Incipient Melting Research Articles

Contact Metamorphism of Pelitic Rocks Constrains the Depth of Emplacement of the Re di Castello Intrusion (Adamello Batholith, Italy)

ABSTRACTRe di Castello is the southernmost unit of the composite Adamello batholith (central Alps, Italy). It was emplaced between 43 and 40 Ma, yet its depth of emplacement is not well constrained. We have studied the contact metamorphism of pelitic country rocks in two localities: the upper Caffaro Valley and the Borzago Valley in the southern and the northern sub‐units of the Re di Castello, respectively. In the Caffaro Valley, the country rocks comprise the Triassic sedimentary Lozio Shale, a carbon‐rich slate–siltstone, which was intruded by quartz‐dioritic magma. Near the contact, the hornfelses display the mineralogical assemblage Kfs‐Crd‐Bt‐Ms‐Pl‐Qz‐Gr, with rare fibrolitic sillimanite in a single sample. Andalusite was never observed throughout the aureole at this locality. In the Borzago Valley, contact metamorphism developed on schists of the pre‐Permian basement with a Variscan, regional metamorphic overprint. The sequence of contact metamorphic mineral assemblages progresses from And‐Bt‐bearing parageneses to the Sil‐Crd‐Bt‐Kfs‐Ms‐Pl‐Qz that characterizes peak conditions, at which incipient melting is also observed. K‐feldspar forms slightly upgrade the first appearance of sillimanite. For samples at both localities, thermodynamic modelling in the NCKFMASHT system failed at predicting a stability field for the sequences of mineral assemblages developed during contact metamorphism. Also the Ti‐in biotite thermometer did not constrain temperatures adequately. The P–T conditions at the thermal peak were thus evaluated by an alternative bathograd‐like approach, considering phase relationships in the simplified NKASH‐C system. To form sillimanite only as product of the incomplete Ms‐Qz breakdown—divariant for the presence of Na in Ms and Kfs and shifted to lower temperature due to the presence of graphite (in the Lozio Shale)—an isobaric path typical of contact metamophism must have crossed above the Msss‐Qz‐Kfsss‐Sil‐And‐fluid invariant point. This constrains an emplacement pressure >3.3 kbar in the Caffaro Valley, and >3.2 kbar in the Borzago Valley. Concerning temperature, the same univariant point also constrains the minimum temperature in the Caffaro Valley as >615°C–620°C, consistent with results of RSCM thermometry on graphite from the Lozio Shale. The evidence of incipient melting in some samples from the Borzago Valley indicates higher temperatures, probably approaching 670°C, near the contact to the intrusion. Assuming an average crust density of 2.7 g cm−3, the estimated pressures correspond to a minimum paleo‐depth of emplacement of about 12 km. These depths are somewhat greater than normally considered and should be regarded as revised constraints on models of the emplacement dynamics of the Adamello batholith and on paleogeographic reconstructions of this part of the Southalpine domain.

Evaluation of critical temperatures of as-cast Co-based superalloys as a function of alloying elements and their effect on microstructure

Co-based superalloys are of great interest in the aeronautical and aerospace industry due to their superior thermal performance compared to Ni-based superalloys. This work explores the influence of elemental additions (Ni, Al, Cr, Si, and Ti) on the critical temperatures (liquidus, solidus, solvus, and eutectic) and microstructural features of several Co systems fabricated by vacuum induction melting until the final Co-30Ni-10Al-4Cr-2Si-2Ti (wt%) composition was obtained. Differential Scanning Calorimetry results indicated high thermal stability concerning the γ' phase formation. Si and Ti additions suppressed the eutectic presence in the proposed superalloy, which is desirable to avoid the incipient melting of components. Instead, the present work exhibits a low-density Co-based superalloy conformed of a continuous γ/β microstructure with γ' precipitating uniformly. These characteristics make the Co-30Ni-10Al-4Cr-2Si and Co-30Ni-10Al-4Cr-2Si-2Ti superalloys promising candidates to be subjected to high temperatures in comparison with current Co-based superalloys.

