An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in thetemperature range 1150–950°C at oxygen fugacitiescorresponding to the quartz–fayalite–magnetite (QFM)and MnO–Mn₃O₄ buffers to assess the role of water andf_O2 on phase relations and differentiation trends in mid-oceanridge basalt (MORB) systems. Starting from a primitive (MgO9·8 wt %) and an evolved MORB (MgO 6·49 wt %),crystallization paths with four different water contents (0·35–4·7wt % H₂O) have been investigated. In primitive MORB, olivineis the liquidus phase followed by plagioclase + clinopyroxene.Amphibole is present only at water-saturated conditions below1000°C, but not all fluid-saturated runs contain amphibole.Magnetite and orthopyroxene are not stable at low f_O2 (QFM buffer).Residual liquids obtained at low f_O2 show a tholeiitic differentiationtrend. The crystallization of magnetite at high f_O2 (MnO–Mn₃O₄buffer) results in a decrease of melt FeO^*/MgO ratio, causinga calc-alkaline differentiation trend. Because the magnetitecrystallization temperature is nearly independent of the H₂Ocontent, in contrast to silicate minerals, the calc-alkalinedifferentiation trend is more pronounced at high water contents.Residual melts at 950°C in a primitive MORB system havecompositions approaching those of oceanic plagiogranites interms of SiO₂ and K₂O, but have Ca/Na ratios and FeO^* contentsthat are too high compared with the natural rocks, implyingthat fractionation processes are necessary to reach typicalcompositions of natural oceanic plagiogranites. KEY WORDS: differentiation; MORB; oxygen fugacity; water activity; oceanic plagiogranite     

Theoritical and Experimental Studies on the Phase Equilibria and Phase Diagrams on the Salt-Water Systems

正1 Introduction Salt lakes are widely distributed in the world,and salt lakes in China are mainly located in the area of the Qinghai-Xizang(Tibet),and the Autonomous Regions of Xinjiang and Inner Mongolia.There are more than 700salt lakes,each with an area larger than 1 km2,in the     

不同温度、压强、氧逸度条件下斜方辉石含水性的实验研究

斜方辉石的含水性显著影响着上地幔的物理化学性质。前人对于斜方辉石的水溶解度的制约因素研究并不充分,特别是氧逸度的影响很少被单独考虑。为了厘清斜方辉石的水溶解度的制约因素,我们借助高温高压设备(MAVO LPC250型活塞圆筒压机)在1.0～3.0 GPa, 800～1100℃和两种典型的氧逸度条件控制下开展了系统性地实验研究,即Ni+Ni O (NNO),Fe+FeO (IW)。通过傅里叶变换红外光谱测量(FTIR),斜方辉石的天然和实验样品显示出7组典型的OH-吸收峰。含水量结果表明,在上地幔浅部条件下,斜方辉石的水溶解度均随温度或压强升高而增大,而随氧逸度值升高而减小,仅氧逸度变化其它条件不变时,还原条件比相对氧化条件高达50%,这与橄榄石等其它名义上无水矿物的氧逸度效应存在不一致。通过估算,橄榄石和斜方辉石间水的分配系数在不同氧逸度条件下可以相差55%。因此,与名义上无水矿物中结构水相关的实验研究,未有效控制氧逸度的结果需要被重新考虑。     

Kiglapait Mineralogy II: Fe-Ti Oxide Minerals and the Activities of Oxygen and Silica

