首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 485 毫秒
1.
Abstract— Opaque minerals in the Qingzhen (EH3) and MacAlpine Hills (MAC) 88136 (EL3) enstatite chondrites were studied and compared with other EH and EL chondrites. All opaque minerals usually occur in multi‐sulfide‐metal clasts and nodules in the matrix between chondrules (El Goresy et al., 1988). The higher abundance of opaque minerals, the occurrence of niningerite and various alkali‐sulfides (e.g., caswellsilverite, phases A and B, djerfisherite) are diagnostic criteria for EH chondrites, while alabandite is characteristic for EL chondrites. In addition, EH chondrites are characterized by enrichments of Si in both kamacite and perryite, and alkali elements in sphalerite and chalcopyrite. The Mn contents of daubreelite and sphalerite are lower in EH than in EL chondrites. These are consistent with lower oxygen fugacity and higher H2S fugacity of EH than EL chondrites. In contrast, the discovery of sphalerite and Zn‐rich daubreelite in MAC 88136 indicates that their absence in EL6 chondrites is probably related to thermal metamorphism in the parent body. Schreibersite microspherules are commonly enclosed in most sulfides in Qingzhen, but are absent in MAC 88136. They were once molten, and probably predated all sulfide host phases. The petrographic setting and chemical compositions of the sulfide hosts of the schreibersite microspherules in EH3 chondrites are consistent with formation by condensation. The earliest sulfide condensates oldhamite and niningerite occupy the interiors of the clasts and nodules, whereas the rims consist of troilite and djerfisherite. In addition, in Qingzhen, some other troilite, djerfisherite and sphalerite assemblages coexist with perryite. They were produced by sulfurization of metallic Fe‐Ni in the nebula. In MAC 88136, sulfurization of Si‐bearing Fe‐Ni metal is less pronounced, and it produced troilite, schreibersite and less abundant perryite. Two kinds of normal zoning and a reverse zoning trends of niningerite, and both normal and reverse zoning of sphalerite were found in clasts and nodules in Qingzhen. The coexistence of normal and reverse zoning profiles in niningerite grains in the same meteorite strongly suggests that they formed before accretion in the parent body, because an asteroidal metamorphic or an impact event in the parent body would have erased these contrasting profiles and destroyed the textural settings. In contrast, alabandite in MAC 88136 shows only normal zoning, with the FeS content decreasing to 9.3 mol% toward troilite, indicating very slow cooling at low temperature.  相似文献   

2.
Abstract— –Literature data show that, among EH chondrites, the Abee impact‐melt breccia exhibits unusual mineralogical characteristics. These include very low MnO in enstatite (<0.04 wt%), higher Mn in troilite (0.24 wt%) and oldhamite (0.36 wt%) than in EH4 Indarch and EH3 Kota‐Kota (which are not impact‐melt breccias), low Mn in keilite (3.6–4.3 wt%), high modal abundances of keilite (11.2 wt%) and silica (～7 wt%, but ranging up to 16 wt% in some regions), low modal abundances of total silicates (58.8 wt%) and troilite (5.8 wt%), and the presence of acicular grains of the amphibole, fluor‐richterite. These features result from Abee's complex history of shock melting and crystallization. Impact heating was responsible for the loss of MnO from enstatite and the concomitant sulfidation of Mn. Troilite and oldhamite grains that crystallized from the impact melt acquired relatively high Mn contents. Abundant keilite and silica also crystallized from the melt; these phases (along with metallic Fe) were produced at the expense of enstatite, niningerite and troilite. Melting of the latter two phases produced a S‐rich liquid with higher Fe/Mg and Fe/Mn ratios than in the original niningerite, allowing the crystallization of keilite. Prior to impact melting, F was distributed throughout Abee, perhaps in part adsorbed onto grain surfaces; after impact melting, most of the F that was not volatilized was incorporated into crystallizing grains of fluor‐richterite. Other EH‐chondrite impact‐melt breccias and impact‐melt rocks exhibit some of these mineralogical features and must have experienced broadly similar thermal histories.  相似文献   

