Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle

Up to 10 per cent of the ocean floor consists of plateaux--regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.     

安徽女山幔源橄榄岩捕虏体Re-Os 同位素地球化学

Re-Os同位素体系可以为研究大陆岩石圈地幔形成和演化提供重要制约.女山12个地幔橄榄岩全岩样品测得Re含量为19×10-12~306×10-12,Os含量为0.81×10-9~2.42×10-9,187Os/188Os比值为0.114 90~0.131 02,187Re/188Os比值为0.037~1.517.两个石榴子石-尖晶石二辉橄榄岩的Os同位素组成与原始上地幔(PUM)的现代值0.129 6相似,其余的尖晶石二辉橄榄岩、方辉橄榄岩,和角闪石、金云母-尖晶石二辉橄榄岩等的Os同位素组成都低于PUM现代值,Os同位素模式年龄为早元古代.实性和隐性地幔交代作用对女山幔源橄榄岩捕虏体的Os同位素组成没有明显影响.橄榄岩捕虏体代表的女山地区岩石圈地幔是元古代岩石圈地幔经过减薄作用后的残留部分.橄榄岩中Re-Os体系扰动是近期事件.     

Tungsten isotope evidence that mantle plumes contain no contribution from the Earth's core

Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets (187)Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earth's outer core produces the isotope anomalies and, as a result of coupled (186)Os-(187)Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile ('iron-loving') element that preferentially partitioned into the Earth's core during core formation but is also 'incompatible' during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted (186)Os-(187)Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core-mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.     

High-precision uranium-series dating of lacustrine carbonates from Daihai Lake

Ages of small aragonite samples retrieved from sediments from the Daihai Lake are determined by using the Thermal Ionization Mass Spectrometry (TIMS) technique. The ages of the samples range from (8740±601) to (10446 ±489) a, with a precision of 2%–3% for non-corrected dating ages and 4.7%-9.8% for corrected ages respectively, and are consistent within the deviation range. At about 10.5 ka BP, the grain-size of the sediments coarsens sharply, the frequency curves of grain-size distribution show coarse grain dominated single-double kurtosis, indicating cooling, decreased hydrodynamic conditions and increased wind influence procedure. This is in good agreement with previous results obtained from the Daihai Lake, which suggests that the climate was cold and the lake surface was shrinking. The TIMS ages of the sediments are therefore believed to be credible.     

Multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes

The hypothesis that a single mushroom-like mantle plume head can generate a large igneous province within a few million years has been widely accepted. The Siberian Traps at the Permian-Triassic boundary and the Deccan Traps at the Cretaceous-Tertiary boundary were probably erupted within one million years. These large eruptions have been linked to mass extinctions. But recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years. This observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal, single-stage plume model suggests. Here we present numerical experiments to demonstrate that the entrainment of a dense eclogite-derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces. The characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations. Such a process may contribute to multiple episodes of large igneous provinces.     

Insights into hominid evolution from the gorilla genome sequence

Gorillas are humans' closest living relatives after chimpanzees, and are of comparable importance for the study of human origins and evolution. Here we present the assembly and analysis of a genome sequence for the western lowland gorilla, and compare the whole genomes of all extant great ape genera. We propose a synthesis of genetic and fossil evidence consistent with placing the human-chimpanzee and human-chimpanzee-gorilla speciation events at approximately 6 and 10 million years ago. In 30% of the genome, gorilla is closer to human or chimpanzee than the latter are to each other; this is rarer around coding genes, indicating pervasive selection throughout great ape evolution, and has functional consequences in gene expression. A comparison of protein coding genes reveals approximately 500 genes showing accelerated evolution on each of the gorilla, human and chimpanzee lineages, and evidence for parallel acceleration, particularly of genes involved in hearing. We also compare the western and eastern gorilla species, estimating an average sequence divergence time 1.75 million years ago, but with evidence for more recent genetic exchange and a population bottleneck in the eastern species. The use of the genome sequence in these and future analyses will promote a deeper understanding of great ape biology and evolution.     

