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Carbonate rocks that have suffered early near-surface dolomitization followed by extended meteoric exposure commonly undergo partial delithification, a process that results in the formation of dolomitic silts and sands, herein termed dologrus. Dologrus is interpreted to form as a result of diffuse dissolution in porous and permeable dolostones prior to burial and compaction. Such dissolution occurs at the crystal–pore-water interface, causing individual crystals to corrode along interfacial boundaries, eventually leading to delithification and the formation of sediment composed of corroded crystals. The resulting sediment grain size is likely controlled in part by the crystal size of the precursor dolostone. Loss of rock competency through the process of dologrus formation in the shallow subsurface can lead to collapse of overlying bedrock and the formation of dolines common to karstic carbonate landscapes. 相似文献
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Yan-Wei Xie Jing-Bo Wang George Hobbs Jane Kaczmarek Di Li Jie Zhang Shi Dai Andrew Cameron Lei Zhang Chen-Chen Miao Mao Yuan Shen Wang Song-Bo Zhang Heng Xu Ren-Xin Xu 《天文和天体物理学研究(英文版)》2019,(7)
Flux densities are fundamental observational parameters that describe a pulsar. In the current pulsar catalogue, 27% of the listed radio pulsars have no flux density measurement in the 20 cm observing band. Here, we present the first such measurements for 32 pulsars observed employing the Parkes radio telescope. We have used both archival and new observations to make these measurements. Various schemes exist for measuring flux densities and we show how the measured flux densities vary between these methods and how the presence of radio-frequency interference will bias the flux density measurements. 相似文献
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In this paper we present new data on the spatial variability of peridotite composition across a kilometer-scale mantle shear
zone within the Lanzo massif (Western Alps, Italy). The shear zone separates the central from the northern part of the massif.
Plagioclase peridotite shows gradually increasing deformation towards the shear zone, from porphyroclastic to mylonitic textures
in the central body, while the northern body is composed of porphyroclastic rocks. The peridotite displays a large range of
compositions, from fertile peridotite to refractory harzburgite and dunite. Deformed peridotites (proto-mylonite and mylonites)
tend to be compositionally more homogeneous and fertile than weakly deformed peridotites. The composition of most plagioclase
peridotites show rather high and constant (Ce/Yb)
N
ratios, and Yb
N
that cannot be explained by any simple melting model. Instead, refertilization modeling, consisting of melt increments from
spinel peridotite sources, particularly with E-MORB melt, reasonably reproduces the plagioclase peridotite whole rock composition.
Combined with constraints from Ce–Nb and Ce–Th systematics, we speculate that peridotites such as those from Lanzo record
pervasive refertilization processes in the thermal boundary layer. In this scenario, mantle shear zones might act as important
areas of melt focusing in the upper mantle that separates the thermal boundary layer from the conductively cooled mantle. 相似文献
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M.-A. Kaczmarek O. Müntener D. Rubatto 《Contributions to Mineralogy and Petrology》2008,155(3):295-312
The U–Pb ages and the trace element content of zircon U–Pb along with major and trace element whole rock data on gabbroic
dikes from the Lanzo lherzolitic massif, N-Italy, have been determined to constrain crustal accretion in ocean–continent transition
zones. Three Fe–Ti gabbros were dated from the central and the southern part of the massif providing middle Jurassic ages
of 161 ± 2, 158 ± 2 and 163 ± 1 Ma, which argue for magmatic activity over few millions of years. Zircon crystals are characterized
by high but variable Th/U ratios, rare earth element patterns enriched in heavy rare earths, pronounced positive Ce and negative
Eu-anomalies consistent with crystallization after substantial plagioclase fractionation. The zircon trace element composition
coupled with whole rock chemistry was used to reconstruct the crystallization history of the gabbros. A number of gabbros
crystallized in situ, and zircon precipitated from trapped, intercumulus liquid, while other gabbros represent residual liquids
that were extracted from a cumulus pile and crystallized along syn-magmatic shear zones. We propose a model in which the emplacement
mechanism of gabbroic rocks in ocean–continent transition zones evolves from in situ crystallization to stratified crystallization
with efficient extraction of residual liquid along syn-magmatic shear zones. Such an evolution of the crystallization history
is probably related to the thermal evolution of the underlying mantle lithosphere.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
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Palygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg2+ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form. 相似文献