The unique lunar composition and its bearing on the origin of the Moon     

《Geochimica et cosmochimica acta》1987,51(5):1297-1309

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The bulk composition of Mars     

G. Jeffrey Taylor 《Chemie der Erde / Geochemistry》2013

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.  相似文献   

Noble gases in E-chondrites     

Jane Crabb  Edward Anders 《Geochimica et cosmochimica acta》1981,45(12):2443-2464

Noble gas data are reported for 12 E-chondrites. Combined with literature data, they show that K-Ar ages are >4 Æ for 14 out of 18 meteorites, yet U, Th-He ages are often shorter, perhaps due to late, mild reheating. Cosmic-ray exposure ages differ systematically between types 4 and 6, with E4's mostly below 16 Myr and E6's above 30 Myr. This may mean that the E-chondrite parent body contains predominantly a single petrologic type on the (~ 1 km) scale of individual impacts, in contrast to the more thoroughly mixed parent bodies of the ordinary chondrites.The heavy noble gases consist of at least two primordial components: the usual planetary component (${}^{36}\text{Ar}{}^{132}\text{Xe}\text{~ 80}$) and a less fractionated, ‘subsolar’ component ($\text{2700 ≤}{}^{36}\text{Ar}{}^{132}\text{Xe}\text{≤ 3800}$). The latter is found in highest concentration in the E4 chondrite South Oman (³⁶Ar = 760 × 10^?8cc/g, ${}^{36}\text{Ar}{}^{132}\text{Xe}\text{= 2700}$). The isotopic compositions of both components are similar to typical planetary values, indicating that some factor other than mass controlled the noble gas elemental ratios. The heavy Xe isotopes occasionally show some of the lowest ${}^{134}\text{Xe}{}^{132}\text{Xe}$ and ${}^{136}\text{Xe}{}^{132}\text{Xe}$ ratios measured in bulk chondrites, suggestive of nearly fission-free Xe (e.g. ${}^{136}\text{Xe}{}^{132}\text{Xe}\text{= 0.3095 ± 0.0020}$). Amounts of planetary gas in E4 E6 chondrites fall in the range for ordinary chondrites of types 4–6, but, in contrast to the ordinary chondrites. fail to correlate with petrologic type or volatile trace element contents. Another unusual feature of E-chondrites is that primordial Ne is present even in most 4's and 5's (²⁰Ne_p ~ 1 to 7 × 10^?8cc/g). with an isotopic composition consistent with planetary Ne.Analyses of mineral separates show that the planetary gases are concentrated in an HF- and HCl-insoluble mineral similar to phase Q, the poorly characterized, HNO₃-soluble carrier of primordial gases in carbonaceous and ordinary chondrites. The subsolar gases, on the other hand, are located in an HCl- and HNO₃-resistant phase, possibly enstatite or a minor phase included in enstatite. Much of the ¹²⁹Xe_r (50% for E4's, > 70% for E6's) is in HCl-resistant but HF-soluble sites, suggestive of a silicate.A similar subsolar component may be responsible for the high ${}^{36}\text{Ar}{}^{132}\text{Xe}$ ratios of some C3's, unequilibrated ordinary chondrites, and the unique aubrite Shallowater. The planet Venus also has a high $\text{Ar}\text{Kr}$ ratio, well above the planetary range, and hence may have acquired its noble gases from an E-chondrite-like material, similar to South Oman.  相似文献   

“Mysterite” : a late condensate from the solar nebula     

H. Higuchi  R. Ganapathy  John W. Morgan  Edward Anders 《Geochimica et cosmochimica acta》1977,41(7):843-852

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 ($\text{Tl}\text{Bi}\text{= 7.2}$ 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   

Lithium,sodium and potassium abundances in carbonaceous chondrites     

Walter Nichiporuk  Carleton B Moore 《Geochimica et cosmochimica acta》1974,38(11):1691-1701

