Alkali Basalts Research Articles

Petrology of Mafic Dykes from the Njimom Area (West-Cameroon): A Contribution to the Characterization of Late Paleozoic and Mesozoic Magmatism in the Southern Continental Part of the Cameroon Volcanic Line

In the western Cameroon, crop out several dyke swarms of Paleozoic–Mesozoic age. These dykes intrude the Precambrian basement in the southern continental part of the Cretaceous Cameroon Volcanic Line. In the Njimom area, two groups of mafic dykes that crosscut the Neoproterozoic basement rocks have been observed. A first group intrudes the mylonites whereas the second group intrudes the granites. The dykes are alkaline basalts and hawaiites. The mineralogical assemblage of both groups of dykes consists of plagioclase, clinopyroxene, altered olivine, and opaque oxides. The dykes that cross-cut the Precambrian mylonitic gneisses show moderate TiO2 (1.7–2.0 wt.%), low MgO (4.4–7.1 wt.%), and compatible trace element concentrations (e.g., Cr = 70–180 ppm; Ni = 30–110 ppm). The dykes that intrude the granites have TiO2 contents between 2.3 and 2.5 wt.% and moderate compatible trace element concentrations (e.g., Cr = 260–280 ppm; Ni = 170–230 ppm). MgO varies from 5.9 to 9.2 wt.%. All mafic dykes are enriched in light lanthanide element and show moderate Zr/Nb and high Zr/Y, Nb/Yb, and Ti/V ratios similar to those of average ocean island basalt (OIB)-type magmas. Some dykes that intrude the mylonites show evidence of contamination by continental crust. The composition of the clinopyroxenes of the dykes that intrude the mylonites clearly indicate different and unrelated parental magmas from dykes that intrude the granites. Contents and fractionation of the least and the most incompatible elements suggest low degrees of partial melting (3–5%) of heterogeneous source slightly enriched in incompatible elements in the spinel stability field. The geochemical features of Njimom dykes (in particular the dykes that intrude the granites) are similar to those of Paleozoic and Mesozoic dykes recorded in the southern continental part of the Cameroon Volcanic Line, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of the South Atlantic Ocean.

Role of Late Cretaceous volcanic-sedimentary melanges, specifically the Aladağ melange, E Turkey, in the rift-drift-subduction-accretion-emplacement history of the Tethyan Inner Tauride ocean

ABSTRACT The Aladağ melange is located between Mesozoic Tauride carbonate platform units and the Late Cretaceous Pozantı-Karsantı ophiolite and its metamorphic sole. The melange comprises a lower, mainly sedimentary unit, composed of variably disrupted thrust sheets of deep-sea sediments, with a localized matrix of mass-flow and turbidity current deposits. The upper, volcanic-sedimentary melange comprises highly disrupted basalt-pelagic carbonate-radiolarite units, rarely associated with gravity-flow deposits. Tholeiitic to alkaline basalts in the melange range from N-MORB, to E-MORB and to OIB, and are dated as Middle Jurassic to Early Cretaceous using Radiolaria from interbedded pelagic sediments. Comparisons with accretionary melanges throughout southern Turkey and elsewhere indicate a similar geochemical pattern of variable ‘enrichment’, commonly coupled with a subduction influence (e.g. negative Nb, Ta anomalies). Geochemically, the subduction-influenced basalts are consistent with an oceanic island arc origin. However, the geological setting of Triassic basalts within some of the melanges (e.g. Mersin) is indicative of a rifted continental margin setting. The Mesozoic basalts within the Turkish melanges considered here erupted along the distal, outer northern margin of the Tauride continent, followed by Late Cretaceous subduction-accretion and supra-subduction zone ophiolite emplacement. Regional to global chemical comparisons suggest that the subduction influence could represent geochemical subduction inheritance rather than contemporaneous subduction.

Major and trace element compositions of basaltic lavas from western margin of central main Ethiopian rift: enriched asthenosphere vs. mantle plume contribution

Major and trace element data are presented for basaltic lavas from western rift margin of central Main Ethiopian Rift located at Kella area to investigate the processes involved in the petrogenesis of the erupted magmas and the nature of mantle source compositions. Kella area is composed of Quaternary (<1.6 Ma), Miocene (10.6–8.3 Ma) and Oligocene basalts (30-29 Ma) ranging from alkaline to tholeiitic in composition. The geochemical variations of basaltic samples from Kella area exhibit two compositionally distinct basaltic groups. The Oligocene tholeiitic basalts display low MgO (5.29–6.11 wt.%), TiO2 (2.15–2.47 wt.%), P2O5 (0.28–0.34 wt.%), and high ratio of CaO/Al2O3 (0.68–0.72) and La/Nb (0.89–1.10). Whereas Quaternary and Miocene alkaline basalts display high MgO (7.40–8.86 wt.%), TiO2 (2.4–2.53 wt.%), P2O5 (0.44–0.52 wt.%) and low ratio of CaO/Al2O3 (0.62–0.66) and La/Nb (0.71–0.76). The contrasting incompatible element ratios (e.g., K/Nb, La/Nb, Rb/Zr and Zr/Nb) between tholeiite and alkaline basalts reflect differences in their mantle sources. Major and trace element variations, therefore, reflect the involvement of two geochemically distinct mantle sources in the petrogenesis of Kella basaltic lavas: i) the Oligocene tholeiite basaltic melts derived from enriched asthenosphere mantle source (E-MORB) and ii) the Miocene and Quaternary alkali basaltic melts show a close similarity with ocean island basalts (OIBs) geochemistry, and this end member ascribed to the arrival of Afar plume head. The geochemical modeling reveals that the Oligocene tholeiite basaltic melts produced by an equilibrium melting with 3–5 % degree of partial melting in spinel lherzolite mantle source, whereas the alkali basalts were produced with ∼2% degree of partial melting within spinel-garnet lherzolite transition zone mantle sources.

