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1.
<正>GRV 020175 is an Antarctic mesosiderite,containing about 43 vol%silicates and 57 vol% metal.Metal occurs in a variety of textures from irregular large masses,to veins penetrating silicates, and to matrix fine grains.The metallic portion contains kamacite,troilite and minor taenite.Terrestrial weathering is evident as partial replacement of the metal and troilite veins by Fe oxides.Silicate phases exhibit a porphyritic texture with pyroxene,plagioclase,minor silica and rare olivine phenocrysts embedded in a fine-grained groundmass.The matrix is ophitic and consists mainly of pyroxene and plagioclase grains.Some orthopyroxene phenocrysts occur as euhedral crystals with chemical zoning from a magnesian core to a ferroan overgrowth;others are characterized by many fine inclusions of plagioclase composition.Pigeonite has almost inverted to its orthopyroxene host with augite lamellae, enclosed by more magnesian rims.Olivine occurs as subhedral crystals,surrounded by a necklace of tiny chromite grains(about 2-3μm).Plagioclase has a heterogeneous composition without zoning. Pyroxene geothermometry of GRV 020175 gives a peak metamorphic temperature(～1000℃) and a closure temperature(～875℃).Molar Fe/Mn ratios(19-32) of pyroxenes are consistent with mesosiderite pyroxenes(16-35) and most plagioclase compositions(An_(87.5_96.6)) are within the range of mesosiderite plagioclase grains(An_(88-95)).Olivine composition(Fo_(53.8)) is only slightly lower than the range of olivine compositions in mesosiderites(Fo_(55-90)).All petrographic characteristics and chemical compositions of GRV 020175 are consistent with those of mesosiderite and based on its matrix texture and relatively abundant plagioclase,it can be further classified as a type 3A mesosiderite.Mineralogical, penological,and geochemical studies of GRV 020175 imply a complex formation history starting as rapid crystallization from a magma in a lava flow on the surface or as a shallow intrusion.Following primary igneous crystallization,the silicate underwent varying degrees of reheating.It was reheated to 1000℃,followed by rapid cooling to 875℃.Subsequently,metal mixed with silicate,during or after which,reduction of silicates occurred;the reducing agent is likely to have been sulfur.After redox reaction,the sample underwent thermal metamorphism,which produced the corona on the olivine, rims on the inverted pigeonite phenocrysts and overgrowths on the orthopyroxene phenocrysts,and homogenized matrix pyroxenes.Nevertheless,metamorphism was not extensive enough to completely reequilibrate the GRV 020175 materials. 相似文献
2.
Twenty-nine unbrecciated eucrites have been thoroughly characterized in terms of the petrologic factors that affect their spectra, such as mineral chemistry, modal adundances, grain sizes, and textures. We have conducted a combined petrologic and spectral study designed to provide insight into the petrogenesis of the basaltic crust of Vesta and the variety of rock-types that exist within it, as well as aid in the petrologic interpretation of spectra to be collected by the Dawn orbiting spacecraft. This paper details the petrology part of the study. Unbrecciated eucrite samples were selected to avoid the complications of lithologic mixing in the accompanying spectral study. A wide variety of textural types are seen within the basaltic eucrites, encompassing quenched, coarse-grained, and granoblastic samples. Zoned pyroxenes in eucrites and those that preserve a history of initial rapid cooling are rare. Nearly all eucrite samples have been thermally metamorphosed and would commonly be classified as equilibrated; however, this term reflects only the quadrilateral (Mg, Fe, and Ca) compositions of pyroxenes, and considerable variations are seen within the minor elements (Al, Ti, and Cr) in pyroxenes as well as plagioclase compositions. Determination of both pyroxene and plagioclase compositions together with pyroxene geothermometry provides a better estimate for the relative degree of thermal metamorphism a eucrite has experienced. The petrologic differences observed here might allow different eucrites to be distinguished spectrally. This is especially true for the varying pyroxene compositions as the spectra of eucrites are dominated by absorption features attributed to pyroxene. 相似文献
3.
