Experimental study of liquid evolution in an Fe-rich,layered mafic intrusion: constraints of Fe-Ti oxide precipitation on the T-fO 2 and T-ϱ paths of tholeiitic magmas

The Newark Island layered intrusion, a composite intrusion displaying a similar fractionation sequence to the Skaergaard, has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber. This study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from QFM (quartz-fayalite-magnetite) to 0.5 log₁₀ units above IW (iron-wustite). These experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the Fe–Ti oxide minerals. Titanomagnetite saturates prior to ilmenite at QFM, but the order is reversed at lower oxygen fugacities. In the layered series of the Newark Island intrusion, ilmenite arrives shortly before titanomagnetite and the titanomagnetite/ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite. Comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to QFM before titanomagnetite saturation and decreased afterward, but always remained between the QFM and IW buffers. Similar trends in the modes of the Fe–Ti oxides (ilmenite and titanomagnetite) in the Skaergaard, Kiglapait, and Somerset Dam intrusions along with Fe₂O₃/FeO ratios in MORBs suggest that such a temperature-oxygen fugacity path may be typical of tholeiitic magma differentiation. Calculations of the temperature-density paths of the experimental liquids indicate that, at all possible oxygen fugacities, the density must have decreased abruptly after Fe–Ti oxide saturation. Accordingly, liquids replenishing the intrusion after Fe–Ti oxide saturation should pond at the bottom of the chamber, quenching against older cumulates. Field observation at the Newark Island intrusion confirm this prediction. The similarities in the fractionation paths of several other layered intrusions to that of the Newark Island intrusion suggest that the density of the liquids in these intrusions also decreased after Fe–Ti oxide saturation. Experiments on a suggested initial Skaergaard liquid are consistent with this model.     

Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid

 Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl₂Si₂O₈-CaFeSi₃O₈ and CaAl₂Si₂O₈-CaMgSi₃O₈. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe³⁺ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe³⁺/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of X _Fo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts X _Fo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature. Received: 27 March 1998 / Accepted: 30 September 1999     

Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pinciná village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An_20–55 plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, ±K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6–7% of total iron as Fe³⁺ in corundum-normative intergranular dacitic glass) and contains ilmenite with 8–10 mol% Fe₂O₃ and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/cm³) primary CO₂ inclusions with minor H₂, CH₄, H₂S, CO and N₂ (<2 mol% total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx–Ilm–Glass assemblage and fluid density correspond to 1,170±50°C, 5.6±0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25–27% of total iron as Fe³⁺ in diopside-normative intergranular glass of rhyolite–trachyte–dacite composition) and contains Fe₂O₃-rich ilmenite (17–29 mol%) associated with enstatite. Fluid inclusions are composed of CO₂–H₂O mixtures with up to 38 mol% H₂O. Raman spectroscopy revealed H₂S along with dominant CO₂ in the carbonic phase. Equilibrium PT parameters for the intergranular Opx–Ilm–Glass assemblage correspond to 740±15°C, 2.8±0.1 kbar, respectively. Reducing gas species (<2 mol% total) in the CO₂-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH₂ gradient imposed during decrease of redox potential from the log fO₂ values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO₂ values changed from ~QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. REE 300–320, Nb 55–57, Th 4–31, Zr 240–300 ppm, FeO_tot/MgO up to 11), and Sr–O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (⁸⁷Sr/⁸⁶Sr>0.705–0.706, ¹⁸O>9 V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10–11 km. Low water activity in the pincinite parental melt was caused by CO₂-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.     

The influence of primary magma composition,H2O and pressure on mid-ocean ridge basalt differentiation

MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al₂O₃ and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H₂O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H₂O and lower Al₂O₃ than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al₂O₃ and higher H₂O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al₂O₃ and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al₂O₃ is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe³⁺/Fe_total is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important.     