Alteration of high alkaline and alkaline basaltic rocks: parent rocks in the Lava Durian orchard, Sisaket Province, NE Thailand

Lava Durian Sisaket is the first geographically identified (GI) fruit related to the volcano in Thailand and distributed in three districts of Sisaket Province, the southernmost edge of the Khorat Plateau. The parent rocks of orchards are important for the description of soil and rock relation with respect to mineralogical and geochemical characteristics. This work aims to study lithology, mineralogy, and geochemistry of basaltic rocks, parent rocks of in situ soil in these orchards, and delineate the existing basaltic soil models. The several orchards are covered by reddish-brown to brown in situ soils, weathered from mafic volcanic rocks: porphyritic olivine basalt, vesicular olivine basalt, and nephelinite. The microscopic image analysis, XRD, and MiniSEM-EDS are used to classify mineralogy, while XRF and analysis of large and rare elements in ICP-MS/ICP-OES were used to determine parental rocks geochemistry and alteration. The olivine basalts comprise forsterite microphenocrysts associated with bytownite, diopside, augite, pigeonite, and ilmenite groundmass, while nephelinite is composed of nepheline groundmass and bytownite-labradorite, diopside, augite, pigeonite, and ilmenite crystals. In addition, these basalts display high alteration rates, especially olivine highly altered to iddingsite. According to the geochemical data, Sisaket's basalts were identified as alkali basalt and nepheline basanite with high LILEs and LREEs (La, Nd, Pr, Gd, Eu). The kaolinite, smectite, and illite are altered from felsic minerals, while the chlorite and iddingsite are from mafic minerals. The mineralogical analyses classified secondary phyllosilicates related to low-moderate temperature hydrothermal fluid, very high cation exchange capacity (H+, K+, Ca2+, Mg2+), and tropical weathering. The alkaline and high alkaline basalts, presenting as parent rocks, are one of the parameters that produced good nitisal soil of Sisaket's agricultural areas.

Na/K Diversity of Primary Basaltic Magmas Induced by the Separation of Slab-derived Supercritical Liquid: Implications from Alkali Basaltic Lavas from Rishiri Volcano, Southern Kuril Arc

Abstract Primary basaltic magmas in subduction zones exhibit chemical diversity even in individual volcanoes. In this study, we aim to elucidate the dominant factors that determine the chemical composition, especially the Na/K, of primary basaltic magmas by conducting a comparative petrological study of the high-Na/K Horyosawa lava and low-Na/K Nozuka lava from Rishiri Volcano in the southern Kuril Arc. Both lava flows are basaltic and are suitable for estimating the magma generation conditions. The whole-rock SiO2 content of the high-Na/K Horyosawa lava, the main target of this study, ranges from 51·9 to 53·4 wt%. The petrological and geochemical features of this lava indicate that the effects of magma mixing and assimilation of crustal materials were limited, and the low-SiO2 magmas were essentially derived via olivine fractionation from the primary magma. The primary magma is estimated to have been generated by ~2·4 % partial melting of the source mantle with ~0·10 wt% water at ~1300 °C and ~2·4 GPa. By comparing the results of the high-Na/K Horyosawa lavas with those published for the low-Na/K basalts (Nozuka lava), we find that (1) the slab-derived fluids involved in their formation were supercritical liquids with similar compositions, including Na/K, when released from the subducting slab, and (2) the Horyosawa primary magma was generated at a shallower level in the mantle than the Nozuka primary magma. From these observations, it is inferred that the primary magma of the Horyosawa lava was formed by the influx melting of aqueous fluid separated from the supercritical liquid at depths shallower than the critical point, whereas the Nozuka lava was generated by supercritical liquid-fluxed melting at depths deeper than the critical point. These findings show that elemental partitioning during the separation of slab-derived supercritical liquid into aqueous fluid and hydrous melt can induce chemical diversity within the primary basaltic magmas of a single volcanic system.

