Whole-rock REE Patterns Research Articles

The kabr El-Bonaya peridotites, Southeastern Sinai, Egypt: petrology, geochemistry, and metamorphism of Neoproterozoic arc ultramafic cumulates

Two small, isolated ultramafic masses in the northeastern part of the Wadi Kid area, southeast Sinai, are composed of variably serpentinized harzburgite and lherzolite with minor talc-anthophyllite rock. The primary phases are dominantly olivine, orthopyroxene and Cr-spinel; clinopyroxene, amphibole, and phlogopite are also found in lherzolite samples. The whole-rock Mg# of harzburgite samples (89–91) is higher than that of lherzolite (average 82). The harzburgite samples contain olivine with higher Mg and Ni contents, orthopyroxene with higher Mg#, and Cr-spinel with higher Cr content than do the lherzolite samples. The REE patterns of clinopyroxene and amphibole in lherzolite are most consistent with a cumulate origin. Although several compositional characteristics of the harzburgites resemble those of residual mantle, in detail the Cr₂O₃ and Al₂O₃ contents of fresh Cr-spinel in harzburgite are different from those found in mantle samples or in any of the Neoproterozoic ophiolitic peridotites throughout the Arabian-Nubian Shield. Thus, all the ultramafic rocks at Kabr El-Bonaya are best explained as ultramafic cumulates, with harzburgite consisting of early-formed cumulate phases and lherzolite containing later-formed cumulate phases with higher REE abundances, primary hydrous minerals, evolved primary silicates, and high TiO₂ (0.77 wt.%) and Al₂O₃ (18 wt.%) contents in Cr-spinel. The trace-element characteristics of the rocks indicate a subduction-related parental magma: whole-rock chondrite-normalized REE patterns are LREE-enriched; calculated <i>f</i>O₂ values are elevated (+2.47 to +3.39 log units above the fayalite-magnetite-quartz buffer); and computed N-MORB-normalized trace element patterns for melts in equilibrium with clinopyroxene and amphibole have negative Nb-Ta anomalies and enrichment in large-ion lithophile elements. The low Al₂O₃/SiO₂ ratios (0.007–0.040) of harzburgite samples and the low TiO₂ contents and high Cr# of their Cr-spinel indicate derivation from a mantle source that experienced high-degree partial melting. From these characteristics, we infer a boninitic parental melt for the harzburgite. We offer an illustrative quantitative fractionation model that can explain the successive derivation of harzburgite and lherzolite cumulates along a single equilibrium, polybaric cooling path. We conclude that the Kabr El-Bonaya ultramafic cumulates represent the exposed roots of a Neoproterozoic island arc that was caught in the collision between East and West Gondwana.

Eclogites of the North Atlantic Craton: insights from the Chidliak eclogite xenoliths (S. Baffin Island, Canada)

The 156–138 Ma Chidliak kimberlites on the Eastern Hall peninsula (EHP) of Baffin Island entrained mantle xenoliths interpreted to have been a part of the Archean North Atlantic Craton (NAC) lithospheric mantle. We studied 19 Chidliak eclogite xenoliths that comprise 10 bimineralic, 5 rutile-bearing, 3 orthopyroxene-bearing and 1 kyanite-bearing eclogites. We report major and trace element compositions of the minerals, calculated bulk compositions, pressures and temperatures of the rock formation and model melt extraction from viable protoliths. The eclogite samples are classified into three groups of HREE-enriched, LREE-depleted and metasomatized based on their reconstructed whole-rock REE patterns. PT parameters of the eclogites were calculated by projecting garnet–clinopyroxene temperatures onto the local P–T arrays for 65 Chidliak peridotite xenoliths. All Chidliak eclogites are equilibrated in the diamond P–T field and cluster in two groups, low-temperature (n = 5, 840–990 °C at 4.1–5.0 GPa) and high-temperature (n = 11, T > 1320 °C at P > 7.0 GPa). The reconstructed Mg-rich major element bulk compositions and trace elements patterns are similar to Archean basalts from the North Atlantic and Superior cratons and the oceanic gabbros. The LREE-depleted Chidliak eclogites could be residues after 15–55% partial melting of Archean basalt at the eclogite facies of metamorphism that led to extraction of a tonalite–trondhjemite–granodiorite melt from the EHP. The HREE-depleted eclogites may have experienced a lower degree ( 95%. The former are more magnesian, less ferrous and calcic, contain more magnesian and less calcic garnets, and lower proportions of group C eclogites. The contrast may relate to the stronger NAC metasomatism by silicate–carbonate melt observed in Chidliak peridotitic mantle, or to the different formation ages of the eclogites beneath the two cratons.

