Mica-Amphibole-Rutile-Ilmenite-Diopside Research Articles

Recycling process and proto-kimberlite melt metasomatism in the lithosphere-asthenosphere boundary beneath the Amazonian Craton recorded by garnet xenocrysts and mantle xenoliths from the Carolina kimberlite

Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP–PP–PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855–1102 °C and 3.6–7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (LaN/YbN = 5–62; CeN/SmN = 1–3; where N = primitive mantle normalized values), they have high Ca/Al ratios (10–410), low to medium Ti/Eu ratios (742–2840), and low Zr/Hf ratios (13–26), which suggest they were formed by metasomatic reactions with CO2-rich silicate melts. Phlogopite with high TiO2 (>2.0 wt.%), Al2O3 (>12.0 wt.%), and FeOt (5.0–13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO2 (<1.0 wt.%) and lower Al2O3 (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high 143Nd/144Nd (0.51287–0.51371) and εNd (+4.55 to +20.85) accompanied with enriched 87Sr/86Sr (0.70405–0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ± 5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.

The Newly Discovered Neoproterozoic Aillikite Occurrence in Vinoren (Southern Norway): Age, Geodynamic Position and Mineralogical Evidence of Diamond-Bearing Mantle Source

During the period 750–600 Ma ago, prior to the final break-up of the supercontinent Rodinia, the crust of both the North American Craton and Baltica was intruded by significant amounts of rift-related magmas originating from the mantle. In the Proterozoic crust of Southern Norway, the 580 Ma old Fen carbonatite-ultramafic complex is a representative of this type of rocks. In this paper, we report the occurrence of an ultramafic lamprophyre dyke which possibly is linked to the Fen complex, although 40Ar/39Ar data from phenocrystic phlogopite from the dyke gave an age of 686 ± 9 Ma. The lamprophyre dyke was recently discovered in one of the Kongsberg silver mines at Vinoren, Norway. Whole rock geochemistry, geochronological and mineralogical data from the ultramafic lamprophyre dyke are presented aiming to elucidate its origin and possible geodynamic setting. From the whole-rock composition of the Vinoren dyke, the rock could be recognized as transitional between carbonatite and kimberlite-II (orangeite). From its diagnostic mineralogy, the rock is classified as aillikite. The compositions and xenocrystic nature of several of the major and accessory minerals from the Vinoren aillikite are characteristic for diamondiferous rocks (kimberlites/lamproites/UML): Phlogopite with kinoshitalite-rich rims, chromite-spinel-ulvöspinel series, Mg- and Mn-rich ilmenites, rutile and lucasite-(Ce). We suggest that the aillikite melt formed during partial melting of a MARID (mica-amphibole-rutile-ilmenite-diopside)-like source under CO2 fluxing. The pre-rifting geodynamic setting of the Vinoren aillikite before the Rodinia supercontinent breakup suggests a relatively thick SCLM (Subcontinental Lithospheric Mantle) during this stage and might indicate a diamond-bearing source for the parental melt. This is in contrast to the about 100 Ma younger Fen complex, which were derived from a thin SCLM.

Siderophile and chalcophile elements in spinels, sulphides and native Ni in strongly metasomatised xenoliths from the Bultfontein kimberlite (South Africa)

The metasomatised continental mantle may play a key role in the generation of some ore deposits, in particular mineral systems enriched in platinum-group elements (PGE) and Au. The cratonic lithosphere is the longest-lived potential source for these elements, but the processes that facilitate their pre-concentration in the mantle and their later remobilisation to the crust are not yet well-established. Here, we report new results on the petrography, major-element, and siderophile- and chalcophile-element composition of native Ni, base metal sulphides (BMS), and spinels in a suite of well-characterised, highly metasomatised and weakly serpentinised peridotite xenoliths from the Bultfontein kimberlite in the Kaapvaal Craton, and integrate these data with published analyses. Pentlandite in polymict breccias (failed kimberlite intrusions at mantle depth) has lower trace-element contents (e.g., median total PGE 0.72 ppm) than pentlandite in phlogopite peridotites and Mica-Amphibole-Rutile-Ilmenite-Diopside (MARID) rocks (median 1.6 ppm). Spinel is an insignificant host for all elements except Zn, and BMS and native Ni account for typically <25% of the bulk-rock PGE and Au. High bulk-rock Te/S suggest a role for PGE-bearing tellurides, which, along with other compounds of metasomatic origin, may host the missing As, Ag, Cd, Sb, Te and, in part, Bi that are unaccounted for by the main assemblage.The close spatial relationship between BMS and metasomatic minerals (e.g., phlogopite, ilmenite) indicates that the lithospheric mantle beneath Bultfontein was resulphidised by metasomatism after initial melt depletion during stabilisation of the cratonic lithosphere. Newly-formed BMS are markedly PGE-poor, as total PGE contents are <4.2 ppm in pentlandite from seven samples, compared to >26 ppm in BMS in other peridotite xenoliths from the Kaapvaal craton. This represents a strong dilution of the original PGE abundances at the mineral scale, perhaps starting from precursor PGE alloy and small volumes of residual BMS. The latter may have been the precursor to native Ni, which occurs in an unusual Ni-enriched zone in a harzburgite and displays strongly variable, but overall high PGE abundances (up to 81 ppm). In strongly metasomatised peridotites, Au is enriched relative to Pd, and was probably added along with S. A combination of net introduction of S, Au +/− PGE from the asthenosphere and intra-lithospheric redistribution, in part sourced from subducted materials, during metasomatic events may have led to sulphide precipitation at ~80–120 km beneath Bultfontein. This process locally enhanced the metallogenic fertility of this lithospheric reservoir. Further mobilisation of the metal budget stored in these S-rich domains and upwards transport into the crust may require interaction with sulphide-undersaturated melts that can dissolve sulphides along with the metals they store.

