Chlorine partitioning between melt and aqueous chloride fluid phase during granite magma degassing. Part II. Crystallization-induced degassing of melts

An empirical computer model was developed to describe granite magma degassing and the partitioning behavior of Cl between melts and aqueous chloride fluids that formed during eutectic isobaric crystallization of magmas at pressures from 4 to 0.4 kbar and a temperature of 800 ± 25°C. This model is the extensions of the earlier model describing the decompression degassing of granite melts (Lukanin, 2015). The numerical modeling was performed for both closed-system conditions, when fluid remains in the system, and open-system conditions, when fluid is removed from the system. The results of numerical modeling revealed the main factors controlling the behavior of Cl during crystallization-induced degassing, such as the initial contents of Cl and H₂O of the melts, pressure, and the degree of system openness. At high pressures (>1.6 kbar), isobaric crystallization is accompanied by a decrease in the concentrations of Cl in the melt (C_Cl^m) and fluid phase (C_Cl^fl). This tendency becomes even more pronounced in an open-system with increasing pressure and initial Cl content. A decrease in pressure in the range of 1.62–0.85 kbar results in a drastic change in the Cl behavior: the trend of C_Cl^fl and C_Cl^fl decrease dominating during crystallization at high pressures changes to the opposite. At low pressures (<0.85 kbar), the enrichment of the residual melts and released fluids in Cl leads at a certain stage of crystallization to the formation of a heterogeneous fluid consisting of two immiscible aqueous chloride phases, a waterdominated aqueous phase and a chloride-rich liquid (brine).     

Partitioning of rare earth elements between garnet and andesite melt: an autoradiographic study of P-T-X effects

High-pressure equilibrium studies were conducted in piston-cylinder apparatus to determine rare earth element (REE) partitioning between garnet and H₂O-vapor-saturated liquidus, from 20 kbar/980°C to 30 kbar/1060 °C. Ag capsules were employed to eliminate loss of Fe. Partition coefficients (K_D's) were determinined with autoradiographic techniques employing beta-active isotopes of Ce, Sm, and Tm. Major elements in garnet were determined by microprobe analysis. Synthesis and reversal runs of 24 hr or greater duration were used to bracket values of K_D's within analytical uncertainty.The K_D values for all three REE are constant over the radiogenic concentration range of 1 to 350 ppm, suggesting that the high abundance of natural REE in the starting materials may suppress possible deviations from Henry's Law behavior reported in similar autoradiographic studies of synthetic systems with no natural REE. Changes in K_D with increasing pressure and temperature at near-liquidus conditions suggest that the dominant control of K_D is the average size of cations occupying the 8-fold sites in garnet. Specifically, the substitution of 8-fold Ca for Mg and Fe causes an increase in K_D values greater than that attributable to the coincident effects of pressure and temperature. The inverse correlation of increasing K_D with REE ionic radius supports the interpretation that the average size of the 8-fold cation controls the relative variation of K_D among REE.These experimental K_D values produce less relative fractionation in melts between light REE and heavy REE than do previous K_D's derived from data on coexisting natural garnet phenocryst/rockmatrix pairs. Models for the derivation of orogenic andesites from partial melting of subducted basaltic eclogite are qualitatively improved by these new K_D's. Existing calculations of K_D values necessary for the viability of the eclogite fractionation are also in good agreement with these experimental K_D values.     

Partitioning of rare earth elements between phosphate-rich carbonatite melts and mantle peridotites

Summary Near solidus equilibria in the system mantle peridotite-carbonate-phosphate doped with Ce and Yb have been studied at 20 kbar and 950°C. Carbonatitic melts in this system may be quenched into homogeneous glasses. Such melts intensely extract REE from rock-forming mantle minerals, and their migration may cause processes of mantle metasomatism.