Insights into phase transition kinetics from colloid science

Colloids display intriguing transitions between gas, liquid, solid and liquid crystalline phases. Such phase transitions are ubiquitous in nature and have been studied for decades. However, the predictions of phase diagrams are not always realized; systems often become undercooled, supersaturated, or trapped in gel-like states. In many cases the end products strongly depend on the starting position in the phase diagram and discrepancies between predictions and actual observations are due to the intricacies of the dynamics of phase transitions. Colloid science aims to understand the underlying mechanisms of these transitions. Important advances have been made, for example, with new imaging techniques that allow direct observation of individual colloidal particles undergoing phase transitions, revealing some of the secrets of the complex pathways involved.     

Insights into microtubule nucleation from the crystal structure of human gamma-tubulin

Microtubules are hollow polymers of alphabeta-tubulin that show GTP-dependent assembly dynamics and comprise a critical part of the eukaryotic cytoskeleton. Initiation of new microtubules in vivo requires gamma-tubulin, organized as an oligomer within the 2.2-MDa gamma-tubulin ring complex (gamma-TuRC) of higher eukaryotes. Structural insight is lacking regarding gamma-tubulin, its oligomerization and how it promotes microtubule assembly. Here we report the 2.7-A crystal structure of human gamma-tubulin bound to GTP-gammaS (a non-hydrolysable GTP analogue). We observe a 'curved' conformation for gamma-tubulin-GTPgammaS, similar to that seen for GDP-bound, unpolymerized alphabeta-tubulin. Tubulins are thought to represent a distinct class of GTP-binding proteins, and conformational switching in gamma-tubulin might differ from the nucleotide-dependent switching of signalling GTPases. A crystal packing interaction replicates the lateral contacts between alpha- and beta-tubulins in the microtubule, and this association probably forms the basis for gamma-tubulin oligomerization within the gamma-TuRC. Laterally associated gamma-tubulins in the gamma-TuRC might promote microtubule nucleation by providing a template that enhances the intrinsically weak lateral interaction between alphabeta-tubulin heterodimers. Because they are dimeric, alphabeta-tubulins cannot form microtubule-like lateral associations in the curved conformation. The lateral array of gamma-tubulins we observe in the crystal reveals a unique functional property of a monomeric tubulin.     

Insights into social insects from the genome of the honeybee Apis mellifera

Here we report the genome sequence of the honeybee Apis mellifera, a key model for social behaviour and essential to global ecology through pollination. Compared with other sequenced insect genomes, the A. mellifera genome has high A+T and CpG contents, lacks major transposon families, evolves more slowly, and is more similar to vertebrates for circadian rhythm, RNA interference and DNA methylation genes, among others. Furthermore, A. mellifera has fewer genes for innate immunity, detoxification enzymes, cuticle-forming proteins and gustatory receptors, more genes for odorant receptors, and novel genes for nectar and pollen utilization, consistent with its ecology and social organization. Compared to Drosophila, genes in early developmental pathways differ in Apis, whereas similarities exist for functions that differ markedly, such as sex determination, brain function and behaviour. Population genetics suggests a novel African origin for the species A. mellifera and insights into whether Africanized bees spread throughout the New World via hybridization or displacement.     

The influence of a chemical boundary layer on the fixity,spacing and lifetime of mantle plumes

Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of 'ultra-low' seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.     

现代图式理论对英语词汇联想记忆的启示

图式是一种记忆结构。图式理论与英语词汇联想记忆有密切的关系。本文拟从认知心理学的角度来探讨现代图式理论与英语词汇联想记忆的关系,提出英语词汇联想记忆的策略,以帮助学习者提高英语词汇记忆的效率。     

Insights into assembly from structural analysis of bacteriophage PRD1

The structure of the membrane-containing bacteriophage PRD1 has been determined by X-ray crystallography at about 4 A resolution. Here we describe the structure and location of proteins P3, P16, P30 and P31. Different structural proteins seem to have specialist roles in controlling virus assembly. The linearly extended P30 appears to nucleate the formation of the icosahedral facets (composed of trimers of the major capsid protein, P3) and acts as a molecular tape-measure, defining the size of the virus and cementing the facets together. Pentamers of P31 form the vertex base, interlocking with subunits of P3 and interacting with the membrane protein P16. The architectural similarities with adenovirus and one of the largest known virus particles PBCV-1 support the notion that the mechanism of assembly of PRD1 is scaleable and applies across the major viral lineage formed by these viruses.     

