Phase Equilibrium Considerations Research Articles

High temperature oxidation regime transitions in hafnium carbide

AbstractUnderstanding the oxidation behavior of hafnium carbide is crucial to its application in extreme environments. In this work, the transition in high‐temperature oxidation kinetics regimes in hafnium carbide is explained based on phase equilibria considerations supported by observed changes in oxide scale microstructure evolution associated with different transformation pathways. Below, a composition‐dependent critical temperature and oxygen pressure, hafnium carbide first transforms to an amorphous material with nominal composition HfO2C followed by phase separation into carbon and hafnia domains. Subsequently, gaseous transport through a nanometric pore network formed by oxidative removal of phase‐separated carbon becomes rate‐limiting. Above this critical point, the oxidation sequence involves a direct transformation from hafnium carbide to hafnia and gaseous products, leading to dissimilar scale morphologies responsible for the reported transition from gaseous to solid‐state diffusion‐limited oxidation regimes at ultra‐high temperatures.

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

In this work, the influence of selected alloying variations on Zn-assisted liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) is investigated. Cold-rolled AHSS alloys of different carbon (C), manganese (Mn), silicon (Si), and aluminum (Al) concentrations were continuous-annealed to generate a third generation AHSS microstructure (composed of martensite and retained austenite) via quenching and partitioning. High temperature tension tests using simulated spot-weld thermomechanical cycles revealed no significant influence of C and Mn variations on the Zn-LME susceptibility of AHSS. On the other hand, Zn-LME susceptibility was strongly correlated with the Si content of AHSS. A direct comparison of the (reacted) coating microstructures of the Si-alloyed and Low-Si AHSS variants revealed that Si in the AHSS substrate suppresses Fe–Zn alloying reactions and retards the nucleation and growth of Fe-Zn intermetallic phases at the coating-substrate interface in these spot weld simulations. The suppressed intermetallic formation at elevated Si concentrations is consistent with phase equilibria considerations in the Fe-Zn-Si ternary system. In the context of Zn-assisted LME, therefore, Si is hypothesized to aggravate LME behavior by increasing the liquid Zn availability for embrittlement and promoting direct contact between liquid Zn and the AHSS steel substrate.

Effect of the CaO/SiO2 ratio on the controlled solidification of ‘Fe2O3’-CaO-SiO2 melts in air

ABSTRACT The solidification of liquids having bulk compositions in the high iron region of the ‘Fe2O3’-CaO-SiO2 system having selected CaO/SiO2 ratios has been investigated. The compositions investigated were selected such that the bulk compositions were within the hematite primary phase field, fully liquid at temperature (1350°C) and pass through the SFC primary phase field on cooling. Specifically, three CaO/SiO2 ratios (wt/wt) were investigated, 2.78, 3.46 (Nicol S, Jak E, Hayes P. 2019b. Microstructure evolution during controlled solidification of “Fe2O3”-CaO-SiO2 liquids in air. Met. Trans. B. 50:2706–2722) and 4.75 with bulk compositions comprising 69.23, 72.74 and 74.94 wt% Fe2O3 respectively. The phases present and their composition was determined with electron probe microanalysis (EPMA) and X-ray diffraction (XRD). During cooling, four stages of solidification were observed at all three CaO/SiO2 ratios, and an additional two stages at the highest CaO/SiO2 ratio. Non-equilibrium phase and liquidus compositions were observed. Significantly, the silico ferrite of calcium phase (SFC) was not observed in any of the samples, although the formation of this phase was anticipated from phase equilibrium considerations.

Large H2O solubility in dense silica and its implications for the interiors of water-rich planets

Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in [Formula: see text]O-rich setting at high pressures and temperatures ([Formula: see text]) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between [Formula: see text] and [Formula: see text]O as archetypal materials for rock and ice, respectively, at high [Formula: see text] We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the [Formula: see text]-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of ([Formula: see text])[Formula: see text] At pressures above 60 GPa, [Formula: see text]O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

Natural gas hydrate as a potential energy resource: From occurrence to production

Natural gas hydrate reservoirs have been strongly suggested as a potential energy resource. However, this potential is expected to be limited by geological factors, reservoir properties, and phase-equilibria considerations. Accordingly, sufficient understanding and accurate analyses for the complex surroundings in a natural gas hydrate system have to occur before methane recovery. In this paper, we discuss the formation and structure patterns of global natural gas hydrate, including the origins of hydrocarbon, crystal structures, and unique structure transition. We also summarize two important anomalies related to methane occupancy and chlorinity which were revealed very recently. Furthermore, we review the geological and chemical surroundings of the shallow hydrate deposits, the so-called brine patch discovered in the Cascadia Margin and Ulleung Basin, which are significantly related to tectonic conduits for methane gas and positive chlorinity.

