Enriched Mantle I Research Articles

Petrogenesis of the Hoy lava field, a long-lived continental mafic volcanic province in eastern Australia

Small-scale continental basaltic fields can erupt with little warning and bring deep undegassed magmas to the surface rapidly. To explore the lifetime, petrogenesis and plumbing system architecture feeding such basaltic lava fields and compare them with large-scale shield volcanoes, we have focused on the Hoy lava field, central Queensland, Australia. 40Ar/39Ar geochronology, elemental and isotopic whole-rock geochemistry and mineral chemistry on selected Hoy samples reveal long-lived volcanism of ca 50 Ma and magma storage at mantle depths, notably different from the comparatively short duration (3–5 Myr) and crustal magma storage depths of shield volcanoes. In this study, four Hoy lava-field eruptive intervals spanning ca 50 Ma were investigated: 67.5 ± 0.3 Ma, 32.3 ± 0.6–31.6 ± 0.7 Ma, 21.9 ± 0.5 Ma and 18.1 ± 0.3 Ma. In all four eruptive events, samples are porphyritic alkali basalts and trachybasalts (11.41–6.45 wt% MgO) with incompatible element concentrations and Sr–Nd–Pb isotope ratios dominantly derived from a metasomatised sub-continental lithospheric mantle (SCLM) source with an enriched mantle I (EMI) signature. Complex crystal populations show major- and trace-element variations reflecting fractional crystallisation, magma recharge, magma mixing and mantle xenocryst entrainment. Clinopyroxene–melt thermobarometry indicates magma storage in SCLM reservoirs at ∼30–47 km depths. The nearby larger but shorter-lived (3–5 Myr) Buckland central volcano has similar source compositions; however, magma storage is limited and concentrated in the crust, resulting in increased crustal contamination. The results suggest that basaltic centres of contrasting scale and longevity are linked to distinct magma production mechanisms, fluxes, ascent and differentiation.

Abrolhos Magmatic Province petrogenesis and its link with the Vitória-Trindade Ridge, Southeast Brazilian Margin, South Atlantic Ocean

The Abrolhos Magmatic Province (AMP) is an example of an igneous province with about 63,000 km2 located at the Continent-Ocean Boundary (COB), into the Southeast Brazilian Margin. The AMP emerges as five small islands (Santa Bárbara, Redonda, Siriba, Sueste, and Guarita) which integrate the Abrolhos Archipelago located about 55 km offshore Brazil. The studied rocks belong to a transitional basalt series of alkaline affinity, with evolved rocks with high TiO2 contents. This work presents new detailed fieldwork mapping, petrographic, and whole-rock chemistry data, besides Sr–Nd isotopic compositions from AMP rocks. Mapped magmatic rocks in the Abrolhos Islands have been described as extrusive rocks, but we point out that they are shallow intrusions, mostly sills, and should be grouped into diabase units based on fieldwork and textural features. The AMP lithogeochemical and isotopic data suggest the operation of differentiation processes more complex than simple fractional crystallization or AFC. Our model mixing calculations based on new and compiled Sr-Nd-Pb isotopic data suggest a mixture with a prevailing depleted asthenosphere mantle source (represented by DMM; ca. 80%) metasomatized by an enriched mantle I (EMI) component (≤ 10%) and a possible HIMU-type constituent (up to 10%) in the AMP genesis. The assimilation of subducted slabs of the oceanic crust associated with the HIMU signatures is possibly linked to the Brasiliano Event due to the range of the AMP Nd TDM model ages, from 407 to 767 Ma. A viable mechanism for the EMI end-member contribution could either be a physical detachment of the South American subcontinental lithospheric mantle during the breakup of the Gondwana or lithospheric delamination of the South American plate caused by an edge-driven convection mechanism. The AMP and the Vitória-Trindade Ridge (VTR) show an eastward decreasing age pattern from the older ca. 60 Ma Abrolhos Province to the younger Martin Vaz and Trindade Islands (ca. 4 – 0.1 Ma), located ca. 1200 km away from the Brazilian coastline. The volcanic alignment between the VTR and AMP, along with the overlap of geochemical and isotopic data of their different igneous rocks, cannot be a random feature but instead represent the sampling of similar shallow mantle reservoirs, thus suggesting a cogenetic relationship. Finally, a possible petrogenetic link between the AMP and VTR magmatism is discussed.

