The role of carbonate-fluoride melt immiscibility in shallow REE deposit evolution

The Lugiin Gol nepheline syenite intrusion, Mongolia, hosts a range of carbonatite dikes mineralized in rare-earth elements(REE). Both carbonatites and nepheline syenite-fluorite-calcite veinlets are host to a previously unreported macroscale texture involving pseudo-graphic intergrowths of fluorite and calcite. The inclusions within calcite occur as either pure fluorite, with associated REE minerals within the surrounding calcite, or as mixed calcite-fluorite inclusions, with associated zirconosilicate minerals. Consideration of the nature of the texture, and the proportions of fluorite and calcite present(～29 and 71 mol%,respectively), indicates that these textures most likely formed either through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonatefluoride melt. Laser ablation ICP-MS analyses show the pure fluorite inclusions to be depleted in REE relative to the calcite. A model is proposed, in which a carbonate-fluoride melt phase enriched in Zr and the REE, separated from a phonolitic melt, and then either unmixed or underwent cotectic crystallization to generate an REE-rich carbonate melt and an REE-poor fluoride phase. The separation of the fluoride phase(either solid or melt) may have contributed to the enrichment of the carbonate melt in REE, and ultimately its saturation with REE minerals. Previous data have suggested that carbonate melts separated from silicate melts are relatively depleted in the REE, and thus melt immiscibility cannot result in the formation of REE-enriched carbonatites. The observations presented here provide a mechanism by which this could occur, as under either model the textures imply initial separation of a mixed carbonate-fluoride melt from a silicate magma. The separation of an REEenriched carbonate-fluoride melt from phonolitic magma is a hitherto unrecognized mechanism for REE-enrichment in carbonatites, and may play an important role in the formation of shallow magmatic REE deposits.     

Rare Earth Element Behaviour During Evolution and Alteration of the Dartmoor Granite, SW England

The distribution of rare earth elements (REE) within the compositionallyzoned Dartmoor pluton is used to constrain models of graniteevolution and to assess the effects of pervasive hydrothermalalteration on REE mobility. The main process of magma evolutionwas crystal fractionation of early plagioclase, biotite, andaccessory minerals (apatite, monazite, zircon, and xenotime).Concentrations of REE (particularly LREE and Eu) and other elements(Fe₂O₃t, MgO, CaO, TiO₂, Zr, Ba, and Sr) decrease strongly withevolution of the pluton from 71 to 74% SiO2. These trends, andthe inward zoning of the pluton, are compatible with differentiationby crystal fractionation at the level of emplacement, a processthat gave rise to a marginal cumulate granite (CGM) modifiedby country rock assimilation, a body of inner granite (PM),and a late-stage evolved granite (FG) that intruded the earliertypes. REE modelling of the Dartmoor granite types by fractionalcrystallization of REE-enriched accessory minerals from a parentPM-granite shows that the FG-granite cannot have formed froma residual liquid left by crystallization of the CGM-granite.Two discrete stages of crystallization occurred; side-wall cumulateCGM-granite crystallization dominated by LREE-en-riched monazitefractionation followed by a late-stage mobile residual FG-granitein which fractionation was dominated by HREE-enriched apatiteand zircon. Modelling supports the idea that large-scale assimilationof country rock was not the dominant process during Dartmoorgranite evolution. Pervasive hydrothermal alteration locally affected all Dartmoorgranite types, altering primary plagioclase, biotite, apatite,monazite, and, to a lesser extent, zircon and xenotime. Duringpervasive sericitization, chloritization, and tourmalinization,REE were mobilized over distances of centimetres only and redistributedinto the secondary alteration products seridte, chlorite, tourmaline,allanite, and sphene. Whole-rock REE abundances were not affected     

Magmatic-to-hydrothermal crystallization in the W–Sn mineralized Mole Granite (NSW, Australia): Part II: Evolving zircon and thorite trace element chemistry