We report an electron probe investigation of the eucrite Moore County. Moore County consists predominantly of four pyroxene phases and plagioclase, with minor tridymite, ilmenite, chromite, troilite, iron metal and possibly apatite. Temperatures have been computed for Moore County and other eucrites using three independent techniques for coexisting pairs of pyroxenes. These computed temperatures are all subsolidus and approach those calculated for slowly cooled terrestrial igneous and metamorphic rocks. These temperatures conflict with the conclusions of earlier workers that Moore County was catastrophically removed from its cumulate environment at high temperatures. The eucrites cooled to temperatures substantially below the solidus on their parent body.Our results are consistent with the conclusion that Moore County resided for an extended period of time on the eucrite parent body, perhaps from the time of its crystallization from a basaltic melt (～-4.5AE) until a few tens of millions of years ago. This extended residence time is consistent with the conclusion that the eucrite parent body is still intact, but raises dynamical objections to the tentative identification of the eucrite parent body as asteroid 4 Vesta.An alternative scenario involves ejection from the eucrite parent body, at times significantly older than those indicated by cosmic ray exposure ages, of meteoroids of sufficient size that their interiors were shielded from cosmic rays. These meteoroids were removed by low-probability mechanisms into Earth-crossing orbits where they were disrupted and ultimately sampled by the Earth. This proposal appears to remove the dynamical objections to the tentative identification of the eucrite parent body as Vesta. 相似文献
4.
《Chemie der Erde / Geochemistry》2019,79(4):125541
Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ˜0.9–˜10 GPa, ˜1300–˜1730 °C, and < 1 ms (5–50 μm in size), respectively. 相似文献
5.
6.
《Chemie der Erde / Geochemistry》2019,79(4):125536
Although acapulcoites and lodranites played a key role in understanding partial differentiation of asteroids, the lack of samples of the chondritic precursor limits our understanding of the processes that formed these meteorites. Grove Mountains (GRV) 020043 is a type 4 chondrite, with abundant, well-delineated, pyroxene-rich chondrules with an average diameter of 690 μm, microcrystalline mesostasis, polysynthetically striated low-Ca pyroxene, and slightly heterogeneous plagioclase compositions. Similarities in mineralogy, mineral composition, and oxygen isotopic composition link GRV 020043 to the acapulcoite-lodranite clan. These features include a high low-Ca pyroxene to olivine ratio, high kamacite to taenite ratio, and relatively FeO-poor mafic silicates (Fa10.3, Fs10.4) relative to ordinary chondrites, as well as the presence of ubiquitous metal and sulfide inclusions in low-Ca pyroxene and ƒO2 typical of acapulcoites. GRV 020043 shows that evidence of partial melting is not an essential feature for classification within the acapulcoite-lodranite clan. GRV 020043 experienced modest thermal metamorphism similar to type 4 ordinary chondrites. GRV 020043 suggests a range of peak temperatures on the acapulcoite-lodranite parent body similar to that of ordinary chondrites, but shifted to higher temperatures, perhaps consistent with earlier accretion. The mineralogy and mineral compositions of GRV 020043, despite modest thermal metamorphism, suggests that most features of acapulcoites previously attributed to reduction were, instead, inherited from the precursor chondrite. Although partial melting was widespread on the acapulcoite-lodranite parent body, ubiquitous Fe,Ni-FeS blebs in the cores of silicates were not implanted by shock or trapped during silicate melting, but were inherited from the precursor chondrite with subsequent overgrowths during metamorphism. 相似文献
7.
