Niobium/Uranium Evidence for Early Formation of the Continental Crust

Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Archean mantle beneath Western Australia underwent a level of melt extraction, resulting in formation of the continental crust, comparable to that seen in the present mantle. The implication is either that (i) the amount of continental crust that formed before 2.7 x 10(9) years ago was much greater than generally thought or (ii) crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle.     

Breakup of pangaea and isolation of relict mammals in australia, South america, and madagascar

The composition of aboriginal land mammal faunas in Australia and New Guinea (prototherians and metatherians), South America (metatherians and eutherians) and Madagascar (eutherians only) is reconsidered in light of continental drift reconstructions of Mesozoic-Tertiary world paleogeography It is proposed that these three faunas represent successively detached samples of the evolving world mammal fauna as it existed when each of these land masses became faunally isolated from the rest of the world as a result of the progressive fragmentation of Pangaea. Isolation of aboriginal prototherians and metatherians in Australia and New Guinea may date from the Upper JurassicLower Cretaceous; isolation of aboriginal metatherians and eutherians in South America may date from the Middle Cretaceous-Upper Cretaceous; isolation of aboriginal eutherians in Madagascar may date from the Paleocene-Eocene.     

Reconstructing Farallon plate subduction beneath North America back to the Late Cretaceous

Using an inverse mantle convection model that assimilates seismic structure and plate motions, we reconstruct Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow-dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten. The results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized.     

Long-term sea-level fluctuations driven by ocean basin dynamics

Earth's long-term sea-level history is characterized by widespread continental flooding in the Cretaceous period (approximately 145 to 65 million years ago), followed by gradual regression of inland seas. However, published estimates of the Late Cretaceous sea-level high differ by half an order of magnitude, from approximately 40 to approximately 250 meters above the present level. The low estimate is based on the stratigraphy of the New Jersey margin. By assimilating marine geophysical data into reconstructions of ancient ocean basins, we model a Late Cretaceous sea level that is 170 (85 to 270) meters higher than it is today. We use a mantle convection model to suggest that New Jersey subsided by 105 to 180 meters in the past 70 million years because of North America's westward passage over the subducted Farallon plate. This mechanism reconciles New Jersey margin-based sea-level estimates with ocean basin reconstructions.     

Evidence from the lamarck granodiorite for rapid late cretaceous crust formation in california

Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.     

The Emperor Seamounts: southward motion of the Hawaiian hotspot plume in Earth's mantle

The Hawaiian-Emperor hotspot track has a prominent bend, which has served as the basis for the theory that the Hawaiian hotspot, fixed in the deep mantle, traced a change in plate motion. However, paleomagnetic and radiometric age data from samples recovered by ocean drilling define an age-progressive paleolatitude history, indicating that the Emperor Seamount trend was principally formed by the rapid motion (over 40 millimeters per year) of the Hawaiian hotspot plume during Late Cretaceous to early-Tertiary times (81 to 47 million years ago). Evidence for motion of the Hawaiian plume affects models of mantle convection and plate tectonics, changing our understanding of terrestrial dynamics.     

Timing of hot spot--related volcanism and the breakup of madagascar and India

Widespread basalts and rhyolites were erupted in Madagascar during the Late Cretaceous. These are considered to be related to the Marion hot spot and the breakup of Madagascar and Greater India. Seventeen argon-40/argon-39 age determinations reveal that volcanic rocks and dikes from the 1500-kilometer-long rifted eastern margin of Madagascar were emplaced rapidly (mean age = 87.6 +/- 0.6 million years ago) and that the entire duration of Cretaceous volcanism on the island was no more than 6 million years. The evidence suggests that the thick lava pile at Volcan de l'Androy in the south of the island marks the focal point of the Marion hot spot at approximately 88 million years ago and that this mantle plume was instrumental in causing continental breakup.     

Extensive volcanism associated with the separation of australia and antarctica

Alternating hard and soft layers characterize the Gull Rock and Tuit Members of the late Eocene Blanche Point Formation, South Australia. Originally the formation was mainly a mixture of volcanic ash, sponge spicules, and calcareous fossil remains, with hard layers produced later by selective silicification. It resembles Cretaceous sediments from western Europe and the eastern coast of the United States, and in each case it appears that alteration of volcanic ash produced smectite and clinoptilolite with release of silica that subsequently crystallized as opal-CT. The occurrence of similar deposits from New Zealand to as far west as Albany, Western Australia, indicates extensive volcanic activity south of Australia in the late Eocene resulting from rifting and separation from Antarctica.     

Terminal cretaceous environmental events

The geologic record of terminal Cretaceous environmental events indicates that iridium and other associated elements were not deposited instantaneously but during a time interval spanning some 10,000 to 100,000 years. The available geologic evidence favors a mantle rather than meteoritic origin for these elements. These results are in accord with the scenario of a series of intense eruptive volcanic events occurring during a relatively short geologic time interval and not with the scenario of a single large asteroid impact event.     

