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1.
Reported in this paper are structural and compositional data as the basis for the classification of 35 iron meteorites. The Xingjiang iron meteorite, previously labelled IIIAB, is reclassified as IIIE on the basis of its lower Ga/Ni and Ge/Ni ratios, its wider and swollen kamacite bands and the ubiquitous presence of haxonite, (Fe, Ni)23C. IIICD Dongling appears not to be a new meteorite, but to be paired with Nandan. Four Antarctic iron meteorites IAB Allan Hills A77250, A77263, A77289 and A77290 are classified as paired meteorites based on their similarities in structure, and the concentrations of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, W, Re, Ir and Au. It is found that Cu shares certain properties with Ga and Ge, which makes it an excellent taxonomic parameter. BecauseK Cu is near unity, Cu displays a small range of variation within most magmatic groups (less than a factor of 2.2) and, because of its high volatility, large variations can be noticed among groups.  相似文献   

2.
We report structural and compositional data leading to the classification of 41 iron meteorites, increasing the number of classified independent iron meteorites to 576. We also obtained data on a new metal-rich mesosiderite and on two new iron masses that are paired with previously studied irons. For the first time in this series we also report concentrations of Cr, Co, Cu, As, Sb, W, Re and Au in each of these 44 meteorites. We determined 7 of these elements (all except Sb) in 30 previously studied ungrouped or unusual irons, and obtained Cu data on 104 irons, 21 pallasites, and 3 meteorite phases previously studied by E. Scott. We show that Cu possesses characteristics well suited to a taxonomic element: a siderophile nature, a large range among all irons, and a low range within magmatic groups. For the first time we report the partial resolution of the C-rich group IIIE from its populous twin group IIIAB on element-Ni diagrams other than Ir-Ni. Cachiyuyal previously classified ungrouped and Armanty (Xinjiang) previously classified IIIAB are reclassified IIIE. Despite the addition of 3 new irons and the reanalysis of 3 previously studied irons the members of the set of 15 ungrouped irons having very low Ga (<3 μg/g) and Ge (<0.7 μg/g) contents remain individualists. The same is generally true for irons having 100 ≤ Ni ≤ 180 mg/g and compositional similarities to IIICD, but A80104 increases the Garden Head trio to a quartet. Algoma is reclassified from ungrouped to IIICD-an and Hassi-Jekna and Magnesia from IIICD to IIICD-an. The metal of Horse Creek and Mount Egerton is compositionally closely related to metal from EH chondrites. We suggest that the P-rich Bellsbank trio irons formed in the IIAB core in topographic lows filled with an immiscible, P-rich second liquid.  相似文献   

3.
Structural observations and concentrations of Ni, Ga, Ge and Ir allow the classification of 57 iron meteorites in addition to those described in the previous papers in this series; the number of classified independent iron meteorites is now 535. INAA for an additional six elements indicates that five previously studied irons having very high GeGa ratios are compositionally closely related and can be gathered together as group IIF. A previously unstudied iron, Dehesa, has the highest GeGa ratio known in an iron meteorite, a ratio 18 × higher than that in CI chondrites. Although such high GeGa ratios are found in the metal grains of oxidized unequilibrated chondrites, their preservation during core formation requires disequilibrium melting or significant compositional and temperature effects on metal/silicate distribution constants and/or activity coefficients. In terms of GeGa ratios and various other properties group IIF shows genetic links to the Eagle Station pallasites and COCV chondrites. Klamath Falls is a new high-Ni, low-Ir member of group IIIF that extends the concentration ranges in this group and makes these comparable to the ranges in large igneous groups such as IIIAB. Groups IAB and IIICD have been revised to extend the lower Ni boundary of group IIICD down to 62 mg/g. The iron having by far the highest known Ni concentration (585 mg/g), Oktibbeha County, is a member of group IAB and extends the concentration ranges of all elements in this nonmagmatic group. Morasko, a IAB iron associated with a crater field in Poland, is paired with the Seeläsgen iron discovered 100 km away. All explosion craters from which meteorites have been recovered were produced by IAB and IIIAB irons.  相似文献   

