Lower Solidus Temperature Research Articles

Microstructures and Thermal Properties of Sn–Bi–Zn–Ga Alloys as Heat Transfer and Heat Storage Materials

Low melting point alloy is a potential high-temperature heat transfer medium because of the high thermal conductivity, low solidus temperature and wide range of use temperature. Consequently, we investigated the possibility of using Sn–Bi–Zn–Ga alloys as heat storage and heat transfer material. Moreover, we investigated the microstructure and phase compositions by electron probe micro-analysis (EPMA) and X-ray diffusion (XRD). Results show that the new structures and phases are formed in the alloy matrix with Ga additions, which lead to the improvement of the thermal properties. An extensive thermophysical characterization of the Sn–Bi–Zn–Ga alloys has been performed by differential scanning calorimeter (DSC) analysis. The addition of Ga lowers the peak temperature and increases the heat capacity of the alloys. The thermal expansion of the test alloys increases with increasing temperature and the densities decreases with Ga additions. As the density, specific heat capacity and thermal diffusivity change with temperature and physical state, the thermal conductivity of the alloys first decreases and then increases. These results demonstrate the feasibility of using Sn–Bi–Zn–Ga alloys as the high-temperature heat transfer fluid.

Microstructures and thermal properties of Sn–Bi–Pb–Zn alloys as heat storage and transfer materials

Low-melting-point alloy has the characteristics of high thermal conductivity, low solidus temperature and the wide range of use temperature, which is a potential heat transfer medium. The microstructure and thermal properties of Sn–Bi–Pb–Zn alloys as heat transfer and storage material were investigated in this paper. The phase compositions, microstructure and thermal properties were investigated by X-ray diffusion (XRD), electron probe microanalysis (EPMA) and differential scanning calorimeter (DSC) analysis, respectively. The results show that the phases of Sn–Bi–Zn and Sn–Pb–Zn alloys are mainly eutectic formed by solid solution, while the formation of Pb7Bi3 intermetallic compounds decreases the melting point of Sn–Bi–Pb and Bi–Pb–Zn. The thermal properties of the zinc-containing alloys are better than that of Sn–Bi–Pb, but the weight of the zinc-containing alloys significantly reduces above 900 °C. As the density, specific heat capacity and thermal diffusivity change with temperature and physical state, the thermal conductivity of the alloys first decreases and then increases. These results demonstrate the feasibility of using low-melting alloys as the heat transfer and storage material.

Microstructure evolution and mechanical properties of 6N01S-T5 aluminum alloy joints with semi-solid solders by ultrasonic soldering

The article investigated the properties of 6N01 aluminum alloy joints with Sn-80Zn semi-solid solders by ultrasonic soldering. It was found that semi-solid solders consisted of Zn-rich phase with short rod-like dentrite, which had the lower solidus temperatures, while Zn-rich phase with long dentritic grains used the conventional solders. The joints soldered with Sn-80Zn conventional filler metal reached maximum shear strength of 73 MPa at the soldering temperature of 390[Formula: see text]C. The joint using Sn-80Zn semi-solid solder obtained higher shear strength of 90 MPa at the lower temperature of 370[Formula: see text]C. The effects of the ultrasonic time and the soldering temperature on the mechanical properties of the joints were also investigated with the Sn-80Zn semi-solid solders.

Episodic bimodal magmatism at an active continental margin due to Paleo-Pacific Plate subduction: A case study from the southern segment of the Tan-Lu Fault Zone, eastern China

