Rare Earth Element Abundance Patterns Research Articles

Water-sulfur-rich, oxidised adakite magmas are likely porphyry copper progenitors

The world’s largest current Cu resource is volcanic arc-hosted, porphyry copper deposits. Whether unusual parental magmas or fortuitous combinations of processes accompanying emplacement of common parental arc magmas (e.g., basalt) is required for ore deposit formation, remains unclear. Spatial and tectonic associations of adakite (andesite with high La/Yb, Sr/Y) with porphyries exist, but genetic links are debated. Delayed saturation with Cu-bearing sulfides consequent to elevated redox state seems essential for late-stage exsolution of Cu-bearing hydrothermal fluids. Partial melting of igneous layers of subducted, hydrothermally altered oceanic crust in the eclogite stability field are invoked to account for andesitic compositions, residual garnet signatures, and the putative oxidised character of adakites. Alternative petrogeneses include partial melting of lower crustal, garnet-bearing sources and extensive intra-crustal amphibole fractionation. Here we demonstrate mineral-hosted, adakite glass (formerly melt) inclusions in lavas erupted subaqueously in the New Hebrides arc are oxidised relative to island arc (and mid-ocean ridge) basalts, are H2O-S-Cl-rich, and moderately enriched in Cu. Polynomial fitting of chondrite-normalised, rare earth element abundance patterns shows the precursors of these erupted adakites were unequivocally derived from partial melting of subducted slab, and represent optimal porphyry copper progenitors.

Probing the history of ultra‐high temperature metamorphism through rare earth element diffusion in zircon

AbstractThe extent to which solid‐state volume diffusion modifies rare earth element (REE) abundances in accessory minerals during high‐temperature metamorphism governs our ability to link recorded trace‐element compositions to particular thermal events. We model diffusion of REE in zircon under different temperature–time conditions and show that, for both short‐lived (e.g. 1100°C for 1–5 Ma) and more prolonged (e.g. 1050°C for 10–30 Ma or 1000°C for 200 Ma) episodes of ultra‐high‐temperature (UHT) metamorphism, REE diffusion in igneous zircon is sufficiently rapid for REE in a ~50‐μm grain to equilibrate with the new metamorphic mineral assemblage of the host rock. By contrast, unless diffusion is accelerated by recrystallization, the presence of fluids or other processes at temperatures below 900°C zircon will largely retain its original pre‐metamorphic REE abundance pattern, even when the thermal event is long lived (≥100 Ma). Where volume diffusion is dominant, for instance, in the absence of a fluid phase, the sensitivity of REE mobility to temperature can help constrain the temperature–time path of high‐grade metamorphic rocks. Modelling of well‐characterized natural samples from the regional‐scale aureole surrounding the Rogaland Igneous Complex (RIC) in SW Norway shows that variations in REE concentration patterns in zircon indicate a T–t evolution that is consistent with independent P–T–t estimates for regional metamorphism based on phase equilibrium modelling (850–950°C at 7–8 kbar for ~100 Ma). Greater modification of REE abundance patterns in zircons within 2 km of the RIC contact, however, indicates that UHT conditions persisted for ~150 Ma close to the intrusion, with a temperature of ~1100°C for 1–5 Ma at the RIC contact. Thermal modelling suggests that the inferred T–t histories of samples from different distances from the RIC contact are best explained if the complex was emplaced incrementally over 1–5 Ma.

Accumulation patterns and species-specific characteristics of yttrium and rare earth elements (YREEs) in biological matrices from Maluan Bay, China: Implications for biomonitoring

The critical usage of rare earth elements (REEs) in a variety of industrial applications has increased their release to the environment as emerging contaminants, while little is known about the fate and transport of REEs in coastal aquatic biota. In the present study, seven common species were collected and the concentrations of 15 naturally occurring REEs (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) were determined. Significant differences in total REEs concentrations were found among species even in the same taxa or phylum, suggesting that REEs bioaccumulation patterns appeared to be species- and element-dependent even in the same taxa or phylum, but with limited potential for bio-magnification based on the nitrogen isotope signatures (δ15N). Except for occasional anomalies for redox-sensitive elements of Ce and Eu, the abundance patterns of REEs normalized to chondrite revealed similar REE distribution trends, indicating a common source of REEs in all samples. Additionally, the abundances of light REEs (from La to Eu) were much higher than those of heavy REEs (from Gd to Lu and Y), demonstrating the fractionation between the light and heavy REEs. Furthermore, REEs concentrations in molluscs were notably higher than other species, implying their potential as bio-indicators of REEs due to the habitat and specific feeding behavior. Overall, this is not only the first study to focus on distribution levels, accumulation characteristics, geochemical and fractionation patterns of REEs in coastal species from identical area, but quantifying and tracing REE behavior will contribute to better evaluating the possible environmental impacts of REEs enrichment for future biomonitoring research.

