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1.
Chromite in ordinary chondrites (OC) can be used as a shock indicator. A survey of 76 equilibrated H, L and LL chondrites shows that unshocked chromite grains occur in equant, subhedral and rounded morphologies surrounded by silicate or intergrown with metallic Fe-Ni and/or troilite. Some unmelted chromite grains are fractured or crushed during whole-rock brecciation. Others are transected by opaque veins; the veins form when impacts cause localized heating of metal-troilite intergrowths above the Fe-FeS eutectic (988°C), mobilization of metal-troilite melts, and penetration of the melt into fractures in chromite grains. Chromite-plagioclase assemblages occur in nearly every shock-stage S3-S6 OC; the assemblages range in size from 20-300 μm and consist of 0.2-20-μm-size euhedral, subhedral, anhedral and rounded chromite grains surrounded by plagioclase or glass of plagioclase composition. Plagioclase has a low impedance to shock compression. Heat from shock-melted plagioclase caused adjacent chromite grains to melt; chromite grains crystallized from this melt. Those chromite grains in the assemblages that are completely surrounded by plagioclase are generally richer in Al2O3 than unmelted, matrix chromite grains in the same meteorite. Chromite veinlets (typically 0.5-2 μm thick and 10-300 μm long) occur typically in the vicinity of chromite-plagioclase assemblages. The veinlets formed from chromite-plagioclase melts that were injected into fractures in neighboring silicate grains; chromite crystallized in the fractures and the residual plagioclase-rich melt continued to flow, eventually pooling to form plagioclase-rich melt pockets. Chromite-rich “chondrules” (consisting mainly of olivine, plagioclase-normative mesostasis, and 5-15 vol.% chromite) occur in many shocked OC and OC regolith breccias but they are absent from primitive type-3 OC. They may have formed by impact melting chromite, plagioclase and adjacent mafic silicates during higher-energy shock events. The melt was jetted from the impact site and formed droplets due to surface tension. Crystallization of these droplets may have commenced in flight, prior to landing on the parent-body surface.Chromite-plagioclase assemblages and chromite veinlets occur in 25 out of 25 shock-stage S1 OC of petrologic type 5 and 6 that I examined. Although these rocks contain unstrained olivine with sharp optical extinction, most possess other shock indicators such as extensive silicate darkening, numerous occurrences of metallic Cu, polycrystalline troilite, and opaque veins. It seems likely that these rocks were shocked to levels at least as high as shock-stage S3 and then annealed by heat generated during the shock event. During annealing, the olivine crystal lattices healed but other shock indicators survived. Published Ar-Ar age data for some S1 OC indicate that many shock and annealing events occurred very early in the history of the parent asteroids. The common occurrence of shocked and annealed OC is consistent with collisions being a major mechanism responsible for metamorphosing OC.  相似文献   

2.
MIL 99301 is an LL chondrite that has experienced successive episodes of thermal metamorphism, shock metamorphism and annealing. The first recognizable petrogenetic episode resulted in thermal metamorphism of the rock to petrologic type 6 (as indicated by homogeneous olivine compositions, significant textural recrystallization, and the presence of coarse grains of plagioclase, metallic Fe-Ni and troilite). The source of heat for this thermal episode is not identified. The rock also experienced shock metamorphism to shock stage ∼S4 as indicated by extensive silicate darkening (caused by the dispersion within silicate grains of thin chromite melt veins and trails of metallic Fe-Ni and troilite blebs), polycrystalline troilite, myrmekitic plessite, a relatively high occurrence abundance (OA) of metallic Cu (3.6), the presence of numerous chromite-plagioclase assemblages, and coarse grains of low-Ca clinopyroxene with polysynthetic twinning. The shock event responsible for these effects must have occurred after the epoch of thermal metamorphism to type-6 levels; otherwise the polycrystallinity of the troilite would have disappeared and the low-Ca clinopyroxene would have transformed into orthopyroxene. Despite abundant evidence of strong shock, olivine and plagioclase in MIL 99301 exhibit sharp optical extinction, consistent with shock stage S1 and characteristic of an unshocked rock. This implies that an episode of post-shock annealing healed the damaged olivine and plagioclase crystal lattices and thereby changed undulose extinction into sharp extinction. The rock was probably annealed to metamorphic levels approximating petrologic type 4; more significant heating would have transformed the low-Ca clinopyroxene into orthopyroxene. It is not plausible that an episode of annealing occurring after the epoch of thermal metamorphism could have been caused by the decay of 26Al because this isotope would have decayed away by that time. Impact heating is a more plausible source of post-metamorphic annealing of rocks in the vicinity of impact craters on low-density, high-porosity asteroids with rubble-pile structures.  相似文献   