---

Verteilung von Seltenen Erden zwischen phosphatreichen karbonatitischen Schmelzen und Mantel-Peridotiten

Zusammenfassung Gleichgewichte nahe dem Solidus im System Mantel-Peridotit-Karbonat-Phosphat, das mit Ce und Yb dotiert wurde, wurden bei 20 kbar und 950°C untersucht. Karbonatitische Schmelzen in diesem System können zu homogenen Gläsern abgeschreckt werden. Solche Schmelzen extrahieren SEE aus gesteinsbildenden Mantelmineralen und ihre Migration könnte für Vorgänge der Mantel-Metasomatose verantwortlich sein.

---

---

With 2 figures     

Partitioning of F between aqueous fluids and albite granite melt and its petrogenetic and metallogenetic significance

The fluid/melt partitioning experiments on fluorine were carried out in the system albite-H₂O-HF atP = 100 MPa, 770°C ≤T≤800°C: and wt = 2% −6% conditions. The concentrations of fluorine in quenched glasses (melt) were determined by electron microprobe and those of fluorine in the coexisting aqueous fluid were calculated by the method of mass balance. The result shows that the fluorine was concentrated in granitic melt relative to the coexisting fluid. The partition coefficient D_F(wt _F ^F1 /wt _F ^Mt ) ranges from 0.35 to 0.89. It increases with increasing fluorine content in the system. This means that there is not just one single value of partition coefficient for fluorine in the granitic melt-fluid system. The partitioning behavior of fluorine in this system depends critically on fluorine and proton (H⁺) concentrations. Our data suggest that F-rich granitic melts exist in nature and that fluorine may not be an important complexing agent of metal elements in F-bearing fluids. The project was financially supported by both the National Natural Science Foundation of China (No. 49603048) and the State Key Laboratory of Mineral Deposit Research, Nanjing University.     

Partitioning of rare earth and high field strength elements between titanite and phonolitic liquid

We present the results of a LA–ICPMS study of titanites and associated glasses from the mixed-magma phonolitic Fasnia Member of the Diego Hernández Formation, Tenerife, Canary Islands. We employ a method of identifying equilibrium mineral–melt pairs from natural samples using REE contents and a linear form of the lattice strain model equation (Blundy and Wood, 1994), where the Young's modulus (E_M) for the 7-fold coordinated site is an output variable. For felsic magmas that contain crystals potentially derived from a variety of environments within the system, this approach is more rigorous than the use of solely textural criteria such as mineral–glass proximity. We then estimate titanite/melt partition coefficients for Y, Zr, Nb, REE, Hf, Ta, U and Th. In common with prior studies, we find that middle REE partition more strongly into titanite than either light or heavy REE, and that REE partitioning behavior in titanite is reasonably predicted by the lattice strain model. Titanite also fractionates Y from Ho, Zr from Hf, and Nb from Ta. Comparison with experimental data indicates that melt structure effects on partitioning are significant, most particularly in very highly polymerized melts. We use the data to estimate 7-fold coordination radii for trivalent Pr, Nd, Ho, Tm and Lu, and to make approximate predictions of titanite/melt partitioning of Ra, Ac and Pa. Interpolation of data for heavy REE does not predict the behavior of Y, indicating that factors other than charge and radius are involved in partitioning. Variations in Y/Ho induced by magmatic processes appear to be negatively correlated with temperature, and are expected to be greatest in near-minimum melts.     

Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite

The solubility of fluorapatite in 17 silica-rich melts in the system Na₂O-K₂O-Al₂O₃-SiO₂ (with and without CaO or CaF₂) was determined at 1 kbar water pressure and 750 900°C. Apatite saturation occurs at levels of dissolved P₂O₅ ranging between 0.04 (± 0.02) and 0.28 (± 0.13) wt%. with only 4 values outside the 0.09–0.20 wt% range.The results demonstrate not only that apatite is a common liquidus phase in felsic melts, but also that, under most circumstances, it remains in the residue during episodes of partial fusion of the crust. Given a solubility limit of 0.14 wt% dissolved P₂O₅ (the mean of the experimental values) a source containing as little as 0.05% P₂O₅ must be 35% melted before apatite is lost from the residue and no longer buffers the melt P₂O₅ concentration at the saturation value. Higher abundances of P₂O₅ in the source postpone the loss of residual apatite to still higher degrees of melting, and if the source P₂O₅ content exceeds 0.14 wt%, apatite must be residual for all degrees of melting, increasing in abundance as melting proceeds.The generally secondary influence of apatite on the rare earth element (REE) patterns of melt and residue is most apparent when garnet and/or amphibole is minor or lacking in the residue. Fractional crystallization of intermediate (e.g. andesitic) magmas toward felsic compositions invariably results in saturation in apatite and some consequent depletion of REE in the melt.     

