Conditions of condensate rim formation on the surface of lunar regolith particles

Condensate objects observed in the lunar regolith are distinctly separated on the basis of morpho-logical and chemical characteristics into droplets condensed during the expansion of an impact-generated vapor cloud and films condensed on the relatively cold surface of mineral particles. Using the analyses of both condensate forms and experimental data on the evaporation of melt corresponding to a typical lunar highland rock of the gabbro-anorthosite composition from Apollo 16 sample 68415.40, the temperature conditions of vapor condensation during lunar impact events were estimated. The comparison of condensate compositions with the analyses of vapors from the evaporation experiment showed that, compared with the compositions of droplet-type condensates, the condensate rims were formed from a vapor with high contents of refractory CaO and Al₂O₃ and at very different condensation temperatures. The enrichment of vapor in CaO and Al₂O₃ could be attained only at high temperatures of melt evaporation (higher than ∼ 1850°C according to experimental data). The estimated condensation temperatures of droplets are significantly lower, ∼1750–1500°C. Rim-type condensates were produced by vapor quenching on the relatively cold surface of a solid mineral particle, which resulted in almost complete precipitation of all major components of the silicate vapor without fractionation in accordance with their individual volatilities.     

Volatile element chemistry during accretion of the earth

We review some issues relevant to volatile element chemistry during accretion of the Earth with an emphasis on historical development of ideas during the past century and on issues we think are important. These ideas and issues include the following: (1) whether or not the Earth accreted hot and the geochemical evidence for high temperatures during its formation, (2) some chemical consequences of the Earth’s formation before dissipation of solar nebular gas, (3) the building blocks of the Earth, (4) the composition of the Earth and its lithophile volatility trend, (5) chemistry of silicate vapor and steam atmospheres during Earth’s formation, (6) vapor - melt partitioning and possible loss of volatile elements, (7) insights from hot rocky extrasolar planets. We include tabulated chemical kinetic data for high-temperature elementary reactions in silicate vapor and steam atmospheres. We finish with a summary of the known and unknown issues along with suggestions for future work.     

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.     

Melt-vapor solubilities and elemental partitioning in peraluminous granite-pegmatite systems: experimental results with Macusani glass at 200 MPa

Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H₂O and silicate melt components. The H₂O content of melt at saturation (11.5+-0.5 wt.%) is almost twice that predicted by existing melt speciation models. The corresponding solubility of melt components in vapor decreases from 15 wt.% dissolved solids (750°–775° C) to 9 wt.% at 600° C. With regard to major and most minor components, macusanite melt dissolves congruently in vapor. Among the elements studied (B, P, F, Li, Rb, Cs, Be, Sr, Ba, Nb, Zr, Hf, Y, Pb, Th, U, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), only boron has a vapor/melt partition coefficient (D[B]) consistently 1 at superliquidus temperatures (>645° C). Phosphorus and fluorine behave similarly, with D[P] and D[F]<0.5. Little or no significant vapor/melt fractionation is evident among most periodic groups (alkalis, alkaline earths, Zr/Hf, or the REE). The temperature dependence of vapor/melt partition coefficients is generally greatest for cations with charge +3 (except Nb and U); most vapor/melt partition coefficients for trace elements increase with decreasing temperature to the liquidus. Crystallization proceeds by condensation of crystalline phases from vapor; most coexisting melts are aphyric. Changes in the major element content of melt are dominated by the mineral assemblage crystallized from vapor, which includes subequal proportions of white mica, quartz, albite, and orthoclase. The volumetric proportion of (mica + or-thoclase)/albite increases slightly with decreasing T, creating a sodic, alkaline vapor. Vapor deposition of topaz (T500° C), which consumes F from melt, returns K/Na ratios of melt to near unity with the vapor-deposition of albite. The abundances of most trace elements in residual melt change little with the crystallization of major phases, but in some cases are strongly controlled by the deposition of accessory phases including apatite (T550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.     

