REE Data Research Articles

Detrital zircon REE and tectonic settings

Earth's long-term history is preserved in the rock archive, yet this record is incomplete, hindering our understanding of how tectonic processes have evolved and shaped our planet's evolution. The physio-chemical resilience of zircon and its ability to be rapidly analyzed for age, isotopic, and elemental data has made it a key phase in unraveling Earth's long-term evolution. The europium anomaly (Eu/Eu*) and the differentiation degree between light and heavy rare earth elements (LREEN¯/HREEN¯) of zircons positively correlate with the pressure of magma crystallization because high pressure inhibits plagioclase crystallization and promotes garnet growth. However, this correlation can be influenced by fluctuations in redox conditions, protolith composition, and water content of magma, whose stability varies in different tectonic settings. Therefore, the correlation coefficient between Eu/Eu* and LREEN¯/HREEN¯ of detrital zircons (rDz) provides a new proxy to help distinguish tectonic settings. We evaluate this new proxy using available global detrital zircon REE data from different tectonic settings in modern and deep times. The data from well-constrained simple tectonic settings suggest that rDz is higher in convergent settings (0.53–0.85) than in collisional settings (0.12–0.51). Convergent settings can also yield relatively low rDz (0.28–0.55) when there is a contribution of a mantle plume lying below the overriding plate of a subduction zone.When studying a detrital zircon archive derived from a large distributive province that includes sources from a variety of settings, known provenance information should be used to disentangle mixed source signals. Also, only considering detrital zircon grains that formed a short time (e.g., 50 or 100 Myr) prior to deposition can reduce the likelihood of multi-cycle distal mixing of contemporaneous but tectonically distinct zircons in a sample. Additionally, greater confidence in interpretations of changes in tectonic settings may be derived from evolution patterns of rDz instead of isolated values. A decreasing trend can likely be interpreted as a transition from convergent to collisional or mantle plume settings, whereas an increasing trend implies an opposite tectonic transition.

The tectonic setting of the late Ediacaran eastern Arabian basement (ca. 550 Ma): New geochronological and geochemical constraints from the basements of Oman and the United Arab Emirates

The thick Phanerozoic sedimentary cover lying above the Arabian Platform conceals extensive basement between the western Neoproterozoic domains of the Arabian Nubian Shield (ANS) and of the eastern Omani basement. The Neoproterozoic rocks entrained within salt domes that crop out within the United Arab Emirates (UAE) and Qatar are the only direct samples that allow us to investigate the tectonic history of the concealed Arabian basement domains before c. 550 Ma. This study focuses on the U-Pb geochronological and REE geochemical analysis of detrital zircon, apatite, titanite and rutile grains from the Zirku turbidites brought to the surface by halokinetic movements offshore UAE. These data are compared with new U-Pb and REE data from detrital heavy minerals of three metasedimentary members of the Omani Neoproterozoic Hatat Formation collected in the Saih Hatat window. The Omani metasedimentary units yield detrital titanite, zircon and apatite U–Pb ages that span from c. 2500 to 690 Ma, with an age maxima at c. 850 Ma, while the UAE unit yield ages from c. 2550 to 550 Ma with major age peaks at c. 1000, 820, 650 Ma. The analysis of the Omani stratigraphic sequence in the Saih Hatat window revealed that the rocks of the Neoproterozoic Hatat Formation yield similar age distributions of the Abu Mahara Group (lower Huqf Supergroup, exposed in central and southern Oman), and that the REE geochemical composition of the Hatat detrital apatite grains shares akin REE signature with the c. 550 detrital apatite of the Zirku Formation, with both being sourced from volcanic-arcs. These new geochronological and geochemical data, together with recent geophysical evidences, suggest that the basement of the UAE is originally part of the Oman tectonic domain and evolved as part of a volcanic-arc in an active margin at c. 550 Ma, which we interpret as part of the retro-arc of the Cadomian collision in between the Arabian and the Cadomian Taurides, Iran and Afghanistan terranes.

