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1.
Microprobe studies of unzoned plagioclases (An92–96) from crystal tuff of Mutnovskii Volcano and allivalite nodules of rocks from the Ksudach, Malyi Semyachik, and Golovnina Volcanoes revealed small inclusions of a dark-colored mineral that was later identified as spinel. Microprobe analyses showed that the grains are unzoned and spinel inclusions of different chemical compositions may occur in one plagioclase crystal. The spinel compositions form a clear extended single trend corresponding to the solvus zone of a solid solution that has not been described in the literature. The existence of this spinel trend in the solvus zone might have been due to early capturing of spinel grains by growing plagioclase crystals and their rapid cooling soon after eruption, resulting in hardening of the metastable solution. These spinels are supposed to form synchronously with plagioclase crystallization. The diversity of spinel compositions is explained by thermo diffusive equalizing of originally zonal spinel crystals after they were captured by plagioclase crystals or by their growth in crystallization haloes of anorthite.  相似文献   

2.
We present analyses of spheres magnetically extracted from mid-Pacific abyssal clays 0–500,000 years old. The concentration of spheres >200 μm is a few times 10 ppb. The spheres were divided into three groups using their dominant mineralogy, and are named iron, glassy, and silicate. Most spheres were formed from particles that completely melted as they separated from their parent meteoroids during the ablation process. However, some of the silicate spheres contain relict grains of the parent meteoroids that did not experience any melting. Typically, these relict grains are olivine crystals whose cores are Mg-rich (Fo89–99). Commonly the outer rims of these grains were altered during heating. Other relict mineral grains include enstatite, ferrous spinel, chromite, and pentlandite.The three groups of spheres may possibly indicate some genetic significance. It seems reasonable to expect iron-rich spheres to be produced during ablation of iron and metal-rich silicate meteoroids. Metal spheres are probably not produced by ablation of predominantly silicate meteoroids because studies of fusion crusts and laboratory ablated silicate materials have never yielded separate metal spheres, but rather have produced spheres with intergrown iron oxide and silicate phases. The iron spheres possess identical mineralogy with the fusion crusts of Boguslavka, Norfork, and N'Kandhla iron meteorites as well as with the ablation debris created in the laboratory using iron and nickel-iron samples.The glassy spheres are considerably more Fe-rich than the silicate spheres. They consist of magnetite and a Fe-rich glass which is relatively low in Si. Some of these spheres may have experienced pronounced volatile depletion during the ablation process and could have been derived from silicate or metal-rich silicate meteoroids.The silicate spheres are undoubtedly derived from ablation of stony meteoroids. Two of the mineral assemblages occurring in these spheres (olivine-magnetite-glass and sulfide) are identical to those described in the natural fusion crusts of Allende, Orgueil, and Murchison meteorites, laboratory-made ablation debris, and melted interplanetary dust collected from the stratosphere. Bulk compositions and relict grains are useful for determining the parent meteoroid types for the silicate spheres. Bulk analyses of spheres have non-volatile elemental abundances similar to chondritic abundances. Analyses of relict grains identified high-temperature minerals which often occur as larger crystals in a fine-grained matrix that is characterized by voids. These voids were caused by escaping volatiles as minerals decomposed during ablation. Because larger crystals of higher-temperature minerals are associated with fine-grained, low-temperature, volatile-rich matrix, the obvious candidates for parent meteoroids of the silicate spheres containing relict grains are carbonaceous chondrites.  相似文献   

3.
Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high‐grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote‐amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle‐derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted.  相似文献   