Provenance and magmatic-tectonic setting of Campanian-aged volcaniclastic sandstones of the Kannaviou Formation in western Cyprus: Evidence for a South-Neotethyan continental margin volcanic arc

Regionally developed late Cretaceous subduction-related magmatism in the eastern Mediterranean records progressive closure of the Southern Neotethys. In western Cyprus, the late Cretaceous (c. 90 Ma) Troodos ophiolite is depositionally overlain by volcaniclastic sandstones (up to 750 m thick) that are dominated by redeposited pyroclastic fallout, interbedded with both non-calcareous and calcareous radiolarian-bearing mudstones. The sands were mainly deposited by channelised mass-flow processes and to a lesser extent by turbidity currents in a deep-water forearc basin. The succession was folded and locally thrust-deformed related to latest Cretaceous emplacement of adjacent Mesozoic continental margin and oceanic units (Mamonia Complex). SIMS U-Pb analysis of euhedral to subhedral magmatic zircon crystals yielded a weighted mean 206Pb/238U age of 80.1 ± 1.1 Ma (Campanian). Sandstone petrography including compositional framework (Q12F21L67) suggests sediment supply from a volcanic arc. Subordinate continentally-derived detritus suggests a continental margin arc setting for the mafic to felsic fallout ash. SIMS analysis of little-altered volcanic glass indicates high Th/Nb and Th/La ratios that suggest the involvement of continental crust and/or subducted terrigenous sediments in magma genesis. Whole-rock chondrite-normalised REE patterns have comparable trends to modern and ancient forearc basin volcaniclastic sands and sandstones (e.g., northwest Pacific region). Trace element chemistry, although scattered towards the continental island arc fields on some geochemical diagrams (e.g., Th-Sc-Zr), mainly suggests an oceanic island arc source for the Kannaviou Formation sandstones, with variable enrichments in V-Cr-Ni-Sc, depletion in Nb-Ta, and relatively low trace element ratios (e.g., La/Co, Th/Co, La/Sc, Th/Sc). An explanation for this apparent discrepancy is that the volcaniclastic sandstones were derived from a relatively primitive arc constructed on previously depleted, rifted (thinned) Neotethyan continental crust. The inferred continental margin arc developed during early-stage northward subduction of the Southern Neotethys beneath a Tauride microcontinent to the north. Large volumes of volcanic ash were derived from continental margin arc volcanism, probably in the vicinity of the Kyrenia Range in northern Cyprus.

Major and trace element geochemistry of tourmalines from Archean orogenic gold deposits: Proxies for the origin of gold mineralizing fluids?

Orogenic style gold mineralizations in the Archean Hattu Schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary and volcanogenic rocks, as well as felsic intrusives. The gold mineralizations occur as dissemination in altered wall rocks and within hydrothermal quartz veins. Hydrothermal tourmalines are often associated with the gold mineralizations occurring in the quartz veins, as alteration minerals in contacts of the metasedimentary host rocks and quartz veins, but also within mineral assemblages of the metasedimentary and magmatic rocks. We characterize and compare the major, trace and rare earth element chemistry of these tourmalines in order to evaluate their suitability as a petrogenetic tool for tracing the fluid sources of the hydrothermal system. By comparing the chemistry of tourmalines from gold mineralization with those from metamorphic and magmatic host rocks, we test whether tourmaline composition can be used to identify the source rocks of the gold transporting hydrothermal fluids. All analyzed tourmalines belong to the alkali super group and plot along the schorl-dravite join. The hydrothermal tourmalines have Li, Sr and V concentrations comparable to metamorphic tourmalines and clearly distinct from magmatic tourmalines. Because the concentrations of Ni, Pb, Cr, Mn, Ga, Zn and Sn show a wide overlap between magmatic, metamorphic and hydrothermal tourmalines, these elements do not permit to discriminate between different source rocks. The co-variations of many elements in hydrothermal tourmalines, when plotted against Li, show more similarity to metamorphic tourmalines and the whole-rock compositions of metasedimentary and metavolcanic host rocks than to their magmatic counterparts. This indicates that the major and trace element composition of hydrothermal tourmalines in the Hattu Schist Belt is predominantly controlled by the host rocks and local fluid-rock interactions, and does not reflect the distal fluid sources. The effect of local fluid-rock interaction is also manifested by the REE patterns of tourmalines. The magmatic tourmalines have distinct LREE enriched patterns resembling the whole-rock REE patterns of granitic intrusives, while the metamorphic and hydrothermal tourmalines have flat or weakly fractionated patterns similar to metavolcanic and metasedimentary host rocks. Taken together, the tourmaline data suggest that hydrothermal tourmalines associated to gold mineralizing fluids are most likely genetically related to metamorphic rock sources without important contributions of magmatic fluids, and that local fluid-rock interaction exerted a major control on tourmaline chemistry.

