Rare Earth Element Abundances Research Articles

Pulsed volcanism and rapid oceanic deoxygenation during Oceanic Anoxic Event 1a

Abstract Widespread oceanic anoxia, biological crises, and volcanic activity are associated with the onset of Early Aptian (ca. 120 Ma) Oceanic Anoxic Event 1a (OAE1a). Reconstructions of oceanic deoxygenation and its links to broadly contemporaneous volcanism, however, remain poorly resolved. We use geochemical data, including δ53Cr ratios and rare Earth element abundances, to define the timing and tempo of submarine volcanism and global oceanic deoxygenation across this event. Pacific Ocean sediments deposited in the run up to OAE1a record multiple phases of marine volcanism associated with the emplacement of Ontong Java Plateau lavas. Rapid oceanic deoxygenation followed the initial phases of volcanism and a biocalcification crisis. Large swaths of the oceans likely became anoxic from the Tethys to the Pacific Oceans in <30 k.y. Oceanic anoxia persisted for almost one million years after this and was likely sustained through intensified continental and submarine weathering. These results paint a new picture of OAE1a in which volcanism, biological crisis, and oceanic deoxygenation are separated in time and linked through Earth system responses that operate on time scales of tens of thousands of years.

Quantitative estimation of rare earth element abundances in compositionally distinct carbonatites: Implications for proximal remote-sensing prospection of critical elements

Rare earth elements (REE) became a strategic raw material in the 21st century and carbonatite-related deposits frequently carry potentially economic levels of these critical metals. To determine concentrations of major and trace elements (including REE) conventional analysis of bulk rock geochemical samples by means of inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) is mostly applied. However, these analytical methods are labor-intensive, costly and time-consuming. In contrast, modern Remote Sensing (RS) – e.g. proximal remote sensing or imaging spectroscopy – has become a novel tool for detecting and quantifying geological materials as they offer an efficient, non-destructive and cost-effective way not only to identify different minerals and/or chemical elements but also model their abundances. In this study, we investigated whether Partial Least Squares Regression (PLSR) models linking the laboratory sample reflectance to mineral/geochemical property could be employed for quantitative predictions of bulk LREE and HREE concentrations. The suite included samples from four different geographic regions in India, East Africa and the USA. Our results showed that for such samples the quantitative approaches have some limitations.First, our models required to exclude samples with high modal abundance of hematite and samples with too low REE contents (LREE < 500 μg/g, HREE < 100 μg/g). Furthermore, it was revealed that the sample mineralogy has a significant impact on the PLSR predictions. To achieve reliable models the sample suite had to be divided into two datasets following their mineralogy and geochemistry. The first dataset comprised rocks with increased amounts of strontium (Sr, DatasetSr), where REE were predominantly bound in Sr-carbonates. On the other hand, REE mainly bound in Ca–REE-carbonates characterized the second dataset (DatasetOther). Following this division, we were able to construct valid prediction models for bulk LREE and HREE concentrations using PLSR. However, the detected spectral assignments associated with the REE presence indicated that the predictions were mainly indirect, based on the present mineral phases rather than direct absorptions related to LREE and HREE. This illustrates the difficulty and limitations for further model generalization and the ability to be further transferred to other lithologically diverse carbonatite sites.We posit that this topic requires future systematic investigations using the extended carbonatite datasets – samples having a wide range of REE abundances – collected from lithologically diverse regions. This would enable further validation and re-calibration of the constructed prediction models.

Enrichment of rare earth elements in the early Cambrian Zhijin phosphorite deposit, SW China: Evidence from francolite micro-petrography and geochemistry

