Similar Rare Earth Element Patterns Research Articles

The mechanism of dolomitization in a stromatolite mound in the late Cambrian Chaomidian Formation, Shandong Province, China

Clarifying the mechanism of dolomitization is of pivotal importance and represents a burning issue among geologists. A comprehensive model to define the genesis of dolomitization in the Cambrian Chaomidian Formation carbonates is still lacking. In the highly focused study, the advanced integration of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), clumped isotope and carbon/oxygen isotope analyses was used to reveal complex interactions between the genesis and dolomitization of a stromatolite mound in the late Cambrian Chaomidian Formation of China. The results show that the dolomite content increases from bottom to top, along with a shift in lithology from limestone to dolostone. Additionally, as we moved from the base to the summit of the stromatolite mound, the Fe, Mn, and Na contents increase with increasing Mg levels, whereas the δ13C and δ18O values became more positive. Conversely, the levels of Sr and total rare earth elements (REEs) gradually decreased. The dolomite grains in the stromatolite mound are micritic and euhedral to subhedral, with foggy cores and bright edges and low cation ordering. The stromatolite limestone, dolostone, and underlying lime mudstone samples exhibit similar REE patterns: light REE enrichment, heavy REE loss, negative δEu anomalies, and weak negative δCe anomalies. The δ13C and δ18O values fall within the late Cambrian seawater range, suggesting that dolomitizing fluids originated mainly from concentrated seawater and migrated from top to bottom. The δ18O values indicate lower dolomite formation temperatures than calcite formation temperatures, supporting dolomitization during the penecontemporaneous diagenetic stage. Petrographic evidence reveals the presence of pyrite, indicating sulfate-reducing bacterial activity during diagenesis. XPS analysis revealed similar organic functional groups in both the stromatolite limestone and the dolostone, including C-(C = O)-O, C–C/C-(C–H), C-O and C-N. However, the dolostone spectrum exhibited a larger C-OH/C-O peak area, suggesting significant microbial organic matter involvement in dolomite formation. While a kinetic obstacle is recognized as a primary cause of dolomite formation, this study also suggested that microbial activity weakened this obstacle during diagenesis. Thus, microbial metabolism or cyanobacterial decomposition facilitates dolomite formation during the penecontemporaneous diagenetic stage.

Geochemical Study of the Osumi Granodiorite, Southwestern Japan

The Osumi Granodiorite, located on the Osumi Peninsula in southwest Japan, is an example of outer zone granites that were formed during a limited period (13–15 Ma) in response to the subduction of the Philippine Sea Plate. This event, which is linked to the separation of southwest Japan from continental Asia, resulted in unique igneous activity. The Osumi Granodiorite is the largest Miocene granite body in the region. It intrudes into the Mesozoic to Paleogene accretionary complex of the Shimanto Belt and affects contact metamorphism. Despite considerable research on the Osumi Granodiorite, limited geochemical studies, especially on trace and rare earth element (REE) analyses, have been conducted. Furthermore, there are insufficient data on the Rb–Sr isotopic system, leaving the formation process unclear. This study presents whole-rock geochemical and Rb-Sr isotopic data to investigate the petrogenesis of the Osumi Granodiorite. The results suggest a common magma origin for this pluton, as indicated by linear trends on the Harker diagrams and similar REE patterns. The presence of a relatively large Eu anomaly implies formation under a reducing environment. The AKF diagram indicates predominant contamination by pelitic rocks of the Shimanto Belt during magma formation. The Rb–Sr whole-rock isochron diagram and SrI–1000/Sr diagram suggest that the Osumi Granodiorite body was formed by heterogeneous assimilation of magma into the Shimanto Belt. Furthermore, the whole-rock isochron age is 64.3 Ma, which differs by approximately 50 My from the previously reported biotite K–Ar age (14–22 Ma). This age is considered to be a pseudo-isochron age, rather than the consolidation age. During the middle Miocene, the compressive stress field in the outer zone south of the Butsuzo Tectonic Line made it difficult for magma to rise. As a result, it reacted with the sedimentary rocks of the Shimanto Belt to various degrees. The Osumi Granodiorite underwent magma differentiation upon intrusion into the Shimanto Belt. It subsequently ascended, cooled, and interacted with pelitic rocks under stable geological conditions.

Constraining the provenance of black shales using rare earth elements and neodymium isotopes, Midcontinent United States

We conducted research to determine neodymium model dates (TDM), εNd values, and the relative proportions of rare earth elements (REEs) of Paleozoic black shales from the Midcontinent United States to constrain their sediment sources. Twenty-one Cambrian to Pennsylvanian shale formations of the Illinois, Cherokee, Forest City, and Arkoma basins, the Ozark Dome, and the Ouachita Mountains were examined. Findings reveal that these midcontinental shales consist primarily of felsic detrital minerals that originated from the craton. The Cambrian Mt. Simon, Eau Claire, and Tunnel City shales of the northern Illinois Basin exhibit REE patterns and Nd isotopic signatures similar to those of the Ordovician Mazarn and Womble shales of the Ouachita Mountains, which indicates derivation from a similar sediment source. Sediment was likely derived from the Superior and/or Trans-Hudson cratonic provinces. The dominant sediment source shifted during deposition of the Middle Ordovician Womble Shale due to the uplift of the Appalachian Mountains during the Taconic orogeny, as suggested by the younger TDM dates, less negative εNd values, and similar REE patterns compared to those of the older Cambrian and Early Ordovician shales. The Grenville province and Appalachian Basin remained the primary sediment source into the Pennsylvanian.

