The partitioning of trace elements between clinopyroxene and trachybasaltic melt during rapid cooling and crystal growth

We present the variation in trace element partition coefficients measured at the interface between rapidly cooled clinopyroxene crystals and co-existing melts. Results indicate that, as the cooling rate is increased, clinopyroxene crystals are progressively depleted in Si, Ca and Mg counterbalanced by enrichments in Al (mainly tetrahedral Al^iv), Na and Ti. Partition coefficients (Ds) for rare earth elements (REE), high field strength elements (HFSE) and transition elements (TE) increase with increasing cooling rate, in response to clinopyroxene compositional variations. The entry of REE into the M2 site is facilitated by a coupled substitution where either Na substitutes for Ca on the M2 site or Al^iv substitutes for Si in the tetrahedral site. The latter substitution reflects an increased ease of locally balancing the excess charge at M2 as the number of surrounding Al^iv atoms increases. Due to the lower concentration of Ca in rapidly cooled clinopyroxenes, divalent large ion lithophile elements (LILE) on M2 decrease at the expense of monovalent cations. Conversely, higher concentrations of HFSE and TE on the M1 site are facilitated as the average charge on this site increases with the replacement of divalent-charged cations by Al^vi. Although crystallization kinetics modify clinopyroxene composition, deviations from equilibrium partitioning are insufficient to change the tendency of a trace element to be compatible or incompatible. Consequently, there are regular relationships between ionic radius, valence of the trace element and D. At both equilibrium and cooling rate conditions, Ds for isovalent cations define parabola-like curves when plotted against ionic radius, consistent with the lattice strain model, demonstrating that the partitioning of trace elements is driven by charge balance mechanisms; cation substitution reactions can be treated in terms of the energetics of the various charge-imbalanced configurations.     

Pre-eruptive melt composition and constraints on degassing of a water-rich pantellerite magma,Fantale volcano,Ethiopia

To determine the pre-eruptive composition of peralkaline magma at Frantale volcano, Ethiopia, we have studied glass inclusions in phenocrysts from a lateceupting, glassy pantelleritic lava flow. Matrix glass and crystal-free glass inclusions in quartz were analyzed for all major and most minor elements by electron microprobe and for H₂O and 15 lithophile trace elements by ion microprobe (SIMS). Compositions of inclusions may have been slightly modified by post-trapping quartz crystallization, the average concentrations of all constituents but silica may be artificially high by 10% relative. Glass inclusions contain extreme enrichments in H₂O (mean of 4.6 to 4.9 wt%) and several lithophile trace elements, which suggest that the lava erupted from a highly evolved, water-rich fraction of magma. The pre-eruptive concentration of water was much higher than that generally considered to occur in pantellerite magmas. Trends observed for lithophile elements in whole-rock samples from pre-,syn-and post-caldera eruptive units are mimicked in glass inclusions from the studied pantellerite lava; concentrations of Rb, Y, Zr, Nb, and Ce±Cl increase with progressive differentiation. With the exception of Cl and H₂O contents, the composition of matrix glass is similar to that of glass inclusions suggesting: that few constituents exsolved from magma or cooling glass; eruption and quench of the lava occurred rapidly; and the matrix glass is, largely, compositionally representative of melt. Higher average abundances of Cl and H₂O in glass inclusions suggest that these volatiles exsolved after melt entrapment; degassing could have occurred as either an equilibrium or disequilibrium process.     

Crystallization kinetics of alkali feldspars in cooling and decompression-induced crystallization experiments in trachytic melt