3.
Abstract— The Ca isotopic compositions of 32 oldhamite (CaS) grains from the Qingzhen (EH3), MAC88136 (EL3), and Indarch (EH4) enstatite chondrites were determined by ion microprobe mass spectrometry. Also measured were the S isotopic compositions of eight oldhamite, two niningerite (MgS), and seven troilite (FeS) grains. The S isotopic compositions of all minerals are normal, but oldhamite grains of the first two meteorites exhibit apparent small 48Ca excesses and deficits that are correlated with isotopic mass fractionation as determined from the 40Ca-44Ca pair. The interpretation of these results is complicated by the fact that none of the established mass fractionation laws can account for the data in the Norton County oldhamite standard. The method of analysis is carefully scrutinized for experimental artifacts. Neither interferences nor any known mass fractionation effect can satisfactorily explain the observed small deviations from normal isotopic composition. If these are truly isotopic anomalies, they are much smaller than those observed in hibonite. The nucleosynthetic origin of Ca isotopes is discussed.  相似文献   

4.
Abstract— Enstatite is the primary silicate phase of equilibrated enstatite chondrites (EECs). The CaO contents of these enstatites lie close to or on the enstatite-diopside phase boundary, yet, curiously, diopside has always been absent from EEC assemblages. In contrast, aubrites contain abundant diopside even though they are thought to be derived from an E chondrite-like protolith. A phase equilibrium analysis of the Ca-Mg-Fe-Mn-Si-O-S system under reducing conditions solves this enigma and shows that diopside-bearing EECs should commonly be found. When S fugacity is sufficiently high (e.g., Fe-FeS buffer), low O fugacity limits the stability of diopside in favor of oldhamite. Under such conditions, the relative stability of diopside and oldhamite is described by the reaction: CaMgSi2O6 + MgS = CaS + Mg2Si2O6 A large bulk compositional field exists where diopside and oldhamite are simultaneously stable. The existence of oldhamite does not preclude the stability of diopside. Phase diagram topology demonstrates that bulk compositions lying in the enstatite-oldhamite field and enstatite-oldhamite-alabandite field have enstatite CaO contents nearly identical to that of enstatite in equilibrium with diopside alone. This explains the high enstatite CaO contents of all EECs that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET 90102. Elephant Moraine 90102 has a typical EL6 texture and contains the assemblage: enstatite, diopside, albite, kamacite, troilite, sinoite, and graphite. Trace quantities of alabandite, oldhamite and daubreelite are also present. Diopside is stable in EET 90102 because its bulk composition lies within either the enstatite-diopside-oldhamite-alabandite or diopside-alabandite-enstatite stability fields. In contrast, all other EECs analyzed to date have bulk compositions lying in the enstatite-oldhamite-alabandite stability field. The discovery of diopside in EET 90102 helps confirm the predictions of the phase equilibrium analysis. Elephant Moraine 90102 experienced a high-temperature metamorphic equilibration from which it was quenched. The enstatite-diopside, CaS in alabandite and Fe in alabandite, geothermometers yield temperatures of last equilibration of ～900 °C. The absence of daubreelite and schreibersite along with high troilite Cr contents and high kamacite P contents confirm a high-temperature metamorphic quench. The EET 90102 chondrite experienced a somewhat different cooling history and has a slightly different bulk composition than all other EECs studied to date; however, the close mineralogic, petrologic and textural similarities between EET 90102 and nominal EL6 chondrites signify that it should be classified as a diopside- and sinoite-bearing EL6 chondrite. Assuming that the aubrites formed from an E chondrite-like protolith, a source rock similar to that of a diopside-bearing EEC offers a clear advantage for aubrite formation. Melting of a diopside-saturated EEC protolith would not require conversion of CaS to achieve diopside-saturation upon cooling.  相似文献   

