Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The ¹⁸⁷Os/¹⁸⁸Os ratios in lavas with <30 ppt Os (n = 4) are elevated compared to ratios in coexisting olivine and appear to be contaminated at a shallow level. The ¹⁸⁷Os/¹⁸⁸Os ratios in the remaining lavas with >30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with ¹⁸⁷Os/¹⁸⁸Os of 0.1296). These low ¹⁸⁷Os/¹⁸⁸Os preclude significant shallow-level contamination from oceanic crust. The ¹⁸⁷Os/¹⁸⁸Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between ¹⁴³Nd/¹⁴⁴Nd and ¹⁸⁷Os/¹⁸⁸Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re-Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ³He/⁴He and ¹⁸⁷Os/¹⁸⁸Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic ¹⁸⁷Os/¹⁸⁸Os and ¹⁴³Nd/¹⁴⁴Nd, low ³He/⁴He and high ⁸⁷Sr/⁸⁶Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high ³He/⁴He (∼28 Ra), high ¹⁴³Nd/¹⁴⁴Nd (∼0.5132), high ¹⁸⁷Os/¹⁸⁸Os (∼0.14) and low ⁸⁷Sr/⁸⁶Sr (∼0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland-Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.     

The nature of the sub-continental mantle: constraints from the major-element composition of continental flood basalts

Many continental flood basalts (CFB) have isotope and trace-element signatures that differ from those of oceanic basalts and much interest concerns the extent to which these reflect differences in their upper mantle source regions. A review of selected data sets from the Mesozoic and Tertiary CFB confirms significant differences in their major- and trace-element compositions compared with those of basalts erupted through oceanic lithosphere. In general, those CFB suites characterised by low Nb/La, high (⁸⁷Sr/⁸⁶Sr)_i and low εNd_i tend to exhibit relatively low TiO₂, CaO/Al₂O₃, Na₂O and/or Fe₂O₃, and relatively high SiO₂. In contrast, those which have high Nb/La, low (⁸⁷Sr/⁸⁶Sr)_i and high εNd_i ratios, like the upper units in the Deccan Traps, have major- and trace-element compositions similar to oceanic basalts. It would appear that those CFB that have distinctive isotope and trace-element ratios also exhibit distinctive major-element contents, suggesting that major and trace elements have not been decoupled significantly during magma generation and differentiation.

When compared (at 8% MgO) with oceanic basalt trends, the displacement of many CFB to lower Na₂O, Fe₂O₃^*, TiO₂ and CaO/Al₂O₃, but higher SiO₂, at similar Mg#, is not readily explicable by crustal contamination. Rather, it reflects source composition and/or the effects of the melting processes. The model compositions of melts produced by decompression of mantle plumes beneath continental lithosphere have relatively low SiO₂ and high Fe₂O₃^*. In contrast, the available experimental data indicate that partial melts of peridotite have low TiO₂, Na₂O and Fe₂O₃^*CaO/Al₂O₃, if the peridotite has been previously depleted by melt extraction. Moreover, melting of hydrated, depleted peridotite yields SiO₂-rich, Fe₂O₃- and CaO-poor melts. Since anhydrous, depleted peridotite has a high-temperature solidus, it is argued that the source of these CFB was variably melt depleted and hydrated mantle, inferred to be within the lithosphere. Isotope data suggest these source regions were often old and relatively enriched in incompatible trace elements, and it is envisaged that H₂O±CO₂ were added at the same time as the incompatible elements. An implication is that a significant proportion of the new continental crust generated since the Permian reflected multistage processes involving mobilization of continental mantle lithosphere that was enriched in minor and trace elements during the Proterozoic.     

The nature and origin of geochemical variation in Mid-Atlantic Ridge basalts from the Central North Atlantic

