Initial Melt Composition Research Articles

Genetics, age, and crystallization history of group IC iron meteorites

The IC iron meteorite group is characterized utilizing nucleosynthetic mass-independent isotopic compositions and 182W age constraints, coupled with siderophile element concentration measurements and modeling of crystal-liquid fractionation processes. The six IC irons analyzed, Arispe, Bendego, Chihuahua City, Nocoleche, NWA 2743, and Winburg have indistinguishable Mo and W genetic isotopic compositions and are consistent with derivation from the same parent body, which formed in the non-carbonaceous (NC) nebular reservoir. A pre-exposure µ182W value (parts-per-million deviations in isotopic ratios from terrestrial standards) for the six IC irons of −337 ± 5 corresponds to a metal-silicate segregation age of 1.0 ± 0.4 Myr after calcium-aluminum-rich inclusion (CAI) formation. This age is similar to those determined for other NC iron groups. Siderophile element abundances of the IC irons are generally similar and characterized by minor depletions in the more volatile siderophile elements. Highly siderophile element (HSE) distributions among the IC group suggest that the initial parent body core was S-rich, with preferred model results indicating an initial melt composition with ∼ 18 wt% S, 2 wt% P and 0.03 wt% C. Processes in addition to fractional crystallization, such as late-stage parent body modification, possibly as a result of impacts, and subsequent metal-melt mixing, are required within the first 100 Myr of Solar System history to explain the range of HSE abundances.

The Effects of Source Mixing and Fractional Crystallization on the Composition of Eocene Granites in the Himalayan Orogen

Abstract Granites are generally the final products of crustal anatexis. The composition of the initial melts may be changed by fractional crystallization during magma evolution. Thus, it is crucial to retrieve the temperature and pressure conditions of crustal anatexis on the basis of the composition of the initial melts rather than the evolved melts. Here we use a suite of ∼46–41 Ma granites from the Himalayan orogen to address this issue. These rocks can be divided into two groups in terms of their petrological and geochemical features. One group has high maficity (MgO + FeOt = 2–4 wt%) and mainly consists of two-mica granites, and is characterized by apparent adakite geochemical signatures, including high Sr concentrations and Sr/Y and La/Yb ratios, and low concentrations of heavy rare earth elements and Y. The other group has low maficity (MgO + FeOt <1 wt%) and consists of subvolcanic porphyritic granites and garnet/tourmaline-bearing leucogranites. This group does not possess apparent adakite signatures. The low-maficity group (LMG) has lower MgO + FeOt contents and the high-maficity group (HMG) has higher Mg# compared with initial anatectic melts determined by experimental petrology and melt inclusions studies. Petrological observations indicate that the HMG and the LMG can be explained as a crystal-rich cumulate and its fractionated melt, respectively, such that the initial anatectic melt is best represented by an intermediate composition. Such a cogenetic relationship is supported by the comparable Sr–Nd isotopic compositions of the two coeval groups. However, these compositions are also highly variable, pointing to a mixed source that was composed of amphibolite and metapelite with contrasting isotope compositions. We model the major and trace element compositions of anatectic melts generated by partial melting of a mixed source at four apparent thermobarometric ratios of 600, 800, 1000 and 1200 °C GPa–1. Modeling results indicate that melt produced at 1000 °C GPa–1 best matches the major and trace element compositions of the inferred initial melt compositions. In particular, a binary mixture generated from 10 vol% partial melting of amphibolite and 30 vol% melting of metapelite at 850 ± 50 °C and 8·5 ± 0·5 kbar gives the best match. Therefore, this study highlights that high thermobarometric ratios and subsequent fractional crystallization are responsible for the generation of the apparent adakitic geochemical signatures, rather than melting at the base of the thickened crust as previously proposed. The thermal anomaly responsible for the Eocene magmatism in the Himalayan orogen was probably related to asthenosphere upwelling in response to rollback of the subducting Neo-Tethyan oceanic slab at the terminal stage of continental collision between India and Asia. As such, a transition in dynamic regime from compression to extension is necessary for the generation of high thermobarometric ratios in continental collision zone. Therefore, after correcting for potential effects of fractional crystallization and crystal accumulation on melt composition, granite geochemistry coupled with thermodynamic modeling can better elucidate the petrogenesis of granites and the geodynamic mechanisms associated with anatexis at convergent plate boundaries.

