Showing papers in "Contributions to Mineralogy and Petrology in 1991"

The compressibility of silicate liquids containing Fe 2 O 3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states

Abstract: Ultrasonic longitudinal acoustic velocities in oxidized silicate liquids indicate that the pressure derivative of the partial-molar volume of Fe2O3 is the same in iron-rich alkali-, alkaline earth- and natural silicate melt compositions at 1 bar. The dV/dP for multicomponent silicate liquids can be expressed as a linear combination of partial-molar constants plus a positive excess term for Na2O−Al2O3 mixing. Partial-molar properties for FeO and Fe2O3 components allow extension of the empirical expression of Sack et al. (1980) to permit the calculation of Fe-redox equilibrium in a natural silicate liquid as a function of composition, temperature, fo2 and pressure; a more formal thermodynamic expression is presented in the Appendix. The predicted equilibrium fo2 of natural silicate melts, of fixed oxygen content, closely parallels that defined by the metastable assemblage fayalite+magnetite+β-quartz (FMQ), in pressure-temperature space. A silicate melt initially equilibrated at 3 GPa and FMQ, will remain within approximately 0.5 log10 units of FMQ during its closed-system ascent. Thus, for magmas closed to oxygen, iron-redox equilibrium in crystal-poor pristine glassy lavas represents an excellent probe of the relative oxidation state of their source regions.

Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites

Abstract: Peraluminous granitoid magmas are a characteristic product of ultrametamorphism leading to anatexis of aluminous metasedimentary rocks in the continental crust. The mechanisms and characteristic length-scales over which these magmas can be mobilized depend strongly on their melt fraction, because of their high viscosities. Thus, it is of fundamental importance to understand the controls exerted by pressure, temperature and bulk composition of the source material on melt productivity. We have studied experimentally the vapour-absent melting behaviour of a natural metapelitic rock and our results differ greatly from those of previous experimental and theoretical investigations of melt productivity from metamorphic rocks. Under H2O-undersaturated conditions, bulk composition of the source material is the overriding factor controlling melt fraction at temperatures on the order of 850–900° C. Granitoid melts formed in this temperature interval by the peritectic dehydration-melting reaction: $$\begin{gathered} Biotite + plagioclase + aluminosilicate + quartz \hfill \\ = melt + garnet \hfill \\ \end{gathered} $$ have a restricted compositional range. As a consequence, melt fractions will be maximized from protoliths whose modes coincide with the stoichiometry of the melting reaction. This “optimum mode” (approximately 38% biotite, 32% quartz, 22% plagioclase and 8% aluminosilicate) reflects the fact that generation of low-temperature granitoid liquids requires both fusible quartzo-feldspathic components and H2O (from hydrous minerals). Metapelitic rocks rich in mica and aluminosilicate and poor in plagioclase contain an excess of refractory material (Al2O3, FeO, MgO) with low solubility in low-temperature silicic melts, and will therefore be poor magma sources. Melt fraction varies inversely with pressure in the range 7–13 kbar, but the effect is not strong: the decrease (at constant temperature) over this pressure range is of at most 15 vol% (absolute). The liquids produced in our experiments are silicarich (68–73 wt% SiO2), strongly peraluminous (2–5 wt% normative corundum) and very felsic (MgO+FeO* +TiO2 less than 3 wt%, even at temperatures above 1000° C). The last observation suggests that peraluminous granitoids with more than 10% mafic minerals (biotite, cordierite, garnet) contain some entrained restite. Furthermore, because liquids are also remarkably constant in composition, we believe that restite separation is more important than fractional crystallization in controlling the variability within and among peraluminous granitoids. We present liquidus phase diagrams that allow us to follow the phase relationships of melting of silica-and alumina-saturated rocks at pressures corresponding to the mid- to deep-continental crust. Garnet, aluminosilicate, quartz and ilmenite are the predominant restitic phases at temperatures of about 900° C, but Ti-rich biotite or calcic plagioclase can also be present, depending on the bulk composition of the protolith. At temperatures above 950–1050° C (depending on the pressure) the restitic assemblage is: hercynitic spinel+ilmenite+quartz±aluminosilicate. Our results therefore support the concept that aluminous granulites (garnet-spinel-plagioclase-aluminosilicate-quartz) can be the refractory residuum of anatectic events.

High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle

Abstract: Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under controlled oxygen fugacity (fO2). fO2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO2-C, Ni−NiO, and Fe3O4−Fe2O3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe3+/ΣFe in spinel is a linear function of fO2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog(fO2), but decreases slightly with increasing Cr content in spinel. The Fe3+/ΣFe ratio falls with increasing pressure at given Δlog(fO2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing fO2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from the fayalite-magnetite-quartz (FMQ) buffer: $$\Delta log (f_{O_2 } )^{FMQ} = 0.27 + 2505/T - 400P/T - 6 log(X_{Fe}^{olv} ) - 3200(1 - X_{Fe}^{olv} )^2 /T + 2 log(X_{Fe^{2 + } }^{sp} ) + 4 log(X_{Fe^{3 + } }^{sp} ) + 2630(X_{Al}^{sp} )^2 /T.$$

Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions

Abstract: A large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase. As a consequence, H2O-undersaturated or fluid-absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events. In an attempt to determine the systematics of melting under fluid-absent conditions, a series of piston-cylinder experiments was conducted on two natural amphibolites; one, a meta-alkali basalt (ABA) with a total water content of ∼ wt% contained in hornblende, and the other, a meta-island-arc tholeiite (IAT) which has ∼1–1.3 wt% water contained in hornblende, cummingtonite and biotite. The experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures, and sta temperature interval over which amphibole was stable, were controlled by the amphibolites' different bulk compositions and their resulting metamorphic assemblages. The volume % of melt produced by melting of the two amphibolites were compared with estimated amounts, based on Burnham's (1979) water-melt solubility model and the fluid-absent melting model presented by Clemens and Vielzeuf (1987). The observed melt volumes were greater than estimated. As the water content of melt largely detemines the volume % of melt produced, independent measurements of the water-content of the glass formed during partial melting in the ABA were made by thermogravimetric analyses. The water content of the ABA glass is ∼2 wt%, which is less than the assumed “melt-water” content (water content of the melt) used in previous modeling of fluidabsent anatexis in mafic lithologies. As a consequence, more melt can be expected during fluid-absent partial melting of mafic lower crust, as is observed in the experiments. A modification of the Clemens and Vielzeuf (1987) fluid-absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5, 8 and 10 kbar. Tectonic scenarios in which the crust is thickened (i.e. by collision) then undergoes extension or where a previously thinned crust is later rethickened, provide enough heat so that amphibolite melting under fluid-absent conditions can become importan and hence responsible for some melts produced during post-collisional magmatism. The results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains.

The redox states of basic and silicic magmas: a reflection of their source regions?

Abstract: At present the best estimates of the oxygen fugacity of spinel-lherzolites that could be the source material of basic magmas is about five log units below the Ni−NiO buffer to one above it. However partially glassy basic lavas, ranging from MORBs to minettes, all with olivine on their liquidus, cover a wider range, and may have oxygen fugacities that extend to four log units above NNO. Surprisingly the range of oxygen fugacities observed in silicic lavas and ashflows with quartz phenocrysts is smaller, despite a crustal dominated evolution. The peralkaline silicic lava type pantellerite is the most reduced, equivalent to MORBs, whereas the large volume ashflows with phenocrysts of hornblende and/or sphene are the most oxidised. As the concentration of water in the basic lavas is correlated with increase in their redox state, it would seem that water could be the agent of this increase. That this is unlikely is seen in the behavior of silicic ashflows and lavas, particularly those of Yellowstone. Here the silicic magmas of the last 2Ma contain about 2 wt% FeO(total), and typically phenocrysts of fayalite, quartz and Fe−Ti oxides. Despite extensive exchange of the 18O of the magma with meteoric water after caldera collapse (Hildreth et al. 1984), there is no displacement of the redox equilibria. Thus the thermal dissociation of molecular H2O to H2, and its subsequent diffusive loss to cause oxidation must have been minimal. This could only be so if the activity of water was small, as it would be if H2O reacted with the silicate liquid to form OH groups (Stolper 1982). The conclusion is that silicic magmas with small amounts of iron and large amounts of water do not have their redox states reset, which in turn presumably reflect their generation. By analogy basic magmas with large amounts of iron and far less water are even less likely to have their redox equilibria disturbed, so that their oxygen fugacities will also reflect their source regions. The effect of pressure on the ferric-ferrous equilibrium in basic magmas can be calculated from experimental measurements of the partial molar volumes of FeO and Fe2O3, and their pressure derivatives ϖV/ϖP, in silicate liquids. The effect of pressure is found to be about the same on the liquid as it is for various solid oxygen buffers. Accordingly there should be mantle source regions covering the same range of oxygen fugacity as that found in basic lavas, but so far samples of spinel-lherzolite of equivalent oxygen fugacity to minettes or other potassic lavas have not been found. Whether or not the redox state of phlogopite-pyroxenites is equivalent to these potassic lavas cannot be established without experiment.

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

Abstract: 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 log10fO2-T relations are reflected in coexisting ferromagnesian mineral assemblages. Volcanics with the lowest relative oxygen fugacity (Δlog10fO2) are characterized by the assemblage olivine-quartz, those with slightly higher Δ log10fO2 s, by the assemblage orthopyroxene-quartz. The sequence proceeds with the necessary phases biotite-feldspar, then hornblende-quartz-clinopyroxene, and finally at the highest Δ log10fO2 s, sphene-quartz-clinopyroxene. Quantitative analysis of these trends, utilizing thermodynamic data for the constituent phases, establishes that, in most cases, the T-log10fO2value 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-log10fO2trends 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.