Activities and volatilities of trace components in silicate melts: a novel use of metal–silicate partitioning data

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as SiO₂, led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, γ_FeO in the silicate. We obtained γ_FeO as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known f_O2. The compositional dependence of γ_FeO is weak, but, when calculated in the system Diopside–Anorthite–Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that γ_NiO/γ_FeO is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace MoO₂ and WO₃ dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside–Anorthite–Forsterite, both MoO₂ and WO₃ exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, γ_MoO2 and γ_WO3 varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal–silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, CuO_0.5 and InO_1.5 in a silica-saturated melt at 1,650 °C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for CuO_0.5 is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933–1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under ‘normal’ terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal–silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures.     

Rare earth partitioning between immiscible carbonate and silicate liquids and CO2 vapor: Results and implications for the formation of light rare earth-enriched rocks

Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt.Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions.Minimum REE partition coefficients for CO₂ vapor/carbonate melt and CO₂ vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO₂ vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO₂ vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar.Mantle metasomatism by a CO₂-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO₂-rich fluids under both mantle and crustal conditions.     

Estimation of temperature conditions for the formation of HASP and GASP glasses from the lunar regolith

Impact cratering on the Moon’s surface was accompanied by the high-temperature melting of rocks, melt evaporation, and silicate vapor condensation. Evidence for the extensive evaporative fractionation of melts was found in HASP (High-Alumina Silica-Poor) glasses from the lunar regolith. Numerous objects of condensation origin were found in the Apollo 14 regolith breccia. They are referred to as GASP (Gas-Associated Spheroidal Precipitates). With respect to chemical characteristics, namely FeO and SiO₂ contents, GASP were subdivided into Fe-rich (FeGASP) and Si-rich (SiGASP) condensates. Based on experimental data on the evaporation of aluminous basalt sample 68415.40 from the Apollo 16 collection and the calculated compositions of residual melts and complementary vapors at various temperatures, we compared the obtained compositions with the chemical analyses of the HASP glasses and GASP condensates. The comparison was aimed at estimating the temperature conditions of HASP and GASP formation. The comparison showed that the compositions of the HASP glasses and GASP condensates are consistent with the compositions obtained in the equilibrium experiment. In accordance with the experiment, the temperature range of the evaporation of HASP glasses was estimated as ∼1750–1870°C. The temperature interval of condensation, with allowance for the effect of vapor supercooling, is ∼1700–1500°C for FeGASP and no higher than 1700–1750°C for SiGASP. This paper discusses the problems of establishing interphase thermodynamic equilibrium during the dispersion of a vapor-melt cloud, vapor supercooling during its condensation, and the influence of the curvature of melt and condensate particles on the character of evaporation and condensation.     

Effect of oxygen fugacity on the etching rate of diamond crystals in silicate melt

Experimental data on the etching of diamond crystals in basaltic melt at 1130°C with variable oxygen fugacity in the environment are considered. The oxygen fugacity was set with the HM and NNO buffers. The study was carried out on a 0.6–0.8 mm fraction (powder) of natural diamond crystals. It has been established that, at the same temperature, the rate of diamond etching (oxidation) in silicate melt depends on the oxygen fugacity in the environment. The etching rate decreases with decline in the oxygen fugacity from the case where the melt comes into contact with atmospheric air to the conditions controlled by the HM and NNO buffers. Under the conditions of the HM and NNO buffers, oxidation was accompanied by surface graphitization of diamond crystals.     