Origins of mass-dependent and mass-independent Ca isotope variations in meteoritic components and meteorites

The Ca isotope composition of meteorites and their components may vary due to mass-dependent and/or -independent isotope effects. In order to evaluate the origin of these effects, five amoeboid olivine aggregates (AOAs), three calcium aluminum inclusions (CAIs), five chondrules (C), a dark inclusion from Allende (CV3), two dark fragments from North West Africa 753 (NWA 753; R3.9), and a whole rock sample of Orgueil (CI1) were analyzed. This is the first coupled mass-dependent and -independent Ca isotope dataset to include AOAs, a dark inclusion, and dark fragments. Where sample masses permit, Ca isotope data are reported with corresponding petrographic analyses and rare earth element (REE) relative abundance patterns. The CAIs and AOAs are enriched in light Ca isotopes (δ44/40Ca −5.32 to +0.72, where δ44/40Ca is reported relative to SRM 915a). Samples CAI 5 and AOA 1 have anomalous Group II REE patterns. These REE and δ44/40Ca data suggest that the CAI 5 and AOA 1 compositions were set via kinetic isotope fractionation during condensation and evaporation. The remaining samples show mass-dependent Ca isotope variations which cluster between δ44/40Ca +0.53 and +1.59, some of which are coupled with unfractionated REE abundance patterns. These meteoritic components likely formed through the coaccretion of the evaporative residue and condensate following Group II CAI formation or their chemical and isotopic signatures were decoupled (e.g., via nebular or parent-body alteration). The whole rock sample of Orgueil has a δ44/40Ca +0.67 ± 0.18 which is in agreement with most published data. Parent-body alteration, terrestrial alteration, and variable sampling of Ca-rich meteoritic components can have an effect on δ44/40Ca compositions in whole rock meteorites.Samples AOA 1, CAI 5, C 2, and C 4 display mass-independent 48/44Ca anomalies (ε48/44Ca +6 to +12) which are resolved from the standard composition. Other samples measured for these effects (AOA 5, CAI 1, CAI 2, C 3, D 1, D 2, D 3) possess the same 48/44Ca isotope composition as the standard within analytical uncertainty. These data indicate a heterogeneous distribution of 48Ca in the early solar nebula during formation of CAIs, AOAs, and chondrules. In a ε48/44Ca vs. δ44/40Ca plot, no strong correlation is evident which suggests that the thermal processing event which caused a heterogeneous distribution of ε48/44Ca in the solar nebula is unlikely to be directly related to the thermal processing event that caused coupled REE and Ca mass-dependent isotopic fractionation in meteoritic components.

Sources of rare earth elements in shells and soft-tissues of bivalves from Tokyo Bay

Abstract Five species of bivalves (Japanese littleneck, blue mussel, trough shell, Stimpson's quahog, and Japanese dosinia) were collected from three different sites in Tokyo Bay and their shells and soft tissues were analyzed for rare earth element (REE) concentrations. Their REE compositions were compared with those of sediment and seawater at corresponding sites. To estimate the digestive strength of the bivalves, different acids were used to extract the elements in sediment for comparison of REE compositions. The REE concentrations of soft tissues were higher than those of shells. The REE abundance patterns of both shells and soft tissues resembled those of sediments or suspended particles rather than those dissolved in seawater, except for mussels. REE composites calculated by mixing particulate and dissolved REEs at much greater ratios for particulate REEs can explain the REE composition of both shells and soft-tissues. In the case of mussel shells, the contribution of REEs dissolved in seawater is important, but despite this most of mussel REEs similarly originate from suspended particles in seawater. This implies that a simple projection to seawater REEs from the REE composition of shells would be difficult. The apparent partitioning coefficients of REEs over Ca in shells ranged from 0.1 to 10 against REEs/Ca of seawater.

