The Cocos and Carnegie Aseismic Ridges: a Trace Element Record of Long-term Plume-Spreading Center Interaction

The aseismic Cocos and Carnegie Ridges, two prominent bathymetricfeatures in the eastern Pacific, record 20 Myr of interactionbetween the Galápagos hotspot and the adjacent GalápagosSpreading Center. Trace element data determined by inductivelycoupled plasma-mass spectrometry in >90 dredged seamountlavas are used to estimate melt generation conditions and mantlesource compositions along the ridges. Lavas from seamount provinceson the Cocos Ridge are alkalic and more enriched in incompatibletrace elements than any in the Galápagos archipelagotoday. The seamount lavas are effectively modeled as small degreemelts of a Galápagos plume source. Their eruption immediatelyfollows the failure of a rift zone at each seamount province'slocation. Thus the anomalously young alkalic lavas of the CocosRidge, including Cocos Island, are probably caused by post-abandonmentvolcanism following either a ridge jump or rift failure, andnot the direct activity of the Galápagos plume. The seamountshave plume-like signatures because they tap underlying mantlepreviously infused with Galápagos plume material. Whereasplume heterogeneities appear to be long-lived, tectonic rearrangementsof the ridge plate boundary may be the dominant factor in controllingregional eruptive behavior and compositional variations. KEY WORDS: mantle plume; mid-ocean ridge; Galápagos; abandoned rift; partial melting of the mantle     

Petrology and Geochemistry of Volcan Cerro Azul: Petrologic Diversity among the Western Galapagos Volcanoes

Cerro Azul, one of the large shield volcanoes in the westernGalápagos archipelago, has erupted a wide range of tholeiiticto alkalic basalts. These diverse compositions include someof the most primitive yet reported from the western archipelagoand are unlike those of the other, well-studied, neighboringvolcanoes of Sierra Negra and Alcedo, which have erupted basaltof fairly uniform composition. Major- and trace-element modelingshows that Cerro Azul, Alcedo and Sierra Negra share a similardepth of melting and source composition. Modeling also revealsthat there are small, systematic differences in the extent ofpartial melting between the volcanoes that can be related totheir distance from the proposed plume center below the westernmostisland of Fernandina. However, even though melts segregatingfrom the plume in the western Galápagos reflect a narrowrange of temperatures and source compositions, there are widevariations in the enrichments of major and trace elements betweenCerro Azul, Alcedo and Sierra Negra that cannot be attributedto mantle processes. We believe the observed intershield geochemicaldifferences result from magma supply and cooling rates thatare unique to each volcano, and reflect the variations in lithospherictransport and storage processes across the western archipelago. KEY WORDS: basalt; Galápagos; magma supply; mantle plume; ocean island     

Relationships between Mafic and Peralkaline Silicic Magmatism in Continental Rift Settings: a Petrological, Geochemical and Isotopic Study of the Gedemsa Volcano, Central Ethiopian Rift

Petrological and geochemical data are reported for basalts andsilicic peralkaline rocks from the Quaternary Gedemsa volcano,northern Ethiopian rift, with the aim of discussing the petrogenesisof peralkaline magmas and the significance of the Daly Gap occurringat local and regional scales. Incompatible element vs incompatibleelement diagrams display smooth positive trends; the isotoperatios of the silicic rocks (⁸⁷Sr/⁸⁶Sr = 0·70406–0·70719;¹⁴³Nd/¹⁴⁴Nd = 0·51274–0·51279) encompassthose of the mafic rocks. These data suggest a genetic linkbetween rhyolites and basalts, but are not definitive in establishingwhether silicic rocks are related to basalts through fractionalcrystallization or partial melting. Geochemical modelling ofincompatible vs compatible elements excludes the possibilitythat peralkaline rhyolites are generated by melting of basalticrocks, and indicates a derivation by fractional crystallizationplus moderate assimilation of wall rocks (AFC) starting fromtrachytes; the latter have exceedingly low contents of compatibleelements, which precludes a derivation by basalt melting. ContinuousAFC from basalt to rhyolite, with small rates of crustal assimilation,best explains the geochemical data. This process generated azoned magma chamber whose silicic upper part acted as a densityfilter for mafic magmas and was preferentially tapped; maficmagmas, ponding at the bottom, were erupted only during post-calderastages, intensively mingled with silicic melts. The large numberof caldera depressions found in the northern Ethiopian riftand their coincidence with zones of positive gravity anomaliessuggest the occurrence of numerous magma chambers where evolutionaryprocesses generated silicic peralkaline melts starting frommafic parental magmas. This suggests that the petrological andvolcanological model proposed for Gedemsa may have regionalsignificance, thus furnishing an explanation for the large-volumeperalkaline ignimbrites in the Ethiopian rift. KEY WORDS: peralkaline rhyolites; geochemistry; Daly Gap; Gedemsa volcano; Ethiopian rift     

