Metasomatized Mantle Xenoliths Research Articles

Mineralogical characterization of rejuvenated magmatism at Burton Guyot, Louisville Seamount trail

Volcaniclastic sequences drilled during IODP Expedition 330 on top of Burton Guyot preserve a unique record of rejuvenated magmatic activity along the Louisville Seamount trail. Geochemical analysis of clinopyroxenes in primary volcaniclastic deposits of this rejuvenated phase allows the reconstruction of magmatic evolution from the shield to post-erosional phases of a Louisville seamount, and to compare this evolution to that of Hawaiian volcanoes. Our results reveal the occurrence of three main types of clinopyroxenes in the rejuvenated volcaniclastic deposits at Burton Guyot, with a Na (and Al)-poor phenocrystic clinopyroxene and two types of Na-rich clinopyroxenes from disaggregated ultramafic xenoliths. The rejuvenated Na-poor phenocrysts have the same compositional range as clinopyroxenes associated with the shield stage of the volcano, indicating an overlap in shield and rejuvenated magma compositions. The dominant type of Na-rich clinopyroxene (Type 1) is very similar to clinopyroxenes in Hawaiian pyroxenitic xenoliths thought to represent high pressure cumulates. Their relatively low Mg/(Mg + Fe), Cr, and Sc contents, similar trace element abundances and high Al(vi):Al(iv) to Hawaiian cumulates indicates that they too are cumulates. This contrasts with lower Al(vi):Al(iv) of the Na-poor phenocrysts that crystallized between 6–7 kbars and 1150–1200 °C. Type 2 clinopyroxenes are Mg-rich, and have major and trace element compositions very similar to clinopyroxenes in Hawaiian peridotites. These clinopyroxenes are interpreted as fragments of mantle xenoliths. They show intermediate amounts of incompatible element depletion, between more enriched Hawaiian peridotites and strongly depleted abyssal peridotites. Some grains exhibit the effects of mantle metasomatism, having spoon-shaped, chondrite-normalized REE patterns like those of Hawaiian peridotite xenoliths. The occurrence of disaggregated pyroxenitic cumulates and metasomatized mantle xenoliths in rejuvenated magmas of both Burton Guyot and Hawaiian islands suggests that the plumbing system of these volcanic systems share significant similarities. However, consistently with previous geochemical studies of the Louisville seamounts, geochemical consistency of shield and rejuvenated clinopyroxenes at Burton Guyot show that this volcano experienced similar alkaline magmatism from shield to rejuvenated stages. This is an important difference with the evolution of Hawaiian volcanoes that includes a dominantly tholeiitic shield stages and alkaline post-shield and rejuvenated stages, which suggests that the model of Hawaiian island formation may not be fully applicable to Louisville seamounts.

Titanates of the lindsleyite–mathiasite (LIMA) group reveal isotope disequilibrium associated with metasomatism in the mantle beneath Kimberley (South Africa)

Radiogenic isotope variations unrelated to radiogenic ingrowth are common between minerals found in metasomatised mantle xenoliths entrained in kimberlite, basalts and related magmas. As the metasomatic minerals are assumed to have been in isotopic equilibrium originally, such variations are typically attributed to contamination by the magma host and/or interaction with mantle fluids during or before xenolith transport to surface. However, the increasing evidence of metasomatism by multiple, compositionally distinct fluids permeating the lithospheric mantle, coeval with specific magmatic events, suggests that isotopic disequilibrium might be a consequence of discrete, though complex, metasomatic events.Here we provide clear evidence of elemental and Sr isotope heterogeneity between coeval Ti-rich LIMA (lindsleyite–mathiasite) minerals at the time of their formation in the mantle. LIMA minerals occur in close textural association with clinopyroxene and phlogopite in low-temperature (∼800–900 °C), strongly metasomatised mantle xenoliths from the ∼84 Ma Bultfontein kimberlite (South Africa). Previous U/Pb dating of the LIMA phases was used to argue that each xenolith recorded a single event of LIMA crystallisation at ∼180–190 Ma, coeval with the emplacement of Karoo magmas. SEM imaging reveals that up to four types of LIMA phases coexist in each xenolith, and occasionally in a single LIMA grain. Major element and in situ Sr isotope analyses of the different LIMA types show that each phase has a distinct elemental composition and initial 87Sr/86Sr ratio (e.g., 0.7068–0.7086 and 0.7115–0.7129 for two LIMA types in a single xenolith; 0.7053-0.7131 across the entire sample suite).These combined age and isotopic constraints require that multiple fluids metasomatised these rocks at broadly the same time (i.e. within a few thousands to millions of years), and produced similar mineralogical features. Elemental and isotopic variations between different LIMA types could be due to interaction between one (or more) Karoo-related Ti-rich silicate melts and previously metasomatised, phlogopite-rich lithospheric mantle. This study demonstrates that mantle metasomatic assemblages seemingly generated in a single event may instead result from the infiltration of broadly coeval fluids with variable compositions. This in turn implies that the isotopic variations recorded in mantle rocks may be an inherent feature of metasomatism, and that hot fluids infiltrating a rock do not necessarily cause equilibration at the cm scale, as has been assumed previously. Simple modelling of solid-state diffusion in mantle minerals shows that isotopic disequilibrium may be preserved for up to hundreds of Myr at mantle lithosphere temperatures (≤1100–1200 °C), unless subsequently affected by transient heating and/or fluid infiltration events. Radiogenic isotope disequilibrium associated with mantle metasomatism may therefore be a common feature of mantle xenoliths.

