Tertiary basalts and peridotite xenoliths from the Hessian Depression (NW Germany), reflecting mantle compositions low in radiogenic Nd and Sr

A total of 17 alkali basalts (alkali olivine basalt, limburgite, olivine nephelinite) and quartz tholeiites, and of 10 peridotite xenoliths (or their clinopyroxenes) were analyzed for Nd and Sr isotopes. ¹⁴³Nd/¹⁴⁴Nd ratios and ⁸⁷Sr/⁸⁶Sr ratios of all basalts and of the majority of ultramafic xenoliths plot below the mantle array with a large variation in Nd isotopes and a smaller variation in Sr isotopes. The tholeiites were less radiogenic in Nd than the alkali basalts. Volcanics from the Eifel and Massif Central regions contain Nd and Sr, which is more radiogenic than that of the basalts from the Hessian Depression. Nd and Sr isotopic compositions of all rocks from the latter area, with the exception of one tholeiite and one peridotite plot in the same field of isotope ratios as the Ronda ultramafic tectonite (SW Spain), which ranges in composition from garnet to plagioclase peridotite. The alkali basaltic rocks are products of smaller degrees of partial melting of depleted peridotite, which has undergone a larger metasomatic alteration compared with the source rock of tholeiitic magmas. For the peridotite xenoliths such metasomatic alteration is indicated by the correlation of their K contents and isotopic compositions. We assume that the upper mantle locally can acquire isotopic signatures low in radiogenic Nd and Sr from the introduction of delaminated crust. Such granulites low in radiogenic Nd and Sr are products of early REE fractionation and granite (Rb) separation.     

Geochemistry and origin of the Mirny field kimberlites,Siberia

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

     

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths.

The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.     

Spinel peridotite xenoliths from the Atsagin-Dush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle

Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO₂-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ³⁴S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996     

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) (La_N/Yb_N = 5–62; Ce_N/Sm_N = 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 FeO_t (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.     

Characterising modal metasomatic processes in young continental lithospheric mantle: a microsampling isotopic and trace element study on xenoliths from the Middle Atlas Mountains,Morocco

Clinopyroxene is a major host for lithophile elements in the mantle lithosphere, and therefore it is critical whether we are to understand the constraints that this mineral puts on mantle evolution and melt generation. This study presents a detailed in situ trace element and Sr isotope study of clinopyroxene, amphibole and melt from two spinel lherzolites from the Middle Atlas Mountains, Morocco. The results show that there is limited, but discernable, Sr isotopic variation between clinopyroxene crystals within these xenoliths [⁸⁷Sr/⁸⁶Sr ranging from 0.703416 (±11 2SE) to 0.703681 (±12 2SE)]. Trace element patterns show similar interelement fractionation with LREE enrichment, but there is a considerable range in terms of elemental concentration (e.g. over 100 ppm in Sr concentrations). Observed modal clinopyroxene is far more abundant than that predicted from estimates of melt depletion. This along with isotope and trace element variability found in these xenoliths supports a multistage metasomatic process in which clinopyroxene and amphibole are recent secondary additions to the lithospheric mantle. Elemental systematics indicate that the metasomatic mineral assemblage has most recently equilibrated with a carbonatitic melt prior to inclusion in the host basalt. The clinopyroxene from this study is typical of global off-craton clinopyroxene in terms of Sr isotope composition, suggesting that the majority of clinopyroxene in off-craton settings may have a recent metasomatic origin. These findings indicate that caution is required when using peridotite xenoliths to estimate the degree of elemental enrichment in the subcontinental lithosphere.     

Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes

A suite of metasomatised xenoliths from the Letlhakane kimberlite (Botswana) forms a metasomatic sequence from garnet peridotite to garnet phlogopite peridotite to phlogopite peridotite. Before the modal metasomatism, most of the Letlhakane xenoliths were depleted harzburgites that had been subjected to an earlier cryptic metasomatic event. Modal phlogopite and clinopyroxene - Cr-spinel increase at the expense of garnet and orthopyroxene with increasing degrees of metasomatism. The most metasomatised xenolith is a wehrlite. With progressive modal metasomatism, the clinopyroxene becomes enriched in Sr, Sc and the LREE, orthopyroxene becomes depleted in Ca and Ni, but enriched in Al and Mn, and olivine becomes depleted in Al and V. Garnet chemical composition largely remains unchanged. The garnet replacement reaction seen in most xenoliths allows the measurement of the flux of trace elements through detailed modal analysis of the pseudomorphs. Mass balance calculations show that the modally metasomatised rocks became enriched in incompatible elements such as Sr, Na, K, the LREE and the HFSE (Ti, Zr and Nb). Major elements (Al, Cr and Fe) and garnet-compatible trace elements (V, Y, Sc, and the HREE) were removed during this metasomatic process. The modal metasomatism caused a strong depletion in Al, and the results challenge previous suggestions that this metasomatic process merely occurred within an Al-poor environment. The data suggest that the xenoliths represent the mantle wallrock adjacent to a major conduit for an alkaline basic silicate melt (with high contents of volatile and incompatible elements). The volatile and incompatible element-enriched component of this melt percolated into the wallrock along a strong temperature gradient and caused the observed range of metasomatism.     

Sr-Nd-Pb Isotopic Compositions of Peridotite Xenoliths from Spitsbergen: Numerical Modelling Indicates Sr-Nd Decoupling in the Mantle by Melt Percolation Metasomatism

Several spinel peridotite xenoliths from Spitsbergen have Sr–Ndisotopic compositions that plot to the right of the ‘mantlearray’ defined by oceanic basalts and the DM end-member(depleted mantle, with low ⁸⁷Sr/⁸⁶Sr and high ¹⁴³Nd/¹⁴⁴Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to Sr–Nd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. ‘Chromatographic’effects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing Sr–Nd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, Sr–Nd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling     

Geochemical Records of Mantle Processes in Mantle Xenoliths from Three Cenozoic Basaltic Volcanoes in Eastern China

Rare earth element (REE) contents, and Sr and Nd isotopic compositions were measured for three suites of mantle xenoliths from the Kuandian, Hannuoba and Huinan volcanoes in the north of the Sino-Korean Platform. From the correlations of Yb contents with Al/Si and Ca/Si ratios, the peridotites are considered to be the residues of partial melting of the primitive mantle. The chondrite-normalized REE compositions are diverse, varying from strongly LREE-depleted to LREE-enriched, with various types of REE patterns. Metasomatic alteration by small-volume silicate melts, of mantle peridotites previously variably depleted due to fractional melting in the spinel peridotite field, can account for the diversity of REE patterns. The Sr/ Ba versus La/Ba correlation indicates that the metasomatic agent was enriched in Ba over Sr and La, suggestive of its volatile-rich signature and an origin by fluid-triggered melting in an ancient subduction zone. The Sr and Nd isotopic compositions of these xenoliths, even from     

Petrological, geochemical and isotopic characteristics of the lithospheric mantle beneath Sardinia (Italy) as indicated by ultramafic xenoliths enclosed in alkaline lavas

Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)_N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (⁸⁷Sr/⁸⁶Sr from 0.70385 to 0.70568 and ¹⁴³Nd/¹⁴⁴Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.     

Phlogopite in mantle xenoliths and kimberlite from the Grib pipe,Arkhangelsk province,Russia:Evidence for multi-stage mantle metasomatism and origin of phlogopite in kimberlite

We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.     