The Zhijin phosphorite deposit is a giant early Cambrian rare earth element (REE) deposit, with particular enrichment in heavy REEs, and is of high economic significance. The REEs of the deposit occur almost entirely in apatite, which, on the basis of petrographic characteristics, can be divided into francolite grains (90%), amorphous apatite, and phosphate fossils. In this paper, we report an investigation into francolite grains in different types of phosphorite from ore sections of the Zhijin deposit. Detailed petrographic and transmission electron microscopy observations reveal that the francolite grains are composed of nano-apatite crystals measuring 120–160 nm long. Trace-element contents of pyrite and whole-rock geochemical indicators (such as Cr/Zr and V/Ni ratios and Th/U–Co/Ni plots) suggest that the Zhijin phosphorite formed in an anoxic hydrothermal sedimentary environment. However, the negative Ce/Ce* and Eu/Eu* anomalies indicate that the diagenetic environment of the francolite grains was oxidic. Different diagenetic redox environments and the presence of transportation fractures in francolite grains identified under back-scattered electron imaging reveal that the francolite grains underwent prolonged abrasion and transportation in seawater before sedimentation. Petrographic observations and laser ablation–mass spectrometry element surface scanning analyses indicate that the REEs are uniformly distributed in nano-apatite in isomorphic form, and no nano-inclusions or REE minerals were found, implying that late mineralization was not the cause of REE enrichment. The higher (La/Yb)N ratios of francolite grains relative to seawater indicate that the francolite grains captured abundant REEs during the early diagenetic stage. We infer that prolonged exposure to seawater and the high specific surface area of francolite grains are the key factors that led to the enrichment in REEs. Combining the timing of the metallogenic epoch (543 Ma), Sr–Nd isotopic data, and the Y/Ho ratio, which is close to those of modern seawater, indicates that the REEs were sourced from paleo-seawater. Paleogeographic information suggests that during the early Cambrian, extensive continental collisions caused great amounts of continental weathering products to enter the paleo-ocean, leading to the production of abundant amounts of hydrothermal–sedimentary phosphorus and REEs in shallow seas, thus forming an ideal sedimentary environment for generating the REE resources of the Zhijin phosphorite deposit.

Geochemistry and Genesis of Beryl Crystals in the LCT Pegmatite Type, Ebrahim-Attar Mountain, Western Iran

Ebrahim-Attar granitic pegmatite, which is distributed in southwest Ghorveh, western Iran, is strongly peraluminous and contains minor beryl crystals. Pale-green to white beryl grains are crystallized in the rim and central parts of the granite body. The beryl grains are characterized by low contents of alkali oxides (Na2O = 0.24–0.41 wt.%, K2O = 0.05–0.17 wt.%, Li2O = 0.03–0.04 wt.%, and Cs2O = 0.01–0.03 wt.%) and high contents of Be2O oxide (10.0 to 11.9 wt.%). The low contents of alkali elements (oxides), low Na/Li (apfu) ratios (2.94 to 5.75), and variations in iron oxide (FeO= 0.28–1.18 wt.%) reveal a poorly evolved magmatic source of the beryl grains. Low abundances of rare earth elements (ΣREE = 0.8–4.9 ppm) with high 87Sr/86Sr(i) ratios of 0.739 ±0.036 for the beryl grains and 0.7081 for the host granites infer that the primary magma was directly produced by partial melting of the upper continental crust (UCC). The crystallization temperature of the Ebrahim-Attar granitic pegmatite changes from 586 to 755 °C (average = 629 °C), as calculated based on the zircon saturation index. Furthermore, the quartz geobarometer calculation shows that crystallization occurred at pressures of approximately 233–246 MPa. This pressure range is a promising condition for saturation of Be in magma. During granitic magma crystallization, the melt was gradually saturated with Be, and then beryl crystallized in the assemblage of the main minerals such as quartz and feldspar. Likewise, the host granite is characterized by high ratios of Nb/Ta (4.79–16.3) and Zr/Hf (12.2–19.1), and peraluminous signatures are compatible with Be-bearing LCT (Li-Ce and Ta) pegmatites.

State-of-the-art and perspectives in the heavy mineral industry of Sri Lanka

Heavy minerals have become an essential commodity in modern-day technology. The objective of this study is to examine the current status of the heavy mineral industry of Sri Lanka. The present study also examines the economic viability of potential heavy mineral placer deposits, based on geochemical data. Field investigations suggested the occurrence of heavy minerals as beach-placers, and inland/raised beach deposits along the coastline of Sri Lanka. Geochemical results showed the enrichment of TiO2, Fe2O3, Al2O3, and trace elements, and depletion of other major oxides such as Na2O, K2O, MnO, MgO, CaO, and P2O5. Consequently, it suggests the abundance of ilmenite, leucoxene, rutile, sillimanite, garnet, and rare earth element (REE)-bearing heavy minerals in potential placer deposits. In contrast, this study illustrates the exported quantity (in the weight ratio of 35:3:1) of ilmenite, rutile, and zircon minerals during the last decade. Ilmenite is the dominant heavy mineral in Sri Lanka, and thus, its production and price volatilisations are essential to understand the current status and predict the future behaviour of the heavy mineral industry. Statistical observations suggest that the global production and unit price variations of ilmenite have increased gradually from 1950 to 2015. Besides, the forecasted variations in annual global production and unit price changes of ilmenite indicate healthy compound annual growth rates of 1.27% and 1.43%, respectively. However, Sri Lanka has less contribution (< 1%) to global ilmenite production. Consequently, the value addition of ilmenite must be highly encouraged with at least an immediate stage (e.g. synthetic rutile). The present study also highlights possible value addition stages of available heavy mineral resources (low-, intermediate-, or high-level value addition), and discusses action plans to improve the quality of the heavy mineral industry of Sri Lanka. In recommendations, imposing sustainable regulations and policies would uplift the heavy mineral industry of Sri Lanka. For example, value addition to local heavy minerals and terminating raw commodity export are highly recommended.

