Distinct Mantle Domains Research Articles

Rifting evolution of the lithospheric subcontinental mantle: New insights from the External Ligurian ophiolites (Northern Apennine, Italy)

The present study focuses on the petrographic and petrological characteristics of mantle bodies included in Upper Cretaceous sedimentary melanges of the External Ligurian units (Northern Appennine), within the Monte Gavi and Monte Sant'Agostino areas. Two distinct pyroxenite-bearing mantle sections were recognized, mostly based on their plagioclase-facies evolution. The Monte Gavi mantle section is nearly undeformed and records a process of melt infiltration and reaction under plagioclase-facies conditions. The melt-rock interaction event involved both peridotites (mostly harzburgites) and enclosed spinel pyroxenite layers, and is estimated to have occurred at 0.7–0.8 GPa. In the Monte Gavi peridotites and pyroxenites, the spinel-facies clinopyroxene was partially replaced by plagioclase and new orthopyroxene (± secondary clinopyroxene). The reactive melt migration led to relatively high TiO2 contents in relict clinopyroxene and spinel (up to 2.3 wt% and 1.0 wt%, respectively, in the pyroxenites), with the latter also having high Cr# (up to 35 in the peridotites). The Monte Sant'Agostino mantle section displays a widespread ductile shearing and no evidence for melt-rock interaction under plagioclase-facies conditions. The main deformation phase recorded by the Monte Sant'Agostino peridotites (mostly lherzolites) is estimated to have occurred at 750–780 °C and 0.3–0.6 GPa, and gave rise to protomylonitic to ultramylonitic textures characterized by 10–50 μm neoblasts. The enclosed pyroxenite layers yielded relatively high temperature and pressure estimates (870–930 °C and 0.8–0.9 GPa). Presumably, in the Monte Sant'Agostino mantle section, plagioclase crystallization occurred earlier in the pyroxenites than in enclosing lherzolites, thereby enhancing strain localization and formation of mylonite shear zones in the entire mantle section. We propose that subcontinental mantle section from the External Ligurian units consists of three distinct mantle domains, developed in response to the rifting evolution that ultimately formed a Middle Jurassic ocean-continent transition: (1) a spinel tectonite domain that underwent no significant deformation and melt-rock reaction under plagioclase-facies conditions, characterized by static plagioclase development, (2) a plagioclase mylonite domain experiencing melt-absent deformation, and (3) a nearly undeformed domain that underwent melt infiltration and reaction under plagioclase-facies conditions. We relate mantle domains (1,2) to a rifting-driven uplift in the late Triassic accommodated by large-scale shear zones consisting of plagioclase mylonites.

Sulfide mantle source heterogeneity recorded in basaltic lavas from the Azores

New highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and Os isotopic compositions, along with major- and trace-element abundance and Sr-Nd-Hf-Pb isotope data are reported for high-MgO (>8–17 wt.%) lavas from the islands of São Miguel, Terceira, Pico and Faial in the Azores archipelago. The lavas span the range of Sr-Nd-Hf-Pb isotope and trace-element abundance variability reported previously for the islands. Pico and Faial lavas preserve distinct absolute and relative HSE abundances and 187Os/188Os (ƩHSE = 2 ± 2 ng g−1; 2SD; 187Os/188Os in lavas with > 50 pg g−1 Os = 0.1255 ± 31; 2SD) compared with São Miguel and Terceira lavas (ƩHSE = 5 ± 3 ng g−1; 187Os/188Os50pg = 0.1284 ± 48). Although HSE abundance variations in volcanic rocks from individual Azorean islands can be explained by sulfide crystallization, those between Pico and Faial and São Miguel and Terceira can only reasonably be attributed to partial melting of distinct mantle sources.Pico and Faial lava compositions are consistent with high degrees of sulfide melting (90–100%) from a sulfide-depleted mantle source (<100 ppm) with relatively unradiogenic 187Os/188Os (<0.126). São Miguel and Terceira lavas likely derive from a sulfide-enriched (300–600 ppm) mantle source that underwent lesser degrees of sulfide melting (1–15%), with more radiogenic 187Os/188Os (>0.128). Radiogenic sulfides are not solely linked to the enriched lithophile isotope signature found in eastern São Miguel. Instead, São Miguel and Terceira magmatism may collectively result from metasomatism associated with the recycling of old subducted oceanic lithosphere. Abundances of sulfide-hosted HSE, and 187Os/188Os, are decoupled from silicate-hosted Sr-Nd-Hf-Pb isotope compositions, in the mantle. Compositional variations of the HSE among global OIB suggest that, while distinct mantle domains are represented by heterogeneous sulfide mineralogy, comprising common metasomatic (enriched) and magmatic (depleted) compositions, these sulfide populations are distinct beneath different OIB due to their origin in different lithospheric materials (e.g., oceanic crust versus pelagic sediment), and to the degree of partial melting caused by variable mantle potential temperature or lithospheric thickness.