The assemblage orthopyroxene+cummingtonite+quartz. The low-temperature stability limit

The monovariant reaction Opx+H₂O Cum+Ol+Q and the Cum+Opx+Q stability field were studied under hydrothermal conditions at P _total=2940, 4900 bar and the oxygen fugacity of the QFM buffer. Under these conditions, the Opx lower stability brackets were 730°±10° and 740°±5° C, respectively. The kinetics of the reactions in the Cum+ Opx+Q mixture showed that there were only minor differences in the equilibrium compositions of the coexisting Opx and Cum over the 740°–780° C range. At T=780°, 760° and 740° C, the FeO/FeO+MgO ratio, in mol% was: Opx_52.5–Cum_49.5, Opx₆₂–Cum₅₇, Opx₇₂–Cum₆₆ (P=2940 bar) and Opx₆₂–Cum_58.5 Opx_71.5–Cum_66.5, Opx₈₀–Cum₇₅ (P= 4900 bar). The results are in good agreement with earlier studies in the Opx+Ol+Q and Cum+Ol+Q assemblages.Abbreviations Opx Orthopyroxene - Ol olivine - Cum cummingtonite - Mt magnetite - Q quartz - tk talc     

Graphite oxidation in the Apollo 17 orange glass magma: Implications for the generation of a lunar volcanic gas phase

An Apollo 17 picritic orange glass composition has been used to experimentally investigate the conditions at which graphite would oxidize to form a CO-rich gas, and ultimately produce lunar fire-fountain eruptions. Isothermal decompression experiments run above the A17 orange glass liquidus temperature (>1350 °C) suggest that the initial CO-rich gas phase produced by graphite oxidation would be generated during magma ascent at a pressure of 40 MPa, 8.5 km beneath the lunar surface. Additional experiments with 2000 ppm S and 1000 ppm Cl showed that the presence of these dissolved gas species would not affect the depth of graphite oxidation, verifying that the first volcanic gas phase would be generated by the oxidation of graphite.A simple ideal chemical mixing model for calculating melt FeO activity in a Fe-metal/silicate melt system was tested with a series of 0.1 MPa controlled oxygen fugacity experiments. Agreement between the model and experiments allows the model to be used to calculate oxygen fugacity in picritic lunar glass compositions such as the A17 orange glass. Using this model in a reanalysis of chemical equilibria between the natural A17 orange glass melt and the metal spherules (Fe₈₅Ni₁₄Co₁) trapped within the glass beads indicates a log oxygen fugacity of −11.2, 0.7 log units, more oxidized than previous estimates. At the A17 orange glass liquidus temperature (1350 ± 5 °C), this f_O2 corresponds to a minimum pressure of 41 MPa on the graphite–C–O surface. The fact that the same critical graphite oxidation pressure was determined in decompression experiments and from the Fe–FeO activity model for the natural A17 orange glass–metal assemblage strongly supports this pressure (8.5 km depth) for volcanic gas formation in lunar basalts. Generation of a gas by oxidation of C in ascending magma is likely to have been important in getting dense lunar magmas to the surface as well as in generating fire-fountain eruptions. The vesicles common in many lunar basalts and the ubiquitous Fe-metal in these rocks are also likely generated by the oxidation of carbon. The presence of carbon in the lunar basalts and the recent discovery of ppm levels of water in lunar basalts indicate that at least parts of the lunar interior still contained volatiles at 3.9 bybp.     

The effects of temperature,oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts

Chromite was equilibrated with two natural basic liquids and one natural ultrabasic liquid at temperatures and oxygen fugacities appropriate to geological conditions. The experiments were designed to document changes in mineral and glass compositions between the iron-wüstite and nickel-nickel oxide buffers, with special emphasis on conditions along quartz-fayalite-magnetite. The Cr contents of the melts at chromite saturation increase strongly with increasing temperature and with decreasing oxygen fugacity.A relationship is described which accounts for the compositional dependence of the partitioning of Cr between spinels and silicate melts by considering the exchange of FeCr₂O₄ component between the crystalline and melt phases. Interpretation of the data in terms of this exchange suggests that Cr³⁺ in metaluminous melts occurs in octahedrally coordinated sites, and that it does not depend on charge-balancing by monovalent cations. In this model, Cr³⁺ is proposed to behave like network-modifying Al³⁺ and Fe³⁺, i.e., the excess aluminum and ferric iron which do not participate in tetrahedrally coordinated matrix or network-forming complexes.The results can also be applied to the problem of the formation of massive chromitites of great lateral extent in basic layered intrusions. The data are consistent with a model in which the crystallization of chromite is initiated through magma mixing, in combination with the rapid heat loss associated with periodic influxes of magma into a chamber. An alternative model, in which chromite crystallization is initiated by repeated fluctuations in oxygen fugacity, is possible only if the magma f_O2 is not controlled by an oxygen buffer such as QFM.     