Evaluation of Mantle Processes in an Extensional Regime: Insight from In Situ O and Sr Isotope Systematics of Mantle Xenoliths from Ethiopia

AbstractNew O and Sr isotope compositions of minerals from well-characterized and variably metasomatized mantle xenoliths in Cenozoic basalts from the northwestern plateau (Gundeweyn area) and southern rift zone (Dillo and Megado areas) of Ethiopia were acquired to further evaluate the nature and effects of metasomatic processes affecting the lithospheric mantle beneath the East African Rift System (EARS), where a transition from plume impingement to rifting is recorded. The studied samples are dominantly fresh spinel lherzolites with subordinate spinel harzburgites and clinopyroxenites. The in situ δ18O composition of olivine is largely homogeneous, with average values of 5.31‰ for Gundeweyn, 5.22‰ for Dillo, and 5.25‰ for Megado xenoliths. The values are within the range of normal mantle olivine (δ18O = 5.1‰–5.4‰), and there is no systematic variation of δ18O values with petrographic characteristics, elemental compositions, Sr isotopic ratios, or tectonic setting (i.e., plateau [Gundeweyn] vs. rift [Dil...

Sulfur isotope composition of metasomatised mantle xenoliths from the Bultfontein kimberlite (Kimberley, South Africa): Contribution from subducted sediments and the effect of sulfide alteration on S isotope systematics

Abstract Sulfur isotopes are a powerful geochemical tracer in high-temperature processes, but have rarely been applied to the study of mantle metasomatism. In addition, there are very limited S isotope data on sub-continental lithospheric mantle (SCLM) material. For cratonic regions, these data are restricted to sulfide inclusions in diamonds. To provide new constraints on the S isotope composition of the SCLM and on the source(s) of mantle metasomatic fluids beneath the diamondiferous Kimberley region (South Africa), we investigated the S isotope systematics of five metasomatised mantle xenoliths from the Bultfontein kimberlite. Pentlandite and chalcopyrite in these xenoliths were analysed by in situ secondary-ion mass spectrometry (SIMS), with bulk-rock material measured by gas source isotope ratio mass spectrometry techniques. Based on previous studies, the xenoliths experienced different types of metasomatism to one another at distinct times (∼180 and ∼90–80 Ma). Contained pentlandite grains show variable alteration to heazlewoodite (i.e. Ni sulfide) + magnetite. The in situ S isotope analyses of pentlandite exhibit a relatively restricted range between −5.9 and − 1.4 ‰ δ 34 S (compared to VCDT), with no statistically meaningful differences between samples. Chalcopyrite only occurs in one sample and shows δ 34 S values between −5.4 and − 1.0 ‰ . The bulk-rock Ssulfide isotope analyses vary between −3.4 and + 0.8 ‰ δ 34 S . Importantly, the only sample hosting dominantly fresh sulfides shows a bulk-rock δ 34 S value consistent with the mean value for the sulfides, whereas the other samples exhibit higher bulk 34S/32S ratios. The differences between bulk-rock and average in situ δ 34 S values are directly correlated with the degree of sulfide alteration. This evidence indicates that the elevated 34S/32S ratios in the bulk samples are not due to the introduction of heavy S (commonly as sulfates) and are best explained by isotopic fractionation coupled with the removal of light S during serpentinisation, when pentlandite is altered to S-poor mixtures of heazlewoodite and magnetite. Available bulk Ssulfide isotopic data for SCLM peridotite xenoliths are dominated by positive δ 34 S values, which contrasts with the negative values of the sulfides. These results imply that the mantle S isotope values from bulk peridotite samples are commonly modified by isotopic fractionation during serpentinisation. Therefore, the S isotopic composition of the SCLM may require revision. The limited isotopic variability shown by sulfides in the Bultfontein mantle xenoliths is probably due to intermittent tapping of a mantle source with a relatively restricted S isotope composition. While the asthenospheric mantle ( δ 34 S ≤ − 1.4 ‰ ) is a viable candidate, δ 34 S values as low as − 5.9 ‰ require input from recycled crustal material, possibly represented by the sulfur reservoir with negative δ 34 S signature that is missing in the >500 Ma sedimentary record and could have been subducted and stored in the Earth's mantle.