Cambrian rift magmatism recorded in subvolcanic sills of the Ediacaran-Cambrian La Ciénega Formation, NW Mexico

Mafic-ultramafic subvolcanic metabasalt sills intrude Ediacaran and Cambrian strata of the La Ciénega Formation of Sonora, Mexico. Sills are typically 1–7.5 m thick, with minor associated thin (∼10 cm thick) intrusions. Sills are typically bed parallel with upper and/or lower tempered (quenched) borders and porphyritic textures characterized by abundant olivine and pyroxene pseudomorphs that have been transformed to clay and carbonate minerals. Metamorphic halos in adjacent dolostones and quartzites occur at the bottom and top contacts, and metabasalt sills contain carbonate veins typical of hydrothermal alteration and mobilization. Unlike surface flows of the overlying Cerro Rajón Formation, all sills in the La Ciénega Formation lack evidence of crystal settling, yet evidence for assimilation of country rocks is present. Assimilation is more typical of sills rather than flows and is diagnostic of sills in the El Arpa Formation, a stratigraphically lower unit in the succession. Actinolite-chlorite-albite metamorphic mineral paragenesis suggests low-grade greenschist facies metamorphism for these metabasaltic rocks. Although their primary mineralogy is obscured due to alteration, olivine and clinopyroxene pseudomorphs are present and their major and trace element geochemistry suggest they were derived from alkaline basaltic magmas with low (normalized) SiO2N (22.7–43.1 wt%), high MgON (7.25–16.6 wt%), and high TiO2N (4.04–6.27 wt%). Immobile trace elements suggest these sills were originally alkaline rift-related basalts with OIB-type signatures. Tectonic discrimination diagrams suggest continental rift magmatism with low geochemical evolution ratios and relatively low mantle melting rates when compared to contemporaneous rift related magmatism. Sr-Hf-Nd-Pb isotopes suggest the metabasalts originated from an Enriched Mantle I (EMI) reservoir and melting occurred during adiabatic decompression. These geochemical characteristics are remarkably similar to volcanic rocks of the Cerro Rajón Formation. The inferred magmatic composition, mantle melting rates, petrography and field relations of the La Ciénega Formation metabasalt are consistent with a low viscosity feeder system like that envisaged to have sourced volcanic rocks of the overlying Cambrian (Terreneuvian) Cerro Rajón Formation. Together, evidence from these two volcanic units is consistent with a Cambrian continental rift event close to the onset of the Sauk transgression.

Geochemistry and zircon ages of the Yushigou diabase in the Longshoushan area, Alxa Block: implications for crust–mantle interaction and tectonic evolution

AbstractThe North Qilian orogenic belt in North China has been defined as a subduction–collision zone between the Alxa Block and the Qilian Block. We present petrography, zircon U–Pb geochronology, major- and trace-element geochemistry, and Sr–Nd–Pb–Hf isotope analysis for the Yushigou diabase from the Longshoushan area, which is located SW of the Alxa Block, aiming to understand its petrogenetic link to subduction processes. The Yushigou diabase belongs to the tholeiite series, and shows enrichment in light rare earth and large-ion lithophile elements, and a depletion in heavy rare earth and high-field-strength elements. Laser ablation – inductively coupled plasma – mass spectrometry U–Pb zircon dating yielded an emplacement age of 414 ± 9 Ma, with an ϵHf(t) value in the range of −10.3 to 1.8. The whole-rock initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the diabase range over 16.811–17.157, 15.331–15.422 and 37.768–37.895, respectively. The (87Sr/86Sr)i ratios vary between 0.7086 and 0.7106, and ϵNd(t) values vary between −14.4 and −13.4, which are significantly higher than the ϵHf(t) value (Nd–Hf decoupling). An interpretation of the elemental and isotopic data suggests that the Yushigou diabase was derived from partial melting of an enriched mantle I (EM-I) -type lithospheric mantle in the spinel–garnet transitional zone. Based on the geochemical features and previous regional geological data, we propose that the Silurian magmatism was most likely triggered by slab break-off after the closure of the North Qilian Ocean, and ancient continental materials from the subduction slab metasomatized the overlying lithospheric mantle during exhumation.