Evaluating the Precise 39Ar/40Ar Dating of Multiple Mineral Potassic Phases in Ultra‐alkaline Rocks: Applications to Mantle Systematics

AbstractA suite of potassium‐bearing minerals from the Walgidee Hills lamproite intrusion in the Kimberley region of Western Australia was selected for 39Ar/40Ar dating. These included wadeite, jeppeite, priderite, potassium richterite, and phlogopite. All recorded excellent plateau ages, with the mean age of the combined data set being 17.3±0.3 Ma. Phlogopite recorded the largest uncertainty, whereas, of the other minerals, wadeite gave the best precision. Although rare to absent in common magmatic rocks, these minerals are widely distributed in alkaline complexes and in lamproite, kimberlite and orangeite intrusions. The results indicate this suite of minerals is excellent for 39Ar/40Ar dating and that they can be used singly or in combination to obtain the precise magmatic crystallization ages of ultra‐alkaline rocks. Because of the stability of potassium richterite at mantle depths, 39Ar/40Ar dating of MARID (mica‐amphibole‐rutile‐ilmenite‐diopside) xenoliths should be a more widely‐applied technique to investigating mantle geodynamics.

Sr–Nd–O isotopic evidence of variable sources of mantle metasomatism in the subcratonic lithospheric mantle beneath the Grib kimberlite, northwestern Russia

To provide new insights into the type and extent of mantle metasomatism in the subcratonic lithospheric mantle, we examined the Sr–Nd–O isotopic compositions of orthopyroxene, clinopyroxene, garnet, ilmenite and phlogopite from sheared garnet lherzolite, granular garnet harzburgites and lherzolites and clinopyroxene-phlogopite rocks from the Grib kimberlite in the Arkhangelsk diamond province in northwestern Russia.Clinopyroxene and orthopyroxene from sheared garnet lherzolite initially have a close value of 87Sr/86Sr(t) (~0.7034) and close weak positive εNd. Orthopyroxene and clinopyroxene are in oxygen isotope equilibrium with coexisting olivine. Clinopyroxene from a garnet harzburgite has a low 87Sr/86Sr(t) isotope ratio of 0.70266. Clinopyroxene from granular garnet lherzolites has a relatively narrow variation in 87Sr/86Sr(t) (0.70456–0.70582) and considerably larger variations in εNd (−4.3 − +1.0) isotope ratios. Garnet displays elevated initial 87Sr/86Sr(t) values (0.70540–0.70633). Ilmenite shows a narrow range in 87Sr/86Sr(t) (0.70497–0.70522) coupled with εNd values of +0.4 and +3.5. These isotopic data suggest granular garnet lherzolite of mantle metasomatism took place during the interaction of kimberlite melts with SCLM that contained mica-amphibole-rutile-ilmenite-diopside (MARID)-type metasomes. Clinopyroxenes from clinopyroxene-phlogopite (phlogopite wehrlite) xenoliths display a broader range in 87Sr/86Sr(t) (0.70486–0.70813) that is significantly higher than the kimberlite values and a circa-chondritic εNd (−0.1 − +1.3) with a restricted δ18O range (5.11‰–5.33‰). More radiogenic Sr isotopic composition decoupled from Nd isotopes could have been induced by metasomatic melt/fluid related to a subducted material.The isotopic compositions of mantle minerals preserve Sr–Nd isotopic evidence of pre-kimberlite metasomatic events that were probably due to incomplete reequilibration with ultramafic carbonated melt. Based on mineral pairs Rb–Sr isochrons and a clinopyroxene-based Sm–Nd errochron, these mantle metasomatic events correspond to ~550–600 Ma and ~1200 Ma episodes of magmatic–thermal activity.

