The distribution of trace elements in carbonaceous chondrites     

Hans-Josef Knab 《Geochimica et cosmochimica acta》1981,45(9):1563-1572

12 carbonaceous chondrites, amongst them representatives of nearly all known petrologic types were analyzed for twenty trace elements by spark source mass spectrography combined with the isotope dilution method. Data on different element groups (refractory, moderately volatile and volatile) show that the distribution of the trace elements in the carbonaceous chondrites, with the exception of Renazzo, can be well explained by Anders' two-component model. This is also valid for the highly metamorphosed CV5 chondrite Karoonda.Furthermore, it is observed that the $\text{Zr}\text{Hf}$-ratios in the carbonaceous chondrites increase with increasing petrologic type which is interpreted as the result of mixing two components with different $\text{Zr}\text{Hf}$-ratios  相似文献   

New data on the abundance of palladium in meteorites     

N Mermelengas  J.R De Laeter  K.J.R Rosman 《Geochimica et cosmochimica acta》1979,43(5):747-753

The concentration of Pd in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites, 29 iron meteorites and other samples has been determined by stable isotope dilution using solid source mass spectrometry. The Cl chondrite Orgueil gives a ‘cosmic’ abundance for Pd of 1.5 (Si = 10⁶ atoms), in good agreement with the currently accepted value.The concentration of Pd shows little variation among the carbonaceous chondrites, but in ordinary chondrites decreases from the H to L to LL groups. Pd in achondrites is approx 100 times lower than in chondrites. Data for iron meteorites plot around the ‘cosmic’ $\text{Pd}\text{Ni}$ ratio; however the Pd data falls into distinct groups, corresponding to the chemical group classification. These results support the hypothesis that at least two fractionation processes have occurred during the formation of iron meteorites.  相似文献   

Electron microprobe study of a ‘mysterite’-bearing inclusion from the Krymka LL-chondrite     

Lawrence Grossman  John M. Allen  Glenn J. MacPherson 《Geochimica et cosmochimica acta》1980,44(2):211-216

The black inclusion from the Krymka LL3 chondrite previously found to contain ‘mysterite’ by Lewiset al. (1979) belongs to a hitherto unknown class of carbonaceous chondrites. Its olivine and pyroxene compositions. Fo 97–99 and En 96, respectively, are characteristic of carbonaceous chondrites and its plagioclase composition. An 100, is characteristic of C3's. It contains a peculiar group of Co-, Cr-rich metal grains whose compositions are similar, but not identical, to those in C2 chondrites and which also bear some similarities to those in Renazzo. Its weight ratios of total $\text{Fe}\text{SiO}{}_2$ and $\text{solSiO}{}_2\text{MgO}$ are 0.74 and 1.43, respectively, and its atomic ratio of $\text{Si}\text{Al}$ is 10.7, exactly the same as in carbonaceous chondrites. Its bulk chemical composition is very close to that of the Murchison C2 chondrite. The association of mysterite with a special type of carbonaceous chondrite material suggests that mysterite formed by low-temperature condensation in a different region of the nebula from other carbonaceous chondrites.  相似文献   

Are C1 chondrites chemically fractionated? a trace element study     

Mitsuru Ebihara  Rainer Wolf  Edward Anders 《Geochimica et cosmochimica acta》1982,46(10):1849-1861

