Partitioning of Ca and Al between forsterite and silicate melt in dynamic systems with implications for the origin of Ca,Al‐rich forsterites in primitive meteorites

Abstract— We report the results of dynamic crystallization experiments that were specifically designed to study the dependence of Ca and Al partitioning between forsterite and melt in rapidly cooling Caand Al‐rich melts. The partitioning of Ca between olivine and silicate melt is found to be independent of the cooling rate within the range of 1.5 to 1000°C/hr and at CaO contents of up to 25 wt%. Within analytical uncertainty, our data plot on the equilibrium partitioning curve obtained by Libourel (1999). The partitioning behavior of Al at high cooling rates is more complex. Aluminum is much more heterogenously distributed in the olivine and the co‐existing melt than Ca. But, no systematic trend of Al partition coefficient with cooling rate is observed. We apply the results of the experiments to the formation of meteoritic forsterites with relatively high contents of Ca and Al. Although these forsterites are found frequently inside chondrules, the Ca contents of their host chondrules are far too low to crystallize these high Ca‐forsterites. This is also true for very rapid cooling of chondrule melts. The parental melt of these forsterites requires CaO contents above 20 wt%.     

Partitioning of Na between olivine and melt: An experimental study with application to the formation of meteoritic Na2O-rich chondrule glass and refractory forsterite grains

We report experimentally determined 1 atm olivine/melt D_Na partitioning data for low f_O2, a variety of melt compositions and a temperature range of 1325-1522 °C. We demonstrated that high-current electron microprobe analyses (EPMA, I = 500 nA, 600 s on the peak) allow quantitative determination of Na₂O in olivine down to ∼10 μg/g. The mean olivine/melt D_Na from 12 experimental runs is 0.0031 ± 0.0007 (1σ). This is the recommended value for low pressures and a wide range of natural compositions.This result is applied to the problem of the origin of alkalis in chondrules and the formation of chondritic refractory forsterite grains. The data on Semarkona (LL3.0) chondrules show that Na₂O is primordial and was present during olivine crystallization. For refractory forsterite grains from Murchison (CM2), we demonstrate that high CaO contents are not a result of equilibration with Na₂O-rich melts, but require high activities of CaO during their formation.     

Origin of SiO2-rich components in ordinary chondrites

Silica-rich objects are common minor components in ordinary chondrites (OC), occurring as fragments and as chondrules. Their typical paragenesis is orthopyroxene + SiO₂ (with bulk SiO₂ >65 wt%) and occasionally with additional olivine and/or spinel. Individual silica-rich components (SRC) have previously been studied in various types of OCs, although there is only one comprehensive study of these objects by Brigham et al. [Brigham, C.A., Murrell, M.T., Yabuki, H., Ouyang, Z., El Goresy, A., 1986. Silica-bearing chondrules and clasts in ordinary chondrites. Geochim. Cosmochim. Acta 50, 1655-1666]. Several different explanations of how SRCs formed have been published. The main question is how silica-enrichment was achieved, because CI-chondritic atomic Mg/Si-ratio is 1.07 and as a consequence only olivine and pyroxene, but no free silica should be stable. There are two basic possibilities for the SiO₂-enrichment: (1) a RedOx-mechanism or magmatic fractionation on the parent body and (2) fractional condensation or recycling of chondrule mesostasis in the solar nebula. To better constrain the origin of these objects, we measured major and rare earth elements in SRCs of various types of ordinary chondrites, and in addition, we studied silica polymorphism in these objects using an in situ micro-Raman technique. Bulk chondrule compositions define mixing lines between the compositions of olivine and pyroxene. The SRCs extend these lines to an SiO₂ end member. In contrast, magmatic trends grossly deviate from these mixing lines. Concentrations of CaO, Al₂O₃, and REE in the pyroxenes of the SRCs are low (0.01 to 1× CI) and the CI-normalized REE-patterns are virtually flat, typical of bulk chondrules, but untypical of magmatic trends. We therefore conclude that SiO₂-rich objects are not of magmatic origin. They are the result of fractional condensation in the solar nebula. The silica in SRCs occurs mainly as tridymite and sometimes as cristobalite or—in very rare cases—as quartz. Some SiO₂-phases yielded a yet unknown micro-Raman spectrum, which we were unable to identify. The often chondrule-like shape of SRCs as well as the presence of high-temperature SiO₂-polymorphs lead to the following model for the origin of SRCs: formation of SiO₂-rich precursors in the solar nebula by fractional condensation, reheating to temperatures between 1140 and >1968 K, thereby forming the SRCs,—probably during the chondrule-forming process—followed by rapid cooling.     