Paul C. Buchanan Takaaki Noguchi Donald D. Bogard Ikuo Katayama 《Geochimica et cosmochimica acta》2005,69(7):1883-1898
The unequilibrated eucrite Yamato 82202 (Y82202) contains a network of relatively thick (up to 1 mm in width) glass veins. The host of the meteorite represents a monomict breccia composed of volcanic rock that crystallized > 4.3 Ga ago as a lava flow on the surface of 4 Vesta. The veins formed 3.90 ± 0.04 Ga ago, probably as a result of frictional melting associated with impact, under conditions of low effective oxygen fugacity and higher sulfur fugacity. The glass contains disordered structural elements characteristic of pyroxene and feldspar, reminiscent of the eucritic target lithology. The unequilibrated pyroxenes of the volcanic host and the pristine character of the glass indicate that this meteoritic material did not experience significant thermal metamorphism after initial crystallization of the lava. Hence, it was not affected by regional metamorphism caused by burial to a significant depth or by long-term contact metamorphism associated with a thick lava flow, large intrusion, or hot layer of impact ejecta. The meteorite resided at a shallow depth (though not at the surface) on 4 Vesta or on one of the vestoids until it was ejected and traveled to Earth, probably with other HED materials that have 36Ar exposure ages of ∼13 Ma. These data suggest that the surface of the asteroid 4 Vesta has significant, though probably small, proportions of glass, as well as unequilibrated volcanic rock. 相似文献
8.
《Chemie der Erde / Geochemistry》2015,75(2):155-183
The howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocks and impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-based telescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compelling case that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted. Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addition to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites are gabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth within the crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills, and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites, and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiation occurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxygen isotope compositions and siderophile element contents favor a model of extensive melting of Vesta forming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield a metallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneous lithologies were subjected to post-crystallization thermal processing, and most eucrites show textural and mineral-compositional evidence for metamorphism. The cause of this common metamorphism is unclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusion rates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts, and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack of mixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymict breccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbers of meteorites, compositions of howardites, and models of magma ocean solidification, cumulate eucrites and their residual ferroan mafic melts are minor components of the vestan crust. 相似文献
9.
Ya. V. Bychkova E. V. Koptev-Dvornikov N. N. Kononkova E. E. Kameneva 《Geochemistry International》2007,45(2):131-151
Data obtained on the compositions of rock-forming minerals from the Kivakka layered intrusion indicate that the #Fe of olivine increases up the vertical section of the intrusion, and the #Mg of the pyroxenes and the anorthite contents of the plagioclase simultaneously decrease, which reflects the fractionation of the parental melt. Emphasis is placed onto the compositional variability of minerals in the layered unit. A method is developed for the reliable calculation of the composition of pyroxenes before their exoslution (separately for ortho-and clinopyroxene). The composition of minerals in the zone of intercalating melano-, meso-, and leucocratic norites remains practically unchanged, which is generally consistent with the results of mathematical simulations: compositional variations of the bronzite (#Mg) and plagioclase (An content) during the crystallization of the bronzite-plagioclase cotectic were no greater than 5 mol %. The variations in the concentrations of trace elements in the cumulus and intercumulus clinopyroxene occurred to be the most informative. It was determined that the Cr concentration in the cumulus augite is statistically significantly lower than that in the intercumulus augite, which is at variance with fractionation laws and led us to suggest that the rocks with cumulus pyroxenes were formed by a melt with a higher degree of fractionation than those of the intercumulus melts of the adjacent layers. 相似文献
10.
The lherzolites have recrystallized to plagioclase lherzolites consisting of olvine, pyroxenes, chromian spinel, plagioclase and pargasite at a depth of 20 to 25 km in the uppermost part of the mantle. It is believed that the garnet lherzolites and spinel lherzolites were originally derived from depths of 50–75 km and 30–50 km respectively. The clinopyroxenes contained about 10 mol. % of jadeite and Tschermak's molecules, respectively and the orthopyroxenes also included about 5–10% of Tschermak's component. Transported upward, the garnet was transformed through pyroxene-spinel symplectite to olivine, plagioclase and spinel aggregates, and most of the jadeite amd some Tschermak's components in the pyroxenes formed secondary pyroxenes and pargasite, and finally plagioclase under isochemical conditions. 相似文献
11.
