Setouchi high-Mg andesites revisited: geochemical evidence for melting of subducting sediments

In order to evaluate the mechanism of production of unusual high-Mg andesite (HMA) magmas, Pb–Nd–Sr isotopic compositions were determined for HMAs and basalts from the Miocene Setouchi volcanic belt in the SW Japan arc. The isotopic compositions of Setouchi rocks form mixing lines between local oceanic sediments and Japan Sea backarc basin basalts, suggesting a significant contribution of the subducting sediment component to the HMA magma generation. Mixing calculations using compositions of an inferred original mantle and local oceanic sediments suggest that a sediment-derived melt, neither an H₂O-rich fluid nor an amphibolite/eclogite-derived melt, could have been produced first and served as a plausible metasomatic agent for the HMA magma source. The unusual tectonic setting, including subduction of a newly-borne hence hot plate, may be responsible for melting of subducting sediments.     

Tectonic setting and formation of the Setouchi volcanic rocks in the western Seto Inland Sea,Japan

The Setouchi volcanic rocks include high-Mg andesites (HMAs) and garnet-bearing dacite–rhyolite, and are sporadically distributed along the Median Tectonic Line, Japan. New U–Pb zircon ages and geological and geochemical data are presented for those rocks in the Western Setouchi region (W-Setouchi). Previous studies referred to the altered andesite in the W-Setouchi as “pre-Setouchi volcanic rocks.” However, on the basis of the new U–Pb age (14.4 Ma ± 0.3 Ma) and geochemical characteristics, we redefine it as the Jikamuro Formation, part of the Setouchi volcanic rocks. Incompatible elements are more enriched in the Jikamuro Formation rocks than in the Setouchi HMAs. The characteristic element compositions may be explained by mixing of compositionally different magmas, including subducted sediment melts, plus a contribution from crustal contamination. A stress-inversion technique with Bingham distribution method was applied to the orientations of felsic and mafic dikes within the Setouchi volcanic rocks, and indicates paleo-stress conditions during the period of Setouchi volcanism in the W-Setouchi. The analysis reveals NNW-extensional stresses and a strike-slip stress. We infer that the former represents extensional conditions during the main period of volcanism and the latter represents a stress transition during the most recent period of volcanism (after 12 Ma).     

The deep layers of a Paleozoic arc: geochemistry of the Copley-Balaklala series, northern California

REE, Zr, Nb concentrations and Sr, Nd isotope compositions have been measured in Copley basalts and andesites, Balaklala rhyolites, and Mule Mountain trondhjemites (northern California) which represent the deep layers of a well preserved intra-oceanic island arc of Siluro-Devonian age.⁸⁷Sr/⁸⁶Sr is shifted towards high values (up to 0.707) whereas Ce is preferentially removed from rhyolites. A large proportion of the analyzed samples including some acidic rocks shows a pronounced depletion in light REE. The ε_Nd(T) values of most Copley, Balaklala, and Mule Mountain rocks fall in the range +6 to +8 which suggests that they originated from a normal MORB-type source (ε_Nd(T) ≈ +9) contaminated with either sediments or an OIB-type component.In modern island arcs, only the shallow levels are accessible: comparison with the Copley-Balaklala-Mule Mountain Series suggests that, at depth, an immature island arc is likely to comprise thick layers of LILE-depleted tholeiites and rhyolites intensely altered by pervasive circulation of seawater. Least-square solutions of trace element models suggest that rhyolites and trondhjemites represent remelting of mafic volcanics from the arc basement rather than residual melts of basalt-andesite differentiation.     

Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models

We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better. This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic rhyolites.     

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.     

Origin and evolution of mid-Cretaceous, garnet-bearing, intermediate and silicic volcanics from Canterbury, New Zealand