Formation of metal and silicate globules in Gujba: a new Bencubbin-like meteorite fall

Gujba is a coarse-grained meteorite fall composed of 41 vol% large kamacite globules, 20 vol% large light-colored silicate globules with cryptocrystalline, barred pyroxene and barred olivine textures, 39 vol% dark-colored, silicate-rich matrix, and rare refractory inclusions. Gujba resembles Bencubbin and Weatherford in texture, oxygen-isotopic composition and in having high bulk δ¹⁵N values (∼+685‰). The ³He cosmic-ray exposure age of Gujba (26 ± 7 Ma) is essentially identical to that of Bencubbin, suggesting that they were both reduced to meter-size fragments in the same parent-body collision. The Gujba metal globules exhibit metal-troilite quench textures and vary in their abundances of troilite and volatile siderophile elements. We suggest that the metal globules formed as liquid droplets either via condensation in an impact-generated vapor plume or by evaporation of preexisting metal particles in a plume. The lower the abundance of volatile elements in the metal globules, the higher the globule quench temperature. We infer that the large silicate globules also formed from completely molten droplets; their low volatile-element abundances indicate that they also formed at high temperatures, probably by processes analogous to those that formed the metal globules. The coarse-grained Bencubbin-Weatherford-Gujba meteorites may represent a depositional component from the vapor cloud enriched in coarse and dense particles. A second class of Bencubbin-like meteorites (represented by Hammadah al Hamra 237 and QUE 94411) may be a finer fraction derived from the same vapor cloud.     

A periclase-dolomite-calcite carbonatite from the Oka complex,Quebec, and its calculated volatile composition

The eutectic mineral assemblage calcite-dolomite-periclase-apatite-forsterite-magnesioferrite-pyrrhotite-alabandite in a carbonatite dike within the Oka complex, Quebec, buffers the fugacities (and partial pressures) of all gas species in C-O-H-S-F, assuming vapor saturation. At the inferred eutectic (640° C, 1 kbar), the most important gas species and their partial pressures (bars) were: H₂O, 882; CO₂, 110; H₂, 4.6; H₂S, 2.7; CO, 0.5; and CH₄, 0.1. Oxygen fugacity was near the QFM buffer, logf(O₂)=–18.6, and sulfur fugacity was near the QFM-pyrrhotite buffer, logf(S₂)=–5.9. Fluorine fugacity was low, logf(F₂)=–43.9, consistent with the absence of fluoride minerals other than apatite. Presence of a water-rich gas phase is consistent with experiments on synthetic carbonatite systems (e.g. Fanelli et al. 1981), although compositions of the gas phase in published experiments cannot be determined exactly.Contribution no. 390 from the Mineralogical Laboratory, The University of Michigan     

Native silicon and iron silicides in the Dhofar 280 lunar meteorite

The Dhofar 280 lunar highland meteorite is the first one in which native silicon was identified in association with iron silicides. This association is surrounded by silicate material enriched in Si, Na, K, and S and occurs within an impact-melt matrix. Compared to the meteorite matrix, the objects with native Si and the silicate material around them show high Al-normalized concentrations of volatile elements and/or elements with low sensitivity to oxygen but are not any significantly enriched in refractory lithophile elements. Some lithophile elements (V, U, Sm, Eu, and Yb) seem to be contained in reduced forms, and this predetermines REE proportions atypical of lunar rocks and a very low Th/U ratio. The admixture of siderophile elements (Ni, Co, Ge, and Sb) suggests that the Si-bearing objects were contaminated with meteorite material and were produced by the impact reworking of lunar rocks. The high concentrations of volatile elements suggest that the genesis of these objects could be related to the condensation of silicate vapor generated during meteorite impacts. The reduction of silicon and other elements could take place in an impact vapor cloud, with the subsequent condensation of these elements together with volatile components. On the other hand, condensates of silicate vapor could be reduced by impact reworking of impact breccias. Impact-induced vaporization and condensation seem not to play any significant role in forming the composition of the lunar crust, but the contents of the products of such processes can be locally relatively high. The greatest amounts of silicate vapor were generated during significant impact events. For example, more than 70% of the total mass of lunar material evaporated in the course of impact events should have resulted from the collision of the Moon with a cosmic body that produced the Moon??s largest South Pole-Aitken basin.     