Magma Evolution of the Sete Cidades Volcano, Sao Miguel, Azores

The Sete Cidades volcano (São Miguel, Azores) is situatedat the eastern end of the ultraslow spreading Terceira riftaxis. The volcano comprises several dominantly basaltic pre-calderaeruptions, a trachytic caldera-forming stage and a post-calderastage consisting of alternating trachytic and basaltic eruptions.The post-caldera flank lavas are more primitive (>5 wt %MgO) than the pre-caldera lavas, implying extended fractionalcrystallization and longer crustal residence times for the pre-caldera,shield-building lavas. Thermobarometric estimates show thatthe ascending alkali basaltic magmas stagnated and crystallizedat the crust–mantle boundary (15 km depth), whereas themore evolved magmas mainly fractionated in the upper crust (3km depth). The caldera-forming eruption was triggered by a basalticinjection into a shallow trachytic magma chamber. Lavas fromall stages follow a single, continuous liquid line of descentfrom alkali basalt to trachyte, although slight differencesin incompatible element (e.g. Ba/Nb, La/Nb) and Sr isotope ratiosimply some heterogeneity of the mantle source. Major and traceelement data suggest similar partial melting processes throughoutthe evolution of the volcano. Slight geochemical differencesbetween post- and pre-caldera stage lavas from the Sete Cidadesvolcanic system indicate a variation in the mantle source compositionwith time. The oxygen fugacity increased from the pre-calderato the post-caldera stage lavas, probably as a result of theassimilation of crustal rocks; this is supported by the presenceof crustal xenoliths in the lavas of the flank vents. The lavasfrom the Sete Cidades volcano generally have low Sr isotoperatios; however, rocks from one post-caldera vent on the westernflank indicate mixing with magmas resembling the lavas fromthe neighbouring Agua de Pau volcano, having higher Sr isotoperatios. The different magma sources at Sete Cidades and theadjacent Agua de Pau volcano imply that, despite their closeproximity, there is only limited interaction between them. KEY WORDS: crystallization depth; fractionation; stratigraphy; Terceira rift; volcanic stages     

Evolution of Holocene Dacite and Compositionally Zoned Magma, Volcan San Pedro, Southern Volcanic Zone, Chile

Volcán San Pedro in the Andean Southern Volcanic Zone(SVZ) Chile, comprises Holocene basaltic to dacitic lavas withtrace element and strontium isotope ratios more variable thanthose of most Pleistocene lavas of the underlying Tatara–SanPedro complex. Older Holocene activity built a composite coneof basaltic andesitic and silicic andesitic lavas with traceelement ratios distinct from those of younger lavas. Collapseof the ancestral volcano triggered the Younger Holocene eruptivephase including a sequence of lava flows zoned from high-K calc-alkalinehornblende–biotite dacite to two-pyroxene andesite. Notably,hornblende–phlogopite gabbroic xenoliths in the daciticlava have relatively low ⁸⁷Sr/⁸⁶Sr ratios identical to theirhost, whereas abundant quenched basaltic inclusions are moreradiogenic than any silicic lava. The latest volcanism rebuiltthe modern 3621 m high summit cone from basaltic andesite thatis also more radiogenic than the dacitic lavas. We propose thefollowing model for the zoned magma: (1) generation of hornblende–biotitedacite by dehydration partial melting of phlogopite-bearingrock similar to the gabbroic xenoliths; (2) forceful intrusionof basaltic magma into the dacite, producing quenched basalticinclusions and dispersion of olivine and plagioclase xenocryststhroughout the dacite; (3) cooling and crystallization–differentiationof the basalt to basaltic andesite; (4) mixing of the basalticandesite with dacite to form a small volume of two-pyroxenehybrid andesite. The modern volcano comprises basaltic andesitethat developed independently from the zoned magma reservoir.Evolution of dacitic and andesitic magma during the Holoceneand over the past 350 kyr reflects the intrusion of multiplemafic magmas that on occasion partially melted or assimilatedhydrous gabbro within the shallow crust. The chemical and isotopiczoning of Holocene magma at Volcán San Pedro is paralleledby that of historically erupted magma at neighboring VolcánQuizapu. Consequently, the role of young, unradiogenic hydrousgabbro in generating dacite and contaminating basalt may beunderappreciated in the SVZ. KEY WORDS: Andes; dacite; gabbro; Holocene; strontium isotopes     

