Granitoids as categorized by tectonic environment are (1) island arc granitoids (IAG), (2) continental arc granitoids (CAG), (3) continental collision granitoids (CCG), (4) postorogenic granitoids (POG), (5) rift-related granitoids (RRG), (6) continental epeirogenic uplift granitoids (CEUG), and (7) oceanic plagiogranites (OP). Of these, the IAG, CAG, CCG, and POG are considered orogenic granitoids, and the RRG, CEUG, and OP are considered anorogenic granitoids.

The discrimination of granitoids is based on the major-element chemistry. Various discrimination plots are presented which sequentially discriminate the different tectonic environments. OP are separated from all other granitoids on the K2O versus SiO2 plot. Discrimination between group I (IAG + CAG + CCG), group II (RRG + CEUG), and group III (POG) granitoids can be achieved by using plots of Al2O3 versus SiO2, FeO(T)/ [FeO(T) + MgO] versus SiO2, and AFM and ACF ternary diagrams. In the figures, group I and group II plot in individual fields. Identification of group III is different, in that group III does not have a unique field in which it plots. Group III is identified because it consistently displays characteristics of both group I and group II. Further discrimination within group I can be accomplished on the basis of Shand's index. Only CCG have A/CNK [AL2O3/(CaO + Na2O + K2O)] values greater than 1.15. It is not possible to discriminate between IAG and CAG. Further discrimination within group II is done using the TiO2 versus SiO2 plot.

The proposed discrimination scheme is applied to the Proterozoic granitoids of the midcontinent of the United States. It is shown that the Arbuckle granitoids are not anorogenic as previously thought.

Tectonic discrimination of granitoids

Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria:
 For A and B all possible within-site and cross-site interactions among the species □−K−Na−Ca−Mg−Fe2+−Fe3+−Al−Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.

Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry

Electromagnetically induced transparency1,2,3 is a quantum interference effect that permits the propagation of light through an otherwise opaque atomic medium; a ‘coupling’ laser is used to create the interference necessary to allow the transmission of resonant pulses from a ‘probe’ laser. This technique has been used4,5,6 to slow and spatially compress light pulses by seven orders of magnitude, resulting in their complete localization and containment within an atomic cloud4. Here we use electromagnetically induced transparency to bring laser pulses to a complete stop in a magnetically trapped, cold cloud of sodium atoms. Within the spatially localized pulse region, the atoms are in a superposition state determined by the amplitudes and phases of the coupling and probe laser fields. Upon sudden turn-off of the coupling laser, the compressed probe pulse is effectively stopped; coherent information initially contained in the laser fields is ‘frozen’ in the atomic medium for up to 1 ms. The coupling laser is turned back on at a later time and the probe pulse is regenerated: the stored coherence is read out and transferred back into the radiation field. We present a theoretical model that reveals that the system is self-adjusting to minimize dissipative loss during the ‘read’ and ‘write’ operations. We anticipate applications of this phenomenon for quantum information processing.

Observation of coherent optical information storage in an atomic medium using halted light pulses

Knowledge of temperature and pressure, however qualitative, has been central to our views of geology since at least the early 19th century. In 1822, for example, Charles Daubeny presented what may be the very first “Geological Thermometer,” comparing temperatures of various geologic processes (Torrens 2006). Daubeny (1835) may even have been the first to measure the temperature of a lava flow, by laying a thermometer on the top of a flow at Vesuvius—albeit several months following the eruption, after intervening rain (his estimate was 390°F). In any case, pressure ( P ) and temperature ( T ) estimation lie at the heart of fundamental questions: How hot is Earth, and at what rate has the planet cooled. Are volcanoes the products of thermally driven mantle plumes? Where are magmas stored, and how are they transported to the surface—and how do storage and transport relate to plate tectonics? Well-calibrated thermometers and barometers are essential tools if we are to fully appreciate the driving forces and inner workings of volcanic systems.

This chapter presents methods to estimate the P-T conditions of volcanic and other igneous processes. The coverage includes a review of existing geothermometers and geobarometers, and a presentation of approximately 30 new models, including a new plagioclase-liquid hygrometer. Our emphasis is on experimentally calibrated “thermobarometers,” based on analytic expressions using P or T as dependent variables. For numerical reasons (touched on below) such expressions will always provide the most accurate means of P-T estimation, and are also most easily employed. Analytical expressions also allow error to be ascertained; in the absence of estimates of error, P-T estimates are nearly meaningless. This chapter is intended to complement the chapters by Anderson et al. (2008), who cover granitic systems, and by Blundy and Cashman (2008) and Hansteen and Klugel (2008), who consider additional methods …

