The Fe/Mn ratio in MORB and OIB determined by ICP-MS

Variations in the abundance of iron in the mantle may have important consequences for mantle dynamics and geochemistry. The abundance of iron in lavas derived from mantle source regions varies during partial melting and subsequent fractionation, so that source heterogeneities are not easily resolved in iron abundances alone. However, manganese is a geochemically similar element, so that the planetary Fe/Mn ratio is approximately constant. Here, we report new Fe/Mn results for mid-oceanic ridge basalts (MORBs), oceanic island basalts (OIBs) and komatiites using a precise inductively coupled plasma mass spectrometric (ICP-MS) method to measure Fe/Mn to better than 0.5% (2σ). As a measure of reproducibility of Fe/Mn, five olivine and five orthopyroxene grains from a Kilbourne Hole peridotite xenolith yielded Fe/Mn 69.8 ± 0.4 and 44.3 ± 0.2, respectively. To avoid ubiquitous secondary Fe-Mn oxides, Fe/Mn ratios in Pacific, Atlantic, and Indian MORBs were determined by Laser Ablation ICP-MS. MORB Fe/Mn (53-56) corrected for crystal fractionation yielded a value of 54.0 ± 1.2 (1σ). Icelandic basalts and picrites (MgO 10-29%) had Fe/Mn ratios 56-61, with a single exception. Six relatively fresh komatiites from Belingwe (MgO 20-29%) yielded Fe/Mn values of 58.3 ± 0.2 (1σ). Basalts from Tahiti and Reunion exhibited high Fe/Mn (>65), like Hawaii. This implies that the mantle source regions of Tahiti and Reunion lavas may have been enriched in Fe relative to other mantle reservoirs (e.g., MORBs, Iceland, Belingwe). Combined with previous results for Hawaii, we now find that Fe/Mn > 65 is characteristic of at least two plumes from the Pacific Superswell. It is conceivable that this is evidence for excess Fe due to core-mantle interaction in these mantle plumes, although partial melting of secondary pyroxenites may cause similar variations in Fe/Mn. Heterogeneity of Fe/Mn in mantle-derived lavas is now clearly documented.     

Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

Basalt geochemistry as a diagnostic indicator of tectonic setting

Basalt geochemistry can be used as a diagnostic indicator for determining the tectonic setting of origin, because specific plate tectonic settings often impart distinctive geochemical characteristics. For example: (1) mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) have clearly distinguishable trace element and Sr-Nd isotope geochemical characteristics; (2) arc related basalts, including IOAB (intra-oceanic arc basalts), IAB (island arc basalts) and CAB (continental arc basalts), exhibit following distinguishing features: all are characterized by low Nb/La ratios (<0.85) and negative Nb, Ta and Ti anomalies; most exhibit low Nb concentrations (<8 ppm), high positive ɛ_Nd values and low enrichment of incompatible elements except the continental arc shoshonitic basalts that possess high concentrations of incompatible trace elements and lower to negative ɛ_Nd values; (3) although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated continental intraplate basalts as arc related, there are still some essential differences between continental intraplate basalts and arc related ones; such as: uncontaminated continental intraplate basalts have high Nb concentrations, Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛ_Nd values that distinguish them from the arc related ones; whereas, the contaminated continental intraplate basalts are characterized by pronounced negative Nb, Ta and Ti anomalies, but their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones; (4) an important difference between back-arc basin basalts (BABB) and the MORB is that the former exhibit both MORB-like and arc-like geochemical characteristics; (5) most oceanic plateau basalts (OPB) show diagnostic geochemical characteristics of enriched MORB (E-MORB) to transitional MORB (T-MORB); only the Kerguelen Plateau is an exception; the early (pre 90 Ma) volcanism of the Kerguelen Plateau is associated with the Early Cretaceous break-up of Gondwana and displays features of continental flood basaltic volcanism; with time, the tectonic setting of the Kerguelen plume-derived volcanism changed from a rifted continental margin setting (133–118 Ma) through a young, widening ocean (118–40 Ma), finally to an oceanic intraplate setting (~40 Ma to the present).Tectonic discrimination diagrams should not be used in isolation, but can still be useful as part of holistic geochemical characterization. For example: (1) MORB and OIB are distinguishable from each other in the 3Tb-Th-2Ta diagram; (2) the arc related basalts, including IOAB, IAB and CAB, constantly plot in the arc-related basalts fields in the Th/Yb-Ta/Yb diagram; (3) the 3Tb-Th-2Ta diagram can be utilized to fully illustrate both MORB-like and arc-like characteristics of BABB; (4) some discriminant diagrams (such as Zr/Y-Zr, Th/Yb-Ta/Yb, 3Tb-Th-2Ta and Hf/3-Th-Nb/16 diagrams) can be used to distinguish continental intra plate basalts from arc related ones; (5) although there are not any discrimination diagrams published that delineate an OPB field, some trace element diagrams can still reveal diagnostic characteristics of the OPB.     