Rhenium–osmium isotopes in pervasively metasomatized mantle xenoliths from the Bohemian Massif and implications for the reliability of Os model ages

Peridotite xenoliths brought to the surface by basaltic lavas attest to a variety of mantle processes, including partial melting, melt percolation or refertilization. The whole rock Re–Os concentrations and Os isotopic compositions were determined for 30 xenoliths collected from 11 localities across the northern Bohemian Massif in order to evaluate the Os model ages and attempt to relate the results to major crustal tectonic events during the history. Most samples were affected by variable extent of metasomatic overprint, which is commonly paralleled by very low Os concentrations (<1ppb). Rhenium concentrations in the whole suite are below the primitive mantle value. A subset of samples shows evidence for Re addition from host basaltic rocks, consistent with the presence of abundant melt pockets with secondary sulphides. The 187Os/188Os ratios range from 0.1162 to 0.1330 and cannot be directly related to individual mantle domains, implying the inability of more recent tectonic events to reset the original Os isotopic systematics. The calculated mantle extraction ages (TMA) range from <0.1 to 2.1Ga, whereas future ages obtained for nine samples indicate metasomatic overprints. The Re depletion ages (TRD) vary between <0.1 and ~1.6Ga. However, the TRD is not well suited for direct comparison with crustal ages because it represents a minimum age limit rather than specific age estimate. Therefore, a modified model age (TRDII) was calculated assuming a non-zero Re content during the pre-metasomatic stage and using a composition of the most depleted sample in our suite. A prominent peak in the calculated TRDII ages ranges between 0.5 and 0.6Ga which corresponds to the Cadomian orogenic cycle.

Metasomatized Mantle Xenoliths as a Record of the Lithospheric Mantle Evolution of the Northern Edge of the Ahaggar Swell, In Teria (Algeria)

Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.

In-situ geochemistry of sulfides in highly metasomatized mantle xenoliths from Kerguelen, southern Indian Ocean

Mantle xenoliths from the Kerguelen Archipelago record a complex multistage history involving a high degree (15 to 25%) partial melting that created a harzburgitic mantle completely stripped of Base Metal Sulfides (BMS), followed by pervasive melt-rock reaction with alkaline melts above the Kerguelen mantle plume. Subsequent reaction of the highly refractory protolith with small volumes of carbonate-rich silicate melts led to a re-enrichment in BMS (up to 0.05wt.%). Two BMS precipitation mechanisms are suggested: immiscibility from the silicate-carbonate melt and sulfidation reactions from a CO2-rich supercritical fluid. In-situ analyses of chalcophile and siderophile elements (major and trace levels) in the BMS shed new light on their origin. The BMS phases that precipitated via immiscibility are metal-rich sulfide melts which progressively evolved toward Ni and Cu-rich end-members by cumulate fractionation of monosulfide solid solution (mss) during percolation inside the peridotites. Some cumulate mss have elevated and fractionated IPGE contents (200–900× C1-Chondrite abundances), indicating random digestion of preexisting Os, Ir, Ru-rich PGM by the percolating sulfide melt. The BMS that precipitated by sulfidation reactions from a CO2-rich vapour phase are subsolidus exsolution products from Cu-bearing but Ni-poorer mss. They have the highest concentrations of PGEs and show selective enrichment in S, Pd, Pt and Os over Cu, S, Ir, Ru and Rh. Their PGE compositions confirm experimental data, which demonstrate that S, Pd, Pt and Os can be efficiently transported in a CO2-rich supercritical fluid. Superchondritic (S/Se), (Os/Ir) and (Pd/Pt) in both bulk-rocks and individual sulfides are inferred to be the geochemical fingerprints of sulfide crystallisation from a CO2-rich vapour exsolved from a highly evolved carbonate-rich metasomatic melt.

