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1.
Application of 182Hf-182W chronometry to constrain the duration of early solar system processes requires the precise knowledge of the initial Hf and W isotope compositions of the solar system. To determine these values, we investigated the Hf-W isotopic systematics of bulk samples and mineral separates from several Ca,Al-rich inclusions (CAIs) from the CV3 chondrites Allende and NWA 2364. Most of the investigated CAIs have relative proportions of 183W, 184W, and 186W that are indistinguishable from those of bulk chondrites and the terrestrial standard. In contrast, one of the investigated Allende CAIs has a lower 184W/183W ratio, most likely reflecting an overabundance of r-process relative to s-process isotopes of W. All other bulk CAIs have similar 180Hf/184W and 182W/184W ratios that are elevated relative to average carbonaceous chondrites, probably reflecting Hf-W fractionation in the solar nebula within the first ∼3 Myr. The limited spread in 180Hf/184W ratios among the bulk CAIs precludes determination of a CAI whole-rock isochron but the fassaites have high 180Hf/184W and radiogenic 182W/184W ratios up to ∼14 ε units higher than the bulk rock. This makes it possible to obtain precise internal Hf-W isochrons for CAIs. There is evidence of disturbed Hf-W systematics in one of the CAIs but all other investigated CAIs show no detectable effects of parent body processes such as alteration and thermal metamorphism. Except for two fractions from one Allende CAI, all fractions from the investigated CAIs plot on a single well-defined isochron, which defines the initial ε182W = −3.28 ± 0.12 and 182Hf/180Hf = (9.72 ± 0.44) × 10−5 at the time of CAI formation. The initial 182Hf/180Hf and 26Al/27Al ratios of the angrites D’Orbigny and Sahara 99555 are consistent with the decay from initial abundances of 182Hf and 26Al as measured in CAIs, suggesting that these two nuclides were homogeneously distributed throughout the solar system. However, the uncertainties on the initial 182Hf/180Hf and 26Al/27Al ratios are too large to exclude that some 26Al in CAIs was produced locally by particle irradiation close to an early active Sun. The initial 182Hf/180Hf of CAIs corresponds to an absolute age of 4568.3 ± 0.7 Ma, which may be defined as the age of the solar system. This age is 0.5-2 Myr older than the most precise 207Pb-206Pb age of Efremovka CAI 60, which does not seem to date CAI formation. Tungsten model ages for magmatic iron meteorites, calculated relative to the newly and more precisely defined initial ε182W of CAIs, indicate that core formation in their parent bodies occurred in less than ∼1 Myr after CAI formation. This confirms earlier conclusions that the accretion of the parent bodies of magmatic iron meteorites predated chondrule formation and that their differentiation was triggered by heating from decay of abundant 26Al. A more precise dating of core formation in iron meteorite parent bodies requires precise quantification of cosmic-ray effects on W isotopes but this has not been established yet.  相似文献   

2.
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   

3.
Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured 92Zr and 91Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in 182W vs. Hf/W and 142Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the 182W and 142Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the 182W vs. Hf/W and 142Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar 182Hf-182W and 146Sm-142Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.  相似文献   

