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A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to − 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

A suite of 12 peridotite xenoliths from the Wesselton kimberlite was studied and found to sample the subcratonic lithospheric mantle over a pressure range from 3.6 to 4.7 GPa and a temperature range of 880 to 1120 °C. Major, minor and trace element compositions indicate that both metasomatised and un-metasomatised samples are present over this pressure range. Fe3 +/∑ Fe in garnet from four xenoliths was determined using Fe K-edge XANES spectroscopy, enabling the redox state of the sampled subcratonic mantle to be determined for three garnet bearing samples. ΔlogfO2[FMQ] varied from 0 to − 3.3 over the sampled pressure interval, with the un-metasomatised samples falling within the global trend of decreasing ΔlogfO2[FMQ] with increasing depth. Superimposed on this was an oxidation trend, at higher pressures (≥ 4.5 GPa), with ΔlogfO2 increasing by 1.5 to 2 units in the metasomatically enriched samples, indicating a clear link between metasomatism and oxidation. One potential source of this oxidation is a carbonated silicate melt, which will increase in carbonate content as ΔlogfO2 increases. Mantle minerals affected by such a melt have the potential to shift from the field of diamond stability into that of carbonate, threatening the stability of diamond.

Kimberlite is a rare volcanic rock renowned as the major host of diamonds and originated at the base of the subcontinental lithospheric mantle. Although kimberlite magmas are dense in crystals and deeply-derived rock fragments, they ascend to the surface extremely rapidly, enabling diamonds to survive. The unique physical properties of kimberlite magmas depend on the specific compositions of their parental melts that, in absence of historical eruptions and due to pervasive alteration of kimberlite rocks, remain highly debatable. We explain exceptionally rapid ascent of kimberlite magma from mantle depths by combining empirical data on the essentially carbonatite composition of the kimberlite primary melts and experimental evidence on interaction of the carbonate liquids with mantle minerals. Our experimental study shows that orthopyroxene is completely dissolved in a Na2CO3 melt at 2.0–5.0 GPa and 1000–1200 °C. The dissolution of orthopyroxene results in homogeneous silicate–carbonate melt at 5.0 GPa and 1200 °C, and is followed by unmixing of carbonate and carbonated silicate melts and formation of stable magmatic emulsion at lower pressures and temperatures. The dispersed silicate melt has a significant capacity for storing a carbonate component in the deep mantle (13 wt% CO2 at 2.0 GPa). We envisage that this component reaches saturation and is gradually released as CO2 bubbles, as the silicate melt globules are transported upwards through the lithosphere by the carbonatite magma. The globules of unmixed, CO2-rich silicate melt are continuously produced upon further reaction between the natrocarbonatite melt and mantle peridotite. On decompression the dispersed silicate melt phase ensures a continuous supply of CO2 bubbles that decrease density and increase buoyancy and promote rapid ascent of the magmatic emulsion.

Kimberlite is a rare volcanic rock renowned as the major host of diamonds and originated at the base of the subcontinental lithospheric mantle. Although kimberlite magmas are dense in crystals and deeply-derived rock fragments, they ascend to the surface extremely rapidly, enabling diamonds to survive. The unique physical properties of kimberlite magmas depend on the specific compositions of their parental melts that, in absence of historical eruptions and due to pervasive alteration of kimberlite rocks, remain highly debatable. We explain exceptionally rapid ascent of kimberlite magma from mantle depths by combining empirical data on the essentially carbonatite composition of the kimberlite primary melts and experimental evidence on interaction of the carbonate liquids with mantle minerals. Our experimental study shows that orthopyroxene is completely dissolved in a Na2CO3 melt at 2.0–5.0 GPa and 1000–1200 °C. The dissolution of orthopyroxene results in homogeneous silicate–carbonate melt at 5.0 GPa and 1200 °C, and is followed by unmixing of carbonate and carbonated silicate melts and formation of stable magmatic emulsion at lower pressures and temperatures. The dispersed silicate melt has a significant capacity for storing a carbonate component in the deep mantle (13 wt% CO2 at 2.0 GPa). We envisage that this component reaches saturation and is gradually released as CO2 bubbles, as the silicate melt globules are transported upwards through the lithosphere by the carbonatite magma. The globules of unmixed, CO2-rich silicate melt are continuously produced upon further reaction between the natrocarbonatite melt and mantle peridotite. On decompression the dispersed silicate melt phase ensures a continuous supply of CO2 bubbles that decrease density and increase buoyancy and promote rapid ascent of the magmatic emulsion.

Plate tectonic processes introduce basaltic crust ( as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts ( 10 to 30%), ocean island and continental basalts ( many > 60%), and komatiites ( 20 to 30%). These results imply involvement of 2 to 20% ( up to 28%) of recycled crust in mantle melting.

