Geology topics

S.R. Bohlen

Publications and source records attributed to S.R. Bohlen.

16 recordsLinked to original sources

USGS2007

On the formation of granulites

The tectonic settings for the formation and evolution of regional granulite terranes and the lowermost continental crust can be deduced from pressure–temperature–time ( P–T –time) paths and constrained by petrological and geophysical considerations. P–T conditions deduced for regional granulites require transient, average geothermal gradients of greater than 35°C km −1 , implying minimum heat flow in excess of 100 mWm −2 . Such high heat flow is probably caused by magmatic heating. Tectonic settings wherein such conditions are found include convergent plate margins, continental rifts, hot spots and at the margins of large, deep-seated batholiths. However, particular P–T –time paths do not allow specific tectonic settings to be distinguished at this time. Under different conditions, both clockwise, CW ( P max attained before T max ), and anticlockwise, ACW ( P max attained slightly after T max ), paths are possible in the same tectonic setting. Both CW and ACW end-member paths can yield nearly isobaric cooling, IBC, paths. Such cooling paths are clearly not an artefact of thermobarometry, but can be constrained by solid–solid and devolatilization equilibria and geophysical modelling. In terms of understanding the evolution of the deep crust, a potentially significant group of regional granulite terranes are those that show evidence for ACW-IBC paths. Such paths are the likely result of: (i) episodic igneous activity resulting in intrusions within all levels of the crust, (ii) thickening of the crust by magmatic underplating, (iii) slow uplift as a result of the formation of a deep, garnet-rich crustal root and (iv) excavation resulting from a later tectonic event unrelated to that resulting in the formation of the granulites. The later event might be triggered by the delamination of the garnet-rich, lowermost crust.

Journal of Metamorphic Geology↗

USGS1998

Heat capacity and thermodynamic properties for coesite and jadeite, reexamination of the quartz-coesite equilibrium boundary

The heat capacities of synthetic coesite and jadeite were measured between about 15 and 850 K by adiabatic and differential scanning calorimetry. The experimental data were smoothed and estimates were made of heat capacities to 1800 K. The following equations represent our estimate of the heat capacities of coesite and jadeite between 298.15 and 1800 K: [see original article for formula]. Tables of thermodynamic values for coesite and jadeite to 1800 K are presented. The entropies of coesite and jadeite are 40.38 ?? 0.12 and 136.5 ?? 0.32 J/(mol.K), respectively, at 298.15 K. The entropy for coesite derived here confirms the value published earlier by Holm et al. (1967). We have derived an equation to describe the quartz-coesite boundary over the temperature range of 600 to 1500 K, P(GPa) = 1.76 + 0.001T(K). Our results are in agreement with the enthalpy of transition reported by Akaogi and Navrotsky (1984) and yield -907.6 ?? 1.4 kJ/mol for the enthalpy of formation of coesite from the elements at 298.15 K and 1 bar, in agreement with the value recommended by CODATA (Khodakovsky et al. 1995). Several sources of uncertainty remain unacceptably high, including: the heat capacities of coesite at temperatures above about 1000 K; the heat capacities and volumetric properties of ?? quartz at higher pressures and at temperatures above 844 K; the pressure corrections for the piston cylinder apparatus used to determine the quartz-coesite equilibrium boundary.