Migration and accumulation of hydrous mantle incipient melt in the Earth's asthenosphere: Constraints from in-situ falling sphere viscometry measurements

The Earth's oceanic lithosphere-asthenosphere boundary (LAB) is marked by a notable decrease in seismic wave velocities and an increase in attenuation. This phenomenon is likely attributed to the accumulation of partial melt at the top of the asthenosphere. Nevertheless, the process involving the upward migration and aggregation of low-degree partial melts, highly likely to be mantle incipient melts in the asthenosphere, remains underexplored. Viscosity is a key factor controlling the flow of melts, thus in this study, we used In-situ X-ray falling sphere viscometry experiments to determine the viscosity of the mantle incipient melt containing 1.8–4.0 wt.% water at 1.5–6 GPa and 2100 K. We found that water and pressure can effectively decrease the viscosity of the mantle incipient melt. Therefore, the mantle incipient melts are highly mobile with a high segregation velocity in the deeper part of the asthenosphere. However, during ascent, their mobility diminishes due to an increase in viscosity and a decrease in water content. These mechanisms potentially contribute to the accumulation of partial melt at the top of the asthenosphere.

Phase-field approach assisted solution heat treatment efficient design for novel multicomponent Co-based superalloys

Fast homogenization without incipient melting is the key to designing solution heat treatment (SHT) process for single crystal superalloys. In this paper, a multicomponent phase-field model coupled with solidus temperature is developed for dynamically tracking incipient melting temperature, TIM, of alloy during SHT. The simulation process runs completely automatically only by inputting the composition information of as-cast specimen. Based on the obtained TIM-time profile, the holding temperature and the time of SHT are determined. Applying this approach to Co–30Ni–11Al–4W–1Ta–4Ti–5Cr single crystal specimen, a gradual increased TIM from 1247 °C to 1293 °C within 72,000 s is obtained, and thus a stepwise SHT process with a short time of less than 20 h is designed. The experimental verification shows that after SHT, the segregation coefficient of the slowest diffusing element W decreased from more than 2 to 1.065, the content of harmful secondary phase has decreased to below 0.1%, and no incipient melting occurs.

Incipient carbonate melting drives metal and sulfur mobilization in the mantle.

We present results from high-pressure, high-temperature experiments that generate incipient carbonate melts at mantle conditions (~90 kilometers depth and temperatures between 750° and 1050°C). We show that these primitive carbonate melts can sequester sulfur in its oxidized form of sulfate, as well as base and precious metals from mantle lithologies of peridotite and pyroxenite. It is proposed that these carbonate sulfur-rich melts may be more widespread than previously thought and that they may play a first-order role in the metallogenic enhancement of localized lithospheric domains. They act as effective agents to dissolve, redistribute, and concentrate metals within discrete domains of the mantle and into shallower regions within Earth, where dynamic physicochemical processes can lead to ore genesis at various crustal depths.

Silicon isotopes in an Archaean migmatite confirm seawater silicification of TTG sources

Unraveling ancient melting processes is key to understanding how the earliest, tonalite-trondhjemite-granodiorite (TTG)-dominated continental crust formed from partial melting of amphibolite. Application of silicon isotope analysis to ancient crust reveals that Archaean TTGs exhibit consistently high Si isotope values (δ30Si) compared to modern granitoids, attributed to seawater-derived silica introduced by either (a) partial melting of variably silicified basalts or (b) assimilation of authigenic silica-rich marine lithologies in the melt source. However, both mechanisms can introduce highly variable δ30Si, conflicting with the strikingly consistent δ30Si compositions of Archaean TTGs. This study investigates an alternative model, whereby the distinct mineralogy and chemistry of TTG melt sources impart a distinct silicon isotope composition to the melt, compared with “modern” granitoids. We measured δ30Si in component parts (melanosome and leucosome) of an Archaean (2.7 Ga) mafic migmatite and coeval amphibolites and mafic granulites from the Kapuskasing uplift, Canada, to explore how Si isotopes fractionate during incipient TTG melt formation. Our data reveal leucosome (i.e., melt) exhibits consistently high δ30Si values compared to a relatively isotopically lighter melanosome (i.e., residuum). We also derive inter-mineral silicon isotope fractionation factors for mineral separates that agree well with those of ab initio estimates for the same minerals and show that the magnitude of equilibrium fractionation between TTG source rock and melt replicates that in Phanerozoic granitoids. We conclude the effects of magmatic differentiation on δ30Si have remained consistent throughout Earth history, meaning that Archaean TTGs must require a source isotopically heavier than unaltered basalt, as reflected by our amphibolites and mafic migmatite components. The consistently heavy δ30Si of seawater through Earth history, and the high SiO2 content of amphibolites relative to coeval leucosome-free granulites in our study area, imply seawater silicification is the source of the observed high δ30Si. Thus, the consistently heavy Si isotope compositions measured in Archaean melt products define a unique aspect of ancient crust formation: that of the silicification of TTG source rock, implying the intrinsic involvement of a primeval hydrosphere.