Microstructure evolution and sequence of phase transition reactions through the solidification of Mo-Si-B alloy; a phase-field study

The present study is focused on the phase evolution pattern and phase transition reactions between melt and arising solid phases during solidification of three different Mo-Si-B alloy compositions. To carry out this work, three alloy compositions from the Mo3Si, Mo5SiB2 and MoSS primary solidification regions, in the vicinity of ternary eutectic composition of Mo-Si-B system were selected. To investigate phase evolution and solidification reactions, a phase-field simulation is conducted with MicressTM applying the thermodynamic data generated with FactsageTM. The simulations confirmed the presence of binary and ternary eutectic reactions. It was observed that, depending on the initial melt composition, a primary phase emanates at the beginning of solidification which causes a composition fluctuation in the residual melt. Depending on the composition of the residual melt, thermodynamically feasible phases tend to perform a binary reaction until the rest of liquid undergoes the ternary MoSS-Mo5SiB2-Mo3Si eutectic reaction. However, exploring the solidification path of the considered compositions implied the uncertainties in considered thermodynamic parameters which requires the revision of thermodynamic database.

Through the impact glass: Insight into the evolution of melt at the Mistastin Lake impact structure

AbstractBy analyzing impact glass, the evolution of the impact melt at the Mistastin Lake impact structure was investigated. Impact glass clasts are present in a range of impactites, including polymict breccias and clast‐rich impact melt rock, and from a variety of settings within the crater. From the glass clasts analyzed, three petrographic subtypes of impact glass were identified based on their clast content, prevalence of schlieren, color, texture, and habit. Several alteration phases were also observed replacing glass and infilling vesicles; however, textural observations and quantified compositional data allowed for the identification of pristine impact glass. Although the various types of glasses show significant overlap in their major oxide composition, several subtle variations in the major oxide chemistry of the glass were observed. To investigate this variation, a least‐squares mixing model was implemented utilizing the composition of the glass and the known target rock chemistry to model the initial melt composition. Additionally, image analysis of the glass clasts was used to investigate whether the compositional variations correlated to textural difference in the lithologies. We propose that the textural and compositional dichotomy observed is a product of the evolution, assimilation, and emplacement of the glass. The dichotomy is reflective of the melt either being ballistically emplaced (group 2 glasses: occurring in melt‐poor polymict breccias at lowermost stratigraphic position outside the transient crater) or the result of late‐stage melt flows (group 1 glasses, occurring in melt‐bearing polymict breccias and impact melt rocks at higher stratigraphic positions outside the transient crater).

Влияние кристаллизационной дифференциации на состав остаточного расплава для плагиоклаза при различных p-t условиях

Crystallization differentiation processes in the melt volume for albite-anorthite solid solution series have been studied. For the albite-anorthite system, the change in the melt composition due to crystallization differentiation is calculated for pressure values P = 6,3 kbar and 1 bar and temperature T = 1410 °C, 1350 and 1300 °C. A calculation technique is presented for composition of the melt remaining after settling of plagioclase particles. The residual melt compositions have been calculated for different initial melt compositions and different P-T parameters. The change in composition due to crystallization differentiation of the melt is the difference in the percentage composition for each oxide on the liquidus line and the initial melt composition. The dimensionless ratios (similarity criteria) for the initial melt composition and the change of the oxide content , , , have been obtained. The change of each oxide percentage is calculated in weight percents and in the dimensionless form (as values of above-mentioned similarity criteria). The initial melt is depleted in different components. The depletion is due to settling of plagioclase particles and melt volume reduction. The latter is the sum of the solid particles and the melt volumes in the intercrystalline spaces of the settled particles’ layer. It is shown that the processes of crystallization differentiation are the sum total of hydrodynamic (geodynamic) and petrological processes. These processes can be studied using the methods of similarity theory. The compositional change in the melt due to crystallization differentiation can be represented in the form of an analytical relationship between the petrological similarity criteria

Kinetic study of vacuum evaporation of elements from ternary melts; case of dilute solution of P in Si-Al melts