3.
Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.  相似文献   

4.
Enstatite-rich meteorites include EH and EL chondrites, rare ungrouped enstatite chondrites, aubrites, a few metal-rich meteorites (possibly derived from the mantle of the aubrite parent body), various impact-melt breccias and impact-melt rocks, and a few samples that may be partial-melt residues ultimately derived from enstatite chondrites. Members of these sets of rocks exhibit a wide range of impact features including mineral-lattice deformation, whole-rock brecciation, petrofabrics, opaque veins, rare high-pressure phases, silicate darkening, silicate-rich melt veins and melt pockets, shock-produced diamonds, euhedral enstatite grains, nucleation of enstatite on relict grains and chondrules, low MnO in enstatite, high Mn in troilite and oldhamite, grains of keilite, abundant silica, euhedral graphite, euhedral sinoite, F-rich amphibole and mica, and impact-melt globules and spherules. No single meteorite possesses all of these features, although many possess several. Impacts can also cause bulk REE fractionations due to melting and loss of oldhamite (CaS) – the main REE carrier in enstatite meteorites. The Shallowater aubrite can be modeled as an impact-melt rock derived from a large cratering event on a porous enstatite chondritic asteroid; it may have been shock melted at depth, slowly cooled and then excavated and quenched. Mount Egerton may share a broadly similar shock and thermal history; it could be from the same parent body as Shallowater. Many aubrites contain large pyroxene grains that exhibit weak mosaic extinction, consistent with shock-stage S4; in contrast, small olivine grains in some of these same aubrites have sharp or undulose extinction, consistent with shock stage S1 to S2. Because elemental diffusion is much faster in olivine than pyroxene, it seems likely that these aubrites experienced mild post-shock annealing, perhaps due to relatively shallow burial after an energetic impact event. There are correlations among EH and EL chondrites between petrologic type and the degree of shock, consistent with the hypothesis that collisional heating is mainly responsible for enstatite-chondrite thermal metamorphism. Nevertheless, the apparent shock stages of EL6 and EH6 chondrites tend to be lower than EL3-5 and EH3-5 chondrites, suggesting that the type-6 enstatite chondrites (many of which possess impact-produced features) were shocked and annealed. The relatively young Ar–Ar ages of enstatite chondrites record heating events that occurred long after any 26Al that may have been present initially had decayed away. Impacts remain the only plausible heat source at these late dates. Some enstatite meteorites accreted to other celestial bodies: Hadley Rille (EH) was partly melted when it struck the Moon; Galim (b), also an EH chondrite, was shocked and partly oxidized when it accreted to the LL parent asteroid. EH, EL and aubrite-like clasts also occur in the polymict breccias Kaidun (a carbonaceous chondrite) and Almahata Sitta (an anomalous ureilite). The EH and EL clasts in Kaidun appear unshocked; some clasts in Almahata Sitta may have been extensively shocked on their parent bodies prior to being incorporated into the Almahata Sitta host.  相似文献   

