Degree Of LREE Enrichment Research Articles

Petrological constraints on evolution of continental lithospheric mantle beneath the northwestern Ethiopian plateau: Insight from mantle xenoliths from the Gundeweyn area, East Gojam, Ethiopia

Detailed petrographical observations and in-situ major- and trace-element data for minerals from ten spinel peridotite xenoliths from a new locality in Gundeweyn area, East Gojam, have been examined in order to understand the composition, equilibrium temperature and pressure conditions as well as depletion and enrichment processes of continental lithospheric mantle beneath the Ethiopian plateau. The peridotite samples are very fresh and, with the exception of one spinel harzburgite, are all spinel lherzolites. Texturally, the xenoliths can be divided into two groups as primary and secondary textures. Primary textures are protogranular and porphyroclastic while secondary ones include reaction, spongy and lamellae textures. The Fo content of olivine and Cr# of spinel ranges from 86.5 to 90.5 and 7.7 to 14.1 in the lherzolites, respectively and are 89.8 and 49.8, respectively, in the harzburgite. All of the lherzolites fall into the lower Cr# and Fo region in the olivine–spinel mantle array than the harzburgite, which indicates that they are fertile peridotites that experienced low degrees of partial melting and melt extraction. Orthopyroxene and clinopyroxene show variable Cr2O3 and Al2O3 contents regardless of their lithology. The Mg# of orthopyroxene and clinopyroxene are 87.3 to 90.1 and 85.8 to 90.5 for lherzolite and 90.4 and 91.2 for harzburgite, respectively. The peridotites have been equilibrated at a temperature and pressure ranging from 850 to 1100°C and 10.2 to 30kbar, respectively, with the highest pressure record from the harzburgite. They record high mantle heat flow between 60 and 150mW/m2, which is not typical for continental environments (40mW/m2). Such a high geotherm in continental area shows the presence of active mantle upwelling beneath the Ethiopian plateau, which is consistent with the tectonic setting of nearby area of the Afar plume. Clinopyroxene of five lherzolites and one harzburgite samples have a LREE enriched pattern and the rest exhibit LREE depletion relative to HREE. These suggest that the lithospheric mantle of the Ethiopian plateau has experienced at least two major processes, specifically, partial melting and metasomatism that produce LREE-depleted and -enriched signature of continental lithospheric mantle, respectively. There is also no clear relationship between degree of LREE enrichment and petrography of the studied peridotite.Based on our data, we conclude that the lithospheric mantle beneath Gundeweyn has experienced melt extraction during and/or before pan-African orogeny and then interacted with various degrees of asthenospheric melt. The interaction is probably related to mantle upwelling, which is mainly focused beneath East Africa rift system (EARS).

Temperature effect over garnet effect on uptake of trace elements in zircon of TTG-like rocks

To assess the garnet effect on uptake of rare earth elements in TTG petrogenesis, zircon trace elements were analyzed for TTG-like rocks in a Neoproterozoic composite batholith at Huangling in the Yangtze Gorge, South China. A TTG-like suite has higher whole-rock La/Yb and Sr/Y ratios, lower whole-rock Nb/La ratios and ε Nd( t) values, and lower zircon ε Hf( t) values than the other three non-TTG suites. Zircons from this suite have higher Hf, Nb and Ta contents than those from the other three non-TTG suites. However, REE profiles and ratios cannot be unambiguously distinguished between residued cores and co-magmatic domains despite significant differences in their U–Pb ages and Lu–Hf isotopes. All zircons from the four suites are enriched in HREE but depleted in LREE, with positive Ce anomalies. They display no to very weak Eu anomalies, contrasting to the typical zircon of magmatic origin. This suggests that plagioclase was neither a stable phase during partial melting nor a removed phase during fractional crystallization. Most of the zircons from the TTG-like rocks have Ti contents lower than 30 ppm, yielding a range of Ti-in-zircon temperatures from 600 to 880 °C. While a part of zircons from all granitoids show various degrees of LREE enrichment, the absence of flat HREE patterns is evident for all zircons, including both residued cores and co-magmatic domains in the TTG-like rocks. This suggests that there is no preferential uptake of HREE in garnet relative to zircon even if garnet could exist as a stable phase during partial melting to form the TTG and their source rocks. Because partition coefficients for garnet/melt, zircon/melt and zircon/garnet decrease with increasing temperature during partial melting, comparison of calculated apparent D HREE values with experimental D HREE values between zircon and melt allows estimate of temperatures for zircon crystallization. This yields a range of 750 to 800 °C for the TTG-like rocks, which is consistent with whole-rock zircon saturation temperatures of 756 to 793 °C. However, they are significantly lower than the crossover temperatures of 900 to 950 °C for preferential partition of HREE in the zircon–garnet–melt system. Thus, the temperature is suggested as a key cause for the lack of garnet effect on the uptake of HREE in zircons from the TTG-like rocks. Therefore, the temperature effect can play a critical role in dictating the partition of HREE in the zircon–garnet–melt system of adakitic and TTG-type rocks.