Seventy-two basalts from 58 dredge stations located along the Mid-Atlantic Ridge from 29°N to 59°N have been analyzed for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and for K, Rb, Cc, Sr and Ba. The Sr-isotope profile along the ridge has three distinct maxima, one coinciding with the Azores platform (0.70345), one at 45°N (0.70340) and the third at 35°N, in the vicinity of the Oceanographer Fracture Zone. Basalts from ridge segments between 29°N and 33°N, and 49°N and 59°N have ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios typical of ‘normal’ mid-ocean ridge basalts (0.70230–0.70280). Profiles of K, Rb, Cs, Sr, Bz, Rb/Sr and Ba/Sr are similar to the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ profile, but Rb/K, Cs/K and Ba/K show broad maxima between 35°N and 45°N.These variations result from chemical and isotopic heterogeneity in the mantle, and are interpreted as caused by a mantle plume beneath the Azores which mixes with the LIL-element-depleted asthenosphere. Additional plumes may exist beneath 45°N and 35°N.Compared to the LIL-element-depleted asthenosphere, the Azores mantle plume is 10 to 30 times enriched in LIL elements with very small (? 0.1) bulk crystal/melt partition coefficients (Rb, Cs, Ba, La). Mildly incompatible elements (0.1 < D < 1) (Sr, Sm, Yb) are only 0.8–3 times enriched. These, observations suggest that LIL element differences between these two mantle reservoirs resulted from processes involving solid-liquid equilibria and not vapor-solid or vapor-liquid equilibria. Isotope systematics indicate that neither mantle reservoir remained a closed system since the formation of the Earth, but it is not possible to determine the time at which heterogeneity first developed.     

Iceland and the North Atlantic: A review

Iceland is a special volcanic island in an anomalous ocean basin. A review of the unusual features shows that among others topography and gravity are broadly positive, spreading has been and still is complex, seismicity is slightly diffuse and the chemistry of the basalts is different from that at normal ridges. In summary we observe a tendency of lithospheric dispersal and spreading in the North Atlantic and its surroundings. These observations together with what is known about Icelandic crust, heat flow, tectonic history, etc., point to a hot mantle upwelling beneath Iceland. The shape of the upwelling currents is not known. Although at present much speculation is possible, the authors prefers to think of a broad rising region uplifting the lithospheric plates such that they tend to slide away from Iceland more forcefully than is the case from normal spreading ridges.     

Seismicity and volcanism of Jan Mayen Island

The small, arctic Jan Mayen Island, site of the World's northernmost active volcano, Beerenberg, is part of the mid-Atlantic ridge system and located along the Jan Mayen Fracture Zone (JMFZ). Recent data from the local seismic network, and fault plane solutions from the global network, indicate that the island is located at a ridge-crest intersection, which might explain the origin of the island and the associated volcanism. Moreover, the new data suggest a series of offset segments of the Mohn's Ridge, overlapping in an en echelon pattern. In January 1985, a flank eruption was for the first time observed with the local seismic network. Volcanic tremors and low-frequency events were observed on 5 January at 2230 h and 10 hours later the first large earthquake occurred. No visual confirmation of the eruption was made until 6 January at 1630 h. The seismic observations rule out the possibility that the large earthquake caused the eruption; it seems more likely that the changes in local stress conditions triggered the earthquake and that the eruption started before the first large earthquake. Recent observations show that the local network provides an efficient tool for monitoring and warning of volcanic activity. However, since there was no change in the local seismicity in the days or months before the 1985 eruption, it seems to be difficult to make long-term predictions of Beerenberg flank eruptions without using other techniques such as observations of tilt.     

Rayleigh wave tomography in the North Atlantic: high resolution images of the Iceland, Azores and Eifel mantle plumes

Presented in this paper is a high resolution S_v-wave velocity and azimuthal anisotropy model for the upper mantle beneath the North Atlantic and surrounding region derived from the analysis of about 9000 fundamental and higher-mode Rayleigh waveforms. Much of the dataset comes from global and national digital seismic networks, but to improve the path coverage a number of instruments at coastal sites in northwest Europe, Iceland and eastern Greenland was deployed by us and a number of collaborators. The dense path coverage, the wide azimuthal distribution and the substantial higher-mode content of the dataset, as well as the relatively short path-lengths in the dataset have enabled us to build an upper mantle model with a horizontal resolution of a few hundred kilometers extending to 400 km depth. Low upper mantle velocities exist beneath three major hotspots: Iceland, the Azores and Eifel. The best depth resolution in the model occurs in NW Europe and in this area low S_v-velocities in the vicinity of the Eifel hotspot extend to about 400 km depth. Major negative velocity anomalies exist in the North Atlantic upper mantle beneath both Iceland and the Azores hotspots. Both anomalies are, above 200 km depth, 4–7% slow with respect to PREM and elongated along the mid-Atlantic Ridge. Low velocities extend to the south of Iceland beneath the Reykjanes Ridge where other geophysical and geochemical observations indicate the presence of hot plume material. The low velocities also extend beneath the Kolbeinsey Ridge north of Iceland, where there is also supporting geochemical evidence for the presence of hot plume material. The low-velocity upper mantle beneath the Kolbeinsey Ridge may also be associated with a plume beneath Jan Mayen. The anomaly associated with the Azores extends from about 25°N to 45°N along the ridge axis, which is in agreement with the area influenced by the Azores Plume, predicted from geophysical and geochemical observations. Compared to the anomaly associated with Iceland, the Azores anomaly is elongated further along the ridge, is shallower and decays more rapidly with depth. The fast propagation direction of horizontally propagating S_v-waves in the Atlantic south of Iceland correlates well with the east–west ridge-spreading direction at all depths and changes to a direction close to NS in the vicinity of Iceland.     