5.
2013年2月15日,俄罗斯车里雅宾斯克(Chelyabinsk)发生了伴随罕见的空中爆炸的大规模陨石雨事件。本文对3块代表不同冲击变质程度的车里雅宾斯克陨石碎块进行了研究。它们都具有部分熔壳,其中1块仅出现碎裂,1块含有冲击熔融细脉,1块基本由冲击熔融囊和冲击熔脉组成。冲击变质程度最低的样品,代表了该陨石母体小行星的原始岩石矿物学特征:即具有粗粒的岩石结构和均一的矿物化学组成,但仍保留一些残余球粒,表明受到了明显的热变质作用,其岩石类型可划分为5型。铁镁质硅酸盐高的Fe O含量(橄榄石Fa:27.9mol%~28.2mol%,辉石Fs值:23.3mol%~23.7mol%)、以及较低的Fe-Ni金属含量,表明其化学群属于低铁低金属的LL群。我们所分析的样品与前人报导的结果相似,未发现不同岩性的岩屑,表明车里雅宾斯克陨石的原始岩矿特征较为均一。3块陨石碎块中,随着冲击程度的增强,其冲击变质特征依次表现为硅酸盐矿物的破碎、熔长石化更为普遍、陨硫铁与铁镍合金共熔、硅酸盐熔脉的形成、铬铁矿与长石共熔、以及大量熔融囊的发育等。但是,在冲击熔融囊和熔脉中,以及相邻围岩中均未发现高压矿物相。熔脉中的橄榄石晶屑和相邻围岩的橄榄石颗粒表现为化学成分的不均一,在背散射电子图像中呈不同灰度的结构。这与其他强烈冲击变质陨石中橄榄石的林伍德石或瓦茨利石相变相似。该陨石中林伍德石或瓦茨利石的缺失很可能是由于强烈撞击后高温产生的退变质。这也表明车里雅宾斯克陨石的母体小行星可能遭受了非常强烈的撞击事件。  相似文献   

6.
The Dergaon fall represents a shock-melted H4-5(S5) ordinary chondrite which includes at least ten textural varieties of chondrules and belongs to the high chondrule-matrix ratio type.Our study reveals that the chondrules are of diverse mineralogy with variable olivine-pyroxene ratios(Type Ⅱ),igneous melt textures developed under variable cooling rates and formed through melt fractionations from two different melt reservoirs.Based on the experimental analogues,mineralogical associations and phase compositions,it is suggested that the Dergaon chondrules reflect two contrasting environments:a hot,dust-enriched and highly oxidized nebular environment through melting,without significant evaporation,and an arrested reducing environment concomitant with major evaporation loss of alkali and highly volatile trace elements.Coexistence of chlorapatite and merrillite suggests formation of the Dergaon matrix in an acidic accretionary environment.Textural integration and chemical homogenization occurred at ~ 1 atmospheric pressure and a mean temperature of 765 C mark the radiogenic thermal event.Equilibrated shock features(olivine mosaicism,diaplectic plagioclase,polycrystalline troilite) due to an impact-induced thermal event reflect a shock pressure 45 GPa and temperature of 600 C.By contrast,the local disequilibrium shock features(silicate melt veins comprising of olivine crystallites,troilite melt veins and metal droplets) correspond to a shock pressure up to 75 GPa and temperature950 ℃.  相似文献   

7.
The microtexture and mineralogy of a 580-μm-wide melt vein in the Tenham L6 chondrite were investigated using field-emission scanning electron microscopy and transmission electron microscopy to better understand the shock conditions. The melt vein consists of a matrix of silicate plus metal-sulfide grains that crystallized from immiscible melts, and sub-rounded fragments of the host chondrite that have been entrained in the melt and transformed to polycrystalline high-pressure silicates. The melt-vein matrix contains two distinct textures and mineral assemblages corresponding to the vein edge and interior. The 30-μm-wide vein edge consists of vitrified silicate perovskite + ringwoodite + akimotoite + majorite with minor metal-sulfide. The 520-μm-wide vein interior consists of majorite + magnesiowüstite with irregular metal-sulfide blebs. Although these mineral assemblages are distinctly different, the pressure stabilities of both assemblages are consistent with crystallization from similar pressure conditions: the melt-vein edge crystallized at about 23-25 GPa and the vein interior crystallized at about 21-25 GPa. This relatively narrow pressure range suggests that the melt vein either crystallized at a constant equilibrium shock pressure of ∼25 GPa or during a relatively slow pressure release. Using a finite element heat transfer program to model the thermal history of this melt vein during shock, we estimate that the time required to quench this 580-μm-wide vein was ∼40 ms. Because the entire vein contains high-pressure minerals that crystallized from the melt, the shock-pressure duration was at least 40 ms. Using a synthetic Hugoniot for Tenham and assuming that the sample experienced a peak-shock pressure of 25 GPa near the impact site, we estimate that the Tenham parent body experienced an impact with collision velocity ∼2 km/s. Based on a one-dimensional planar impact model, we estimate that the projectile size was >150 m in thickness.  相似文献   