Partitioning of Cu, Sn, Mo, W, U, and Th between melt and aqueous fluid in the systems haplogranite-H2O−HCl and haplogranite-H2O−HF

Abstract: The partition coefficients KD=cfluid/cmelt of Cu, Sn, Mo, W, U, and Th between aqueous fluid and melt were measured in the systems haplogranite-H2O−HCl and haplogranite-H2O−HF at 2kbars, 750°C, and Ni−NiO buffer conditions using rapid-quench cold seal bombs, with many reversed runs. Concentrations of trace elements (1–1000 ppm) in the quenched aqueous fluid and in the glass were determined by plasma emission spectrometry (DCP). KD of F is close to 1 in the system studied. KD of Cu and Sn strongly increases with increasing Cl concentration due to the formation of chloride complexes in the aqueous fluid, while HF has no effect. However, in 2M HCl, KD of Cu approaches 100, while KD of Sn is below 0.1 under the same conditions. The partition coefficients of Mo and W are high if water is the only volatile present (Mo: 5.5, W: 3.5), but strongly decrease with increasing HCl and HF, due to the destabilization of hydroxy complexes. KD of U and Th is very low in the absence of complexing agents, but strongly increases with increasing HF concentration. KD of U also increases with increasing HCl concentration and with increasing CO2 concentration in the system haplogranite-H2O−CO2, indicating the stability of chloride and carbonate complexes of U at magmatic temperatures. The data suggest a stoichiometric ratio of Cl: U=3:1 and of F:U=2:1 in these complexes. Cl-rich fluids are responsible for the formation of porphyry Cu deposits, but are much less effective in the transport of Sn. F appears not to be essential for the concentration of Mo and W in fluids evolving from a granitic magma. The different complexing behavior of U and Th in aqueous fluids may account for their fractionation during magma genesis.

Trace-element-rich brines in eclogitic veins: implications for fluid composition and transport during subduction

Abstract: Primary and pseudosecondary fluid inclusions occur in oscillatory-and sector-zoned omphacite in eclogitic veins from the Monviso ophiolitic complex in the Western Alps. The inclusions contain aqueous brines and daughter crystals of halite, sylvite, calcite, dolomite, albite, anhydrite and/or gypsum, barite, baddeleyite, rutile, sphene, Fe oxides, pyrite and monazite. This daughter mineral suite indicates high solubilites of Na, K, Ca, Mg, Fe, Zr, Ti, P, Ba, Ce, La, Th, and S species and provides direct evidence for transport of high-fieldstrenght, large-ion-lithophile, and light-rare-earth elements as dissolved species during subduction. Fluid-inclusion heterogeneities preserved within and between adjacent grains in the veins, however, suggest that the scale of fluid equilibration was small. A crack-seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins. From these observations, we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure; the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow. Vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites, Small-scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow, and hence limited metasomatism. We conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase. At Monviso, despite evidence for high trace element solubilities in saline brines, the elements were not removed from the downgoing slab prior to teaching depths of ∼40 km.

Solubility of Ca and Al in orthopyroxene from spinel peridotite: an improved version of an empirical geothermometer

Abstract: Geothermometric equations for spinel peridotites by Fujii (1976), Gasparik and Newton (1984), and Chatterjee, and Terhart (1985) based on the reaction enstatite (en)+spinel (sp)→Mg−Tschermaks (mats)+forsterite (fo) were tested using a nearly isothermal suite of mantle xenoliths from the Eifel, West Germany. In spite of using activities of MgAl2O4, en, and mats to allow for the non-ideal solution behaviour of the constituent phases, temperatures calculated from these equations systematically change as a function of Cr/(Cr+AL+Fe3+) in spinel. We propose an improved version of the empirical geothermometer for spinel peridotites of Sachtleben and Seck (1981) derived from the evaluation of the solubilities of Ca and Al in orthopyroxene from more than 100 spinel peridotites from the Rhenish Volcanic Province. A least squares regression yielded a smooth correlation between $${\text{(}}X_{{\text{Ca}}} {\text{)}}^{{\text{opx}}} {\text{ and 1n K}}_{\text{D}} = 1n\frac{{(x_{{\text{Fo}}}^{{\text{Ol}}} )^2 {\text{ * }}(x_{{\text{Al}}}^{{\text{Ml}}} )^{opx} }}{{(x_{{\text{Al}}}^{{\text{Sp}}} )^2 {\text{ * }}(x_{{\text{Mg}}}^{{\text{Sp}}} )^2 {\text{ * }}(x_{{\text{Mg}}}^{{\text{M1}}} )^{opx} }}$$ as measure of Al in orthopyroxene, if the mole fraction of (Mg,Fe)Cr2O4 [Y Sp Cr ] in spinel is allowed for. Fitting (X Al)opx and (X Cr)opx or ln KD to temperatures derived from the new thermometer of Brey and Koehler (1990) based on (X Ca)opx leads to the following equations: $$\begin{gathered} T{\text{(}}^\circ {\text{C)}} = 2248.25 + 991.58*1n{\text{ K}}_{\text{D}} + 153.32*(1n{\text{ K}}_{\text{D}} )^2 + 539.05*y_{Cr}^{Sp} \hfill \\ {\text{ }} - 2005.74*(y_{Cr}^{Sp} )^2 \hfill \\ T{\text{(}}^\circ {\text{C)}} = 636.54 + 2088.21*x_{{\text{Al}}}^{{\text{M1}}} + 14527.32*x_{{\text{Cr}}}^{{\text{M1}}} \hfill \\ \end{gathered}$$ The last of these is more suitable for practical use but is applicable only over a limited range of (X Al)M1 and (X Cr)M1 in orthopyroxene. The lack of steady-state equilibrium conditions due to young thermal perturbations or fast cooling may lead to discrepancies of temperatures calculated from (X Ca)opx and (X Al)opx. Spinel peridotite xenoliths from Central Asia are shown to demonstrate this.

