Chemical composition of Mars

The composition of Mars has been calculated from the cosmochemical model of Ganapathy and Anders (1974) which assumes that planets and chondrites underwent the same 4 fractionation processes in the solar nebula. Because elements of similar volatility stay together in these processes, only 4 index elements (U, Fe, K and Tl or Ar³⁶) are needed to calculate the abundances of all 83 elements in the planet. The values chosen are U = 28 ppb, K = 62 ppm (based on $\text{K}\text{U}\text{= 2200}$ from orbital γ-spectrometry and on thermal history calculations by Toksöz and Hsui (1978) Fe = 26.72% (from geophysical data), and Tl = 0.14 ppb (from the Ar³⁶ and Ar⁴⁰ abundances measured by Viking).The mantle of Mars is an iron-rich [Mg/(Mg + Fe) = 0.77] garnet wehrlite (ρ = 3.52?3.54 g/cm³), similar to McGetchin and Smyth's (1978) estimate but containing more Ca and Al. It is nearly identical to the bulk Moon composition of Morgan et al. (1978b). The core makes up 0.19 of the planet and contains 3.5% S—much less than estimated by other models. Volatiles have nearly Moon-like abundances, being depleted relative to the Earth by factors of 0.36 (K-group, T_cond = 600–1300 K) or 0.029 (Tl group, T_cond < 600 K). The water abundance corresponds to a 9 m layer, but could be higher by as much as a factor of 11.Comparison of model compositions for 5 differentiated planets (Earth, Venus, Mars, Moon, and eucrite parent body) suggests that volatile depletion correlates mainly with size rather than with radial distance from the Sun. However, the relatively high volatile content of shergottites and some chondrites shows that the correlation is not simple; other factors must also be involved.     

The bulk composition of Mars

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.     

The major element composition of the upper mantle estimated from the composition of lherzolites

The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.     

The molecular composition of ambers

Bulk (elemental composition, IR, CP/MAS ¹³C NMR) and molecular (GC-MS) analyses have been performed on a series of ambers and resins derived from different locations (Dominican Republic, Philippines, Canada, Israel, New Zealand, Chile) having diverse botanical affinities (Araucariaceae, Hymenaea) and variable age (from Holocene to Early Cretaceous). No major differences have been observed from the elemental composition and the spectroscopic data; however, the molecular analyses of the solvent extractable fraction show that a specific mixture of components is present in each sample. These are mainly diterpenoid products that in general are also found abundantly in the higher plants from which the ambers and resins originate. Nevertheless, a direct relationship between major terpenoid constituents in fossil resins and precursor plant materials can only be established for the younger samples.Irrespective of the geographical or botanical origin of the ambers and resins, several common age-dependent molecular transformation trends can be recognized: (1) progressive loss of olefinic bonds (especially those located in exocyclic positions), (2) decrease of functionalized products, and (3) increasing proportion of aromatized components. However, even in the samples of older age (Cretaceous) the degree of aromatization is very low when compared with that of other higher-plant related materials such as fossilized woods or low rank coals. This indicates that maturation must involve essentially olefin polymerization processes instead of extensive aromatization.     

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.     

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.     

Organic composition of Seville aerosols

The organic species present in the aerosols of the Seville atmosphere, sampled at different periods of the year, under distinct traffic regimes, are presented. The compounds are related to incomplete fuel combustion in vehicular exhausts.     

Mineralogical composition of atmospheric airborne particulates

A massive increase in public concern in the last 20 years about the health effects of atmospheric airborne particulates has caused a surge in research activities and has led to ever more stringent government air-quality standards.     

The major-ion composition of silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg²⁺, Na⁺, and SO₄²⁻, and much higher concentrations of Ca²⁺ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca²⁺ in excess of SO₄²⁻. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO₄-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na⁺ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg²⁺/Ca²⁺ ratio of Silurian seawater was ∼1.4, and the K⁺/Ca²⁺ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg²⁺/Ca²⁺ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg²⁺/Ca²⁺ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg²⁺/Ca²⁺ ratio of seawater, not the pCO₂ of the Silurian atmosphere.     

Mineralogical composition of the Nyiragongo rocks

The author reviews briefly the application of the AMS calculation of rock analyses developed byAlfred Rittmann to the lavas of the Nyiragongo volcano in North Kivu, Republic of Congo (Kinsasha). In that volcanic field illustrated in Fig. 1, two petrographic provinces can be distinguished: the Nyamuragira area with volcanics of the basanite group (moderate degree of undersaturation) and the Nyiragongo area with those of the leucitite-nephelinite-melilitite group (high degree of undersaturation).In contrast to the Al-rich titanian clinopyroxene of the leucitites and nephelinites, that of the typical nepheline melilitites is poor in Al and Ti with relatively small amount of the acmite component. The melilitites show a significant excess of the alkalies over aluminum. At least a part of this excess is contained in the glassy material found in the rocks. It is suggested that the genesis of the Nyiragongo magma, mainly melilititic in composition, is possibly connected with carbonatic fractions in the magma basin and enrichment of the alkalies by gaseous transfer in the form of carbonates.