Rare Earth Elements Behaviour at West Coast of Peninsular Malaysia Rocky Shore Ecosystem Using Saccostrea cucullata as Bioindicator

This study focuses on the assessment of rare earth elements (REEs) in rocky shore ecosystem along west coast region of Peninsular Malaysia coastal waters, describing their partitioning in selected body part of Saccostrea cucullata and deliberating on interspatial variability. Samples were digested using Teflon Bomb technique and concentrations of 14 naturally occurring REEs were measured using ICP-MS technique, along with selected trace metals for additional data. The REEs fractionation patterns normalized to chondrite were remarkably similar, indicating a common source of the REEs. Consistent REEs abundance patterns were found in all samples, with enrichment of LREE over HREE, which implies that REEs are transported as a coherent group through west coast of Peninsular Malaysia aquatic ecosystems. Quantification of anomalies calculated using values normalized to chondrite data presented ratios higher than unity for Ce and ratios lower than unity for Eu in all sites. Filter feeder S. cucullata is considered to be highly potential as a bioindicator for REEs, undoubtedly due to its feeding behaviour that is much related to suspended particles as REEs sources. Comparative results showed that the values are considerably below the safety limit, with the exception of Ce and Nd in the soft tissue of S. cucullata.

Paleoenvironments revealed by rare-earth element systematics in vertebrate bioapatite from the Lower Devonian of Svalbard

In situ rare-earth element (REE) compositions have been measured in early vertebrate microremains from the Lower Devonian basin of Andrée Land (Svalbard), with the aim of obtaining information about their early depositional environment and potential reworking. Vertebrate microremains with different histology were used for the analyses, sourced from two different localities of marginal marine to freshwater sediments from geographically distant parts of the Grey Hœk Formation (Skamdalen and Tavlefjellet members). We selected thelodont and undescribed ?chondrichthyan scales, which allowed us to define potential taxonomic, histological, and taphonomic variables of the REE uptake. Results showed REE concentrations to be relatively uniform within the scales of each taxon, but apparent discrepancies were visible between the studied localities and separate taxa. The compilation of REE abundance patterns as well as REE ratios have revealed that thelodont and ?chondrichthyan originating from the same locality must have had different burial and early diagenetic histories. The shapes of the REE profiles, together with the presence and absence of the Eu and Ce anomalies, equally suggested different depositional and diagenetic environments for these two sympatric taxa resulting from either stratigraphical or long-distance reworking. The REE concentrations appear to have visible differences between separate dental tissues, particularly between enameloid and dentine of thelodonts, emphasizing the importance of in situ measurements in microfossil biomineral geochemistry.

HETEROGENEOUS ISOTOPIC ANOMALIES OF SM AND GD IN THE NORTON COUNTY METEORITE: EVIDENCE FOR IRRADIATION FROM THE ACTIVE EARLY SUN

Large and heterogeneous isotopic variations of 150Sm/149Sm and 158Gd/157Gd due to neutron capture reactions caused by cosmic-ray irradiation were found in chemical and mineral separates from the Norton County meteorite. The light-colored separates, consisting mainly of enstatite (Mg2Si2O6), have a very large neutron fluence of 1.98?? 1017?n?cm?2, which is 10 times higher than that of the whole rock. Furthermore, four chemical separates showed a large variation in neutron fluences, ranging from 1.82?? 1016 to 1.87?? 1017?n?cm?2. The variable amounts of neutron fluences from a small single fragment of the Norton County meteorite cannot be simply explained by single-stage cosmic-ray irradiation in space. Rare earth element (REE) analyses revealed that the fractions with high neutron fluences have similar chemical properties to those in the early condensates in the solar system, showing depletions of Eu and Yb in their REE abundance patterns. The data provide evidence for an activity of the early Sun (T Tauri), suggesting the migration of early and intense irradiation materials into the Norton County meteorite's parent body.

Fractionation of rare earth elements in refractory inclusions from the Ningqiang meteorite: Origin of positive anomalies in Ce, Eu, and Yb