Geochemistry and tectonic significance of the Stony Mountain gabbro, North Carolina: Implications for the Early Paleozoic evolution of Carolinia

Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia. The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO 2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas. Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~ 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.

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Carolinia comprises a collection of Neoproterozoic–Early Paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to Laurentia in the early to middle Paleozoic. In central North Carolina, mafic rocks of the Stony Mountain gabbro intrude sub-aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the Albemarle Group. The age of the Stony Mountain gabbro is constrained to the Early Cambrian–Middle Ordovician. Field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the Neoproterozoic–earliest Cambrian Albemarle Group, yet the gabbro pre-dates regional metamorphism and tectonism related to the Late Ordovician accretion of Carolinia to Laurentia.The Stony Mountain gabbro has a sub-alkaline basaltic composition, variable TiO2, MgO and Ni/Cr values. The rocks have a geochemical signature typical of island-arcs; the degree of LREE enrichment, prominent negative Nb anomalies and Nb/Th ratios are all features of low-K to medium-K tholeiitic basalts in modern island-arc, subduction-related lavas.Isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of ~ 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the rocks of the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving Early Paleozoic island arc–back arc rift–basin system. The presence of an Early Cambrian arc–back arc rift system in Carolinia is broadly coeval with arc–back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.

Apatite as an indicator mineral for mineral exploration: trace-element compositions and their relationship to host rock type

Over 700 apatite grains from a range of rock types have been analysed by laser-ablation microprobe ICPMS for 28 trace elements, to investigate the potential usefulness of apatite as an indicator mineral in mineral exploration. Apatites derived from different rock types have distinctive absolute and relative abundances of many trace elements (including rare-earth elements (REE), Sr, Y, Mn, Th), and chondrite-normalised trace-element patterns. The slope of chondrite-normalised REE patterns varies systematically from ultramafic through mafic/intermediate to highly fractionated granitoid rock types. (Ce/Yb) cn is very high in apatites from carbonatites and mantle-derived lherzolites (over 100 and over 200, respectively), while (Ce/Yb) cn values in apatites from granitic pegmatites are generally less than 1, reflecting both HREE enrichment and LREE depletion. Within a large suite of apatites from granitoid rocks, chemical composition is closely related to both the degree of fractionation and the oxidation state of the magma, two important parameters in determining the mineral potential of the magmatic system. Apatite can accept high levels of transition and chalcophile elements and As, making it feasible to recognise apatite associated with specific types of mineralisation. Multivariate statistical analysis has provided a user-friendly scheme to distinguish apatites from different rock types, based on contents of Sr, Y, Mn and total REE, the degree of LREE enrichment and the size of the Eu anomaly. The scheme can be used for the recognition of apatites from specific rock types or styles of mineralisation, so that the provenance of apatite grains in heavy mineral concentrates can be determined and used in geochemical exploration.

