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1.
Neutron activation analysis was used to determine As, Au, Bi, Cd, Co, Cu, Ga, In, Sb, Se, Te, Tl and Zn in 13 different unequilibrated ordinary chondrites (UOC), i.e. those having chemicallyinhomogeneous silicates. This study together with prior data completes our coverage of this group of 23 primitive chondrites. Four elements are quite variable in UOC (Cd—20 x, In—30 x, Bi—300 x and Tl—1300 x), the others varying by 2–8 x. Three highly-depleted elements—Bi, In and Tl—are richer by 5–35 x in unequilibrated chondrites than in their equilibrated congeners. All 3 elements vary directly in characteristic fashion with disequilibrium parameters for olivine and pyroxene in UOC and generally with petrologic type 3 > 4 > 5 > 6. The data do not provide unambiguous evidence for nebular fractionation of siderophile elements. Examination of statistically-significant interelement relationships among various ordinary chondrite populations involving 34 elements reveals patterns distinct from those of other chondritic groups. These patterns reflect nebular metal-silicate fractionation which preceded or accompanied thermal fractionation. The results point to significant differences in the formation of primitive carbonaceous, enstatite and ordinary chondrites.  相似文献   

2.
The results of a search for significant (95 % confidence level) inter-element relationships among 13 trace elements in carbonaceous chondrites and 26 elements and the disequilibrium parameter for silicate phases in unequilibrated ordinary chondrites (UOC) indicate pronounced differences in the formation processes of these two sorts of primitive chondrites. Twenty-six pairs of elements are correlated in carbonaceous chondrites and these correlations lend support to a model involving mixing in different ratios of material differing in thermal history.Comparison of the 26 elements in UOC shows that 39 pairs of elements are significantly related and only very volatile elements are correlated with the disequilibrium parameter. Each of the inter-element relationships can be specifically ascribed to a metal-silicate fractionation in the solar nebula or to a thermal fractionation. These relationships are about equally consistent with the metamorphism, two-component condensation and simultaneous accretion-condensation models for the origin, of the ordinary chondrites, each requiring adoption of specific ad hoc assumptions for complete consistency.  相似文献   

3.
Neutron activation analysis was used to determine As, Au, Bi, Cd, Co, Cu, Ga, In, Sb, Se, Te, Tl and Zn in 11 samples representing 9 chondrites of grades E4–6. These chondrites exhibit systematic intra- and inter-grade differences particularly for highly-variable elements, the differences being E4 ? E3 > E6 ? E5. The abundance pattern for these 13 and an additional 16 elements in E3-6 chondrites differs from those of other primitive meteorites—the carbonaceous and unequilibrated ordinary chondrites. A search for statistically-significant interelement relationships among the 13 elements (for grades E4–6) reveal that 40 elementpairs are linearly and/or exponentially correlated. Similar consideration of data for 37 elements in 12 chondrites (grades E3–6) reveals that 191 element-pairs exhibit such relationships, 170 involving linear and/or exponential correlations, the remainder involving anti-correlations. The patterns depicting these relationships—i.e. the correlation profiles—and elemental abundance patterns, factor analysis and two-element correlation diagrams are consistent with all enstatite chondrites representing a single evolutionary sequence. The primary process responsible for the chemical trends of these chondrites involved thermal fractionation accompanied by geochemical fractionation of sulfide-plus-metal from silicate, probably during condensation and accretion of solid material from the solar nebula. Chalcophile elements may have been fractionated during condensation or, after accretion, during thermal metamorphism in the parent body. No genetic model proposed thus far accounts for the detailed chemical trends, although the constrained equilibrium theory and two-component condensation theories qualitatively seem most satisfactory. The correlation profiles of enstatite, carbonaceous and unequilibrated ordinary chondrites are distinctly different, pointing to major differences in the formation conditions of these different sorts of primitive meteorites.  相似文献   