High 3He/4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes

The high 3He/4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic 3He/4He ratios yet recorded. A strong correlation between 3He/4He and 87Sr/86Sr, 143Nd/144Nd and trace element ratios demonstrate that the 3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest 3He/4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3He/4He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.     

Insights into DNA recombination from the structure of a RAD51-BRCA2 complex

The breast cancer susceptibility protein BRCA2 controls the function of RAD51, a recombinase enzyme, in pathways for DNA repair by homologous recombination. We report here the structure of a complex between an evolutionarily conserved sequence in BRCA2 (the BRC repeat) and the RecA-homology domain of RAD51. The BRC repeat mimics a motif in RAD51 that serves as an interface for oligomerization between individual RAD51 monomers, thus enabling BRCA2 to control the assembly of the RAD51 nucleoprotein filament, which is essential for strand-pairing reactions during DNA recombination. The RAD51 oligomerization motif is highly conserved among RecA-like recombinases, highlighting a common evolutionary origin for the mechanism of nucleoprotein filament formation, mirrored in the BRC repeat. Cancer-associated mutations that affect the BRC repeat disrupt its predicted interaction with RAD51, yielding structural insight into mechanisms for cancer susceptibility.     

Insights into SCF ubiquitin ligases from the structure of the Skp1-Skp2 complex

F-box proteins are members of a large family that regulates the cell cycle, the immune response, signalling cascades and developmental programmes by targeting proteins, such as cyclins, cyclin-dependent kinase inhibitors, IkappaBalpha and beta-catenin, for ubiquitination (reviewed in refs 1-3). F-box proteins are the substrate-recognition components of SCF (Skp1-Cullin-F-box protein) ubiquitin-protein ligases. They bind the SCF constant catalytic core by means of the F-box motif interacting with Skp1, and they bind substrates through their variable protein-protein interaction domains. The large number of F-box proteins is thought to allow ubiquitination of numerous, diverse substrates. Most organisms have several Skp1 family members, but the function of these Skp1 homologues and the rules of recognition between different F-box and Skp1 proteins remain unknown. Here we describe the crystal structure of the human F-box protein Skp2 bound to Skp1. Skp1 recruits the F-box protein through a bipartite interface involving both the F-box and the substrate-recognition domain. The structure raises the possibility that different Skp1 family members evolved to function with different subsets of F-box proteins, and suggests that the F-box protein may not only recruit substrate, but may also position it optimally for the ubiquitination reaction.     

Insights into the ubiquitin transfer cascade from the structure of the activating enzyme for NEDD8

Post-translational modification by ubiquitin-like proteins (Ublps) is an essential cellular regulatory mechanism. The Ublp NEDD8 regulates cell division, signalling and embryogenesis. Ublps are conjugated to their targets by the sequential action of E1, E2 and often E3 enzymes. Each Ublp has a dedicated E1, or activating enzyme, that initiates its conjugation cascade. First, E1 associates with the Ublp and catalyses adenylation of the carboxy terminus of the Ublp. Second, E1 forms a thioester between its catalytic cysteine and the Ublp. Next, E1 is loaded with a second Ublp molecule, adenylating the C terminus of this second Ublp while still carrying the first thioester-bound Ublp. Last, E1 binds E2 and promotes Ublp transfer to the catalytic cysteine of E2. We report here the structure and mutational analysis of human APPBP1-UBA3, the heterodimeric E1 enzyme for NEDD8 (ref. 11). Each E1 activity is specified by a domain: an adenylation domain resembling bacterial adenylating enzymes, an E1-specific domain organized around the catalytic cysteine, and a domain involved in E2 recognition resembling ubiquitin. The domains are arranged around two clefts that coordinate protein and nucleotide binding so that each of E1's reactions drives the next, in an assembly-line fashion.     

