Potassium and Associated Elements in Tholeiitic Basalts

Selected average analyses of tholeiitic basalts, which havemarkedly differing contents of K₂O, show a broad coherence betweenK, P, Ti, Ba, Rb, Sr, and Zr. The content of K₂O in the averageanalyses of 43 tholeiitic provinces shows a wide range (0.07–1.61per cent) and has a positively skewed frequency distribution.K₂O in individual tholeiitic basalts and dolerites has a similardistribution pattern. The relationship between K₂O and MgO in the average analysesindicates that the range of K₂O content cannot be solely theresult of near-surface differentiation, nor, indeed, any differentiationprocess which is indexed by the MgO content of the differentiates. Processes which could effect an enrichment of K and associatedelements in tholeiites are assessed and it is concluded thatdegree of melting, eclogite fractionation at high pressure,and mantle wall-rock reaction are likely to be significant factors.Crustal contamination is considered not to be a generally significantfactor in the development of continental tholeiites. The concept that all other tholeiites are derived from a parentalmagma with the composition of low-K deep oceanic tholeiitesis rejected.     

Liquid Immiscibility and the Evolution of Basaltic Magma

This experimental study examines relationships between alternativeevolution paths of basaltic liquids (the so-called Bowen andFenner trends), and silicate liquid immiscibility. Syntheticanalogues of natural immiscible systems exhibited in volcanicglasses and melt inclusions were used as starting mixtures.Conventional quench experiments in 1 atm gas mixing furnacesproved unable to reproduce unmixing of ferrobasaltic melts,yielding instead either turbid, opalescent glasses, or crystallizationof tridymite and pyroxenes. In contrast, experiments involvingin situ high-temperature centrifugation at 1000g (g = 9·8m/s²) did yield macroscopic unmixing and phase separation. Centrifugationfor 3–4 h was insufficient to complete phase segregation,and resulted in sub-micron immiscible emulsions in quenchedglasses. For a model liquid composition of the Middle Zone ofthe Skaergaard intrusion at super-liquidus temperatures of 1110–1120°C,centrifugation produced a thin, silicic layer (64·5 wt%SiO₂ and 7·4 wt% FeO) at the top of the main Fe-richglass (46 wt% SiO₂ and 21 wt% FeO). The divergent compositionsat the top and bottom were shown in a series of static runsto crystallize very similar crystal assemblages of plagioclase,pyroxene, olivine, and Fe–Ti oxides. We infer from theseresults that unmixing of complex aluminosilicate liquids maybe seriously kinetically hampered (presumably by a nucleationbarrier), and thus conventional static experiments may not correctlyreproduce it. In the light of our centrifuge experiments, immiscibilityin the Skaergaard intrusion could have started already at thetransition from the Lower to the Middle Zone. Thus, magma unmixingmight be an important factor in the development of the Fe-enrichmenttrend documented in the cumulates of the Skaergaard LayeredSeries. KEY WORDS: liquid immiscibility; Skaergaard; layered intrusions; experimental petrology     

Silicate glass and sulfides in ultramafic xenoliths,Newer Basalts,Victoria, Australia

The Newer Basalts of Victoria contain an abundance of inclusions, the majority being Iherzolites of an accidental origin. During ascent of the host magma a CO₂-rich vapor phase ‘boiled’ off and penetrated the inclusions, resulting in ubiquitous fluid inclusions and partial melting along some fractures and intergranular boundaries. Fe+Ni+Cu sulfide globules distributed within the glass and along planes in the primary silicates apparently formed during ascent and originated with a sulfur component in the invading vapor and not with remobilization of free sulfides in the xenoliths themselves.     