Ion microprobe analyses of rare earth elements (REEs), Ba, and Hf were performed for various types of refractory inclusions including amoeboid olivine aggregates (AOAs) from the Ningqiang ungrouped carbonaceous chondrite to search for possible relationships between REE abundance patterns and bulk chemical compositions of the inclusions. Four types of CI-normalized REE patterns were recognized: (1) nearly flat (unfractionated) pattern with or without Eu (and Yb) anomalies (Groups I, III, or V), (2) depletions of ultrarefractory heavy REEs (HREEs) relative to light REEs (LREEs), and depletions of Eu and Yb ( Group II, but without depletion of Yb in some cases), (3) depletions of ultrarefractory HREEs with positive anomalies in Ce, (Eu), and Yb ( Modified Group II), and (4) nearly flat pattern with positive anomalies in Ce, (Eu), and Yb ( Modified Group I). No systematic correlation was found between bulk chemical compositions and REE patterns of the inclusions. This suggests that the observed REE fractionations occurred prior to condensation of major elements (e.g., Mg and Si) which defined bulk chemical compositions of the inclusions. It is remarkable that 7 out of 19 inclusions show positive anomalies in Ce, Yb, and in some cases, Eu as well (Modified Group I and Modified Group II), suggesting that such anomalies are rather common among inclusions in the Ningqiang and possibly in other primitive meteorites. Two possible mechanisms are considered for the formation of Modified Group II and Modified Group I patterns. In Model 1, Modified Group II is formed by a process similar to that produced Group II but removal of ultrarefractory dust occurred at slightly lower temperatures, where not only ultrarefractory HREEs but some fraction of LREEs had been condensed and removed from the system. Modified Group I may be explained by addition of an unfractionated component to the Modified Group II component, or alternatively, by partial removal of ultrarefractory dust from the system. In Model 2, Modified Group II is formed by later addition of Ce, (Eu), and Yb onto fine-grained dust or inclusions having HREE-depleted, Group II-like REE patterns. Similarly, Modified Group I is explained by later addition of Ce, (Eu), and Yb onto those with almost unfractionated REE patterns. The observed REE data show that both the degree of HREE-depletion (e.g., Er-depletion) and that of fractionation among HREEs (e.g., depletion in the Er/Gd ratio) for Modified Group II are very similar to those for Group II. Model 1 predicts almost complete removal of ultrarefractory HREEs from the system, resulting in much higher HREE-depletion for Modified Group II, which is not consistent with the present observations. Addition of an unfractionated component may explain moderate depletion of HREEs in Modified Group II, but it will diminish fractionation among HREEs, which is not consistent with the present observations. In contrast, Model 2 predicts no correlations between Ce–(Eu)–Yb-enrichment and HREE-depletion or between Ce–(Eu)–Yb-enrichment and fractionation among HREEs, consistent with the present observations. Hence, Model 2 seems more likely. If this is the case, at least two distinct regions with different REE characteristics are required for the formation of Modified Group II inclusions: one is a high temperature region where Group II-like (HREE-depleted) inclusions or their precursors are formed by condensation from a fractionated gas after removal of ultrarefractory dust, and another is a low temperature region enriched in Ce, Eu, and Yb in the gas phase. Abundant occurrence of positive Ce–(Eu)–Yb anomalies suggests that migration of solid materials from one region to another occurs rather frequently in the solar nebula. The most likely place satisfying such conditions for the formation of these inclusions may be the innermost part of the protoplanetary disk.

Tetrad effects in REE abundance patterns of chondrules from CM meteorites: Implications for aqueous alteration on the CM parent asteroid

Lanthanide tetrad effect in bulk chondrules from two moderately altered CM chondrites, Murchison and Yamato-793321 (Y-793321), are reported for the first time. Twenty-three chondrules were petrographically characterized and analyzed for 10 rare earth elements (REE) and other trace and major elements (Ba, Sr, Rb, K, Ca, Mg and Fe) using the precise isotope dilution technique. The results indicate systematic depletion (several times) of alkali and alkaline earths compared to CV and CO chondrules. Most of the porphyritic olivine (8 PO) and olivine-pyroxene (4 POP), porphyritic and radial pyroxene (2 PP, 1 RP), and granular olivine (1 GO) chondrules show a light-REE (L-REE) depleted, heavy-REE (H-REE) smoothly fractionated pattern composed of four (upward convex) segments possessing a relatively large negative Eu anomaly (CI-normalized La/Sm, Lu/Er and Eu/Eu* ratios = 0.3–1: Eu*, normal value). On the other hand, all barred-olivine (5 BO) chondrules, a few PO and POP indicate almost a flat L-REE pattern. In addition, regardless of their textural types, nearly half of the chondrules have a variable degree of Ce and Yb anomalies, and/or L/H-REE discontinuity, which is similar to CV and CO chondrules. The observed L- and H-convex REE patterns accompanied with the negative Eu anomaly is the first known case for chondrules as well as meteoritic materials, but have been previously reported for geological samples such as sedimentary rocks, late stage igneous and metamorphic rocks, and are explained as the lanthanide tetrad effect, which plausibly results from fluid–rock interaction. We suggest that the marked REE fractionations occurred by the selective incorporation of L-, H-REEs and Eu into alteration products in the matrix during alteration processes on the CM parent body, but that the gas/solid REE fractionation characteristics established in the nebula have basically remained unchanged. We suggest that the tetrad effects observed here represent a new index of physico-chemical conditions of fluid–rock interactions prevalent on the CM parent body.