Rare Earth Element and Incompatible Trace Element Abundances in Emeralds Reveal Their Formation Environments

Emeralds require the unusual association of typically compatible elements (Cr, V), with incompatible Be to form, and occur in complex tectonic settings associated with sediments (type IIB; Colombia) or, more commonly, with magmatism and regional metamorphism (IA). Precise rare earth element (REE) and incompatible trace element abundances are reported for a global suite of emeralds, enabling the identification of the environments in which they formed. Type IIB emeralds have nearly flat continental crust normalized REE patterns (La/YbCC = ~2), consistent with a sedimentary source origin. Type IA emerald REE patterns have upturns in the heavy REE (La/YbCC = ~0.3), a feature also shared with South African emeralds occurring in Archaean host rocks. Modeling of type IA emerald compositions indicates that they form from magmatic fluids of sedimentary (S)-type granite melts interacting with Cr, V-rich mafic–ultramafic crustal protoliths. This geochemical signature links emerald formation with continental suture zones. Diamonds, rubies, and sapphires have been considered as ‘plate tectonic gemstones’ based on mineral inclusions within them, or associations with plate tectonic indicators. Emeralds are distinct plate tectonic gemstones, recording geochemical evidence for origin within their mineral structure, and indicating that plate tectonic processes have led to emerald deposit formation since at least the Archaean.

Biogeochemical status of the Paleo-Pacific Ocean: clues from the early Cambrian of South Australia

The Ediacaran–Cambrian transition was a time of profound reorganisation of the biosphere, coinciding with the assembly of Gondwana and increasing atmospheric and oceanic oxygenation. The lower Cambrian marine sediments of the Stansbury Basin, South Australia, were deposited at low northern paleolatitudes on the western margin of the emerging Pacific Ocean. Here we report results of a multi-pronged investigation employing trace and rare earth element (REE) abundances, total organic carbon (TOC) contents and pyritic sulfur isotopic compositions (δ34Spy) in carbonaceous shales from three formations within the Normanville and Kanmantoo groups (sequences Є1.2 to Є2.2). TOC ranges from ≤0.5 wt% in the Emu Bay Shale, through 0.4–1.9 wt% in the Talisker Formation, to 0.2–2.6 wt% in the Heatherdale Shale. Covariance is demonstrated between trace elements and organic matter, with comparative uranium enrichment in the Heatherdale Shale likely linked to increased primary productivity. Heavy REEs and yttrium are typically more depleted than light REEs. Provenance appears to evolve through the upper Normanville sediments into the Kanmantoo Group, corroborating published detrital zircon interpretations that suggest an increasing dominance of southerly derived sources. The prevailing paleoredox regime for the Heatherdale Shale and the Talisker Formation was dysoxic, evolving into progressively more reducing conditions up section, the latter exhibiting a secular decline in δ34Spy (from +10 to −11‰ VCDT). Conversely, redox proxies concur in demonstrating that the basal fossiliferous Emu Bay Shale accumulated beneath an oxic water column. Comparison of trace element and REE distributions with those of slightly older sequences on the Yangtze Platform, South China, reveals striking similarities, implying that the trace element chemistry of the Paleo-Pacific and Asian oceans was homogeneous. KEY POINTS Trace and rare earth element, total organic carbon and sulfur isotopic data provide a multi-faceted profile of shales from three formations in the Stansbury Basin. The prevailing paleoredox regime for the Heatherdale Shale and Talisker Formation was dysoxic, whereas the basal Emu Bay Shale accumulated beneath an oxic water column. The strong covariance between TOC and normalised Mo, V and U in these shales highlights the evolving bioproductivity and oxygenation of the early Cambrian ocean. Similar element signatures in lower Cambrian black shales, South China, reflect the homogeneous biogeochemistry of the Paleo-Pacific and Asian oceans.

