The compositional classification of chondrites: III. Ungrouped carbonaceous chondrites

Seven carbonaceous chondrites (Allan Hills A77307, Adelaide, Al Rais, Coolidge, Grosnaja, Karoonda and Renazzo) with uncertain classifications were analyzed by instrumental and radiochemical neutron activation analysis for 29 elements: Na, Mg, Al, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Se, Br, Ru, Cd, Sb, La, Sm, Eu, Yb, Lu, Os, Ir and Au. Five of these chondrites (A77307, Adelaide, Al Rais, Karoonda and Renazzo) are unique ‘grouplets’, not closely related to other groups or to each other. Only Coolidge (CV4) and Grosnaja (CV3-an) are members of previously established groups. A77307 and Adelaide have refractory lithophile abundances similar to those in the CM-CO clan; A77307 probably is a member of that clan, but Adelaide, which shows CV-like petrographic characteristics, cannot as yet be assigned to a clan. Al Rais and Renazzo have similar refractory lithophile abundances (essentially at CI levels) and probably belong to the same clan, i.e., formed in the same region of the nebula. There are insufficient data to determine whether they formed at the same general region as the CI chondrites, but separates having O-isotope compositions near the terrestrial fractionation line indicate that this is plausible. Karoonda has refractory lithophile abundances ~ 1.21 × CI and appears to belong to a new clan distinct from CM-CO (1.11 × CI) and CV (1.34×).     

Glasses in the D'Orbigny angrite

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.     

Abundance patterns of thirteen trace elements in primitive carbonaceous and unequilibrated ordinary chondrites

A neutron activation analysis technique was used to determine Au, Re, Co, Mo, As, Sb, Ga, Se, Te, Hg, Zn, Bi and Tl in 11 carbonaceous chondrites, 12 unequilibrated ordinary chondrites (UOC), and 4 equilibrated ordinary chondrites. The first 6 elements are ‘undepleted’, the next 3 ‘normally-depleted’ and the last 4 ‘strongly-depleted’. Except for Hg, ‘depleted-element’ abundances in carbonaceous chondrites lead to mean relative ratios of C1:C2:C3 = 1.00:0.53:0.29, i.e. those predicted by a two-component (mixing of high-temperature and low-temperature fractions) model. The last 4 nominally ‘undepleted’ elements are somewhat depleted in ordinary chondrites, As and Sb showing partial depletion in C3 and the latter in C2 chondrites as well. This requires a modification of the two-component model to indicate that deposition of elements during condensation of high temperature material was not an all-or-nothing process.Apart from Bi and Tl, the elements studied have similar abundances in unequilibrated and equilibrated ordinary chondrites and only the former are unquestionably correlated with the degree of disequilibrium in silicate minerals. Only some ‘strongly-depleted’ elements exhibit at least one of the following—proportional depletion in UOC, progressive depletion in petrographic grades 3–6 ordinary chondrites and enrichment in the gas-containing dark portion of gas-rich, light-dark meteorites—indicating that such depletion does not ensure that an element will exhibit these trends. Partly or completely siderophile As, Au, Co, Ga, Mo, Re and Sb vary with chemical type in the same manner in both unequilibrated and equilibrated ordinary chondrites and doubtless reflect a process involving fractionation of metallic iron.     

