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1.
The highly siderophile elements (HSE) pose a challenge for planetary geochemistry. They are normally strongly partitioned into metal relative to silicate. Consequently, planetary core segregation might be expected to essentially quantitatively remove these elements from planetary mantles. Yet the abundances of these elements estimated for Earth's primitive upper mantle (PUM) and the martian mantle are broadly similar, and only about 200 times lower than those of chondritic meteorites. In contrast, although problematic to estimate, abundances in the lunar mantle may be more than twenty times lower than in the terrestrial PUM. The generally chondritic Os isotopic compositions estimated for the terrestrial, lunar and martian mantles require that their long-term Re/Os ratios were within the range of chondritic meteorites. Further, most HSE in the terrestrial PUM also appear to be present in chondritic relative abundances, although Ru/Ir and Pd/Ir ratios are slightly suprachondritic. Similarly suprachondritic Ru/Ir and Pd/Ir ratios have also been reported for some lunar impact melt breccias that were created via large basin forming events.Numerous hypotheses have been proposed to account for the HSE present in Earth's mantle. These hypotheses include inefficient core formation, lowered metal-silicate D values resulting from metal segregation at elevated temperatures and pressures (as may occur at the base of a deep magma ocean), and late accretion of materials with chondritic bulk compositions after the cessation of core segregation. Synthesis of the large database now available for HSE in the terrestrial mantle, lunar samples, and martian meteorites reveals that each of the main hypotheses has flaws. Most difficult to explain is the similarity between HSE in the Earth's PUM and estimates for the martian mantle, coupled with the striking differences between the PUM and estimates for the lunar mantle. More complex, hybrid models that may include aspects of inefficient core formation, HSE partitioning at elevated temperatures and pressures, and late accretion may ultimately be necessary to account for all of the observed HSE characteristics. Participation of aspects of each process may not be surprising as it is difficult to envision the growth of a planet, like Earth, without the involvement of each.  相似文献   

2.
Undifferentiated meteorites (chondrites) have the same relative abundances of refractory lithophile elements (Ca, Al, Ti, Sc, REE, etc.), despite variable absolute concentrations. The reasonable assumption of chondritic ratios among refractory elements in the bulk Earth is used to constrain the chemical composition of the upper mantle in the following way: Correlations of the compatible refractory elements Ca, Al, Ti, Sc and Yb with MgO are worldwide very similar in suites of spinel-lherzolite xenoliths from basaltic rocks. Such suites represent upper mantle material depleted to differing degrees by extraction of partial melts. From these refractory elements vs. MgO correlations, ratios of pairs of refractory elements were calculated at various MgO contents. Chondritic AlTi and ScTi ratios were only obtained for MgO contents below 36%. A chrondritic ScYb ratio requires an MgO content above 35%. We therefore accept 35.5% as the most reasonable MgO content of undepleted upper mantle. This MgO content is slightly below the spinel-lherzolite with the lowest measured MgO content (36.22%). The corresponding Al2O3 content of 4.75% is higher than in previous estimates of upper mantle composition. The concentrations of other elements were obtained from similar correlations at a MgO content of 35.5%. The resulting present upper mantle composition is enriched in refractory elements by a factor of 1.49 relative to Si and Cl and by a factor of 1.12 for Mg relative to Si and Cl. These enrichments are in the same range as those for the Vigarano type carbonaceous chondrites. The Mg/Mg + Fe ratio of 89 is slightly lower than previous estimates.The CaAl ratio in spinel lherzolite suites is, however, uniformly higher worldwide than the chondritic ratio by about 15%. Orogenic peridotites as well as komatiites appear to have similar non-chondritic CaAl ratios. It is therefore suggested that this non-chondritic CaAl ratio is a characteristic of the upper mantle, possibly since the Archean. A minor fractionation of about 4% of garnet in an early, global melting event (deep magma ocean?) is presented as the most likely cause for the high CaAl-ratio. In this case the addition of 4% of such a garnet component to the undepleted present upper mantle would be required to obtain the composition of the primordial upper mantle. The CaAl-ratio of this primordial mantle would be 15% higher than that of the undepleted present upper mantle, resulting in an enrichment of refractory elements of 1.70 (AlSi relative to Cl) for the primordial upper mantle.  相似文献   

3.
We have conducted petrographic, chemical and in-situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated R-chondrite. Refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low FeO and high refractory lithophile element (RLE) contents. Refractory forsterites are typically enriched in 16O relative to ‘normal’ olivine independent of the bulk O-isotope ratios of the parent meteorites. Analyses of refractory forsterites spread along a Δ17O mixing line with Δ17O ranging from +2 to −10‰. Due to similarities in oxygen isotopes and chemical compositions, we conclude that refractory forsterites of various types of chondrites come from a single common reservoir. Implications of this hypothesis for the chemical and O-isotope evolution of silicates in the early solar nebular are discussed.  相似文献   