American Mineralogist↗

USGS1997

Kinetics of the coesite to quartz transformation

The survival of coesite in ultrahigh-pressure (UHP) rocks has important implications for the exhumation of subducted crustal rocks. We have conducted experiments to study the mechanism and rate of the coesite→quartz transformation using polycrystalline coesite aggregates, fabricated by devitrifying silica glass cylinders containing 2850 H/10 6 Si at 1000°C and 3.6 GPa for 24 h. Conditions were adjusted following synthesis to transform the samples at 700–1000°C at pressures 190–410 MPa below the quartz–coesite equilibrium boundary. Reaction proceeds via grain-boundary nucleation and interface-controlled growth, with characteristic reaction textures remarkably similar to those seen in natural UHP rocks. We infer that the experimental reaction mechanism is identical to that in nature, a prerequisite for reliable extrapolation of the rate data. Growth rates obtained by direct measurement differ by up to two orders of magnitude from those estimated by fitting a rate equation to the transformation–time data. Fitting the rates to Turnbull's equation for growth therefore yields two distinct sets of parameters with similar activation energies (242 or 269 kJ/mol) but significantly different pre-exponential constants. Extrapolation based on either set of growth rates suggests that coesite should not be preserved on geologic time scales if it reaches the quartz stability field at temperatures above 375–400°C. The survival of coesite has previously been linked to its inclusion in strong phases, such as garnet, that can sustain a high internal pressure during decompression. Other factors that may play a crucial role in preservation are low fluid availability — possibly even less than that of our nominally “dry” experiments — and the development of transformation stress, which inhibits nucleation and growth. These issues are discussed in the context of our experiments as well as recent observations from natural rocks.

Earth and Planetary Science Letters↗

USGS1996

Stability of hydrous phases in subducting oceanic crust

Experiments in the basalt-H 2 O system at 600–950°C and 0.8–3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T ≥650°C; the dehydration H 2 O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H 2 O is stored in lawsonite at 600°C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7°C/km, H 2 O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.

Earth and Planetary Science Letters↗

USGS1995

Mixing properties and stability of jadeite-acmite pyroxene in the presence of albite and quartz

The stability of synthetic jadeite-acmite pyroxene coexisting with albite and quartz has been determined at 600, 700, and 900?? C. The end-member reaction: albite = jadeite + quartz has been determined to lie between 1.67 and 1.70 GPa at 600?? C, 1.88 and 1.90 GPa at 700?? C, and 2.44 and 2.48 GPa at 900?? C. Jd78Acm22 + quartz is stable above 1.58, 1.78, and 2.33 GPa at 600, 700, and 900?? C, respectively. Jd61Acm39 + quartz is stable above 1.47, 1.67, and 2.18 GPa at 600, 700, and 900?? C, respectively. Addition of as much as 40% of acmite component in jadeite extends pyroxene stability by less than 300 MPa at 900?? C. Unit-cell parameters measured for synthetic jadeite-acmite pyroxenes indicate linear volume-composition relations. The data are consistent with ideal mixing in jadeite-acmite solutions. ?? 1995 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1995

Experimentally determined solidi in the Ca-bearing granite system NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O-CO2

The phase relationships of melting of synthetic granite in the presence of an H2O-CO2 fluid were determined. These results provide constraints on the maximum temperatures of regional metamorphism attainable in vapor-saturated metapelitic and quartzofeldspathic rocks that escaped widespread melting. At pressures below 10 kbar, a fluid phase of XH2O = 0.75, 0.5, and 0.25 limits temperatures to below ~700-725, ~800-825, and ~850-875??C, respectively. As a consequence, the formation of granulite does not require CO2 concentrations in a coexisting fluid to exceed an XCO2 of 0.25-0.5. -from Authors

American Mineralogist↗

USGS1993

Experimental investigation and application of the equilibrium rutile + orthopyroxene = quartz + ilmenite