The Sn–W mineralized Mole Granite in Eastern Australia hosts zircon populations that crystallized at several stages during a protracted magmatic to hydrothermal evolution. Thirty-four elements have been quantified by laser-ablation inductively-coupled-plasma mass-spectrometric microanalysis with the aim of relating the chemistry of zircon to its growth environment. Trace element contents are highly variable for all textural occurrences. Zircon inclusions in earliest quartz phenocryst suggest that zircon was a liquidus phase that crystallized probably deep in the crust. Trace element contents are conspicuously high, showing only a slight positive Ce anomaly but a pronounced negative Eu-anomaly. Successive crystallization stages of magmatic zircon are characterized by progressive depletion in trace element contents, notably the rare earth elements, with an increasingly important positive Ce-anomaly. This evolution reflects saturation of REE accepting minerals such as monazite, thorite, xenotime and possibly apatite and is affected little by the exsolution of a magmatic–hydrothermal fluid. Zircon that is interpreted to have precipitated from aqueous fluids in Sn–W-bearing quartz veins shows REE patterns indistinguishable from those of late magmatic zircon. When combined with experimental evidence on the fluid–melt partitioning of REE, it indicates that the REE distribution coefficients for zircon/melt and zircon/fluid are largely comparable.

The second example of hydrothermal zircon crystallized some 2 My after the host granite. These crystals reveal an intragranular zonation of increasing trace element concentrations from core to rim. Therefore, REE abundances and patterns alone are not conclusive indicators of the geological environment in which zircon crystallized. Nevertheless, variations in trace element contents of zircon that relate to the chemistry of the melt or fluid from which zircon crystallized, as measured in cogenetic melt and fluid inclusions, are promising for future petrogenetic modeling.

Lead and Cs are strongly incompatible in hydrothermal zircon, with estimated zircon–fluid distribution coefficients D ≤ 0.001, while Sn and Li are moderately incompatible, D_Sn  0.6 and D_Li  0.1, and Ce is compatible, D_Ce  14. Moreover, hydrothermal zircon has a more pronounced negative Eu-anomaly and higher Ta/Nb and U/Th ratios than the magmatic zircons of the Mole Granite.     

Rare earth element partition between sphene,apatite and other coexisting minerals of the Kangerdlugssuaq intrusion,E. Greenland

Cumulus apatite, sphene, feldspar, amphibole and biotite from the pulaskite of the Kangerdlugssuaq alkaline intrusion have been analysed for rare earth elements (REE) by instrumental neutron activation analysis. The apatite is particularly rich in REE, contains 3.6% Ce and shows a steep, light REE-enriched, chondrite-normalised pattern. The other minerals have light REE enrichment but with sphene showing a peak at Ce on a chondrite-normalised plot. REE partition coefficient values show that the light REE are preferentially accommodated by apatite relative to sphene. The differences in these coefficients result from differences in the co-ordination of the REE in the two minerals.     

Geochemistry and U–Pb age of zircons from the Vurechuaivench massif,Monchegorsk complex,Kola region

The paper presents data on the geochemical and geochronological characteristics of zircons from mafic rocks of part of the Monchegorsk layered complex represented by the Vurechuaivench massif. Ages of zircons (SHRIMP-II) from samples V-l-09 (anorthosite) and V-2-09 (gabbronorite) are dated back to 2508 ± 7 and 2504 ± 8 Ma, respectively. The chondrite-normalized REE patterns confirm the magmatic nature of zircons. The data unequivocally indicate that the U–Pb age of zircon from both gabbronorite and anorthosite corresponds to the age of melt crystallization in a magmatic chamber. The mantle origin of gabbroic rocks of the Vurechuaivench massif is confirmed by the REE patterns of three zircon generations with different crystallization sequences. The wide range of the Ce/Ce* ratio (9.96–105.24) established for zircons from gabbroic rocks of the Vurechuaivench massif indicates sharply oxidative conditions of zircon crystallization. For deepseated mantle rocks, these data can only be explained by significant contamination of the melt with country rock material.     