Detecting homogenous clusters using whole-rock chemical compositions and REE patterns: A graph-based geochemical approach

The rock chemical composition may be affected by a wide variety of primary and secondary geological processes. Analyzing geochemical datasets of highly altered rocks is usually faced with the challenges in detecting the relationships among objects. Using traditional clustering methods in such datasets with high-dimensional data and various types of attributes commonly leads to poor quality results. Hence, a graph-based geochemical approach was proposed in this study to solve this problem. In order to determine the relationship between objects, various similarity measures related to whole-rock composition, REE pattern, and the geographical location were employed in combination to weight the edges of a similarity graph. A spectral method was effectively used to identify clusters (communities) representing rock groups, geological units, or geochemical zones in the weighted similarity graph. It could also recognize the corresponding groups being compositionally and genetically similar to each other and distinguish sub-groups or anomalous samples in the dataset with regard to the different levels of clustering. Firstly, the performance and effectiveness of the proposed approach was evaluated by testing on a GEOROC11Geochemistry of Rocks of the Oceans and Continents. dataset based on some graph clustering quality functions. Then the approach was applied to the geochemical dataset of Choghart orebody comprising various altered rocks. The obtained clusters were visualized by a k-nearest neighbor classification technique to represent geochemical zones as a continuous map.

Geochemistry of Precordillera serpentinites, western Argentina: evidence for multistage hydrothermal alteration and tectonic implications for the Neoproterozoic–early Paleozoic.

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes during their tectonic emplacement. Exposures of this type of rocks can be found in the Argentine Precordillera (Cuyania terrane) and Frontal Cordillera, both located in central-western Argentina, within the Central Andes. In these regions are outcrops of a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills and pillow lavas (with an E- to N-MORB geochemical signature) and mafic granulites, spatially associated with marine metasedimentary rocks. The serpentinite bodies consist of lizardite/chrysotile + brucite + magnetite, with scarce pentlandite and anhedral reddish-brown Cr-spinel (picotite, pleonaste and spinel sensu stricto ) as relict magmatic phases. The original peridotites were moderately-depleted harzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridge and an arc-related ophiolite. Whole-rock REE patterns of serpentinites exhibit enriched REE patterns ((La/Yb) CN =13-59) regarding CI chondrite with positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid and serpentinites, in which moderate temperature (350o-400oC), CO 2 -rich, mildly basic hydrothermal fluid was involved and was responsible for the addition of Ca, Sr and REE to serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to infer another fluid metasomatism, where low-temperatures (<250oC), highly-oxidized, highly-acid fluid lead to the precipitation of silica. The association of these metasomatized serpentinite bodies with neoproterozoic continental margin sucessions and MORB magmatism at the suture zone of the Cuyania and Chilenia terranes suggests the development of an oceanic basin between them during the Neoproterozoic-early Paleozoic.