8.
Smyer is an H-chondrite impact-melt breccia containing ∼20 vol% 0.5- to 13-mm-thick silicate-rich melt veins surrounding unmelted subrounded chondritic clasts up to 7 cm in maximum dimension. At the interface between some of the melt veins and chondritic clasts, there are troilite-rich regions consisting of unmelted, crushed 0.2- to 140-μm-size angular silicate grains and chondrule fragments surrounded by troilite and transected by thin troilite veins. Troilite fills every available fracture in the silicates, including some as thin as 0.1 μm. Little metallic Fe-Ni is present in these regions: the FeS/Fe modal ratio ranges from ∼25:1 to ∼500:1, far higher than the eutectic weight ratio of 7.5:1. The texture of these regions indicates that the sulfide formed from a fluid of very low viscosity. The moderately high viscosity (0.2 poise) and large surface tension of liquid FeS, its inability to wet silicate grain surfaces at low oxygen fugacities, and the supereutectic FeS/Fe ratios in the troilite-rich regions indicate that the fluid was a vapor. It seems likely that during the shock event that melted Smyer, many silicates adjacent to the melt veins were crushed. Upon release of shock pressure, some of the troilite evaporated and dissociated. Molecules of S2 were transported and condensed into fractures and around tiny silicate grains; there, they combined with Fe from small adjacent metallic Fe-Ni grains to form troilite. The Ni content at the edges of some of these metal grains increased significantly; Co from these Ni-rich grains diffused into nearby kamacite. Impact-induced S volatilization may have played a major role in depleting the surface of 433 Eros (and other chondritic asteroids) in S.  相似文献   

9.
GRA 95209 may provide our best opportunity to date to understand the earliest stages of core formation in asteroidal bodies. This lodranite preserves a physically, chemically, and mineralogically complex set of metal-sulfide veins. High-resolution X-ray computed tomography revealed three distinct lithologies. The dominant mixed metal-silicate-sulfide matrix is cut by metal-rich, graphite-bearing veins exceeding 1 cm in width and grades into a volumetrically minor metal-poor region. Silicate compositions and modal abundances are typical for lodranites, while the mineralogy of the metal-sulfide component is complex and differs among the three lithologies. Kamacite and troilite occur with chromite, tetrataenite, schreibersite, graphite, and a range of phosphates. An 39Ar-40Ar age of 4.521 ± 0.006 Ga measures the time of closure of the K-Ar system. Carbon rosettes within the metal-rich vein are nitrogen-poor, well crystallized, include kamacite sub-grains of composition comparable to the host metal, and are essentially isotopically homogeneous (δ13C ∼ −33‰). In contrast, carbon rosettes within metal of the metal-poor lithology are N-poor, poorly crystallized, include kamacite grains that are Ni-poor compared to their host metal, and are isotopically heterogeneous (δ13C ranging from −50 to +80‰) even within a single metal grain. The silicate portion of GRA 95209 is similar to the lodranite EET 84302, sharing a common texture, silicate mineral compositions, and Ar-Ar age. GRA 95209 and EET 84302 are intermediate between acapulcoites and lodranites. Both experienced Fe,Ni-FeS melting with extensive melt migration, but record only the onset of silicate partial melting with limited migration of silicate melt. The complex metal-sulfide veins in GRA 95209 resulted from low-degree partial melting and melt migration and intruded the matrix lithology. Reactions between solid minerals and melt, including oxidation-reduction reactions, produced the array of phosphates, schreibersite, and tetrataenite. Extensive reduction in the metal-rich vein resulted from its origin in a hotter portion of the asteroid. This difference in thermal history is supported by the graphite structures and isotopic compositions. The graphite rosettes in the metal-rich vein are consistent with high-temperature igneous processing. In contrast, the carbon in the metal-poor lithology appears to preserve a record of formation in the nebula prior to parent-body formation. Carbon incorporated from the solar nebula into a differentiating asteroid is preferentially incorporated in metal-sulfide melts that form a core, but does not achieve isotopic homogeneity until extensive thermal processing occurs.  相似文献   