贵州省梵净山地区玄武质与流纹质岩浆液态不混溶的发现

正液态不混溶(liquid immiscibility)作为最重要的成岩成矿机制之一早就引起了学术界的关注(罗照华等,2007)。但是,自从阿波罗11号采集的月岩中首次发现自然条件下硅酸盐液态不混溶现象(Roedder et al.,1971)以来,地球火成岩中成规模的液态不混溶现象并不多见,涉及液态不混溶过程的论述大多数是基于其最终产物的解释。笔者等于2016年3月在贵州省梵净山地区(图1)进行野外地质工作中发     

Immiscibility in Ca-amphiboles

The literature data of nine different occurrences of coexistingmineral pairs of Ca-amphibole have been studied and the bulkvectors, spanning the miscibility gap, derived. The additivecomponent is always impure Mg-tremolite accompanied by someglaucophane and cummingtonite component. The four major exchangecomponents required to describe the compositional variationin coexisting mineral pairs are the edenite (ED), tschermak's(TS), FeMg_–1 and Fe³⁺-tschermak's (FeTs) vector. Trivalentiron is postulated on the basis of excess charges in the bulkvector the size of which coincides with residuals in Al^tet,–Si, Fe and –Mg. The four cations have equal sizes,forming the vector Fe³⁺ Al^tet Mg_–1Si_–1. This distributionscheme is consistent for all the different occurrences and setsthe basis for a comparison. Deviations from the scheme wouldradically complicate the proposed exchange pattern. The ratioTS:ED in most mineral samples fluctuates between one and two.Projection of the data points in the vector space TS–EDonto the line 1ED: 2TS (Tr–Hbl) or 1ED:1TS (Tr–Prg)provides the projected tremolite content (= 1–X_Hbl or = 1–X_prg). This parameter,applied to coexisting pairs, and plotted against the ratio Mg/(Mg+ Fe) shows some characteristic features about the miscibilitygap. In the Mg-pure system the solvus is almost symmetric andlocated in the temperature range between 800 and 870C. Smallamounts (0.10 pfu) of Fe²⁺ in the M(4) -sites and replacingCa have a dramatic effect, forcing the solvus to much lowertemperatures of 650C. An increase in the ratio Fe/(Fe + Mg)causes a shift of the solvus towards more tremolitic compositionswith temperatures 500–650C. The maximum asymmetry ofthe solvus is reached where the Al-poor member (tremolite) hasa composition of =1.0 and Mg/ (Mg + Fe) 0.6. The corresponding Al-rich member has =0.5 and Mg/ (Mg + Fe) 0.4. An anomalyof the solous is observed at Mg/ (Mg + Fe)=0.8. It manifestsas a kind of highly asymmetric ‘sub-gap’ in thetremolite-rich composition range. This is explained by the partitioningof Fe²⁺ into the single M(3) -site and is characterized by athermal hump to 650–700C. KEY WORDS: tremolite; hornblende; pargasite; immiscibility; solous     

Evidence of Liquid Immiscibility in Alkaline Ultrabasic Dikes at Callander Bay, Ontario

Monchiquitic and camptonitic dikes radiating from an alkalinecarbonatite intrusion display sharply bounded, ellipsoidal,coarse-grained segregations of predominantly leucocratic minerals,identical in both composition and zoning to minerals in thematrix. These oeelli are not found in chilled margins, but elsewhereoccur in layers or segregations within the intrusives. Similarphenomena are seen in rocks of similar composition, both plutonicand volcantic, the world over. Field relations, detailed mineralogy,and macro- and micro-chemical studies strongly suggest thatthey result from liquid immiscibility in ultra basic alkalinemagma. Hydrothermal melting experiments show that olivine-bearing dikeswith carbonate-rich ocelli melt to a homogeneous silicate liquid,and an immiscible CO2-rich fluid phase. Kaersutite-bearing lamprophyrewith felsic ocelli yields a homogeneous silicate liquid on firstmelting, but on further heating this liquid dissociates intotwo immiscsible portions. The data strongly suggest that aninitial carbonated nepheliniti magma could gives rise by successiveimmiscibility to carbonatite, melteigite, and nepheline syeniteliquids.     