Petrology and geochemistry of Li-bearing pegmatites and related granitic rocks in southern Thailand: implications for petrogenesis and lithium potential in Thailand

Lithium (Li) can be found in many minerals, including lepidolite. Lepidolite is found in pegmatite-related tin deposits in the Phang Nga area in southern Thailand. According to their field occurrence, petrography, mineral chemistry, and whole-rock geochemistry, the Li-bearing pegmatites and the granitic rocks in the study area can be linked to tin deposits in southern Thailand as part of the SE Asian tin belt. The Li-bearing pegmatites are characterized by an abundance of lepidolite, K-feldspar, plagioclase, and quartz with some accessory minerals of fluorite, cassiterite, apatite, monazite, and beryl. The granitic rocks show various compositions, including porphyritic biotite–muscovite granite, biotite granite, and muscovite—tourmaline granite with different proportions of K-feldspar, plagioclase, quartz, biotite, muscovite, and tourmaline. Whole-rock geochemistry indicates that both the Li-bearing pegmatites and granitic rocks have a close relationship rooted in their peraluminous S-type granite affinity. The Li-bearing pegmatites evolved from highly fractionated S-type granitic rocks comparable with the Western Belt Granite of Thailand. The enrichment of large-ion lithophile elements (e.g., Rb and K) and the depletion of Ba, Nb, and Ti together with similar rare Earth element patterns reflect the collisional setting indicating the Sibumasu–West Burma and West Burma—Indo-Burma collisions during the Cretaceous to the Eocene. The crystallization pressure—temperature conditions of these rocks were 3.49–4.25 kbar and 622°C–675°C, respectively, with an emplacement depth of 13–15 km. The Li-bearing pegmatites had a relatively high average Li grade compared with those of other Li-bearing pegmatites in the world.

Mica in a sedimentary feature as evidence of humid conditions for a classic desert in Paraná Basin, southern Brazil

In Paraná Basin, oriented mica appears into horizontal layered fine sediments feature defined as geodes with sediments. Mica is identified as biotite in a volcano-sedimentary context of Botucatu and Serra Geral formations. Throughout the accomplishment of multiple analyses such as petrography, rock chemical evaluation, Rare Earth Elements (REE) values and also the Chemical Index of Alteration (CIA) some contributions for local paleoenvironment and paleoweathering for the classic Botucatu paleodesert is proposed. Since biotite is not regular in sediments of a desert environment, the mica presence suggests that it was transported by aqueous input and deposited within the provided spaces. The arrangement of these sediments as planar cross-stratification also confirms that this process is water dependent, otherwise the preservation of the layers' horizontality would not be preserved. The REE results of negative Ce (Cerium) anomaly in some features, geochemically implies the contribution of water column during deposition. Some geodes are intensely silicified, and in these, there is a relative preservation of Ce attributed to an intense siliceous cementation, avoiding the oxidation of Ce. Despite some differences in Ce values, these features hold similar REE patterns and CIA values indicating similar provenance for the geodes’ sediments and they were probably deposited under similar chemical conditions. The Ce difference between the slightly silicified and intensely silicified geodes shows local conditions for the values, influenced by the presence of water presence and silicification.

Genetic Mechanism of Structurally Controlled Dolomites Derived from Seawater-Hydrothermal Mixed Fluids—A Case Study from Middle Permian, Central Sichuan Basin, South China