Geochemistry of Etendeka magmatism: Spatial heterogeneity in the Tristan-Gough plume head

The origin and distribution of geochemically distinct source components in continental flood volcanism (generally associated with the initial phase of a mantle plume head) are poorly understood. Here we present new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data from the Etendeka flood basalts and associated dikes from northern and central Namibia that are believed to have been produced during the initial stage of the Tristan-Gough hotspot. Following earlier studies, the Etendeka lava flows and dikes are divided into high-Ti and low-Ti groups. The trace element and isotopic composition of the high-Ti tholeiitic basalts, exclusively outcropping in northern Etendeka (northwestern Namibia), are similar to the Gough-type enriched mantle I (EMI) composition found on the Walvis Ridge (the Atlantic type locality for the EMI end member). The low-Ti tholeiitic basalts, primarily outcropping in Southern Etendeka (central western Namibia), have higher 143Nd/144Nd and 207Pb/204Pb but lower 208Pb/204Pb ratios than the Gough composition. Combining our data with newly published 3He/4He data and estimates of the magma source's potential temperature from 1520-1680°C, we conclude that the source of the low-Ti basalts was also intrinsic to the Tristan-Gough plume, consistent with a spatially-zoned plume head. The low-Ti basalts were derived from a distinct EMI-type source component that has thus far only been detected in the initial Tristan-Gough plume head (∼132 Ma), but not the later submarine hotspot track.

Temporal constraints on magma generation and differentiation in a continental volcano: Buckland, eastern Australia

The eastern margin of the Australian continent hosts a large number of Cenozoic intraplate volcanoes along a 2000 km long track. Here, we study mafic lavas from the Buckland volcano, Queensland, located in the northern (older) segment of this track, to assess magma generation and differentiation through time. The rocks are aphanitic to microporphyritic basalts, trachy-basalts and basanites. Incompatible element geochemistry together with Sr-Nd-Pb isotope ratios indicate that magmas formed from an enriched mantle I (EMI)-like garnet-bearing source with variable degrees of crustal contamination. Whole rock elemental variations suggest fractionation of olivine, plagioclase, clinopyroxene and/or magnetite. There is no petrographic or geochemical evidence of magma mixing in the studied rocks (e.g., lack of recycled minerals), suggesting a relatively quick ascent from the source to the surface without major storage at shallow levels.40Ar/39Ar geochronology reveals two stages of volcanism: 30.3 ± 0.1 Ma and 27.4 ± 0.2 Ma. The Old Buckland (30.3 ± 0.1 Ma) melts have negative K anomalies, and incompatible element ratios suggest the occurrence of residual hydrous minerals in a metasomatised mantle source. We therefore infer that at the onset of volcanism, deep-mantle-derived magmas interacted with metasomatised sub-continental lithospheric mantle (SCLM). Major and trace element data, clinopyroxene thermobarometry and thermodynamic modelling indicate magma evolution by assimilation and fractional crystallisation (AFC) during ascent through the crust. Following a hiatus in volcanic activity of ~2.5 Ma, eruption of Young Buckland (27.4 ± 0.2 Ma) lavas marked a shift towards more alkaline compositions. Trace element compositions indicate lower degrees of partial melting and a lack of interaction with metasomatic components. Young Buckland lavas become progressively more SiO2-saturated up stratigraphy, suggesting an increase in the degree of partial melting with time. Young Buckland lavas also have more radiogenic 87Sr/86Sr and 207Pb/204Pb ratios and less radiogenic 143Nd/144Nd ratios up stratigraphy. These isotopic variations, together with coupled increases in Pb and K and decreases in Ce/Pb (27.22 to 11.09) and Nb/U (68.30 to 29.96), suggest that crustal contamination also increased with time.By placing absolute age and stratigraphic constraints on the Buckland lavas, we have been able to ascertain differentiation signatures imposed on mantle-derived melts during ascent through the continental lithosphere over 3 Ma. Our study provides new constraints on magma generation and differentiation in continental intraplate volcanic systems.

