LREE-enriched Melt Research Articles

Compositional signatures of clinopyroxenes from the mafic rocks of the Nagaland ophiolite (NE India): Inferences for melting of diverse source regions in a transitional arc-type setting

The Nagaland-Manipur ophiolite belt (NMOB) represents remnants of the Neo-Tethyan Ocean that evolved during the accretion of the Indian and Myanmar lithospheric plates. We studied clinopyroxenes of cumulate gabbro and basalt rocks from the northern section of the belt to decode the mechanism that controlled their petrogenesis and geotectonic setting. Clinopyroxenes (augite-diopside) exhibit compositional zoning and significant variations in their major and trace element contents. Zoned clinopyroxene from a single volcanic rock sample (2d-3-6-11) show higher concentrations of TiO2 (1.44 to 5.13 wt%) and Al (0.128–0.359 apfu) and lower Si (1.658–1.889 apfu) compared to clinopyroxenes from the other mafic rocks. The mg# [100 × Mg/(Mg + Fe2+)] values of clinopyroxenes range between 60.1 and 85.7. The trace element contents and the chondrite-normalised REE patterns of clinopyroxenes imply diverse geochemical affinities of both depleted mantle typical of tholeiite rocks and a less depleted LREE-enriched melt for the Nagaland mafic rocks. Pressure and temperature of crystallisation were estimated using single clinopyroxene geothermobarometry and were found to vary between 1–8 kbar and 840–1179 °C, respectively. In discrimination diagrams, the zoned clinopyroxenes show an alkaline composition consistent with a within-plate environment. On the other hand, clinopyroxenes from the rest of the mafic samples show a sub-alkaline composition typical of magmas produced in arc-related settings. Our study indicates the existence of diverse magma sources in an island arc-to-back arc setting during the formation of magmatic rocks in the NMOB.

Compositional variations of chromian spinels from peridotites of the Spontang ophiolite complex, Ladakh, NW Himalayas, India: petrogenetic implications

The Spontang ophiolite complex exposed along the Indus Tsangpo Suture Zone (ITSZ) represents a fragment of oceanic lithosphere emplaced after the closure of the Neo-Tethyan Ocean. The complex lying south of the ITSZ forms the highest tectonic thrust slice above the Mesozoic–Early Tertiary continental margin in the Ladakh-Zanskar Himalaya. The complex consists of a well-preserved ophiolite sequence dominated by peridotites, gabbros and ultramafic cumulates along with highly tectonized sheeted dykes and pillow lavas. The mantle suite of rocks is represented by minor lherzolites, harzburgites and dunites. Chromian spinel is brown to reddish, and its morphology and textural relationship with coexisting silicates varies with strain. Spinel occurs as blebs and vermicular exsolutions within orthopyroxene to spherical inclusions within olivine, characteristic of which is the elongate holly leaf shape. Chrome spinels are characterized by low TiO2 and high Cr2O3 indicative of their depleted nature. Cr# [= atomic ratio Cr/(Cr + Al)] in the studied spinels are characterized by a small decrease in TiO2 for a larger increase in Cr# consistent with observations for spinels aligned along the Luobusa trend of the Yarlung Zangpo Suture Zone (YZSZ) ophiolites. Variations in Cr-spinel Cr# and Mg# observed in the investigated peridotites may have resulted from a wide range of degrees of mantle melting during their evolution. Mineral and whole-rock chemistry of the Spontang peridotites is characterized by interaction between depleted magma and pre-existing oceanic lithosphere, typical of supra-subduction zone settings. The Spontang peridotites have olivine, clinopyroxene and orthopyroxene compositions similar to those from both abyssal and fore-arc peridotites and display spoon shaped REE profiles characteristic of interaction between LREE-enriched melt, derived from the subducting slab and LREEdepleted mantle residues. Equilibration temperatures calculated for the above rocks indicate that the studied samples represent typical mantle peridotites formed within the spinel stability field.