4.
Iron meteorites were analysed for nineteen siderophile and chalcophile elements by conventional inductively coupled plasma-mass spectrometry with the specific aim of demonstrating that this technique is an effective alternative to the more routine, yet complex, methodologies adopted in this field such as instrumental or radiochemical neutron activation analysis. Two aliquots of each meteorite sample, in the form of small shavings, were dissolved, one in 6 mol l-1 HNO3 and the other in aqua regia , and diluted to a final concentration of 1 mg sample per 1 ml of solution, without pre-concentrating the analytes. Nitric acid solutions were used for the determination of the elements Cr, Co, Ni, Cu, Ga, Ge and As; aqua regia solutions were analysed for the elements Mo, Ru, Rh, Pd, In, Sn, Sb, W, Re, Ir, Pt and Au. Samples were analysed by external calibration, carried out using synthetic multi-elemental solutions, and internal standardisation (with Be, Rb and Bi selected as internal standards). The results obtained from the analyses of nine geochemically well-characterized iron meteorites (Canyon Diablo, Odessa, Toluca, Coahuila, Sikhote-Alin, Buenaventura, Tambo Quemado, Gibeon, NWA 859) with widely variable chemical compositions are in good agreement with literature values for most elements. Detection limits were generally below the lowest concentration observed in iron meteorites. The most notable exception is for Ge, which cannot be successfully determined in the low-Ge meteorites of groups IVA, IVB and IIIF and a number of ungrouped irons. A test of the overall reproducibility of the adopted method, undertaken by repeatedly analysing the same Odessa IAB meteorite specimen, yielded relative standard deviations (1 s ) of between 1 and 6% for all elements except Cr (40%).  相似文献   

5.
We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal-liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.Phosphate inclusions in EET show Δ17O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal-liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.  相似文献   

6.
We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.  相似文献   

7.
Twenty-one iron meteorites with Ge contents below 1 μg/g, including nine belonging to groups IIIF and IVB, have been analyzed by instrumental neutron activation analysis (INAA) for the elements Co, Cr, As, Au, Re, Ir and W. Groups IIIF and IVB show positive correlations of Au, As and Co (IIIF only) with published Ni analyses, and negative correlations of Ir, Re, Cr (IVB only) and W (IIIF only) with Ni. On element-Ni plots, the gradients of the least squares lines are similar to those of many other groups, excluding IAB and IIICD. With the inclusion of a new member, Klamath Falls, group IIIF has the widest range of Au, As and Co contents of any group and the steepest gradients on plots of these elements against Ni. It is likely that these trends in groups IIIF and IVB were produced by fractionation of elements between solid and liquid metal, probably during fractional crystallization.It has been suggested that some of the 15 irons with <l μg/g Ge which lie outside the groups might be related. However, the INAA data indicate that no two are as strongly related as two group members. These low-Ge irons and the members of groups IIIF, IVA and IVB tend to have low concentrations of As, Au and P, low CoNi ratios and high Cr contents. The depletion of the more volatile elements probably results from incomplete condensation into the metal from the solar nebula.The structures of low-Ge irons generally reflect fast cooling rates (20–2000 K Myr?1). When data for all iron meteorites are plotted on a logarithmic graph of cooling rate against Ge concentration and results for related irons are averaged, there is a significant negative correlation. This suggests that metal grains which inefficiently condensed Ge and other volatile elements tended to accrete into small parent bodies.  相似文献   

8.
San Cristobal is an unusual group IB ataxite with 25 per cent Ni, composed of taenite grains 2–3 cm in diameter and silicate-troilite-graphite nodules concentrated on the grain boundaries. Silicate compositions are typical of group IAB: olivine Fa3.3, orthopyroxene Fs6.9 and feldspar Ab88. Plagioclase shows peristerite unmixing, previously unrecorded in meteorites, and occasional K-rich feldspar grains have an unusual antiperthite exsolution. Brianite Na2CaMg(PO4)2 and haxonite (Fe, Ni)23C6 are common in nodules and matrix, respectively, while cohenite is rare. Part of the matrix contains a pearlitic kamacite precipitate instead of the usual oriented platelets.San Cristobal has extreme concentrations of many elements; e.g. the highest published Ag, Cu, In and Sb contents and the lowest Mo and Pt in irons. These data and the mineralogy show that San Cristobal has many characteristics of both groups IB and IIID, but that it fits group IB trends better. Ratios of refractory element abundances to those in Cl chondrites (both normalized to Ni) decrease through IB from l in IA to 0.03 in San Cristobal, but the other siderophilic elements have a small range of abundance ratios, 0.5–2, throughout IAB. We suggest that IB grains either formed in a part of the solar nebula where refractories had been previously removed, or else failed to equilibrate with a refractory-rich, high-temperature condensate. After condensation of the volatiles, Fe was partially removed, perhaps by oxidation. Group IIICD seems to have experienced similar fractionations. Unlike other iron meteorite groups, neither IAB nor IIICD appears to have been fully molten.  相似文献   