4.
We report RNAA results for Co, Au, Sb, Ga, Rb, Cs, Se, Ag, Te, Zn, In, Bi, Tl and Cd (in increasing order of metamorphic mobility) in 22 Antarctic unequilibrated ordinary chondrites (UOC). This brings to 38 the number of UOC for which data for highly volatile elements are known. For elements of lesser mobility (Co to Se, omitting Cs) overall variability in UOC are low, relative standard deviations (one sigma) being no more than a factor of two. For Ag, Te and Zn, relative standard deviations are 2-4×, while for Cs and the four most volatile elements, the variabilities are 8-110×. Elemental abundances do not vary with chemical type (H, L and LL) nor with UOC subtype (3.0-3.9). Contents of all elements reach levels up to, even exceeding, cosmic and all but Cd and the two alkalis, seem unaffected by post-accretionary processes. Contents of highly volatile elements are consistent with the idea that source regions producing contemporary falls and older Antarctic UOC differed in thermal histories. The presence or absence of carbide magnetite assemblages (CMA) generally accords with high or low Cd contents, respectively. This relationship accords with the prior suggestion that CMA formed by alteration of Fe-Ni metal by C-O-H-containing fluids at temperatures <700 K, generated by thermal metamorphism in parent body interiors. The absence of CMA in most UOC (and OC), may indicate that they were subsequently destroyed as metamorphic intensity increased. The high, often supercosmic, Rb and Cs levels in UOC may result from their high solubility in liquid water signalling their redistribution by C-O-H-containing fluid while in the liquid water field. Because of its uniquely high mobility, Cd could have been enriched by the C-O-H fluids and should have been lost from parent regions during later, higher temperature anhydrous metamorphism at temperatures in the 500-600 °C range.  相似文献   

5.
Bulk compositions of metallic Fe-Ni from two equilibrated ordinary chondrites, Jilin (H5) and Anlong (H5), and two unequilibrated ones, GRV 9919 (L3) and GRV 021603 (H3), were analyzed by inductively coupled plasma mass spectrometry (ICP-MS). The CI-, Co-normalized abundances of siderophile and chalcophile elements of metallic Fe-Ni from the unequilibrated ordinary chondrites correlate with 50% condensation temperatures (i.e., volatility) of the elements. The refractory siderophile elements (i.e., platinum group elements, Re), Au, Ni and Co show a flat pattern (1.01×CI Co-normalized), while moderate elements (As, Cu, Ag, Ga, Ge, Zn) decrease with volatility from 0.63×CI (Co-normalized, As) to 0.05×CI (Co-normalized, Zn). Cr and Mn show deficit relative to the trend, probably due to their main partition in silicates and sulfides (nonmagnetic). Metallic Fe-Ni from the equilibrated ordinary chondrites shows similar patterns, except for strong deficit of Cr, Mn, Ag and Zn. It is indicated that these elements were almost all partitioned into silicates and/or sulfides during thermal metamorphism. The similar deficit of Cr, Mn, Ag and Zn was also found in iron meteorites. Our analyses demonstrate similar behaviors of W and Mo as refractory siderophile elements during condensation of the solar nebula, except for slight depletion of Mo in the L3 and H5 chondrites. The Mo-depletion of metallic Fe-Ni from GRV 9919 (L3) relative to GRV 021603 (H3) could be due to a more oxidizing condition of the former than the latter in the solar nebula. In contrast, the Mo-depletion of the metallic Fe-Ni from the H5 chondrites may reflect partition of Mo from metal to silicates and/or sulfides during thermal metamorphism in the asteroidal body.  相似文献   