A quantitative investigation into the temporal and spatial variations in the emergence of adult blackflies (Diptera: Simuliidae) from the Breitenbach, a small upland stream in Germany

As a contribution to the long-term emergence studies carried out as part of the “Breitenbach ecosystem project”, this paper presents the results obtained on emergence patterns and population dynamics of blackflies from 1984 to 1988. The Breitenbach is a small first order stream near Schlitz/Hesse, running into the Fulda river. Adult blackflies were caught in 4 greenhouse emergence traps, each spanning the whole width of the stream for a length of 6 m. A manual and then a partly automated method for collecting trapped insects was applied for three years and one year, respectively. Sixteen blackfly-species of different origin were found in the traps: a) autochthonous species: Prosimulium tomosvaryi, Simulium vernum s.l., S. cryophilum s.l., S. ornatum s.l. (common); S. costatum, S. trifasciatum, S. monticola, S. argyreatum (rare); b) species of doubtful origin: S. lundstromi, S. angustitarse, S. angustipes, S. aureum (very rare); c) allochthonous species: S. lineatum, S. equinum, S. erythrocephalum, S. noelleri. It was shown that the last 4 species had not emerged from the Breitenbach but had flown into the traps as adults. In addition, females of autochthonous species with blood or with mature eggs were trapped, which were also considered to have flown in. Besides revealing a limitation of the trap construction, they supported the detailed interpretation of some intricated patterns of appearance. Variations in emergence patterns and specimen numbers between years and traps were pronounced, but only in a few cases could they be attributed to changes in abiotic factors such as water temperature or discharge. P. tomosvaryi had the most simple and regular life cycle, with one well synchronised emergence peak annually from April to May (or even to June). No gradient of specimen numbers along the stream was evident. The two closely related species S. vernum and S. cryophilum had quite similar emergence patterns: There were two broad peaks per year, extending mainly from March to June and from July to October or November. In some cases the number and separation of consecutive generations was not clear. The abundance of both species clearly decreased downstream, more so for S. cryophilum than for S. vernum. In a trap closest to a tributary spring, S. cryophilum was the dominant species during three of the four years examined. The adults of S. ornatum displayed an intricate pattern of appearance, with very low specimen numbers in spring and medium to very high numbers in July/August and September/October. S. ornatum is the only blackfly species that inhabits both the Breitenbach and the adjacent section of the Fulda river. It was shown that females emerging from the Fulda river regularly invade the Breitenbach valley in greatly varying numbers and oviposit there. This leads to overlapping larval cohorts with corresponding emergence peaks. Although S. ornatum was the most abundant species in one year in the lower traps (60 to 80% of all individuals), it remains uncertain whether it is a long-term, permanent member of the autochthonous blackfly fauna of the Breitenbach. Estimates of total numbers of flown-in adults, actual emergence, dry weight biomass, the ecological separation of the species and their life cycle strategies are discussed.     