Petrology and geochemistry of a peridotite body in Central- Carpathian Paleogene sediments (Sedlice, eastern Slovakia)

Abstract We studied representative samples from a peridotite body situated NE of Sedlice village within the Central- Carpathian Paleogene sediments in the Central Western Carpathians. The relationship of the peridotite to the surrounding Paleogene sediments is not clear. The fractures of the brecciated peridotite margin are healed with secondary magnesite and calcite. On the basis of the presented bulk-rock and electron microprobe data, the wt. % amounts of mineral phases were calculated. Most of calculated “modal” compositions of this peridotite corresponds to harzburgites composed of olivine (∼70-80 wt. %), orthopyroxene (∼17-24 wt. %), clinopyroxene ( &lt; 5 wt. %) and minor spinel ( &lt; 1 wt. %). Harzburgites could originate from lherzolitic protoliths due to a higher degree of partial melting. Rare lherzolites contain porphyroclastic 1-2 mm across orthopyroxene (up to 25 wt. %), clinopyroxene (∼ 5-8 wt. %) and minor spinel ( &lt; 0.75 wt. %). On the other hand, rare, olivine-rich dunites with scarce orthopyroxene porphyroclasts are associated with harzburgites. Metamorphic mineral assemblage of low-Al clinopyroxene (3), tremolite, chrysotile, andradite, Cr-spinel to chromite and magnetite, and an increase of fayalite component in part of olivine, indicate low-temperature metamorphic overprint. The Primitive Mantle normalized whole-rock REE patterns suggest a depleted mantle rock-suite. An increase in LREE and a positive Eu anomaly may be consequence of interactive metamorphic fluids during serpentinization. Similar rocks have been reported from the Meliatic Bôrka Nappe overlying the Central Western Carpathians orogenic wedge since the Late Cretaceous, and they could be a potential source of these peridotite blocks in the Paleogene sediments.

Geochemistry and Genesis of the Sodium Charnockitic Gneisses, Eastern Hebei Province, China

Abstract High in sodium and low in potassium (Na2O/K2O > 1), the charnockitic gneiss series in the Santunying- Taipingzhai area, eastern Hebei province, consists of hypersthene- quartz- diorite, hypersthene-granodiorite and hypersthene-plagioclase-granite. Geological, petrological and large ion lithophile element (LILE), high field strength element (HFSE) and REE geochemical studies suggest that the medium-coarse-grained hypersthene-granodiorite is the product of crystallization of anatectic magmas of the same composition. Under granulite facies conditions, the equilibrium crystallization differentiation of the magmas yielded the early crystallization phase—high- SiO2, LILE-depleted, low-ΣREE, positive Eu anomaly and REE- saturated hypersthene- plagioclase- granite. The residual phase, coarse- grained to pegmatitic hypersthene-granodiorite, is marked by low SiO2, LILE-enrichment, high ΣREE and REE-undersaturation. These rocks and hypersthene-quartz-diorite enclaves constitute the sodium-charnockitic gneiss series in eastern Hebei province. Model calculation for trace elements in the granitoids was applied. On the basis of a systematic geological study, the equation for calculation was chosen, the source magma was determined and the partition coefficients were obtained. The resulting curves are entirely consistent with those observed in the patterns of actual rocks. The study indicates that whole-rock REE patterns can not be used directly in the comparison of the sources and genesis of granitoids.

Origin and significance of spinel and garnet pyroxenites in the shallow lithospheric mantle: Ultramafic massifs in orogenic belts in Western Europe and NW Africa