The Mt Somers Volcanics are part of a suite of mid-Cretaceous (89 ± 2 Ma) intermediate to silicic volcanics, erupted onto an eroded surface of Torlesse sediments. Rock types vary from basaltic andesite to high-silica rhyolite. Andesites are medium- to high-K with phenocrysts of plagioclase, orthopyroxene and pigeonite. Dacites are peraluminous and commonly contain granulite facies xenoliths and garnet xenocrysts. Equilibrium mineral assemblages indicate metamorphic pressures of close to 6 kbar at 800°C. Rhyolites are peraluminous with phenocrysts of quartz, sanidine, plagioclase, biotite, garnet and orthopyroxene. The ferromagnesian phases show textural evidence of magmatic crystallization and are chemically distinct from xenocryst phases in dacites. Equilibrium assemblages indicate that early magmatic crystallization occurred at close to 7 kbar (20 km depth) at above 850°C, with melt-water contents of less than 3.5%. Major-element contents, trace-element contents and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7085 indicate that the rhyolites formed by partial melting of dominantly quartzo-feldspathic Torlesse sediments, leaving a granulite-facies residue. The chemical variation displayed by the rhyolites is best explained by fractional crystallization of the observed high-pressure phenocryst assemblage. Most elements show a compositional gap between rhyolite and dacite. The major-element, trace-element and Sr isotope compositions of the intermediate lavas are best explained by assimilation of lower crustal material combined with fractional crystallization in mantle-derived tholeiitic magmas. Magmatism was the result of heat and magma flux from the mantle, during the change from compressive to extensional tectonics after the culmination of the Rangitata Orogeny.     

Slab melt as metasomatic agent in island arc magma mantle sources, Negros and Batan (Philippines)

Abstract Two new cases of association of adakites with ‘normal’ island arc lavas and transitional adakites are recognized in the islands of Batan and Negros in northern and central Philippines, respectively. The Batan lavas are related to the subduction of the middle Miocene portion of the South China Sea basin along the Manila trench; those of Negros come from the almost aseismic subduction of the middle Miocene Sulu Sea crust along the Negros trench. The occurrence of the Batan adakites is consistent with previous findings showing adakitic glass inclusions within minerals of mantle xenoliths associated with Batan arc lavas. The similarity of adakite ages (1.09 Ma) and that of the metasomatized xenoliths (1 Ma) suggests that both are linked to the same slab‐melting and metasomatic event. Earlier Sr, Pb and Nd‐isotopic studies, however, also reveal the presence of an important sediment contribution to the Batan lava geochemistry. Thus, the role played by slab melts, assumed to have mid‐ocean ridge basalts‐like (MORB) isotopic characteristics, in enriching the Batan subarc mantle is largely masked by the sediment input. The Negros adakites are present only in Mount Cuernos, the volcanic center nearest to the Negros trench. Batch partial melting calculations show that the Negros adakites could be derived from a garnet amphibolitic source with normal‐MORB (N‐MORB) geochemistry. This is supported by the MORB‐like isotopic characteristics of the Mount Cuernos lavas. The volcanic rocks from the other volcanoes consist of normal arc and transitional adakitic lavas that have slightly higher Sr‐ and Pb‐isotopic ratios, probably due to slight sediment input. Mixing of adakites and normal arc lavas to produce transitional adakites is only partly supported by trace element geochemistry and not by field evidence. The transitional adakites can be modeled as partial melts of an adakite‐enriched mantle. Trace element enrichment of non‐adakitic lavas could reflect the interaction of their mantle source with uprising slab melts, as metasomatic mantle minerals scavenge certain trace elements from the adakitic fluids. Therefore, in arcs beneath which thick (up to 2 km) continent‐derived detrital sediments are involved in subduction, like in Batan, the sediment signature can overwhelm the slab melt input. In arcs like Negros where slow subduction could cause a more efficient scraping of thinner (approximately 1 km) detrital sediments, the contribution of slab melts is easier to detect.     

Flux versus decompression melting at stratovolcanoes in southeastern Guatemala

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua–Pacaya and Tecuamburro–Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral–aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)_N ratios of ∼1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting.     

Trace element characteristics of partial melts produced by melting of metabasalts at high pressures: Constraints on the formation condition of adakitic melts

Modern adakite, Archean tonalite-trondhjemite- granodiorite (TTG) and adakitic rocks derived from lower continental crust are high Na and Al felsic rocks and are characterized by strong heavy REE and Y de- pletion and high Sr/Y and La/Yb ratios, which sug…     

Temporal evolution of the Mariana arc during rifting of the Mariana Trough traced through the volcaniclastic record

The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.     

