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A fast algorithm is presented for numerical evaluation of forward and inverse Radon transforms. The algorithm does not perform exact one-to-one mapping as the discrete Fourier transform but, due to the use of band-limited basis functions, it is robust and sufficiently accurate for seismic applications. By rewriting the transform as a convolution, a computational speed is obtained similar to the speed of the 2D fast Fourier transform. 相似文献
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Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry 总被引:2,自引:0,他引:2
The spinelgarnet transition in Cr/Al-enriched peridotiticbulk compositions is known from experimental investigationsto occur at 2070 kbar, within the pressure range sampledby kimberlites. We show that the Cr2O3CaO compositionsof concentrate garnets from kimberlite have maximum Cr/Ca arrayscharacterized by Cr2O3/CaO 0·960·81, andinterpret the arrays as primary evidence of chromitegarnetcoexistence in Cr-rich harzburgitic or lherzolitic bulk compositionsderived from depth within the lithosphere. Under Cr-saturatedconditions on a known geotherm, each Cr/Ca array implicitlydelineates an isobar inside a garnet Cr2O3CaO diagram.This simplification invites a graphical approach to calibratean empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromitegarnetharzburgite xenoliths from the Roberts Victor kimberlite tightlybracket a graphitediamond constraint (GDC) located atCr2O3 = 0·94CaO + 5·0 (wt %), representing a pivotalcalibration corresponding to 43 kbar on a 38 mW/m2 conductivegeotherm. Additional calibration points are established at 14,17·4 and 59·1 kbar by judiciously projecting garnetcompositions from simple-system experiments onto the same geotherm.The garnet Cr/Ca barometer is then simply formulated as follows(in wt %):
- if Cr2O3 0·94CaO + 5, then P38 (kbar) = 26·9+ 3·22Cr2O3 3·03CaO, or
- if Cr2O3 <0·94CaO + 5, then P38 (kbar) = 9·2+ 36[(Cr2O3+ 1·6)/(CaO + 7·02)].
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BRUECKNER HANNES K.; VAN ROERMUND HERMAN L. M.; PEARSON NORMAN J. 《Journal of Petrology》2004,45(2):415-437
Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M1 and M2) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M1 garnet and the crystallization of low-Cr spinel1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M2 assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobeinductively coupled plasma mass spectrometryReOs analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an ReOs isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene SrNd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model SmNdages (TDM = 1·12·2 Ga) indicate that fertilizationtook place in Proterozoic times. SmNd garnet2clinopyroxene2wholerock ± orthopyroxene2 mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2 相似文献
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The total mineralization of solutions squeezed out of montmorillonite clay saturated in sea water was determined at different overburden pressures. The subsequent fractions of expelled solutions were also analysed for various anions (Cl?, SO2-4, HCO?3, F?) and cations (Na+, K+, Mg2+, Ca2+, B3+). The results indicate that the concentrations of squeezed-out solutions during the initial stages of compaction (at pressures up to 35 kg/cm2) are slightly higher than that of interstitial solution present initially. The concentration of squeezed-out solution goes through a maximum, or at least remains constant, before starting to decrease with increasing overburden pressure. 相似文献
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