A precise olivine-augite Mg-Fe-exchange geothermometer

 Olivine and augite that were experimentally equilibrated in the temperature interval 1175–1080°C at 1 bar in natural basaltic and andesitic bulk compositions are used to calibrate an Mg-Fe²⁺ cation-exchange geothermometer. Within its temperature interval of experimental calibration, and over a broad range in olivine Mg/Fe ratio, the geothermometer has a standard error of ±6°C. In compositionally simpler synthetic systems, the same calibration retrieves appropriate experimental temperatures up to at least 1250°C. In application to intermediate and felsic volcanic rocks erupted at ∼1080 –800°C (below the range of experimental calibration), calculated olivine-augite temperatures are in good agreement with Fe-Ti oxide thermometry in the same samples. These results encourage confidence in the olivine-augite geothermometer over at least the 800–1250°C interval at low pressures. Sparse experimental data up to 1250°C at higher pressures for olivine + augite in the assemblage olivine + plagioclase + augite ± pigeonite or orthopyroxene suggest that the low-pressure calibration recovers experimental temperatures without systematic bias to pressures of 10 kbar. Examples illustrate applications to determining igneous equilibration temperatures in holocrystalline extrusive and intrusive rocks, and to estimating intratelluric H₂O content dissolved in magmas. Received: 24 February 1995 / Accepted: 1 March 1996     

18O/16O ratios of anorthosites and related rocks from the Rogaland Complex (SW Norway)

The South Rogaland Complex (South Western Norway) consists of several anorthositic intrusions emplaced in granulite facies metamorphic rocks. The anorthosites and related norites and jotunites have δ ¹⁸O values of 5.2 to 7‰ suggesting a mantle origin for these rocks, in agreement with the strontium isotopic evidence. The acidic rocks, mostly charnockitic, associated with the anorthosites have similar δ ¹⁸O values and thus a comagmatic relation between these two rock types is inferred. Small departures from mantle values are explained in terms of crustal contamination by surrounding gneisses that have δ ¹⁸O values between 4.3 and 10‰ Locally, this corresponds to important anatexis as has been suggested for the Farsund charnockite on the basis of strontium isotope and REE geochemistry. The isotopic temperatures calculated from the isotopic fractionations are in the range 500°–700° C, lower than the orthomagmatic temperatures and probably due to subsolidus isotopic exchange during the slow cooling of these plutonic rocks, either during a late magmatic deuteric stage or during a slow, postorogenic ascent under wet conditions.     

Mineral chemistry and Ti in zircon thermometry: Insights into magmatic evolution of the Sangan igneous rocks,NE Iran

The Sangan Magmatic complex (SMC) is, a large I-type magmatic complex, located in the northeastern Iran. Zircons extracted from the intrusive and volcanic rocks within the SMC record a similar Hf compositions and REE patterns, indicating that these chemical signatures have likely been inherited from the same source and simple history of magmatic crystallization during the evolution of the orogeny. The zircon from volcanic rocks yield Ti-in-zircon crystallization temperatures of 667–1145?°C with average temperatures of 934?°C while those from granitoids indicate crystallization temperatures of 614–898?°C with an average of 812?°C. Ti-in-zircon, Ti in biotite thermometries also indicates that the crystallization temperatures of volcanic rocks are relatively higher than those of granitoids. The biotite chemistry studies reveal that this mineral crystallized at approximately 725°–800?°C and 758° to 816?°C for granitoid and volcanic rocks, respectively, which is similar to obtained temperatures by Zir-saturation of Eq. (1). T_zicsat and T_magma trend lines on the T-SiO₂ diagram cross at high silica contents of ～68?wt.%, at which temperature the magma becomes zircon-saturated and new zircons are crystallized. The zircon REE data including Ce/Ce*, Eu/Eu*, and Th/U ratios suggest that SMC igneous rocks are formed from oxidized magma. However, the zircon Th/U and Hf data suggest that the SMC became progressively more oxidized and also indicate lower temperatures from volcanic and plutonic rock with decreasing time.     