Mesozoic high-Mg andesites from the Daohugou area, Inner Mongolia: Upper-crustal fractional crystallization of parental melt derived from metasomatized lithospheric mantle wedge

Mineral chemistry, major- and trace-element data, zircon U–Pb ages, and Sr–Nd isotopic data are presented for a suite of volcanic rocks from the Daohugou area, Ningcheng City, Inner Mongolia, on the northern margin of the North China Craton. Samples from the suite are of basaltic andesite to rhyolite in composition, with the rocks containing <60 wt% SiO2 have high MgO, Cr, and Ni contents, and classify as high-Mg andesites (HMAs). Zircons from a rhyolite yielded weighted mean 206Pb/238U age of 164 ± 1 Ma, indicating that the Daohugou volcanic suite is coeval with the Tiaojishan Formation of northern Hebei and western Liaoning Province. The HMAs have similar enriched-mantle I (EMI)-type isotopic compositions to each other, with low εNd (t) values, moderate (87Sr/86Sr) i ratios, enrichment in LREEs relative to LILEs, and depletion in HFSEs (e.g., Nb, Ta, Ti), indicating formation through protracted fractional crystallization of a common parental magma. The unusually low CaO contents and CaO/FeO ratios of olivine phenocrysts in the HMAs suggest that the parental melt was subduction-related. The results of Rhyolite-MELTS modelling indicates that HMAs may form through upper-crustal fractional crystallization from arc basalts. Therefore, the Daohugou HMAs were most likely formed through fractional crystallization of a parental melt derived from metasomatized lithospheric mantle at crustal depths. The addition of “water” to the cratonic keel may have played a key role in the destruction of the North China Craton.

Collision-induced post-plateau volcanism: Evidence from a seamount on Ontong Java Plateau

Many seamounts on the Ontong Java Plateau (OJP) occur near the Stewart Arch, a topographic high that extends parallel to the North Solomon Trench along the southern margins of the plateau. Despite the thick sediment cover, several volcanic cones with strong acoustic reflection were discovered on the submarine flank of the Nuugurigia Seamount. From such volcanic cones, basalts were successfully sampled by dredging. Radiometric dating of basalts and ferromanganese encrustation indicate eruption age of 20–25Ma, significantly younger than the ~122Ma main OJP plateau and post-plateau basalts. The age range coincides with the collision of the OJP with the Solomon Arc. The Nuugurigia basalts geochemically differ from any other rocks sampled on the OJP so far. They are alkali basalts with elevated Sr, low Zr and Hf, and Enriched Mantle-I (EMI)-like isotopic composition. Parental magmas of these alkali basalts may have formed by small-degree melting of peridotitic mantle impregnated with recycled pyroxenite material having enriched geochemical composition in the OJP's mantle root. We conclude that small-volume alkali basalts from the enriched mantle root migrated through faults or fractures caused by the collision along the Stewart Arch to form the seamount. Our results suggest that the collision of the OJP with the Solomon arc played an important role in the origin of similar post-plateau seamounts along the Stewart Arch.

Reappraisal of Early Cretaceous highly enriched mantle beneath the Dabie Orogen: evidence from mineralogy and geochemistry of the Xiaohekou complex

An Early Cretaceous highly enriched mantle beneath the Dabie Orogen, previously proposed on the basis of exceedingly enriched isotopic signatures documented in intermediate–mafic–ultramafic complexes, has been re-evaluated by new field, mineralogical and geochemical data from the best exposed Xiaohekou complex. The Xiaohekou complex consists of three pyroxenite cores intruded by olivine gabbronorite dykes and successively surrounded outward by gabbroic rocks, leucogabbros and monzonites. Textural and compositional data for pyroxenes and plagioclase as well as whole-rock geochemistry suggest two-stage crystallizations. The Sr–Nd isotopic values correlate with the SiO 2 and MgO contents, and one gabbronorite sample from the margin of the complex has sodic plagioclase and highly enriched Sr–Nd isotopic ratios, also pointing to fractional crystallizations and crustal contamination. One gabbro sample from the centre of the complex and three from olivine gabbronorite dykes have the lowest Sr but highest Nd isotopic values close to those of typical Enriched Mantle I (EMI). These data together with isotopic and MELTS modelling suggest that an EMI-like mantle source plus crustal assimilation and crystallization can explain all field, mineralogical and geochemical characteristics of the Xiaohekou complex, without the need to invoke a mantle source more enriched than EMI beneath the Dabie Orogen. Supplementary materials: Compositions of pyroxenes, feldspars, hornblende, olivine and biotite, and whole-rock major element oxides, trace element and Sr-Nd isotopic compositions from selected samples in the Xiaohekou complex are provided as .rtf files at http://www.geolsoc.org.uk/SUP18856 .