Lateral Mixing Processes in the Hadean

AbstractSamples of ancient Eoarchean volcanism suggest the existence of isotopically distinct mantle domains. 142Nd evidence suggests that these domains formed within the first several hundred million years of Earth's history and were preserved until circa 3.7 Ga—a span of almost 800 Myr. Here we explore evolving mantle convection models under Hadean conditions to constrain mantle mixing dynamics and in particular focus on the generation, or preservation, of upper mantle hemispheric compositional anomalies. We find that initially radially homogenous models fail to produce hemispheric anomalies spontaneously due to mixing processes and that rheological and buoyancy effects, or asymmetric processes such as plate tectonics or meteorite impacts, do not generate long‐lived heterogeneities in upper mantle composition from homogenous initial conditions. In contrast, models initiated with compositional variations can preserve these hemispheric differences of periods &gt;800 Myr despite vigorous internal convection, as a function of the proclivity of these models to exhibit stagnant‐lid behavior, resulting in isolated convection cells with restricted lateral transport.

Subducted slab-plume interaction traced by magnesium isotopes in the northern margin of the Tarim Large Igneous Province

Incorporation of subducted slabs may account for the geochemical and isotopic variations of large igneous provinces (LIPs). However, the mechanism and process by which subducted slabs are involved into magmas is still highly debated. Here, we report a set of high resolution Mg isotopes for a suite of alkaline and Fe-rich rocks (including basalts, mafic–ultramafic layered intrusions, diabase dykes and mantle xenoliths in the kimberlitic rocks) from Tarim Large Igneous Province (TLIP). We observed that δ26Mg values of basalts range from −0.29 to −0.45‰, −0.31 to −0.42‰ for mafic–ultramafic layered intrusions, −0.28 to −0.31‰ for diabase dykes and −0.29 to −0.44‰ for pyroxenite xenoliths from the kimberlitic rocks, typically lighter than the normal mantle source (−0.25‰±0.04, 2 SD). After carefully precluding other possibilities, we propose that the light Mg isotopic compositions and high FeO contents should be ascribed to the involvement of recycled sedimentary carbonate rocks and pyroxenite/eclogite. Moreover, from basalts, through layered intrusions to diabase dykes, (87Sr/86Sr)i values and δ18OV-SMOW declined, whereas ε(Nd)t and δ26Mg values increased with progressive partial melting of mantle, indicating that components of carbonate rock and pyroxenite/eclogite in the mantle sources were waning over time. In combination with the previous reported Mg isotopes for carbonatite, nephelinite and kimberlitic rocks in TLIP, two distinct mantle domains are recognized for this province: 1) a lithospheric mantle source for basalts and mafic–ultramafic layered intrusions which were modified by calcite/dolomite and eclogite-derived high-Si melts, as evidenced by enriched Sr–Nd–O and light Mg isotopic compositions; 2) a plume source for carbonatite, nephelinite and kimberlitic rocks which were related to magnesite or periclase/perovskite involvement as reflected by depleted Sr–Nd–O and extremely light Mg isotopes. Ultimately, our study suggests that subducted slabs could make important contributions to LIP generation, and establishes a potential linkage between plate tectonics and mantle plume.