Tracing provenance of eolian sands in the northern Ulan Buh Desert, China: Evidence from the REE geochemical characteristics

Desertification is a primary threat to crop farming, the livestock industry, and the eco-environment in northwestern China. Through field research and satellite imagery, it has been discovered that there are 4–5 sand belts in the shape of handclasps connecting major deserts in the Alxa Plateau region of northern China. These sand belts have been expanding for several decades. However, eolian sand transport between major deserts through these expanding sand belts is still underestimated. Identifying the source areas of eolian sands in deserts provide essential knowledge to prevent desertification. In this study, we collected samples from the northern Ulan Buh Desert (UBD), the Yamalik Desert (YD), and the Langshan Mountains and analyzed their REE contents in the <125 μm fraction. We compared REE geochemical characteristics of fine-grained sands in the UBD with other recently published REE data for potential source areas. Our findings indicate that the primary origins of fine-grained sands in the northern UBD are from the Qilian Mountains (QM) and Badain Jaran Desert (BJD), which are transported through the YD. Contributions from the Gobi-Altay Mountains to the north are relatively minor. The Langshan Mountains and Hawula Mountains are not considered as the primary sources of fine-grained sands in the UBD. In contrast, surface sediments in the southern UBD have a local mixture of the paleo-lacustrine deposits, the Gobi sediments, and minor external inputs from the BJD and QM. Our findings demonstrate that sand belts connecting these deserts play a crucial role in the eolian transport of fine-grained sands. Therefore, both the Chinese government and scientists should pay close attention to them in future efforts to combat desertification.

Negative δ13Ccarb excursions within early part of the Lomagundi event recorded in the Paleoproterozoic sedimentary carbonates, Aravalli Supergroup, Rajasthan India: Chemostratigraphy and basin evolution

High resolution stable isotopic (C and O) and geochemical studies of Paleoproterozoic carbonates were carried out in the Umra sub-basin of the Aravalli Supergroup, Rajasthan, India. All the sections, Umra-Dedkiya (UM), Umra-Jampa (BD), and Matoon (MU and GU), have yielded characteristic Lomagundi type positive values, with δ13Ccarb > 4 ‰ VPDB (maximum up to 6.86 ‰). Samples of two sections (UM and BD) have also yielded several distinct negative δ13Ccarb values (up to −6.14 ‰ VPDB). Geochemical validation of trace element and REE data indicate both these types of excursions to be of pristine character, with no signature of organic-remineralisation or hypersaline depositional condition. Mineralogical assemblages lack hypersalinity indicator minerals, such as Na-tourmaline, Na-Albite or scapolite, in the positive δ13C bearing carbonate samples. This supports the inference regarding the observed positive δ13C excursions mimicking the global Paleoproterozoic Lomagundi event. Chemostratigraphic correlation within the basin and with one of the global strato-type Paleoproterozoic carbonate unit from Paso Severino Formation, Uruguay, indicate interruptions in the initial part of the Lomagundi episode marked by several negative δ13C excursion events before achieving a stable positive δ13C character. This phenomenon is also evident from variation in paleoproductivity proxies, such as TOC, TN, Ba and P values, consistent with δ13C fluctuations. Mathematical model calculations have corroborated such a conclusion. All these observations motivated us to present a modified version of the global Paleoproterozoic δ13Ccarb curve and a six stage basin evolution model to explain the evolution of δ13C vis-à-vis variation in atmospheric oxygen.

Trondhjemites from the Western Dharwar Craton, Southern India: Implications for Mesoarchean crustal growth

Trondhjemite, together with tonalite and granodiorite (TTG) are important building blocks of the Archean continental crust and their studies provide insights into the formation of the early continents. The Western Dharwar Craton (WDC) in southern India contains voluminous TTG rocks that record prolonged Archean crustal evolution history with evidence for multiple episodes of magmatic accretion and metamorphic events from the Paleoarchean (∼3600 Ma) to Archean-Proterozoic transition (∼2500 Ma ago). In this study, we investigate trondhjemites from different typical locations in the WDC. Petrological, geochemical and zircon UPb, REE and LuHf data are presented and evaluated to understand the timing of magma emplacement and petrogenetic history. The trondhjemites in our study were generated from partial melting of juvenile hydrous basalt with arc features. The melting depth is variable, mainly shallow, while some melts originated at greater depths with variable plagioclase, amphibole with or without garnet, ilmenite or rutile in source residue. The age data presented here suggest that the crust accretion of the Mesoarchean Western Dharwar Craton occurred dominantly at 3.13–3.27 Ma followed by reworking of lower crustal basement 3.0–2.9 Ga.