Concentrations of lithium, sodium, and potassium in 18 carbonaceous chondrites were determined in the same sample solution by atomic absorption. Mean abundances in carbonaceous Type I chondrites are, in atoms 10⁶ Si: Li = 60.1, Na = 5800, K = 3700. Relative to Type I carbonaceous chondrites, abundances in Type II's are: Li = 0.87, Na = 0.61, K = 0.58; and in Type III's Li = 0.82, Na = 0.49, K = 0.36. Evidently there is a differential depletion of potassium relative to sodium in Type III's, suggesting a fractionation after accretion.  相似文献   

Derivation of a heterogeneous lithic fragment in the Bovedy L-group chondrite from impact-melted porphyritic chondrules     

Alan E Rubin  Klaus Keil  G.Jeffrey Taylor  M.-S Ma  R.A Schmitt  D.D Bogard 《Geochimica et cosmochimica acta》1981,45(11):2213-2228

The Bovedy L-group chondrite contains a light-colored poikilitic lithic fragment with olivine, low-Ca pyroxene and kamacite compositions characteristic of porphyritic chondrules from unequilibrated ordinary chondrites. Its texture, compositional similarities to porphyritic chondrules, and low Na₂O, K₂O and P₂O₅ content indicate that the fragment represents a solidified, slightly fractionated impact melt formed from a source that was rich in porphyritic chondrules. The fragment is heterogeneous, with a progressive increase in the bulk $\text{MgO}\text{FeO}$ ratio and in MgO content of olivines and low-Ca pyroxenes across its length. ${}^{39}\text{Ar}{}^{40}\text{Ar}$ analyses of the fragment and host indicate that the meteorite experienced extensive degassing due to reheating. The approximate age of 0.5–0.94 Byr dates the reheating event and not the formation of the lithic fragment or the Bovedy breccia. This reheating event renders the fragment's and host's metallographic cooling rate of ~ 5 C/Myr (through 500°C) imprecise. However, the absence of martensite and the presence of kamacite. zoned taenite and tetrataenite in the fragment and host are consistent with such slow cooling through 500°C. This cooling rate must have resulted from burial of the fragment-host assemblage beneath insulating material on the Bovedy parent body. If the thermal diffusivity (κ) of this overburden was approximately comparable to that of the lunar regolith (10^?4cm²/sec), then the fragment was buried at a depth ≌ 6.5 km; if K = 10^?2 cm²/sec (similar to chondritic material), then the fragment was buried at a depth ?65 km.  相似文献   

Chemical studies of L chondrites—II. Shock-induced trace element mobilization     

Thomas M. Walsh  Michael E. Lipschutz 《Geochimica et cosmochimica acta》1982,46(12):2491-2500

Previous studies of chondrites heated in the laboratory for extended periods under conditions approximating those in shock-heated collisional debris indicate that Au, Co, Se, Ga, Rb, Cs, Te, Bi, In, Ag, Zn, Tl and Cd progress in mobility. We report data for these 13 trace elements in 14 L4–6 chondrites of established shock history and discuss these and 13 additional chondrites studied earlier. Trace element contents vary with petrologic type, $\text{S}\text{Fe}$ sub-group and shock history, the last dominating strongly. Absolute abundances and interelement relationships for the 6 or 7 most mobile elements vary with degree of shock-loading (i.e. residual temperatures) established from mineralogic/petrologic study. A tertiary process, shock-heating, previously known to have affected radiogenic ⁴⁰Ar and/or ⁴He in meteorites but not other elements, apparently was at least as effective as other open-system processes (secondary [parent body] and primary [nebular and/or accretionary] episodes) in establishing mobile trace element contents of L chondrites and probably others. If conditions during early genetic episodes are to be deduced from compositional information, shocked meteorites should be avoided or effects of later processes should be compensated for.  相似文献   

Al-Sm-Eu-Sr systematics of eucrites and Moon rocks: Implications for planetary bulk compositions     