Dolomite bodies in the Middle Permian of the central Sichuan Basin have been reported as favorable natural gas reservoirs. The Middle Permian dolomite consists of three types of recrystallized dolomite (Rd1, Rd2, and Rd3) and one type of dolomite cement (Sd). Rd1 might be formed as the primary mineral along the calcite in the original sea-water. Its δ13C value and 87Sr/86Sr ratio, consistent with those of marine limestone and Permian seawater, support that the dolomitizing fluid for Rd1 was Permian seawater preserved in the strata. Rd2 consists of fine to medium (50 μm to 250 μm) and planar to curved crystals. Geochemical indicators (slightly high 87Sr/86Sr ratio, similar rare earth element patterns, negative δ18O, slightly high salinity) confirm that the dolomitizing fluid of Rd2 was mainly Permian seawater during shallow burial, with a small number of hydrothermal fluids. Rd3 and Sd are featured by very large (>250 μm), curved crystals, and high-temperature, high-salinity, and obviously positive Eu anomalies, suggesting that their diagenetic fluids were mainly hydrothermal fluids from deep. Additionally, inherited carbon sources and the 87Sr/86Sr ratios of some samples fall within the range of Permian seawater distribution, confirming the contribution of Permian seawater. ELIP activity caused the formation of this dolomite through the mixing of seawater and hydrothermal fluids. The main fluid circulation channels were activated basement faults, epigenetic karst pores, and shallowly buried high-permeability strata. During the peak period of ELIP activity, the continuous upwelling of deep hydrothermal fluids led to the continuous formation of Rd2, Rd3, and Sd. The dolomitization fluid of Rd2 was mainly composed of seawater and featured a certain lateral extension, which was away from faults. Rd3 and Sd are mainly distributed along the fault system, and excessive dolomitization caused by the hydrothermal activity, to some extent, inhibited the lateral movement of hydrothermal fluids. This study provides a good example for exploring the genetic mechanism and distribution pattern of structurally controlled dolomites under a volcanic activity background.

In Situ U—Pb Dating and Trace Element Analysis of Garnet in the Tongshanling Cu Polymetallic Deposit, South China

The grossular–andradite garnet is an ideal mineral for indicating the formation age of skarn, which also pretends to constrain skarn processes because of its higher REE (rare earth elements) content. The Tongshanling deposit is a medium-sized reduced skarn Cu–W–Pb–Zn deposit associated with a highly evolved I-type granodiorite intrusion in the Nanling metallogenic belt, South China. Different mineral assemblages, microscopic characteristics, and BSE images distinguish two kinds of garnets in the prograde and retrograde skarn stages. The garnet grains from the prograde skarn stage have a U–Pb isochron age of 165.4 ± 3.8 Ma (MSWD = 0.7) and that from the retrograde skarn stage have a U–Pb isochron age of 159.5 ± 1.7 Ma (MSWD = 1.8), implying that the thermal metamorphism and hydrothermal metasomatism mainly occurred in the middle Upper Jurassic. The total amount of rare earth elements (∑REE+Y) in the garnet gradually decreased and the REE patterns shifted from enriched HREE with Eu negative anomaly to HREE-depleted with Eu positive anomaly. The decreasing U content and increasing Eu anomaly in the retrograde skarn stage indicate a redox environment change from oxidation to reduction. However, garnet from different elevations within the same stage (+90 m, +5 m, −35 m, −200 m, and −400 m) exhibit similar REE patterns, despite weak cooling and significant depressurization processes confirmed by fluid inclusion microthermography. As a result, the REE content and patterns are dominated by the REE species of parent fluids, which are changed over time by symbiotic REE-enriched mineral precipitation and the redox environment, while being slightly affected by the fluid pressure. Grossular garnets, rich in U and REEs, and found in reduced skarn deposits, can constrain chronology and reveal the spatio-temporal zonal characteristics.

Mineralization and magmatic origin of the 2.1 Ga Zhaoanzhuang magnetite-apatite deposit, North China Craton: Constraints on the formation of the Paleoproterozoic iron oxide-apatite deposits

Iron oxide-apatite (IOA) deposits provide large amounts of iron for steel industry worldwide, but their origins are still in dispute. Although IOA deposits were commonly genetically related to intermediate-mafic igneous rocks, the occurrences of IOA deposits in ultramafic rocks are scarce. The Zhaoanzhuang iron deposit in the North China Craton is characterized by comprising abundant low-Ti magnetite, apatite and ultramafic minerals (e.g., olivine, orthopyroxene), but its origin has long been controversial. These ultramafic minerals mainly occur as aggregates surrounded by magnetite in the iron ores and display typical cumulate textures and high Fo values, indicative of magmatic origin. The δ56Fe and δ18O values of magnetite from the ores range from 0.22 ± 0.03‰ to 0.32± 0.03‰ and from 2.6‰ to 4.5‰, respectively, akin to those in magmatic rocks and IOA deposits but distinct from those of metamorphic BIFs. Both types of apatite occurring in the ultramafic aggregates and the iron ores contain 2.07 to 2.69 wt% F, less than 0.65 wt% Cl, up to 7900 ppm rare earth elements (REE) and greatly variable Sr-Nd isotopic compositions, reflecting that both types are of magmatic-hydrothermal origin. The crust-like εNd(t) values (-7.1 to −5.2) of all apatite grains record reset of their Sm-Nd system, which is resulted from the involvement of carbonate fluids based on their similar REE patterns. In situ Sm-Nd isotope analyses of apatite from the ultramafic aggregates and the iron ores have yielded similar Sm-Nd isochron ages at 2100 ± 81 Ma (MSWD = 0.84) and 2096 ± 240 Ma (MSWD = 0.85), respectively, which can constrain the minimum age for the Fe mineralization. Combined with sedimentary age of the strata, the ore-forming age of the deposit was 2.3–2.1 Ga. The negative εNd(t) values of the ultramafic aggregates (-1.2 to −1.0), the iron ores (-1.8 to −1.5) and magnetite (-1.6 to −0.8) from the ores indicate that the ore-forming magma has a slightly enriched signature. Combined with linear relationship between their εNd(t) values and MgO contents, an involved mantle-derived ultramafic magma is most likely to be contaminated by the crustal material during ascending. Therefore, the Zhaoanzhuang deposit is a good Paleoproterozoic case of the rare IOA deposits associated with ultramafic magmas.