Experimental determination of the solubility of Au in silicate melts

Summary We report here new data on the solubility of Au in silicate melts of anorthite-diopside eutectic composition at a wide range of oxygen fugacities, from pure oxygen to 10^–8 atm, and at a temperature range of 1300 °C to 1480 °C. Because experiments were done with metal loops at temperatures above the Au-melting temperature, PdAu-metal-alloys had to be used. Pd-solubility data derived from the same set of experiments agree with earlier data obtained from experiments with pure Pd-metal (Borisov et al., 1994a). The results of the present experiments show that Pd-solubilities are by a factor of 2 to 6 higher than Au-solubilities. Both, Au and Pd solubilities decrease with decreasing oxygen fugacity. At oxygen fugacities below the iron-wiistite buffer (IW) Au solubility increases with decreasing fO₂ probably reflecting formation of Au-silicides at such reducing conditions. Compared to Pd, Au has higher activity coefficients in Fe-metal and lower solubility in silicate melts. This leads to similar metal-silicate partition coefficients for both elements. At a temperature of 1350 °C and an oxygen fugacity corresponding to IW-2 D^Au (met/sil) is about 2.5 · 10⁷ and D^Pd (met/sil) about 1.6 · 10⁷. Thus similar behavior is expected during metal separation in planetary bodies including core formation in the Earth. The metal/silicate partition coefficient of Ir is, however, by several orders of magnitudes higher (Borisov and Palme, 1995a). Equilibration with chondritic metal will therefore lead to grossly non-chondritic Pd/Ir or Au/Ir ratios in coexisting silicate phases. Chondritic ratios are thus indicative of the presence of unfractionated meteoritic components. Samples from the upper mantle of the Earth, for example, reflect the admixture of a late unfractionated (chondritic) veneer (e.g.,Kimura et al., 1974;Jagoutz et al., 1979).Solubilities of Pd and Au in silicate melts are much higher than the contents in terrestrial basalts implying that the abundances of these two elements are not buffered by residual PGE- and Au-containing alloys. The most likely process for fractionating PGEs in terrestrial magmas are mineral-melt (e.g., olivine/melt) equilibria.

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Experimentelle Bestimmung der Löslichkeit von Au in Silikatschmelzen

Zusammenfassung In der vorliegenden Arbeit wird über die Ergebnisse der Bestimmung der Löslichkeit von Au in Silikatschmelzen mit der Zusammensetzung des Anorthit-Diopsid Eutektikums berichtet. Die Versuche wurden mittels Metallschlaufe über einen weiten Sauerstoffpartialdruckbereich, von reinem Sauerstoff bis zu 10^–8 atm und in einem Temperaturbereich von 1300 °C bis 1480 °C, durchgeführt. Da diese Temperaturen jedoch den Au-Schmelzpunkt überschreiten, wurde mit AuPd-Legierungen gearbeitet. Die Ergebnisse der dadurch zusätzlich erhaltenen Pd-Versuche stimmen mit früher bestimmten, mit reinen Pd-Schlaufen durchgeführten Pd-Löslichkkeiten überein (Borisov et al., 1994a). Die auf reine Metalle zurückgerechneten Löslichkeiten von Pd sind um einen Faktor 2 bis 6 mal höher als die entsprechenden Au-Löslichkeiten. Die Löslichkeiten beider Metalle nehmen mit abnehmendem Sauerstoffpartialdruck ab. Unter noch stärker reduzierenden Bedingungen (Eisen-Wüstit Gleichgewicht) nimmt die Löslichkeit von Au jedoch zu. Dies könnte auf die Bildung von Au-Siliziden zurückzuführen sein.Im Vergleich zu Pd sind die Aktivitätskoeffizienten von Au in metallischem Eisen höher, die Löslichkeiten in Silikatschmelzen jedoch niedriger. Das führt zu ähnlichen Metall/Silikat Verteilungskoeffizienten von Au und Pd. Bei einer Temperatur von 1350 °C und einer Sauerstoffugazität von IW-2 ergeben sich für D^Au (met/sil) 2.5 · 10⁷ und für D^Pd (met/sil) 1.6* 10⁷. Der Metall/Silikat-Verteilungskoeffizient von Ir ist jedoch unter den gleichen Bedingungen um mehrere Größenordnungen höher (Borisov andPalme, 1995a). Ein chondritisches Pd/Ir- oder Au/Ir-Verhältnis kann also auf die Anwesenheit einer unfraktionierten chondritischen Komponente zurückgeführt werden. Dies gilt beispielsweise für Proben aus dem oberen Erdmantel. Hier handelt es sich vermutlich um Zumischung einer späten chondritischen Akkretionskomponente, die sich nicht mehr mit einer metallischen Phase (Kern) ins Gleichgewicht gesetzt hat (z.B.Kimura et al., 1974,Jagoutz et al., 1979).Die Löslichkeiten von Pd und Au in Silikatschmelzen sind wesentlich höher als ihre Gehalte in basaltischen und komatiitischen Laven. Dies bedeutet, daß Au und Pd in Schmelzen aus dem Erdmantel nicht durch residuale Au- und/oder Pd-haltige Metall phasen bestimmt sind. Gleichgewichte zwischen Schmelze und Mineralen (z.B. Olivin) sind die wahrscheinlichsten Fraktionierungsmechanismen für Platingruppenelemente in terrestrischen Magmen.

