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1.
The 182Hf-182W systematics of meteoritic and planetary samples provide firm constraints on the chronology of the accretion and earliest evolution of asteroids and terrestrial planets and lead to the following succession and duration of events in the earliest solar system. Formation of Ca,Al-rich inclusions (CAIs) at 4568.3 ± 0.7 Ma was followed by the accretion and differentiation of the parent bodies of some magmatic iron meteorites within less than ∼1 Myr. Chondrules from H chondrites formed 1.7 ± 0.7 Myr after CAIs, about contemporaneously with chondrules from L and LL chondrites as shown by their 26Al-26Mg ages. Some magmatism on the parent bodies of angrites, eucrites, and mesosiderites started as soon as ∼3 Myr after CAI formation and may have continued until ∼10 Myr. A similar timescale is obtained for the high-temperature metamorphic evolution of the H chondrite parent body. Thermal modeling combined with these age constraints reveals that the different thermal histories of meteorite parent bodies primarily reflect their initial abundance of 26Al, which is determined by their accretion age. Impact-related processes were important in the subsequent evolution of asteroids but do not appear to have induced large-scale melting. For instance, Hf-W ages for eucrite metals postdate CAI formation by ∼20 Myr and may reflect impact-triggered thermal metamorphism in the crust of the eucrite parent body. Likewise, the Hf-W systematics of some non-magmatic iron meteorites were modified by impact-related processes but the timing of this event(s) remains poorly constrained.The strong fractionation of lithophile Hf from siderophile W during core formation makes the Hf-W system an ideal chronometer for this major differentiation event. However, for larger planets such as the terrestrial planets the calculated Hf-W ages are particularly sensitive to the occurrence of large impacts, the degree to which impactor cores re-equilibrated with the target mantle during large collisions, and changes in the metal-silicate partition coefficients of W due to changing fO2 in differentiating planetary bodies. Calculated core formation ages for Mars range from 0 to 20 Myr after CAI formation and currently cannot distinguish between scenarios where Mars formed by runaway growth and where its formation was more protracted. Tungsten model ages for core formation in Earth range from ∼30 Myr to >100 Myr after CAIs and hence do not provide a unique age for the formation of Earth. However, the identical 182W/184W ratios of the lunar and terrestrial mantles provide powerful evidence that the Moon-forming giant impact and the final stage of Earth’s core formation occurred after extinction of 182Hf (i.e., more than ∼50 Myr after CAIs), unless the Hf/W ratios of the bulk silicate Moon and Earth are identical to within less than ∼10%. Furthermore, the identical 182W/184W of the lunar and terrestrial mantles is difficult to explain unless either the Moon consists predominantly of terrestrial material or the W in the proto-lunar magma disk isotopically equilibrated with the Earth’s mantle.Hafnium-tungsten chronometry also provides constraints on the duration of magma ocean solidification in terrestrial planets. Variations in the 182W/184W ratios of martian meteorites reflect an early differentiation of the martian mantle during the effective lifetime of 182Hf. In contrast, no 182W variations exist in the lunar mantle, demonstrating magma ocean solidification later than ∼60 Myr, in agreement with 147Sm-143Nd ages for ferroan anorthosites. The Moon-forming giant impact most likely erased any evidence of a prior differentiation of Earth’s mantle, consistent with a 146Sm-142Nd age of 50-200 Myr for the earliest differentiation of Earth’s mantle. However, the Hf-W chronology of the formation of Earth’s core and the Moon-forming impact is difficult to reconcile with the preservation of 146Sm-142Nd evidence for an early (<30 Myr after CAIs) differentiation of a chondritic Earth’s mantle. Instead, the combined 182W-142Nd evidence suggests that bulk Earth may have superchondritic Sm/Nd and Hf/W ratios, in which case formation of its core must have terminated more than ∼42 Myr after formation of CAIs, consistent with the Hf-W age for the formation of the Moon.  相似文献   