Determination of zircon/melt trace element partition coefficients from SIMS analysis of melt inclusions in zircon

Partition coefficients (^zircon/meltD_M) for rare earth elements (REE) (La, Ce, Nd, Sm, Dy, Er and Yb) and other trace elements (Ba, Rb, B, Sr, Ti, Y and Nb) between zircon and melt have been calculated from secondary ion mass spectrometric (SIMS) analyses of zircon/melt inclusion pairs. The melt inclusion-mineral (MIM) technique shows that D_REE increase in compatibility with increasing atomic number, similar to results of previous studies. However, D_REE determined using the MIM technique are, in general, lower than previously reported values. Calculated D_REE indicate that light REE with atomic numbers less than Sm are incompatible in zircon and become more incompatible with decreasing atomic number. This behavior is in contrast to most previously published results which indicate D > 1 and define a flat partitioning pattern for elements from La through Sm. The partition coefficients for the heavy REE determined using the MIM technique are lower than previously published results by factors of ≈15 to 20 but follow a similar trend. These differences are thought to reflect the effects of mineral and/or glass contaminants in samples from earlier studies which employed bulk analysis techniques.D_REE determined using the MIM technique agree well with values predicted using the equations of Brice (1975), which are based on the size and elasticity of crystallographic sites. The presence of Ce⁴⁺ in the melt results in elevated D_Ce compared to neighboring REE due to the similar valence and size of Ce⁴⁺ and Zr⁴⁺. Predicted ^zircon/meltD values for Ce⁴⁺ and Ce³⁺ indicate that the Ce⁴⁺/Ce³⁺ ratios of the melt ranged from about 10⁻³ to 10⁻². Partition coefficients for other trace elements determined in this study increase in compatibility in the order Ba < Rb < B < Sr < Ti < Y < Nb, with Ba, Rb, B and Sr showing incompatible behavior (D_M < 1.0), and Ti, Y and Nb showing compatible behavior (D_M > 1.0).The effect of partition coefficients on melt evolution during petrogenetic modeling was examined using partition coefficients determined in this study and compared to trends obtained using published partition coefficients. The lower D_REE determined in this study result in smaller REE bulk distribution coefficients, for a given mineral assemblage, compared to those calculated using previously reported values. As an example, fractional crystallization of an assemblage composed of 35% hornblende, 64.5% plagioclase and 0.5% zircon produces a melt that becomes increasingly more enriched in Yb using the D_Yb from this study. Using D_Yb from Fujimaki (1986) results in a melt that becomes progressively depleted in Yb during crystallization.     

Zircon from charnockite gneiss,charnockite, and leucosome of migmatite in the Nimnyr Block of the Aldan Shield

The microgeochemistry of zircon was studied in three samples: charnockite gneiss (1594), charnockite (1594a), and migmatite leucosome Lc4 (1594c). Prismatic (Zrn I) and oval (Zrn II) zircon morphotypes are distinguished in the first two samples. Most zircon grains consist of two-phase cores and overgrowth rims variable in thickness. The average weighted concordant U–Pb age of Zrn II cores from charnockite gneiss is 2436 ± 10 Ma. The concordant ages of Zrn I and Zrn II cores from charnockite are 2402 ± 16 Ma and 2453 ± 14 Ma, respectively. Some overgrowth rims are 1.9–2.1 Ga in age. In leucosome Lc4, all measured prismatic zircon crystals yielded a discordant age of 1942 ± 11 Ma (the upper intersection of discordia with concordia). These zircons are strongly altered and anomalously enriched in U and Th. Zrn I grains are enriched relative to Zrn II in REE, Li, Ca, Sr, Ba, Hf, Th, and U. Zrn I is considered to be a product of melt crystallization or subsolidus recrystallization in the presence of melt. Zrn II is relict or crystallizing from melt and then partly fused again. Zrn I from charnockite gneiss and especially from charnockite are markedly altered and have a more discordant age than Zrn II. This is probably related to concentration of fluid in the residual melt left after zircon crystallization.     