Chemical composition and origin of the earth's primitive mantle

An attempt has been made to estimate the chemical composition of the earth's primitive mantle by a critical evaluation of data derived from ultramafic mantle samples and partial melting model calculations for mafic and ultramafic magmas of various ages.Compatible (Al, Ca, Si, Mg, Fe) and moderately incompatible (Ti, Zr, heavy and middle rare earth) elements in basaltic magma sources have not changed significantly since the early Archaean (~3.5 Byr). Estimated abundances for refractory lithophile elements (such as Al, Ca, Ti, Zr, Y, Se, REE etc.) in the primitive mantle are about 2.0 times ordinary chondrites (~ 1.1 times Cl chondrites relative to Mg). Highly incompatible volatile elements (K, Rb, Cs, Tl, Pb etc.) are depleted in the mantle throughout geological time. Abundances of Fe, Ni and Co are obtained on the basis of values for ultramafic nodules and model calculations using komatiites of various ages. The results show little (? 20%?) dispersion and there is no obvious secular variation since 3.5 Byr. Noble metals show similar effects. These data permit constraints to be placed on the timing of core formation.The estimated elemental abundances for the primitive mantle are normalized to Cl chondrites relative to Mg and plotted against the solar condensation temperature at 10^?4 atm. Above 700 K there are two parallel trends which are defined by lithophile elements (Al, Ca, REE, Ti, Mg, Si, Cr, Mn, Na, K, Rb, F, Zn etc.) and siderophile elements (W, Ni, Co, P, As, Ag, Sb and Ge) respectively. The depletion factor for the siderophile trend relative to the lithophile trend is about 0.085. Within each trend there is a continuous depletion towards lower temperature. A third trend is defined by noble metals (Ir, Os, Re, Pd, Pt and Au) with a depletion factor of about 0.003 relative to Cl chondrites. These trends are interpreted in terms of core-mantle differentiation and volatility-controlled processes operating before and during earth accretion.     

Elemental composition of individual chondrules from carbonaceous chondrites,including Allende

Major and trace element analyses of over 180 individual chondrules from 12 carbonaceous chondrites are reported, including individual analyses of 60 chondrules from Pueblito de Allende. Siderophile elements in most chondrules are depleted, compared to the whole chondrite. Correlations of Al-Ir and Ir-Sc among chondrules high in Ca and Al were observed. A Cu-Mn correlation was also found for chondrules from some meteorites. No correlation was observed between Au and other siderophile elements (Fe, Ni, Co and Ir). It is suggested that these elemental associations were present in the material from which the chondrules formed. Compositionally, chondrules appear to be a multicomponent mixture of remelted dust. One component displaying an Al-Ir correlation is identified as Allende-type white aggregates. The other components are a material chemically similar to the present matrix and sulfides-plus-metal material. Abundances of the REE (rare earth elements) were measured in ‘ordinary’ Allende chondrules and were 50% higher than REE abundances in Mokoia chondrules; REE abundances in Ca-Al rich chondrules were similar to REE abundances in Ca-rich white aggregates.     

Formation of the Bencubbin polymict meteoritic breccia

The Bencubbin meteorite is a polymict breccia consisting of a host fraction of ~60% metal and ~40% ferromagnesian silicates and a selection of carbonaceous, ordinary and ‘enstatite’ chondritic clasts. Concentrations of 27 elements were determined by neutron activation in replicate samples of the host silicates and the ordinary and carbonaceous chondritic clasts; 12 elements were determined in the host metal. Compositional data for the ordinary chondrite clast indicate a classification of LL4 ± 1. Refractory element data for the carbonaceous chondrite clast indicate that it belongs to the CI-CM-CO clan; its volatile element abundances are intermediate between those of CM and CO chondrites. Abundances of nonvolatile elements in the silicate host are similar to those in the carbonaceous chondrite clast and in CM chondrites; the rare earths are unfractionated. We conclude that it is not achondritic as previously designated, but chondritic and that it is probably related to the CI-CM-CO clan; its volatile abundances are lower than those in CO chondrites. Oxygen isotope data are consistent with these classifications. Host metal in Bencubbin and in the closely related Weatherford meteorite has low abundances of moderately volatile siderophiles; among iron meteorite groups its nearest relative is group IIIF.We suggest that Bencubbin and Weatherford formed as a result of an impact event on a carbonaceous chondrite regolith. The impact generated an ‘instant magma’ that trapped and surrounded regolithic clasts to form the polymict breccia. The parent of this ‘magma’ was probably the regolith itself, perhaps mainly consisting of the so-called ‘enstatite’ chondrite materials. Accretion of such a variety of materials to a small parent body was probably only possible in the asteroid belt.     

The composition of the Earth

Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an Fe---Ni core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an Fe---Ni alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence ( 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk Fe/Al of 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth ( 14%) and the core ( 86%). Chondritic meteorites display a range of Fe/Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.     