The influence of Fe<Superscript>3+</Superscript> on garnet–orthopyroxene and garnet–olivine geothermometers

Applying Fe²⁺–Mg exchange geothermometers to natural samples may lead to incorrect temperature estimates if significant Fe³⁺ is present. In order to quantify this effect, high-pressure experiments were carried out in a belt apparatus in a natural system close to CFMAS at 5 GPa and 1,100–1,400 °C. The oxygen fugacity in the experiments was at or below the Re–ReO₂ buffer. This is at significantly more oxidized conditions than in previous experiments, and, as consequence, higher Fe³⁺/Fe²⁺ ratios were generated. The Fe³⁺ content of garnet in the experiments was quantified by electron microprobe using the flank method. Making the usual assumption that Fe_total = Fe²⁺, the two-pyroxene thermometer of Brey and Köhler (J Pet 31:1353–1378, 1990) reproduced the experimental temperature to ±35 °C and the garnet–clinopyroxene Fe²⁺–Mg exchange thermometer of Krogh (Contrib Miner Pet 99:44–48, 1988) overestimated the temperatures on average by only 25 °C. On the other hand, application of the garnet–olivine (O’Neill and Wood in Contrib Miner Pet 70:59–70, 1979) and garnet–orthopyroxene (Harley in Contrib Miner Pet 86:359–373, 1984) exchange geothermometers yielded an underestimation in calculated temperatures of >200 °C. However, making explicit accounting for Fe³⁺ in garnet (i.e. using only measured Fe²⁺) leads to a vast improvement in the agreement between calculated and experimental temperatures, generally to within ±70 °C for the garnet–orthopyroxene geothermometer as well as noticeable improvement of calculated temperatures for the garnet–olivine geothermometer. Our results demonstrate that the two-pyroxene and garnet–clinopyroxene thermometers are rather insensitive to the presence of Fe³⁺ whilst direct accounting of Fe³⁺ in garnet is essential when applying the garnet–olivine and garnet–orthopyroxene thermometers.     

Partitioning of fluorine and chlorine between biotite and granitic melt: experimental calibration at 200 MPa H2O

Experiments from 640 to 680?°C, 200 MPa H₂O at?f _O2?≈?NNO, employing a natural?F-rich?vitrophyric rhyolite from Spor Mountain, Utah, assessed the effect of variable Mg′ [100Mg/(Mg?+?Mn?+?Fe)] on the partitioning of fluorine and chlorine between biotite (Bt) and melt. Over this temperature interval, Bt (?±?fluorite, ?±?quartz) is the sole liquidus phase. Partition coefficients for fluorine between biotite and glass (D_F ^Bt/melt) show a strong dependence on the Mg′ of Bt.?D_F ^Bt/melt varies from???1.5 to 7.2 over the range of Mg′ from 21 to 76. A strong linear correlation between?D_F ^Bt/melt?and Mg′ has a slope of 9.4 and extrapolates through the origin (i.e., D_F ^Bt/melt?≈?0 at Mg′?=?0, an annite-siderophyllite solid solution in these experiments). D_Cl ^Bt/melt values (???1 to 6) in the same experiments vary inversely with Mg′. The Al-content of biotite does not vary with the aluminum saturation index (ASI?=?molar Al₂O₃/Σ alkali and alkaline earth oxides) of melt, but two exchange mechanisms involving Al appear to operate in these micas: (1) Al^vi?+?Al^iv?? Si^iv?+?Mg^iv, and Mg^iv?+?2Al^iv? 2Si^iv?+?□^iv. The effects of other components such as Li or other intensive parameters including f _O2 have yet to be evaluated?systematically. At comparable Mg′ of Bt, however, the Spor Mountain rhyolite yields higher D_F ^Bt/melt values than an Li-rich, strongly peraluminous melt previously investigated. The results indicate that the Mg′ of Bt exerts the principal control on halogen partitioning, with ASI and T as second-order variables. The experimental partition coefficients compare well with other experimental results but not with most volcanic rocks. Magmatic Bt from most rhyolites records higher D_F ^Bt/melt due to reequilibration with degassed (H₂O-depleted) magma and perhaps with F₂O_?1 exchange that may accompany oxidation ([Fe³⁺O] [Fe²⁺OH]_?1). This behavior is evident in magmatic biotite from a zoned peraluminous rhyolite complex near Morococala, Bolivia: Bt is sharply zoned with F-rich rims, but Bt(core)-melt inclusion pairs fall on our experimental curve for D_F ^Bt/melt. These experimental data can be used in part to assess the preservation of magmatic volatile contents in plutonic or volcanic silicic rocks. For plutonic rocks, the actual F-content of melt, not a relative activity ratio involving HF species, can be reasonably estimated if the mica has not undergone subsolidus reequilibration. This information is potentially useful for some shallow-level Ca-poor magmas that are thought to be rich in F (e.g., A- and S-type granites) but do not conserve F well as rocks.     

