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.     

The Solar System's Earliest Chemistry: Systematics of Refractory Inclusions

Refractory inclusions, or CAIs (calcium-aluminium-rich inclusions) are a unique ingredient in chondritic meteorites. As the name suggests, they are enriched in refractory elements, essentially reflecting a condensation sequence of phases from a cooling gas of solar composition. However, the widespread preservation of diverse isotopic anomalies is not compatible with the inclusions having been in a gaseous form. Rather, the CAIs appear to represent mixtures of condensate and refractory residue materials. The condensates formed from cooling solar gas and fractionation of that gas produced variations in the abundances of refractory elements according to volatility. Solar condensate has isotopically normal Ca and Ti isotopic compositions and has ²⁶Al/²⁷Al of the canonical value for the solar system at 5 × 10?⁵. Residues of material falling in toward the Sun are probably aluminous oxides such as corundum and hibonite, and preserve diverse Ca and Ti isotopic anomalies. Meteoritic inclusions from the Murchison meteorite show the best polarization of these components. Spinel-hibonite-perovskite inclusions (SHIBs) predominantly have normal Ca and Ti isotopes, ²⁶Al/²⁷Al at 5 × 10?⁵, and ultrarefractory fractionated REE patterns. Single hibonite crystal fragments (PLACs) have diverse Ca and Ti isotopic compositions and low ²⁶Al/²⁷Al because of the initially high proportion of ²⁷Al in the residue. REE patterns in PLACs are variable in terms of the ultrarefractory fractionation of their REE patterns, as indicated by Tm/Tm?, but are dominated by depletion in the less refractory REE Eu and Yb. Both PLACs and SHIBs homogenized with ¹⁶O-rich gas, enriched relative to terrestrial O by up to 7%, thus removing any isotopic heterogeneity from the PLAC precursors. CAIs formed close to the Sun where condensation and re-evaporation of REE was possible, and were then ejected back to planetary radii where they were eventually accreted onto planetesimals.     

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Concentrations of the REE, Sc, Co, Fe, Zn, Ir, Na and Cr were determined by instrumental neutron activation and mass spectrometric isotope dilution analysis for mineral separates of the coarseand fine-grained types (group I and II of Martin and Mason's classification) of the Allende inclusions.These data, combined with data on mineral/liquid partition coefficients, oxygen isotope distributions and diffusion calculations, suggest the following: (1) Minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements. On the other hand, differences in oxygen isotope distributions among minerals preclude a totally molten stage in the history of the inclusion. Group I inclusions were formed by rapid condensation (either to liquid or solid) in a supercooled solar nebula; extrasolar pyroxene and spinel dust were included but not melted in the condensing inclusions, thus preserving their extrasolar oxygen isotope composition. REE were distributed by diffusion during the subsequent heating at subsolidus temperatures; because oxygen diffuses much more slowly at these temperatures, the oxygen isotope anomalies were preserved. (2) The fine-grained (group II) inclusions were also formed by condensation from a super-cooled nebular gas; however, REE-rich clinopyroxene and spinel were formed early and REE-poor sodalite and nepheline were formed later and mechanically mixed with clinopyroxene and spinel to form the inclusions. The REE patterns of the bulk inclusions and the mineral separates are fractionated, indicating that REE abundances in the gaseous phase were already fractionated at the time of condensation of the minerals. (3) Pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage thus resetting the ²⁶Al-²⁶Mg chronometer.     

Fractionation in the solar nebula: condensation of yttrium and the rare earth elements

The condensation of Y and the rare earth elements (REE) from the solar nebula may be controlled by thermodynamic equilibrium between gas and condensed solids. Highly fractionated REE patterns may result if condensates are removed from the gas before condensation is complete. It is found that the fractionation is not a smooth function of REE ionic radius but varies in an extremely irregular pattern. Both Yb and Eu are predicted to be extremely depleted in the early condensate without the requirement of condensation in the divalent state. The model is discussed with respect to a highly fractionated pattern observed by Tanaka and Masuda (Icarus19, 523–530 1973), in a pink Ca-Al-rich inclusion from the Allende meteorite and can account for the abundances of each REE determined. According to the model this inclusion represents a condensate from a previously fractionated gas rather than from a gas of solar composition. Before the condensation of this inclusion, an earlier condensate was formed and was removed from equilibrium with the gas.     