4.
The abundance of C in carbonaceous and ordinary chondrites decreases exponentially with increasing shock pressure as inferred from the petrologic shock classification of Scott et al. [Scott, E.R.D., Keil, K., Stoffler, D., 1992. Shock metamorphism of carbonaceous chondrites. Geochim. Cosmochim. Acta 56, 4281-4293] and Stoffler et al. [Stoffler, D., Keil, K., Scott, E.R.D., 1991. Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845-3867]. This confirms the experimental results of Tyburczy et al. [Tyburczy, J.A., Frisch, B., Ahrens, T.J., 1986. Shock-induced volatile loss from a carbonaceous chondrite: implications for planetary accretion. Earth Planet. Sci. Lett. 80, 201-207] on shock-induced devolatization of the Murchison meteorite showing that carbonaceous chondrites appear to be completely devolatilized at impact velocities greater than 2 km s-1. Both of these results suggest that C incorporation would have been most efficient in the early stages of accretion, and that the primordial C content of the Earth was between 10(24) and 10(25) g C (1-10% efficiency of incorporation). This estimate agrees well with the value of 3-7 x 10(24) g C based on the atmospheric abundance of 36Ar and the chondritic C/36Ar (Marty and Jambon, 1987). Several observations suggest that C likely was incorporated into the Earth's core during accretion. (1) Graphite and carbides are commonly present in iron meteorites, and those iron meteorites with Widmanstatten patterns reflecting the slowest cooling rates (mostly Group I and IIIb) contain the highest C abundances. The C abundance-cooling rate correlation is consistent with dissolution of C into Fe-Ni liquids that segregated to form the cores of the iron meteorite parent bodies. (2) The carbon isotopic composition of graphite in iron meteorites exhibits a uniform value of -5% [Deines, P., Wickman, F.E. 1973. The isotopic composition of 'graphitic' carbon from iron meteorites and some remarks on the troilitic sulfur of iron meteorites. Geochim. Cosmochim. Acta 37, 1295-1319; Deines, P., Wickman, F.E., 1975. A contribution to the stable carbon isotope geochemistry of iron meteorites. Geochim. Cosmochim. Acta 39, 547-557] identical to the mode in the distribution found in diamonds, carbonatites and oceanic basalts [Mattey, D.P., 1987. Carbon isotopes in the mantle. Terra Cognita 7, 31-37]. (3) The room pressure solubility of C in molten iron is 4.3 wt% C. Phase equilibria confirm that the Fe-C eutectic persists to 12 GPa, and thermochemical calculations for the Fe-C-S system by Wood [Wood, B.J., 1993. Carbon in the core. Earth Planet. Sci. Lett. 117, 593-607] predict that C is soluble in Fe liquids at core pressures. The abundance of 36Ar in chondrites decreases exponentially with increasing shock pressure as observed for C. It is well known that noble gases are positively correlated and physically associated with C in meteorites [e.g. Otting, W., Zahringer J., 1967. Total carbon content and primordial rare gases in chondrites. Geochim. Cosmochim. Acta 31, 1949-1960; Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta, 42, 1775-1797]. This suggests a mechanism by which primordial He and other noble gases may have incorporated into the Earth during accretion. The abundance of He in the primordial Earth required to sustain the modern He flux for 4 Ga (assuming a planetary 3 He/4 He; Reynolds et al. [Reynolds, J.H., Frick, U., Niel, J.M., Phinney, D.L., 1978. Rare-gas-rich separates from carbonaceous chondrites. Geochim. Cosmochim. Acta 42, 1775-1797] is calculated to be > or = 10(-8) cm3 g-1. This minimum estimate is consistent with a 1-10% efficiency of noble gas retention during accretion and the observed abundance of He in carbonaceous chondrites (10(-5) to 10(-4) cm3 g-1 excluding spallogenic contributions).  相似文献   

