EMO extrusive rocks have high εNd(t) (+8 to +8.9) and isotopically are similar to the Oman lavas. This isotopic signature indicates a depleted mid-ocean ridge basalt (MORB) mantle source for the genesis of these rocks, except isotopic gabbros containing lower εNd(t) (+5.1 to +5.7) and thus show higher contribution of subducted slab components in their mantle source. High ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios for the EMO igneous rocks also suggest considerable involvement of slab-derived components into the mantle source of these rocks. The variable geochemical signatures of the EMO lavas are mostly similar to Zagros and Oman ophiolite magmatic rocks, although the Pb isotopic composition shows similarity to the isotopic characteristic of inner Zagros ophiolite belt. This study postulates that the EMO formed during the early stages of Neo-Tethyan subduction initiation beneath the Lut block in a proto-forearc basin. We suggest subduction initiation caused asthenospheric upwelling and thereafter melting to generate the MORB-like melts. This event left the harzburgitic residues and the MORB-like melts interacted with the surrounding peridotites to generate the impregnated lherzolites, which are quite abundant in the EMO. Therefore, these lherzolites formed due to the refertilization of mantle rocks through porous flows of MORB-like melts. The inception of subduction caused mantle wedge to be enriched slightly by the slab components. Melting of these metasomatized mantle generated isotropic gabbros and basaltic to andesitic lavas with FAB-like signature. At the later stage, higher contribution of the slab-derived components into the overlying mantle wedge causes formation of diabase dikes with supra-subduction zone – or IAT-like signatures. Trachy-basalts were probably the result of late-stage magmatism fed by the melts originated from an OIB source asthenospheric mantle due to slab break-off. This occurred after emplacement of EMO and the formation of tectonic melange.     

Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks,Roman Magmatic Province,Southern Italy

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite-bearing rocks are confined to the pre-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones and domes both within the caldera and on the external flanks of the pre-caldera Roccamonfina volcano. On the basis of new trace element and Sr–Nd–Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the pre-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.70926 to 0.70999, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788–18.851 for ²⁰⁶Pb/²⁰⁴Pb, 15.685–15.701 for ²⁰⁷Pb/²⁰⁴Pb, and 39.048–39.076 for ²⁰⁸Pb/²⁰⁴Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than pre-caldera ultrapotassic leucite-bearing rocks. ⁸⁷Sr/⁸⁶Sr ranges from 0.70665 to 0.70745, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.51234 to 0.51238, ²⁰⁶Pb/²⁰⁴Pb ranges from 18.924 to 19.153, ²⁰⁷Pb/²⁰⁴Pb ranges from 15.661 to 15.694, and ²⁰⁸Pb/²⁰⁴Pb ranges from 39.084 to 39.212. High-K calc-alkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced pre-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post-collisional isotherms relaxation or increasing T°C due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10–20% of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Petrogenesis of the Neogene volcanic units in the NE–SW-trending basins in western Anatolia,Turkey

The western Anatolian volcanic province formed during Eocene to Recent times is one of the major volcanic belts in the Aegean–western Anatolian region. We present new chemical (whole-rock major and trace elements, and Sr, Nd, Pb and O isotopes) and new Ar/Ar age data from the Miocene volcanic rocks in the NE–SW-trending Neogene basins that formed on the northern part of the Menderes Massif during its exhumation as a core complex. The early-middle Miocene volcanic rocks are classified as high-K calc-alkaline (HKVR), shoshonitic (SHVR) and ultrapotassic (UKVR), with the Late Miocene basalts being transitional between the early-middle Miocene volcanics and the Na-alkaline Quaternary Kula volcanics (QKV). The early-middle Miocene volcanic rocks are strongly enriched in large ion lithophile elements (LILE), have high ⁸⁷Sr/⁸⁶Sr_(i) (0.70631–0.71001), low ¹⁴³Nd/¹⁴⁴Nd_(i) (0.512145–0.512488) and high Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 18.838–19.148; ²⁰⁷Pb/²⁰⁴Pb = 15.672–15.725; ²⁰⁸Pb/²⁰⁴Pb = 38.904–39.172). The high field strength element (HFSE) ratios of the most primitive early-middle Miocene volcanic rocks indicate that they were derived from a mantle source with a primitive mantle (PM)-like composition. The HFSE ratios of the late Miocene basalts and QKV, on the other hand, indicate an OIB-like mantle origin—a hypothesis that is supported by their trace element patterns and isotopic compositions. The HFSE ratios of the early-middle Miocene volcanic rocks also indicate that their mantle source was distinct from those of the Eocene volcanic rocks located further north, and of the other volcanic provinces in the region. The mantle source of the SHVR and UKVR was influenced by (1) trace element and isotopic enrichment by subduction-related metasomatic events and (2) trace element enrichment by “multi-stage melting and melt percolation” processes in the lithospheric mantle. The contemporaneous SHVR and UKVR show little effect of upper crustal contamination. Trace element ratios of the HKVR indicate that they were derived mainly from lower continental crustal melts which then mixed with mantle-derived lavas (~20–40%). The HKVR then underwent differentiation from andesites to rhyolites via nearly pure fractional crystallization processes in the upper crust, such that have undergone a two-stage petrogenetic evolution.     