Kimberlite is a rare volcanic rock renowned as the major host of diamonds and originated at the base of the subcontinental lithospheric mantle. Although kimberlite magmas are dense in crystals and deeply-derived rock fragments, they ascend to the surface extremely rapidly, enabling diamonds to survive. The unique physical properties of kimberlite magmas depend on the specific compositions of their parental melts that, in absence of historical eruptions and due to pervasive alteration of kimberlite rocks, remain highly debatable. We explain exceptionally rapid ascent of kimberlite magma from mantle depths by combining empirical data on the essentially carbonatite composition of the kimberlite primary melts and experimental evidence on interaction of the carbonate liquids with mantle minerals. Our experimental study shows that orthopyroxene is completely dissolved in a Na2CO3 melt at 2.0–5.0 GPa and 1000–1200 °C. The dissolution of orthopyroxene results in homogeneous silicate–carbonate melt at 5.0 GPa and 1200 °C, and is followed by unmixing of carbonate and carbonated silicate melts and formation of stable magmatic emulsion at lower pressures and temperatures. The dispersed silicate melt has a significant capacity for storing a carbonate component in the deep mantle (13 wt% CO2 at 2.0 GPa). We envisage that this component reaches saturation and is gradually released as CO2 bubbles, as the silicate melt globules are transported upwards through the lithosphere by the carbonatite magma. The globules of unmixed, CO2-rich silicate melt are continuously produced upon further reaction between the natrocarbonatite melt and mantle peridotite. On decompression the dispersed silicate melt phase ensures a continuous supply of CO2 bubbles that decrease density and increase buoyancy and promote rapid ascent of the magmatic emulsion.

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, exotic metasomatic melts influxing into the lithosphere, or breakdown of amphibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyroxene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, Iherzolite and harzburgite xenoliths from southeastern Australia. The majority of glass compositions are best explained by melting of amphibole (usually complete, as amphibole is now absent from most samples) with varying but significant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete modal melting of phlogopite, or apatite. The data confirm our earlier model, that the glass present in patches in these samples derives from high-temperature, transport-related breakdown of a metasomatic phase assemblage (amphibole + clinopyroxene + phlogopite + apatite) present in the xenoliths prior to their entrainment in the host magmas.

Mantle-derived xenoliths from western Victoria, Australia, contain glass in patches and veinlets, and as secondary inclusions within xenolith minerals. Glass patches are commonly associated with primary clinopyroxene, spinel and in some cases relict pargasitic amphibole, phlogopite, apatite or calcitic carbonate. The patches and veins are filled with secondary microphenocrysts of olivine, clinopyroxene and spinel. The siliceous, aluminous, alkali-rich glasses display ranges in major element compositions which are similar to those of glasses in mantle xenoliths worldwide. Textures and compositions of glass and associated phases in the Victorian xenolith suite argue strongly against origins involving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts.Metasomatism of the southeastern Australian lithosphere is expressed by formation of amphibole, phlogopite, and apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in Iherzolite or harzburgite. Partial or complete melting of this metasomatic assemblage occurred immediately prior to, or during, entrainment of the xenolith in the host magmas, and locally produced disequilibrium liquids, now preserved as glass. Melting in the xenoliths was caused by the thermal and decompressional effects of transport in the host magma, or by thermal and metasomatic effects in the lithosphere associated with adjacent intrusion of magmas. Large inter-xenolithic variation in glass compositions was caused by variations in the nature and proportions of the precursor metasomatic assemblage, by reaction of the melts with primary orthopyroxene in the Iherzolites and harzburgites, and by rapid, disequilibrium crystallization from the melts of an assemblage of olivine, clinopyroxene and spinel in ail samples.

The composition of olivine phenocrysts in Hawaiian tholeiitic picrites and in Mid-Ocean Ridge picrites vary up to Mg#91.3 and Mg#92.1 respectively. The compositions and liquidus temperatures of the magmas crystallizing the most magnesian phenocrysts can be estimated and we find that anhydrous liquidus temperatures (at 1 bar pressure) of Hawaiian tholeiitic picrites average 1365°C, for E-MOR picrites average 1355°C, and for N-MOR picrites average 1335°C. Water contents of the magmas decrease in the order Hawaiian picrites, E-MOR picrites to N-MOR picrites, and consideration of liquidus depression by these water contents leads to the conclusion that magma temperatures for all types were approximately 1325°C at ~ 1 bar. The data from parental or primary magmas suggests that the temperature contrast between 'Hot-Spot' and MOR magmas is 20°C. Application of information from partial melting studies of lherzolites and liquidus studies of the Hot-Spot and MOR picrites leads to the conclusion that both 'Hot-Spot' and MOR primary basalts are derived from mantle with potential temperature Tp ~ 1430°C. Insofar as primitive magmas may be used to infer the potential temperature of their sources, there is no evidence for a temperature contrast of Tp = 100-250°C between 'Hot-Spot' or 'Deep Mantle Plume' sources and ambient (MOR source) asthenospheric mantle. Although magma temperatures are similar, the residual mantle compositions for Hawaiian picrites are refractory harzburgites, more refractory (including Cr/Cr+Al ratio) than the lherzolite to harzburgite residue from MOR picrite extraction. It is argued that the buoyancy plume and geophysically anomalous mantle beneath the Hawaiian Arch is due to compositional and not temperature contrasts in the upper mantle. The four-component mixing identified in the Hawaiian source is attributed to interaction between old subducted lithospheric slabs, buoyant or suspended in the upper mantle, and surrounding ambient mantle at Tp = 1430°C.