4.
Peridotite inclusions, crystal fragments, and kimberlite breccia at Green Knobs, New Mexico, have been studied to evaluate compositions and processes in the upper mantle below the Colorado Plateau. Most peridotite inclusions are spinel lherzolites and harzburgites, or their partly hydrated equivalents, in the Cr-diopside group. Orthopyroxene-rich websterites and olivine websterites comprise 3% of the peridotites and formed as cumulates. Typical anhydrous or slightly hydrated peridotites contain aluminous, calcic diopside (5–7% Al2O3), aluminous orthopyroxene (3–6% Al2O3), spinel, and olivine (near Fa9). Geothermometers based on different mineral pairs yield temperatures from above 1100°C to below 700°C in single rocks. High values, derived from pyroxenes with included exsolution lamellae, may approximate temperatures of primary crystallization. Low values, based on olivine-spinel and olivine-clinopyroxene pairs, approach upper mantle temperatures before eruption. In rare samples, some spinel grains are rimmed by garnet while others are not rimmed; garnet formation was controlled by nucleation kinetics. About one-third of the peridotites were deformed shortly before eruption, with effects ranging from mild cataclasis to the production of ultramylonites.Discrete crystals of garnet, olivine (near Fa8), and Cr-diopside represent garnet peridotite. Eclogites were not found. The garnet peridotite is more depleted than overlying spinel peridotite, and it is not a likely source for the minettes associated with the kimberlites.The mantle below Green Knobs consists of spinel peridotite from 45 to perhaps 60 km depth immediately underlain by more-depleted garnet peridotite. The position of the spinel-garnet transition may be fixed by kinetics. The kimberlite may have been produced when heat from ascending minette magma released volatiles from otherwise depleted garnet peridotite. Resulting gas-solid mixtures erupted along zones of deformation associated with Colorado Plateau monoclines. Sheared lherzolites formed during renewed movement along these zones.  相似文献   

5.
Abundant fluid inclusions in olivine of dunite xenoliths (～1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO2 (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions.Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO2 liquid and gas, with an aggregate ? = ～ 0.5–0.75 g cm?3, and represent trapped globules of dense supercritical CO2 (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO2 blebs. Spherical sulfide blebs having widely variable volume ratios to CO2 and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present.Assuming olivine growth at ～ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO2P-V-T data indicates that the primary inclusions were trapped at ～ 220–470 MPa (2200–4700 bars), or ～ 8–17 km depth in basalt magma of ? = 2.7 g cm?3. Because the temperature cannot change much during the rise to eruption, the range of CO2 densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.  相似文献   

6.
A single garnet clinopyroxenite xenolith found at the Dish Hill basanite cone near Ludlow, California, has well developed unmixing and reaction textures like those found in garnet pyroxenite inclusions in Hawaiian, African and Australian basalts and like those of pyroxenites in some European alpine peridotites. Reconstructed pyroxene compositions suggest that before unmixing the rock consisted of clinopyroxene and about 10% garnet plus spinel, but all of the garnet may have been dissolved in clinopyroxene. Most or all of the garnet formed by exsolution from clinopyroxene and by reaction between clinopyroxene and spinel in an open system. Following exsolution, the rock was deformed and partly recrystallized in the solid state. Similarity of compositions of exsolved and recrystallized minerals suggests recrystallization at P-T conditions similar to those of exsolution.The rock is not the chemical equivalent of the host basanite and cannot represent magma of basanitic composition crystallized in the mantle. Its history of deformation and recrystallization, like that of accompanying spinel lherzolite inclusions, supports the idea that the garnet clinopyroxenite is an accidental inclusion derived from the upper mantle.  相似文献   

7.
The data on the geochemistry of the rocks of Kizimen Volcano and results of microprobe studies of major and trace elements in plagioclase grains from acid lavas and basalt inclusions are presented. The characteristics of the Kizimen Volcano are the following: (1) basalt inclusions are abundant in acid lavas; (2) banded, mixed lavas occur; (3) the distribution curves of rare-earth elements of acidic lavas and basalt inclusions intersect; (4) Sr-Nd isotope systematics of the rocks and inclusions do not indicate mixture with crustal material; (5) plagioclase phenocrysts are of direct and reverse zonation; (6) olivine and hornblende, as well as acid and mafic plagioclases, coexist in the rocks. The studies revealed that the rocks are of a hybrid nature and originated in the course of repeated mixture of acid and mafic melts either with chemical and thermal interaction of melts or exclusively thermal ones. Study of the major- and trace-element distribution in zonal minerals provides an informative tool for understanding the history of the generation and evolution of melts in a magma chamber.  相似文献   