4.
The Earth’s tungsten budget during mantle melting and crust formation   总被引：1，自引：0，他引：1  
During silicate melting on Earth, W is one of the most incompatible trace elements, similar to Th, Ba or U. As W is also moderately siderophile during metal segregation, ratios of W and the lithophile Th and U in silicate rocks have therefore been used to constrain the W abundance of the Earth’s mantle and the Hf-W age of core formation. This study presents high-precision W concentration data obtained by isotope dilution for samples covering important silicate reservoirs on Earth. The data reveal significant fractionations of W from other highly incompatible lithophile elements such as Th, U, and Ta. Many arc lavas exhibit a selective enrichment of W relative to Th, U, and Nb-Ta, reflecting W enrichment in the sub-arc mantle via fluid-like components derived from subducting plates. In contrast, during enrichment by melt-like subduction components, W is generally slightly depleted relative to Th and U, but is still enriched relative to Ta. Hence, all arc rocks and the continental crust exhibit uniformly low Ta/W (ca. 1), whereas W/Th and W/U may show opposite fractionation trends, depending on the role of fluid- and melt-like subduction components. Further high-precision W data for OIBs and MORBs reveal a systematic depletion of W in both rock types relative to other HFSE, resulting in high Ta/W that are complementary to the low Ta/W observed in arc rocks and the continental crust. Similar to previous interpretations based on Nb/U and Ce/Pb systematics, our Ta/W data confirm a depletion of the depleted upper mantle (DM) in fluid mobile elements relative to the primitive mantle (PRIMA). The abundance of W in the depleted upper mantle relative to other immobile and highly incompatible elements such as Nb and Ta is therefore not representative of the bulk silicate Earth. Based on mass balance calculations using Ta-W systematics in the major silicate reservoirs, the W abundance of the Earth’s primitive mantle can be constrained to 12 ppb, resulting in revised ratios of W-U and W-Th of 0.53 and 0.14, respectively. The newly constrained Hf-W ratio of the silicate Earth is 25.8, significantly higher than previously estimated (18.7) and overlaps within error the Hf-W ratio proposed for the Moon (ca. 24.9). The 182Hf-182W model age for the formation of the Earth’s core that is inferred from the 182W abundance and the Hf/W of the silicate Earth is therefore younger than previously calculated, by up to 5 Myrs after solar system formation depending on the accretion models used. The similar Hf/W ratios and 182W compositions of the Earth and the silicate Moon suggest a strong link between the Moon forming giant impact and final metal-silicate equilibration on the Earth.  相似文献   

5.
The 182Hf-182W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the 182Hf/180Hf and εW at the time of CAI formation to (1.07 ± 0.10) × 10−4 and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial εW values that are similar to or slightly lower than the initial value of CAIs. These low εW values may in part reflect 182W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of 26Al, which was sufficiently abundant (initial 26Al/27Al >1.4 × 10−5) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.  相似文献   

6.
Recent 182Hf-182W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial ε182W (−3.47 ± 0.20, 2σ) identical to that of magmatic iron meteorites after correction of cosmogenic 182W burn-out (−3.47 ± 0.35, 2σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred <1 m.y. after CAI formation. To assess the extent of isotopic disturbance, we have analyzed the elemental distribution of Hf and W in two CAIs, Ef2 from Efremovka (CV3 reduced), and Golfball from Allende (CV3 oxidized). Fassaite is the sole host of Hf (10-25 ppm) and, therefore, of radiogenic W in CAIs, with 180Hf/184W > 103, which is lowered by the ubiquitous presence of metal inclusions to 180Hf/184W > 10 in bulk fassaite. Metal alloy (Ni ∼ 50%) is the sole host of W (∼500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has 180Hf/184W < 0.006. Melilite has both Hf and W below detection limits (<0.01 ppm), but the presence of numerous metallic inclusions or OAs makes melilite a carrier for W, with 180Hf/184W < 1 in bulk melilite. Secondary processes had little impact on the 182Hf-182W systematics of Ef2, but a vein cross-cutting fassaite in Golfball has >100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the 182Hf-182W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new 182Hf-182W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.  相似文献   

7.
We present new ultra-high precision 142Nd/144Nd measurements of early Archaean rocks using the new generation thermal ionization mass spectrometer Triton. Repeated measurements of the Ames Nd standard demonstrate that the 142Nd/144Nd ratio can be determined with external precision of 2 ppm (2σ), allowing confident resolution of anomalies as small as 5 ppm. A major analytical improvement lies in the elimination of the double normalization procedure required to correct our former measurements from a secondary mass fractionation effect. Our new results indicate that metasediments, metabasalts, and orthogneisses from the 3.6 to 3.8 Ga West Greenland craton display positive 142Nd anomalies ranging from 8 to 15 ppm. Using a simple two-stage model with an initial ε143Nd value of 1.9 ± 0.6 ε-units, coupled 147Sm-143Nd and 146Sm-142Nd chronometry constrains mantle differentiation to 50-200 Ma after formation of the solar system. This chronological constraint is consistent with differentiation of the Earth’s mantle during the late stage of crystallization of a magma ocean. We have developed a two-box model describing 142Nd and 143Nd isotopic evolution of depleted mantle during the subsequent evolution of the crust-mantle system. Our results indicate that early terrestrial protocrust had a lifetime of ca. 0.7-1 Ga in order to produce the observed Nd isotope signature of Archaean rocks. In the context of this two box mantle-crust system, we model the evolution of isotopic and chemical heterogeneity of depleted mantle as a function of the mantle stirring time. Using the dispersion of 142Nd/144Nd and 143Nd/144Nd ratios observed in early Archaean rocks, we constrain the stirring time of early Earth’s mantle to 100-250 Ma, a factor of 5 shorter than the stirring time inferred from modern oceanic basalts.  相似文献   