Paul H Warren 《Geochimica et cosmochimica acta》1983,47(9):1559-1571

Analysis of the Eu and Sr “anomalies” of eucrites and lunar rocks allows constraints to be placed on the bulk compositions of the eucrite parent body (EPB) and the Moon. The elements Al, REE, and Sr, all are essentially incompatible with the major minerals of these small, low-?(O₂) bodies, except for plagioclase, into which Al, Sr, and Eu tend to concentrate. Therefore, the hypothesis that Al, REE, and Sr in the EPB and the Moon are all in proportions close to those in the bulk solar system (i.e., chondrites) leads to certain predictions about the concentrations of these elements in samples affected by plagioclase fractionation. The predictions are almost ideally fulfilled by eucrites and lunar samples. For the EPB, the ratios $\text{REE}\text{Al}$, $\text{Sr}\text{Al}$, and $\text{Sr}\text{REE}$ are constrained to be probably within 10%, almost certainly within 20%, of the chondritic ratios. For the more complicated Moon, the constraints are less precise: $\text{REE}\text{Al}$ is very probably within 25% of chondritic; $\text{Sr}\text{Al}$ and $\text{Sr}\text{REE}$ are probably within 35% of chondritic. These findings are proof that there is a strong similarity between the bulk compositions of the planets and the compositions of chondritic meteorites.The eucrites' Sm-Eu-Sr systematics are also valuable sources of constraints on the distribution coefficients for Eu and Sr into plagioclase, at low ?(O₂). From the slope of data for noncumulate eucrites on a Eu-Sm plot, D(Eu,pl/liq) can be inferred to be 1.1_?0.1^0.2. From the slope on a Sr-Sm plot, D(Sr,pl/liq)) can be inferred to be 1.5 ± 0.3. In the case of D(Eu), this is in excellent agreement with experimental data. In the case of D(Sr), the empirical result is probably more appropriate for eucritic systems than most experimental data, which, due to compositional effects, scatter widely.  相似文献   

${}^{40}\text{Ar}{}^{39}\text{Ar}$ dating,Ar diffusion properties,and cooling rate determinations of severely shocked chondrites     

D.D. Bogard  W.C. Hirsch 《Geochimica et cosmochimica acta》1980,44(11):1667-1682

Determinations of ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex ${}^{40}\text{Ar}{}^{39}\text{Ar}$ release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of ⁴⁰Ar by the event, but retained from ~ 2 to ~45% of their ⁴⁰Ar. When calculated values of the diffusion parameter, $\text{D}\text{a}{}^2$, for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct $\text{K}\text{Ca}$ ratios and different average temperatures for Ar release. The experimentally determined values of $\text{D}\text{a}{}^2$ for the high temperature phase of several severely shocked chondrites are ~10^?7 to 10^?5sec^?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, $\text{D}\text{a}{}^2$ values, and fraction of ⁴⁰Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10^?2 10^?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from ⁴⁰Ar and Ni disagree by a factor of ~10⁵. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time.  相似文献   

Chemical fractionations in meteorites—VII. Cosmothermometry and cosmobarometry     

John W. Larimer 《Geochimica et cosmochimica acta》1973,37(6):1603-1623

The fractional condensation of Bi, Cd, In, Pb and Tl from a cooling gas of cosmic composition is calculated. Predicted absolute and relative abundances of the elements are in good to excellent agreement with the analytical data. This strongly suggests that the presently observed abundances were established at the time of accretion. There is no need to invoke non-equilibrium during condensation or element redistribution after accretion to explain the observations. The elements may therefore be used as cosmothermometers to predict accretion temperatures.Calculated accretion temperatures fall in the range of 420 to 540°K. But a large percentage of each chrondrite group (H, L, LL and E) fall within much narrower intervals, ≤20°K. This implies that the bulk of each group accreted over a narrow temperature range which is consistent with their uniform oxidation states and $\text{O}{}^{18}\text{O}{}^{16}$ ratios. In fact, temperatures inferred from the oxidation state and oxygen isotopes are in excellent agreement with the trace element data.The condensation curves of all these elements are pressure-dependent but are confined to fall in the temperature interval 400 to 600°K owing to the absence of Fe₃O₄ and the presence of FeS in ordinary chrondrites. Absolute upper and lower limits on the total pressure can thus be deduced: 10^?3 to 10^?6 atm. In addition, the condensation curves for Bi and In cross over at T = 462°K and P_t = 2 × 10^?5 atm. The observed relative abundances of Bi and In suggest the L-group formed at slightly lower and the H-group at slightly higher P and T.  相似文献   