Chemical compositions of refractory inclusions in the Murchison C2 chondrite

Samples from ten refractory inclusions in Murchison, some of which are splits of inclusions whose mineralogical and petrographic characteristics are known, have been analysed for thirty-six elements by neutron activation. Six inclusions have group II or group III patterns or variants of such patterns. Two inclusions, BB-5 and MUCH-1, have large negative Yb anomalies unaccompanied by correspondingly large negative Eu anomalies. It is possible that the latter condensed originally with group III patterns and preferentially took up Eu in later exchange processes under reducing conditions. One inclusion, SH-2, has heavy REE enrichment factors that increase with the refractoriness of the REE, indicating the presence of an extremely high-temperature, or ultrarefractory, REE condensate, but it also has a heavy REE/light REE ratio that indicates mixing of that component with a lower-temperature REE condensate. The frequency of highly fractionated REE patterns and absence of group I patterns suggest that refractory inclusions in Murchison stopped equilibrating with the nebular gas at higher temperatures than most Allende coarse-grained inclusions. The lower Ir/Os and Ru/Re ratios of some Murchison inclusions compared to those of Allende coarse-grained inclusions indicate that condensate alloys that contributed noble metals to the former also stopped equilibrating with the nebular gas at higher temperatures than those that contributed noble metals to the latter. Murchison inclusions tend to be lower in non-refractory elements than Allende coarse-grained inclusions, suggesting that, on average, the former underwent less severe secondary alteration than the latter.     

Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones

Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements (n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd_SN/Yb_SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement (n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd_SN/Yb_SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modern seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans.     

碳酸岩型稀土矿床成矿流体演化机制研究现状及展望

与碱性岩有关的碳酸岩型内生稀土矿床在中国乃至世界上轻稀土资源储量中占有极为重要的地位,诸如我国内蒙古的白云鄂博稀土矿床、川西冕宁—德昌稀土成矿带中的牦牛坪、大陆槽等稀土矿床、山东微山县郗山稀土矿床以及美国的Mountain Pass稀土矿床等都属于这种类型的稀土矿床.当前,对于这类稀土矿床的成矿流体演化机制,学界主要存...     

Rare-earth abundances in chondritic meteorites

Fifteen chondrites, including eight carbonaceous chondrites, have been analyzed for rare earth element (REE) abundances by isotope dilution. These analyses complement and extend earlier isotope dilution REE determinations in chondrites, performed in other laboratories, so that coverage of major chondrite classes is now complete. An examination of this body of precise and comparable REE data from individual chondrites reveals that only a small proportion of the analyses have flat, unfractionated REE patterns within experimental error. A statistical procedure is used to derive revised chondritic abundances of REE by selection of unfractionated patterns. A number of the remaining analyses show Eu anomalies and fractionated patterns consistent with magmatic fractionation as encountered in the products of planetary differentiation. However, many patterns exhibit features not readily explicable by known magmatic processes; in particular, positive Ce anomalies are often encountered. Abundance anomalies can be quantitatively determined by the use of a least-squares curve fitting procedure. The wide variety of anomalous patterns and the uncertainties in model parameters preclude detailed modeling of the origin of anomalies, but it is probable that at least some arise from fractional condensation in the solar nebula, as has been demonstrated for Allende inclusions. Elemental abundance anomalies are found in all major chondrite classes. If these anomalies are ignored, the range and nature of variation within chondrite classes are consistent with a parent body model, in which solid-liquid or solid-solid equilibria induce variations from an unfractionated bulk composition. Absolute abundances in the H, L and LL parent bodies are almost twice those of the E parent body.The persistence of anomalies in chondritic materials relatively removed from direct condensational processes implies that anomalous components are resistant to equilibration or were introduced at a late stage of chondrite formation. Large scale segregation of gas and condensate is also implied, and raises the possibility of bulk variations in REE abundances between planetary bodies.     