Rare Earth Element Inversions and Percolation Models for Hawaii

A detailed study has been made of the evolution with time ofthe mantle source and degree of melting required to producea typical Hawaiian volcano. The preshield stage involves a changefrom a depleted to a primitive mantle source and an increasein the melt fraction. Important intra- and inter-volcano variationsoccur during the shield stage which may be related to melt transportprocesses. With time, Mauna Loa's lavas show depletion in increasinglycompatible elements, a result which is consistent with percolationprocesses. Postshield magmas are generated by very small degreesof melting of a depleted mantle source and may be importantin the development of posterosional volcanism. Posterosionallavas can be generated by variable degrees of melting of anamphibole-bearing source that has been enriched by postshieldmagma. A simple percolation model is consistent with the isotopic evolutionof Hawaiian volcanoes. All of the observed isotopic variationseen between Hawaiian volcanoes may be produced from only twomantle sources by small variations in the percolation processes. ^* Present address: Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK.     

The Geochemistry of Lavas from the Gomores Archipelago, Western Indian Ocean: Petrogenesis and Mantle Source Region Characteristics

New mineral and bulk-rock analyses, as well as Nd, Sr and Pbisotope compositions are presented for lavas from Grande Comore,Moheli and Mayotte, thru of the four main islands of the ComoresArchipelago in the western Indian Ocean, and these data an usedto evaluate the petrogenesis, evolution and mantle source regioncharacteristics of Comorean lavas. The typically silica-undersaturated,alkaline lavas from all three islands can be grouped into twodistinct types: La Grille-type (LGT) lavas, which display strongrelative depletions in K, and Karthala-type (KT) lavas, whichdo not. With the exception of the lavas erupted by La Grillevolcano on Grande Comore, which exhibit the petrographic andgeochemical characteristics expected of primary mantle-derivedmagmas, all Comorean lavas analysed have experienced compositionalmodifications after they segregated from their source regions.Much of this variation can be explained quantitatively by fractionalcrystallization processes dominated by the fractionation ofolivineand clinopyroxene. Semi-quantitative modelling shows that theconsistent and fundamental difference in composition betweenK-depleted LGT lavas and normal KT lavas can be attributed topartial melting processes, provided amphibole is a residualmantle phase after extraction of LGT magmas at low degrees ofmelting. Low absolute abundances of the heavy rare earth elementsin LGT magmas are interpreted to reflect partial melting withinthe garnet stability field In contrast, KT magmas, which donot show relative K depletions, are considered to be the productsof somewhat larger degrees of partial melting of an amphibolefreesource at comparatively shallower depths. Whereas the Nd andSr isotopic compositions of Comorean lavas (which show a significantrange: ⁸⁷Sr/⁸⁶Sr = 0.70319–0.70393; ¹⁴³Nd/Nd = 0.51263–0.51288)bear evidence for a time-averaged depletion in incompatibleelements, the high incompatible element abundances of the lavasare interpreted to reflect the effects of a recent mantle enrichmentevent. At depths well within the garnet stability field thismantle enrichment is interpreted to have taken the form of modalmetasomatism with the introduction of amphibole (giving riseto the source of LGT magmas), whereas cryptic metasomatism tookplace at shallower levels (giving rise to the source of KT lavas).The Nd, Sr and Pb isotope signature of the majority of Comoreanlavas (both LGT and KT) is proposed to be the result of predominant4contributions from a somewhat heterogeneous source4 4 4 presentativeof the ambient sub-Comorean mantle, comprising a mixture betweena HIMU component and a component on the depleted portion ofthe mantle array (possibly the source of Indian Ocean MORB),with only limited contributions from an EM I plume component.The lavas erupted by Karthala volcano (the youngest Comoreanlavas), however, have significantly different isotopic compositionsfrom all other Comorean lavas (lower ¹⁴³Nd/¹⁴⁴Nd and higher⁸⁷Sr/⁸⁶Sr), suggesting increased contributions from the EM Icomponent. KEY WORDS: basalt petrogenesis; Comores; mantle geochemistry; ocean island basalts ^*Telephone: 27-21-6502921. Fax: 27-21-6503781 e-mail: alr{at}geology.uct.ac.za.     