Relict olivine grains, chondrule recycling, and implications for the chemical, thermal, and mechanical processing of nebular materials

Chondrules and isolated forsterites in five low-subtype ordinary chondrites [NWA 3127 (LL3.1), Sahara 97210 (LL3.2), Wells (LL3.3), Chainpur (LL3.4), and Sahara 98175 (LL3.5)] were studied using petrographic, EMPA, and SIMS techniques to better constrain the origin of chondrules and the olivine grains within them. Our results imply that igneous crystallization, vapor fractionation, redox effects, and open-system behavior were important processes. All olivine grains, including normal, relict, and isolated forsterite grains, show evidence for igneous fractionation under disequilibrium conditions, with olivine crystallizing during rapid cooling (closer to 2000 °C/h than to 100 °C/h). Vapor fractionation is manifested by anti-correlated abundances between refractory elements (Al, Sc, Y, Ti, Ca, V) and volatile elements (Cr, Mn, P, Rb, Fe) in olivine. Redox effects are evidenced in various ways, and imply that Fe, Co, Ni, and P were partitioned more into metal, and V was partitioned more into olivine, under reducing conditions in the most FeO-poor melts. There is no obvious evidence for systematic variations in olivine composition according to meteorite subtype, but shock melting in Sahara 97210 resulted in the injection of glass-derived melt into olivine, resulting in artificially high abundances of Ba, Sr, Na, Ti, and some other incompatible elements in olivine. Terrestrial weathering in a hot desert environment may have mobilized Ba and Sr in some glasses.Our data suggest that chondrules in ordinary chondrites experienced repeated thermal, chemical, and mechanical processing during a “recycling” process over an extended time period, which involved multiple episodes of melting under fluctuating redox and heating conditions, and multiple episodes of chondrule break-up in some cases. Forsterite grains, including normal grains in forsterite-bearing type I chondrules, the cores of isolated forsterites, and relict forsterite in type II chondrules, all crystallized from similar, refractory melts under reducing conditions; relict Mg-olivine and isolated forsterite grains were thus derived from type I chondrules. Olivine in type II chondrules, including normal grains and ferroan overgrowths on relict Mg-olivine, crystallized from more volatile-rich, oxidized, and relatively unfractionated melts. Relict dusty olivine grains in type I chondrules were derived from type II chondrules during incomplete melting episodes involving reduction and some vaporization, with clear (non-dusty) grains in dusty olivine-bearing chondrules crystallizing from the reduced and partly vaporized melts. Melt compositions parental to normal olivine grains in type I and II chondrules are systematically enriched in refractory elements compared to bulk chondrule compositions, implying that chondrules often experienced open-system exchange with more volatile-rich surroundings after some olivine had crystallized, possibly while the chondrules were still partly molten. Type II chondrules could have been derived from type I chondrules by the addition of relatively volatile-rich material, followed by re-melting and little evaporation under oxidizing conditions. In contrast, type I chondrules could have been derived from type II chondrules by re-melting involving more-or-less evaporation under reducing conditions. Chemical, oxygen isotope, and petrographic data are best accommodated by a model in which there were several (>2-3, sometimes ?4-5) melting episodes for most chondrules in ordinary chondrites.     

The compositional classification of chondrites—I. The carbonaceous chondrite groups

Twenty carbonaceous chondrites were analyzed by instrumental and radiochemical neutron activation analysis for Na, Mg, Al, K, Ca, Sc, V. Cr, Mn. Fe, Co, Ni, Zn, Ga, Ge, As, Se. Br. Ru, Cd, In, Sb, La, Sm, Eu, Yb, Lu, Os, Ir, and Au. Analysis of 2 or more samples of all but 2 chondrites has helped yield a high precision that allowed the resolution of numerous previously unrecognized trends. Refractory lithophile abundances decrease through the sequence CV (1.33 × CI), CM-CO (1.11 × CI) and CI. The abundances of the common siderophiles Fe, Ni and Co follow the order CI >CM >CO >CV, with CV chondrites depleted about 15% relative to CI. Volatile lithophile (Mn to K) and volatile siderophile (As to Ge) abundances decrease in the order CI >CM >CO >CV. The volatile trends in CO and CV chondrites reverse for the more volatile elements (Br to Cd) producing the sequence CI >CM >CV >CO. These three different sequences in the ordering of group elemental abundances can be used to resolve compositionally the four carbonaceous chondrite groups.We define clans to consist of one or more groups formed at a narrow range of heliocentric distances. Quantization of refractory lithophile abundances indicates the existence of three carbonaceous chondrite clans: CI, CM-CO, and CV. Despite similarities in parameters such as volatile abundances and O-isotope compositions differences in chondrule size and refractory abundances suggest that CO and CV chondrites are indeed best placed in separate clans. The relative heliocentric distance at which CI chondrites formed cannot be inferred, thus it seems safer to assign them to a separate clan.     