中国东南沿海中-新生代玄武岩微量元素和Nd-Sr-Pb同位素研究

本文对中国东南沿海不含幔源包体的中生代玄武岩和含幔源包体的新生代玄武岩进行了微量元素和Nd-Sr-Pb同位素对比研究。中生代玄武岩呈Ta、Nb和Hf负异常,低Ce/Pb、Nb/U比值和高La/Nb比值,与岛弧火山岩和陆壳岩石的微量元素特征相类似,说明在岩浆生成和上升过程中,幔源组分受到了陆壳组分的混染。新生代玄武岩呈Ta、Nb正异常和Pb负异常,高Ce/Pb、Nb/U比值和低La/Nb比值,与海岛玄武岩(OIB)相类似,Nd-Sr同位素成分与夏威夷玄武岩类似,因而它们未受明显的陆壳混染。143Nd/144Nd与206Pb/204Pb之间的负相关关系和87Sr/86Sr与206Pb/204Pb之间的正相关关系说明本区新生代玄武岩起源于中等亏损程度的软流圈地幔,并与EMII富集地幔组分发生了混合。     

Evidence for high-pressure core-mantle differentiation from the metal-silicate partitioning of lithophile and weakly-siderophile elements

Liquid Fe metal-liquid silicate partition coefficients for the lithophile and weakly-siderophile elements Ta, Nb, V, Cr, Si, Mn, Ga, In and Zn have been measured in multianvil experiments performed from 2 to 24 GPa, 2023-2873 K and at oxygen fugacities of −1.3 to −4.2 log units relative to the iron-wüstite buffer. Compositional effects of light elements dissolved in the metal liquid (S, C) have been examined and experiments were performed in both graphite and MgO capsules, specifically to address the effect of C solubility in Fe-metal on siderophile element partitioning. The results were used to examine whether there is categorical evidence that a significant portion of metal-silicate equilibration occurred under very high pressures during core-mantle fractionation on Earth. Although the depletion of V from the mantle due to core formation is significantly greater than that of Nb, our results indicate that both elements have similar siderophile tendencies under reducing conditions at low pressures. With increasing pressure, however, Nb becomes less siderophile than V, implying that average metal-silicate equilibration pressures of at least 10-40 GPa are required to explain the Nb/V ratio of the mantle. Similarly the moderately-siderophile, volatile element ratios Ga/Mn and In/Zn are chondritic in the mantle but both volatility and core-mantle equilibration at low pressure would render these ratios strongly sub-chondritic. Our results indicate that pressures of metal-silicate partitioning exceeding 30-60 GPa would be required to render these element ratios chondritic in the mantle. These observations strongly indicate that metal-silicate equilibration must have occurred at high pressures, and therefore support core-formation models that involve deep magma oceans. Moreover, our results allow us to exclude models that envisage primarily low-pressure (<1 GPa) equilibration in relatively small planetary bodies. We also argue that the core cannot contain significant U as this would require metal-silicate equilibration at oxygen fugacities low enough for significant amounts of Ta to have also been extracted from the mantle. Likewise, as In is more siderophile than Pb but similarly volatile and also quite chalcophile it would have been difficult for Pb to enter the core without reversing the relative depletions of these elements in the mantle unless metal-silicate equilibration occurred at high pressures >20 GPa.     

