Thermal Stability of Chloritoid at High Pressure and relatively High Oxygen Fugacity

The equilibrium conditions for the oxidation reaction of chloritoidto staurolite, magnetite, quartz, and vapor have been determinedexperimentally in the pressure range 10-25 kb using the hematite-magnetitebuffer. At 10 kb total pressure the reaction is in equilibriumat 575 C. At 5 kb a value of 544 C was obtained by extrapolationof the high pressure results with an adjustment for the changingthermodynamic properties of water in the low pressure range. At oxygen fugacities along the nickel-nickel oxide buffer, thethermal stability of chloritoid is promoted by 50-60 C abovethe hematite-magnetite values. Here the breakdown products staurolite,almandine, and magnetite are probably more stable. The recently produced petrographic evidence for the oxidationbreakdown of chloritoid and the relative lack of sensitivityof the reaction to moderate changes in total pressure and oxygenfugacity indicate that it may have potential as an importantindicator on the temperature scale of progressive metamorphism.Several independent temperature estimates of the chloritoidbreakdown event by workers in the field agree very well withthe present value of about 550 C for the oxidation reactionof chloritoid to staurolite, magnetite, quartz, and vapor atthe hematite-magnetite buffer in the pressure range 5-10 kb.     

Potassium diffusion in melilite: Experimental studies and constraints on the thermal history and size of planetesimals hosting CAIs

Abstract— Among the calcium‐aluminum‐rich inclusions (CAIs), excess ⁴¹K (⁴¹K*), which was produced by the decay of the short‐lived radionuclide ⁴¹Ca (t_1/2 = 0.1 Myr), has so far been detected in fassaite and in two grains of melilites. These observations could be used to provide important constraints on the thermal history and size of the planetesimals into which the CAIs were incorporated, provided the diffusion kinetic properties of K in these minerals are known. Thus, we have experimentally determined K diffusion kinetics in the melilite end‐members, åkermanite and gehlenite, as a function of temperature (900–1200 °C) and crystallographic orientation at 1 bar pressure. The closure temperature of K diffusion in melilite, T_c(K:mel), for the observed grain size of melilite in the CAIs and cooling rate of 10–100 °C/Myr, as calculated from our diffusion data, is much higher than that of Mg in anorthite. The latter was calculated from the available Mg diffusion data in anorthite. Assuming that the planetesimals were heated by the decay of ²⁶Al and ⁶⁰Fe, we have calculated the size of a planetesimal as a function of the accretion time t_f such that the peak temperature at a specified radial distance r_c equals T_c(K:mel). The ratio (r_c/R)³ defines the planetesimal volume fraction within which ⁴¹K* in melilite grains would be at least partly disturbed, if these were randomly distributed within a planetesimal. A similar calculation was also carried out to define R versus t_f relation such that ²⁶Mg* was lost from ?50% of randomly distributed anorthite grains, as seems to be suggested by the observational data. These calculations suggest that ?60% of melilite grains should retain ⁴¹K* if ?50% of anorthite grains had retained ²⁶Mg*. Assuming that t_f was not smaller than the time of chondrule formation, our calculations yield minimum planetesimal radius of ?20–30 km, depending on the choice of planetesimal surface temperature and initial abundance of the heat producing isotope ⁶⁰Fe.     

Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications

We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe²⁺. A mixture of PbO with about 55 mol per cent PbF₂ provedvery effective as a flux. The experimental results do not suggest any significant dependenceof K_D on Fe/Mg ratio at T 1000?C. The lnK_D vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to K_D and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr³⁺ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe²⁺ in garnetand about 1 wt. per cent Al₂O₃ in orthopyroxene.     

Thermal perturbation during charnockitization and granulite facies metamorphism in southern India

Abstract We have deduced the steady-state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P-T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m² at 1–2.5 Ga. Comparison of the calculated steady-state geotherm with the available P-T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km. Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO₂ influx from a deep-seated source, possibly the mantle. In this work, we have evaluated both the transient and steady-state thermal effects of the heat convected by CO₂ outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady-state geotherm, and that a flux of CO₂ of ±90 mol/m² yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO₂ flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value. Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35-km-thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre-uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non-erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO₂ fluxing from a deep-seated source as the cause for the formation of charnockites in this zone. The required focusing of CO₂ in this case is c. 40% of that estimated in the model where CO₂ fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO₂ fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.     

Compositional properties of coexisting orthopyroxene and spinel in some Antarctic diogenites: Implications for thermal history

Abstract— We analyzed the compositional profiles of coexisting orthopyroxenes and spinels in six diogenite samples from the Antarctic meteorite collection and used the data to constrain their thermal histories. The closure temperatures of Fe²⁺‐Mg exchange between spinel and orthopyroxene in these samples vary between ～630 and 830 °C. However, those in other diogenite samples, for which the compositional data are available in the literature, extend up to ～1125 °C. This wide range of closure temperatures suggests repeated excavation of the diogenites from their original sites over a long time interval during cooling. The orthopyroxene grains were found to be homogeneous in composition while two of the relatively large spinel grains in the samples Elephant Moraine (EET) 87530 and Thiel Mountains (TIL) 82410 showed compositional zoning near the rim. Modeling of the spinel zoning in TIL 82410 suggests that it developed during cooling under a regolith or ejecta blanket, possibly at a depth of ～80–120 m, and that the spinel composition was homogeneous at ～900 °C. A nonlinear cooling model in which the cooling rate is given by ηT(K)², with η = 5.8 times 10^?3 K^?1Ma^?1, leads to simulated retrograde zoning profile in spinel which match the observed profile in TIL 82410 very well.     

Relationship between cooling rate and cooling age of a mineral: Theory and applications to meteorites

Abstract— We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb‐Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ～90 Ma younger Sm‐Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb‐Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an “impulsive heating” event, as suggested earlier.     

Geochemistry and petrogenesis of Neoproterozoic Mylliem granitoids,Meghalaya Plateau,northeastern India

The Mylliem granitoids of the Meghalaya Plateau, northeastern India, represent one of the disharmonic Neoproterozoic igneous plutons, which are intrusive into low-grade Shillong Group of metasediments. Field studies indicate that the Mylliem granitoids cover an area of about 40 km² and is characterized by development of variable attitude of primary foliations mostly marked along the margin of the pluton. Xenoliths of both Shillong Group of metasediments and mafic rocks have been found to occur within Mylliem granitoids. Structural study of the primary foliation is suggestive of funnel-shaped intrusion of Mylliem granitoids with no appreciable evidence of shearing. Petrographically, Mylliem granitoids are characterized by pink to white phenocrysts of prismatic microcline/perthite and lath-shaped plagioclase (An₂₀–An₂₉). Groundmass material is characterized by quartz, microcline, plagioclase, muscovite and biotite. Sphene and apatite occur as accessory minerals. Petrographically Mylliem granitoids have been discriminated as granite and granodiorite according to IUGS system of classification.