Investigation of Freeze-Linings in Copper-Containing Slag Systems: Part I. Preliminary Experiments

Slag freeze-linings are increasingly used in industrial pyrometallurgical processes to insure that furnace integrity is maintained in aggressive high-temperature environments. Most previous studies of freeze-linings have analyzed the formation of slag deposits based solely on heat-transfer models. The focus of the present research is to determine the impact of slag chemistry and local process conditions on the microstructures, thickness, stability, and heat-transfer characteristics of the frozen deposit at steady-state conditions. The formation of the freeze-linings is studied under controlled laboratory conditions using an air-cooled “cold-finger” technique for Cu-Fe-Si-Al-O slag at equilibrium with metallic copper relevant to the industrial copper smelting processes. The phase assemblages and microstructures of the deposits formed in the cold-finger experiments differ significantly from those expected from phase equilibrium considerations. The freeze-lining deposits have been found, in general, to consist of several layers. Starting from the cold finger, these layers consist of glass; glass with microcrystalline precipitates; closed crystalline layer; and open crystalline layer. Even at steady-state conditions, there was no primary phase sealing layer of delafossite [Cu2O · (Al, Fe)2O3] present at the deposit/liquid interface—these observations differ markedly from those expected from phase equilibrium considerations. The findings have significant practical implications, and potential for the improved design and operation of industrial metallurgical furnaces.

Supercritical fluid processing of Nafion® membranes: Methanol permeability and proton conductivity

AbstractNafion® membranes commonly used in direct methanol fuel cells (DMFC) are typically limited by high methanol permeability. These membranes have phase‐segregated sulfonated ionic domains in a perfluorinated backbone, which make processing difficult and limited by phase equilibria considerations. This study used supercritical fluids (SCFs) as a processing alternative, since the gas‐like mass transport properties of SCFs allow for better penetration into the membranes and the use of polar cosolvents could also influence their morphology, thus fine‐tuning their physical and transport properties. The SCF processing was performed at 40°C and 200 bars using pure CO2 and CO2 with several polar cosolvents of different size and chemical functionalities like: acetic acid, acetone, acetonitrile, cyclohexanone, dichloromethane, ethanol, isopropanol, methanol, and tetrahydrofuran. Methanol permeability measurements revealed that the SCF processed membranes reduced the permeation of methanol by several orders of magnitude, especially with the use of some small polar cosolvents. Proton conductivity measurements, using AC electrochemical impedance spectroscopy, were on the order of 0.03–0.09 S/cm, which indicates that processing with SCF CO2 plus some cosolvents maintained the high proton conductivity while reducing the methanol permeability. The results are explained using XRD and SAXS. XRD analysis of the SCF processed samples revealed an increasing pattern in the crystallinity, which influenced the transport properties of the membrane. SAXS measurements confirmed the morphological differences that led to the changes in transport properties of the SCF processed membranes. Finally, processing flow direction (parallel versus perpendicular flow) played a major role in the morphological changes of this anisotropic membrane. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Phase selection and transition in Hf-rich hafnia-titania nanolaminates

Hf-rich hafnia-titania nanolaminate films with five HfO2-TiO2 bilayer architectures (0.64 to 0.94 Hf atom fraction) were sputter deposited on unheated fused silica substrates, annealed post-deposition from 573 to 1273 K, and analyzed by x-ray diffraction to study phase selection and transition. Isochronal annealing for 1 h intervals from 573 to 1173 K produces weak crystallization into monoclinic (m) HfO2 doped with Ti, i.e., m-Hf1-xTixO2. The amount of Ti incorporated into m-HfO2 depends upon both architecture and overall stoichiometry, but in all but the coarsest architecture, exceeds the bulk solubility limit of x = 0.05. Initial annealing at 1273 K produces significant crystallization into a biphasic structure, m-Hf1-xTixO2 and orthorhombic (o) HfTiO4. From bulk phase equilibrium considerations, o-HfTiO4 is expected to crystallize under conditions of interfacial bilayer mixing. However, upon further annealing at 1273 K, o-HfTiO4 proves to be unstable. o-HfTiO4 demixing inevitably occurs independent of architecture and stoichiometry, resulting in final crystallization products after 96 h at 1273 K that are m-Hf1-xTixO2 with x ≈ 0.05 and TiO2 doped with Hf. We suggest that o-HfTiO4 instability arises from a driving force to form domains similar to those found in the low temperature in/commensurate structures of ZrTiO4. A detailed crystallographic group-subgroup analysis of the o (Pbcn) → m (P21/c) transition shows that these domains can be represented by an orientation twin in the latter structure and their creation can be achieved by a single step second-order phase transition.

