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141.
Fresh basalt glasses from the North Chile Ridge (NCR) in the southeastern Pacific have Ne isotopic compositions distinctly different from typical mid-ocean ridge basalts (MORB). In a three-isotope plot of 20Ne/22Ne vs. 21Ne/22Ne, the NCR data define a correlation line with a slope smaller than that of the MORB correlation line, i.e. their Ne composition is more nucleogenic than that of MORB. 3He/4He ratios are slightly lower than the MORB average, whereas in a few stepwise heating fractions very high 40Ar/36Ar ratios up to 28,000 are found. One model to explain the data assumes contamination of the NCR mantle source by material from the continental or oceanic crust, but in addition to difficulties with quantitatively reconciling the noble gas patterns with such a model it seems unable to account for some geochemical characteristics of NCR basalts reported earlier [Bach et al., Earth Planet. Sci. Lett. 142 (1996) 223–240], such as depletions in highly incompatible elements and unradiogenic Sr isotope compositions. Therefore we favor the scenario of a mantle source which was depleted and degassed previously, possibly as a residue from mantle melting beneath the southern East Pacific Rise that was transported to the NCR and melted again. The time during which such a depleted reservoir would have to be separated from the MORB mantle is estimated at 10–100 Ma based on U/Th–Ne systematics, in reasonable agreement with the time scale deduced from the formation history of the NCR and the temporal evolution of the southeast Pacific. 相似文献
142.
143.
Bruno Ribbat Wolfgang Roether Karl Otto Münnich 《Earth and Planetary Science Letters》1976,32(2):331-341
A14C balance for the Eastern Caribbean deep water indicates the average inflow of Atlantic water into the basin to be 2.3 × 105 m3/sec (±30%), or about 2–4 times the values estimated previously. The balance uses a model representation of the deep-water turnover, and is based on14C concentrations at a station in the Venezuelan Basin which average Δ14C= 89‰ below 800 m depth with a total range of only 9‰, as well as on a14C concentration of the Atlantic inflow of Δ14C= ?71%. as obtained from measurements outside the Antilles Arch. The turnover time of the basin water below 2500 m depth is 55 years, which corresponds to an average upwelling velocity at this depth of about 35 m/year. With such upwelling, the temperature profile below 1800 m (the depth of the sill determining the inflow of new water) requires a vertical eddy diffusivity of about 5 cm2/sec. The oxygen consumption, and silica and CO2 regeneration, rates below 2500 m depth are obtained as ?0.18, + 0.08, and + 0.2 μmole kg?1 yr?1, respectively. The CO2 regeneration has but a negligible effect on the14C balance. 相似文献
144.
Wolfgang Roether 《Earth and Planetary Science Letters》1974,23(1):108-115
Tritium and14C data of the “GOGO I” station at 28.5°N, 121.6°W in November 1971 are reported. The tritium decline between 150 and 350 m depth is as pronounced as was observed on a previous occupation of the same position, station “Geosecs I” in September 1969, and the tritium concentrations below 200 m are unchanged.14C data from the depth range of tritium decline are corrected for fallout14C contribution. The correcting procedure requires simultaneous measurements of14C, ΣCO2, and tritium. It is concluded that the natural absolute14C concentration attains a maximum near 400 m depth, of 7.5% excess over that of surface water. 相似文献
145.
We determined U–Pb ages on zircons from Ladakh granitoid samples of three previously undated plutons and deduced four distinct age groups between c. 67 and c. 45 Ma (66.6 ± 2.1, 57.6 ± 1.4, 53.4 ± 1.8, 52.50 ± 0.53 and 45.27 ± 0.56 Ma). This suggests that the Ladakh batholith grew by addition of at least four distinct subduction‐related magma pulses at c. 67, 58, 53 and 45 Ma, thus indicating that the belt was continuously active throughout the Palaeocene and the Middle Eocene (Lutetian). The 45.27 ± 0.56 Ma pluton at Daah‐Hanu is the last major calcalkaline arc magmatic pulse in the Ladakh batholith. Thereafter, the subduction‐related major plutonism gradually waned. The earlier estimate for the youngest pluton within the Ladakh batholith is 49.8 ± 0.8 Ma for the Leh pluton ( J. Geol., 2000, 108 , 303 ). 相似文献
146.
Pierre‐Yves Pascal Claude Bellemare Wolfgang Sterrer Henricus T. S. Boschker Silvina Gonzalez‐Rizzo Olivier Gros 《Marine Ecology》2015,36(2):246-257
Haplognathia ruberrima is a cosmopolitan gnathostomulid species found in sulfur bacterial mats in mangroves in Guadeloupe (French West Indies). Haplognathia ruberrima presents a δ13C value lower than all measured meiofaunal grazers and lower than the available measured food sources of this environment. This low δ13C value can not be due to specific ingestion of 13C‐depleted methanogenic bacteria because abundances of those bacteria are reduced in surficial and deep sediments as revealed by δ13C of bacterial fatty acid. According to scanning electron microscope observations, no bacterial ectosymbionts were observed at the surface of the gnathostomulids, and transmission electron microscope views revealed the absence of bacterial endosymbionts. Energy‐dispersive X‐ray spectroscopy analysis detected low levels of sulfur (0.32%±0.8) in biological tissues of H. ruberrima, confirming the absence of thioautotrophic bacterial symbionts in these animals. Consequently, the low δ13C value of H. ruberrima can not be due to the presence of sulfur‐oxidizing symbionts but more probably to the selective and exclusive consumption of free‐living, sulfur‐oxidizing bacteria. 相似文献
147.