This research is devoted to study phosphorus removal from Si-Al alloys by vacuum refining of the ternary system of dilute solutions of P in Si-20 wt%Al. The experiments were carried out in an induction furnace and after the refining process, the melt was characterized by ICP-MS technique to trace the concentration change of the volatile elements. The experimental results show that P removal from Si-Al-P melts takes place faster compared to Si-P melts and Al evaporates during the vacuum refining as well. The empirical kinetics of P and Al evaporation is discussed and the apparent activation energy for P and Al evaporation from Si-Al melts is obtained as EP=249.4kJ·mol-1and EAl=144.8kJ·mol-1 respectively. Results show that the composition of the melt changes continuously during the refining process due to rapid Al evaporation. In order to investigate the evaporation kinetics of the melt constituents, we developed a numerical approach by applying the Hertz-Knudsen-Langmuir equation for evaporation. This approach can be applied to model the evaporation of the melt constituents in a ternary system whose composition changes during the vacuum refining process. The model is validated with performed experimental results and it can be applied to discuss the effect of temperature, pressure, initial melt composition on the time of vacuum refining.

Fluid-present anatexis of Neoarchean tonalite and amphibolite in the Western Shandong Province

Metatonalite and amphibolite from the Taishan region of the Western Shandong Province in the North China Craton record c. 2.60 Ga fluid-present partial melting via the breakdown of biotite, plagioclase and quartz to produce peritectic hornblende and anatectic melt. Eight paired leucosome–melanosome samples from metatonalite and three paired samples from amphibolite were investigated to evaluate the composition of the melt. Hornblende, biotite and plagioclase in the leucosomes and hornblende and plagioclase in melanosomes from both rock types have similar compositions. Two leucosome samples from the metatonalite were influenced by the removal of heavy rare earth element-rich hornblende and the accumulation of plagioclase. The other leucosomes are interpreted to represent near initial melt compositions with a minor component of peritectic hornblende and are enriched in Si, Na and Sr and depleted in K, Ca, Ba and Rb relative to melanosomes. The enrichment of Na in the melt is inconsistent with experimental results of fluid-present melting of tonalite, but is broadly consistent with the experimental results of fluid-fluxed melting of amphibolite. The absence of K-feldspar in both rock types is a critical control on the composition of anatectic melt and initial melt compositions were probably similar for both rock types. Leucosomes inherited rare earth element patterns from their sources, which suggests that some trace element diagrams used to infer tectonic settings and depths of melting are not appropriate for reworked components of Archean grey gneisses. Whole-rockδ18O suggest that the fluids responsible for inducing local melting were derived from the intrusion and crystallization of ~2.60 Ga tonalites and trondhjemites in the Taishan region. One amphibolite sample has a relatively low δ18O suggestive of interaction with meteoric water or seawater possibly related to crustal extension and asthenosphere upwelling at ~2.60 Ga. Fluid-present partial melting reworked 2.75–2.60 Ga tonalites and amphibolites, generating ~2.60 Ga sodium-rich components of grey gneisses in the Western Shandong Province.

Origin of Holocene trachyte lavas of the Quetrupillán volcanic complex, Chile: Examples of residual melts in a rejuvenated crystalline mush reservoir

The Quetrupillán Volcanic Complex (QVC) is a stratovolcano placed in the center of a NW-SE volcanic chain, between Villarrica volcano and Lanín volcano, in the Central Southern Volcanic Zone of the Andes. Its youngest effusive products are dominated by crystal-poor (most samples with <9 vol% phenocrysts), crystal clot-bearing trachytes (from 64.6 up to 66.2 wt% SiO2), whereas the oldest units are mainly basaltic andesites. Two-stage generation of QVC trachytes by differentiation at shallow depth (<1 kbar) and NNO-QFM oxidation conditions were obtained from initial melt compositions equivalent to the Huililco basalts, a small eruptive centre located ca. 12 km NE of the QVC main vent. Pyroxene-bearing crystal clots, locally abundant in the trachytes, were formed at 900–960 °C (±55 °C) and represent a dismembered crystal mush from which interstitial trachytic melts were extracted and transported upward before eruption. Heating of the crystal mush by a hotter magma recharge is inferred from complex zoned plagioclases formed at higher crystallization temperatures (50–90 °C) than those obtained from pyroxene. Ca-rich plagioclase overgrowths around more albitic cores, followed by an external rim of similar composition to the core are interpreted as restoration to the initial conditions of plagioclase crystallization after the mentioned heating event. Additionally, a late heating of up to 150 °C just prior to eruption is recorded by Fe-Ti oxide thermometry.