Plate tectonic processes introduce basaltic crust ( as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts ( 10 to 30%), ocean island and continental basalts ( many > 60%), and komatiites ( 20 to 30%). These results imply involvement of 2 to 20% ( up to 28%) of recycled crust in mantle melting.

Plate tectonic processes introduce basaltic crust ( as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts ( 10 to 30%), ocean island and continental basalts ( many > 60%), and komatiites ( 20 to 30%). These results imply involvement of 2 to 20% ( up to 28%) of recycled crust in mantle melting.

The Baffin Island picrites are highly magnesian (less than or equal to22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modi. cation of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modi. cation, the inclusions are systematically slightly lower in Al2O3, and significantly higher in SiO2, K2O and P2O5 than the lavas' fractionation trends. CaO, Na2O and TiO2 contents lie within the lavas' fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to approximate to 15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (< 1%), suggesting that the inclusions' compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions' compositions are frequently more heterogeneous than those of the lavas that host them.

Kimberlite is a rare volcanic rock renowned as the major host of diamonds and originated at the base of the subcontinental lithospheric mantle. Although kimberlite magmas are dense in crystals and deeply-derived rock fragments, they ascend to the surface extremely rapidly, enabling diamonds to survive. The unique physical properties of kimberlite magmas depend on the specific compositions of their parental melts that, in absence of historical eruptions and due to pervasive alteration of kimberlite rocks, remain highly debatable. We explain exceptionally rapid ascent of kimberlite magma from mantle depths by combining empirical data on the essentially carbonatite composition of the kimberlite primary melts and experimental evidence on interaction of the carbonate liquids with mantle minerals. Our experimental study shows that orthopyroxene is completely dissolved in a Na2CO3 melt at 2.0–5.0 GPa and 1000–1200 °C. The dissolution of orthopyroxene results in homogeneous silicate–carbonate melt at 5.0 GPa and 1200 °C, and is followed by unmixing of carbonate and carbonated silicate melts and formation of stable magmatic emulsion at lower pressures and temperatures. The dispersed silicate melt has a significant capacity for storing a carbonate component in the deep mantle (13 wt% CO2 at 2.0 GPa). We envisage that this component reaches saturation and is gradually released as CO2 bubbles, as the silicate melt globules are transported upwards through the lithosphere by the carbonatite magma. The globules of unmixed, CO2-rich silicate melt are continuously produced upon further reaction between the natrocarbonatite melt and mantle peridotite. On decompression the dispersed silicate melt phase ensures a continuous supply of CO2 bubbles that decrease density and increase buoyancy and promote rapid ascent of the magmatic emulsion.

The Baffin Island picrites are highly magnesian (less than or equal to22 wt% MgO) olivine tholeiites, erupted through felsic continental crust. Plots of most major and minor element oxides against MgO for the lavas define very tight trends consistent with modi. cation of melts parental to the erupted suite by olivine fractionation or accumulation. However, melt inclusions trapped in primitive olivine phenocrysts in these lavas have much more diverse compositions. After correction for post-entrapment modi. cation, the inclusions are systematically slightly lower in Al2O3, and significantly higher in SiO2, K2O and P2O5 than the lavas' fractionation trends. CaO, Na2O and TiO2 contents lie within the lavas' fractionation trends. Similarly, most inclusions are higher in Sr/Nd, K/Nb, Rb/Ba, Rb/Sr, U/Nb and Ba/Th than the lavas. These characteristics resulted from up to approximate to 15% contamination of evolving picritic-basaltic liquids by locally-derived, broadly granitic partial melts of the quartz + feldspar-rich crust through which the picrites erupted. Contamination was minor in the bulk lavas (< 1%), suggesting that the inclusions' compositions partly reflect a link between wall rock reaction and precipitation of liquidus olivine. Rapid crystallisation of liquidus olivine from the picrites, along with melting of felsic crustal wall rocks of magma chambers or conduits, were likely during emplacement of hot picritic magmas into cooler felsic crust. Inclusion compositions may thus reflect mixing trends or may be constrained to phase boundaries between olivine and a phase being resorbed, for example, an olivine-plagioclase cotectic. The extent of contamination was probably a complex function of diffusion rates of components in the magmas, and phenocryst growth rates and proximity to wall rock. These results bear on the common observation that melt inclusions' compositions are frequently more heterogeneous than those of the lavas that host them.