10.
The Chassigny meteorite is a moderately shocked olivine achondrite or chassignite with features indicative of a cumulate origin with some subsolidus annealing. Chassigny is an iron-rich dunite (Fo68) with minor amounts of Ca-rich and Ca-poor pyroxene, alkalic feldspar, chromite, and melt inclusions in olivine. Accessory phases include chlorapatite, troilite, marcasite, kaersutite amphibole, pentlandite, ilmenite, rutile and baddeleyite. The meteorite experienced shock pressures of ~150–200 kbar as evidenced by planar and irregular fractures in olivine, local recrystallization in pyroxene and reduced birefringence and rare deformation lamallae in feldspar. Kaersutitic amphibole (K0.05 Na0.45)0.50 (Ca1.71 Na0.29)2.00 (Mg2.73 ‘Fe’1.19 Ti0.73 A10.23 Cr0.08 Mn0.03)4.99 (Si6.05Al1.95)8.00 O22 (OH, F)2 containing hydrogen and lesser amounts of fluorine represents the first extraterrestrial occurrence of hydrous amphibole and the first meteoritic amphibole type other than fluorichterite. Kaersutite is found only in melt inclusions.Melt inclusion bulk compositional data suggest crystallization from a low-Ca melt that may have been similar in major element abundances to the silicate portion of LL group chondrites. However, Chassigny has a fractionated pattern for REE and the lack of metallic iron, possible presence of minor Ni in the olivine and Fe3+ in the chromites indicates that Chassigny formed under relatively more oxidizing conditions than most other achondrites. Therefore its parental melt could not have been directly derived from a chondritic composition in a simple single-stage process. The iron-rich bulk composition, cumulate texture and abundance as well as alkalic nature of the interstitial feldspar indicate that Chassigny could not have generated eucritic magmas. This places further constraints on its relationship to other meteorites and the parent body from which it is derived. The Brachina meteorite is similar to Chassigny except that it is finer grained, more feldspathic and is unshocked. It extends the fractionation range of this group which now represents two unusual meteorites.  相似文献   

11.
The Kenna ureilite was found in February, 1972 near the town of Kenna, Roosevelt County, New Mexico U.S.A., weighed 10.9 kg, and measured 26.7 × 14.7 × 14.2 cm; it is the seventh known ureilite. The meteorite is composed of xenoblastic olivine (Fo79.2), commonly rimmed by forsterite (Fo99), and pigeonite (En73Wo9Fs18), in a volumetric ratio of 3:1, set in a matrix of three carbon polymorphs (graphite, lonsdaleite, and diamond) plus nickel-iron metal and troilite. Some thin metalliferous veins penetrating silicate grains contain secondary inclusions of melt with high-calcium clinopyroxene (high-Ca, Mg-rich augite to augite), andesine, K-feldspar, chromite, and siliceous CaO- and alkali-rich glasses of variable compositions.Textural, mineralogical and fabric information suggest a complex history for Kenna, involving igneous, metamorphic and shock processes. The rock appears to have originated as an ultramafic cumulate whose texture and structure was modified by adcumulus processes and by solution and redeposition in a weak deviatoric stress field. A strong mineral elongation lineation was produced during this high-temperature phase accompanied by mild plastic deformation of olivine on the system 0kl[100]. Superimposed on this original texture and fabric are processes resulting from light to moderate (50–250 kbar) shock deformation, as manifested by fracturing of the silicates, slip parallel to (001) in olivine, and twin and translation gliding parallel to (100) in the clinopyroxene. Lonsdaleite and diamond probably formed during this shock phase, which may be associated with the break-up of the parent body, but the relative time of introduction of the carbon-rich matrix is still unresolved.  相似文献   