Extreme serpentinization and desulfurization in an early Earth setting

The ca. 3.5 Ga Bon Accord Ni deposit was located in the Barberton Greenstone Belt of the Kaapvaal Craton, South Africa, and contained a unique assemblage of Ni-rich minerals including trevorite (NiFe2O4). It was initially interpreted as a metamorphosed extraterrestrial body, recently ruled out by Cr isotope data, and subsequently as a fragment of the Archean Earth’s core. More recent suggestions have highlighted similarities between Bon Accord and Archean magmatic sulfide mineralization. We present a geochemical (rare earth element [REE], halogen, and highly siderophile element [HSE] abundance) and isotopic (noble gases, Zn, Cr, and Re-Os) study to elucidate the origin of this enigmatic body. Bon Accord is enriched in the REE relative to primitive mantle (PM), with a pattern resembling that of Al-depleted komatiites. The bulk material has &amp;gt;10 × PM Os and Ir, &amp;gt;100 × PM Ru, Pt, Pd, and Re, and radiogenic 187Os/188Os. Trevorite, silicates, and two bulk-rock samples are consistent with chondritic to sub-chondritic initial 187Os/188Os at the time of formation. The new REE data implicate a komatiite precursor in the formation of Bon Accord, and the HSE data bear striking similarities to those of Ni-enriched Archean magmatic sulfide deposits. Enrichment in the heavier Zn isotopes supports desulfurization of a sulfide (isotopically light) deposit during serpentinization to produce the trevorite-dominated body. We conservatively estimate this process could have mobilized as much as ∼9.2 × 104 tonnes of H2S, offering an intriguing possibility for sourcing of the sulfur that fixes the historically important gold mineralization in the Barberton Greenstone Belt.

Late Carboniferous palaeodepositional changes recorded by inorganic proxies and REE data from the coal-bearing strata: An example on the Czech part of the Upper Silesian Coal basin (USCB)

In the Czech Republic, coal-bearing siliciclastic sediments have been deposited during the Serpukhovian and Bashkirian (Carboniferous). Until now, no attention has been paid to inorganic geochemical assessment of the coals and associated non-coal rocks from the mixed shallow-marine to continental sediments (Ostrava Formation), and continental non-marine settings (Karviná Formation). Samples were collected from a 750 m deep coal exploration borehole at the ČSM Mine. The bulk parameters, total organic carbon TOC, total inorganic carbon TIC, total sulphur TS, major elements, trace elements, and REEs were measured on these samples, and their mineral associations have been investigated using microscopy combined with the principal component analysis (PCA). Common redox proxies V/Cr, U/Th, Ni/Co, Mo/U, and the ratio S/TOC have been tested on the samples to investigate their usefulness for studying anoxia. Research concludes that redox proxies such as U/Th, Ni/Co and V/Cr have been strongly influenced by the clastic input and carbonates, which it hinders for them to be reliable indicators of anoxia. On the basis of Eu anomaly and REEs distribution, the primary source of detrital elements comes from the parent rock, being governed more by physical than redox processes.

The Devonian-Carboniferous transition at Borkewehr near Wocklum (northern Rhenish Massif, Germany) – a potential GSSP section