The stable carbon isotopes in enstatite chondrites and Cumberland Falls

The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average ¹³C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest ¹³C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.     

The Compositions of Six Chinese Ordinary Chondrites and Element Distributions in Their Different Phases

Six Chinese ordinary chondrites (four of them have fallen in recent years and the trace element abundances have not yet been reported for the other two) were examined.The contents of 21 elements (Na,Cr,Mn,Sc,Se,Zn,Br,Ni,Fe,Co,Ir,Cu,Ga,As,Au,Sb,Os,W,Re,Pt,and Ru)in the magnetic fractions and 20 elements (Na,K,Ca,Sc,Cr,Mn,Fe,Co,Ni,Zn,Se,Br,La,Sm,Eu,Yb,Lu,Ir,Au,and As) in the non-magnetic fractions were de-termined by INAA. The results indicate that the 5 H-group chondrites show almost no difference in composition,but they are different from the Zhaodong L-group chondrite in elemental abundance.As a normalized element(relative to CI),the concentrations of Ga in the magnetic fractions can be used to classify ordinary chondrites(H-,L- and LL-group).The bulk composition and modal weight of each component calculated from element concentrations in different phases are in good agreement with the bulk rock analyses presented in the literature.     

Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in δD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of ∼0‰ (least altered, most volatile rich) and −200‰ (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-δD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a δD value of ∼−158‰, consistent with initial accretion of CM parent bodies at ∼4 AU.     

Reflectance spectrophotometry extended to u.v. for terrestrial,lunar and meteoritic samples

Extension of remote sensing of planetary bodies to the ultraviolet is now feasiable up to 2000 Å from earth-orbiting telescopes and spacecraft. The benefits of this extension is analysed on the basis of laboratory spectra taken on a large variety of terrestrial, lunar and meteoritic samples. Knowledge of the albedo for two wavelengths at 2300 and 6500 Å permits classification of a surface into one of the following types: lunar, carbonaceous chondrites, ordinary chondrites, achondrites or acidic rocks, basaltic rocks, irons. For lunar-type surfaces, a simple albedo measurement at 6500 Å can be converted into quantitative abundance determinations of silicate, aluminium oxide and iron; a large amount of telescopic lunar photometry data is available for mapping these abundances. Extension of the photometry to 2300 Å permits quantitative measurement of TiO₂ abundances. For asteroids and non-icy satellites, rock-type classification and constraints in chemical abundances of Si, Al, Fe and Ti can be derived from photometry at 2300 and 6500 Å. The IUE telescope already orbiting the earth, the Space Telescope to come, the lunar polar orbiter and other spacecraft under prospect are potentially available to provide the photometric observations at 6500 and 2300 Å required.     

太平洋北部铁锰结核富集区沉积物的元素地球化学特征

本文对太平洋北部铁锰结核富集区沉积物的元素地球化学作了较为详细的研究。因子分析提供的信息表明,元素的分布主要受三个因子控制:（1）粘土及Fe、Mn氧化物水化物胶体的吸附作用;（2）生物化学作用过程有关的自生沉积作用;（3）海底页岩风化及附近海区的火山喷发作用。元素的来源:（1）Fe、Mn、Cu、Co、Ni、Zn、Cr、Cr、Mg、Al、Ti、K共生,主要来自粘土吸附;（2）C_有机、N、Sr、Na及Si、Ca、Sr主要来自生物化学过程沉积;（3）Pb主要来源于岩石碎屑（火山喷发碎屑）。     