Zr-rich accessory minerals (titanite, perrierite, zirconolite, baddeleyite) record strong oxidation associated with magma mixing in the south Peruvian potassic province

The Oroscocha Quaternary volcano, in the Inner Arc Domain of the Andean Cordillera (southern Peru), emitted peraluminous rhyolites and trachydacites that entrained decimetric to millimetric lamprophyric blobs. These latter show kersantite modal compositions (equal proportion of groundmass plagioclase and K-feldspar) and potassic bulk-rock compositions (1<K₂O/Na₂O<2; 6.7–7.2 wt.% CaO). Kersantite blobs have shapes and microstructures consistent with an origin from a mixing process between mafic potassic melts and rhyolitic melts. Both melts did exchange their phenocrysts during the mixing process. In addition to index minerals of lamprophyres (Ba–Ti–phlogopite, F-rich apatite, andesine and Ca-rich sanidine), the groundmass of kersantite blobs displays essenite-rich diopside (up to 22 mol%), Ti-poor magnetite microlites, Ti-poor hematite microlites and a series of Ca–Ti–Zr- and REE-rich accessory minerals that have never been reported from lamprophyres. Titanite [up to 5.3 wt.% ZrO₂ and 5.2 wt.% (Y₂O₃ + REE₂O₃)] and Zr- and Ca-rich perrierite (up to 7.2 wt.% ZrO₂ and 10.8 wt.% CaO) predate LREE- and iron-rich zirconolite and Fe-, Ti-, Hf-, Nb- and Ce-rich baddeleyite (up to 5.3 wt.% Fe₂O₃, 3.2 wt.% TiO₂, 1.5 wt.% HfO₂, 1.2 wt.% Nb₂O₅, 0.25 wt.% CeO₂) in the crystallization order of the groundmass. Isomorphic substitutions suggest iron to occur as Fe³⁺ in all the accessory phases. This feature, the essenitic substitution in the clinopyroxene and the occurrence of hematite microlites, all indicate a drastic increase of the oxygen fugacity (from FMQ − 1 to FMQ + 5 log units) well above the HM synthetic buffer within a narrow temperature range (1100–1000 °C). Such a late-magmatic oxidation is ascribed to assimilation of water from the felsic melts during magma mixing, followed by rapid degassing and water dissociation during eruption of host felsic lavas. Thus, magma mixing involving felsic melt end-members provides a mechanism for mafic potassic melts to be oxidized beyond the HM synthetic buffer curve.     