The Borkewehr section near Balve at the southeastern end of the Remscheid-Altena Anticline is currently the best and most complete Devonian/Carboniferous Boundary (DCB) succession of the Rhenish Massif, Germany. Based on a multidisciplinary approach, focusing on refined conodont and ammonoid biostratigraphy, microfacies analyses, sequence stratigraphy, carbon isotopes (Corg and Ccarb), trace element geochemistry, including REE data, and cyclostratigraphy, the section is proposed as a candidate for the new basal Carboniferous GSSP. The succession represents a pelagic “seamount” setting. It spans the upper Famennian to lower Viséan, with a condensed development of most Rhenish lithostratigraphic units and sedimentary sequences. It is the type-section of the oxic and strongly cyclic, fossiliferous Wocklum Limestone and of important uppermost Famennian and basal Carboniferous index species. The First Appearance Datum (FAD) of Protognathodus kockeli s.str., in a phylogenetic succession from the ancestral Pr. semikockeli n. sp., is proposed as future GSSP level. This FAD is sandwiched between many other marker levels for global correlation. Below are, from base to top, the transgressive base of the at least partly anoxic Hangenberg Black Shale (base Lower Hangenberg Crisis Interval), the Hangenberg Regression (onset of siltstones of the basal Middle Crisis Interval), the level of maximum regression, indicated by maximum Zr/Al values, and the initial transgression of the basal Upper Crisis Interval, marked by the re-onset of carbonate deposition. Above follow the locally cryptogenic FAD of Siphonodella (Eosiphonodella) sulcata s.l. at the base of the post-Crisis Interval (Hangenberg Limestone), associated with a conodont biofacies shift, the FAD of Si. (Eo.) bransoni, and, much higher, the sharp Lower Alum Shale Event at the base of the classical middle Tournaisian. The significance of the section is greatly enlarged by its precise correlation with numerous other Rhenish DCB sections that provide important additional data on faunas, geochemistry, geochronological ages, and magnetic susceptibility. The new Borkewehr data suggest changes of weathering intensity, based on Rb/K and K/Al trends, and Milankovitch cyclicity associated with the climate changes of the Hangenberg Crisis. The local conodont extinction rate at the top of the Wocklum Limestone lies at 57 % while there are no survivors in ammonoids, trilobites, or deep-water corals. Positive carbon isotopic shifts of carbonate in the Upper Crisis Interval and of Corg in the Middle Crisis Interval are decoupled, which suggests complex underlying processes that are not yet understood.

NanoSIMS Analysis of Rare Earth Elements in Silicate Glass and Zircon: Implications for Partition Coefficients

We have developed a method to analyze all rare earth elements in silicate glasses and zircon minerals using a high lateral resolution secondary ion mass spectrometer (NanoSIMS). A 2nA O− primary beam was used to sputter a 7–8-μm diameter crater on the sample surface, and secondary positive ions were extracted for mass analysis using an accelerating voltage of 8 kV. A high mass resolving power of 9,400 at 10% peak height was attained to separate heavy REE from oxide of light REE. A multi-collector system combined with peak-jumping by magnetic field was adjusted to detect REEs and silicon-30 for calibration. Based on results of NIST SRM610 glass, sensitivities of REEs vary from 3 cps/ppm/nA of Lu to 13 cps/ppm/nA of Eu. Reproducibility of REE/Si ratios is better than 18% at 2σ. Secondary ion yields of REEs show positive relationships with their ionization potential of second valence. REEs of AS3, QGNG, and Torihama zircons were measured and calibrated against those of 91500 standard zircon. SIYs of REEs of zircon are identical to those of the glass standard. AS3 and QGNG data are generally consistent with those of previous work. Torihama REE data combined with the whole rock data provide partition coefficients of REEs between silicate melt and zircon. The relationship between these coefficients and ionic radius is explained by an elastic moduli model.

The Paleoproterozoic magmatic arc of Trivandrum Block, southern India: From Columbia to Gondwana

The southern part of Peninsular India is designated as the ‘Southern Granulite Terrane’ (SGT), the southern domain of which is composed of three Paleoproterozoic crustal segments – the Madras, Trivandrum and Nagercoil blocks. The Trivandrum block is mostly covered by metasupracrustals, termed as khondalites, representing granulite facies metapelites with the basement rocks occurring as NW-SE trending ribbon-like exposures and previously termed as massive charnockites. Here we investigate these basement rocks and identify that they are dominantly composed of charnockites (felsic) and enderbites (intermediate) corresponding to orthopyroxene bearing granulite facies granitoids, together with (meta) gabbro and diorite. We present petrologic, geochemical and zircon U-Pb-Hf and REE data on these rocks. The gabbros are mostly ferroan and calc-alkaline to calcic affinity with tholeiitic parentage. Their REE patterns are relatively flat and both REE and trace element plots indicate an E-MBORB source. The diorites are ferroan and tholeiitic with calcic to calc-alkaline nature and are slightly metaluminous The enderbite corresponds to granodioritic composition with ferroan signature and tholeiitic lineage. The charnockites are high silica granitoids with a transitional feature between magnesian and ferroan. Marked LREE enrichment and steep LREE to HREE slope, HFSE depletion, and other geochemical features suggest arc magmatic affinity in a subduction-related setting. Trace element discrimination plots also suggest subduction-related origin for all the rock types. Zircon U-Pb data show that the gabbro, charnockite and enderbite are broadly coeval with magma emplacement during the Paleoproterozoic at 1990–1962 Ma, and the dominantly positive εHf(t) values (up to + 7.0) indicate formation of new crust from juvenile magmas. The diorites represent a slightly younger phase of magmatism at 1808–1794 Ma involving crustal recycling as indicated by the negative εHf(t) values (-5.8 to −10.5). The construction of the Trivandrum block correlates with the timing of assembly of Columbia. Zircon U-Pb data also date the timing of metamorphism as 590–496 Ma, suggesting long-lived heat input and metamorphism, coinciding with Gondwana amalgamation. A compilation of the zircon U-Pb-Hf data from this block shows a major pulse of magmatism at ca. 2.1 to 1.9 Ga that involved building of new crust from juvenile magmatic sources. In contrast, the younger pulse which lasted up to 1.7 Ga dominantly involved the reworking of Paleoproterozoic as well as the vestiges of ancient Meso-Neoarchean crust. Thus, the Trivandrum Block records the gradual maturity of a subduction-related magmatic arc within a Paleoproterozoic convergent margin regime. The lithologic assemblages from the basement of Trivandrum block trace the history from the Paleoproterozoic Columbia to the late Neoproterozoic – Cambrian Gondwana supercontinents.