The geophysical signature of a continental intraplate volcanic system: From surface to mantle source

The structure of continental intraplate volcanic systems — which occur far from tectonic boundaries, unlike the majority of Earth's volcanism — is enigmatic and not fully understood, as are the underlying mechanisms responsible, due in part to a lack of high-resolution geophysical data. Central Mongolia contains Quaternary–Neogene aged alkaline basalt flows and volcanic cones, thousands of kilometres from active tectonic margins, in addition to an abundance of geochemical and petrological data — making this a natural laboratory to study intraplate volcanism. Using a recently collected, high-resolution, multi-scale, magnetotelluric dataset acquired across central Mongolia, we generate and analyze electrical resistivity models of the structure beneath the Tariat and Chuluut volcanic zones with the goal of imaging the volcanic system from surface to mantle source. The models reveal narrow, subvertical, lower resistivity anomalies in the middle-upper crust that are conspicuously located beneath surface expressions of volcanism. The lower crust (depths of 25–50 km) is characterized by the widespread distribution of isolated low-resistivity zones. A local low-resistivity zone is imaged in the mantle (depths of 60–90 km) above a broad, homogenous, doming low-resistivity feature. Considering the available evidence, we propose that the low-resistivity anomalies in the middle-upper crust are the remnant signatures of past transient magma pathways (or collection of pathways), caused by metasomatic alteration during the ascent of hot magmatic fluids. The lower crustal anomalies are interpreted to be domains of saline fluids in a thermally perturbed lower crust. In the mantle, the low-resistivity structure is explained by a broad mantle upwelling and thermal anomaly with a local zone of low-percent partial melt — the source for intraplate volcanism. The geophysical images are consistent with geochemical and petrological evidence from erupted lavas that indicates a single common mantle source region, limited crustal contamination, and rapid direct ascent, making crustal magma storage unlikely. Thus the geophysical models show remarkable and unique translithospheric images of a continental intraplate volcanic system, from surface to mantle source, with the results relevant to other continental regions.

Basaltic dykes and their xenoliths from the Gerroa–Kiama region, southern Sydney Basin, New South Wales: evidence for multiple intrusive episodes

The orientation, petrography and geochemistry of numerous basaltic dykes between Kiama and Gerroa were investigated in this study. Geochemical analysis shows that the dykes range from sub-alkaline to alkaline in composition. The dykes fit into three distinct geochemical groups, with most being transitional from alkali basalts to foidites, followed by alkali basalts and lastly andesites/basaltic andesites. We suggest that three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes contain phenocrysts of olivine, plagioclase and clinopyroxene in a fine-grained groundmass. Multiple microtextures were found, including porphyritic, glomeroporphyritic, amygdaloidal, aphanitic and intergranular. Most dykes strike northwest–southeast, two strike east–west, and four strike northeast–southwest. One dyke to the south of Boat Harbour Reserve contains numerous xenoliths, including gabbroic, granitic and charnockitic types. The gabbroic xenoliths were likely sourced from the mid-crust, whereas the granitic xenoliths were likely derived from the upper crust. Joints in the study area strike northwest–southeast, north–south, east–west and northeast–southwest. The dominant joint set strikes northwest–southeast, and secondary joint sets strike northeast–southwest. The least pervasive joint sets strike east–west and approximately north–south. The dykes intruded along long-lived inherent structural weaknesses in the host rocks, as their orientations match the orientations of dominant joint sets. Geochemically, the dykes are distinct in having an abundance of foidites, which are rare within the Sydney Basin. They are clearly different from the nearby upper Permian K-rich Gerringong Volcanics, but many of the alkali basaltic dykes are like those of the Sydney Basin, and the foidites are similar in composition to the lower Eocene Kulnura basalt on the NSW Central Coast. The orientation, geographic distribution and geochemistry of the dykes strongly suggest that the basaltic magmas have been injected through the same upper-crustal weaknesses over a protracted period. KEY POINTS Dykes occurring between Kiama and Gerroa fall under three separate geochemical groups: alkali basalts to foidites, alkali basalts and andesites/basaltic andesites. Three intrusive episodes occurred based on petrographic analysis and geochemical classification. The dykes intruded along long-lived inherent structural weaknesses in the host rocks.

Geochemistry of Late Permian basalts from boreholes in the Sichuan Basin, SW China: Implications for an extension of the Emeishan large igneous province

The emplacement of the Late Permian Emeishan large igneous province (ELIP) in the Upper Yangtze craton, SW China, is considered to be a crucial factor that triggered the end-Guadalupian biodiversity crisis. However, the contribution of ELIP remains debated because of its relatively modest known size. The restricted assessment of its areal extent is, in part, due to the lack of study of the potential volcanic rocks concealed beneath younger strata. Here we present the first geochemical and isotopic variations and detailed spatial distribution of Late Permian basaltic rocks penetrated by deep exploratory boreholes in the Sichuan Basin. The Late Permian basaltic rocks are mainly distributed in the western and northeastern parts of the Sichuan Basin and are positioned at the same stratigraphic level as the Emeishan flood basalts. Petrography, whole-rock trace elements and Sr-Nd-Pb isotopic compositions demonstrate that these rocks are alkaline basalts and have oceanic island basalt - like characteristics, indicating a similar origin as the Emeishan flood basalts. The trace element and isotope ratios suggest that the basaltic magmas in the western Sichuan Basin were derived by low degrees of partial melting of a garnet-bearing source. In contrast, the basalts in the northeastern part of the basin were generated by higher degrees of melting of a shallower mantle source in the spinel-garnet transition zone. Varying amounts of contamination by lower continental crustal melts are suggested by rough correlations between isotope and immobile trace element ratios. The spatial changes in composition and origin of the basaltic rocks in the Sichuan Basin can be interpreted as the result of varying extents of Emeishan mantle plume-lithosphere interaction under different lithospheric stress backgrounds, such that the reactivation of pre-existing basement faults controlled the emplacement of basaltic magma in the periphery of the ELIP. Our work reveals a substantially broader extent of the ELIP in buried sedimentary strata, which might have contributed to the abrupt end-Guadalupian climatic change.