5.
We present petrologic and isotopic data on Northwest Africa (NWA) 4799, NWA 7809, NWA 7214, and NWA 11071 meteorites, which were previously classified as aubrites. These four meteorites contain between 31 and 56 vol% of equigranular, nearly endmember enstatite, Fe,Ni metal, plagioclase, terrestrial alteration products, and sulfides, such as troilite, niningerite, daubréelite, oldhamite, and caswellsilverite. The equigranular texture of the enstatite and the presence of the metal surrounding enstatite indicate that these rocks were not formed through igneous processes like the aubrites, but rather by impact processes. In addition, the presence of pre‐terrestrially weathered metal (7.1–14 vol%), undifferentiated modal abundances compared to enstatite chondrites, presence of graphite, absence of diopside and forsterite, low Ti in troilite, and high Si in Fe,Ni metals suggest that these rocks formed through impact melting on chondritic and not aubritic parent bodies. Formation of these meteorites on a parent body with similar properties to the EHa enstatite chondrite parent body is suggested by their mineralogy. These parent bodies have undergone impact events from at least 4.5 Ga (NWA 11071) until at least 4.2 Ga (NWA 4799) according to 39Ar‐40Ar ages, indicating that this region of the solar system was heavily bombarded early in its history. By comparing NWA enstatite chondrite impact melts to Mercury, we infer that they represent imperfect petrological analogs to this planet given their high metal abundances, but they could represent important geochemical analogs for the behavior and geochemical affinities of elements on Mercury. Furthermore, the enstatite chondrite impact melts represent an important petrological analog for understanding high‐temperature processes and impact processes on Mercury, due to their similar mineralogies, Fe‐metal‐rich and FeO‐poor silicate abundances, and low oxygen fugacity.  相似文献   

6.
Abstract– Mineral grains that comprise dust particles in circumstellar, interstellar, and protostellar environments can potentially undergo amorphization and other solid‐state transformations from exposure to energetic ions from space plasmas. The Fe‐sulfide minerals troilite (FeS) and pyrrhotite (Fe1?xS) are important known dust components, but their potential to undergo structural changes, including amorphization, from space radiation processing in dusty space environments has not been experimentally evaluated relative to silicates. We used a transmission electron microscope (TEM) with capabilities for in situ ion irradiation to precisely follow structural changes in troilite and pyrrhotite exposed to 1.0 MeV Kr++ ions selected to optimize the probability of inducing amorphization from nuclear elastic collisional processes. No indication of amorphization was found in either mineral up to an experimentally practical ion dose of 1 × 1016 Kr++ ions cm?2, indicating that both structures can remain crystalline up to a modeled collisional damage level of at least 26 displacements‐per‐atom. This behavior matches that of some of the most radiation‐resistant nonmetallic phases known, and is two orders of magnitude higher than the levels at which Mg‐rich olivine and enstatite become amorphous under the same irradiation conditions. Although pyrrhotite retained short‐range crystalline order during irradiation, its longer range vacancy‐ordered superstructure is removed at modeled damage levels equivalent to those at which olivine and enstatite become amorphous. This suggests that space radiation conditions sufficient to amorphize olivine and enstatite in circumstellar and interstellar environments would convert coexisting pyrrhotite to its disordered structural form, thereby changing magnetic and possibly other properties that determine how pyrrhotite will behave in these environments.  相似文献   

7.
Abstract— The origin of the aubrite parent body (APB) and its relation to the enstatite chondrites is still unclear. Therefore we began a detailed chemical study of the aubrite Peña Blanca Spring. Bulk samples and mineral separates (oldhamite, troilite, alabandite, pyroxene) of Peña Blanca Spring were analyzed for major and trace elements by instrumental neutron activation analysis (INAA). In addition, a leaching experiment was performed on a powdered bulk sample to study the distribution of trace elements in aubrite minerals. The elemental abundances in Peña Blanca Spring are compared to abundances in EH-chondrites and EL-chondrites in an attempt to distinguish volatility related fractionations (evaporation, condensation) from planetary differentiation (melting and core formation). Low abundances of siderophile (e.g., Ir) and chalcophile (e.g., V) elements in bulk samples indicate that 25% (by mass) metal and about 6% (by mass) sulfide separated from an enstatite chondrite like-parent body to form a core and a residual mantle with aubrite composition. We argue that the high observed rare earth element (REE) abundances in oldhamite (>100 × EH-chondrite normalized) reflect REE incorporation into oldhamite during nebular condensation. Thus, oldhamite in aubrites is, at least in part, a relict phase as originally proposed by Lodders and Palme (1990). Some re-equilibration of CaS with silicates has, however, occurred, leading to partial redistribution of REE, as exemplified by the uptake of Eu by plagioclase. The distribution of the REE among aubritic minerals cannot be the result of fractional crystallization, which would occur if high degrees of partial melting took place on the APB. Instead, the REE distributions indicate incomplete equilibrium of oldhamite and other phases. Therefore, a short non-equlibrium melting episode led to segregation of metal and sulfides.  相似文献   