Volcanic rocks in the southern part of the Tan-Lu Fault Zone (TLFZ), eastern China, provide key evidence for the connection between spatially and temporally related pulses of Cretaceous magmatism, activity on the TLFZ, and the dynamics of the Paleo-Pacific Plate. We identify two stages of bimodal volcanism in the Chaohu–Lujiang segment, southern TLFZ, based on lithological assemblages and precise isotopic ages. The Early Cretaceous (125–121 Ma) volcanic rocks are fractionated aluminous A-type rhyolites and basalts, whereas the Late Cretaceous volcanic rocks (97–83 Ma) range in composition from porphyritic andesites and trachydacites to rhyolites. The Early Cretaceous rhyolites display strongly negative Nd-Hf isotopic compositions (εNd(t) = −16.3 to −16.1, εHf(t) = −24.1 to −17.9). The most likely source rocks of the fractionated aluminous A-type rhyolites are metaigneous rocks of the Feidong Complex. The Early Cretaceous potassic basalts (K2O/Na2O = 1.5–3.4, εNd(t) = −9.0) exhibit more enriched isotopic compositions than the Late Cretaceous sodic porphyritic andesites (K2O/Na2O = 0.6–0.8, εNd(t) = −5.6, εHf(t) = −15.1 to −6.8), indicating derivation from heterogeneously enriched lithospheric mantle. This heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures. Modeling of selected elements reveals that the Late Cretaceous trachydacites (εNd(t) = −12.2 to −8.2, εHf(t) = −16.5 to −6.4) originated by mixing between coeval mafic and felsic magmas. The most negative (εNd(t) = −18.8 to −17.1, εHf(t) = −20.5 to −4.7) isotopic compositions, along with Neoproterozoic inherited zircons within the Late Cretaceous rhyolites, indicate derivation from lower crust of the South China Block. We suggest that the Early Cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting Izanagi slab. However, the TLFZ became a focus of Late Cretaceous bimodal volcanism due to weak far-field extension caused by the slow northward motion of the Paleo-Pacific Plate. Moreover, the high velocity and low angle of subduction of the Izanagi slab were the main driving mechanisms behind an Early Cretaceous transpressional event that separated the two stages of bimodal volcanism.

Properties of composites with La(Fe, Si)13 hydrides and sodium silicate

Abstract We adopted sodium silicate to prepare composites of La(Fe, Si)13 hydrides in this work. A homogenous and continuous matrix of sodium silicate was formed at a relatively low solidification temperature. No counter diffusion between the matrix and the grains of La(Fe, Si)13 hydrides was observed. The magnetocaloric effect of pure La(Fe, Si)13 hydrides is largely preserved in the composites, showing a high tolerance to sodium silicate. These composites possess high thermal conductivity and decent compressive strength compared with the epoxy resin bonded samples. These results suggest that sodium silicate is a promising binder material for producing bulk La(Fe, Si)13 hydrides for their application in magnetic refrigeration.

Three-Layer Clad Strip Casting Using a Vertical Type Tandem Twin Roll Caster

This paper shows improvements made to a vertical type tandem twin roll caster and the appropriate casting conditions necessary to cast three-layer clad strips, the base strip of which has a lower solidification temperature than the overlay strip. In experiments, 4045 aluminum alloy was used for the base strip and 3003 aluminum alloy was used for the overlay strips. The roll speed was 30 m/min. By connecting the overlay strips to the base strip one at a time and cooling the base strip to between 450 and 530°C after applying the first overlay strip, a sound three-layer clad strip – defined as one in which the interfaces between strips are clear and do not separate during bending-to-failure tests – could be cast. The tensile shear testing between the base and second overlay strip was improved as the base-strip temperature was increased to 450-530°C range.

Trace element systematics and Nd, Sr and Pb isotopes of Pliocene flood basalt magmas (Ethiopian rift): A case for Afar plume-lithosphere interaction

We present high-precision trace element and Sr-Nd-Pb isotope data for Pliocene flood basalts located at Sire (eastern rift wall) and Chefe Donsa (western rift wall) sections from the main Ethiopian rift (MER). These basalts are tholeiitic to mildly alkaline and were derived by moderate degrees of melting (~10–20%) of a spinel lherzolite containing minor amounts of amphibole within the lithospheric mantle. Strontium, Nd and Pb isotopic compositions (87Sr/86Sr: 0.7040–0.7057, 143Nd/144Nd: 0.52164–0.51287, 206Pb/204Pb: 17.75–19.20, 207Pb/204Pb: 15.53–15.64 and 208Pb/204Pb: 38.12–38.86) are between the inferred composition of the Afar mantle plume and an unspecified enriched lithospheric component, suggesting interaction of melts from asthenospheric and lithospheric mantle domains rather than melting of homogeneous mixed sources or magma mixing. It is proposed that in the Oligocene the impinging Afar plume creates zones of metasomatically enriched mantle. Later, lithospheric thinning as a result of incipient rifting leads to decompression and melting of this metasomatically enriched mantle due to its lower solidus temperature. The chemistry of the Pliocene lavas therefore reflect two associated scenarios namely (i) upwelling plume material from the advancing Afar plume may have induced metasomatism in the lithospheric mantle (ii) mobilization and melting of this enriched lithospheric mantle material resulted in melts characterized by variable mixtures of the two mantle reservoirs.