Genesis of ferromanganese deposits from the Central Anatolian province, Yozgat-Aşağı Eğerci village, Turkey: Geochemical properties and fluid inclusions

Eğerci village is located 16 km southwest of Yerköy (Yozgat) area which has a ferromanganese deposit that formed at the contact between basalt and limestone and shows that a banded stockwork structure occurs occasionally within the limestone. The mineralization consists of pyrolusite, goethite, and ramsdellite, together with a lesser quantity of magnetite. Gangue minerals are determined as calcite and quartz. REE data from the mineral samples demonstrate a range from 2.70 - 63.70 ppm and the average value is 28.00. These results permit a comparison to be made with mineralization in hydrothermal deposits. Moreover, mineral samples show a positive Eu anomaly 0.88-48.10 ppm (ave. 9.94 ppm). The Ce anomaly values vary between 0.02 and 0.88 ppm (ave. 0.58 ppm). It is possible that the mineralization may be affected by the mixing of seawater and hydrothermal fluids. The value of the positive Eu anomaly is evidence of modern oceanic hydrothermal manganese deposits. Insight of previous fluid inclusion, studies can be easily inferred that mineralization can occur at three different stages. The temperature of the first stage ranged from 338 oC to 428 oC and other stages vary from 269 oC - 317 oC and 143 oC - 236 oC, respectively. As a comparison, calculated salinity is higher in Type I fluid inclusions (1.9-14.7 wt.% NaCl equiv.) than Type II and III fluid inclusions (1.9-5.1 wt.%NaCl (equiv.) It is possible that the mineralization was formed by the mixing of magmatic and meteoric waters.

Late Cretaceous volcanism and fluid circulation in the South Atlantic: Insights from continental carbonates in the onshore Namibe Basin (Angola)

The Namibe Basin developed in the Early Cretaceous as part of the South Atlantic rift system between Africa and South America. Magmatic activity occurred during the syn-rift (Valanginian-Hauterivian) and post-rift (Coniacian-Early Campanian) phases, with the latter triggering fluid circulation events that had significant impacts on reservoir quality and hydrocarbon prospectivity. The continental carbonates of the Mariquita Member (Mb.) developed during the Late Cretaceous magmatic pulse and record the associated fluid circulation. Thus, study of these carbonates provides insights into paleo-fluid sources and circulation pathways, which can ultimately help to constrain diagenetic phases, their effects on reservoir quality, and hydrocarbon re-mobilisation.This study analyses the tectonostratigraphic setting, facies and diagenesis of the Mariquita Mb. carbonates by integrating field, petrographic and geochemical data. The carbonates are stratigraphically bound by Upper Cretaceous magmatic rocks, and constitute a series of fault-associated spring mound systems that pass laterally into lacustrine sediments. The main diagenetic events include pervasive dolomitisation, silicification and multi-stage fracturing. Fluid temperatures during dolomitisation were between ∼32 and 56 °C, while silicification occurred between ∼70 and > 260 °C. Fluids likely followed fault-controlled convective circulation pathways. The fluid compositions comprised infiltrated/modified meteoric and marine waters, as suggested by negative carbon isotopes, enriched oxygen and strontium isotopes, and REE data. Fluid modification occurred via interaction with subsurface rock units, such as basement, volcanic/siliciclastic aquifers and evaporites. However, higher temperatures during silicification could suggest late contributions from magmatic fluids too. Bitumen within porosity possibly reflects regional hydrocarbon mobilisation processes driven by magmatic fluids.