Equilibria in the Sirf (Silica-Ilmenite-Rutile-Ferrosilite) system: {Mathematical expression} have been calibrated in the range 800-1100?? C and 12-26 kbar using a piston-cylinder apparatus to assess the potential of the equilibria for geobarometry in granulite facies assemblages that lack garnet. Thermodynamic calculations indicate that the two end-member equilibria involving quartz + geikielite = rutile + enstatite, and quartz + ilmenite = rutile + ferrosilite, are metastable. We therefore reversed equilibria over the compositional range Fs40-70, using Ag80Pd20 capsules with {Mathematical expression} buffered at or near iron-wu??stite. Ilmenite compositions coexisting with orthopyroxene are {Mathematical expression} of 0.06 to 0.15 and {Mathematical expression} of 0.00 to 0.01, corresponding to KD values of 13.3, 10.2, 9.0 and 8.0 (??0.5) at 800, 900, 1000 and 1100?? C, respectively, where KD=(XMg/XFe)Opx/(XMg/XFe)Ilm. Pressures have been calculated using equilibria in the Sirf system for granulites from the Grenville Province of Ontario and for granulite facies xenoliths from central Mexico. Pressures are consistent with other well-calibrated geobarometers for orthopyroxeneilmenite pairs from two Mexican samples in which oxide textures appear to represent equilibrium. Geologically unreasonable pressures are obtained, however, where oxide textures are complex. Application of data from this study on the equilibrium distribution of iron and magnesium between ilmenite and orthopyroxene suggests that some ilmenite in deep crustal xenoliths is not equilibrated with coexisting pyroxene, while assemblages from exposed granulite terranes have reequilibrated during retrogression. The Sirf equilibria are sensitive to small changes in composition and may be used for determination of activity/composition (a/X) relations of orthopyroxene if an ilmenite model is specified. A symmetric regular solution model has been used for orthopyroxene in conjunction with activity models for ilmenite available from the literature to calculate a/X relations in orthopyroxene of intermediate composition. Data from this study indicate that FeSiO3-MgSiO3 orthopyroxene exhibits small, positive deviations from ideality over the range 800-1100??C. ?? 1993 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1993

Heat capacity and phase equilibria of almandine, Fe3Al2Si3O12

The heat capacity of a synthetic almandine, Fe 3 Al 2 Si 3 O 12 , was measured from 6 to 350 K using equilibrium, intermittent-heating quasi-adiabatic calorimetry and from 420 to 1000 K using differential scanning calorimetry. These measurements yield Cp 298 = 342.80 ± 1.4 J/mol · K and S 298 o = 342.60 J/mol · K. Mössbauer characterizations show the almandine to contain less than 2 ± 1% of the total iron as Fe 3+ . X-ray diffraction studies of this synthetic almandine yield a = 11.521 ± 0.001 Å and V 298 o = 115.11 +- 0.01 cm 3 /mol, somewhat smaller than previously reported. The low-temperature Cp data indicate a lambda transition at 8.7 K related to an antiferromagnetic-paramagnetic transition with T N = 7.5 K . Modeling of the lattice contribution to the total entropy suggests the presence of entropy in excess of that attributable to the effects of lattice vibrations and the magnetic transition. This probably arises from a low-temperature electronic transition (Schottky contribution). Combination of the Cp data with existing thermodynamic and phase equilibrium data on almandine yields ΔG f ,298 o = −4938.3 kJ/mol and ΔH f ,298 o = —5261.3 kJ/mol for almandine when calculated from the elements. The equilibrium almandine = hercynite + fayalite + quartz limits the upper T P "> TP for almandine and is metastably located at ca. 570°C at P = 1 bar, with a dP dT "> dPdT of +17 bars/°C. This agrees well with reversed experiments on almandine stability when they are corrected for magnetite and hercynite solid-solutions. In ‖ O 2- T space, almandine oxidizes near QFM by the reactions almandine + O 2 = magnetite + sillimanite + quartz and almandine + 02 = hercynite + magnetite + quartz . With suitable correction for reduced activities of solid phases, these equilibria provide useful oxygen barometers for medium- to high-grade metamorphic rocks.

Geochimica et Cosmochimica Acta↗

USGS1992

Antiferromagnetism in pressure-amorphized Fe2SiO4

Amorphous Fe 2 SiO 4 synthesized at elevated pressures exhibits a Néel transition at a temperature identical to that observed in the crystalline form, T N = 65 (±2) kelvin at zero pressure. This behavior contrasts sharply with observations on other disordered systems, such as spin glasses, which characteristically exhibit strong "frustration" of the spins and consequent marked suppression of the Néel transition.