Mid-Cretaceous Wake seamounts in NW Pacific originate from secondary mantle plumes with Arago hotspot composition

The geochemistry of oceanic intraplate (primarily oceanic island and seamount) lavas can provide essential information on the composition and evolution of their mantle source and geodynamics. Due to very limited rock sampling of the many mid-Cretaceous Wake seamounts in NW Pacific, the mantle source lithology and petrogenesis of their lavas, and the geodynamic mechanism responsible for generating the lavas have not been fully delineated. In order to help resolve these issues, here we present whole-rock major-trace element and Sr-Nd-Pb-Hf isotopic data for twenty-one lava samples collected from Lamont, Dacheng, Xufu, Penglai, Niulang, Zhinyu, and Zhanlu seamounts in the Southern Wake seamount trail (WST). These lava samples are silica-undersaturated alkali basalt and basanites/nephelinite. They have high CaO, FeOT and TiO2 contents and CaO/Al2O3 ratios, consistent with their derivation from partial melting of a carbonated peridotite or reaction between carbonated MORB-eclogite-derived silicate melts and fertile peridotite. High Zr/Hf and negative Zr-Hf-Ti anomalies in the most mafic lavas further suggest a contribution from carbonated components in their mantle source. These lavas show FOZO (focal zone)-HIMU (high μ = 238U/204Pb)-like Sr-Nd-Pb-Hf isotopic compositions (e.g., (206Pb/204Pb)i = 19.36–20.72), falling within the Arago (also known as “Young Rurutu” or “Atiu”) hotspot field. Combined with the sparse previous age and geochemical data, we propose that WST lavas were most likely derived from partial melting of secondary plume clusters emanating from the top of Arago mantle plume trapped at the mantle transition zone. Alternatively, WST lavas could have come from a number of secondary plumelets emanating from the top of the Pacific Large Low Shear Velocity Province (LLSVP). The simultaneously upwelling secondary plumes or plumelets generated chronologically overlapping, compositionally similar and closely-spaced Wake seamounts atop the moving Pacific plate.

Various sources of rare earth element enrichment at Manamas volcanic rock, Timor Island

Manamas volcanic rock formed due to crustal thinning in fore arc setting. This research aims to provide information and the enrichment process of rare earth elements in Manamas Formation on the Timor Island and their tectonic implication. Manamas volcanic rock exposed in Bihati River, Baun, Timor consists of two different types of basalts, namely alkaline basalt and sub alkaline basalt. Analysis using ICP-MS method shows enrichment in large ion lithophile element and high field strength element. Subalkaline basalt has N-MORB patterns and alkaline basalt have OIB patterns. The Nb element is relatively impoverished that indicates influence of subduction activities. Thorium and uranium elements also show significant enrichment, due to sedimentary rocks contamination or continental crust or directly from the asthenosphere due to magma upwelling. The two distinctive patterns interpreted due to slab tear phenomenon beneath Timor Island during Australia oceanic plate subduction and recycled oceanic crust beneath Banda Arc.

A Plate‐Mantle Convection System in the West Pacific Revealed by Tertiary Ultramafic‐Mafic Volcanic Rocks in Southeast China

AbstractCenozoic volcanic rocks in the East Asian continental margin usually form because of asthenosphere upwelling and resultant decompression melting, and their origins are important in deciphering mantle geodynamics attributed to intraplate volcanism. In this study, Oligocene basanite‐tephrite rocks were found in continental interior outcrops, and Miocene to Pliocene basanite, alkaline basalt, and olivine tholeiite were abundant in coastal areas in Zhejiang, Southeast China. Episodic magmatic activities appeared to migrate oceanward with an ∼100‐km spatial gap, which was temporally and spatially synchronized with repeated retreat and rollback of the Tertiary westward subducting Pacific slab. All rocks had OIB‐like trace‐element patterns, but the basanite‐tephrite rocks contained higher LILE, HFSE, and LREE than the alkaline basalt and olivine tholeiite. Geochemical modeling indicated the basanite‐tephrite and the alkaline and olivine tholeiite basalts had low‐degree (7%–15%) and slightly higher degree (20%–30%) partial melting of heterogeneous pyroxenite sources, respectively. In addition, their magma sources showed a progressive weakening of carbonatite metasomatism and decreasing melting depths. Those features were tightly associated with punctuated upwelling of asthenospheric materials, most likely metasomatized by the stagnant Pacific slab‐derived silica/carbonatite melts. A plate‐mantle convection system is proposed in which asthenosphere materials pulled by the subducting Pacific slab explain the intraplate volcanism in East Asia as asthenospheric upwelling, which formed by escaping from either slab tears or slab front edges in the mantle transition zone.