Geochemical and geochronological evidence of meteorite impact excavating the Archean basement at Lonar Crater, Central India

The effect of meteorite impact on basement rocks remains widely debated at Lonar Crater, Central India. We investigate an impact melt-bearing rock from the ejecta layer of Lonar to assess the potential interaction of the projectile with the underlying Archean basement. The investigated sample is a melanocratic, aphanitic melt rock comprising glassy inclusions, and plagioclase and quartz clasts. Plagioclase and quartz show shock-induced features including diaplectic glass and well-developed planar deformation features (PDFs), respectively. The decrease in shock pressure is thus established with the plagioclase of overlying basalt showing shock features like diaplectic glass whereas the quartz from lower Archean basement showing PDF development. Laser Raman spectroscopy reveals the presence of cristobalite. The impact melt-bearing rock shows major element geochemistry similar to that of Deccan basalt with a tholeiitic trend, but the Harker variation plots reveal an independent field for the melt rock between the two end members: Deccan basalt and Archean basement represented by Peninsular gneiss. The REE geochemistry also reveals more fractionation for the melt rock. Zircon U-Pb data indicate an upper intercept age around 3.0 to 3.1 Ga. Chondrite normalized zircon REE data display a positive Eu anomaly indicating a reheating during the impact event. This indicates that the melt rock sample was generated by the incorporation of Archean basement gneiss components, which is compatible with the estimated depth of excavation for an impact structure of this size. This specifies a depth to basement in the range between 522 and 570 m.

Seawater‐Particle Interactions of Rare Earth Elements and Neodymium Isotopes in the Deep Central Arctic Ocean

AbstractIn the central Arctic Ocean, dissolved rare earth element concentrations ([dREE]) and the neodymium (Nd) isotope compositions are constant throughout the deep water column (&gt;1,000 m water depth), indicating unique conditions among the ocean basins and therefore requiring an investigation of seawater‐particle interactions. Here, we present the first high‐resolution particulate REE and Nd isotope data from the Arctic Ocean and discuss the possible seawater‐particle processes affecting the Arctic Ocean. Our results show that particulate [REE] are on the same order of magnitude as in other ocean basins, suggesting that particle composition is the main cause for a lack of pREE release to the dissolved pool. The lithogenic fraction dominates throughout the water column while the biogenic material contribution is very small. This paucity of biogenic material results in reduced particle‐seawater exchanges of REEs and Nd isotopes. Moreover, we note only slight differences in the dissolved Nd isotope composition between the Eurasian and Canadian Basins. This is due to the different source regions supplying different dissolved and particulate Nd isotope signatures to both basins. The dissolved [REE] and Nd isotope composition of Atlantic waters are modified during their flow paths through contributions from the Kara Sea, lowering the salinity and increasing [dREE] and dNd isotope compositions. Hydrothermal influence from the Gakkel Ridge on dissolved and particulate [REE] and Nd isotopes could not be detected.

Constraints on the origins of iron silicide spherules in ultrahigh‐temperature distal impact ejecta

AbstractTerrestrially occurring iron silicide spherules, described in the geological literature for 160 years as cosmogenic and approved as “extraterrestrial” minerals by IMA CNMMN in 1984, so far have escaped any serious examination by meteoriticists. Our isotopic and REE data, obtained for silicide spherules for the first time, disagree with the meteoritic origin of gupeiite (Fe3Si) and xifengite (Fe5Si3) spherules from two continents. Despite departures from terrestrial norms (87Rb/86Sr—0.0174; 87Sr/86Sr—0.700181; 3He/4He—7.57 × 10−6; 40Ar/36Ar—325.9), the compositions of 143Nd/144Nd (0.512034) and 147Sm/144Nd (0.06357), as well as REE abundances, clarify provenance from upper crust sediments for samples with U/Pb age of 121–314 ka from the Ala‐Tau range in the Urals. However, the morphology of flanged button shapes, ring waves, and eccentro‐radiating ridges reliably constrains the origin of silicide spherules to distal meteoritic impact ejecta. Arc jet ablation experiments have previously demonstrated that similar morphologies, observed on australite tektites, reflect aerodynamic ablation rates corresponding to flight velocities well into orbital range. These features are generally accepted as conclusive evidence for hypervelocity atmospheric entry from space. Internal structure, consistent with accretion through the coalescence of 3–5 µm droplets, and composition, closely corresponding to 1893–1154 K span of C‐type condensation sequences, indicate a high probability of processing through recondensation of ejecta vapor.