《Chemie der Erde / Geochemistry》2015,75(3):273-285
More than 200 years after its discovery, asteroid (4) Vesta is thought to be the parent body for the howardite, eucrite and diogenite (HED) meteorites. The Dawn spacecraft spent ∼14 months in orbit around this largest, intact differentiated asteroid to study its internal structure, geology, mineralogy and chemistry. Carrying a suite of instruments that included two framing cameras, a visible-near infrared spectrometer, and a gamma-ray and neutron detector, coupled with radio tracking for gravity, Dawn revealed a geologically and geochemically complex world. A constrained core size of ∼110–130 km radius is consistent with predictions based on differentiation models for the HED meteorite parent body. Hubble Space Telescope observations had already shown that Vesta is scarred by a south polar basin comparable in diameter to that of the asteroid itself. Dawn showed that the south polar Rheasilvia basin dominates the asteroid, with a central uplift that rivals the large shield volcanoes of the Solar System in height. An older basin, Veneneia, partially underlies Rheasilvia. A series of graben-like equatorial and northern troughs were created during these massive impact events 1–2 Ga ago. These events also resurfaced much of the southern hemisphere and exposed deeper-seated diogenitic lithologies. Although the mineralogy and geochemistry vary across the surface for rock-forming elements and minerals, the range is small, suggesting that impact processes have efficiently homogenized the surface of Vesta at scales observed by the instruments on the Dawn spacecraft. The distribution of hydrogen is correlated with surface age, which likely results from the admixture of exogenic carbonaceous chondrites with Vesta's basaltic surface. Clasts of such material are observed within the surficial howardite meteorites in our collections. Dawn significantly strengthened the link between (4) Vesta and the HED meteorites, but the pervasive mixing, lack of a convincing and widespread detection of olivine, and poorly-constrained lateral and vertical extents of units leaves unanswered the central question of whether Vesta once had a magma ocean. Dawn is continuing its mission to the presumed ice-rich asteroid (1) Ceres. 相似文献
12.
《Chemie der Erde / Geochemistry》2019,79(4):125526
Quantifying the amounts of various igneous lithologies in Vesta’s crust allows the estimation of petrologic ratios that describe the asteroid’s global differentiation and subsequent magmatic history. The eucrite:diogenite (Euc:Diog) ratio measures the relative proportions of mafic and ultramafic components. The intrusive:extrusive (I:E) ratio assesses the effectiveness of magma ascent and eruption. We estimate these ratios by counting numbers and masses of eucrites, cumulate eucrites, and diogenites in the world’s meteorite collections, and by calculating their proportions as components of crustal polymict breccias (howardites) using chemical mixing diagrams and petrologic mapping of multiple thin sections. The latter two methods yield a Euc:Diog ratio of ∼2:1, although meteorite numbers and masses give slightly higher ratios. Surface lithologic maps compiled from spectra of Dawn spacecraft instruments (VIR and GRaND) yield Euc:Diog ratios that bracket estimates of Euc:Diog from the meteorites. The I:E ratios from HEDs lie between 0.5–2.1:1, due to uncertainties in identifying cumulate eucrite. Gravity mapping of Vesta by the Dawn spacecraft supports the existence of diogenite plutons in the crust. Quantifying the proportion of high-density diogenitic crust in the gravity map yields I:E ratios of 0.8-1:2:1, values which are bracketed by calculations based on HEDs. The I:E ratio for Vesta is lower than for Earth and Mars, consistent with physical modeling of asteroid-size bodies. Nevertheless, it indicates a significant role for pluton emplacement during the formation of Vesta’s crust. These results are inconsistent with simple differentiation models that produce the crust by crystallization of a global magma ocean, unless residual melts are extracted into crustal magma chambers. 相似文献
13.