Moon and Earth : compositional differences inferred from siderophiles,volatiles, and alkalis in basalts     

Rainer Wolf  Edward Anders 《Geochimica et cosmochimica acta》1980,44(12):2111-2124

We have compared RNAA analyses of 18 trace elements in 25 low-Ti lunar and 10 terrestrial oceanic basalts. According to Ringwood and Kesson, the abundance ratio in basalts for most of these elements approximates the ratio in the two planets.Volatiles (Ag, Bi, Br, Cd, In, Sb, Sn, Tl, Zn) are depleted in lunar basalts by a nearly constant factor of 0.026 ± 0.013, relative to terrestrial basalts. Given the differences in volatility among these elements, this constancy is not consistent with models that derive the Moon's volatiles from partial recondensation of the Earth's mantle or from partial degassing of a captured body. It is consistent with models that derive planetary volatiles from a thin veneer (or a residuum) of C-chondrite material; apparently the Moon received only 2.6% of the Earth's endowment of such material per unit mass.Chalcogens (Se and Te) have virtually constant and identical abundances in lunar and terrestrial basalts, probably reflecting saturation with Fe(S, Se, Te) in the source regions.Siderophiles show diverse trends. Ni is relatively abundant in lunar basalts (4 × 10^?3 × Cl-chondrites), whereas Ir, Re, Ge, Au are depleted to 10^?4?10^?5× Cl. Except for Ir, these elements are consistently enriched in terrestrial basalts: Ni 3 × , Re 370 ×, Ge 330 × , Au 9 × . This difference apparently reflects the presence of nickel-iron phase in the lunar mantle, which sequesters these metals. On Earth, where such metal is absent, these elements partition into the crust to a greater degree. Though no lunar mantle rock is known, an analogue is provided by the siderophile-rich dunite 72417 (~0.1% metal) and the complementary, siderophile-poor troctolite 76535. The implied metal-siderophile distribution coefficients range from 10⁴ to 10⁶, and are consistent with available laboratory data.The evidence does not support the alternative explanation advanced by Ringwood—that Re was volatilized during the Moon's formation, and is an incompatible element (like La or W⁴⁺) in igneous processes. Re is much more depleted than elements of far greater volatility: (Re/U)_Cl～- 4 × 10^?6 vs (T1/U)_Cl = 1.3 × 10^?4, and Re does not correlate with La or other incompatibles.Heavy alkalis (K, Rb, Cs) show increasing depletion with atomic number. Cs/Rb ratios in lunar basalts, eucrites, and shergottites are 0.44, 0.36, and 0.65 × Cl, whereas the value for the bulk Earth is 0.15–0.26. These ratios fall within the range observed in LL and E6 chondrites. supporting the suggestion that the alkali depletion in planets, as in chondrites, was caused by localized remelting of nebular dust (= chondrule formation). Indeed, the small fractionation of K, Rb and Cs, despite their great differences in volatility, suggests that the planets, like the chondrites, formed from a mixture of depleted and undepleted material, not from a single, partially devolatilized material.  相似文献   

On the calculation of cosmic-ray exposure ages of stone meteorites     

Philip J. Cressy  Donald D. Bogard 《Geochimica et cosmochimica acta》1976,40(7):749-762

Abundances of cosmic ray-produced noble gases and ²⁶Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of < 3 × 10⁶ yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Séverin chondrites are utilized to derive equations for normalizing the production rates of cosmogenic ³He, ²¹Ne, and ³⁸Ar in chondrites to an adopted ‘average’ shielding: ${}^{22}\text{Ne}{}^{21}\text{Ne}\text{= 1.114.}$ The measured unsaturated ²⁶Al concentrations and the calculated equilibrium ²⁶Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistry- and shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10^?8 cm³/g 10⁶ yr) are: ³He = 2.45,²¹Ne = 0.47, and ³⁸Ar = 0.069, which are ~ 25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.  相似文献   