Experimental determination of REE partition coefficients between zircon,garnet and melt: a key to understanding high‐T crustal processes

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt was determined using synthetic compositions designed to represent partial melts formed in the lower crust during anatexis. The experiments, performed using internally heated gas pressure vessels at 7 kbar and 900–1000 °C, represent equilibrium partitioning of the middle to heavy REE between zircon and garnet during high‐grade metamorphism in the mid to lower crust. The D_REE (zircon/garnet) values show a clear partitioning signature close to unity from Gd to Lu. Because the light REE have low concentrations in both minerals, values are calculated from strain modelling of the middle to heavy REE experimental data; these results show that zircon is favoured over garnet by up to two orders of magnitude. The resulting general concave‐up shape to the partitioning pattern across the REE reflects the preferential incorporation of middle REE into garnet, with D_Gd (zircon/garnet) ranging from 0.7 to 1.1, D_Ho (zircon/garnet) from 0.4 to 0.7 and D_Lu (zircon/garnet) from 0.6 to 1.3. There is no significant temperature dependence in the zircon–garnet REE partitioning at 7 kbar and 900–1000 °C, suggesting that these values can be applied to the interpretation of zircon–garnet equilibrium and timing relationships in the ultrahigh‐T metamorphism of low‐Ca pelitic and aluminous granulites.     

Trace element composition of igneous zircon: a thermal and compositional record of the accumulation and evolution of a large silicic batholith, Spirit Mountain, Nevada

Hafnium, U, Th, and REE content of zircons from the Spirit Mountain batholith in southern Nevada correlate with calculated temperatures from the Ti-in-zircon thermometer to support field and petrologic evidence of rejuvenation of crystal mush and melt extraction events during the 2-million year accumulation of the granitoid batholith. Marked variation in zircon composition from sample to sample, from grain to grain within individual samples, and from zone to zone within individual grains documents in detail a history of fluctuating conditions with repeated episodes of replenishment, reheating, crystal mush rejuvenation, fractional crystallization, and melt segregation. The zircons exhibit compositional and thermal variability indicative of variations in host melt composition due to (1) melt rejuvenation, mixing, and fractionation (2) coeval growth of other REE-rich accessory minerals, and possibly (3) fluctuation in fO₂.     

Meteoritic zircon – Occurrence and chemical characteristics

In common with the remarkable variation in the bulk rock Zr content of distinct meteorite groups, ranging from <1 ppm to >800 ppm, the occurrence and abundance of accessory zircon is also highly diverse and limited to certain meteorite classes. A detailed literature study on the occurrence of meteoritic zircon, along with other Zr-bearing phases reveals that lunar rocks, eucrites and mesosiderites are the prime sources of meteoritic zircon. Rare zircon grains occur in chondrites, silicate-bearing iron meteorites and Martian meteorites, with grain sizes of >5 μm allowing chemical and chronological studies at high spatial resolution using secondary ion mass spectrometry (SIMS) technique. Grain sizes, crystal habits, structural and chemical characteristics of zircon grains derived from various meteorite types, including their REE abundances, minor element concentrations, and Zr/Hf values is diverse. Superchondritic Zr/Hf values (47 ± 8; s.d. with n = 97), i.e., typical for zircon in eucrites and mesosiderites, indicate crystallization from a fractionated, incompatible-element-rich (residual) melt. Differences in REE abundances, occurrence or absence of Ce- and Eu-anomalies, and overall REE patterns that are often fractionated with a depletion in LREE, might be primarily controlled by variable formation conditions of individual grains and/or differences in the residual melt compositions on a small, local scale within single samples. Subsequent fractionation/modification of the chemical fingerprint of meteoritic zircon can involve high-temperature annealing processes during thermal metamorphic reactions and/or impact events along with mixing of lithic fragments since many samples are breccias.     

Trace element and Nd isotope analyses of apatite in granitoids and metamorphosed granitoids from the eastern Central Asian Orogenic Belt: Implications for petrogenesis and post-magmatic alteration

We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. ε_Nd(t) values of the these apatites are decoupled from zircon ε_Hf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO₂ contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.     

New data on distribution of REEs in sulfide minerals and Sm-Nd dating of ore genesis of layered basic intrusions

This paper presents data on the distribution of REEs in sulfide minerals from ore-bearing gabbronorites in the Penikat layered intrusion and the results of their isotopic-geochronological Sm-Nd study. A new procedure for determination of REEs in the samples without preliminary separating and concentrating was tested on standard samples to be further used for analysis of sulfide minerals. Analysis of the spectra of the REE distribution in sulfides represents a distribution trend that is similar to the already studied bulk rock and allows deducing that the character of the REE distribution in sulfide minerals from gabbronorites in the Penikat layered intrusion was inherited from the parent magma melt; while the formation of sulfides took place at the stage of rock crystallization. The performed complex studies allow considering that sulfides can be successfully used together with the rock-forming minerals in Sm-Nd dating of ore-bearing mafite-ultramafite intrusions.     