Microlites (minute spherulitic, dendritic, skeletal, acicular and poikilitic crystals) diagnostic of crystallization in quenched melt or glass in fault rocks have been used to infer fossil earthquakes. High‐P microlites and crystallites are described here in a variably eclogitized gabbro, the wallrock to the coesite‐bearing eclogite breccia at Yangkou in the Chinese Su‐Lu high‐P metamorphic belt. The studied hand specimens are free of discernible shear deformation, although microfractures are not uncommon under the microscope. In the least eclogitized gabbro, the metagabbro, stellate growths of high‐P minerals on the relict igneous minerals are common. Dendritic garnet crystals (<1?5 μm) grew around rutile and/or phengite replacing ilmenite and biotite, respectively. Skeletal garnet also rims broken flakes of igneous biotite and mechanically twinned augite. Radial intergrowths of omphacite and quartz developed around relict igneous orthopyroxene and are rimmed by skeletal or poikilitic garnet where a Ti‐bearing mineral relict is present. Acicular epidote, kyanite and phengite crystallites are randomly distributed in a matrix of Na‐rich plagioclase, forming the pseudomorphs after igneous plagioclase. In the more eclogitized gabbro, the coronitic eclogite located closer to the eclogite breccia, all the igneous minerals broke down into high‐P assemblages. Thick coronas of poikilitic garnet grew between the pseudomorphs after igneous plagioclase and ferromagnesian minerals. The igneous plagioclase is replaced by omphacite crystallites, with minor amounts of phengite and kyanite. Thermodynamic modelling of the plagioclase pseudomorphs shows an increase in P–T in the wallrock from the metagabbro to the coronitic eclogite, and the P–T variation is unrelated to H2O content. The fluid‐poor pressure overstepping scenario is unsupported both by phase diagram modelling and by whole‐rock chemical data, which show that the various types of eclogitized gabbro are all fairly dry. A large pressure difference of >2 GPa between the metagabbro and the coesite‐bearing eclogites ~20 m apart cannot be explained by the subduction hypothesis because this would require a depth difference of >60 km. The microlites and crystallites are evidence for dynamic crystallization due to rapid cooling because constitutional supercooling was unlikely for the plagioclase pseudomorphs. The lack of annealing of the broken biotite and augite overgrown by strain free skeletal garnet is consistent with a transient high‐P–T event at a low ambient temperature (<300 °C), probably in the crust. Therefore, the eclogitization of the wallrock to the eclogite breccia was also coseismic, as proposed earlier for the eclogite facies fault rocks. The outcrop‐scale P–T variation and the transient nature of the high‐P–T event are inconsistent with the other existing tectonic models for high‐P metamorphism. The fact that the less refractory but denser biotite is largely preserved while the more refractory but less dense plagioclase broke down completely into high‐P microlite assemblages in the metagabbro indicates a significant rise in pressure rather than temperature. Given that the metamorphic temperatures are far below the melting temperatures of most of the gabbroic minerals under fluid‐absent conditions, stress‐induced amorphization appears to be the more likely mechanism of the coseismic high‐P metamorphism. 相似文献
14.
Arch M. Reid J.L. Warner W.I. Ridley R.W. Brown 《Geochimica et cosmochimica acta》1973,37(4):1011-1030
Glass compositions in the Luna 20 soil indicate a minor contribution of mare rocks and a major contribution of highly feldspathic highland material. Glasses with the composition of Highland basalt (anorthositic gabbro or norite) predominate in a range of highly aluminous glasses.The analyses of minerals in the soil show that the highland rocks have a unique assemblage of minerals that can readily be distinguished from the mineral assemblages of either mare or KREEP basalts. The soils are characterized by abundant anorthitic (An92–99), low-Fe plagioclase. Highly magnesian orthopyroxenes, pigeonites and augites are the most prominent pyroxenes. Unlike mare basalt pyroxenes, clinopyroxenes with intermediate Ca values are not abundant, but extreme iron enrichment towards pyroxferroite does occur. Olivines are more abundant than at other sites and are Mg-rich, low in Ca and Cr. Spinels with compositions approaching MgAl2O4 predominate over pleonastes and chromites. Ilmenite and metal are present but not abundant.These data establish the unique nature of the minerals in the highland soils. The mineral compositions are consistent with derivation from a suite of highly feldspathic rocks in which Highland basalt compositions predominate. Some of the mineral data, particularly from the pyroxenes, are suggestive of surface or near-surface processes, rather than plutonic crystallization. 相似文献
15.