Tectonic evolution of the Iceland region,North Atlantic

Major hypotheses on the formation of the Iceland region are considered. It is noted that plate- and plume-tectonic genesis is the most substantiated hypothesis for this region. Model estimations of the effect of hot plume on the formation of genetically different oceanic ridges are obtained. Computer calculations are performed for the thermal subsidence rate of aseismic ridges (Ninetyeast and Hawaiian-Emperor) in the asthenosphere of the Indian and Pacific oceans. Comparative analysis of the calculated subsidence rates of these ridges with those in the Iceland region (Reykjanes and Kolbeinsey ridges) is performed. The results suggest that the thermophysical processes of formation of the spreading Reykjanes and Kolbeinsey ridges were similar to those of the aseismic Ninetyeast and Hawaiian-Emperor ridges: the genesis of all these ridges is related to the functioning of a hotspot. Analysis of the heat flux distribution in the Iceland Island and Hawaiian Rise areas is carried out. Analysis and numerical calculations indicate that the genesis of Iceland was initially characterized by the plume-tectonic transformation of a continental rather than oceanic lithosphere. The level of geothermal regime near Iceland was two times higher (100 mW/m²) relative to the Hawaiian Rise area (50 mW/m²) because the average lithosphere thickness of the Reykjanes and Kolbeinsey ridges near the Iceland was approximately two times less (40 km) relative to the thickness of the Pacific Plate (80 km) in the Hawaiian area. The main stages of evolution of the Iceland region are based on geological and geothermal data and numerical thermophysical modeling. The Cenozoic tectonic evolution of the region is considered. Paleogeodynamic reconstructions of the North Atlantic in the hotspot system at 60, 50, and 20 Ma are obtained.     

Changing compositions in the Iceland plume; Isotopic and elemental constraints from the Paleogene Faroe flood basalts

Elemental and Sr, Nd, Hf and high precision Pb isotopic data are presented from 59 low-Ti and high-Ti lavas from the syn-break up part of the Faroe Flood Basalt Province. The depleted MORB-like low-Ti lavas erupted in the rift zone between the Faroe Islands and central East Greenland around the time of break up of the North Atlantic have isotopic end-member compositions different from the depleted Iceland lavas. We suggest that the main low-Ti mantle component is NAEM (North Atlantic End-Member (Ellam and Stuart, 2000, J. Petrol. 41, 919) and that the ²⁰⁷Pb/²⁰⁴Pb value of the component should be 15.35 and ε_Hf = + 16.5. NAEM is the main depleted component in the early Iceland plume. This is supported by high mantle potential temperatures (up to 1550 °C) calculated for the source of the low-Ti basalts. The unique mantle isotopic composition of NAEM with low ²⁰⁶Pb/²⁰⁴Pb (17.5) and Δ7/4Pb (? 3.8) precludes a derivation from recycled MORB lithosphere. Instead we suggest that NAEM represents a plume component of recycled depleted Archean lithospheric mantle that was further depleted ~ 500 Ma ago, possibly in connection with the recycling process. Two other isotopic end-members are required to explain the variation of the Faroe low-Ti basalts: (1) The Faroe depleted component (FDC), with ⁸⁷Sr/⁸⁶Sr = 0.7025, ε_Nd = + 11, ε_Hf = + 19.5, ²⁰⁶Pb/²⁰⁴Pb = 18.2, ²⁰⁷Pb/²⁰⁴Pb = 15.454 and ²⁰⁸Pb/²⁰⁴Pb = 37.75, which is similar in composition to some Atlantic MORB and is regarded as a local upper mantle source. (2) An enriched EM-type component similar in geochemistry to the Icelandic Öræfajökull lavas. This component is believed to be recycled pelagic sediments in the plume but it can alternatively be a local crustal or lithospheric mantle component. The enriched Faroe high-Ti lavas erupted inland from the rift have isotopic compositions very similar to the enriched Icelandic neo-volcanics and these lava suites apparently share the two enriched plume end-members IE1 and IE2 (Geochim. Cosmochim. Acta 68, 2, 2004). The lack of mixing between high and low-Ti melts at the time of break up, is explained by a zoned plume where only low-Ti sources were present beneath the rift zone surrounded by high-Ti sources on both sides of the rift. The enriched plume components in the high-Ti lava sequences on the Faroe Islands and central East Greenland changed rapidly on a ka-scale which implies, from geophysical modelling, that this area was positioned above the center of the plume, and that the Iceland plume was centered under the Atlantic ridge already from the Paleocene.     