Calcareous inclusions in lavas and agglomerates of Santorini volcano

Thermally metamorphosed and metasomatised fragments of basement actinolite-chlorite-calcite-quartz schists and quartz-bearing marbles are found as inclusions in Quaternary agglomerates and historic (197 B. C.—1950) dacitic lavas of Santorini volcano, Greece.Inclusions in agglomerates preserve the structure of parent schists in the alternation of bands rich in diopside or salite with bands rich in plagioclase. By contrast, inclusions in historic dacites are not banded. Most develop a thin zone of hybrid material at the contact with enclosing lava. The assemblage calcic clinopyroxene-wollastonite-plagioclase is commonly developed. The clinopyroxene is a Fe³⁺-rich salite or ferrosalite. Andradite-rich garnet and sphene are accessory minerals. Most examples carry interstitial siliceous glass of distinctive chemical composition, and several show minor olivine, augite, hypersthene and calcic plagioclase of magmatic origin.Other inclusions exhibit the assemblage anhydrite-calcic clinopyroxene, the latter mineral ranging widely in Al content. A single example has been observed to develop two distinct assemblages, the first coarsely crystalline melilite-wollastonite-magnetite, the second finely intergrown melilite-wollastonite-andraditic garnet (-xonotlite).Stability data for hedenbergite and andradite as constituents of skarn assemblages suggest that the clinopyroxene-rich assemblages of inclusions in historic dacites formed at temperatures near to or above 800° C and oxygen fugacity (fO₂) considerably greater than that which could be imposed upon the inclusions by dacite magma (T 900° C, fO₂10^–13 atm.). Thermal breakdown of original carbonates of the inclusions probably supplied the necessary oxygen. T-fO₂ data for the reaction 4 Magnetite+18 Wollastonite 6 Andradite indicate that the assemblage melilite-wollastonite-magnetite of the last inclusion described formed at higher T and/or lower fO₂ than the assemblage melilite-wollastonite-garnet. The latter assemblage undoubtedly formed during inclusion of the fragment by dacite magma, while metamorphism by a more basic, high temperature magma may have produced the former. Temperature data for reactions limiting the stability of melilite in the system CaO-Al₂O₃-SiO₂-H₂O indicate a minimum temperature of around 800° C for formation of both assemblages.     

Precambrian crustal components,plutonic associations,plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study

Chichi-jima, Bonin Islands, consists of dominant Eocene submarine volcanic rocks, comprising boninites, andesites and dacites, and subordinate sedimentary rocks. The dacites occur frequently in breccias and pillows overlying a boninite pillow lava sequence. The boninite pillows are intruded by a multiple dike, in which a core boninite is chilled against outer dacites. A density-stratified chamber may have been capped by a dacite magma. The dacites, which can be divided into quartz dacite and quartz-free dacite, are differentiates from the boninite-forming magmas, because they vary continuously in composition from boninites through andesites. The quartz dacites, corresponding to rhyolite in SiO₂, are lower in Na₂O and K₂O than most orogenic dacites. Some of the dacites are characterized by ferropigeonite (Wo_7–16En_23–39Fs_68-54) phenocrysts and are clearly ferrodacite, producing variable amounts of Fs-rich normative pyroxenes. The relation of SiO₂ to total FeO/MgO ratio indicates that many of both types of dacites, with glasses in boninites, are enriched in total FeO despite the strong calc-alkalic affinity of boninites. The crystallization temperature of ferropigeonite with Mg value 30 in a quartz dacite is estimated to be 900° C and that in a quartz-free dacite to be 1050° C, which are unusually high for differentiated silicic rocks. Some Chichi-jima rocks are fresh, having a low ratio of Fe₂O₃ to FeO. On the basis of the experimental study of magmatic ferric-ferrous equilibria at 1 bar, the oxygen fugacities are calculated as 10^–13.6 bars at 900° C for a ferropigeonite quartz dacite and 10^–8.9 bars at 1200° C for a boninite with the lowest Fe³⁺/Fe²⁺. Both values lie below the quartz-fayalite-magnetite buffer line. The boninite series volcanic rocks have preserved low oxygen fugacities as well as high temperatures until the latest differentiation stage. The ferropigeonite phenocrysts have crystallized from the dacite magmas under the conditions of moderately high temperatures, very low oxygen fugacities and high total FeO and SiO₂ concentrations.     

新疆西天山备战基性-超基性岩矿物地球化学研究及其对铁成矿作用的制约

文章对备战铁矿区内的基性-超基性岩、中基性岩脉、矿体围岩和铁矿石中的辉石、橄榄石、金云母、铁钛氧化物进行了电子探针分析与显微特征研究。金云母辉石橄榄岩中辉石的化学成分在w(SiO₂)-w(Al₂O₃)图中均落于亚碱性系列区域,在w(Al₂O₃)-w(Na₂O)-w(TiO₂)图中,辉石主要落在拉斑玄武岩系列区域,表明该区岩浆经历了拉斑玄武岩系列演化。备战金云母辉石橄榄岩中橄榄石的w(FeO)较低,介于19.22%～23.79%,w(MgO)较高,介于37.35%～41.30%,Fo变化介于0.74～0.79,属于贵橄榄石。橄榄石中较低的w(FeO)表明其形成于较高的氧逸度环境,而岩浆的拉斑玄武岩系列演化一般发生于低氧逸度条件,综合分析岩浆经历了从低氧逸度到高氧逸度变化的过程。随着岩浆的演化,橄榄石Fo与w(Ni)由负相关变为正相关又变为负相关关系,其中负相关关系表明在岩浆演化过程中橄榄石与粒间硫化物熔浆发生过Ni-Fe交换反应,从另一...     