Mantle-derived xenoliths from western Victoria, Australia, contain glass in patches and veinlets, and as secondary inclusions within xenolith minerals. Glass patches are commonly associated with primary clinopyroxene, spinel and in some cases relict pargasitic amphibole, phlogopite, apatite or calcitic carbonate. The patches and veins are filled with secondary microphenocrysts of olivine, clinopyroxene and spinel. The siliceous, aluminous, alkali-rich glasses display ranges in major element compositions which are similar to those of glasses in mantle xenoliths worldwide. Textures and compositions of glass and associated phases in the Victorian xenolith suite argue strongly against origins involving equilibrium partial melting of spinel peridotite, or migration of exotic metasomatic melts.Metasomatism of the southeastern Australian lithosphere is expressed by formation of amphibole, phlogopite, and apatite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in Iherzolite or harzburgite. Partial or complete melting of this metasomatic assemblage occurred immediately prior to, or during, entrainment of the xenolith in the host magmas, and locally produced disequilibrium liquids, now preserved as glass. Melting in the xenoliths was caused by the thermal and decompressional effects of transport in the host magma, or by thermal and metasomatic effects in the lithosphere associated with adjacent intrusion of magmas. Large inter-xenolithic variation in glass compositions was caused by variations in the nature and proportions of the precursor metasomatic assemblage, by reaction of the melts with primary orthopyroxene in the Iherzolites and harzburgites, and by rapid, disequilibrium crystallization from the melts of an assemblage of olivine, clinopyroxene and spinel in ail samples.

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, exotic metasomatic melts influxing into the lithosphere, or breakdown of amphibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyroxene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, Iherzolite and harzburgite xenoliths from southeastern Australia. The majority of glass compositions are best explained by melting of amphibole (usually complete, as amphibole is now absent from most samples) with varying but significant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete modal melting of phlogopite, or apatite. The data confirm our earlier model, that the glass present in patches in these samples derives from high-temperature, transport-related breakdown of a metasomatic phase assemblage (amphibole + clinopyroxene + phlogopite + apatite) present in the xenoliths prior to their entrainment in the host magmas.

The composition of olivine phenocrysts in Hawaiian tholeiitic picrites and in Mid-Ocean Ridge picrites vary up to Mg#91.3 and Mg#92.1 respectively. The compositions and liquidus temperatures of the magmas crystallizing the most magnesian phenocrysts can be estimated and we find that anhydrous liquidus temperatures (at 1 bar pressure) of Hawaiian tholeiitic picrites average 1365°C, for E-MOR picrites average 1355°C, and for N-MOR picrites average 1335°C. Water contents of the magmas decrease in the order Hawaiian picrites, E-MOR picrites to N-MOR picrites, and consideration of liquidus depression by these water contents leads to the conclusion that magma temperatures for all types were approximately 1325°C at ~ 1 bar. The data from parental or primary magmas suggests that the temperature contrast between 'Hot-Spot' and MOR magmas is 20°C. Application of information from partial melting studies of lherzolites and liquidus studies of the Hot-Spot and MOR picrites leads to the conclusion that both 'Hot-Spot' and MOR primary basalts are derived from mantle with potential temperature Tp ~ 1430°C. Insofar as primitive magmas may be used to infer the potential temperature of their sources, there is no evidence for a temperature contrast of Tp = 100-250°C between 'Hot-Spot' or 'Deep Mantle Plume' sources and ambient (MOR source) asthenospheric mantle. Although magma temperatures are similar, the residual mantle compositions for Hawaiian picrites are refractory harzburgites, more refractory (including Cr/Cr+Al ratio) than the lherzolite to harzburgite residue from MOR picrite extraction. It is argued that the buoyancy plume and geophysically anomalous mantle beneath the Hawaiian Arch is due to compositional and not temperature contrasts in the upper mantle. The four-component mixing identified in the Hawaiian source is attributed to interaction between old subducted lithospheric slabs, buoyant or suspended in the upper mantle, and surrounding ambient mantle at Tp = 1430°C.

Plate tectonic processes introduce basaltic crust ( as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts ( 10 to 30%), ocean island and continental basalts ( many > 60%), and komatiites ( 20 to 30%). These results imply involvement of 2 to 20% ( up to 28%) of recycled crust in mantle melting.