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF), the site of the Valles caldera, lies at the intersectionof the Jemez lineament, a Proterozoic suture, and the CenozoicRio Grande rift. Parental magmas are of two types: K-depletedsilica-undersaturated, derived from the partial melting of lithosphericmantle with residual amphibole, and tholeiitic, derived fromeither asthenospheric or lithospheric mantle. Variability insilica-undersaturated basalts reflects contributions of meltsderived from lherzolitic and pyroxenitic mantle, representingheterogeneous lithosphere associated with the suture. The Kdepletion is inherited by fractionated, crustally contaminatedderivatives (hawaiites and mugearites), leading to distinctiveincompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta)greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth.These compositions dominate the mafic and intermediate lavas,and the JMVF is therefore derived largely, and perhaps entirely,from melting of fertile continental Jemez lineament lithosphereduring rift-related extension. Significant variations in Pband Nd isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17·20–18·93;¹⁴³Nd/¹⁴⁴Nd = 0·51244–0·51272) result fromcrustal contamination, whereas ⁸⁷Sr/⁸⁶Sr is low and relativelyuniform (0·7040–0·7048). We compare theeffects of contamination by low-⁸⁷Sr/⁸⁶Sr crust with assimilationof high-⁸⁷Sr/⁸⁶Sr granitoid by partial melting, with Sr retainedin a feldspathic residue. Both models satisfactorily reproducethe isotopic features of the rocks, but the lack of a measurableEu anomaly in most JMVF mafic lavas is difficult to reconcilewith a major role for residual plagioclase during petrogenesis. KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes     

Petrogenesis of Tertiary Continental Intra-plate Lavas from the Westerwald Region, Germany

Tertiary volcanic rocks from the Westerwald region range frombasanites and alkali basalts to trachytes, whereas lavas fromthe margin of the Vogelsberg volcanic field consist of morealkaline basanites and alkali basalts. Heavy rare earth elementfractionation indicates that the primitive Westerwald magmasprobably represent melts of garnet peridotite. The Vogelsbergmelts formed in the spinel–garnet peridotite transitionregion with residual amphibole for some magmas suggesting meltingof relatively cold mantle. Assimilation of lower-crustal rocksand fractional crystallization altered the composition of lavasfrom the Westerwald and Vogelsberg region significantly. Thecontaminating lower crust beneath the Rhenish Massif has a differentisotopic composition from the lower continental crust beneaththe Hessian Depression and Vogelsberg, implying a compositionalboundary between the two crustal domains. The mantle sourceof the lavas from the Rhenish Massif has higher ²⁰⁶Pb/²⁰⁴Pband ⁸⁷Sr/⁸⁶Sr than the mantle source beneath the Vogelsbergand Hessian Depression. The 30–20 Ma volcanism of theWesterwald apparently had the same mantle source as the QuaternaryEifel lavas, suggesting that the magmas probably formed in apulsing mantle plume with a maximum excess temperature of 100°Cbeneath the Rhenish Massif. The relatively shallow melting ofamphibole-bearing peridotite beneath the Vogelsberg and HessianDepression may indicate an origin from a metasomatized portionof the thermal boundary layer. KEY WORDS: continental rift volcanism; basanites; trachytes; assimilation; fractional crystallization; partial melting     