High-temperature condensates in chondrites and the environment in which they formed

Chemical compositions of melilitee and titaniferous pyroxenes in calcium- and aluminum-rich inclusions in carbonaceous chondrites are consistent with their origin as hightemperature condensates from a gas of solar composition. Thermodynamic calculations indicate that the highest temperature minerals equilibrated with the gas at temperatures in excess of 1400°K. The lack of evidence for direct condensation of gas to liquid enables us to set an upper limit to the pressure when the inclusions formed which may be as low as 2.2 × 10^?3atm. Glasses, which are commonly found in chondrules, are interpreted as quench products of liquids formed by secondary reheating of primary solid condensates. The high-temperature inclusions constitute evidence that accretion of grains to cm-sized objects occurred at a very early stage in the evolution of the solar nebula.     

Origin of compact type A refractory inclusions from CV3 carbonaceous chondrites

Compact type A (CTA) inclusions are one of the major types of coarse-grained refractory inclusions found in carbonaceous chondrites. They have not been studied in a systematic fashion, leading to some uncertainties and unproven assumptions about their origin. To address this situation, we studied a total of eight CTAs from Allende, Efremovka and Axtell by scanning electron-microscopic and electron and ion-microprobe techniques. These inclusions are very melilite-rich, ranging from ∼60 vol% to nearly monomineralic. Also present are Mg–Al spinel (5–20%), perovskite (trace–∼3%) and, in some samples, Ti-rich (∼17 wt% TiO₂^tot) fassaite (trace–∼20%), and rhönite (≤1%). Melilite compositions are mostly between Åk₁₅ and Åk₄₀. Chondrite-normalized REE abundance patterns for melilite (flat at ∼10 × CI with positive Eu anomalies) and fassaite (slight HREE enrichment relative to LREE and negative Eu anomalies) are like those for their counterparts in once-molten type B inclusions. The patterns for rhönite have positive slopes from La through Lu and abundances <10 × CI for La and 35–60 × CI for Lu. Features of CTAs that suggest that they were once molten include: rounded inclusion shapes; positively correlated Sc and V abundances in fassaite; radially oriented melilite laths at inclusion rims; and the distribution of trace elements among the phases. Fractional crystallization models show that, with one exception, the REE contents of perovskite and fassaite arose by crystallization of these phases from late, residual liquids that would have resulted from prior crystallization of the observed proportions of melilite and spinel from liquids having the bulk compositions of the inclusions. One Allende CTA (TS32), however, has several features (irregular shape, reversely zoned melilite, fassaite REE contents) that are not readily explained by crystallization from a melt. This inclusion may have undergone little melting and may be dominated by relict grains.     

Thermodynamic model of the formation of ore bodies at the Akchatau wolframite greisen-vein deposit

The Akchatau wolframite deposit in central Kazakhstan is a typical greisen deposit. Extensive geological and geochemical data, including those on numerous geochemical signatures (isotopic composition of O, H, C, noble gases, data on fluid inclusions, REE, and others) allowed us to decipher the physicochemical conditions and main factors that caused metasomatism and ore formation. Physicochemical modeling by the HCh program package (designed by Yu.B. Shvarov) was applied to reconstruct the composition of the greisenizing solution, cooling, boiling, interaction with granites; condensation of the gas phase; and fluid mixing. The predominant species of W transfer, (NaHWO _{4 aq} ⁰ ), and precipitation factors were determined. In small ore bodies, precipitation was caused by a temperature decrease. The precipitation of wolframite in near-vein greisens is related to the interaction of boiling highly mineralized solutions with host granites. Boiling does not affect wolframite precipitation but increases the content and ore potential of the greisenizing fluids, facilitating the formation of high-grade wolframite ores. In the filling veins of these bodies, ore precipitation is related to the dilution of solutions by weakly mineralized exogenic waters and the condensate of the gas phase. Tungsten mineralization of the Akchatau deposit was formed in an oxidizing environment, which is controlled by granite minerals during mobilization of ore components.     