Trace element studies of silicate‐rich inclusions in the Guin (UNGR) and Kodaikanal (IIE) iron meteorites

Abstract— A devitrified glass inclusion from the Guin (UNGR) iron consists of cryptocrystalline feldspars, pyroxenes, and silica and is rich in SiO2, Al2O3, and Na2O. It contains a rutile grain and is in contact with a large Cl apatite. The latter is very rich in rare earth elements (REEs) (∼80 × CI), which display a flat abundance pattern, except for Eu and Yb, which are underabundant. The devitrified glass is very poor in REEs (<0.1 × CI), except for Eu and Yb, which have positive abundance anomalies. Devitrified glass and Cl apatite are out of chemical equilibrium and their complementary REE patterns indicate a genesis via condensation under reducing conditions.Inclusion 1 in the Kodaikanal (IIE) iron consists of glass only, whereas inclusion 2 consists of clinopyroxene, which is partly overgrown by low‐Ca pyroxene, and apatite embedded in devitrified glass. All minerals are euhedral or have skeletal habits indicating crystallization from the liquid precursor of the glass. Pyroxenes and the apatite are rich in trace elements, indicating crystallization from a liquid that had 10–50 × CI abundances of REEs and refractory lithophile elements (RLEs). The co‐existing glass is poor in REEs (∼0.1–1 × CI) and, consequently, a liquid of such chemical composition cannot have crystallized the phenocrysts. Glasses have variable chemical compositions but are rich in SiO2, Al2O3, Na2O, and K2O as well as in HFSEs, Be, B, and Rb. The REE abundance patterns are mostly flat, except for the glass‐only inclusion, which has heavy rare earth elements (HREEs) > light rare earth elements (LREEs) and deficits in Eu and Yb—an ultrarefractory pattern.The genetic models suggested so far cannot explain what is observed and, consequently, we offer a new model for silicate inclusion formation in IIE and related irons. Nebular processes and a relationship with E meteorites (Guin) or Ca‐Al‐rich inclusions (CAIs) (Kodaikanal) are indicated. A sequence of condensation (CaS, TiN or refractory pyroxene‐rich liquids) and vapor‐solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions (phosphate, rutile formation, alkali and Fe2+ metasomatism, metasomatic loss of REEs from glass).

Drill core LB‐08A, Bosumtwi impact structure, Ghana: Geochemistry of fallback breccia and basement samples from the central uplift

Abstract— The 1.07 Myr old Bosumtwi impact structure in Ghana (West Africa), which measures 10.5 km in diameter and is largely filled by Lake Bosumtwi, is associated with one of four currently known tektite strewn fields. Two boreholes were drilled to acquire hard‐rock samples of the deep crater moat and from the flank of the central uplift (LB‐07A and LB‐08A, respectively) during a recent ICDP‐sponsored drilling project.Here we present results of major and trace element analysis of 112 samples from drill core LB‐08A. This core, which was recovered between 235.6 and 451.33 m depth below lake level, contains polymict lithic breccia intercalated with suevite, which overlies fractured/brecciated metasediment. The basement is dominated by meta‐graywacke (from fine‐grained to gritty), but also includes some phyllite and slate, as well as suevite dikelets and a few units of a distinct light greenish gray, medium‐grained meta‐graywacke.Most of the variations of the major and trace element abundances in the different lithologies result from the initial compositional variations of the various target rock types, as well as from aqueous alteration processes, which have undeniably affected the different rocks.Suevite from core LB‐08A (fallback suevite) and fallout suevite samples (from outside the northern crater rim) display some differences in major (mainly in MgO, CaO, and Na2O contents) and minor (mainly Cr and Ni) element abundances that could be related to the higher degree of alteration of fallback suevites, but also result from differences in the clast populations of the two suevite populations. For example, granite clasts are present in fallout suevite but not in fallback breccia, and calcite clasts are present in fallback breccia and not in fallout suevite. Chondrite‐normalized rare earth element abundance patterns for polymict impact breccia and basement samples are very similar to each other.Siderophile element contents in the impact breccias are not significantly different from those of the metasediments, or compared to target rocks from outside the crater rim. So far, no evidence for a meteoritic component has been detected in polymict impact breccias during this study, in agreement with previous work.