Tracing the 850-Ma continental flood basalts from a piece of subducted continental crust in the North Qaidam UHPM belt,NW China

A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Qaidam Basin, North Tibetan Plateau. Eclogites in the Yuka eclogite terrane, northwest part of this UHPM belt, occur as blocks or layers of varying size intercalated with granitic and pelitic gneisses. These eclogites have protoliths geochemically similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB). On the basis of Ti/Y ratios, they can be divided into low-Ti and high-Ti groups. The low-Ti group (LTG) eclogites exhibit relatively low TiO₂ (most <2.5 wt%) and Ti/Y (<500) but comparatively high Mg# (48–55), whereas the high-Ti group (HTG) eclogites have high TiO₂ (most >2.5 wt%) and Ti/Y (>500) but lower Mg# (46–52). Zircons from two eclogite samples gave a magmatic crystallization (protolith) age of ∼850 Ma and a UHPM age of ∼433 Ma. The occurrence, geochemical features and age data of the Yuka eclogites suggest that their protoliths are segments of continental flood basalts (CFBs) with a mantle plume origin, similar to most typical CFBs. Our observation, together with the tectonic history and regional geologic context, lend support for the large scale onset of mantle plume within the Rodinia supercontinent at ∼850 Ma. The Qaidam block is probably one of the fragments of the Rodinia supercontinent with a volcanic-rifted passive margin. The latter may have been dragged to mantle depths by its subducting leading edge of the oceanic lithosphere in the Early Paleozoic.     

High-precision high field strength element partitioning between garnet, amphibole and alkaline melt from Kakanui, New Zealand

The high field strength elements (HFSE: Zr, Hf, Nb, Ta, and W) are an important group of chemical tracers that are increasingly used to investigate magmatic differentiation processes. Successful modeling of these processes requires the availability of accurate mineral-melt partition coefficients (D). To date, these have largely been determined by ion microprobe or laser ablation-ICP-MS analyses of the run products of high-pressure, high-temperature experiments. Since HFSE are (highly) incompatible, relatively immobile, high-charge, and difficult to ionize, these experiments and their analysis are challenging. Here we explore whether high-precision analyses of natural mineral-melt systems can provide additional constraints on HFSE partitioning.The HFSE concentrations in natural garnet and amphibole and their alkaline host melt from Kakanui, New Zealand are determined with high precision isotope dilution on a multi-collector-ICP-MS. Major and trace element compositions combined with Lu-Hf isotopic systematics and detailed petrographic sample analysis are used to assess mineral-melt equilibrium and to provide context for the HFSE D measurements. The whole-rock nephelinite, ∼1 mm sized amphiboles in the nephelinite, and garnet megacrysts have similar initial Hf isotope ratios with a mean initial ¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282900 ± 0.000026 (2σ). In contrast, the amphibole megacrysts are isotopically distinct (¹⁷⁶Hf/¹⁷⁷Hf_{(34 Ma)} = 0.282830 ± 0.000011). Rare earth element D values for garnet megacryst-nephelinite melt and ∼1 mm amphibole-nephelinite melt plotted as a function of ionic radii show classic near-parabolic trends that are in excellent agreement with crystal lattice-strain models. These observations are consistent with equilibrium between the whole-rock nephelinite, the ∼1 mm amphibole grains within the nephelinite and the garnet megacrysts.High-precision isotope dilution results for Zr and Hf in garnet (D_Zr = 0.220 ± 0.007 and D_Hf = 0.216 ± 0.005 [2σ]), and for all HFSE in amphibole are consistent with previous experimental findings. However, our measurements for Nb and Ta in garnet (D_Nb = 0.0007 ± 0.0001 and D_Ta = 0.0011 ± 0.0006 [2σ]) show that conventional methods may overestimate Nb and Ta concentrations, thereby overestimating both Nb and Ta absolute D values for garnet by up to 3 orders of magnitude and underestimating D_Nb/D_Ta by greater than a factor of 100. As a consequence, the role of residual garnet in imposing Nb/Ta fractionation may be less important than previously thought. Moreover, garnet D_Hf/D_W = 17 and D_Nb/D_Zr = 0.003 imply fractionation of Hf from W and Nb from Zr upon garnet crystallization, which may have influenced short-lived ¹⁸²Hf-¹⁸²W and ⁹²Nb-⁹²Zr isotopic systems in Hadean time.     

A high field strength element perspective on early lunar differentiation

Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured ⁹²Zr and ⁹¹Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the ¹⁸²W and ¹⁴²Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the ¹⁸²W vs. Hf/W and ¹⁴²Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar ¹⁸²Hf-¹⁸²W and ¹⁴⁶Sm-¹⁴²Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.     