Cumulus titanomagnetite and subordinate ilmenite first appearin the Upper Zone of the Kiglapait intrusion. They arrive graduallyand then reach abnormal abundances before falling to a sustainedcotectic mode near seven volume per-cent. The most Ti-rich titanomagnetites(to Usp ₆₆) are preserved in ore bands (layers) which solidifiedby adcumulus growth leading to the complete expulsion of interstitialsilicate liquid. Analyses from three of these ore bands, appliedto the solution model of Lindsley (1977), form a single lineararray in f_o2 versus 1/T ?K, with log f_o2 = (–28,283 ?89)/T + 11.03 ? 0.25. This array implies log f_o2 = –9.65at 1094 ?C, the model temperature of the Main Ore Band, consistentwith primary mineral compositions Ilm₈₉, Usp₇₉ and a weightmode of 18 per cent ilmenite. Silicate rocks yield another linear array, i.e. log f_o2 = (–37,910? 102)/T + 22.57 ? 0.61. This array is ascribed to closure ofsubsolidus reactions from initial compositions near Usp_79–80Ilm₉₀. The center of gravity of the data falling on this arraysuggests a primary mode of about 50 per cent ilmenite for thesilicate rocks, implying somewhat more reducing conditions ofcrystallization than for the ore bands. The modal overproductionrepresented by the ore bands is attributed to super-saturationin oxygen, which is demonstrated by the Al-depleted compositionsof titanomagnetite in ore bands, by direct evidence for elevatedf_o2 at the top of the Main Ore Band, and by abnormally magnesiansilicate mineral compositions in and near the ore bands. The primary titanomagnetite composition for average rocks isestimated at Usp₈₀ for the base, and Usp₇₃ for the top of theUpper Zone, from rock and mineral chemistry and observed textures.The idealized magma path for the Upper Zone runs from (T andlog f_o2) 1154 ?C, –9.0 to 960 ?C, –12.2. The orebands lie above this path and are interpreted as lying on themetastable extension of the Lower Zone path, which originatesat 1250 ?C, –8.1, on the WM buffer at 4 kbar total pressure. Silica activity is estimated from mineral compositions nearthe ore bands as applied to the FMQ equilibrium, and mappedfor the Lower Zone by an adjustment downward from the En-Fo-Silequilibrium, with resultant values near 0.55 relative to quartz= 1.0. The logarithmic oxygen/silica activity ratio (OSAR) coincideswith that of the Skaergaard intrusion in the Lower Zones. TheSkaergaard OSAR is offset downward from the Kiglapait trendduring MZ time, and remains below it at the end of crystallization.The more highly silicated Skaergaard magma was initially moreoxidized than the Kiglapait magma, but this relation was reversedafter the loss of olivine in the Skaergaard intrusion, as couldhave been predicted from theory and the mineralogy of the twointrusions.     

Phase Equilibria Constraints on Relations of Ore-bearing Intrusionswith Flood Basalts in the Panxi Region, Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small uitramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-PI-OI-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and iow-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limabe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basaits. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.     

Subduction Influence on Oxygen Fugacity and Trace and Volatile Elements in Basalts Across the Cascade Volcanic Arc

Fluids or melts derived from a subducting plate are often citedas a mechanism for the oxidation of arc magmas. What remainsunclear is the link between the fluid, oxygen fugacity, andother major and trace components, as well as the spatial distributionof the impact of those fluids. To test the potential effectsof addition of a subduction-derived fluid or melt to the sub-arcmantle, olivine-hosted melt inclusions from primitive basalticlavas sampled from across the central Oregon Cascades (43°–45°N)have been analyzed for major, trace and volatile elements andfO₂. Oxygen fugacity was determined in melt inclusions fromsulfur speciation determined by electron microprobe and fromolivine–chromite oxygen geobarometry. The overall rangein fO₂ based on sulfur speciation measurements is from <–0·25log units to + 1·9 log units (FMQ, where FMQ is fayalite–magnetite–quartzbuffer). Oxygen fugacity is positively correlated with fluid-mobiletrace element and light rare earth element contents in basaltsgenerated by relatively low-degree partial melting. Establishinga further correlation between fO₂ and fluid-mobile trace elementabundances with position along the arc requires the basaltsto be subdivided into shoshonitic, calc-alkaline, low-K tholeiiteand enriched intraplate basalt groups. Melt inclusions fromenriched intraplate and shoshonitic lavas show increasing fO₂and trace element abundances closer to the trench, whereas calc-alkalinemelt inclusions exhibit no significant across-arc variations.Low-K tholeiitic melt inclusions record an increase in incompatibletrace elements closer to the trench; however, there is no correlatedincrease in fO₂. The correlation observed in enriched intraplateand shoshonitic melt inclusions is interpreted to reflect aprogressively greater proportion of a fluid-rich, oxidized subductioncomponent in magmas generated nearer the subduction zone. Significantly,calc-alkaline melt inclusions with high ratios of large ionlithophile elements to high field strength elements, characteristicof ‘typical’ arc magmas, have oxidation states indistinguishablefrom low-K tholeiite and enriched intraplate basalt melt inclusions.The lack of across-arc geochemical variation in calc-alkalinemelt inclusions may suggest that these basalts are not necessarilythe most appropriate magmas for examining recent addition ofa subduction component to the sub-arc mantle. Flux and batchmelt model results produce a wide range of predicted amountsof melting and subduction component added to the mantle source;however, general trends characterized by increased melting andproportion of the subduction component from enriched intraplate,to low-K tholeiite, to calc-alkaline are robust. The model resultsdo not require enriched intraplate, low-K tholeiite and calc-alkalinemagmas to be produced from the same more fertile mantle source.However, enriched intraplate magmas, in contrast to calc-alkalineand low-K tholeiite magmas, cannot be generated from a depletedmantle source. Flux or batch melting of either the more fertileor depleted mantle sources used to generate the low-K tholeiite,calc-alkaline, and enriched intraplate magmas cannot reproduceshoshonitic compositions, which require a significantly depletedmantle source strongly metasomatized by a subduction component.The potential mantle source for shoshonitic basalts has a predictedfO₂ (after oxidation) from + 0·3 to + 2·4 logunits (FMQ) whereas the mantle source for low-K tholeiite, calc-alkaline,and enriched intraplate magmas may range from –1·1to + 0·7 log units (FMQ). KEY WORDS: basalt; Cascades; melt inclusions; oxidation state; volatiles     