9.
本文对南丹IIICD铁陨石的矿物学特征进行了研究,并与同为铁陨石但化学分类不同的阿根廷IAB铁陨石和西伯利亚IIB铁陨石进行了对比,重点探讨了风化作用对铁陨石矿物学特征的影响.首先用偏光显微镜、静水称重、扫描电镜观察了样品的基本矿物学特征和微形貌特征,然后用振动式样品磁强计、X射线衍射与电子探针能谱半定量测试研究了样品的磁学性质、物相和化学组成.研究结果表明,南丹铁陨石在较强的自然风化作用下,光泽变弱为土状光泽,相对密度降低;风化产生的反铁磁性物质会使陨石的磁性下降;另外,样品表面物相组成也发生较大变化,以针铁矿(FeOOH)和磁铁矿(Fe3O4)等铁的次生矿物为主;但风化壳以下的矿物物相及化学成分均未发生明显变化,以Fe、Ni为主的铁纹石、镍纹石物相存在.  相似文献   

10.
Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by 26Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.  相似文献   

11.
Helium,neon, and argon in the iron meteorites Dongling,Nantan and Ningbo   总被引：1，自引：0，他引：1  
The light noble gases He, Ne and Ar have been measured in the iron meteorites Dongling, Nantan and Ningbo. Dongling and Ningbo show a deficit of cosmic-ray that produced3He of ca. 30% and 10%, respectively, which is argued to be caused by the loss of3H (tritium) from the meteoroids during the time of their exposure to the cosmic radiation. Nantan has the lowest content of noble gases as yet reported for any iron meteorite. Cosmogenic3He and38Ar are only about 1/5000 of those in Dongling which has particularly interesting implications if the two meteorites belong to the same fall[2]. In addition, Nantan contains nonspallogenic4He which we believe to be of radiogenic origin. This radiogenic4He, together with a U-content of 2.6×10−11 g/g[20] yields a4He retention age close to the cosmic-ray exposure age of Dongling. If Dongling and Nantan were part of the same meteoroid[2], this result would indicate that He retention in the meteoroid age were 4,500 Ma, a U-content of less than 7.2×10−13 g/g is required to explain the non-cosmogenic4He present. An upper limit to the number of transuranium or superheavy-element atoms which have decayed by α-emission in Nantan since onset of He retention is 2×1010 per gram.  相似文献   

12.
Electron optical techniques were employed to investigate the plessite structure and composition of four IIICD fine octahedrites. These meteorites have a similar thermal history and differences in plessite structure can be ascribed to varying bulk Ni content and/or localized differences in carbon content. Microdiffraction patterns from regions as small as 20 nm dia. were obtained for the first time from plessite structures. It was established that transformation twins in clear taenite I have the conventional fcc twin relationship, individual kamacite and taenite cells in the cloudy zone have the Kurdjumov-Sachs orientation and fine γ rods in the decomposed martensite zone display both the Nishiyama and Kurdjumov-Sachs relation with the matrix-α. All the IIICD irons contain cloudy zone and martensitic plessite. Except for Dayton, martensitic plessite shows further decomposition into α + λ at low temperatures. Using STEM X-ray microanalysis with a spatial resolution of ~ 50 nm, Ni composition profiles in taenite from all the IIICD irons showed a maximum of ~48 wt% Ni. The structural and compositional data indicate that plessite formation occurs at quite low temperatures (~ 200–300°C) during the cooling history of the IIICD irons.  相似文献   