Melt–peridotite interactions: Links between garnet pyroxenite and high-Mg# signature of continental crust

Abundant lherzolite, garnet pyroxenite and granulite xenoliths are found in the Neogene Hannuoba basalt of the North China craton. Garnet pyroxenites generally occur as veins/layers in spinel lherzolites. There is a gradual decrease in olivine and an increase in orthopyroxene mode going from the lherzolite to the pyroxenite, suggesting that orthopyroxene may be forming at the expense of olivine. Garnet pyroxenites are enriched in the highly incompatible elements (e.g., Rb, K, Na, Sr, Ba, Nb and Ta) but have high and uniform Ni contents and Mg#s (83–90). This set of geochemical observations is paradoxical because the enrichments in highly incompatible elements signify derivation from a melt having either an evolved character or a significant fluid component, but the high Ni contents and high Mg#s suggest a much more primitive origin. A somewhat similar paradox is observed in the granulite xenoliths. Many of the granulite xenoliths have intermediate compositions, characterized by SiO 2 > 50 wt.%, high Al 2O 3, Na 2O, and Sr contents, low Y and heavy rare-earth contents, and high Sr/Y, La/Yb and Na 2O/K 2O ratios. However, these intermediate granulites have unusually high Mg#s (54–71) and high Ni (21–147 ppm) contents for their SiO 2 contents and would otherwise suggest that these granulites are more primitive than their SiO 2 contents indicate. It has been hypothesized that continuous melt–rock reaction between a silicic melt and ultramafic country rock (lherzolite) can convert olivine to orthopyroxene, ultimately resulting in the formation of a high Mg# garnet pyroxenite, similar to what is seen in the Hannuoba garnet pyroxenite composite xenoliths. In addition, silicic melts that have reacted with mantle peridotite would be predicted to have anomalously high Mg#s and Ni contents (due to the strong buffering capacity of peridotite for Mg and Ni), producing melts having compositions similar to the intermediate–mafic granulite xenoliths in this study. It is thus possible that the Hannuoba garnet pyroxenites and intermediate–mafic granulites share a common petrogenetic origin. Such a link is further supported by the fact that the rare-earth element abundance patterns of melts calculated to be in equilibrium with the garnet pyroxenites roughly coincide with that of the intermediate granulites. It is concluded that the Hannuoba garnet pyroxenite-bearing composite xenoliths may present the first physical evidence for the hypothesized melt–rock reaction necessary for generating evolved magmas with high Mg# and hence, high-Mg# andesitic signature of the continental crust.

Ion microprobe U‐Th‐Pb dating and REE analyses of phosphates in the nakhlites Lafayette and Yamato‐000593/000749

Abstract— U, Th, and Pb isotopes and rare earth elements (REEs) in individual phosphate grains from martian meteorites Lafayette and Yamato‐000593/000749 were measured using a sensitive high‐resolution ion microprobe (SHRIMP). Observed U‐Pb data of 12 apatite grains from Yamato (Y‐) 000593/000749 are well represented by linear regressions in both “conventional” 2D isochron plots and the 3D U‐Pb plot (total Pb/U isochron), indicating that the formation age of this meteorite is 1.53 ± 0.46 Ga (2σ). On the other hand, the data of nine apatite grains from Lafayette are well represented by planar regression rather than linear regression, indicating that its formation age is 1.15 ± 0.34 Ga (2σ) and that a secondary alteration process slightly disturbed its U‐Pb systematics as discussed in the literature regarding Nakhla. The observed REE abundance patterns of the apatites in Lafayette and Yamato‐000749, normalized to CI chondrites, are characterized by a progressive depletion of heavy REEs (HREEs), a negative Eu anomaly, similarity to each other, and consistency with previously reported data for Nakhla. Considering the extensive data from other radiometric systems such as Sm‐Nd, Rb‐Sr, Ar‐Ar, and trace elements, our results suggest that the parent magmas of the nakhlites, including the newly found Y‐000593/000749, are similar and that their crystallization ages are ˜1.3 Ga.

REE abundances in the matrix of the Allende (CV) meteorite: Implications for matrix origin

Abstract— The trace element distributions in the matrix of primitive chondrites were examined using four least‐contaminated matrix specimens from the polished sections of the Allende (CV) meteorite. Analysis of rare earth element (REE), Ba, Sr, Rb, and K abundances by isotope dilution mass spectrometry revealed that the elemental abundances of lithophile elements except for alkali metals (K, Rb) in the specimens of the Allende matrix studied here are nearly CI (carbonaceous Orgueil) chondritic (∼1 × CI). Compared to refractory elements, all the matrix samples exhibited systematic depletion of the moderately volatile elements K and Rb (0.1–0.5 × CI). We suggest that the matrix precursor material did not carry significant amounts of alkali metals or that the alkalis were removed from the matrix precursor material during the parent body process and/or before matrix formation and accretion. The matrix specimens displayed slightly fractionated REE abundance patterns with positive Ce anomalies (CI‐normalized La/Yb ratio = 1.32–1.65; Ce/Ce* = 1.16–1.28; Eu/Eu* = 0.98–1.10). The REE features of the Allende matrix do not indicate a direct relationship with chondrules or calcium‐aluminum‐rich inclusions (CAIs), which in turn suggests that the matrix was not formed from materials produced by the breakage and disaggregation of the chondrules or CAIs. Therefore, we infer that the Allende matrix retains the REE features acquired during the condensation process in the nebula gas.