Mantle source of the Archean Panozero pluton, Karelia: evidence from isotope-geochemical study of rocks and minerals

This work considers geochemical and isotopic characteristics of the source of the Archean Panozero pluton derived from LILEand LREE-enriched lithospheric mantle. Sr and Nd isotopic data on clinopyroxenes and augites define a source with Sri = 0.7017 and ɛNd(t) varying within a narrow range from + 0.7 to + 1.4 (averaging + 1.1), which is close to previously obtained whole-rock isotopic data. Similar ɛNd(t) were obtained for the Archean alkaline rocks of Canada, whereas the Archean mafic rocks of the Baltic and Canadian Shields formed from depleted mantle have ɛNd(t) ∼2. Lead isotope measurements on K-feldspars (KFsp) and monzonite showed that the source of the pluton has μ = 8.98 for the Stacey-Kramers two-stage model, at low U/Pb and high Th/U ratios. Different lead isotope composition corresponding to μ = 10.43 was determined in KFsp from quartz monzonites. Diverse interpretations of obtained data have been proposed. It was noted that the Pb-Pb isotopic system was disturbed by a later (∼ 1.9 Ga) thermal event. The ratios of elements of similar compatibility were used to determine the geochemical specifics of source of the Panozero pluton. Their comparison with numerous literature data on metasomatized mantle xenoliths and minerals in them showed that the mantle source strongly differed from primitive mantle in ratios of elements, whose mineralmelt partitioning coefficients considerably differs from mineral-fluid partitioning, for instance, Nb/La. Mantle source that was responsible for geochemical peculiarities of the Panozero pluton was made up of Phl, CPx, and Ap.

Geology and petrology of the archean high-K and high-Mg Panozero massif, Central Karelia

The high-K and high-Mg Panozero central-type intrusion is located on the shore of Lake Segozero, Central Karelia, and has an age of 2737 ± 10 Ma. Detailed mapping and petrological study showed that it was formed in three magmatic cycles that were separated by lamprophyre dikes. The first cycle is composed mainly of mafic rocks (layered complex: pyroxenites-honblendites-monzogabbro) and monzonites 1; the second cycle includes monzonites 2, and the third cycle comprises monzonites 3 and quartz monzonites. The massif is cut by numerous lamprophyre dikes and breccia zones. As compared to calc-alkaline series, the studied rocks are enriched in K, Ba, Sr, P, LREE, have high mg# (mg# = 0.5–0.65), and elevated contents of Cr and Ni. The parent composition of the layered complex was determined to be monzogabbro. Model calculations showed that the compositional variations of the Panozero Complex are consistent with the fractional crystallization of monzogabbro. The melts were fractionated in an intermediate chamber and during the flowing and crystallization of the magma. The parent melt of the intrusion was formed by the partial melting of mantle enriched in some LILE, LREE, and volatiles (CO2 and H2O). The volatile enrichment of the melt manifests itself in the mineral composition of the rocks, the presence of primary gas inclusions in apatite, and diverse structural features. The comparison of the rocks of the Panozero Massif with metasomatized mantle xenoliths in the variation diagrams for incompatible elements showed that the mantle source of the Panozero Complex was metasomatized by fluid consisting of H2O and CO2 of different origin.

Origin and geodynamic significance of Upper Cretaceous lamprophyres from the Villány Mts (S Hungary)

In the Villany Mts of southern Hungary, ocelli-bearing porphyritic lamprophyre dykes and sills of Upper Cretaceous age occur sporadically, intruding Mesozoic carbonate rocks. They at places contain metasomatised mantle xenoliths and quartz xenocrysts of crustal origin. They are moderately fractionated with significant LILE and LREE enrichments and a notable Nb–Ta negative anomaly. Trace elements indicate that they formed in an intraplate environment by very low degree partial melting of a metasomatised garnet lherzolite mantle source that was enriched by earlier subduction. Based on petrography, geochemistry and age constraints, they differ from other Mesozoic basic rocks of the Tisza block (Mecsek Mts and Slavonian basalts); however, they show a significant geochemical similarity to the Upper Cretaceous lamprophyre dyke swarm from NE Transdanubia (northwestern Hungary) situated on the Alcapa microplate. Thus we suggest that lamprophyres from the Villany Mts and NE Transdanubia could have originated from the same or similar enriched asthenospheric mantle sources.