Petrology and isotope-geochemistry of San Vincenzo rhyolites (Tuscany,Italy)

Two groups of rhyolites have been recognized at San Vincenzo (Tuscany, Italy). Group A rhyolites are characterized by plagioclase, quartz, biotite, sanidine and cordierite mineral assemblages. They show constant MgO and variable CaO and Na₂O contents. Initial⁸⁷Sr/⁸⁶Sr ratios in group A samples range between 0.71950 and 0.72535, whereas the Nd isotopic compositions are relatively constant (0.51215–0.51222). Group B rhyolites are characterized by orthopyroxene and clinopyroxene as additional minerals, and show textural, mineralogical and chemical evidence of interaction with more mafic magmas. The Sr and Nd isotopic ratios range between 0.71283–0.71542 and 0.51224–0.51227 respectively. Magmatic inclusions of variable size (1 mm to 10 cm) were found in groups B rhyolites. These inclusions consist mainly of diopsidic clinopyroxene and minor olivine and biotite. They are latitic in composition and represent blobs of hybrid intermediate magmas entrained in the rhyolitic melts. These magmatic inclusions have relatively high Sr contents (996–1529 ppm) and Sr and Nd isotope-ratios of 0.70807–0.70830 and 0.51245–0.51252 respectively.⁸⁷Sr/⁸⁷Sr data on minerals separated from both group A and B rhyolites and magmatic inclusions reveal strong isotopic disequilibria due to the presence of both restitic and newly crystallized phases in group A rhyolites and due to interaction of rhyolites with a mantle-de-rived magma in group B rhyolites. Isotopic data on whole rocks and minerals allow us to interpret the group A rhyolites as representative of different degrees of melting of an isotopically fairly homogeneous pelitic source; conversely, group B rhyolites underwent interactions with a mantle-derived magma. The crustal source as inferred from isotopic systematics would be characterized by⁸⁷Sr/⁸⁶Sr and¹⁴³Nd/¹⁴⁴Nd ratios close to 0.7194 and 0.51216 respectively. The sub-crustal magma would have Sr isotopic composition close to 0.7077 and a¹⁴³Nd/¹⁴⁴Nd ratio greater than or equal to 0.51252. These isotopic features are different from those reported for the parental magmas postulated for Vulsini and Alban Hills in the nearby Roman Magmatic Province, and are similar to those of the Vesuvius and Ischia magmas.     

A hafnium isotope and trace element perspective on melting of the depleted mantle

New Hf isotope and trace element data on mid-ocean ridge basalts (MORB) from the Pacific Ocean basin are remarkably uniform (¹⁷⁶Hf/¹⁷⁷Hf≈0.28313–0.28326) and comparable to previously published data [Salters, Earth Planet. Sci. Lett. 141 (1996) 109–123; Patchett, Lithos 16 (1983) 47–51]. Atlantic MORB have ¹⁷⁶Hf/¹⁷⁷Hf ranging from 0.28302 to 0.28335 confirming the wide range originally identified by Patchett and Tatsumoto [Geophys. Res. Lett. 7 (1980) 1077–1080]. Indian MORB define an even wider range, from 0.28277 to 0.28337, but three exotic samples have very unradiogenic Hf isotope compositions. Their very low ¹⁷⁶Hf/¹⁷⁷Hf ratios, together with their trace element characteristics, require the presence of unusual plume-type material beneath the Indian ridge. All other Indian MORB have uniform Hf isotope compositions at about 0.2832, and define a small field displaced to the right of other MORB in Hf–Nd isotope space. The distinct nature of Indian MORB is best explained by the presence in Indian depleted mantle of old recycled oceanic crust and pelagic sediments. Sm/Hf ratios calculated from new high-precision rare earth element and Hf trace element data do not vary in MORB in the same way as in ocean island basalts (OIB): ratios are constant in OIB, but decrease with increasing Sm contents in MORB. The constancy of Sm/Hf in OIB is probably due to an overwhelming influence of residual garnet during melting. By contrast, the decrease of Sm/Hf in MORB is due to the effect of clinopyroxene in the residue of melting beneath ridges, an interpretation confirmed by quantitative modeling of melting. The relationship between Sm/Nd and Lu/Hf ratios in MORB does not require the presence of garnet in the residual mineralogy. The decoupling of Lu/Hf ratios and Hf isotope compositions – the so-called Hf paradox [Salters and Hart, EOS Trans. Am. Geophys. Union 70 (1989) 510] – can be explained by melting dominantly in the spinel field at shallow depths beneath mid-ocean ridges.     