Anhydrous spinel and garnet pyroxenites form a small (< 10%) but integral part of the shallow sub-continental lithospheric mantle. They occur as layers or dykes within tectonically-emplaced ultramafic massifs and display lithologies that range from orthopyroxenite through websterites to clinopyroxenite, with or without olivine. The ultramafic massifs in orogenic belts of western Europe and Morocco contain many different varieties of mantle pyroxenites that show a wide range of major and trace element compositions. Their bulk compositions tend to vary between at least three end-members: two of these are similar to the consitutent clinopyroxene and orthopyroxene, while the third component has concentrations of MgO, CaO, Al 2O 3, TiO 2 and Na 2O that are close to basaltic levels and may indicate the former presence of trapped basaltic melt. These pyroxenites are probably cumulates in the broadest sense. Nevertheless, some pyroxenites from Beni Bousera and Ronda have whole rock compositions that are similar to that of bulk oceanic crust. Most mantle pyroxenites have LREE-depleted whole-rock REE patterns, although with lower absolute concentrations than MORB or bulk oceanic crust, and most lack Eu anomalies. Trace element patterns in clinopyroxenes from pyroxenites are also usually LREE-depleted. Most clinopyroxenes show depletions in Zr, Hf and Sr, although those from the Cabo Ortegal complex (NW Spain) display LREE-enrichment and significant Sr peaks, attributed to the influence of subduction-related fluids or melts. δ 18O values for clinopyroxenes from mantle pyroxenites in ultramafic massifs tend to be in the range 5.0–5.8‰, i.e. typical mantle values. Rare garnet pyroxenite layers from a few massifs show higher values that reflect recycling of a crustal component. Similarly, most Sr, Nd and Pb isotopic ratios from pyroxenites fall in the depleted mantle field, although with a slightly wider range of values than is typical for shallow sub-continental mantle peridotites from Europe. Most mantle pyroxenites are best explained as products of crystal accumulation of mantle-derived magmas, together with variable amounts of trapped interstitial magma. Others may be the products of interaction between magma and peridotite or older pyroxenite wall-rock. Only samples from a few massifs (e.g. garnet pyroxenites from Beni Bousera and Ronda) display strong evidence of being related to recycled oceanic crust or melts of oceanic crust. Even fewer mantle pyroxenites show good evidence for derivation either by incipient melting of the host peridotite or by metamorphic segregation. Pyroxenites are low-MgO, high CaO, high Al 2O 3 lithologies that provide a well-documented example of enrichment of the lithospheric mantle. If such material were recycled into the asthenosphere, mantle pyroxenites might melt preferentially to the host peridotite. However, the precise role of mantle pyroxenites in magmagenesis could be difficult to track, as many of their isotope and trace element compositions (e.g Sm/Nd ratios) are often similar to those of their host mantle peridotites.

Assessing the impact of black shale processes on REE and the U–Pb isotope system in the southern Appalachian Basin

Upper Devonian Java and Huron shales from the southern Appalachian Basin exhibit evidence of REE redistribution and resetting of the U–Pb isotopic system at the time of deposition (ca. 365 Ma). This alteration to the primary detrital signature of these shales is indicative of black shale diagenetic/depositional processes that in effect obscure paleoenvironmental and provenance information that may be recorded. The most direct evidence of these black shale processes is the rotated nature of the whole-rock REE patterns from this sequence of presumably source related shales. The neo-formation of REE-bearing diagenetic phases such as apatite and possibly carbonate has led to an unusually large range in the whole-rock 147Sm/ 144Nd and the Ce/Ce* for this sequence of shales. Changes to these whole-rock ratios will effectively alter the Nd model age and create a Ce/Ce* that records evidence of diagenetic processes rather than information about the paleooceanographic conditions of the basin. Simultaneously, the sequestration of U at the time of deposition and subsequent fixation in diagenetic phases such as apatite has effectively reset the 238U– 206Pb isotopic system at 340±50 Ma overprinting the Pb isotopic signature of the source area. The trace-element and isotopic signatures recorded by the Java and Huron formation shales from the study area reveal a syn-/postdepositional history dominated by what can be referred to as black shale processes. The diagenetic conditions that develop at and below the sediment water interface during the deposition of organic-rich shale are unique and consequently leave behind a unique geochemical fingerprint that can potentially overprint paleoenvironmental and provenance information.

Boron metasomatism and behaviour of rare earth elements during formation of tourmaline rocks in the eastern Arunta Inlier, central Australia