吐哈盆地早二叠世玄武岩原始岩浆性质:来自熔融包裹体成分的制约

东天山地区的二叠纪玄武岩沿着区域的北东东向断裂呈脉状分布,吐哈盆地玄武岩的40Ar-39Ar坪年龄为298.2±3.8Ma,为早二叠世,与前人的玄武岩年龄结果在误差范围内一致。可能与东天山地区二叠纪岩浆铜镍矿床镁铁-超镁铁岩有密切的成因联系。吐哈玄武岩的主微量成分显示其为岛弧拉斑、大陆弧玄武岩,轻稀土富集和Nb、Ta负异常,指示源区可能经历过俯冲作用的改造。吐哈盆地二叠纪玄武岩含有新鲜的橄榄石和长石斑晶,橄榄石斑晶中熔融包裹体较发育。熔融包裹体为玻璃质、气相和玻璃质、气相、固相两种类型。包裹体中不透明矿物主要为磁铁矿,说明捕获包裹体时岩浆的氧逸度和Fe含量较高。熔融包裹体分为高MgO和低MgO含量两种。高MgO含量的包体同时具有低SiO_2、低微量和稀土元素含量的特征,可能为地幔高部分熔融的产物,且经历过深部演化程度较弱。该高MgO熔体的微量元素显示Nb、Ta亏损的特征,具有N-MORB特征的微量和稀土元素分配模式,预示该熔体为受到俯冲交代的地幔熔融形成。熔融包裹体相对玄武岩具有低的Th和Ta含量、相对弱的Nb和Ta的负异常的特征,指示熔融包裹体的成分经受改造程度低于玄武岩,暗示可能为经历过较少后期作用改造的相对原始的熔体。熔体中Cu含量(12.4×10~(-6)~299×10~(-6))在正常玄武质岩浆含量范围内,而Ni含量(236×10~(-6)~697×10~(-6))高于高镁溢流科马提岩和洋中脊玄武岩。该Cu、Ni含量略显解耦的熔体可能代表了经历过深部少量的硫化物熔离,带走小部分Cu和Ni等成矿元素之后所捕获的岩浆。如果将该熔体视为东天山地区二叠纪岩浆铜镍硫化物矿床的母岩浆,该母岩浆中Ni含量相对较高可能是岩浆铜镍硫化物矿床中矿石的Ni/Cu比值大多大于1.0的主要因素。     

The Composition and Age of the Mesoarchean Gabbro in the South Vygozersky and Kamennoozersky Greenstone Structures,Karelia

The geochemical and zircon geochronological (U-Pb, SHRIMP-II) study of Mesoarchean gabbros of the South Vygozersky and Kamennoozersky greenstone structures of Central Karelia made it possible to distinguish four gabbro types: (1) Fe–Ti gabbro, 2869 ± 12 Ma, (2) gabbro compositionally close to tholeiitic basalts, 2857 ± 7 Ma, (3) leucogbabbro, 2840 ± 5 Ma; and (4) melanogabbro, 2818 ± 14 Ma. From the early to late gabbros, the rocks are depleted in Ti, Fe, V, Y, Zr, Nb, Hf, REE and enriched in Mg, Ca, Cr, Ni. According to the systematics (Condie, 2005), the Nb/Y, Zr/Y, Zr/Nb ratios in the studied Late Archean gabbros are close to those of primitive mantle, while the gabbros in composition are similar to those of plumederived ocean-plateau basalts. Their magma sources were derived from different mantle reservoirs. The leucogabbro and melanogabbro with similar εNd = +4 were derived from a depleted mantle source (DM). The gabbro close in composition to tholeiitic basalts and having the elevated positive ε_Nd (+4.9) was derived from a strongly depleted mantle source. Insignificant admixture of crustal material or lithospheric mantle is inferred in a source of the Fe–Ti gabbro (with lowest ε_Nd = +2.1).     