Relationships among the Geordie Lake Cu-Pd deposit, alkaline basalt, and syenites in the Coldwell Complex, Midcontinent Rift, Canada

ABSTRACT The Geordie Lake Cu-Pd deposit is associated with troctolite at the base of the Geordie Lake intrusion, located near the center of the Coldwell Complex (1106.5 + 1.2 Ma). It is the only platinum group element deposit in the Midcontinent Rift associated with alkaline rocks. This study focuses on the long-standing questions regarding genetic relationships among the Geordie Lake gabbros, the Wolfcamp basalt, and the various syenites that make up the east-central portion of the Coldwell Complex. Primitive mantle-normalized trace-element patterns for the Geordie Lake intrusion are nearly flat from Th to Ce and show negative Sr, Eu, and Zr anomalies. Characteristic ratios for the Geordie Lake gabbro and troctolite include Th/Nb (0.12), La/Nb (1.1), La/Lu (150), La/Sm (6.9), Zr/Sm (18), and Gd/Yb (2.8). Trace-element patterns that are useful for determining petrogenesis for gabbros are similar to the Wolfcamp basalt and augite syenite with some key exceptions, notably the middle rare earth element and Zr abundances. Affects due to metasomatism or crustal contamination in Wolfcamp basalt and Geordie Lake gabbros and syenites are negligible. Results of Rayleigh fractionation modeling show (1) the Geordie Lake intrusion and Wolfcamp basalt are very similar but not directly related by crystallization, (2) the gabbros and basalt are not related to the syenites, (3) the lower augite syenite can be related to the upper augite syenite and amphibole quartz syenite by fractionation of a hypothetical crystal cumulate composed of orthoclase (78%), clinopyroxene (15%), olivine (1%), and titanomagnetite (6%). We conclude that the Geordie Lake intrusion, Wolfcamp basalt, and saturated syenites in the Coldwell were derived by separate partial melting events in a common mantle source. The origin of the sulfide mineralization is enigmatic because it exhibits characteristics of both magmatic and hydrothermal processes. The sulfide assemblage changes from disseminated bornite and chalcopyrite in the basal zone to pyrrhotite plus chalcopyrite in the upper zones. Sulfides occur as coarse blebs interstitial to fresh or partly altered silicates, or as very fine grains intergrown with clusters of biotite and actinolite. Primitive mantle-normalized platinum group element patterns exhibit a W-shape for Pd-Pt-Rh-Ir-Ni, indicating a relative depletion of Pt and Ir. The Cu/Pd ratios in the mineralized zones are within the range of mantle values (1000–10,000), Pd/Pt is 14–19, Pd/Rh is 91 + 37, and Pd/Ir &amp;gt;16,000. The Pd/Pt, Pd/Rh, and Pd/Ir are considerably higher than in the Wolfcamp basalt (&amp;lt;1, 17, and 75, respectively). If the sulfides are magmatic in origin, then either the Geordie Lake magma was, unlike the Wolfcamp basalt magma, depleted in Pt, Rh, and Ir, or these elements were selectively removed from the sulfide assemblage. Alternatively, Pd was enriched by late-stage hydrothermal processes. Additional work is recommended to constrain petrogenesis of the sulfides by detailed base-metal and TABS (Te, As, Bi, Sb, and Sn) element analysis.

Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes

Basanite-alkali basalt-tholeiite suites have unique chemical and isotopic signatures associated with recycled crustal materials. However, the relative contribution of recycled carbonates and silicates is unclear owing to the inability of traditional isotopic approaches (e.g., Sr, Nd, Pb, and O isotopes) to fully identify trace amounts of recycled crustal materials. On the other hand, the identification of the types of recycled components would help decipher origin of these magmatic suites. Since the zinc isotopic ratios (reported as the per mil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard, δ66Zn) are distinct between marine carbonates and silicate reservoirs (e.g., mantle, oceanic mafic crust, and siliciclastic sediments), they could be used to identify the different source endmembers in magmatic rocks. In this work, we used the Zn and Sr-Nd isotopic ratios of Cenozoic continental basalts from the Central North China Craton (also known as Trans-North China Orogen, TNCO) to identify the nature of the recycled materials present in the mantle source of these basanite-alkali basalt-tholeiite suite. We found that, basanites and alkali basalts have high δ66Zn values (ranging from 0.32‰ to 0.46‰), while tholeiites have low δ66Zn values (0.28 ± 0.04‰, 2sd) similar to mid-oceanic ridge basalts (MORB) (δ66Zn = 0.27 ± 0.05‰, 2sd). Given the limited extent of Zn isotopic fractionation (<0.1‰) during crystallization and <20% partial melting, the elevated δ66Zn values observed in the basanites and alkali basalts reflect the involvement of recycled marine carbonates (which have an average δ66Zn of ∼0.91‰) in the mantle sources. Moreover, the δ66Zn values of these basalts show clear correlations with carbonate metasomatism indexes (such as, Ti/Eu and Zr/Hf ratios). Mixing models for δ66Zn values vs. 87Sr/86Sr ratios and εNd(t) values demonstrate that the mantle sources of basanites, alkali basalts, and tholeiites were metasomatized by different proportions of carbonates and siliceous sediments. Furthermore, both isotopic and geochemical compositions exhibit symmetrical variations from the central zone towards the south and the north along the TNCO, that is, δ66Zn values and 143Nd/144Nd and Ce/Pb ratios decrease, while SiO2 content and 87Sr/86Sr ratios increase. We suggest that more carbonates and fewer siliceous sediments are released from the subducted slab, with an increase in the depth of subduction. We further propose that this process is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.