Tourmaline rocks of previously unclear genesis and spatially associated with W- (Cu)-bearing calc-silicate rocks occur in Palaeoproterozoic supracrustal and felsic intrusive rocks in the Bonya Hills in the eastern Arunta Inlier, central Australia. Tourmalinisation of metapelitic host rocks postdates the peak of regional low-pressure metamorphism (M1/D1, ~500 °C, ~0.2 GPa), and occurred synkinematically between the two main deformation events D1 and D2, coeval with emplacement of Late Strangways (~1.73 Ga) tourmaline-bearing leucogranites and pegmatites. Tourmaline is classified as schorl to dravite in tourmaline–quartz rocks and surrounding tourmaline-rich alteration zones, and as Fe-rich schorl to foitite in the leucogranites. Boron metasomatism resulted in systematic depletion of K, Li, Rb, Cs, Mn and enrichment of B, and in some samples of Na and Ca, in the tourmaline rocks compared to unaltered metasedimentary host rocks. Whole-rock REE concentrations and patterns of unaltered schist, tourmalinised schist and tourmaline–quartz veins—the latter were the zones of influx of the boron-rich hydrothermal fluid—are comparable to those of post-Archaean shales. Thus, the whole-rock REE patterns of these rocks are mostly controlled by the metapelitic precursor. In contrast, REE concentrations of leucogranitic rocks are low (≤10 times chondritic), and their flat REE patterns with pronounced negative Eu anomalies are typical for fractionated granitic melts coexisting with a fluid phase. REE patterns for tourmalines separated from metapelite-hosted tourmaline–quartz veins and tourmaline-bearing granites are very different from one another but each tourmaline pattern mirrors the REE distribution of its immediate host rock. Tourmalines occurring in tourmaline–quartz veins within tourmalinised metasediments have LREE-enriched (LaN/YbN=6.3–55), shale-like patterns with higher ΣREE (54–108 ppm). In contrast, those formed in evolved leucogranites exhibit flat REE patterns (LaN/YbN=1.0–5.6) with pronounced negative Eu anomalies and are lower in ΣREE (5.6–30 ppm). We therefore conclude that REE concentrations and patterns of tourmaline from the different tourmaline rocks studied are controlled by the host rock and not by the hydrothermal fluid causing boron metasomatism. From the similarity of the REE pattern of separated tourmaline with the host rock, we further conclude that incorporation of REEs in tourmaline is not intrinsically controlled (i.e. by crystal chemical factors). Tourmaline does not preferentially fractionate specific REEs or groups of REEs during crystallisation from evolved boron- and fluid-rich granitic melts or during alteration of clastic metasediments by boron-rich magmatic-hydrothermal fluids.

Light rare earth element enrichments in ureilites: A detailed ion microprobe study

Abstract— This paper explores the possible origin of the light rare earth element (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C‐rich matrix areas of the three ureilites with the most striking V‐shaped whole‐rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe.All olivines and clinopyroxenes measured have LREE‐depleted patterns with little variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched for but did not find any minor mineral phases that carry LREEs. The only exception is one Ti‐rich area (∼20μm) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundances in this area are high, ranging from La ≅ 400 × CI to Lu ≅ 40 × CI. In contrast, all ion microprobe analyses of C‐rich matrix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In addition, C‐rich matrix areas in the three polymict ureilites, Elephant Moraine (EET) 83309, EET 87720, and North Haig, which have less pronounced V‐shaped whole‐rock REE patterns, show smaller but distinct LREE‐enrichments. The C‐rich matrix in Antarctic ureilites tends to have much lower LREE concentrations than the matrix in non‐Antarctic ureilites. There is no obvious association of the LREEs with other major or minor elements in the C‐rich areas. Ion images further show that the LREE enrichments are homogeneously distributed on a microscale in most C‐rich matrix areas of Kenna, Goalpara, and Novo Urei. These observations suggest that the LREEs in ureilites most probably are absorbed on the surface of fine‐grained amorphous graphite in the C‐rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We favor LREE introduction by terrestrial contamination.

Composition and Sm-Nd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite

Quaternary volcanics from San Luis Potosí (SLP) in central Mexico sampled a xenolith suite comprising uppermantle nodules (spinell lherzolites, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xenoliths. The latter consist of granulite-facies, mafic to intermediate metaigneous rocks and high-grade metasediments. Precursors of the metaigneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulates and fractionated melts.Mafic xenoliths consist of plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± amphibole ± apatite ± rutile ± scapolite. Garnet has reacted extensively with clinopyroxene, forming corona textures of secondary plagioclase, orthopyroxene and amphibole. Intermediate xenoliths are garnet-free and quartz-bearing. All samples display similar Ca-Al zoning patterns documented in orthopyroxene, clinopyroxene and plagioclase which are, along with the presence of coronas around garnet, interpreted as representing partial re-equilibration stages from 940 ± 60°C and 7–11 kbar down to lower temperatures and pressures. There is no indication of a secondary heating event in these xenoliths.Sm-Nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal that the hornblende pyroxenites are high-pressure cumulates linked to the host basanites. Four mainly intermediate metaigneous granulites from a single vent, which are free of secondary amphibole, and two spinel pyroxenites, regarded as the most primitive member of the metaigneous suite, define a regression line in a Sm-Nd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initial ϵNd-value of +2.3 Metaigneous xenoliths containing secondary amphibole plot significantly above the regression line, which is probably due to the introduction of externally derived Nd during amphibole formation under retrograde p-T conditions. Discussing the geological significance of the calculated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This gives evidence for formation of lower crust of basic to intermediate compositions during the mid-Proterozoic, now situated beneath SLP.