13.
We present new data for iron meteorites that are members of group IAB or are closely related to this large group, and we have also reevaluated some of our earlier data for these irons. In the past it was not possible to distinguish IAB and IIICD irons on the basis of their positions on element-Ni diagrams, but we now show that plotting the new and revised data yields six sets of compact fields on element-Au diagrams, each set corresponding to a compositional group. The largest set includes the majority (≈70) of irons previously designated IA; we christened this set the IAB main group. The remaining five sets we designate “subgroups” within the IAB complex. Three of these subgroups have Au contents similar to the main group, and form parallel trends on most element-Ni diagrams. The groups originally designated IIIC and IIID are two of these subgroups; they are now well resolved from each other and from the main group. The other low-Au subgroup has Ni contents just above the main group. Two other IAB subgroups have appreciably higher Au contents than the main group and show weaker compositional links to it. We have named these five subgroups on the basis of their Au and Ni contents. The three subgroups having Au contents similar to the main group are the low-Au (L) subgroups, the two others the high-Au (H) subgroups. The Ni contents are designated high (H), medium (M), or low (L). Thus the old group IIID is now the sLH subgroup, the old group IIIC is the sLM subgroup. In addition, eight irons assigned to two grouplets plot between sLL and sLM on most element-Au diagrams. A large number (27) of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the IAB complex.Many of these irons contain coarse silicates having similar properties. Most are roughly chondritic in composition; the mafic silicates show evidence of reduction during metamorphism. In each case the silicate O-isotopic composition is within the carbonaceous chondrite range (Δ17O ≤ −0.3‰). In all but four cases these are within the so-called IAB range, −0.30 ≥ Δ17O ≥ −0.68‰. Fine silicates appear to be ubiquitous in the main group and low-Au subgroups; this requires that viscosities in the parental melt reached high values before buoyancy could separate these.The well-defined main-group trends on element-Au diagrams provide constraints for evaluating possible models; we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium. The main arguments against the main group having formed by fractional crystallization are: a) the small range in Ir, and b) the evidence for rapid crystallization and a high cooling rate through the γ-iron stability field. The evidence for the latter are the small sizes of the γ-iron crystals parental to the Widmanstätten pattern and the limited thermal effects recorded in the silicates (including retention of albitic plagioclase and abundant primordial rare gases). In contrast, crystal segregation in a cooling metallic melt (and related processes such as incomplete melting and melt migration) can produce the observed trends in the main group. We infer that this melt was formed by impact heating on a porous chondritic body, and that the melt was initially hotter than the combined mix of silicates and metal in the local region; the melt cooled rapidly by heat conduction into the cooler surroundings (mainly silicates). We suggest that the close compositional relationships between the main group and the low-Au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids.  相似文献   

14.
We have investigated the partitioning of Ir. Ge, Ga, W, Cr, Au, P, and Ni between solid metal and metallic liquid as a function of temperature and S-concentration of the metallic liquid. Partition coefficients for siderophile elements such as Ir, W, Ga and Ge increase by factors of 10–100 as the Sconcentration of the metallic liquid increases from 0–30 wt%. Partition coefficients for other siderophile elements such as Ni, Au and P increase by only factors of 2–3. In contrast, partition coefficients for the more chalcophile element Cr decrease. These experimentally-determined partition coefficients have been used in conjunction with a fractional crystallization model to reproduce the geochemical behavior of Ni, P, Au and Ir during the magmatic evolution of groups IIAB, IIIAB, IVA and IVB iron meteorites. The mean S-concentration for each group increases in the order IVB, IVA, IIIAB, IIAB, in accord with cosmochemical prediction. However, we are unable to reproduce the geochemical behavior of Ge, Ga, W and Cr in an internally consistent way. We conclude that the magmatic histories of these iron meteorite groups are more complex than has been generally assumed.  相似文献   

15.
During the past three decades many iron meteorites have been collected from the deserts of North Africa. Almost all are now characterized, and the distribution among classes is found to be very different from those that were in museums prior to the collection of meteorites from hot and cold (Antarctica) deserts. Similar to the iron meteorites from Antarctica, the irons from Northwest Africa include a high fraction of ungrouped irons and of minor subgroups of group IAB. The different distribution is attributed to the small median size of the desert meteorites (∼1.3 kg in North African irons, ∼30 kg in non-desert irons). It appears that a sizable fraction of these small (several centimeter) masses constitute melt pockets produced by impacts in chondritic regoliths; they were never part of a large (meter-to-kilometer) magma bodies. As a result, a meter-size fragment ejected from the regolith of the asteroid may contain several of these small metallic masses. It may be that such stochastic sampling effects enhanced the fraction of IAB-sHL irons among the irons from Northwest Africa.The variety observed in small meteoroids is also enhanced because (relative to large) small fragments are more efficiently ejected from asteroids and because the orbital parameters of small meteoroids are more strongly affected by collisions and drag effects, they evolve to have Earth-crossing perihelia more rapidly than large meteoroids; as a result, the set of small meteoroids tends to sample a larger number of parent asteroids than does the set of larger meteoroids.  相似文献   