REE AND SR-PB-ND ISOTOPE GEOCHEMISTRY OF TIBETAN PERIDOTITES: IMPLICATIONS FOR MELTING PROCESSES

It is ubiquitously accepted that mantle peridotite are residue after melting and are formed after extraction of variable amount of MORB type melt (Prinzhofer and Allegre, 1985; Jonhson et al., 1990; Snow et al., 1994). For this reason, they are a key in understanding melting in the mantle and melt extraction processes. They also provide information about mantle source heterogeneity and constraints on temporal evolution of the mantle. Nevertheless petrogenetic studies of ophiolite complexes generally concentrate on the magmatic oceanic crust. Trace elements and isotopic studies of highly depleted peridotite are rare partly due to their extensive alteration and weathering. Moreover Radiogenic and REE are found in very low concentration in mantle peridotites meaning that it is difficult to obtain high quality data. The Tibetan ophiolites provide a unique opportunity to study mantle processes because they are fresh (<10% serpentinisation) and well exposed. We have used ID ICP-MS and NdO techniques to measure REE and Nd isotopes in these highly depleted peridotites. We present major, trace element and Nd isotope data of 15 harzburgites from Luobusa ophiolite massif, Tibet. The aim of this study is to obtain a better understanding of the mantle processes that lead to the geochemical characteristics of these residues and to the formation of the oceanic crust. The Luobusa peridotite massif is situated in the Yarlung Zangbo Suture Zone and is assumed to have the same formation and emplacement ages as the Xigaze ophiolite (respectively 110 and 50 Ma). The massif is 43 km by 3-5 km. It has a mantle sequence mainly composed of diopside-bearing harzburgites with abundant dunite lenses and podiform chromitites that are residual after extraction of tholeitic (MORB type) magmas (Zhou et al., 1996). Remains of the oceanic crust (cumulates rocks, pillow lava, marine sediments) are present in a melange zone tectonically overlying the mantle peridotites (Zhou et al., 1996). The major element compositions of Luobusa peridotites show a large range of variation: CaO and MgO contents vary between 0.6 and 2 wt% and 42.1 and 45.1 wt% respectively. There is an excellent correlation between major element contents (e.g., Al2O3 or CaO) and HREE. Similar relationships have been reported elsewhere and are consistent with HREE depletion being associated with the loss of Aland Ca-rich phases during melting. Luobusa peridotites have REE patterns with subparallel HREE and constant MREE depletion. LREE are significantly more variable. Some samples are highly depleted in LREE, with minimum values of La at 0.004 x chondrite. Other have a “spoon-shaped” REE patterns, with a marked inflection at Sm. Their (Ce/Sm)N ratios varies between 0.2 and 0.9 and reflected different degree of LREE enrichment. The (143Nd/144Nd)i ratios vary between 0.5120 and 0.5130 and Sm/Nd from 1.1 to 3. (143Nd/144Nd)i also correlates positively with (Ce/Sm)N. The degree of LREE depletion in the peridotites is controlled by stratigraphic position: shallow peridotites being characterised by the highest (Ce/Sm)N and the highest depletion in HREE . Together with the “spoonshaped” REE patterns, these data imply that either melt was trapped in the residua during the extraction process or that the residua have been subsequently LREE enriched by melt addition. Similar observations have been made for other peridotites massif (Voykar and Trinity) and several possible mechanisms have been put forward to explain the geochemical data. The high degree of depletion of the Tibetan peridotites implies that they have recorded a multistage melting event. The samples with LREE depleted and high Nd ratios probably represent the residue after MORB type melt extraction from a previous depleted source. The LREE enrichment can be explained by 2 alternatives models: 1)- Melt/Residue interaction in a chromatographic column (Navon and Stolper, 1987; Bodinier et al., 1990 and others) or 2)- Crustal contamination either in a supra-subduction environment or during emplacement of the ophiolite (Sharma and Wasserburg, 1996; Gruau et al., 1998). The large range in Nd isotopic composition and the presence of samples showing less radiogenic Nd ratios associated with the highest degree of LREE enrichment tend towards the second alternative: a second melting event in a supra-subduction zone. This idea is also supported by the composition of the chromite pods present in the Luobusa massif that show boninite affinity (Zhou et al., 1996). This preliminary study show that the LREE enrichment is not as extensive as reported for other massif and tend to suggest that the geochemical characteristics are mantle melting related feature and not due to secondary alteration by infiltrating groundwater. The detailed REE and Sr/Pb/Nd isotope analysis of minerals are underway and will constrain the nature and the provenance of the melt involved and the succession of processes that occurred in the melting column and the geodynamic environment in which the massif was formed.

Evolution of the upper mantle beneath the southern Baikal rift zone: an Sr-Nd isotope study of xenoliths from the Bartoy volcanoes

Anhydrous and amphibole-bearing peridotite xenoliths occur in roughly equal quantitites in the Bartoy volcanic field about 100 km south of the southern tip of Lake Baikal in Siberia (Russia). Whole-rock samples and pure mineral separates from nine xenoliths have been analyzed for Sr and Nd isotopes in order to characterize the upper mantle beneath the southern Baikal rift zone. In an Sr-Nd isotope diagram both dry and hydrous xenoliths from Bartoy plot at the junction between the fields of MORB and ocean island basalts. This contrasts with data available on two other localities around Lake Baikal (Tariat and Vitim) where peridotites typically have Sr−Nd isotope compositions indicative of strong long-term depletion in incompatible elements. Our data indicate significant chemical and isotopic heterogeneity in the mantle beneath Bartoy that may be attributed to its position close to an ancient suture zone separating the Siberian Platform from the Mongol-Okhotsk mobile belt and occupied now by the Baikal rift. Two peridotites have clinopyroxenes depleted in light rare earth elements (LREE) with Sr and Nd model ages of about 2 Ga and seem to retain the trace element and isotopic signatures of old depleted lithospheric mantle, while all other xenoliths show different degrees of LREE-enrichment. Amphiboles and clinopyroxenes in the hydrous peridotites are in Sr−Nd isotopic disequilibrium. If this reflects in situ decay of 147Sm and 87Rb rather than heterogeneities produced by recent metasomatic formation of amphiboles then 300–400 Ma have passed since the minerals were last in equilibrium. This age range then indicates an old enrichment episode or repeated events during the Paleozoic in the lithospheric mantle initially depleted maybe ∼2 Ga ago. The Bartoy hydrous and enriched dry peridotites, therefore, are unlikely to represent fragments of a young asthenospheric bulge which, according to seismic reflection studies, reached the Moho at the axis of the Baikal rift zone a few Ma ago. By contrast, hydrous veins in peridotites may be associated with rift formation processes.