Magmatic-Hydrothermal Evolution in the “Bottoms” of Porphyry Copper Systems: Evidence from Silicate Melt and Aqueous Fluid Inclusions in Granitoid Intrusions in the Gyeongsang Basin,South Korea

The Gyeongsang Basin of southeastern Korea contains numerous Cretaceous-early Tertiary (120–40 Ma) granitoid intrusions formed at a convergent plate boundary. The geotectonic setting is similar to that associated with porphyry-type mineralization elsewhere in the Circumpacific region. However, erosion has removed higher-level economic mineralization and exposed deeper levels of the granitoids, representing the poorly mineralized “bottoms” of porphyry copper systems. The intrusions of the Gyeongsang Basin thus provide a unique opportunity to advance our understanding of magmatic-hydrothermal evolution in the roots of porphyry-type systems, below the level of economic mineralization.

The physical and chemical environment during crystallization of the magmas has been characterized through studies of silicate melt and aqueous fluid inclusions in the granitoids. Two different types of silicate melt inclusions were recognized based on occurrence and room-temperature appearance. Type-I inclusions contain one or more crystalline phases and vapor; type-II inclusions consist of a cluster of small crystals, partially devitrified glass, and vapor. Petrographic and Raman analyses indicate that most silicate melt inclusions contain muscovite daughter crystals. Some also contain feldspar. Solidus temperatures of type-I inclusions in quartz phenocrysts range from ≈630to 650°C, whereas solidus temperatures of type-I and type-II inclusions in vug quartz are slightly higher (640–670°C). Liquidus temperatures span a much wider range compared to solidus temperatures, with maximum liquidus temperatures of melts in phenocrysts being slightly higher (≤930°C) than those in vug quartz (≤910°C).

Three types of aqueous inclusions were observed based on occurrence and room temperature phase proportions. Type-A inclusions are liquid rich and low to moderate in salinity; type-B inclusions are vapor rich and low in salinity; type-C inclusions are liquid rich and contain a halite daughter mineral. Some type- A inclusions with a salinity of approximately 25 wt% NaCl equivalent are spatially associated with silicate melt inclusions in phenocrysts, where they occur as three-dimensional clusters of tiny inclusions surrounding the silicate melt inclusion. Type-A inclusions also occur along fractures in quartz phenocrysts. Non-fracture-controlled type-C inclusions are rare in phenocryst quartz, but are common in vug quartz, where they are associated with silicate melt inclusions. Type-C inclusions that coexist with silicate melt inclusions generally homogenize by halite dissolution after the vapor bubble and show a wide range in salinity, from about 30 to >60 wt% NaCl equivalent. Coexisting halite-bearing (Type-C) and vapor-rich (Type-B) inclusions in phenocryst quartz suggest local immiscibility in the late-or post-magmatic fluid.

Pressure-temperature conditions during the final stages of magmatic-hydrothermal activity associated with the granitoid intrusions of the Gyeongsang Basin were approximately 630° to 670° C and 1.9 to 2.5 kbars. These results suggest that the granitoids do not contain economic porphyry coppertype mineralization because the magmas crystallized at high pressures (relative to typical porphyry copper magmas) and did not become saturated in water until a relatively late stage in the crystallization history. Failure to reach water saturation resulted in most of the copper in the original melt being sequestered as a trace component in earlier-crystallizing silicate and sulfide phases to produce anomalous but subeconomic copper grades. Furthermore, owing to the depth of emplacement, less energy was available to fracture the rocks when water did exsolve from the magma, and the pressure remained too high for aqueous fluid immiscibility to be an important metal-concentrating or depositing mechanism. Geological, petrographic, and geochemical characteristics suggest that the granitoid rocks of the Gyeongsang Basin represent ethroot zones of porphyry-type systems, and any higher-grade mineralization that may have been present higher in the system has since been removed by erosion.     