Distribution of titanium and the rare earth elements between peridotitic minerals

The concentrations of titanium and rare earth elements (REE) in olivines, orthopyroxenes, clinopyroxenes and spinels from four anhydrous, spinel-bearing peridotite xenoliths have been determined. The distribution of titanium (used as an analogue for the high field strength elements: HFSE) relative to the REE between clinopyroxenes and orthopyroxenes varies as a function of the whole rock composition and modal mineralogy. The distribution coefficients for titanium and the REE in these peridotites do not reflect mineral-melt equilibria. It is believed that subsolidus distribution coefficients for HFSE relative to REE vary with temperature. Ratios of various incompatible elements (e.g., Ti/Eu, Zr/Sm, Hf/Sm and P/Nd) in peridotite minerals differ from those in most primary basalts. However, the abundance ratios of incompatible elements in the bulk peridotite are comparable to those found in modern basalts. Given this and the differing contribution of melt from each phase during melting, near constant ratios of such incompatible elements in primary and primitive basalts and komatiites reflect the buffering of the melt by its residue. These ratios are fixed in the magma during the initial stages of melting because of similar and low distribution coefficients between melt and bulk residue for these element pairs. Differences in the relative abundances of titanium and REE in clinopyroxenes and orthopyroxenes demonstrate that mantle normalized abundance patterns for clinopyroxene are not equivalent to those of the whole rock. Therefore, claims of a widespread HFSE-depleted reservoir in the upper mantle base solely on the relative abundances of incompatible elements in peridotitic clinopyroxenes are invalid.     

Partitioning of elements between majorite garnet and melt and implications for petrogenesis of komatiite

Partitioning of elements between majorite garnet and ultrabasic melt has been studied at 16 GPa and 1950° C. Ca, Ti, La, Sm, Gd, Zr, Hf, Fe, Ni, Mn, K, and Na are enriched in the melt, whereas Al, Cr, V, Sc and Yb are concentrated in majorite garnet. Thus, majorite garnet fractionation by partial melting could produce chemical heterogeneities in these elements deviating from chondritic abundance. Using the partitioning behaviour of elements between majorite garnet and ultrabasic melt, the petrogenesis of komatiite is discussed. A simple model to explain the chemical varieties of komatiites is as follows. Aluminadepleted komatiite was generated by partial melting of the primitive mantle at 200–650 km depth, and alumina-enriched komatiite is the product of remelting of the residual solid at the same depths, whereas alumina-undepleted komatiite was formed by partial melting of the primitive upper mantle at depths shallower than 200 km. We suggest the possibility of large-scale chemical layering or heterogeneity in the early Archean upper mantle as an alternative model for komatiite genesis; shallower mantle depleted in majorite garnet and the underlying mantle enriched in majorite garnet. Alumina-depleted and alumina-enriched komatiites in the early Archean might be generated by a high degree of partial melting of the layered mantle. Such chemical layering could have been homogenized by the late Archean. This explains the observations that alumina-depleted and alumina-enriched komatiites were generally formed in the early Archean but alumina-undepleted komatiite was erupted in the late Archean.     