Metamorphic gabbro and basalt in ophiolitic and continental nappes of the Zermatt region (Western Alps)

The composition of meta-gabbro and meta-basalt occurring abundant and widespread in all nappes of the nappe stack exposed in the Zermatt region of the Western Alp shows distinct patterns related to the geodynamic origin of metamorphic basic rocks. Eclogitic meta-basalts of the ophiolitic Zermatt-Saas Unit (ZSU) show enriched MORB signatures. The meta-basalts (eclogites) of the continental fragment of the Theodul Glacier Unit (TGU) derive from pre-Alpine metamorphic continental intraplate basalts. Meta-basalts (eclogites) from the continental basement of the Siviez-Mischabel nappe (SMN) derive from MORB thus a genetic relation to the TGU eclogites can be excluded. All basic igneous rocks experienced post-magmatic alteration by fluid-rock interaction ranging from processes at the seafloor, in the shallow crust, during subduction zone hydration, in the exhumation channel and late Alpine regional metamorphisms. The consequences of these alteration processes can be identified at various levels in the rock composition data. It was found that the REE data are little affected by fluid-rock alteration. Some trace elements, notably Cs, Rb, and Ba are typically massively altered relative to igneous compositions in all three groups of meta-basalts. Generally, meta-basalts from the TGU and the SMN preserved the features of the original composition whilst the ZSU meta-volcanic rocks experienced massive alteration. For the ZSU meta-volcanic rocks it is evident that Zr was gained and Y lost during high-pressure fluid-rock interaction indicating a mobile behavior of the two elements during HP-metamorphism in contrast to their behavior in hydrothermal near-surface fluid-rock interaction.

Geochemistry of late paleoproterozoic Anjana and Amet granites of the Aravalli craton with affinities to sanukitoid series granitoids: Implications for petrogenetic and geodynamic processes

The Mangalwar Complex of the Aravalli craton is marked by the presence of late Paleoproterozoic granites referred to as Anjana Granite and Amet Granite. These granites occur as 1.64 Ga old plutons intruding greenstone sequences and migmatitic gneisses of Mangalwar Complex which comprises parts of BGC of the Aravalli craton. In the present contribution major, trace and REE data of these granites along with associated microgranular mafic enclaves (MMEs) are presented and discussed. Geochemically these granites are quartz monzonite, metaluminous, sub-alkaline and high-K calc-alkaline rocks. The most important characteristics of Anjana and Amet granites are low SiO2, high MgO, Mg#, K2O, Ba, and low Na2O/K2O ratios. In addition, the REEs show moderate to high fractionation, with (La/Yb) ratios up to 22 and 23 of the Anjana and Amet granites respectively, with no or positive europium anomalies. In the primitive mantle-normalized trace element diagrams both granites show depletion in high-field strength elements (HFSE) such as Nb, Ta, P, Ti and enrichment in LILEs. Most of these features are comparable to those of sanukitoid series rocks. Geochemically both granites are distinguished as high-Ti sanukitoids. Geochemical characteristics of MMEs suggest that they are similar to Anjana and Amet granites and in turn to sanukitoids with lower SiO2 content. They display LREE enriched patterns with low values (avg. 13) of (La/Yb)N, negative Eu anomalies and high HREE contents (58 ppm). It is suggested that the parental magma of Anjana and Amet granitic plutons originated through a four stage process (1) Generation of magmatic melts produced by partial melting of terrigeneous sediments of subducting slab in an arc setting; (2) interaction of those melts with the overlying mantle wedge, and total consumption of slab-derived melts during the reaction resulting in production of a metasomatized mantle; (3) tectonothermal event, possibly related to the slab break-off, causing asthenospheric mantle upwelling. This may have induced the melting of the metasomatized mantle and the generation of sanukitoid magmas. The parental magmas of Anjana and Amet granites and their mafic enclaves were generated at lower and higher lithospheric levels respectively (4) Granitic magma ascended due to viscosity and gravity instabilities and interacted with enclave magma at higher mantle level. Both magmas ascended towards upper crust and evolved through fractional crystallisation. Existing data suggest that in the Mangalwar Complex, the formation of sanukitoid magma started even during Mesoarchaean times and continued till late Paleoproterozoic. Formation of sanukitoid magma during this time indicates that in northern Indian shield the multi-stage subduction- accretionary orogenic processes continued for a protracted geological period and played a major role in the origin and evolution of early continental crust.