Thermometers and Barometers for Volcanic Systems

The thermodynamic properties of 254 end-members, including 210 mineral end-members, 18 silicate liquid end-members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end-members have been added, including several deep mantle phases and, for the first time, sulphur-bearing minerals. Silicate liquid end-members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine-bearing equilibria, sulphur-bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

A new geobarometer based on the Al content of igneous hornblendes in equilibrium with melt, fluid, biotite, quartz, sanidine, plagioclase, sphene, and magnetite or ilmenite has been calibrated experimentally. The calibration was performed by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780C, and then analyzing euhedral hornblendes in equilibrium with glass (melt). Experiments were performed on natural samples of both volcanic and plutonic rocks. Earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks. The experimental calibration differs from the empirical calibrations, especially above 5 kbar, and shows that the Al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought. The geobarometers uncertainty is dramatically reduced. The derived equation is P ({plus minus}0.5 kbar) = 3 {minus}3.46 ({plus minus}0.24) + 4.23 ({plus minus}0.13) (Al{sup T}). The geobarometer is applied to post-Bishop Tuff volcanic rocks from Long Valley caldera, California, and reveals that most rhyodacites in this complex erupted from depths of about 6 km. These eruptions occurred over 500,000 yr, suggesting that the rhyodacitic magma reservoir beneath Longmore » Valley had reached a steady P (depth)-T state.« less

Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks

Recent 1980–1986 Mount St. Helens dacites contain the phenocryst assemblage, plagioclase, amphibole, low-Ca pyroxene, magnetite, ilmenite, and rare high-Ca pyroxene, which indicates that they all originated from an 8 km deep reservoir at 900°±20°C with XH2O= 0.67 in fluid according to experimental data. Iron-titanium oxide phenocryst compositions indicate that all post May 18 dacitic magmas erupted at 900°±20°C except for the final lava extrusion in October 1986; the magma reservoir may have cooled to 866°C by October 1986. Amphiboles in the post May 18, 1980, magma contain one or more amphibole populations characterized by reaction rims of different thicknesses. The development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir. Constant P and T and isothermal decompression experiments show that during a 900°C constant rate decompression from 8 km to the surface, no reaction rim develops on amphibole in 4 days, a 10-μm rim develops in 10 days, and a 35-μm rim develops in 20 days. These experimental data and histograms of rim widths in 1980–1986 Mount St. Helens dacites show that post May 18 eruptions are composed in large part of magma represented by a population of thin-rimmed amphiboles, magma which ascended from the deep (8 km) reservoir in 6 to 10 days. The remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 μm, magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly (millimeter scale) into the erupting magma. The mixing in a viscous, slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population. The experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post May 18 dacites of about 15–30 m/h for magma represented by the thick-rimmed amphiboles and from 35 to 50 m/hr for magma represented by the thin-rimmed crystals. An ascent rate of >66 m/h is indicated for the May 18, 1980, eruption, which contains amphiboles with no reaction rims. The volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath Mount St. Helens in 1981–1982 was equivalent to a cylinder 8 km long and 8–9 m in radius.

Magma ascent rates from amphibole breakdown: An experimental study applied to the 1980–1986 Mount St. Helens eruptions

Three methods of estimating H20 contents of geologic glasses are compared: (1) ion microprobe analysis (secondary ion mass spectrometry), (2) Fourier-transform infrared spectroscopy(FTIR), and (3) electron microprobe analysis using the Na decay-curve method. Each analytical method has its own advantages under certain conditions, depending on the relative importance of analytical accuracy, precision, sensitivity, spatial resolution, and convenience, and each is capable of providing reasonably accurate estimates of the H20, or total volatile, content of geologicglasses.The accuracy of ion microprobe analyses depends critically on the availability of well-characterized hydrous standard glasses. Precision is often better than 0,2 wt% (10). The method provides good spatial resolution (-15 #m) and the capability to determine simultaneously the abundance of other volatile species of interest (e.g., F, B). FTIR spectroscopy provides excellent analytical sensitivity (-10 ppm), accuracy and precision «0.1 wt%),and the capability to determine the abundance of H20 and C02 species (H20, OH-, C02' eOj-) in analyzed glasses, although the spatial resolution (> 25-35 #m) is not as good as that of the ion microprobe. The main advantages of the estimation of H20 contents of hydrous glasses using the electron microprobe are excellent spatial resolution (- 10 #m) and analytical convenience. The disadvantages are that accuracy and precision (>0.5 wt%) are not as good as those associated with the other methods, but, for certain applications, these uncertainties may be acceptable for the estimation of H20 contents of H20-rich (> 1 wt%) samples.