Origin of Dolomite in Lacustrine Organic-Rich Shale: A Case Study in the Shahejie Formation of the Dongying Sag, Bohai Bay Basin

In most organic-rich shale reservoirs, dolomite is widely distributed and has different types and crystal sizes. However, the characteristics and formation mechanism of the dolomites in organic-rich shale are still poorly understood. Petrographic and geochemical analyses were performed to interpret the formation of dolomite in the lacustrine organic-rich shale of the Shahejie Formation, Dongying Sag, Bohai Bay Basin. Four types of dolomites, which represent episodic recrystallization, were classified based on crystal size and shape: 1) micritic dolomite (Dol-1), 2) sub-to euhedral (cloudy cores with clear rims) dolomite (planar-e) (Dol-2), 3) anhedral dolomite (coarse planar-s to nonplanar crystals) in phosphatic particles (Dol-3), and 4) fracture-filling anhedral dolomite (Dol-4). Dol-1 has nonplanar mosaic micritic crystals with irregular intercrystalline boundaries and dull cathodoluminescence (CL), suggesting dolomitization during the early burial stage. Dol-1 tends to occur under high paleosalinity and warm conditions. Furthermore, the syngenetic relationship, with abundant framboidal pyrite and gypsum, suggests that bacterial sulfate reduction (BSR) may influence the formation of Dol-1. The high content of Sr and low content of Mn/Sr also indicate less influence on burial. The Dol-2 crystals show cloudy cores with clear rims attributed to progressive dolomitization during burial. Dol-2 is always associated with the organic matter within the organic matter-rich lamina. The anhedral crystals and undulate (sweeping) extinction of Dol-3, which is usually encased by phosphatic particles in the organic-rich lamina, reflect the recrystallization affected by bacteria and the subsequent thermal evolution of organic matter. Dol-4 fulfills the abnormal pressure fractures crosscutting the earlier phases (Dol-1 and Dol-2) with undulate (sweeping) extinction and different rare earth element (REE) patterns. Dol-4 may be affected by hydrothermal fluids, which are influenced by the thermal evolution of organic matter. The 87Sr/86Sr values of the four types of dolomites similarly demonstrate the same dolomitizing fluids. Dol-1 to Dol-3 have similar REE patterns but are different from Dol-4, suggesting that Dol-4 likely resulted from circulation through basinal sediments instead of different fluids. Focusing on the origin of dolomite has been instrumental in understanding the diagenetic evolution, fluid flow, and organic-inorganic interactions in organic-rich shale and, hence, the reservoir formation of shale oil.

Tungsten mineralization process and oxygen fugacity evolution in the Weijia W deposit, South China: Constraints from the microstructures, geochemistry, and oxygen isotopes of scheelite, garnet, and calcite

Scheelite is a major (W)-bearing mineral enriched in rare earth elements (REEs) and is widely used to investigate the mineralization process, fluid source and redox environment of W deposits. In the Weijia skarn-type W deposit in South China, scheelite was formed during the greisen, retrograde skarn, oxidation, and sulfide-quartz vein stages. Various characteristics of cathodoluminescence (CL) images of scheelite grains (e.g., light to dark color, core-rim texture and oscillatory zonation) have recorded their detailed crystallization process. Scheelite grains formed in the greisen and retrograde skarn stages have similar LREE patterns, and the gradual fractionation in HREEs may be a result of the precipitation of garnet (And38-70Gr29-52Pr1-3) in the prograde skarn stage. In addition, the similar REE patterns of scheelite from the greisen stage and the granite porphyry pluton suggest a genetic relationship. The oxygen isotopic compositions of scheelite and quartz are all plotted near the edge of the magmatic fluid field, confirming their derivation from the same magmatic hydrothermal fluid. Elemental mapping and spot analyses using an electron probe microanalyzer indicated that extensive isomorphic substitution of W by molybdenum (Mo) occurs in scheelite, and the darker color in the CL images corresponds to a higher Mo content and lower W content. The Mo content of scheelite gradually increased from the greisen stage (mean = 0.46%), through the retrograde stage (mean = 2.89%), to the oxidation stage (mean = 19.8%), and then dropped sharply in the sulfide-quartz vein stage (mean = 0.20%), indicating a change in oxygen fugacity. Moreover, scheelite formed in the sulfide-quartz vein stage has an oscillating Mo content, thus implying a slight or periodical change in oxygen fugacity during the same mineralization stage.

