Degrees Of Metasomatism Research Articles

Reaction between volatile-bearing eclogite and harzburgite as a function of degree of interaction: Experimental constraints at 4 GPa

Abstract The mantle is known to be heterogeneous, mainly composed of peridotite and eclogite. Eclogite-derived hydrous melts may interact with harzburgite at the slab-mantle interface in subduction zones or in the sub-continental lithospheric mantle. In this study, such interactions were simulated by performing hybridization experiments in which a layer of eclogite was juxtaposed to a layer of harzburgite in the presence of H2O-CO2 at 4 GPa and 1200 °C, conditions where eclogite is super-solidus while harzburgite is sub-solidus. A diamond trap was placed in between the two layers to trap the fluid or melt phase, allowing direct determination of their composition. The multi-anvil was rotated at different frequencies to examine the effect of increasing degree of interaction on the melt composition as well as the mineral compositions. The interaction of eclogite-derived hydrous melt and harzburgite results in a reaction layer at the interface between the two lithologies, composed of Opx and garnet. The harzburgite above the reaction layer is metasomatized, containing various amounts of olivine, Opx, Cpx, and garnet. The eclogitic melt is modified during this interaction. With increasing interaction, a thicker reaction layer is formed. Both the eclogitic and the peridotitic garnet compositions approach each other and become intermediate between the composition of the garnet in the eclogite+H2O+CO2 system and the garnet in the harzburgite+H2O+CO2 system at these conditions. The Mg# of the peridotitic olivine and Opx decreases with increasing interaction. The initial basaltic melt in equilibrium with eclogite is metaluminous, turning to a peralkaline melt with increasing interaction with the harzburgite. The metasomatizing effect of the eclogite-derived hydrous melt on the harzburgite is observed by increasing the mode of the peridotitic Opx, Cpx, and garnet at the expense of peridotitic olivine and eclogitic garnet. A slight increase in melt fraction occurs as well. This interaction also results in a gradient in the log fO2. Relatively more oxidizing conditions occur near the reaction layer, becoming more reduced into the peridotite, suggesting that the reaction zones act as partial barriers for the melt to travel through the peridotite. Increased interaction leads to higher log fO2 values. These experiments demonstrate the influence of the degree of interaction on the range of melt compositions found in volcanic arcs as well as the degree of metasomatism in the mantle found in the sub continental lithospheric mantle.

Subduction-related mantle accretion and makeover revealed by mantle xenoliths at the Pacific margin of NE Eurasia

Subduction is thought to be a major factor in the origin and makeover of continental lithospheric mantle, but essential aspects of these processes continue to be debated. Here, we report textural, chemical and Re Os isotope data on mantle xenoliths from far eastern Russia located at different distances from the Asian Pacific coast to examine how the proximity to subduction zones affects lithospheric mantle compositions. The xenoliths come from the Khanka block close to the border with China and several sites in the Sikhote-Alin orogenic belt (SAOB) near the Japan Sea coast. The inboard Khanka xenoliths are spinel lherzolites with chemical compositions, suggesting that they are residues of low- to moderate-degrees of melt extraction from fertile mantle and essentially unaffected by later events for 1.9 Ga. Such rocks are also common among peridotites in the SAOB, located closer to the subduction margins, but the SAOB suite also includes: a) heterogeneous rocks with veins and patches of pyroxenites or hydrous minerals, and b) rocks enriched in iron (8.8–13.5 wt%), Al 2 O 3 and incompatible minor elements. The positive correlation between elevated 187 Os/ 188 Os ratios (>0.135) and FeO contents in some Fe-rich SAOB xenoliths is interpreted to indicate that the metasomatic melts were derived from the asthenosphere hybridized with recycled materials from the Pacific slab. The lack of correlation between Re -depletion Os model ages (T RD ) and depletion indices, as well as the similarity of Os isotopic compositions between the SAOB peridotites and abyssal peridotites are interpreted to show that the SAOB mantle possibly formed and accreted to Eurasia from the asthenosphere during eastward migration of the Paleo-Pacific subduction trench. Based these results, we infer that the SAOB lithospheric mantle experienced multi-stage metasomatism after its formation, the most significant of which is localized infiltration of subduction-related basaltic melts with recycled characteristics induced by the extension of Japan Sea and release of volatiles from the slab. By contrast, the inboard Khanka lithospheric mantle shows little, if any, evidence for metasomatism. Such contrasting metasomatic modification implies different degrees and styles of makeover processes at different distances relative to the subduction margins. • The Khanka mantle was little affected by metasomatism since it was formed at ~1.9 Ga. • The Sikhote-Alin lithospheric mantle was formed by subduction and accretion. • Mantle metasomatism beneath Sikhote-Alin was caused by melts with recycled signature. • The nature and degrees of metasomatism are related to the distance from subduction front.