8.
We present here the Raman spectroscopic study of silicate and carbonaceous minerals in three ordinary chondrites with the aim to improve our understanding the impact process including the peak metamorphic pressures present in carbon‐bearing ordinary chondites. The characteristic Raman vibrational peaks of olivines, pyroxenes, and plagioclase have been determined on three ordinary chondrites from India, Dergaon (H5), Mahadevpur (H4/5), and Kamargaon (L6). The Raman spectra of these meteorite samples show the presence of nanodiamonds at 1334–1345 cm?1 and 1591–1619 cm?1. The full‐width at half maximum (FWHM) of Raman peaks for Mahadevpur and Dergaon reflect the nature of shock metamorphism in these meteorites. The frequency shift in Raman spectra might be because of shock effects during the formation of the diamond/graphite grains.  相似文献   

9.
Abstract— This study presents compositional data and 57Fe Mössbauer spectra, taken at 295 K and 85 K, of two fragments of the enstatite (EL6) chondrite Neuschwanstein that fell near the famous Neuschwanstein castle (Bavaria, southern Germany) on April 6, 2002. Main silicate minerals are enstatite (Fs 2) and plagioclase (An 20), the main opaque minerals are kamacite and troilite. Small amounts of oldhamite, daubreelite, and schreibersite have been found. The presented Mössbauer data are the first data gathered for an EL6 chondrite. The dominant parts of each Mössbauer spectrum consist of two six‐line patterns due to the presence of ferromagnetic phases kamacite and troilite. In contrast to other chondrites, peaks of other iron species in the central parts of the spectra are missing due to an extremely low content of Fe‐bearing paramagnetic components. The hyperfine interaction parameters for kamacite are internal magnetic hyperfine field Hhf = 333.2 kOe, isomer shift (relative to a metallic Fe foil) IS = 0.01 mm/s, quadrupole splitting QS = 0 mm/s, line width W = 0.41 mm/s. The data for troilite are Hhf = 305.5 kOe, IS = 0.75 mm/s, QS = ?0.85 mm/s, W = 0.34 mm/s.  相似文献   

10.
Abstract— Correlated in situ analyses of the oxygen and magnesium isotopic compositions of aluminum‐rich chondrules from unequilibrated enstatite chondrites were obtained using an ion microprobe. Among eleven aluminum‐rich chondrules and two plagioclase fragments measured for 26Al‐26Mg systematics, only one aluminum‐rich chondrule contains excess 26Mg from the in situ decay of 26Al; the inferred initial ratio (26Al/27Al)o = (6.8 ± 2.4) × 10?6 is consistent with ratios observed in chondrules from carbonaceous chondrites and unequilibrated ordinary chondrites. The oxygen isotopic compositions of five aluminum‐rich chondrules and one plagioclase fragment define a line of slope ?0.6 ± 0.1 on a three‐oxygen‐isotope diagram, overlapping the field defined by ferromagnesian chondrules in enstatite chondrites but extending to more 16O‐rich compositions with a range in δ18O of about ?12‰. Based on their oxygen isotopic compositions, aluminum‐rich chondrules in unequilibrated enstatite chondrites are probably genetically related to ferromagnesian chondrules and are not simple mixtures of materials from ferromagnesian chondrules and calcium‐aluminum‐rich inclusions (CAIs). Relative to their counterparts from unequilibrated ordinary chondrites, aluminum‐rich chondrules from unequilibrated enstatite chondrites show a narrower oxygen isotopic range and much less resolvable excess 26Mg from the in situ decay of 26Al, probably resulting from higher degrees of equilibration and isotopic exchange during post‐crystallization metamorphism. However, the presence of 26Al‐bearing chondrules within the primitive ordinary, carbonaceous, and now enstatite chondrites suggests that 26Al was at least approximately homogeneously distributed across the chondrite‐forming region.  相似文献   