Six C1 chondrite samples and a C2 xenolith from the Plainview H5 chondrite were analyzed by radiochemical neutron activation for the elements Ag, Au, Bi, Br, Cd, Ce, Cs, Eu, Ge, In, Ir, Lu, Nd, Ni, Os, Pd, Pt, Rb, Re, Sb, Se, Sn, Tb, Te, Tl, Yb, and Zn. The data were combined with 9 earlier analyses from this laboratory and examined for evidence of chemical fractionation in C1 chondrites.A number of elements (Br, Rb, Cs, Au, Re, Os, Ni, Pd, Sb, Bi, In, Te) show small but correlated variations. Those of the first 8 probably reflect hydrothermal alteration in the meteorite parent body, whereas those of Sb, Bi, In, and Te may at least in part involve nebular processes. Br and Au show systematic abundance differences from meteorite to meteorite, which suggests hydrothermal transport on a kilometer scale. The remaining elements vary from sample to sample, suggesting transport on a centimeter scale.There is no conclusive evidence for nebular fractionation affecting C1 's. Though C1 chondrites have lower $\text{Zr}\text{Hf}$ and $\text{Ir}\text{Re}$ ratios than do other chondrite classes, these ratios vary in other classes, suggesting that those classes rather than C1's are fractionated. Three fractionation-prone REE—Ce, Eu, and Yb have essentially the same relative abundances in C1's and all other chondrite classes, and hence apparently are not fractionated in C1's. We did not confirm the large Tb and Yb variations in C1's reported by other workers.We present revised mean C1 abundances for 35 elements, based on the new data and a critical selection of literature data. Changes are generally less than 10%, except for Br, Rb, Ag, Sb, Te, Au, and the REE.The Plainview C2 xenolith has normal trace element abundances, except for 3 elements falling appreciably above the C2 range: Rb, Cs, and Bi. Hydrothermal alteration may be the reason for all 3, though nebular fractionation remains a possibility for Bi.  相似文献   

The compositional classification of chondrites: IV. Ungrouped chondritic meteorites and clasts     

Gregory W. Kallemeyn  John T. Wasson 《Geochimica et cosmochimica acta》1985,49(1):261-270

Six specimens of unusual chondritic materials were analyzed by neutron activation for 30 elements in order to assess their degree of chondritic compositional pristinity and to search for evidence of genetic links to other chondrites. Five have highly recrystallized textures; the other, the Cumberland Falls chondrite, has suffered minor metamorphic recrystallization. Acapulco and Allan Hills A77081, are closely related and have subpristine compositions; they are more distantly related to Enon which has an altered composition. Udei Station appears to be a IAB meteorite even though its $\text{FeO}\text{(FeO + MgO)}$ ratio is slightly above the IAB field. The highly weathered meteorite Tierra Bianca is closely related to IAB but has a δ¹⁸O value 5 standard deviations higher than the IAB mean and is designated ungrouped. Udei Station and Tierra Bianca have altered compositions; rare earth element patterns indicate loss of a phosphate phase. The elemental composition of the Cumberland Falls chondrite is virtually identical to that of LL chondrites and its O-isotope composition is closely similar to those of some unequilibrated ordinary chondrites including LL Semarkona. The $\text{FeO}\text{(FeO + MgO)}$ ratios in its olivine are generally much lower than those in pyroxene, a relationship we show to be indicative of in situ reduction resulting from exchange with the aubritic host. The names winonaites and forsterite chondrites have no taxonomic utility.  相似文献   

Ries impact crater,southern Germany: search for meteoritic material     

John W. Morgan  Marie-josée Janssens  Jan Hertogen  Jacques Gros  Hiroshi Takahashi 《Geochimica et cosmochimica acta》1979,43(6):803-815