Reinforcing the origin of volcanic rocks from the Massif Central through the isotopic composition of lead and strontium

The Massif Central in France hosts a large volume of Tertiary to Recent continental alkaline volcanism, with evidence of crustal contamination of the differentiated magmas. Detailed investigations on feldspathic basalts and basanite rocks from the Cantal Volcano exposed in the Allanche basin by lead and strontium isotopes i) reveal the role of the mantle components Depleted Mantle (DM) and Low Velocity Component (LVC, corresponding to the European asthenospheric reservoir) end-members based on 208Pb/204Pb and 206Pb/204Pb isotopic ratios, and ii) show contamination by meta-igneous granulites from the lower crust when using 207Pb/204Pb vs. 206Pb/204Pb ratios, as only part of the samples plot along the Northern Hemisphere Reference Line (NHRL) mixing line between DM and HIMU (High U/Pb Mantle) end-members. Cross plotting of Pb–Sr isotopes rules out recycled sediment and/or igneous granulite as potential crustal contaminant for the Cantal basalts, but suggests mixing between the low-velocity component (LVC) and EMI (Enriched Mantle I)-pelagic sediments. Replaced in the regional context of the Massif Central, we confirm the existence of several mantle sources and different crustal contaminations in the magmas of the Cantal Volcano.

An Ordovician intra-oceanic subduction system influenced by ridge subduction in the West Junggar, Northwest China

We report elemental and Nd–Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase–clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14–15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial 87Sr/86Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ϵNdt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt–andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar.

Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling

Abstract A palaeogeographical reconstruction of the South American and African continents back to anomaly C34 (84 Ma) brings together the Rio Grande Rise (RGR) and the central portion of the Walvis Ridge (WR), thus the RGR–WR aseismic ridges may have a common origin. If the construction of the RGR–WR basaltic plateau took place mainly between 89 and 78 Ma, as indicated by the ages of the basalts sampled by DSDP wells, then the basaltic magmas are the result of an ‘on-ridge’ volcanism. Once separated, the normal sea-floor spreading and thermal subsidence of the RGR and WR ridges continued until approximately 47 Ma when an Eocene magmatism took place in the RGR. In the WR, a younger volcanism is observed in the Guyot Province. The available geochemical and isotope data of the WR–RGR basalts do not indicate the participation of the continental crust melting component. Incompatible trace element ratios and isotope signatures of the basalts from the RGR–WR ridges are distinct from the present-day Tristan da Cunha alkaline rocks, and are nearly identical to the high-Ti Paraná Magmatic Province (PMP) tholeiites (133–132 Ma). Both the high-Ti PMP and the WR–RGR basalts are characterized by moderate initial 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb isotope ratios [Enriched Mantle I (EMI) mantle component], suggesting melting from a common source, with significant participation of sub-continental lithospheric mantle (SCLM). A three-dimensional (3D) flexural modelling of the RGR and WR was conducted using ETOPO1 digital topography/bathymetry and EGM2008-derived free-air anomalies as a constraint. The best fit between the observed and calculated free-air anomalies was obtained for an elastic plate with elastic plate thickness ( T e ) of less than 5 km, consistent with an ‘on-ridge’ initial construction of the RGR–WR. The modelling of the crust–mantle interface depths indicates a total crustal thickness of up to 30 km in the RGR–WR. Flexural analysis reinforces the geological evidence that RGR was constructed during two main magmatic episodes, the tholeiitic basalts in the Santonian–Conician times and the alkaline magmatism in the Eocene. Geochemical and geophysical evidence, which rules out the classical deep-mantle plume model in explaining the generation of basalts of these volcanic provinces, is presented. Finally, three models to explain the geochemical and isotope signatures of RGR–WR basalts are reviewed: (1) thermal erosion of SCLM owing to edge-driven convection; (2) melting of fragmented or detached SCLM and lower crust; and (3) thermal erosion at the base of the SCLM with lateral transport of enriched components by mantle flow.