6.
The Plainview. Texas, meteorite is a polymict-brecciated H-group chondrite composed of recrystallized light-colored portions embedded in a well-compacted, dense, somewhat recrystallized, dark-colored matrix. Both portions consist of equilibrated silicates (H5 classification), but a small number of silicate grains and unequilibrated lithic fragments not compatible with equilibrated ordinary H-group material are present in the dark-colored matrix. Lithic fragments include: (i) dark-colored, more or less altered, type II carbonaceous chondrites. (ii) unequilibrated ordinary chondrites and (iii) light-colored, unequilibrated and equilibrated fragments, some of which are compositionally similar to the host. Also present are fragment-like dark areas that are highly-shocked host material and not true lithic fragments (pseudo-fragments). Conclusions: Plainview represents a complex regolith breccia formed by repeated impact episodes. Recrystallized, light-colored portions represent surface or near-surface material of a small (asteroidal-sized) parent body. Impacts broke up this material to form fine-grained, dark material which enclosed light-colored protolith. Lithic fragments (i-iii) and some unequilibrated silicate grains and chondrules (apparently derived from unequilibrated chondrites) were embedded in the dark matrix during these repeated impacts. Xenolitlils of carbonaceous and unequilibrated ordinary chondrites are either residues of projectiles that impacted the Plainview parent body, or material from coexisting regoliths impact-splashed into Plainview regolith. Chondrules and silicate grains in the dark matrix which differ from H-group material are likely related to these xenoliths and their regoliths. Light-colored lithic fragments may represent shock-melted chondritic material, sometimes compositionally-modified, or new, achondritic meteoritic types. Unequilibrated and carbonaceous lithic fragments in the dark-colored host matrix indicate that equilibration of the host occurred before incorporation of the fragments and that compaction and lithification of the Plainview regolith to form a coherent meteorite must have occurred at temperatures below 300°C and/or on a short time scale.  相似文献   

7.
Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.  相似文献   

8.
Seventeen trace elements (Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Rb, Re, Sb, Se, Te, Tl, U and Zn) were measured by neutron activation analysis in 8 C1 samples (1 Alais, 3 Ivuna, 4 Orgueil) and in 3 C2 samples (one each of Mighei, Murchison, Murray). The results show far less scatter than earlier literature data. The standard deviation of a single measurement from the mean of 8 C1 samples lies between 2 and 14 per cent, except for the following 4 elements: Au ±18 per cent, Ag ±22 per cent, Rb ±19 per cent and Br ±33 per cent. The first two probably reflect contamination and sample heterogeneity, the last two, analytical error. Apparently C1 chondrites have a far more uniform composition than some authors have claimed.The new data suggest significant revisions in cosmic abundance for the following elements (old values in parentheses): Zn 1250 (1500), Cd 1.51 (2.12), Ir 0.72 (0.43) atoms/106 Si atoms. The Br value is also lower, 6.8 vs 20.6, but may be affected by analytical error.Relative to C1 chondrites, the C2 chondrites Mighei, Murchison and Murray are depleted in volatile elements by a factor of 0.508 ± 0.038, much more constant than indicated by oldor data. Ordinary chondrites also show a more uniform depletion relative to the new C1 data. The mean depletion factor of Sb, F, Cu, Ga, Ge, Sn, S, Se, Te and Ag is 0.227 ± 0.027 in H-chondrites. This constancy further strengthens the case for the two-component model of chondrite formation.  相似文献   

9.
Three of the most highly metamorphosed meteorites of their respective classes, Shaw (LL7), Karoonda (C5), and Coolidge (C4), were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Te, Tl, U, and Zn. Comparison with data by Lipschutz and coworkers on artificially heated primitive meteorites shows that the natural metamorphism of meteorites cannot have taken place in a system open to volatiles. Shaw, metamorphosed at 1300°C for >106 yr, is less depleted in In, Bi, Ag, Te, Zn, and Tl than Krymka heated at 1000°C for 1 week. Karoonda, metamorphosed at 600°C for many millennia, is less depleted in Bi and Tl than Allende heated at 600°C for 1 week.Data on primordial noble gases also show that the volatile-element patterns of ordinary and carbonaceous chondrites were established by nebular condensation, and changed little if at all during metamorphism. For enstatite chondrites, the evidence is still incomplete, but seems to favor a nebular origin of the volatile pattern.The general constancy of Tl/Rb, Tl/Cs and Tl/U ratios in terrestrial and lunar rocks suggests that loss of volatile metals such as Tl is rare during normal magmatism or metamorphism. Only impact melts show such loss with any frequency.  相似文献   