Deep and persistent melt layer in the Archaean mantle

The transition from the Archaean to the Proterozoic eon ended a period of great instability at the Earth’s surface. The origin of this transition could be a change in the dynamic regime of the Earth’s interior. Here we use laboratory experiments to investigate the solidus of samples representative of the Archaean upper mantle. Our two complementary in situ measurements of the melting curve reveal a solidus that is 200–250 K lower than previously reported at depths higher than about 100 km. Such a lower solidus temperature makes partial melting today easier than previously thought, particularly in the presence of volatiles (H2O and CO2). A lower solidus could also account for the early high production of melts such as komatiites. For an Archaean mantle that was 200–300 K hotter than today, significant melting is expected at depths from 100–150 km to more than 400 km. Thus, a persistent layer of melt may have existed in the Archaean upper mantle. This shell of molten material may have progressively disappeared because of secular cooling of the mantle. Crystallization would have increased the upper mantle viscosity and could have enhanced mechanical coupling between the lithosphere and the asthenosphere. Such a change might explain the transition from surface dynamics dominated by a stagnant lid on the early Earth to modern-like plate tectonics with deep slab subduction. A persistent melt layer may have existed in the Archaean upper mantle, according to experimental analyses. The melt layer could have decoupled the mantle from the overlying lithosphere, hindering plate tectonics.

Effect of Processing Conditions and Interfacial Geometry on Reaction Metallurgical Joining of Copper

Reaction metallurgical joining (RMJ) was recently developed as a new method for copper-to-copper joining. In this study, a cold metal transfer (CMT) process was investigated to pre-place a low-melting-temperature reaction material between the copper parts to be joined prior to the RMJ process. The joint area is heated via an applied electrical current so that the reaction material dissolves the base metal surface to form a reaction product with a lower solidus temperature than copper. The applied load causes this reaction product, a liquid and/or liquid-solid mixture, to be displaced from the joint, during which time it carries away surface oxides and contaminants. The metallurgically clean opposing surfaces are brought into contact to form a continuous metallurgical bond. The combination of CMT and RMJ methods will potentially provide a robust copper joining process.

Microstructure and mechanical properties of friction stir welds on unmodified and P-modified Al-Mg2Si-Si alloys

Welded joints formed by friction stir welding (FSW) consist of three distinct zones: a base material zone (BMZ), a thermo-mechanically affected zone (TMAZ), and a weld nugget (WN). Primary Mg2Si phases are identified as equiaxed crystals and polygonal particles in unmodified and P-modified Al-Mg2Si-Si alloys in the BMZ, respectively. In the WN, the equiaxed primary Mg2Si crystals in the unmodified alloys are transformed to significantly smaller polygonal/irregular particles; the corners of the polygonal primary Mg2Si particles in the modified alloys become smoother and smaller. The segregation of the primary Mg2Si phase is reduced for both the unmodified and modified alloys in the WN. Both unmodified and modified alloys have a lower solidus temperature in the WN. The ultimate tensile strengths (UTSs) of the welded joints are enhanced by 5% and 8% for the unmodified and modified alloys, respectively, in comparison with the parent material. The UTS of the welded joints in the modified alloy is ∼20% higher than in the unmodified alloy.

Effect of rare earth addition on solidification characteristics and microstructure of ZRE1 magnesium cast alloy

The thermal parameters of ZRE1 cast alloy with 1.0 wt.% of praseodymium (Pr) and 1.0 wt.% of erbium (Er) were evaluated by the computer-aided cooling curve thermal analysis (CA-CCTA), whereas the microstructure analysis was investigated by the optical microscope and field emission scanning electron microscopy. The results of the cooling curve and microstructure analyses showed that Pr and Er reduced the grain size. The thermal analysis result recorded a faster freezing time and lower solidification temperature with Pr and Er additions and 15.5 and 11.2% decrease in the grain size, respectively. Scanning electron microscopy results showed that Er formed a granular particles of Er-containing phases existed in the α-Mg matrix, while Pr formed Mg–Zn–Pr–Ce phase with secondary phase which distributed along the grain boundaries.