5.
Don Francis   《Lithos》2003,71(2-4):135-152
The Earth's continents are cored by Archean cratons underlain by seismically fast mantle roots descending to depths of 200+ km that appear to be both more refractory and colder than the surrounding asthenospheric mantle. Low-temperature mantle xenoliths from kimberlite pipes indicate that the shallow parts of these cratonic mantle roots are dominated by refractory harzburgites that are very old (3+ Ga). A fundamental mass balance problem arises, however, when attempts are made to relate Archean high-Mg lavas to a refractory restite equivalent to the refractory lithospheric mantle roots beneath Archean cratons. The majority of high-Mg Archean magmas are too low in Al and high in Si to leave behind a refractory residue with the composition of the harzburgite xenoliths that constitute the Archean mantle roots beneath continental cratons, if a Pyrolitic primitive mantle source is assumed. The problem is particularly acute for 3+ Ga Al-depleted komatiites and the Si-rich harzburgites of the Kaapvaal and Slave cratons, but remains for cratonic harzburgites that are not anomalously rich in orthopyroxene and many Al-undepleted komatiites. This problem would disappear if fertile Archean mantle was richer in Fe and Si, more similar in composition to chondritic meteorites than the present Pyrolitic upper mantle of the Earth. Accepting the possibility that the Earth's convecting upper mantle has become poorer in Fe and Si over geologic time not only provides a simpler way of relating Archean high-Mg lavas to the lithospheric mantle roots that underlie Archean cratons, but could lead to new models for the nature Archean magmatism and the lower mantle sources of modern hot-spot volcanism.  相似文献   

6.
The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO2 and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.  相似文献   

7.
According to their genesis, meteorites are classified into heliocentric (which originate from the asteroid belt) and planetocentric (which are fragments of the satellites of giant planets, including the Proto-Earth). Heliocentric meteorites (chondrites and primitive meteorites genetically related to them) used in this study as a characteristic of initial phases of the origin of the terrestrial planets. Synthesis of information on planetocentric meteorites (achondrites and iron meteorites) provides the basis for a model for the genesis of the satellites of giant planets and the Moon. The origin and primary layering of the Earth was initially analogously to that of planets of the HH chondritic type, as follows from similarities between the Earth’s primary crust and mantle and the chondrules of Fe-richest chondrites. The development of the Earth’s mantle and crust precluded its explosive breakup during the transition from its protoplanetary to planetary evolutionary stage, whereas chondritic planets underwent explosive breakup into asteroids. Lunar silicate rocks are poorer in Fe than achondrites, and this is explained in the model for the genesis of the Moon by the separation of a small metallic core, which sometime (at 3–4 Ga) induced the planet’s magnetic field. Iron from this core was involved into the generation of lunar depressions (lunar maria) filled with Fe- and Ti-rich rocks. In contrast to the parent planets of achondrites, the Moon has a olivine mantle, and this fact predetermined the isotopically heavier oxygen isotopic composition of lunar rocks. This effect also predetermined the specifics of the Earth’s rocks, whose oxygen became systematically isotopically heavier from the Precambrian to Paleozoic and Mesozoic in the course of olivinization of the peridotite mantle, a processes that formed the so-called roots of continents.  相似文献   

8.
对流上地幔Os同位素组成的准确估算是运用Re-Os同位素体系探讨地幔演化的基础。前人研究主要是以地幔橄榄岩为研究对象,由于地幔橄榄岩Os同位素存在明显的不均一性,因而直接影响估算值的准确性。对流上地幔中包含的不同亏损程度的难熔组分在部分熔融过程中难以熔融,对形成的熔体相的Os同位素组成贡献很少或者没有。因此,与对流上地幔具有相同的Os同位素组成初始值的早期分离结晶岩石(如堆晶岩),结合堆晶岩中锆石的准确定年,可以用来估算对流上地幔Os同位素组成。本文根据这一方法测试了那曲地区弧后盆地堆晶岩的Os同位素组成和锆石U-Pb年龄,推测那曲地区新特提斯洋对流上地幔Os同位素组成为碳质球粒陨石型的。根据这一模型,对比了罗布莎和东巧铬铁矿岩、含矿围岩以及不含矿围岩的Os同位素特征,揭示出矿石及围岩均具有古老大陆岩石圈地幔信息,而不含矿围岩(泽当岩体)的Os同位素组成为碳质球粒陨石型的,无古老大陆岩石圈地幔信息。  相似文献   