Twenty-three samples from the Ries crater, representing a wide range of shock metamorphism, were analyzed for seven siderophile elements (Au, Ge, Ir, Ni, Os, Pd, Re) and five volatile elements (Ag, Cd, Sb, Se, Zn). Taking Ir as an example, we found siderophile enrichments over the indigenous level of 0.015 ppb Ir occur in only eight samples. The excess is very modest; even the most enriched samples (a weakly shocked biotite gneiss and a metal-impregnated amphibolite) have Ir, Os corresponding to ~4 × 10^?4 C1 chondrite abundances. Of five flädle glasses analyzed only one shows excess Ir. Suevite matrix and vesicular glass have slight enrichment, but homogenous glass from the same rock does not. In flädle glasses, Ni and Se are strongly correlated and apparently reside in Ir, Os-poor Sulfides [pyrrhotite, chalcopyrite, pentlandite(?)]of terrestrial, probably sedimentary, origin. The Ir, Os and Ni enrichments of the metal-bearing amphibolite are compatible with chondritic ratios, but these are ill-defined because of uncertainty in Ni. In the other samples enriched in siderophiles Ir(Os), Ni and Se are mutually correlated; $\text{Ni}\text{Ir}$ and $\text{Ni}\text{Os}$ ~ 11 × C1 and are much higher than any chondritic ratios; $\text{Se}\text{Ni}\text{~ 2 × C1}$ and suggests a sulfide phase, rather than metal may be the host of the correlated elements. Lacking a plausible local source, this material is apparently meteoritic in origin. The unusual elemental ratios, coupled with the very low enrichments, tend to exclude chondrites and most irons as likely projectile material. Of the achondrites, aubrites seem slightly preferable. Ratios of excess siderophiles in Ries materiel match tolerably those of an aubrite (possibly atypical) occurring as an inclusion in the Bencubbin meteorite, Australia. The Hungaria group of Mars-crossing asteroids may be a source of aubritic projectiles.  相似文献   

Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution     

P.Jonathan Patchett 《Geochimica et cosmochimica acta》1983,47(1):81-91

The ¹⁷⁶Lu-¹⁷⁶Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes ${}^{176}\text{Hf}{}^{177}\text{Hf}$ a powerful geochemical tracer. In general, proportional variations of ${}^{176}\text{Hf}{}^{177}\text{Hf}$ exceed those of ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}$ by factors of 1.5–3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in ${}^{176}\text{Hf}{}^{177}\text{Hf}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ and ${}^{143}\text{Nd}{}^{\mathrm{l44}}\text{Nd}\text{-}{}^{87}\text{Sr}{}^{86}\text{Sr}$ diagrams, ${}^{176}\text{Hf}{}^{177}\text{Hf}$ and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ display a single, linear isotopic variation in the suboceanic mantle. These discordant ${}^{87}\text{Sr}{}^{86}\text{Sr}$ relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased ${}^{176}\text{Hf}{}^{177}\text{Hf}$, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial ${}^{176}\text{Hf}{}^{177}\text{Hf}$. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find wide application in pure chronological studies.  相似文献   

Primary fractionation and secondary alteration within an Archean ultramafic lava flow     

A. E. Beswick 《Contributions to Mineralogy and Petrology》1983,82(2-3):221-231

Fifteen samples across a 4 m thick komatiite flow from the Val d'Or region in the southeast portion of the Abitibi greenstone belt have been analysed for major oxides and trace elements including the rare earth elements (REE). The flow has been subjected to low grade regional metamorphism: virtually all primary mineralogy (olivine and clinopyroxene) has been obliterated although primary textures are well preserved. Compositional differences between the upper, spinifex textured portion and the underlying massive portion of the flow are largely consistent with the primary fractionation of approximately 30% olivine with a composition close to Fo92. Variations in incompatible element ratios across the flow and in enrichments between the spinifex and massive units suggest that Si, Ce and possibly Lu have been lost to, and Ca, Sr and possibly Y have been gained from, the surroundings. Remobilization of other elements (e.g. Zr, Hf, and alkali metals and most of the REE) appears to have been confined within the flow. AI, Ti, V and Sc appear to have been immobile during alteration of the flow. For the flow as a whole values for many element ratios (e.g. Al/Zr, Al/Sc, Sc/Yb, Zr/Hf, K/Rb) are very similar to chondritic values. If the flow represents a 40% melt approximately, and if the residue was essentially dunitic then the source abundance for most elements was close to chondritic; exceptions are the REE (1.5 to 2×chondrite), Ti (enriched relative to chondrites), and V (depleted relative to chondrites).  相似文献   