Origin of the rhyolitic rocks of the taupo volcanic zone, New Zealand

Major element, Rb, Sr, Ba, Cr and V analyses as well as 13 new rare earth element (REE) analyses are presented for the greywacke basement surrounding the Taupo Volcanic Zone (TVZ). On this basis the basement rocks are divided into a Western Basement of approximately andesitic composition ( 62% SiO₂) and an Eastern Basement of approximately granodiorite composition ( 75% SiO₂). These analyses, 5 new REE analyses for the rhyolites, and published data for the volcanic rocks of TVZ are used to investigate the petrogenesis of rhyolitic rocks in the area.Least-squares mixing calculations for major elements show that 88% fractional crystallisation of high-alumina basalt produces a liquid of rhyolitic bulk composition, but Rayleigh fractionation models show that the trace element concentrations of the rhyolites are inconsistent with basalt fractionation. 57% fractionation of the assemblage plagioclase (35.6%), orthopyroxene (9.7%), clinopyroxene (7.8%), ilmenite (0.6%) and magnetite (3.4%) from a plagioclase-pyroxene andesite can produce liquids of rhyolitic bulk composition. REE concentrations produced by this model are consistent with those observed in the rhyolites but predicted Ba and Rb values are lower and V concentrations are higher than those in the rhyolites. Andesite fractionation also produces an unrealistic fractionation of the Cr/V ratio.A non-modal melting model involving 35% melting of a granulitic assemblage (plagioclase + quartz + clinopyroxene + orthopyroxene + biotite + magnetite + cordierite) with a bulk composition equivalent to the Western Basement can reproduce the REE pattern of the rhyolites as well as the concentrations of Rb and Ba. Sr values remain anomalously high, but the Cr/V ratio does not indicate fractionation. Absolute values of Cr and V are within the uncertainties of published crystal—liquid partition coefficients. The rhyolites have relatively flat REE patterns (La/Yb 7.5), as do the greywackes (La/Yb 8.2), so it is therefore unlikely that the rhyolites equilibrated with a garnet or amphibole-bearing assemblage.     

Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust.The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low-¹⁸O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.     

Plume-lithosphere interactions in the ocean basins: constraints from the source mineralogy

Trace element relationships of near-primary alkalic lavas from La Grille volcano, Grande Comore, in the Indian Ocean, as well as those of the Honolulu volcanic series, Oahu, Hawaii, show that their sources contain amphibole and/or phlogopite. Small amounts of each mineral (2% amphibole in the source of La Grille and 0.5% phlogopite plus some amphibole in the source of the Honolulu volcanics) and a range of absolute degrees of partial melting from 1 to 5% for both series are consistent with the observed trace element variation. Amphibole and phlogopite are not stable at the temperatures of convecting upper mantle or upwelling thermal plumes from the deep mantle; however, they are stable at pressure-temperature conditions of the oceanic lithospheric mantle. Therefore, the presence of amphibole and/or phlogopite in the magma source region of volcanics is strong evidence for lithospheric melting, and we conclude that the La Grille and the Honolulu series formed by melting of the oceanic lithospheric mantle.

The identification of amphibole ± phlogopite in the source region of both series implies that the metasomatism by fluids or volatile-rich melts occurred prior to melting. The presence of hydrous phases results in a lower solidus temperature of the lithospheric mantle, which can be reached by conductive heating by the thermal plumes. Isotope ratios of the La Grille and the Honolulu series display a restricted range in composition and represent compositional end-members for each island. Larger isotopic variations in shield lavas, represented by the contemporaneous Karthala volcano on Grande Comore and the older Koolau series on Oahu, reflect interaction of the upwelling thermal plumes with the lithospheric mantle rather than the heterogeneity of deep-seated mantle plume sources or entrainment of mantle material in the rising plume. Literature OsSr isotope ratio covariations constrain the process of plume-lithosphere interaction as occurring through mixing of plume melts and low-degree melts from the metasomatized oceanic lithospheric mantle.

The characterization of the lithospheric mantle signature allows the isotopic composition of the deep mantle plume components to be identified, and mixing relationships show that the Karthala and Koolau plume end-members have nearly uniform isotopic compositions. Based on independent arguments, isotopic variations on Heard and Easter islands have been shown to be a result of mixing between deep plume sources having distinct isotopic compositions with lithosphere or shallow asthenospheric mantle. To the extent that these case studies are representative of oceanic island volcanism, they indicate that interaction with oceanic lithospheric mantle plays an important role in the compositions of lavas erupted during the shield-building stage of plume magmatism, and that isotopic compositions of deep mantle plume sources are nearly uniform on the scale that they are sampled by melting.     

