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151.
Contributors to chromium isotope variation of meteorites 总被引:3,自引:0,他引:3
Liping Qin Conel M.O’D. Alexander Mary F. Horan 《Geochimica et cosmochimica acta》2010,74(3):1122-400
We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of our samples, they were first fused with lithium borate-tetraborate at 1050-1000 °C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in 54Cr/52Cr from 0.4 to 1.6ε (1ε = 1 part in 10,000), and ordinary chondrites display a common 54Cr/52Cr deficit of ∼0.4ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54Cr-enriched carrier, which is characterized by a large excess in 54Cr/52Cr (up to 200ε) associated with a very small deficit in 53Cr/52Cr (<2ε). We did not find 54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the 54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in 54Cr/52Cr. The 54Cr/52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a 54Cr-rich component in the inner Solar System.We confirm the correlated excesses in 54Cr/52Cr and 53Cr/52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn-Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn-Cr isochron reported by Shukolyukov and Lugmair (2006) and Moynier et al. (2007), mostly because we find much smaller total variations in ε53Cr (∼0.2). All bulk chondrites have small ε53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The ε53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System 53Mn/55Mn value of 5.4(±2.4) × 10−6, corresponding to an absolute age of 4566.4 (±2.2) Ma.Nuclear interactions with cosmic rays result in coupled excesses in ε54Cr and ε53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in ε54Cr of up to 140ε. These new results show that the Mn-Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages. 相似文献
152.
Thomas J. Zega Conel M. O’D. Alexander Larry R. Nittler Rhonda M. Stroud 《Geochimica et cosmochimica acta》2010,74(20):5966-5983
We report a coordinated analytical study of matrix material in the Tagish Lake carbonaceous chondrite in which the same small (?20 μm) fragments were measured by secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), and X-ray absorption near-edge spectroscopy (XANES). SIMS analysis reveals H and N isotopic anomalies (hotspots), ranging from hundreds to thousands of nanometers in size, which are present throughout the fragments. Although the differences in spatial resolution of the SIMS techniques we have used introduce some uncertainty into the exact location of the hotspots, in general, the H and N isotopic anomalies are spatially correlated with C enrichments, suggesting an organic carrier. TEM analysis, enabled by site-specific extraction using a focused-ion-beam scanning-electron microscope, shows that the hotspots contain an amorphous component, Fe-Ni sulfides, serpentine, and mixed-cation carbonates. TEM imaging reveals that the amorphous component occurs in solid and porous forms, EDS indicates that it contains abundant C, and EELS and XANES at the C K edge reveal that it is largely aromatic. This amorphous component is probably macromolecular C, likely the carrier of the isotopic anomalies, and similar to the material extracted from bulk samples as insoluble organic matter. However, given the large sizes of some of the hotspots, the disparity in spatial resolution among the various techniques employed in our study, and the phases with which they are associated, we cannot entirely rule out that some of the isotopic anomalies are carried by inorganic material, e.g., sheet silicates. The isotopic composition of the organic matter points to an initially primitive origin, quite possibly within cold interstellar clouds or the outer reaches of the solar protoplanetary disk. The association of organic material with secondary phases, e.g., serpentine and carbonates, suggests that the organic matter was susceptible to parent-body processing, and thus, isotopic dilution. 相似文献
153.
Alexander G. Sokol Yury N. Palyanov Igor N. Kupriyanov Mariya P. Polovinka 《Geochimica et cosmochimica acta》2010,74(16):4793-4806
High pressure experiments have been performed in the systems Mg2SiO4-C-O-H and Mg2SiO4-K2CO3-C at 6.3 GPa and 1200 to 1600 °C using a split-sphere multi-anvil apparatus. In the Mg2SiO4-C-O-H system the composition of fluid was modeled by adding different amounts of water and stearic acid. The fO2 was controlled by the Mo-MoO2 or Fe-FeO oxygen buffers. Several experiments in the Mg2SiO4-C-O-H system and all experiments in the Mg2SiO4-K2CO3-C system have been conducted without buffering the fO2. Forsterite in the system Mg2SiO4-K2CO3-C does not reveal OH absorption bands in the IR spectra, while forsterite coexisting with carbon-bearing fluid and silicate melt at logfO2 from FMQ-2 to FMQ-5 (from 2 to 5 log units below fayalite-magnetite-quartz oxygen buffer) contains 800-1850 wt. ppm H2O. The maximum concentrations were detected at 1400 °C and FMQ-3.5. We observed an increase in the solidus temperature in the system Mg2SiO4-C-O-H from 1200 to above 1600 °C with log fO2 decreasing from FMQ-2 to FMQ-5. The increase of the solidus temperature and the broadening of the stability field of the H2O-H2-CH4 subsolidus fluid phase at 1400-1600 °C explain the high H2O storage capacity of forsterite relative to that crystallized from carbon-free, oxidized, hydrous, silicic melt. At temperatures above 1400 °C liquidus forsterite precipitated along with diamond from oxidized (FMQ-1) carbonate-silicate melt and from silicate melt dissolving the moderately reduced C-O-H fluid (from FMQ-2 to FMQ-3.5). Formation of diamond was not detected under ultra-reduced conditions (FMQ-5) at 1200-1600 °C. Olivine co-precipitating with diamond from dry carbonate-silicate or hydrous-silicic fluid/melt can provide information on the H2O contents and speciation of the diamond-forming media in the mantle. The conditions for minimum post-crystallization alteration of olivine and its hydrogen content are discussed. 相似文献
154.