Isotopic analyses of clinopyroxenes demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle

The trace element and radiogenic isotope systematics of clinopyroxene have frequently been used to characterise mantle metasomatic processes, because it is the main host of most lithophile elements in the lithospheric mantle. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100–200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40–75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369–0.70383; εNdi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to being in equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples in this study indicate equilibration depths of 135–160 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites in this study, which derive from shallower depths (<135 km), and have higher Sr (~350–1000 ppm) and LREE contents, corresponding to higher La/Zr of > ~ 0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the “enriched mantle” towards those of Type 1 rocks (e.g., εNdi = −12.7 to −4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526–0.71177), which are correlated with Sr contents and La/Zr ratios, and which might be explained by the interaction between peridotite and melts from different enriched sources within the lithospheric mantle. Most notably, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that a pervasive, alkaline metasomatic event caused MARID to be generated and harzburgites to be converted to lherzolite in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.

Thallium isotopic composition of phlogopite in kimberlite-hosted MARID and PIC mantle xenoliths

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are rare mantle xenoliths entrained by kimberlites. Their high phlogopite modes (15 to ∼100vol.%) and consequent enrichments in alkali metals and H2O suggest a metasomatic origin. Phlogopite also has high concentrations (>0.2μg/g) of thallium (Tl) relative to mantle abundances (<3ng/g). Thallium isotope ratios have proven useful in tracing the input of Tl-rich materials, such as pelagic sediments and altered oceanic crust, to mantle sources because of their distinct isotopic compositions compared to the peridotitic mantle. This study presents the first Tl isotopic compositions of well-characterised phlogopite separates from MARID and PIC samples to further our understanding of their genesis. The PIC rocks in this study were previously interpreted as the products of kimberlite melt metasomatism, whereas the radiogenic and stable N–O isotope systematics of MARID rocks suggest a parental metasomatic agent containing a recycled component.The ε205Tl values of phlogopite in both PIC (–2.7±0.8; 2s.d., n=4) and MARID samples (–2.5±1.3; 2s.d., n=21) overlap with the estimated mantle composition (–2.0±1.0). PIC phlogopite Tl contents (∼0.4μg/g) are suggestive of equilibrium with kimberlite melts (0.1–0.6μg/g Tl), based on partitioning experiments in other silica-undersaturated melts. Kimberlite Tl-ε205Tl systematics suggest their genesis does not require a recycled contribution: however, high temperature-altered oceanic crust cannot be ruled out as a component of the Kimberley kimberlites’ source. Mantle-like ε205Tl values in MARID samples also seem to contradict previous suggestions of a recycled contribution towards their genesis. Recycled components with isotopic compositions close to mantle values (e.g., high temperature-altered oceanic crust) are still permitted. Moreover, mass balance mixing models indicate that incorporation into the primitive mantle of 1–30% of a low temperature-altered oceanic crust+continental crust recycled component or 1–50% of continental crust alone could be accommodated by the Tl–ε205Tl systematics of the MARID parental melt. This scenario is consistent with experimental evidence and existing isotopic data. One PIC phlogopite separate has an extremely light Tl isotopic composition of –9.9, interpreted to result from kinetic isotopic fractionation. Overall, phlogopite is the main host mineral for Tl in metasomatised mantle and shows a very restricted range in Tl isotopic composition, which overlaps with estimates of the mantle composition. These results strongly suggest that negligible high temperature equilibrium Tl isotopic fractionation occurs during metasomatism and reinforces previous estimates of the mantle’s Tl isotopic composition.

Evidence for subduction-related signatures in the southern African lithosphere from the N-O isotopic composition of metasomatic mantle minerals