Petrology and geochemistry of peridotite xenoliths from the Letlhakane kimberlites, Botswana

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m² continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996     

Ancient melt depletion overprinted by young carbonatitic metasomatism in the New Zealand lithospheric mantle

Spinel facies dunite, harzburgite, lherzolite and wehrlite mantle xenoliths from a cluster of Miocene volcanoes in southern New Zealand preserve evidence of the complex evolution of the underlying continental mantle lithosphere. Spinel Cr# records melt extraction with some values indicative of near complete removal of clinopyroxene. LREE-enriched, low Ti/Eu and low Al₂O₃ clinopyroxene and rare F-, LREE-rich apatite indicates subsequent interaction between peridotite and a metasomatising carbonatitic melt. The clearest metasomatic signature occurs in the formerly highly depleted samples because there was little or no pre-existing clinopyroxene to dilute the carbonatite signature. For the same reason, the isotopic character of the metasomatising agent is best observed in the formerly highly depleted peridotites (⁸⁷Sr/⁸⁶Sr = 0.7028–0.7031; ¹⁴³Nd/¹⁴⁴Nd = 0.5129; ²⁰⁶Pb/²⁰⁴Pb = 20.2–20.3). These isotope ratios are very close to, but slightly less radiogenic than, the HIMU end-member mantle reservoir. Nd isotope data imply carbonatite metasomatism occurred within the last several hundred million years, with ubiquitous pyroxene core-to-rim Al diffusion zoning indicating that it must pre-date cooling of the lithospheric mantle following Late Cretaceous–Eocene rifting of Zealandia from Gondwana. Metasomatism was significantly younger than ancient Re-depletion ages of ~2 Ga and shows that decoupling of peridotite isotope systems has occurred.     

Phase Relations in the Harzburgite–Hydrous Carbonate Melt at 5.5–7.5 GPa and 1200–1350°С

Phase relations are studied experimentally in the harzburgite–hydrous carbonate melt system, the bulk composition of which represents primary kimberlite. Experiments were carried out at 5.5 and 7.5 GPa, 1200–1350°С, and \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) = 0.39–0.57, and lasted 60 hours. It is established that olivine–orthopyroxene–garnet–magnesite–melt assemblage is stable within the entire range of the studied parameters. With increase of temperature and \({{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}\) in the system, Ca# in the melt and the olivine fraction in the peridotite matrix significantly decrease. The composition of silicate phases in run products is close to those of high-temperature mantle peridotite. Analysis of obtained data suggest that magnesite at the base of subcontinental lithosphere could be derived by metasomatic alteration of peridotite by asthenospheric hydrous carbonate melts. The process is possible in the temperature range typical of heat flux of 40–45 mW/m², which corresponds to the conditions of formation of the deepest peridotite xenoliths. Crystallization of magnesite during interaction with peridotite matrix can be considered as experimentally substantiated mechanism of CO₂ accumulation in subcratonic lithosphere.     

Ultramafic Xenoliths in Plio-Pleistocene Alkali Basalts from the Eastern Transylvanian Basin: Depleted Mantle Enriched by Vein Metasomatism

Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. ⁸⁷Sr/⁸⁶Srand ¹⁴³Nd/^l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower ^l43Nd/^l44Nd and higher ⁸⁷Sr/Sr⁸⁶values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has ⁸⁷Sr/Sr⁸⁶ and ¹⁴³Sr/Sr¹⁴⁴ ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal ^*corresponding author     

Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province,Russia)

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO₂, Cr₂O₃, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.     

Unravelling the effects of melt depletion and secondary infiltration on mantle Re-Os isotopes beneath the French Massif Central

Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt/fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7-46.1 wt.%; CaO 0.9-3.6 wt.%), and many samples yield unradiogenic bulk-rock Os isotope ratios (¹⁸⁷Os/¹⁸⁸Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt/fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and/or homogenised average of these multiple events.Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from two populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic ¹⁸⁷Os/¹⁸⁸Os, and (ii) primary sulfides with high [Os] and unradiogenic ¹⁸⁷Os/¹⁸⁸Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for >97% of bulk-rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).     

Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism

The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low ⁸⁷Sr/⁸⁶Sr (0.70294 to 0.70342) and high ɛ_Nd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low ⁸⁷Sr/⁸⁶Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO₂-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000     

Radiogenic helium in xenoliths from Simcoe, Washington, USA: implications for metasomatic processes in the mantle wedge above subduction zones

We report ³He/⁴He ratios from 10 peridotite xenoliths considered to represent samples of the uppermost mantle wedge above the downgoing Juan de Fuca Plate. Helium isotopic ratios in all but two of the xenoliths are similar to many arc magmas, roughly 7 Ra (1 Ra=atmospheric value). Based on decoupling of He from Sr, Nd and Os in these samples, similar He ratios in olivines from rims of larger xenoliths, and modeling of helium exchange between xenoliths and magmas, we interpret this ratio as that of helium in the host basalt. ³He/⁴He ratios as low as 4.2 Ra are found in olivines from the cores of the two largest xenoliths. These results cannot be reasonably explained by interaction with crustal material or post-eruptive ingrowth of ⁴He, but have been produced by interaction between mantle peridotite and a ⁴He-rich melt or fluid. Either ⁴He already present in the subducting oceanic crust has been retained to significant depths below Simcoe and then directly released behind the arc to interact with the mantle wedge, or, more likely, ⁴He has been produced by decay of U and Th in metasomatized mantle directly above the slab; a He-rich fluid or melt from this source has then ascended and modified the region of mantle represented by the xenoliths. This latter model is supported by estimates of residence time for the Simcoe metasomatic agent from U–Th–Pb isotopic systematics of pyroxenes from the Simcoe peridotites, estimated U and Th concentrations in the source of the fluid or melt, and commonly assumed patterns of helium behavior. This model is also consistent with higher ³He/⁴He ratios typically measured in arc samples; the portion of sub-arc mantle with such low He isotope ratios may be quite small, but the Simcoe xenoliths record a much larger volumetric contribution of the He-rich metasomatic agent than do arc lavas.     

Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt,Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths

In situ trace element analyses of constituent minerals in mantle xenoliths occurring in an alnöite diatreme and in nephelinite plugs emplaced within the central zone of the Damara Belt have been determined by laser ablation ICP-MS. Primitive mantle-normalized trace element patterns of clinopyroxene and amphibole indicate the presence of both depleted MORB-like mantle and variably enriched mantle beneath this region. Clinopyroxenes showing geochemical depletion have low La/Sm_n ratios (0.02–0.2), whereas those showing variable enrichment have La/Sm_n ranging up to 3.8 and La/Yb_n to 9.1. The most enriched clinopyroxenes coexist with amphibole showing similar REE patterns (La/Sm_n = 1.3–4.1; La/Yb_n = 4.5–9). Primitive mantle-normalized trace element patterns allow further groups to be distinguished amongst the variably enriched clinopyroxenes: one having strong relative depletion in Rb–Ba, Ta–Nb and relative enrichment in Th–U; another with similar characteristics but with additional strong relative depletion in Zr–Hf; and one showing no significant anomalies. Amphiboles show similar normalized trace element patterns to co-existing clinopyroxene. Clinopyroxene and amphiboles showing LREE_N enrichment have high Sr and low Nd isotope ratios compared to clinopyroxene with LREE-depleted patterns. Numerical simulation of melt percolation through the mantle via reactive porous flow is used to show that the chromatographic affect associated with such a melt migration process is able to account for the fractionation seen in La–Ce–Nd in cryptically metasomatized clinopyroxenes in Type 1 xenoliths, where melt–matrix interactions occur near the percolation front, whereas REE patterns in clinopyroxenes proximal to the source of metasomatic melt/fluid match those found in modally metasomatized Type 2 xenoliths. The strong fractionation between Rb–Ba, Th–U and Ta–Nb shown by some cryptically metasomatized xenoliths can be also accounted for by reactive porous flow, provided amphibole crystallizes from the percolating melt/fluid close to its source. The presence of amphibole in vein-like structures in some xenoliths is consistent with this interpretation. The strong depletion in Zr–Hf in clinopyroxene and amphibole in some xenoliths cannot be accounted for by melt migration processes and requires metasomatism by a separate carbonate-rich melt/fluid. When taken together with published isotope data on these same xenoliths, the source of metasomatic enrichment of the previously depleted (MORB-like) sub-Damaran lithospheric mantle is attributed to the upwelling Tristan plume head at the time of continental breakup.