What can we learn from REE abundances in clinopyroxene and orthopyroxene in residual mantle peridotites?

Clinopyroxene and orthopyroxene are the two major repositories of rare-earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis have made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between clinopyroxene and orthopyroxene at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith. In general, HREE in clinopyroxene and LREE in orthopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.

Geochemistry of ultramafic and mafic rocks from the northern Central Asian Orogenic Belt (Tuva, Central Asia) - constraints on lower and middle arc crust formation linked to late Proterozoic intra-oceanic subduction

The Agardagh Tes-Chem complex (ATCC) in Tuva, Central Asia (50.5°N, 95°E) exposes a rare mafic to ultramafic crust-mantle fragment that developed within a late Neoproterozoic (~570 Ma) intra-oceanic island arc system that was accreted to the Tuva-Mongolian microcontinent during the formation of the Central Asian Orogenic Belt.Residual mantle rocks (harzburgites and dunites) are highly refractory with high Cr# (0.59–0.83) and intermediate Mg# (0.46–0.52) in spinel and experienced high degrees of total melt extraction (up to 25%). In ultramafic cumulate rocks (wehrlites and pyroxenites), Cr# and Mg# in spinel are distinctly lower (0.22–0.45 and 0.34–0.37), and rare earth element (REE) abundances of clinopyroxenes range between 0.1 and 1-times chondritic with chondrite normalized La/Yb of ~0.04. Trace element abundances in wehrlite clinopyroxene are lower than in pyroxenite clinopyroxene, whereas all have positive Sr but pronounced negative Zr anomalies. Calculated equilibrium melts of wehrlites and clinopyroxenites suggest strongly depleted hydrous high-MgO high-SiO2 primary magmas (picrites to boninites) from which these rocks have evolved.Incompatible trace element concentrations in mafic (i.e. gabbroic to dioritic) rocks vary strongly with REE contents between ~0.7 and 20-times chondritic, and chondrite normalized La/Yb between 0.25 and 0.98. Initial εNd varies between +4.8 and +7.1. These features, along with the broad textural and modal variations indicate that the mafic rocks developed from a single intrusive body that underwent substantial crustal assimilation. Cr and Ni contents in the mafic rocks are low and indicate their formation from already differentiated magmas.The evolution of the ATCC prior to amalgamation is explained by a two stage magmatic process. During the initial stages of melting low degree magmas evolved from a refractory mantle wedge fluxed by hydrous fluids and sediments or sediment derived melts to form enriched upper crustal shallow level intrusive and extrusive rocks. Later stage melting from a less hydrous and more depleted but SiO2-enriched source produced the parental picrites/boninites from which the lower and middle arc crustal rocks formed. These magmas differentiated within at least two distinct magma reservoirs at lowermost crustal levels, where cumulate wehrlites formed mostly independently from cumulate pyroxenites. This is evident from distinctly different trace-element compositions of these two rock types. The wehrlite derivative magmas subsequently intruded to mid-crustal levels where they crystallized the mafic rocks within a single intrusion, but accompanied by assimilation of preexisting enriched arc crust.

Rare Earth Elements of Sediments in Rivers and Estuaries of the East Coast of India

The rare earth elements (REE) in the clay fraction of sediments in 15 rivers and their estuaries along the east coast of India were analysed in this study. The total REE content (ΣREE) varied from 130.98 to 289.85 μg/g and from 70.89 to 352.61 μg/g in rivers and estuaries respectively. The ΣREEs of estuarine clays (except the Brahmani and Baitarani) was lower than in rivers. The Post-Archean average Australian Shale-normalized REE patterns in rivers and estuaries were similar and categorized into three types. The REE patterns reflect the composition of dominant geological formations in river basins and extent of sediment mixing from different sources during transport. Hydrodynamic conditions controlled the abundance and fractionation of REE in the estuaries. The Sm/Nd ratios of clays were largely controlled by mineral composition and Y/Ho ratios were affected by sedimentary processes in the estuaries.