Evidence for fluoride melts in Earth's mantle formed by liquid immiscibility

Hitherto unknown fluoride melts were found in metasomatized mantle xenoliths from New Zealand. These fluoride melts were only preserved in rock fragments that were carefully polished using nonhydrous polishing liquids. The fluoride melts were derived by liquid immiscibility from a precursor alkali silicate melt, and their geochemical signature suggests an origin within a subduction-zone setting.

Os isotopes in mantle xenoliths from the Eifel volcanic field and the Vogelsberg (Germany): age constraints on the lithospheric mantle

The effects of mantle metasomatism on the sulfide phase in mantle xenoliths in general, and on the Os isotopic system in particular, have received increased attention in recent years. Here, we report on Os isotopic systematics of metasomatized mantle xenoliths from the late Quaternary Eifel (Dreiser Weiher and Meerfelder Maar) and neighboring Vogelsberg volcanic fields, which provide insight into the effects of melt extraction and metasomatism on Os isotopes and place constraints on the evolution of the lithospheric mantle component beneath central Europe. Sixteen harzburgite, lherzolite, and pyroxenite xenoliths from the Eifel and two lherzolite xenoliths from the Vogelsberg were analyzed for Os isotopes. Samples from the anhydrous peridotite suite (Ib) are highly variable in their Os isotopes, ranging from subchondritic values (187Os/188Os=0.1236) to suprachondritic values (187Os/188Os=0.1420), indicating that some of these samples have been overprinted by the addition of radiogenic Os and have lost the primary mantle Os that was presumably present. The suprachondritic values suggest a source for this Os in a reservoir with a time-integrated Re/Os ratio greater than that of the bulk Earth. Eifel samples with Os contents >1.5 ng/g from the hydrous suite (Ia) have relatively unradiogenic Os isotope compositions (187Os/188Os=0.1208–0.1237) and Al2O3–Os isotopic systematics consistent with ancient melt depletion and isolation from the convecting asthenospheric mantle for time periods similar to the age of the overlying crust (~1.5 Ga) as well with results from peridotite massifs in the European region. The LREE-metasomatism and the enrichment of Os (up to 6.47 ng/g) and As (sulfide metasomatism?) in the hydrous suite is strongly inversely correlated with the Os isotope ratios, demonstrating that mantle processes such as metasomatism can significantly modify the Os isotope chemistry of mantle xenoliths.

Metasomatic Mantle Xenoliths from the Bismarck Microplate (Papua New Guinea)--Thermal Evolution, Geochemistry and Extent of Slab-induced Metasomatism

formation at a mid-ocean ridge. Cooling rates in the range of 1°C/ geochemistry; Bismarck Archipelago‐Papua New Guinea Ma were calculated from zoning of Ca in olivine using the Lasaga algorithm. Subsequent to this cooling, a strong metasomatism affected the mantle peridotites when metasomatic agents emerged from the underlying slab of a subduction zone, which was stalled about 15 my ago. This resulted in the formation of orthopyroxene-, INTRODUCTION clinopyroxene-, phlogopite- and hornblende-bearing veins crosscutting Evidence for metasomatic processes in the Earth’s mantle spinel peridotites and olivine clinopyroxenites, as well as pervasively is provided by trace element and isotope variations in metasomatized plagioclase lherzolites. The metasomatic xenoliths volcanic rocks (e.g. Hawkesworth et al., 1987), by metareveal strong chemical disequilibria between the metasomatic minerals somatized sections of ultramafic massifs (e.g. Bonatti et and the adjacent, unaltered host rock minerals, which are especially al., 1981) and by metasomatized mantle xenoliths of prominent in the veined samples. Temperatures during the meta- mainly peridotitic composition, which are transported to somatic overprint, estimated using spinel‐olivine thermometry, range the surface by mafic or ultramafic magmas (e.g. Jagoutz between 660 and 950°C. Oxygen barometry reveals an elevated et al., 1979). Two different styles of metasomatism are oxygen fugacity, with log(fo2) FMQ values of 0·4 to >4. A geo- recorded in these rocks. Some ultramafic rocks show chemical study of the ultramafic rocks shows that all types of xenoliths cryptic metasomatism, resulting in trace element enhave been metasomatized. Pervasively metasomatized plagioclase richment without any visible mineral or textural changes lherzolites and cumulate olivine clinopyroxenites have high contents (Dawson, 1984). The second type of metasomatism is of middle and heavy rare earth elements compared with veined characterized by petrographically well-recognizable reperidotites. Cryptic metasomatism, indicated by increased light rare placement textures and/or the presence of secondary, earth elements and Nd concentrations, results from LREE-rich hydrous minerals (e.g. phlogopite, hornblende) as well as secondary clinopyroxene, plagioclase, and accessory hydrous fluid circulation. The investigated peridotites underwent a