An experimental determination of rare earth partition coefficients between a chloride containing vapor phase and silicate melts

The partitioning behavior of cerium, europium, gadolinium and ytterbium between an aqueous “vapor” phase and water saturated silicate melt have been experimentally examined using a new experimental approach employing radioactive tracers and a double-capsule technique. Equilibrium was established by reversing the partition coefficient¹ and by betatrack autoradiography. Aqueous solution compositions were varied by adding different amounts of chloride and in some cases fluoride or carbon dioxide. The H₂O contents of the Spruce Pine pegmatite melts were varied by conducting experiments at 4.0 kb, 800°C and at 1.25 kb, 800°C. A jadeite-nepheline composition (75 wt% Jadeite) also was employed at 4.0 kb, 800°C.The chloride experiments (Spruce Pine 4 kb, 800°C) show a linear relationship between the cube of the chloride molality and the partition coefficients of the trivalent rare earths. Europium, under the experimental fO₂ conditions (quartz-fayalite-magnetite buffer), varied linearly as the fifth power of the chloride molality. At the chloride molalities examined (<1.1 mC1), all the rare earths partitioned preferentially into the melt phase (K_P^RE <1). Relative to pure water, the presence of chloride and fluoride fon increased the partitioning of the individual rare earths into the vapor phase, while carbon dioxide did not. Europium anomalies were recorded 1n all experiments, particularly those involving the Spruce P1ne melt at 4.0 kb and 800°C which displayed a large positive europium anomaly at all chloride molalities. Furthermore, a relative fractionation of the trivalent rare earths was also observed in these experiments, such that K_P^Ce>K_P^Gd>K_P^Yb. The smaller ytterbium ion was consistently concentrated in the melt phase relative to the other rare earths in all experiments on the Spruce Pine composition. Experiments on the jadeite-nepheline composition showed no relative fractionation and a positive europium anomaly. The 1.25 kb experiment on the Spruce Pine composition showed a negative europium anomaly in plots of K_p^RE vs. REE.The overall rare earth partitioning at a constant chloride molality (mCl = .914) was such that K_P^SP(1.25 kb) > K_P^SP(4.0 kb) > K_P^Jd-Ne(4.0 kb), where SP = Spruce Pine, Jd-Ne = jadeitenepheli Using the model of Burnnam (1975), It is suggested that the trivalent rare earth partitioning is related to the cube of the melt octahedral site concentration; a property which 1n hydrous melts 1s dependent on melt composition and hydroxyl molality. Excellent agreement was found for the Spruce Pine melt, whereas the jadeite-nepheline melt gave apparent hydroxyl molalities which were too high for the measured partition coefficient. Additional octahedral sites are proposed for this unusual composition perhaps due to some aluminum in 6-fold coordination. The apparent compositional variation of europium partitioning at a constant oxygen fugacity is believed to be related to both the octahedral melt site concentration for trlvalent europium and an 8-coordinated site concentration for divalent europium. Any parameter which affects the numbers of these sites (_PH₂O, melt composition) will affect the rare earth partitioning. The observed dependency of the partition coefficient on the structural state of the melt could be as significant as its dependency on crystalline structural constraints. Furthermore, since _PH₂O can drastically effect the melt structural state, its effects could be reflected in melt/crystal partition coefficients.     

Behaviors of trace elements and relationships between the compositions of solid phase and melt during fractional crystallization

Given r_i =f _i(F) andK _i = g_i(F), wherer _i is the mineral mass fraction in an instantaneous solid phase,K _i the partition coefficient, andf _i(F) andg _i(F) respectively the continuous, sectionally smooth functions of solidification degreeF, we have: $$C^1 = \frac{{C_0 }}{{1 - F}} \cdot \exp \left( { - \int_0^F {\frac{{\Sigma fi(F) \cdot g_i (F)}}{{1 - F}} \cdot dF} } \right)and \bar C^s = \frac{{C_o }}{F}\left[ {1 - (1 - F) \cdot \frac{{C^1 }}{{C^0 }}} \right]$$ Also presented in this paper are other equations describing the compositional changes of solid and liquid phases in magmatic processes (partial melting). The following understanding is attained based on the application of these equations to the Hushan pluton of Mt. Taishan in the west of Shandong Province. 1) For granitoids changes in rock-forming minerals may not be taken into consideration in theoretical modelling of REE distribution, but accessory minerals rich in REE, especially those highly rich in some REE must be taken into account. 2) In the case of well differentiated granitoids, it can be assumed thatFε [0,1] may exert only little influence on the calculations. 3) In practice the (La / Sm)-La diagram should be applied with care.     