Lower crustal melting via magma underplating: Elemental and Sr–Nd–Pb isotopic constraints from late Mesozoic intermediate–felsic volcanic rocks in the northeastern North China Block

Ar–Ar dating, major and trace element analyses, and Sr–Nd–Pb isotope results of two groups of Lower Cretaceous (erupted at 126 and 119 Ma, respectively) intermediate–felsic lava from the northeastern North China Block (NCB) suggest their derivation from melting of mixtures between the heterogeneous lower crust and underplated basalts. Both groups exhibit high‐K calc‐alkaline to shoshonitic affinities, characterized by light rare earth element (LREE) and large ion lithophile element (LILE) enrichment and variable high field strength element (HFSE, e.g. Nb, Ta and Ti) depletion, and moderately radiogenic Sr and unradiogenic Nd and Pb isotopic compositions. Compared with Group 2, Group 1 rocks have relatively higher K₂O and Al₂O₃/(CaO + K₂O + Na₂O) in molar ratio, higher HFSE concentrations and lower Nb/Ta ratios, and higher Sr–Nd–Pb isotope ratios. Group 1 rocks were derived from a mixture of an enriched mantle‐derived magma and a lower crust that has developed radiogenic Sr and unradiogenic Nd and Pb isotopic compositions, whereas the Group 2 magmas were melts of another mixture between the same mantle‐derived component and another type of lower crust having even lower Sr, Nd, and Pb isotopic ratios. Shift in source region from Group 1 to Group 2 coincided with a change in melting conditions: hydrous melting of both the underplated basalt and the lower crust produced the earlier high‐Nb and low‐Nb/Ta melts with little or no residual Ti‐rich phases; while the younger low‐Nb and high‐Nb/Ta magmas were melted under a water‐deficient system, in which Ti‐rich phases were retained in the source. Generation of the two groups of intermediate–felsic volcanic rocks was genetically linked with the contemporaneous magma underplating event as a result of lithospheric thinning in the eastern NCB.     

Sr and Nd isotope geochemistry of coexisting alkaline magma series,Cantal, Massif Central,France

Sr and Nd isotope analyses are presented for Tertiary continental alkaline volcanics from Cantal, Massif Central, France. The volcanics belong to two main magma series, silica-saturated and silica-undersaturated (with rare nephelinites). Trace element and isotopic data indicate a common source for the basic parental magmas of both major series; the nephelinites in contrast must have been derived from a mantle source which is isotopically and chemically distinct from that which gave rise to the basalts and basanites.⁸⁷Sr/⁸⁶Sr initial ratios range from 0.7034 to 0.7056 in the main magma series (excluding rhyolites) and¹⁴³Nd/¹⁴⁴Nd ratios vary between 0.512927 and 0.512669; both are correlated with increasing SiO₂ in the lavas. The data can be explained by a model of crustal contamination linked with fractional crystallisation. This indicates that crustal magma chambers are the sites of differentiation since only rarely do evolved magmas not show a crustal isotopic signature and conversely basic magmas have primitive isotopic ratios unless they contain obvious crustal-derived xenocrysts. Potential contaminants include lower crustal granulites or partial melts of upper crustal units. Equal amounts of contamination are required for both magma series, refuting hypotheses of selective contamination of the silica-saturated series.The isotopic characteristics of the apparently primary nephelinite lavas demonstrates widespread heterogeneity in the mantle beneath Cantal. Some rhyolites, previously thought to be extremely contaminated or to be crustally derived, are shown to have undergone post-emplacement hydrothermal alteration.     

Comparison of rhyolites from continental rift,continental arc and oceanic island arc: Implication for the mechanism of silicic magma generation