Geochemical Features of a Subduction Initiation Ophiolite Type in Habana‐Matanzas Region (Western Cuba)

We present new geochemical data for the upper mantle and crustal sections (whole‐rock major and trace element compositions) as well as mineral chemical data, from the Northern Carbibbean ophiolites in the Habana‐Matanzas region in Western Cuba. These ophiolites are part of the Northern Cuban Ophiolitic Belt (NCOB), extending for more than 1000 km along the island. The upper mantle peridotites are composed mainly of refractory harzburgite with tectonite textures, and show convex‐downward patterns depleted in MREE normalized to chondrite values (McDonough and Sun, 1995). These geochemical trends are characteristic for depleted mantle wedge peridotites metasomatized by slab‐derived, LREE enriched melts. The NCOB also includes abyssal peridotites having lower LREE/HREE ratios and displaying relatively homogeneous and flat patterns from MREE to HREE. These peridotites represent fragments accreted into the continental margin from a subducted oceanic lithosphere. Gabbro and dolerite units in the NCOB are systematically depleted in High Field Strength Elements (HFSE: Nb, Ta, Hf, Ti) and REE with respect to N‐MORB (<1 X N‐MORB). Their melt evolution was affected by subduction input. Spatially associated granitic rocks have a volcanic arc geochemical affinity. Some mafic extrusive rocks within the NCOB exhibit boninitic signatures, and may represent the products of subduction initiation magmatism, whereas other extrusive rock occurrences display N‐MORB to E‐MORB geochemical fingerprints, slightly modified by subduction derived fluids. Using these geochemical data and constraints, we present a tectonomagmatic model for the evolution of the NCOB within the framework of the Caribbean geology.

Mineralogical and geochemical constraints on the shallow origin, ancient veining, and multi-stage modification of the Lherz peridotite

New major- and trace-element data of bulk-rocks and constituent minerals, and whole-rock Re–Os isotopic compositions of samples from the Lherz Massif, French Pyrenees, reveal complex petrological relationships between the dominant lithologies of lherzolite ± olivine-websterite and harzburgite. The Lherz peridotite body contains elongate, foliation parallel, lithological strips of harzburgite, lherzolite, and olivine-websterite cross-cut by later veins of hornblende-bearing pyroxenites. Peridotite lithologies are markedly bimodal, with a clear compositional gap between harzburgites and lherzolites ± olivine-websterite. Bulk-rock and mineral major-element oxide (Mg–Fe–Si–Cr) compositions show that harzburgites are highly-depleted and result from ∼20-25 wt.% melt extraction at pressures <2 GPa. Incompatible and moderately-compatible trace-element abundances of hornblendite-free harzburgites are analogous to some mantle-wedge peridotites. In contrast, lherzolites ± olivine-websterite overlap estimates of primitive mantle composition, yet these materials are composite samples that represent physical mixtures of residual lherzolites and clinopyroxene dominated cumulates equilibrated with a LREE-enriched tholeiitic melt. Trace-element compositions of harzburgite, and some lherzolite bulk-rocks and pyroxenes have been modified by; (1) wide–spread interaction with a low-volume LREE-enriched melt +/− fluid that has disturbed highly-incompatible elements (e.g., LREEs, Zr) without enrichment of alkali- and Ti-contents; and (2) intrusion of relatively recent, small-volume, hornblendite-forming, basanitic melts linked to modal and cryptic metasomatism resulting in whole-rock and pyroxene Ti, Na and MREE enrichment. Rhenium-Os isotope systematics of Lherz samples are also compositionally bimodal; lherzolites ± olivine-websterite have chondritc to suprachondritic 187Os/ 188Os and 187Re/ 188Os values that overlap the range reported for Earth’s primitive upper mantle, whereas harzburgites have sub-chondritic 187Os/ 188Os and 187Re/ 188Os values. Various Os-model age calculations indicate that harzburgites, lherzolites, and olivine-websterites have been isolated from convective homogenisation since the Meso-Proterozoic and this broadly coincides with the time of melt extraction controlled by harzburgite Os-isotope compositions. The association between harzburgites resulting from melting in mantle-wedge environments and Os-rich trace-phases (laurite–erlichmanite sulphides and Pt–Os–Ir-alloys) suggests that a significant portion of persistent refractory anomalies in the present-day convecting mantle of Earth may be linked to ancient large-scale melting events related to wide-spread subduction-zone processing.