Cooling and decompression experiments have been carried out on trachytic melts in order to investigate crystallization kinetics of alkali feldspar, the effect of the degree of undercooling (ΔT = T _liquidus ? T _experimental) and time on nucleation and crystal growth process. This experimental work gives us new data about crystallization kinetics of trachytic melts, and it that will be useful to better understand the natural system of Campi Flegrei volcanoes. Experiments have been conducted using cold seal pressure vessel apparatus, at pressure between 30 and 200 MPa, temperature between 750 and 855 °C, time between 7,200 and 57,600 s and redox condition close to the NNO +0.8 buffer. These conditions are ideal to reproducing pre- and syn-eruptive conditions of the Campi Flegrei volcanoes, where the “conditions” pertain to the complete range of pressures, temperatures and time at which the experiments were performed. Alkali feldspar is the main phase present in this trachyte, and its abundance can strongly vary with small changes in pressure, temperature and water content in the melt, implying appreciable variations in the textures and in the crystallization kinetics. The obtained results show that crystallization kinetics are strictly related to ΔT, time, final pressure, superheating (?ΔT) and water content in the melt. ΔT is the driving force of the crystallization, and it has a strong influence on nucleation and growth processes. In fact, the growth process dominates crystallization at small ΔT, whereas the nucleation dominates crystallization at large ΔT. Time also is an important variable during crystallization process, because long experiment durations involve more nucleation events of alkali feldspar than short experiment durations. This is an important aspect to understand magma evolution in the magma chamber and in the conduit, which in turn has strong effects on magma rheology.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites

 Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher ⁸⁷Sr/⁸⁶Sr_i and lower ɛ_Nd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high ⁸⁷Sr/⁸⁶Sr, low ɛ_Nd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale. Received: 28 September 1998 / Accepted: 29 December 1999     

Chlorine variations in the magma of Soufrière Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions

The Soufrière Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)_m. An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)_m. Melt inclusion H₂O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with D_Cl^fl-m between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)_m with temperature. We suggest that the zoning can be caused by accumulation of CO₂-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H₂O fugacity and/or increasing Cl_m result in increasing (Cl/OH)_m while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO₂-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO₂ emissions at the surface.     

Supersaturation and crystal growth in the roof-zone of the Skaergaard magma chamber

The coarse-grained Upper Border Series rocks of the Skaergaard intrusion contain abundant skeletal crystals of magnetite and ilmenite, skeletal and hopper crystals of apatite, and less abundant sector-zoned augite crystals and hopper zircon crystals. In addition, the melanogranophyres which occur as pods and lenses in the lower part of the Upper Border Series and the upper part of the Layered Series are characterized by very coarse-grained dendritic ferrohedenbergite crystals. Skeletal, hopper, and sectorzoned crystals are not present in the Layered Series gabbros. The development of these unusual crystal morphologies in the Upper Border Series requires that the roof-zone magma was intermittently supersaturated and indicates that the Skaergaard magma chamber was compositionally zoned and that heat loss through the roof maintained a temperature gradient in the magma that was greater than the adiabatic gradient. It is suggested that supersaturation developed in the roof-zone of the intrusion as a result of convective overturn and magma mixing during the early stages of crystallization, and as a result of sudden volatile loss during the later stages of crystallization when the Upper Border Series rocks became rigid enough to fracture.     

Laser-ablation ICP-MS analysis of silicate and sulfide melt inclusions in an andesitic complex II: evidence for magma mixing and magma chamber evolution

Laser-ablation microanalysis of a large suite of silicate and sulfide melt inclusions from the deeply eroded, Cu-Au-mineralizing Farallón Negro Volcanic Complex (NW Argentina) shows that most phenocrysts in a given rock sample were not formed in equilibrium with each other. Phenocrysts in the andesitic volcano were brought together in dominantly andesitic—dacitic extrusive and intrusive rocks by intense magma mixing. This hybridization process is not apparent from macroscopic mingling textures, but is clearly recorded by systematically contrasting melt inclusions in different minerals from a given sample. Amphibole (and rare pyroxene) phenocrysts consistently contain inclusions of a mafic melt from which they crystallized before and during magma mixing. Most plagioclase and quartz phenocrysts contain melt inclusions of more felsic composition than the host rock. The endmember components of this mixing process are a rhyodacite magma with a likely crustal component, and a very mafic mantle-derived magma similar in composition to lamprophyre dykes emplaced early in the evolution of the complex. The resulting magmas are dominantly andesitic, in sharp contrast to the prominently bimodal distribution of mafic and felsic melts recorded by the inclusions. These results severely limit the use of mineral assemblages to derive information on the conditions of magma formation. Observed mineral associations are primarily the result of the mixing of partially crystallized magmas. The most mafic melt is trapped only in amphibole, suggesting pressures exceeding 350 MPa, temperatures of around 1,000 °C and water contents in excess on 6 wt%. Upon mixing, amphibole crystallized with plagioclase from andesitic magma in the source region of porphyry intrusions at 250 MPa, 950 °C and water contents of 5.5 wt%. During ascent of the extrusive magmas, pyroxene and plagioclase crystallized together, as a result of magma degassing at low pressures (150 MPa). Protracted extrusive activity built a large stratovolcano over the total lifetime of the magmatic complex (>3 m.y.). The mixing process probably triggered eruptions as a result of volatile exsolution.Electronic Supplementary Material Supplementary material (eTable 1and eFigure 1) is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: T.L. Grove     