Major and rare earth element mineral chemistry of low-grade assemblages inform dynamics of hydrothermal ocean-floor metamorphism in the Dinaridic Neotethys

AbstractThis contribution provides insights into ocean-floor hydrothermal metamorphism of the fast-evolving Dinaridic Neotethys. Mineralogical, geochemical and Sr isotope data collected from altered ophiolites and non-ophiolite basalts/andesites and tuffs of the active continental margin are consistent with hydrothermal alteration trajectories that reflect the host-rock composition. This suggests that hydrothermal fluxes were restricted within a simple closed seawater-fed system. Based on the initial isotopic ratios of Sr, two fluid–rock interaction trends are established: (a) low-to-medium degrees of metasomatism in pre-Middle Jurassic anorogenic ophiolites that progressively abated, and (b) increased intensities of metasomatism in post-Middle Jurassic orogenic ophiolites. This agrees with chlorite thermometry and Ca-Al-(Fe)-silicate phase chemistry. The metamorphic assemblages belong to the zeolite, prehnite-pumpellyite, prehnite-actinolite and greenschist facies. The facies is reliant on the temperature of hydrothermal systems and their fluid chemistry. Rare earth element (REE) phase geochemistry shows (a) variable fluid–rock ratios in chlorite and pumpellyite dependent on fluid temperatures, (b) prominent Eu and Ce anomalies that reflect the fluid oxidation state, (c) light REE/heavy REE mobilization attributed to prevalent ligand complexation, and (d) multi-phase fluid percolation across reaction zones of heterogeneous permeability. This study proposes initiation of simple hydrothermal system(s) at or near a spreading centre(s) in the infancy of the Dinaridic Neotethys. Such a system became more complex during Middle Jurassic and Early Cretaceous time with reactive hydrothermal fluids passing the recharge area and reaching the hot reaction zone. An abrupt obliteration of the established high-temperature regime ensued, following the final closure of the Neotethys.

The Relationship Between Kimberlitic Magmatism and Electrical Conductivity Anomalies in the Mantle

AbstractKimberlites are igneous rocks whose formation remains enigmatic due to their severely altered nature, highly variable compositions and rapid ascent through the lithosphere. The spatiotemporal distribution of kimberlites suggests that mantle metasomatism may play an essential role in their emplacement. Since the magnetotelluric (MT) method is sensitive to metasomatic mantle modification in the forms of interconnected minerals and water, we compiled MT models from Australia, Brazil, Botswana, Canada, Namibia, South Africa and the USA, calculated water content variations and compared them to the distribution of kimberlites. Results indicate that kimberlites mostly ascend through hydrated/metasomatized lithosphere and avoid both dry and heavily metasomatized lithosphere. Diamondiferous kimberlites preferentially occur on moderately metasomatized lithosphere, with the diamondiferous rate increasing with the degree of metasomatism in Archean terranes and decreasing in Proterozoic terranes. These results link geophysical observations of mantle metasomatism to kimberlite magmatism and will improve mineral systems models for diamond exploration.

Geology and genesis of the Cihai mafic intrusions in Beishan Terrane, Xinjiang, Northwest China: Implication for iron mineralization and tectonic setting