Deposition from “Waxy” Mixtures under Turbulent Flow in Pipelines: Inclusion of a Viscoplastic Deformation Model for Deposit Aging

A mathematical model is described and used to predict solids deposition from multicomponent paraffinic “waxy” mixtures under turbulent flow in pipelines. The model is based on the moving boundary problem formulation, in which the deposit formation and growth is modeled primarily as a heat-transfer process with phase change. The effects of shear stress (or Reynolds number) and deposition time (or deposit aging) are incorporated via a viscoplastic model, which is based on one-dimensional deformation of a cubical cage that squeezes out a fraction of the liquid phase from the deposit. Numerical solutions were obtained for the radial and axial growth of the deposit with time at Reynolds numbers (Re) of 10 000−25 000. The predicted trends are in agreement with the experimental results from recent laboratory deposition studies. The steady-state deposit thickness under turbulent flow is predicted to be considerably smaller than that under laminar flow, and it decreased with an increase in Re. The average wax content of the deposit is predicted to also increase with Re and deposition time (or aging), causing the deposit to become enriched in heavier paraffins and depleted in lighter paraffins. The results indicate that, although an increase in Re and deposition time causes wax enrichment in the deposit, the deposit thickness is dependent on heat-transfer and thermodynamic phase equilibrium considerations.

A multi‐objective optimization approach to polygeneration energy systems design

AbstractPolygeneration, typically involving co‐production of methanol and electricity, is a promising energy conversion technology which provides opportunities for high energy utilization efficiency and low/zero emissions. The optimal design of such a complex, large‐scale and highly nonlinear process system poses significant challenges. In this article, we present a multiobjective optimization model for the optimal design of a methanol/electricity polygeneration plant. Economic and environmental criteria are simultaneously optimized over a superstructure capturing a number of possible combinations of technologies and types of equipment. Aggregated models are considered, including a detailed methanol synthesis step with chemical kinetics and phase equilibrium considerations. The resulting model is formulated as a non‐convex mixed‐integer nonlinear programming problem. Global optimization and parallel computation techniques are employed to generate an optimal Pareto frontier. © 2009 American Institute of Chemical Engineers AIChE J, 2010

Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge

Aqueous dihydrogen (H 2,aq) is produced in copious amounts when seawater interacts with peridotite and H 2O oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly understood in this process is the partitioning of iron and its oxidation state in serpentine, although both impose an important control on dihydrogen production. We present results of detailed petrographic, mineral chemical, magnetic and Mößbauer analyses of partially to fully serpentinized peridotites from the Ocean Drilling Program (ODP) Leg 209, Mid-Atlantic Ridge (MAR) 15°N area. These results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite–seawater reaction path models. In samples from Hole 1274A, mesh-rims reveal a distinct in-to-out zoning from brucite at the interface with primary olivine, followed by a zone of serpentine + brucite ± magnetite and finally serpentine + magnetite in the outermost mesh-rim. The compositions of coexisting serpentine (Mg# 95) and brucite (Mg# 80) vary little throughout the core. About 30–50% of the iron in serpentine/brucite mesh-rims is trivalent, irrespective of subbasement depth and protolith (harzburgite versus dunite). Model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water-to-rock ratio during serpentinization. At temperatures above 330 °C the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen depends on the availability of an external silica source. At these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen, hence conditions are not reducing enough to form awaruite. At T < 330 °C, hydrogen generation is facilitated by the formation of brucite, as dissolution of olivine to form serpentine, magnetite and brucite requires no addition of silica. The model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from <150 to 250 °C and bulk water-to-rock ratios between 0.1 and 5. These conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization. The development of the common brucite rims around olivine is either due to an arrested reaction olivine → brucite → serpentine + brucite, or reflects metastable olivine–brucite equilibria developing in the strong gradient in silica activity between orthopyroxene (talc–serpentine) and olivine (serpentine–brucite).