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene,garnet, and basaltic melt at high pressures

Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought. Received: 1 July 1997 / Accepted: 11 May 1998     

长江三峡地区成冰纪-埃迪卡拉纪转换时期微量元素和稀土元素地球化学特征

通过微量元素和稀土元素地球化学分析,对长江三峡地区陡山沱组层型剖面--田家园子剖面成冰系南沱组顶部和埃迪卡拉系陡山沱组下部102个岩石样品进行了地球化学研究。重点分析了氧化还原敏感元素(Zn,Co,U,Mo,Ni,V)的富集特征,并探讨其可能成因机制以及三峡地区成冰纪-埃迪卡拉纪转换时期的水体特征。结果表明:在南沱组顶部仅Zn和Co富集;在盖帽白云岩下部,氧化还原敏感元素均富集,而在盖帽白云岩上部,除Zn和V外,其他氧化还原敏感元素均亏损;在陡山沱组Ⅱ段下部,氧化还原敏感元素由最初的亏损,逐渐变为较稳定的富集。在陡山沱组下部,出现2次明显的富集峰值,分别出现在剖面的0.4m处(盖帽白云岩中间)和6.5m处(陡山沱组Ⅱ段下部)。整个剖面大部分样品具有Eu的轻微正异常(Eu/Eu^*＜1.6), 而在剖面0.4m和6.5m处,Eu具有明显的正异常,结合稀土配分类型、Y/Ho值、La异常、Ce异常等指标,推测这2次异常均可能受到深海热液流的影响,而缺氧海水的上涌造成水体缺氧,导致这些元素出现富集峰值。U/Th、V/(V+Ni)以及稀土元素指标综合指示,三峡地区南沱组顶部冰碛岩应为氧化环境下的沉积物;随着冰川消融,冰融淡水注入古海洋,陡山沱组盖帽碳酸盐的沉积受冰融淡水的影响,深部缺氧海水的上涌使沉积水体经历氧化-缺氧-氧化的转变,海水的分层性较强;而陡山沱组Ⅱ段下部沉积环境以分层性较弱的弱氧化环境为主。     

The behaviour of the rare earth elements during mixing of river and sea waters

Rare earth element concentrations have been measured in organic-rich Luce river water and coastal sea water. Concentrations (e.g. ～350?1850 pmol/kg Nd in the Water of Luce and ～45?350 pmol/ kg Nd in Luce Bay) are related to the presence of particles, with 30–60% of the REE associated with >0.4?0.7 μm particles, and to riverine Fe concentrations. REE fractionation occurs in the river water the submicrometre river water is heavy REE enriched whereas the coarser fraction has a more shale-like REE pattern.Laboratory experiments show that the REE in organic-rich river waters are chiefly associated with Feorganic matter colloids which flocculate during estuarine mixing. Preferential removal of heavy REE (～95%) relative to light REE (～60%) occurs, but no Ce anomaly is developed. In contrast, no REE removal occurs during estuarine mixing with organic-poor river water.     