Coexistence of enriched and modern-like 142Nd signatures in Archean igneous rocks of the eastern Kaapvaal Craton, southern Africa

The short-lived 146Sm–142Nd isotope system is an important tool for tracing Hadean crust–mantle differentiation processes and constraining their imprint on much younger rocks from Archean cratons. We report the first comprehensive set of high-precision 142Nd analyses for granitoids and amphibolites of the Ancient Gneiss Complex (AGC; Swaziland) and the oldest metavolcanic units of the Barberton Greenstone Belt (BGB; South Africa). The investigated samples span an age range from 3.66 Ga to 3.22 Ga and are representative of major geological units of the AGC and the lower Onverwacht Group of the BGB. Measured samples yielded μ142Nd values in the range from −8 ppm to +3 ppm relative to the JNdi-1 terrestrial standard, with typical errors smaller than 4.4 ppm. The distribution of the μ142Nd values for these 17 measured samples is bimodal with ten samples showing a tendency towards slightly negative μ142Nd anomalies, whereas seven samples have 142Nd similar to the terrestrial reference. The only confidently resolvable μ142Nd anomalies were found in a 3.44 Ga Ngwane Gneiss sample and in amphibolites of the ca. 3.45 Ga Dwalile Greenstone Remnant, revealing μ142Nd values ranging from −7.9±4.4 to −6.1±4.3 ppm. The μ142Nd deficits do not correlate with age, lithological unit, or sample locality. Instead, our results reveal that two distinct mantle domains were involved in the formation of the AGC crust. The two reservoirs can be distinguished by their μ142Nd signatures. Mantle-derived rocks tapped the enriched reservoir with negative μ142Nd at least until 3.46 Ga, whereas the granitoids preserved a negative μ142Nd signature that formed by incorporation of older AGC crust at least until 3.22 Ga. The oldest gneisses with no μ142Nd anomaly are up to 3.64 Ga in age, indicating that a modern terrestrial 142Nd reservoir was already present by early Archean times.

Helium isotope systematics in the vicinity of the Azores triple junction: Constraints on the Azores geodynamics

We present new He isotopic data from subaerial and submarine samples collected along the Terceira Rift (Azores Plateau) in order to constrain the sources involved in the Azores magmatism in the unique geodynamic context of the Azores triple junction. Lower than MORB 4He/3He isotopic ratios were determined on olivines from the Terceira Island and the adjacent Serreta submarine ridge (down to ≈69,000; ≈10.5 R/Ra), which in addition to the Ne isotopes previously published in Madureira et al. (2005) [Primitive helium and neon isotopes in Terceira Island: Constraints on the origin of the Azores archipelago. Earth and Planetary Science Letters 233, 429–440], support the geochemical models that consider the involvement of the lower mantle in the Azores magmatism. The analysis of He data published for the Mid-Atlantic Ridge (MAR) points to the existence of chemically distinct mantle domains underneath Eurasia and Nubia lithospheric plates, which can result from an heterogeneous distribution of asthenosphere metasomatism during the initial stages of plume activity in the Azores region. At the Terceira Rift, lavas from Graciosa Island, as well as from the western end of São Miguel Island and D. João de Castro Bank, display 4He/3He ratios similar to those observed along the MAR segments located to the north of the Azores triple junction area. Conversely, samples from the south Hirondelle Basin and also Faial display a He isotopic signature similar to that of MORB erupted along MAR segments located to the south of the Azores Plateau. The Terceira Rift and the Azores triple junction area are thus characterized by the mingling of two different asthenospheric domains referred as “Eurasia” and “Nubia” type and by the presence of mantle plume derived material having primitive helium signature.