10.
The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.  相似文献   

11.
High-precision Zn isotopic variations are reported for carbonaceous chondrites (CC), equilibrated (EOC) and unequilibrated (UOC) ordinary chondrites, iron meteorites from the IAB-IIICD (nonmagmatic) and IIIA (magmatic) groups, and metal from the Brenham pallasite. For irons, δ65Cu values are also reported. Data have also been obtained on a coarse-grained type-B calcium-, aluminum-rich refractory inclusion (CAI) from Allende and on acid leaches of Allende (CV3), Krymka (LL3), and Charsonville (H6). Variations expressed as δ66Zn (deviation in parts per thousand of 66Zn/64Zn in samples relative to a standard) spread over a range of 0.3‰ for carbonaceous chondrites, 2‰ for ordinary chondrites, and 4‰ for irons.The measured 66Zn/64Zn, 67Zn/64Zn, and 68Zn/64Zn ratios vary linearly with mass difference and define a common isotope fractionation line with terrestrial samples, which demonstrates that Zn was derived from an initially single homogeneous reservoir. The δ66Zn values are correlated with meteorite compositions and slightly decrease in the order CI, CM, CV-CO, and to UOC. The isotopically light Zn of Allende CAI and the acid-resistant residues of Allende and Krymka show that the light component is associated with refractory material, presumably minerals from the spinel-group. This, together with the reverse correlation between relative abundances of light Zn isotopes and volatile element abundances, suggests that Zn depletion in planetary bodies with respect to CI cannot be ascribed to devolatilization of CI-like material. These observations rather suggest that refractory material reacted with a gas phase enriched in the lighter Zn isotopes. Alternatively, chondrules with their associated rims should carry a light Zn isotopic signature. The δ66Zn values of unequilibrated chondrites are rather uniform, whereas equilibrated chondrites show distinctly more isotopic variability.The values of δ65Cu-δ66Zn in irons define two trends. The moderate and positively correlated Cu and Zn isotope variations in IIIA and pallasite samples probably reflect crystallization of silicate, sulfide, and solid metal from the liquid metal. The range of δ66Zn values of the IAB-IIICD group is large (>3‰) and contrasts with the moderate fractionation of Cu isotopes. We interpret this feature and the negative δ66Zn-δ65Cu correlation as reflecting mixing, possibly achieved by percolation, between metals from a regolith devolatilized at low temperature (enriched in heavy zinc) and metallic liquids formed within the parent body.  相似文献   

12.
The abundances of Fe, Ni, Co, Au, Ir, Ga, As and Mg have been determined by instrumental neutron activation analysis in 38 type 3 ordinary chondrites (10 of which may be paired) and 15 equilibrated chondrites. Classification of type 3 ordinary chondrites into the H, L and LL classes using oxygen isotopes and parameters which reflect oxidation state (Fa and Fs in the olivine and pyroxene and Co in kamacite) is difficult or impossible. Bulk compositional parameters, based on the equilibrated chondrites, have therefore been used to classify the type 3 chondrites. The distribution of the type 3 ordinary chondrites over the classes is very different from that of the equilibrated chondrites, the LL chondrites being more heavily represented. The type 3 ordinary chondrites contain 5 to 15 percent lower abundances of siderophile elements and a compilation of the present data and literature data indicates a small, systematic decrease in siderophile element concentration with decreasing petrologic type. The type 3 ordinary chondrites have, like the equilibrated ordinary chondrites, suffered a fractionation of their siderophile elements, but the loss of Ni in comparison with Au and Ir is greater for the type 3 chondrites. These siderophile element trends were established at the nebula phase of chondritic history and the co-variation with petrologic type implies onion-shell structures for the ordinary chondrite parent bodies. It is also clear that the relationship between the type 3 and the equilibrated ordinary chondrites involves more than simple, closed-system metamorphism.  相似文献   