Secondary Ca–Al silicates in plutonic rocks: implications for their cooling history

Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early post-magmatic stage of the intermediate to basic Hercynian plutonic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgebirge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, prehnite, pumpellyite, epidote and laumontite form lenses within unaltered or only slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 °C. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rather narrow ranges of temperature (200-280 °C) and pressure (2-3 kbar). Laumontite is formed at the end of Ca-Al silicate crystallization (180-260 °C, 1-3 kbar), mostly in small fractures in association with prehnite and adularia. The observed crystallization sequence of the Ca-Al silicates and their stabilities define a retrograde alteration path for the plutonic rocks. The Ca-Al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low X_CO2 fluids during post-magmatic cooling. Subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 °C (prehnite-pumpellyite paragenesis). This shows that Ca-Al silicate assemblages may be a unique tool to constrain the P-T conditions of the subsolidus cooling of intermediate to basic plutonic bodies.     

A new formulation of garnet–clinopyroxene geothermometer based on accumulation and statistical analysis of a large experimental data set

Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet–clinopyroxene geothermometer based on 333 garnet–clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe³⁺ content of clinopyroxene. For the calibration, a published subregular‐solution model was adopted to express the non‐ideality of garnet. The magnitude of the Fe–Mg excess interaction parameter for clinopyroxene (W_FeMg^Cpx), and differences in enthalpy and entropy of the Fe–Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe–Mg excess interaction parameter of clinopyroxene (W_FeMg^Cpx = ? 3843 J mol^?1). The pressure correction is simply treated as linear, and the difference in volume of the Fe–Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be ?120.72 (J kbar^?1 mol^?1). The regressed and obtained thermometer formulation is as follows: where T = temperature, P = pressure (kbar), A = 0.5 X_grs (X_prp ? X_alm ? X_sps), B = 0.5 X_grs (X_prp ? X_alm + X_sps), C = 0.5 (X_grs + X_sps) (X_prp ? X_alm), X_prp = Mg/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_alm = Fe/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_sps = Mn/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_grs = Ca/(Fe²⁺ + Mn + Mg + Ca)^Grt, X_Mg^Cpx = Mg/(Al + Fe^total + Mg)^Cpx, X_Fe^Cpx = Fe²⁺/(Al + Fe^total + Mg)^Cpx, K_D = (Fe²⁺/Mg)^Grt/(Fe²⁺/Mg)^Cpx, Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800–1820 °C), with a standard deviation (1 sigma) of 74 °C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100 °C at >1300 °C and overestimate by 100–200 °C at <1300 °C. In addition, they tend to overestimate temperatures for high‐Ca garnet (X_grs ≈ 0.30–0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20–100 °C lower than previous formulations.     

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An_80?81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: $\text{D}\text{?}\text{= 10.99 (}\text{cm}{}^2\text{/sec) exp}\text{(?123.4(}\text{kcal/mol}\text{)/RT), (T}$ in °K). This value is for diffusion perpendicular to the (03 1?) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures.The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids.The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

Geochemistry of granitization and magmatic replacement of basic volcanic rocks in the contact aureole of the Yurchik gabbronorite intrusion, Ganal Range, Kamchatka

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO₂, Al₂O₃, Na₂O, K₂O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of ¹⁴³Nd/¹⁴⁴Nd and increase of ⁸⁷Sr/⁸⁶Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites. It is suggested that the processes of metamorphism, metasomatism, and magmatic replacement were caused by highly mineralized mantle fluids, which percolated along magmatic channels serving as pathways for gabbroid magma.     

Interdiffusion of Mg–Fe in olivine at 1,400–1,600 °C and 1 atm total pressure

The interdiffusion coefficient of Mg–Fe in olivine (D _Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10^?3.5–10^?2 Pa using a diffusion couple technique. The D _Mg–Fe shows the anisotropy (largest along the [001] direction and smallest along the [100] direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D _Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe³⁺/Fe²⁺ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe³⁺/Fe²⁺ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D _Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.     