Two tales of the continental lithospheric mantle prior to the destruction of the North China Craton: Insights from Early Cretaceous mafic intrusions in western Shandong, East China

Weakened lithospheric zones such as the Dabie–Sulu orogenic belt and Tan-Lu fault zone played important roles in the destruction of the North China Craton (NCC) during the late Mesozoic. Early Cretaceous mafic intrusions in western Shandong, contemporary with extensive magmatism during the destruction of the NCC, delineate two spatially distinct mantle domains (EM1- and EM2-like) beneath the craton’s interior and weakened lithospheric zones, respectively. The Jinan and Zouping gabbros from the craton interior (∼128Ma) show fractionated LREE and nearly flat HREE patterns ([La/Yb]N=2.94–8.95; [Dy/Yb]N=1.23–1.69) with notable negative Ta, Nb and Ti anomalies. They have strong negative ɛNd(t) (−15.7 to −7.1), low initial 87Sr/86Sr (0.7039–0.7060) and negative zircon ɛHf(t) of −20.0 to −6.2. These “crustal fingerprints” cannot be explained by crustal contamination, but were likely derived from a hybrid mantle source. Crustal delamination or detachment during the Early Paleoproterozoic might be responsible for the involvement of Early Precambrian crustal materials in the Mesozoic mantle source beneath the southeastern NCC. In comparison, the Early Cretaceous mafic igneous rocks from regions (e.g., Yinan, Mengyin and Fangcheng) adjacent to the Dabie–Sulu orogenic belt and Tan-Lu fault zone have higher 87Sr/86Sr ratios (0.7059–0.7119), suggesting modification of the lithospheric mantle by melts/fluids derived from the Yangtze crust. The Mesozoic crustal delamination may have triggered the destruction of the lithospheric root beneath the Dabie–Sulu orogenic belt, whereas the lithospheric thinning beneath the interior of the southeastern NCC is attributed to the thermo-mechanical erosion by lateral convective asthenosphere.

우리나라 상부암석권 맨틀: 페리도타이트 포획암으로부터의 고찰

우리나라 백령도, 제주도, 보은, 아산, 평택, 간성일대에는 맨틀 페리도타이트들이 알칼리 현무암류에 포획되어 온 형태로 분포하고 있다. K-Ar 전암연대측정 자료에 의하면 이들 화산암류는 약 0.1-18.9Ma에 분출하였다. 페리도타이트의 광물조합은 감람석-사방휘석-단사휘석-첨정석이며, 함수광물이나 석류석이 보고된 바는 없다. 암석의 종류는 레어조라이트에서 하즈버가이트에 걸쳐있다. 감람석은 <TEX>$Fo_{88.4-92.0}$</TEX>이고, 단사휘석은 투휘석, 사방휘석은 엔스터타이트가 풍부하며, 첨정석은 Cr이 풍부하다(Cr# = 7.8-53.6). 주성분 원소함량에 의하면 이들 페리도타이트는 부화된 중앙해령현무암 근원 맨틀로부터 대략 26%까지의 부분용융을 겪은 후에 남은 잔류물로 추정된다. 그러나 맨틀에서의 이차적인 변성교대작용의 흔적이 미량원소의 함량에 기록되어 있는 경우도 있다. 양휘석 지온계에 의하면 우리나라 페리도타이트 포획암의 평형온도는 대략 <TEX>$850-1050^{\circ}C$</TEX> 범위이다. Sr-Nd 동위원소비는 결핍된 중앙해령현무암의 근원맨틀 성분(DMM)에서부터 지구전암(BSE) 성분 값까지 넓은 범위에 걸쳐있어, 오랜 시간의 성장과 진화의 역사를 대변하고 있다. Sr-Nd-Pb 동위원소비에 의하면 우리나라 암석권 맨틀은 대개 맨틀 단성분 중 DMM과 EM2의 혼합으로 설명되며, 상부 대륙지각과 연계될 수 있는 남북 내지는 동서방향으로의 지역성을 기록하고 있지는 않다. 이는 동아시아의 신생대후기 판내부기원 현무암류들이 기록하고 있는 남북간의 이분성과는 매우 대조적인 것으로, 동아시아 연약권 맨틀의 지역성을 시사하고 있다. Os 모델연대에 기초하면 우리나라 암석권 맨틀이 연약권으로부터 분리된 시기는 약 1.8-1.9 Ga로 추정된다. Peridotite xenoliths hosted by alkali basalts from South Korea occur in Baengnyeong Island, Jeju Island, Boeun, Asan, Pyeongtaek and Ganseong areas. K-Ar whole-rock ages of the basaltic rocks range from 0.1 to 18.9 Ma. The peridotites are dominantly lherzolites and magnesian harzburgites, and the constituent minerals are Fo-rich olivine (<TEX>$Fo_{88.4-92.0}$</TEX>), En-rich orthopyroxene, Di-rich clinopyroxene, and Cr-rich spinel (Cr# = 7.8-53.6). Hydrous minerals, such as pargasite and phlogopite, or garnet have not been reported yet. The Korean peridotites are residues after variable degree of partial melting (up to 26%) and melt extraction from fertile MORB mantle. However, some samples (usually refractory harzburgites) exhibit metasomatic enrichment of the highly incompatible elements, such as LREE. Equilibration temperatures estimated using two-pyroxene geothermometry range from ca. 850 to <TEX>$1050^{\circ}C$</TEX>. Sr and Nd isotopic compositions in clinopyroxene separates from the Korean peridotites show trends between depleted MORB-like mantle (DMM) and bulk silicate earth (BSE), which can be explained by secondary metasomatic overprinting of a precursor time-integrated depleted mantle. The Korean peridotite clinopyroxenes define mixing trends between DMM and EM2 end members on Sr-Pb and Nd-Pb isotopic correlation diagrams, without any corresponding changes in the basement. This is contrary to what we observe in late Cenozoic intraplate volcanism in East Asia which shows two distinct mantle sources such as a DMM-EM1 array for NE China including Baengnyeong Island and a DMM-EM2 array for Southeast Asia including Jeju Island. This observation suggests the existence of large-scale two distinct mantle domains in the shallow asthenosphere beneath East Asia. The Re-Os model ages on Korean peridotites indicate that they have been isolated from convecting mantle between ca. 1.8 and 1.9 Ga.

