A comparative study of melilite and fassaite in Types B1 and B2 refractory inclusions     

S.B. Simon  L. Grossman 《Geochimica et cosmochimica acta》2006,70(3):780-798

Most of the petrologic data available for Type B inclusions comes from Type B1s. Relatively little comes from the B2s, and there has not been a systematic comparison of the properties of their two most abundant minerals. In this work, we document the compositions and zoning patterns of melilite and fassaite in Type B2 inclusions, and compare and contrast them with the features of their counterparts in Type B1 inclusions. We find that melilite compositions in Type B2 inclusions are similar to those of Type B1s, with maximum Åk contents of ∼75 mol % and a positive correlation between Åk and Na₂O contents. Asymmetrically zoned melilite is common in Type B2s as are melilite grains with reversely zoned regions, and the reversely zoned portions of crystals are thicker than in B1s. In B2s, like B1s, fassaite is zoned with decreasing Ti, Sc, and V oxide contents from cores to rims of grains. Approximately half of the Ti is trivalent, but unlike that in B1s, within fassaite grains in B2s the Ti³⁺/(Ti³⁺ + Ti⁴⁺) ratio does not decrease from core to rim, and sharp enrichments (“spikes”) in Ti³⁺ and V are not observed. Sector-zoned fassaite is much more common in B2s than in B1s. The differences we observed can be accounted for by the differences in bulk compositions between B1s and B2s. Type B2 inclusions tend to have higher SiO₂ contents, hence higher An/Ge component ratios, than Type B1s. Phase equilibria show that, compared to B1s, in B2s less melilite should crystallize prior to the appearance of fassaite, so that in B2s a higher proportion of melilite cocrystallizes with fassaite, causing more of the crystals to be reversely zoned; more melilite crystallizes while adjacent to other crystals, leading to asymmetrical zoning; and with more liquid available, transport of components to growing fassaite occurs more readily than in B1s, facilitating crystal growth and giving rise to sector zoning. The lack of zoning with respect to Ti³⁺/Ti^tot and the absence of Ti³⁺-, V-rich spikes suggest that Type B2 melts maintained equilibrium with the nebular gas throughout crystallization, while the interiors of B1s were probably isolated from the gas, perhaps by their melilite mantles. This makes the similarity of Na-Åk relationships in B1 and B2 melilite difficult to understand, but apparently enclosure by melilite mantles was not necessary for the retention of Na₂O during crystallization of Type B refractory inclusions.  相似文献   

New high-pressure and high-temperature metal/silicate partitioning of U and Pb: Implications for the cores of the Earth and Mars     

Valérie Malavergne  Martine Tarrida  Hélène Bureau  Craig Schwandt 《Geochimica et cosmochimica acta》2007,71(10):2637-2655

In order to quantify possible fractionation of U and Pb into a metallic core, we have performed piston cylinder and multi-anvil press experiments at high pressure (up to 20 GPa) and high temperature (up to 2400 °C) and obtained the distribution coefficient D_{metal-silicate} and the exchange partition coefficient K_{metal-silicate} for these elements between metal and silicates (mineral or liquid). and depend strongly on the S content of the metallic phase, and also on the oxygen fugacity, in agreement with an effective valence state of 4 for U in silicates and 2 for Pb in silicates. and show no discernable pressure and temperature trend. U remains lithophile even at high pressure and high temperature but its lithophile nature decreases at very low oxygen fugacity. From our experimental data, it was possible to calculate the U and Pb contents of the cores of Mars and Earth under core-mantle equilibrium conditions at high pressure and high temperature. From the D_{metal-silicate} of the present study, we obtained that: 0.008 ppm < Pb_{in the core} <4.4 ppm, and 0.0003 ppb < U_{in the core} < 0.63 ppb, depending on whether the metal is S-free or S-saturated respectively, and if the mantle was molten or solid during the segregation process of the Earth’s core around ΔIW-2. For Mars, based on a core segregation process around ΔIW-1, we obtained that: 0.005 ppm < Pb_{in the core} < 3 ppm, and 0.00002 ppb < U_{in the core} < 0.05 ppb, depending on the metallic composition: S-free or S-saturated respectively.Our results suggest that the low concentration of Pb in the terrestrial mantle could not be explained by an early Pb sequestration in the Earth’s core even if S is the dominant light element of the core. If we assume a magma ocean scenario, U might produced a maximum value of 1.5% of the total heat budget of the core with a segregation occurring below ΔIW-3. The values found in the present study for U in the Martian core suggest that the magnetic field activity of Mars before ∼0.5 b.y. after its formation would be difficult to ascribe to the decay of U alone.  相似文献   