The Cihai Fe deposit (100 Mt @ 48% Fe) represents an economically important iron deposit in Beishan Terrane, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Many Late Carboniferous to Permian Fe(-Cu) deposits (e.g., Yamansu, Shaquanzi, and Weiya) are also present in adjacent Eastern Tianshan orogenic belt. The Cihai deposit is hosted in mafic dykes (i.e., diabase) and in the contact zone between the mafic intrusion and Mesoproterozoic schist. New zircon U–Pb isotope dating in this study gives a crystallization age of 306.9 ± 3.2 Ma for the ore-bearing diabase, which is the first age-constrained Late Carboniferous mafic magmatic activity at Cihai district. In combination with previous Ar–Ar amphibole dating (281.9 ± 2.2 Ma) for the massive ores, it is suggested that the main iron mineralization and associated mafic magmatic activities occurred at ca. 306–282 Ma, coeval with the large-scale iron mineralization epoch (ca. 324–280 Ma) in Eastern Tianshan. The Cihai diabase samples are LILEs-rich (e.g., K, Rb, and Sr) and HFSEs-depleted (e.g., Nb, Ta, and Ti) calc-alkaline series, and exhibit high Mg# values (51–63), as well as low compatible element concentrations (Ni = 14–53 ppm; Co = 18–43 ppm; and Cr = 26–219 ppm), indicating a mantle wedge source. They have elevated Ba/La and Sr/Nd values with limited Th/Yb ratios (<1), and display slightly Eu anomalies (0.90–1.19). Isotopically, they are characterized by positive εHf(t) values (+13.0 to + 16.1) and young Hf model ages (300 to 420 Ma), which are broadly similar to the isotopic compositions of the depleted mantle and within the ranges of most other mafic–ultramafic intrusions in Beishan-Tianshan orogen. These features suggest that the Cihai diabase intrusions were most likely formed by partial melting of a mantle wedge with some degree of metasomatism by slab-derived fluids. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the earliest generation of mafic dykes formed in an arc setting related to southward subduction of the paleo-Tianshan Ocean at ca. 306 Ma, and the main iron mineralization and intensive mafic magmatism occurred in a subsequent post-collisional regime during the Early Permian.

MANTLE TERRANES OF THE SIBERIAN CRATON: THEIR INTERACTION WITH PLUME MELTS BASED ON THERMOBAROMETRY AND GEOCHEMISTRY OF MANTLE XENOCRYSTS

We have studied variations in the structure and composition of minerals from the pipes of the Yakutian kimberlite province (YKP) in different mantle terranes of the Siberian craton. The study was based on an extensive database, including the microprobe analysis datasets consolidated by IGM, IG, IEC and IGDNM SB RAS and ALROSA and geochemical analysis of minerals performed by LA‐ICP‐MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). The reconstruction shows layering under the tubes, including 6–7 slab that were probably formed due to subduction; the slabs are separated by pyroxenitic, eclogitic and metasomatic layers and dunite lenses. Transects and mantle profiles across kimberlite fields are constructed. Within the limits of the revealed tectonic terranes, we assume a collage of microplates formed in the early – middle Archean. Extended submeridional structures of the tectonic terranes are not always confirmed at the mantle level. Beneath the Anabar and Aldan shields, the mantle sections show more coarse layers and 3–4 large horizons of dunites with garnet and pyroxene nests separated by ilmenite‐ phlogopite metasomatites and pyroxenites. Terranes representing the suture zones between the protocratons (e.g. Khapchan) are often saturated with eclogites and pyroxenites that may occur as leghthy ascending bodies of magmatic eclogites penetrating through the mantle lithosphere structure (ML). A nearly ubiquitous pyroxenite layer at the level of 3.5–4.5 GPa formed probably in the early Archean with a high heat flux during melting of eclogites and was subsequently traced by plume melts. Within the early Archean protocratons – granite‐greenstone terranes (Tungus, Markha, Berekta, and Sharyzhalgai, ~3.8–3.0 Gyr [ Gladkochub et al., 2019 ], the mantle lithosphere is less depleted and largely metasomatized. The ML structure of the Daldyn and Magan granulite‐orthogneiss terranes is layered with folding revealed in the north‐to‐south sections from the Udachnaya pipe to the Krasnopresnenskaya pipe, which is less pronounced in the latitudinal direction. From the Daldyn field to the Alakit field, there is an increase in the degree of metasomatism, and higher alkalinity of pyroxenes and larger amounts of phlogopite are noted. The most productive Aikhal and Yubileinaya pipes are confined to a dunite core, which is accompanied by a change in the specialization of high‐charge elements Ta‐Nb to Zr‐Hf. Within the limits of the Magan terrane, the thin‐layer structure of the middle and upper parts of the craton keel is replaced with a sharply depleted productive horizon at its base. The mantle of the granite‐greenstone Markha terrrein comprises eclogite (often pelitic) horizons, which suggests subduction of the continental lithosphere or sediments. In the central and northern parts of the Siberian craton, most structures in the mantle are sinking to the west at small angles. The geochemistry features of garnets and pyroxenes from various mantle terranes are considered in detail.