A Compensated-Redlich-Kwong (CORK) equation for volumes and fugacities of CO 2 and H 2 O in the range 1 bar to 50 kbar and 100 1600°C

Abstract: We present a simple virial-type extension to the modified Redlich-Kwong (MRK) equation for calculation of the volumes and fugacities of H2O and CO2 over the pressure range 0.001–50 kbar and 100 to 1400°C (H2O) and 100 to 1600°C (CO2). This extension has been designed to: (a) compensate for the tendency of the MRK equation to overestimate volumes at high pressures, and (b) accommodate the volume behaviour of coexisting gas and liquid phases along the saturation curve. The equation developed for CO2 may be used to derive volumes and fugacities of CO, H2, CH4, N2, O2 and other gases which conform to the corresponding states principle. For H2O the measured volumes of Burnham et al. are significantly higher in the range 4–10 kbar than those presented by other workers. For CO2 the volume behaviour at high pressures derived from published MRK equations are very different (larger volumes, steeper (∂P/∂T)V, and hence larger fugacities) from the virial-type equations of Saxena and Fei. Our CORK equation for CO2 yields fugacities which are in closer agreement with the available high pressure experimental decarbonation reactions.

The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps ; Detailed petrography, mineral chemistry and PT-path

Abstract: Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.

Beginning of melting and composition of first melts in the system Qz-Ab-Or-H 2 O-CO 2

Abstract: Solidus temperatures have been determined for minimum melt compositions in the system Qz(SiO2)-Ab(NaAlSi3O8)-Or(KAlSi3O8) at P(fluid)=2,5 and 10 kbar and at various water activities. The dry solidus was investigated in a dry argon environment. Water activities (aH2O) between 0.0 and 1.0 were obtained by using H2O-CO2 mixtures. The Or/Ab+Or ratio of first melts increases considerably with decreasing water activity. At 10 kbar it is 0.28 in the water-saturated system and 0.56 at water activity 0.1. The Qz-content does not change with changing water activities. The Ab-content of minimum melts formed at high pressures and low aH2O may remain almost constant in ascending magmas that are cooling and crystallizing. Qz-content increases at the expense of the Or-component. Solidus temperatures decrease considerably when aH2O increases slightly from zero. At 10 kbar, the temperature difference between dry melting and the solidus for aH2O=0.1 is 120°C. The influence of pure CO2 on the solidus is very limited in the investigated P-T range. The solidus is approximatively 760°C at aH2O=0.5 between 2 and 10 kbar and approximatively 830°C at aH2O=0.3. This means that melting of quartz-feldspar assemblages may induce dehydration reactions at P-T conditions of the granulite facies.

Partition coefficients for olivine-melt and orthopyroxene-melt systems

Abstract: Thermodynamic analysis shows that olivinemelt and orthopyroxene-melt partition coefficients for many elements should be approximately linear functions of DMg. These simple relationships can be combined with the constraint of mineral stoichiometry to allow the direct calculation of partition coefficients for these elements if the major element chemistry of the melt phase is known. A large dataset of published and unpublished experimental mineral-melt pairs for compositions in the range komatiite to andesite has allowed the determination of the empirical constants required for this calculation. The precision of these parameterisations is demonstrated by comparing the values calculated with those observed. Comparison of phenocryst-matrix partition coefficients with those measured from experimental mineral-melt pairs demonstrates that experimentally determined partition coefficients are equivalent to those in magmatic processes. There are therefore no significant kinetic factors precluding magmatic partitioning being reproduced on an experimental timescale. The model provides a set of simple tests for equilibrium and enables the chemical evolution of a magma fractionating olivine or orthopyroxene to be modelled. An empirical equation for distinguishing orthopyroxene from other low-Ca pyroxenes in chemical analyses of experimental runs is also presented.

High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano, northern California

Abstract: Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K2O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.

An internally consistent model for the thermodynamic properties of Fe−Mg-titanomagnetite-aluminate spinels