Late Archean to Early Proterozoic lithospheric mantle beneath the western North China craton: Sr–Nd–Os isotopes of peridotite xenoliths from Yangyuan and Fansi

Sr–Nd–Os isotopic analyses are presented for peridotite xenoliths from Tertiary alkali basalts in Yangyuan and Fansi with the aim of identifying and characterizing the relics of ancient lithospheric mantle that survived lithospheric removal in the western North China Craton (NCC). The analyzed samples are residual lherzolites and harzburgites, ranging from fertile to highly depleted (0.36–4.0 wt% Al 2O 3) composition. Some LREE-enriched samples are characterized by moderate 87Sr/ 86Sr (0.7044 to 0.7047) and low ε Nd (− 6.9 to − 10.6), pointing to an EMI-type signature. This is distinct from the predominant depleted isotopic composition in mantle xenoliths from eastern China. Os isotopic ratios range from 0.1106 to 0.1325. The lower limit is the most unradiogenic value measured so far for Cenozoic basalt-borne xenoliths from eastern China. Two samples show radiogenic Os ratios higher than that of the primitive upper mantle, one sample has an anomalously high Os concentration (> 9 ppb). These samples also show high La/Yb, consistent with the addition of radiogenic components during the infiltration of asthenosphere-derived and/or subduction-related melts in the lithospheric mantle. The remaining samples define positive correlations between 187Os/ 188Os and indices of melt extraction, which yield a model age of ∼ 2.6 Ga. This age of melt extraction overlaps with the Nd model age of the overlying crust, indicating a coupled crust–mantle system in the western NCC. This contrasts with the decoupled nature in the eastern NCC, suggesting distinct mantle domains underneath the NCC. Such a heterogeneous age structure of the upper mantle is compatible with the view that the lithospheric removal was largely limited to the eastern NCC.

Adakite-like porphyries from the southern Tibetan continental collision zones: evidence for slab melt metasomatism

We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dyn/Ybn and Lan/Ybn. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial 87Sr/86Sr and 206Pb/204Pb ratios, and the highest 144Nd/143Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.

Early Cretaceous gabbroic complex from Yinan, Shandong Province: petrogenesis and mantle domains beneath the North China Craton

Sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO2, low FeO and TiO2 contents and a strong enrichment of light rare earth elements [(La/Yb)n=11–50], but no Eu anomaly. They have low Nb/La (0.07–0.29), radiogenic 87Sr/86Sr (0.710) and unradiogenic eNd(t) (−15 to −13). These “crustal fingerprints” cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (>20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low 87Sr/86Sr) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high 87Sr/86Sr) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros (127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.

Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N): age, source, and relationship to the Iceland and putative Jan Mayen plumes

The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71°N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope compositions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr=0.70344–0.70352, 143Nd/144Nd=0.51283–0.51288, 206Pb/204Pb=18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb=18.95) together with relatively radiogenic Nd (143Nd/144Nd=0.51295) and low-radiogenic Sr (87Sr/86Sr=0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6°N, apparently reflecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge.

Geochemical and isotopic (Sr, Nd, Pb) evidence for plume–lithosphere interactions in the genesis of Grande Comore magmas (Indian Ocean)

Three groups are distinguished among the Grande Comore lavas: Old Karthala, Recent Karthala and La Grille. Major and trace element data allow a clear distinction between Karthala saturated to slightly undersaturated lavas (shield-stage) and La Grille strongly undersaturated primary melts (post-shield stage). All these lavas are strongly enriched in incompatible elements, with relative enrichments in Ba, Nb, La and relative depletions in Rb, K, Sr, Sm. Low HREE contents and strongly fractionated REE patterns indicate that garnet is residual in their sources. La Grille lavas exhibit the highest and most variable degrees of incompatible element enrichment, the strongest relative K depletion and a wide range of the most incompatible element ratios. These lavas are low-degree melts of an amphibole-bearing source. Ratios of the most incompatible elements are homogeneous among Karthala lavas. They were produced by slightly higher degrees of melting, with much smaller amounts of residual garnet and amphibole, probably at shallower depth, than La Grille lavas. La Grille lavas exhibit the most radiogenic 143Nd/ 144Nd (0.51283–0.51287) and the least radiogenic 87Sr/ 86Sr (0.70315–0.70336), 206Pb/ 204Pb (18.98–19.31) and 208Pb/ 204Pb (38.80–39.13). Old Karthala lavas display the most radiogenic 206Pb/ 204Pb (19.47–19.86) of the data set and intermediate 87Sr/ 86Sr and 143Nd/ 144Nd. Recent Karthala lavas exhibit the most radiogenic 87Sr/ 86Sr (0.70362–0.70396), the least radiogenic 143Nd/ 144Nd (0.51265–0.51272) and intermediate 206Pb/ 204Pb. Karthala lavas having `LoNd compositions' require the mixing of closely associated EM I and HIMU components. Recent Karthala lavas reflect a stronger contribution of the EM I component whereas Old Karthala lavas are the most enriched in the HIMU component and contain a additional DM component. La Grille lavas reflect the significant involvement of a DM component, in addition to HIMU and EM I components. A strong coupling between isotopes and incompatible element ratios is observed among La Grille lavas with the most isotopically depleted samples exhibiting the most enriched trace element signature. The geochemical and isotope variability recorded in Grande Comore lavas is a consequence of small and variable degrees of melting of the heterogeneous underlying mantle. We propose that the EM I and HIMU components are closely associated within the mantle plume and that the DM component is located in the oceanic lithosphere. Karthala lavas, corresponding to shallower, higher-degree melts, give the best estimate of the plume composition and must have been produced close to the plume axis. La Grille low-degree melts require lower temperatures for their genesis and must have been generated at the periphery of the plume. The two volcanoes belong to two distinct magmatic systems. The atypical `LoNd' signature of the Grande Comore lavas, contrasting with the Indian Ocean Dupal signature, may be explained by the specific location of the Comores Archipelago, at the junction of two distinct oceanic crusts, in the transition zone between two distinct mantle domains.

