The Amsterdam-St. Paul volcanic province,and the formation of low Al tholeiitic andesites

The island of St. Paul in the Indian Ocean is located on the axis of the southeastern branch of the Indian Ocean Rise, as is the similar volcano of Amsterdam, 80 km to the north. Both volcanoes and possibly part of the local ridge are formed of a high-alumina plagioclase tholeiite making this a distinctive volcanic province. Lavas with Al₂O₂ contents of 12 to 22% are directly related by plagioclase fractionation. Residual rocks are a distinctive low silica, low alumina, potassic andesite with andesine and even olligoclase feldspar. Parallel lines of evolution exist in the sea floor plagioclase tholeiites, which differ only in slightly lower tenor of Sr and potassium group elements, and in the Steens Mountain tholeiites of Oregon, which differ in their significantly higher Sr and K group elements     

Cambrian–early Ordovician volcanism across the South Armorican and Occitan domains of the Variscan Belt in France: Continental break-up and rifting of the northern Gondwana margin

The Cambrian–lower Ordovician volcanic units of the South Armorican and Occitan domains are analysed in a tectonostratigraphic survey of the French Variscan Belt. The South Armorican lavas consist of continental tholeiites in middle Cambrian–Furongian sequences related to continental break-up. A significant volcanic activity occurred in the Tremadocian, dominated by crustal melted rhyolitic lavas and initial rifting tholeiites. The Occitan lavas are distributed into five volcanic phases: (1) basal Cambrian rhyolites, (2) upper lower Cambrian Mg-rich tholeiites close to N-MORBs but crustal contaminated, (3) upper lower–middle Cambrian continental tholeiites, (4) Tremadocian rhyolites, and (5) upper lower Ordovician initial rift tholeiites. A rifting event linked to asthenosphere upwelling took place in the late early Cambrian but did not evolve. It renewed in the Tremadocian with abundant crustal melting due to underplating of mixed asthenospheric and lithospheric magmas. This main tectono-magmatic continental rift is termed the “Tremadocian Tectonic Belt” underlined by a chain of rhyolitic volcanoes from Occitan and South Armorican domains to Central Iberia. It evolved with the setting of syn-rift coarse siliciclastic deposits overlain by post-rift deep water shales in a suite of sedimentary basins that forecasted the South Armorican–Medio-European Ocean as a part of the Palaeotethys Ocean.     

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.     

Geochemistry of tholeiites from Lanai,Hawaii

Lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the Hawaiian hotspot. This volcano apparently became extinct during the shield-building stage of volcanism, as shown by the absence of both alkalic cap and post-erosional lavas. Major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that Lanai tholeiites span a large range in composition. Some Lanai lavas are unique geochemically among Hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any Hawaiian lavas and well-developed positive Eu anomalies. The geochemical characteristics of these low-abundance Lanai tholeiites are not the result of alteration, differences in mantle source modal mineralogy, the presence of residual accessory mantle phases or fractional crystallization of such phases, assimilation of depleted [MORB] wall-rock, or accumulation/resorption of phenocrysts or xenocrysts. Incompatible trace element ratios (e.g., Nb/La, Nb/Th, La/Th, La/Hf, Ce/Pb) in Lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements. Large variations in La/Sm, La/Yb, and absolute REE abundances at constant MgO suggest that Lanai tholeiites formed by variable amounts of partial melting. However, large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source, but must reflect geochemical heterogeneities in the Lanai source. Partial melting modeling indicates that the mixed Lanai source is probably LREE-enriched [i.e., (La/Yb)_CN>1]. One component in the Lanai source, exemplified by the low-abundance tholeiites, has markedly lower REE/HFSE, Th/HFSE, alkali/HFSE, and Ce/Pb ratios than other Lanai or Hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the Hawaiian plume source. The positive Eu anomalies that characterize the low-abundance Lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component. Progressive temporal geochemical variations in Lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time. This change could have resulted from a progressive decrease in the extent of partial melting of the Lanai source. The compositional variability of Lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting. Lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger-volume Hawaiian shields. Therefore, volume differences between Hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting. The premature cessation of eruptive activity at Lanai may be attributed to relatively large degrees of partial melting of a small plume.     

Peridotitic and gabbroic rocks associated with the shield-forming lavas of réunion

The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons.     

Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

Mayon is the most active volcano along the east margin of southernLuzon, Philippines. Petrographic and major element data indicatethat Mayon has produced a basaltic to andesitic lava seriesby fractional crystallization and magma mixing. Trace elementdata indicate that the parental basalts came from a heterogeneousmantle source. The unmodified composition of the mantle wedgeis similar to that beneath the Indian Ocean. To this mantlewas added a subduction component consisting of melt from subductedpelagic sediment and aqueous fluid dehydrated from the subductedbasaltic crust. Lavas from the highly active Taal Volcano onthe west margin of southern Luzon are compositionally more variablethan Mayon lavas. Taal lavas also originated from a mantle wedgemetasomatized by aqueous fluid dehydrated from the subductedbasaltic crust and melt plus fluid derived from the subductedterrigenous sediment. More sediment is involved in the generationof Taal lavas. Lead isotopes argue against crustal contamination.Some heterogeneity of the unmodified mantle wedge and differencesin whether the sediment signature is transferred into the lavasource through an aqueous fluid or melt phase are needed toexplain the regional compositional variation of Philippine arclavas. KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano     

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.     

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.     

Geochemistry of an oceanite-ankaramite-basalt suite from East Island,Crozet Archipelago

The petrology and geochemistry of East Island have been investigated for the first time. The island is a deeply dissected remnant of a Pleistocene shield volcano, one of several emerging from an oceanic rise forming part of the southwest branch of the Indian Ocean ridge system. The lavas form a flat-lying sequence of oceanites, ankaramites, olivine basalts and feldsparphyric basalts, the ankaramites containing 1 cm phenocrysts of diopsidic clinopyroxene. X-Ray fluorescence analyses were made of 43 lavas for the major elements plus Cr, Ni, Rb, Sr, Ba, Pb, and Th and the minerals were analysed by electron microprobe. The elements Mg, Cr, and Ni are strongly concentrated in spinel, olivine and clinopyroxene phases and in the ankaramites and oceanite lavas with maximum concentrations of 18% MgO, 1,000 ppm Cr, 380 ppm Ni, while Al, Ti, K, Rb, Ba, Th, Na, P, Sr concentrate in the groundmass and in the feldspathic and aphyric basalts. The elements Si, Ca, Fe and Mn remain virtually constant throughout the series.Correlations of +0.95 or better exist between the concentrations of elements within the two groups given above, and negative correlations between elements in different groups. The fractionation trends are unique with respect to the constant Al/Ti ratio and K/Sr ratio, but all trends may be reproduced by calculating the effect of subtraction of suitable amounts of chromite, olivine and low Ti clinopyroxene from an alkaline olivine basalt parent. Either fractionation has taken place involving these three phases under low pressure conditions or it is the result of different degrees of partial melting of mantle material.A complex magnesian chrome spinel is found in the ankaramites and is often jacketed by a chromian titanomagnetite. A complete series of intermediate compositions appears to exist between the two end members.     

Temporal evolution of the kerguelen plume: Geochemical evidence from 38 to 82 ma lavas forming the Ninetyeast ridge

Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have²⁰⁶pb/²⁰⁴Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher²⁰⁶Pb/²⁰⁴Pb, such as that expressed in lavas from St. Paul Island.     

Basalt geochemistry used to investigate past tectonic environments on Cyprus

Study of the geochemical fingerprints of four geologically distinct suites of volcanic rocks on Cyprus are used to sketch a tectonic history of the island. Lavas from the Mamonia complex resemble alkalic within-plate basalts; lower pillow lavas and diabases of the Troodos Massif have features both of ocean-floor and island-arc tholeiites and could have been erupted in an interarc basin; the upper pillow lavas of the Troodos Massif resemble primitive tholeiitic basalts from island arcs; lavas from the Kyrenia range resemble transitional to alkalic within-plate basalts. The low TiO₂ concentrations from the Troodos Massif may indicate a slow spreading rate. The Sr concentrations in the upper pillow lavas indicate an eruption at a maximum distance of 80 km above a Benioff zone. The results suggest formation of the Troodos Massif in the Campanian by spreading in an interarc basin followed by eruption of island-arc tholeiites. Obduction of continental material and ocean islands may have taken place in the Maestrichtian and Middle Miocene.     

Geochemistry of Kauai volcanics and a mixing model for the origin of Hawaiian alkali basalts

A comprehensive model is developed to explain the major, trace element and strontium and neodymium isotopic characteristics of alkali basalts from Hawaii. The model is similar to that of Chen and Frey (1983) in that it requires mixing of a small melt fraction of MORB-source material with another component to generate the alkalic suite of a particular Hawaiian volcano. It differs from the Chen and Frey model in that the other end-member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data. Alkali basalts and tholeiites from Kauai analyzed in this study show a nearly complete transition in Sr and Nd isotopes. There is a relatively well-constrained array on a Nd-Sr isotope correlation plot that can be explained by two-component mixing of Kauai tholeiite magma and a small amount of melt of East Pacific Rise source rock. After corrections are made for fractional crystallization (involving primarily clinopyroxene and olivine), the Sr and Ba concentrations of Kauai lavas plot along mixing curves defined by the above sources, providing positive tests of the mixing hypothesis. Implications of this model are: (1) the main source of Hawaiian shield-building tholeiites is a mixture of subducted crust, primitive mantle and depleted asthenosphere that has been homogenized prior to melting, (2) early alkalic volcanism (as at Loihi seamount) will be characterized by greater isotopic heterogeneity than will late-stage alkali basalt production, and (3) there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given Hawaiian volcano. One represents smaller degrees of melting of the same source that generated shield-building tholeiites (Kohala-type); the other derives from the mixed source discussed in this paper (Haleakala-, Kauai-type).     