Molybdenum, tungsten and manganese partitioning in the system pyrrhotite-Fe-S-O melt-rhyolite melt: Impact of sulfide segregation on arc magma evolution   总被引：3，自引：0，他引：3  

M.J. Mengason 《Geochimica et cosmochimica acta》2011,75(22):7018-7030

Experiments were performed to determine the partitioning of molybdenum, tungsten and manganese among a rhyolitic melt (melt), pyrrhotite (po), and an immiscible Fe-S-O melt (Fe-S-O). Sulfide phases such as these may be isolated from a silicate melt along with other crystallizing phases during the evolution of arc magma, and partition coefficients are required to model the effect of this process on molybdenum and tungsten budgets.We developed an experimental design to take advantage of properties of the phases under study. Careful control of temperature allowed pyrrhotite and magnetite to be stable along with an Fe-S-O melt, and this phase assemblage allowed the composition of run-product pyrrhotite to be used to calculate both fS₂ and fO₂ for the experiments. At run temperature, (1042 ± 2 °C), a rhyolitic melt can be formed at low pressure, under nominally dry conditions, which removed the need for confining pressure as well as externally imposed fugacities. The silica-saturated melt allowed the charges to be contained in sealed evacuated silica tubes without danger of reaction, and with closed system behavior for molybdenum and tungsten.Experiments were run for durations up to 2000 min. Molybdenite (mb) and wolframite (wo) were added to the experiments as sources for molybdenum and tungsten, respectively. Manganese was added to the system as both a component of the starting rhyolitic pumice, and of Mn-bearing wolframite. Oxygen fugacity in these experiments was fixed at the Ni-NiO oxygen fugacity buffer. Sulfur fugacity was 10⁻¹ bar. Run products were analyzed by EPMA and LA-ICP-MS. Analysis of the run products yielded ( standard deviation of the mean): , , , and . The partition coefficients for manganese in this system are and .Simple Rayleigh fractionation modeling suggests that oxidized felsic melts produced through fractional crystallization may have lost as much as 14% of their initial molybdenum, but only 2% of their initial tungsten, through the removal of an Fe-S-O melt along with crystalline phases. Modeling consistent with conditions of oxygen and sulfur fugacity influenced by assimilation of sulfide (with low concentrations of molybdenum and tungsten) from, for example, sedimentary rock, results in evolved magmas significantly depleted in molybdenum, but only moderately depleted in tungsten. The molybdenum:tungsten ratio can vary by two orders of magnitude. These systematics may help to explain some of the variability in metal ratios of intrusion-related hydrothermal ore deposits.  相似文献   

Ca-Al-rich inclusions from the Ningqiang meteorite: continuous assemblages of nebular condensates and genetic link to Type B inclusions   总被引：1，自引：0，他引：1  

Y Lin  M Kimura 《Geochimica et cosmochimica acta》2003,67(12):2251-2267

The petrography and mineral chemistry of 110 Ca-, Al-rich inclusions (CAIs) and 9 Ca- and/or Al-rich amoeboid olivine aggregates (AOAs) from the Ningqiang carbonaceous chondrite are reported. These CAIs are referred to as hibonite-bearing and hibonite-free melilite-spinel-rich (Type A), and spinel-pyroxene inclusions. Melilite is more gehlenitic in the hibonite-bearing Type As than in the other two types, and all of them vary within a range of Åk_0-30. Modal compositions of the three types of CAIs overlap with each other, and make up a continuum with wide ranges of melilite: spinel: diopside. The diopside occurs as rims on the CAIs or their individual concentric objects. The 9 AOAs contain spinel ± diopside ± anorthite in the centers of the aggregates; the spinel grains rimmed by diopside in the centers are similar to the spinel-pyroxene inclusions. Bulk compositions of these CAIs vary along the condensation trajectory, with the hibonite-bearing Type As plotting at the beginning followed by hibonite-free Type As then by spinel-pyroxene inclusions as temperature decreases. Bulk compositions of the AOAs are close to the lowest temperature condensation trajectory. Except for a few with compact textures, most of the Type As and spinel-pyroxene inclusions are fluffy aggregates, probably pristine vapor-solid condensates of the nebula.The bulk compositions of the Type As appear to overlap with the range of most melilite-Ti-Al-clinopyroxene-rich (Type B) inclusions. Hence, crystallization of liquids produced by melting the Type As can form Type B inclusions, without significant evaporative loss of MgO or SiO₂. A few Type Bs have bulk compositions deviating from the range of their proposed precursors, and may have suffered significant evaporation, as suggested in previous studies.  相似文献   