Geochemical variations in lavas from Kahoolawe volcano,Hawaii: evidence for open system evolution of plume-derived magmas

The Kahoolawe shield volcano produced precaldera and caldera-filling tholeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield volcanoes. However, Kahoolawe tholeiites display wide ranges in incompatible trace element ratios (e.g., Nb/Th=9–24, Th/Ta=0.6–1.3), ⁸⁷Sr/⁸⁶Sr (0.70379–0.70440), ¹⁴³Nd/¹⁴⁴Nd (0.51273–0.51298), and ²⁰⁶Pb/²⁰⁴Pb (17.92–18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaiian tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas. Ratios of highly incompatible trace elements and Sr, Nd, and Pb isotopic ratios from coherent sub-trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (23.5) ascending plume, possibly comprising ancient subducted oceanic lithosphere. Slightly evolved tholeiites experienced combined crystal fractionation and assimilation (AFC) of material derived from a distinct reservoir (Nb/Th 9) of asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocean ridge basalt-like isotopic ratios but enriched OIB-like trace element ratios, representing part of the lithospheric mantle (or melts thereof). Periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in the suboeanic mantle, and the processes by which Hawaiian tholeiites form and evolve.     

Origin of the absarokite–banakite association of the Damavand volcano (Iran): trace elements and Sr,Nd, Pb isotope constraints

The activity of the Damavand volcano (Central Alborz, northern Iran) began 1.8 Ma ago and continued up to 7 ka BP. Although the volcanic suite is clearly of shoshonitic affinity, only two petrographic types can be distinguished in the studied lavas: (1) weakly differentiated absarokites (49 < %SiO₂ < 51), scattered around the volcano but with a regional extension, (2) highly differentiated banakites (59 < %SiO₂ < 63), which form the bulk of the 4,000 m thick volcanic pile. All lavas are alkalic (3.7 < %K₂O < 5), REE and LILE-rich (e.g., 85 < La < 148 ppm; 9 < Th < 32 ppm) and show highly fractionated REE patterns (69 < La/Yb < 115) and pronounced Nb–Ta negative anomalies. The absarokites are characterised by Sr (0.7045–0.7046) and Nd (0.51266–0.51269) isotope compositions close to the Bulk Earth values, and distinct from those of the banakites (0.7047 < ⁸⁷Sr/⁸⁶Sr < 0.7049, 0.51258 < ¹⁴³Nd/¹⁴⁴Nd < 0.51262). The Pb isotope ratios are also slightly lower in the absarokites than in the banakites (18.71 < ²⁰⁶Pb/²⁰⁴Pb < 18.77, 15.62 < ²⁰⁷Pb/²⁰⁴Pb < 15.63, 38.85 < ²⁰⁸Pb/²⁰⁴Pb < 38.91, and 18.77 < ²⁰⁶Pb/²⁰⁴Pb < 18.84, 15.62 < ²⁰⁷Pb/²⁰⁴Pb < 15.64, 38.94 < ²⁰⁸Pb/²⁰⁴Pb < 39.06, respectively). Overall, there is a clear tendency towards higher Sr, Pb and lower Nd isotope ratios with increasing degree of differentiation. This study suggests that the absarokites result from a low degree of partial melting (∼5%) of a highly metasomatized mantle source, which inherited its characteristics from an old subduction setting. The initiation of volcanic activity 1.8 Ma ago results from variations in the lithospheric thermal regime, probably related to lithospheric delamination as proposed for Anatolia (Pearce et al. 1990). The banakites are mainly generated by extensive fractional crystallisation (∼70%) of the absarokitic magma, with a limited amount (a few percents) of assimilation of an old crustal component, in the form of bulk assimilation or AFC processes, which both can explain the Sr, Nd and Pb isotope data.     

Signatures of the source for the Emeishan flood basalts in the Ertan area： Pb isotope evidence

The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.     

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.     

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.     

Source components of the Gran Canaria (Canary Islands) shield stage magmas: evidence from olivine composition and Sr–Nd–Pb isotopes

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (⁸⁷Sr/⁸⁶Sr = 0.70315–0.70331, ¹⁴³Nd/¹⁴⁴Nd = 0.51288–0.51292, ²⁰⁶Pb/²⁰⁴Pb = 19.55–19.93, ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.63, ²⁰⁸Pb/²⁰⁴Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with ⁸⁷Sr/⁸⁶Sr = 0.70337, ¹⁴³Nd/¹⁴⁴Nd = 0.51291, ²⁰⁶Pb/²⁰⁴Pb = 19.36, ²⁰⁷Pb/²⁰⁴Pb = 15.61 and ²⁰⁸Pb/²⁰⁴Pb = 39.07 (EM-type end member), and pyroxenite with ⁸⁷Sr/⁸⁶Sr = 0.70309, ¹⁴³Nd/¹⁴⁴Nd = 0.51289, ²⁰⁶Pb/²⁰⁴Pb = 20.03, ²⁰⁷Pb/²⁰⁴Pb = 15.62 and ²⁰⁸Pb/²⁰⁴Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.     