Phase Equilibria of the Aqueous Systems Containing Lithium and Carbonate Ions

正1 Introduction Alkaline lakes are widely distributed in the area of the Qinghai-Tibet Plateau.Most of the salt lakes are famous for their high concentration of lithium,potassium,magnesium,boron(Ma,2000).In recent years,as a new energy material,lithium and its compounds are widely used in the new area,such as aerospace industry,nuclear     

高压水热体系中氧逸度/活度测量与控制技术的回顾与展望

高压水热体系在科学实验、工业过程和自然界中广泛存在.氧逸度/活度是高压水热体系最基本的物理化学参数之一,经常控制着体系的物质形态、性状以及过程的发生与发展,因此对高压水热体系氧逸度/活度的测量与控制具有极为重要的意义.本文首先对目前国际上流行的各种高压水热体系氧逸度/活度测量与控制技术从原理和基本特点等方面作了详细的介绍,接着对各种技术的优、缺点作了比较与评价,文末对未来高压水热科学与技术领域氧逸度/活度测量与控制技术的发展趋势作出了展望.     

The Olivine--Orthopyroxene--Spinel Oxygen Geobarometer, the Nickel Precipitation Curve, and the Oxygen Fugacity of the Earth's Upper Mantle

The common upper mantle assemblage olivine-orthopyroxene-spinelmay be used to calculate the oxygen fugacity at which mantle-derivedperidotites have equilibrated. The equilibrium has been calibratedusing the large amount of existing data on the thermodynamicproperties of each phase in this assemblage. A by-product ofthis procedure is a new calibration of the olivine-spinel Mg-Fe²⁺exchange geothermometer. Application of the equilibrium to avariety of peridotite xenoliths indicates that the oxygen fugacityof the upper mantle lies between the quartz-fayalite-magnetite(QFM) and w?stite-magnetite (WM) oxygen buffers; the few apparentexceptions to this rule may be due to analytical error, particularlyin the Fe³⁺ content of the spinet phase. In fact, the determinationof Fe³⁺ in spinet is at present the limiting factor in the accurateapplication of the method: within this limitation, the presentlyavailable evidence suggests that the oxygen fugacity of themantle may be laterally homogeneous over wide regions, but mayalso show small differences between these regions. The fluidspecies in the system C-H-O at such oxygen fugacities are predominantlyCO₂ and/or H₂O, and not CH₄/H₂ The minimum possible oxygen fugacity of the mantle is givenby the nickel content of olivine in equilibrium with orthopyroxene;for typical mantle compositions this minimum curve is virtuallycoincident with the iron-w?stite (IW) oxygen buffer.     

Phase Equilibria of Salt-water Systems Containing Magnesium and Borate Ions

正1 Introduction A salt lake is a naturally occurring complex body of water and salt interaction.More than 700 salt lakes are widely distributed in the area of the Qinghai-Tibet Plateau.Most of the salt lakes are famous for their abundance of lithium,potassium,magnesium,and boron resources.It is     

Magmatic Oxygen Fugacity and Significance of the Pulang Ore-bearing Prophyry,in Yunnan Province, China

Please refer to the attachment(s) for more details     

Differentiation of Ferro-Basaltic Magmas under Conditions Open and Closed to Oxygen: Implications for the Skaergaard Intrusion and Other Natural Systems