Abstract: A model is developed for the thermodynamic properties of Fe2+−Mg2+-aluminate-titanate-ferrite spinels of space group Fd3m. The model incorporates an expression for the configurational entropy of mixing which accounts for long-range order over tetrahedral and octahedral sites. Short-range order or departures from cubic symmetry are not considered. The non-configurational Gibbs energy is formulated as a second degree Taylor expansion in six linearly independent composition and ordering variables. The model parameters are calibrated to reproduce miscibility gap constraints, order-disorder phenomena in MgAl2O4 and MgFe2O4, and Fe2+−Mg2+ partitioning data between olivine and: (1) aluminate spinels; (2) ferrite spinels; (3) titanate spinels; (4) mixed aluminate-ferrite spinels. This calibration is achieved without invoking non-configurational excess entropies of mixing. The model predicts that the ordering state of FeAl2O4 is more normal than that of MgAl2O4. It also successfully accounts for heat of solution measurements and activity-composition relations in the constituent binaries. Phase equilibrium constraints require that the structure of Fe3O4 is more inverse than random at all temperatures and that Mg2+ has a strong tetrahedral site preference with respect to that of Fe2+. The analysis suggests that in the titanates short range order on octahedral sites may be significant at temperatures as high as 1300° C. Constraints developed from calibrating the thermodynamic properties of Fe2+−Mg2+-aluminatetitanate-ferrite spinel solid solutions permit extension of the database of Berman (1988) to include estimates of the end-member properties of hercynite (FeAl2O4), ulvospinel (Fe2TiO4), MgFe2O4 and cubic Mg2TiO4. In constructing these estimates, provision is made for low-temperature magnetic entropy contributions and the energetic consequences of disordering the aluminates and the ferrites. These estimates are consistent with all of the available low-temperature adiabatic calorimetry, high-temperature heat content, and heat of solution measurements on the end-members. The analysis implies that there is a substantial heat capacity anomaly in the range 300°–900° C associated with disordering of the MgAl2O4 structure while that in FeAl2O4 becomes significant at temperatures above 700° C. The same heat capacity response in the ferrites indicates that the order/disorder transformation is coupled to the antiferromagnetic-paramagnetic transition in MgFe2O4 but takes place well above the ferrimagnetic-paramagnetic transition in magnetite. The proposed model is internally consistent with solution theory reported elsewhere for Fe2+−Mg2+ olivines and orthopyroxenes (Sack and Ghiorso 1989), rhombohedral oxides (Ghiorso 1990a) and the remaining end-member properties of Berman (1988).

Effects of diffusional modification of garnet growth zoning on P-T path calculations

Abstract: The effect of intragranular diffusion on chemical zoning in garnet and on P-T paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth. Syn-and post-growth diffusion of Mg−Fe−Mn−Ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635°C. Compositions from zoned garnet, as modified by diffusion, hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure-temperature (P-T) paths from isobaric and polybaric model histories. P-T path calculations, based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635°C, show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths. Retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path. The magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition. The effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating-only models that reached peak temperatures approximately 15–30°C higher. Compositions of garnets with radii less than 1 mm, that reached actual peak temperatures of 605°C along temperature-time histories characteristic of regional metamorphism, experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories. Path distortions were significant at heating/cooling rates less than 10°C/Ma, but rapidly diminished for rates faster than this. In polybaric simulations diffusion effects were least noticeable when the actual pressure-temperature conditions changed in a clockwise sense (i.e., convex to higher P and higher T), but apprecciable modification was seen in path models that underwent counterclockwise changes in P and T. Reequilibration of garnet rim compositions occurred during cooling on all paths, and temperature maxima obtained from garnet-biotite geothermometry underestimated actual peak conditions by 40 to 70°C. Calculations suggest that P-T path trajectories calculated from garnets of at least 1 mm size, and that experienced actual thermal maxima below 585°C, are not likely to be distorted by diffusional effects during regional metamorphism. However, P-T path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history. The partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling, seriously complicating the task of quantitatively estimating diffusion effects on path calculations.

Residence of trace elements in metasomatized spinel lherzolite xenoliths: a proton-microprobe study

Abstract: Minerals occurring in dry and modally metasomatized spinel lherzolites from western Victoria have been analysed by proton microprobe for Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, Pb, Br, rare-earth elements (REE), Th and U. Mass-balance calculations demonstrate that these trace elements are contained in specific acceptor minerals and do not occur in significant concentrations at clean grain boundaries. The level of particular trace elements in the rock depends on the presence of specific phases: for example high levels of REE, Sr (and U, Th, Br) require apatite, while Ba, Nb and Ta are strongly concentrated in amphibole±mica. Mantle metasomatism in these spinel lherzolites is inferred to result from an open-system process involving infiltration of fluids released by crystallizing silicate melts. This process produces metasomatic zones with different modal mineralogy and hence greatly different trace-element signatures. The data demonstrate that large-ion-lithophile (LIL) and high-field strength (HFS) elements in metasomatized spinel lherzolites are strongly concentrated in non-refractory phases, which will break down easily in heated volumes such as the walls of magma conduits. The heterogeneity observed in trace-element patterns of intraplate alkali basaltic rocks may not reflect source heterogeneity, but may result largely from contamination by metasomatized mantle wall rock. The KDs for most trace elements show little temperature dependence except for KDSr between orthopyroxene and clinopyroxene where KD decreases with increasing temperature. The generally uniform KDs can be used to test for disequilibrium in such assemblages.

Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?

Abstract: Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young.