Magmatic processes in the Bishop Tuff rhyolitic magma based on trace elements in melt inclusions and pumice matrix glass

To investigate the origin of compositional zonation in the Bishop Tuff magma body, we have analyzed trace elements in the matrix glass of pumice clasts and in quartz-hosted melt inclusions. Our results show contrasting patterns for quartz in different parts of the Bishop Tuff. In all samples from the early part of the eruption, trace element compositions of matrix glasses are similar to but slightly more evolved than quartz-hosted melt inclusions. This indicates a cogenetic relationship between quartz crystals and their surrounding matrix glass, consistent with in situ crystallization. The range of incompatible element concentrations in melt inclusions and matrix glass from single pumice clasts requires 16–20 wt% in situ crystallization. This is greater than the actual crystal content of the pumices (<15 % crystals). In contrast to the pattern for the early pumices, pyroclastic flow samples from the middle part of the eruption show contrasting trends: In some clasts, the matrix is more evolved than the inclusions, whereas in other clasts, the matrix is less evolved. In the late Bishop Tuff, all crystal-rich samples have matrix glasses that are less evolved than the melt inclusions. Trace element abundances indicate that the cores of quartz in the late Bishop Tuff crystallized from more differentiated rhyolitic magma that was similar in many ways, yet distinct from the early-erupted Bishop Tuff. Our results are compatible with a model of secular incremental zoning (Hildreth and Wilson in Compositional zoning of the Bishop Tuff. J Petrol 48(5):951–999, 2007), in which melt batches from underlying crystal mush rise to various levels in a growing magma body according to their buoyancy. Early- and middle-erupted quartz crystallized from highly evolved rhyolitic melt, but then some parts of the middle-erupted magma were invaded by less differentiated rhyolite such that the matrix melt at the time of eruption was less evolved than the melt inclusions. A similar process occurred but to a greater extent in magma that erupted to form the late Bishop Tuff. In addition, there was a final, major magma mixing event in the late magma that formed Ti-rich rims on quartz and Ba-rich rims on sanidine, trapped less evolved rhyolitic melt inclusions, and resulted in dark and swirly crystal-poor pumice that is a rare type throughout much of the Bishop Tuff.     

Competing effects of crystal chemistry and silicate melt composition on trace element behavior in magmatic systems: insights from crystal/silicate melt partitioning of the REE,HFSE, Sn,In, Ga,Ba, Pt and Rh

We present new partition coefficients for the REE, HFSE, Sn, In, Ga, Ba, Pt and Rh between clinopyroxene, olivine and basaltic melt as a function of crystal chemistry and melt composition at temperatures of 1190–1300 °C and 1-bar pressure. Two components, namely \(\mathrm {Al_2O_3}\) and \(\mathrm {Na_2O}\), were chosen to be investigated since they are known to affect the structure of silicate melts and especially clinopyroxene crystal chemistry. The amount of \(^{[4]}\mathrm{Al}\) in clinopyroxene will result in an increase of \(D_i^\mathrm{{cpx/melt}}\) even after applying a correction factor to account for the effect of melt polymerization. Moreover, the positive correlation between \(^{[4]}\mathrm{Al}\) and \(D_i^\mathrm{{cpx/melt}}\) is not restricted to the REE, but also applies for Sn, Ga, In, and Ba. The addition of up to 2.6 wt% \(\mathrm {Na_2O}\) to the silicate melt universally increases the \(D_i^\mathrm{{cpx/melt}}\) without any concomitant change in crystal chemistry or a significant effect in melt polymerization. This compositional effect is likely due to the ability of Na to break REE–Al complexes in the melt. Our results emphasize the importance of considering all variables that affect the behavior of trace elements in magmatic systems before applying the lattice strain model and derive meaningful results for the changes in the parameters of the crystallographic sites.     