Podiform chromitites and mantle peridotites of the Antalya ophiolite, Isparta Angle (SW Turkey): Implications for partial melting and melt–rock interaction in oceanic and subduction-related settings

The Antalya ophiolites (AO) in SW Turkey comprise remnants from the southern branch of the Neotethyan oceanic basins. The mantle peridotites of the AO include spinel lherzolite, harzburgite, and dunite. Harzburgites are dominant over spinel lherzolites in the AO, whereas dunites occur exclusively as ‘envelopes’ surrounding chromitite pods hosted by harzburgite. Petrographic and geochemical data from the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in bulk rock contents of Al 2O 3, CaO, SiO 2, TiO 2, Sc, V and Y, modal clinopyroxene, Al 2O 3 content of pyroxenes, and an increase in mafic phase Mg#s and spinel Cr#s (Cr#: 18.0–39.6% in spinel lherzolites, 33.2–77.8% in harzburgites, and 68.7–72.2% in dunites). The chondrite-normalized REE patterns of the AO spinel lherzolites have LREE depleted signature showing steep slopes from HREE to LREE, and are compatible with the residue of a low degree (8–18%) of partial melting of N type MOR mantle. Their mineral composition, whole-rock chemistry and oxygen fugacities between FMQ −1.01 and FMQ −0.57 are consistent with mid-ocean ridge-type mantle. On the other hand, the AO harzburgites have slight enrichment in LREE relative to the patterns expected for residues of partial malting thereby indicating reaction with a LREE-enriched melt. Thus, their chondrite-normalized REE patterns show ‘V-shaped’ REE distributions similar to those of forearc harzburgites. The AO harzburgites have oxygen fugacities between FMQ − 1.08 and FMQ +0.72. The correlation of spinel Cr# and oxygen fugacity values indicates polygenetic association in the AO harzburgites as encountered in modern forearcs (e.g. Izu–Bonin–Mariana and South Sandwich forearc complexes). The spinel Cr# and oxygen fugacity correlation and the LREE enrichment, despite their ultra-refractory character, confirm the notion that the AO harzburgites could be residue from interaction of a previously melted mantle source (the AO spinel lherzolites) with an ascending LREE-enriched melt in a supra-subduction environment. The interaction must have caused modest melt fractions of the pre-existing mantle lithosphere. Therefore, the harzburgites experienced higher degree partial melting (17 to 25%) than the spinel lherzolites. The AO dunites, the most ‘refractory’ of the peridotites, appear to have formed at oxygen fugacities between FMQ +0.72 and FMQ +1.72, and show V-shaped REE distributions similar to those of supra-subduction zone (SSZ) dunites. Phase equilibria, field evidence and petrographic features show that the AO dunites are formed by dissolution of pyroxene from host harzburgites during the melt–mantle interaction. Focused melt flow caused the highest of melt fractions around the reaction zone. The AO podiform chromitites composed of a very refractory olivine–spinel assemblage (Fo: 94.8 and 96.1 – Cr#: 72.2–81.2%), corroborating a boninitic parentage. Therefore, the chromitites are considered to be the product of boninitic melt evolved by melt–rock interaction in a supra-subduction environment. Our results support a model involving both melt–rock interaction and differential partial melting to explain (a) precipitation of podiform chromitite from boninitic magma and (b) peridotites showing a sequence from MORB-like to subduction-related affinity.