Magma genesis and crustal contamination of continental intraplate lavas in northwestern Syria

The Miocene to Quaternary lavas of northwestern Syria range from basanite, alkali basalts, and tholeiites to basaltic andesites, hawaiites, and mugearites. Crustal assimilation and fractional crystallization processes (AFC) modified the composition of the mantle derived magmas. Crustal assimilation is indicated by decreasing Nb/U (52.8–17.9) and increasing Pb/Nd (0.09–0.21) and by variable isotopic compositions of the lavas (⁸⁷Sr/⁸⁶Sr: 0.7036–0.7048, ¹⁴³Nd/¹⁴⁴Nd: 0.51294–0.51269, ²⁰⁶Pb/²⁰⁴Pb: 18.98–18.60) throughout the differentiation. Modeling of the AFC processes indicates that the magmas have assimilated up to 25% of continental upper crust. The stratigraphy of the lavas reveals decreasing degrees and increasing depths of melting with time and the strongly fractionated heavy rare earth elements indicate melt generation in the garnet stability field. Modeling of melt formation based on trace element contents suggests that 8–10% melting of the asthenospheric mantle source produced the tholeiites, whereas basanite and alkali basalts are formed by 2–4% melting of a similar source.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Major events in evolution of the Kazbek neovolcanic center,Greater Caucasus: Isotope-geochronological data

The products of the activity of the Late Quaternary Kazbek neovolcanic center in the Greater Caucasus are studied by isotopic-geochronological methods. It is found that the youngest magmatism evolved during the last 400–450 k.y. over four discrete phases: 395–435, 200–250, 90–120, and less than 50 ka. The petrological-geochemical and published isotopic data point to the mixed mantle-crustal origin of the Kazbek lavas with the leading role of crystallization differentiation of deep magmas and assimilation of the crustal material. We recorded two episodes (～100 and less than 50 ka) of replenishment of the subsurface magmatic chamber under the Kazbek center by the main mantle melt and its mixing with the relict dacite magma that led to the formation of highly mobile hybrid andesite lavas and served as a trigger of the renewal of volcanic activity. Reactivation of the mantle source of the Kazbek center at the end of the Neopleistocene and the Holocene age of the last eruptions indicate the potential danger of this region because of the renewal of the volcanic activity. The medium Devdoraki copper deposit is located in the vicinity of the Kazbek volcano. It represents a unique polychronous, currently evolved ore-magmatic system that originated in the Jurassic.     

Petrology and geochemistry of Camiguin Island, southern Philippines: insights to the source of adakites and other lavas in a complex arc setting

Camiguin is a small volcanic island located 12 km north of Mindanao Island in southern Philippines. The island consists of four volcanic centers which have erupted basaltic to rhyolitic calcalkaline lavas during the last ∼400 ka. Major element, trace element and Sr, Nd and Pb isotopic data indicate that the volcanic centers have produced a single lava series from a common mantle source. Modeling results indicate that Camiguin lavas were produced by periodic injection of a parental magma into shallow magma chambers allowing assimilation and fractional crystallization (AFC) processes to take place. The chemical and isotopic composition of Camiguin lavas bears strong resemblance to the majority of lavas from the central Mindanao volcanic field confirming that Camiguin is an extension of the tectonically complex Central Mindanao Arc (CMA). The most likely source of Camiguin and most CMA magmas is the mantle wedge metasomatized by fluids dehydrated from a subducted slab. Some Camiguin high-silica lavas are similar to high-silica lavas from Mindanao, which have been identified as “adakites” derived from direct melting of a subducted basaltic crust. More detailed comparison of Camiguin and Mindanao adakites with silicic slab-derived melts and magnesian andesites from the western Aleutians, southernmost Chile and Batan Island in northern Philippines indicates that the Mindanao adakites are not pure slab melts. Rather, the CMA adakites are similar to Camiguin high-silica lavas which are products of an AFC process and have negligible connection to melting of subducted basaltic crust. Received: 27 February 1998 / Accepted: 27 August 1998     

Geochemistry of lamprophyres from the Western Alps,Italy: implications for the origin of an enriched isotopic component in the Italian mantle

Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected ⁸⁷Sr/⁸⁶Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: ²⁰⁶Pb/²⁰⁴Pb = 18.669–18.895, ²⁰⁷Pb/²⁰⁴Pb = 15.605–15.689 and ²⁰⁸Pb/²⁰⁴Pb = 38.224–39.134. ⁸⁷Sr/⁸⁶Sr ratios show a negative correlation with ¹⁴³Nd/¹⁴⁴Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.