From mantle peridotites to hybrid troctolites: Textural and chemical evolution during melt-rock interaction history (Mt. Maggiore, Corsica, France)

Recent studies investigate the replacive formation of hybrid troctolites from mantle peridotites after multiple stages of melt-rock reaction. However, these studies are not conducted in a field-controlled geological setting displaying the clear evolution from the protolith to the end-product of the reactions. The Mt. Maggiore peridotitic body exposes a clear evolution from spinel lherzolite to plagioclase-bearing lithotypes (plagioclase peridotites, olivine-rich troctolites and troctolites) during two continuous episodes of melt-rock interaction. In the spinel facies, the reactive percolation of a LREE-depleted melt leads to the dissolution of mantle pyroxenes and the growth of olivine crystals, forming replacive spinel dunites. The progressive evolution from spinel lherzolite to harzburgite to replacive dunite is accompanied by a change of olivine Crystallographic Preferred Orientation (CPO), from axial-[100] in the lherzolite to axial-[010] olivine CPO in the dunites, indicative of deformation in presence of melt. The initial percolating melt composition is consistent with single melt increments after 6% partial melting of a depleted mantle source. Reactive melt percolation leads to a progressive enrichment in the melt M-HREE absolute concentrations, while preserving its LREE depletion, consistent with the enriched analyzed HREE composition of olivine in the spinel dunite.In the shallower plagioclase facies, the melts modified by reactive melt percolation impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene in the peridotites. This impregnation stage is also observed in the spinel dunites, forming hybrid olivine-rich troctolites and troctolites. The dissolution-precipitation reactions forming hybrid troctolites cause a progressive textural evolution of the olivine matrix, with the disruption of deformed coarse grains into undeformed small rounded grains. This textural evolution is not accompanied by clear changes in the olivine CPO, indicating low instantaneous melt/rock ratios during the impregnation process. Olivine, plagioclase and clinopyroxene REE compositions analyzed in troctolite fit a process of impregnation with a progressive closure of the porosity (at decreasing melt mass), leading to the crystallization of trapped melt and REE enrichments during the last crystallization increments.

Fragmentation-driven grain refinement in directional solidification of AlCu10wt-% alloy at low pulling speeds

The formation of the grain structure in Al-10 wt%Cu alloy during directional solidification was investigated experimentally. The alloy composition was chosen because of its special feature that both the initial melt composition and the solidifying primary Al dendrites have almost identical densities. Therefore, gravity-related effects such as buoyancy or sedimentation acting on nucleated or fragmented solid particles in the melt are expected to be minor. In 3D bulk samples at low solidification velocities, unexpected equiaxed grain growth was found instead of columnar growth. This behavior was investigated in accompanying solidification experiments with thin samples using in-situ X-ray diagnostics. It is demonstrated that fragments detach from the dendrite tip region and move slightly ahead of the solid-liquid interface as they grow. As a result, a dendritic microstructure consisting of elongated equiaxed grains is developed. Accordingly, fragmentation was identified as responsible for grain refinement in the given parameter range.

Solubility of water in lunar basalt at low pH2O

We report the solubility of water in Apollo 15 basaltic “Yellow Glass” and an iron-free basaltic analog composition at 1atm and 1350°C. We equilibrated melts in a 1-atm furnace with flowing H2/CO2 gas mixtures that spanned ∼8 orders of magnitude in fO2 (from three orders of magnitude more reducing than the iron-wüstite buffer, IW−3.0, to IW+4.8) and ∼4 orders of magnitude in pH2/pH2O (from 0.003 to 24). Based on Fourier transform infrared spectroscopy (FTIR), our quenched experimental glasses contain 69–425ppm total water (by weight). Our results demonstrate that under the conditions of our experiments: (1) hydroxyl is the only H-bearing species detected by FTIR; (2) the solubility of water is proportional to the square root of pH2O in the furnace atmosphere and is independent of fO2 and pH2/pH2O; (3) the solubility of water is very similar in both melt compositions; (4) the concentration of H2 in our iron-free experiments is <∼4ppm, even at oxygen fugacities as low as IW−2.3 and pH2/pH2O as high as 11; (5) Secondary ion mass spectrometry (SIMS) analyses of water in iron-rich glasses equilibrated under variable fO2 conditions may be strongly influenced by matrix effects, even when the concentration of water in the glasses is low; and (6) Our results can be used to constrain the entrapment pressure of lunar melt inclusions and the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads. We find that the most water-rich melt inclusion of Hauri et al. (2011) would be in equilibrium with a vapor with pH2O∼3bar and pH2 ∼8bar. We constrain the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads to be 0.0005bar and 0.0011bar respectively. We calculate that batch degassing of lunar magmas containing initial volatile contents of 1200ppm H2O (dissolved primarily as hydroxyl) and 4–64ppm C would produce enough vapor to reach the critical vapor volume fraction thought to be required for magma fragmentation (∼65–75vol.%) at a total pressure of ∼5bar (corresponding to a depth beneath the lunar surface of ∼120m). At a fragmentation pressure of ∼5bar, the calculated vapor composition is dominated by H2, supporting the hypothesis that H2, rather than CO, was the primary propellant of the lunar fire fountain eruptions. The results of our batch degassing model suggest that initial melt compositions with >∼200ppm C would be required for the vapor composition to be dominated by CO rather than H2 at 65–75% vesicularity.