Metagranodiorite samples from the Brossasco‐Isasca Unit, Dora‐Maira Massif, western Alps, show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra‐high pressure (UHP) metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K‐feldspar, biotite and minor phases. During UHP metamorphism, the plagioclase (site P) was replaced by zoisite, jadeite, quartz, K‐feldspar and kyanite, and coronitic reactions developed between biotite and adjacent minerals. At the original igneous biotite–quartz contact (site A), a single corona of poorly zoned garnet is developed, whereas at the biotite–K‐feldspar (site B) and biotite–plagioclase (site C) contacts, composite coronas are formed. Integration of results from petrographic observations, calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions, which reached as high as 24 kbar and 650 °C. Accurate microanalysis by energy‐dispersive spectroscopy (EDS) and statistical analysis of the data allowed us to identify, for the first time in a natural Na‐pyroxene of metagranitoid rocks, the end‐member Ca‐Eskola. 相似文献
16.
Cosmic ray exposure age and time of formation of a new meteorite,Dhofar 007 achondrite from Oman: A rock fragment from the asteroid Vesta 总被引：1，自引：0，他引：1
The isotopic composition of noble gases was investigated in the Dhofar 007 meteorite. Petrographic and mineralogical observations suggested that it is a brecciated cumulate eucrite with high contents of siderophile elements. The concentrations of noble gases in Dhofar 007 are identical to those of other eucrites. Its cosmic ray exposure age was estimated as 11.8 ± 0.8 Ma, which coincides with a maximum on the histogram of comic ray exposure ages of eucrite meteorites. It can be supposed that, similar to other eucrites, Dhofar 007 was ejected from the surface of their parent body (presumably, asteroid Vesta) about 12.0 Ma ago. The crystallization age of the Dhofar 007 eucrite was estimated from the ratio of plutonogenic Xe to Nd as 4476 ± 22 Ma. The potassium-argon age is much younger, 3.7–4.1 Ga, which indicates partial loss of radiogenic argon during the history of the meteorite, most likely related to impact metamorphic events. 相似文献
17.
Chemical and physical weathering in a hot‐arid,tectonically active alluvial system of Anza Borrego Desert,California 
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下载免费PDF全文 Investigation of chemical and physical weathering of bedrock and alluvial sediment in the Anza Borrego Desert, California, sheds light on weathering processes in hot‐arid systems and clarifies interpretations of climate from alluvial sediment. All of the alluvial sediment in the study area emanates from Cretaceous tonalite of the Peninsular Range, enabling exploration of the effects of external variables – climate, transport distance and tectonics – on the physical and chemical properties of the sediment. Chemical weathering in this area is dominated by plagioclase alteration observed in both bedrock outcrops and sediment, evinced most clearly by changes in the Eu anomaly. Biotite chemical weathering, manifested by interlayer K+ loss, is not evident in bedrock, but clearly observed in the sediment. Despite the weak intensity of chemical weathering (Chemical Index of Alteration = 56 to 62), fine‐grained (<63 μm) sediment displays a clear weathering trend in A–CN–K space and contains up to 25% clay minerals. Physical abrasion and grain‐size reduction in biotite during transport predominates in the sediment, whereas physical (insolation) weathering affecting bedrock is inferred from estimates of differential thermal expansion of mineral phases in response to extreme temperature changes in the study area. Chemical alteration and Brunauer–Emmett–Teller surface area both increase within the active Elsinore fault zone at the distal end of the depositional transect, reflecting tectonic‐induced fracturing and associated accelerated weathering. Extensive fracturing, together with a more humid Pleistocene climate, probably facilitated in situ bedrock weathering, preceding arid alluvial deposition in the Holocene. This study demonstrates that both climate and tectonic processes can affect chemical and physical weathering, resulting in alteration of plagioclase, leaching of K+ from biotite in the sediment and formation of clay minerals, even in hot, arid systems. 相似文献
18.