Immiscibility between Fe- and Si-Rich Silicate Melts in Mesoproterozoic Ferrobasalt of the Ladoga Graben,Karelia, Russia

Doklady Earth Sciences - Evidence for liquid immiscibility between Fe-rich and Si-rich silicate melts in ferrobasalt of the Ladoga Graben in the Baltic Shield belonging to the Mesoproterozoic...     

富氟花岗岩浆液态不混溶作用及其成岩成矿效应

除了结晶分异模式外,富氟花岗岩浆液态不混溶作用也是伟晶岩成岩成矿的重要机制。熔体包裹体和实验研究表明,富氟花岗岩浆的液态不混溶会形成富挥发份的贫硅熔体和与其共轭的富硅酸盐熔体。花岗岩中的异离体型伟晶岩和贯入状脉型伟晶岩,挥发份、助熔剂等元素和同位素组成在不混溶相间的突变性是该类不混溶作用的主要标志。富氟花岗岩浆不混溶作用不但可以解释伟晶岩的特征性矿物分带,对传统的伟晶岩成岩理论提出了挑战;还对稀有金属有高度富集作用,这主要归功于F、B、P等挥发份对稀有金属元素的亲和力。然而,目前该类不混溶作用成矿效应的研究还比较薄弱,这主要要归因于没有理想的地质对象。甲基卡矿床是亚洲最大的固体锂矿床,初步研究表明该矿床发生了富氟花岗岩浆液态不混溶作用,是研究氟花岗岩浆液态不混溶过程中稀有金属的分配、迁移、富集规律和机制的理想对象。     

Fractionation and Liquid Immiscibility in an Anorthositic Pluton of the Nain Complex, Labrador

Fine-grained anorthositic dikes are associated with a massiveleuconorite pluton (Cl = 15) which is exposed over an area ofabout 200 km². Internally, the pluton shows little compositionalvariation; average plagioclase composition ranges from An₅₂to An₄₈. The dikes are nearly uniform in composition and similarto the estimated bulk composition of the pluton (55 per centSiO₂). They therefore appear to represent the parental magmaof the leuconorite pluton. A small body of granite (10 km²) was emplaced within and priorto the complete solidification of the leuconorite. The graniticintrusion caused local deformation of the leuconorite and filter-pressingof its late stage interstitial liquids. These liquids occurin the younger hydrous granite as very finegrained, chilledpillows of nearly anhydrous Fe-rich diorite and granite. Mostof the pillows are diorites with approximately 55 per cent SiO₂.On oxide plots these lie approximately on a plagioclase controlline passing through the composition of the leuconorite dikes.The entire group of chilled pillows ranges in composition from45 to 71 per cent SiO₂ with a gap between 57 and 63 per centSiO₂. On oxide plots they produce a smooth trend which is obliqueto and truncates the plagioclase control line. Variation inthe pillows can best be explained by late-stage liquid immiscibility. Fractionation in the interstitial magma was controlled earlyby crystallization of plagioclase and later by plagioclase pluspyroxene. Very late stage differentiation was controlled mainlyby liquid immiscibility and produced FeO- and SiO₂-rich liquids.     

The Gronnedal-Ika Carbonatite-Syenite Complex, South Greenland: Carbonatite Formation by Liquid Immiscibility

The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (La_CN–Yb_CN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. _Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and _Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has ¹⁸O values of 7·8 to 8·6and ¹³C values of –3·9 to –4·6. ¹⁸Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in ¹⁸O betweencalculated syenitic melts and carbonatites, and the overlapin _Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite     