In-situ assimilation of mantle minerals by kimberlitic magmas — Direct evidence from a garnet wehrlite xenolith entrained in the Bultfontein kimberlite (Kimberley, South Africa)

The lack of consensus on the possible range of initial kimberlite melt compositions and their evolution as they ascend through and interact with mantle and crustal wall rocks, hampers a complete understanding of kimberlite petrogenesis. Attempts to resolve these issues are complicated by the fact that kimberlite rocks are mixtures of magmatic, xenocrystic and antecrystic components and, hence, are not directly representative of their parental melt composition. Furthermore, there is a lack of direct evidence of the assimilation processes that may characterise kimberlitic melts during ascent, which makes understanding their melt evolution difficult.In this contribution we provide novel constraints on the interaction between precursor kimberlite melts and lithospheric mantle wall rocks. We present detailed textural and geochemical data for a carbonate-rich vein assemblage that traverses a garnet wehrlite xenolith [equilibrated at ~1060°C and 43kbar (~140–145km)] from the Bultfontein kimberlite (Kimberley, South Africa). This vein assemblage is dominated by Ca–Mg carbonates, with subordinate oxide minerals, olivine, sulphides, and apatite. Vein phases have highly variable compositions indicating formation under disequilibrium conditions. Primary inclusions in the vein minerals and secondary inclusion trails in host wehrlite minerals contain abundant alkali-bearing phases (e.g., Na–K bearing carbonates, Mg-freudenbergite, Na-bearing apatite and phlogopite). The Sr-isotope composition of vein carbonates overlaps those of groundmass calcite from the Bultfontein kimberlite, as well as perovskite from the other kimberlites in the Kimberley area. Clinopyroxene and garnet in the host wehrlite are resorbed and have Si-rich reaction mantles where in contact with the carbonate-rich veins. Within some veins, the carbonates occur as droplet-like, globular segregations, separated from a similarly shaped Si-rich phase by a thin meniscus of Mg-magnetite. These textures are interpreted to represent immiscibility between carbonate and silicate melts.The preservation of reaction mantles, immiscibility textures and disequilibrium in the vein assemblage, suggests quenching, probably triggered by entrainment and rapid transport toward the Earth's surface in the host kimberlite magma. Based on the Sr-isotope systematics of vein carbonate minerals, and the close temporal relationship between carbonate-rich metasomatism and kimberlite magmatism, we suggest that the carbonate-rich vein assemblage was produced by the interaction between a melt genetically related to the Bultfontein kimberlite and wehrlitic mantle wall rock. If correct, this unique xenolith sample provides a rare snapshot of the assimilation processes that might characterise parental kimberlite melts during their ascent through the lithospheric mantle.