Because processes such as fractional crystallization and crystallizationunder conditions closed to oxygen are difficult to simulatein the laboratory there is a need for quantitative models ofmagma crystallization behaviour. Comparison of experimentaldata on an iron-rich basaltic composition with predictions ofthe MELTS free energy minimization algorithm shows that althoughliquidus temperatures and silicate mineral equilibria are predictedrelatively well, the saturation of Fe–Ti oxides is notWe have used the same experimental data to construct an alternativecrystallization model based on known equilibrium phase relations,mineral–melt partitioning of major elements, and massbalance constraints. The model is used to explore the consequencesof equilibrium and fractional crystallization in systems openand closed to oxygen. Liquid lines of descent for perfect equilibriumand perfect fractional crystallization are predicted to be verysimilar. In a system open to oxygen the model predicts thatmagnetite saturation leads to strongly decreasing iron and increasingsilica contents of residual liquids, whereas systems closedto oxygen crystallize less abundant magnetite, leading to aless pronounced iron depletion in the liquid. Predicted bulksolid compositions and variations of f_o2, with falling temperatureagree well with those observed or inferred from the cumulatesof the Skaergaard intrusion, but none of the predicted liquidlines of descent are consistent with the extreme iron enrichmentproposed for this intrusion based on mass balance calculations.Compositional factors such as water and phosphorus are not thoughtto be the source of the discrepancy as the cumulates of theBasistoppen sill (which closely resemble those of Skaergaard)may be used to calculate a liquid line of descent in agreementwith that predicted by the model for fractional crystallizationclosed to oxygen. A comparison of the predicted T-f_o2, pathsand liquid lines of descent with those inferred from naturalsystems suggests that volcanic centres such as Iceland and Hawaiievolve under conditions open to oxygen, whereas evidence fromplutonic environments (e.g. Skaergaard and Kiglapait layeredintrusions) suggests that they evolved under conditions moreclosed to oxygen. The compositional evolution of the melt phasein volcanic and plutonic systems may therefore be different,although the results of this study suggest that magnetite saturationwill limit Fe enrichment in all environments to <20wt% FeO^*,consistent with enrichments reported for volcanic glasses. KEY WORDS: Skaergaard; ferro-basalt; iron enrichment; oxygen fugacity ^*Bayerisches Geoinstitut, Universitt Bayreuth, D-95440 Bayreuth, Germany. Telephone: $ 49-921-553718. Fax $ 49-921-553769. e-mail: mike.toplis{at}uni-bayreuth.de     

Phase Relations of Basalts in their Melting Range at PH2O = 5 kb as a Function of Oxygen Fugacity: Part I. Mafic Phases

The phase relations of three basalts, the Picture Gorge tholeiite,the 1921 Kilauea olivine tholeiite, and the 1801 Hualalai alkalibasalt, were studied at 5 kb water pressure, 680–1000°C,at the oxygen fugacities of the quartz-fayalite-magnetite (QFM)and hematite-magnetite (HM) buffers. In the range 680–850 °C, the crystalline assemblageon the QFM buffer is dominantly hornblende+ plagioclase, ±ilmenite, magnetite, sphene, fayalitic olivine, and phlogopiticmica. From 875 to 1000 °C the crystalline assemblage ishornblende+ olivine± augite+ ilmenite± magnetite.A melt phase is present from 700 to 1000 °C; a vapor phasewas present in all charges. The hornblendes formed on the QFM buffer range in compositionfrom common green hornblendes at low temperatures to kaersutitichornblendes at 1000 °C. A1(IV) and Ti increase temperature.AI(VI) passes through a maximum near 825 °C, decreasingboth above and below this temperature. AI(IV) is proportionalto the sum A1(VI)+2Ti. There is a positive linear correlationof approximately 3 : 1 between AI(IV) and the number of cationsin the A-site. The most likely explanation for this correlationat present is that the substitution of AI(VI) or Ti⁺⁴for a divalentcation creates local charge imbalances in the amphibole structurewhich can be compensated only by further A-site substitution.There also appears to be a correlation between the a-cell dimensionof hornblende and the A-site occupancy. Above a thresh holdvalue of approxmately 0.5 cations in A, a increases as A-siteoccupancy increases. Phase relations on the hematite-magnetite buffer are considerablysimpler. The hornblendes show relatively little change in compositionas temperature increases, and in the tholelitic compositionsbreak down at or below 970 °C 35–60 °C above thefirst appearance of augite±olivine. The melting of hornblendeis incongruent in all cases. The Fe-Ti oxides are pseudo-brookiteand titanohematite; at 1000 °C these oxides make up 10 percent by weight of the assemblage and contain most of the Tio₂and FeO in the charge. The patterns of hornblende variation observed in this studycompare closely with those reported in a wide range of experimentaland field data. The appearance of high-TiO₂ kaersutitic hornblendesin the tholeities at 1000° C, P_H2O= 5 kb on the QFM bufferimplies that the restricted occurence of kaersutite in nature(where it is associated only with mafic to intermediate alkalicrocks) is controlled by volatile content (H₂O_,F₂)rather thanby differences in condensed bulk composition.     