Chemical versus Temporal Controls on the Evolution of Tholeiitic and Calc-alkaline Magmas at Two Volcanoes in the Alaska-Aleutian Arc

The Alaska–Aleutian island arc is well known for eruptingboth tholeiitic and calc-alkaline magmas. To investigate therelative roles of chemical and temporal controls in generatingthese contrasting liquid lines of descent we have undertakena detailed study of tholeiitic lavas from Akutan volcano inthe oceanic Aleutian arc and calc-alkaline products from Aniakchakvolcano on the continental Alaskan Peninsula. The differencesdo not appear to be linked to parental magma composition. TheAkutan lavas can be explained by closed-system magmatic evolution,whereas curvilinear trace element trends and a large range in⁸⁷Sr/⁸⁶Sr isotope ratios in the Aniakchak data appear to requirethe combined effects of fractional crystallization, assimilationand magma mixing. Both magmatic suites preserve a similar rangein ²²⁶Ra–²³⁰Th disequilibria, which suggests that thetime scale of crustal residence of magmas beneath both thesevolcanoes was similar, and of the order of several thousandyears. This is consistent with numerical estimates of the timescales for crystallization caused by cooling in convecting crustalmagma chambers. During that time interval the tholeiitic Akutanmagmas underwent restricted, closed-system, compositional evolution.In contrast, the calc-alkaline magmas beneath Aniakchak volcanounderwent significant open-system compositional evolution. Combiningthese results with data from other studies we suggest that differentiationis faster in calc-alkaline and potassic magma series than intholeiitic series, owing to a combination of greater extentsof assimilation, magma mixing and cooling. KEY WORDS: uranium-series; Aleutian arc; magma differentiation; time scales     

Assimilation and Fractional Crystallization Controlled by Transport Process of Crustal Melt: Implications from an Alkali Basalt-Dacite Suite from Rishiri Volcano, Japan

Mechanisms of fractional crystallization with simultaneous crustalassimilation (AFC) are examined for the Kutsugata and Tanetomilavas, an alkali basalt–dacite suite erupted sequentiallyfrom Rishiri Volcano, northern Japan. The major element variationswithin the suite can be explained by boundary layer fractionation;that is, mixing of a magma in the main part of the magma bodywith a fractionated interstitial melt transported from the mushyboundary layer at the floor. Systematic variations in SiO₂ correlatewith variations in the Pb, Sr and Nd isotopic compositions ofthe lavas. The geochemical variations of the lavas are explainedby a constant and relatively low ratio of assimilated mass tocrystallized mass (‘r value’). In the magma chamberin which the Kutsugata and Tanetomi magmas evolved, a strongthermal gradient was present and it is suggested that the marginalpart of the reservoir was completely solidified. The assimilantwas transported by crack flow from the partially fused floorcrust to the partially crystallized floor mush zone throughfractures in the solidified margin, formed mainly by thermalstresses resulting from cooling of the solidified margin andheating of the crust. The crustal melt was then mixed with thefractionated interstitial melt in the mushy zone, and the mixedmelt was further transported by compositional convection tothe main magma, causing its geochemical evolution to be characteristicof AFC. The volume flux of the assimilant from the crust tothe magma chamber is suggested to have decreased progressivelywith time (proportional to t^–1/2), and was about 3 x 10^–2m/year at t = 10 years and 1 x 10^–2 m/year at t = 100years. It has been commonly considered that the heat balancebetween magmas and the surrounding crust controls the couplingof assimilation and fractional crystallization processes (i.e.absolute value of r). However, it is inferred from this studythat the ratio of assimilated mass to crystallized mass canbe controlled by the transport process of the assimilant fromthe crust to magma chambers. KEY WORDS: assimilation and fractional crystallization; mass balance model; magma chamber; melt transport; Pb isotope     

Geochemical characteristics of the La Réunion mantle plume source inferred from olivine-hosted melt inclusions from the adventive cones of Piton de la Fournaise volcano (La Réunion Island)