Toxicity of representative mixture of five rare earth elements in juvenile rainbow trout (Oncorhynchus mykiss) juveniles

Rare earth elements (REEs) are contaminants of increasing interest due to intense mining activities for commercial purposes and ultimately released in the environment. We exposed juvenile rainbow trout (Oncorhynchus mykiss) to a representative mixture of the five most abundant REEs for 96 h at concentrations similar found in lakes contaminated by mining activities at 0.1, 1, 10, and 100X whereas the 1x mixture contained cerium (Ce, 280 μg/L), lanthanum (La, 140 μg/L), neodymium (Nd, 120 μg/L), praseodymium (Pr, 28 μg/L), and samarium (Sm, 23 μg/L). We investigated the expression of 14 genes involved in oxidative stress, DNA repair, tissue growth/proliferation, protein chaperoning, xenobiotic biotransformation, and ammonia metabolism in the liver. In addition, DNA damage, oxidative stress (lipid peroxidation or LPO), inflammation (cyclooxygenase or COX activity), detoxification mechanisms (glutathione-S-transferase activity or GST), and labile zinc were determined in gills. The data revealed that genes involved in oxidative stress-catalase (cat), heat shock proteins 70 (hsp70), and glutamate dehydrogenase (glud) were upregulated while glutathione S-transferase (gst) and metallothionein (mt) gene expressions were downregulated. The mixture was genotoxic and increased labile Zn in gills of exposed trout. These changes occurred at concentrations 600 times lower than the LC50 for this mixture indicating effects below the 1X concentration. Based on principal component analysis and concentration-dependent reponses, the following sublethal effects were considered the most important/significant: DNA strand breaks (genotoxicity), labile Zn, cat, gst, hsp70, sparc, mt, and glud. These effects of fish juveniles are likely to occur in environments under the influence of mining activities.

Boron isotope compositions establish the origin of marble from metamorphic complexes: Québec, New York, and Sri Lanka

Abstract The origin of an array of marble samples found in both the Grenville Province and southwestern Sri Lanka remains uncertain, whether magmatic, sedimentary, or mixed, due to their proximity to both carbonatite bodies and carbonate-rich metasedimentary rocks. This study reports boron and trace element abundances, in addition to carbon, oxygen, boron, and strontium isotopic compositions, to determine the petrogenesis of these carbonate-rich samples. Boron abundances for all of the samples are relatively high and variable (1.48–71.1 ppm) compared to those for carbonatites worldwide (≤1 ppm), and mostly overlap those documented for sedimentary sources (up to 54 ppm). The rare earth element (REE) abundances (0.5–1068 ppm) for the marbles studied are similar to those for local sedimentary units and thus contain, in general, lower REE contents than both the average worldwide calciocarbonatite and respective neighboring carbonatite bodies. The δ13CV-PDB and δ18OV-SMOW compositions for all of the samples range between –2.9 to +3.2 ± 0.1‰ and +14.3 to +25.8 ± 0.2‰, respectively, and are considerably heavier than those reported for magmatic or metamorphosed carbonatites. The 87Sr/86Sr ratios reported here range from 0.70417 to 0.70672, which are more radiogenic than the average 87Sr/86Sr (~0.70345) reported for carbonatites included for comparison in this study. Importantly, the boron isotopic compositions (δ11B‰) for samples from the Grenville Province range from +7.5 to +15.7 ± 0.5‰, which are consistent with those reported for biogenic carbonate (+4.9 to +35.1‰). In contrast, δ11B values for the samples of marble from Sri Lanka vary from –9.8 to –14.3 ± 0.5‰ overlapping with those estimated for average bulk continental crust (–9.1 ± 2.4‰). Together, the boron compositions, chemical data, stable (C, O), and radiogenic Sr isotopic data overwhelmingly point to a sedimentary origin for the marble samples examined here. Specifically, the samples from the Grenville Province represent marble formed during high-temperature regional metamorphism of limestone units. The Sri Lankan samples were formed from carbonate-rich and 11B-poor fluids derived from a crustal source. The boron isotopic compositions for the samples studied here are also compared to those reported for mantle-derived carbonate (i.e., carbonatites) worldwide, along with their associated δ13CV-PDB and 87Sr/86Sr values. This comparison results in defining three isotopically distinct fields; mantle-derived carbonates, sedimentary carbonates derived from heterogeneous limestone protoliths, and carbonates derived from meteoric water interacting with crustal material. This work establishes the effective use of boron isotopic compositions in determining the origin of carbonate-rich rocks of contentious petrogenesis.