Geochemistry of Ore‐bearing Lamprophyre from the Cu‐Ni Deposit in Dhi Samir, Yemen

Abstract:Magmatic Cu‐Ni sulfide deposits are generally associated with mafic‐ultramafic rocks and it has not been reported that lamprophyre is one of the surrounding rocks of Cu‐Ni sulfide deposits. The Dhi Samir deposit in Yemen, however, is a rare example of Cu‐Ni deposits which are hosted in lamprophyre dikes. In this paper, comprehensive research is made on petrology, petrochemistry and isotope geochemistry for Cu‐Ni‐bearing rocks in the Dhi Samir area and the results show that dark rocks related to Cu‐Ni orebodies are sodium‐weak potassium and belong to calc‐alkaline series lamprophyre, especially camptonite, characterized by enriched alkali, iron and titanium. In these rocks large‐ion‐lithophile elements are obviously concentrated, while high field strength elements slightly depleted, showing clear negative anomalies of Ta and Nb, and weak deficiency of Ti. The ΣREE is very high (225.67–290.05 ppm) and the REE partition curves are flat and right‐inclined, featuring a LREE‐enriched pattern with low negative Eu anomalies. Study of magmatic source areas indicates that the rocks have low (87Sr/86Sr) and high εNd(t), and the magmas were probably derived from the enriched mantle I (EM‐I) end‐member. Based on the LA‐ICPMS on zircon U‐Pb isotope dating, the lamprophyre in the Dhi Samir mining area has an age of 602±2.6 Ma, indicating that the rock was formed in the late Proterozoic and in an intraplate setting due to magmatism of an extensional environment in the post‐Pan‐Africa orogeny.

An ancient metasomatic source for the Walvis Ridge basalts

The proposed origins for the Enriched Mantle I component are many and various and some require an arbitrary addition of an exotic component, be it pure sediment or an enriched melt from the subcontinental lithosphere. With Pitcairn, Walvis Ridge is the ‘type-locality’ for the Enriched Mantle I (EMI) component. We analyzed basalts from DSDP Site 525A, Site 527 and Site 528 on the Walvis Ridge with the aim to constrain the history of its source. The isotopic compositions we measured for the three sites overlap with the values obtained by Richardson et al. (1982a) and extend towards less radiogenic Sr and more radiogenic Pb and Nd isotopic compositions. We used our new trace element and radiogenic isotope (Hf, Nd, Pb and Sr) characterization in combination with the literature data to produce the simplest possible model that satisfies the trace element and isotopic constraints. Although the elevated 207Pb/ 204Pb with respect to 206Pb/ 204Pb predicts an ancient origin for EMI, none of the proposed origins had modeled it as such. The data is consistent with the EMI composition being formed by the addition of a melt to a mantle with bulk Earth-like composition followed by melt extraction of a low degree melt. The timing of these two events is such that the metasomatism has to have taken place prior to 4 Ga and the subsequent melt removal before 3.5 Ga. This confirms the expectation of an ancient character for the EMI component. The Walvis Ridge data shows two distinct two component mixing trends: one formed by the less enriched Site 527 and Site 528 basalts and one formed by the Site 525A basalts. The two trends have the EMI endmember in common. The less depleted end of the Site 527–Site 528 basalts is FOZO-like and can be explained by the addition of a recycled component (basaltic oceanic crust plus sediment). This recycled component was altered during subduction. The sense and magnitude of the chemical fractionation resulting from the subduction alteration are in agreement with dehydration experiments on basalts and sediment. Compared to other EMI like basalts the Walvis Ridge basalts have flatter REE patterns and show less fractionation between large ion lithophile and heavy REE elements. Using the isotopic compositions as constrains for the parent–daughter ratios we were able to model the trace element patterns of the basalts as melting between 5 and 10% for Site 525A and between 10 and 15% for the depleted end of the Site 528–Site 527 array. In all cases a significant portion of melting takes place in the garnet stability field.