Mantle source heterogeneity in subduction zones: constraints from elemental and isotope (Sr, Nd, and Pb) data on Vulcano Island, Aeolian Archipelago, Italy

Vulcano is part of the Aeolian volcanic arc in the southern Tyrrhenian Sea. Its products were emplaced through multiple episodes of edifice building and collapse since about 120 ka B.P. to present. A major discontinuity in the activity occurred after about 28 ka, while the focus of volcanism moved from SE to NW. The older products are basalts to shoshonites, and have lower K2O than the younger ones, shoshonites to rhyolites. Between these two groups, Lentia latites-rhyolites, Spiaggia Lunga basalts, and Quadrara shoshonite-trachytes, erupted along the western side of Vulcano Island. The Spiaggia Lunga basalts (i) are the most primitive magmas erupted at Vulcano after 28 ka (ii) mark the change between older and younger phases, and (iii) overlap geochemically with a monzogabbroic intrusion of similar age. This work focuses on the Spiaggia Lunga products, discussed within a large dataset of geochemical and radiogenic isotope analyses on the entire Vulcano sequence. Older products have more primitive geochemical and isotope characteristics, and lower incompatible element contents, than younger ones. The Spiaggia Lunga basalts exhibit intermediate geochemical characteristics between the older and the younger groups, and can likely be regarded as a third magmatic phase, which represents a distinct mantle reservoir active during the magmatic history of Vulcano. Significant variations of Sr, Nd, and Pb isotope ratios, and isotopic disequilibrium between phenocrysts and groundmass, are present among the Vulcano products. This variability suggests crustal assimilation in shallow-level magma chambers, which also accounts for the formation of evolved products by combined assimilation and fractional crystallization, particularly in the younger series. Considering only the mafic products, incompatible element patterns with high LILE/HFSE and enriched signatures of Sr, Nd, and Pb isotope ratios, indicate enriched mantle sources. Besides, chemical and isotope variability among older, younger, and Spiaggia Lunga mafic rocks, suggests an origin from geochemically diverse primary melts, derived from distinct mantle reservoirs. Their parent magmas, based on geochemical and isotope patterns, were from both MORB- and OIB-type mantle sources, subject to variable degrees of metasomatism by subducted sediments.

Near-Infrared Spectroscopy of Hydrothermal versus Low-Grade Metamorphic Chlorites

Although the composition of chlorite group minerals represents a known proxy for conditions in various geological environments, few comparative studies of chlorites from different geological environments have been carried out. In this study, we compare chlorites from a hydrothermal system with those formed under low-grade metamorphic conditions. Both sets of samples were collected from the Pilbara Craton, Western Australia. Near-infrared (NIR) spectroscopy was used to determine if spectral differences record compositional differences between chlorites from the two geological environments. The spectra showed a significant difference in the Mg-OH absorption wavelength, near 2350 nm, with the hydrothermal group showing longer absorption wavelengths than the metamorphic one. A comparison of the spectral data with geochemical analyses showed a relation between the absorption wavelength and the magnesium-to-iron ratio (magnesium number) of chlorite, as well as the bulk rock composition. Metamorphic rocks have a higher magnesium-to-iron ratio than the hydrothermal ones, predominantly explained by differences in the degree of metasomatism. In the hydrothermal system, mass transfer changes the bulk rock composition while for the metamorphic samples the original bulk chemistry determines the current composition of the rock.

Alakit and Daldyn kimberlite fields, Siberia, Russia: Two types of mantle sub-terranes beneath central Yakutia?