Ba-Mg isotopic evidence from an OIB-type diabase for a big mantle wedge beneath East Asia in the Early Cretaceous

Beneath Eastern Asia, the world's spatially most extensive, and possibly extremely long-lived big mantle wedge (BMW) occurs, formed by the Izanagi-Pacific slab residing in the mantle transition zone (MTZ). It provides a fascinating model accounting for the intraplate magmatism and earthquake >1800 km distal from the trench which are incompletely considered by classic plate tectonics. Since when the BMW exists is crucial for global and regional geodynamic reconstructions and modeling of material transfer. However, temporal constraints from the geophysical, geodynamic model, tectonic, and magmatic studies yield conflicting results. To contribute to this endeavor, we have characterized BaMg isotopic features of the ∼123 Ma-old, OIB-type Daixi diabase in Eastern China, which is located ∼300 km west of the inferred Izanagi subduction zone. Results show that the diabase has δ138/134Ba (−0.08‰ to 0.04‰) slightly lower than the terrestrial mantle (∼0.03 to 0.05‰). The δ138/134Ba values are negatively correlated with (87Sr/86Sr)i, K/Ti, K/La, and K/Na of the diabase, pointing to the incorporation of subducted sediments into the mantle source. In addition, the diabase also has lower δ26Mg (−0.67‰ to −0.35‰) than the terrestrial mantle (−0.25 ± 0.04‰) and low (87Sr/86Sr)i (0.704254–0.706300), reflecting source contributions of marine Mg‑carbonates. Thus, BaMg isotopes record the source contribution of carbonate-bearing sediments which must have melted in the MTZ (20–22 GPa) according to experimental investigations. High K/U (∼30,500) and Hf/Hf* (∼1.68) ratios of the melts derived from the enriched source moreover require partial melting in the presence of peritectic liebermannite and majorite. These two peritectic phases are formed in the MTZ (410–660 km) after early incipient melting of carbonate-bearing sediments during which the loss of Na-rich, Ca‑carbonate melts has resulted in the removal of Th, U, and some LREE and retention of K, Ba, Zr, and Hf. These findings provide robust evidence that the Daixi diabase originated from an enriched mantle source metasomatized by melts/fluids derived from the stagnant slab at the MTZ. They also place important constraints on the existence of the Izanagi BMW beneath eastern Asia in the Early Cretaceous (∼123 Ma). It reconciles magmatic with previous tectonic and metallogenetic models that showed lithospheric thinning (peak at ∼125 Ma) and gold mineralization flare-up (peak at ∼120 Ma) at the Eastern China continental margin. Thus, our study is important to understand not only the materials cycling and geodynamic processes within a BMW system but also the tectonic-magmatic effects and interactions between the subducted slab and the overlying continental plate.