Major and trace element compositions were obtained for bulk rocks and melt inclusions hosted in olivine crystals (Fo > 85) from the adventive cones of the Piton de La Fournaise volcano (La Réunion Island). Ratios of highly incompatible trace elements for these magmas are used to identify the nature of the La Réunion mantle plume source. Although adventive cone lavas display unusual major element compositions compared to the historical lavas of the volcano (e.g., lower CaO/Al₂O₃), trace element data suggest that the magmas emitted by the adventive cones originate from a common chemical source. This source may correspond to either a homogeneous mixed source of different mantle components or a near-primitive less-differentiated mantle source. The melt inclusions display ratios of highly incompatible elements (e.g., Th/La, Nb/La) which are similar to primitive mantle values, and lower Nb/U ratios compared to most oceanic basalts. These results and previous isotopic and trace element data suggest that La Réunion plume samples a source which is intermediate between a primitive-like mantle domain and a slightly depleted one almost unaffected by the recycling processes. This source could have originated from early depletion of the primitive mantle. Assuming a depletion 4.45 Gyr ago, ~10% melting of this slightly depleted source could explain the enriched trace element concentrations of the melt inclusions.     

New volcanological and volatile data provide strong support for the continuous existence of Galápagos Islands over the past 17 million years

The first systematic rock sampling of volcanoes along the Galápagos hotspot tracks (the aseismic Cocos, Carnegie, Malpelo and Coiba ridges and adjacent seamounts) in the area between the Galápagos Islands and Central and South America was carried out on R/V Sonne cruise 144-3. Guyot-shaped seamounts, paleo-beach or intertidal wave-cut platform deposits, the structure and texture of volcanic rocks, and low sulfur contents of fresh glasses dredged at these volcanoes imply that ocean islands existed continuously above the Galápagos hotspot for at least the past 17 million years. These new data significantly extend the time period over which the unique endemic Galápagos fauna could have evolved, providing a complete solution to the long-standing enigma of the evolution of Galápagos land and marine iguanas.     

Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)

In the Northern Volcanic Zone of the Andes, the Cayambe VolcanicComplex consists of: (1) a basal, mostly effusive volcano, theViejo Cayambe, whose lavas (andesites and subordinate dacitesand rhyolites) are typically calc-alkaline; and (2) a younger,essentially dacitic, composite edifice, the Nevado Cayambe,characterized by lavas with adakitic signatures and explosiveeruptive styles. The construction of Viejo Cayambe began >1·1Myr ago and ended at 1·0 Ma. The young and still activeNevado Cayambe grew after a period of quiescence of about 0·6Myr, from 0·4 Ma to Holocene. Its complex history isdivided into at least three large construction phases (Angurealcone, Main Summit cone and Secondary Summit cone) and compriseslarge pyroclastic events, debris avalanches, as well as periodsof dome activity. Geochemical data indicate that fractionalcrystallization and crustal assimilation processes have a limitedrole in the genesis of each suite. On the contrary, field observations,and mineralogical and geochemical data show the increasing importanceof magma mixing during the evolution of the volcanic complex.The adakitic signature of Nevado Cayambe magmas is related topartial melting of a basaltic source, which could be the lowercrust or the subducted slab. However, reliable geophysical andgeochemical evidence indicates that the source of adakitic componentis the subducted slab. Thus, the Viejo Cayambe magmas are inferredto come from a mantle wedge source metasomatized by slab-derivedmelts (adakites), whereas the Nevado Cayambe magmas indicatea greater involvement of adakitic melts in their petrogenesis.This temporal evolution can be related to the presence of thesubducted Carnegie Ridge, modifying the geothermal gradientalong the Wadati–Benioff zone and favouring slab partialmelting. KEY WORDS: adakites; ⁴⁰Ar/³⁹Ar dating; Cayambe volcano; Ecuador; mantle metasomatism; Andes     

Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas

Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (⁸⁷Sr/⁸⁶Sr_initial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr_initial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr_initial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy     

Constraints on the Source Components of Lavas Forming the Hawaiian North Arch and Honolulu Volcanics