8.
The mineralogical and elemental variations across the main shear zone of the Saltville thrust at Sharp Gap in Knoxville, Tennessee, U.S.A., were studied in a suite of deformed and undefromed carbonate rock samples using X-ray diffraction and electron microprobe methods. An examination of the samples for deformation effects at mesoscopic scale and under the optical microscope reveals familiar cataclastic deformation features including foliated cataclasites and microbreccias occurring in a well-defined, 1–2 m wide zone of intense deformation, plus evidence of hydrofracturing and extensive syndeformational pressure solution. There exists a clear correlation between the observed cataclastic deformation and mineral and elemental distribution which we interpret to result from a deformation-induced dolomite to calcite transformation in the shear zone. The transformation has resulted in removal of Mg from the shear zone, selective deposition of calcite as an intergranular cement in cataclasite/microbreccia units and a relative increase in the concentration of detrital quartz and feldspars.The compositional difference between the shear zone and wall rocks is explained in connection with cataclastic deformation features in terms of a model in which a dual pressure-solution/cataclastic flow mechanism leads to a gradual cementation-hardening of segments of the shear zone. Instabilities could occur via permeability reduction and increased pore pressure within these segments. Hydrofracturing of the hardened segments along with high strain rate reordering of the shear zone materials reset the ruptured zone back to the dual deformation mechanism regime. As a long-term effect, the compositional transformation of the shear zone is expected to prolong periods of creep and cause smaller coseismic stress drops since under the imposed conditions calcite is more ductile and soluble than dolomite.  相似文献   

9.
Minerals that occur as incrustations in cooling lavas are formed by fractional condensation of volatile halides. Sulfates are of secondary origin formed during oxidation of the degassing volatiles. The trace metal compositions of the incrustations are controlled by the F/Cl ratio of the transporting gas phase rather than by element abundances in the silicate melt. The F/Cl ratio of the gas phase, inferred from incrustation mineral equilibria, indicates regional differences in the halogen contents of Icelandic magmas. The rift zone volcanism (tholeiites) is distinguished by low halogen content and very low F/Cl ratios, while the off-rift volcanism (alkaline rocks) is associated with high halogen content and high F/Cl ratios.  相似文献   

10.
Melt inclusions from four individual lava samples representing the HIMU (Mangaia Island), EMI (Pitcairn Island) and EMII (Tahaa Island) end member components, have heterogeneous Pb isotopic composition larger than that defined by the erupted lavas in each island. The broad linear trend in 207Pb/206Pb–208Pb/206Pb space produced by the melt inclusions from Mangaia, Tahaa and fPitcairn samples reproduces the entire trend defined by the Austral chain, the Society islands and the Pitcairn island and seamount groups. The inclusions preserve a record of melt composition of far greater isotopic diversity than that sampled in whole rock basalts. These results can be explained by mixing of a common depleted component with the HIMU, EMI and EMII lavas, respectively. We favor a model that considers the oceanic lithosphere to be that common component. We suggest that the Pb isotopic compositions of the melt inclusions reflect wall rock reaction of HIMU, EMI and EMII melts during their percolation through the oceanic lithosphere. Under these conditions, the localized rapid crystallization of olivine from primitive basalt near the reaction zone would allow the entrapment of melt inclusions with different isotopic composition.  相似文献   

11.
Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An80-An94), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide.  相似文献   

12.
Co-existing fluid and silicate inclusions in mantle diamond   总被引：1，自引：0，他引：1  
We document the compositions of co-existing silicate macro-inclusions and fluid micro-inclusions in the fibrous coats of eight coated diamonds from the Panda kimberlite (Canada). The mineral inclusions in the diamond coats come from either the peridotite suite (Cr-pyrope, orthopyroxene, olivine and Cr-diopside) or the eclogite suite (omphacite). Therefore, fibrous diamonds grow in the same paragenetic environments as octahedral diamonds. The inclusions document a more fertile source composition (lower Mg# and higher CaO) than for equivalent phases in octahedral diamonds from Panda and worldwide. However, moderate to high Cr2O3 contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process. Geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 °C. The co-existing fluid micro-inclusions are dominated by H2O, carbonate and KCl. Fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between Fe–Ca–Mg carbonate and KCl. Inclusions in the one eclogitic sample also contain quartz. We suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid, and so provide a snap shot of a metasomatic event in the mantle.  相似文献   