Genetic link between EMI and EMII: An adakite connection

Geochemical modeling of the origin of enriched mantle I (EMI) and enriched mantle II (EMII) is conducted from the perspective of adakite production. For the model, the average composition of adakites is re-estimated from published data for eighteen trace elements. Although the concentrations determined for highly incompatible elements are very high (about 100 times of primitive mantle), these high concentrations can be explained by melting of oceanic crust without sediment contribution. The compiled data further suggest that the mantle–slab melt reaction would play a major role in the production of basic adakites. In addition, crystal fractionation in the magma chamber should produce additional chemical variations in adakites, in particular for acidic adakites. To examine the effect of chemical variations on the isotopic composition of recycled adakites, broad correlations between trace elements and SiO 2 concentrations, and the MELTS program are employed. The results suggest that recycling of a basic adakite (SiO 2 = 55%) can account for EMI isotopic signatures with storage times of about 2.0 Gyr. The isotopic compositions of less-basic adakites and their evolved magmas shift towards EMII values with increasing SiO 2 concentrations. In particular, evolved acidic adakite can yield EMII isotopic signatures. These lines of evidence suggest that the recycling of adakites at various stages of evolution can conceivably produce the entire isotopic range between EMI and EMII reservoirs. Consequently, adakite recycling via sediment subduction or subduction erosion can account for the origins of EMI and EMII reservoirs. In this context, residual garnet under high pressure and plagioclase fractionation at low pressure might play an essential role in producing the chemical variations among adakites that ultimately govern the isotopic compositions of these geochemical reservoirs.

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back‐Arc Granite: Miocene Tsushima Pluton, Southwest Japan

AbstractThe Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial 87Sr/86Sr ratios (0.7065–0.7085) and lower εNd(t) (−7.70 to −4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and −0.53 to −5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and −3.75 to −6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.

Características Isotópicas (Nd e Sr), Geoquímicas e Petrográficas da Intrusão Alcalina do Morro de São João: Implicações Geodinâmicas e Sobre a Composição do Manto Sublitosférico

The Morro de São João Alkaline Complex (MSJ) is located in the NE part of Rio de Janeiro State and shows pronounced topography, with the following lithology: coarse-grained felsic syenites (K-feldspar, nepheline, hornblende, titanite and pseudoleucite), and mafic syenites (with K-feldspar, hornblende and piroxene). Petrography and lithogeochemistry data suggest a bimodal suite. Felsic and mafic magma mixing and mingling textures are locally observed indicating the presence of coeval magmas. Nd and Sr isotope compositions correspond to EMI (Enriched Mantle I) mantle reservoir signatures, suggesting an asthenospheric enriched mantle source with low 87Sr/86Sr values (from 0.7049 to 0.7061) and low 143Nd/144Nd (from 0.512361 to 0.512428). The &#949;Nd values range from -4.03 to -5.54, indicating an anomalous enriched mantle reservoir, confirmed by the high T DM values between 730 - 830 Ma, very different from the 72 - 56 Ma K-Ar cooling ages. The comparison of Sr and Nd signatures with related hotspot traces (Trindade, St. Helena and Tristan da Cunha) indicates that they are similar to those of Tristan da Cunha, but somewhat different from those of Trindade and St. Helena.

Geochemistry of a new enriched mantle type locality in the northern hemisphere: Implications for the origin of the EM-I source

Late Cretaceous (66.2 ± 0.5 Ma amphibole and 66.7 ± 0.2 Ma phlogopite 40Ar/ 39Ar ages) nephelinitic volcanic rocks from Godzilla Seamount in the eastern North Atlantic (34°N latitude) have trace element and Sr–Nd–Pb–Hf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low 207Pb/ 204Pb relative to 206Pb/ 204Pb ratios ( 206Pb/ 204Pb in = 17.7, 207Pb/ 204Pb in = 15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like ( δ 18O = 5.6–5.8‰). Age and location of the isolated Godzilla Seamount, however, preclude it from being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of 207Pb/ 204Pb ratios (and Δ7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in 207Pb/ 204Pb ratios and Δ7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high μ ( 238U/ 204Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower μ). In accordance with this model, the global variations in 207Pb/ 204Pb ratios and Δ7/4 could reflect geographic differences in μ and/or the age at which the transition from stages 1 to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low 206Pb/ 204Pb, high 208Pb/ 204Pb (positive Δ8/4) but variable 207Pb/ 204Pb (positive and negative Δ7/4).