Experimental REE partitioning between calcite and aqueous solution at 25.DEG.C. and 1atm: Constraints on the incorporation of seawater REE into seamount-type limestones

Partitioning experiments of rare earth elements (REEs) between calcite and aqueous (CaCl2 + NaCl) solution at 25°C and 1 atm have been made in order to elucidate the incorporation process of seawater REE into marine limestones. Calcite-supersaturated solution doped with REE was constantly pumped into a reactor, in which calcite seeds and the solution were mixed and CO2 + N2 gas was bubbled to allow calcite overgrowths. Absolute values of REE partition coefficients, Kd(REE) = (XREE/XCa)calcite/([REE]total/[Ca])solution, could not be determined because of very small calcite overgrowths and difficulty in evaluating XCa. The relative values of Kd(REE) like Kd(REE)/Kd(Gd), however, were determined successfully. Kd(REE) given by [REE]total depends on REE(III)-carbonate complexation in solution, and does not correspond to a reaction incorporating a specific dissolved REE species into calcite. Hence, here we used Kd(REE)ω = (XREE/XCa)calcite/([REECO3+]/[Ca])solution, which is given by Kd(REE) and REE(III)-carbonate complexation constants. Kd(REE)ω relates directly to ΔGr. for the reaction incorporating REECO3+(aq) into calcite. The series variation of experimental logKd(REE)ω shows a convex tetrad effect, suggesting larger Racah parameters of REECO3+(aq) relative to REE(III) in calcite. The variation also shows a kink at around Pm, Sm and Eu, possibly reflecting a coordination change of REE(III) in calcite across the series. Yttrium is definitely enriched in solution relative to heavy REEs. Using the relative values of experimental logKd(REE)ω and REE analyses of upper Paleozoic Japanese Ishimaki and Tahara limestones, REE abundance patterns for seawater coexistent with the marine limestones have been calculated provided that pH, ΣCO2 and salinity are the same in the ancient and present times. The calculated patterns are quite similar to those for relatively deep waters at around 400 to 1000 m in the modern Pacific. Ishimaki and Tahara limestones are of the seamount-type. Hence, the calculated seawater REE patterns with large negative Ce anomalies suggest that the incorporation of seawater REE into the limestones occurred in moderately deep water, probably because of subsidence of volcanic seamounts that were capped by Ishimaki and Tahara limestones.

In situ U–Pb dating and REE analyses of phosphates in extraterrestrial materials

U and Pb isotopes and rare earth elements (REEs) of phosphate grains in three H-chondrites of different petrologic types were measured by the SHRIMP instrument. The observed ages of 4554±39 Ma (2 σ) of Y-7301 (H6) and 4535±120 Ma of Y-74371 (H4) are consistent with previous work using TIMS. In contrast, those of 4084±350 Ma of ALH-77294 (H5) is apparently younger than previous study on other H5 chondrites, suggesting that the U–Pb system was affected by secondary thermal event such as an impact heating. Observed isotopic compositions of common lead of these chondrites indicate that model age using the Pb composition of Canyon Diablo FeS as initial lead might not be valid for some Antarctic meteorites. The REE abundance patterns of the whitlockites normalized to CI chondrite are characterized by an almost constant enrichment at 200–400 times higher than CI chondrite, and a striking negative Eu anomaly. The REE abundances of the apatites are lower than those of whitlockite grains, and vary appreciably among different grains. The significant correlation between the Eu anomaly (Eu/Eu ∗) and concentration of REE is confirmed.