Fluid-rock interaction and fluid mixing in the large Furong tin deposit, South China: New insights from tourmaline and apatite chemistry and in situ B-Nd-Sr isotope composition

AbstractThe Furong tin deposit (South China) is genetically associated with the multiphase Qitianling batholith that consists of main-phase and minor, but more fractionated, late-phase granites. Several tourmaline and apatite generations are distinguished. Tourmaline (Tur) variants comprise pre-ore Tur-1 as disseminations and nodules in the late-phase granite, pre- to syn-ore Tur-2 as replacements in nodules and as veins crosscutting the late-phase granite and nodules, syn-ore Tur-3 in tin greisens, pre- to syn-ore Tur-4 as veins in the altered main-phase granite, and syn-ore Tur-5 from tin skarns in a distinct Ca-rich environment. Apatite (Ap) generations include accessory Ap-G in the main-phase granite, and Ap-I to Ap-III from three stages related to skarn-type mineralization (garnet-diopside stage-I, pargasite-phlogopite-cassiterite stage-II, and sulfide-rich stage-III). Textural and compositional features suggest that all tourmaline variants are hydrothermal in origin with alkali and schorl to foitite composition and minor extensions to calcic and X-site vacant tourmaline groups, whereas all apatite generations belong to fluorapatite with Ap-G crystallizing from the magma and Ap-I to Ap-III being hydrothermal in origin. The narrow range of tourmaline δ11B values (–14.8 to –10.4‰) suggests a single magmatic boron source in the ore-forming fluids. The similar rare earth element patterns and εNd(t) values (–8.2 to –5.9 for Ap-G and –8.0 to –7.3 for Ap-I) between magmatic and hydrothermal apatite indicate that the skarn-forming fluids are dominantly derived from granites. The 87Sr/86Sr ratios of Ap-I to Ap-III (0.70733–0.70795) are similar to the carbonate wall rocks, but distinctly different from the more radiogenic granites, indicating Sr exchange with carbonate rocks. Integrating previous H-O isotopic data, the tourmaline and apatite elemental and B-Sr-Nd isotope results suggest that the greisen-type ore formed by interaction of B-, Na-, Li-, Zn-, and Sn-rich magmatic fluids with the late-phase granite in a closed and reduced feldspar-destructive environment, whereas the tin skarns resulted from mixing of magmatic fluids with meteoric water and interaction with the carbonate wall rocks in an open system where oxygen fugacity changed from reduced to oxidized conditions. During fluid-rock interactions and fluid mixing, considerable Ca, Mg, V, Ni, and Sr from the host rocks were introduced into the ore system. Coupled hydrothermal minerals such as tourmaline and apatite have great potential to fingerprint the nature, source, and evolution of fluids in granite-related ore systems.

Fault-controlled carbonate-hosted barite-fluorite mineral systems: The Shuanghe deposit, Yangtze Block, South China

Fault-controlled carbonate-hosted Ba-F deposits occur in many major orogenic belts of the world and are important repositories of fluorine production required in various fields of chemical, metallurgical glass, ceramic, and cement industries. Here we investigate the poorly documented Shuanghe barite-fluorite deposit in the Yangtze Block of South China. This deposit offers an excellent case for studying the processes of formation of fault-controlled carbonate-hosted barite-fluorite mineral system. The orebodies in Shuanghe are hosted in carbonate rocks belonging to the early Ordovician Honghuayuan Formation and are dominantly controlled by NE-trending faults. Fluorite and barite exhibit similar rare earth element patterns with positive Eu anomalies and negative Ce anomalies. We report fluorite Sm–Nd isotopic data which yielded an isochron age of 103.1 ± 1.3 Ma. The δ18OV‐SMOW values range from +0.2‰ to +1.1‰ and the δDV‐SMOW values show a range of –33.1‰ to −26.3‰. The δ34SV-CDT values for barite range from 31.1‰ to 38.2‰. The bulk and in-situ Sr values for fluorite and barite show a range of 0.709081 to 0.712258. The petrological and geochemical data reported in this study suggest the following processes: (1) derivation of the ore-forming fluid from basinal water mixed with meteoric water; and (2) origin of the ore-forming components from Cambrian carbonate rocks, black shale, and Ordovician limestone. We propose a model that correlates the tectonic background of mineralisation with the Late Cretaceous subduction of the Paleo-Pacific Oceanic plate.