13.
Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?17O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?17O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?17O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies.  相似文献   

14.
Lamellar inclusions of chromian spinel (Cr/Cr + Al> 0.7), clinopyroxene and chromian spinel-clinopyroxene symplectite occur in olivines from alpine-type peridotites which have equilibrated at relatively low temperature (<700°C). They occur most commonly in dunite with very magnesian olivine (Fo93 to Fo95) and discrete grains of Cr-rich spinel. Olivine which initially equilibrated with magnesian and Cr-rich liquid had contained small amounts of calcium and trivalent chromium in the octahedral site, and lamellar chromian spinel and diopsidic clinopyroxene exsolved during the annealing process. The ordinary depletion of chromium or absence of chromian spinel lamellae in olivines in igneous rocks may be partly due to the effective exclusion of chromium from olivine upon cooling.  相似文献   

15.
Magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy, electron diffraction and electron microprobe analysis. Observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although “chains” are also seen. Electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure. Electron microprobe analysis shows the magnetite to be slightly titaniferous. Electron microscopy indicates that the grains, rather than being flake shaped, are parallelepiped crystals with a mean length of99.3 ± 8.7nm, a mean width of62.3 ± 6.1nm yielding a width-to-length ratio of 0.63. These data support the contention that the magnetic bacterial grains are single-domain crystals capable of producing a natural remanent magnetization in sediments.  相似文献   

16.
Ultramafic inclusions from San Carlos, Arizona, are classified into two groups. Group I inclusions are dominated by magnesian (Mg/Mg + ΣFe= 0.86 – 0.91), olivine-rich peridotites containing Cr-rich clinopyroxene and spinel. The less abundant Group I pyroxenites (containing Mg- and Cr-rich pyroxenes) occur as discrete inclusions and as portions of composite inclusions where they have a sharp, planar interface with lherzolite. Group II inclusions are dominated by clinopyroxene-rich peridotites containing Al- and Ti-rich augite and commonly abundant, Al-rich spinel. Compared to Group I inclusions, they are more Fe-rich (Mg/Mg + ΣFe= 0.62 – 0.78) and more hetereogeneous in composition and modal proportions. Similar groups occur at many ultramafic inclusion localities.Our petrographic and geochemical results lead to the following conclusions. Olivine-rich Group I inclusions are not genetically related to the host basanite, and they are formed from two components. Component A is a partial melting residue; it comprises the major portion of these inclusions and determines the modal mineralogy and major and compatible trace element composition. Component B results from a small degree (<5%) of garnet peridotite melting (probably, within the low-velocity zone). This highly LIL-element-enriched melt has migrated upwards into the overlying component A where it crystallized primarily as clinopyroxene and amphibole, and thus, introduced LIL elements into the residual component A. Subsequent cooling and subsolidus recrystallization have removed textural evidence of this mixing. This model has also been proposed for olivine-rich Group I inclusions from Victoria, Australia. At Victoria and San Carlos some relatively clinopyroxene-rich Group I lherzolites are not contaminated by component B, and they represent the best estimates of upper mantle composition prior to melting. Group I orthopyroxenites may be fragments of tectonic layers formed in lherzolite, but they could also be early cumulates (now metamorphosed) from the melt in equilibrium with component A. Group I clinopyroxenites have geochemical features of clinopyroxene in equilibrium with a magma. Thus, they could also represent early cumulates (now metamorphosed) from a magma unrelated to the host basanite. Alternatively, their geochemical characteristics could result from more complex models such as residues from partial remelting of pyroxenite dikes and veins or intradike segregation processes such as filter pressing. All Group II inclusions studied appear to be cumulates derived from a SiO2-undersaturated magma, possibly an early magma in the same volcanic episode which culminated with eruption of the host basanite. The poikilitic texture of amphibole-rich (kaersutite) inclusions is consistent with a cumulate origin. The bulk compositions of Group II inclusions are not equivalent to typical basaltic compositions.  相似文献   