Study on Dissolution of Ba-Containing Inoculant in Ductile Cast Iron Melt and Nucleation of Graphite

The production of high quality ductile cast iron requires different and complex inoculants. Besides iron and silicon, they also contain alloying elements such as zirconium, strontium, barium, calcium and rare earth metals. The addition of these elements reduces carbide solidification and increases the number of eutectic cells, which improves the quality of the cast iron produced. This study investigates the process of incipient melting of a complex barium inoculant and its effects on graphite nucleation. In the study, the sample was prepared by introducing the inoculant grain into the melt of the ductile iron. The region between the inoculant grain and the less inoculated matrix was examined metallographically using light and scanning electron microscopy. We used energy dispersion spectroscopy to determine the phases in the microstructure formed. It was found that graphite particles can already nucleate and grow in solid particles when the inoculant is still dissolving and also from the melt where the Ba and Ca concentrations are high and form BaO·CaO phases that serve as nuclei for graphite growth.

Microstructural Evolution and Mechanical Properties of a Ni-Based Alloy with High Boron Content for the Pre-Sintered Preform (PSP) Application

The pre-sintered preform (PSP) is an advanced technology for repairing the Ni-based superalloy blade in a turbine. In general, boron is added to the Ni-based superalloys in small quantities (<0.1 wt.%) to increase boundary strength and cohesivity. Despite this, the effect of high B content (>1.0 wt.%) on the microstructure evolution and mechanical properties in Ni-based superalloys for the PSP application is rarely studied. The variety, composition and evolution of the precipitates during solution heat treatment in the alloy with high B content were determined by EBSD, EPMA and SEM. The results indicate that Cr, W and Mo-rich M5B3 type borides precipitate from the matrix and its area fraction reaches up to about 8%. The area fraction of boride decreases with the prolonging of solution time and the increase of temperature higher than 1120 °C. The borides nearly disappear after solution treatment at 1160 °C for 2 h. The redissolution of boride and eutectic results in the formation of B-rich area with low incipient melting (about 1189 °C). It can bond metallurgically with the blade under the melting point of the blade, which decreases the precipitation of harmful phases of the blade after PSP repairing. The microhardness within the grain in the PSP work-blank first decreases (lower than 1160 °C) and then increases (higher than 1185 °C) with the increase of solution heat treatment temperature due to the dissolving and precipitation of borides. The tensile strength of the combination of PSP work-blank and Mar-M247 matrix at room temperature after solution treatment is related to the area fraction of boride, incipient melting and the cohesion between PSP work-blank and Mar-M247 matrix.

Model for the Pore Formation During Incipient Melting of Single-Crystal Nickel-Based Superalloys

Model for the Pore Formation During Incipient Melting of Single-Crystal Nickel-Based Superalloys

New insights into the optimisation of the solution heat treatment process and properties of CMSX-4 superalloys

For nickel-based single-crystal superalloys, simultaneously reducing the segregation and optimising the microstructure is a primary challenge in the field of heat treatment. To further improve rupture life, the element partition, microstructure evolution, and creep behaviour of the second-generation nickel-based single-crystal superalloy CMSX-4 under five different solution processes were studied using electron microprobe, scanning electron microscopy, and transmission electron microscopy. Under the same conditions, both continuous heating and remelting heat treatment exhibited higher elements homogenisation efficiency. Moreover, it was found that the improvement of element segregation can make γ′ phase smaller and more uniform. These heat treatments reduced the incipient melting tendency or eliminated the incipient melting eutectic, thereby increasing the peak heat treatment temperature of the CMSX-4 alloy. However, remelting heat treatment results in a significant increase in the porosity of the alloy, leading to a deterioration in performance. Finally, based on the above study, a novel process combining stepwise heating and continuous heating was proposed, which not only increased the peak solid-solution temperature but also prevented the incipient melting and optimized the microstructure. After using the novel process, the durability of the alloy increased by 48 % from 240 to 356 h under the test conditions of 980 °C/250 MPa. The higher proportion of the γ′ phase, narrower matrix channel, and denser dislocation network were the main reasons for the improvement in the rupture life.