Timescales determining the degree of kinetic isotope fractionation by evaporation and condensation

A first order characteristic of the relative abundance of the elements in solar system materials ranging in size from inclusions in primitive meteorites to planetary sized objects such as the Earth and the Moon is that they are very much like that of the Sun for the more refractory elements but systematically depleted to varying degrees in the more volatile elements. This is taken as evidence that evaporation and and/or condensation were important processes in determining the distinctive chemical properties of solar system materials. In some instances there is also isotopic evidence suggesting evaporation in that certain materials are found enriched in the heavy isotopes of their more volatile elements. Here model calculations are used to explore how the relative rates of various key processes determine the relationship between elemental and isotopic fractionation during partial evaporation and partial condensation. The natural measure of time for the systems considered here is the evaporation or condensation timescale defined as the time it would take under the prevailing conditions for evaporation or condensation to completely transfer the element of interest between the two phases of the system. The other timescales considered involve the rate of change of temperature, the rate at which gas is removed from further interaction with the condensed phase, and the rates of diffusion in the condensed and gas phases. The results show that a key determinant of whether or not elemental fractionations have associated isotopic effects is the ratio of the partial pressure of a volatile element (P_i) to its saturation vapor pressure (P_i,sat) over the condensed phase. Systems in which the rate of temperature change or of gas removal are slow compared to the evaporation or condensation timescale will be in the limit P_i ∼ P_i,sat and thus will have little or no isotopic fractionation because at the high temperatures considered here there is negligible equilibrium fractionation of isotopes. If on the other hand the temperature changes are relatively fast, then P_i ≠ P_i,sat and there will be both elemental and isotopic fractionation during partial evaporation or partial condensation. Rapid removal of evolved gas results in P_i ? P_i,sat which will produce isotopically heavy evaporation residues. Diffusion-limited regimes, where transports within a phase are not sufficiently fast to maintain chemical and or isotopic homogeneity, will typically produce less isotopic fractionation than had the phases remained well mixed. The model results are used to suggest a likely explanation for the heavy silicon and magnesium isotopic composition of Type B CAIs (as due to rapid partial melting and subsequent cooling at rates of a few °C per hour), for the uniformity of the potassium isotopic composition of chondrules despite large differences in potassium depletions (as due to volatilization of potassium by reheating in regions of large but variable chondrules per unit volume), and that the remarkable uniformity of the potassium isotopic composition of solar system materials is not a measure of the relative importance of evaporation and condensation but rather due to the solar nebula having evolved sufficiently slowly that materials did not significantly depart from chemical equilibrium.     

Clinopyroxene-orthopyroxene major and rare earth elements partitioning in spinel peridotite xenoliths from Assab (Ethiopia)

Major element and rare earth element (REE) partitioning among coexisting clinopyroxene-orthopyroxene pairs from mantle xenoliths of the Assab Range (Ethiopia) are discussed in terms of crystal-chemistry.Major element partitioning indicates relatively uniform conditions of subsolidus equilibration over a narrow range of temperatures (mean value about 1100 C) in the spinel peridotite stability field. Major element distributions and correlations, moreover, seem to indicate that the mantle material studied underwent slightly different depletions prior to the metamorphic equilibration.In spite of the rather homogeneous major element compositions for both cpx and opx, clinopyroxenes show chondrite-normalized REE patterns which are widely variable both in shape and absolute values, whereas orthopyroxenes exhibit more restricted ranges and concordant profiles.REE activity ratios have been investigated by applying Iiyama's (Bull. Soc. fr. Minéral. Christallogr.97, 143–151) thermodynamic model: the estimated activity patterns exhibit a good coherence for the different pyroxene pairs, in spite of the contrasting features of their REE concentration ratios. The wide ranges in the measured partition values for the same rare earth element in different pyroxene pairs have been related to coupled substitutions involving A1 in the Z site and REE in the M₂ site of clinopyroxene.     