2.
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   

3.
The short-lived 182Hf-182W-isotope system is an ideal clock to trace core formation and accretion processes of planets. Planetary accretion and metal/silicate fractionation chronologies are calculated relative to the chondritic 182Hf-182W-isotope evolution. Here, we report new high-precision W-isotope data for the carbonaceous chondrite Allende that are much less radiogenic than previously reported and are in good agreement with published internal Hf-W chronometry of enstatite chondrites. If the W-isotope composition of terrestrial rocks, representing the bulk silicate Earth, is homogeneous and 2.24 ε182W units more radiogenic than that of the bulk Earth, metal/silicate differentiation of the Earth occurred very early. The new W-isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion. Early terrestrial core formation implies rapid terrestrial accretion, thus permitting formation of the Moon by giant impact while 182Hf was still alive. This could explain why lunar W-isotopes are more radiogenic than the terrestrial value.  相似文献   

4.
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   

5.
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   

6.
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   

7.
The mantles of the Earth and Moon are similarly depleted in V, Cr, and Mn relative to the concentrations of these elements in chondritic meteorites. The similar depletions have been used as evidence that the Moon inherited its mantle from the Earth after a giant impact event. We have conducted liquid metal-liquid silicate partitioning experiments for V, Cr, and Mn from 3 to 14 GPa and 1723 to 2573 K to understand the behavior of these elements during planetary core formation. Our experiments have included systematic studies of the effects of temperature, silicate composition, metallic S-content, metallic C-content, and pressure. Temperature has a significant effect on the partitioning of V, Cr, Mn, with all three elements increasing their partitioning into the metallic liquid with increasing temperature. In contrast, pressure is not observed to affect the partitioning behavior. The experimental results show the partitioning of Cr and Mn are hardly dependent on the silicate composition, whereas V partitions more strongly into depolymerized silicate melts. The addition of either S or C to the metallic liquid causes increased metal-silicate partition coefficients for all three elements. Parameterizing and applying the experimental data, we find that the Earth’s mantle depletions of V, Cr, and possibly Mn can be explained by core formation in a high-temperature magma ocean under oxygen fugacity conditions about two log units below the iron-wüstite buffer, though the depletion of Mn may be due entirely to its volatility. However, more oxidizing conditions proposed in recent core formation models for the Earth cannot account for any of the depletions. Additionally, because we observe no pressure effect on the partitioning behavior, the data do not require the mantle of the Moon to be derived from the Earth’s mantle, although this is not ruled out. All that is required to create depletions of V, Cr, and Mn in a mantle is a planetary body that is hot enough and reducing enough during its core formation. Such conditions could have existed on the Moon-forming impactor.  相似文献   

8.
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   

9.
In this paper, we examine the distribution of incompatible elements in Earth’s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide. A structural model combined with an element partitioning model forms the basis for our analyses. The former separates a bulk peridotite into mineral crystal lattices, interfaces (grain and interphase boundaries), and intra- and inter-granular inclusions as sites for incompatible elements. The latter relates the distribution of elements among these sites based on lattice strain theory. By treating both intra- and inter-granular inclusions as a melt-like phase, the combined models successfully reproduce the relative concentrations of incompatible elements among minerals, clean rock (reconstituted from mineral compositions and mineral mode), and whole rock. The analyses reveal common signatures in the rocks: (1) incompatible elements in the crystal lattices of olivine, orthopyroxene and clinopyroxene achieved chemical equilibrium. (2) Olivine, orthopyroxene and clinopyroxene grains contain similar amounts of an intra-granular, melt-like component possibly in the form of sub-micron inclusions with weight (≈volume) fractions between 5 × 10−5 and 1 × 10−2. (3) All rocks contain an inter-granular melt-like component with a fraction between 10−4 and 10−2, well above the amount expected to be stored along interfaces. (4) Fractions of the inter- and intra-granular components are positively correlated, indicating that they were originated from the same process. (5) The inter- and intra-granular melt-like phases are chemically equilibrated with other structural components. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that the melt-like component formed during ascent. Instead, its ubiquitous appearance, its invisibility to optical microscopy, and its absorption of the incompatible elements in a manner similar to a melt phase even at sub-solidus condition, all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites. From the mineral mode, grain size, and mineral plus whole rock concentrations of incompatible elements, we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium (metasomatic) effect as has been done previously. Although it should be tested by additional analytical studies, our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic (disequilibrium) event is required.  相似文献   