Tholeiitic, calc-alkaline and (?)alkaline igneous rocks of the Shortland Islands, Solomon Islands

The Shortland Islands lie in a northeast-southwest line across the western end of Solomon Islands, immediately adjacent to Bougainville. Three major islands dominate the group.Fauro and surrounding islands, in the northeast, have an altered basement suite comprising tholeiite, icelandite and tholeiitic dacite. This is intruded by a high-level calc-alkaline assemblage of microdiorite, hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano. Pyroclastics comprising high-alumina basalt and pyroxene andesite overlie the volcanogenic sandstones. The tholeiitic basement lavas may be of Late Oligocene to Early Miocene age, and the calc-alkaline rocks are probably also pre-Pliocene in age.Alu, in the centre of the group, also has an altered tholeiitic lava basement, which is intruded by a quartz diorite body and overlain by hypersthene-augite basaltic andesite. Pliocene siltstone and Quaternary shallow marine carbonates cover these igneous rocks over much of the island.Mono, in the southwest, has a small basement exposure of altered pillowed hawaiite, overlain by Miocene pelagic limestone, Pliocene siltstone and Quaternary reef limestone. Isolated clasts of pyroxene andesite and ?benmoreite occur in streams and on beaches.The younger, calc-alkaline suites on all islands were formed in an island arc environment, possibly related to subduction from the southwest beneath the New Britain Trench. The basement lavas on Alu are probably early island arc tholeiites, and both these lavas and the calc-alkaline rocks of Alu share a common trend on variation diagrams. The two igneous suites of Fauro, however, have distinctly different trends. The basement lavas have some chemical similarities with oceanic tholeiites, but an early island arc origin for these lavas cannot be ruled out. The altered hawaiite and benmoreite on Mono probably originated in an oceanic island environment.     

The petrology and geochemistry of the Azores Islands

Forty lavas from the Azores Islands have been analyzed for ⁸⁷Sr/⁸⁶Sr ratios, major elements, first transition series metals, and LIL elements. The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria. Differentiated lavas include both typical trachytes and comenditic trachytes and comendites. Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks. Systematic interisland differences are also apparent in these variation diagrams. LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau. ⁸⁷Sr/⁸⁶Sr ratios in lavas from 6 of the 9 islands range from 0.70332 to 0.70354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau. This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source. However, lavas from the islands of Faial and Pico have ⁸⁷Sr/⁸⁶Sr ratios up to 0.70394 and ratios in Sao Miguel lavas range up to 0.70525, suggesting basalts from these islands are derived from a chemically distinct source. Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy. The alkali basalts are derived by roughly half as much melting as are the tholeiites. Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent.     

Tholeiitic and alkalic basalts of the oldest Pacific Ocean crust

Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.     

Isotope systematics of Li, Sr, Nd, and volatiles in Indian Ocean MORBs of the Rodrigues Triple Junction: Constraints on the origin of the DUPAL anomaly

The DUPAL anomaly, a radiogenic isotope anomaly discovered in the Indian Ocean mantle, has been interpreted as due to a large-scale mantle heterogeneity. To provide new constraints on the DUPAL origin, we analyzed isotope ratios of Li, Sr, and Nd in fresh N-MORB glasses recovered from the Rodrigues Triple Junction in the Indian Ocean, and from the North Atlantic. The Li isotopic compositions of the Indian Ocean DUPAL N-MORBs were comparable to those of the North Atlantic non-DUPAL N-MORBs. The source of the DUPAL signature in Indian Ocean MORBs and the E-MORB-type enriched mantle source have quite different Li isotopic compositions. The ¹⁴³Nd/¹⁴⁴Nd values of both sources are significantly lower than those of the North Atlantic N-MORBs. The δ⁷Li values of most oceanic island basalts with similar low ¹⁴³Nd/¹⁴⁴Nd signatures are also higher than those of the North Atlantic N-MORBs, except for several Koolau lavas. The Li isotope results support the recent proposal that significant amounts of recycled lower continental crust might produce the radiogenic isotope signatures of the Indian Ocean DUPAL source.     