Thermal convection is the motor of Earth dynamics and therefore is the link between plate motions, hotspots, seismic velocity variations in the mantle, and anomalies of the gravity field. Small scale mantle anomalies, such as plumes, do, however, generally escape detection by tomographic methods. It is attempted to approach the problem of detection in a somewhat statistical manner. Correlations are sought between spherical harmonic expansions of the fields under study: the hotspot distribution, mantle velocity variations, gravity, heat flow. Using spherical harmonic representations of global fields implies integration and averaging over the whole globe. Thus, although relationships may remain masked in the space domain by a multitude of effects, tendencies may become visible in the spectra or in appropriate averages.The main results are the following: There is a significant long wavelength (n=2,3) negative correlation between the hotspot density and the P-wave velocity variation in the lower mantle. Positive hotspot density of degree 2 to 9 generally correlates with low seismic velocity in all depths of the upper mantle and with positive gravity. This fits well with plume-type convection. These results are also confirmed regionally for a number of individual mid-ocean ridges and hotspots. The hotspot density and the free air anomalies are distinctly positive above regions of low velocity extending to great depth. The effect is not distinct at ridges with shallow velocity anomalies. In a general way, we suggest that the antipodal upwellings (Pacific, Africa) are divided by downwelling currents around the shrinking Pacific. Plate boundaries can easily move away from their past connections with the deeper mantle. Small scale plume currents seem to be depicted in the hotspot expansion. © 1999 Elsevier Science Ltd. All rights reserved. 相似文献
148.
149.
The OSIRIS camera onboard Rosetta successfully acquired images of asteroid 2867 Steins through a variety of color filters during the flyby on 5 September 2008. The best images of this 5 km diameter asteroid have a resolution of 78 m per pixel. We process the images by deconvolving with the point spread function and enlarging through the Mitchell-Netravali filter. The enhanced set is analyzed by means of various techniques (PCA, band ratios, stereo anaglyphs) to study surface morphology and search for variegation. We identify a landslide, which supports a YORP origin for Steins’ unusual diamond shape. In addition, we find that the interior of one of two large craters on the south pole is bluer than the rest of the body. 相似文献
150.
The Kabanga Ni sulfide deposit, Tanzania: I. Geology, petrography, silicate rock geochemistry, and sulfur and oxygen isotopes 总被引:1,自引:0,他引:1
Wolfgang D. Maier Sarah-Jane Barnes Arindam Sarkar Ed Ripley Chusi Li Tim Livesey 《Mineralium Deposita》2010,45(5):419-441
The Kabanga Ni sulfide deposit represents one of the most significant Ni sulfide discoveries of the last two decades, with
current indicated mineral resources of 23.23 Mt at 2.64% Ni and inferred mineral resources of 28.5 Mt at 2.7% Ni (Nov. 2008).
The sulfides are hosted by a suite of ∼1.4 Ga ultramafic–mafic, sill-like, and chonolithic intrusions that form part of the
approximately 500 km long Kabanga–Musongati–Kapalagulu igneous belt in Tanzania and Burundi. The igneous bodies are up to
about 1 km thick and 4 km long. They crystallized from several compositionally distinct magma pulses emplaced into sulfide-bearing
pelitic schists. The first magma was a siliceous high-magnesium basalt (approximately 13.3% MgO) that formed a network of
fine-grained acicular-textured gabbronoritic and orthopyroxenitic sills (Mg# opx 78–88, An plag 45–88). The magma was highly
enriched in incompatible trace elements (LILE, LREE) and had pronounced negative Nb and Ta anomalies and heavy O isotopic
signatures (δ18O +6 to +8). These compositional features are consistent with about 20% contamination of primitive picrite with the sulfidic
pelitic schists. Subsequent magma pulses were more magnesian (approximately 14–15% MgO) and less contaminated (e.g., δ18O +5.1 to +6.6). They injected into the earlier sills, resulting in the formation of medium-grained harzburgites, olivine
orthopyroxenites and orthopyroxenites (Fo 83–89, Mg# opx 86–89), and magmatic breccias consisting of gabbronorite–orthopyroxenite fragments within an olivine-rich matrix. All intrusions
in the Kabanga area contain abundant sulfides (pyrrhotite, pentlandite, and minor chalcopyrite and pyrite). In the lower portions
and the immediate footwall of two of the intrusions, namely Kabanga North and Kabanga Main, there occur numerous layers, lenses,
and veins of massive Ni sulfides reaching a thickness of several meters. The largest amount of high grade, massive sulfide
occurs in the smallest intrusion (Kabanga North). The sulfides have heavy S isotopic signatures (δ34S wr = +10 to +24) that broadly overlap with those of the country rock sulfides, consistent with significant assimilation
of external sulfur from the Karagwe–Ankolean sedimentary sequence. However, based partly on the relatively homogenous distribution
of disseminated sulfides in many of the intrusive rocks, we propose that the Kabanga magmas reached sulfide saturation prior
to final emplacement, in staging chambers or feeder conduits, followed by entrainment of the sulfides during continued magma
ascent. Oxygen isotope data indicate that the mode of sulfide assimilation changed with time. The heavy δ18O ratios of the early magmas are consistent with ingestion of the sedimentary country rocks in bulk. The relatively light
δ18O ratios of the later magmas indicate less bulk assimilation of the country rocks, but in addition the magmas selectively
assimilated additional S, possibly through devolatization of the country rocks or through cannibalization of magmatic sulfides
deposited in the conduits by preceding magma surges. The intrusions were tilted at ca. 1.37 Ga, during the Kibaran orogeny
and associated synkinematic granite plutonism. This caused solid-state mobilization of ductile sulfides into shear zones,
notably along the base of the intrusions where sulfide-hornfels breccias and lenses and layers of massive sulfides may reach
a thickness of >10 m and can extend for several 10 s to >100 m away from the intrusions. These horizons represent an important
exploration target for additional nickel sulfide deposits. 相似文献