8.
地球早期演化的Hf-W同位素制约   总被引：1，自引：1，他引：0  
梅清风  杨进辉 《岩石学报》2018,34(1):207-216
~(182)Hf-~(182)W作为短周期放射性衰变体系,可有效约束地球早期演化吸积增生和内部分异过程。本文通过系统总结、归纳太阳系形成初期地球核幔分异过程中Hf-W同位素变化规律、月球与地球硅酸盐的W同位素组成,提出利用~(182)Hf-~(182)W体系测定地球核幔分异时间不确定性的主要原因是地球核幔分异的持续性及开放性,大碰撞时间的~(182)Hf-~(182)W同位素限定主要受控于硅酸盐地球和硅酸盐月球的Hf/W比值,讨论了地幔W同位素不均一性的形成机制,与现代地幔不同的~(182)W/~(184)W组成可能代表了后增生作用之前整体硅酸盐地球的W同位素组成,也可能是~(182)Hf未完全灭绝时形成的区域性Hf/W比值差异经~(182)Hf衰变形成的结果。这些结论为探索类地行星形成与演化提供了重要制约。  相似文献   

9.
Super-chondritic 142Nd signatures are ubiquitous in terrestrial, Martian and lunar samples, and indicate that the terrestrial planets may have accreted from material with Sm/Nd ratio higher than chondritic. This contradicts the long-held view that chondrites represent a reference composition for the 147Sm-143Nd system. Using coupled 146Sm-142Nd and 147Sm-143Nd systematics in planetary samples, we have proposed a new set of values for the 147Sm/144Nd and 143Nd/144Nd ratios of the bulk silicate Earth (Caro et al., 2008). Here, we revise the Bulk Silicate Earth estimates for the 87Rb-87Sr and 176Lu-176Hf systems using coupled Sr-Nd-Hf systematics in terrestrial rocks. These estimates are consistent with Hf-Nd systematics in lunar samples. The implications of a slightly non-chondritic silicate Earth with respect to the geochemical evolution of the mantle-crust system are then examined. We show that the Archean mantle has evolved with a composition indistinguishable from that of the primitive mantle until about 2 Gyr. Positive ε143Nd and ε176Hf values ubiquitous in the Archean mantle are thus accounted for by the non-chondritic Sm/Nd and Lu/Hf composition of the primitive mantle rather than by massive early crustal formation, which solves the paradox that early Archean domains only have a limited extension in the present-day continents. The Sm-Nd and Lu-Hf evolution of the depleted mantle for the past 3.5 Gyr can be entirely explained by continuous extraction of the continents from a well-mixed mantle. Thus, in contrast to the chondritic Earth model, Sm-Nd mass balance relationships can be satisfied without the need to call upon hidden reservoirs or layered mantle convection. This new Sm-Nd mass balance yields a scenario of mantle evolution consistent with trace element and noble gas systematics. The high 3He/4He mantle component is associated with 143Nd/144Nd compositions indistinguishable from the bulk silicate Earth, suggesting that the less degassed mantle sources did not experience significant fractionation for moderately incompatible elements.  相似文献   