The radiogenic isotope composition of the high-latitude North Atlantic mantle

Research Article| May 01, 1997 The radiogenic isotope composition of the high-latitude North Atlantic mantle Dieter F. Mertz; Dieter F. Mertz 1Johannes Gutenberg-Universität, Institut für Geowissenschaften, 55099 Mainz, Germany2Max Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Search for other works by this author on: GSW Google Scholar Karsten M. Haase Karsten M. Haase 2Max Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany3Geologisch-Paläontologisches Institut der Universität Kiel, Olshausenstraße 40, 24118 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Dieter F. Mertz 1Johannes Gutenberg-Universität, Institut für Geowissenschaften, 55099 Mainz, Germany2Max Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany Karsten M. Haase 2Max Planck-Institut für Chemie, Abteilung Geochemie, Postfach 3060, 55020 Mainz, Germany3Geologisch-Paläontologisches Institut der Universität Kiel, Olshausenstraße 40, 24118 Kiel, Germany Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1997) 25 (5): 411–414. https://doi.org/10.1130/0091-7613(1997)025<0411:TRICOT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Dieter F. Mertz, Karsten M. Haase; The radiogenic isotope composition of the high-latitude North Atlantic mantle. Geology 1997;; 25 (5): 411–414. doi: https://doi.org/10.1130/0091-7613(1997)025<0411:TRICOT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract New Sr, Nd, and Pb isotope data on North Atlantic high-latitude (about 69°30'N to 76°30'N) intraplate and ridge volcanic rocks, together with published data, are used to evaluate the mantle sources contributing to Iceland and the surrounding ridges. Considerable overlap between Icelandic lavas and depth-filtered North Atlantic mid-oceanic-ridge basalts (MORBs) in all isotope plots confirm that a MORB asthenosphere-plume mixing model can explain the ranges of isotope compositions found on Iceland. Regional isotope patterns north of lat 53°N show two distinct mantle domains. Lavas forming a low-radiogenic-Pb, high-radiogenic-Sr array occur north of central Iceland, whereas volcanic rocks forming a high-radiogenic-Pb, low-radiogenic-Sr array mainly erupt on and south of Iceland. The depleted end members of each array most likely represent MORB source material. The enriched end member of the high-radiogenic-Pb, low-radiogenic-Sr array appears to reside in the Iceland plume. The other enriched end member also occurs in on-axis and mainly intraplate magmas north of about lat 71°N. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trace-element and isotopic characteristics of small-degree melts of the asthenosphere: Evidence from the alkalic basalts of the Antarctic Peninsula

Miocene-Recent continental alkalic basalts were erupted along the Antarctic Peninsula as a result of decompressional melting of the asthenosphere caused by the formation of slab-windows beneath the continental margin following the cessation of subduction. The basalts appear not to be related to a period of major lithospheric attenuation, nor were they formed as a result of the influence of a mantle plume. They exhibit strong trace-element and isotopic affinities with OIB, Sr- and Nd-isotope compositions ranging from 0.70269 to 0.70343 and 0.512863 to 0.51300, respectively, similar to the composition of HIMU OIB. However, new Pb-isotope analyses show that 206 Pb 204 Pb ratios (18.79–19.28) fall within the range for E-type MORB with Δ8 4 and Δ7 4 varying from −28 to +26 and from +1 to +10, respectively. Δ8 4- values , Sr-isotope ratios and some LILE/HFSE ratios exhibit negative covariations with La n Yb n and Nb/Y ratios implying some control of degree of partial melting on geochemical composition. Nb/U ratios (14–40) are considerably lower than most OIB and MORB. The basalts also have unusually low absolute abundances of Rb and Ba and high K/Ba and K/Rb ratios (50–140 and 400–1500, respectively). Correlated PbSrNd isotope and trace-element behaviour suggests that the asthenosphere from which these basalts were derived was subjected to multiple melt extraction/depletion events. One period of melt extraction was ancient (∼ 1.7 Ga) and similar to that affecting MORB source mantle, and was followed by a more recent (?Mesozoic) event. This more recent event resulted in increased U/Pb, U/Nb and U/Th ratios and further depletion in ultra-incompatible element such as Rb and Ba, causing high K/Rb and K/Ba ratios in the erupted lavas. This implies that the asthenosphere beneath the Antarctic Peninsula is heterogeneous on a small scale. Small-degree melts are capable of sampling geochemically, and possibly mineralogically, distinct mantle domains from larger-degree melts. During larger degrees of partial melting, the scale of melting approaches the scale of heterogeneity and integration of melts from different geochemical domains occurs.