Evidence for adiabatic decompression melting in the Southern Mariana Arc from high-Mg lavas and melt inclusions

Unusually magnesian (Mg# ∼76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al₂O₃ from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17–17.86 wt%, 0.35–0.5% TiO₂, 42–50% SiO₂ and 1.66–3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182–334 ppm) and H₂O contents (0.11–0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log ΔFMQ of + 1.53–1.66) which lie at the low end of the range estimates for arc basalts and picrites (ΔFMQ = + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367°C at 1–1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Basalt basement from the Kerguelen Plateau and the trail of a Dupal plume

The first samples of volcanic basement recovered from the Kerguelen Plateau are Lower Cretaceous transitional tholeiites. Isotope and incompatible element abundance ratios for these rocks are similar to ocean island basalts from the southern hemisphere Dupal anomaly region, and geochemical, geological and geophysical data are consistent with volcanic activity associated with a mantle plume. A reconstruction of plate motions suggests that the Kerguelen Plateau formed above a mantle plume in the interval 118-95 Ma, during the opening of the Indian Ocean between India and Australia-Antarctica. This plume was the source of other plateaus and ridges of the eastern Indian Ocean and possibly the Bunbury Basalt of southwestern Australia, and is now beneath Heard Island.     

Petrology of the Western Reykjanes Peninsula, Iceland

The active tholeiitic volcanic zone of the Reykjanes Peninsulaconsists of five volcanic fissure swarms, the two westernmostof which are the subject of this petrological study. The recent(less than 12,000 years) extrusives of the swarms group morphologicallyand petrographically into small picrite basalt lava shields,large olivine tholeiite lava shields and tholeiite fissure lavas;formed in that chronological succession. The picrite basalts exhibit a primitive mineralogy with chromite,olivine (Fo 89) and plagioclase (An 90) as phenocrysts and mayrepresent a primary liquid from the mantle. Simultaneous crystallizationof olivine, plagioclase and augite to form glomerocrysts inthe fissure lavas indicate low pressure cotectic crystallizationconditions. Twenty-eight new major element chemical analyses of the lavasare presented. They are generally characterized by a low contentof alkalies and high CaO. The lavas constitute two main suites,a lava shield suite and a fissure lava suite. There is a positivecorrelation between the volume of individual lavas and the contentof incompatible elements of the lavas within each group. Likewisethere is an overall chemical trend through time demonstrated,for example, by a rise in K₂O from about 0.02 per cent to 0.24per cent during the last, approximately, 12,000 years. There is an apparent chemical zoning within each volcanic swarmsuch that the most evolved and youngest lavas are found in thecentral axial area of the swarm. This central area is also characterizedby graben subsidence, high magnetic anomalies and high temperaturethermal areas, all indicative of shallow magma reservoir(s).In spite of indications of fractional crystallization in theevolution of the olivine tholeiites and tholeiites, some otherprocesses must be sought to explain the volume chemistry relations.Cyclic volcanic activity is tentatively suggested to explainthe observed regular temporal variations within the swarm, eachcycle starting with the formation of picrite basalts.     

Geochemical evolution of Indian Ocean basaltic magmatism

A comparison of new and published geochemical characteristics of magmatism in the western and eastern Indian Ocean at the initial and recent stages of its evolution revealed several important differences between the mantle sources of basaltic melts from this ocean.

1. The sources of basalts, from ancient rises and from flanks of the modern Central Indian Ridge within the western Indian Ocean contain an enriched component similar in composition to the source of the Réunion basalts (with radiogenic Pb and Sr and unradiogenic Nd), except for basalts from the Comores Islands, which exhibit a contribution from an enriched HIMU-like component.
2. The modern rift lavas of spreading ridges display generally similar geochemical compositions. Several local isotopic anomalies are characterized by the presence of an EM2-like component. However, two anomalous areas with distinctly different enriched mantle sources were recognized in the westernmost part of the Southwestern Indian Ridge (SWIR). The enriched mantle source of the western SWIR tholeiites in the vicinity of the Bouvet Triple Junction has the isotopic ratios indicating a mixture of HIMU + EM2 in the source. The rift anomaly distinguished at 40° E displays the EM1 signature in the mantle source, which is characterized by relatively low ²⁰⁶Pb/²⁰⁴Pb (up to 17.0) and high ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb and ⁸⁷Sr/⁸⁶Sr. This source may be due to mixing with material from the continental lithosphere of the ancient continent Gondwana. The material from this source can be distinguished in magmas related to the Mesozoic plume activity in Antarctica, as well as in basalts from the eastern Indian Ocean rises, which were formed by the Kerguelen plume at 100–90 Ma.
3. The geochemical heterogeneities identified in the ancient and present-day magmatic products from the western and eastern Indian Ocean are thought to reflect the geodynamic evolution of the region. In the eastern part of the ocean, the interaction of the evolving Kerguelen plume with the rift zones produced magmas with specific geochemical characteristics during the early opening of the ocean; such a dispersion of magma composition was not recognized in the western part of the ocean.