Crystal accumulation in a 4.2 Ga lunar impact melt

The ages of lunar impact basins and the role of fractional crystallization in producing compositional heterogeneity of lunar melt sheets are long-standing problems with significant implications for solar system dynamics and the petrologic evolution of the lunar crust. Here we document the formation of a basin-scale impact on the Moon at 4.20±0.07Ga based on the 147Sm–143Nd isochron age of a magnesian, noritic anorthosite melt rock from lunar breccia 67955. Major element compositions of plagioclase and mafic silicates in the melt rock imply a substantial component of primary Mg-suite cumulates or related lithologies in the pre-impact crustal stratigraphy. Trace element compositions of the plagioclase, including diagnostic ratios such as Sr/Ba, are also mostly similar to those in plagioclase from Mg-suite cumulates, with a small number of grains trending toward compositions observed in ferroan anorthosites. Mineral-melt distribution coefficients applied to trace element compositions of the 67955 plagioclase and pyroxene predict parental melt compositions that contrast strongly with the bulk rock. Compared to the whole rock, parental melts calculated from the plagioclase are enriched in REE (ΣREELa–Yb=131–885, average 619ppm vs. 39.8ppm) and they have more fractionated REE patterns (La/Ybn=1.2–9.8, average 4.9 vs. 1.5) with deep negative Eu anomalies (Eu/Eu∗=0.09–0.40 vs. 1.36). Trace element data for the pyroxenes also imply incompatible-element enriched parental melts. Subsolidus equilibration between the plagioclase and the pyroxene apparently rotated the REE patterns, but the conclusion that the parental melt was highly enriched in REE relative to the whole rock appears robust. Quantitative modeling shows that fractional crystallization of the 67955 whole rock composition cannot reproduce the range of Ba, Sr, Ti, and La concentrations measured in the 67955 plagioclase. Rather, the models require an initial melt composition that was strongly enriched in these elements, and they suggest that fractional crystallization became less efficient as crystallization proceeded. The contrast between the inferred parental melt composition and the bulk rock implies formation of the 67955 noritic anorthosite as a crystal cumulate, and that the cogenetic residual melt was strongly enriched in incompatible elements. If so, this would be the first documented example of fractional crystallization in a lunar impact melt sheet. The petrological and geochemical characteristics of the 67955 noritic anorthosite suggest that it formed by an impact in the Procellarum-KREEP Terrane, and was transported to the Apollo 16 site as Imbrium ejecta. Inheritance of ejecta related to this pre-Imbrium basin may contribute to the common occurrence of ∼4.2Ga 40Ar–39Ar plateau ages in breccia clasts and regolith fragments from the rim of North Ray crater. In that case, those data may provide no constraints on the age of the Nectaris basin, despite its proximity to the Apollo 16 site.

Prototypic corium analysis: a round robin for SEM and EDS characterisation

In case of a nuclear reactor severe accident, the core could melt, forming a high temperature mixture called corium. A uranium oxide-containing sample from a corium-concrete interaction test has been analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) at three different laboratories. In all the measured windows, the composition lay on or close to the line connecting the initial corium melt composition to the concrete average composition. The round robin exercise confirmed that SEM and EDS analysis can be used confidently to perform fast and good quality local corium analyses and provide local quantitative compositions in the metallic elements of corium with an uncertainty of 10 % of the measured value. Differences up to 25 % of the measured value were found for the oxygen content, although two of the laboratories provided very close results.

Amphibole-bearing Multiphase Solid Inclusions in Olivine and Plagioclase from a Layered Gabbro: Origin of the Trapped Melts

This study presents new data regarding the petrographic and mineralogical characteristics of amphibole-bearing multiphase solid inclusions (MSI) enclosed in olivine and plagioclase from a well-characterized Japanese layered gabbro, the Murotomisaki Gabbroic Intrusion. Bulk compositions of the MSI were obtained using wavelength-dispersive (WDS) and energy-dispersive (EDS) techniques combined with modal analysis. Many MSI in olivines are spherical or polygonal in shape and are mainly composed of pargasitic amphibole, orthopyroxene, and mica (biotite and aspidolite). The MSI in olivine phenocrysts in the chilled margins of the intrusion are composed of plagioclase and hornblende rather than pargasite. Many of them are found to be enriched in olivine components in their bulk compositions, and therefore they cannot be regarded as simple trapped basaltic melts or their derivatives. The MSI in plagioclase are more irregular in shape and occur in the resorbed part of calcic plagioclase cores; they are mainly composed of pargasite and sodic plagioclase. Attempts were made to obtain the original composition of the MSI melts by applying a correction for Fe^Mg exchange reactions with the host olivine. It was found that after the correction, the MSI in the chilled margin were closest to the published initial basaltic melt composition, whereas the olivine-hosted MSI from the olivine gabbros were still fairly high in olivine components despite the correction. Such post-entrapment modification was probably minimal for the plagioclase-hosted MSI because plagioclase is essentially free of Mg and Fe, and therefore plagioclase-hosted MSI are believed to represent the compositions of the original trapped melts. These are characteristically enriched in plagioclase components. We believe that the anomalous compositions of the trapped melts (after the corrections) were generated by local dissolution of olivine and plagioclase caused by the introduction of water from lower horizons of the crystallization boundary layer in a magma chamber. The MSI are therefore considered to provide new evidence for the dissolution of silicates by hydrous fluxing in a lower crystallizing boundary layer of a magma reservoir during solidification of basaltic magma.