Experimental rare earth element partition coefficients for garnet,clinopyroxene and amphibole coexisting with andesitic and basaltic liquids

The partitioning of La, Sm, Dy, Ho and Yb between garnet, calcic clinopyroxene, calcic amphibole and andesitic and basaltic liquids has been studied experimentally. Glasses containing one or more REE in concentrations of 500–2000 ppm were crystallized at pressures of 10–35 kbar, and temperatures of 900–1520°C. Water was added to stabilize amphibole and to allow study of partition coefficients over wide temperature ranges. Major element and REE contents of crystal rims and adjacent glass were determined by EPMA, with limits of detection for individual REE of 100–180 ppm. Measured partition coefficients, D_REE^Cryst-liq, are independent of REE concentration over the concentration ranges used.D-values show an inverse dependence on temperature, best illustrated for garnet. At a given temperature, they are almost always higher for equilibria involving andesitic liquid. Garnet shows by far the greatest range of D-values, with e.g. D_La < 0.05 and D_Yb ~ 44 for andesitic liquid at 940°C. D_Yb falls to ~ 12 at 1420°C. D_Sm^Ga-liq also correlates negatively with temperature and positively with the grossular content of garnet. Patterns of D_ree^Cryst-Liq for calcic clinopyroxenes and amphiboles are sub-parallel, with D-values for amphibole generally higher. Both individual D-values and patterns for the crystalline phases studied are comparable with those determined for phenocryst-matrix pairs in natural dacites, andesites and basalts.D-values and patterns are interpreted in terms of the entry of REE³⁺ cations into mineral structures and liquids of contrasted major element compositions. The significance of the partition coefficients for models of the genesis of andesitic and Hy-normative basaltic magmas is assessed. Most magmas of these types in island arcs are unlikely to be produced by melting of garnet-bearing sources such as eclogite or garnet lherzolite.     

Trace element partitioning between carbonatitic melts and mantle transition zone minerals: Implications for the source of carbonatites

The occurrence of CO₂-rich lavas (carbonatites, kimberlites) and carbonate-rich xenoliths provide evidence for the existence of carbonatitic melts in the mantle. To model the chemical composition of such melts in the deep mantle, we experimentally determined partition coefficients for 23 trace elements (including REE, U-Th, HFSE, LILE) between deep mantle minerals and carbonatite liquids at 20 and 25 GPa and 1600 °C. Under these conditions, majoritic garnet and CaSiO₃ perovskite are the main reservoirs for trace elements. This study used both femtosecond LA-ICP-MS and SIMS techniques to measure reliable trace element concentrations. Comparison of the two techniques shows a general agreement, except for Sc and Ba. Our experimentally determined partition coefficients are consistent with the lattice strain model. The data suggest an effect of melt structure on partition coefficients in this pressure range. For instance, strain-free partition coefficient (D₀) for majorite-carbonatite melts do not follow the order of cation valence, , observed for majorite-CO₂-free silicate melts. The newly determined partition coefficients were combined with trace element composition of majoritic garnets found as inclusions in diamond to model trace element patterns of deep-seated carbonatites. The result compares favorably with natural carbonatites. This suggests that carbonatites can originate from the mantle transition zone.     