11.
Abstract— Partition coefficients of the rare-earth-elements (REE) between sulfides (FeS or CaS) and silicate melt were determined experimentally at 1200–1300 °C. The REE sulfide/silicate partition coefficients (D) are ≤1 under the experimental O and S fugacities, which demonstrates that the REE are mainly located in the silicate phase. Rare-earth-element partition coefficients in the FeS/silicate system decrease from light to heavy REE, while the opposite behavior is found for the CaS/silicate system, where partition coefficients increase from light to heavy REE. In both sulfide systems, Eu is preferentially incorporated into the sulfide phases, as also expected from thermodynamic calculations. The Eu sulfide/silicate partition coefficient is about a factor of ten higher than that of neighboring Sm and Gd, in accordance with thermodynamic predictions of REE sulfide/silicate partition coefficients. The low sulfide/silicate partition coefficients indicate that CaS (oldhamite) in enstatite achondrites (aubrites) cannot have gained its high REE concentrations during igneous differentiation processes. The high REE concentrations and the REE patterns in aubritic oldhamite are more plausibly explained by REE condensation into refractory CaS. The refractory nature of CaS prevented major exchange reactions of the oldhamite with other aubritic minerals during the short differentiation and metamorphism period on the aubrite parent body. Thus, oldhamite in aubrites may be relict condensates altered to different degrees during short heating events, as originally suggested by Lodders and Palme (1990).  相似文献   

12.
In this study we present a review of low-temperature magnetic properties of alabandite (Fe, Mn)S, daubreelite FeCr2S4, pyrrhotite Fe1−xS and troilite FeS updated with new experimental data. The results indicate that besides FeNi alloys mainly daubreelite with its Curie temperature TC ∼ 150 K and strong induced and remanent magnetizations may be a significant magnetic mineral in cold environments and may complement that of FeNi or even dominate magnetic properties of sulfide rich bodies at temperatures below TC.Comets are known to contain iron-bearing sulfides within dusty fraction and their surfaces are subject to temperature variations in the range of 100-200 K down to the depth of several meters while the cometary interior is thermally stable at several tens of Kelvin which is within the temperature range where alabandite, daubreelite or troilite are “magnetic”. Thus not only FeNi alloys, but also sulfides have to be considered in the interpretation of magnetic data from cometary objects such as will be delivered by Rosetta mission. Modeling indicates that magnetic interactions between cometary nucleus containing iron-bearing sulfides and interplanetary magnetic field would be difficult, but not impossible, to detect from orbit. Rosetta’s Philae lander present on the surface would provide more reliable signal.  相似文献   

13.
Abstract— We studied 2 enstatite aggregates (En >99), with sizes of 0.5 and 1.5 mm, embedded in the carbonaceous matrix of Kaidun. They contain sulfide inclusions up to 650 μm in length, which consist mainly of niningerite but contain numerous grains of heideite as well as oldhamite and some secondary phases (complex Fe, Ti, S hydroxides and Ca carbonate). Both niningerite and heideite are enriched in all trace elements relative to the co‐existing enstatite except for Be and Sc. The niningerite has the highest Ca content (about 5 wt%) of all niningerites analyzed so far in any meteorite and is the phase richest in trace elements. The REE pattern is fractionated, with the CI‐normalized abundance of Lu being higher by 2 orders of magnitude than that of La, and has a strong negative Eu anomaly. Heideite is, on average, poorer in trace elements except for Zr, La, and Li. Its REE pattern is flat at about 0.5 × CI, and it also has a strong negative Eu anomaly. The enstatite is very poor in trace elements. Its Ce content is about 0.01 that of niningerite, but Li, Be, Ti, and Sc have between 0.1 and 1 × CI abundances. The preferential partitioning of typical lithophile elements into sulfides indicates highly O‐deficient and S‐dominated formation conditions for the aggregates. The minimum temperature of niningerite formation is estimated to be ?850–900 °C. The texture and the chemical characteristics of the phases in the aggregates suggest formation by aggregation and subsequent sintering before incorporation into the Kaidun breccia. The trace element data obtained for heideite, the first on record, show that this mineral, in addition to oldhamite and niningerite, is also a significant carrier of trace elements in enstatite meteorites.  相似文献   