10.
Tungsten is a moderately siderophile high-field-strength element that is hydrophile and widely regarded as highly incompatible during mantle melting. In an effort to extend empirical knowledge regarding the behaviour of W during the latter process, we report new high-precision trace element data (W, Th, U, Ba, La, Sm) that represent both terrestrial and planetary reservoirs: MORB (11), abyssal peridotites (8), eucrite basalts (3), and carbonaceous chondrites (8). A full trace element suite is also reported for Cordilleran Permian ophiolite peridotites (12) to better constrain the behaviour of W in the upper mantle. In addition, we report our long-term averages for a number of USGS (BIR-1, BHVO-1, BHVO-2, PCC-1, DTS-1) and GSJ (JA-3, JP-1) standard reference materials, some of which we conclude to be heterogeneous and contaminated with respect to W. The most significant finding of this study is that many of the highly depleted upper mantle peridotites contain far higher W concentrations than expected. In the absence of convincing indications for alteration, re-enrichment or contamination, we propose that the W excess was caused by retention in an Os-Ir alloy phase, whose stability is dependent on fO2 of the mantle source region. This explanation could help to account for the particularly low W content of N-MORB and implies that the lithophile behaviour of W in basaltic rocks is not an accurate representation of the behaviour in the melt source. These findings then become relevant to the interpretation of W-isotopic data for achondrites, where the fractionation of Hf from W during melting is used to infer the Hf/W of the parent body mantle. This is exemplified by the differentiation chronology of the eucrite parent body (EPB), which has been modeled with a melt source with high Hf/W. By contrast, we explore the alternative scenario with a low mantle Hf/W on the EPB. Using available eucrite literature data, a maximum core segregation age of 1.2 ± 1.2 Myr after the closure of CAIs is calculated with a more prolonged time between core formation and mantle fractionation of ca. 2 Myr. This timeline is consistent with most recent published chronologies of the EPB differentiation based on the 53Mn-53Cr and 26Al-26Mg systems.  相似文献   

11.
We report here the results of an investigation of W and Nd isotopes in the SNC (Shergottite-Nakhlite-Chassignite (martian)) meteorites. We have determined that ε182W values in the nakhlites are uniform within analytical uncertainties and have an average value of ∼3. Also, while ε182W values in the shergottites have a limited range (from 0.3-0.7), their ε142Nd values vary considerably (from −0.2-0.9). There appears to be no correlation between ε182W and ε142Nd in the nakhlites and shergottites. These results shed new light on early differentiation processes on Mars, particularly on the timing and nature of fractionation in silicate reservoirs. Assuming a two-stage model, the metallic core is estimated to have formed at ∼12 Myr after the beginning of the solar system. Major silicate differentiation established the nakhlite source reservoir before ∼4542 Ma and the shergottite source reservoirs at 4525 Ma. These ages imply that, within the uncertainties afforded by the 182Hf-182W and 146Sm-142Nd chronometers, the silicate differentiation events that established the source reservoirs of the nakhlites and shergottites may have occurred contemporaneously, possibly during crystallization of a global magma ocean. The distinct 182W-142Nd isotope systematics in the nakhlites and the shergottites imply the presence of at least three isotopically distinct silicate reservoirs on Mars, two of which are depleted in incompatible lithophile elements relative to chondrites, and the third is enriched. The two depleted silicate reservoirs most likely reside in the Martian mantle, while the enriched reservoir could be either in the crust or the mantle. Therefore, the 182W-142Nd isotope systematics indicate that the nakhlites and the shergottites originated from distinct source reservoirs and cannot be petrogenetically related. A further implication is that the source reservoirs of the nakhlites and shergottites on Mars have been isolated since their establishment before ∼4.5 Ga. Therefore, there has been no giant impact or efficient global mantle convection to thoroughly homogenize the Martian mantle following the establishment of the SNC source reservoirs.  相似文献   

12.
Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived 182Hf-182W radionuclide system. Here we present new 182Hf-182W data for major types of primitive achondrites including acapulcoites, winonaites and one lodranite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites.For acapulcoites we derived a 182Hf-182W age of ΔtCAI = 4.1 +1.2/−1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of ΔtCAI = 4.8 +3.1/−2.6 Ma (assuming a bulk Hf/W ratio of ∼1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of ΔtCAI = 14.3 +2.7/−2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between ∼1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf-W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between ∼35 and ∼4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were ∼40 K/Ma to ∼720 K and then ∼3 K/Ma to ∼550 K.Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse correlation between accretion-age of asteroids and intensity of heating in their interiors as expected from heating by 26Al and 60Fe decay. However, the early accretion of the parent asteroid of primitive IAB silicates (∼1.0 Ma after CAIs; Schulz et al., 2009) and the possibly impact-induced melting-history of winonaites show that this concept is too simplistic. Parent body size, impact-driven melting as well as heat-insulating regolith cover also need to be considered in the early history of asteroid differentiation.  相似文献   