16.
Magmatic iron meteorites are thought to be samples of the central metallic cores of asteroid-sized parent bodies. Sulfur is believed to have been an important constituent of these parental cores, but due to the low solubility of S in solid metal, initial S-contents for the magmatic groups cannot be determined through direct measurements of the iron meteorites. However, experimental solid metal-liquid metal partition coefficients show a strong dependence on the S-content of the metallic liquid. Thus, by using the experimental partition coefficients to model the fractional crystallization trends within magmatic iron meteorite groups, the S-contents of the parental cores can be indirectly estimated. Modeling the Au, Ga, Ge, and Ir fractionations in four of the largest magmatic iron meteorite groups leads to best estimates for the S-contents of the parental cores of 12 ± 1.5 wt% for the IIIAB group, 17 ± 1.5 wt% for the IIAB group, and 1 ± 1 wt% for the IVB group. The IVA elemental fractionations are not adequately fit by a simple fractional crystallization model with a unique initial S-content. These S-content estimates are much higher than those recently inferred from crystallization models involving trapped melt. The discrepancy is due largely to the different partition coefficients that are used by the two models. When only partition coefficients that are consistent with the experimental data are used, the trapped melt model, and the low S-contents it advocates, cannot match the Ge and Ir fractionations that are observed in IIIAB iron meteorites.  相似文献   

17.
The addition of two meteorites to the iron meteorite grouplet originally known as the Bellsbank trio brings the population to five, the minimum number for group status. With Ga and Ge contents in the general “II” range, the new group has been designated IIG. The members of this group have low-Ni contents in the metal and large amounts of coarse schreibersite ((Fe,NI)3P); their bulk P contents are 17-21 mg/g, the highest known in iron meteorites. Their S contents are exceptionally low, ranging from 0.2 to 2 mg/g. We report neutron-activation-analysis data for metal samples; the data generally show smooth trends on element-Au diagrams. The low Ir and high Au contents suggest formation during the late crystallization of a magma.Because on element-Au or element-Ni diagrams the IIG fields of the important taxonomic elements Ni, Ga, Ge and As are offset from those of the IIAB irons, past researchers have concluded that the IIG irons could not have formed from the same magma, and thus that the two groups originated on separate parent bodies. However, on most element-Au diagrams the IIG fields plot close to extensions of IIAB trends to higher Au concentrations.There is general agreement that immiscibility led to the formation of an upper S-rich and a lower P-rich magma in the IIAB core. We suggest that the IIG irons formed from the P-rich magma, and that schreibersite was a liquidus phase during the final stages of crystallization. The offsets in Ni and As (and possibly other elements) may result from solid-state elemental redistribution between metal and schreibersite during slow cooling. For example, it is well established that the equilibrium Ni content is >2× higher in late-formed relative to early-formed schreibersite. It is plausible that As substitutes nearly ideally for P in schreibersite at eutectic temperatures but becomes incompatible at low temperatures.[Wasson J. T., Huber, H. and Malvin, D. J. (2007) Formation of IIAB iron meteorites. Geochim. Cosmochim. Acta71, 760-781] argued that, in the most evolved IIAB irons, the amount of trapped melt was high. The high P contents of IIG irons also require high contents of trapped melt but the local geometry seems to have allowed the S-rich immiscible melt to escape as it formed. The escaping melt may have selectively depleted elements such as Au and Ge.  相似文献   

18.
Group IID is the fifth largest group of iron meteorites and the fourth largest magmatic group (i.e., that formed by fractional crystallization). We report neutron-activation data for 19 (of 21 known) IID irons. These confirm earlier studies showing that the group has a relatively limited range in Ir concentrations, a factor of 5. This limited range is not mainly due to incomplete sampling; Instead, it seems to indicate low solid/liquid distribution coefficients reflecting very low S contents of the parental magma, the same explanation responsible for the limited range in group IVA. Despite this similarity, these two groups have very different volatile patterns. Group IVA has very low abundances of the volatile elements Ga, Sb and Ge whereas in group IID Ga and Sb abundances are the highest known in a magmatic group of iron meteorites and Ge abundances are the second highest (after group IIAB). Group IID appears to be the only large magmatic group having high volatile abundances but low S. In the volatile-depleted groups IVA and IVB it is plausible that S was lost as a volatile from a chondritic precursor material. Because group IID seems to have experienced minimal loss of volatiles, we suggest that S was lost as an early melt having a composition near that of the Fe–FeS eutectic (315 mg/g S). When temperatures had risen 400–500 K higher P-rich melts formed, became gravitationally unstable, and drained through the first melt to form an inner core that was parental to the IID irons. As discussed by [Kracher, A., Wasson, J.T., 1982. The role of S in the evolution of the parental cores of the iron meteorites. Geochim. Cosmochim. Acta 46, 2419–2426], it is plausible that a metal-rich inner core and a S-rich outer core could coexist metastably because stratification near the interface permitted only diffusional mixing. The initial liquidus temperature of the inner, P-rich core is estimated to have been 1740 K; after >60% crystallization the increase in P and the decrease in temperature may have permitted immiscibility with the S-rich outer core. We have not recognized samples of the outer core.  相似文献   