Mn-Cr systematics of the early Solar System revisited     

A. Trinquier  J.-L. Birck  C. Göpel 《Geochimica et cosmochimica acta》2008,72(20):5146-5163

We have reinvestigated the Mn-Cr systematics in a number of primitive meteorites, differentiated planetesimals and terrestrial planets in order to address the chronology of the early stages of protoplanetary disk evolution and planetary formation. Our analytical procedure is based on the assumption of terrestrial abundances for ⁵⁰Cr and ⁵²Cr only; recognizing that a data reduction scheme based on Earth-like ⁵⁴Cr/⁵²Cr abundances in all meteorites is not tenable. Here we show that initial ε_⁵³Cr compositions of ⁵⁴Cr-rich and ⁵⁴Cr-poor acid leach fractions in the primitive carbonaceous chondrite Orgueil differ by 0.9ε, reflecting primordial mineral-scale heterogeneity. However, asteroidal processing effectively homogenized any ε_⁵³Cr variations on the planetesimal scale, providing a uniform present-day solar ε_⁵³Cr=0.20±0.10. Thus, our ⁵³Mn-⁵³Cr data argue against the previously suggested ⁵³Mn heliocentric gradient. Instead, we suggest that inner Solar System objects possessed an initially homogeneous ⁵³Mn/⁵⁵Mn composition, which determined by two independent means is estimated at (6.28 ± 0.66) × 10⁻⁶. Our revised Mn-Cr age for Ste. Marguerite (SM) metamorphism of 4562.9 ± 1.0 Ma is identical to the Pb-Pb age of SM phosphates. Using this age, we confirm that mantle differentiation of the eucrite parent body occurred 4564.9 ± 1.1 Ma ago, and revise the time interval between this event and CAI formation to 2.2 ± 1.1 Ma. We also constrain metamorphism in carbonaceous chondrites of type 2 and 3 to have occurred between 1 and 6 Ma after CAI formation. The ⁵³Mn-⁵³Cr correlation among chondrites, planetesimals and terrestrial planets (the eucrite parent body, Mars and Earth) provides evidence for Mn/Cr fractionation within the protoplanetary disk recorded by all precursor materials of the terrestrial planets and primitive asteroids. This fractionation appears to have occurred within 2 Ma of CAI formation.  相似文献   

The abundances of titanium,zirconium and hafnium in stony meteorites     

Masako Shima 《Geochimica et cosmochimica acta》1979,43(3):353-362

The concentrations of Ti, Zr and Hf have been determined, by a stable isotope dilution method, in 27 chondrites, seven achondrites and standard rock samples BCR-1 and W-1.Among all chondrites investigated, enstatite chondrite Abee is lowest in Ti atomic ratio compared with Si while all carbonaceous chondrites show higher values. The Zr contents are higher in CII and CIII chondrites, relative to the other groups of chondrites. There is a clustering of Ti and Zr within each group. The $\text{Zr}\text{Hf}$ ratios in CII, CIII. E and H chondrites are essentially the same, while that in the CI chondrite is lower and in L, LL and unequilibrated chondrites are higher.The concentrations of Ti, Zr, Hf and $\text{Ti}\text{Zr}$, $\text{Zr}\text{Hf}$ ratios in achondrites are variable, even among members of the same group.Based on these results, condensation models for these elements are discussed. The variable results for Ti, Zr and Hf in achondrites may be due to the reheating recrystallization and metamorphic processes.‘Cosmic atomic abundances’ of Ti, Zr and Hf are calculated as 2470, 11.2 and 0.185. respectively for Si = 10⁶ atoms.  相似文献   

Hf-W isotope systematics of chondrites, eucrites, and martian meteorites: Chronology of core formation and early mantle differentiation in Vesta and Mars     