13.
Origin and chronology of chondritic components: A review   总被引：1，自引：0，他引：1  
Mineralogical observations, chemical and oxygen-isotope compositions, absolute 207Pb-206Pb ages and short-lived isotope systematics (7Be-7Li, 10Be-10B, 26Al-26Mg, 36Cl-36S, 41Ca-41K, 53Mn-53Cr, 60Fe-60Ni, 182Hf-182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides (7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca-41K, and 60Fe-60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (Δ17O ∼ −24 ± 2‰) nebular gas. The 26Al-poor [(26Al/27Al)0 < 1 × 10−5], 16O-rich (Δ17O ∼ −24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (Δ17O > −10‰) and have (26Al/27Al)0 similar to those in chondrules (<1 × 10−5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (Δ17O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.  相似文献   

14.
Zircons from the Simmern H5 chondrite and Pomozdino eucrite have been analyzed for their Hf-W isotope systematics. Zircons have high intrinsic Hf contents and coupled with low W, make them ideal Hf-W chronometers. However, measurements of (182Hf/180Hf)0 are far from straightforward with low signals of radiogenic 182W and difficulties in calibrating Hf/W ratios. Zircons were analyzed from the Simmern chondrite, and the Pomozdino eucrite. The Simmern zircon has an ultrarefractory-enriched trace element pattern, a feature commonly associated with refractory inclusions. Analyses of Simmern zircon show variable Hf/W that is likely due to surface contamination. Simmern chondrite zircon yields a (182Hf/180Hf)0 of 7.2 (± 4.5) × 10−5. Pomozdino eucrite zircon analyses show very high Hf/W values indicating (182Hf/180Hf)0 of 1.7 (± 1.1) × 10−5. The Simmern value is in good agreement with the initial value of the solar system indicated from Hf-W systematics of chondrites. The Pomozdino value is lower than expected if eucrites formed within several million years of refractory inclusions as suggested from Al-Mg systematics of eucrites.  相似文献   

15.
Excesses of 182W have previously been measured in samples from the Moon and Mars, and can be derived from high Hf/W regions in their interiors during their early histories. Although planetary mantles will have superchondritic Hf/W after core formation, the extent to which high Hf/W regions could be generated by magmatic fractionation has not been evaluated. In order to address the latter possibility, we have carried out experiments from 100 MPa to 10.0 GPa, 1150 to 1850°C, at oxygen fugacities near the IW (iron-wüstite) buffer, and measured partition coefficients for W and Hf for plagioclase-liquid, olivine-liquid, orthopyroxene-liquid, clinopyroxene-liquid, garnet-liquid, and metal-liquid pairs. Clinopyroxene and garnet are both capable of fractionating Hf from W during magmatic crystallization or mantle melting, and minor variations in the measured D’s can be attributed to crystal chemical effects. Excesses of 182W and 142Nd in lunar samples can be explained by fractionation of Hf from W, and Sm from Nd (by ilmenite and clinopyroxene) during crystallization of the latest stages of a lunar magma ocean. Correlations of εW with εNd in martian samples could be a result of early silicate fractionation in the martian mantle (clinopyroxene and/or garnet).  相似文献   