广东三水盆地古近系心组生油层的元素地球化学特征

沉积物的元素地球化学特征是对沉积盆地水体环境以及古气候条件变化的响应。本文根据元素（Al、Fe、Mg、Ca、K、Na、P、V、Ni、Co、Cr、Cu、Zn、Sr、Ba、Cd、Li、Mn、Pb、Ti）的含量及其比值（Al/Ti、Fe/Mn、Sr/Ba、Mg/Ca、Sr/Ca、Na/Ca、V/Cr、Ni/Co、Ni/V）的变化，对三水盆地古近系心组红岗段生油层的沉积条件进行了系统分析。心组红岗段下部（亚段A）表现为较稳定的地球化学特征。各元素丰度及其比值指示这一时期陆源输入持续较高、且物源组成变化不大。由于海水入侵的影响，湖盆水体盐度相对较高，底部水体以弱氧化条件为主，O₂-H₂S界面位于水/沉积物界面附近。红岗段中上部（亚段B、C）的元素地球化学特征变化较为频繁且幅度很大，反映古气候和湖盆沉积条件的迅速变迁。在潮湿气候条件下，沉积物的地球化学特征表现为以Al、Ti为代表的外源元素含量及其比值较高，而Mg、Ca等盆内化学沉积元素含量较低。古氧气指标指示底部水体为还原环境，有利于有机质保存。因而，相应于较高的有机碳含量。在间歇性干旱时期，陆源输入减少，外源元素含量及其比值显著降低。随着蒸发作用的加强，水体盐度加大，内源元素丰度以及Mg/Ca、Sr/Ba、Sr/Ca和Na/Ca比值大幅度上升。底部水体为氧化环境，O₂-H₂S界面多位于水/沉积物界面或沉积物中。上述两种气候条件在红岗段中上部沉积时期交替出现。红岗段沉积后期由于淡水的长期输入，湖水出现逐渐淡化趋势。     

A revision of the meteorite based cosmic abundance of boron

Analyses of meteorites for B abundances have shown that many chondrites are contaminated with terrestrial B, producing erroneously high meteoritic abundances of this element. Boron concentrations in freshly prepared interior samples are significantly lower than they are in samples with unknown or unspecified terrestrial histories. An estimate of the cosmic abundance based upon the analyses of 8 interior samples of 2 carbonaceous chondrites and 1 interior sample of each of 8 ordinary chondrites is a factor of 6.7 less than the previous low estimate. Our revised value, 3.0 B/10¹⁰H, is in excellent agreement with estimates based on observations of the solar photosphere. There is no longer a need to consider processes that enrich B in carbonaceous chondrites or deplete it in the sun. Relative meteoritic abundances of Li, Be and B are now in general agreement with models of nucleosynthesis of these light elements by galactic cosmic ray induced spallation.     

The elemental composition of plankton

Phytoplankton samples, collected in Monterey Bay, California, were analyzed for their Pb, Hg, Cd, Co, Ag, Cr, Ti, V, Mn, Ni, Cu, Fe, Zn, AI, Mo, Ba, Sr, K, Ca, Mg, Na and SiO₂ content. The results of these analyses were categorized on a chemical basis and the sample data were placed in three groups: Group I, Ti not detected; Group II, Ti detected; and Group III, Sr concentrators present. Levels of most elements were higher in Groups II and III for a variety of reasons that are discussed in the text. The siliceous frustules, remaining after organic-matter digestion, were also analyzed for the elements listed above. Significant amounts of Al, Ti, Fe, Mn, Cu and Zn were found.Zooplankton and microplankton samples, collected in Monterey Bay, California; off the coast of Oregon; and on a transect between Hawaii and Monterey, were also analyzed for the elements listed above (except Si). In general, element levels in the inshore and offshore zoo-plankton were similar; however, the microplankton samples, in which strontium was highly concentrated, were almost always higher in Pb, Hg, Cu, Fe and Zn.     