Current understanding of the fate of subducted material (and related fluids) in the deep Earth can be improved by combining major and trace element geochemistry with stable isotopic compositions of mantle rocks or minerals. Limited isotopic fractionation during high temperature processes means that significant deviations from mantle-like isotope ratios in mantle rocks probably result from recycling of surficial material. To determine the effects and origins of mantle metasomatic fluids/melts, new δ15N and δ18O data have been collected for thirteen mantle xenoliths – harzburgites, wehrlites, lherzolites, and MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) rocks – from the Bultfontein kimberlite (Kimberley, South Africa), which show varying degrees of metasomatism. The δ18O values of olivine and orthopyroxene in phlogopite-free harzburgites match the mantle composition (δ18Oolivine = +5.2 ± 0.3‰; δ18Oorthopyroxene = +5.7 ± 0.3‰; 2 s.d.), consistent with previous inferences that harzburgites were formed by interaction with ancient silica-rich melts unrelated to subduction processes. Wehrlite samples display mineral compositional characteristics (e.g., low La/Zr in clinopyroxene) resembling those of other products of kimberlite melt metasomatism, such as PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks. The inferred interaction with kimberlite melts may be responsible for O isotopic disequilibrium between clinopyroxene and olivine (Δ18O = +0.2‰) in the wehrlites of this study. In contrast with broadly mantle-like δ18O values, the δ15N value of phlogopite in a wehrlite sample (+5.9‰) differs from the mantle composition (δ15N = −5 ± 2‰). This unusual N isotopic composition in kimberlite-related mantle products might indicate that a recycled crustal component occurred in the source of the Kimberley kimberlites, or was assimilated during interaction with the lithospheric mantle.Similar major and trace element characteristics in clinopyroxene from phlogopite-lherzolite and MARID samples suggest metasomatism by fluids of similar composition. Lherzolite and MARID clinopyroxene δ18O values (as low as +4.4‰) extend below those reported in mantle peridotites (i.e. δ18Oclinopyroxene = +5.6 ± 0.3‰; 2 s.d.), and strong negative correlations are found between mineral δ18O values and major element compositions (e.g., Na2O contents in clinopyroxene). Furthermore, phlogopite δ15N values (+4 to +7‰) in the studied lherzolite and MARID samples are higher than mantle values. Combined, the low δ18O-high δ15N isotopic signatures of MARID and lherzolite samples suggest progressive mantle metasomatism by a melt containing a recycled oceanic crust (eclogitic) component. This study demonstrates that progressive enrichment of the subcontinental lithospheric mantle may be inextricably linked to plate tectonics via recycling of subducted crustal material into the deep mantle.

K-rich hydrous mantle lithosphere beneath the Ontong Java Plateau: Significance for the genesis of oceanic basalts and Archean continents

Olivine-free metasomatic mantle-derived xenoliths, frequently recovered from kimberlite and lamprophyre intrusions on the continents, are seldom described from oceanic settings. We report the mineralogy, geochemistry and Sr-Nd-Hf-Pb-Os isotopic compositions of a unique K-rich, hydrous mantle nodule sourced from a 34 Ma ultramafic lamprophyre (alnöite) pipe on the island of Malaita, Ontong Java Plateau, SW Pacific. The Ontong Java Plateau, the most voluminous oceanic large igeous province, has a >100 km thick lithospheric root reminiscent of thick lithospheres that characterize cratonic settings. The phlogopite, amphibole, clinopyroxene and ilmenite-rich nature of the Malaita metasomatic nodule bears striking similarity to cratonic mica-amphibole-rutile-ilmenite-diopside (MARID) suite xenoliths, and it provides a tangible example of heavily overprinted oceanic mantle lithosphere. The nodule phlogopite 40Ar/39Ar age of 44.7 ± 1.8 Ma (95% confidence level) predates the 34 Ma alnöites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnöite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnöite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnöite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (87Sr/86Sr45Ma = 0.70419–0.70423; εNd45Ma = +3.5; εHf45Ma = +5.3; 206Pb/204Pb45Ma = 18.66–18.71; 207Pb/204Pb45Ma = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high 187Os/188Os45Ma (0.1824–0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. K-rich hydrous metasomatized mantle lithosphere could represent an enriched source component that is required for the formation of many oceanic intraplate basaltic provinces. As such, strongly metasomatized oceanic lithosphere provides an alternative to recycled crust components within the convecting mantle to explain enriched mantle signatures observed in oceanic basalts worldwide. However, constraining the volumetric abundance of strongly metasomatized oceanic lithosphere, possibly revealed by seismic mid-lithospheric discontinuities, is required before its importance can be established. The existence of K-rich metasomatic components within oceanic plateau lithosphere may also have implications for Archean continental crust formation models. The K-rich mantle component required to explain sanukitoid magma formation associated with continental crust building TTG magmatism may not be restricted to a subduction channel environment, but it can equally reside within overthickened oceanic lithosphere. Thus, oceanic plateaus away from subduction zones could have served as nucleation points for growth of continental crust during the Archean.

Progressive metasomatism of the mantle by kimberlite melts: Sr–Nd–Hf–Pb isotope compositions of MARID and PIC minerals