川西呷村超大黑矿型矿床成矿流体烯土元素组成

本文用ＩＣＰ－ＭＳ首次测定了呷村银多金属黑矿型矿床矿石流体包裹体中的稀土元素含量,研究表明,主成矿期流体稀土元素配分模式均为轻稀土富集,Ｅｕ具明显正异常,通过初步对比,本区主成矿期流体与东太平洋脊、大西洋脊等现代高温酸性地热系统热液具有相似的稀土模式,反映了它们物化条件的相似性;但前者∑ＰＥＥ高于后者,且两者Ｅｕ／Ｅｕ＾＊值不同,经过分析,本区成矿流体Ｅｕ正异常主要为Ｔ、ｐＨ、ｆｏ２控制,另外,围     

The world-class Natalka gold deposit, northeast Russia: REE patterns, fluid inclusions, stable oxygen isotopes, and formation conditions of ore

REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H₂O-CO₂-NaCl-MgCl₂ with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO₂-CH₄, and (3) H₂O-NaCl-MgCl₂ with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H₂S and destabilization of the Au(HS)^2? complex. The fluid with the third composition arose due to the boiling of the H₂O-CO₂-CH₄-NaCl-MgCl₂ liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ¹⁸O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by $ \delta ^{18} O_{H_2 O} REE patterns of hydrothermally altered rocks, fluid inclusions, and stable oxygen isotopes of quartz were studied at the Natalka gold deposit. Metasomatic rocks formed under decompression reveal gradual depletion in LREE and HREE relative to siltstone of the protolith. The HREE patterns of metasomatic rocks formed under decompression are uniform; an insignificant removal of LREE can be noted. The progressive extraction of REE with increasing alteration of rocks could have been due to the effect of magmatogenic or meteoric fluid. Because a Ce anomaly is absent, the participation of oxidized meteoric water was limited. The inverse correlation between the total REE content and the Eu anomaly value in altered rocks indicates a substantial role of magmatogenic fluid. The REE patterns of altered rocks formed under compression show that the role of metamorphic fluid was not great. All metasomatic rocks are enriched in LREE, so that the enrichment of fluid in LREE as well may be suggested. Three fluid compositions were captured as fluid inclusions: (1) H₂O-CO₂-NaCl-MgCl₂ with a salinity of 1.0–4.9 wt % NaCl equiv, (2) CO₂-CH₄, and (3) H₂O-NaCl-MgCl₂ with a salinity of 7.0–5.6 wt % NaCl equiv. Compositions (1) and (2) coexisted in the mineral-forming system at 250–350°C and 1.1–2.4 kbar as products of phase separation under conditions of decreasing P and T. The interaction of this fluid with host rocks resulted in the formation of extensive halos of beresitized rocks with sulfide disseminations. The precipitation of arsenopyrite and pyrite led to the substantial depletion of mineral-forming fluid in H₂S and destabilization of the Au(HS)²⁻ complex. The fluid with the third composition arose due to the boiling of the H₂O-CO₂-CH₄-NaCl-MgCl₂ liquid and was responsible for metasomatic alteration of host rocks. The late mineral assemblages were deposited from this fluid at the initial stage of ore formation. The high methane concentrations in the ore-forming fluid were likely caused by interaction of hydrothermal ore-bearing solutions with carbonaceous host rocks. The δ¹⁸O values of quartz from quartz-scheelite-pyrite-arsenopyrite and sulfide-sulfosalt mineral assemblages vary from +11.6 to +14.1‰ and +11.2 to +13.5‰, respectively. The parental fluids of the early and late mineral assemblages probably were derived from a magmatic source and were characterized by = +6.3 to +8.8‰ at 350°C and +3.6 to +5.9‰ at 280°C, respectively. The narrow interval of oxygen isotopic compositions shows that this source was homogeneous. The data obtained allow us to suggest that the deposit formation was related to magmatic activity, including the direct supply of ore components from a magma chamber and mobilization of these components in the processes of dehydration and decarbonation during contact and regional metamorphism. Original Russian Text ? N.A. Goryachev, O.V. Vikent’eva, N.S. Bortnikov, V.Yu. Prokof’ev, V.A. Alpatov, V.V. Golub, 2008, published in Geologiya Rudnykh Mestorozhdenii, 2008, Vol. 50, No. 5, pp. 414–444.     