华北地幔橄榄岩锂同位素研究进展

作为非传统稳定同位素的代表,Li同位素已被广泛应用于示踪与熔体和流体相关的地质过程。地幔橄榄岩高度不均一的Li含量和Li同位素组成特征是熔/流体与地幔橄榄岩相互作用过程中同位素分馏作用及温度下降过程中Li在矿物之间扩散分馏的结果,同位素扩散现象通常被矿物颗粒边部的组成特征所记录。由于Li在橄榄石中的扩散速度比在辉石中慢,所以大颗粒橄榄石核部的Li含量和同位素组成更能反映岩石圈地幔的组成,而辉石的组成特征更多地记录了后期的过程。大量地幔捕虏体的锂同位素组成特征研究表明,华北克拉通岩石圈地幔经历了蚀变洋壳在俯冲过程中、残留俯冲板片在深部地幔、以及上涌的软流圈所释放的不同组成的熔/流体的改造过程。由于改造作用的多阶段性和改造程度的不同,华北克拉通东部与中部地幔橄榄岩的组成特征具有系统性的差异。Li同位素与其他地球化学指标的联合示踪是未来地幔地球化学研究的发展趋势。

     

Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle

Pb, O, Nd, and Sr isotopic data for the Columbia River basalts paint a complex picture for the origin of this flood basalt province. At least 3 distinct mantle sources appear to have been involved, superimposed upon which are the effects of crystal fractionation and mass exchange with evolved crustal wallrocks. To a large degree, the initiation of Columbia River volcanism and the geochemical characteristics of the basalts appear to have been influenced by subduction of the Juan de Fuca plate beneath the North American plate in a manner analogous to the origin of back-arc basins. The physical structure of the crust appears to have influenced the late stage evolution of the magmas by directing the locus of eruption to the border between the ancient continental interior and much younger crust to the south and west. This proximity to the continental interior also allowed old enriched subcontinental mantle to become involved in the very late stages of Columbia River volcanism. An important consequence of the existence of enriched mantle regions beneath continents is that the combination, crust plus enriched mantle, requires more incompatible elements to have been extracted from the remainder of the mantle than would be the case if no enriched mantle existed.     

Os isotopes in mantle xenoliths from the Eifel volcanic field and the Vogelsberg (Germany): age constraints on the lithospheric mantle

The effects of mantle metasomatism on the sulfide phase in mantle xenoliths in general, and on the Os isotopic system in particular, have received increased attention in recent years. Here, we report on Os isotopic systematics of metasomatized mantle xenoliths from the late Quaternary Eifel (Dreiser Weiher and Meerfelder Maar) and neighboring Vogelsberg volcanic fields, which provide insight into the effects of melt extraction and metasomatism on Os isotopes and place constraints on the evolution of the lithospheric mantle component beneath central Europe. Sixteen harzburgite, lherzolite, and pyroxenite xenoliths from the Eifel and two lherzolite xenoliths from the Vogelsberg were analyzed for Os isotopes. Samples from the anhydrous peridotite suite (Ib) are highly variable in their Os isotopes, ranging from subchondritic values (¹⁸⁷Os/¹⁸⁸Os=0.1236) to suprachondritic values (¹⁸⁷Os/¹⁸⁸Os=0.1420), indicating that some of these samples have been overprinted by the addition of radiogenic Os and have lost the primary mantle Os that was presumably present. The suprachondritic values suggest a source for this Os in a reservoir with a time-integrated Re/Os ratio greater than that of the bulk Earth. Eifel samples with Os contents >1.5 ng/g from the hydrous suite (Ia) have relatively unradiogenic Os isotope compositions (¹⁸⁷Os/¹⁸⁸Os=0.1208-0.1237) and Al₂O₃-Os isotopic systematics consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust (~1.5 Ga) as well with results from peridotite massifs in the European region. The LREE-metasomatism and the enrichment of Os (up to 6.47 ng/g) and As (sulfide metasomatism?) in the hydrous suite is strongly inversely correlated with the Os isotope ratios, demonstrating that mantle processes such as metasomatism can significantly modify the Os isotope chemistry of mantle xenoliths.     