13.
We determined ten trace elements by neutron activation analysis in Tieschitz (H3) chondrite powder heated in a low-pressure environment (initially ~ 10?5 atm H2) for 1 week at 100°C increments from 400–1000°C. Of these, Co seems unaffected by heating, 20% of Ga is lost only at 1000°C and losses of other elements progress with temperature to extremes of 25% for Se, 75% for Cs and 90–97% for Ag, Bi, In, Te, Tl and Zn. Treating elemental mobilization as kinetically-controlled by diffusion from spherical grains of uniform size, Ag, Cs, In and Se are lost from a single site by a single process while Bi, Te, Tl and Zn are lost from two sites or from one site by different processes at high and low temperatures. Magnitudes of apparent activation energies for loss of the first four elements at all temperatures and the last four at low temperatures are consistent with volume diffusion; at high temperatures Bi, Te, Tl and Zn are lost by a low-energy process, like desorption.We compared trace element abundances, patterns of statistically-significant correlations, factor analysis and two-element correlations between Tieschitz and heated Krymka (L3) and, except for factor analysis, “as-received” H3–6 chondrites. Trends for heated ordinary chondrites are similar though small differences occur; those for Tieschitz and H3–6 chondrites differ markedly indicating that H3–6 chondrites—unlike E3–6 chondrites—probably escaped substantial open-system metamorphism. Sharp contrasts in pictures for E-, L- and H-group chondrites indicate substantial differences in genetic histories.  相似文献   

14.
The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and 187Os/188Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, 187Os/188Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.  相似文献   

15.
We have attempted to clarify the nature of “mysterite”, a material that had been postulated to explain the overabundance of Tl, Bi and Ag in certain chondrites. Four dark clasts and a vein sample from the H6 chondrite Supuhee were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Br, Cd, Cs, Ge, In, Ir, Ni, Os, Rb, Re, Sb, Se, Te, Tl and Zn. One of the clasts is enriched in all volatile elements, while the other 4 samples are enriched only in the siderophile volatiles Ag, Bi and Tl. The enrichments range up to 100 times typical H6 chondrite abundances. The proportions of Ag, Bi, Tl suggest the presence of at least two, Tl-rich and Tl-poor, varieties of mysterite (TlBi = 7.2 and <0.1). The former seems to dominate in Supuhee and Krymka, and the latter in Mezö-Madaras. Apparently mysterite is a late condensate from the solar nebula that collected volatiles left behind by earlier generations of chondrites. It was incorporated in Supuhee and perhaps in other chondrites (mainly of low petrologic types) during brecciation events.  相似文献   

16.
Six C2M chondrites (Boriskino, Cold Bokkeveld, Erakot, Essebi, Haripura and Santa Cruz) and the C2R chondrite Al Rais were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Sn, Te, Tl, U, and Zn. Abundances (relative to Cl chondrites) show a systematic dependence on volatility, apparently reflecting volatile loss during formation of chondrules and other high-T components. Elements of nebular condensation temperature (Tc) > 1200 K are undepleted, those of Tc < 700 K are depleted by a constant factor (0.482 ± 0.049 for C2M's) and elements of intermediate volatility are depleted by intermediate factors. The abundances do not “tend to fall monotonically as a function of [Tc],” as previously claimed by Wai and Wasson (1977) for a more restricted temperature range. For meteorites that have suffered little aqueous alteration (Mighei, Murchison, Murray), the mean abundance of volatiles agrees with the matrix content, but for the more altered meteorites, matrix contents are 20–30% higher. Only a few meteorites deviate appreciably from the mean abundance pattern. Al Rais, a C2R chondrite with a significant metal content, is systematically lower in 12 volatiles, but is enriched in Ni and Pd. Haripura and Erakot are enriched in Bi and Tl, possibly from the late condensate, mysterite.  相似文献   