$\text{Ca}\text{Al}$ ratio and composition of the Earth's upper mantle     

H Palme  K.G Nickel 《Geochimica et cosmochimica acta》1985,49(10):2123-2132

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 $\text{Al}\text{Ti}$ and $\text{Sc}\text{Ti}$ ratios were only obtained for MgO contents below 36%. A chrondritic $\text{Sc}\text{Yb}$ 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 $\text{Ca}\text{Al}$ 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 $\text{Ca}\text{Al}$ ratios. It is therefore suggested that this non-chondritic $\text{Ca}\text{Al}$ 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 $\text{Ca}\text{Al}\text{-}\text{ratio}$. 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 $\text{Ca}\text{Al}\text{-}\text{ratio}$ 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 ($\text{Al}\text{Si}$ relative to Cl) for the primordial upper mantle.  相似文献   

Melt composition control of Zr/Hf fractionation in magmatic processes   总被引：9，自引：0，他引：9  

Robert L. Linnen  Hans Keppler 《Geochimica et cosmochimica acta》2002,66(18):3293-3301

Zircon (ZrSiO₄) and hafnon (HfSiO₄) solubilities in water-saturated granitic melts have been determined as a function of melt composition at 800° and 1035°C at 200 MPa. The solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt. Moreover, the molar ratio of zircon and hafnon solubility is a function of melt composition. Although the solubilities are nearly identical in peralkaline melts, zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts. Accordingly, calculated partition coefficients of Zr and Hf between zircon and melt are nearly equal for the peralkaline melts, whereas for metaluminous and peraluminous melts D_Hf/D_Zr for zircon is 0.5 to 0.2. Consequently, zircon fractionation will strongly decrease Zr/Hf in some granites, whereas it has little effect on the Zr/Hf ratio in alkaline melts or similar depolymerized melt compositions.The ratio of the molar solubilities of zircon and hafnon for a given melt composition, temperature, and pressure is proportional to the Hf/Zr activity coefficient ratio in the melt. The data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts. However, it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached. For most ferromagnesian minerals in equilibrium with a depolymerized melt, D_Hf > D_Zr. Typical values of D_Hf/D_Zr range from 1.5 to 2.5 for clinopyroxene, amphibole, and titanite. Because of the change in the Hf/Zr activity ratio in the melt, the relative fractionation of Zr and Hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite. It is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in Zr/Hf, whereas fractional crystallization of depolymerized melts tends to increase Zr/Hf. There is no need to invoke fluid metasomatism to explain these effects. Results demonstrate that for ions with identical charge and nearly identical radius, crystal chemistry does not alone determine relative compatibilities. Rather, the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice.  相似文献   

Chemistry of Zircon in Rare Metal Granitoids and Associated Rocks, Eastern Desert, Egypt     

Hamdy M. Abdalla    Hossam Helba    Hiroharu Matsueda 《Resource Geology》2009,59(1):51-68

Typological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon‐bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb‐enriched, A₁‐granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y‐enriched, A₂‐type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ≥ Nb + Sn + Be ± W‐enriched, Li‐albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± W‐enriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well‐developed pyramidal faces and short prisms (i.e. pseudo‐octahedral form) with length/width ratios in the range of 2:1–1:1. It is of Zr_0.990Hf_0.007SiO₄ composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr_0.99Hf_0.01SiO₄ composition and is associated with sub‐ to transolvus assemblage of K‐feldspar, quartz, plagioclase and annite‐siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1–3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li‐albite granite ranges between Zr_0.925Hf_0.075SiO₄ and Zr_0.705Hf_0.295SiO₄. It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1–2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li‐white mica), indicates a higher solubility of Hf‐fluoride complexes and their more stabilized state at lower temperature in Li‐ and F‐rich sodic melts. Zircon of apogranite association ranges in composition between Zr_0.967Hf_0.013SiO₄ in the lower unaltered alkali feldspar granite zone and Zr_0.805Hf_0.064(Y, U, Th, heavy rare‐earth elements) [HREE])_0.125SiO₄ in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to reflect a roofward increasing in μ_KF, μ_NaF, and μ_HF of the exsolved fluids. Columbite, xenotime, thorite, cassiterite, beryl and fluorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1–0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, reflecting disequilibrium conditions (e.g. sudden change in P, T, salinity, and pH) between the growing crystals and the exsolved fluids.  相似文献   