青藏高原南部地幔包体的发现及其意义

尽管青藏高原碰撞后超钾质岩石代表了上地幔低度部分熔融的产物,增加了对地幔的了解,但是对青藏高原陆下岩石圈地幔的性质依然缺乏清楚认识,其中一个最主要的问题是高原腹地的超钾质岩石中一直缺少幔源包体和巨晶.本文报导了青藏高原南部赛利普粗面安山岩(具有典型的超钾质岩石特征,年龄约为17Ma)中产出的地幔包体的矿物主量元素成分.包体大小为0.5cm～1.5cm,主要为两类,一类是辉石岩(0px Cpx),另一类为二辉橄榄岩(Ol Opx Cpx±Phl±sp).包体中橄榄石(Mg#=89～90,CaO=0.05%～0.12%,TiO2<0.03%,NiO=0.29%～0.80%),单斜辉石(Mg#=88～91,Al2O3=5.5%～7%),斜方辉石(TiO2=0.05%～0.15%,Al2O3=2%～5%)和尖晶石(Mg#=58～76,Cr#=10～44,Cr2O3=9%～35%,MnO=0.09%～0.24%,FeO=11%～18%,Al2O3=29%～57%,MgO=16%～21%)的成分与中国东部新生代玄武岩中的地幔包体特征一致.包体的温度为990～1140℃、压力为16～20kb,显示的地温曲线与中国东部、东非肯尼亚等世界典型裂谷区的上地幔特征一致,表明青藏高原南部在中新世尽管处于印度与亚洲大陆的挤压汇聚状态,但是仍具有区域性伸展作用存在,这与藏南广泛发育的南北向裂谷和地表高热流是吻合的.包体中含水金云母与石英的出现以及尖晶石成分不均一性等揭示了包体中多期交代作用过程.与金云母共生的尖晶石后期改造作用表明导致藏南上地幔改造的交代流体应是与寄主火山岩成分接近的富K,Si和H2O的流体.藏南地幔包体的深入研究将对揭示青藏高原地幔的成分、状态与深部作用过程以及为更好解释超钾质岩石的成因提供更多的证据.     

华北龙岗第四纪玄武岩：岩石成因和源区性质

华北克拉通东北缘龙岗第四纪玄武岩的地球化学研究为大陆碱性玄武岩的成因以及源区的性质提供了重要的依据.龙岗第四纪玄武岩为碱性玄武岩,具有类似OIB的REE和微量元素分配特征.岩石的Sr-Nd同位素轻度亏损（87Sr/86Sr =0.7044～0.7048,εNd=0.6～2.1）,具有Dupal异常的高放射性成因Pb同位素组成（^206 Pb/^204 Pb=17.734～18.194,^207 Pb/^204 Pb=15.553～15.594,^208 Pb/%204 Pb=38.322～38.707）.这种地球化学特征指示了原始岩浆起源于＜70km深度的地幔,并经历了一定程度的橄榄岩、单斜辉石和钛.铁氧化物的结晶分异.岩浆源区中以来类似MORB软流圈物质的熔体为主,另外有少量来自EM Ⅰ性质的富集岩石圈地幔以及俯冲流体/熔体的物质贡献,显示了深部岩石圈-软流圈一定程度的相互作用以及太平洋板块俯冲的影响.岩浆源区多种端元组分的存在表明该地区岩石圈的减薄/置换受到多种因素的影响.     