10.
New isotopic studies of 142Nd, the daughter product of the short-lived and now extinct isotope 146Sm, have revealed that the accessible part of the silicate Earth (e.g., upper mantle and crust) is more radiogenic in 142Nd/144Nd than that of chondritic meteorites. The positive 142Nd anomaly of the Earth’s mantle implies that the Sm/Nd ratio of the mantle was fractionated early in Earth’s history and that the complementary low 142Nd reservoir has remained isolated from the mantle since its formation. This has led to the suggestion that an early enriched reservoir, formed within Earth’s first hundred million years (the Hadean), resides permanently in the deep interior of the Earth. One hypothesis for a permanently isolated reservoir is that there may be an Fe-rich, and hence intrinsically dense, chemical boundary layer at the core-mantle boundary. The protoliths of this chemical boundary layer could have originated at upper mantle pressures during extreme fractional crystallization of a global magma ocean during the Hadean but testing this hypothesis is difficult because samples of this early enriched reservoir do not exist. This hypothesis, however, is potentially refutable. Here, we investigate a post-Archean magnetite-sulfide magma formed by extreme magmatic differentiation to test whether residual Fe-rich liquids of any kind have the necessary trace-element signatures to satisfy certain global geochemical imbalances. The magnetite-sulfide magma is found to have high Pb contents (and low U/Pb ratios), high Re/Os ratios, and anti-correlated Sm/Nd and Lu/Hf fractionations. Permanent segregation of such a magma would (1) provide a means of early Pb sequestration, resulting in the high U/Pb ratio of the bulk silicate Earth, (2) be a source of radiogenic 187Os in the source regions of plumes, and (3) provide an explanation for decoupled Hf and Nd isotopic evolution in the early Archean, which is not easily produced by silicate fractionation. However, the magnetite-sulfide magma is not highly enriched in K, and thus, at face value, this magma analog would not serve as a repository for all of the heat producing elements. Nevertheless, other Fe-O-S liquids reported elsewhere are enriched in apatite, which carries high concentrations of K, U and Th. Given some promising geochemical fractionations of the Fe-rich liquids investigated here, the notion of a Hadean Fe-rich residual liquid deserves continued consideration from additional experimental or analog studies.  相似文献   

11.
New analyses and critical re-evaluation of older data permit another estimate of the abundances of some 42 elements in the silicate shells of the Earth, excluding the core.These data and estimations of volcanic rock production lead to a series of estimates of the mass-fractions of depleted and undepleted mantle. Most likely estimates of the latter lie in the range 0.2–0.4. The best estimates are obtained for the most incompatible elements K, Rb, Sr, Ba, Nb, Th, U.The development of the continental crust and the change in tectonic processes over time is consistent with these interpretations.  相似文献   