The Metastable Phase Equilibria of the MeX-MgCl_2-H_2O Systems

正1 Introduction The Pingluoba brine,which characterized as high concentration with sodium,potassium,boron,lithium,and rubidium,possess great development value.The main composition of the brine can be summarized to the     

Zoisite--Anorthite--Calcite Stability Relations in H2O--CO2 Fluids at 5000 Bars: An Experimental and SEM Study

The reaction 2 zoisite + CO₂ = 3 anorthite + calcite + H₂O hasbeen reversed experimentally in cold-seal pressure vessels usingnatural phases and H2O–C02 fluids generated by water-silveroxalate mixtures. Equilibrium has been determined at 5000 50bars, 599 9 °C and 0–075 ± 0–010 X_{CO2.Extrapolation using the MRK equation of Kerrick & Jacobs(1981) gives an equilibrium curve of negative T–X slopeconsistent with bracketing runs at 500, 550 and 650 °C.The curve agrees only with a new bracket of Nitsch (in Hoschek,1980), and is at higher XCo2} than all other experimental determinationsand at lower XCO₂ than those calculated from the thermodynamicdata of Helgeson et al. (1978). Discrepancies are attributedto differences in starting materials and small errors in thethermodynamic properties of the phases. Reaction direction and equilibrium have been determined by observingsurface textures of run products by SEM. Growth and solutiontextures are non-equivalent, permitting unequivocal determinationof reaction direction even where the extent of reaction is small,an advantage over conventional and insensitive XRD methods whichmeasure bulk changes in the charge. Dissolution features ofanorthite and zoisite are defect-related indicating controlby surface reaction, whereas calcite dissolves by both surfacereaction and diffusion controlled processes. Margarite forms in most runs below 585 °C. Textural features,its restriction to the margarite stability field and comparisonwith feldspar solubility data demonstrate it is an equilibriumphase formed by incongruent solubility of anorthite and zoisitein H₂O-CO₂ fluids. Quench phases formed from the solute areconsequently silica-rich, with implications for metasomaticprocesses in feldspar–epidote–bearing rock and fluidsystems. Absence of margarite from runs with anorthite, zoisiteand calcite in the zoisite stability field is apparently dueto the fast growth rate of zoisite. The full equilibrium assemblageis zoisite–anorthite–calcite–margarite atthese temperatures, and the degeneracy of the model system isunobtainable in experiments, and presumably, in nature.     

赤铁矿—水体系氧同位素分馏的实验研究

本文分别以3种不同的可溶性三价铁盐作为Fe~(3+)源物质的强迫水解法和以针铁矿和四方纤铁矿为起始物质的溶液转化法,在90～315℃范围内合成赤铁矿,测定了赤铁矿与水之间的氧同位素分馏。矿物合成实验和氧同位素分析结果显示,在90～225℃范围内,实验获得的赤铁矿与水之间氧同位素分馏为亚稳态分馏,并且不同合成实验方法得到不同的分馏关系,前者相对于后者富集(18)O约为2‰,即:10~31nα_(赤铁矿-水)=1.17±0.02×10~6/T~2-9.14±0.20(强迫水解法);10~31nα_(赤铁矿-水)=1.46±0.18×10~6/T~2-14.52±0.03(溶液转化法)。但温度在315℃以下,无论强迫水解法还是溶液转化法,在实验误差范围内实验测定的分馏值几乎不可区分,并且与增量方法的理论预测相近,表明该温度下获得的赤铁矿与水之间氧同位素分馏代表了赤铁矿-水体系氧同位素平衡分馏。此外,两种不同方法获得了不同的分馏关系,显示低温下赤铁矿-水体系氧同位素分馏不仅依赖于赤铁矿形成的温度,而且取决于赤铁矿的成因机制,因此对应于不同形成环境下的动力学亚稳态平衡,这对解释低温环境中赤铁矿的氧同位素数据具有重要意义。     

Phase Equilibria Constraints on Relations of Ore‐bearing Intrusions with Flood Basalts in the Panxi Region,Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-Pl-Ol-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basalts. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.     