Enrichment of Rh,Ru, Ir and Os in Cr spinels from oxidized magmas: Evidence from the Ambae volcano,Vanuatu

Experimental studies, performed under oxidized conditions (fO₂ > QFM + 2, where QFM is quartz–fayalite–magnetite oxygen buffer), have shown that Rh, Ru, Ir and Os are strongly compatible with Cr spinel, whereas empirical studies of Cr spinels from ultramafic–mafic rocks suggest that the experimental results may overestimate the partition coefficients. We report laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of platinum-group elements (PGE), Au and Re abundances in Cr spinels from the Ambae volcano, Vanuatu (fO₂ = QFM + 2.5), the Jimberlana layered intrusion, western Australia, and the Bushveld complex, South Africa (fO₂ ～ QFM). The results show that Rh and IPGEs (Iridium-group PGE; Ru, Ir, Os) partition strongly into the Cr spinels that crystallized from the oxidized Ambae lavas whereas most of the Cr spinels from the more reduced Jimberlana layered intrusion and the Bushveld complex contain no detectable PGE, Au or Re, with exception of ～10 ppb of Ir in some Jimberlana Cr spinels. In the Ambae Cr spinels, Rh, Ru and, to lesser extent Os, are positively correlated with Fe³⁺, Ni and V. The homogeneous distribution of Rh and IPGEs in LA-ICP-MS time-resolved spectra indicates that these elements are in solid solution in Cr spinels. Pt–Fe alloys occur as inclusions within the Ambae Cr spinels, which indicate that the Ambae melt was saturated with Pt.Our results show that partitioning of Rh, Ru and Ir into Cr spinels increases with increasing oxygen fugacity, which suggests that the high concentrations of these elements in the Ambae Cr spinels are due to the high oxygen fugacity of the host magma. Therefore, Cr spinels may play an important role in controlling the concentrations of Rh and IPGEs during fractional crystallization of oxidized ultramafic–mafic magmas and during partial melting of oxidized arc mantle.     

Fe-Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas

A new thermodynamic formulation of the Fe–Ti oxide geothermometer/oxygen barometer is developed. The method is based upon recently calibrated models for spinel solid solutions in the quinary system (Fe²⁺, Mg)(Al,Fe³⁺,Cr)₂O₄–(Fe²⁺, Mg)₂TiO₄ by Sack and Ghiorso, and rhombohedral oxides in the quaternary system (Fe²⁺,Mg,Mn)TiO₃–Fe₂O₃ (this paper). The formulation is internally consistent with thermodynamic models for (Fe²⁺,Mg)-olivine and -orthopyroxene solid solutions and end-member thermodynamic properties tabulated by Berman. The constituent expressions account for compositional and temperature dependent cation ordering and reproduce miscibility gap features in all of the component binaries. The calibration does not account for the excess Gibbs energy resulting from compositional and temperature dependent magnetic ordering in either phase. This limits application of the method to assemblages that equilibrated at temperatures above 600° C. Practical implementation of the proposed geothermometer/oxygen barometer requires minimal use of projection algorthms in accommodating compositions of naturally occurring phases. The new formulation is applied to the estimation of temperature and oxygen fugacity in a wide variety of intermediate to silicic volcanic rocks. In combination with previous work on olivine and orthopyroxene thermodynamics, equilibration pressures are computed for a subset of these volcanics that contain the assemblage quartz, oxides and either ferromagnesian silicate. The calculated log₁₀ f _{O 2}-T relations are reflected in coexisting ferromagnesian mineral assemblages. Volcanics with the lowest relative oxygen fugacity (log₁₀ f _{O 2}) are characterized by the assemblage olivine-quartz, those with slightly higher log₁₀ f _{O 2} s, by the assemblage orthopyroxene-quartz. The sequence proceeds with the necessary phases biotite-feldspar, then hornblende-quartz-clinopyroxene, and finally at the highest log₁₀ f _{O 2} s, sphene-quartz-clinopyroxene. Quantitative analysis of these trends, utilizing thermodynamic data for the constituent phases, establishes that, in most cases, the T-log₁₀ f _{O 2}value computed from the oxides is consistent with the compositions of coexisting silicate phases, indicating that phenocryst equilibrium was achieved prior to eruption. There is, however, considerable evidence of oxide-silicate disequilibrium in samples collected from more slowly cooled domes and obsidians. In addition, T-log₁₀ f _{O 2}trends from volcanic rocks that contain biotite and orthopyroxene are interpreted to imply a condition of Fe²⁺–Mg exchange disequilibrium between orthopyroxene and coexisting ferromagnesian silicates and melt. It is suspected that many biotite-feldspar-quartz-orthopyroxene bearing low temperature volcanic rocks inherit orthopyroxene xenocrysts which crystallized earlier in the cooling history of the magma body.The problem is probably at least as complex as that of the feldspars... A.F. Buddington (1956)     