Influence of SiC Particles on the Grain Refinement of an Mg-Al Alloy

SiC particles are effective grain refiners in Mg-Al alloys. Several investigations, from different researchers, into their effect on a range of alloys with different Al contents has been undertaken and it has been found that the greatest reduction in grain size occurs in alloys having low Al contents. Performing grain refinement studies on a range of alloy contents also allows for further investigation into the mechanisms of grain refinement. It was found that the size of the SiC particles is also important in magnesium grain refinement. However, the presence of Mg2Si in the microstructure and the consideration of phase equilibria suggest that SiC can transform into other binary or ternary carbides. If such carbides are formed, they may also act as an effective grain refiner for Mg-Al alloys. In this study, the possibility of formation of new carbides (Al4C3, Al2MgC2, Mn7C3, Mg2C, Mg2C3, Al2CO etc.) and their ability to be good grain refiners for Mg-Al alloys is discussed.

Modeling the Effect of Shear Stress on the Composition and Growth of the Deposit Layer from “Waxy” Mixtures under Laminar Flow in a Pipeline

A mathematical model based on the moving boundary problem formulation for solids deposition was modified to account for the effect of shear stress in laminar flow of paraffinic mixtures. The effect of shear stress on the composition and growth of the deposit layer was incorporated via a recently proposed approach involving one-dimensional deformation of a cubical cage, leading to the release of a fraction of the liquid from the deposit. Numerical solutions were obtained for the growth of the deposit layer with time, both radially and axially. Predictions were obtained for the effect of different cubical-cage deformation angles on the composition and solid/liquid phase ratio in the deposit layer. Whereas an increase in the deformation angle was predicted to cause wax enrichment in the deposit, the deposit-layer thickness was primarily dependent upon the heat-transfer and phase equilibrium considerations. The predictions also indicated that an increase in the deformation angle delayed the deposition process, because of a corresponding increase in the average solid-phase fraction within the deposit, and it caused the deposit to become enriched in heavier n-alkanes and depleted in lighter n-alkanes.

Petrology, fluid inclusions and metamorphic history of Bhopalpatnam granulites, central India

The Bhopalpatnam granulite belt (BGB) flanks the Pranhita–Godavari (PG) rift basin with ages ranging from 1.6 to 1.9 Ga, representing a major Mesoproterozoic thermal regime in central India. We present here new results on the metamorphic P–T conditions from garnet-bearing enderbitic charnockite and pelitic gneisses, and document the fluid regimes from detailed fluid inclusions studies in corundum and quartz in corundum-bearing gneiss and garnet–sillimanite–rutile gneiss, respectively, from this granulite belt. A peak pressure–temperature window of 8–9.5 kbar and 720–800 °C is defined by phase equilibrium considerations on garnet–clinopyroxene–plagioclase–quartz assemblages in garnet–bearing charnockite. The garnet–plagioclase–sillimanite–quartz assemblages in pelitic gneiss yield slightly lower P–T estimates of 660–720 °C at 4.7–6.5 kbar. Primary fluid inclusions in corundum comprise monophase carbonic inclusions with melting temperatures varying from −56.6 to −57.4 °C, indicating that the dominant part of the trapped fluid is pure CO 2. The inclusions homogenize into the liquid phase at temperatures in the range of −18.5 to −5.9 °C (1.024–0.961 g/cm 3). Fluid inclusions in quartz from garnet–sillimanite–rutile gneiss correspond to two categories; Group I (primary) and Group II (pseudosecondary). Both the groups show melting temperatures in the range of −55.9 to −62.9 °C, suggesting the presence of traces of other volatiles (CH 4/N 2) in addition to CO 2. The homogenization temperature of Group I inclusions ranges from −46.3 to −0.1 °C, the extreme low homogenizations indicating very high-density (up to 1.140 g/cm 3) for the carbonic fluid. Group II inclusions show higher homogenization temperatures of up to 16.2 °C. The estimated CO 2 isochore for primary inclusions in corundum and quartz intersects the peak P–T condition of the BGB derived from mineral phase equilibria. We therefore, infer that CO 2 was the dominant fluid species that was trapped at or near the peak metamorphic conditions in BGB. The metamorphic and fluid regimes, as well as the exhumation history of this granulite belt are thus constrained from a combination of petrologic and fluid inclusion studies.