An Experimental Study of Trace Element Partitioning between Peridotite Minerals and Alkaline Basaltic Melts at 1250°C and 1 GPa: Crystal and Melt Composition Impacts on Partition Coefficients

Abstract Peridotite–magma interaction is important in establishing magma pathways through the mantle and in metasomatism of the lithospheric mantle. Reactions that consume orthopyroxene and produce olivine and clinopyroxene are of particular interest because these reactions should lead to a redistribution of trace elements between the solid and melt phases at equilibrium. This study examines interaction of a silica-undersaturated alkaline basalt (basanite) with a range of peridotite compositions from dunite, through harzburgite to wehrlite at 1250°C and 1 GPa. Our experiments used the natural concentration of trace elements in the starting materials which allowed us to measure mineral—olivine partition coefficients for Rb, Ca, Co, Sr, Sc, Ct, Y, Ti, V and Zr. For orthopyroxene—and clinopyroxene—melt we additionally measured partitioning of Cs, Ba, all rare earth elements (REE; except Pm), Hf, Th, U, Nb and Ta. We show that there are subtle variations in the partition coefficients, particularly of the REEs that are related to the bulk composition of the system. We also show that with the exception of cations that can have multiple valence states, e.g. vanadium, the lattice strain model and in particular the double fit routine gives excellent agreement between the calculated and experimentally determined partition coefficients. The double fit model allows us to examine the effect of mineral composition on partitioning such that we can show preference of trace elements for the M1 and M2 sites in the pyroxenes. Although our results are similar to those of previous studies, there are two main differences: first we have a complete set of partition coefficients for every trace element that is measurable by LA-ICPMS in our starting material, where previous studies may be missing one or more elements in particular one or more of the middle REE in the pyroxenes Second, we show that although partition coefficients for trace elements in orthopyroxene are comparable between this and previous studies, the REE in clinopyroxene are typically a factor of 2–3 lower in this study. We also note that are correlations between partition coefficient and the composition of olivine, orthopyroxene, clinopyroxene and glass (melt). The relation of partitioning to melt composition suggests that some further development of the lattice strain model is needed. Finally, we show that there is agreement between our measured partition coefficients and those predicted from parameterized models of clinopyroxene–melt partitioning, however, there are unresolved differences that may result from differences in the substitution mechanisms of trace elements in M1 vs. M2 sites in clinopyroxene that are in part related to the composition of the coexisting melt.

Radiogenic (Sr-Nd-Hf-Os) isotope data for neogene volcanic rocks from the rif–betic orogen and lanzarote (Canary Islands) in the western Mediterranean

In this communication we present new radiogenic Sr-Nd-Hf-Os isotope data for Neogene volcanic rocks from the Rif-Betic Orogen and Lanzarote Island in the western Mediterranean along with some previously published data. Major and trace element data for the Rif samples are provided as a data supplement. Geological context and petrographic details for equivalent rocks have been published previously but the new data, especially the Hf and Os isotope data should be of use for future appraisals of the distribution of sub-crustal mantle domains in the region. Upon inspection, it would appear that alkali basalts, shoshonites and lamproites sample very different sub-continental lithospheric mantle sources in Spain and Morocco whereas there are close similarities between Lanzarote basalts and mantle xenoliths from the Atlas Mountains in Morocco.

Alkalinity of ocean island lavas decoupled from enriched source components: A case study from the EM1-PREMA Tasmantid mantle plume