Science↗

USGS1992

Time and metamorphic petrology: Calcite to aragonite experiments

Although the equilibrium phase relations of many mineral systems are generally well established, the rates of transformations, particularly in polycrystalline rocks, are not. The results of experiments on the calcite to aragonite transformation in polycrystalline marble are different from those for earlier experiments on powdered and single-crystal calcite. The transformation in the polycrystalline samples occurs by different mechanisms, with a different temperature dependence, and at a markedly slower rate. This work demonstrates the importance of kinetic studies on fully dense polycrystalline aggregates for understanding mineralogic phase changes in nature. Extrapolation of these results to geological time scales suggests that transformation of calcite to aragonite does not occur in the absence of volatiles at temperatures below 200°C. Kinetic hindrance is likely to extend to higher temperatures in more complex transformations.

Science↗

USGS1992

Solution properties of almandine-pyrope garnet as determined by phase equilibrium experiments

The thermodynamic mixing properties of almandine-pyrope garnet were derived from phase equilibrium experiments at temperatures of 900 and 1000??C and pressures from 8 to 14 kbar. Almandine has essentially ideal behavior in almandine-pyrope garnet over the composition range Alm89-Alm61 at the above experimental conditions. In all experimental products a systematic partitioning of Fe and Mg between garnet and ilmenite was seen with ln Kd ??? 1.59 which was not temperature sensitive. The results support the use of garnet mixing models that incorporate ideal or nearly ideal Fe-Mg parameters. -from Authors

American Mineralogist↗

USGS1991

The Reaction Titanite+Kyanite=Anorthite+Rutile and Titanite-Rutile Barometry in Eclogites

Titanite and rutile are a common mineral pair in eclogites, and many equilibria involving these phases are potentially useful in estimating pressures of metamorphism. We have reversed one such reaction, {Mathematical expression} using a piston-cylinder apparatus. Titanite+kyanite is the high-pressure assemblage and our results locate the equilibrium between 15.5 15.9, 17.7-17.9, 18.8-19.0, and 20.0-20.2 kb at 900, 1000, 1050, and 1100??C, respectively. The experiments require a positive dP/dT of between 20.5 and 23.5 bars/??C for the reaction. We use the reversed equilibrium and two other reactions, {Mathematical expression} and {Mathematical expression} to calculate metamorphic conditions for three eclogite localities. Using these reactions in conjunction with garnet-clinopyroxene Fe2+-Mg exchange equilibria, conditions of metamorphism were 16 kb and 750??C for kyaniteeclogites from Glenelg, Scotland, 21 kb and 625??C for eclogite-facies mica schists from the Tauern Window, Austria, and 46 kb and 850??C for eclogite-facies biotite gneisses from the Kokchetav Massif, USSR. For the Scottish and Austrian eclogites, the pressures derived from the titanite-rutile reactions provide additional constraints on pressures for these localities, leading to precise estimates of metamorphic conditions. In the case of the Soviet Union eclogites, the results show that the silicate-oxide assemblage is consistent with the remarkable occurrence of diamond inclusions in the garnets. The results of this study suggest that titanite and rutile stably coexist in many eclogites and that titanite solid solutions are ideal or nearly so. ?? 1991 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1990

The Stillwater Complex and its anorthosites: an accident of magmatic underplating?