Coupled Cu and O excesses in chondrites

Recent developments in multiple-collector magnetic-sector ICP-MS (inductively coupled plasma-mass spectrometry) have permitted the relative abundances of the two isotopes 63 and 65 of copper to be measured with unprecedented precision (40 ppm). Here, we report Cu isotopic variations among eight carbonaceous chondrites (CCs) from the CI, CM, CO, and CV groups and the presently ungrouped Tagish Lake, and 10 ordinary chondrites (OCs) from the H, L, and LL groups. The widest isotopic range of ∼0.8‰ per a.m.u. is observed for the carbonaceous chondrites. Copper in carbonaceous chondrites becomes isotopically lighter with petrologic type in the order 1 to 3 but seems extremely homogeneous for each type. The Cu isotopic composition of Tagish Lake confirms its other characteristics that are intermediate between CI and CM. In three of the groups (CI-CM-CO), as well as for Tagish Lake, ⁶³Cu excess over terrestrial mantle abundances correlates well with ¹⁶O excess. For all four groups, ⁶³Cu excess also correlates remarkably well with elemental refractory/volatile ratios (e.g., Ca/Mn). For ordinary chondrites, small differences exist between the H, L, and LL groups, with Cu becoming isotopically heavier in that order. Equilibrated and unequilibrated samples, however, exhibit the same Cu isotopic signature within each group. Although the range of Cu isotopic compositions in ordinary chondrites is smaller than in carbonaceous chondrites, ⁶³Cu excesses still correlate with ¹⁶O excesses. The observed trends of isotopic variation seem incompatible with a single-stage fractionation process by either volatilization or low-temperature metamorphism. The correlations between ⁶³Cu excesses and ¹⁶O excesses suggest the presence of at least two and perhaps three isotopically distinct Cu reservoirs in the early Solar System: (1) an Earth-like reservoir common to the CI and LL probably representing the main Cu stock of the inner Solar System, (2) a reservoir present in all carbonaceous chondrites, but most abundant in CV, with large ⁶³Cu and ¹⁶O excesses (this reservoir is probably hosted in refractory material), and (3) possibly a third reservoir present in ordinary chondrites. The OC trend may also be explained as a mixture of the first two Cu reservoirs if its oxygen was first equilibrated with nebular gas. The coexistence of ⁶³Cu and ¹⁶O excesses in the same component raises the issue of how volatile Cu was preserved in refractory material. A strong correlation between ⁶³Cu/⁶⁵Cu and Ni/Cu ratios suggests that ⁶³Cu excess may have originated as more refractory ⁶³Ni (T_1/2 = 100 yr) upon irradiation of refractory grains by electromagnetic flares and particle bursts during the T-Tauri phase of the Sun.     

Condensation in the primitive solar nebula

The distribution of the major elements between vapor and solid has been calculated for a cooling gas of cosmic composition. The assumption is made that high temperature condensates remain in equilibrium with the vapor, affecting the temperatures of appearance of successively less refractory phases. The model suggests that the major textural features and mineralogical composition of the Ca, Al-rich inclusions in the C3 chondrites were produced during condensation in the nebula characterized by slight departures from chemical equilibrium due to incomplete reaction of high temperature condensates. Fractionation of such a phase assemblage is sufficient to produce part of the lithophile element depletion of the ordinary chondrites relative to the cosmic abundances.     

Chemical composition of stars in the galactic halo

The chemical compositions of the atmospheres of six metal-poor stars are analyzed. Spectra with signal-to-noise ratios of no less than 100 and a resolution of R≈17 000 were obtained using the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The abundances of Li, O, α-process elements (Mg, Si, Ca, Ti), Na, K, Sc, iron-peak elements (Cr, Mn, Fe, Ni, Cu, Zn), and s-process elements (Y, Ba) are derived. The star G251-54 ([Fe/H]=?1.55, T _eff=5541 K, logg=3.58) is deficient in some elements compared to both stars with similar metallicities and the Sun. The atmosphere of G251-54 has the following elemental abundances relative to iron: [O/Fe]=+0.47, [α/Fe]≈?0.3, [Na/Fe]=?0.60, [Sc/Fe]=?0.57, [Cr, Ni, Fe]≈0, [Zn/Fe]=+0.16, [Cu/Fe]=?0.66, [Y/Fe]=?0.70, and [Ba/Fe]=?1.35. The remaining five stars have metallicities in the range ?1.6<[Fe/H]相似文献