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks occur as mantle-derived xenoliths in kimberlites and other alkaline volcanic rocks. Both rock types are alkaline and ultramafic in composition. The H2O and alkali metal enrichments in MARID and PIC rocks, reflected in abundant phlogopite, have been suggested to be caused by extreme mantle metasomatism. Radiogenic (Sr–Nd–Hf–Pb) isotope and trace element compositions for mineral separates from MARID (clinopyroxene and amphibole) and PIC (clinopyroxene only) samples derived from Cretaceous kimberlites (Kimberley) and orangeites (Newlands) from South Africa are used here to examine the source(s) of mantle metasomatism. PIC clinopyroxene (n=4) is relatively homogeneous, with narrow ranges in initial isotopic composition (calculated to the emplacement age of the host Bultfontein kimberlite; 87Sr/86Sri: 0.7037–0.7041; εNdi: +3.0 to +3.6; εHfi: +2.2 to +2.5; 206Pb/204Pbi: 19.72–19.94) similar to kimberlite values. This is consistent with PIC rocks representing peridotites modified by intense metasomatic interaction with kimberlite melts.The MARID clinopyroxene (n=9) and amphibole separates (n=11) studied here display broader ranges in isotope composition (e.g., 87Sr/86Sri: 0.705–0.711; εNdi: −11.0 to −1.0; εHfi: −17.9 to −8.5; 206Pb/204Pbi: 17.33–18.72) than observed in previous studies of MARID rocks. The Nd–Hf isotope compositions of kimberlite-derived MARID samples fall below the mantle array (ΔεHfi between −13.0 and −2.4), a feature reported widely for kimberlites and other alkaline magmas. We propose that such displacements in MARID minerals result from metasomatic alteration of an initial “enriched mantle” MARID composition (i.e., 87Sr/86Sri = 0.711; εNdi = −11.0; εHfi = −17.9; and 206Pb/204Pbi = 17.3) by the entraining kimberlite magma (87Sr/86Sr∼i0.704; εNd∼i+3.3; εHf∼i+2.3; 206Pb/204Pb∼i19.7). A model simulating the flow of kimberlite magma through a mantle column, thereby gradually equilibrating the isotopic and chemical compositions of the MARID wall-rock with those of the kimberlite magma, broadly reproduces the Sr–Nd–Hf–Pb isotope compositions of the MARID minerals analysed here. This model also suggests that assimilation of MARID components could be responsible for negative ΔεHfi values in kimberlites. The isotopic composition of the inferred initial MARID end-member, with high 87Sr/86Sr and low εNd, εHf, and 206Pb/204Pb, resembles those found in orangeites, supporting previous inferences of a genetic link between MARID-veined mantle and orangeites. The metasomatic agent that produced such compositions in MARID rocks must be more extreme than the EM-II mantle component and may relate to recycled material that experienced long-term storage in the lithospheric mantle.

New geochemical constraints on the origins of MARID and PIC rocks: Implications for mantle metasomatism and mantle-derived potassic magmatism

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are unusual mantle samples entrained by kimberlites and other alkaline volcanic rocks. The formation of MARID rocks remains hotly debated. Although the incompatible element (for example, large ion lithophile element) enrichment in these rocks suggests that they formed by mantle metasomatism, the layered textures of some MARID samples (and MARID veins in composite xenoliths) are more indicative of formation by magmatic processes. MARID lithologies have also been implicated as an important source component in the genesis of intraplate ultramafic potassic magmas (e.g., lamproites, orangeites, ultramafic lamprophyres), due to similarities in their geochemical and isotopic signatures. To determine the origins of MARID and PIC xenoliths and to understand how they relate to alkaline magmatism, this study presents new mineral major and trace element data and bulk-rock reconstructions for 26 MARID and PIC samples from the Kimberley-Barkly West area in South Africa. Similarities between compositions of PIC minerals and corresponding phases in metasomatised mantle peridotites are indicative of PIC formation by pervasive metasomatic alteration of peridotites. MARID genesis remains a complicated issue, with no definitive evidence precluding either the magmatic or metasomatic model. MARID minerals exhibit broad ranges in Mg# (e.g., clinopyroxene Mg# from 82 to 91), which may be indicative of fractionation processes occurring in the MARID-forming fluid/melt. Finally, two quantitative modelling approaches were used to determine the compositions of theoretical melts in equilibrium with MARID rocks. Both models indicate that MARID-derived melts have trace element patterns resembling mantle-derived potassic magma compositions (e.g., lamproites, orangeites, ultramafic lamprophyres), supporting inferences that these magmas may originate from MARID-rich mantle sources.