Composition and conditions of crystallization of zircon from the rare-metal ores of the Gremyakha–Vyrmes Massif,Kola Peninsula

The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.     

REE mineralogy and geochemistry of the Western Keivy peralkaline granite massif,Kola Peninsula,Russia

The authors have studied the geology, geochemistry, petrology and mineralogy of the rare earth elements (REE) occurring in the Western Keivy peralkaline granite massif (Kola Peninsula, NW Russia) aged 2674 ± 6 Ma. The massif hosts Zr- and REE-rich areas with economic potential (e.g. the Yumperuaiv and Large Pedestal Zr-REE deposits), where 25% of ΣREE are represented by heavy REE (HREE). The main REE minerals are: chevkinite-(Ce), britholite-(Y) and products of their metamict decay, bastnäsite-(Ce), allanite-(Ce), fergusonite-(Y), monazite-(Ce), and others. The areas contain also significant quantities of zircon reaching potentially economic levels. We have discovered that behavior of REE and Zr is controlled by alkalinity of melt/solution, which, in turn, is controlled by crystallization of alkaline pyroxenes (predominantly aegirine) and amphiboles (predominantly arfvedsonite) at a late magmatic stage. Crystallization of mafic minerals leads to a sharp increase of K₂O content and decrease of SiO₂ content that cause a decrease of melt viscosity and REE and Zr solubility in the liquid. Therefore, REE and zirconium immediately precipitate as zircon and REE-minerals. There are numerous pod- and lens-like granitic pegmatites within the massif. Pegmatites in the REE-rich areas are also enriched in REE, but HREE prevails over light REE (LREE), about 88% of REE sum.     

REE Tetrad Effects in Rare—metal Granites

Described in this paper are the characteristics of tetrad effects of REE in rare-metal granites.Based on the analytical data and experimental geochemical data available,it is pointed out that the tetrad effects of REE in the granites are produced in the metal-fluid system.Intense fractional crystallization of granitic melt(containing REE accessary minerals)and its interaction with volatile-rich(F,Cl)fluid are the major factors leading to the tetrad effects of REE.From this,this paper presents a composite genetic model for high-degree fractional crystallization-volatile-rich fluid metasomatism of rare-metal granites.With the model,quantitative calculations have been made.Meanwhile,it is pointed out that the tetrad effects of REE can be used as an important indicator to distinguish mineralized granites from barren ones.     

中国北方元古宙沉积岩中自生稀土矿物特征及其意义--以北京、大连地区为例

大连震旦系十三里台组首次发现自生稀土元素矿物独居石后 ,在北京十三陵中元古代长城系常州沟组、串岭沟组和大红峪组又发现了自生的独居石以及其他磷酸盐和硅酸盐稀土矿物。自生稀土矿物的形成和岩石中稀土元素含量较高有关 ;电子探针背散射图像和 P、Th、L a、Ce、Nd、Y等元素面分布图像研究表明 ,沉积岩中的自生稀土矿物与岩浆岩、变质岩和碎屑成因的截然不同。本文首次报道了元古宙长城系常州沟组沉积岩中碎屑锆石边部成岩过程中形成的自生磷钇矿。中国北方元古宙泥砂质碎屑沉积岩普遍具有高稀土组合 ,许多地区都可能存在独居石等自生稀土矿物 ,如天津蓟县、辽西、辽南和宣化等地 ,为利用离子探针 (SHRIMP)确定其同位素地质年代提供了可能。此外 ,本文对比了大连震旦系自生独居石和内蒙白云鄂博矿区东矿的独居石晶形和化学成分的相似性 ,再一次提出中国北方元古宙富稀土地层可能是内蒙古白云鄂博巨型稀土元素矿床的矿源层问题。     

Comment on “Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex,southern West Greenland: Nature of the parent magma” by B. L. Weaver,J. Tarney and B. Windley

Evidence that REE have been metamorphically redistributed between plagioclase feldspar and mafic minerals, as suggested by Weaveret al. (1981) is not forthcoming from the six Fiskenaesset rocks used in the study. REE patterns of the separated feldspars and of two rocks containing small modal amounts of mafic minerals are consistent with a light REE-enriched magma during the formation of the Fiskenaesset Complex.     