The geochemistry of Neoarchean (ca. 2700 Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic and geodynamic processes

Neoarchean (ca. 2700 Ma) Wawa greenstone belts, Superior Province, contain two lithologically and geochemically distinct volcanic associations: (1) a plume-derived oceanic plateau association; and (2) a subduction-derived oceanic island arc association. The oceanic plateau association is composed of three groups: (1) voluminous tholeiitic basalts with near-flat REE patterns; (2) volumetrically minor moderately LREE-depleted tholeiitic basalts and Al-undepleted komatiites; and (3) volumetrically minor moderately to strongly LREE-enriched transitional to alkaline basalts and Al-depleted komatiites. This geochemical diversity is comparable to those of Phanerozoic oceanic plateaus and ocean islands. The oceanic island arc association is characterized mainly by variably LREE-enriched and Nb-depleted (relative to Th and La) tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites, with minor adakites, magnesian andesites, and Nb-enriched basalts. All these lavas are intruded by subduction-derived late- to post-tectonic gabbros, tonalites, trondhjemites, granodiorites, and monzonites. Within the Wawa subprovince the Schreiber–Hemlo greenstone belt is dominated by plume-derived oceanic plateau volcanic rocks (60–80%), whereas the Winston Lake and Manitouwadge greenstone belts are composed dominantly of subduction-derived (70–90%) volcanic rocks. New major and trace element, and Sm–Nd isotope data are presented for volcanic rocks from the Schreiber–Hemlo, Winston Lake, and Manitouwadge greenstone belts, to address their petrogenetic origin. In addition, new major and trace element data are reported for subduction-derived gabbros from the Schreiber–Hemlo greenstone belt. The geochemical diversity of the oceanic plateau association is consistent with a compositionally heterogeneous mantle plume source. Depleted initial Nd isotopic compositions ( ɛ Nd = +1.7 to +2.2) indicate that transitional to alkaline basalts were derived from a long-term LREE-depleted mantle source. It is shown that the origin of transitional to alkaline lavas can be explained by mixing of LREE-depleted and LREE-enriched melts in an upwelling mantle plume. Depleted to enriched LREE patterns in the plume-derived association are consistent with variable degrees of melting that took place over a range of temperatures and pressures as the plume ascended towards the surface. On the basis of field relationships and geochemical data, it is suggested that the geodynamic evolution of the Schreiber–Hemlo greenstone belt began with the initiation of a subduction zone at the margin of an oceanic plateau at about 2725 Ma, forming an intra-oceanic island arc. In response to plate reorganization and subducted slab rollback, the island arc underwent extension leading to the opening of a backarc basin. Hydrothermal alteration of backarc oceanic crust resulted in the formation of volcanogenic massive sulphide (VMS) deposits in the Winston Lake and Manitouwadge greenstone belts.

Evidence for multistage evolution in a xenolith of diamond-bearing eclogite from the Udachnaya kimberlite pipe

We present the results of a detailed investigation of large (18 cm long, 6.9 kg) diamondiferous eclogite xenolith from the Udachnaya kimberlite pipe. We have studied the spatial distribution of diamonds, their relationship with the matrix minerals, mineral inclusions and their major- and trace-element compositions. The xenolith is composed of pale green clinopyroxene, round garnet grains and phlogopite, and is characterized by irregular distribution of minerals and development of zones enriched in phlogopite and garnet. Minerals in diamondiferous xenoliths show major-element compositions transitional between Udachnaya eclogites and those from peridotites. The host clinopyroxene are enriched relative to pyroxenes from Group A eclogites and websterites in respect K 2O and LREE. We believe that such peculiarities reflect the interaction between mantle peridotites and LREE-enriched melts. The sources of the melts are uncertain. The distribution of diamonds (1 to 5 mm in size) in the xenolith is extremely irregular: they are more abundant in the garnet- and phlogopite-enriched zones. Diamonds often make up veins and schlieren. Nitrogen aggregation characteristics suggest that the xenolith contains diamonds of at least two generations. High degrees of aggregation in colorless diamonds of the earlier generation indicates that they formed at least 1 Ga before kimberlite eruption, whereas yellow diamonds of the second generation (type IaA) were formed only a short time before kimberlite emplacement. The presence of K-Clinopyroxene, coesite, CaCO 3, and high-silica mica inclusions in diamonds of the second generation, and their internal and external morphology, suggest that these diamonds crystallized during interaction of the eclogitic substrate with high-density fluids compositionally intermediate between carbonatitic and hydrous-silicic end members rich in Si, K, Ti.