16.
The estimation of the time of Earth??s core formation on the basis of isotopic systems with short-lived and long-lived parent nuclides gives significantly different results. Isotopic data for the 182Hf-182W system with a 182Hf half-life of approximately 9 Myr can be interpreted in such a way that the core was formed 34 Myr after the origin of the solar system assuming complete core-mantle equilibrium. Similar estimates on the basis of the U-Pb isotopic system suggest a significantly longer mean time of core formation of approximately 120 Myr. If the Earth??s core were formed instantaneously, both isotopic systems would have shown identical values corresponding to the true age. The discrepancy between the U-Pb and Hf-W systems can be resolved assuming prolonged differentiation of prototerrestrial material into silicate and metallic phases, which occurred not simultaneously and uniformly in different parts of the mantle. This resulted in the isotopic heterogeneity of the mantle, and its subsequent isotopic homogenization occurred slowly. Under such conditions, the mean isotopic compositions of W and Pb in the mantle do not correspond to the mean time of the separation of silicate and metallic phases. This is related to the fact that the exponential function of radioactive decay is strongly nonlinear at high values of the argument, and its mean value does not correspond to the mean value of the function. There are compelling reasons to believe that the early mantle was heterogeneous with respect to W isotopic composition and was subsequently homogenized by convective mixing. This follows from the fact that the lifetime of isotopic heterogeneities in the mantle is close to 1.8 Gyr for various long-lived isotopic systems. There is also no equilibrium between the mantle and the core with respect to the contents of siderophile elements. Because of this, the mean isotopic ratios of W and Pb cannot be used for the direct computation of the time of metal-silicate differentiation in the Earth. Such estimation requires more sophisticated models accounting for the duration of the differentiation process using several isotope pairs. Given the prolonged core formation, which has probably continued up to now, the question about its age becomes ambiguous, and only the most probable growth rate of the core can be estimated. The combined use of the U-Pb and Hf-W systems constrains the time of formation of 90% of the core mass between 0.12 and 2.7 billion years. These model estimates could have been realistic under the condition of complete disequilibrium between the silicate and metallic phases, which is as improbable as the suggestion of complete equilibrium between them on the whole Earth scale.  相似文献   

17.
The origin of the observed niobium deficit in the bulk silicate Earth (BSE) compared to chondritic meteorites constitutes a long-standing problem in geochemistry. The deficit requires a large-scale process fractionating niobium from tantalum, and a super-chondritic Nb/Ta reservoir hidden in the deep silicate Earth and/or in the metallic core. The only voluminous super-chondritic Nb/Ta silicate reservoir analysed to date is found in lunar basalts that assimilated highly evolved Fe-rich rocks associated with anorthosites in the lunar crust. These Fe-rich rocks, enriched in incompatible elements, are thought to represent the last fractions of melt remaining at the end of lunar magma ocean crystallization. Here we report high-precision Nb-Ta data for a Fe-rich, late-stage rock suite associated with a terrestrial anorthosite from the Proterozoic Bolangir complex in India. The geochemical characteristics of this rock suite resemble those expected for late-stage residual melts from a terrestrial magma ocean. Samples show extreme, super-chondritic Nb/Ta up to 31.1 and highly elevated Nb concentrations up to 338 ppm. We argue that formation of an early enriched crustal reservoir (EECR) with these characteristics (high Fe, high Nb, superchondritic Nb/Ta) is likely in the course of Hadean late-stage terrestrial magma ocean solidification. Subduction and subsequent permanent deep mantle storage in the D′′ layer of a minor amount (∼0.5% of the BSE mass) of this EECR can readily explain the terrestrial Nb deficit, without the need to invoke core Nb storage. Our model is consistent with short-lived 142Nd and long-lived 176Hf-143Nd isotope models for early differentiation of the Earth’s crust. In addition, the inferred Lu/Hf of this EECR implies that this reservoir can also balance the offset of terrestrial Hf isotope ratios compared to the chondritic reservoir. As such, late-stage magma ocean residual melts may constitute the enigmatic parental reservoir of Hadean zircons with low time-integrated Hf isotope compositions.  相似文献   