Mineral data from Yakutian kimberlites allow reconstruction of the history of lithospheric mantle. Differences occur in compositions of mantle pyropes and clinopyroxenes from large kimberlite pipes in the Alakit and Daldyn fields. In the Alakit field, Cr-diopsides are alkaline, and Stykanskaya and some other pipes contain more sub-calcic pyropes and dunitic-type diamond inclusions, while in the Daldyn field harzburgitic pyropes are frequent. The eclogitic diamond inclusions in the Alakit field are sharply divided in types and conditions, while in the Daldyn field they show varying compositions and often continuous Pressure–Temperature (P–T) ranges with increasing Fe# with decreasing pressures. In Alakit, Cr-pargasites to richterites were found in all pipes, while in Daldyn, pargasites are rare Dalnyaya and Zarnitsa pipes. Cr-diopsides from the Alakit region show higher levels of light Rare Earth Elements (LREE) and stronger REE-slopes, and enrichment in light Rare Earth Elements (LREE), sometimes Th-U, and small troughs in Nb-Ta-Zr. In the Daldyn field, the High Field Strength Elements HFSE troughs are more common in clinopyroxenes with low REE abundances, while those from sheared and refertilized peridotites have smooth patterns. Garnets from Alakit show HREE minima, but those from Daldyn often have a trough at Y and high U and Pb. PTXfO2 diagrams from both regions show similarities, suggesting similar layering and structures. The degree of metasomatism is often higher for pipes which show dispersion in P–Fe# trends for garnets. In the mantle beneath Udachnaya and Aykhal, pipes show 6–7 linear arrays of P–Fe# in the lower part of the mantle section at 7.5–3.0 GPa, probably reflecting primary subduction horizons. Beneath the Sytykanskaya pipe, there are several horizons with opposite inclinations which reflect metasomatic processes. The high dispersion of the P–Fe# trend indicating widespread metasomatism is associated with decreased diamond grades. Possible explanation of the differences in mineralogy and geochemistry of the mantle sections may relate to their tectonic positions during growth of the lithospheric keel. Enrichment in volatiles and alkalis possibly corresponds to interaction with subduction-related fluids and melts in the craton margins. Incorporation of island arc peridotites from an eroded arc is a possible scenario.

Trace element partitioning and Lu–Hf isotope systematics in spinel peridotites from the Rio Grande Rift and Colorado Plateau: Towards improved age assessment of clinopyroxene Lu/Hf–176Hf/177Hf in SCLM peridotite

Our study of Colorado Plateau and Rio Grande rift spinel peridotite xenoliths determined bulk Lu–Hf budgets and cpx, opx, and whole rock 176Hf/177Hf to evaluate the potential age significance of Lu/Hf–Hf isotope correlations in sub-continental lithospheric mantle (SCLM) derived xenoliths. The samples have fertilities (spinel Cr# range from 0.1 to 0.5), and equilibration temperatures (950–1050°C) that overlap with those of spinel peridotites commonly used in Lu–Hf dating studies. The Lu/Hf of clinopyroxene (cpx) and its associated whole rock (WR) are similar in fertile samples, but cpx has a lower Lu/Hf than that of the WR in refractory samples. Orthopyroxene (opx) has systematically higher Lu/Hf than cpx, but in many samples, the opx and cpx have identical 176Hf/177Hf, suggesting that the Hf isotope composition of these minerals is supported by the Lu/Hf of the WR rather than that of the individual minerals. Many opx fractions and whole rocks have less radiogenic 176Hf/177Hf than their corresponding cpx (but similar to that of the host magma), reflecting contamination. Because the 176Hf/177Hf of cpx is supported by the WR Lu/Hf, it is necessary to use the latter rather than the cpx Lu/Hf for isochron dating. However, because the actual WR Lu/Hf values are susceptible to secondary overprinting, we recommend using model WR Lu/Hf values (determined from mineral Lu and Hf concentrations and modal abundances) to construct external cpx isochrons. Model WR and measured cpx 176Hf/177Hf are correlated (r2=0.98) in a suite of eastern Colorado Plateau xenoliths and yield an apparent age of 1.2Ga (versus 1.5Ga when – incorrectly – using cpx Lu/Hf). Despite the good correlation between Lu/Hf and 176Hf/177Hf, we do not find correlations between indicators of melt depletion and Lu/Hf or 176Hf/177Hf. This suggests that Lu/Hf–176Hf/177Hf correlations are not the result of melt depletion. Instead, we propose that Lu/Hf–176Hf/177Hf correlations are the result of mixing between depleted and enriched components (e.g. metasomatism of depleted lithospheric mantle). We emphasize that additional data should be utilized (e.g. whole rock Lu–Hf budget, degree of metasomatism, opx 176Hf/177Hf) when interpreting apparent Lu–Hf isochrons for age significance.