Importance of melt circulation and crust-mantle interaction in the lithospheric evolution beneath the North China Craton: Evidence from Mesozoic basalt-borne clinopyroxene xenocrysts and pyroxenite xenoliths

Mesozoic Fangcheng basalts from the North China Craton contain many clinopyroxene xenocrysts and pyroxenite xenoliths, which provide important information about melt circulation and crust-mantle interaction in the evolution of the sub-continental lithosphere beneath the region. All the xenocrysts show textural and chemical zoning. The zoning is mostly simple (simply-zoned), but some show complex patterns (complexly-zoned). In-situ major and trace element analyses suggest that all the simply-zoned xenocrysts may have been disaggregated from pyroxenite veins at mantle depth. The zoning is interpreted to result from chemical exchange with the host magma after their entrainment. The complexly-zoned xenocrysts record a complicated history of the lithospheric evolution: their cores could preserve information of high-temperature granulite-facies metamorphism in the lower crust and their intermediary zones might record metamorphic overgrowth in the mantle spinel-facies stability field. Therefore, the cores of the complexly-zoned xenocrysts have probably been derived from the Archean lower crust or newly-accreted lower crust and the intermediary zones could be formed through crust–mantle interaction. All the pyroxenite xenoliths are cumulates and were crystallized from the LREE enriched melts at mantle depth or crust–mantle transitional zone. Thus the clinopyroxene xenocrysts and pyroxenite xenoliths provide evidence for the existence of considerable crust–mantle interaction and melt circulation in the lithospheric mantle, which led to rapid lithospheric enrichment.

Harzburgite–dunite–orthopyroxenite suite as a record of supra-subduction zone setting for the Oman ophiolite mantle

We found a harzburgite–dunite–orthopyroxenite suite from the West Jizi block in the northern Oman ophiolite as a record of supra-subduction zone magmatism in the mantle. The orthopyroxenite has features of parallel dikes, each dike being 10–30 cm thick and in direct contact with the host harzburgite. The orthopyroxenite is composed of orthopyroxene (> 95 vol.%), clinopyroxene (< 5 vol.%) and trace spinel (< 1 vol.%), and is free from olivine. All rocks from the suite are characterized by high Cr# [= Cr / (Cr + Al) atomic ratio] of spinel (Cr# ≥ 0.65). The Fo content of olivine is relatively low, 89.9 to 91.1, in harzburgite and dunite. Clinopyroxenes are very low in incompatible elements and show LREE enrichment in orthopyroxenite and host harzburgite. Similar trace element concentrations of clinopyroxenes in the harzburgite and orthopyroxenite suggest their genetic linkage: the harzburgite is a residue of a partial melting, and its extracted melt formed orthopyroxenite dike. The melt in equilibrium with clinopyroxene is similar in trace-element characteristics to boninite. The mineral chemistries indicate that the harzburgite experienced higher degree of partial melting than abyssal peridotite. LREE-enriched melt and/or fluid assisted the high-degree melting. These lines of evidence indicate an involvement of arc magma in the rock suite genesis, i.e. a supra-subduction zone setting experienced by the Oman ophiolite mantle.