Calculated diffusion coefficients and the growth rate of olivine in a basalt magma

Concentration gradients in glass adjacent to skeletal olivines in a DSDP basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170°C, the diffusion coefficient of Mg²⁺ ions in the basalt is 4·5.10^?9 cm²/s. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates of 2–6.10^?7 cm/s. This is too slow for olivine to have grown in situ during quenching. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.     

Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature

We have studied the crystallization sequences, mineral chemistries, and textures that develop when an average Type B Ca-Al-rich inclusion composition is cooled in air from 1275–1580° C to below 1000°C at rates between 0.5 and 1000°C/hr. Crystallization sequences, the textures of all the major phases, pyroxene chemistry, and melilite zoning patterns are functions of both the cooling rate and the temperature from which cooling begins. Determination of the order of pyroxene and plagioclase crystallization has been identified as an important goal for petrographic studies of CAIs because it can be used to set constraints on the cooling rate experienced by an individual inclusion. Overall textures plus melilite zoning patterns and pyroxene chemistry can give important clues as to whether pyroxene or plagioclase began to crystallize first. Melilite texture and chemistry appear to yield the most valuable information on the maximum temperature to which an inclusion was raised prior to cooling.Comparison of our experimental results with petrographic observations of Type B CAIs suggests that most inclusions were partially melted and then cooled at rates on the order of a few tenths to tens of degrees per hour. Maximum temperatures of about 1400°C appear most likely for intermediate Type B Allende inclusions. Our results do not support the suggestion that the textures observed in these inclusions formed by crystallization of supercooled, metastable melt droplets condensed from nebular gas. The slow cooling rates we infer for CAIs are difficult to reconcile with models for their origin that imply simple radiative cooling of individual molten or partially molten droplets in a cold, low density environment. On the other hand, cooling rates of the nebular cloud are believed to have been much slower than those we have inferred for Type B CAIs. Scenarios that could be reconciled with the thermal history that we have inferred include drag heating of particles falling through nebular gas, heating by intense radiation (e.g., via flares) from the early sun, heating in nebular shock fronts, or other thermal heterogeneities in the early nebula allowing time scales for cooling (and heating) of CAIs much shorter than those for the nebular cloud as a whole. Successful models for the origin of Type B CAIs must account for the fact that most Type B CAIs cooled relatively slowly from a partially molten state.     

Garnet fractionation,progressive melt loss and bulk composition variations in anatectic metabasites:Complications for interpreting the geodynamic significance of TTGs

Tonalite-trondhjemite-granodiorite(TTG) suites constitute a large proportion of the Archean geological record;however,the geodynamic processes that generated them,and Archean continental crust in general,remain a subject of debate.The concentrations and ratios of Sr,Y,La,Yb,Nb,and Ta in TTGs are commonly used to determine the depth of melting of their metabasic sources.The trace element composition of melt produced by metabasic source rocks during anatexis is strongly affected by the presence and abundance of pressure-sensitive minerals,such as plagioclase(Sr-bearing),garnet(Y-and HREE-bearing),and rutile(Nb-and Ta-bearing).Elevated Sr/Y and La/Yb ratios and low concentrations of Nb and Ta in TTGs are generally considered to indicate melting at high pressures(≥2.0 GPa).The depth of melting is a key factor in determining the origin of TTGs as this provides critical information on the tectonic setting of their generation.We use phase equilibrium and trace element modelling to explore the effects of three potential influences on TTG trace element compositions:fractionation of trace elements into peritectic garnet cores,progressive melt loss from the source,and source bulk composition.We model three different compositions of Archean basalts along thermal gradients of 500℃/GPa,750℃/GPa,and 1000℃/GPa.The models produce maj or and trace element melt compositions that are generally consistent with measured compositions of TTGs.Although Sr/Y,La/Yb,Nb,and Ta exhibit pressure-dependent behaviour,other factors also affect these values.Garnet fractionation causes Sr/Y and La/Yb to reach much greater values and in this scenario,the values also increase with increasing temperature.Source bulk composition has an effect in all scenarios and most strongly influences La/Yb,Nb,and Ta.Overall,these results show that Sr/Y,La/Yb,Nb,and Ta can reach values generally considered to be indicative of high pressure melting at a range of P-T conditions including P 2.0 GPa.Consequently,trace element compositions of TTGs alone may provide a misleading impression of the depth of melting of metabasites and the geodynamic environment of Archean crustal growth and reworking.     