12.
We have observed vesicles filled with heavy nitrogen gas and water vapor in three settings in the Bencubbin CB chondrite: in the mesostasis of the silicate clasts, in the mesostasis of the chondrules of an ordinary chondrite (OC) xenolith, and in grains we refer to as bubble grains, and interpret as remelted OC chondrule mesostasis. In our view, these bubbles are a local phenomenon and formed as a consequence of the impact of the OC fragment onto the Bencubbin parent body. The heavy nitrogen in the bubbles came from one or several of its carrier phases in Bencubbin, and the water came from hydrous silicates. As formulated by Meibom et al. (Meibom A., Righter K., Chabot N., Dehn G., Antignano A., McCoy T. J., Krot A. N., Zolensky M. E., Petaev M. I. and Keil K. (2005) Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: remains of the missing matrix? Meteorit. Planet. Sci.40, 1377-1391) these hydrous phases, similar to the hydrated clasts now found in CH and CBb chondrites, were probably common in the Bencubbin parent body at that time. They were later almost totally destroyed by a large scale shock event, and contributed to form the impact melt that now fills space in between the large clasts of Bencubbin. Our observations indirectly confirm this hypothesis. From our composition measurements, we infer that the silicate part of the impact melt was made in roughly equal proportions of melted phyllosilicates and melted anhydrous silicates. The oxygen isotopic composition of the impact melt is much heavier than that of the silicate clasts, probably reflecting the composition of the water at the origin of the phyllosilicates. The O isotope measurements of the OC inclusion chondrules present some features that seem to be common in OCs: a composition of the chondrule crystals slightly lighter than that of whole chondrules, and one olivine crystal with a very light composition.  相似文献   

13.
Samples of the unshocked, equilibrated chondrite, Kernouve (H6), have been annealed for 1–100 hours at 500–1200°C, their thermoluminescence sensitivity measured and Na, K, Mn, Ca and Sc determined by instrumental neutron activation analysis. The TL sensitivity decreased with temperature until by 1000°C it had fallen by 40%. The process responsible has an activation energy of ~8 kcal/mole and probably involves diffusion. Samples annealed 1000–1200°C had TL sensitivities 10?2 times the unannealed values, most of the decrease occurring ~1100°C. This process has an activation energy of ~100 kcal/mole and is probably related to the melting of the TL phosphor, feldspar, with some decomposition and loss of Cs, Na and K. Meteorites whose petrography indicates healing > 1100°C by natural shock heating events (shock facies d-f). have TL sensitivities similar to samples annealed > 1100°C. Our own and literature compositional data indicate that TL is more stable to annealing than Ag, In, Tl, Bi, Zn and Te and less stable than Na, K, Mn, Ca, Se and Co, while the TL decrease resembles very closely the pattern of Cs loss on annealing.  相似文献   

14.
The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: (1) Fe-enrichment along the cracks that cross cut the pyroxene crystals (“Fe-metasomatism”); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; (2) deposits of Fe-rich olivine (Fa64-86) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An97-98) is associated with the fayalitic olivine; (3) at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites.  相似文献   

15.
Seventeen shock-recovery experiments were performed on powder mixtures of one part (by weight) olivine (St. John's forsterite) plus two parts silica glass (pure vitreous silica) in order to characterize the physical and chemical interaction of two chemically incompatible components during shock. Powders of <45 m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6.2 to 64.2 GPa (1 GPa= 10kbar).Petrographie features observed in thin section attest to mechanical and thermal metamorphism. Samples shocked to pressures from 6.2 to 39.3 GPa form compacted, mosaic, granular aggregates with fractured and strained grains. Samples shocked to pressures from 42.9 to 64.2 GPa form vesicular, mixed melts containing flow schlieren and relict olivine fragments. Petrographic disequilibrium is manifested in cataclastic textures showing deformational anisotropy and in thermal effects showing non-uniform intergranular melting. This disequilibrium is caused by an irregular pressure distribution resulting from the rapid collapse of pore spaces.The chemical composition of the shock melts are similar in each of six samples shocked to pressures of 42.9 to 64.2 GPa. Melt chemistry is bimodal in each sample. Colorless melts are 99.9% SiO2 and represent pure silica glass melts; pale to dark green melts range in composition from 47% to 64% SiO2 and represent a progressive mixture of olivine melt (41% SiO2) with silica glass melt. Surprisingly, the compositions of the colored glasses are intermediate between the composition of pure olivine and the bulk composition of the original starting material (79% SiO2) and are similar to enstatitic pyroxene compositions (50% to 57% SiO2; 33% to 37% MgO). Although bulk compositions of shocked samples are unchanged, the creation of melts with pyroxene compositions instead of bulk sample compositions may indicate that an incipient eutectic-type fusion may have occurred in small olivine-normative domains surrounding individual olivine grains. Chemical disequilibrium is evidenced by the creation of these olivine-normative melts from a quartz-normative starting compositions and by the chemical heterogeneity in the melts.  相似文献   