Numerical Analysis of Block Caving-Induced Instability in Large Open Pit Slopes: A Finite Element/Discrete Element Approach 总被引:3,自引:2,他引:1
This paper addresses one of the most challenging problems in mining rock engineering—the interaction between block cave mining
and a large overlying open pit. The finite element modeling/discrete element modeling (FEM/DEM) approach was utilized in the
analysis of block caving-induced step-path failure development in a large open pit slope. The analysis indicated that there
is a threshold percentage of critical intact rock bridges along a step-path failure plane that may ensure the stability of
an open pit throughout caving operations. Transition from open pit to underground mining at Palabora mine presents an important
example of a pit wall instability triggered by caving. Using combined FEM/DEM-DFN (discrete fracture network) modeling, it
was possible to investigate the formation of a basal failure surface within an open pit slope as a direct result of cave mining.
The modeling of Palabora highlighted the importance of rock mass tensile strength and its influence on caving-induced slope
response. 相似文献
155.
In the Kor-, Saualpe and Pohorje regions of the Eastern Alps eclogite bodies occur within metapelitic gneisses. The bodies are between 1 meter and several hundreds of meters in size and some of them were defined by Haüy (1822) as the type locality for the rock type “eclogite”. A growing body of petrological work has documented the metamorphic evolution of the metapelites surrounding the eclogites. However, few phase diagrams have been constructed for the eclogite bodies themselves. Here we use recently available activity models for amphiboles to present new thermodynamic pseudosections for the Hohl locality of the Koralpe eclogites. We show that this eclogite reached peak conditions in a narrow PT field obliquely oriented in PT space between 16.5 and 20.5 kbar and 620°C to 720°C and that its metamorphic evolution was likely to have occurred under water saturated conditions. We conclude that eclogite and the surrounding metapelites have certainly undergone the same metamorphic peak in Eo-alpine time. Comparison of our results with different PT estimates on the eclogite from Pohorje, suggest that a PT gradient from Koralpe to Pohorje is likely. 相似文献
156.
Hayfaa Abdul Aziz Madelaine B?hme Alexander Rocholl Jerome Prieto Jan R. Wijbrans Valerian Bachtadse Albert Ulbig 《International Journal of Earth Sciences》2010,99(8):1859-1886
A detailed integrated stratigraphic study (biostratigraphy and magnetostratigraphy) was carried out on five sections from
the western part of the Bavarian Upper Freshwater Molasse of the North Alpine Foreland Basin (NAFB), greatly improving the
chronostratigraphy of these sediments. The sections belong to the lithostratigraphic units Limnische Untere Serie (UL) and Fluviatile Untere Serie (UF) and contain 19 (mostly new) small-mammal bearing levels, significantly refining the local biostratigraphy. Radiometric
ages obtained from glass shards from tuff horizons are used together with the biostratigraphic information for constructing
and confirming the magnetostratigraphic correlation of the studied sections to the Astronomical Tuned Time Scale (ANTS04;
Lourens et al. in Geologic Time Scale 2004, Cambridge University Press, 2004). This correlation implies that the UL lithostratigraphic
unit corresponds to the latest Ottnangian and the Early Karpatian, whereas the UF corresponds to the Karpatian and the Early
Badenian. This indicates that the Brackish- to Freshwater Molasse transition already occurred during the late Ottnangian.
The pre-Riesian hiatus occurred in the latest Karpatian and lower Early Badenian in Eastern Bavaria and Bohemia and in the
Late Karpatian and earliest Badenian in Western Bavaria. The geochemical and Ar–Ar data of volcanic ashes suggest that highly
evolved silicic magmas from a single volcano or volcanic center, characterized by a uniform Nd isotopic composition, erupted
repetitively over the course of at least 1.6 Myr. Three phases of eruptive activity were identified at 16.1 ± 0.2 Ma (Zahling-2),
15.6 ± 0.4 Ma (Krumbad), and 14.5 ± 0.2 Ma (Heilsberg, Hegau). The correlation of the local biostratigraphic zonation to the
ANTS04 enables further the characterization of both the Ottnangian–Karpatian and Karpatian–Badenian boundaries in the NAFB
by small-mammal biostratigraphy. According to these results the Ottnangian–Karpatian boundary is contemporaneous with the
first appearance datum of Megacricetodon
bavaricus (in the size of the type population) and the first common occurrence of Keramidomys thaleri, whereas Ligerimys florancei, Melissiodon dominans and Prodeinotherium aff. bavaricum have been already disappeared during the late Ottnangian. The Karpatian–Badenian boundary is characterized by a significant
size increase of the large Megacricetodon lineage and possibly a (re-)immigration of Prodeinotherium bavaricum. 相似文献
157.