Iron meteorites: Crystallization,thermal history,parent bodies,and origin

We review the crystallization of the iron meteorite chemical groups, the thermal history of the irons as revealed by the metallographic cooling rates, the ages of the iron meteorites and their relationships with other meteorite types, and the formation of the iron meteorite parent bodies. Within most iron meteorite groups, chemical trends are broadly consistent with fractional crystallization, implying that each group formed from a single molten metallic pool or core. However, these pools or cores differed considerably in their S concentrations, which affect partition coefficients and crystallization conditions significantly. The silicate-bearing iron meteorite groups, IAB and IIE, have textures and poorly defined elemental trends suggesting that impacts mixed molten metal and silicates and that neither group formed from a single isolated metallic melt. Advances in the understanding of the generation of the Widmanstätten pattern, and especially the importance of P during the nucleation and growth of kamacite, have led to improved measurements of the cooling rates of iron meteorites. Typical cooling rates from fractionally crystallized iron meteorite groups at 500–700 °C are about 100–10,000 °C/Myr, with total cooling times of 10 Myr or less. The measured cooling rates vary from 60 to 300 °C/Myr for the IIIAB group and 100–6600 °C/Myr for the IVA group. The wide range of cooling rates for IVA irons and their inverse correlation with bulk Ni concentration show that they crystallized and cooled not in a mantled core but in a large metallic body of radius 150±50 km with scarcely any silicate insulation. This body may have formed in a grazing protoplanetary impact. The fractionally crystallized groups, according to Hf–W isotopic systematics, are derived originally from bodies that accreted and melted to form cores early in the history of the solar system, <1 Myr after CAI formation. The ungrouped irons likely come from at least 50 distinct parent bodies that formed in analogous ways to the fractionally crystallized groups. Contrary to traditional views about their origin, iron meteorites may have been derived originally from bodies as large as 1000 km or more in size. Most iron meteorites come directly or indirectly from bodies that accreted before the chondrites, possibly at 1–2 AU rather than in the asteroid belt. Many of these bodies may have been disrupted by impacts soon after they formed and their fragments were scattered into the asteroid belt by protoplanets.     

Dependence of clinopyroxene composition on cooling rate in basaltic magmas: Implications for thermobarometry

The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5 °C/min from 1250 down to 1000 °C) and at isothermal conditions of 1000 °C from a basaltic composition at a pressure of 500 MPa under anhydrous and hydrous (H₂O = 1.3 wt.%) conditions. The clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe²⁺ and Si and enriched in Na, Fe³⁺, Al (mainly Al^IV), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH = Di + Hd and TE = CaTs + CaFeTs + En) are calculated to describe the effect of cooling rate on the clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process induced by rapid cooling in basic rocks under hydrous and anhydrous conditions.Dynamic crystallization conditions affect the partitioning of major elements between clinopyroxene and melt; with increasing cooling rate, the value of crystal–melt partition coefficient departs from that obtained at the isothermal condition. However, in spite of these variations, the values of ^cpx–meltKd_Fe–Mg remain almost constant. Therefore, the Fe²–Mg exchange between clinopyroxene and melt is not suitable to prove the (dis)equilibrium conditions in basaltic cooling magmas, giving rise to possible mismatches in the application of thermobarometers. The results of our study are consistent with that observed at the margin of dikes or in the exterior portions of lavas, where the cooling rate is maximized and disequilibrium compositions of clinopyroxene have been found.     