Ion microprobe U‐Pb dating and REE analyses of phosphates in H4‐chondrite, Yamato‐74371

U and Pb isotopes and rare earth elements (REEs) in individual phosphate grains from H4‐chondrites, Y‐74371 were measured by using an ion microprobe. Nine analyses of whitlockite (merrillite) and five analyses of apatites indicate a total Pb/U isochron age of 4533 ± 40 Ma (2 sigma), which are consistent with the results from previous using Thermal ionization mass spectrometer (TIMS). The observed apatites age of 4525 ± 30 Ma (2 sigma), however, is younger than those of previous works. On the other hand, the time difference between the whitlockite age of 4601 ± 94 Ma (2 sigma) and those of apatites are not observed, because of the large uncertainties. The REE abundance patterns of the whitlockites normalized to CI chondrite are characterized by an almost constant enrichment of 200–800 times CI abundance, and a striking negative Eu anomaly. The REE abundances of the apatites are lower than those of whitlockite grains, and vary appreciably among different grains. The remarkable correlation between the Eu anomaly (Eu/Eu*) and total concentration of REE is confirmed, but significant correlation with Eu anomaly and formation age of individual grains are not observed.

Redistribution of fissiogenic and non-fissiogenic REE, Th and U in and around natural fission reactors at Oklo and Bangombé, Gabon

Elemental abundances and isotopic compositions of rare earth elements (REE), Th and U of twenty eight samples from four kinds of natural fission reactors at the Oklo and Bangombé uranium ores were determined by mass spectrometry. Fissiogenic and non-fissiogenic REE components of each sample were calculated from the isotopic compositions. The geochemical distribution of both fissiogenic and non-fissiogenic REE differs among the four reactors. Each reactor shows the following features possibly associated with the local geochemical conditions. (1) Reactor 9 at the shallow depth of the Oklo deposit contains higher amounts of non-fissiogenic REE than the other reactors. The distribution of non-fissiogenic REE does not reflect the primordial REE abundance patterns, and might have been disturbed in and around the reactor due to supergene alteration. Our results suggest that significant amounts of fissiogenic lighter REE (LREE), such as La, Ce and Pr, were mobile in the reactor; (2) Reactors 10 and 13 located deep underground have much lower contents of non-fissiogenic REE than reactor 9. All fissiogenic REE show high retention in the reactors. Non-fissiogenic REE of reactor 10 inform of the primordial ore formation without any geochemical disturbance. Little redistribution of both fissiogenic and non-fissiogenic REE in the reactor 10 was observed; and (3) The Bangombé reactor located at shallow depth has apparently well preserved fissiogenic REE. Non-fissiogenic REE in the Bangombé reactor might have been disturbed, possibly by hydrothermal processes. The distribution behavior of fissiogenic REE is considered to be related with physico-chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor, but clear relationship between nuclear parameters and the REE distribution could not be obtained in this study.

Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan.

We have measured U concentrations, and elemental ratio of 238 U/ 206 Pb, and isotopic compositions of Pb in thirteen zircons extracted from a volcaniclastic rock of the Kamiaso conglomerate in the Mino terrane and sixty three analyses in fifty zircons from the Amo gneiss, a metamorphic rock originated from an intermediate volcanic rock in the Hida terrane of central Japan by means of an ion microprobe. Zircons from the Kamiaso were first analyzed in Australian National University (ANU) and then measured again by Hiroshima University (HU) Sensitive High Resolution Ion MicroProbe. Both radiometric ages of 238 U/ 206 Pb* and 207 Pb*/ 206 Pb* obtained by the ANU instrument agree well with those by HU except for one sample. There are six geochronological episodes in the Kamiaso sample; about 2550 Ma, 2000 Ma, 1300 Ma, 920 Ma, 250 and 220 Ma. U-Pb zircon geochronology of the Amo sample indicates that there are six concordant zircon episodes and three concordia intercept episodes; about 3420 Ma, 2560 Ma, 1840 Ma, 1130 Ma, 580 Ma, 400 Ma, 360 Ma, 285 Ma, and 250 Ma. The 3420 Ma is the oldest age so far reported in Japan. The 2560 Ma event may have a relation to the period of the evolution of the Sino-Korean craton between 2.7‐2.5 Ga. The third age peak of about 1840 Ma suggests that the zircons were derived from the Ryongnam massif in South Korea with the similar formation age. The younger ages of 1130 Ma, 580 Ma, 400 Ma, 360 Ma and 285 Ma may be minor events recorded in the Hida metamorphic belt. The latest age of 250 Ma may be the most important metamorphic event in Hida terrane. Rare earth element abundance patterns of the Amo zircons show progressive increase with atomic number together with positive Ce anomaly, which is a typical zircon signature. Small negative anomaly of Eu is found in zircons with the age of 2560 Ma and 250 Ma while zircons with intermediate ages show large anomaly. There is no correlation between Ce and Eu anomaly, suggesting that both anomalies may not have been simply controlled by the oxidation state during zircon crystallization.