Silica- and LREE-enriched spinel peridotite xenoliths from the Quaternary intraplate alkali basalt, Jeju Island, South Korea: Old subarc fragments?

Abstract Spinel harzburgite to lherzolite xenoliths are entrapped in Quaternary intraplate alkali basalts on Jeju Island, South Korea. These xenoliths are unusual in containing late-stage secondary orthopyroxene, free of deformation and exsolution that is replacing olivine as the main pervasive metasomatic mineral. These xenoliths are characterized by high Mg# in olivine, orthopyroxene, and clinopyroxene (89–93) and variable Cr# of spinel (9–53), representing residues left after variable degrees of melt extraction (~ 25%). In contrast to their depleted major-element compositions, clinopyroxenes in the xenoliths are enriched in most incompatible trace elements. Clinopyroxenes display enrichment in light rare earth elements (LREE) or spoon-shaped REE with a general enrichment in La over Ce, and depletion in high field strength elements (HFSE; e.g., Nb-Ta, Zr-Hf, Ti). Orthopyroxenes (either primary or secondary) are characterized by low TiO2, high Al2O3, and moderate CaO contents, and resemble those of sub-continental arc peridotites from the eastern Pacific. The geochemical evidence, in addition to the formation of secondary orthopyroxene, indicates that Jeju peridotite xenoliths have been subjected to different degrees of metasomatism by subduction-related silica- and LREE-enriched fluids (or melts). However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. We therefore propose that the Jeju peridotite xenoliths went through a two-stage evolution, with their composition primarily controlled by early fractional melt extraction, which was subsequently modified by residual slab-derived fluids (or melts). Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of the subcontinental lithospheric mantle, resulting in the enriched mantle domains, such as EMI or EMII.

Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine–garnet-websterite, garnet-websterite, orthopyroxene–phlogopite–garnet-websterite, coarse-grained phlogopite–garnet-websterite, phlogopite–garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U–Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734–0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation.

Along-arc variations in lithospheric mantle compositions in Kamchatka, Russia: First trace element data on mantle xenoliths from the Klyuchevskoy Group volcanoes

We provide results of a detailed study of the first peridotite xenoliths of proven mantle origin reported from Bezymyanny volcano in the Klyuchevskoy Group, northern Kamchatka arc. The xenoliths are coarse spinel harzburgites made up mainly of Mg-rich olivine as well as subhedral orthopyroxene (opx) and Cr-rich spinel, and also contain fine-grained interstitial pyroxenes, amphibole and feldspar. The samples are unique in preserving the evidence for both initial arc mantle substrate produced by high-degree melt extraction and subsequent enrichment events. We show that the textures, modal and major oxide compositions of the Bezymyanny xenoliths are generally similar to those of spinel harzburgite xenoliths from Avacha volcano in southern Kamchatka. However, coarse opx from the Bezymyanny harzburgites has higher abundances of light and medium rare earth elements and other highly incompatible elements than coarse opx from the Avacha harzburgites. We infer that (1) the sub-arc lithospheric mantle beneath both Avacha and Bezymyanny (and possibly between these volcanoes) consists predominantly of harzburgitic melting residues, which experienced metasomatism by slab-related fluids or low-fraction, fluid-rich melts and (2) the degrees of metasomatism are higher beneath Bezymyanny. By contrast, xenolith suites from Shiveluch and Kharchinsky volcanoes 50–100km north of the Klyuchevskoy Group include abundant cumulates and products of reaction of mantle rocks with silicate melts at high melt/rock ratios. The high melt flux through the lithospheric mantle beneath Shiveluch and Kharchinsky may be related to the asthenospheric flow around the northern edge of the sinking Pacific plate; lateral propagation of fluids in the mantle wedge south of the plate edge may contribute to metasomatism in the mantle lithosphere beneath the Klyuchevskoy Group volcanoes.

Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

Noble gases in pyroxenites and metasomatised peridotites from the Newer Volcanics, southeastern Australia: implications for mantle metasomatism

The elemental and isotopic compositions of five noble gases (He, Ne, Ar, Kr and Xe) have been determined in selected, well-documented, ultramafic xenoliths from southeastern Australia. These xenoliths include both spinel-bearing peridotites with an apparent metasomatic overprint and garnet-bearing pyroxenites. In general, helium, neon, argon and xenon isotopic ratios from gases trapped in fluid inclusions of the samples are mid-ocean ridge basalt (MORB)-like, as has previously been found in anhydrous lherzolite xenoliths from the same area. In addition to the MORB-like components, radiogenic ( 4He*) and nucleogenic ( 21Ne* and 22Ne*) components were found in the present samples, reflecting relatively high U and Th contents in the metasomatic minerals such as amphibole, apatite and clinopyroxene. These components are only released upon melting, thus are likely to be trapped in the crystal lattices of the minerals. The MORB-like noble gas component found in fluid inclusions (invariably CO 2-rich) of these samples probably was introduced into the lithospheric mantle by metasomatising melts derived from the upper mantle. The noble gases dissolved in the metasomatising melt are likely to be effectively decoupled from incompatible elements during segregation of a CO 2-rich fluid in the ascending melt. In the metasomatising melt, the noble gases would partition into the CO 2-rich fluid whereas the incompatible elements would remain in the melt phase. As a result, the noble gas composition in fluid inclusions of the minerals appears to be independent of their degree of metasomatism indicated by their host rock mineralogy and trace elemental geochemistry. Once noble gases are trapped in CO 2-rich fluid inclusions within the minerals, they would preserve their source signatures without being affected significantly by the ingrowth of radiogenic and nucleogenic products, because the U, Th and K would remain in the melt. The predominance of MORB-like noble gas signatures in fluid inclusions of the xenoliths suggests that the metasomatising components were derived from the asthenospheric mantle underlying southeastern Australia.

Extreme Mantle Heterogeneity beneath the Jingpohu Area, Northeastern China—Geochemical Evidence of Holocene Basaltic Rock

AbstractHolocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, their petrological characteristics are very different: alkaline olivine basalt without any megacrysts in the former, and leucite tephrite with Ti‐amphibole, phlogopite and anorthoclasite megacrysts in the latter. On the basis of their geochemical characteristics, the two types of basaltic rocks should belong to weakly sodian alkaline basalts. But leucite tephrite is characterized by higher Al2O3, Na2O and K2O, higher enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), lower MgO and CaO, compatible elements and moderately compatible elements and lower Mg*values and Na/K ratios in comparison with alkaline olivine basalt. However, the two types of basaltic rocks have similar Sr, Nd, Pb isotopic compositions, which suggests that the mantle beneath the Jingpohu area was homogeneous before undergoing some geological processes about 3490 years ago. As the activity of the mantle plume led to different degrees of metasomatism, extreme mantle source heterogeneities occurred beneath the Jingpohu area. In comparison with alkaline olivine basalt, the leucite tephrite was derived from the more enriched mantle source region and resulted from strong metasomatism.

Geochemical evolution of the Ivigtut granite, South Greenland: a fluorine-rich “A-type” intrusion

The Ivigtut alkali granite stock lies within the Proterozoic Gardar alkaline igneous province of South Greenland. This small (300 m across) granite body once contained the world's largest body of cryolite, now mined out. The granite surrounding the cryolite body has been extensively metasomatised by F- and CO 2-rich fluids, leading to zonal enrichment in HFSE and REE. Variations in the Rb–Sr and Sm–Nd systematics of the granite can be correlated with the degree of metasomatism. Unaltered granites have highly variable initial 87 Sr/ 86 Sr , and initial ε Nd ratios of around −3, suggesting formation through crustal contamination of a mantle-derived magma. Metasomatised granites show higher initial ε Nd ratios, which can be interpreted to suggest that the metasomatic fluid was derived from a mantle source.

Origin of Sr, Nd and Pb isotopic systematics in high-Sr basalts from central Arizona

Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, ɛNd= −9.2 to −10.7); alkali olivine basalts (87Sr/ 86Sr = 0.7049 to 0.7054, ɛNd= −2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, ɛNd= −3.5 to −7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, ɛNd= −1.4 to −2.6). Post-faulting lavas have lower 87Sr/86Sr (<0.7045) and ɛNd from −3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.