12.
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   

13.
Analytical data on major elements and 31 trace elements in olivine nephelinites, nepheline basanites, basanitic alkali olivine basalts and their differentiates (tephrites, hawaiites, mugearites, benmoreites, latites, phonolites and trachytes) from Hegau, Kaiserstuhl, Rhön, Hessian Depression, Vogelsberg, Westerwald, Siebengebirge, E Eifel and Hocheifel are evaluated. They were based on 400 samples with new or unpublished data on about one third of the rocks. The Sr–Nd isotopic compositions for 78 rocks are included. The alkali basaltic volcanism is caused by adiabatic decompression of asthenospheric mantle updomed to a minimum depth of 50 km in connection with the Alpine continent collision. The chemical compositions of the primary basaltic melts from the different areas are similar containing about one hundred-fold enrichment of highly incompatible elements relative to the primitive mantle from partial melting of depleted and secondarily enriched peridotite. The elements Cs, K, Pb and Ti are specifically depleted in the basalts partly because of phlogopite being residual at partial melting. The Tertiary alkali basalts range in Nd-isotopic composition from 0.51288 to 0.51273 and in Sr-isotopic ratios from 0.7032 to 0.7042. These ranges indicate mixtures of HIMU, depleted and enriched mantle components in the metasomatically altered peridotite source which resembles that of certain ocean islands. The Nd-Sr-isotopic compositions of the Quaternary E Eifel are close to bulk Earth ratios. East and W Eifel plots differ distinctly from the Tertiary Hocheifel which is geographically intermediate. This isotopic difference, beside specific K/Na ratios, is probably caused by separate metasomatic pulses that immediately preceded the respective periods of volcanism. The metasomatically altered mantle had partly primitive mantle signatures (Nb/Ta, Zr/Sm and Th/U ratios) and partly ocean island (or MORB) source properties (Rb/Cs). A MORB source can be excluded because of the low K/Rb and high Th/U ratios. A correlation of D with 87Sr/86Sr in amphibole and phlogopite and a slightly larger 18O than in MORB is conformable with a seawater and crustal impact on the source of alkali basalts. Slightly higher than average water concentrations in the source of certain primary basaltic melts (indicated by amphibole phenocrysts in their basalts) are required for differentiation of these basalts in magma chambers of the upper crust. Model calculations are presented to explain compositions of differentiates which range from about 60% to about 20% residual melt. The latter are represented by phonolites and trachytes. The Nd- and Sr-isotopic signatures of the majority of differentiates indicate contamination by a granitic partial melt from the wall rocks of magma chambers. Olivine nephelinite magma was the common source of contaminated differentiates.  相似文献   

14.
A decoupling in MORB of measured Th/U (κ = 2.5) from that calculated by Pb isotopes (κ = 3.8) for the depleted asthenosphere is well established, and has been referred to as the second Pb paradox (Kramers, J.D., and Tolstikhin, I.N., 1997. Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust. Chem. Geol., 139, 75–110.) or the kappa conundrum (Elliott, T., Zindler, A., and Bourdon, B., 1999. Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle. Earth Planet. Sci. Lett., 169, 129–145.). More controversial has been the cause and timing of this phenomenon, although a higher return flux of U6+ relative to Th4+ and(or) the recycling of crustal Pb into the mantle have become the preferred explanations of most workers. Such a combined mechanism effectively operating over the past 2.5 Ga was modelled in plumbotectonics (Zartman, R.E., and Haines, S., 1988. The plumbotectonics model for Pb isotopic systematics among major terrestrial reservoirs—a case for bi–directional transport. Geochim. Cosmochim. Acta, 52, 1327–1339. 709.33.), and found to be quantitatively feasible.A large TIMS, SIMS and LA-ICPMS database of Th and U concentrations for kimberlite-hosted zircon, particularly from Cr-poor megacrystic suites, now exists (Kinny et al., 1989, Berryman et al., 1999, Griffin et al., 2000 and Spetsius et al., 2002; Appendix A and Appendix B, this work). Six suites comprising 10 or more zircon grains with ages between 90 and 2550 Ma reveal consistent patterns when plotted on Th/U vs. U diagrams. We interpret these patterns as resulting from fractional crystallization of a melt with kimberlite affinity presumably derived from the asthenosphere, permitting the extrapolation to an initial Th/U at the time zircon crystallization began. A two-fold decrease is seen in this ratio over the past 2.5 Ga, suggesting that during this time a similar change has occurred in the parent silicate melt. Estimates of Th and U distribution coefficients between zircon and coexisting melt permit calculation of Th/U in the melt, which, for these highly incompatible elements, presumably is the same as for its mantle source rock. Kimberlitic zircon may thus indeed give evidence of a reduction in κ, tentatively calculated as from 4 to 2, since the Archean for the depleted asthenosphere.  相似文献   