The effect of Si on metal-silicate partitioning of siderophile elements and implications for the conditions of core formation

We have determined the liquid metal-liquid silicate partitioning of Ni, Co, Mo, W, V, Cr and Nb at 1.5 GPa/1923 K and 6 GPa/2123 K under conditions of constant silicate melt composition with variable amounts of Si in the Fe-rich metallic liquid. Partitioning of Ni, Co, Mo, W and V is sensitive to the Si content of the metal with, in all five cases, increasing Si tending to make the element more lithophile than for conditions where the metal is Si-free. In contrast, metal-silicate partitioning of Cr and Nb is, at constant silicate melt composition, insensitive to the Si content of the metal.The implications of our data are that if, as indicated by the Si isotopic composition of the silicate Earth ( [Georg et al., 2007] and [Fitoussi et al., 2009]), the core contains significant amounts of Si, the important siderophile elements Ni, Co, W and Mo were more lithophile during accretion and core formation than previously believed.We use our new data in conjunction with published metal-silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal-silicate partitioning of siderophile elements. We find that the effect of Si on the siderophile characteristics of Ni, Co and W means that the pressures of core segregation estimated from these elements are ∼5 GPa lower than those derived from experiments in which the metal contained negligible Si (e.g., Wade and Wood, 2005). The core-mantle partitioning of Cr and Nb requires that most of Earth accretion took place under conditions which were much more reducing than those implied by the current FeO content of the mantle and that the oxidation took place late in the accretionary process. Paths of terrestrial accretion, oxidation state and partitioning which are consistent with the current mantle contents of Ni, Co, W, V, Cr and Nb lead to Si and O contents of the core of ∼4.3 wt.% and 0.15%, respectively.     

Genesis of kalsilite melilitite at Cupaello,Central Italy: Evidence from melt inclusions

The paper presents data on primary carbonate–silicate melt inclusions hosted in diopside phenocrysts from kalsilite melilitite of Cupaello volcano in Central Italy. The melt inclusions are partly crystalline and contain kalsilite, phlogopite, pectolite, combeite, calcite, Ba–Sr carbonate, baryte, halite, apatite, residual glass, and a gas phase. Daughter pectolite and combeite identified in the inclusions are the first finds of these minerals in kamafugite rocks from central Italy. Our detailed data on the melt inclusions in minerals indicate that the diopside phenocrysts crystallized at 1170–1190°C from a homogeneous melilitite magma enriched in volatile components (CO₂, 0.5–0.6 wt % H₂O, and 0.1–0.2 wt % F). In the process of crystallization at the small variation in P-T parameters two-phase silicate-carbonate liquid immiscibility occurred at lower temperatures (below 1080–1150°C), when spatially separated melilitite silicate and Sr-Ba-rich alkalicarbonate melts already existed. The silicate–carbonate immiscibility was definitely responsible for the formation of the carbonatite tuff at the volcano. The melilitite melt was rich in incompatible elements, first of all, LILE and LREE. This specific enrichment of the melt in these elements and the previously established high isotopic ratios are common to all Italian kamafugites and seem to be related to the specific ITEM mantle source, which underwent metasomatism and enrichment in incompatible elements.     

Experimental testing of olivine–melt equilibrium models at high temperatures

Data are presented on the equilibrium compositions of olivine and melts in the products of 101 experiments performed at 1300–1600°C, atmospheric pressure, and controlled oxygen fugacity by means of new equipment at the Vernadsky Institute. It was shown that the available models of the olivine–melt equilibrium describe with insufficient adequacy the natural systems at temperatures over 1400°C. The most adequate is the model by Ford et al. (1983). However, this model overestimates systematically the equilibrium temperature with underestimating by 20–40°C at 1450–1600°C. These data point to the need for developing a new, improved quantitative model of the olivine–melt equilibrium for high-temperature magnesian melts, as well as to the possibility of these studies on the basis of the equipment presented.     