The magma forming the 20 m thick crinanitic/picrodoleritic Dun Raisburgh sill, part of the Little Minch Sill Complex of NW Scotland, comprised a mafic carrier liquid with a crystal cargo of plagioclase and olivine (1 vol%). The olivine component of the cargo settled on the floor of the intrusion while the more buoyant plagioclase component remained suspended during solidification, resulting in a relatively high plagioclase content in the centre of the sill. The settled olivine grains form a lower fining-upwards sequence overlain by a poorly sorted accumulation formed of grains that grew within the convecting magma. The accumulation of olivine on the sill floor occurred over 5–10 weeks, synchronous with the upwards-propagation of a solidification front comprising a porous (~?70 vol% interstitial liquid) plagioclase-rich crystal mush. 相似文献
19.
Two sections subsequently sawed from meteorite fragment Kaidun #d(4–5)C were examined under an optical and scanning electron
microscope. The major minerals of the fragment are pigeonite and anorthite; the accessory minerals are spinels and occasional
grains of ilmenite, Ca phosphate, and Fe sulfide. The compositions of the minerals broadly vary. The texture of the fragment,
its mineralogy, and mineral chemistries definitely indicate that the fragment originated from a crystalline rock affected
by partial melting during an impact event. The chemical compositions of both minerals (the concentration of the anorthite
end member of plagioclase An99 and the Fe/Mn atomic ratio of 44 in the pyroxenes) differ from those in other known meteorite samples and suggest that fragment
d(4–5)C of the Kaidun meteorite represents a new differentiated parent body that has never been sampled before. 相似文献
20.
Experimental modelling of corona textures 总被引：1，自引：0，他引：1
Formation of corona textures along olivine–plagioclase and orthopyroxene–plagioclase interfaces has been experimentally reproduced at 670 and 700 °C and 5 kbar with either a pure H2O fluid phase or 0.1 and 37 m NaCl–H2O solution fluid. In these experiments, we investigate the interaction of primary olivine and/or orthopyroxene and plagioclase in powders and polished crystals, and in small samples of a natural gabbro. The experiments result in the formation of corona textures with several layers of different assemblages (according to the experimental conditions) consisting of garnet (grossular), clinopyroxene, orthopyroxene, amphibole, chlorite and phlogopite. The experiments show major differences in the number of layers, the mineral assemblages and mineral composition, and in the trends of composition of plagioclase in coronas around olivine and orthopyroxene. The fluid phase composition influences the corona assemblages and the composition of the minerals in the experimental coronas; for example, garnet appears in the coronas in the second experiment where the NaCl–H2O ratio is low. Experimental modelling of corona textures confirms a model of simultaneous growth of layers by the mechanism of diffusion metasomatism with participation of a fluid phase through which mass is transferred. Zoning in the experimental coronas shows opposing diffusion of Al and Ca from plagioclase and Mg and Fe from olivine/orthopyroxene; difference in the mobility of the components is inferred from observations in the coronas. The experimental corona textures are compared with natural coronas from the Belomorian belt (Baltic shield), developed at 670–690 °C and 7–8 kbar, and the Marun‐Keu complex (Polar Urals), developed at 670–700 °C and 14–16 kbar, where the corona textures correspond to a transitional stage of the gabbro‐to‐eclogite transformation. 相似文献