Revisiting the provenance of the Stonehenge bluestones: Refining the provenance of the Group 2 non-spotted dolerites using rare earth element geochemistry

The doleritic bluestones of Stonehenge, sourced from the Mynydd Preseli in west Wales, have been previously classified into three geochemical groups on the basis of compatible element geochemistry (Bevins et al., 2014). The majority of Group 1 (spotted) dolerites were identified as coming from the outcrop of Carn Goedog, Group 3 (spotted) dolerites were linked to the outcrops Carn Breseb, Carn Gyfrwy, outcrops in the vicinity of Carn Alw and an un-named outcrop west of Carn Ddafad-las and Group 2 (non-spotted) dolerites were identified as coming from either Cerrigmarchogion or Craig Talfynydd. A sub-set of the samples used by Bevins et al. (2014) have been re-analysed by solution nebulisation ICP-MS, including analyses of the rare earth elements (REE).Analysis of the REE data reveals that Groups 1 and 3 dolerites from both Stonehenge and the Preseli have very similar REE patterns which strongly suggests that they are derived from a single intrusive body. Group 2 non-spotted dolerites are now divided, on the basis of their REE contents, into four Preseli and two Stonehenge sub-groups, (Groups 2i-2iv and Groups 2v-2vi, respectively) while Stonehenge orthostat sample SH44 plots apart from all other Stonehenge and Preseli samples in all discriminant diagrams used. The new data show that Preseli Group 2i dolerites have very distinct concave down “humped” patterns and bear no resemblance to any analysed Stonehenge dolerites. The source of Stonehenge Group 2v dolerites remains equivocal; they plot close to Preseli Group 2ii dolerites from Carn Ddafad-las and Garn Ddu Fach and have in common the presence of notable positive Eu anomalies, but they show minor differences, especially in relation to their Gdn/Ybn ratios. However, Stonehenge orthostat sample SH45 shows a near identical REE composition to Preseli Group 2iii dolerites from Cerrigmarchogion.In terms of the interpretation of REE contents and chondrite-normalized patterns we found no differences whether using the ‘standard’ techniques used by geochemists, based on chondrite-normalized elemental ratios and values, or the quantitative approach using shape factors derived from polynomial curve fitting.

Middle Permian dolomites of the SW Sichuan Basin and the role of the Emeishan Large Igneous Province in their origin

Dolomitization of the Middle Permian carbonate platform in South China occurred in successive phases, partially influenced by the emplacement of the Emeishan Large Igneous Province. The main replacement dolomites are: (1) finely crystalline (˂50 μm) Md I, (2) mesocrystalline (50–250 μm), non-planar-a to non-planar-s Md II; and (3) coarse crystalline (250–500 μm), non-planar-s to non-planar-a Md III. Dolomite cement (Cd) is characterized by pores and/or fractures filled with saddle crystals within the Md III dolomites. The Md I dolomites share similar rare earth element patterns and carbon and strontium isotopes to host limestones and δ18O values that are in equilibrium with the limestone. These characteristics indicate that Md I dolomite was formed via the seepage/reflux dolomitization of evaporated seawater during the penecontemporaneous phase. The Md II dolomites share δ13C values and 87Sr/86Sr ratios similar to those of typical Permian carbonates, characterized by a range of low δ18O values. Analyses of fluid inclusions in these Md II dolomites showed homogenization temperatures of 90–130 °C and salinities of 4–9 wt% NaCl equivalent. These results suggest that Md II represents typical replacive dolomitization occurring within convective half-cells under geothermal conditions. They are particularly pervasive when the platform is heated by magmatic underplating and/or igneous emplacement to form thermal convection. The coarser Md III dolomites are characterized by high porosity and subsequently partially filled with saddle dolomite cement (Cd). These Md III and Cd dolomites are rich in radiogenic strontium, have low δ13C values, very low δ18O values, positive Eu anomalies, and contain fluid inclusions indicating high homogenization temperatures and salinities. These characteristics suggest that the Md III and Cd dolomites precipitated from high-temperature hydrothermal fluids, potentially associated with the emplacement of the Emeishan Large Igneous Province (LIP). Regional heating from the Emeishan LIP induced basin uplift and faulting. These were important in channeling the upwelling deep brine fluids into the overlying platform carbonates, leading to the localized intense dolomitization. This suite of Middle Permian dolomites is an excellent example of the successive phases of different dolomitization processes, including that associated with hydrothermal and thermal convection during the emplacement of a LIP.

Geochemistry of surface soil in the Eastern Pamirs and its implications for Muztagata ice core dust provenance

Determining the geochemical composition of mineral dust is highly effective for provenance tracing. Studies on the geochemistry of surface soils in the Pamirs, especially with different size fractions, are relatively limited. This study investigated the major, trace and rare earth elements (REEs) of surface soil samples collected on the Eastern Pamirs. The Fe/Al (~0.60), Mg/Al (~0.30) and Ti/Al (~0.05) ratios clearly highlight its clayey nature, while the La/Th (~2.60) and Th/U (~4.00) ratios indicate it is typical of the upper continental crust (UCC). The La-Th-Sc and Zr/10-Th-Sc ternary diagrams show the source rock of the Eastern Pamirs surface soil to be mainly felsic. The bulk and <20 μm fractions were separated and analysed, showing similar REE patterns. However, the LaN/SmN, δCe, Ce/Nd, Th/Tb and Y/Er ratios of the <20 μm fraction differ from those of the bulk, suggesting particle size sorting has a significant impact on the elemental composition of surface soil samples. The Taklimakan Desert sand samples partly display similar tracing indices while the Eastern Pamirs surface soil samples generally show different provenance indexes compared to the Muztagata ice core and aerosol dust, indicating that the Taklimakan Desert might be an important contributor to the Muztagata ice core/aerosol dust and the local contribution is minor. Analysis of the MODIS AOD fields, wind streamline fields and HYSPLIT backward trajectory model demonstrates certain atmospheric circulation patterns and dust transport pathways in the Eastern Pamirs, which allows for sourcing of Muztagata dust from the Taklimakan Desert, Central Asian deserts, and northern Afghanistan and Pakistan.