Reequilibration of chromite within Kilauea Iki lava lake, Hawaii

Abstract: Chromite mainly occurs as tiny inclusions within or at the edges of olivine phenocrysts in the 1959 Kilauea Iki lava lake. Liquilus chromite compositions are only preserved in scoria that was rapidly quenched from eruption temperatures. Analyses of drill core taken from the lava lake in 1960, 1961, 1975, 1979, and 1981 show that chromite becomes richer in Fe+2, Fe+3, Ti and poorer in Mg, Al, Cr than the liquidus chromite. The amount of compositional change depends on the time elapsed since eruption, the cooling history of the sample, the extent of differentiation of the interstitial melt, and the position of the chromite inclusion within the olivine phenocryst. Compositional changes of the chromite inclusions are thought to be a result of reequilibration with the residual melt by cationic diffusion (Mg, Al, Cr outwards and Fe+2, Fe+3, Ti inwards) through olivine. The changing chemical potential gradients produced as the residual melt cools, crystallizes and differentiates drives the reequilibration process. Major and minor element zoning profiles in olivine phenocrysts suggest that volume diffusion through olivine may have been the major mechanism of cationic transport through olivine. The dramatic compositional changes observed in chromite over the 22 years between eruption and 1981 has major implications for othe molten bodies.

Magma sources for Mesozoic anorogenic granites of the White Mountain magma series, New England, USA

Abstract: The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm−Nd and Rb−Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<∼1 in eNd). Overall, Mesozoic granite plutons range in eNd [T] from +4.2 to -2.3, with most +2 to 0, and in initial 87Sr/86Sr from 0.7031 to ∼0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at eNd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas.

A refined garnet - biotite Fe−Mg exchange geothermometer and its application in amphibolites and granulites

Abstract: A new formulation of garnet-biotite Fe−Mg exchange thermometer has been developed through statistical regression of the reversed experimental data of Ferry and Spear. Input parameters include available thermo-chemical data for quaternary Fe−Mg−Ca−Mn garnet solid solution and for excess free energy terms, associated with mixing of Al and Ti, in octahedral sites, in biotite solid solution. The regression indicates that Fe−Mg mixing in biotite approximates a symmetrical regular solution model showing positive deviation from ideality withW FeMg bi =1073±490 cal/mol. ΔH r and ΔS r for the garnet-biotite exchange equilibrium were derived to be 4301 cal and 1.85 cal respectively. The resultant thermometer gives consistent results for rocks with a much wider compositional range than can be accommodated by earlier formulations.

The tholeiite to alkalic basalt transition at Haleakala Volcano, Maui, Hawaii

Abstract: Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (∼1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of ∼10% to 15%, while the intercalated alkalic lavas reflect a range of ∼6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from ∼0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at ∼0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.

A model for coupled fluid-flow and mixed-volatile mineral reactions with applications to regional metamorphism

Abstract: The effect of fluid flow on mixed-volatile reactions in metamorphic rocks is described by an expression derived from the standard equation for coupled chemical-reaction and fluid-flow in porous media. If local mineral-fluid equilibrium is assumed, the expression quantitatively relates the time-integrated flux at any point in a flow-system to the progress of devolatilization reactions and the temperature- and pressure-gradients along the direction of flow. Model calculations indicate that rocks are generally devolatilized by fluids flowing uptemperature and/or down-pressure. Flow down-temperature typically results in hydration and carbonation of rocks. Time-integrated fluid fluxes implied by visible amounts of mineral products of devolatilization reactions are on the order of 5·102–5·104 mol/cm2. The model was applied to regionally metamorphosed impure carbonate rocks from south-central Maine, USA, to obtain estimates of fluid flux, flow-direction, and in-situ metamorphic-rock permeability from petrologic data. Calculated time-integrated fluxes are 104–106 cm3/cm2 at 400°–450° C, 3,500 bars. Fluid flowed from regions of low temperature to regions of high temperature at the peak of the metamorphic event. Using Darcy's Law and estimates for the duration of metamorphism and hydrologic head, calculated fluxes are 0.1–20·10-4 m/year and minimum permeabilities are 10-10–10-6 Darcy. The range of inferred permeability is in good agreement with published laboratory measurements of the permeability of metamorphic rocks.

Compositional changes of minerals associated with dynamic recrystallizatin

Abstract: The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.

Composition gaps, critical crystallinity, and fractional crystallization in orogenic (calc-alkaline) magmatic systems

Abstract: The recognition of a three-way correlation between magmatic SiO2 content, critical crystallinity, and the size (magnitude) of crystal fractionation-generated composition gaps in calc-alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps. To explain this correlation, it is proposed that fractionation-generated composition gaps are caused by: (1) simultaneous interior (i.e. non-substrate) crystallization and vigorous chamberwide convection which leads to progessive crystal suspension; (2) cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma, and; (3) eventual buoyancy-driven crystal-liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap. This mechanism implies that many, if not most magma bodies are characterized by interior crystallization and vigorous convection, conditions which are not universally agreed upon at present. Given that such conditions characterize natural magma bodies, fractional crystallization through crystal settling in low-velocity boundary layers should be an important mechanism of fractional crystallization. In a crystallizing and convecting body of magma, composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention. As a given volcanic plumbing system matures with time, average settling/retention ratios within individual magma bodies should increase due to higher average wall-rock temperatures. It follows that, within a given volcanic center, early-stage volcanism should be more likely to display fractionation-generated composition gaps than later-stage volcanism. Such a temporal evolution has been documented at at least two Aleutian calc-alkaline volcanic centers.