Noble gas and halogen geochemistry of mantle fluids: comparison of African and Canadian diamonds

Volatile-bearing fluids in diamond have been characterised using extension of the 40Ar- 39Ar technique to simultaneously measure noble gas isotopes, halogens (Cl, Br and I), K and U. Samples investigated include opaque cubic and fibrous diamonds from the North West Territories, Canada, the Democratic Republic of Congo, (DRC, formerly Zaire), and Jwaneng, Botswana. These are compared with results obtained from metasomatised mantle xenoliths from Bultfontein, South Africa. Diamonds and xenoliths show a narrow range of 40Ar∗/Cl values between 506–1347 × 10 −6 molar (M) with mean values that overlap for African diamonds and xenoliths (831 ± 218 × 10 −6 M) and Canadian diamonds (965 ± 273 × 10 −6 M). These values are consistent with the estimated MORB value and support the presence of a widespread 40Ar and Cl-rich fluid in the mantle. Canadian diamonds have high and variable halogen ratios, with Br/Cl = 1.3–63.0 × 10 −3 M and I/Cl = 9.8–1703.5 × 10 −6 M. In contrast, African diamonds, have less variable Br/Cl = 1.0–2.0 × 10 −4 M and I/Cl = 13.6−176.4 × 10 −6 M (with most I/Cl between 20–70 × 10 −6 M). Fluids in diamonds from DRC and Botswana, have Ar and halogen compositions close to those estimated for the present-day MORB source. The concentrations of noble gases (Ar, Kr and Xe), halogens, K and U in Canadian coated stones are 10–30 times higher than in African coated stones probably due to a higher inclusion population density. The large variation in halogen ratios measured in Canadian diamonds is the first evidence for significant halogen fractionation in the mantle. The Br/Cl ratios are notably above the range reported for crustal fluids. The high halogen ratios in Canadian diamonds are consistent with crystallisation of a Cl-bearing mineral, possibly involving apatite, from a fluid with starting composition similar to that in African diamonds.

Features of the Early Palaeozoic Mantle beneath Langao County and Its Formation Mechanism

AbstractPhlogopite‐amphibole pyroxenite xenoliths contained in an Early Palaeozoic alkali subvolcanic lamprophyre complex in Langao County, Shaanxi Province, are metasomatized mantle xenoliths, composed mainly of clinopyroxene, amphibole, phlogopite, apatite, pervoskite, ilmenite and sphene with well‐developed subsolidus metamorphism‐deformation textures, such as “triple points” and “cataclastic boundaries”. Minerological studies indicate that clinopyroxene is rich in SiO2 and MgO and poor in TiO2 and Al2O3, which is notably different from magmatogenic deep‐seated megacrysts and phenocrysts formed in the range of mantle pressure. Amphibole and phlogopite have the compositional feature of mantle‐derived amphibole and phlogopite. Sm‐Nd isotope studies suggest that the metasomatized mantle beneath Langao County is the product of metasomatism of primitive mantle by melt (fluid) derived from the mantle plume, and the mantle metasomatism occurred 650 Ma ago. The process of mantle metasomatism changed from mantle metasomatism induced by CO2‐, H2O‐ and CH4‐rich fluid in the early stage to that induced by melt and P‐, Ti‐, Ca‐ and Fe‐rich fluid in the late stage.