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Zusammenfassung Verf. überblickt kurz die Anwendung der vonAlfred Rittmann ausgearbeiteten AMS-Berechnungsmethode von Gesteinsanalysen auf die Laven des Nyiragongo-Vulkans in Nord Kivu, Republik Congo (Kinsasha). In dem betr. vulkanischen Gebiet (Fig. 1) können zwei petrographische Provinzen unterschieden werden: das Nyamuragira-Gebiet mit Vulkaniten der Basanitgruppe (mäßig untersättigt) und das Nyiragongo-Gebiet mit Gesteinen der Leuzitit-Nephelinit-Melilitithgruppe (stark untersättigt).Im Gegensatz zu dem Al-reichen Titanklinopyroxen der Leuzitite und Nephelinite, ist derjenige der typischen Nephelin-Melilitithen arm an Al und Ti mit einem relativ niedrigen Gehalt an Akmitkomponente. Die Melilitithe zeigen einen bedeutenden Überschuß von Alkalien über Aluminium. Wenigstens ein Teil dieses Überschusses ist in dem glasigen Material enthalten, das in den Gesteinen vorkommt. Es wird darauf hingewiesen, daß die Entstehung des hauptsächlich melilitithischen Nyiragongo-Magmas möglicherweise mit karbonatischen Anteilen der Magmaherde und mit Anreicherung von Alkalien durch Gastransport in Form von Karbonaten verknüpft ist.

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Résumé L'auteur étudie brièvement la méthode AMS deAlfred Rittmann pour la calculation d'analyses de roches, appliquée aux laves du Nyiragongo volcan en Kivu nord, République Congo (Kinsasha). Dans ce terrain volcanique (voir table 1) deux provinces pétrographiques peuvent etre distinguées: la région Nyamuragira avec des volcans du groupe basanite (peu sous-saturés) et la région Nyiragongo avec des volcans du groupe leucitite-nephelinite-melilitite (fort sous-saturés).Au contraire du clinopyroxène titanien, riche en Al, des leucitites et des nephelinites, celui des typiques nephelines-melilitites est pauvre en Al et Ti avec un degré peu considérable d'acmite. Les melilitites montrent un excès considérable d'alcalis sur aluminium. Au moins une partie de cet excès se trouve dans la matière vitreuse qui existe dans les roches. Il est indiqué que la genèse du Nyiragongo magma, qui est principalement melilitique, est peut-être reliée avec des fraction carbonatiques des centres magmatiques et avec l'enrichissement des alcalis par le transport de gaz.

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. . Nyiragongo . : 1. Nyamuragira ; 2. Nyiragongo , . , Nyiragongo, , , .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Isotopic composition of silicon in meteorites

Samples of bulk meteorites show only mass-dependent fractionation of silicon isotopes. No isotopic anomalies were found. The variation of the ratios ²⁹Si/²⁸Si and ³⁰Si/²⁸Si over the meteorite classes is small; 1%. per mass unit difference. The average Si isotopic composition for each class of meteorites is identical, within analytical uncertainties. This is quite unlike O, whose anomalous isotopic abundances in bulk samples differentiate among the classes of meteorites. The overlapping abundance ranges of Si isotopes among many classes of meteorites suggest closed-system behavior for this element prior to meteorite accretion and allow calculation of an average solar system Si isotope composition.     

The composition of carbonaceous chondrite matrix

Matrix compositions of 32 carbonaceous chondrites have been analyzed by an electron microprobe defocussed-beam technique. Except in those chondrites that show evidence of metamorphism, matrices are compositionally similar and have correlation coefficients of +0.96 or greater. Weight per cent Mg/Si in matrices is constant (0.82 ± 0.05) but less than ratios derived from bulk analyses. Matrices in metamorphosed meteorites are Mg-depleted relative to those of other chondrites. Al Rais and Renazzo (anomalous by any classification scheme) have Mg-enriched matrices. Average matrix compositions cluster into chemical subgroups similar to those based on bulk chemical and petrographie criteria [C1, C2, C3(0), C3(V)]. C1 matrices are particularly variable in composition from point to point within the same meteorite, but points within individual breccia clasts appear to be more compositionally uniform. Cl matrices are depleted in Na, S, and Ca relative to solar and C2 matrix values, probably as a result of leaching. Matrix Ca/A1 ratios are highly variable and generally fall below the accepted meteoritic value. The only strong interelement correlation is for Fe, Ni, and S in C2 matrices, suggesting mixing of variable proportions of two components: Mg-rich phyllosilicate and a Ni-bearing chalcophile phase. The amount of magnetite associated with C2 matrix appears to vary systematically with matrix composition. Isotopic, chemical, and mineralogical constraints suggest that matrix, although appreciably altered in some meteorites, is chiefly a solar system condensation product which contains an admixture of unprocessed interstellar dust.     