Kimberlite-related metasomatism recorded in MARID and PIC mantle xenoliths

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) xenoliths are thought to be formed by intense “primary” mantle metasomatism. These rocks also display secondary features, such as cross-cutting veins and geochemical zonation of matrix minerals, which probably reflect later metasomatic events. To investigate the nature and origin(s) of these secondary features, 28 MARID and PIC xenoliths from southern African kimberlites and orangeites have been studied. MARID-hosted veins contain both carbonate and Ti-rich phases (e.g., titanite, phlogopite), suggesting that they formed by the infiltration of a carbonated silicate melt. Elevated TiO2 contents in MARID matrix mineral rims are spatially associated with carbonate-dominated veins, suggesting a genetic relationship between vein formation and geochemical zonation. Spongy rims around primary MARID and PIC clinopyroxene are depleted in Na2O and Al2O3 relative to their cores, possibly reflecting mineral dissolution in the xenoliths during ascent and emplacement of the entraining kimberlite. The preservation of compositional differences between primary and secondary phases in MARID and PIC xenoliths indicates that metasomatism occurred shortly before, or broadly coeval with, kimberlite/orangeite magmatism; otherwise, at typical mantle temperatures, such features would have quickly re-equilibrated. Increased Na2O in some mineral rims (e.g., K-richterite) may therefore reflect equilibration with a more Na-enriched primitive kimberlite melt composition than is commonly suggested. Vein-hosted clinopyroxene 87Sr/86Sri (0.70539 ± 0.00079) in one MARID sample is intermediate between primary clinopyroxene in the sample (0.70814 ± 0.00002) and the host Bultfontein kimberlite (0.70432 ± 0.00005), suggesting that vein minerals are derived from interactions between primary MARID phases and kimberlite-related melts/fluids. Sulfur isotope compositions of barite (δ34SVCDT = +4.69 ‰) and sulfides (δ34SVCDT = −0.69 ‰) in carbonate veins reflect equilibration at temperatures of 850–900 °C, consistent with sulfur-rich melt/fluid infiltration in the lithospheric mantle. In contrast, vein carbonate C-O isotope systematics (δ13CVPDB = −9.18 ‰; δ18OVSMOW = +17.22 ‰) are not typical of kimberlites or other mantle carbonates (δ13CVPDB = −3 to −8 ‰; δ18OVSMOW = 6 to 9 ‰), and may represent post-emplacement hydrothermal interactions of the cooling kimberlite with crustal fluids. These constraints suggest protracted metasomatism of MARID rocks shortly before and during entrainment by the host kimberlite.

Oxygen and hydrogen isotopic composition of phlogopites and amphiboles in diamond-bearing kimberlite hosted MARID xenoliths: Constraints on fluid-rock interaction and recycled crustal material in the deep continental lithospheric mantle

Abstract MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenoliths are transported from the deep-cratonic lithosphere to the Earth's surface by Cretaceous kimberlites emplaced in the Kimberley area of the Kaapvaal Craton. MARID xenoliths have high modal abundances (70–80 vol%) of mica and amphibole, indicating their origin from a hydrous source. The δ18O values (4.7 ≤ δ18O ≤ 6.9‰) of phlogopite micas from 14 MARID samples indicate that these minerals are both 18O–enriched and 18O–depleted with respect to the average upper mantle δ18O value of 5.8 ± 0.6‰. The range of δ2H values of phlogopites (−83 ≤ δ2H ≤ −53‰, n = 14) of MARID xenoliths are slightly larger than the average mantle δ2H value (−70 ± 10‰). The oxygen (Δ18Ophlogopites-amphibole = −0.4 and 0.4‰) and hydrogen (Δ2Hphlogopite-amphibole = 14 and 36‰) isotopic disequilibrium recorded from two MARID xenoliths suggests the duration of the last isotopic exchange, possibly just before the kimberlite emplacement, between these minerals and metasomatic fluids was too short to reach isotopic equilibrium. Our model calculation indicates that the phlogopites of MARID xenoliths underwent isotopic exchange with fluids of δ18O = 5.5 to 10‰, δ2H = −62 to −90‰. The range of δ18O value of the calculated metasomatic fluids resembles the oxygen isotopic composition of the primary mantle carbonate (~ 6–9‰) suggesting interaction between carbonatic melt and MARID xenoliths in the continental lithospheric mantle. Furthermore, δ18O values of phlogopites together with previously published nitrogen isotope data (−11 ≤ δ15N ≤ 9‰; Banerjee et al., 2015) indicates incorporation of inhomogeneously distributed recycled crustal material from subducted crust within their source magma. Therefore, O-H-N isotope data for MARID xenoliths indicates their crystallization from geochemically heterogeneous magma in the upper continental mantle and subsequent metasomatism with mantle fluids.

Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?

Kimberlites and orangeites (previously named Group-II kimberlites) are small-volume igneous rocks occurring in diatremes, sills and dykes. They are the main hosts for diamonds and are of scientific importance because they contain fragments of entrained mantle and crustal rocks, thus providing key information about the subcontinental lithosphere. Orangeites are ultrapotassic, H2O and CO2-rich rocks hosting minerals such as phlogopite, olivine, calcite and apatite. The major, trace element and isotopic compositions of orangeites resemble those of intensely metasomatized mantle of the type represented by MARID (mica-amphibole-rutile-ilmenite-diopside) xenoliths. Here we report new data for two MARID xenoliths from the Bultfontein kimberlite (Kimberley, South Africa) and we show that MARID-veined mantle has mineralogical (carbonate-apatite) and geochemical (Sr-Nd-Hf-O isotopes) characteristics compatible with orangeite melt generation from a MARID-rich source. This interpretation is supported by U-Pb zircon ages in MARID xenoliths from the Kimberley kimberlites, which confirm MARID rock formation before orangeite magmatism in the area.