On the dolomite reservoirs formed by dissolution: Differential eogenetic versus hydrothermal in the lower Permian Sichuan Basin, southwestern China

Dolomite reservoirs formed by dissolution are significant for hydrocarbon exploration, but their origins are poorly understood because of their complexity. This study investigated the lower Permian Qixia Formation of the Sichuan Basin (northern Longmenshan Mountains), southwestern China, which has recently been the location of giant natural gas discoveries. Petrological and geochemical analyses were undertaken, including trace elements and C, O, and Sr isotopes. The results indicate that the reservoirs were formed by differential eogenetic karstification and hydrothermal dissolution of grain dolomites. Core and thin-section observations identified three types of dolomite based on grain size: fine- to medium-crystalline dolomite (Dol-1), medium- to coarse-crystalline dolomite (Dol-2), and coarse- to megacrystalline saddle dolomite (Dol-3). The Dol-2 types can be further divided into three subtypes according to the grain structure and degree of pore development: euhedral medium- to coarse-crystalline dolomite with pores (Dol-2a), porous subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2b), and tight subhedral–anhedral medium- to coarse-crystalline dolomite (Dol-2c). The Dol-1 types appear to have formed because of the release of and subsequent metasomatism by Mg2+ in trapped seawater along stylolites at shallow burial conditions (500–1000 m). The Dol-2a types resulted mainly from eogenetic karstification of original limestone and subsequent dolomitization by marine-related fluids. These dolomites are geochemically characterized by relatively flat rare-earth element patterns and relatively low Y/Ho values (mean e 37.1). The Dol-2b and Dol-3 types were formed mainly by hydrothermal fluids and have similar rare-earth element patterns with large positive Eu anomalies (mean e 5.7). The Dol-2c types experienced metasomatism by marine-related fluids mixed with minor hydrothermal fluids. All these formation mechanisms reflect the influence of multistage diagenetic and hydrothermal fluids. The Dol-2a types form the most favorable reservoirs in the study area, followed by the Dol-2b types. The distribution of the Dol-2a types was controlled by eogenetic karstification of grain shoals, whereas that of the Dol-2b types reflects the influence of tectonically derived hydrothermal fluid dissolution of grain shoals. As such, both primary sedimentary features and later differential dissolution (eogenetic and hydrothermal) contributed to reservoir development, which involved factors such as the sedimentary facies (grain shoal), sea-level fluctuations (eogenetic karst development), and faulting (hydrothermal dissolution). Therefore, the formation of dolomite and associated reservoirs is complex. The results of this study provide an improved understanding of the origins and complexity of dolomite reservoirs formed by dissolution, which is relevant to exploration for such reservoirs.

Rare Earth Elements in Dinosaur Bones Across the Embryo-Adult Spectrum

There is broad consensus that many bones of extinct vertebrates contain Rare Earth Elements (REE) and other trace minerals that have been incorporated and enriched into the fossil during diagenesis. During fossilization, apatite minerals in vertebrate bones recrystallize from metastable biogenic forms to thermodynamically more stable inorganic apatite minerals while incorporating REEs from their environment. More than ~95% of the REE in fossil bones are diagenetically incorporated postmortem and bones in different environments have different and distinct REE signatures, generally viewed to be controlled by sedimentological and taphonomic processes. The REE pattern in fossils is generally stable and the normalized concentration patterns provide unique “fingerprints” or signatures that have been used for various geological and paleontological investigations. The discovery of embryos and their adults at the same site, a previously unrecorded occurrence in the fossil record, allowed us to compare for the first time the relationship between REE concentrations in the same taxon at widely differing ontogenetic stages. Here we compare REE patterns in bones from two distinct sites in Yunnan, China, both preserving embryonic and adult bones of Early Jurassic sauropodomorph dinosaurs. These dinosaurs are closely related to each other and close in geological age, and their bones show very similar REE patterns. However, the embryonic bones have significantly lower levels of total REEs when compared to their adult counterparts. We attribute the tenfold difference to the level of ossification of the bones and the dramatically lower concentration of apatites in the embryonic than in the adult bones. We conclude that the ontogenetic stage of fossil materials can have a significant impact on REE concentrations, and discuss the impact of these results on future work.