9.
The mineralogy and bulk compositions of the matrices of the CR chondrites MET 00426 and QUE 99177 have been studied using a combination of SEM, EPMA, and TEM techniques. The matrices of these two chondrites are texturally, chemically, and mineralogically similar and are characterized by significant FeO-enrichments with respect to other CR chondrite matrices, nearly flat refractory lithophile patterns, variable volatile element patterns, and a simple mineral assemblage dominated by amorphous silicate material and Fe,Ni sulfides. Fine-grained, crystalline silicate phases such as olivine and pyroxene appear to be extremely rare in the matrices of both meteorites. Instead, the mineralogy of matrices and fine-grained rims of both meteorites consists of abundant amorphous FeO-rich silicate material, containing nanoparticles of Fe,Ni sulfides (troilite, pyrrhotite, and pentlandite). Secondary alteration minerals that are characteristic of other CR chondrites (e.g., Renazzo and Al Rais), such as phyllosilicates, magnetite, and calcite are also rare. The texture and mineralogy of the matrices of MET 00426 and QUE 99177 share many features with matrices in the primitive carbonaceous chondrites ALH A77307 (CO3.0) and Acfer 094 (unique). These observations show that MET 00426 and QUE 99177 are very low petrologic type 3 chondrites that have escaped the effects of aqueous alteration, unlike other CR chondrites, which are typically classified as petrologic type 2. We suggest that these meteorites represent additional samples of highly primitive, but extremely rare carbonaceous chondrites of petrologic type 3.00, according to the classification scheme of Grossman and Brearley (2005). The highly pristine nature of MET 00426 and QUE 99177 provides important additional insights into the origins of fine-grained materials in carbonaceous chondrites. Based on our new observations, we infer that the amorphous silicate material and nanosulfide particles that dominate the matrices of these meteorites formed in the solar nebula by rapid condensation of material following high-temperature events, such as those that formed chondrules.  相似文献   

10.
《Geochimica et cosmochimica acta》1999,63(13-14):2105-2122
We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature data are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composition of ALH84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity with EET79001-B for siderophile as well as lithophile elements; both are extraordinarily depleted in the “noblest” siderophiles (Os and Ir), to roughly 0.00001 × CI chondrites. As in terrestrial igneous rocks, among martian rocks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. For Mars, Re shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2–3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks generally show less clear-cut trends (relatively few MgO-rich samples are available). These trends are exploited to infer the compositions of the primitive Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth’s, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth’s mantle. This similarity constitutes a key constraint on the style of core-mantle differentiation in both Mars and Earth. Successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles (“high” compared to the lunar and HED mantles, and to models of simple partitioning at typical low-pressure magmatic temperatures), but only predict high Ni for the Earth’s mantle. Models that engender the noble siderophile excess in Earth’s mantle through a uniquely terrestrial process, such as a Moon-forming giant impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth’s size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final ∼ 1% of accretion these bodies acquired sufficient H2O to oxidize most of the later-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core.  相似文献   

11.
Previous estimates of the composition of the EPB have tacitly assumed chondritic abundances of refractory LIL elements. A model is presented here which utilizes refractory element ratios instead. Consideration of these ratios and of known olivine-liquid partition coefficients for Sc, Mg and Si imply that the EPB mantle may be accurately modeled as a simple mixture of 25% eucrite + 75% olivine. The model also implies that eucrites such as Juvinas and Sioux County were derived by large degrees of partial melting (20–30%). Comparison of the model EPB mantle to known meteorite classes indicates a similarity between the element abundances of the EPB and C3 chondrites.  相似文献   

12.
Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk's lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth's mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth's mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.  相似文献   

13.
Chondritic Mg isotope composition of the Earth   总被引：2，自引：0，他引：2  
The processes of planetary accretion and differentiation have potentially been recorded as variations in the stable isotope ratios of the major elements between planetary objects. However, the magnitude of observed isotopic variations for several elements (Mg, Fe, Si) is at the limit of what current analytical precision and accuracy are able to resolve. Here, we present a comprehensive data set of Mg isotope ratios measured in ocean island and mid-ocean ridge basalts, peridotites and chondrites. The precision and accuracy were verified by isotopic standard addition for two samples, one carbonaceous chondrite (Murchison) and one continental flood basalt (BCR-1). In contrast with some previous studies, our data from terrestrial and chondritic materials have invariant Mg isotope ratios within the uncertainty of the method (0.1‰ for the 26Mg/24Mg ratio, 2SD). Although isotopic variations of less than about 0.1‰ could still be present, the data demonstrate that, at this level of uncertainty, the bulk silicate Earth and chondritic Mg reservoir have a homogeneous δ26Mg = −0.23‰ (26Mg/24Mg ratio of the sample relative to the DSM3 standard set to zero by definition). This implies that neither planetary accretion processes nor partial mantle melting and subsequent shallow-level differentiation have fractionated Mg isotope ratios. These observations imply in particular that the formation of the Earth cannot stem from preferential sorting of chondrite constituents that would have been fractionated in their Mg isotope composition. It also implies that unlike oxygen isotopes, there was no zonation in Mg isotopes in the inner solar system.  相似文献   