The Yarlung Zangbo Suture Zone ophiolitic mélange (southern Tibet): new insights from geochemistry of ultramafic rocks

The southern contact of the Yarlung-Zangbo Suture Zone ophiolitic belt is marked by a highly sheared serpentinite mélange containing ultramafic blocks. These peridotites can be divided into three main groups. (1) Lherzolites and Cpx-harzburgites contain brownish spinel with Mg# of 0.7–0.75 and Cr# of 0.15–0.27. They resemble fertile abyssal peridotites with generally smooth LREE-depleted and fairly flat MREE–HREE profiles. (2) Transitional harzburgites contain reddish spinels with Mg# of 0.57–0.66 and Cr# of 0.35–0.46. They resemble depleted abyssal or supra-subduction zone peridotites in that MREE–HREE profiles have positive slopes indicative of high degrees of partial melting. LREE profiles vary from depleted to slightly enriched, consistent with some interacting melt. (3) Harzburgites and dunites contain dark reddish spinels with Mg# of 0.47–0.68 and Cr# of 0.40–0.63. They have U-shaped profiles characteristics of interaction between LREE-enriched melt and REE-depleted mantle residues. Fractional melting modelling indicates that Cpx-harburgites may be the residues from 5 to 15% melting, transitional harzburgites from 15 to 23% melting, and harzburgites and dunites from 22 to 29% melting. The South Sandwich arc-basin system is considered a modern analog of the initial geodynamic setting.

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al 2 O 3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al 2 O 3 and TiO 2 ) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0·5121 (close to the host minette values) to 0·5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0·5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas. The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in large ion lithophile elements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr–Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallization at mantle depths of minette magmas in Eocene times, that caused further metasomatic enrichment of the lithosphere.

No significant element transfer from the oceanic plate to the mantle wedge during subduction and exhumation of the Tethys lithosphere (Western Alps)

In the Western Alps, Mesozoic ophiolites attest to the former existence of a slow-spreading rate ocean. Several units of this oceanic lithosphere, slightly to highly metamorphosed (up to the eclogite facies), are included in a nappe stacking formed during Tertiary subduction- and collision-related processes. Mantle and magmatic rock samples from five tectonic units, which have equilibrated at various metamorphic grades, were examined and analysed in order to estimate their prograde and retrograde metamorphic evolution and to evaluate the resulting chemical changes. Alpine metamorphism appears to be isochemical on the sample scale, or even on the mineral scale when the primary mineral is only pseudomorphosed at low-strain conditions. No significant loss or gain of trace elements, nor of Sr and Nd isotope exchange, appear to have occurred since alteration on the oceanic floor. As in the present oceans, the Sr isotopes recorded seawater contamination during sea floor spreading, whereas most trace element contents and the Nd isotopic signature of the magmatic and mantle protoliths remain unchanged. These results lead us to consider that no significant transfer of element occurred to the mantle wedge during subduction of the Tethyan units, even during the formation of eclogites at pressures up to 13 kbar. Serpentinized mantle peridotites retained the signature of a depleted mantle metasomatized by U-, Th- and LREE-enriched melts, similar to the European subcontinental mantle. As in the Ligurian ophiolites, gabbros and basalts cannot be related by fractional crystallization, and both originate from a same residual mantle source below the suboceanic mantle.

Rb-Sr and Sm-Nd isotopes in garnet pyroxenite xenoliths from Siberian kimberlites: an insight into lithospheric mantle.

Rb-Sr and Sm-Nd isotope compositions of seven garnet pyroxenite xenoliths from Udachnaya and Mir kimberlites, Siberia, are presented. Five of them are websterites, and the other two are clinopyroxenites. The websterite suite is divided into two groups (Group A and Group B) according to their mineral chemistry. Group B websterites are characterized by presence of jadeite-rich clinopyroxene and pyrope-poor garnet. The Garnet-orthopyroxene geobarometry indicates that Group A were produced in deep positions of the cratonic mantle (up to diamond stability field) and Group B websterites are formed at lower pressure (ca. 19 kb). Probably, the Rb-Sr isotope system was re-equilibrated at the time of kimberlite emplacement at 367 Ma. The initial Sr isotope compositions differs from one group to the other. The initial ratio 87Sr/86Sr (0.70272-0.70338) for Group A is lower than that of Group B (0.70485-0.70737). The Sm-Nd isotope system in all samples gives apparent isochron ages considerably older than that of the emplacement of host kimberlites. Group A websterites and clinopyroxenites are apparently younger (1223-582 Ma) than those of Group B (1550-1465 Ma). The origin of garnet pyroxenites is suggested as crystal accumulation from LREE-enriched melts which episodically added into peridotite-dominated lithospheric mantle. These melts played also a role of metasomatic agent to the host peridotites. A delaminated lower crustal protolith has probably been involved in Group B origin. The Sm-Nd isotope study reveals a systematic increase of initial 143Nd/144Nd with time. A vairable Sm-Nd isotope data suggest that peridotites and pyroxenites in lithospheric mantle beneath the Udachnaya area were in isotopic equilibrium, but not beneath the Mir area where peridotites depleted in initial Nd isotope composition.