Markov Chain Monte Carlo Predictions of Neutron-rich Lanthanide Properties as a Probe of r-process Dynamics

Abstract Lanthanide element signatures are key to understanding many astrophysical observables, from merger kilonova light curves to stellar and solar abundances. To learn about the lanthanide element synthesis that enriched our solar system, we apply the statistical method of Markov Chain Monte Carlo to examine the nuclear masses capable of forming the r-process rare-earth abundance peak. We describe the physical constraints we implement with this statistical approach and demonstrate the use of the parallel chains method to explore the multidimensional parameter space. We apply our procedure to three moderately neutron-rich astrophysical outflows with distinct types of r-process dynamics. We show that the mass solutions found are dependent on outflow conditions and are related to the r-process path. We describe in detail the mechanism behind peak formation in each case. We then compare our mass predictions for neutron-rich neodymium and samarium isotopes to the latest experimental data from the CPT at CARIBU. We find our mass predictions given outflows that undergo an extended (n,γ)⇄(γ,n) equilibrium to be those most compatible with both observational solar abundances and neutron-rich mass measurements.

Long-range transport of aeolian deposits during the last 32 kyr inferred from rare earth elements and grain-size analysis of sediments from Lake Lugu, Southwestern China

Accumulations of aeolian deposits in a range of sedimentary environments provide evidence for past changes in atmospheric circulation patterns over a range of spatiotemporal scales. As yet, there have been relatively few studies of aeolian records in Southwestern (SW) China, and thus there is hitherto untapped potential of using such records to reveal regional atmospheric circulation. Rare earth element (REE) and grain-size analyses were carried out on a well-dated sediment core from Lake Lugu, SW China, to establish the provenance of aeolian deposits preserved within the lake during the last 32 kyr. The core sediments (LGH2) possess high δEuN values (0.73–0.87) and exhibit a moderate REE abundance (156–274 ppm). The δEuN-∑REE pattern of the Lake Lugu sediments are distinct from those of adjacent geographical areas, but are shown to overlap with the sediments from the Kashmir Valley and northwestern (NW) Deccan Plateau indicating a long-distance dust source. The grain-size endmember for the aeolian deposits shows a gradually increasing trend from 32 kyr B.P. to the onset of the Holocene, followed by a sharp decrease around 9 kyr B.P., with very low values observed during the Holocene optimum (8.3–2 kyr B.P.). Aeolian deposition resumed from 2 kyr B.P. and has continued to the present day. Comparisons with both local and regional palaeoclimate records show that both the local climate and East Asian monsoon (EAM) have not influenced aeolian deposition in Lake Lugu. Rather, changing vegetation cover in the north India and NW Deccan Plateau provided the sediment supply, with entrainment governed by variations in the Southern Winter Westerlies in the dry season (north India), and the South Asian summer monsoon (SASM) in the rainy season (NW Deccan Plateau).

Mafic Microgranular Enclaves Formed by Gas-driven Filter Pressing During Rapid Cooling: an Example from the Gangdese Batholith in Southern Tibet

AbstractMafic microgranular enclaves (MMEs), widespread in intermediate to felsic arc plutons, carry significant information on the genesis and evolution of arc magmas, yet their origin remains debatable. Here, we examine MME-host diorite pairs from the c.200 Ma Cuijiu Igneous Complex in the eastern Gangdese Batholith, southern Tibet, to constrain the petrogenesis of MMEs and the evolution of arc magmas. Within the complex, MMEs are essentially similar to their host diorites with similar emplacement ages (∼200 Ma), mineral assemblages and mineral compositions, as well as whole-rock Sr–Nd–Hf and zircon Hf isotopic compositions. However, MMEs have higher modal contents of hornblende and biotite, and are enriched in compatible elements and depleted in incompatible elements. Zircons from some MME samples are characterized by dark cathodoluminescence (CL) cores overgrown by light-CL rims of varying thickness. The dark-CL cores show higher Th, U and rare earth elements (REE) abundances than the light-CL rims. Based on comparison with co-genetic mafic melts and mass-balance calculations, we propose that the MMEs were early-crystallized cumulates (autoliths) related to their host diorites. The chilled textures, flow microstructures and pillow shapes suggest that the MMEs experienced rapid cooling before being captured by the host magmas. The rapid cooling may result from contact between ascending diorite magmas and cooler wall rocks. As the magmas quickly crystallized, they reached second boiling and vesiculation, and separated into fine-grained crystal-rich margins and melt-rich centres. Gradients in crystallinity and pressure expelled interstitial melts from the crystal-rich margins to the crystal-poor centres, leading to crystal-liquid separation (gas-driven filter pressing). The dark-CL zircon cores with high Th and U abundances may crystallize from highly evolved interstitial melts within the crystal-rich margins. The fine-grained crystal-rich margins were subsequently captured and dragged as MMEs before their complete crystallization by later ascending host magmas. This differentiation process could have occurred over several kilometres of magma ascent, and have played an important role in the polybaric fractional crystallization of the Cuijiu Igneous Complex, feeding more differentiated andesitic magmas to upper crustal mushes.