巴尔哲超大型稀有稀土矿床成矿机制研究

巴尔哲矿床中的矿化和非矿化碱性花岗岩主要造岩矿物均为微斜长石、石英、钠闪石和钠长石,但其相对含量及颗粒大小明显不同,且两类岩石中包裹体的组成特征及锆石的结晶习性也有显著差异.主量元素分析显示,矿化与非矿化碱性花岗岩均以富硅、富碱、贫镁和钙为特征,为较典型的非造山A型花岗岩.尽管矿化碱性花岗岩中K_2O和Na_2O的含量均没有明显的增加,但其Na+K/Al、Na_2O+K_2O/CaO、FeO~*/MgO及K_2O/MgO等岩石化学参数与非矿化碱性花岗岩明显不同.在矿化碱性花岗岩中除了矿化的稀土元素及Nb、Zr强烈富集外,U、Th及Y也明显富集,而Ba、Sr、P、Eu和Ti表现为强烈的亏损.在非矿化碱性花岗岩中除了大离子亲石元素Rb略有富集外,稀土元素、Nb、Zr、U、Th、Ta及Y并无明显富集,虽然Sr、P、Eu和Ti也表现为亏损,但与矿化碱性花岗岩相比其亏损程度明显降低.岩相学、岩石化学及微量元素地球化学特征显示,矿化碱性花岗岩不可能是非矿化碱性花岗岩硅化和钠长石化作用的产物,二者应是同一岩浆体系不同演化阶段熔体固结的产物.K/Rb、Rb/Sr及δEu等地球化学参数显示,矿化碱性花岗岩是高演化A型花岗质熔体固结的产物;而岩石学、包裹体及地球化学特征则显示,这种高演化的A型花岗质熔体已经进入了岩浆一热液过渡阶段.巴尔哲矿床稀有稀土元素的超常富集和成矿与A型花岗岩的高演化过程密切相关.     

State of rare earth elements in the rare earth deposits of Northwest Guizhou,China

We studied the states of rare earth elements in ore of the Xianglushan rare earth deposit. Rare earth ore samples were tested and examined by scanning electron microscope, electron probe, and chemical leaching. No independent rare earth minerals were detected by scanning electron microscope. Elements detected by the electronic probe for the in situ micro-zone of the sample included: O, Al, Si, Ca, Mg, Fe, Ti, K, Na, S, Cl, C, Cu, Cr, V, and Pt. Rare earth elements were not detected by electron probe. (NH4)₂SO₄, (NH₄)Cl, NaCl, and H₂SO₄ were used as reagents in chemical leaching experiments that easily leached out rare earth elements under the action of 10% reagent, indicating that the rare earth elements in ore are mainly in the ionic state rather than present as rare earth minerals.     

Water–granite interaction: Clues from strontium,neodymium and rare earth elements in soil and waters

Strontium-, Nd-, and rare-earth-element-isotope data are presented from rock, weathered rock (arene) and saprolite, sediment and soil, shallow and deep groundwater (e.g. mineral-water springs), and surface waters in the Margeride massif, located in the French Massif Central. Granitoid rock and gneiss are the main lithologies encountered in the Margeride, which corresponds to a large and 5-km-deep laccolith. Compared to bedrock, the Sr isotopes in arene, regolith, sediment and soil strongly diverge with a linear increase in the ⁸⁷Sr/⁸⁶Sr and Rb/Sr ratios. Neodymium isotopes fluctuate least between bedrock and the weathering products. In order to characterise the theoretical Sr isotopic signature IRf(Sr) of water interacting with granite, a dissolution model was applied, based on the hypothesis that most of the Sr comes from the dissolution of plagioclase, K-feldspar and biotite. Similar to the Sr model, an approach was developed for modelling the theoretical Nd isotopic signature IRf(Nd) of water interacting with a granite, assuming that most Nd originates from dissolution of the same minerals as those that yield Sr, plus apatite. The IRf(Sr) ratio of water after equilibration with the Sr derived from minerals was calculated for the Margeride granite and compared to values measured in surface- and groundwaters. Comparison of the results shows agreement between the calculated IRf(Sr) and the observed ⁸⁷Sr/⁸⁶Sr ratios. When calculating the IRf(Nd) ratio of water after equilibration with the Nd derived from minerals of the Margeride granite, the results indicated good agreement with surface-water values, whereas mineralised waters analysed within the Margeride hydrosystem could not be directly linked to weathering of the granite alone. Because the recharge area of deep groundwater is located on the Margeride massif, very deep circulation involving interaction with other rocks (e.g. shales) at depths of >5 km must be considered.