T. Kleine  K. Mezger  H. Palme 《Geochimica et cosmochimica acta》2004,68(13):2935-2946

The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct ¹⁸²Hf-¹⁸²W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised ¹⁸²W/¹⁸⁴W of chondrites, and the combined chondrite data yield an improved ?_W value for chondrites of −1.9 ± 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 ± 1.4 Ma and suggest that core formation took place 0.9 ± 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ∼5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published ¹⁸²W and ¹⁴²Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic ¹⁴²Nd abundances and elevated ?_W values of ∼0.4. The ¹⁸²W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled ¹⁴²Nd-¹⁸²W excesses of 0.5-1 and 2-3 ? units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated ¹⁴²Nd/¹⁴⁴Nd ratios of 0.5-0.7 ? units and ?_W values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ∼15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ∼5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The ¹⁸²W/¹⁸⁴W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic ¹⁸²W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived ²⁶Al and ⁶⁰Fe were important heat sources for melting and core formation in asteroids.  相似文献   

Unequilibrated ordinary chondrites: A tentative subclassification based on volatile-element content     

Edward Anders  M.G. Zadnik 《Geochimica et cosmochimica acta》1985,49(5):1281-1291

For unequilibrated ordinary chondrites (= UOC), two measures of primitiveness are available: volatile content, in principle reflecting accretion conditions from the solar nebula, and metamorphism, reflecting reheating in the parent bodies. These two measures do not always correlate, and we have therefore developed a tentative classification scheme based on volatile content that complements the Searset al. (1980) scheme based on metamorphism. Like the latter, it subdivides type 3 chondrites on a scale of 3.0 to 3.9; the notation 3.4/0 indicates a meteorite that is subtype 3.4 according to metamorphism and 3.0 according to volatile content.The classification is based mainly on C and Xe—two elements that are little affected by shock-induced reheating—and to a lesser extent on Ar³⁶,Bi,In, and Tl. Of 22 meteorites considered, the majority have concordant classifications (±0.2) on the two scales. However, 5 meteorites are richer in volatiles than their metamorphic grade indicates: Sharps 3.4/0, ALHA 77011 3.5/0, Ngawi 3.6/3, ALHA 77299 3.7/4, and Mezö-Madaras 3.7/3. It remains to be seen whether these differences indeed denote a more primitive nature.Some new clues to the formation of chondrites may eventually come from Xe and C. Their concentrations in UOC's vary by more than 5×, but the $\text{Xe}\text{C}$ ratio remains nearly constant at 3.4 × 10^?3 of the solar-system ratio. Even the ratios for other chondrite classes differ only slightly from that for UOC's, e.g., C3O (1.5×) and E3,4 (0.4×). Either the 4 factors determining this ratio (T, t, P, and internal surface area of the carbon) varied in complementary fashion, or—more probably—they varied only slightly in the entire source region of chondrites.  相似文献   

Isotopic anomalies of noble gases in meteorites and their origins—IV. C3 (Ornans) carbonaceous chondrites     

Leo Alaerts  Roy S Lewis  Edward Anders 《Geochimica et cosmochimica acta》1979,43(9):1421-1432