Textural evolution and metamorphism of pillow basalts from the Franciscan Complex,western Marin County,California

Petrogenesis of Franciscan pillow basalts from the Franciscan Complex of western Marin County California entails both dynamic crystallization of tholeiitic magma and subsequent low-temperature metamorphism. Brittle deformation during tectonic emplacement of pillow basalts into a chert greywacke terrain is manifested by the shearing of interpillow matrix and polishing of pillow rims, but the igneous textures within pillows are well preserved.The cooling history of pillow basalts can be understood through analysis of morphologic variations of primary olivine and plagioclase from rim to core of the pillow. Crystal sizes and plagioclase dendrite spacings are consisted with a cooling rate which generally decreases inward. Some pillows show a marked asymmetry in plagioclase and olivine morphology suggesting lower cooling rates caused by asymmetric cooling of the pillows. Olivine morphologies, primarily hopper and chain forms, are consistent with cooling rates of 2–10 °C/h for pillow cores and 50–75 °C/h for pillow rims.Low temperature hydrothermal alteration has produced secondary minerals indicative of zeolite facies conditions. Pillow matrix is either chloritic or zeolitic (in part laumontized). Pillow rims display incomplete replacement of calcic palagonite by pumpellyite (Fe₂O₃=9–21 wt%), prehnite (Fe₂O₃=5–7 wt%), sphene and quartz. Metamorphism of pillow interiors, manifested by: (1) veins of quartz, pumpellyite, calcite, or harmotome (BaO=15 wt%); (2) amygdules containing analcime, chlorite or quartz; and (3) replacement of olivine by pumpellyite or smectite/illite, of plagioclase by albite (An3)+sericite, and of glassy groundmass by fine-grained chlorite. Primary augite (Wo3₃₉En₁₃Fs₄₈) was not altered. The described paragenesis may be attributed to oceanfloor and/or Franciscan-type metamorphism.     

A hydrothermal investigation of the system FeOFe2O3TiO2: A discussion with new data

New experimental data on the system FeOFe₂O₂TiO₂ using a new oxygen buffer are presented. It is found that there is a narrow range of oxygen fugacity above 550°C within which hematite and ilmenite coexist stably.     

Chromite deposits in the northern Oman ophiolite: Mineralogical constraints

Chromite deposits in the northern Oman ophiolitic complex occur in three structural contexts, i.e., (1) at the base of the cumulate series, (2) in the top kilometer of the mantle sequence, and (3) in the deeper parts of the mantle. Types 1 and 2 are characterized by the diversity of interstitial silicates where in decreasing order of abundance olivine, clinopyroxene, orthopyroxene, plagioclase, and amphibole occur, as opposed to type 3 which contains only olivine. They differ however in ore texture. Similar silicates also occur as euhedral inclusions in chromite crystals, but their proportions are reversed. The composition of the interstitial silicates is comparable to that found in early cumulates. Type-1 and type-2 chromite deposits crystallized from a magma similar to that from which the basal cumulates formed (Al₂O₃, 15.1–16.1 wt%; FeO/MgO, 0.55–0.60). The type-3 chromites were derived from a magma of much lower Al₂O₃ content (12.5 wt%). It is considered that they belong to an older episode in the magmatic evolution of the complex.     