Interdiffusion of hydrous dacitic and rhyolitic melts and the efficacy of rhyolite contamination of dacitic enclaves

Abstract: Effective binary diffusion coefficients of Si during the interdiffusion of hydrous, 3 and 6% H2O, dacitic and rhyolitic melts have been determined at 1.0 GPa, 1100°–1400°C. Water is shown to enhance diffusivities by one to two orders of magnitude above dry Si diffusivities in the same compositional system for SiO2 compositions 65–75wt%. The effect of silica content on diffusion is small and typically within experimental error. With 3% H2O in the melts the Arrhenius equation for Si diffusion at 70% SiO2 is: $${\text{D = }}2.583\operatorname{x} 10^{ - {\text{ }}8} {\text{ }}\exp ( - 126.5/R{\text{T}})$$ where D is the diffusivity in m2/s, the activation energy (126.5) is in kJ/mol, R is in J/mol and T in Kelvin. Although less-well constrained, the Si diffusivity at 70% SiO2 with 6% H2O in the melts can be described by: $${\text{D = }}2.692\operatorname{x} 10^{ - {\text{ }}7} {\text{ }}\exp ( - 131.4/R{\text{T}})$$ The activation energies for diffusion are substantially below the activation energy of 236.4kJ/mol measured during anhydrous interdiffusion in the same system (Baker 1990). The decrease in activation energy with the initial addition of 3% water and the relative insensitivity of the activation energy to the additional water is related to the abundance of OH species in the melt, and the reduction of (Si,Al)-O bond strengths due to the interaction of hydroxyls with the (Si,Al)-O network. Changes in the pre-exponential factor of Arrhenius equations are attributed to the abundance of H2O species in the melts. No decoupling of non-alkalies from SiO2 during interdiffusion of the two melts was observed, although alkalies diffuse much more rapidly than non-alkalies (but were not measured quantitatively in this study) and can become decoupled. Interdiffusion of Si and all non-alkalies is demonstrated to be predictable, at least to within a factor of ten, by the Eyring equation. Using the diffusion data of this study for nonalkalies and of other studies for alkalies and Sr isotopes the contamination of a host rhyolitic magma by dacitic enclaves, 5 and 50 cm radius, has been modeled for temperatures of 1000°, 900°, and 800° C with water contents of 3 and 6%. Even when the effects of phenocrysts on diffusion in the dacitic enclaves are estimated the results of the modeling demonstrate that significant contamination is possible in the case of small enclaves, and even large enclaves have the potential to affect the composition of their host magma in geologically short times.

Petrogenesis of the Merensky Reef in the Rustenburg section of the Bushveld Complex

Abstract: A petrogenetic model for the Merensky Reef in the Rustenburg section of the Bushveld Complex has been developed based on detailed field and petrographic observations and electron microprobe data. The model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage. The model is devised to account for several key observations: (1) Although the dominant rock type in the Rusterburg sections is pegmatoidal feldspathic pyroxenite, there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite, and small amounts of olivine are present in nearly all pegmatoids. (2) The pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter-thick layer of anorthosite. The thicknesses of the two layers exhibit a well-defined, positive correlation. (3) Inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid. The leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef. (4) Chromite in seams adjacent to plagioclase-rich rocks is characterized by higher Mg/Mg+Fe and Al/R3 and lower Cr/R3 than that in seams adjacent to pyroxene-rich rocks. Similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ. The chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates. It is proposed that the present reef was originally a melt-rich horizon in norite immediately overlain by relatively crystallized pyroxenite. Magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef. The melt-rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water-undersaturated. Hydration of the intercumulus melt was accompanied by melting, and the hydration/melting front migrated downward into the footwall and upward into the hanging wall. In the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase. With continued hydration chromite was stabilized as melt alumina content increased. The regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front. The stratigraphic sequence downward through the base of the reef or pegmatoid (melt)-chromitite-anorthosite-norite represents the sequence of stable mineral assemblages across the hydration/melting front. The sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures. With cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite-olivine-pyroxene-plagioclase, with peritectic loss of some olivine. Calculations of mass balance indicate that a significant proportion of the melt was lost from the melt-rich horizon. Variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the Merensky Reef resulted because the processes of hydration, melting and melt loss operated to varying extents.

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano, Panama

Abstract: The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.