The existing procedures for the selection of runout model parameters from back-analyses do not allow integrating different
types of runout criteria and generally lack a systematic approach. A new method based on receiver operating characteristic
(ROC) analyses and aimed at overcoming these limitations is herein proposed. The method consists of estimating discrete classifiers
for every runout simulation associated with a set of model parameters. The set of parameters that yields the best prediction
is selected using ROC metrics and space. The procedure is illustrated with the back-analyses of a rainfall-triggered debris
flow that killed 300–500 people in the Metropolitan Area of San Salvador in 1982. The selected model parameters are used to
estimate forward predictions for scenarios that correspond to different return periods. The proposed procedure may be useful
in the assessment of areas potentially affected by landslides. In turn, this information can be used in the production or
updating of land use plans and zonations, similar to that currently being carried out by the Office for Urban Planning of
the Metropolitan Area of San Salvador in El Salvador. 相似文献
158.
Worldwide, the ammonoid genus Prolobites is only known from a few localities, and from these fossil beds almost all of the specimens are adults as shown by the presence
of a terminal growth stage. This is in marked contrast to the co-occurring ammonoid genera such as Sporadoceras, Prionoceras, and Platyclymenia. Size distribution of specimens of Prolobites from three studied localities show that, unlike in the co-occurring ammonoid species, most of the material belongs to adult
individuals. The morphometric analysis of Prolobites delphinus (Sandberger & Sandberger 1851) demonstrates the intraspecific variability including variants with elliptical coiling and that dimorphism is not detectable.
The Prolobites material shows close resemblance to spawning populations of Recent coleoids such as the squid Todarodes filippovae Adam
1975. Possible mass spawning events are discussed in the context of the size distribution and limited geographic range of Prolobites. Finally, the potential fecundity and brooding behaviour of Prolobites is hypothesized using the examples of post spawning egg care in Recent coleoids. 相似文献
159.
Jürgen M. Reitner Wilfried Gruber Alexander Römer Rainer Morawetz 《Swiss Journal of Geoscience》2010,103(3):385-405
We present a study of the inneralpine basin of Hopfgarten focused on the analysis of basin fill in order to reveal its formation
in relation to paleo-ice flow and tectonics. The study is based on geological mapping as well as seismic (reflection and refraction)
and geoelectrical surveys. The oldest sequence in the basin, identified by seismic stratigraphy at 400 m below surface, consists
of coarse grained sediments of supposedly Oligocene to Miocene age, which subsided along faults linked to the Inn fault. Three
superimposed sequences, each displaying baselaps in contact with a subglacially formed unconformity and sigmoid foresets,
show pleniglacial conditions followed by a glaciolacustrine environment. The uppermost of these three sequences lies on top
of last glacial maximum till (LGM; Würmian Pleniglacial; MIS 2) and represents Termination I. The middle sequence is classified
as Termination II following the Rissian Pleniglacial (MIS 6). The oldest glacial sequence cannot be constrained chronostratigraphically
but might correlate with Termination V following the major glaciation of MIS 12. Limited glacial erosion during the LGM occurred
only during the ice build-up phase. Further overdeepening was impeded due to topographic barrier and mutual blockades of glaciers
within this highly dissected landscape. The occurrence and relative timing of the impediment was controlled by the onset of
transfluences and thus by the altitude of coles. The higher amount of overdeepening during older glacial periods is explained
by longer phases of free ice advance in the ice build up phase due to higher transfluences routes at that time. Thus, the
preservation of older Pleistocene sequences within the basin may be the result of the lowering of watersheds from one glaciation
to the next. Our model of an inverse relationship between glacial shaping of the surface and the subsurface may apply to similar
Alpine landscapes as well. 相似文献
160.
Thilo Wrona Alexander C. Whittaker Rebecca E. Bell Robert L. Gawthorpe Haakon Fossen Christopher A.-L. Jackson Marit Stokke Bauck 《Basin Research》2023,35(2):744-761
Our understanding of continental rifting is, in large parts, derived from the stratigraphic record. This record is, however, incomplete as it does not often capture the geomorphic and erosional signal of rifting. New 3D seismic reflection data reveal a Late Permian-Early Triassic landscape incised into the pre-rift basement of the northern North Sea. This landscape, which covers at least 542 km2, preserves a drainage system bound by two major tectonic faults. A quantitative geomorphic analysis of the drainage system reveals 68 catchments, with channel steepness and knickpoint analysis of catchment-hosted palaeo-rivers showing that the landscape preserved a >2 Myr long period of transient tectonics. We interpret that this landscape records a punctuated uplift of the footwall of a major rift-related normal fault (Vette Fault) at the onset of rifting. The landscape was preserved by a combination of relatively rapid subsidence in the hangingwall of a younger fault (Øygarden Fault) and burial by post-incision sediments. As such, we show how and why erosional landscapes are preserved in the stratigraphic record, and how they can help us understand the tectono-stratigraphic evolution of ancient continental rifts. 相似文献