Origin of early Cretaceous calc-alkaline lamprophyres from the Sulu orogen in eastern China: implications for enrichment processes beneath continental collisional belt

K-Ar dating, major- and trace-element and Sr-Nd isotopic analyses were carried out for early Cretaceous (122-127 Ma) lamprophyres from the Sulu orogen in eastern China. The results show strong fractionation in rare-earth elements with (LREE) >100 times chondrite, but HREE <10 times chondrite, indicating the presence of residual garnet in the melting source. These rocks are characterized by significant LILE and LREE enrichment but Nb and Ta depletion with moderate Zr/Hf (39.8-50.8 with regard to 36 for primitive mantle) and Nb/Ta (17.8-23.0, compared with 17.5 for primitive mantle) fractionations, probably as a consequence of carbonate- and rutile-rich melt metasomatism induced by dehydration and/or melting of subducted continental slab at mantle depths. Age-correlated Sr-Nd isotope ratios show moderate ranges of ⁸⁷Sr/⁸⁶Sr(i) from 0.70787 to 0.70934 and −17.2 to −11.6 of ε_Nd(t). The lamprophyres from the Sulu orogen were derived from decompression melting of such a metasomatized lithospheric mantle that was mainly composed of phlogophite garnet peridotites and experienced crystal fractionation of a mineral assemblage of olivine+clinopyroxene±plagioclase en route to the surface. Such geochemical and isotopic signatures are also prevalent in the contemporaneous basaltic lavas in the Dabie-Sulu belt, suggesting predominant enrichment processes by carbonate- and rutile-rich metasomatic assemblage beneath the continental collisional belt.     

Boninites as windows into trace element mobility in subduction zones

Boninites are subduction-related rocks originating from re-melting of highly depleted mantle sources left after extraction of tholeiitic melts. Due to their depleted nature, the incompatible trace element inventory of boninites is virtually entirely inherited from slab components without a significant contribution from the refractory mantle wedge. Thus, boninites constitute an excellent window into processes controlling trace element mobilization at the slab-mantle wedge interface. In order to constrain the behaviour of trace elements in subduction zones with a special emphasis on high field strength elements, we analyzed low-Ca boninites and associated tholeiitic basalts from Cape Vogel, Papua New Guinea (PNG) and compare them with compositions of high-Ca boninites and associated tholeiitic basalts from Cyprus. High-precision HFSE (Nb, Ta, Zr, Hf, W) concentration data of the boninites and associated tholeiitic basalts were obtained by isotope dilution. Major, trace element, and Sr-Nd-Hf-Pb isotope compositions clearly document a significant contribution of slab-derived melts involved in the petrogenesis of the PNG boninites, whereas only fluid-like subduction components were involved in the petrogenesis of the PNG basalts and the Cyprus suite. Low-Ca boninites from PNG are derived from a more refractory mantle source (∼21% depletion) than the high-Ca boninites from Cyprus (∼11% depletion) and their respective tholeiitic precursors (<10% depletion). In agreement with the more depleted nature of their mantle source, boninites exhibit a significantly stronger overprint by slab components. High-precision HFSE data indicate that, in comparison to LILE, a somewhat lower but measurable mobilization of all investigated HFSE in both slab-derived fluids and melts is evident. Modelling calculations demonstrate that the subduction components dominate the LILE budget and also largely control LREE and HFSE abundances in the boninite sources. Notably, the increasing influence of slab-derived fluids results in a decrease of the negative Nb-Ta anomaly, most likely reflecting a similar mobility of Nb-Ta and LREE at higher pressures near the critical point of fluid-melt miscibility. Ratios of Zr/Hf and Nb/Ta in the melt-like slab components dominating in the sources of the PNG boninites were probably fractionated in equilibrium with garnet-amphibolitic mafic oceanic crust. HFSE ratios in the Cyprus boninites are best explained by dehydration of subducted pelagic sediments in the absence of Ti-rich phases such as rutile. Our results also confirm previous assertions that the mobility of HFSE decreases in the order Sb > W-Mo > Nb-Ta > Zr-Hf. Furthermore, Mo-W systematics may provide a potential novel tracer for the amount, composition and redox state of subducted pelagic sediments that contribute to the geochemical budget of intra-oceanic arc systems.     