Nitrogen isotopic compositions and concentrations in MARID xenoliths

Nitrogen isotopic compositions and concentrations in micas from kimberlite-hosted MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths from the Kimberley area of the Kaapvaal Craton, South Africa, were measured using continuous-flow isotope-ratio mass spectrometry and Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy data suggest that the nitrogen in MARID phlogopite is from ammonium in the mica structure. The replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites. The δ15N values of micas from MARID xenoliths range from −11 to +9‰, and nitrogen contents range between 113 and 272ppm. The 15N enrichment of most samples relative to the average upper mantle δ15N value of −5‰ suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region. The presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub-continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle.

The formation of saline mantle fluids by open-system crystallization of hydrous silicate-rich vein assemblages – Evidence from fluid inclusions and their host phases in MARID xenoliths from the central Kaapvaal Craton, South Africa

The composition of texturally primary fluid inclusions and their host phases clinopyroxene, K-richterite, and zircon were investigated in two MARID-type (mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths sampled by the Kimberley cluster of Cretaceous kimberlites erupted in the central Kaapvaal Craton, South Africa. P–T conditions of crystallization for the MARID assemblages of 4.2GPa and 960°C were estimated based on Ca-in-opx thermometry and the assumption of a 40mW/m2 geotherm applied to two orthopyroxene-bearing MARIDs collected from the same locality. Cooling/heating stage measurements and Raman spectroscopy indicate a fluid system dominated by H2O–NaCl–MgCl2 with variable total salinities in the range ⩽6.4–32.4mass% and minor amounts of MgCl2, the latter inferred from the crystallization of MgCl2×12H2O during cooling of the inclusions. In addition to liquid and vapour, enstatite, baddeleyite, barite, calcite and a K–Ba–Fe–Cr-titanate were identified as solid phases in opened fluid inclusions, indicating high LIL-(HFS) element concentrations in the saline hydrous fluids prior to solid phase precipitation. The Cl contents of apatite (⩽0.35wt.%), phlogopite (⩽0.09wt.%) and K-richterite (⩽0.025wt.%) follow the enrichment pattern Clap≫Clphl>ClKr which is typical for upper mantle rocks. Fluid inclusion-bearing clinopyroxenes show very low H2O contents of ∼45μg/g which is consistent with a reduced aH2O of the fluids due to the presence of NaCl–MgCl2–SiO2–LILE combined with high fO2 and very low Al3+ contents of the clinopyoxenes. The zircons show a complex compositional zoning with variable and positively correlated Y (⩽1260μg/g), P (⩽1870μg/g) and Sc (⩽1373μg/g) contents, indicating a pretulite-type substitution Si4++Zr4+=P5++(Sc, Y, REE)3+. The Sc contents of the zircons are amongst the highest Sc concentrations reported so far for upper mantle silicates. Oxygen isotope analyses of selected zircons yields δ18O values as high as 6.7±0.2 (2σ)‰ and an overall average δ18O of 6.1±0.2‰ which is slightly higher than that of zircon megacryst δ18O of 5.3±0.3‰ reported from worldwide occurrences. The O-isotopic composition of the zircons is consistent with an open-system fractional crystallization of the MARID assemblage in an olivine-absent environment from very small batches of an alkali-rich hydrous saline high-density fluid or fluid-saturated melt. The hydrous saline fluids with LILE-(HFSE)-rich daughter crystals preserved in the two MARID xenoliths indicates that open-system fractional crystallization of MARID-type assemblages can generate small volumes of highly saline hydrous and incompatible trace element-enriched high-density fluids from larger volumes of alkali-rich hydrous melts of group-II kimberlite affinity. MARID crystallization is thought to be just one very specific example of a more general case of open-system crystallization in mantle veins of hydrous silicate-rich assemblages that can concentrate Cl in residual hydrous fluids to brine strength. Inclusions of K–Cl–P–LILE-rich fluids trapped in diamonds from many locations worldwide provide evidence for a widespread operation of these processes in the subcontinental lithospheric mantle.