SIMS analysis of REE in pyroxenes and amphiboles from the Proterozoic Ikasaulak intrusive complex (SE Greenland): implications for LREE enrichment processes during post-orogenic plutonism

Rare-earth and selected trace elements (Ti, Cr, V, Sr and Zr) have been analyzed by secondary ion mass spectrometry (SIMS) in pyroxenes and amphiboles from both the lower and upper zones of the post-orogenic Proterozoic Ikasaulak intrusive complex (Nagssugtoqidian mobile belt of southeast Greenland). The orthopyroxenes from the harzburgites and olivine-orthopyroxenites (lower zone) are LREE-depleted. In contrast. orthopyroxenes from gabbros (the most mafic members of the upper zone) display U-shaped chondrite-normalized patterns and REE contents which increase from core to rim. In the lower zone, clinopyroxene coexists with pargasitic amphibole, both exhibiting unusually high LREE, Sr and Zr contents. In the upper zone, clinopyroxene is less LREE-enriched and is replaced by late- or post-magmatic hornblende. The variations observed in both the major- and trace-element composition of the ferromagnesian phases indicate that the Ikasaulak intrusive complex did not result from a single magma injection. An early injection of magma was responsible for the lower zone and strongly interacted with the country rocks. Fractional crystallization of this magma was probably accompanied by metasomatism effected by highly LREE-enriched hydrous fluids (Ce abundances up to 1300 × chondrite). The most mafic members from the upper zone were derived from a late injection of magma. This magma probably mixed with the residual liquid of the earlier injection, thus retaining the geochemical signature of the contamination processes which had occurred in the lower zone. Mixing and assimilation processes led the pyroxenes to concentrate REE contents higher than those theoretically predicted on the basis of the usual distribution coefficients for basaltic systems. The late growth of hornblende is unrelated to the LREE-enrichment processes. The ion microprobe analyses of the main mafic rock types of the Ikasaulak intrusive complex indicate that: (i) the parental magmas (tholeiites probably marked by a slight LREE-enrichment) were contaminated during the early cumulus processes; (ii) the degree of LREE-enrichment and HFSE (high field strength elements) depletion in parental magmas, as deduced from the whole-rock chemistry, may be overestimated.

