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911.
Lunar swirls are albedo anomalies associated with strong crustal magnetic fields. Swirls exhibit distinctive spectral properties at both highland and mare locations that are plausibly explained by fine-grained dust sorting. The sorting may result from two processes that are fairly well established on the Moon, but have not been previously considered together. The first process is the vertical electrostatic lofting of charged fine dust. The second process is the development of electrostatic potentials at magnetic anomalies as solar wind protons penetrate more deeply into the magnetic field than electrons. The electrostatic potential can attract or repel charged fine-grained dust that has been lofted. Since the finest fraction of the lunar soil is bright and contributes significantly to the spectral properties of the lunar regolith, the horizontal accumulation or removal of fine dust can change a surface’s spectral properties. This mechanism can explain some of the spectral properties of swirls, accommodates their association with magnetic fields, and permits aspects of weathering by micrometeoroids and the solar wind. 相似文献
912.
Mark S. Bentley Andrew J. Ball Ian P. Wright John C. Zarnecki 《Planetary and Space Science》2011,59(1):79-91
Space weathering is now commonly accepted to modify the optical and magnetic properties of airless body regoliths throughout the Solar System. Although the precise formation processes are not well understood, the presence of ubiquitous sub-microscopic metallic iron (SMFe) grains in lunar soils and corresponding spectral analyses have explained both the unique optical and magnetic properties of such soils. More recently, a variety of ion irradiation, laser melting and vaporisation and impact experiments have been shown to reproduce these effects in the laboratory. Such experiments are crucial to the study of the formation of SMFe under controlled conditions. To date, more emphasis has been placed on optical analyses of laboratory samples, as these address directly the mineralogical interpretation of remote sensing data. However, the magnetic analyses performed on the Apollo and Luna samples have provided useful qualitative and quantitative evaluation of regolith metallic iron content. These techniques are reviewed here, demonstrated on pulsed laser irradiated olivine powder, and their utility for determining the quantity and size distribution of this metallic iron discussed. Ferromagnetic resonance, multi-frequency magnetic susceptibility, vibrating sample magnetometry and thermomagnetic measurements were carried out. Each showed trends expected for the conversion of paramagnetic Fe2+ in olivine to fine-grained Fe0, with some grains in the superparamagnetic size range. Although evidence for superparamagnetic iron was found, the quantity of sub-microscopic metallic iron produced in these experiments proved insufficient to make conclusive measurements of either the quantity or size distribution of this iron. Improvements to both the experimental and analytical procedures are discussed to better enable such measurements in the future. 相似文献
913.
Joshua F. SNAPE Katherine H. JOY Ian A. CRAWFORD 《Meteoritics & planetary science》2011,46(9):1288-1312
Abstract– Lunar meteorite Northeast Africa (NEA) 001 is a feldspathic regolith breccia. This study presents the results of electron microprobe and LA‐ICP‐MS analyses of a section of NEA 001. We identify a range of lunar lithologies including feldspathic impact melt, ferroan noritic anorthosite and magnesian feldspathic clasts, and several very‐low titanium (VLT) basalt clasts. The largest of these basalt clasts has a rare earth element (REE) pattern with light‐REE (LREE) depletion and a positive Euanomaly. This clast also exhibits low incompatible trace element (ITE) concentrations (e.g., <0.1 ppm Th, <0.5 ppm Sm), indicating that it has originated from a parent melt that did not assimilate KREEP material. Positive Eu‐anomalies and such low‐ITE concentrations are uncharacteristic of most basalts returned by the Apollo and Luna missions, and basaltic lunar meteorite samples. We suggest that these features are consistent with the VLT clasts crystallizing from a parent melt which was derived from early mantle cumulates that formed prior to the separation of plagioclase in the lunar magma ocean, as has previously been proposed for some other lunar VLT basalts. Feldspathic impact melts within the sample are found to be more mafic than estimations for the composition of the upper feldspathic lunar crust, suggesting that they may have melted and incorporated material from the lower lunar crust (possibly in large basin‐forming events). The generally feldspathic nature of the impact melt clasts, lack of a KREEP component, and the compositions of the basaltic clasts, leads us to suggest that the meteorite has been sourced from the Outer‐Feldspathic Highlands Terrane (FHT‐O), probably on the lunar farside and within about 1000 km of sources of both Low‐Ti and VLT basalts, the latter possibly existing as cryptomaria deposits. 相似文献
914.