长英质岩浆中液态不混溶与成矿作用关系的实验研究

本文的实验研究表明：１２５０℃（１０５Ｐａ）条件下呈均一状态的花岗岩－ＫＢＦ４－Ｎａ２ＭｏＯ４体系,在１０００℃条件下发生液态不混溶形成三种熔体：相对偏酸性的液滴、相对偏基性的熔体和成矿熔体。成矿熔体中富含ＣａＯ、ＭｇＯ和ＭｏＯ２组分。红外光谱研究表明：硅酸盐熔体结构以［ＳｉＯ４］基团为主,而成矿熔体结构中存在Ｃａ—Ｆ、Ｃａ—Ｏ—Ｍｏ、Ｈ—Ｏ—Ｈ以及Ｘ─ＯＨ（Ｘ＝阳离子）基团,说明Ｈ２Ｏ和Ｆ富集在成矿熔体中。液态不混溶作用产生的三种不混溶熔体的结构差异明显,主要表现在结构单元中桥氧数目的变化,相对偏酸性的Ａ类小球体的结构单元中桥氧数最高,基底玻璃熔体中桥氧数则相对较少,而成矿熔体中不存在桥氧。本实验研究结果表明长英质岩浆中的液态不混溶可直接导致成矿熔体的形成。这意味着长英质岩浆中的液态不混溶可直接导致斑岩矿床的形成。     

Trace-Element Modeling and Source Constraints for Tholeiitic and Cale-alkaline Basalts from a Depleted Asthenospheric Mantle Source,Mt. Erciyes Stratovolcano,Turkey

The Mt. Erciyes stratovolcano was built up in an intraplate tectonic environment as a consequence of Eurasian and Afro-Arabian continental collision. However, the volcanic products generally exhibit a calc-alkaline character; minor amounts of tholeiitic basalts are also present. Tholeiitic basalts show high Fe₂O₃, MgO, CaO, low K₂O, and depleted Ba, Nb, and especially Rb (2.3-5.97 ppm) contents, low ⁸⁷Sr/⁸⁶Sr (0.703344-0.703964), and high ¹⁴³Nd/¹⁴⁴Nd (0.512920-0.512780) isotopic ratios. These compositional features show that they were derived from a depleted asthenospheric mantle source, possibly a MORB-like source component. In contrast, calc-alkaline basaltic rocks exhibit relatively high large-ion-lithophile and high-field-strength elements, high ⁸⁷Sr/⁸⁶Sr (0.704591-0.70507) and low ¹⁴³Nd/¹⁴⁴Nd (0.51272-0.512394) isotopic ratios.

The bulk-rock chemistry of the tholeiitic basalts reflects the chemical composition of the extracted source component. Furthermore, trace-element concentrations may be calculated from an accepted mantle source component (starting composition) for different degrees of partial melting. These calculations also provide a sensitive approach to the origin of tholeiitic basalts. Modeled trace-element compositions of tholeiitic basalts are calculated from a primitive mantle composition. Calculated trace-element compositions imply that tholeiitic basalts are derived by minor fractional melting (1-1.5 %), in the absence of assimilation or deep-crustal melting. The calc-alkaline basalts were subsequently produced from initially tholeiitic basalts by the way of an AFC (assimilation-fractional crystallization) process, with a crustal assimilation of 10-15 %.

The geochemical data, partial melting, and AFC modeling all indicate that basaltic products have a complex evolutionary history involving partial melting from a MORB-like mantle source. The assimilation and fractional crystallization processes are considered as providing an example for the chemical evolution of basaltic products, from tholeiitic to calc-alkaline, in an intraplate environment.     

煌斑岩—硫化物熔体液态不混溶作用的高温高压实验研究

本文以云南老王寨金矿区煌斑岩和黄铁矿为初始物,在1.5～3.0GPa、1162～1560℃条件下进行了煌斑岩—硫化物熔体液态不混溶作用实验。结果表明,煌斑岩硫化物熔体的液态不混溶作用可能发生于岩浆演化的早期（高压条件）,但如有硫存在,在岩浆演化的各个阶段（高压条件→低压条件）均可能出现煌斑岩硫化物熔体的液态不混溶作用;与煌斑岩熔体分离的硫化物熔体在温度场、压力场中作定向运移,逐渐聚集于相对低温、低压区;由于硫化物熔体的密度相对较大,因而与煌斑岩熔体之间出现明显的分层现象。     

A Natural High-lime Silicate Liquid more Basic than Basalt

In some picritic minor intrusions a natural removal of olivinephenocrysts during flowage differentiation has resulted in theformation of a phenocryst-free facies. This facies is not basalticin chemical composition: chemical analyses of this facies fromwidely separated localities indicate consistently that it isricher in lime and poorer in alkalis. Chemically similar aresome aphyric dikes cutting picritic minor intrusions in theCuillins (Skye). The existence of a natural high-lime silicateliquid is established and its origin discussed.     