Liquation cracking in laser powder bed fusion-fabricated Inconel718 of as-built, stress-relieved, and hot isostatic pressed conditions

A critical factor of the laser powder bed fusion (LPBF) additive manufacturing processed Inconel718 is its susceptibility to liquation cracking, primarily caused by the incipient melting of the grain boundary Laves phase or carbide (Nb-rich intermetallic phases). In this study, the liquation cracking behavior of the LPBF-processed Inconel718 was investigated using Gleeble® hot-ductility tests. In particular, the effect of commonly employed post-LPBF heat treatments, such as stress relieving, and hot isostatic pressing (HIP), on the liquation cracking resistance was examined. While the as-built Inconel718 exhibited good liquation cracking resistance, its liquation cracking resistance decreased after the stress relieving and HIP treatment. The variation in liquation cracking susceptibility was attributed to microstructural differences, including grain size, size and volume fraction of the low-melting carbides phase, and the segregation pattern of the solute element Nb. Good liquation cracking resistance of the as-built provides more flexibility in process design involving high temperatures. Meanwhile, HIP treatment can reduce internal defects but presents a significant disadvantage of increased liquation cracking susceptibility. These findings underscore the importance of carefully designing post-processing conditions for additively manufactured parts.

Effect of solidification direction on microstructure and mechanical property of single crystal superalloy CMSX-4

The microstructure and mechanical property of single crystal superalloy CMSX-4 fabricated by downward direction solidification (DWS) and upward direction solidification (UWS) were systematically investigated by optical microscope (OM), scanning electron microscope (SEM), electron probe microanalyzer (EPMA), nano-indentation instrument and finite element simulation method. Meanwhile, the effect of remelting heat treatment on microstructure evolution and mechanical property of the superalloy was also explored. Compared to the traditional UWS process, the DWS process was superior in inhibiting the formation of freckle defects and alleviating the segregation of difficult-diffusion elements. Moreover, the expected application of thin shell casting in the DWS process was also more beneficial to the control of freckles and the refinement of microstructure. The change of solidification direction would alter the solute distribution in the mushy zone. Different convection patterns were then formed under the action of gravity, thereby affecting the microstructure of the casting. In the following process of remelting heat treatment, the generation and elimination of incipient melting structures occurred in both UWS and DWS samples. However, more incipient melting structures in the DWS samples made longer elimination time required. After remelting heat treatment, the mechanical property of the DWS sample was comparable with that of the UWS one, since the element segregation in the final microstructure was reduced, and the size, morphology and uniformity of γ' phase were also adjusted.

Early Paleozoic crustal anatexis during Wuyi-Yunkai orogenesis: Insights from zircon of Fuhuling migmatites in the Yunkai region, South China

Abstract Crustal anatexis is an important process in the tectonic evolution of many orogenic systems. In the Wuyi-Yunkai orogen in the South China block, the duration of partial melting and its relationship with orogenesis are poorly constrained. In this study, we present a multifaceted approach to determine the timing of anatexis and unravel the petrogenesis of Fuhuling migmatites in the Yunkai region of the southwestern South China block. Geochemical analyses indicate that the migmatites have (meta-)sedimentary protoliths. The absence of anhydrous peritectic minerals but the presence of microstructural and outcrop-scale indicators of partial melting suggest that the Fuhuling migmatites experienced fluid-present melting. Complex zoning and variable trace element concentrations in newly formed zircons in migmatites reflect their evolutionary histories during partial melting. Anatectic melt was present at Fuhuling in the Yunkai region from ca. 449–427 Ma during early Paleozoic Wuyi-Yunkai orogenesis. The wide variety of morphologies observed in the Fuhuling migmatites implies that migmatites in the Yunkai region experienced incipient partial melting, melt segregation, and melt migration. Combining new and previous results, we argue that the Yunkai region experienced two stages of crustal anatexis during the early Paleozoic, which may have been triggered by crustal thickening followed by rapid exhumation and orogenic collapse during the intra-plate Wuyi-Yunkai orogeny in the South China block.