The alkalinity of ocean island basalts (OIB), which form by upwelling thermo-chemical instabilities in the mantle, is often associated with the degree of melting. Yet it remains to be tested if alkalinity and the degree of melting are systematically associated with enriched mantle components. The Tasmantid Seamounts, which are fossil remnants of the Tasmantid mantle plume, comprise a north-south age-progressive submarine volcanic chain in the Coral and Tasman Seas East of Australia. Dredged seafloor lavas from nine seamounts along the seamount chain, ranging in age from 50 to 6.5 Ma, show a dichotomy in alkalinity, similar to those observed in other hotspot areas such as Hawai’i or Pitcairn. In radiogenic Sr-Nd-Hf-Pb isotope systematics, Tasmantid Seamount lavas form a continuum between EM1 and PREMA and are thus part of the prevalent EM1-PREMA association in the Pacific. However, their radiogenic Pb isotope systematics mark the Tasmantid plume as an individuum with no resemblance to any other plume. In stable Fe isotopes (expressed as δ57FePrim, which is the Fe isotopic composition calculated to primitive lavas along a liquid line of descent) alkali basalts are, on average, isotopically heavier than tholeiitic lavas, ranging in δ57FePrim from +0.10 to +0.21‰, unrelated to radiogenic isotope systematics, but with co-variations of δ57FePrim and Ti* (which is the primitive melt Ti content). These systematics point towards residual garnet as a key factor and thus indicate a solely petrologic relation between degrees of melting and Fe isotopes. Tholeiitic lavas exhibit a near-depleted mantle like isotopic composition (ranging in δ57FePrim from −0.01 to +0.22‰, median at δ57FePrim + 0.04‰), yet with values of up to δ57FePrim + 0.22‰ that require an isotopically heavy source of unknown origin.The lack of systematics between alkalinity and radiogenic isotope signatures indicates that temperature is the likely driving force for variable melting degrees in some samples over others. Based on these observations, we surmise that tholeiitic lavas form in the plume centre whereas alkaline lavas form at the cooler rim of the conduit. Mixing between melts and associated enriched components in the Tasmantid mantle plume may occur but, at least for alkaline lavas, only on a small scale.

Metallogenic fingerprint of a metasomatized lithospheric mantle feeding gold endowment in the western Mediterranean basin

AbstractSpinel peridotite xenoliths (one plagioclase-bearing) hosted in alkaline basalts from Tallante (southeast Spain) record the mineralogical and geochemical fingerprint of the subcontinental lithospheric mantle (SCLM) evolution beneath the southern Iberian margin. Mantle metasomatism in fertile lherzolites caused the crystallization of clinopyroxene + orthopyroxene + spinel clusters through the percolation of Miocene subalkaline melts during the westward migration of the subduction front in the western Mediterranean. In the Pliocene, heat and volatiles provided by alkaline host-magmas triggered very low melting degrees of metasomatic pyroxene-spinel assemblages, producing melt quenched to silicate glass and reactive spongy coronae around clinopyroxene and spinel. Refertilization of the Tallante peridotites induced the precipitation of base-metal sulfides (BMS) included in metasomatic clino- and orthopyroxene. These sulfides consist of pentlandite ± chalcopyrite ± bornite aggregates with homogeneous composition in terms of major elements (Ni, Fe, Cu) and semi-metals (Se, As, Te, Sb, Bi), but with wide variability of platinum-group elements (PGE) fractionation (0.14 &amp;lt; PdN/IrN &amp;lt; 30.74). Heterogeneous PGE signatures, as well as the presence of euhedral Pt-Pd-Sn-rich platinum-group minerals (PGM) and/or Au-particles within BMS, cannot be explained by conventional models of chalcophile partitioning from sulfide melt. Alternatively, we suggest that they reflect the incorporation of distinct populations of BMS, PGM, and metal nanoparticles (especially of Pt, Pd, and Au) during mantle melting and/or melt percolation. Therefore, we conclude that Miocene subalkaline melts released by asthenosphere upwelling upon slab tearing of the Iberian continental margin effectively stored metals in metasomatized domains of this sector of the SCLM. Remarkably high Au concentrations in Tallante BMS (median 1.78 ppm) support that these metasomatized domains provided a fertile source of metals, especially gold, for the ore-productive Miocene magmatism of the westernmost Mediterranean.

Post-spreading volcanism triggered by CO2 along the South China Sea fossil spreading axis

The post-spreading seamount chain formed along the fossil ridge of the South China Sea (SCS) consists of carbonated silicate melts and alkaline basalts. These temporally and spatially linked samples give an intriguing case where CO2-rich magmas and associated basaltic rocks occurred upon the ultra-thin oceanic lithosphere, and thus they could carry robust information on the generation of alkaline magma via melting of carbonated mantle source. Here we present magnesium isotopic data (δ26Mg), and elemental and radiogenic isotope data for the lavas from the on-axis SCS seamount chain. Both the early- and late-stage samples classified based on their formation ages exhibit remarkable δ26Mg variations (−0.53‰ to −0.11‰ and − 0.38‰ to −0.23‰, respectively). The low-δ26Mg signatures found in two groups of the SCS samples indicate a recycled origin for the source carbon inventory. However, the low-δ26Mg endmember of the early-stage samples is characterized by low SiO2 and enrichment in rare earth elements (REEs), whereas the late-stage samples display a low-δ26Mg endmember that produced high-SiO2 and REE-depleted melts. The Mg-Nd isotope systematics suggest that the early-stage lavas were yielded by CO2-enhanced melting of sediment-rich oceanic crust (εNd = 4.2), whereas the late-stage lavas have sourced the residual and carbon-deficient oceanic crust (εNd = 8.7). Our model demonstrates that the melting of carbonated component can produce compositionally distinct carbonatitic and silicate melts in sequence due to different solidus temperatures. Owe to the relatively thin lithosphere, these two types of melt could individually appear along the fossil ridge of the SCS.