Late Archaean intrusive complexes in the Olekma granite-greenstone terrain (eastern Siberia): geochemical and isotopic study

Two granitoid intrusions, the Amnunnakta and Oldongso massifs in the Olekma granite-greenstone terrain of the Aldan shield have been studied using Sm-Nd, Rb-Sr and Pb-Pb whole rock (WR) and U-Pb zircon methods. The isotopic composition of Pb in plagioclase, as well as whole-rock oxygen isotope compositions and REE patterns were also measured. The following ages were obtained for the Amnunnakta massif: U-Pb zircon 2984±22 Ma (2σ), Pb-Pb WR 2991±57 Ma, Sm-Nd WR 3094±430 Ma and for the Oldongso massif: U-Pb zircon 2999±51 Ma, Pb-Pb WR 2855±170 Ma, Sm-Nd WR 2923±144 Ma. The similar ages suggest penecontemporaneous formation of the two intrusive series. The observed resetting of the Rb-Sr WR systems (2767±116 Ma for Amnunnakta massif and 2597±82 Ma for the Oldongso massif) and opening of the plagioclase Pb isotopic systems are associated with superimposed metamorphic processes of the greenschist and epidote-amphibolite facies. Source regions of the massifs are characterized by long-term depletion in incompatible elements: the initial ϵ Nd(T) value is +2.4±0.4 for both massifs. The μ 1 values are mantle-like at 7.68 for the Amnunnakta and 7.71 for the Oldongso massifs and the initial Sr composition is low (Sr i=0.7015±5) for the Oldongso massif. Basic rocks of the Amnunnakta massif are characterized by low δ 18O values (2.7–4.7) and elevated Sr i=0.7052±7 which may be caused by both a primary low δ 18O, high rm 87Sr 86Sr magma or later interaction with a hydrothermal system. There is no evidence for recycled sedimentary material in the granitoids of either massif. One of the rocks, a fine-grained garnet-bearing trondhjemite, previously assigned to the Amnunnakta massif, yielded a T Nd model age of 3700 Ma and reflects the presence of older sialic material. Moderately fractionated REE patterns with well pronounced negative Eu anomalies in low-alumina trondhjemites of the Amnunnakta massif suggest their formation by a process of differentiation of high-alumina basaltic melts with fractionation of plagioclase. High-alumina tonalite-trondhjemites of the Oldongso massif are characterized by fractionated and depleted HREE patterns suggesting the residual presence of garnet and hornblende during partial melting of a basic source.

Rare-earth element mobility during hydrothermal and metamorphic fluid-rock interaction and the significance of the oxidation state of europium

The Eu 3+ Eu 2+ redox boundary for temperatures up to 600°C and pressures up to 4000 bar has been calculated using the “revised HKF model” for the prediction of the molal Gibbs free energy of formation at elevated pressures and temperatures. The results indicate that the oxygen fugacity ( f O 2 ) at which trivalent Eu is reduced to Eu 2+ increases with increasing temperature. Thus, in high-temperature regimes Eu can be fractionated from the other REE. If in fluid-rock interaction under mildly acidic conditions the REE pattern of the fluid is controlled by sorption processes, the decreasing ionic radius from La to Lu leads to ( La Lu ) cn > 1 , and, provided Eu occurs as Eu 2+, to a positive Eu anomaly (compared to the source-rock for the REE) in the fluid. Under nearly neutral to mildly basic conditions the REE pattern of the same fluid is governed by complexation mechanisms with carbonate, fluoride and hydroxide complexes being the most important REE species. This leads to ( La Lu ) cn < 1 . Because complexation extends the stability of Eu 3+ towards lower f O 2 , reduction to Eu 2+ under mildly basic conditions is limited to more reducing environments. Since the REE content of alteration fluids is extremely low, alteration of whole-rock REE patterns during hydrothermal or metamorphic fluid-rock interaction is rather ineffective, unless the water/rock ratio is ⪢ 10 2–10 3 or infiltration metasomatism is severe. Apart from these situations, REE systematics should not be affected by hydrothermal or metamorphic fluid-rock interaction.