Influence of heat treatments on incipient melting structures of DD5 nickel-based single crystal superalloy

Influence of heat treatments on incipient melting structures of DD5 nickel-based single crystal superalloy

Dependence of impact regime boundaries on the initial temperatures of projectiles and targets

Towards higher impact velocities, ballistic events are increasingly determined by the material temperatures. Related effects might range from moderate thermal softening to full phase transition. In particular, it is of great interest to quantify the conditions for incipient or full melting of metals during impact interactions, which result in a transition from still strength-affected to hydrodynamic material behavior. In this work, we investigate to which extent the respective melting thresholds are also dependent on the initial, and generally elevated, temperatures of projectiles and targets before impact. This is studied through the application of a model developed recently by the authors to characterize the transition regime between high-velocity and hypervelocity impact, for which the melting thresholds of materials were used as the defining quantities. The obtained results are expected to be of general interest for ballistic application cases where projectiles or targets are preheated. Such conditions might result, for example, from aerodynamic forces acting onto a projectile during atmospheric flight, explosive shaped-charge-jet formation or armor exposure to environmental conditions. The performed analyses also broaden the scientific understanding of the relevance of temperature in penetration events, generally known since the 1960s, but often not considered thoroughly in impact studies.

Meso-Neoproterozoic basic-intermediate mafic granulites (metabasites) from the ∼1 Ga granulitic southwestern Oaxacan complex, Mexico, crustal evolution and phase equilibria modeling

The basement of eastern Mexico is made up of Grenvillian granulite-facies metamorphic rocks, which constitute an inferred microcontinent named Oaxaquia. Proto-Oaxaquia oceanic arc reconstructions are related to the transition from Columbia to Rodinia supercontinents (∼1.5 Ga). This study focuses on the petrography, geochemistry, and phase equilibria modeling of metabasites from the southwestern Oaxacan Complex, the largest Oaxaquia outcrop. Southwestern Oaxacan Complex protoliths mainly correspond to gabbro/basalt. They constitute an assemblage apart from the rest of the Oaxacan Complex metabasites due to their Ti enrichment and SiO2 depletion related to post-magmatic/metamorphic processes. Southern study site metabasites correspond to the primitive root of the arc, mostly related to a divergent geotectonic setting. The other metabasites are related to the magmatic back/forearc active margin between proto-Oaxaquia and Amazonia craton. Phase equilibria modeling point to Zapotecan (last Grenvillian cycle) metamorphic peak conditions of 850 ± 25 °C and 1.0 ± 0.15 GPa from one garnet-amphibole bearing metabasite, and a later hydration episode (post-Grenvillian) of 760 ± 10 °C and 0.8 ± 0.20 GPa derived from one amphibole bearing metabasite. The time of incipient partial melting of the southwestern Oaxacan Complex is a pre-Zapotecan event (∼1 Ga). Mineral textures do not display significant processes related to the post-Zapotecan metamorphic retrogression, and only a few whole-rock light rare element patterns display strong enrichments.

In-situ observation of the incipient melting of borides and its effect on the hot-workability of Ni-based superalloys

The excellent creep properties of advanced wrought Ni-based superalloys are achieved by adding grain boundary strengthening elements such as B and C. Their low solubility in the γ matrix may promote precipitation of micrometre-sized borides and carbides which potentially deteriorate hot-workability, e.g. through incipient melting. However, their incipient melting has not yet been directly linked to a decrease in hot-workability in Ni-based superalloys. Here, we study the susceptibility of two Ni-based superalloy variants with main differences in B and C contents (high B & C variant with>triple B and C contents) to incipient melting and its effect on hot-workability. We show that incipient melting causes severe intergranular cracking at low strains during compression at 1200 °C in the high B & C variant only. Compression to higher strains separates grain boundaries fully, partially pulverising the macroscopic specimen. Metallographic specimens already show incipient melting at heating rates they experience when put into a pre-heated furnace. During subsequent cooling, the melt re-solidifies and forms lamellar structures consisting of various phases rich in Mo-Cr-B, Zr, and Ti, the constituents of M3B2 and MC. In-situ observations using a customised high temperature confocal laser scanning microscope set-up unambiguously confirmed the origin of these lamellar structures. We demonstrate for the first time directly the constituent liquation of M3B2. This liquation is assumed to be aided by the partial dissolution of adjacent MC carbides into the melt. These results will help to improve thermo-mechanical simulations and microstructural control during industrial processing of advanced wrought superalloy parts.