Preparation, Microstructure and Properties of Environmentally Friendly Sn-58Bi Eutectic Solder Ribbon for Electronic Package

Sn-58Bi eutectic solder ribbon was successfully prepared using an improved single-roller rapid solidification device with the optimum process parameters. The microstructure, melting behavior and wettability of Sn-58Bi eutectic solder ribbon prepared by rapid solidification technology (RST) and conventional casting-rolling process (CRP) respectively were investigated by optical microscopy (OM), differential scanning calorimetry (DSC) and wettability tests. The results show that the forming effect of solder ribbons can be obviously improved by mainly optimizing roller line speed, nozzle-to-roller spacing and heating temperature. Sn-58Bi eutectic solder ribbon prepared by RST has finer microstructure, lower solidus temperature and more excellent wettability than that of the ribbon prepared by CRP. Therefore, the study of Sn-58Bi eutectic solder ribbon is conducive to cut cost, meet growing performance requirements, and reduce lead contamination for electronic package industry.

Reduction in secondary dendrite arm spacing in cast eutectic Al–Si piston alloys by cerium addition

The effects of Ce on the secondary dendrite arm spacing (SDAS) and mechanical behavior of Al–Si–Cu–Mg alloys were investigated. The reduction of SDAS at different Ce concentrations was evaluated in a directional solidification experiment via computer-aided cooling curve thermal analysis (CA‒CCTA). The results showed that 0.1wt%–1.0wt% Ce addition resulted in a rapid solidification time, Δts, and low solidification temperature, ΔTS, whereas 0.1wt% Ce resulted in a fast solidification time, Δta–Al, of the α-Al phase. Furthermore, Ce addition refined the SDAS, which was reduced to approximately 36%. The mechanical properties of the alloys with and without Ce were investigated using tensile and hardness tests. The quality index (Q) and ultimate tensile strength of (UTS) Al–Si–Cu–Mg alloys significantly improved with the addition of 0.1wt% Ce. Moreover, the base alloy hardness was improved with increasing Ce concentration.

Mantle melting versus mantle metasomatism – “The chicken or the egg” dilemma

Most eclogitic mantle xenoliths brought to the surface exhibit a certain degree of enrichment with incompatible elements, usually attributed to the effect of mantle metasomatism by a putative metasomatic fluid. The metasomatic overprint is represented mainly by enrichments in Na, K, Ba, Ti and LREE and the original source of this fluid remains unknown. In this paper, we present a detailed petrological study of a typical eclogitic mantle xenolith from the Roberts Victor kimberlite mine in South Africa. We find that its textural and mineralogical features present strong evidence for incipient melting. The melting assemblage we observe did not necessarily require introduction of additional components, that is: in-situ melting alone could produce highly incompatible element enriched melt without involvement of a hypothetical and speculative “metasomatic event”. Due to the higher abundance in incompatible elements and lower solidus temperature than peridotites, mantle eclogites, some of which represent previously subducted oceanic crust, are much more plausible sources of mantle metasomatism, but on the other hand, they can be considered as highly metasomatised themselves. This brings us to the “chicken or egg” dilemma – was the secondary mineral assemblage in mantle lithologies a result or a source of mantle metasomatism?

Oxygen isotope and trace element geochemistry of zircons from porphyry copper system: Implications for Late Triassic metallogenesis within the Yidun Terrane, southeastern Tibetan Plateau

It is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore-bearing ones from barren ones, especially by examining the oxygen isotopic and trace element compositions of zircon. The oxygen isotopic composition and trace element abundances of zircons from the mineralized Pulang porphyries and the coeval Disuga and Lannitang andesites of the Yidun Terrane, eastern Tibetan Plateau, have been determined by secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) in this study, respectively. Analyzed zircons have relatively high but invariant δ18O values (~5.8‰–6.9‰) that are indicative of derivation from homogeneous mantle or mantle-derived magmas together with contribution of mature crustal materials. We propose a model for the Late Triassic magmatism in the Yidun Terrane involving magmas derived from partial melting of subduction-metasomatized mantle peridotites that subsequently experienced melting–assimilation–storage–homogenization (MASH) processes. The low calculated Ti-in-zircon temperatures (644°C–863°C, generally between 640°C and 690°C) for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas, and partly controlled by the crystallization of amphibole. Zircon grains from porphyry copper system display small negative europium anomalies (mostly Eu/Eu*>0.5), contrasting with pronounced negative Eu anomalies in zircons (mostly Eu/Eu*<0.5) associated with magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W deposits. Calculated REE compositions of the melt that was in equilibrium with zircons from porphyry copper system display depletion in the middle rare earth elements (MREE) without negative Eu anomaly, while that of zircons from magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W system show depletion in the MREE with negative Eu anomaly. It is speculated that the parental melts of zircon have decreasing magmatic oxidation state in the following sequence: porphyry copper system>magmato-hydrothermal quartz-vein Cu-Mo-W deposit>magmato-hydrothermal quartz-vein Mo-W deposit. This study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits.