Features of mineral and chemical composition of the Khamambettu Carbonatites, Tamil Nadu

The paper presents mineralogical features and EPMA results of the Khamambettu carbonatites. The mineralogical data suggest that these rocks have been generated in magmatic and hydrothermal stages. Mineral geothermometer for carbonatite give temperatures of 790°–980°C. Fluid inclusion measurements in monazite (hydrothermal stage) give temperatures of 220°–290°C. One of the features of the carbonatites is high content of magnesia that is defined by the presence of dolomite, olivine, spinel, phlogopite, Mg-rich ilmenite. Chloritization, serpentinization, amphibolization, silicification processes and occurrence of barite, monazite-(Ce), strontianite, celestine are related to hydrothermal stage. Hydrothermal minerals at the Khamambettu were formed by recrystallization of primary carbonatite minerals in the presence of Ba, (SO₄)^2?, REE and Si carried in solution by the hydrothermal fluid.     

A Precambrian fayalite granite from the south coast of Western Australia

A Precambrian fayalite granite outcropping at Lower King, near Albany, Western Australia, is interpreted as a high-Fe²⁺/(Fe²⁺ + Mg) analogue of charnockite. Calculation of the original titanomagnetite composition from analytical data on ilmenomagnetite ‘exsolution’ intergrowths suggests initial crystallisation of opaque oxides at about 940°C and 10^?12 bars f_o2. This result indicates a magmatic origin for the rock. Other determinable points on the T-f_o2 cooling curve of the fayalite granite pluton include crystallisation of biotite at roughly 800–820°C and 10^?14.5 bars f_o2, and final equilibration of opaque oxides below 550°C and 10^?23 bars f_o2. Mineralogical data on nearby granulite facies country rocks suggest a regional total pressure of roughly 5 kb, and hence the depth of pluton emplacement was probably around 18–19 km. Thus the Lower King fayalite granite is believed to have crystallised from water-deficient, high-T melt or partial melt generated, possibly from metasedimentary rocks, deep in the crust under granulite facies conditions.     

Formation of banded gneisses by deformation of igneous rocks

The development of finely layered gneisses by progressive deformation of a variety of granitoid intrusions, leucogabbroa, anorthosites, and basic and intermediate volcanic rocks is described and illustrated.Gneissose layering developed by the tectonic distortion of: magmatic and volcanic layering; particles such as pillow lavas, igneous crystals, and rock fragments in volcanic and plutonic breccias; patchy igneous textures; and vein networks. The products of extreme deformation during amphibolite facies metamorphism are uniformly layered gneisses.     

Petrogenetic implications of mineral chemical data for the Permian Baima igneous complex, SW China

The Late Permian Fe-Ti oxide ore-bearing Baima igneous complex (BIC) is one of three gabbro-granitoid complexes with the Emeishan large igneous province. Mineral compositions are determined for the BIC layered gabbro in order to constrain subsolidus and magma chamber processes. The averaged compositions of cumulus olivine, clinopyroxene and plagioclase within individual samples range from Fo_65-76, Mg# = 75 to 82 and An_49-64 but they are not correlative. The observed mineral compositions are consistent with those modeled using the pHMELTS program. Highly variable magnetite compositions are consistent with extensive subsolidus re-equilibration and exsolution. The occurrence of reversely-zoned granular olivine in Fe-Ti oxide ores is a manifestation of Mg transfer between Fe-Ti oxides and olivine at relatively high (<1150?°C) subsolidus temperatures. The primary oxide is inferred to be an aluminous titanomagnetite. Similar to other layered intrusions in the region, the gabbroic unit of the BIC displays Zr depletion which is consistent with loss of a residual liquid during magma differentiation. If the most Zr-rich syenites of the complex are taken into account, the Zr budgets between the combined gabbro-syenite and the basalts are similar. This indicates that the BIC most likely represents a closed system in terms of magma extraction.     

Assessment of the Garnet-Clinopyroxene Thermometer

A thermometer based on the MgFe_?1 exchange equilibrium between garnet and clinopyroxene is formulated by using new experimental data measured at 600° to 950°C, 0.8 to 3.0 GPa, and f(O₂) defined by the fayalite-quartz-magnetite buffer in the basalt-H₂O system. The new formulation is T = 3820 / 1.828 + lnK_D (1 + a(2.2 ? p)), where T is temperature (K), P is pressure (GPa), KD is the Fe-Mg partition coefficient between garnet and clino-pyroxene, defined as KD = (Fe²+/Mg)garnet/(Fe²+/Mg) clinopyroxene, and a = 132/T. Application of the thermometer to rocks in amphibolite, granulite, and eclogite terranes yields temperatures that are in reasonable agreement with other well-calibrated thermometers and the experimental calibrations by Ellis and Green (1979) and Pattison and Newton (1989).     