Hawaiian volcanoes, dominantly shields of tholeiitic basalt,form as the Pacific Plate migrates over a hotspot in the mantle.As these shields migrate away from the hotspot, highly alkaliclavas, forming the rejuvenated stage of volcanism, may eruptafter an interval of erosion lasting for 0·25–2·5Myr. Alkalic lavas with geochemical characteristics similarto rejuvenated- stage lavas erupted on the sea floor north ofOahu along the Hawaiian Arch. The variable Tb/Yb, Sr/Ce, K/Ce,Rb/La, Ba/La, Ti/Eu and Zr/Sm ratios in lavas forming the NorthArch and the rejuvenated-stage Honolulu Volcanics were controlledduring partial melting by residual garnet, clinopyroxene, Fe–Tioxides and phlogopite. However, the distinctively high Ba/Thand Sr/Nd ratios of lava forming the North Arch and HonoluluVolcanics reflect source characteristics. These characteristicsare also associated with shield tholeiitic basalt; hence theyarise from the Hawaiian hotspot, which is interpreted to bea mantle plume. Inversion of the batch melting equation usingabundances of highly incompatible elements, such as Th and La,requires enriched sources with 10–55% clinopyroxene and5–25% garnet for North Arch lavas. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios in lavas forming the North Arch and Honolulu Volcanicsare consistent with mixing between the Hawaiian plume and adepleted component related to mid-ocean ridge basalts. Specifically,the enrichment of incompatible elements coupled with low ⁸⁷Sr/⁸⁶Srand high ¹⁴³Nd/¹⁴⁴Nd relative to bulk Earth ratios is best explainedby derivation from depleted lithosphere recently metasomatizedby incipient melt (<2% melting) from the Hawaiian plume.In this metasomatized source, the incompatible element abundances,as well as Sr and Nd isotopic ratios, are controlled by incipientmelts. In contrast, the large range of published ¹⁸⁷Os/¹⁸⁸Osdata (0·134–0·176) reflects heterogeneitycaused by various proportions of pyroxenite veins residing ina depleted peridotite matrix. KEY WORDS: Hawaiian plume; Honolulu Volcanics; North Arch; plume–lithosphere interaction; rejuvenated stage; trace element geochemistry; alkalic lavas     

Tertiary Mafic Lavas of Turkana, Kenya: Constraints on East African Plume Structure and the Occurrence of High-{micro} Volcanism in Africa

The East African Rift System is important to understanding plume-initiatedrifting as manifest in the geochemistry of mafic lavas eruptedalong the rift throughout its evolution. We present new datafrom high-MgO Tertiary lavas from Turkana, northern Kenya, toinvestigate regional melt source components, to identify thedepths and degrees of melting, and to characterize spatiallyand temporally the chemical structure of the underlying mantle.The Turkana area is a region of high lithospheric extensionthat sits between two topographic uplifts thought to be surfaceexpressions of one or more upwelling mantle plumes. Thinningof local crust is believed to be accompanied by widespread removalof the mantle lithosphere, causing the asthenosphere to be inclose contact with the overlying crust. New geochemical dataon basanites, picrites and basalts (MgO >7 wt %) tightlyconstrain the primary melt source regions of Tertiary volcanism.Initial isotopic signatures (¹⁴³Nd/¹⁴⁴Nd = 0·51267–0·51283,⁸⁷Sr/⁸⁶Sr = 0·7031–0·7036) and trace elementabundances (Ce/Pb 30, La/Nb = 0·6–0·8 andBa/Nb = 3–10) in these lavas are consistent with derivationfrom sub-lithospheric sources. Basalts and picrites eruptedbetween 23 and 20 Ma have Sr–Nd–Pb–He isotopiccharacteristics indicative of high-µ influence, recordhigh depths and degrees of partial melting, and are associatedwith rift propagation to the north and south. Accordingly, theselavas sample a source region that is geochemically distinctfrom that reflected both in Oligocene Ethiopian flood basaltsand in the modern Afar region. The geochemical data supportnumerical and theoretical models as well as tomographic resultsproviding for a complex thermal structure in the mantle beneathEast Africa and are interpreted to reflect isotopically distinctplume heads beneath Tanzania and Afar that are derived fromthe chemically heterogeneous South African superplume. KEY WORDS: East African Rift System; mantle plumes; HIMU; geochemistry; Afar     

Constraints on the Genesis of Potassium-rich Italian Volcanic Rocks from U/Th Disequilibrium