Spilitic Degradation of a Tholeiitic Basalt

Transition from black tholeiite to green albite-chlorite spilitewithin a single Deccan flow unit at Bombay, India, providesdirect evidence of the ancestry of a spilite. Clinopyroxenesoccur throughout the sequence and are chemically similar inboth basalt and spilite. All are low-Ca augites that separatedfrom a tholeiitic melt undergoing fractionation until finalquenching supervened. The spilites are secondary products formedfrom solidified tholeiite by local hydrothermal metamorphism.Two alkali feldspars are generated in the spilites and bothbecome progressively ordered with advancing reconstitution ofthe host rocks. The Na-feldspar finally becomes borderline low-albite,K-feldspar is ordered to adularia. The occurrence affords ameans of evaluating relict pyroxenes as indicators of pristinecharacter in degraded mafic volcanic rocks.     

Fractional Crystallization and Liquid Immiscibility Processes in the Alkaline-Carbonatite Complex of Juqui (So Paulo, Brazil)

The Juqui circular intrusion, which is Cretaceous in age (130–135Ma), crops out in the Precambrian gneissic basement in Brazilover an area of 14 km2. It consists of olivine clinopyroxen-itecumulates (with minor olivine gabbros) in the northeastern sector(74 vol.%), whereas ijolites-melteigites-urtites (4%) and nephelinesyenites with minor essexites and syenodiorites (21%) outlinesubannular concentric patterns with an Mg-carbonatite core (1%), in the southwestern part of the complex. Petrographical, bulk rock, and mineral compositional trendsindicate that the origin of the complex can be largely accountedfor by shallow-level fractional crystallization of a carbonatedbasanitic parental magma. Such a magma was generated deep inthe subcontinental lithosphere by low-degree partial meltingof a garnet-phlogopite peridotite source. Mass-balance calculations in agreement with field volume estimatespermit definition of several fractionation stages of the magmaticevolution under nearly closed-system conditions, with inwarddevelopment of zonally arranged side-wall cumulates. These stagesinvolved: (1) fractionation from basanite to essexite magma(liquid fraction F = 33–5%) by crystallization of olivineclinopyroxenite plus minor olivine alkali gabbro cumulates;(2) derivation of the least differentiated mafic nepheline syenite(F = 5–5 %) from essexitic magma by subtraction of a syenodioriteassemblage; (3) exsolution of a carbonatite liquid (5%) froma CO₂-enriched mafic nepheline syenite magma, which also underwentcontinuous fractionation giving rise to ijolite-melteigite-urtitecumulates. The proportion of cumulus clinopyroxene and biotiteand intercumulus nepheline and alkali feldspar in these lastrocks, as well as the absence of alkalis in carbonatite, maybe attributed, at least in part, to loss of alkali-rich hydrousfluids released during and after the unmixing formation of thetwo conjugate liquids. The KD values determined for Mg-carbonatite/nepheline syeniteare lower (1–4–2–9) for light rare earth elements(LREE) than for REE from Eu to Yb (4–6–7–8),in contrast to recent experimental results (Hamilton et al.,1989). A possible explanation is that Juquia Mg-carbonatiterepresents an alreadydifferentiated magma, which underwent extensivefractionation of LREE-enriched calcite. In this way, the highvariability of K₀ REE patterns observed in several alkaline-carbonatitecomplexes can also be accounted for. The remarkably constant initial ⁸⁷Sr/⁸⁶Sr ratios (mostly between0–7052 and 0–7057) support the interpretation ofthe intrusion as having been generated by fractrional crystallizationand liquid immiscibility from a common parental magma. Iligherisotopic ratios (0–7060–0–7078), found mainlyin dykes and in the border facies of the intrusion, may be dueto contamination by the gecissic basement.     