Minor and trace elements in some meteoritic minerals     

Ralph O. Allen  Brian Mason 《Geochimica et cosmochimica acta》1973,37(6):1435-1456

The mineral phases including olivine, orthopyroxene, clinopyroxene, troilite, nickel-iron, plagioclase, chromite and the phosphates were separated from several meteorites. These were a hypersthene chondrite (Modoc), a bronzite chondrite (Guareña), an enstatite chondrite (Khairpur), and two eucrites (Haraiya and Moore County); diopside was separated from the Nakhla achondrite. The purified minerals were analyzed for trace and minor elements by spark source mass spectrometry and instrumental neutron activation analysis. On the meteorites examined our results show that Co, Ni, Cu, Ge, As, Ru, Rh, Pd, Sn, Sb, W, Re, Os, Ir, Pt and Au are entirely or almost entirely siderophile; Na, Rb, Sr, Y, Ba and the rare earth elements lithophile; Se chalcophile. The transition elements So, Ti, V, Cr and Mn are lithophile in most stony meteorites, but show chalcophile affinities in the enstatite chondrites (and enstatite achondrites), as do Zn, Zr and Nb. In the ordinary chondrites Ga shows both lithophile and siderophile affinities, but becomes entirely siderophile in the enstatite chondrites. Molybdenum and tellurium show strong siderophile and weaker chalcophile affinity. The lithophile elements are distributed among the minerals according to the crystallochemical factors, the most effective controlling factor being ionic size.  相似文献   

Absolute isotopic abundances of Ti in meteorites     

F.R Niederer  D.A Papanastassiou  G.J Wasserburg 《Geochimica et cosmochimica acta》1985,49(3):835-851

The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using ${}^{46}\text{Ti}{}^{48}\text{Ti}$. Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. We provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components. The absolute Ti and Ca isotopic compositions still support the correlation of ⁵⁰Ti and ⁴⁸Ca effects in the FUN inclusions and imply contributions from neutron-rich equilibrium or quasi-equilibrium nucleosynthesis. The present identification of endemic effects at ⁴⁶Ti, for the absolute composition, implies a shortfall of an explosive-oxygen component or reflects significant isotope fractionation. Additional nucleosynthetic components are required by ⁴⁷Ti and ⁴⁹Ti effects. Components are also defined in which ⁴⁸Ti is enhanced.Bulk samples of carbonaceous meteorites (C2 and C3 types) show distinct excesses at ⁵⁰Ti but no nonlinear effects at the other Ti isotopes. Other chondrites, including Orgueil (Cl), show no nonlinear effects. Relative to terrestrial Ti, a small isotope fractionation is found for only an enstatite chondrite. The Ti absolute compositions in Ca-Al-rich inclusions show significant isotope fractionation effects corresponding to an enhancement in the heavier isotopes relative to the lighter isotopes as compared to Ti in a TiO₂ standard and in chondrites. The absence of a correlation of Ti isotope fractionation effects with those for Ca and Mg is indicative of multiple processes of condensation, volatilization and recondensation; however, the mechanisms causing the isotope fractionation are not well understood.  相似文献   

Zircon Lu-Hf systematics and the evolution of the Archean crust in the southern Superior Province,Canada   总被引：2，自引：0，他引：2  

Patrick E. Smith  Mitsunobo Tatsumoto  Ronald M. Farquhar 《Contributions to Mineralogy and Petrology》1987,97(1):93-104