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)     

The isotopic and chemical composition of CO2-rich thermal waters in the Mont-Dore region (Massif-Central,France)

Chemical and isotope compositions of fluid samples, collected between 1974 and 1986 from 52 springs or shallow boreholes located in the Mont-Dore region (Massif Central, France), were examined. Some springs and wells were sampled several times during this period. The fluids emerge from Quaternary volcanic rocks or Paleozoic granite at temperatures between 4 and 62°C, and the origin of the H₂O is meteoric. The waters can be classified into three groups: bicarbonate fluids, mixed bicarbonate-chloride fluids (with a mineralization up to 8 g/l), and acid-sulfate fluids. Only two fluids contain sufficient Cl⁻ to be considered as ‘mature’ waters. Previous work has demonstrated that they all contain partly mantle-derived CO₂ gas, and that the CO₂-rich gas phase and bicarbonate-chloride waters are separated at substantial depth.Mineralized fluids circulate at depth and undergo several processes, such as cooling or dilution with recent freshwater, during their ascent to the surface. Therefore, the CO₂-rich gas phase can be partly dissolved in the freshwater, or in deep fluids after their dilution. This process leads to the dissolution of surrounding rocks; such dissolution is discussed on the basis of major-element concentrations (Na, K, Ca, Mg), as well as the Sr 87/86 isotope ratio. Dissolution of S-bearing minerals has also been demonstrated. The presence of the CO₂-rich gas phase also leads to isotope exchange between CO₂ and H₂O. Some mineralized fluids are less affected by these processes than others, in which case they display the chemical and isotopic characteristics of the original deep fluids.It was shown that the applicability of geothermometer calculations for these waters is hampered by several processes that modify the chemical composition. However, some geothermometers can be used for estimating the temperature of the deep fluids using the chemical composition of the less modified fluids. They indicate that fluids emerging from volcanic rocks in the Dordogne valley reach temperatures of around 100–130°C at depth, while the temperature of the fluid that issues from the granite at Saint-Nectaire is 160–175°C at depth.     

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.     

Fe-rich tholeiitic liquids and their cumulate products in the Pleasant Bay layered intrusion, coastal Maine

Fe-rich tholeiitic liquids are preserved as chilled pillows and as the chilled base of a 27 meter thick macrorhythmic layer in the Pleasant Bay mafic-silicic layered intrusion. The compositions of olivine (Fo₁) and plagioclase (An₁₃₋₈) in these extremely fine grained rocks suggest that they represent nearly end stage liquids that formed by fractionation of tholeiitic basalt. Their major element compositions (∼17.5 wt% FeO_T and 54 wt%SiO₂) closely resemble highly evolved glasses in the Loch Ba ring dike and some recent estimates of end-stage liquids related to the Skaergaard layered intrusion, and are consistent with recent experimental studies of tholeiite fractionation. Their trace element compositions are consistent with extensive earlier fractionation of plagioclase, olivine, clinopyroxene, ilmenite, magnetite and apatite. The mineral assemblage of the chilled rocks (olivine, clinopyroxene, quartz, ilmenite and magnetite), apatite saturation temperatures, and very low Fe³⁺/Fe²⁺indicate conditions of crystallization at temperatures of about 950 °C and f _{O 2} about two log units below FMQ. Cumulates that lie about 3 meters above the chilled base of the macrorhythmic layer contain cumulus plagioclase, olivine, clinopyroxene, ilmenite, apatite and zircon. This mineral assemblage and the Fe-Mg ratio in clinopyroxene cores suggest that this cumulate was in equilibrium with a liquid having a composition identical to that of the chilled margin which lies directly beneath it. The high FeO_T and low SiO₂ concentrations of this cumulate (23.3 and 45.8 wt%, respectively) are comparable to those in late stage cumulates of the Skaergaard and Kiglapait intrusions. This association of a chilled liquid and cumulate in the Pleasant Bay intrusion suggests that late stage liquids in tholeiitic layered intrusions may have been more SiO₂-rich than field-based models suggest and lends support to recent experimental studies of tholeiite fractionation at low f _O2 which indicate that saturation of an Fe-Ti oxide phase should cause FeO_T to decrease in the remaining liquid. Received: 17 January 1997 / Accepted: 10 June 1997