Silicate--Carbonate Immiscibility at Oldoinyo Lengai

For approximately the last 50 years eruptions at Oldoinyo Lengaihave produced passive natrocarbonatite lavas interspersed withmixed silicate-natrocarbonatite events approximately every 15–25years. In 1993 an unusual blocky lava erupted and preserveddetailed mixed silicate-natrocarbonatite textures clearly indicatingan immiscible origin. The 1993 blocky flow consists of natrocarbonatitewith small silicate crystal aggregates which constitute 2–5%of the rock. These inclusions are composed of nepheline, melanite,clinopyroxene and wollastonite occurring both as isolated crystalsand ijolite micro-xenoliths. Most significantly, these ijoliticinclusions are surrounded by ‘globules’ of a fine-grainedintergrowth of nepheline, wollastonite and gregoryite, interpretedas quenched melt. Petrographic textures are characteristic ofliquid immiscibility between coexisting natrocarbonatite andsilicate melts. The presence of gregoryite within the silicatemelt globules is particularly important as it represents thecommon liquidus phase between the silicate and natrocarbonatitemelts theoretically required to demonstrate immiscibility betweentwo conjugate liquids. This is the first time that liquid immiscibilityhas been so clearly demonstrated in natural rock samples fromOldoinyo Lengai and agrees very closely with recent experimentalwork. Our detailed model for the petrogenesis of the natrocarbonatitesat Oldoinyo Lengai involves extensive fractionation of a carbonate-richalkaline silicate magma followed by immiscible separation ofnatrocarbonatite at low pressures. KEY WORDS: Oldoinyo Lengai; natrocarbonatite; silicate-carbonate immiscibility; East Africa ^*Corresponding author. Present address: Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW_{7 5}BD, UK     

Liquid Immiscibility in the Join NaAlSi3O8-CaCO3 to 2.5 GPa and the Origin of Calciocarbonatite Magmas

Field evidence from intrusive and effusive carbonatites supportsthe existence of calciocarbonatite magmas. Published experimentalevidence in the model system Na₂O–CaO–Al₂O₃–SiO₂–CO₂indicated the formation of nearly pure (99%) CaCO₃ immiscibleliquids from a carbonated silicate liquid. This evidence hasbeen used to support interpretations of extremely CaCO₃-richcalciocarbonatite magmas, and immiscible liquids with compositionsof almost pure CaCO₃ in metasomatized mantle peridotite andeclogite. Detailed phase relationships are constructed in themodel system, based on phase fields intersected by the joinNaAlSi₃O₈–CaCO₃ (Ab-CC) at 1.0, 1.5, and 2.5 GPa between1100 and 1500C, and analyzed immiscible liquids. The miscibilitygap between silicate-rich liquid and carbonate-rich liquid intersectedby the join Ab-CC contracts considerably with decreasing pressure:2.5 GPa, between Ab₁₀CC₉₀ (by wt%) and Ab₆₅CC₃₅ above 1310C;1.5 GPa, betweenAb₂₃CC₇₇ and Ab₄₃CC₅₇ above 1285C; 1.0 GPa,not intersected. The liquidus piercing point between calciteand silicates becomes enriched in CaCO₃ with decreasing pressure,from Ab₈₀CC₂₀ at 2.5 GPa to Ab₄₇CC₅₃ at 1.0 GPa. No immiscibleliquid contains more than 80% dissolved CaCO₃, and all containat least 5% Na₂CO₃. A round CaCO₃ phase exhibiting morphologysimilar to that displayed by immiscible liquid globules is determinedto be crystalline calcite under experimental conditions. Thetopology of the phase fields and field boundaries illustratesthe kinds of processes and controls existing in magmatic systems.Calciocarbonatite magmas cannot be produced by equilibrium immiscibilityprocess in the mantle. Carbonated silicate magmas in the crustyield residual calciocarbonatite magmas by fractionation alongthe silicate-calcite field boundary, reached either directlyfrom the silicate liquidus or more commonly via the miscibilitygap. Immiscible carbonaterich magmas when freed from the silicateparent cool down a sleep silicate liquidus until they reacha silicate-carbonate field boundary. There is no experimentalevidence for immiscible calciocarbonatite magmas with > 80%CaCO₃, and calcite lapilli cannot be formed from 99% CaCO₃ magmas.Sovites are surely cumulates. KEY WORDS: carbonatite; join NaAlSi₃O₈–CaCO₃; liquid immiscibility; sovite ^* Corresponding author. Telephone: (818)-395–6239. Fax: (818)-568–0935. e-mail: wjl{at}gpi.caltech.edu     