REE fractionation and human Gd footprint along the continuum between Paraguaçu River to coastal South Atlantic waters

In this study, we evaluated the fractionation of dissolved Rare Earth Elements (REE) from the continuum between the Paraguaçu Estuary through Todos os Santos Bay (BTS), Bahia, Brazil and during a 12h tidal cycle in a pier station. The results show that REE were scavenged in the estuarine low salinity region (<5) following the order Light>Medium>Heavy (LREE>MREE>HREE), likely due to colloids coagulation/precipitation. At mid to high salinities, REE concentrations gradually increased, starting with HREE, followed by MREE and LREE. Within Todos os Santos Bay, a clear difference between the eastern and western sides was observed: 1) The western side presented very similar REE patterns, clearly influenced by the fluvial input (Paraguaçu and Subaé Rivers), 2) the eastern side of the bay showed more scattered REE patterns, possibly due to the influence of different sources, such as submarine groundwater discharge (SGD) and the mixing with coastal seawater. The salinity at a pier station did not vary throughout the tidal cycle experiment (S≅33.4), but the REE pattern changed from a terrigenous (flat) at low tide to a marine-like (HREE enriched) pattern at the flood tide. Anthropogenic Gd anomalies (Gd/Gd*>1.4) were observed at the estuary and also at high tide and during ebb tide along the tidal cycle experiment. Taken together, these observations show that the REE behavior along the continent-ocean continuum is quite variable. Moreover, the occurrence of anthropogenic Gd indicates that anthropogenic inputs of REE may complicate their use as a tracer for natural processes in the future.

The role of clinopyroxene in amphibole fractionation of arc magmas: Evidence from mafic intrusive rocks within the Gangdese arc, southern Tibet

Amphibole is thought to be a key control on the differentiation of arc magmas but the fractionation of this mineral remains enigmatic because it is rarely present in erupted lavas. This study presents detailed petrological, mineralogical, and geochronological data for newly identified Late Triassic (217–215 Ma) mafic intrusive rocks from the Gangdese arc of southern Tibet. These gabbros host an amphibole-rich cumulate enclave and clinopyroxene crystal clots, including two mineral assemblages in chemical disequilibrium: (1) amphibole and Type-1 clinopyroxene within the enclave and clots; and (2) small crystals of Type-2 clinopyroxene, plagioclase, and biotite within the host. Textural and geochemical evidence indicate that the enclave and clots represent captured fragments of amphibole- and clinopyroxene-dominated lower crustal crystal mushes, respectively, whereas small host crystals formed by the shallow crystallization of evolved melts extracted from these crystal mushes. The compositional variations between the two types of clinopyroxene also reveal this melt segregation process. Amphibole with similar rare earth element patterns to associated Type-1 clinopyroxene inclusions suggest that the former formed through peritectic reactions consuming clinopyroxene. Enhanced amphibole fractionation may also have occurred as a result of the presence of high-density clinopyroxene inclusions. This indicates that the fractionation of amphibole together with precursory clinopyroxene inclusions within the deep crust controls the early chemical evolution of mafic arc magmas, a process that is consistent with the geochemical variations in lavas from three other active arc volcanoes. The associated cumulates can efficiently delaminate back into the mantle, ultimately leading to the generation of SiO2-rich continental crustal material.

The characteristics, origin, and significance of mafic microgranular enclaves in the granitoids from the Bailingshan complex, Eastern Tianshan, NW China

The Bailingshan complex is located to the south of the Jueluotage tectonic belt of Eastern Tianshan, consisting of quartz monzodiorite, granodiorite, monzogranite, and granite. Mafic microgranular enclaves (MME) are commonly found in granodiorites and monzogranites and have sharp contacts with the host rocks. Sometimes, the fine veins of the host rocks cut through the enclaves. The MMEs are mainly fine‐grained dioritic rocks, including diorite and pyroxene diorite with igneous textures, and having condensing edges with rich fine dark minerals. Among the enclaves, the orthoclase xenocryst with hornblende aggregation rim at its edge, acicular apatites in plagioclase, sometimes late‐crystallized biotites between the oscillatory zonings of plagioclase, and hornblende metasomatic rims occur around the pyroxene phenocrysts can be observed. Both the host rocks and the enclaves are metaluminous and calc‐alkaline to high‐K calc‐alkaline rocks. The enclaves have lower SiO2 and higher Al2O3, FeOT, MgO, CaO, and TiO2 concentrations, as well as higher amounts of the trace elements Cr, Sc, and Th than the host rocks. The host rocks and the enclaves exhibit similar rare earth elements patterns and linear relation of oxides. The sulphur isotopes (δ34S(V‐CDT) = −1.7‰ to +0.7‰) from the iron ores in the Bailingshan area indicate that the mineralization materials originated from the upper mantle. All of these lithology and geochemistry characteristics demonstrate that the enclaves were formed by the mixture of mantle‐derived mafic magma and crust‐derived felsic magma. The iron ore mineralization in the Bailingshan area may be associated with the magma mixing.