16.
A set of troilite-silicate-metal (TSM) inclusions and chondrule rims in the Bishunpur (LL3.1) chondrite provide information regarding impact brecciation of small bodies in the early solar system. The TSM inclusions and chondrule rims consist of numerous angular to subrounded silicate grains that are individually enclosed by fine networks of troilite. FeNi metal also occurs in the troilite matrix. The silicates include olivine (Fo55-98), low-Ca pyroxene (En78-98), and high-Ca pyroxene (En48-68Wo11-32). Al- and Si-rich glass coexists with the silicates. Relatively coarse silicate grains are apparently fragments of chondrules typical of petrologic type-3 chondrites. Troilite fills all available cracks and pores in the silicate grains. Some of the TSM inclusions and rims are themselves surrounded by fine-grained silicate-rich rims (FGR).The TSM inclusions and rims texturally resemble the troilite-rich regions in the Smyer H-chondrite breccia. They probably formed by shock-induced mobilization of troilite during an impact event on a primitive asteroidal body. Because silicates in the TSM inclusions and rims have highly unequilibrated compositions, their precursor was presumably type-3 chondritic material like Bishunpur itself. The TSM inclusions and the chondrules with the TSM rims were fragmented and dispersed after the impact-induced compaction, then reaccreted onto the Bishunpur parent body. FGR probably formed around the TSM inclusions and rims, as well as around some chondrules, during the reaccumulation process. Components of most type-2 and 3 chondrites probably experienced similar processing, i.e., dispersal of unconsolidated materials and subsequent reaccumulation.  相似文献   

17.
The Suizhou meteorite is an L6 chondrite. This meteorite is consisted of olivine, low-Ca pyroxene, plagioclase, FeNi metal, troilite, whitlockite, chlorapatite, chromite and ilmenite. Olivine and pyroxene grains display shock-induced mosaic texture, and most plagioclase grains were melted and transformed to maskelynite. This meteorite contains a few very thin shock-produced melt veins ranging from 20 to 100 μm in width. They are chondritic in composition and contain abundant high-pressure minerals in two assemblages. One is the coarse-grained assemblage of ringwoodite, majorite, lingunite with minor amount of tuite, xieite, the CF-phase, akimotoite and amorphized perovskite, and the fine-grained assemblage (the melt vein matrix) composed of majorite-pyrope garnet, magnesiowüstite. FeNi metal and troilite in the Suizhou shock veins were molten and occur as small intergrowth grains or veinlets filling the interstices of garnet crystals or cracks in the vein matrix. It was revealed that olivine, pyroxene and plagioclase in the Suizhou shock veins have transformed in solid state to their high-pressure polymorphs ringwoodite, majorite, and lingunite, respectively, without change in their chemical compositions.  相似文献   

18.
王桂琴  缪秉魁  林杨挺 《岩石学报》2015,31(9):2724-2736
荷叶塘为一块我国降落的原始3型普通球粒陨石,因此具有重要研究意义。本文对荷叶塘陨石光薄片及粉末样品的岩石学、矿物学和全岩组成地球化学特征进行研究,为这块陨石的深入研究提供重要基础数据。研究表明荷叶塘陨石具L3型陨石岩石学特征,具典型的球粒陨石结构,球粒清晰,球粒结构类型多,基质重结晶程度低,组成模式为:球粒80vol%,金属和硫化物含量为5vol%,基质15vol%。矿物化学成分表明,该陨石球粒以Ⅰ型(贫铁型)球粒为主,橄榄石Fa0.41-34.1(PMD=51),低钙辉石Fs1.82-27.2(PMD=88),Wo0.18-3.13(PMD=103),铁纹石中Co含量平均0.62%(PMD=20),矿物成份不均一程度高,橄榄石矿物结晶颗粒内部化学成分变化大,呈正环带分布,与岩浆型结晶顺序一致,球粒与基质及间隙物成分明显不同,表现为不同物质来源。化学成分全岩分析结果显示,荷叶塘陨石亲石、亲铁元素含量均为L型陨石特征。依据以上岩石矿物学和化学组成特征,依照陨石亚分类参数,将其类型划分为L3.4型普通球粒陨石。冲击变质程度S2,风化程度W1。研究结果表明荷叶塘陨石为一块受后期水、热蚀变和风化影响较少的原始类型陨石。组成矿物成分极不均一,在矿物晶体内部,球粒内部及球粒与基质间均有明显变化。  相似文献   