17.
We determined Ag, Bi, Cd, Co, Cs, Ga, In, Se, Te, Tl and Zn in the 6 ureilite achondrites by neutron activation analysis. All 11 elements are depleted below Cl levels and their characteristic abundance pattern differs substantially from those of chondritic groups. Thus ureilites do not represent a simple mixture of volatile-rich chondrites with achondritic material but perhaps cosmochemically-fractionated achondritic material and a late ‘distillate’ of mobile elements.  相似文献   

18.
We used neutron activation analysis to determine ten trace elements retained in Abee (E4) samples heated at 400–1000°C for 1 week in a low-pressure (initially ~ 10?5atm H2) environment. Eight elements generally are lost progressively with increasing temperature although gas(es) evolved from the samples apparently affect retention of some elements. In the extreme, ‘open-system’ losses are: Se—23%, Cs—40%; Te—87%; Ag, Bi, In, Tl, Zn— ≥93%. Under these conditions Co is not lost; Ga is lost only at 1000°C. At 900°C elements are lost from Abee chips in the same relative order as from Abee powder but the loss is somewhat less facile. Three of the most mobile elements—Bi, In, Tl—are lost more readily from Abee than from Allende (C3), the only other primitive chondrite studied to date. Assuming that elemental loss is a kinetic process involving mobilization from spherical grains, Bi, In, Se, Tl and Zn have different activation energies at high and low temperatures either because each element was originally present in two different sites or each has more than one loss mechanism (diffusion or desorption) in different temperature ranges.Comparison of elemental abundance patterns, patterns of statistically-significant correlations, factor analysis results and two-element correlation diagrams indicate strong similarities between heated Abee and ‘as-received’ enstatite chondrites for mobile elements. These results are consistent with a two-stage evolutionary model for enstatite chondrites involving condensation of cosmochemically fractionated primitive nebular material and subsequent loss of mobile elements from parent material by metamorphism.  相似文献   

19.
Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1–2 per cent). Certain carbonaceous chondrite specimens showed small positive irregularities in Yb abundance. The Yb ‘anomaly’ (approximately + 5 per cent relative to the average of 10 ordinary chondrites) in Orgueil may relate to high temperature components. The REE pattern of Guareña (H6) exhibits comparatively extensive fractionation (about factor 2) with a negative anomaly for Eu (17 ± 1 percent) compared to the average H chondrite. This could be interpreted in terms of extensive thermal metamorphism leading to melting.Apart from absolute abundance differences, there appears to be small but recognizable fractionation among the average relative REE abundances of Cl, E, H and L chondrites. However, individual chondrites within these groups showed more or less fractionated REE patterns relative to each other. The distinction between H and L chondrites was well demonstrated in Eu-Sm correlation curves and absolute abundance differences of REE and major elements.Si-normalized atomic ratios of the REE abundances in different kinds of chondrites to those in Orgueil (Cl) chondrite were 0.58 (E), 0.75 (H), 0.81 (L), 1.07 (C2) and 1.32 (C3).  相似文献   

20.
Six Chinese ordinary chondrites (four of them have fallen in recent years and the trace element abundances have not yet been reported for the other two) were examined.The contents of 21 elements (Na,Cr,Mn,Sc,Se,Zn,Br,Ni,Fe,Co,Ir,Cu,Ga,As,Au,Sb,Os,W,Re,Pt,and Ru)in the magnetic fractions and 20 elements (Na,K,Ca,Sc,Cr,Mn,Fe,Co,Ni,Zn,Se,Br,La,Sm,Eu,Yb,Lu,Ir,Au,and As) in the non-magnetic fractions were de-termined by INAA. The results indicate that the 5 H-group chondrites show almost no difference in composition,but they are different from the Zhaodong L-group chondrite in elemental abundance.As a normalized element(relative to CI),the concentrations of Ga in the magnetic fractions can be used to classify ordinary chondrites(H-,L- and LL-group).The bulk composition and modal weight of each component calculated from element concentrations in different phases are in good agreement with the bulk rock analyses presented in the literature.  相似文献   

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