A combined Lu-Hf and U-Th-Pb isotopic study was made of 25 zircons and 2 whole rocks from the late Archean crust (2,888-2,668 Ma) in the southern Superior Province, Canada. The relative abundances of U, Th, Lu and Hf in zircons from the low grade Michipicoten and Gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks. Zircons from the high grade lower crustal regions adjacent to these belts (Kapuskasing Structural Zone) are distinguished from the low grade zircons by their strong depletions of Lu and Hf. The low Hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the Zr/Hf ratio.Initial Hf isotope ratios for Hf in zircons from the low grade rocks are correlated with silica enrichment of their host rocks. e _Hf varies from +9.2 to –1.3 and data from similar rock types exhibit correlations of e _Hf with time. Whole rock basalt analyses yield e _Hf values of +8.7 and +11.3 suggesting their derivation from a depleted mantle. The basalt data fall on an evolution trend which implies that differentiation from a chondritic mantle occurred at 3,100-2,900 Ma. Low e _Hf values (–1.3 to +1.4) for rhyolites and granites are consistent with a derivation involving remelting of old crust similar to a 2,888 Ma granite with e _HF of +0.5. Significantly higher values (+1.4 to +3.9) are found in zircons from 2,748-2,682 Ma dacites and tonalites suggesting that their parent rocks had higher Lu/Hf ratios. This may indicate that their parent rocks were mafic. However, there is some evidence that the possible lower crustal source reservoirs of these rocks may have undergone processes early in their histories which increased their Lu/ Hf ratios. This would give rise to the higher e _Hf values observed in their derivatives.  相似文献   

The compositional classification of chondrites: II The enstatite chondrite groups     

Derek W. Sears  Gregory W. Kallemeyn  John T. Wasson 《Geochimica et cosmochimica acta》1982,46(4):597-608

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis.Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are $\text{?1.3 ± 0.6}$ Ma for EH chondrites and $\text{2.9 ± 0.5}$ Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun.The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.  相似文献   

Titanium isotopic anomalies in meteorites     

S Niemeyer  G.W Lugmair 《Geochimica et cosmochimica acta》1984,48(7):1401-1416

High-precision analyses of Ti are reported for samples from a variety of meteorite classes. The expanded data base for Allende inclusions still shows Ti isotopic anomalies in every inclusion. All the coarse-grained inclusions give quite similar patterns, but fine-grained inclusions show more variable, and sometimes larger, anomalies. One inclusion, 3675A, was analyzed because others identified it as a possible “FUN” inclusion due to its mass-fractionated Mg. This designation is supported by the significantly more complex Ti isotopic pattern for 3675A compared to all our other Allende inclusions. Available data fail to suggest that any particular Allende mineral phase, including a chromite-carbon fraction from an acid residue, is especially rich in anomalous Ti. We also find anomalous Ti in a bulk sample of a Cl chondrite and in matrix separates from C2 chondrites. The excesses of ⁵⁰Ti are smaller than for Allende inclusions, and subtle differences in Ti isotopic patterns tentatively suggest that parent materials for C1-C2 matrix and Allende inclusions are not directly related. Analyses of chondrules from unequilibrated ordinary chondrites did not yield clear evidence for anomalous Ti, but some “larger than usual” deficits at $\text{50}\text{46}$ give encouragement for future work in this direction. Comparing the magnitude of isotopic shifts at ⁵⁰Ti and ¹⁶O for all these meteorite samples indicates that they are not correlated, but the data do not preclude a correlation between concentrations of “exotic” ⁵⁰Ti and ¹⁶O atoms.Whether or not Allende “FUN” inclusions are considered, at least 4 distinct isotopic components of Ti are required to account for the observed isotopic variations. The Ti data cannot be plausibly explained in terms of an early solar-system particle irradiation; instead, neutron-rich hydrostatic burning within a star is probably responsible for the dominant ⁵⁰Ti anomalies, while s-process mechanisms are viable sources for some of the more subtle Ti variations. We suggest that the Ti anomalies are linked to a diversity of nucleosynthetic sources and the highly refractory behavior of Ti. Therefore, some form of “chemical memory” from the ISM, rather than “late stage supernova injection”, is most likely responsible for the preservation of observed isotopic heterogeneities.  相似文献   

The geochemistry of basalts from a back-arc spreading centre in the East Scotia Sea     

Andrew D. Saunders  John Tarney 《Geochimica et cosmochimica acta》1979,43(4):555-572