15.
海南岛陆缘扩张带蓬莱地区新生代玄武岩中捕获大量尖晶石二辉橄榄岩和方辉橄榄岩幔源包体。激光剥蚀等离子体质谱(LA-ICP-MS)分析结果表明,蓬莱地幔橄榄岩含有三种不同地球化学特征的单斜辉石(Cpx):(1)a类单斜辉石Mg~#=92.3~93.4,来自富集Cpx的二辉橄榄岩,具有极低的LREE和不相容元素含量,HREE平坦,Th、U、La、Sr正异常,经历了7%~10%的尖晶石相部分熔融,仅受到极低程度强不相容元素(Th、U、La、Sr)初期富集交代作用;(2)b类单斜辉石Mg~#=89.9~90.3,来自较富集Cpx的二辉橄榄岩,具有中等的LREE和LILE含量,HREE平坦,微量元素蛛网图上显示Th、U正异常,Rb、Ba、Nb、Ta、Sr、Ti负异常,经历4%~5%的尖晶石相部分熔融,可能受到了含LREE和Th、U等不相容元素的硅酸盐熔体交代;(3)c类单斜辉石Mg~#=91.4~92.8,来自贫Cpx的二辉橄榄岩和方辉橄榄岩,具有富集的LREE和LILE含量,HREE弱分异,微量元素蛛网图上显示Th、U正异常及强烈的Nb、Ta、Ti负异常,经历了8%~20%的尖晶石相部分熔融,其交代熔体可能是来自源区有石榴子石残留的碳酸盐熔体。全岩主、微量元素及模拟计算结果表明,这些幔源包体的主量元素主要受部分熔融程度影响,并且方辉橄榄岩经历的部分熔融程度大于二辉橄榄岩。地幔橄榄岩的Sr-Nd同位素组成表明该区具有MORB-OIB型亏损地幔特征。此外,蓬莱部分地幔橄榄岩包体显示正斜率的HREE分异特征((Gd/Yb)_N=0.4~0.7),暗示该区地幔经历了源自石榴子石稳定区的变压熔融,总体熔融程度为18%以上,指示了较高的地幔潜能温度。综合前人对海南岛新生代玄武岩最新研究成果,我们认为海南地幔柱可能为该区软流圈地幔置换古老岩石圈地幔提供了热源,导致了区域岩石圈地幔的破坏,从而引起包括地幔柱本身、软流圈和富集岩石圈的熔融。岩石圈地幔性质的改变和不均一性可能是海南岛陆缘扩张带新生代岩石圈减薄的主要动力学机制。  相似文献   

16.
Mass balance of incompatible elements was analyzed for different inferred reservoirs of the bulk silicate Earth. It was shown that the lower mantle, as well as the primitive mantle, includes an MORB-source depleted mantle component and material with a high content of incompatible elements. Contribution of the continental crust was found to be subordinate. The predominant part of the enriched mantle formed through differentiation of the mantle itself. Enriched material was supplied into the deep-seated zones of silicate shell through delamination of subcontinental lithospheric blocks affected by mantle metasomatism and their subsequent involvement in the mantle convective system. The osmium isotope composition of the plunged lithospheric material is modified in the lower mantle by the infiltration of sulfide melts.  相似文献   

17.
Tectonomagmatic evolution of the Earth and Moon   总被引：1，自引：0，他引：1  
The Earth and Moon evolved following a similar scenario. The formation of their protocrusts started with upward crystallization of global magmatic oceans. As a result of this process, easily fusible components accumulated in the course of fractional crystallization of melt migrating toward the surface. The protocrusts (granitic in the Earth and anorthositic in the Moon) are retained in ancient continents. The tectonomagmatic activity at the early stage of planet evolution was related to the ascent of mantle plume of the first generation composed of mantle material depleted due to the formation of protocrusts. The regions of extension, rise, and denudation were formed in the Earth above the diffluent heads of such superplumes (Archean granite-greenstone domains and Paleoproterozoic cratons), whereas granulite belts as regions of compression, subsidence, and sedimentation arose above descending mantle flows. The situation may be described in terms of plume tectonics. Gentle uplifts and basins (thalassoids) in lunar continents are probable analogues of these structural elements in the Moon. The period of 2.3–2.0 Ga ago was a turning point in the tectonomagmatic evolution of the Earth, when geochemically enriched Fe-Ti picrites and basalts typical of Phanerozoic within-plate magmatism became widespread. The environmental setting on the Earth’s surface changed at that time, as well. Plate tectonics, currently operating on a global scale, started to develop about ∼2 Ga ago. This turn was related to the origination of thermochemical mantle plumes of the second generation at the interface of the liquid Fe-Ni core and silicate mantle. A similar turning point in the lunar evolution probably occurred 4.2–3.9 Ga ago and completed with the formation of large depressions (seas) with thinned crust and vigorous basaltic magmatism. Such a sequence of events suggests that qualitatively new material previously retained in the planets’ cores was involved in tectonomagmatic processes at the middle stage of planetary evolution. This implies that the considered bodies initially were heterogeneous and were then heated from above to the bottom by propagation of a thermal wave accompanied by cooling of outer shells. Going through the depleted mantle, this wave generated thermal superplumes of the first generation. Cores close to the Fe + FeS eutectics in composition were affected by this wave in the last turn. The melting of the cores resulted in the appearance of thermochemical superplumes and corresponding irreversible rearrangement of geotectonic processes.  相似文献   