Ongonite–elvan magmas of the Kalguty ore-magmatic system ( Gorny Altai): composition, fluid regime, and genesis

The Kalguty ore-magmatic system (OMS) is a complex combination of a granite pluton, a hydrothermal Mo–W deposit, pegmatites, greisens, and a belt of rare-metal (RM) and ultra-rare-metal (URM) elvan and ongonite dikes. Studies of melt inclusions (MI) in quartz phenocrysts in the dike rocks have demonstrated that quenched glass has major element contents close to those of the dike rocks but rare elements (Li, Rb, Be, Cs) and P contents. This suggests that the MI represent magma at the stage preceding the dike emplacement. The MI in quartz from the URM rocks are poorer in Si, Fe, Mg, and REE than those in quartz from the RM rocks but richer in Cs, Rb, Nb, and Ta, like the URM rocks themselves. This indicates that the melts had segregated into RM and URM ones before the studied quartz phenocrysts began to crystallize. The composition of MI glass corresponds to “the albite trend” of differentiation, suggesting that the initial melt compositions were ongonitic, while their K enrichment and formation of elvan magma followed the crystallization of the quartz phenocrysts. According to our estimates, the melt contained 6–7 wt.% H 2O. The quartz phenocrysts crystallized in a heterogeneous medium consisting of a silicate melt and an aqueous fluid. The latter was a high-density supercritical fluid with 3–12 wt.% NaCl equiv. Variations in the gas and salt compositions of the fluid inclusions (FI) are attributed to the interaction between fluids of magmatic and hydrothermal systems. This possibility is confirmed by ample evidence of their coeval formation. Quartz crystallization from the RM melts took place at 630–650 °C, whereas quartz from the URM melts formed at 20–30 °C lower temperatures. Quartz phenocrysts crystallized at 4.5–5.5 kbar. Additional estimates with regard to the mineral composition and quartz compressibility yielded values of 3–6.5 kbar. A petrogenetic model of some crystallization stages of the dike rocks within the Kalguty OMS was constructed on the basis of the results obtained in this study. The melts which formed the dikes of the East Kalguty belt are derivatives of the same magma which formed the major-stage granite pluton. Quartz is present as intratelluric phenocrysts, which crystallized at considerably greater depths than those of the dike emplacement. Differentiation of the parental magma was accompanied by rare element and P accumulation. The compositions of the FI and MI confirm that the magma and hydrothermal system of the Kalguty OMS exchanged their substances. It is associated with the increasing K content of the melts and the subsequent elvan crystallization as well as considerable variations in the salt and gas compositions of the magmatic fluid inclusions.

Magmatic Fractionation by Compositional Convection in a Sheet-like Magma Body: Constraints from the Nosappumisaki Intrusion, Northern Japan