Ferric iron partitioning between plagioclase and silicate liquid: thermodynamics and petrological applications

A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO₂-Al₂O₃-Fe₂O₃-FeO-MgO-CaO-Na₂O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe³⁺,Al³⁺)Si₂O₈-Ca(Fe²⁺,Mg)Si₃O₈ system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeO^t, MgO content and Mg/(Fe^t+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ǂ.34 wt% (relative error =9%) and ǂ.08 wt% (14%) and ǂ.7 (8%) respectively. Calculated Fe³⁺-Al exchange chemical potentials of plagioclase, m_{Fe^{3 +} ( Al )- 1} ^Pl{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} agree with those calculated using reported thermodynamic models for multicomponent spinel, m_{Fe^{3 +} ( Al )- 1} ^Sp{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} and clinopyroxene, m_{Fe^{3 +} ( Al )- 1} ^Cpx{\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} . The FeO^t content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe³⁺/(Fe³⁺+Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase-liquid oxygen barometer are scattered, this study found that plagioclase-spinel-clinopyroxene-oxygen and plagioclase-olivine-oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and f_O2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.     

Ferric-ferrous equilibria in natural silicate liquids at 1 bar

Ferric and ferrous iron concentrations have been measured in 57 silicate liquids equilibrated at temperatures (1,200°–1,330°C) above the liquidus and at oxygen fugacities close to those defined by quartz-fayalite-magnetite. The experimental results reported here span virtually the entire known compositional range of lavas. An empirical equation relating the mole fraction of Fe₂O₃ and FeO to oxygen fugacity, absolute temperature and liquid composition at 1 bar has been formulated, based on the present experimental results and published data. Extrapolating the proposed empirical relationship over several hundred degrees, for instance below the solidus of the 1965 Makaopuhi tholeiitic lava lake, yields calculated oxygen fugacities which are a little lower than those measured directly in drill holes; at 1,100°C this discrepancy is 0.4 log unit. However, the agreement between the oxygen fugacities calculated from our empirical equation and those indicated by the composition of Fe-Ti oxides in andesites and siliceous obsidians is much closer.     

Laser Ablation ICPMS study of trace element partitioning between plagioclase and basaltic melts: an experimental approach

Plagioclase-melt partition coefficients (D) for 34 trace elements at natural concentration levels were determined experimentally in a natural MORB composition at atmospheric pressure using thin Pt-wire loops. Experiments were carried out at three temperatures (1,220, 1,200, and 1,180°C), and at three different oxygen fugacities (fO₂ = IW, QFM, air) in order to assess the effect of fO₂ on the partitioning of elements with multiple valence (Fe, Eu, Cr). Run products were analyzed by laser-ablation ICP-MS. Most trace element Ds increase slightly as temperature decreases, except for D _Zr, D _Fe, D _Eu and D _Cr that vary systematically with fO₂. Applying the Lattice Strain Model to our data suggests the presence of Fe²⁺ entirely in the octahedral site at highly to moderate reducing conditions, while Fe³⁺ was assigned wholly to the tetrahedral site of the plagioclase structure. Furthermore, we provide a new quantitative framework for understanding the partitioning behaviour of Eu, which occurs as both 2+ and 3+ cations, depending on fO₂and confirm the greater compatibility of Eu²⁺, which has an ionic radius similar to Sr, relative to Eu³⁺ in plagioclase and the higher Eu²⁺/ Eu³⁺ under reducing conditions. For petrogenetic basaltic processes, a combined fractionation of Eu²⁺–Sr and Fe–Mg by plagioclase has considerable potential as an oxybarometer for natural magmatic rocks.