Rapid sea floor spreading has taken place over the last 8 Myr behind the South Sandwich island arc, producing a regular set of magnetic lineations. Suites of fresh basalts have been dredged from four widely separated localities along the spreading axis. Dredges 20 and 23 yielded sub-alkaline olivine tholeiites, dredge 22 recovered vesicular tholeiites with minor normative olivine, while dredge 24 contained a fractionated suite of highly vesicular quartz-normative basalts with higher $\text{Fe}\text{Mg}$. The concentrations of the incompatible elements Ti, P, Zr, Hf, Nb, Ta, Y and the REE increase systematically from dredge 24 through dredges 22 and 20 to dredge 23 and there is a comparable increase in $\text{Ce}{}_{\mathrm{N}}\text{Yb}{}_{\mathrm{N}}$. Quantitative modelling suggests that all the basalts can be derived from an essentially similar mantle source (with respect to these elements) through varying degrees of partial melting, but involving some residual clinopyroxene. Basalts from dredge 24 have unusually low concentrations of Ti, P, Zr, Nb, Y, REE and Ni, similar to the values in arc tholeiites, and the more primitive dredge 24 liquids seem to have been generated through high degrees of partial melting (~ 30%) leaving a dunitic residue. Transitional arc tholeiite characteristics are also apparent in the relatively high K, Rb, Ba contents and ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of dredge 24 and 22 basalts, though Nd isotope ratios are uniform. It is considered that fluids derived from the dehydrating subducted slab may have locally penetrated the source regions of the back-arc basalts, carrying K, Rb, Ba and seawater-enriched ⁸⁷Sr, and producing conditions of magma generation similar to that of arc tholeiites. However, it is unlikely that the sources for these and other marginal basin basalts differ fundamentally from the range of mantle sources feeding normal mid-ocean ridges.  相似文献   

“Mysterite” : a late condensate from the solar nebula     

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

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

Geochemical characteristics of boninite series volcanics: implications for their source   总被引：2，自引：0，他引：2  

Rosemary L. Hickey  Frederick A. Frey 《Geochimica et cosmochimica acta》1982,46(11):2099-2115

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A carbonaceous inclusion from the Krymka LL-chondrite: noble gases and trace elements     

Roy S. Lewis  Leo Alaerts  Jan Hertogen  Marie-Josée Janssens  Herbert Palme  Edward Anders 《Geochimica et cosmochimica acta》1979,43(6):897-903

A black inclusion from the Krymka LL3 chondrite was analyzed for 20 trace elements and five noble gases, by radiochemical neutron activation and mass spectrometry. The trace element pattern somewhat resembles that of C1 or C2 chondrites, but with several unique features. Elements of nebular condensation T ? 1000 K (U, Re, Os, Ir, Ni, Pd, Au, Sb and Ge) are essentially undepleted, as in C1 chondrites, but $\text{Re}\text{Ir}$ is 1.49 × higher than the characteristic Cl value. Among elements condensing below 1000 K, Cs, Se, Te, and In are depleted to approximately C2 levels (~0.6 × C1), whereas Ag, Bi, Tl are enriched to ~ 1.6 × C1. Such enrichments are thought to be characteristic of late nebular condensates.The noble-gas pattern also is unique. Gas contents are higher than in C1s, by factors of 2.6 to 19 for Ne through Xe. The $\text{Ar}{}^{36}\text{Xe}{}^{132}$ ratio of 500 is higher than mean values for C1s or C2s (109 or 89) and exceeds even the highest value seen in C3Os, 420, whereas the $\text{He}{}^4\text{Ne}{}^{20}$ ratio of 62 is much lower than the values for C1s and C2s (200–370). The $\text{Xe}{}^{129}\text{Xe}{}^{132}$ and $\text{Xe}{}^{136}\text{Xe}{}^{\mathrm{l32}}$ ratios of 1.040 and 0.320 resemble those of C1 chondrites, and seem to imply typical proportions of radiogenic Xe¹²⁹ and ‘fissiogenic’ xenon.It appears that the inclusion represents a new primitive meteorite type, similar to C-chondrites, but probably a late condensate from a region of higher nebular pressure.  相似文献