18.
We evaluate initial (26Al/27Al)I, (53Mn/55Mn)I, and (182Hf/180Hf)I ratios, together with 207Pb/206Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of 26Al, 53Mn, and 182Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al)SS, I(Mn)SS, and I(Hf)SS, respectively. I(Mn)SS and I(Hf)SS = 9.1 ± 1.7 × 10−6 and 1.07 ± 0.08 × 10−4, respectively, correspond to “canonical” I(Al)SS = 5.1 × 10−5. I(Hf)SS so determined is consistent with I(Hf)SS = 9.72 ± 0.44 × 10−5 directly determined from an internal Hf-W isochron for CAI minerals. I(Mn)SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by 26Al decay had already accreted. The 53Mn incorporated within such asteroids would have been shielded from further “local” spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the 207Pb/206Pb ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for (182Hf/180Hf)I and the 207Pb/206Pb ages of angrites. (182Hf/180Hf)I decreases with decreasing 207Pb/206Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182Hf. The model solar system age thus determined is TSS,Hf-W = 4568.3 ± 0.7 Ma. (26Al/27Al)I and (53Mn/55Mn)I are less consistent with 207Pb/206Pb ages of the corresponding meteorites, but yield TSS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al)SS = 5.1 × 10−5 and a 207Pb/206Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn)SS = 9.1 ± 1.7 × 10−6 is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn)SS = 9.1 ± 1.7 × 10−6, parent body differentiation is found to extend at least to ∼5 Ma post-TSS, i.e., until differentiation of the angrite parent body ∼4563.5 Ma ago, or ∼4564.5 Ma ago using the directly measured 207Pb/206Pb ages of the D’Orbigny-clan angrites. The ∼1 Ma difference is characteristic of a remaining inconsistency for the D’Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the 207Pb/206Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded episodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to ∼10 Ma post-TSS.  相似文献   

19.
A synthetic composition representing the Yamato 980459 martian basalt (shergottite) has been used to carry out phase relation, and rare earth element (REE) olivine and pyroxene partitioning experiments. Yamato 980459 is a sample of primitive basalt derived from a reduced end-member among martian mantle sources. Experiments carried out between 1-2 GPa and 1350-1650 °C simulate the estimated pressure-temperature conditions of basaltic melt generation in the martian mantle. Olivine-melt and orthopyroxene-melt partition coefficients for La, Nd, Sm, Eu, Gd and Yb (DREE values) were determined by LA-ICPMS, and are similar to the published values for terrestrial basaltic systems. We have not detected significant variation in D-values with pressure over the range investigated, and by comparison with previous studies carried out at lower pressure.We apply the experimentally obtained olivine-melt and orthopyroxene-melt DREE values to fractional crystallization and partial melting models to develop a three-stage geochemical model for the evolution of martian meteorites. In our model we propose two ancient (∼4.535 Ga) sources: the Nakhlite Source, located in the shallow mantle, and the Deep Mantle Source, located close to the martian core-mantle boundary. These two sources evolved distinctly on the ε143Nd evolution curve due to their different Sm/Nd ratios. By partially melting the Nakhlite Source at ∼1.3 Ga, we are able to produce a slightly depleted residue (Nakhlite Residue). The Nakhlite Residue is left undisturbed until ∼500 Ma, at which point the depleted Deep Mantle Source is brought up by a plume mechanism carrying with it high heat flow, melts and isotopic signatures of the deep mantle (e.g., ε182W, ε142Nd, etc.). The plume-derived Deep Mantle Source combines with the Nakhlite Residue producing a mixture that becomes a mantle source (herein referred to as “the Y98 source”) for Yamato 980459 and the other depleted shergottites with the characteristic range of Sm/Nd ratios of these meteorites. The same hot plume provides a heat source for the formation of enriched and intermediate shergottites. Our model reproduces the REE patterns of nakhlites and depleted shergottites and can explain high ε143Nd in depleted shergottites. Furthermore, the model results can be used to interpret whole rock Rb-Sr and Sm-Nd ages of shergottites.  相似文献   

20.
Combined 147Sm-143Nd and 176Lu-176Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193 ± 20 Ma (2σ weighted mean). The individual 147Sm-143Nd and 176Lu-176Hf internal isochron ages are 183 ± 12 Ma and 197 ± 29 Ma, respectively, and are concordant with two previously determined 147Sm-143Nd and 87Rb-87Sr internal isochron ages of 190 ± 26 Ma and 207 ± 14 Ma, respectively (Shih et al., 2009). With respect to the 147Sm-143Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported 143Nd relative to its measured 147Sm/144Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1 Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).  相似文献   

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