Vanadium and niobium behavior in rutile as a function of oxygen fugacity: evidence from natural samples

Vanadium occurs in multiple valence states in nature, whereas Nb is exclusively pentavalent. Both are compatible in rutile, but the relationship of V–Nb partitioning and dependence on oxygen fugacity (expressed as fO2) has not yet been systematically investigated. We acquired trace-element concentrations on rutile grains (n = 86) in nine eclogitic samples from the Dabie-Sulu orogenic belt by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and combined them with published results in order to assess the direct and indirect effects of oxygen fugacity on the partitioning of V and Nb into rutile. A well-defined negative correlation between Nb (7–1,200 ppm) and V concentrations (50–3,200 ppm) was found, documenting a competitive relationship in the rutile crystal that does not appear to be controlled by bulk rock or mineral compositions. Based on the published relationship of RtDV and V valence with ∆QFM, we suggest that the priority order of V incorporation into rutile is V4+ > V3+ > V5+. The inferred Nb–V competitive relationship in rutile from the Dabie-Sulu orogenic belt could be explained by decreasing fO2 due to dehydration reactions involving loss of oxidizing fluids during continental subduction: The increased proportion of V3+ (expressed as V3+/∑V) and attendant decrease in RtDV is suggested to lead to an increase in rutile lattice sites available for Nb5+. A similar effect may be observed under more oxidizing conditions. When V5+/∑V increases, RtDV shows a dramatic decline and Nb concentration increases considerably. This is possibly documented by rutile in highly metasomatized and oxidized MARID-type (MARID: mica–amphibole–rutile–ilmenite–diopside) mantle xenoliths from the Kaapvaal craton, which also show a negative V–Nb covariation. In addition, their Nb/Ta covaries with V concentrations: For V concentrations 1,250 ppm, Nb/Ta is considerably lower (5–15). This relationship is mainly controlled by a change in Nb concentrations, suggesting that the indirect dependence of RtDNb on fO2, which is not mirrored in RtDTa, can exert considerable influence on rutile Nb–Ta fractionation.

Hf isotopes of MARID (mica-amphibole-rutile-ilmenite-diopside) rutile trace metasomatic processes in the lithospheric mantle

The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe–multicollector–inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The 176Hf/177Hf ratio shows a wide range, from 0.2811 to 0.2858 (eHf = −55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low 176Hf/177Hf ratios (<0.2812) and Lu/Hf ratios of ∼0.04, that dominated their Hf budget. The MARID rocks were later metasomatized by a fluid and/or melt that had caused the breakdown of eclogitic or lherzolitic garnet with Lu/Hf ratios of ∼0.6, providing a source of highly radiogenic Hf. The low 176Hf/177Hf ratios preserved in some MARID rutiles show that the Nd-Hf isotope systematics of kimberlites and lamproites can be explained by mixing between an asthenospheric melt and the ancient subcontinental lithospheric mantle.

Garnet Lherzolites from the Kaapvaal Craton (South Africa): Trace Element Evidence for a Metasomatic History

Kimberlites from the Kaapvaal craton have sampled numerous mantle garnet lherzolites in addition to garnet harzburgites. Trace element characteristics of constituent clinopyroxenes allow two groups of garnet lherzolites to be distinguished. Trace element compositions of all clinopyroxenes are characterized by enrichment in light rare earth elements (LREE) and large ion lithophile elements and by a relative depletion in Ti, Nb, Ta, and to a lesser extent Zr and Hf. However, the LREE enrichment and the depletion in Nb and Zr (Hf) are less in the Type 1 clinopyroxenes than in the Type 2 clinopyroxenes. Our study suggests that the two melts responsible for the metasomatic imprints observed in the two garnet lherzolite groups are highly alkaline mafic silicate melts. Type 1 clinopyroxenes that have trace element similarities to those of PIC (Phlogopite‐ Ilmenite‐Clinopyroxene) rocks appear to have crystallized from, or been completely equilibrated with, the same melt related to Group I kimberlite magma. The Type 2 clinopyroxenes have trace element similarities to those of MARID (Mica‐ Amphibole‐Rutile‐Ilmenite‐Diopside) rocks and are therefore probably linked to melt related to Group II kimberlite magma.

Australia: Mansfield — kaolin

Nitrogen isotopic compositions and concentrations in micas from kimberlite-hosted MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths from the Kimberley area of the Kaapvaal Craton, South Africa, were measured using continuous-flow isotope-ratio mass spectrometry and Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy data suggest that the nitrogen in MARID phlogopite is from ammonium in the mica structure. The replacement of potassium by ammonium is the major mechanism for the storage of nitrogen within these phlogopites. The δ15N values of micas from MARID xenoliths range from − 11 to + 9‰, and nitrogen contents range between 113 and 272 ppm. The 15N enrichment of most samples relative to the average upper mantle δ15N value of − 5‰ suggests that the nitrogen reflects a recycled reservoir from subducted material within their source region. The presence of nitrogen from a recycled reservoir indicates that sedimentary organic nitrogen in the sub-continental lithosphere is not effectively cycled to the surface resulting in a geochemically heterogeneous upper mantle.