The stability of sapphirine-quartz and hypersthene-sillimanite-quartz assemblages: an experimental investigation in the system FeO−MgO−Al2O3−SiO2 under H2O and CO2 conditions

Abstract: Phase relations and mineral chemistry involving the phases garnet (Gt), spinel (Sp), hypersthene (Hy), sapphirine (Sa), cordierite (Cd), sillimanite (Sil) and quartz (Qz) have been experimentally determined in the system FMAS (FeO−MgO−Al2O2−SiO2) under low fO2 and for various H2O/CO2 conditions. Several compositions were studied with 100 (Mg/Mg+Fe) ratio ranging from 64 to 87 with excess quartz and sillimanite. Our data do not show any differences in Gt−Cd stability and composition as a function of H2O, CO2 and H2O−CO2 (±CH4) content, in good agreement with a previous experimental study at lower temperature (Aranovich and Podlesskii 1983). At 1,000° C and 11 kbar, under CO2-saturated conditions, cordierite grew from a crystalline mix unseeded with cordierite. Thus, under water-absent conditions, cordierite will have a high-P stability field in the presence of CO2. If water has a pressure stabilizing effect on cordierite, then our results would indicate that the effects of H2O and CO2 are of the same magnitude at high temperature. Our data support the theoretical P-T grid proposed by Hensen (1986) for high-T metapelites and are largely consistent with the high-temperature experimental data of Hensen and Green (1973). The univariant boundary Gt+Cd=Hy+Sil+Qz, which marks the disappearance of Hy−Sil−Qz assemblages, has a negative dP/dT slope above 1,000° C and a positive one below this temperature. Extrapolation of our data to iron-free systems shows that the high-P breakdown limit of Mg-cordierite has a negative slope in the range 1,025–1,300° C and probably positive below 1,000° C. This indicates a maximum of stability for Mg-cordierite at around 1,000° C and 13 kbar. Because of the curvature of the univariant reactions En+Sil=Py+Qz, Mg−Cd=En+Sil+Qz and Gt+Cd=Hy+Sil+Qz, the iron-free invariant point involving the phases Py, En, Cd, Sil and Qz probably does not exist. Sapphirine—Qz-bearing assemblages are stable only at temperatures above 1,050° C. At 1,075° C, the joint Gt−Sa is stable up to 11 kbar. At higher pressure, garnet, sapphirine and quartz react according to the reaction Gt+Sa+Qz=Hy+Sil. Reequilibrated sapphirines are more aluminous than the theoretical endmember Mg2Al4SiO10 due to AlAl=MgSi substitutions [100(Al2O3/Al2O3+FeO+MgO) in experimental sapphirines ranges from 50.5 to 52.2]. Sapphirine in the assemblage Sa−Cd−Sil−Qz shows a decrease in Al content with decreasing temperature and pressure, such that the alumina isopleths for sapphirine have a slight negative dP/dT slope. A similar decrease in Al content of sapphirine with temperature is also observed in Sa−Sil−Qz assemblages.

Ion microprobe analysis of oxygen isotope ratios in granulite facies magnetites: diffusive exchange as a guide to cooling history

Abstract: Ion microprobe analysis of magnetites from the Adirondack Mountains, NY, yields oxygen isotope ratios with spatial resolution of 2–8 μm and precision in the range of 1‰ (1 sigma). These analyses represent 11 orders of magnitude reduction in sample size compared to conventional analyses on this material and they are the first report of routinely reproducible precision in the 1 per mil range for analysis of δ18O at this scale. High precision micro-analyses of this sort will permit wide-ranging new applications in stable isotope geochemistry. The analyzed magnetites form nearly spherical grains in a calcite matrix with diopside and monticellite. Textures are characteristic of granulite facies marbles and show no evidence for retrograde recrystallization of magnetite. Magnetites are near to Fe3O4 in composition, and optically and chemically homogeneous. A combination of ion probe plus conventional BrF5 analysis shows that individual grains are homogeneous with δ18O=8.9±1‰ SMOW from the core to near the rim of 0.1–1.2 mm diameter grains. Depth profiling into crystal growth faces of magnetites shows that rims are 9‰ depleted in δ18O. These low δ18O values increase in smooth gradients across the outer 10 μm of magnetite rims in contact with calcite. These are the sharpest intracrystalline gradients measured to date in geological materials. This discovery is confirmed by bulk analysis of 150–350 μm diameter magnetites which average 1.2‰ lower in δ18O than coarse magnetites due to low δ18O rims. Conventional analysis of coexisting calcite yields °18O=18.19, suggesting that bulk Δ18O (Cc-Mt)=9.3‰ and yielding an apparent equilibration “temperature” of 525° C, over 200° C below the temperature of regional metamorphism. Consideration of experimental diffusion data and grain size distribution for magnetite and calcite suggests two contrasting cooling histories. The data for oxygen in calcite under hydrothermal conditions at high P(H2O) indicates that diffusion is faster in magnetite and modelling of the low δ18O rims on magnetite would suggest that the Adirondacks experienced slow cooling after Grenville metamorphism, followed by a brief period of rapid cooling, possibly related to uplift. Conversely, the data for calcite at low P(H2O) show slower oxygen diffusion than in magnetite. Modelling based on these data is consistent with geochronology that shows slow cooling through the blocking temperature of both minerals, suggesting that the low δ18O rims form by exchange with late, low temperature fluids similar to those that infiltrated the rock to serpentinize monticellite and which infiltrated adjacent anorthosite to form late calcite veinlets. In either case, the ion microprobe results indicate that two distinct events are recorded in the post-metamorphic exchange history of these magnetites. Recognition of these events is only possible through microanalysis and has important implications for geothermometry.