Trace element partitioning between type B CAI melts and melilite and spinel: Implications for trace element distribution during CAI formation     

C.C. Lundstrom  A.L. Sutton  W.F. McDonough 《Geochimica et cosmochimica acta》2006,70(13):3421-3435

Calcium- and aluminum-rich inclusions (CAIs), occurring in chondritic meteorites and considered the oldest materials in the solar system, can provide critical information about the environment and time scale of creation of planetary materials. However, interpretation of the trace element and isotope compositions of CAIs, particularly the light elements Li, Be, and B, is hampered by the lack of constraint on melilite-melt and spinel-melt partition coefficients. We determined melilite-melt and spinel-melt partition coefficients for 21 elements by performing controlled cooling rate (2 °C/h) experiments at 1 atmosphere pressure in sealed platinum capsules using a synthetic type B CAI melt. Trace element concentrations were measured by secondary ion mass spectrometry (SIMS) and/or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Melilites vary only slightly in composition, ranging from Åk_31-43. Results for the partitioning of trace elements between melilite and melt in three experiments and between spinel and melt in two experiments show that partition coefficients are independent of trace element concentration, are in good agreement for different analytical techniques (SIMS and LA-ICP-MS), and are in agreement with previous measurements in the literature. Partition coefficients between intermediate composition melilites and CAI melt are the following: Li, 0.5; Be, 1.0; B, 0.22; Rb, 0.012; Sr, 0.68; Zr, 0.004; Nb, 0.003; Cs, 0.002; Ba, 0.018; La, 0.056; Nd, 0.065; Sm, 0.073; Eu, 0.67; Er, 0.037; Yb, 0.018; Hf, 0.001; Ta, 0.003; Pb, 0.15; U, 0.001; Th, 0.002. Site size energetics analysis is used to assess isovalent partitioning into the different cation sites. The Young’s modulus deduced from +2 cations partitioning into the melilite X site agrees well with the bulk modulus of melilite based on X-ray diffraction methods. The changes in light element partitioning as melilite composition varies are predicted and used in several models of fractional crystallization to evaluate if the observed Li, Be, and B systematics in Allende CAI 3529-41 are consistent with crystallization from a melt. Models of crystallization agree reasonably well with observed light element variations in areas previously interpreted to be unperturbed by secondary processes [Chaussidon, M., Robert, F., McKeegan, K.D., 2006. Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived ¹⁰Be and for the possible presence of the short-lived nuclide ⁷Be in the early solar system. Geochim. Cosmochim. Acta70, 224-245], indicating that the trends of light elements could reflect fractional crystallization of a melt. In contrast, areas interpreted to have been affected by alteration processes are not consistent with crystallization models.  相似文献   

Environmental isotopes (N, S, C, O, D) to determine natural attenuation processes in nitrate contaminated waters: Example of Osona (NE Spain)   总被引：1，自引：0，他引：1  

Laura Vitria  Albert Soler  ngels Canals  Neus Otero 《Applied Geochemistry》2008,23(12):3597-3611

Nitrate-contaminated groundwater from an aquifer in the Osona region (NE Spain) was chemically and isotopically (δ¹⁵N_NO3,δ¹⁸O_NO3,δ³⁴S_SO4,δ¹⁸O_SO4, δD, δ¹⁸O_H2O and δ¹³C_DIC) characterized. Diffuse- contamination reached values of 366 mg/L. Nearly 75% of the 37 sampled sites had higher concentrations than the 50 mg/L in limit for drinking water. To identify the source of pollution δ¹⁵N_NO3 and δ¹⁸O_NO3 were used with results, for most samples, in the range of pig manure . Nitrification processes were evaluated by means of the δ¹⁸O of and water. Isotopic data suggested that natural attenuation of was taking place. This process was confirmed using the δ¹⁸O_NO3 coupled with the ratio, avoiding the influence of continuous inputs. A further insight on denitrification processes was obtained by analyzing the ions involved in denitrification reactions. Although the role of organic matter oxidation could neither be confirmed nor discarded, this approach showed a link between denitrification and pyrite oxidation. Therefore, in areas with no adequate infrastructure (e.g. multipiezometers), such as the one studied, this approach could be useful for implementing better water management.  相似文献   