Homogenization of Extrusion Billets of a Novel Al-Mg-Si-Cu Alloy with Increased Copper Content

Within the present work the homogenization of DC-cast (direct chill-cast) extrusion billets of Al-Mg-Si-Cu alloy was investigated. The alloy is characterized by higher Cu content than currently applied in 6xxx series. The aim of the work was analysis of billets homogenization conditions enabling maximum dissolution of soluble phases during heating and soaking as well as their re-precipitation during cooling in form of particles capable for rapid dissolution during subsequent processes. The material was subjected to laboratory homogenization and the microstructural effects were assessed on the basis of DSC (differential scanning calorimetry) tests, SEM/EDS (scanning electron microscopy/energy-dispersive spectroscopy) investigations and XRD (X-ray diffraction) analyses. The proposed homogenization scheme with three soaking stages enabled full dissolution of Q-Al5Cu2Mg8Si6 and θ-Al2Cu phases. The β-Mg2Si phase was not dissolved completely during soaking, but its amount was significantly reduced. Fast cooling from homogenization was needed to refine β-Mg2Si phase particles, but despite this in the microstructure coarse Q-Al5Cu2Mg8Si6 phase particles were found. Thus, rapid billets heating may lead to incipient melting at the temperature of about 545 °C and the careful selection of billets preheating and extrusion conditions was found necessary.

Tracking and dating incipient melting of a new grouplet of primitive achondrites

Primitive achondrites, representing ‘transitional’ samples between chondrites and differentiated achondrites, provide a great opportunity to decode the thermal histories of planetesimals in the early solar system. In this work, we report a comprehensive study of petrography, mineralogy, O-Cr isotopes and 53Mn-53Cr systematics of four ungrouped primitive achondrites, Northwest Africa (NWA) 12869, NWA 3250, NWA 11112, and Tafassasset. High resolution X-ray tomographic microscopy (HR-XTM) observations yield the 3D spatial distributions of metal, sulfide, and plagioclase, providing the direct petrologic evidence for incipient melting. In NWA 12869, NWA 3250, and NWA 11112, only FeNi-FeS eutectic melting occurs, with small fractions of metal and sulfide melt and amalgamate sporadically. As for Tafassasset, in addition to common metal and sulfide melt, plagioclase (±pyroxene) extensively melts and interconnects to form networks, with the generation of basaltic melts. The 53Mn-53Cr systematics in NWA 12869 [initial 53Mn/55Mn of (3.24 ± 0.32) × 10−6] and Tafassasset [initial 53Mn/55Mn of (3.48 ± 0.20) × 10−6] are determined with mineral separates and bulk samples. When anchored to the D’Orbigny angrite, NWA 12869 has a 53Mn-53Cr age of 4563.4 ± 0.6 Ma, while Tafassasset has an age of 4563.8 ± 0.4 Ma. The Mn-Cr ages of NWA 12869 and Tafassasset record the time of incipient melting within their parent bodies.Considering that NWA 12869, NWA 3250, and NWA 11112 exhibit the same olivine-dominated (47–69 vol%) mineral assemblages with recrystallized textures and a great similarity in mineral chemistry (olivine: Fa35–38; pyroxene: Fs27–30Wo3.0–3.5; plagioclase: An47–50; chromite: Cr#: 74–76; Mg#: 14.0–14.6), oxygen fugacity (IW-2 to IW-1), as well as 54Cr nucleosynthetic anomalies (ε54Cr: 1.01 ± 0.09 to 1.44 ± 0.18) and oxygen isotopes (Δ17O: −1.72 to −1.78 ± 0.05‰), we first classify them into a new grouplet of primitive achondrites, different from Tafassasset. This grouplet does not share a parent body with CR chondrites, but rather derives from incipient melting of a carbonaceous chondrite parent body which formed in a nebular reservoir physically close to that sampled by CR chondrites. More chondrite, differentiated achondrite, and/or iron samples associated with this new grouplet would provide further constraints on the rate and onset time of accretion, as well as the interior structure of the planetesimal in the outer solar system.