Effect of kinetics on P removal by Al-Si solvent refining at low solidification temperature

To determine the effect of kinetics on P removal in Al-Si-P system, three sets of experiments with different solidification temperature ranges have been carried out. High P removal rates can be confirmed. An apparent segregation coefficient is introduced to characterize the P removal in this Al-Si-P system, which are determined to be 0.0207, 0.00822 and 0.00679, when the cooling rate is 0.556 mK·s−1 and the Si contents in the melt are 39.1, 29.3, 19.4 at.%, respectively. Theoretical P contents in the primary Si phase controlled by thermodynamic factor (X¯PinprimarySiT) and theoretical P contents in the primary Si phase controlled by kinetic factor (X¯PinprimarySiK) are calculated. The results reveal that the kinetic factors have critical influence on P removal at low solidification temperature.

AlSi12 in-situ alloy formation and residual stress reduction using anchorless selective laser melting

Rapid melt pool formation and solidification during the metal powder bed process Selective Laser Melting (SLM) generates large thermal gradients that can in turn lead to increased residual stress formation within a component. Metal anchors or supports are required to be built in-situ and forcibly hold SLM structures in place and minimise geometric distortion/warpage as a result of this thermal residual stress. Anchors are often costly, difficult and time consuming to remove and limit the geometric freedom of this Additive Manufacturing (AM) process. A novel method known as Anchorless Selective Laser Melting (ASLM) maintains processed material within a stress relieved state throughout the duration of a build. As a result metal components formed using ASLM do not require support structures or anchors. ASLM locally melts two or more powdered materials that alloy under the action of the laser and can form into various combinations of eutectic/hypo/hyper eutectic alloys with a new lower solidification temperature. This new alloy is maintained in a semi-solid or stress reduced state throughout the build with the assistance of elevated powder bed pre-heating. In this paper the ASLM methodology is detailed and investigations into processing of a low temperature eutectic Al-Si binary casting alloy is explored. Two types of Al powders were compared; pre-alloyed AlSi12 and elemental mix Al+12wt% Si. The study established an understanding of the laser in-situ alloying process and confirmed successful alloy formation within the process. Differential thermal analysis, microscopy and X-Ray diffraction were used to further understand the nature of alloying within the process. Residual stress reduction was observed within ASLM processed elemental Al+Si12 and geometries produced without the requirement for anchors.

Mantle refertilization and magmatism in old orogenic regions: The role of late-orogenic pyroxenites

Pyroxenites and garnet pyroxenites are mantle heterogeneities characterized by a lower solidus temperature than the enclosing peridotites; it follows that they are preferentially involved during magma genesis. Constraining their origin, composition, and the interactions they underwent during their subsequent evolution is therefore essential to discuss the sources of magmatism in a given area. Pyroxenites could represent either recycling of crustal rocks in mantle domains or mantle originated rocks (formed either by olivine consuming melt-rock reactions or by crystal fractionation). Petrological and geochemical (major and trace elements, Sr–Nd and O isotopes) features of xenoliths from various occurrences (French Massif-Central, Jordan, Morocco and Cameroon) show that these samples represent cumulates crystallized during melt percolation at mantle conditions. They formed in mantle domains at pressures of 1–2GPa during post-collisional magmatism (possibly Hercynian for the French Massif-Central, and Panafrican for Morocco, Jordan and Cameroon). The thermal re-equilibration of lithospheric domains, typical of the late orogenic exhumation stages, is also recorded by the samples. Most of the samples display a metasomatic overprint that may be either inherited or likely linked to the recent volcanic activity that occurred in the investigated regions.The crystallization of pyroxenites during late orogenic events has implications for the subsequent evolution of the mantle domains. The presence of large amounts of mantle pyroxenites in old orogenic regions indeed imparts peculiar physical and chemical characteristics to these domains. Among others, the global solidus temperature of the whole lithospheric domain will be lowered; in turn, this implies that old orogenic regions are refertilized zones where magmatic activity would be enhanced.