Feldspar recycling across magma mush bodies during the voluminous Half Dome and Cathedral Peak stages of the Tuolumne intrusive complex, Yosemite National Park, California, USA

Abstract Incremental pluton growth can produce sheeted complexes with no magma-magma interaction or large, dynamic magma bodies communicating via crystal and melt exchanges, depending on pulse size and frequency of intrusions. Determining the degree and spatial extent of crystal-melt exchange along and away from plutonic contacts at or near the emplacement level, such as in the large, long-lived Tuolumne intrusive complex (TIC) in California, sheds light onto the process and evolution of incremental growth. This study used field mapping and petrographic and geochemical analysis of plagioclase and K-feldspar populations in the equigranular Half Dome (eHD), porphyritic Half Dome (pHD), and Cathedral Peak (CP) Granodiorites of the southeastern section of the TIC to determine the presence and/or extent of feldspar recycling at interunit contacts. Our results suggest that contacts between major units are predominantly ∼400-m- to 3-km-thick gradational zones. K-feldspar is compositionally distinct in eHD and neighboring gradational zones and shows no evidence of mixing. K-feldspar in a gradational zone between pHD and CP shows evidence of mixing between the two. Plagioclase in eHD and CP display distinct ranges of anorthite content, Sr, and light rare earth element abundances; both populations are observed in pHD. Major oxide and trace element calculations of melts in equilibrium with plagioclase cores indicate that the melts were more silicic, less calcic, and lower in Sr and Rb than corresponding analyzed whole-rock samples. These results suggest that the magmas also underwent plagioclase and biotite accumulation. The presence of two plagioclase populations in pHD is consistent with eHD and CP hybridizing to form pHD in an increasingly maturing and exchanging TIC magmatic system during the eHD-pHD-CP stages but before groundmass and small K-feldspar phenocrysts crystallized.

Sedimentological, Geochemical and Hydrogeochemical Studies of Alluvial Fans for Mineral and Environmental Purposes (Case Study of Southwestern Iran)

The alluvial-fan sediments play a very important role in mineral reserves and underground water resources, though a comprehensive study on such sediments, especially from a geochemical point of view, is still lacking. In this research, a total of 22 particularly important alluvial fans across the Khuzestan Plain in the southwestern Iran were studied through field surveys, petrographic evaluations, and geochemical assessments. These investigations included particle size distribution studies, microscopic observation of 193 samples, subsurface study of 60 boreholes along 10 different sections, facies analysis on 46 different sections, and comprehensive geochemical studies through XRD, XRF, ICP Mass, and wet chemical analyses. Based on these studies, the sediments were found to be dominantly composed of gravel, muddy sand, silt, and clay. The sediments exhibited good roundness and good to fair sorting. The most abundant oxide in the study area being MgO followed by SiO2, Al2O3, and Fe2O3. According to the studies, most of the samples fell in the range of iron-bearing sandstones with rare cases of iron-bearing shales. Investigation of the geochemical indices showed the long sediment transport path from the source, humid climate, and moderate to extensive weathering of the deposit. The depositional setting was found to be dominantly an active continental margin and, to a lesser extent, a continental island arc. Investigating of the studied sediments provenance indicate a felsic to slightly basic source rock. The environmental studies were indicative of relatively low contamination of the region and abundance of rare earth elements including Y, Sc, Nd, La, and Ce, and the abundance of heavy elements including Cr, Zn, Pb, and Cd. Based on the mineralogical studies on the samples, the inter-grain cement in both gravel and conglomerate particles was found to be composed of calcite.