Insights into the subunit interactions of the chloroplast ATP synthase

Subunit interactions of the chloroplast F0F1- ATP synthase were studied using the yeast two-hybrid system. The coding sequences of all the nine subunits of spinach chloroplast ATP synthase were cloned in two-hybrid vectors. The vectors were transformed into the yeast strains HF7c and SFY526 by various pairwise combinations, and the protein interactions were analyzed by measuring the yeast growth on minimal SD medium without serine, lucine and histidine. Interactions of γ Subunit with wild type or two truncated mutants of γ sununit, △εN21 and △εC45, which lose their abilities to inhibit the ATP hydrolysis, were also detected by in vitro and in vivo binding assay. The present results are largely accordant to the common structure model of F0F1-ATP synthase. Different from that in the E. Coli F0F1-ATP synthase, the δ subunit of chloroplast ATP syn- thase could interact with β,γ,ε and all the CF0 subunits in the two-hybrid system. These results suggested that though the chloroplast ATP synthase shares the similar structure and composition of subunits with the enzyme from E. Coli, it may be different in the subunit interactions and con- formational change during catalysis between these two sources of ATP synthase. Based on the present results and our knowledge of structure model of E. Coli ATP synthase, a deduced structure model of chloroplast ATP synthase was proposed.     

Insights into Wnt binding and signalling from the structures of two Frizzled cysteine-rich domains.

Members of the Frizzled family of seven-pass transmembrane proteins serve as receptors for Wnt signalling proteins. Wnt proteins have important roles in the differentiation and patterning of diverse tissues during animal development, and inappropriate activation of Wnt signalling pathways is a key feature of many cancers. An extracellular cysteine-rich domain (CRD) at the amino terminus of Frizzled proteins binds Wnt proteins, as do homologous domains in soluble proteins-termed secreted Frizzled-related proteins-that function as antagonists of Wnt signalling. Recently, an LDL-receptor-related protein has been shown to function as a co-receptor for Wnt proteins and to bind to a Frizzled CRD in a Wnt-dependent manner. To investigate the molecular nature of the Wnt signalling complex, we determined the crystal structures of the CRDs from mouse Frizzled 8 and secreted Frizzled-related protein 3. Here we show a previously unknown protein fold, and the design and interpretation of CRD mutations that identify a Wnt-binding site. CRDs exhibit a conserved dimer interface that may be a feature of Wnt signalling. This work provides a framework for studies of homologous CRDs in proteins including muscle-specific kinase and Smoothened, a component of the Hedgehog signalling pathway.     

Resistance to mantle flow inferred from the electromagnetic strike of the Australian upper mantle

Seismic anisotropy is thought to result from the strain-induced lattice-preferred orientation of mantle minerals, especially olivine, owing to shear waves propagating faster along the a-axis of olivine crystals than along the other axes. This anisotropy results in birefringence, or 'shear-wave splitting', which has been investigated in numerous studies. Although olivine is also anisotropic with respect to electrical conductivity (with the a-axis being most conductive), few studies of the electrical anisotropy of the upper mantle have been undertaken, and these have been limited to relatively shallow depths in the lithospheric upper mantle. Theoretical models of mantle flow have been used to infer that, for progressive simple shear imparted by the motion of an overriding tectonic plate, the a-axes of olivine crystals should align themselves parallel to the direction of plate motion. Here, however, we show that a significant discrepancy exists between the electromagnetic strike of the mantle below Australia and the direction of present-day absolute plate motion. We infer from this discrepancy that the a-axes of olivine crystals are not aligned with the direction of the present-day plate motion of Australia, indicating resistance to deformation of the mantle by plate motion.     

Polybaric partial melting in the continental mantle: Evidence from mantle xenoliths from Qilin Guangdong Province

The extremely low Ti content (160–245 μg/g) in clinopyroxene in some spinel peridotites from Qilin, South China is indicative of high degree of partial melting, inconsistent with their relatively high clinopyroxene modes (7.4%–12.4%). These clinopyroxenes show fractionated HREE patterns ((Gd/Yb)_n<0.2), suggesting the involvement of garnet in the melting regime. These REE patterns can be modeled as residues of 22%–23% fractional melting from a primitive mantle, first in garnet stability field (12%) then continuing in spinel stability field (10%–11%) after breakdown of garnet to pyroxenes and spinel. Such a polybaric melting suggests the lithospheric thinning and rapid mantle upwelling in south China during the Cenozoic. This is consistent with the dominant MORB-OIB isotopic signature and high thermal gradient of the lithospheric mantle in this region, and supports the contention that the formation of South China Sea basin is related to southward migration of continental lithosphere extension, rather than passive back-arc basin.