The C3O chondrites Kainsaz, Lancé and Ornans were studied by an acid dissolution technique, to characterize the noble-gas components in 3 mineral fractions: HF, HCl-solubles (99% of the meteorite), chromite and carbon (0.3–0.9%), and ‘phase Q’, a poorly characterized trace mineral (0.05–0.4%) containing most of the Ar, Kr, Xe. For all fractions, gas contents decline in the order Kainsaz > Lancé > Ornans; this trend parallels volatile contents but not heterogeneity of olivine composition or degree of metamorphism and seems to reflect progressively higher condensation temperatures from the solar nebula.Solubles contain nearly unfractionated Xe, and show ${}^{136}\text{Ar}{}^{132}\text{Xe}$ ratios up to 850. Hence the high $\text{Ar}\text{Xe}$ ratios (200–400) of bulk C3O chondrites must be due to an HF-soluble mineral (possibly magnetite). Phase Q contains ordinary planetary gases and a Ne component of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 10.3 ± 0.4.}$Chromite and carbon contain Ne of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 8.6 ± 0.1}$ and ‘CCF’ xenon (a peculiar component of possibly fissiogenic origin, enriched in the heavy isotopes but accompanied by a component enriched in the light isotopes).In all primitive chondrites, both the amount and the chemical separability of CCFXe parallel the abundance of promordial noble gases and other volatiles, such as C, N, Tl, Bi and In. The close correlation of CCFXe with various properties of undoubtedly local origin (volatile content, petrologic type, presence of ferrichromite and carbon, etc.) is more consistent with a local than with an extrasolar origin of this component. A volatile superheavy element seems to be the most plausible source, but the evidence is not conclusive.  相似文献   

Solar-system abundances of the elements     

Edward Anders  Mitsuru Ebihara 《Geochimica et cosmochimica acta》1982,46(11):2363-2380

A new abundance table has been compiled, based on a critical review of all C1 chondrite analyses up to mid-1982. Where C1 data were inaccurate or lacking, data for other meteorite classes were used, but with allowance for fractionation among classes. In a number of cases, interelement ratios from meteorites or lunar and terrestrial rocks as well as solar wind were used to check and constrain abundances. A few elements were interpolated (Ar, Kr, Xe, Hg) or estimated from astronomical data (H, C, N, O, He, Ne).For most elements, the new abundances differ by less than 20% from those of Cameron (1982a). In 14 cases, the change is between 20 and 50% (He, Ne, Be, Br, Nb, Te, I, Xe, La, Gd, Tb, Yb, Ta and Pb) and in 5 others, it exceeds 50% (B, P, Mo, W, Hg). Some important interelement ratios ($\text{Na}\text{K}$, $\text{Se}\text{Te}$, $\text{Rb}\text{Sr}$, $\text{Kr}\text{Xe}$, $\text{La}\text{W}$, $\text{Th}\text{U}$, $\text{Pb}\text{U}$, etc.) are significantly affected by these changes.Three tests were carried out, to see how closely C1 chondrites approximate primordial solar system abundances. (1) A plot of solar vs Cl abundances shows only 7 discrepancies by more than twice the nominal error of the solar abundance: Ga, Ge, Nb, Ag, Lu, W and Os. Most or all apparently reflect errors in the solar data or f-values. (2) The major cosmochemical groups (refractories, siderophiles, volatiles, etc.) show no significant fractionation between the Sun and C1's, except possibly for a slight enrichment of volatiles in Cl's. (3) Abundances of odd-A nuclides between A = 65 and 209 show an almost perfectly smooth trend, with elemental abundances conforming to the slope defined by isotopic abundances. There is no evidence for systematic fractionation of the major cosmochemical groups from each other. Small irregularities (10–15%) show up in the Ag-Cd-In and Sm-Eu regions; the former may be due to a ~ 15% error in the Ag abundance and the latter, to a 10–20% fractionation of Eu during condensation, to contamination of C1 chondrites with interplanetary dust during regolith exposure, or to a change from s-process to r-process dominance.It appears that the new set of abundances is accurate to at least 10%, as irregularities of 5–10% are readily detectable. Accordingly, Cl chondrites seem to match primordial solar-system matter to ? 10%, with only four exceptions. Br and I are definitely and B is possibly fractionated by hydrothermal alteration, whereas Eu seems to be enriched by nebular condensation or regolith contamination.  相似文献   