Petrology of green polished stone axes of the Jomon period from the Sannai‐Maruyama site, Japan, investigating the origin of source rock

AbstractNumerous green polished stone axes have been excavated from the Sannai‐Maruyama site, one of Japan's largest archeological sites in the Jomon period (5.9–4.2 cal kyr BP). The axes are composed of weakly metamorphosed fine‐grained volcaniclastic rock having a peculiar texture that includes numerous acicular actinolites growing in random directions within a quartz and albite matrix. Cobbles of Aotora stone found along the Nukabira River, Biratori town, southern Hokkaido, are the most likely raw material for these stone axes. Aotora stones have alternate bands of a soft dark‐green picritic layer and a hard SiO2‐rich pale‐green layer. The pale‐green layer has a texture similar to the stone axes. Basaltic and picritic volcanic rocks of the Sorachi‐Yezo Belt occupy the area along the Shidoni River, a tributary of the Nukabira River. Volcaniclastic rocks similar in texture, mineralogy, and bulk rock compositions to the Aotora stone are exposed in the area. These rocks underwent metamorphism under the actinolite‐pumpellyite facies conditions. Their protolith is submarine hyaloclastic rocks that are intercalated with laminated picrite detritus. The stone axes, pale‐green layers of Aotora stone, and those of the volcaniclastic rocks of the Shidoni River area all have high SiO2 (~ 55 wt%), Cr (~ 840 μg/g), and Ni (~ 370 μg/g). The rare earth element patterns with abundant light rare earth elements and depleted heavy rare earth elements of stone axes were also consistent with the pale‐green layers of the outcrop. These pale‐green layers, interleaved with dark‐green layers of picritic detritus, were the likely source rock of the stone axes. The high SiO2 content in the pale‐green layer caused the crystallization of quartz and albite in the matrix, which resulted in high‐quality raw material for making stone axes.

Development of cost-effective nanocrystalline multi-component (Ce,La,Y)-Fe-B permanent magnetic alloys containing no critical rare earth elements of Dy, Tb, Pr and Nd

Here we first report the fully abundant rare earth (RE)-based nanocrystalline multi-component (Ce,La,Y)-Fe-B alloys containing no critical RE elements of Nd, Pr, Dy, and Tb by melt-spinning technique. The roles of La and Y substitutions for Ce have been fully understood. La plays a positive role on both thermal stability and room-temperature (RT) magnetic properties. The enhanced coercivity Hcj by partial substitution of La is attributed to the increases of anisotropy field HA and the formation of continuously distributed grain boundaries resulting from the suppression of CeFe2 phase. Although Y substitution is not benefit for Hcj, both remanent polarization Jr and thermal stability have been effectively improved since Y2Fe14B shows relatively high saturation magnetization Ms and a positive temperature coefficient of HA over a certain temperature range. In addition, RE element segregation has been confirmed, La prefers to enter into the grain boundaries than Ce and Y prefers to remain in the 2:14:1 phase. Based on these understanding, a series of melt-spun (Ce,La,Y)-Fe-B alloys have been designed. A relatively good combination of magnetic properties with maximum energy product (BH)max=7.4 MGOe, Hcj=400 kA/m, and Jr=0.63 T has been obtained in [(Ce0.8La0.2)0.7Y0.3]17Fe78B6 alloy, together with high Curie temperature (Tc=488 K) and low temperature coefficients of remanence (α=-0.255 %/K) and coercivity (β=-0.246 %/K).

Cerium stable isotope analysis of synthetic and terrestrial rock reference materials by MC-ICPMS

Cerium is the most abundant rare earth element (REE) in the solar photosphere, CI chondrites, and the Earth. It has four main stable isotopes (masses: 136,138,140, and 142), with 138Ce being the most studied species, used in geochronology and petrogenesis. In addition, more abundant 140Ce and 142Ce are suggested to be potentially applicable in geochemical investigations. In this work, we developed a modified four-step ion chromatography procedure for Ce chemical separation. Using a MC-ICPMS, we designed a cup configuration to measure 142Ce/140Ce ratio of the samples with an optimized Nd correction equation. A 0.03‰ (2SD) reproducibility was obtained for Ce Ames metal standard. We analyzed ten different igneous and one sedimentary geochemical reference materials. Mean δ142Ce range from −0.07 to 0.32‰. Most of the samples show a heavier Ce isotopic composition than the Ce Ames standard. The majority of rocks have a homogenous δ142Ce. The δ142Ce does not show any correlation with rock chemical composition including their Ce content or rock types. A carbonatite (SARM 40) has a mean δ142Ce of −0.07 ± 0.13‰ (2SD), lower than the other rocks, suggesting the possibility of a pronounced isotopic fractionation. Our work demonstrates the applicability of the developed methodology and the potential of Ce stable isotopes for future geochemical studies. Production of a larger database of δ142Ce values is required to obtain a clearer view on the similarities and differences between different geological material and explaining Ce stable isotope dynamics.