The Stillwater Complex, emplaced 2700??40 Ma, is exposed at the edge of a 4000-km2 block of Late Archean rocks that formed 40 to 110 m.y. yearlier. Voluminous plagioclase cumulates (anorthosites) within the Middle Banded series of the complex are difficult to explain either by in situ fractionation of mafic magma or by popular models for mixing of two magma types. Current models for the evolution of the lowermost continental crust by magmatic underplating suggest that a major crust-forming event of about 100 m.y. duration would satisfy geologic and geochemical constraints for the formation of the Stillwater Complex and the related granitoids. -from Authors

American Mineralogist↗

USGS1989

High-precision UPb ages of metamorphic rutile: Application to the cooling history of high-grade terranes

Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant U Pb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206 Pb/ 204 Pb ratios range from 182 to 22,100 corresponding to 238 U/ 204 Pb ratios of 398–75,100. The proportions of radiogenic 208 Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb. The habits of the rutile crystals range from stubby to acicular, the physical properties vary from opaque/black to transparent/reddish-brown. Separate batches of black and reddish-brown rutile grains from the same samples have similar U and Pb concentrations, Pb-isotope ratios, and yield the same U/Pb ages within analytical uncertainty. No correlation of U concentration and 206 Pb/ 204 Pb ratios with morphology or color of the rutiles was observed among the samples analyzed. Most rutiles yield concordant U Pb ages which are reproducible within analytical uncertainty, i.e. generally ±2 Ma . The U Pb ages for prograde rutile are younger than the time of peak metamorphism given by U Pb ages for garnet and zircon, and also younger than U Pb ages for sphene and monazite, and 40 Ar/ 39 Ar and K Ar ages for hornblende but older than 40 Ar/ 39 Ar and K Ar ages for biotite from the same area. This suggests that the rutile ages reflect cooling below closure temperatures. Within a single hand-specimen, and thus for an identical thermal history, larger rutile grains give older ages than do smaller grains. This suggests that volume diffusion is the most probable mechanism responsible for the ages being younger than the time of peak metamorphism. It also suggests that the dimensions for such diffusion are directly related to the dimensions of the rutile crystal and not to the dimensions of sub-grain domains, as is the case for Ar diffusion in hornblende and feldspar. The concordant U Pb ages as well as the similar ages over a large area suggest that rutile has a well-defined temperature of closure. At a cooling rate of about 0.5–1°C/Ma the closure temperature for U Pb diffusion in rutile is about 420°C for grains with a radius of 0.009–0.021 cm, and about 380°C for grains with a radius of 0.007–0.009 cm. U Pb rutile ages obtained on different grain sizes indicate a cooling rate of about 0.5°C/Ma for the Pikwitonei granulite domain at 2460-2300 Ma. U Pb ages for rutile thus provide high-precision ages for evaluating the cooling history of high-grade terranes and can be critical to the quantitative development of thermal models for crustal evolution.

Earth and Planetary Science Letters↗

USGS1989

Origin of granulite terranes and the formation of the lowermost continental crust

Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystalliztion of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

Science↗

USGS1985

The heat-capacity of ilmenite and phase equilibria in the system Fe-T-O

Low temperature adiabatic calorimetry and high temperature differential scanning calorimetry have been used to measure the heat-capacity of ilmenite (FeTiO 3 ) from 5 to 1000 K. These measurements yield S 298 0 = 108.9 J /( mol · K ). Calculations from published experimental data on the reduction of ilmenite yield Δ 298 0 ( I 1) = −1153.9 kJ /( mol · K ). These new data, combined with available experimental and thermodynamic data for other phases, have been used to calculate phase equilibria in the system Fe-Ti-O. Calculations for the subsystem Ti-O show that extremely low values of ƒO 2 ">ƒ ƒO2 are necessary to stabilize TiO, the mineral hongquiite reported from the Tao district in China. This mineral may not be TiO, and it should be re-examined for substitution of other elements such as N or C. Consideration of solid-solution models for phases in the system Fe-Ti-O allows derivation of a new thermometer/oxybarometer for assemblages of ferropseudobrookite-pseudobrookite ss and hematite-ilmenite ss . Preliminary application of this new thermometer/oxybarometer to lunar and terrestrial lavas gives reasonable estimates of oxygen fugacities, but generally yields subsolidus temperatures, suggesting re-equilibration of one or more phases during cooling.

Geochimica et Cosmochimica Acta↗