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Comparison of microanalytical methods for estimating H2O contents of silicic volcanic glasses

The Mount St. Helens, May 18 pumice is a dacite containing 60% glass by weight and phenocrysts of plagioclase, orthopyroxene, amphibole, titaniferous magnetite, and ilmenite. The glass is uniform in composition, a rhyodacite with 73 wt % SiO2; the phenocrysts are also uniform in composition except for the plagioclase, which has cores averaging An57 and rims averaging An49. Analyses of seven pairs of coexisting Fe-Ti oxides in a representative sample of the light pumice were recast using various mineral calculation procedures; they yielded temperatures ranging from 920° to 940°C and a -log ƒO2 of 10.3–10.1. Electron microprobe analyses of 57 glass inclusions trapped in plagioclase phenocrysts in the light pumice showed little deviation from an average rhyodacitic composition (69.90±0.87 wt % SiO2) when special care was taken to account for Na loss during the analysis. The difference between the average total of these glass inclusion analyses and 100% is 4.6±1 wt %, which is interpreted to be volatiles dissolved in the glass. On an anhydrous basis the average glass inclusion composition is identical to the matrix glass, indicating that neither underwent significant fractionation after melt was trapped by the plagioclase. Experimentally determined phase relations for the representative dacite sample place limits on conditions in the May 18 Mount St. Helens magma chamber, assuming that the dissolved volatiles were 4.6±1 wt % and the temperature was 920°–940°C. Hydrothermal experiments over a range of P, T, and ƒO2 indicate that at no pressure is the observed phase assemblage and residual melt chemistry produced when PH2O = PTotal. Experiments using CO2-H2O fluids to achieve PH2's less than PFluid did reproduce the observed residual melt chemistry and an An50 plagioclase at a specific set of conditions, i.e., atƒO2's between the NNO and MNO buffers, at a PFluid of 220 MPa (2.2 kb), and at a PH2 = 110 MPa (all at 920°–940°C). Amphibole was not stable under these conditions but possibly would be if the PH2 / PFluid ratio was raised to 0.7 or if fluorine were added to the experimental system. It is concluded that just prior to eruption, the upper part of the Mount St. Helens magma chamber was at a pressure of 220±30 MPa corresponding to a depth of 7.2±1 km, PH2 was 0.5 to 0.7 PTotal, and the temperature was 930°±10°C.

The May 18, 1980, eruption of Mount St. Helens: 1. Melt composition and experimental phase equilibria

[1] Experiments were conducted to study the temporal evolution of feldspar crystallization kinetics during isothermal decompression. Pinatubo dacite was held at 780°C, 220 MPa, fO2 = NNO + 2, H2O-saturated conditions for an equilibration period, decompressed to final pressures, Pf, ranging from 175 to 5 MPa, and then held for 0.3–931 hours. According to the plagioclase liquidus curve in PH2O-T space for the relevant melt composition, these decompressions impose effective undercoolings, ΔTeff, of 34–266°C. Growth of preexisting phenocrysts and newly formed sparse microlites dominate crystallization at 75 ≤ Pf < 150 MPa (ΔTeff = 34–93°C), and equilibrium crystal modes are achieved in <168 hours. Microlite nucleation is the dominant transformation process for 10 < Pf < 50 MPa (ΔTeff = 125–241°C), and chemical equilibrium is not attained by 168 hours under these conditions. Slow, steady decompressions typically produced normally zoned, euhedral, and planar-faceted feldspar crystals, although anhedral morphologies were produced at very low Pf. Contrary to expectation, slowly decompressed samples were usually further from chemical equilibrium than rapidly decompressed samples after similar durations below the initial pressure. Although counterintuitive, these trends are consistent with new constraints on the relative rates of feldspar nucleation and growth (controlled by ΔTeff and melt viscosity) experienced during each decompression path. Analysis of liquid to solid transformation kinetics using TTT-style diagrams shows that crystallization occurs most rapidly at ∼100 MPa by a crystal growth mechanism. The next most efficient crystallization conditions are at 25 MPa, in a crystal nucleation-dominated regime.

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Malcolm J. Rutherford

Papers

Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks

Magma ascent rates from amphibole breakdown: An experimental study applied to the 1980–1986 Mount St. Helens eruptions

Comparison of microanalytical methods for estimating H2O contents of silicic volcanic glasses

The May 18, 1980, eruption of Mount St. Helens: 1. Melt composition and experimental phase equilibria

An experimental study of the kinetics of decompression-induced crystallization in silicic melt