19.
A set of 11 impact melt rock samples from the Rochechouart impact structure, France and nine impact melt rock samples from Sääksjärvi impact structure, Finland were studied for their major and trace element compositions, including the abundances of the platinum group elements. The main goal of this study was to identify the projectile type(s) responsible for the formation of these two impact structures. The results confirmed previous studies that suggested extraterrestrial contamination in both the Rochechouart and Sääksjärvi impact melt rocks. The projectile types found for Rochechouart and Sääksjärvi are quite similar, and compatible with the composition of non-magmatic iron meteorites (IA and IIIC). This interpretation is based on: identical platinum group element patterns as well as peculiar Ni/Cr, Ni/Ir and Cr/Ir ratios, which can be explained by mixing of the different components of non-magmatic iron meteorites. This result indicates that, besides ordinary chondrites, non-magmatic iron may be among the most common material impacting the Earth, as they also represent the majority of the projectiles for craters smaller that 1.5 km. The abundance of non-magmatic irons as projectiles as well as their composition (olivine, pyroxene and iron) supports the assumption that a fraction of the S-type asteroids could by related to this type of material.  相似文献   

20.
The textures and mineral chemistries of silicate inclusions in the Udei Station (IAB) and Miles (fractionated IIE) iron meteorites were studied using optical and electron microscopy, SEM, EMPA, and LA-ICP-MS techniques to better understand the origin of silicate-bearing irons. Inclusions in Udei Station include near-chondritic, basaltic/gabbroic, feldspathic orthopyroxenitic, and harzburgitic lithologies. In Miles, most inclusions can be described as feldspathic pyroxenite or pyroxene-enriched basalt/gabbro. The trace-element compositions of both orthopyroxene and plagioclase grains are similar in different lithologies from Udei Station; whereas in different inclusions from Miles, the compositions of orthopyroxene grains are similar, while those of clinopyroxene, plagioclase, and especially Cl-apatite are variable. Orthopyroxene in Miles tends to be enriched in REE compared to that in Udei Station, but the reverse is true for plagioclase and clinopyroxene.The data can be explained by models involving partial melting of chondritic protoliths, silicate melt migration, and redox reactions between silicate and metal components to form phosphate. The extent of heating, melt migration, and phosphate formation were all greater in Miles. Silicates in Miles were formed from liquids produced by ∼30% partial melting of a chondritic precursor brought to a peak temperature of ∼1250 °C. This silicate melt crystallized in two stages. During Stage 1, crystallizing minerals (orthopyroxene, clinopyroxene, chromite, and olivine) were largely in equilibrium with an intercumulus melt that was evolving by igneous fractionation during slow cooling, with a residence time of ∼20 ka at ∼1150 °C. During Stage 2, following probable re-melting of feldspathic materials, and after the silicate “mush” was mixed with molten metal, plagioclase and phosphate fractionally crystallized together during more rapid cooling down to the solidus. In Udei Station, despite a lower peak temperature (<1180 °C) and degree of silicate partial melting (∼3-10%), silicate melt was able to efficiently separate from silicate solid to produce melt residues (harzburgite) and liquids or cumulates (basalt/gabbro, feldspathic orthopyroxenite) prior to final metal emplacement. Olivine was generally out of equilibrium with other minerals, but orthopyroxene and plagioclase largely equilibrated under magmatic conditions, and clinopyroxene in basalt/gabbro crystallized from a more evolved silicate melt.We suggest that a model involving major collisional disruption and mixing of partly molten, endogenically heated planetesimals can best explain the data for IAB and fractionated IIE silicate-bearing irons. The extent of endogenic heating was different (less for the IABs), and the amount of parent body disruption was different (scrambling with collisional unroofing for the IAB/IIICD/winonaite body, more complete destruction for the fractionated IIE body), but both bodies were partly molten and incompletely differentiated at the time of impact. We suggest that the post-impact secondary body for IAB/IIICD/winonaite meteorites was mineralogically zoned with Ni-poor metal in the center, and that the secondary body for fractionated IIE meteorites was a relatively small melt-rich body that had separated from olivine during collisional break-up.  相似文献   

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