Ion-microprobe zircon geochemistry as an indicator of mineral genesis during geochronological studies

This paper considers the distribution of trace elements (including rare earth elements) in zircons dated by the ion-microprobe U-Th-Pb isotope method and its genetic implications. Two problems were addressed on the basis of the investigation of trace element compositions of zircons: (1) genesis of zircons from subalkaline magmatic rocks, sysenites, and sanukitoids and their comparison with tonalites as exemplified by the rocks of the Karelian region, and (2) determination of trace element signatures of zircons from the oldest granulite-facies rocks of the Ukrainian shield. It was shown that the REE distribution patterns of the tonalites, which crystallized in equilibrium with melt, are strictly governed by crystal-chemical laws. The REE distribution patterns show a positive slope with an increase from La to Lu, a positive Ce anomaly, and a negative Eu anomaly. Similar patterns were observed in zircons from the syenites. The trace element contents of zircons are related to those of melts through partition coefficients. Zircons from the sanukitoids show a considerable LREE enrichment, which is inconsistent with the calculated zircon/melt partition coefficients and presumably related to the inherently imperfect zircon structure. Such a structure was formed during zircon crystallization from melt at high temperatures and the anomalous fluid regime that is characteristic, in particular, of sanukitoid melts. The REE distribution patterns of zircons that crystallized under granulite-facies conditions are sharply different from typical distributions in HREE depletion, which was caused by the competitive growth of garnet during zircon crystallization.     

Abundance and distribution of the rare-earth elements and yttrium in the rocks and minerals of the Oka carbonatite complex,Quebec

are-earth (REE) and yttrium abundances were determined, by an ion-exchange-X-ray fluorescence procedure, for whole-rock (14) and mineral (87) samples from the Oka carbonatite complex. Whole-rock and mineral data indicate a trend of total REE + Y enrichment, and relative enrichment in light REE, in the order: ultrafenites < ijolites < okaites. The sövites may show wide variations in total REE + Y concentrations, but relative REE abundance patterns will be similar. The greatest REE and Y concentrations occur in apatite, niocalite, perovskite and pyrochlore. Many of the minerals show europium anomalies (both positive and negative), and these are believed to be the result of closed system competition between the various minerals for divalent Eu. The partition coefficients for mineral pairs are quite variable, indicating that the Oka rocks were emplaced through a wide-range of physicochemical and/or nonequilibrium conditions. A reasonable model for the origin of the complex involves a limited partial melting of mantle material, emplacement of the melt in a magma chamber, crystallization of mafic minerals resulting in a residual liquid which produced ijolite and subsequently okaite, and crystallization of the carbonatites from a volatile-rich, possibly immiscible, phase.     

青藏高原最年轻碱性玄武岩SHRIMP年龄的地质意义

青藏高原北缘康西瓦地区的新生代碱性玄武岩含有大量的岩浆锆石,SHRIMP锆石U-Pb定年结果（3.8Ma）揭示该玄武岩形成于上新世早期（赞克尔期）.与世界典型地区玄武岩相比,该区玄武岩具有富集包括Zr,U,Th在内的不相容元素的特征.根据前人的实验成果,这种富集与这类元素在高温高压条件下在流体相中浓度增加有关.玄武岩中发育黑云母斑晶可以作为熔浆富含挥发分的证据,而导致熔浆富含挥发分的原因则可能是幔源橄榄岩包体中金云母的脱水熔融.当黑云母开始晶出时,水流体的消耗造成熔浆中ZrO4的浓度降低,同始晶出锆石.这表明,含水矿物呈斑晶产出有利于锆石在硅酸不饱和熔浆中的结晶.因此,锆石U-Pb定年方法可以应用于具有类似岩石学特征的火成岩中.另一方面,由于幔源岩浆事件可以作为一个地质旋回或阶段开始的触发事件,建议将该测年结果看作是青藏高原最新一期隆升的起始时间.