Partitioning of Ni between olivine and siliceous eclogite partial melt: experimental constraints on the mantle source of Hawaiian basalts

Olivine is abundant in Earth’s upper mantle and ubiquitous in basaltic lavas, but rarely occurs in eclogite. Partial melts of eclogite are, therefore, not in equilibrium with olivine, and will react with peridotite as they migrate through the upper mantle. If such melts erupt at Earth’s surface, their compositions will be highly modified and they may be olivine-saturated. We investigated experimentally the reaction between olivine and siliceous eclogite partial melt, and determined element partitioning between olivine and the melt produced by this reaction. Our results demonstrate that mixing of reacted eclogite partial melt with primitive basalt is capable of producing the positive correlation between melt SiO₂ content and olivine Ni content observed in some Hawaiian lavas. Experiments were carried out by equilibrating eclogite partial melt or basalt with San Carlos olivine at 1 bar and 1,201–1,350°C. Our results show that eclogite partial melts equilibrated with mantle olivine retain their high SiO₂, low FeO and MgO characteristics. Further, olivine-melt partition coefficients for Ni measured in these experiments are significantly larger than for basalt. Mixing of these melts with primitive Hawaiian tholeiitic lavas results in crystallization of high-Ni olivines similar to those in Makapuu-stage Koolau lavas, even though the mixed magmas have only moderate Ni contents. This results from a hyperbolic increase of the Ni partition coefficient with increasing polymerization of the mixed melt. Note that while eclogite partial melt in contact with peridotite will equilibrate with pyroxene as well as olivine, this will have the effect of buffering the activity of SiO₂ in the reacted melt at a higher level. Therefore, an eclogite partial melt equilibrated with harzburgite will have higher SiO₂ than one equilibrated with dunite, enhancing the effects observed in our experiments. Our results demonstrate that an olivine-free “hybrid” pyroxenite source is not required to explain the presence of high-Ni olivines in Hawaiian lavas and, therefore, indicate that the proportion of eclogite in the Hawaiian plume is less than has been estimated in recent studies.     

扬马延海脊构造-沉积演化及油气潜力

扬马延海脊位于北大西洋的北极圈附近,东格陵兰板块和挪威板块之间,冰岛东北方向。北极地区地域辽阔,油气资源丰富,但是恶劣的环境一直制约油气的勘探进展。在扬马延海脊的沉积演化过程中,扬马延海脊在第三纪前有着和东格陵兰陆架、挪威陆架相似的沉积序列,其构造演化经历了二叠纪陆内裂谷、三叠纪—侏罗纪同裂谷和微陆块漂移、白垩纪至今热沉降和被动陆缘等3个阶段。结合前人研究成果,对搜集的东格陵兰陆架、挪威陆架的油气地质资料分析,认为扬马延海脊可划分为扬马延盆地、扬马延西部构造带、扬马延中部凸起带、扬马延海槽、扬马延东部斜坡、扬马延南部复杂构造带6个构造单元,在其上发育着2套油气系统。同时扬马延海脊发育有伸展构造圈闭、地垒断块圈闭、构造圈闭和地层圈闭,这些圈闭为油气的赋存提供了良好的环境,也有利于划分有利油气勘探区带。研究结果可为进一步分析扬马延海脊构造特征等方面提供基础信息,同时对我国参与研究开发北极油气资源具有重大意义。     

The Little Ice Age glacier maximum in Iceland and the North Atlantic Oscillation: evidence from Lambatungnajökull, southeast Iceland

This article examines the link between late Holocene fluctuations of Lambatungnajökull, an outlet glacier of the Vatnajökull ice cap in Iceland, and variations in climate. Geomorphological evidence is used to reconstruct the pattern of glacier fluctuations, while lichenometry and tephrostratigraphy are used to date glacial landforms deposited over the past ˜400 years. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion occurred shortly before c . AD 1795, probably during the 1780s. Recession over the last 200 years was punctuated by re-advances in the 1810s, 1850s, 1870s, 1890s and c . 1920, 1930 and 1965. Lambatungnajökull receded more rapidly in the 1930s and 1940s than at any other time during the last 200 years. The rate and style of glacier retreat since 1930 compare well with other similar-sized, non-surging, glaciers in southeast Iceland, suggesting that the terminus fluctuations are climatically driven. Furthermore, the pattern of glacier fluctuations over the 20th century broadly reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation (NAO) Advances of Lambatungnajökull generally occur during prolonged periods of negative NAO index. The main implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. Mounting evidence now suggests that the period between AD 1750 and 1800, rather than the late 19th century, represented the culmination of the Little Ice Age in Iceland.     