14.
Abstract— It appears that the mineralogy and chemical properties of type 3 enstatite chondrites could have been established by fractionation processes (removal of a refractory component, and depletion of water) in the solar nebula, and by equilibration with nebular gas at low‐to‐intermediate temperatures (approximately 700–950 K). We describe a model for the origin of type 3 enstatite chondrites that for the first time can simultaneously account for the mineral abundances, bulk‐chemistry, and phase compositions of these chondrites by the operation of plausible processes in the solar nebula. This model, which assumes a representative nebular gas pressure of 10?5 bar, entails three steps: (1) initial removal of 56% of the equilibrium condensed phases in a system of solar composition at 1270 K; (2) an average loss of 80–85% water vapor in the remaining gas; and (3) two different closure temperatures for the condensed phases. The first step involves a “refractory element fractionation” and is needed to account for the overall major element composition of enstatite chondrites, assuming an initial system with a solar composition. The second step, water‐vapor depletion, is needed to stabilize Si‐bearing metal, oldhamite, and niningerite, which are characteristic minerals of the enstatite chondrites. Variations in closure temperatures are suggested by the way in which the bulk chemistry and mineral assemblages of predicted condensates change with temperature, and how these parameters correlate with the observations of enstatite chondrites. In general, most phases in type 3 enstatite chondrites appear to have ceased equilibrating with nebular gas at approximately 900–950 K, except for Fe‐metal, which continued to partially react with nebular gas to temperatures as low as ～700 K.  相似文献   

15.
Yilmia, a new enstatite chondrite contains moderately well defined radiating and granular chondrules. The plagioclase to enstatite ratio is appreciably higher within than outside of the two granular chondrules in our microprobe sections. Osbornite was observed within the granular chondrules, but not in the rayed chondrules or surrounding matrix Major phases include enstatite, plagioclase (Ab80 An16 Or4), silica, silicon-rich kamacite and titanian troilite. Minor phases are many and varied: sinoite, silicon-rich taenite, schreibersite, graphite, osbornite, oldhamite, “normal” and zincian daubreelite, ferroan alabandite and a new FeZnMn monosulfide The new mineral (Fe.538 Zn.246 Mn.159 Mg.004 S) closely resembles albandite and could easily have been overlooked in other meteorites unless a microprobe was used. A new form of oldhamite was also found. Indarch oldhamite, analyzed for comparative purposes, consists of two similar but distinct species: Ca.96 Mn.005 Mg.04 Fe.01 S and Ca1.000 Mn.004 Mg.02 Fe.005 S. These have not been reported from other meteorites Based on its mineralogy and texture this is a type II (E6) enstatite chondrite that is transitional toward the intermediate type (E5). It is unique in its mineralogical complexity, abundance of taenite, diversity of zincian minerals and monosulfides, and restriction of osbornite to certain chondrules  相似文献   