P-T Conditions and the Influence of Graphite on Pelitic Phase Relations in the Ballachulish Aureole, Scotland

Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered a_H2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced a_H2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.     

Phase Equilibria for Calcic Scapolite, and Implications of Variable Al-Si Disorder for P-T, T-XCO2, and a-X Relations

Carbonate scapolite is a potentially powerful mineral for calculatingCO₂ activities in non-calcareous rocks, but an analysis of thethermodynamics and phase equilibria of carbonate scapolite isfirst necessary. This includes an evaluation of Al-Si disorderin meionite, as this has the greatest effect on derived phaserelations. Available experimental data on meionite stability,X-ray diffraction refinements and nuclear magnetic resonancespectra for calcic scapolite do not uniquely constrain the Al-Siordering state of synthetic meionite. However, the data aremost consistent with a high degree of Al-Si disorder and inconsistentwith long-range Al-Si order. An internally consistent thermodynamicdata set was derived and used to calculate P-T and T-X_CO2 equilibriainvolving meionite in the CaO-Al₂O₃-SiO₂-CO₂-H₂O (CASCH) system.The effect of Al-Si disorder is illustrated by calculating thephase equilibria using an ordered, an arbitrary intermediatedisordered, and a completely Al-Si disordered standard statefor meionite. The Gibbs free energy of meionite was calculatedfrom reversals (at 790–815?C, 2–15 kb) on the reaction 3 Anorthite +Calcite =Meionite The _fG?_{m, 298} for each of the standard states is –13 146?6,–13128?8, and –130930kJ/mol, respectively. Becauseof the steep slope of reaction (1) and limited temperature rangeover which it breaks down, meionite used in the experimentsto constrain reaction (1) must possess a limited range of Al-Sidisorder. The P-T slope of reaction (1) increases, and the slopeof meionite decarbonation equilibria changes from positive tonegative in T-X_CO2 and P-T space, as a function of increasingAl-Si disorder. Meionite has a wide stability field at highT in T-X space at 5 and 10 kb (P_Total=P_Fluid), being stableto X_CO2=0?06. Meionite alone breaks down to undersaturated gehleniteand/or corundum-bearing assemblages at 5 kb, and to clinozoisiteat 10 kb. The effect of solid solutions on the T-X stabilityof meionite is similar to that of increasing pressure, stabilizingmeionite to lower temperature. Variable Al-Si disorder doesnot significantly affect the upper limit of meionite stabilityin T-X_CO2 space. Activity-composition relations for meionitein carbonate scapolite were calculated relative to reaction(1) from data on natural scapolite-plagioclase-calcite assemblages.The extent of departure from ideality varies as a function ofAl-Si disorder. Negative deviations from ideality are indicatedfor natural scapolite solid solutions at T<750?C, based ona disordered Al-Si standard state for meionite. This is likelyto reflect a more ordered Al-Si distribution in natural scapolitescompared with the synthetic endmember standard state. Present address: Department of Earth and Space Sciences, State University of New York, Stony Brook, New York 11794-2100     

Experimental Study of Pyroxene Equilibria in the System CaO--MgO--FeO--SiO2

Subsolidus phase equilibria of pyroxenes in the CaO—MgO—FeO—SiO₂system have been experimentally determined at 30 and 15 kb,and mainly at 1200 °C. While phase relations at 30 kb and1200 °C are characterized only by orthopyroxene and clinopyroxene,at 15 kb and 1200 °C phase relations are complicated bythe appearance of the two-clinopyroxene miscibility gap. Thestability of Ca-free clinopyroxene was demonstrated at 15 kband 1200 °C and higher temperatures, and at 30 kb and 1450°C. The new experimental data were combined with the data from theliterature to yield a generalized phase equilibrium scheme ofpyroxenes in temperature-pressure-composition space. Thoughthe majority of naturally occuring pyroxenes can be understoodin the light of the scheme, there are a few samples not fullyconsistent with it. These are briefly mentioned.