The geochemistry of olivine-hosted melt inclusions in a FAMOUS basalt ALV519-4-1

Some 22 melt inclusions hosted by high-Mg olivines (Fo 88–91) from a single basalt lava from the FAMOUS area of the Mid-Atlantic Ridge were analyzed for major and trace elements. The results demonstrate that large chemical variabilities are present in melts that existed in close temporal and spatial proximity at the site of host olivine growth. Major element compositions after corrections for growth of host olivine after entrapment show that they are very primitive (MgO ranging from 9 to 14 wt.%; Mg# around 0.73), and that variations of CaO, FeO and SiO 2 as a function of MgO are much greater than expected from fractionation of olivine from the most magnesian melt. Exactly 14 of the melt inclusions are consistent with equilibration with mantle minerals at 8.7±2 kbar and 1272±18°C. Abundance patterns of rare earth elements (REE) show that light REE (LREE)-enriched transitional mid-ocean ridge basalts (MORB) and LREE-depleted N-type MORB existed in close temporal and spatial proximity. It is argued that co-variations of trace element ratios, especially [La/Sm] N and Ti/Zr can be modeled by critical melting of a depleted MORB source at depths where garnet is stable to form LREE-enriched melts, and at depths of spinel lherzolite residue to form LREE-depleted melts. Abundance patterns of middle to heavy REE (M∼HREE) indicate, however, that important chemical modification has occurred by reactions between deep melts and shallow mantle in an AFC-type mechanism during upward migration of melts. This mechanism also appears to explain some of the major element characteristics, and suggests general importance of melt/mantle reaction during migration of MORB.

Geochemistry of garnet peridotite massifs from lower Austria and the composition of deep lithosphere beneath a Palaeozoic convergent plate margin

In the southern Bohemian massif garnet-bearing high-temperature peridotite massifs have been exhumed during Carboniferous collision of the Baltic plate and rifted fragments of Gondwana. Geological and geochronological data suggest that the peridotites represent fragments of lithosphere derived from a destructive plate margin. Depleted garnet lherzolites, spinel and spinel-garnet harzburgites from lower Austria show major and trace element variations with MgO and HREE abundances which indicate that they underwent moderate (∼ 5%) to high (> 20%) degrees of partial melting in the spinel peridotite facies. Peridotites sampled away from pyroxenite layers have isotopic compositions typical of depleted mantle ( 87Sr 86Sr = 0.7021 to 0.7033 , ϵ Nd (335 Ma) = 8 to 12). Peridotites that occur close to LREE-enriched pyroxenites with negative Eu anomalies yield evidence for a multi-stage history, i.e. they have ( Tb Yb ) n < 1 and ( La Sm ) n > 1 . Re-enrichment in incompatible elements, e.g., high ( La Sm ) n in harzburgites and depleted lherzolites, and the relatively high SiO 2 contents of some of these rocks, can be explained by percolation of the LREE-enriched melts which were parental to nearby pyroxenite layers. Refertilization, bulk REE enrichment and isotope disequilibrium of minerals in some peridotites are related to late physical mixing with garnet pyroxenites during the exhumation process. A second group of peridotites comprises garnet lherzolites with ( La Sm ) n < 1 . These LREE-depleted garnet lherzolites contain thin pyroxenite layers with flat to LREE-enriched REE patterns without Eu anomalies. Both, these peridotites and pyroxenites ( 87Sr 86Sr up to 0.7039, ϵ Nd (335 Ma) from 11 to −1.1) also appear to be isotopically distinct from the pyroxenites with negative Eu anomalies. A third group of lherzolites has high 87Sr 86Sr (up to 0.7049) and moderately high Nd isotopic compositions ( ϵ Nd (335 Ma) from 7.9 to 4.4), and were probably also modified by the interaction with melts. The variability of isotopic compositions in the peridotites may indicate the possible variability in hangingwall lithospheric mantle beneath convergent plate margins. The average major and trace element composition of the peridotites from lower Austria is more depleted than the average of continental spinel peridotite xenoliths, but less depleted compared with garnet peridotite xenoliths from cratonic settings.