Polar standstill of the mid-cretaceous pacific plate and its geodynamic implications

Paleomagnetic data from the Mid-Cretaceous Mountains suggest that Pacific plate motion during the Early to mid-Cretaceous was slow, less than 0.3 degree per year, resembling the polar standstill observed in coeval rocks of Eurasia and North America. There is little evidence for a change in plate motion that could have precipitated the major volcanic episode of the early Aptian that is marked by the formation of the Ontong Java Plateau. During the volcanism, oceanic plates bordering the Pacific plate moved rapidly. Large-scale northward motion of the Pacific plate began after volcanism ceased. This pattern suggests that mantle plume volcanism exerted control on plate tectonics in the Cretaceous Pacific basin.     

Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga

The long-favored paradigm for the development of continental crust is one of progressive growth beginning at approximately 4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. epsilonHf (deviations of 176Hf/177Hf from bulk Earth in parts per 10(4)) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf approximately 0.01), perhaps as early as 4.5 Ga. Positive epsilon(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.     

Coupled 142Nd-143Nd isotopic evidence for Hadean mantle dynamics

The oldest rocks-3.85 billion years old-from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early-formed large-scale chemical heterogeneities in Earth's mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains.     

Rapid formation of ontong java plateau by aptian mantle plume volcanism

The timing of flood basalt volcanism associated with formation of the Ontong Java Plateau (OJP) is estimated from paleomagnetic and paleontologic data. Much of OJP formed rapidly in less than 3 million years during the early Aptian, at the beginning of the Cretaceous Normal Polarity Superchron. Crustal emplacement rates are inferred to have been several times those of the Deccan Traps. These estimates are consistent with an origin of the OJP by impingement at the base of the oceanic lithosphere by the head of a large mantle plume. Formation of the OJP may have led to a rise in sea level that induced global oceanic anoxia. Carbon dioxide emissions likely contributed to the mid-Cretaceous greenhouse climate but did not provoke major biologic extinctions.     

Seismic evidence for olivine phase changes at the 410- and 660-kilometer discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4 spinel and postspinel transformations.     

Bottom-feeding plesiosaurs

Elasmosaurid plesiosaurs were an important part of Cretaceous marine reptile communities and are generally considered to have been predators of small, agile, free-swimming fish and cephalopods. Two elasmosaurid specimens from Aptian and Albian deposits in Queensland, Australia, include fossilized gut contents dominated by benthic invertebrates: bivalves, gastropods, and crustaceans. Both specimens also contained large numbers of gastroliths (stomach stones). These finds point to a wider niche than has previously been supposed for these seemingly specialized predators and may also influence long-running controversy over the question of gastrolith function in plesiosaurs.     

The age of parana flood volcanism, rifting of gondwanaland, and the jurassic-cretaceous boundary

The Paraná-Etendeka flood volcanic event produced approximately 1.5 x 10(6) cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New (40)Ar/(39)Ar data combined with earlier paleomagnetic results indicate that Paraná flood volcanism in southern Brazil began at 133 +/- 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paraná flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.     

大风子科的地理分布及其起源

本文以属为单位,将大风子科84个属划分为27个分布亚型,隶属于11个分布类型。在此基础上对本科植物的地理分布进行了分析,认为大风子科是一个典型的热带性质的且富于岛屿区系成分和与各地区联系广泛的类群;其现代分布中心在旧世界热带地区,特别是热带非洲。根据其地理分布分析,结合板块及大陆漂移学说理论,可推断大风子科起源于古南大陆,特别是热带非洲地区;其起源时间至少不迟于白垩纪末期或第三纪初期,这一点已得到了古植物学资料的证实。     

Olivine-garnet transformation in a meteorite

Garnet has been identified for the first time as a meteorite mineral in the Coorara chondrite from Western Australia. It replaces olivine grains in a 1-millimeter veinlet traversing the body of the meteorite. The associated olivine has abnormally low birefringence, which suggests a highly shocked condition. Microprobe analyses do not distinguish the garnet from the associated olivine, which has the composition (Mg(.75)Fe(.25))(2) SiO(4); the garnet may have the composition Mg(3)Fe(2)Si(3)O(12) but be unresolvable from the accompanying olivine, or alternatively is nonstoichiometric. Transformation of olivine to garnet under high pressure could have significant implications for the phase composition of the Earth's mantle.     

Diamonds and the african lithosphere

Data and inferences drawn from studies of diamond inclusions, xenocrysts, and xenoliths in the kimberlites of southern Africa are combined to characterize the structure of that portion of the Kaapvaal craton that lies within the mantle. The craton has a root composed in large part of peridotites that are strongly depleted in basaltic components. The asthenosphere boundary shelves from depths of 170 to 190 kilometers beneath the craton to approximately 140 kilometers beneath the mobile belts bordering the craton on the south and west. The root formed earlier than 3 billion years ago, and at that time ambient temperatures in it were 900 degrees to 1200 degrees C; these temperatures are near those estimated from data for xenoliths erupted in the Late Cretaceous or from present-day heat-flow measurements. Many of the diamonds in southern Africa are believed to have crystallized in this root in Archean time and were xenocrysts in the kimberlites that brought them to the surface.     

长牡蛎二倍体和三倍体外套膜的差异

本文从外观、组织切片、蛋白质和糖元含量等方面分析了长特蛎二倍体和三倍体外套膜的差异。与二倍体相比,三倍体外套膜厚、不透明、呈乳白色、结弱组织较厚,糖原含量高３．９倍。