16.
Abstract— Aubritic oldhamite (CaS) has been the subject of intense study recently because it is the major rare-earth-element (REE) carrier in aubrites, has a variety of REE patterns comparable to those in unequilibrated enstatite chondrites and has an extraordinarily high melting point as a pure substance (2525 °C). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmelted nebular relict, rather than of igneous origin. We have conducted REE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 °C and 1300 °C and subsolidus annealing experiments. All experiments produced crystalline oldhamite, with a range of compositions, glass and Fe metal, as well as enstatite, SiO2, diopside and troilite in some charges. Rare-earth-element partitioning is strongly dependent on oldhamite composition and temperature. Subsolidus annealing results in larger partition coefficients for some oldhamite grains, particularly those in contact with troilite. All experimental oldhamite/silicate melt partition coefficients are <20 and the vast majority are <5, which is similar to those reported in the literature and is two orders of magnitude less than those inferred for natural aubritic oldhamite. These partition coefficients preclude a simple igneous model, since REE abundances in aubritic oldhamite are greater than would be predicted on the basis of the experimental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, although experimental evidence that suggests melting of oldhamite at temperatures lower than that reached on the aubrite parent body are clearly inconsistent with the nebular model. Our experiments are consistent also with a complex igneous history. Oldhamite REE patterns may reflect a complex process of partial melting, melt removal, fractional crystallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can produce a wide range of REE patterns in aubritic oldhamite, as well as elevated (100–1000 × CI) REE abundances observed in aubritic oldhamite.  相似文献   

17.
Abstract– Queen Alexandra Range (QUE) 94204, an enstatite achondrite, is a coarse‐grained, highly recrystallized, chondrule‐free and unbrecciated rock dominated (about 70 vol%) by anhedral, equigranular crystals of orthoenstatite of nearly endmember composition (Fs0.1–0.4, Wo0.3–0.4) with interstitial plagioclase, kamacite, and troilite. Abundance of approximately 120° triple junctions and the close association of metal–sulfide and plagioclase‐rich melts indicate that QUE 94204 has undergone limited partial melting with inefficient melt extraction. Mineral chemistry indicates a high degree of thermal metamorphism. Kamacite in QUE 94204 contains between 2.09 and 2.55 wt% Si, similar to highly metamorphosed EL chondrites. Plagioclase has between 4.31 and 6.66 wt% CaO, higher than other E chondrites but closer in composition to plagioclase from metamorphosed EL chondrites. QUE 94204 troilite contains up to 2.55 wt% Ti, consistent with extensive thermal metamorphism of an E chondrite‐like precursor. Results presented in this study indicate that QUE 94204 is the result of low degree, (about 5–20 vol%, probably toward the lower end of this range) partial melting of an E chondrite protolith. Textural and chemical evidence suggests that during the metamorphism of QUE 94204, melts formed first at the Fe,Ni‐FeS cotectic near approximately 900 °C, followed by plagioclase‐pyroxene silicate partial melts near approximately 1100 °C. Neither the Fe,Ni‐FeS nor the plagioclase‐pyroxene melts were efficiently segregated or extracted. QUE 94204 belongs to a grouplet of similar “primitive enstatite achondrites” that are analogous to the acapulcoites‐lodranites, but that have resulted from the partial melting of an E chondrite‐like protolith.  相似文献   

18.
Abstract– Raman analyses were performed of individual micrometer‐sized fragments of material returned to Earth by the NASA Stardust mission to comet 81P/Wild 2. The studied fragments originated from grains (C2054,0,35,91,0 and C2092,6,80,51,0) of two different penetration tracks that occurred in two different silica aerogel collector cells. All fragments of both particles have Raman spectra characteristic of amorphous sp2‐bonded carbon that are in general agreement with the results published in previous Stardust particle studies. The present study, however, does not focus on the discussion of specific details of the D and G band parameters, but rather reports on additional information that can be obtained from returned Stardust samples via Raman spectroscopy. Most notably, the Raman spectra show that all analyzed fragments of the particles were contaminated with the capture medium (i.e., aerogel). The silica aerogel is laced with organic aliphatic and aromatic hydrocarbon impurities that resulted in strong bands in the ～ 2900 Δcm?1 spectral range (C‐H stretching modes). Aerogel bands are also found in the 1000–1600 Δcm?1 spectral range, where they overlap with the bands of the amorphous sp2‐bonded carbon. The peaks associated with the aerogel contamination differ between the two grains that originated from two different aerogel cells. In addition to the bands due to aerogel contamination and the always present sp2‐bonded carbon bands, fragments of particle C2092,6,80,51,0 also show Raman peaks for pyrrhotite and Fa30Fo70 olivine. Complete (up to 4000 Δcm?1) raw and baseline‐corrected Raman spectra of the Stardust particles are shown and discussed in detail.  相似文献   