The isotopic composition of plant sulfur

The isotopic composition of sulfur has been studied in plants representative of various regions of the U.S.S.R., two oceanic islands, and atmospheric precipitations on land and in marine areas. In soils, the isotopic composition of sulfur in the atmospheric water varies as a result of sulfate reduction (increase of δ³⁴S of the soil sulfate) and sulfate regeneration from hydrogen sulfide. The sulfur in plants from the oceanic islands has characteristically higher values of δ³⁴S than the sulfur in the plants and in the atmospheric water of the continents. Compared to sea water, the sulfur from the island plants that were studied contains a considerably lesser proportion of the ³⁴S isotope. This can be explained by the significant role in such plants of the sulfur of the atmospheric air masses coming from the continents.     

Hydrogen isotope composition of deep-seated water

D/H ratios of phlogopites and amphiboles from rocks of possible mantle origin and also those of water from (glass?) inclusions in olivines of the olivine nodule and peridotites have been determined. The mantle water seems to have aδD value of —85±10‰ on the basis of results of inclusions in the nodule-olivine.     

The major-ion composition of Permian seawater

The major-ion (Mg²⁺, Ca²⁺, Na⁺, K⁺, SO₄²⁻, and Cl⁻) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274-272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, including SO₄²⁻ > Ca²⁺ at the point of gypsum precipitation, evolution into Mg²⁺-Na⁺-K⁺-SO₄²⁻-Cl⁻ brines, and Mg²⁺/K⁺ ratios ∼5. Permian seawater, however, is slightly depleted in SO₄²⁻ and enriched in Ca²⁺, although modeling results do not rule out Ca²⁺ concentrations close to those in present-day seawater. Na⁺ and Mg²⁺ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg²⁺/Ca²⁺ ratios >2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements.The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296-283 Ma), Lower Permian Artinskian-Kungurian (283-274 Ma), and Upper Permian Tatarian (258-251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO₄²⁻ concentrations are 20 mmol per kg H₂O (mmolal) and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca²⁺ is at or above its concentration in modern seawater throughout the Permian. Mg²⁺ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg²⁺/Ca²⁺ ratios are 3.5 (total range = 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range = 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca²⁺. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production.The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater.     

The sulfur isotope composition of basaltic rocks

The sulfur isotope composition of tholeiitic basalts, olivine alkali basalts and alkalirich undersaturated basalts were investigated. A method of preparation was devised

1. for the extraction of the small amounts of sulfur contained in the rock samples (about 100 ppm S),
2. for the separation of sulfide- and sulfate-sulfur.

Tholeiitic and olivine alkali basalts show a predominance of sulfide-sulfur. Alkali-rich undersaturated basalts show sulfide- and sulfate-sulfur. The oxidation potential of the magma is reflected in the proportions of sulfide- and sulfate-sulfur. Differences in the conditions of oxidation are also the cause of the sulfur isotope fractionation observed. The mean in the isotope composition of the sulfur in the olivine alkali basalts (with the exception of two samples which show extreme deviation) is δ ³⁴S= +1.3 per mil. The values for the olivine alkali basalts are concentrated around this mean in a remarkable way, showing only small deviation for the individual samples. When the tholeiitic basalts deviate from this mean, it is only with a relative enrichment in the ³²S isotope. With a pronounced variation of the individual values, the mean for the sulfide-sulfur is δ ³⁴S=?0.3 per mil. The few sulfate values of both types of basalt are without significance for the discussion of their origin. However, this does not apply to the alkali-rich undersaturated basalts. Due to the higher water content, this basaltic magma had a higher oxygen partial pressure which favoured the formation of SO₂ and SO ₄ ^2? besides H₂S while pressure was released during the ascent of the magma. The sulfur isotope fractionation connected with this oxidation led to a total enrichment of ³⁴S in the rock, (δ ³⁴S for total sulfur: +3.1 per mil) with particular favouring the sulfate (δ ³⁴S=+4.2 per mil). It is accepted that the sulfur of all three types of basalts derives directly from the mantle. The olivine alkali basalts show the least deviation from the mantle value, which, in the place of origin of the basalts from the region investigated, would probably have been δ ³⁴S=+1.3(±0.5) per mil. From this it may be concluded that the olivine alkali basalts — the most frequent type of basalt in this region — had their origin in the partial melting of the mantle without further differentiation. From the sulfur isotope data we concluded that the primary isotope composition of the continental tholeiitic basalts probably corresponds to that of the olivine-alkali basalts, and to that of the mantle. However, due to degasing in the layers near to the surface, some samples lost ³⁴S, which may be related to the formation of SO₂ during the release of pressure. There is no positive indication of a differentiation in shallow depths (<15 km — in the sense of Green and Ringwood, 1967). The reason for the obvious isotopic fractionation of the alkali-rich undersaturated basalts may be seen in their higher primary water content. This is a pronounced indication of the origin of this type of magma. Bultitude and Green (1968) proved by experiment, that the formation of alkali-rich undersaturated basaltic magma is possible in the mantle in the presence of water. Only a small amount of water is available for the formation of magma in the mantle. With a water content higher than normal for basalts, only small amounts of magma can be formed, but at lower temperatures this would allow the melting of a larger fraction of mantle material. By reaction with the wall rock, these magmas could be enriched in those components of mantle minerals which have the lowest melting point. This may help to explain their geochemical characteristics.     