14.
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   

15.
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   

16.
Stable isotopic data of meteorites are critical for understanding the evolution of terrestrial planets. In this study, we report high-precision vanadium (V) isotopic compositions of 11 unequilibrated and equilibrated L chondrites. Our samples show an average δ51V of ??1.25‰?±?0.38‰ (2SD, n?=?11), which is ~?0.5‰ lighter than that of the bulk silicate Earth constrained by mantle peridotites. Isotopic fractionation in type 3 ordinary chondrites vary from ??1.76‰ to ??1.29‰, whereas the δ51V of equilibrated chondrites vary from ??1.37‰ to ??1.08‰. δ51V of L chondrites do not correlate with thermal metamorphism, shock stage, or weathering degree. Future studies are required to explore the reason for V isotope variation in the solar system.  相似文献   

17.
支霞臣  秦协 《地球科学》2006,31(1):23-30
Re-Os同位素体系为研究地幔的成分-结构-演化提供了新的地球化学示踪和定年的工具.上地幔Os同位素组成演化的球粒陨石模型是Re-Os体系用于地幔物质定年的基础, 尤其在采用Re亏损模式年龄和Os同位素代理等时线年龄时.综合了铁陨石和各类球粒陨石、地幔橄榄岩包体和蛇绿岩豆荚状铬铁矿的Re-Os同位素体系研究的近期成果, 为认识对流上地幔Os同位素组成的演化提供了制约.对河北遵化蛇绿岩豆荚状铬铁矿岩的研究, 获得新太古代(2.5 Ga)时形成豆荚状铬铁矿的对流上地幔的187Os/188Os=0.110 2, 与球粒陨石型模式的一致.文献中常用的球粒陨石模式的参数如下: 地球形成时(4.558 Ga)初始值187Os/186Os为0.095 31, 现代值分别采用碳质球粒陨石的187Os/186Os比值为0.127 0和原始上地幔(PUM)的187Os/186Os比值为0.129 6, PUM与普通球粒陨石和顽火球粒陨石的187Os/186Os比值接近.   相似文献   

18.
With the aim to better understand the cause of the iron isotope heterogeneity of mantle-derived bulk peridotites, we compared the petrological, geochemical and iron isotope composition of four xenolith suites from different geodynamic settings; sub-arc mantle (Patagonia); subcontinental lithospheric mantle (Cameroon), oceanic mantle (Kerguelen) and cratonic mantle (South Africa). Although correlations were not easy to obtain and remain scattered because these rocks record successive geological events, those found between δ57Fe, Mg#, some major and trace element contents of rocks and minerals highlight the processes responsible for the Fe isotope heterogeneity. While partial melting processes only account for moderate Fe isotope variations in the mantle (<0.2 ‰, with bulk rock values yielding a range of δ57Fe ± 0.1 ‰ relative to IRMM-14), the main cause of Fe isotope heterogeneity is metasomatism (>0.9 ‰). The kinetic nature of rapid metasomatic exchanges between low viscosity melts/fluids and their wall-rocks peridotite in the mantle is the likely explanation for this large range. There are a variety of responses of Fe isotope signatures depending on the nature of the metasomatic processes, allowing for a more detailed study of metasomatism in the mantle with Fe isotopes. The current database on the iron isotope composition of peridotite xenoliths and mafic eruptive rocks highlights that most basalts have their main source deeper than the lithospheric mantle. Finally, it is concluded that due to a complex geological history, Fe isotope compositions of mantle xenoliths are too scattered to define a mean isotopic composition with enough accuracy to assess whether the bulk silicate Earth has a mean δ57Fe that is chondritic, or if it is ~0.1 ‰ above chondrites as initially proposed.  相似文献   

19.
We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal-liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.Phosphate inclusions in EET show Δ17O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal-liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.  相似文献   

20.
In three brecciated meteorites, Bencubbin, Cumberland Falls and Plainview, the oxygen isotopic compositions of different rock types within each meteorite were determined to seek genetic relationships between them. In all cases the isotopic compositions are not consistent with derivation from a single parent body. There is no evidence that chondrites and achondrites could be derived from a common parent body. The chondritic inclusions in Bencubbin and Cumberland Falls cannot be identified with any of the ordinary chondritic meteorites. The carbonaceous chondritic fragments in Bencubbin are smilar to, but not identical with, C2 meteorites. The achondritic portion of Bencubbin has a very unusual isotopic composition, which, along with its close relative Weatherford, sets it in a class distinctly apart from other achondrites. Lithic fragments in brecciated meteorites provide a wider range of rock types than is represented by known macroscopic meteorites. Collisions between some meteorite parent bodies were of sufficiently low velocity that fragments of both are preserved in breccias.  相似文献   

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