Implications of REE incorporation and host sediment influence on the origin and growth processes of ferromanganese nodules from Central Indian Ocean Basin

This study presents new major, trace and REE data for thirty-five ferromanganese nodules recovered from areas representing three different sediment types (siliceous, red clay and their transition zone) in the Central Indian Ocean Basin (CIOB) to address their genetic aspects, classification, growth rate, nature of host sediments and influence of REE in the processes of nodule formation. The nodules from CIOB are mostly either hydrogenetic (metals coming from oxygenated bottom water) and diagenetic (metals coming from suboxic sediment pore water) or a combination of both, depending on the source of supply of metal. However, a number of biogeochemical processes mediate this supply of metals which again changes from time to time, making the nodule growth process highly dynamic. This study suggests that at the initial stage of nodule growth, host sediments do not play much role in controlling the growth processes for which REEs can enter both Mn and Fe oxyhydroxide phases equally. Thus, the bottom water signature is imprinted in these early formed nodules irrespective of their host sediment substrate but with gradual growth and burial in the sediment, the main mode of metal enrichment becomes diagenetic through sediment pore water. This tends to increase the concentration of Mn, Ni and Cu over other elements which are retained in the sediment fraction. Among the REEs, Ce concentration of the nodules shows significant positive anomaly due to variation in redox potential and hence its magnitude can be used to get an idea about the metal enrichment procedure and the genetic type of the nodules. However, based on host sediment only, not much difference is found in the magnitude of Ce anomaly in these nodules. On the other hand, discrimination diagram, based on HFSE and REY chemistry, indicates that most of these nodules are of diagenetic origin under oxic condition with a trend towards hydrogenetic field. Further, the genetic type of the ferromanganese nodules from the CIOB are more effectively differentiated by a combination of their major and trace element concentrations rather than solely based on their REE or HFSE chemistry or host sediment substrate.

Tetrad effect of rare earth elements caused by fractional crystallization in high-silica granites: An example from central Tibet

Determining the formation mechanism (fractional crystallization vs. melt-fluid interaction) of the REE tetrad effect in high-silica granites is critical for understanding trace elemental behaviors (e.g., enrichment of rare metals) during magmatic and hydrothermal evolution. This study reports new petrography, zircon U-Pb ages, trace elemental and Hf isotopic, and whole-rock geochemical data of high-silica granites from the Gajin Batholith in the central Lhasa Terrane, Tibet. All samples are dated at ca. 100 Ma and likely formed during Lhasa-Qiangtang collision. These samples are syenogranites and alkali-feldspar granites, belonging to high-silica I-type granites (SiO2 = 74.2–78.2 wt%), having high total-alkalis, low magnesian, marked enrichments in Pb, U, Th, and Rb, and marked depletions in Ti, Eu, P, Sr, and Ba. The petrological and geochemical data indicate moderate (0–20%) fractionation of plagioclase + K-feldspar + biotite (5:3:2). The REE data clearly define regular chondrite-normalized patterns (No-tetrad Group) and pronounced tetrad patterns (Tetrad Group). The REE tread effect in Tetrad Group can be reproduced by fractionation of monazite and allanite as indicated by a Rayleigh fractionation modeling. New modeling results for samples reported elsewhere, along with petrographic observations and decoupling between the REE tetrad effect and fractionation of twin-elements, demonstrate that the REE tetrad effect in high-silica granites is inherited from fractional crystallization rather than formed by melt-fluid interaction as suggested widely.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.