19.
文章主要通过电子探针、扫描电镜、激光拉曼光谱、透射电镜等微区微分析技术研究GRV 022115球粒陨石的基础矿物学特征和冲击变质矿物学特征，探讨陨石冲击熔融脉的形成机制和界定其母体的冲击条件。陨石主岩主要由橄榄石、辉石、熔长石、铁镍金属和硫化物等矿物组成。根据主岩的硅酸盐矿物学特征，确定GRV 022115是风化程度较低（W1） 的L6型普通球粒陨石，与前期分类结果一致。根据熔融脉内含有大量林伍德石的现象，修正GRV 022115陨石的冲击级别为S6，比原定的S5高一个级别。GRV 022115球粒陨石中有多条冲击熔融脉，熔融脉由基质和主岩碎块包裹体两类岩相组组成。熔融脉基质的主要组成是微米级粒状镁铁榴石与纳米级的含铁方镁石，是在平衡冲击压力下结晶的产物。冲击熔融脉主岩碎块包裹体中的橄榄石、低钙辉石、长石碎块已部分或全部转为相对应的高压相。橄榄石相变为林伍德石；个别低钙辉石相变为钙钛矿结构布里奇曼石微晶的集合体；长石主要相变为熔长石与玲根石。几乎所有的主岩碎块都有高温熔融的圆滑边界。熔融脉内外同类矿物的主量和微量元素具有一定的差异性，该差异性可以反映高温高压下混溶作用和扩散作用的影响。结合陨石冲击熔融脉形成机制和结晶模型，根据熔脉基质中镁铁榴石+方镁石矿物组合及静态高温高压实验相图，界定该陨石经受的冲击压力为23~27 GPa。  相似文献   

20.
The abundance of metallic iron is highly variable in different kinds of chondrites. The precise mechanism by which metal fractionation occurred and its place in time relative to chondrule formation are unknown. As metallic iron is abundant in most Type I (FeO-poor) chondrules, determining under what conditions metal could form in chondrules is of great interest. Assuming chondrules were formed from low temperature nebular condensate, we heated an anhydrous CI-like material at 1580°C in conditions similar to those of the canonical nebula (PH2 = 1.3 × 10−5 atm). We reproduced many of the characteristics of Type IA and IIA chondrules but none of them contained any iron metal. In these experiments FeO was abundant in charges that were heated for as long as 6 h. At a lower temperature, 1350°C, dendritic/cellular metal crystallized from Fe-FeS melts during the evaporation of S. However, the silicate portion consisted of many relict grains and vesicles, not typical of chondrules.Evaporation experiments conducted at PH2 = 1 atm and 1565°C produced charges containing metallic iron both as melt droplets and inclusions in olivine, similar to those found in chondrules. Formation of iron in these experiments was primarily the result of desulfurization of FeS. With long heating times Fe° was lost by evaporation. Apart from some reduction of FeO by kerogen to make metal inclusions within olivine grains, reduction of FeO to make Fe° in these charges was not observed.This study shows that under canonical nebular conditions FeS and iron-metal are extremely volatile so that metal-rich Type I chondrules could not form by melting “CI.” Under these conditions FeO is lost predominantly by hydrogen stripping and, due to the relative low abundance of hydrogen at low pressures, remains in the melt for as long as 6 h. Conversely, at higher total pressures (1-atm H2) iron metal (produced mainly by the desulfurization of troilite) is less volatile and remains in the melt for longer times (at least 6 h). In addition, due to elevated pressures of hydrogen, FeO is stripped away much faster. These results suggest that chondrule formation occurred in environments with elevated pressures relative to the canonical nebula for iron metal to be present.  相似文献   

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