Grain refinement of Mg–Al(–Mn) alloys by SiC additions

The addition of SiC particles effectively grain refined a range of Mg-Al alloys. The greatest reductions in grain size were found for the alloys with lower Al contents. The presence of Mg2Si in the microstructure after that SiC addition, and consideration of phase equilibria suggested that the SiC transforms to Al4C3, and this is the actual nucleant. The addition of Mn poisoned the grain refining effect of the SiC addition, probably due to the formation of less potent Al-Mn-carbides. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Petrogenesis of Al-rich chondrules: Evidence from bulk compositions and phase equilibria

We measured major, minor, and trace-element compositions for eleven Al-rich chondrules from unequilibrated ordinary chondrites to investigate the relationships between Al-rich chondrules, ferromagnesian chondrules, Ca-, Al-rich inclusions (CAIs), and amoeboid olivine aggregates (AOAs). Phase equilibrium considerations show that, for the most part, mineral assemblages in Al-rich chondrules are those expected from melts of the observed compositions. The diversity of mineral assemblages and Al-rich chondrule types arises mainly from the fact that the array of compositions spans both the spinel-saturated anorthite-forsterite reaction curve and a thermal divide defined by where the anorthite-forsterite join crosses the reaction curve. The reaction curve accounts for the two principal varieties of Al-rich chondrule, plagioclase-phyric and olivine-phyric, with or without aluminous spinel. The thermal divide influences the subsequent evolution of each variety. A third variety of Al-rich chondrule contains abundant sodium-rich glass; trace-element fractionation patterns suggest that these glassy Al-rich chondrules could have been derived from the other two by extensive alteration of plagioclase to nepheline followed by remelting. The bulk compositions of Al-rich chondrules (except sodium-rich ones) are intermediate in a volatility sense between ferromagnesian chondrules and type C CAIs. The combined trend of bulk compositions for CAIs, Al-rich chondrules, and ferromagnesian chondrules mirrors, but does not exactly match, the trend predicted from equilibrium condensation at P T ∼ 10 -3 atm; the observed trend does not match the trend found for evaporation from a liquid of chondritic composition. We thus infer that the bulk compositions of the precursors to CAIs, Al-rich chondrules, were ferromagnesian chondrules were controlled primarily by vapor-solid reactions (condensation or sublimation) in the solar nebula. Some Al-rich chondrules are consistent with an origin by melting of a compound CAI-ferromagnesian chondrule hybrid; others cannot be so explained. Any hybrid model is restricted by the constraint that the CAI precursor consisted dominantly of pyroxene + plagioclase + spinel; melilite cannot have been a significant component. Amoeboid olivine aggregates also have the inferred mineralogical characteristics of Al-rich chondrule precursors—they are mixtures of olivine with plagioclase-spinel-pyroxene-rich CAIs—but the few measured bulk compositions are more olivine-rich than those of Al-rich chondrules.

Aluminothermic reduction of zirconia

Results are presented for the reaction of aluminium metal with zirconia ceramics under (reducing) sintering conditions. During hot-pressing the principal phase formed is zirconium monoxide which is shown to have a rock-salt cubic structure and a unit-cell dimension of 0.46258 nm. When pressureless sintered or hot-pressed using a yttria partially-stabilised zirconia, composite structures of ZrO, alumina and cubic zirconia are produced which are the result of reduction reactions which proceed through the liquid metal phase. The results are interpreted through X-ray diffraction, microstructural and phase equilibria considerations which show that the sintering gas also plays a role in reaction through the oxygen partial pressure. The results allow calculation of an approximate value of the free energy of formation of zirconium monoxide (−294,500 J mol −1) which is consistent with the reported values for the monatomic carbide and nitride of zirconium and with other similar transition metal carbide, nitride and monoxide series.