19.
The common appearance of hygroscopic brine (“sweating”) on ordinary chondrites (OCs) from Oman during storage under room conditions initiated a study on the role of water‐soluble salts on the weathering of OCs. Analyses of leachates from OCs and soils, combined with petrography of alteration features and a 11‐month record of in situ meteorite and soil temperatures, are used to evaluate the role of salts in OC weathering. Main soluble ions in soils are Ca2+, SO42?, HCO3?, Na+, and Cl?, while OC leachates are dominated by Mg2+ (from meteoritic olivine), Ca2+ (from soil), Cl? (from soil), SO42? (from meteoritic troilite and soil), and iron (meteoritic). “Sweating meteorites” mainly contain Mg2+ and Cl?. The median Na/Cl mass ratio of leachates changes from 0.65 in soils to 0.07 in meteorites, indicating the precipitation of a Na‐rich phase or loss of an efflorescent Na‐salt. The total concentrations of water‐soluble ions in bulk OCs ranges from 600 to 9000 μg g?1 (median 2500 μg g?1) as compared to 187–14140 μg g?1 in soils (median 1148 μg g?1). Soil salts dissolved by rain water are soaked up by meteorites by capillary forces. Daily heating (up to 66.3 °C) and cooling of the meteorites cause a pumping effect, resulting in a strong concentration of soluble ions in meteorites over time. The concentrations of water‐soluble ions in meteorites, which are complex mixtures of ions from the soil and from oxidation and hydrolysis of meteoritic material, depend on the degree of weathering and are highest at W3. Input of soil contaminants generally dominates over the ions mobilized from meteorites. Silicate hydrolysis preferentially affects olivine and is enhanced by sulfide oxidation, producing local acidic conditions as evidenced by jarosite. Plagioclase weathering is negligible. After completion of troilite oxidation, the rate of chemical weathering slows down with continuing Ca‐sulfate contamination.  相似文献   

20.
Abstract— Like calcium‐aluminum‐rich inclusions (CAIs) from carbonaceous and ordinary chondrites, enstatite chondrite CAIs are composed of refractory minerals such as spinel, perovskite, Al, Ti‐diopside, melilite, hibonite, and anorthitic plagioclase, which may be partially to completely surrounded by halos of Na‐(±Cl)‐rich minerals. Porous, aggregate, and compact textures of the refractory cores in enstatite chondrite CAIs and rare Wark—Lovering rims are also similar to CAIs from other chondrite groups. However, the small size (<100μm), low abundance (<1% by mode in thin section), occurrence of only spinel or hibonite‐rich types, and presence of primary Ti‐(±V)‐oxides, and secondary geikelite and Ti, Fe‐sulfides distinguish the assemblage of enstatite chondrite CAIs from other groups. The primary mineral assemblage in enstatite chondrite CAIs is devoid of indicators (e.g., oldhamite, osbornite) of low O fugacities. Thus, high‐temperature processing of the CAIs did not occur under the reducing conditions characteristic of enstatite chondrites, implying that either (1) the CAIs are foreign to enstatite‐chondrite‐forming regions or (2) O fugacities fluctuated within the enstatite‐chondrite‐forming region. In contrast, secondary geikelite and Ti‐Fe‐sulfide, which replace perovskite, indicate that alteration of perovskite occurred under reducing conditions distinct from CAIs in the other chondrite groups. We have not ascertained whether the reduced alteration of enstatite chondrite CAIs occurred in a nebular or parent‐body setting. We conclude that each chondrite group is correlated with a unique assemblage of CAIs, indicating spatial or temporal variations in physical conditions during production or dispersal of CAIs.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号