The isotopic composition of carbonatite and kimberlite carbonates and their bearing on the isotopic composition of deep-seated carbon

New carbon and oxygen isotopic compositions of carbonates from 14 carbonatite and 11 kimberlite occurrences are reported. A review of the available data on the carbon isotopic composition ranges of carbonatite and kimberlite carbonates shows that they are similar and overlap that of diamonds. The mean carbon isotopic composition of carbonates from 22 selected carbonatite complexes (?5.1%., s = ±l.4%.vsPDB) is indistinguishable from that of 13 kimberlite pipes (?4.7%. s = ±1.2%.) as well as that of 60 individual diamond analyses (?5.8%., s = 1.8%.). The oxygen isotopic compositions of kimberlite carbonates, however, are enriched in O¹⁸ by several permil with respect to those of carbonates from the subvolcanic type of carbonatite.The data suggest that not all carbonatite, kimberlite and diamond occurrences have the same average carbon isotopic composition and that significant differences exist between them. Carbon isotopic composition measurements available for the East African Rift system suggest geographic and/or tectonic groupings e.g. carbonate lavas, tuffs and intusive carbonatites associated with the Eastern Rift yield a range of δC¹³ values from ?5.8 to ?7.4%., similar to that of the carbonate rocks associated with the Western Rift volcanism (?5.8 to ?7.9%.). In contrast the interrift area encompassing Lakes Victoria, Malawi (Nyasa) and Chilwa, apparently are characterized by carbonatitic carbonates of higher C¹³ content (?2.4 to ?4.4%.).If carbonatite and kimberlite carbonates as well as diamonds represent deep seated carbon, the mean isotopic composition of this carbon is estimated as ?5.2%. and the range is ?2 to ?8%. The selection of any particular value within this range to be used as a criterion of deep-seated origin is at the moment not warranted. Indeed, the choice of any specific composition for such carbon may be meaningless, as the source of kimberlite, carbonatite and diamond carbon may not be isotopically uniform.     

Structure and viscosity of rhyolitic composition melts

The average local structure of a rhyolitic composition glass has been determined at 25°C using X-ray radial distribution analysis (RDA) and quasi-crystalline modelling and is best described as similar to that in a stuffed framework composed principally of six-membered rings of Si and Al tetrahedra (basically a stuffed tridymite-like model). Using this model it is possible to calculate a density (2.41 g/cm³) which compares well with the measured density (2.40 g/cm³); a structural model based on four-membered rings (an albite-like model) results in a substantially higher calculated density (2.60 g/cm³). We suggest that the rhyolite glass structural model is appropriate for rhyolitic melts, based on evidence from the recent literature. New viscosity data for an anhydrous rhyolite composition measured between 1200°C and 1500°C are presented and interpreted in terms of our proposed structural model and previous melt structure models for the major normative components of rhyolite. A mechanism for diffusion and viscous flow in framework silicate melts (including rhyolite composition) is proposed on the basis of recent molecular orbital calculations and molecular dynamics simulations of silicate and fluoride melts.     

The chemical composition of a Greenland glacier

Chemical analyses on water from dated strata of a south Greenland permanent ice sheet revealed that there is a larger amount of sulfate in samples accumulated during the past decade than in those 60 or more years older. This increase is attributed to combustion of fossil fuel. With the exception of mercury, cadmium and possibly copper, the heavy metal distributions in the glacial waters are similar to those in atmospheric dusts. Previously reported higher mercury values in recently deposited strata were not confirmed.