The mechanism of fractionation in a sheet-like magma body was investigated based on observations from the Nosappumisaki intrusion, northern Japan. This is one of a number of late Cretaceous shoshonite sills in the Nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing � 50 vol. % cumulus crystals and a overlying middle zone free of such cumulus crystals. These zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones. It is inferred that massive crystal settling or flotation from a magma initially laden with � 20 vol. % of crystals (primocrysts) in a sheet-like magma body formed a cumulus pile and an overlying crystal-free melt layer. After the formation of the compound structure with partial freezing of the upper and lower marginal zones, the internal part of the intrusion slowly solidified. During this period the melt in the central part of the intrusive sheet underwent fractional crystallization. This is shown by patterns of vertical variation in the composition of the interstitial melt. The overall patterns are inverted (mirrorimage) S-shaped and S-shaped for incompatible and compatible elements, respectively. The incompatible element abundances show maxima near the bottom of the middle zone, which corresponds to the final solidification horizon. They show minima in the cumulate zone, which suggests discharge of an evolved melt from the crystal pile. Fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer.The melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile.This conclusion is based on the following observations. (1) There is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone. (2) Leucocratic pipe-like structures, which represent relict pathways for the evolved melt, are developed in the cumulate zone.These structures increase in diameter, decrease in number density, and increase in abundance of incompatible elements with height. (3) Cumulus augite in the cumulate zone has Ca-rich and Al-poor dissolved rims. The extent of dissolution increases with stratigraphic height, suggesting downwelling of an H2O-rich melt from the overlying melt layer. (4) There is no evidence for extensive compaction accompanying pore melt crystallization in the main part of the cumulate zone. (5) The initial melt composition estimated from the groundmass of the chilled margin rocks and the average composition of the material interstitial to the primocrysts for the whole sill are statistically identical. A mass-balance model for compositional convection was constructed to quantify the melt transportation processes. Model parameters were optimized by fitting the observed vertical variation of the interstitial melt compositions. The optimized results suggest that the evolved melts discharged from the crystal pile and rose through the central melt layer as plumes with minor mingling with the surrounding melt. The initially formed compound structure is a highly preferential environment for compositional convection.

Olivine dissolution in basaltic melt

The main purpose of this work is to understand and quantify diffusive and convective olivine dissolution in basaltic melt. Crystal dissolution and growth in a magma chamber is often accompanied by the descent or ascent of the crystal in the chamber due to gravity. The motion induces convection that enhances mass transport. Such convective dissolution and growth rates have not been quantified before. MgO diffusivity in the melt ( D MgO), MgO concentration of the interface melt ( C 0) and the effective thickness of the compositional boundary layer (δ) are necessary parameters to model the convective dissolution. Experiments of non-convective olivine dissolution in a basaltic melt were conducted at 1271–1480 °C and 0.47–1.42 GPa in a piston–cylinder apparatus. At specific temperature and pressure conditions, multiple experiments of different durations show that the interface melt reaches near-saturation within 2 min. Therefore, diffusion, not interface reaction, is the rate-controlling step for non-convective olivine dissolution in basaltic melt. The compositional profile length and olivine dissolution distance are proportional to the square root of experimental duration, consistent with diffusive dissolution. D MgO and C 0 are obtained from the experimental results. D MgO displays Arrhenian dependence on temperature, but the pressure dependence is small and not resolved. C 0 increases with increasing temperature and decreases with increasing pressure. Comparison with literature data shows that D MgO depends strongly on the initial melt composition, while C 0 does not. δ is estimated from fluid dynamics. D MgO/δ, which characterizes the kinetic and dynamic aspects of convective crystal dissolution, is parameterized as a function of temperature, pressure, and olivine composition. Convective olivine dissolution rate in basaltic melt can be conveniently calculated from the model results. Application to convective crystal growth and xenolith digestion is discussed.

Phase-Field Simulation of Fusion Interface Events during Solidification of Dissimilar Welds: Effect of Composition Inhomogeneity

We investigate the events near the fusion interfaces of dissimilar welds using a phase-field model developed for single-phase solidification of binary alloys. The parameters used here correspond to the dissimilar welding of a Ni/Cu couple. The events at the Ni and the Cu interface are very different, which illustrate the importance of the phase diagram through the slope of the liquidus curves. In the Ni side, where the liquidus temperature decreases with increasing alloying, solutal melting of the base metal takes place; the resolidification, with continuously increasing solid composition, is very sluggish until the interface encounters a homogeneous melt composition. The growth difficulty of the base metal increases with increasing initial melt composition, which is equivalent to a steeper slope of the liquidus curve. In the Cu side, the initial conditions result in a deeply undercooled melt and contributions from both constrained and unconstrained modes of growth are observed. The simulations bring out the possibility of nucleation of a concentrated solid phase from the melt, and a secondary melting of the substrate due to the associated recalescence event. The results for the Ni and Cu interfaces can be used to understand more complex dissimilar weld interfaces involving multiphase solidification.

Impact of process factors on production of aluminum master alloys containing zirconium and scandium

We discuss basic theory and process for synthesis of aluminum master alloys containing rare-earth metals. We study the effect of initial melt composition and synthesis temperature on the concentrations of alloying elements in the [final] alloy. We determine the composition and structure of the resulting intermetallic compounds.