The origin of shoshonites: new insights from the Tertiary high-potassium intrusions of eastern Tibet 总被引:1,自引:0,他引:1
Ian?H.?CampbellEmail author Aleksandr?S.?Stepanov Hua-Ying?Liang Charlotte?M.?Allen Marc?D.?Norman Yu-Qiang?Zhang Ying-Wen?Xie 《Contributions to Mineralogy and Petrology》2014,167(3):983
The shoshonitic intrusions of eastern Tibet, which range in age from 33 to 41 Ma and in composition from ultramafic (SiO2 = 42 %) to felsic (SiO2 = 74 %), were produced during the collision of India with Eurasia. The mafic and ultramafic members of the suite are characterized by phenocrysts of phlogopite, olivine and clinopyroxene, low SiO2, high MgO and Mg/Fe ratios, and olivine forsterite contents of Fo87 to Fo93, indicative of equilibrium with mantle olivine and orthopyroxene. Direct melting of the mantle, on the other hand, could not have produced the felsic members. They have a phenocryst assemblage of plagioclase, amphibole and quartz, high SiO2 and low MgO, with Mg/Fe ratios well below the values expected for a melt in equilibrium with the mantle. Furthermore, the lack of decrease in Cr with increasing SiO2 and decreasing MgO from ultramafic to felsic rocks precludes the possibility that the felsic members were derived by fractional crystallization from the mafic members. Similarly, magma mixing, crustal contamination and crystal accumulation can be excluded as important processes. Yet all members of the suite share similar incompatible element and radiogenic isotope ratios, which suggests a common origin and source. We propose that melting for all members of the shoshonite suite was initiated in continental crust that was thrust into the upper mantle at various points along the transpressional Red River-Ailao Shan-Batang-Lijiang fault system. The melt formed by high-degree, fluid-absent melting reactions at high-T and high-P and at the expense of biotite and phengite. The melts acquired their high concentrations of incompatible elements as a consequence of the complete dissolution of pre-existing accessory minerals. The melts produced were quartz-saturated and reacted with the overlying mantle to produce garnet and pyroxene during their ascent. The felsic magmas reacted little with the adjacent mantle and preserved the essential features of their original chemistry, including their high SiO2, low Ni, Cr and MgO contents, and low Mg/Fe ratio, whereas the mafic and ultramafic magmas are the result of extensive reaction with the mantle. Although the mafic magmas preserved the incompatible element and radiogenic isotope ratios of their crustal source, buffering by olivine and orthopyroxene extensively modified their MgO, Ni, Cr, SiO2 contents and Mg/Fe ratio to values dictated by equilibrium with the mantle. 相似文献
915.
Ten well‐preserved, earthquake‐triggered liquefaction mounds and a carbonate sand volcano have been found in the Mesoproterozoic Wumishan Formation (1550–1400 Ma) in the Beijing area, North China. These features crop out in a roadcut near Zhuanghuwa Village. All ten mounds occur in the same sedimentary layer and have rounded shapes with some concentric and radial fissures arising from the centre. They range from 1.5 to 4 m in diameter and from 10 cm to 30 cm in height. The carbonate sand volcano has a diameter of 110 cm and the ‘crater’ at the top has a depth of about 30 cm. Associated with these mounds and the sand volcano are many ‘normal’ sedimentary structures and numerous soft‐sediment deformation structures. The former include ripple marks, cross‐bedding, stromatolites and desiccation cracks, indicating deposition in a stable shallow‐water peritidal platform environment. The latter include intrastratal faults and folds, seismically formed breccias and carbonate clastic dykes. The morphological features and the genesis of these liquefaction mounds are very similar to mounds formed recently by the great Wenchuan Earthquake of China (2008). Detailed thin‐section study of the mounds found no signs of any kind of biological constructional process; instead it reveals some obvious fluidification and liquefaction characteristics. Comparative studies have shown that these features are probably the products of Mesoproterozoic earthquake activity. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
916.
Shen Liu Wenchao Su Ruizhong Hu Caixia Feng Shan Gao Ian M. Coulson Tao Wang Guangying Feng Yan Tao Yong Xia 《Lithos》2010,114(1-2):253-264
Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon 206Pb/238U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by 40Ar/39Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K2O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial 87Sr/86Sr ratios (0.7060–0.7063) and positive εNd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (~ 10%) by interaction with upper crustal materials. 相似文献
917.
Groundwater geochemical evolution in the northern portion of the Guarani Aquifer System (Brazil) and its relationship to diagenetic features 总被引:1,自引:0,他引:1
The groundwater flow pattern in the northern portion of GAS (Guarani Aquifer System) is characterized by the existence of four regional recharge areas located in São Paulo, Mato Grosso do Sul and Goiás states. From these areas of recharge the regional flow is radial and directed toward the center of the Paraná Sedimentary Basin. Local discharge occurs in portions of outcrop regions. The groundwater has low mineralization and can be classified as Ca or Ca–Mg–HCO3 type, Na–HCO3 type and Na–HCO3/Cl/SO4 type, this sequence represents the hydrochemical evolution. The mechanisms responsible for this evolution are dissolution of feldspars and removal of the carbonate cement from the sandstone mineral framework, followed by ion exchange, responsible for the increase in the Na concentration and decrease of Ca, and, finally, enrichment in Cl and SO4 derived from underlying aquifer units. The hydrochemical evolution is consistent with diagenetic features that are observed in the sandstones, with the presence of siliceous cement in the outcrop areas, and carbonate cement toward the center of Paraná Basin. 相似文献
918.