Nitrogen and water bubbles, oxygen isotopes, shock effects: Deciphering the history of the Bencubbin meteorite breccia

We have observed vesicles filled with heavy nitrogen gas and water vapor in three settings in the Bencubbin CB chondrite: in the mesostasis of the silicate clasts, in the mesostasis of the chondrules of an ordinary chondrite (OC) xenolith, and in grains we refer to as bubble grains, and interpret as remelted OC chondrule mesostasis. In our view, these bubbles are a local phenomenon and formed as a consequence of the impact of the OC fragment onto the Bencubbin parent body. The heavy nitrogen in the bubbles came from one or several of its carrier phases in Bencubbin, and the water came from hydrous silicates. As formulated by Meibom et al. (Meibom A., Righter K., Chabot N., Dehn G., Antignano A., McCoy T. J., Krot A. N., Zolensky M. E., Petaev M. I. and Keil K. (2005) Shock melts in QUE 94411, Hammadah al Hamra 237, and Bencubbin: remains of the missing matrix? Meteorit. Planet. Sci.40, 1377-1391) these hydrous phases, similar to the hydrated clasts now found in CH and CB_b chondrites, were probably common in the Bencubbin parent body at that time. They were later almost totally destroyed by a large scale shock event, and contributed to form the impact melt that now fills space in between the large clasts of Bencubbin. Our observations indirectly confirm this hypothesis. From our composition measurements, we infer that the silicate part of the impact melt was made in roughly equal proportions of melted phyllosilicates and melted anhydrous silicates. The oxygen isotopic composition of the impact melt is much heavier than that of the silicate clasts, probably reflecting the composition of the water at the origin of the phyllosilicates. The O isotope measurements of the OC inclusion chondrules present some features that seem to be common in OCs: a composition of the chondrule crystals slightly lighter than that of whole chondrules, and one olivine crystal with a very light composition.     

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO₃, Ba and Sr. In addition to the presence of SO₃, calculated fO₂ for magnetites indicate a high oxygen fugacity and that Fe⁺³>Fe⁺² in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO₃-rich and characterized by high oxygen fugacity.     

The Moon: A Taylor perspective

We address several current lunar problems. The data suggest that the Moon likely possesses an Fe-rich metallic core a few percent of lunar volume, although definitive proof is lacking. Refractory elements appear to be enriched relative both to the composition of the primordial solar nebula (CI) and the Earth. Very volatile elements appear to be depleted uniformly at high levels. We adopt the single-impact hypothesis for lunar origin, which formed a high-temperature silicate vapor disk, mostly of metal-poor silicate material from an impactor (Theia) that was already depleted in volatiles. We speculate that the curious lunar bulk-composition resulted from condensation from high-temperature vapor at around a few Earth radii. This could produce an enriched refractory-element composition that cut off below 1000 K, producing a uniform depletion in very volatile elements.     

Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated.     

金在花岗质熔体中溶解度的初步实验研究

以合成花岗岩、水、不同浓度的盐酸溶液、氢氟酸溶液作为反应初始物,在850℃,100 MPa,接近于NNO的条件下开展了金在不同花岗质熔体中溶解度的实验研究,实验固液相产物中的金含量使用石墨炉原子吸收法测定。实验结果显示,金在花岗质熔体中的溶解度变化范围为1.87～156.62μg/g,流体相中金的溶解度为0.31～6.92μg/g;金在熔体相中的溶解度较其在共存液相中的高。花岗质熔体相中金的溶解度明显受熔体化学组成的影响,过碱性富钠花岗质熔体中金的溶解度明显高些;金在花岗质熔体中的溶解度随着熔体中Na2O/K2O摩尔比增大而增大;在氟氯共存岩浆体系中,氟含量变化对金在熔体相中的溶解度影响不明显,而液相中氯含量增大有利于提高金在流体相中的含量。