REE, RbSr and SmNd studies of Norwegian eclogites

REE analyses and isotopic studies have been carried out on Norwegian eclogites and garnet peridotites to determine the age of metamorphism and the origin and history of the protoliths. Whole-rock REE patterns of eclogites in gneiss vary greatly, even within single outcrops. Many show positive Eu anomalies, indicating plagioclase fractionation and thus gabbroic protoliths. The irregular REE patterns and the common depletion of La and Ce relative to the middle REE, may reflect metasomatic effects during high- P metamorphism. The distribution of REE between clinopyroxene (cpx) and garnet (gnt) in the garnet peridotites is consistent with equilibration at lower T than the corresponding phases of gnt-lherzolite nodules in kimberlites. If the Norwegian gnt-peridotites are fragments of subcontinental mantle, they have undergone post-emplacement equilibration. Most eclogites that have high T MET (> 700°C) also show equilibrium distribution of REE between cpx and gnt, with large spreads in Sm/Nd. Gnt-cpx pairs from five such eclogites give SmNd ages from 407–447 Ma. In lower- T eclogites (∼ 600°C) the REE have not equilibrated between gnt and cpx; no spread in Sm-Nd is observed, and no dates have been obtained. I Sr varies from 0.7025 to 0.7245 in 21 samples of eclogites in gneisses. Most orthopyroxene (opx)-free eclogites fall in the range 0.7025–0.7060, while 7 or 8 opx-bearing ones have I Sr > 0.715. While most opx-free eclogites may be derived from gabbroic protoliths, the opx-eclogites must either be derived from old Rb-rich protoliths, or have interacted extensively with the surrounding gneisses. The Sr and Nd isotopic data suggest that the rocks studied here all had complex crustal histories prior to the eclogite-forming metamorphism. Mica-cpx or mica-whole rock (WR) RbSr “ages” on Norwegian eclogites range from 1200 to < 400 Ma. Many samples show marked isotopic disequilibrium and the detailed significance of these “ages” is debatable. However, a five-point mineral isochron on one kyanite eclogite gives an isochron age of 398±1 Ma, with I Sr = 0.7034. The eclogite-forming event in western Norway was apparently part of theCaledonian continent-continent collision, in which the Western Norwegian Gneiss terrane was overridden by the Greenland plate. The Wenlockian SmNd mineral ages may refer to the beginning of uplift after this collision, while the RbSr mineral ages probably reflect a later stage of the uplift-cooling cycle.

Rare earth element geochemistry of garnet lherzolite and megacrystalline nodules from minette of the Colorado plateau province

Mineral and whole-rock REE abundances in garnet lherzolite and megacrystalline nodules from The Thumb display broad correlations with major element compositions. Lherzolites with > 12 modal % clinopyroxene plus garnet (“high-Ca Al lherzolites”) have relatively flat chondrite-normalized whole-rock REE patterns. Lherzolites poor in clinopyroxene and garnet (“low-Ca Al lherzolites”) have lower HREE in clinopyroxenes and garnets and higher whole-rock LREE/HREE. It is concluded that the low-Ca Al lherzolites may have undergone LREE metasomatism after depletion of the major element compositions by partial melting and that much of the garnet now present was originally dissolved in aluminous orthopyroxene. The high-Ca Al lherzolites may be interpreted either as primordial mantle samples or as products of equilibration with very LREE-enriched liquids. The “megacrystalline” nodules are medium- to ultracoarse-grained intergrowths and megacrysts with mineral compositions similar to discrete nodule suites in kimberlites. The REE abundances of the megacrystalline minerals are consistent with an origin as cumulates from magma with extremely fractionated REE, similar to minette or kimberlite. The patterns of correlation of REE and major elements in this inclusion suite are similar to the patterns observed in the garnet lherzolite and discrete nodule suites of southern African kimberlites. Both of the subcontinental mantle provinces represented by these suites contain three distinct petrogenetic components: refractory garnet lherzolite enriched in LREE and depleted in HREE, fertile garnet lherzolite with generally chondritic REE abundances, and a suite of ultracoarse minerals precipitated from magma with extremely fractionated REE generally similar to the host magmas.