Phase transition temperature ranges and storage density of paraffin wax phase change materials

Paraffin waxes have been used in many latent thermal energy storage applications because of their advantageous thermal performances. In this paper, the liquid–solid phase diagram of the binary system of tetradecane and hexadecane has been used to obtain information of the phase transition processes for cool storage applications. The analysis of the phase diagram indicates that, except for the minimum-melting point mixture, all mixtures melt and freeze in a temperature range and not at a constant temperature. The latent heat of fusion evolves throughout this temperature range. Differential scanning calorimetry (DSC) was used to determine the thermophysical properties of the binary system. Depending on the DSC settings throughout the measurements, varying results were obtained. For example, when the DSC runs at a high heating/cooling rate, it will lead to erroneous information. Also, the correct phase transition temperature range cannot be obtained simply from DSC measurement. By combining phase equilibrium considerations with DSC measurements, a reliable design method to incorporate both the heat of phase change and the temperature range is presented.

Control of homogeneously catalyzed reactions by phase equilibriaThis work was presented at the Green Solvents for Catalysis Meeting held in Bruchsal, Germany, 13–16th October 2000.

Carbon dioxide was used not only as a reaction medium but also as a renewable C1 feedstock for the palladium catalyzed coupling reaction of butadiene and carbon dioxide. By utilization of nitrile modified phosphine ligands, in the liquid phase complete butadiene conversion and selectivity for the formation of (3E)-3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (1) up to 47% were achieved. Phase equilibrium considerations were used for process development in order to realize single-phase conditions for the reaction. In contrast to a CO2 enriched phase composition, variation of the density only enables moderate conversion rates also with single-phase conditions and without further catalyst modification.

A rare composite xenolith from Salt Lake Crater, Oahu: high-pressure fractionation and implications for kimberlitic melts in the Hawaiian mantle

A composite xenolith of olivine-bearing garnet clinopyroxenite wall rock intruded by two spinels + garnet veins is described. Vein minerals exhibit textural evidence of a reaction relationship with the mineral phases in the wall rock. Wall rock clinopyroxene contains exsolved blebby garnet and very fine lamellar exsolution of orthopyroxene, indicating that this xenolith had undergone considerable subsolidus cooling. Garnet–clinopyroxene thermometry suggests that the xenolith last equilibrated in the mantle at a temperature of about 1,060 (±25 °C). The spinels in the veins are of two kinds: pleonaste (that occurs with vein garnet) and a high-Mg, high-Al titanomagnetite (MAT spinels). Intriguingly, the MAT spinels are chemically very similar to the spinels found as groundmass in kimberlites, are moderately subhedral to euhedral, have a weakly developed cumulate texture, and, at places, show a reaction relation with the pleonaste + garnet (cumulate?) assemblage in the vein. Based on petrographic, chemical, and phase equilibrium considerations, we propose the following evolutionary history of this composite xenolith. (1) In the first stage the olivine-bearing garnet clinopyroxenite formed as crystal extracts (cumulates) as a result of high pressure fractionation of an alkaline melt in the deepest levels of Hawaiian lithosphere/uppermost asthenosphere (100–110 km). (2) In the second stage, igneous veining (the melt composition of this vein is not precisely known but could be kimberlitic) occurs in the already existing wall rock resulting in the precipitation of pleonaste + garnet. A reaction relation between the igneous veins and the wall rock also characterizes this stage. (3) The last igneous episode in this xenolith is recorded by MAT spinels in the wall rock and their precipitation close to the previous pleonaste + garnet veins. The last igneous stage could well be due again to high pressure fractionation of a kimberlitic melt (the residual melt after precipitation of pleonaste + garnet). The time relationship between exsolution and the later igneous veining stages is not known. The MAT spinels are not a result of sub-solidus solvus processes as partial reaction (melt present) between the pleonaste + garnet (from the second igneous stage) and MAT spinel exists, pointing to the igneous nature of the MAT spinel. Based on striking similarity between the MAT spinels in our xenolith and those found as groundmass in kimberlites, we propose that the veining stages could well have been kimberlitic. Thus, even though kimberlitic melts are not seen on the Koolau shield, this particular xenolith clearly shows the existence of such melts at great depths beneath Hawaii. We also propose that the initial wall rock, which represents crystal extracts (even though it does not exhibit definitive cumulate texture) as a result of high-pressure fractionation of an alkaline melt and subsequent veining episodes, are of pre-Koolau age. This implies that the Koolau shield volcano may have had a pre-shield alkalic stage.