Metasomatism of the mantle lithosphere recorded by rare earth elements in garnets

Garnets derived from peridotite and kimberlite concentrates in southern African kimberlites show normal and sinusoidal REE patterns. Both types of REE patterns are interpreted to have resulted from the metasomatism of previously existing mantle garnets by LREE-enriched melts or fluids. In this model, metasomatism proceeded in accordance with garnet-liquid K d s which increase by orders of magnitude from the LREE and MREE to the HREE, thus fractionating the REE into three groups. The development of sinusoidal REE patterns resulted from the HREE lagging behind the LREE during metasomatic readjustment in composition. Sinusoidal REE patterns represent a state of disequilibrium during the re-equilibration process and are retained by garnets of relatively refractory compositions. Garnets from off-craton and on-craton regions show both types of REE patterns, suggesting that the presence of refractory garnets typical of cratonic regions is not restricted to the craton. Calculated liquid compositions for the metasomatic melts resemble kimberlite and lamproite, but were probably unrelated to the host kimberlites. Melts from off-craton regions are less LREE rich than melts from cratonic regions. These results suggest a different lithosphere in the off-craton region, possibly reflecting extensional tectonics and a higher degree of mantle upwelling.

The geochemistry and petrogenesis of volcanics and sheeted dikes from the Hatay (Kizildag) Ophiolite, southern Turkey: Possible formation with the Troodos Ophiolite, Cyprus, along fore-arc spreading centers

Geochemical and petrological investigations of the Hatay (Kizildag) Ophiolite, southern Turkey, indicate that the volcanics and sheeted dikes compositionally range from island-arc tholeiites and basaltic andesites to transitional boninitictype lavas formed within a supra-subduction zone environment. Geochemical modeling indicates that the compositional trends within both the sheeted dike and pillow basalt sections of Hatay cannot be related through fractional crystallization alone but require multiple parental magmas of differing compositions within each unit. The more refractory liquids (higher MgO, Ni and Cr coupled with lower concentrations of Ti, Zr, Y and REE) formed at lower pressures (shallower depths) through greater degrees of partial melting of a more depleted mantle source relative to less-refractory magmas which formed at higher pressures and possibly lower degrees of melting. The Hatay volcanics and dikes span the compositional range of Lower Pillow Lavas (LPLs) and Upper Pillow Lavas (UPLs) from the Troodos Complex, Cyprus, suggesting, along with other evidence, that the two ophiolites may be petrogenetically and tectonically related. Formation of the Hatay and Troodos ophiolites was possibly associated with extension within a fore-arc environment following compression and detachment along an intra-oceanic ridge system and development of a subduction zone. The range of parental liquid compositions for volcanics and dikes may reflect variable mixing of geochemically diverse melt increments generated within a polybaric melting column in the mantle wedge. The melting column may have additionally received contributions of LREE-enriched melts from deeper, more juvenile (fertile) sources. Formation of parental liquids through variable mixing of melt increments appears to be common to both the Hatay and Troodos ophiolites.