18.
Cenozoic lavas from Hainan Island,South China,comprise quartz tholeiite,olivine tholeiite,alkali basalt,and basanite and form a continuous,tholeiite-dominated,compositional spectrum.Highly incompatible elements and their relationships with isotopes in these lavas are shown to be useful in evaluating mantle-source composition,whereas modeling suggests that ratios of elements with bulk partition coefficients significantly larger than those of Nb and Ta may be sensitive to partial melting.Th/Ta and La/Nb ratios of alkali basalts are lower than those of tholeiites,and they are all lower than those of the primitive mantle,These ratios correlate positively with ^207Pb/^204Pb and ^87Sr/^86Sr ratios.Such relationships can be explained by mixing of depleted and enriched source components.A depleted component is indicated by alkali basalt compositions and is similar to some depleted OIB (PREMA).The enriched component,similar to sediment compositions,is indicated by tholeiites with high LILE/HFSE,^207Pb/^204Pb,and ^87Sr/^86Sr ratios.In general,basalts from Hainan and the South China Basin(SCB)share common geochemical characters.e.g.high Rb/Sr,Th/Ta,^207Pb/^206Pb,and low Ba/Th ratios.Such a geochemical trend is comparable to that of EMII-type OIB and best explained as the result of subduction.Occurrence of these characteristics in both continental Hainan basalts and SCB seamout basalts indicates the presence of a South China geochemical domain that exists in the mantle region below the lithosphere.  相似文献   

19.
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   

20.
The depleted mantle and the continental crust are generally thought to balance the budget of refractory and lithophile elements of the Bulk Silicate Earth (BSE), resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) niobium and tantalum appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion.In this study a series of nineteen orogenic MORB-type eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question of whether subducted oceanic crust could form a hidden reservoir to account for the mass imbalance of Nb/Ta between BSE and the chondritic reservoir. Concentrations of HFSE were analyzed with isotope dilution (ID) techniques. Additionally, LA-ICPMS analyses of clinopyroxene, garnet and rutile have been performed. Rutile is by far the major host for Nb and Ta in all analyzed eclogites. However, many rutiles revealed zoning in Nb/Ta ratios, with cores being higher than rims. Accordingly, in situ analyses of rutiles have to be evaluated carefully and rutile cores do not necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.The HFSE concentration data in bulk rocks show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show neither enrichment of Nb compared to similarly incompatible elements (e.g. La), nor fractionation of Nb/Ta ratios relative to MOR-basalts, the likely precursor of these rocks. This indicates that during the conversion of the oceanic crust to eclogites in most cases, (1) HFSE and REE have similar mobility on average, possibly because both element groups remain in the down going slab, and (2) no significant fractionation of Nb/Ta occurs in subducted oceanic crust. With an average Nb/Ta ratio of 14.2 ± 1.4 (2s.e.), the investigated eclogites cannot balance the differences between BSE and chondrite. Additionally, as their average Nb/Ta is indistinguishable from the Nb/Ta of MORB, they are also an unlikely candidate to balance the potentially small differences in Nb/Ta between the continental crust and the mantle.  相似文献   

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