We present new U-series isotope, ⁸⁷Sr/ ⁸⁶Sr, ¹⁴³Nd/ ¹⁴⁴Nd andtrace element data for a set of mafic, K-rich rocks from volcanoesin Central–Southern Italy. These shoshonitic to ultrapotassiclavas display strongly depleted high field strength element(HSFE) abundances with respect to other incompatible trace elementstogether with high but variable ⁸⁷Sr/ ⁸⁶Sr and low but variable¹⁴³Nd/ ¹⁴⁴Nd values. Such characteristics are thought to bedue to addition of subducted crust of variable amount and compositionto their mantle sources prior to magma genesis. Rocks from thenorthernmost region (i.e. Tuscan Magmatic Province and NorthernRoman Magmatic Province) display (²³⁰Th/ ²³⁸U) activity ratiosclose to radioactive equilibrium, suggesting that metasomatismof their sources occurred before 400 ka and recent melting tookplace at shallow depths, in the absence of garnet. A ²³⁸U excessof up to 27% has been measured in rocks from the NeapolitanDistrict. The occurrence of significant U excesses is a featureof arc magmas, but is typically seen in depleted lavas ratherthan in highly enriched rocks such as these (20 ppm Th). Thissignature requires a recent addition of a U-rich component tothe already strongly enriched mantle wedge beneath this regionof Italy. We suggest that a supercritical liquid, from deeplysubducted carbonate-rich sediments of the still-active Ionianslab, is responsible for generating a high-U, low-Th component,which produces the observed disequilibria. A 30% ²³⁰Th excessmeasured in a single unaltered sample from the Lucanian MagmaticProvince, along with a less marked negative HFSE anomaly, suggeststhe contribution of a deeper, garnet-bearing component in thegenesis of these magmas, plausibly related to the upwellingof asthenospheric mantle around the corner of the Ionian slab. KEY WORDS: U/Th disequilibria; potassic and ultrapotassic rocks; subduction: metasomatism; mantle melting; Central and Southern Italy     

Melt Inclusions in Primitive Olivine Phenocrysts: the Role of Localized Reaction Processes in the Origin of Anomalous Compositions

Melt inclusions are small portions of liquid trapped by growingcrystals during magma evolution. Recent studies of melt inclusionshave revealed a large range of unusual major and trace elementcompositions in phenocrysts from primitive mantle-derived magmaticrocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-Anplagioclase]. Inclusions in phenocrysts crystallized from moreevolved magmas (e.g. olivine Fo < 85 mol %), are usuallycompositionally similar to the host lavas. This paper reviewsthe chemistry of melt inclusions in high-Fo olivine phenocrystsfocusing on those with anomalous major and trace element contentsfrom mid-ocean ridge and subduction-related basalts. We suggestthat a significant portion of the anomalous inclusion compositionsreflects localized, grain-scale dissolution–reaction–mixing(DRM) processes within the magmatic plumbing system. The DRMprocesses occur at the margins of primitive magma bodies, wheremagma is in contact with cooler wall rocks and/or pre-existingsemi-solidified crystal mush zones (depending on the specificenvironment). Injection of hotter, more primitive magma causespartial dissolution (incongruent melting) of the mush-zone phases,which are not in equilibrium with the primitive melt, and mixingof the reaction products with the primitive magma. Localizedrapid crystallization of high-Fo olivines from the primitivemagma may lead to entrapment of numerous large melt inclusions,which record the DRM processes in progress. In some magmaticsuites melt inclusions in primitive phenocrysts may be naturallybiased towards the anomalous compositions. The occurrence ofmelt inclusions with unusual compositions does not necessarilyimply the existence of new geologically significant magma typesand/or melt-generation processes, and caution should be exercisedin their interpretation. KEY WORDS: melt inclusions; olivine; geochemistry; mush zones; MORB; subduction-related magmas     

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K₂O: 0.01 to 0.4 oxide wt%, P₂O₅: <0.01 to 0.2 oxide wt%, La_N: 7 to 35, Yb_N: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K₂O: 0.32 to 0.9 oxide wt %, P₂O₅: 0.02 to 0.35 oxide wt%, La_N: 11 to 60, Yb_N: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998