Group IVA irons: New constraints on the crystallization and cooling history of an asteroidal core with a complex history

We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal-liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.Phosphate inclusions in EET show Δ¹⁷O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal-liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body.     

Paleozoic plume-lithospheric processes in northeastern Fennoscandia: Evaluation of the composition of the parental mantle melts and magma generation conditions

The paper reports quantitative evaluations of the modal and chemical composition of the mantle whose Paleozoic activation gave rise to the Kola alkaline province in the northeastern Baltic Shield. The volume of alkaline magmatism within the province and the volume of the mantle melts that were generated in the course of the Paleozoic activation cycle were evaluated by three-dimensional density modeling on the basis of gravimetric data. Our studies involved, along with the sampling of alkaline magmatic rocks in the region, the examination of the deep (to a depth of 22.5 km) structure of all alkaline intrusions in the province and the development of their three-dimensional density models. Concentrations of trace elements were precisely analyzed by the ICP-MS technique, and these data were used in order to calculate the weighted mean concentrations of trace elements in rocks of the province, to simulate the melting of mantle sources, and to evaluate the geodynamic sequences of these mantle processes. Our simulations indicate that the total volume of the Paleozoic mantle melts in the northeastern part of the Baltic Shield amounted to 15000 ± 2700 km³. The calculated composition of the partial melts that could be produced by the mantle of average composition shows the necessity for the significant introduction of certain elements into the mantle source. It is demonstrated that primitive melts in the Kola province were highly probably derived at low degrees of melting of the source (0.3–0.5%), whose composition corresponded to phlogopite-bearing (±amphibole) garnet lherzolite under the conditions of the mantle garnet depth facies. The calculated degree of enrichment of this sources was three times higher than the average concentrations of incompatible elements in the primitive mantle. It is demonstrated that magma generating processes affected much of the lithosphere beneath northeastern Fennoscandia and reached a depth of 120 km, i.e., the depth of the mantle facies of garnet lherzolite. The area of this region corresponds to the area of regional Paleozoic magmatism, and its depth correlates with the estimated P-T conditions under which the mantle xenoliths found in regional diatremes were formed.     

Assimilation and crystal accumulation in a mid-crustal magma chamber: the Sausfjellet pluton, north-central Norway

The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of hornblende-bearing to hornblende-rich gabbro and diorite which is thought to have crystallized from an H₂O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the “annular zone”). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite–hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich calc-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H₂O-poor andesitic parent. These rocks have approximately constant δ¹⁸O of +6.6±0.2‰ and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in δ¹⁸O, which ranges from +6.7‰ to +8.6‰. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks.     

加勒比海小安德列斯岛弧Kick’emJenny海底火山岩的斜长石成分环带:示踪大洋岛弧岩浆房的演化

我们对采自于加勒比海地区小安德列斯岛弧(Lesser Antilles Arc)Kick’em Jenny(KEJ)海底火山玄武岩中的斜长石斑晶进行了矿物形态和成分分析。利用电子探针(EMPA)和LA-ICP-MS测定了具有环带结构的斜长石斑晶中主量元素的空间分布,同时也利用LA-ICP-MS分析了斜长石中Sr的分布。结果表明,在不同的矿物斑晶中,元素含量均表现出和环带结构相联系的空间分布变化。斜长石斑晶中最主要的结构为韵律环带以及熔蚀结构,所测定的矿物边缘都存在An值从由内向外迅速降低的致密韵律环带,可能反映了快速结晶时的不平衡;而晶体内部的稀疏韵律环带结构是由岩浆填充或对流活动导致的。部分斜长石的熔蚀层An值由内向外升高,反映了高Ca岩浆填充的过程。这说明斜长石斑晶的矿物形态和元素环带可以用来制约俯冲带海底火山岩浆从源区上升到岩浆房再到喷发的复杂过程,包括岩浆演化、熔体多次填充、熔体与结晶矿物之间的反应、以及矿物再熔融等。这对于理解海底火山的喷发以及岛弧岩浆岩的演化有重要意义。