Fluid-present melting of ocean crust in subduction zones

Phase equilibria were investigated in the pressure range 400–2000 MPa on the Pinatubo dacite, a representative example of natural Cenozoic adakites (slab melts), to constrain the temperature versus wt% H 2 O conditions of magma genesis. The experimental results suggest that water contents of the melt were at least 10 wt% and temperatures were ∼900 °C during magma ascent from the slab. Natural aluminous amphiboles present in the dacite crystallized at pressures of ∼1000 MPa, fixing an upper temperature limit of 950 °C for the slab melts at the crust-mantle boundary. Amphiboles experimentally crystallized at 1000 MPa are identical to those found in melt inclusions in metasomatized mantle xenoliths. These natural inclusions represent samples of slab melts and contain glasses with a dacitic bulk composition, which requires an H 2 O content in the melt of >10 wt% at 1000 MPa and 900 °C. For conditions of magma generation of 800–900 °C at 2000–2300 MPa, melt H 2 O contents close to 15 wt% are needed to generate melts with dacitic bulk composition. Overall, these constraints are inconsistent with slab melts being produced by dehydration melting of amphibolite, a mechanism unable to produce dacite melt compositions that are both hydrous and cool. Slab melting in modern subduction zones must occur under fluid-present conditions at temperatures below 900 °C, in agreement with the thermal regime of subducted oceanic crust as deduced from numerical simulation of heat transfer using the present-day mantle geotherm. At such low temperatures, Ti-bearing phases are stable during melting; thus any metasomatizing slab melt will be strongly depleted in Nb, Ta, and Ti, a major characteristic of arc magmas produced in the mantle wedge.

Phlogopite-amphibole-pyroxenite xenoliths in Langao, Shaanxi Province, China: Evidence for mantle metasomatism

Phlogopite-amphibole-pyroxenite xenoliths contained in the alkali basic-ultrabasic subvolcanic complex in Langao, Shaanxi Province, are composed of diopside, Ti-rich pargasite, phlogopite apatite, sphene and ilmenite, which have subsolidus metamorphism-deformation textures such as triple-points, cataclastic boundaries and kink-bands. Mineral chemical characteristics show that the diposide, Ti-rich paragasite and phlogopite are derived from the mantle and are the products of mantle metasomatism. Compared with normal mantle-derived spinel-lherzolites, the xenoliths are enriched in TiO2, Fe2O3, CaO, Na2O and K2O, with apparent depletion in MgO. Chondrite-normalized REE patterns and primordial-mantle normalized trace elements data show that they are enriched in REE (especially LREE) and incompatible trace elements. The petrographic, mineralogical and petrochemical characteristics indicate that the xenoliths are metasomatized mantle xenoliths, which offers the evidence for mantle metasomatism and represents the anomalous mantle beneath the Early Paleozoic rift in northern Daba Mountains. The agents of mantle metasomatism are probably derived from the rising of mantle hot plumes. The processes of metasomatism varied from limited-range fluid metasomatism in deep mantle (>90 km) to pervasive metasomatism of silicate melt.

Th–Sr–Nd–Pb isotope and trace element evidence for the origin of the São Miguel, Azores, enriched mantle source

A suite of young alkali basalts from across the island of São Miguel has been analyzed for major and trace element concentrations and Sr, Nd, Pb and Th isotopic compositions in order to document the intra-island variation, and to constrain the processes involved in the formation of the enriched mantle sources of these lavas. As shown in previous studies, large isotopic and trace element variations correlate with geographic location across São Miguel. New Th isotope data show a similar correlation, with ( 230Th)/( 232Th) varying from 1.094 to 0.877. These variations are attributed to two-component mixing, where the western end of the island is similar to the source of the Azores Platform tholeiites, and the eastern end of the island has a more enriched, EMII-type source. Trace element systematics indicate that the São Miguel enriched mantle source is enriched relative to the predominant Azores plume source in elements including K, Rb, Th, U, Pb and Zr, but not Ba, Nb or Ti. Incorporation into the Azores plume of approximately 5% subduction-processed terrigenous sediment can explain many of the trace element and isotopic characteristics of the São Miguel enriched mantle source. However, this mechanism cannot easily explain the constant Cs/Rb ratios across the island, the concave downward trend of Ba/Th vs. Ti/Zr, and the recent decrease in Th/U of the São Miguel enriched mantle source documented by Th and Pb isotope systematics. An alternative model is that the São Miguel enriched mantle source is subcontinental lithospheric mantle. This is supported by the similarity of the trace element signatures of the São Miguel enriched mantle source to those of hydrous metasomatized mantle xenoliths. In particular, the low Ba/Rb signature of the São Miguel enriched mantle, which is unusual for oceanic basalts, is a characteristic of many hydrous metasomatized mantle xenoliths and magmas believed to be derived from such sources. We propose that this subcontinental lithospheric mantle resided originally beneath northwestern Africa or Iberia but was delaminated during rifting upon the opening of the Atlantic Ocean basin, and that it is presently a shallow, localized contaminant that is melted due to an influx of heat from the Azores plume.