Liquid Immiscibility of Boninite in Xiangcheng, Southwestern China, and Its implication to Genetic Relationship between Boninite and Komatiitic Basalt

Boninitic rocks and associated high-magnesian basalt and high-iron tholeiite in the Xiangcheng area constitute the basal horizon of the arc volcanic sequence in the Triassic Yidun Island-Arc, southwestern China. The boninite occurs as pillow, massive and ocellar lavas; the last one possesses well-developed globular structure and alternates with the former two. The boninite is characterized by the absence of phenocrysts of olivine and low-Ca pyroxenes and by low CaO/Al2O3 ratios (<0.67) and high Cr (>1000 ppm) and Ni (>250 ppm). The normalized abundance patterns (NAP) of trace elements to primitive mantle are similar to the NAP of low-Ca modern boninites and SHMB in the Archaean and Proterozoic.As a mechanism of ocellar texture, liquid immiscibility in boninite is supported by the following lines of evidence: (a) sharp contact between ocelli and matrix, (b) constant volumetric ratios of ocelli/matrix and common coalescence of ocelli in ocellar rocks, (c) identical micro-spinifex textures and mineral asse     

Empirical Solution Model for Alkalic to Tholeiitic Basic Magmas

Currently available models to simulate naturally occurring mineral–meltequilibria use mineral components limited to tholeiitic basaltcompositions and thus they cannot be used for alkali-rick basaltsand basanites. To expand mineral–melt equilibria calculationsto alkali-rich composition space at low pressures, we have derivedequations that describe chemical equilibria between olivine–melt,pyroxene–melt, plagioclase–melt, nepheline–meltand leucite–melt components. Excess free energies of reactionsbetween the end-member mineral and melt components at equilibriumhave been expressed as a function of melt composition, temperatureand fo₂. The database used to calculate the mineral–meltexpressions consists of a total of >350 anhydrous experimentsconducted under controlled oxygen fugacity defined by the quartz–fayalite–magnetite(QFM) oxygen buffer. Rocks used in these experiments range frombasanites, nephelinites and alkali olivine basalts, to tholeiiticbasalts and basaltic andesites. Using bulk compositions of startingmaterials both in this experimental database and in others thatwere not incorporated into the regression of modeled parameters,modeled equations successfully predict, at a given temperatureand fo₂, compositions of multiply saturated melts as well asthe compositions of coexisting minerals. Standard deviationsof the calculated mole fractions of mineral components () areas follows: anorthite 002; forsterite 002; clinoenstatite002; enstatite 0003; nepheline 002; and leucite 001. Standarddeviations () of the calculated melt compositions in terms ofweight percent of oxides are: SiO₂ 0•96; Al₂O₃ 132; Fe₂O₃023; FeO 121; MgO 084; CaO 079; Na₂O 058; and K₂O 069.All calculations were carried out using a non-linear Newton–Raphsonnumerical procedure. KEY WORDS: mineral–melt equilibria; alkalic–tholeiitic basalts; equilibrium thermodynamics ^*Corresponding author