制约类地行星大气层和水体发育的主要因素          下载免费PDF全文

欧阳自远  邹永廖  李春来  刘建忠  徐琳 《第四纪研究》2002,22(6):558-567

根据行星探测的资料,综合分析了水星、金星、地球(包括月球)、火星的大气层和水体的发育特征,对比了金星、火星的大气层与水体同地球的差异。类地行星质量小、体积小、密度大、旋转慢、卫星少甚至没有、挥发性元素较类木行星少、距离太阳较近,早期残留的原始大气层已经被早期太阳在金牛变星阶段的强烈太阳风所驱赶,加上巨大而频繁的撞击作用,使原始大气层被驱赶殆尽。现在的大气层是次生的,是由行星内部的去气作用形成的。类地行星的大气层、水体的发育和表生作用的特征与行星的质量大小(表征行星内部能量的大小和构造活动的强烈及持续时间)及行星与太阳的距离等因素有关。在类地行星中,地球和金星质量最大,逃逸速度最大,可将更多的气体“束缚”在它们表面,因此它们的大气有着复杂的组成和较大的密度。火星质量较小,逃逸速度不到地球的一半,在漫长的演化历史中,大气逐渐逸散进入太空,大气密度变得很稀薄。水星质量更小,而且最靠近太阳,不仅太阳风的驱赶作用强烈,而且表面温度高,气体分子的热运动更加剧烈,加剧了大气的逸散,所以水星的大气层极为稀薄,并且主要为太阳风成分。月球质量最小,几乎没有大气层,更没有水体的发育。行星的热演化历史对大气层和水体发育具有重要的制  相似文献   

The solubility of noble gases in water and in NaCl brine     

S.P. Smith  B.M. Kennedy 《Geochimica et cosmochimica acta》1983,47(3):503-515

A direct-sampling, mass-spectrometric technique has been used to measure simultaneously the solubilities of He, Ne, Ar, Kr, and Xe in fresh water and NaCl brine (0 to 5.2 molar) from 0° to 65 °C, and at 1 atm total pressure of moist air. The argon solubility in the most concentrated brines is 4 to 7 times less than in fresh water at 65 °C and 0°C, respectively. The salt effect is parameterized using the Setschenow equation. where M is NaCl moiarity, βⁱ_o(T) and βⁱ(T) the Bunsen solubility coefficients for gas i in fresh water and brine, and Kⁱ_M(T) the empirical salting coefficient. Values of Kⁱ_M(T) are calculated using volumetric concentration units for noble gas and NaCl content and are independent of NaCl molarity. Below about 40°C, temperature coefficients of all Kⁱ_M are negative. The value of K^He_M is a minimum at 40°C. K^Ar_M decreases from about 0.40 at 0°C to 0.28 at 65 °C. The absolute magnitudes of the differences in salting coefficients (relative to K^Ar_M) decrease from 0° to 65°C. Over the range of conditions studied, all noble gases are salted out, and K^He_M ? K^Ne_M < K^Ar_M < K^Kr_M < K^Xe_M.From the solubility data, we calculated $\text{Δ}\text{G}{}^0{}_{\mathrm{tr}}$, $\text{Δ}\text{S}{}^0{}_{\mathrm{tr}}$, $\text{Δ}\text{H}{}^0{}_{\mathrm{tr}}$ and $\text{Δ}\text{C}{}^{\mathrm{O}}{}_{\mathrm{p,tr}}$ for the transfer of noble gases from fresh water to 1 molar NaCl solutions. At low temperatures $\text{Δ}\text{S}{}^0{}_{\mathrm{tr}}$, is positive, but decreases and becomes negative at temperatures ranging from about 25°C for He to 45°C for Xe. At low temperatures, the dissolved electrolyte apparently interferes with the formation of a cage of solvent molecules about the noble gas atom. At higher temperatures, the local environment of the gas atom in the brine appears to be slightly more ordered than in pure water, possibly reflecting the longer effective range of the ionic fields at higher temperature.The measured solubilities can be used to model noble gas partitioning in two-phase geothermal systems at low temperatures. The data can also be used to estimate the temperature and concentration dependence of the salt effect for other alkali halides. Extrapolation of the measured data is not possible due to the incompletely-characterized minima in the temperature dependence of the salting coefficients. The regularities in the data observed at low temperatures suggest relatively few high-temperature data will be required to model the behavior of noble gases in high-temperature geothermal brines.  相似文献