华北地幔捕虏体Fe-Mg-Ca同位素研究进展

近年来,得益于同位素分析技术和质谱仪器性能的提高,使得铁（Fe）、镁（Mg）和钙（Ca）等非传统稳定同位素的高精度测量成为可能,并很快在地球化学、天体化学和生物地球化学等研究领域取得了丰硕的成果。本文通过对比分析来自华北克拉通不同地区不同类型地幔捕虏体的Fe、Mg和Ca位素组成特征,揭示华北克拉通岩石圈地幔Fe、Mg和Ca同位素组成不均一性的成因,并在此基础上,探讨华北大陆岩石圈地幔演化过程如部分熔融、橄榄岩-熔体反应过程、熔体的性质和来源等科学问题,为华北克拉通岩石圈的演化过程提供新证据。

     

华北克拉通中部早白垩世岩石圈地幔性质：来自狐偃山科头钾质正长岩年代学、同位素的限制

华北克拉通东部中生代岩石圈减薄已经取得了大量进展,相比之下对克拉通中部岩石圈演化认识不足。本文对华北克拉通中部狐偃山杂岩体中科头正长岩进行了锆石U-Pb年代学、主量和微量元素、Sr-Nd-Hf同位素地球化学研究。LA-ICP-MS锆石U-Pb定年结果显示,科头正长岩侵位于早白垩世晚期(～111Ma)。岩石样品均为钾质—超钾质,属于碱性系列岩石。这些岩石相对富集轻稀土,亏损重稀土和中稀土,具有明显的正Eu异常(Eu/Eu~*=1.22～1.96)、富集大离子亲石元素LILE(Rb、Sr、Ba),亏损高场强元素HFSE(Nb、Ta、Ti)。所有岩石样品具有相对高的初始~(87)Sr/~(86)Sr比值(0.7058～0.7062)和低的ε_(Nd)(t)(-10.4～—11.1),ε_(Hf)(t)介于—12.2～—5.2之间。详细的元素和同位素地球化学研究表明科头正长岩的原始岩浆可能来源于富集岩石圈地幔中富金云母的尖晶石二辉橄榄岩源区的部分熔融,岩浆演化过程中经历了地壳混染与结晶分异(AFC)过程。结合华北克拉通中部岩浆资料,将华北克拉通中部早白垩世岩浆作用分为早晚两期:早期(123～135Ma)为岩浆作用高峰期,晚期(～114Ma)为最后一期弱的岩浆期;华北克拉通中部陆下岩石圈地幔是富集的、不均一的,其至少在早白垩世(138～111Ma)一直处于减薄状态。     

Composite nature of the North China Granulite-Facies Belt: Tectonothermal and geochronological constraints

Granulite-facies rocks are intermittently exposed in a roughly E–W trending belt that extends for approximately 2000 km across the North China Craton, from the Helanshan, Qianlishan, Wulashan–Daqingshan, Guyang and Jining Complexes in the Western Block, through the Huai'an, Hengshan, Xuanhua and Chengde Complexes in the Trans-North China Orogen, to the Jianping (Western Liaoning), Eastern Hebei, Northern Liaoning and Southern Jilin Complexes in the Eastern Block. The belt is generally referred to as the North China Granulite-Facies Belt, previously interpreted as the lowest part of an obliquely exposed crust of the North China Craton. Recent data indicate that the North China Granulite-Facies Belt is not a single terrane. Instead, it represents components of three separate terranes: the Eastern and Western Blocks and Trans-North China Orogen. Each of these units records different metamorphic histories and reflect the complex tectonic evolution of the NCC during the late Archean and Paleoproterozoic. Mafic granulites in the Eastern Block and the Yinshan Terrane (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.5 Ga, with anticlockwise P–T paths involving near isobaric cooling following peak metamorphism, reflecting an origin related to intrusion and underplating of mantle-derived magmas. Pelitic granulites in the Khondalite Belt (Western Block) underwent medium-pressure granulite-facies metamorphism at about 2.0–1.9 Ga, with clockwise P–T paths, which record the Paleoproterozoic amalgamation of the Yinshan and Ordos Terranes to form the Western Block. Mafic and pelitic granulites in the Trans-North China Orogen experienced high- to medium-pressure granulite-facies metamorphism at 1.85 Ga, with clockwise P–T paths involving nearly isothermal decompression following peak metamorphism, which are in accord with the final collision between the Eastern and Western Blocks to form the North China Craton at 1.8 Ga. The NCGB cannot therefore represent a separate unique terrane; instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times.