Whole-rock/groundmass differentiation trends of rare earth elements in high-silica rhyolites

Whole-rock/groundmass differentiation trends of three series of high-silica rhyolites from northern Mexico and west Texas contrast with that of the Bishop Tuff of California. Together, the four rock series cover a complete spectrum of trends from one where all REE increase with differentiation (peralkaline series of the Chinati Mountains, Texas) to one where all except Ce decrease with fractionation (Batopilas, Mexico). The various differentiation trends reflect different accessory mineral assemblages and different apparent partition coefficient patterns of major mafic phenocrysts. The light REE decrease during differentiation in rock series where mafic phenocrysts and accessory phases are relatively enriched in the light REE (e.g., metaluminous series of the Chinati Mountains and the Bishop Tuff). In series that lack light REE-enriched accessory minerals and contain minerals with maxima in their partition coefficient patterns in the middle and heavy REE (e.g., zircon, apatite, and hornblende), the middle and heavy REE in the rock series decrease with fractionation (e.g., Batopilas). Finally, in rock series that lack abundant REE-enriched accessory minerals and that have relatively small partition coefficients for the major mafic minerals, the REE increase with differentiation (e.g., peralkaline series of the Chinati Mountains).     

Origin of 16O-rich fine-grained Ca-Al-rich inclusions of different mineralogy and texture

A coordinated mineralogical and oxygen isotopic study of four fine-grained calcium-, aluminum-rich inclusions (CAIs) from the ALHA77307 CO3.0 carbonaceous chondrite was conducted. Three of the inclusions studied, 05, 1-65, and 2-119, all have nodular structures that represent three major groups, melilite-rich, spinel-rich, and hibonite-rich, based on their primary core mineral assemblages. A condensation origin was inferred for these CAIs. However, the difference in their primary core mineralogy reflects unique nebular environments in which multiple gas-solid reactions occurred under disequilibrium conditions to form hibonite, spinel, and melilite with minor perovskite and Al,Ti-rich diopside. A common occurrence of a diopside rim on the CAIs records a widespread event that marks the end of their condensation as a result of isolation from a nebular gas. An exception is a rare inclusion 2-112 that contains euhedral spinel crystals embedded in melilite, suggesting this CAI had been re-melted. All of the fine-grained CAIs analyzed in ALHA77307 are ¹⁶O-rich with an average Δ¹⁷O value of ∼−22 ± 5‰ (2σ), indicating no apparent correlation between their textures and oxygen isotopic compositions. We therefore conclude that a prevalent ¹⁶O-rich gas reservoir existed in a region of the solar nebula where CO3 fine-grained CAIs formed, initially by condensation and then later, some of them were reprocessed by melting event(s).     

Trace elements in the Allende meteorite—I. Coarse-grained,Ca-rich inclusions

INAA of ten coarse-grained, melilite-spinel-bearing inclusions in the Allende meteorite for Ca, Sc, Hf, Ta, W, Os, Ir, Ru, La, Ce, Sm, Eu, Tb, Dy, Yb, Fe, Co, Cr and Au reveals that all of the refractory elements are enriched by a mean factor of 18.6 relative to their concentrations in Cl chondrites, consistent with a high-temperature condensation origin for the inclusions. Os, Ir and Ru were probably incorporated by the inclusions as tiny nuggets of an alloy in which they were dissolved in cosmic proportion to one another. Sc and Hf entered the inclusions in a separate phase, also in cosmic proportion, accompanied by a fraction of the REE. Bulk REE abundances are independent of the major minerals in the inclusions; yet, data from mineral separates suggest that the REE were partitioned between coexisting melilite and pyroxene according to crystal structure controls. A two-stage model is proposed in which the REE first entered the inclusions as trace, refractory condensate phases and then re-distributed themselves between the crystallizing major phases after the inclusions were melted in the nebula.