Origin of alkali olivine basalts and hawaiites in the western Mexican arc: Evidence of rapid phenocryst growth and magma mixing during ascent along fractures

Abstract A detailed petrological study is presented to constrain the origin of a suite of alkali olivine basalt and hawaiite (&amp;gt;5 wt% MgO) lavas that were erupted in a rift zone within the western Mexican arc (Trans-Mexican Volcanic Belt), adjacent to the Sangangüey andesitic stratovolcano, together with more evolved lavas (mugearites and benmoreites; &amp;lt;5 wt% MgO). As previously documented in the literature, the Sangangüey mafic lavas are devoid of any arc geochemical signature, despite their location within an arc. In this study, a new olivine-melt thermometer/hygrometer, based on the partitioning behavior of Ni2+ and Mg2+, was applied to the Sangangüey basalts (SB). The results show that the high-MgO (&amp;gt;9 wt%) SB crystallized at higher temperatures and lower melt-water contents (0–1.3 wt%) compared to high-MgO arc basalts (≤5.7 wt% H2O) erupted in the west-central Mexican arc. The Sangangüey lavas with 5–8 wt% MgO display evidence of mixing between high-MgO alkali olivine basalts and low-MgO mugearites. It is proposed that the unique composition of the mugearites (i.e., low SiO2 contents and elevated FeO and TiO2 contents) is the result of partial melting of mafic lower crust driven by the influx of high-MgO intraplate basalts under relatively hot, dry, and reduced conditions. On the basis of crystal textures and compositional zoning patterns, it is shown that both phenocryst growth and magma mixing occurred rapidly, most likely during ascent along fractures, and not slowly during prolonged storage in a crustal magma chamber.

The origin of the earliest Jurassic basaltic rocks in southern Jiangxi Province, southeastern China: Implications for interaction between the asthenosphere and metasomatised lithosphere

Compositions of the earliest Jurassic basalts (190–188 Ma) from the the Baimianshi, Changpu, and Dongkeng basins in southern Jiangxi Province provide insight into the nature of their mantle sources as well as evidence for asthenosphere–lithospheric mantle interaction beneath Southeastern China. We report for the first time elemental and SrNd isotopic data for the basalts collected from new drill cores. The basalts are mainly tholeiitic and subordinate alkaline basalts. The alkaline basalts have high TiO2 contents (3.13–4.19 wt%), and high Ti/Y (596–699) and Nb/Y ratios (0.85–0.92) and Nb/La ratios (0.80–0.95). These basalts are characterised by ocean island basalt (OIB)-like trace element characteristics, and have primitive mantle-like εNd(t) values (−0.85 to +0.33) and moderately elevated (87Sr/86Sr)i ratios (0.7074–0.7082). The tholeiitic basalts have a large range of TiO2 contents (1.26–3.26 wt%) and Nb/La ratios (0.40–0.88), plus relatively low Ti/Y (135–534) and Nb/Y ratios (0.32–0.69). Based on Nb/La ratios, the tholeiitic basaltic rocks can further divided into two sub-types: high Nb/La (Nb/La > 0.70) and low Nb/La (Nb/La < 0.70) tholeiitic basalts. The high Nb/La tholeiitic basalts exhibit slight NbTa depletions (Nb/La = 0.73–0.88), mostly positive εNd(t) values (−0.04 to +2.5) and slightly elevated (87Sr/86Sr)i ratios (0.7061–0.7088), whereas the low Nb/La tholeiitic basalts exhibit moderate to large NbTa depletions (Nb/La = 0.40–0.65), much lower εNd(t) values (−7.4 to −0.60) and higher (87Sr/86Sr)i ratios (0.7082–0.7115). In particular, the two low Nb/La tholeiitic basalts (termed as “high-Ba tholeiitic basalts” in this study) are characterised by island arc-like trace element features, such as significant enrichments in Ba and Th, and depletions in Nb, Ta, P, and Ti (Nb/La = 0.40). The two high-Ba basalts also exhibit significantly lower εNd(t) values (−7.3 to −7.4) and higher (87Sr/86Sr)i ratios (0.7112–0.7115). Based on the results, we propose that the parental magmas of the alkaline basalts were produced by low degrees (& 5%–7%) of partial melting of an asthenospheric mantle source at the depth within the garnet–spinel-bearing mantle with more garnet than spinel, the parental magmas of the high Nb/La tholeiitic basalts were generated by moderate degrees (& 15%–20%) of partial melting of a depleted asthenospheric mantle at a relatively shallow depth within the garnet–spinel stability field, the parental magmas of the high-Ba tholeiitic basalts were derived from lithospheric mantle that had been metasomatised by subducted sediment-derived melts and/or fluids. We envision the formation of tholeiitic to alkaline basaltic magmas in the earliest Jurassic in South China records was the result of interactions between the asthenosphere and metasomatised lithosphere during upwelling of the asthenospheric mantle that was triggered by the foundering and/or delamination of a subducted flat slab beneath this region and steep NW-ward palaeo-Pacific subduction.