Sm-Nd and Rb-Sr isotopic systematics of ureilites

Sm-Nd and Rb-Sr isotopic data are reported for seven ureilites. On the basis of Sm-Nd isotopic systematics these ureilites are divided into three groups: (1) ALH82130, PCA82506, and META78008; (2) Kenna, Novo Urei, and ALHA77257; (3) LEW85440. Group 1: Whole-rock samples of these ureilites are highly depleted assemblages ( 147Sm 144Nd = 0.33–0.35 ) having Sm-Nd model ages consistent with 4.55 Ga. Their Rb-Sr isotopic systematics, however, require that radiogenic Sr (present-day 87Sr 86Sr = 0.716–0.721 ) was introduced into these rocks at some time later than 4.55 Ga. Group 2: Whole-rock samples of Kenna, Novo Urei, and ALHA77257 and a pyroxene separate from Kenna plot on a Sm-Nd isochron, with an age of 3.74 ± 0.02 Ga and an initial 143Nd 144Nd ratio of 0.50945 ± 2. Whole-rock samples of these ureilites are heterogeneous mixtures of an unidentified LREE-enriched component and a LREE-depleted olivine + pyroxene assemblage. The LREE-enriched component is volumetrically minor but has LREE abundances sufficiently high to dominate whole-rock Sm and Nd abundances and Sm Nd ratios. Our best estimate indicates that it has [Nd]= ~8ppm and 147Sm 144Nd = ~0.115 . Variation in the quantity of LREE-enriched component present in whole-rock samples accounts for most of the variation in their Sm/ Nd ratios. Second-order variation is due to varying olivine/pyroxene ratios. The possibility that 3.74 Ga is the crystallization age of Kenna, Novo Urei, and ALHA77257 cannot be ruled out. However, we favour the interpretation that the 3.74 Ga isochron records an event in which LREE-enriched material (a metasomatic fluid which was the precursor of the present LREE-enriched component?) reacted with a preexisting highly depleted ( 147Sm 144Nd ≥ 0.51 ) ultramafic assemblage. The Sr isotopic compositions of eleven samples of Kenna (whole-rocks and various separated fractions) are virtually identical ( 87Sr 86Sr = 0.7086 ), regardless of Rb/Sr ratio. There is a positive correlation between Sr concentration and degree of LREE-enrichment in these samples, suggesting that the LREE-enriched component is also Sr-rich. The Sr data for Kenna could be explained by introduction of radiogenic Sr along with the LREE-enriched material at 3.74 Ga and a subsequent Rb-free history (all present Rb would have to be terrestrial contamination in this interpretation). The Sr isotopic compositions of Novo Urei and ALHA77257 are not, however, the same as that of Kenna. Group 3: LEW85440 neither has a model Sm-Nd age of 4.55 Ga nor plots on the 3.74 Ga isochron of the Kenna group. It might have had an evolution similar to that of the Kenna group but involving different times and/or isotopic compositions. If some samples of group 1 ureilites contain a LREE-enriched component, then this component must have been generated and mobilized on the ureilite parent body or bodies at least twice in the interval 4.55->3.74 Ga. There is no apparent correlation between oxygen isotopic systematics and Sm-Nd or Rb-Sr isotopic systematics.

Preliminary study of REE iron formations in China

Geological and geochemical characteristics of REE iron formation (REEIF), a term proposed by Prof. Tu Kuang-chih to specify a special type of Precambrian iron formations rich in REE, are discussed in this paper with special reference to its REE contents, REE distribution patterns, the formation mechanism, the relationship between its development and the multi-stage evolution of the continental crust in China, and the implications of REE as an indicator of oxidation state for ancient atmosphere. Major conclusions are outlined as follows: (1) REEIFs are characterized by high REE concentrations against the very low REE levels in normal Precambrian iron formations. (2) REEIFs are formed by marine sedimentary-diagenetic processes in miogeosynclines or transition zones during Proterozoic times To some extent, volcanic activity may play an important role in the deposition of ore-forming materials. In a broader sense, REEIFs belong to Fe-bearing dolomite formations. Most REEIFs in China may be superimposed by late geological processes such as hydrothermal-metasomatism, migmatization and metamorphism. Generally, REEIFs have much in common with stratabound ore deposits in respect to their characteristic features. (3) Similar to Precambrian iron formations, REE are enriched in LREE. But, the degree of LREE enrichment is noticed to increase of total REE content. Most REEIFs are characterized by high ratios of σ Ce/σY, (Mg+Fe)/Ca, Na/K, Nb/Ta, Zr/Hf, Th/U, Ba/Sr, etc. (4) The extensive occurrence of REEIFs indicates higher REE abundance in the continental crust of China, thus lending further support to the multi-stage theory regarding the evolution of chemical elements and the differentiation in the continental crust of China. (5) Preliminary data seem to support the time-dependence of REE distribution patterns and relative Eu contents of REEIFs in China.

Rare-earth elements in the rock-forming minerals of melilitic rocks in alkaline-ultrabasic complexes

The rare-earth distributions in melilites and other coexisting minerals of melilite-containing rocks from five alkaline-ultrabasic complexes were analysed by partition paper chromatography and neutron activation. Relatively late differentiates are characterized by elevated relative alkalinity and reduced absolute basicity. The characteristics of melilitic sövites markedly differ from established trends. The minerals studied are richer in LREE than chondrites and REE concentrations in these minerals increase in the late differentiates of the melilitic series. A proportionality between logarithms of La and Sm concentrations in pyroxene and melilite imply that melilite-containing rocks (except melilitic sövite) belong to a similar comagmatic series. The degree of LREE-enrichment in rock-forming minerals directly depends on the alkalinity of the rock. The distribution of REE between coexisting melilite and pyroxene may be used as an additional genetic criterion: p.e. magmatic La Pyx La Mel ratio is 0.91, while in metasomatic rocks this ratio is between 1.80 and 1.97.