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With 5 Figures     

Origin of chondritic forsterite grains

Iron-poor and refractory lithophile element (RLE) rich forsterite grains occur in all major types of unequilibrated chondrites. In our laser ablation inductively coupled mass spectrometry (LA-ICPMS) minor and trace element study we show that refractory forsterites (RF) from carbonaceous (CC), unequilibrated ordinary (UOC) and a Rumuruti chondrite (RC) have similar chemical compositions with high RLE concentrations and low concentrations of Mn, Fe, Co and Ni. Fractionation of RLEs and rare earth elements (REEs) is in agreement with formation by crystallization from a RLE rich silicate melt. Low concentrations and the fractionation of moderately siderophile elements (Fe, Co, Ni) in RFs suggests formation at low oxygen fugacity, possibly in equilibrium with primitive Fe,Ni metal condensates in a gas of solar composition. Anomalously high Ti in the parental melt can be explained by Ti³⁺/Ti⁴⁺ ∼1.5, supporting formation of RF in highly reducing conditions. Low Mn concentrations indicate formation at high temperatures (>∼1160 K). The model of formation of RFs and the accompanying physico-chemical conditions during their formation as well as their relation to non refractory olivine are discussed.     

The conditions of chondrule formation, Part I: Closed system

Mixing was an important process in the early solar nebula and is often used as an argument to explain the compositional scatter among chondrules—mm-sized, once molten silicate spherules in chondritic meteorites. If it is hypothesized that chondrules only acted as closed systems and the scatter in chondrule bulk chemical compositions is only the result of mixing heterogeneous precursor grains—the basic components of chondrules—, it is in turn possible to determine the sizes of the precursor grains using statistical calculations. In order to reproduce the observed compositional scatter in chondrules not more than ∼10 precursor grains should contribute to a single chondrule, each with a diameter of several 100 μm. This finding has important implications for the conditions of chondrule formation and replaces the so far widely accepted model that chondrules formed from fine-grained “dust-balls”. Chondrules rather formed from coarse-grained precursor aggregates with variable amounts of μm-fine matrix material. As a consequence, only chondrite matrix or interstellar material winnows as precursor material. Large grains of variable composition serving as precursor grains must have been formed prior to chondrule formation. Chondrules probably have not been their immediate precursors, as only 1-2 chondrule recycling steps would have homogenized bulk chondrule compositions. Chondrule recycling can therefore only have occurred to a limited extent. Chondrule formation needed at least three steps: (1) production of large and heterogeneous chondrule precursor grains, (2) agglomeration of large precursor grains and fine-grained precursors into aggregates, (3) formation of chondrules during transient heating events. Al-rich chondrules can in this context be explained by the admixture of CAIs to either chondrule precursors or a population of existing chondrules.