δ13C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ18O and δ2H values of water, δ34S values of dissolved SO4, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. δ13CDIC values in the Murray River vary between −9.5 and −4.7‰ with a range of <3‰ within any sampling round. δ13CDIC values of the tributaries are −11.0‰ to −5.1‰. DIC concentrations of the Murray River increase from ∼25 mg/L in the middle and upper reaches of the river to 45–55 mg/L in the lower reaches. However, the mass ratio of DIC as a proportion of the total dissolved solids (TDS) decreases from ∼0.6–0.7 in the headwaters to ∼0.2–0.3 in the lower reaches of the river, with similar downstream changes in DIC/Cl ratios. This precludes simple evaporative concentration of DIC and is interpreted as the river evading CO2; this interpretation is consistent with pCO2 values that are in the range 550–11,200 ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (∼360 ppmv). The δ13CDIC values are similar to those that would be produced by the weathering of marine limestone (δ13C ∼ 0‰). However, the lack of marine limestones cropping out in the Murray–Darling Basin and the relatively uniform δ13CDIC values of the Murray River (even in upland reaches where the dominant rock types are metamorphosed silicates and granites) make this unlikely. Rather the high pCO2 values and δ13CDIC values are best explained by a combination of mineralisation of low δ13C organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and δ13C values. 相似文献
919.
Nigel Harris Alison Hunt Ian Parkinson Andrew Tindle Magisuren Yondon Samantha Hammond 《Contributions to Mineralogy and Petrology》2010,160(1):67-81
Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay
dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent
dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed
in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites
were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed
at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene,
clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na,
low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas
is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from
both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined
by olivine zoning, lies in the range 0.2–0.3 m s−1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the
Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene
geotherm beneath the Baikal rift. 相似文献
920.
Ian Parsons John D. Fitz Gerald James K. W. Lee Tim Ivanic Ute Golla-Schindler 《Contributions to Mineralogy and Petrology》2010,160(2):155-180
Microtextural changes brought about by heating alkali feldspar crystals from the Shap granite, northern England, at atmospheric
pressure, have been studied using transmission and scanning electron microscopy. A typical unheated phenocryst from Shap is
composed of about 70 vol% of tweed orthoclase with strain-controlled coherent or semicoherent micro- and crypto-perthitic
albite lamellae, with maximum lamellar thicknesses <1 μm. Semicoherent lamellae are encircled by nanotunnel loops in two orientations
and cut by pull-apart cracks. The average bulk composition of this microtexture is Ab27.6Or71.8An0.6. The remaining 30 vol% is deuterically coarsened, microporous patch and vein perthite composed of incoherent subgrains of
oligoclase, albite and irregular microcline. The largest subgrains are ~3 μm in diameter. Heating times in the laboratory
were 12 to 6,792 h and T from 300°C into the melting interval at 1,100°C. Most samples were annealed at constant T but two were heated to simulate an 40Ar/39Ar step-heating schedule. Homogenisation of strain-controlled lamellae by Na↔K inter-diffusion was rapid, so that in all run
products at >700°C, and after >48 h at 700°C, all such regions were essentially compositionally homogeneous, as indicated
by X-ray analyses at fine scale in the transmission electron microscope. Changes in lamellar thickness with time at different
T point to an activation energy of ~350 kJmol−1. A lamella which homogenised after 6,800 h at 600°C, therefore, would have required only 0.6 s to do so in the melting interval
at 1,100°C. Subgrains in patch perthite homogenised more slowly than coherent lamellae and chemical gradients in patches persisted
for >5,000 h at 700°C. Homogenisation T is in agreement with experimentally determined solvi for coherent ordered intergrowths, when a 50–100°C increase in T for An1 is applied. Homogenisation of lamellae appears to proceed in an unexpected manner: two smooth interfaces, microstructurally
sharp, advance from the original interfaces toward the mid-line of each twinned, semicoherent lamella. In places, the homogenisation
interfaces have shapes reflecting the local arrangements of nanotunnels or pull-aparts. Analyses confirm that the change in
alkali composition is also relatively sharp at these interfaces. Si–Al disordering is far slower than alkali homogenisation
so that tweed texture in orthoclase, tartan twinning in irregular microcline, and Albite twins in albite lamellae and patches
persisted in all our experiments, including 5,478 h at 700°C, 148 h at 1,000°C and 5 h at 1,100°C, even though the ensemble
in each case was chemically homogeneous. Nanotunnels and pull-aparts were modified after only 50 min at 500°C following the
simulated 40Ar/39Ar step-heating schedule. New features called ‘slots’ developed away from albite lamellae, often with planar traces linking
slots to the closest lamella. Slot arrays were often aligned along ghost-like regions of diffraction contrast which may mark
the original edges of lamellae. We suggest that the slot arrays result from healing of pull-aparts containing fluid. At 700°C
and above, the dominant defects were subspherical ‘bubbles’, which evolved from slots or from regions of deuteric coarsening.
The small degree of partial melting observed after 5 h at 1,100°C was often in the vicinity of bubbles. Larger micropores,
which formed at subgrain boundaries in patch perthite during deuteric coarsening, retain their shape up to the melting point,
as do the subgrain boundaries themselves. It is clear that modification of defects providing potential fast pathways for diffusion
in granitic alkali feldspars begins below 500°C and that defect character progressively changes up to, and beyond, the onset
of melting. 相似文献