The Dual Fluid Reactor – A novel concept for a fast nuclear reactor of high efficiency

The Dual Fluid Reactor, DFR, is a novel concept of a fast heterogeneous nuclear reactor. Its key feature is the employment of two separate liquid cycles, one for fuel and one for the coolant. As opposed to other liquid-fuel concepts like the Molten-Salt Fast Reactor (MSFR), both cycles in the DFR can be separately optimized for their respective purpose, leading to advantageous consequences: A very high power density resulting in remarkable cost savings, and a highly negative temperature feedback coefficient, enabling a self-regulation without any control rods or mechanical parts in the core.In the current reference design the fuel liquid is an undiluted actinide trichloride based on isotope-purified Cl-37, circulating at an operating temperature of 1000°C. It can be processed on-line in a small internal processing unit utilizing fractional distillation or electro refining. Medical radioisotopes like Mo-99/Tc-99m are by-products and can be provided right away. In a more advanced design, an actinide metal alloy melt with an appropriately low solidus temperature is also possible which enables a reduction of the core size and allows a further increase in the operating temperature due to its high heat conductivity.For the reference design, pure Lead as coolant is the best choice. It yields a very hard neutron spectrum, fostering a very good neutron economy and therefore making the DFR a preferred thorium breeder but also a very effective waste incinerator and transmuter. With its high coolant temperature the DFR achieves the same ambitions as the Generation IV concept of the very high temperature reactor (VHTR), with all its advantages like electricity production with high efficiency and the synthesis of carbon-free fuels, but with overall production costs competitive with today’s refined oil.The specific combination of the liquids in the very high temperature regime requires structural materials withstanding corrosive attacks. Because of the small size of the reactor core the utilization of these expensive materials would have no significant impact on the overall energy (and also economic) efficiency, measured by the EROI (Energy Return on Investment), which is more than 20 times higher than for a light-water reactor (LWR).The DFR inherits the positive properties of the lead-cooled reactor (LFR) and of the MSFR, especially its outstanding passive safety features.

Constraints on the mantle mineralogy of an ultra-slow ridge: Hafnium isotopes in abyssal peridotites and basalts from the 9–25°E Southwest Indian Ridge

We report on the Hf isotopic compositions of clinopyroxene mineral separates from eleven abyssal peridotites and Nd and Hf-isotopic compositions of twenty-seven co-located basalts from 9–25°E South West Indian Ridge (SWIR). In Nd–Hf isotope space the SWIR peridotites plot within the global MORB field (εNd=4.5–12.5, εHf=9.6–18.7), with the 15.23°E peridotites being the most radiogenic. The lack of correlation between Hf isotopes and trace or major element systematics including Lu/Hf ratios suggests that the 15.23°E peridotites were recently processed beneath the ridge and therefore participated in the production of the SWIR lavas. The Hf isotopic compositions of 15.23°E peridotites are more radiogenic than all basalts from the 9–25°E ridge, whereas the 9.98°E and 16.64°E peridotites partially overlap with the Hf isotope ratios of the spatially co-located basalts. This indicates the upwelling mantle beneath the SWIR contains material with enriched isotope signatures in addition to an isotopically depleted peridotitic mantle, which is consistent with the SWIR peridotites and basalt Nd isotope systematics from previously published studies. As the enriched isotope signatures are not observed in the peridotites we assume that they are sourced from material with lower solidus temperature than a typical peridotite. This enriched material was consumed during melting, and therefore may be mineralogically distinct (e.g. pyroxenite). Moreover, the variable spatial distribution of the enriched isotope signatures requires preferential sampling of the enriched component at distinct along-axis locations. The Hf–Nd isotope variability of the 9–25°E basalts can be entirely explained by mixing between a depleted peridotitic mantle end-member with the isotope composition of the 15.23°E peridotites and an enriched end-member with the isotope composition of the Narrowgate Segment lavas at 14.6°E. We estimate a maximum of 5% modal abundance of the enriched material in a peridotitic SWIR mantle. Our calculations further suggest that a larger portion (∼40%) of the Hf and Nd budget in the eruptive basalts is dominated by the enriched component. This implies that estimates of the MORB – source composition from the erupted MORB will be biased towards the fertile components and will underestimate the abundance of depleted peridotite in their source. The emerging picture is that of a convective upper mantle that is far more heterogeneous than MORB suggest, and where isotopically depleted peridotitic material may be more abundant than lava compositions suggest. Additional data from abyssal peridotites and other oceanic mantle xenoliths are needed in order to generate more realistic estimates of the upper mantle compositional variability and the processes that produced this picture. In addition, we demonstrate that the contrast in isotopic compositions between the basalts from the magmatic segments and peridotites from the adjacent amagmatic segments is consistent with melt focusing as a response to the variation in lithospheric crustal thickness, where deep low degree melts with larger proportion of enriched component focus in the areas of thin lithosphere.