We discuss the chemical compositions of rhyolites from three distinct tectonic settings: (i) the continental rift from Ethiopia (both Oligocene–Miocene and Quaternary rhyolites); (ii) the early Miocene continental arc of Japan (the Mt Wasso rhyolites related to the rifting of the Japan Sea); and (iii) the oceanic Izu–Bonin Island Arc. The comparison reveals that the oceanic island arc rhyolites have high contents of CaO, Al₂O₃, and Sr, and extremely low abundance of trace elements including K₂O. In contrast, the Ethiopian continental rift rhyolites are characterized by low contents of CaO, Al₂O₃, and Sr, and high contents of K₂O, and are enriched in the whole range of trace elements. The continental arc Mt Wasso rhyolites are apparently low in Nb content, although they display similar chemical trends to those of the Ethiopian rhyolites. This obvious difference in the chemical signatures of the rhyolites from the three tectonic settings is the consequence of their derivation from different sources. The implication of this result is that fractional crystallization processes were dominant in the rift‐related rhyolites both from continental rift and continental arc regardless of the prevailing tectonic setting and the nature of the crust (age, thickness, composition), whereas the oceanic island arc rhyolites may form through partial melting of young, mafic crust.     

Ce anomalies in Gough Island lavas — Trace element characteristics of a recycled sediment component

High precision trace element data obtained by inductively coupled plasma mass spectrometry and Sr–Nd isotope analyses are presented for mafic volcanic rocks from Gough Island, South Atlantic. The new data reveal negative Ce anomalies, with Ce/Ce^? values in Gough lavas extending down to ～ 0.92. Ce is only fractionated from other rare earth elements (REE) due to formation of Ce⁴⁺ under oxidizing conditions of near-surface environments while other REE remain trivalent. Ce anomalies in convergent margin magmas have been shown to indicate a contribution of a subducted sediment component. In contrast, Ce anomalies in intra-plate basalts have been attributed to weathering processes, but can be excluded here based on element–element systematics indicating magmatic trends rather than weathering-induced element mobility. Shallow-level contamination by local marine sediments with negative Ce anomaly inherited from seawater can be excluded because Gough lavas with increasingly negative Ce anomalies do not trend towards low Ce/Pb ratios characterizing such sediments. Rather, it is argued that the negative Ce anomalies in Gough Island lavas are consistent with variable amounts of a sediment component in the mantle plume source. Mixtures between estimates of subducting sediment columns with negative Ce anomaly and mantle capable of giving rise to Gough Island magmas without Ce anomalies reproduce the Gough compositional array with the exception of highly fluid-mobile elements. The calculated trace element composition of the deeply recycled sediment in the Gough plume source is depleted in fluid-mobile elements such as Ba and Pb relative to the composition of some present-day subducting sediments. This loss is attributed to the dehydration or flushing of sediment in the subduction factory, consistent with constraints from arc magmas.     

Lacustrine sediments and lichen transplants: two contrasting and complimentary environmental archives of natural and anthropogenic lead in the South Urals, Russia

Lead (Pb) concentrations and isotope ratios of two different geochemical archives are compared; lake sediment cores and lichens (Hypogymnia physodes, naturally growing and transplanted) from a ca. 80 km-long transect centred on the Cu smelter and former mining town of Karabash, Ural Mountains, Russia. Lead concentrations in sediment cores from 10 lakes were generally low near their base and show an abrupt increase in their upper portions interpreted to coincide with the onset of large-scale smelting operations in 1910. Lead isotope ratios derived from ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb of the bottom layers differed significantly from those of the top. The top sediments have isotope ratios that show distinct end members, one of which was the stack dust from the Karabash smelter, which is similar to the Pb derived from ores from Sibay, a major mine in the Urals. The composition of the bottom sediment layers generally fall slightly off a mixing line between the top sediments and average Earth’s upper crust. Lichens transplanted from a reference site, as well as naturally growing lichens, sampled from southwest of the smelter have isotope ratios similar to those of the stack dust. Lichens to the northeast contained Pb from the smelter, but are increasingly influenced by other sources probably leaded petrol and local soils, and a signature derived from a source enriched in ²⁰⁷Pb. Vegetables collected from local kitchen gardens contained Pb from an additional atmospheric source, possibly coal. Our work confirms that: (1) Pb isotopes in lake sediments provide a long-term record of inputs and allows the characterisation of natural and anthropogenic sources; (2) Pb isotopes in lichens provide a short-term record of local and long-range atmospheric deposition at high spatial resolution and short time scales as they replace their Pb content within a few months; (3) determination of all four stable Pb isotopes is necessary for the identification of the sources of Pb and is extremely sensitive for discerning minor source signatures, even in an area with a dominant source such as a smelter. Particularly significant for the Karabash area is that ore-smelter-derived airborne Pb is a major component in the lake sediments and lichens but its contribution reaches insignificant levels ca. 40 km from the smelter.