17.
Olivine crystals in mare basalts 12004,8 and 12022,12 are normally zoned with Cr-poor rims. The Ni content of rare 2–10-μm metal inclusions in olivine decreases markedly as Fe/Mg in their immediate olivine hosts increases. Each metal grain appears to have been enclosed by late olivine almost immediately after it crystallized. The fractionation trend for the olivine and metal contrasts with the subsolidus equilibration trend for pallasites. For the basalts, not even local equilibration of Fe, Ni and Co at metal/olivine interfaces can be detected by microprobe. Ni and Co concentrations range from about 300 ppm in olivine cores to about 70 ppm in rims. The limits of detection, at 95% confidence, are 36 ppm (Ni) and 25 ppm (Co). The distribution of Ni and Co in olivine, like that of Mg and Cr, records the depletion of these elements in the melt.Fractional solidification models, using the Ni and Co concentrations of the whole rock, and Ni and Co concentrations of the earliest formed olivine, metal and “opaques” as initial compositions, allow metal and olivine compositions to be predicted if the order of crystallization is known. Conversely the order of crystallization can be established if known olivine and metal compositions are reproduced. Calculated Ni and Co contents for metal and olivine in these basalts correspond to observed concentrations only where metal precipitation is delayed until the liquid has crystallized 4–5 wt.% olivine.  相似文献   

18.
The double-torsion test using Aji granite was carried out to investigate the interaction between stress-induced crack path and mineral grains. Crack velocities were controlled at range 10–7 m/s to 10–1 m/s. After the stressed specimens were dyed, we checked the crack path by thin section analysis, using an optical microscope. The stress-induced crack path was divided into two types, transgranular and intergranular cracks, and each path was subdivided with respect to mineral grains. In spite of the extensive range of crack velocities, the ratios between the transgranular and intergranular crack lengths did not change. The crack paths were all jagged, and often showed detour around the grain boundary when faced with obstacles like hard grains or preexisting cracks. That is to say, quartz grain played an important role as an obstacle. Feldspar grain could change the crack path because of its cleavage plane. Biolite grain had a serious effect on the path even if its constitution ratio is very small. Fractal dimensions of the crack paths were calculated by three methods, as indicators of surface roughness. The fractal dimensions were shown in a slight trend with the change of crack velocity. This trend can be explained from the point of limited cracking rate in stress corrosion.  相似文献   

19.
Mechanical weakening due to solid state transformation of mineral phases has long been proposed to be a significant mechanism for localization of deformation in the Earth’s lithosphere and the mantle transition zone. However, experimental observations confirming such a proposition are lacking. Here we present a novel approach to prove the existence of a minimum in the creep strength of quartz at the α–β transition by observing the deformation of fluid inclusions in a quartz crystal using a hydrothermal diamond-anvil cell. Pressure differences required for permanent deformation of the quartz around fluid inclusions were significantly lower at the phase transition than in either the stability fields of α- or β-quartz. These results indicate that transformation plasticity of silicates can indeed cause a considerable localized reduction in the strength of the Earth’s crust and mantle.  相似文献   

20.
对采自藏北无人区西藏金沙江缝合带的基性、超基性岩样品进行了岩石学、岩石化学、稀土和微量元素、同位素等分析测试 ,以及主要矿物成分的电子探针分析。研究结果表明 ,这些样品为辉长岩和辉石岩 ,属于钙碱性至碱性系列 ,其矿物组成没有斜方辉石 ,未见变形显微构造特征。其化学成分以高Al2 O3,FeO ,Na2 O +K2 O和低SiO2 ,MgO含量为特征。稀土元素配分型式以斜率较大的微弱负铕异常的右倾曲线为特征 ,LREE富集。它们具有较高的Sr丰度和87Sr/ 86 Sr比值 (>0 70 6 9)以及高的2 0 7Pb/ 2 0 4 Pb ,2 0 8Pb/ 2 0 4 Pb和2 0 6 Pb/ 2 0 4 Pb比值。上述特征有别于藏北蛇绿岩的基性、超基性岩 ,表明它们不是蛇绿岩套组合 ,其形成可能与金沙江缝合带的弧后扩张有关  相似文献   

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