Formation of refractory inclusions by evaporation of condensate precursors     

Lawrence Grossman  Denton S Ebel  Steven B Simon 《Geochimica et cosmochimica acta》2002,66(1):145-161

Berman’s (1983) activity-composition model for CaO-MgO-Al₂O₃-SiO₂ liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO₂, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of P_H2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at P_H2 = 10⁻⁶ bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO₂ evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO₂ evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO₂-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO₂ at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO₂ and <12% MgO require precursors with higher SiO₂/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.  相似文献   

The role of heterogenetic mantle in the genesis of adakites northeast of Sanandaj,northwestern Iran     

Hossein Azizi  Yoshihiro Asahara  Motohiro Tsuboi  Kento Takemura  Saman Razyani 《Chemie der Erde / Geochemistry》2014

  相似文献   

The partitioning behavior of As and Au in S-free and S-bearing magmatic assemblages   总被引：3，自引：0，他引：3  

Adam C. Simon  Thomas Pettke  Philip M. Piccoli 《Geochimica et cosmochimica acta》2007,71(7):1764-1782

The partitioning of As and Au between rhyolite melt and low-salinity vapor (2 wt% NaCl eq.) in a melt-vapor-Au metal ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 120 MPa and f_O2=NNO. The S-bearing runs have calculated values for the fugacities of H₂S, SO₂ and S₂ of logf_H2S=1.1, logf_SO2=-1.5, and logf_S2=-3.0. The ratio of H₂S to SO₂ is on the order of 400. The experiments constrain the effect of S on the partitioning behavior of As and Au at magmatic conditions. Calculated average Nernst-type partition coefficients (±1σ) for As between vapor and melt, , are 1.0 ± 0.1 and 2.5 ± 0.3 in the S-free and S-bearing assemblages, respectively. These results suggest that sulfur has a small, but statistically meaningful, effect on the mass transfer of As between silicate melt and low-salinity vapor at the experimental conditions. Efficiencies of removal, calculated following Candela and Holland (1986), suggest that the S-free and S-bearing low-salinity vapor can scavenge approximately 41% and 63% As from water-saturated rhyolite melt, respectively, during devolatilization assuming that As is partitioned into magnetite and pyrrhotite during second boiling. The S-free data are consistent with the presence of arsenous acid, As(OH)₃ in the vapor phase. However, the S-bearing data suggest the presence of both arsenous acid and a As-S complex in S-bearing magmatic vapor. Apparent equilibrium constants, , describing the partitioning of As between melt and vapor are −1.3 (0.1) and −1.1 (0.1) for the S-free and S-bearing runs, respectively. The increase in the value of with the addition of S suggests a role for S in complexing and scavenging As from the melt during degassing.The calculated vapor/melt partition coefficients (±1σ) for Au between vapor and melt, , in S-free and S-bearing assemblages are 15 ± 2.5 and 12 ± 0.3, respectively. Efficiencies of removal (Candela and Holland, 1986) for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization (Simon et al., 2003), suggest that magmatic vapor may scavenge on the order of 72% Au from a water-saturated melt. Efficiencies of removal calculated for the S-bearing assemblage, assuming pyrrhotite and magnetite are the dominant Au-sequestering solid phases, indicate that vapor may scavenge on the order of 60% Au from the melt. These model calculations suggest that the loss of pyrrhotite and magnetite from a melt, owing to punctuated differentiation during ascent and emplacement, does not prohibit the ability of a rhyolite melt to generate a large-tonnage Au deposit. Apparent equilibrium constants describing the partitioning of Au between melt and vapor were calculated using the mean values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400 h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for are −4.4 (0.1) and −4.2 (0.2) for the S-free and S-bearing runs, respectively. These data suggest that the presence of S does not affect the mass transfer of Au from degassing silicate melt to an exsolved, low-salinity vapor in a low-f_S2 assemblage (i.e., pyrrhotite-magnetite at NNO) at the experimental conditions reported here. Efficiencies of removal are calculated and used to model the mass transfer of Au from a crystallizing silicate melt to an exsolved, low-salinity vapor phase. The calculations suggest that the model, absolute tonnage of Au scavenged and transported by S-free and S-bearing vapors, from a crystallizing melt, would be comparable and that the time-integrated flux of low-salinity vapor could be responsible for a significant quantity of the Au in magmatic-hydrothermal ore deposits.  相似文献