Experimentally determined trace and minor element partitioning between clinopyroxene and carbonatite melt under upper mantle conditions

Laser ablation microprobe analyses of clinopyroxenes equilibrated with carbonatite melt at 20–22 kbar and 1050–1100°C are used to calculate partition coefficients for an extensive suite of trace elements. Our experiments were performed on a mixture of peridotite minerals and two types of carbonates with differing trace element contents (natural carbonatite and mixtures of sedimentary carbonates). Although trace element concentrations vary by an order of magnitude between the carbonate mixtures, the partition coefficients are similar. Some, however, differ substantially from previously published values. Most of the measured elements have broadly similar partition coefficients of D cpx clq = 0.1–0.4 , including Pr, Nd, Eu, Dy, Er and Hf, which are reported here for the first time. Considerably higher partition values are found for Ti and V which are compatible in clinopyroxene with D cpx lq = 1.4 ± 0.6 for Ti and 2.9 ± 0.9 for V (1σ). The clinopyroxene partitioning data for carbonatite ( D cpx clq ) are remarkably similar to published values for silicate melts ( D cpx slq ), with significant exceptions for the behaviour of Ti, Ba, Nb and perhaps Ta, whereas values for other silicate minerals differ more strongly between carbonatite and silicate melts. From our results, the most sensitive indicator of mantle metasomatism.by carbonatite melt as opposed to silicate melt infiltration should be low Ti Eu in metasomatised peridotites assuming closed-system metasomatism. In contrast to suggestions from some studies of metasomatised mantle xenoliths Zr Hf ratios appear to remain unchanged during caatite metasomatism. These chemical effects only partly agree with those described from natural mantle rocks interpreted to result from carbonatite metasomatism.

Melt geometry, movement and crystallization, in relation to mantle dykes, veins and metasomatism

Consideration of theoretical, experimental and natural rock data show that basic-ultrabasic melt will disperse along mineral grain edges in olivine-rich mantle rock and thereby form a connected three-dimensional network throughout the rock even when present in only small (less than 1%) volumes. The viscosity of such melts will also allow small (less than 1-5%) volumes to move on appropriate geological timescales as a result of gravity-driven compaction. These features mean that small volume basic-ultrabasic melts are capable of infiltrating and metasomatizing mantle peridotites. Modally metasomatized mantle xenoliths are commonly closely associated with an array of dyke-like and vein injection phenomena. Textural, structural and modal characteristics of a wide array of mantle dykes, veins and metasomatic rocks suggest that such rocks have certain features in common with cumulates, and might usefully be distinguished as dyke cumulates and metasomatic infill cumulates . They represent partial crystal precipitates from melt flowing along channelways or pervasively through peridotite, and their bulk rock compositions provide poor guides to actual mantle melt compositions. The crystallization of the minerals in dykes/veins/ metasomites causes differentiation of the melt by crystal fractionation processes, but at the same time the melt may maintain equilibrium with host rock phases (e.g. olivine) and chromatographic column or percolation effects will control the range of transport of different chemical components by the melt. These combined processes are referred to as percolative fractional crystallization . Data on the actual trace element compositions of melt in equilibrium with the minerals of mantle dykes/veins/metasomites are calculated from trace element analyses of the minerals by using partition coefficients. For a wide variety of metasomatic suites, the calculated melt compositions show a progression of trace element abundances from ones similar to primitive asthenospheric OIB-like compositions towards more incompatible element enriched compositions. Thus they support the hypothesis that fractional crystallization and percolative fractional crystallization processes operating upon initial primitive asthenospheric melts may yield melt compositions matching those necessary for wide varieties of mantle metasomatism. The differentiation of the melts and evolution of the metasomatic rocks proceed together. No evidence for the involvement of volatile-rich fluids distinct from melts has been found. The trace element compositions of many kimberlitic and lamproitic melts may also arise by processes of percolative fractional crystallization of initially primitive melts with oIB-like trace element compositions, as a result of flow through mantle peridotite.