Permian volcanic rocks from the Apuseni Mountains (Romania): Geochemistry and tectonic constraints

An Early Permian volcanic assemblage is well exposed in the central-western part of the Apuseni Mountains (Romania). The rocks are represented by rhyolites, basalts and subordinate andesites suggesting a bimodal volcanic activity that is intimately associated with a post-orogenic (Variscan) syn-sedimentary intra-basinal continental molasse sequences. The mafic and mafic-intermediate rocks belong to sub-alkaline tholeiitic series were separated in three groups (I–III) showing a high Th and Pb abundances, depletion in Nb, Ta and Sr, and slightly enriched in LREE patterns (La_N/Yb_N = 1.4–4.4). Isotopically, the rocks of Group I have the initial ratios ⁸⁷Sr/⁸⁶Sr_(i) = 0.709351–0.707112, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512490–0.512588 and high positive ?_Nd270 values from 3.9 to 5.80; the rocks of Group II present for the initial ratios values ⁸⁷Sr/⁸⁶Sr_(i) = 0.709434–0.710092, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512231–0.512210 and for ?_Nd270 the negative values from −1.17 to −1.56; the rocks of Group III display for the initial ratios the values ⁸⁷Sr/⁸⁶Sr_(i) = 0.710751–0.709448, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512347–0.512411 and for ?_Nd270 the positive values from 1.64 to 2.35. The rocks resembling continental tholeiites, suggest a mantle origin and were further affected by fractionation and crustal contamination. In addition, the REE geochemistry (1 > Sm_N/Yb_N < 2.5; 0.9 > La_N/Sm_N < 2.5) suggests that these rocks were generated by high percentage partial melting of a metasomatized mantle in the garnet peridotite facies. The felsic rocks are enriched in Cs, Rb Th and U and depleted in Nb, Ta, Sr, Eu, and Ti. The REE fractionation patterns show a strong negative Eu anomaly (Eu/Eu* = 0.23–0.40). The felsic rocks show the initial ratios the values: ⁸⁷Sr/⁸⁶Sr_(i) = 0.704096–0.707805, ¹⁴³Nd/¹⁴⁴Nd_(i) = 0.512012–0.512021 and for ?_Nd270 the negative values from −5.27 to −5.44. They suggest to be generated within the lower crust during the emplacement of mantle-derived magmas that provided necessary heat to crustal partial melting.     

Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts?

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas.     

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources,plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.     

大兴安岭北部伊列克得组玄武岩的地球化学特征

大兴安岭北部伊列克得组玄岩类具有富碱（Ｋ２Ｏ＋Ｎａ２Ｏ〉６．３７％）、高Ｋ２Ｏ（２．６２％～３．１１％）,高Ｋ２Ｏ／Ｎａ２Ｏ比值（０．７～０．８）和Ｔｈ／Ｔａ、Ｃｅ／Ｎｂ、Ｔａ／Ｎｂ比值,以及强烈富集大离子亲石元素（ＬＩＬＥ）和轻稀土元素（ＬＲＥＥ）的特征,属于钾玄质系列,低Ｓｒ／Ｎｄ比值（１３～１７．５）的特征类似于洋岛玄武岩,但其Ｔａ／Ｈｆ,Ｔｈ／Ｔａ比值特征及与Ａ型花岗岩共生等特征表明,钾玄     

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb =  0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their ⁸⁷Sr/⁸⁶Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the ⁸⁷Sr/⁸⁶Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.     

Zircon U-Pb and Lu-Hf isotopic systematics of the Daping plutonic rocks: Implications for the Neoproterozoic tectonic evolution of the northeastern margin of the Indochina block, Southwest China

The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U-Pb dating on the zircon cores and rims of a hornblende-gabbro yield ²⁰⁶Pb/²³⁸U weighted means ages of 873 ± 9.1 Ma and 769 ± 7 Ma, respectively, and that for cores, mantles and rims of a granodiorite yield ²⁰⁶Pb/²³⁸U weighted means ages of 981-987 Ma, 829 ± 10 Ma and 761 ± 11 Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have ε_Hf^(t) values ranging from − 5.0 to − 5.8 and + 0.6 to + 6.4. Corresponding single-stage model ages range from 1626 to 1662 Ma and 1094 to 1311 Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) ε_Hf^(t) values of + 3.3 to + 12.3 with single-stage Hf model ages of 897 to 1235 Ma; and (2) ε_Hf^(t) values of − 1.9 to − 7.8 with single stage model ages of 1470-1667 Ma. The zircon rims are characterized by positive ε_Hf^(t) values (+ 5.4 to + 8.2) with single-stage model ages ranging from 977 to 1108 Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg^# (52.1-65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.     

Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

Bulk-rock geochemical compositions of hypabyssal kimberlites,emplaced through the Archaean Kaapvaal craton and ProterozoicNamaqua–Natal belt, are used to estimate close-to-primarymagma compositions of Group I kimberlites (Mg-number = 0·82–0·87;22–28 wt % MgO; 21–30 wt % SiO₂; 10–17 wt% CaO; 0·2–1·7 wt % K₂O) and Group II kimberlites(Mg-number = 0·86–0·89; 23–29 wt %MgO; 28–36 wt % SiO₂; 8–13 wt % CaO; 1·6–4·6wt % K₂O). Group I kimberlites are distinguished from GroupII by their lower Ba/Nb (<12), Th/Nb (<1·1) andLa/Nb (<1·1) but higher Ce/Pb (>22) ratios. Thedistinct rare earth element patterns of the two types of kimberlitesindicate a more highly metasomatized source for Group II kimberlites,with more residual clinopyroxene and less residual garnet. Thesimilarity of Sr and Nd isotope ratios and diagnostic traceelement ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group Ikimberlites to ocean island basalts (OIB), but more refractoryMg-numbers and Ni contents, are consistent with derivation ofGroup I kimberlites from subcontinental lithospheric mantle(SCLM) that has been enriched by OIB-like melts or fluids. Sourceenrichment ages and plate reconstructions support a direct associationof these melts or fluids with Mesozoic upwelling beneath southernAfrica of a mantle plume(s), at present located beneath thesouthern South Atlantic Ocean. In contrast, the geochemicalcharacteristics of both on- and off-craton Group II kimberlitesshow strong similarity to calc-alkaline magmas, particularlyin their Nb and Ta depletion and Pb enrichment. It is suggestedthat Group II kimberlites are derived from both Archaean andProterozoic lithospheric mantle source regions metasomatizedby melts or fluids associated with ancient subduction events,unrelated to mantle plume upwelling. The upwelling of mantleplumes beneath southern Africa during the Mesozoic, at the timeof Gondwana break-up, may have acted as a heat source for partialmelting of the SCLM and the generation of both Group I and GroupII kimberlite magmas. KEY WORDS: kimberlite; geochemistry; petrogenesis; mantle plumes; South Africa     

Role of lithosphere-asthenosphere interaction in the genesis of Quaternary alkali and tholeiitic basalts from Datong, western North China Craton

The geochemistry of Quaternary volcanic rocks from Datong provides important constraints on the petrogenesis of continental alkali and tholeiitic basalts and lithospheric evolution in the western North China Craton. Alkali basalts in north Datong have trace element compositions similar to oceanic island basalts (OIB). They show nearly homogenous isotopic compositions (?_Nd = 5.4-6.8 and ⁸⁷Sr / ⁸⁶Sr = 0.7035-0.7037) that resemble the nearby Hannuoba Miocene basalts, indicating that the two lava suites share a similar asthenospheric source. However, Datong basalts have conspicuously lower Al₂O₃ and CaO, higher SiO₂ and HREE contents and Na / Ti ratios, compared to Hannuoba lavas at comparable MgO. This compositional difference is attributable to the combined effect of source difference and temporal decrease in melting depth. The latter reflects Cenozoic lithospheric thinning of the western North China Craton.Tholeiitic basalts in southeast Datong have incompatible element ratios that differ from OIB; they have lower ?_Nd (1.3-3.7) and higher ⁸⁷Sr / ⁸⁶Sr (0.7039-0.7046) compared to alkali basalts. These moderately evolved rocks (MgO < 7%) display unusually high Cr concentrations (> 200 ppm), a nearly flat LREE pattern and a fractionated HREE with the “kink” occurring at Gd. A shallow melting depth (< 60 km), suggested by their Q-normative composition, is in conflict with the residual garnet in the source (> 75 km) as required by REE modeling. This paradox, which is reminiscent of that for Hawaiian tholeiites, can be reconciled if garnet lherzolite melts react with refractory peridotites during which orthopyroxene is dissolved and olivine precipitates. The diagnostic consequence of this melt-rock reaction includes increases in SiO₂ and Cr, decreases in Al₂O₃ and CaO, and formation of “kinked” REE patterns. Involvement of lithospheric mantle in the genesis of Datong tholeiites may be related to the Cenozoic lithospheric thinning/erosion in the western North China Craton. The spatial distribution of Datong alkali and tholeiitic basalts may be related to enhanced extension along the lithospheric boundary between the Western Block of the North China Craton and the Trans-North China Orogen.