Alan Bruce Thompson

Bio: Alan Bruce Thompson is an academic researcher from ETH Zurich. The author has contributed to research in topics: Metamorphism & Solidus. The author has an hindex of 48, co-authored 73 publications receiving 10333 citations. Previous affiliations of Alan Bruce Thompson include University of Zurich & Gifu Shotoku Gakuen University.

Pressure-Temperature-Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

Abstract: The development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional-scale thermal regimes. These are: the total radiogenic heat supply within the thickened crust, the supply of heat from the mantle, the thermal conductivity of the medium and the length and time scales of erosion of the continental crust. The orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening, during which little temperature change occurs in individual rocks, followed by a lengthier phase of erosion, at the end of which the crust is at its original thickness. The principal features of pressure-temperature-time (PTt) paths followed by rocks in this environment are a period of thermal relaxation, during which the temperature rises towards the higher geotherm that would be supported by the thickened crust, followed by a period of cooling as the rock approaches the cold land surface. The temperature increase that occurs is governed by the degree of thickening of the crust, its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary. For much of the parameter range considered, the heating phase encompasses a considerable portion of the exhumation (decompression) part of the PTt path. In addition to the detailed calculation of PTt paths we present an idealized model of the thickening and exhumation process, which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path. An important feature of these PTt paths is that most of them lie within 50 °C of the maximum temperature attained for one third or more of the total duration of their burial and uplift, and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience. Considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm. A companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes. © 1984 Oxford University Press.

Fluid-absent (dehydration) melting of biotite in metapelites in the early stages of crustal anatexis

Abstract: Experimental investigation of the fluid absent melting reaction biotite+plagioclase+A12SiOs+quartz~ garnet + K-feldspar + melt at 10 kbar, indicated that melt- ing began between 760 and 800~ and was extensive at 850 ~ C This reaction apparently has a positive dP/dT at least up to 10 kbar, compared to a calculated backbending in a simplified system Natural biotites are stabilised to higher dehydration melting temperatures probably by Ti Consequently, metapelites undergo two-stages of fluid-ab- sent melting even in thickened, continental crust The two stages firstly as muscovite, and secondly as biotite react, appear to persist to at least 17 kbar, depending upon biotite composition This lies well within the stability field of kya- nite migmatites Thus, complex petrogenetic grids can be computed from the little available data over a wide range of P-T-Xroek-XH20 conditions Microprobe analyses indicate good correlations with the Fe/Mg garnet + biotite exchange thermometer and the garnet +plagioclase +A12SiO5 + quartz barometer at 850 ~ C, 10 kbar Furthermore, micro- probe analyses show that vGar'" Liq ~yBio in fluid-absent ~x Fe f Fe t- l Fe as well as in HzO-saturated pelite anatexis Thus intermedi- ate Fe/Mg compositions, containing Bio, Gar, Als, Plg, Ksp, Qtz will eutectically melt before Fe-richer or Mg-richer metapelites If the partition VG,r/vLiq reverses with increas- " Fe /~x Fe

Pressure—Temperature—Time Paths of Regional Metamorphism II. Their Inference and Interpretation using Mineral Assemblages in Metamorphic Rocks

Abstract: A companion paper (England & Thompson, 1984a) investigates the pressure-temperature—time (PTt) paths followed by rocks undergoing burial metamorphism in continental thickening events. This paper discusses problems involved in inferring such paths from the petrological data available in metamorphic rocks and—once such paths are determined—how they may be interpreted in terms of the thermal budgets of metamorphism. Each of the principal facies series (glaucophane-jadeite, andalusite-sillimanite and kyanite-sillimanite) may be encountered by rocks involved in the thickening and erosion of continental crust in a regime of average continental heat flow. The inference of a minimum thermal budget required for a given metamorphism depends strongly on a knowledge of the PTt paths followed by rocks during the metamorphism. Discrimination between possible thermal regimes is greatly enhanced if portions of PTt paths, rather than single PT points, are available, and additional constraint is possible if these paths are supplemented by geochronological, structural and heat flow data. 1. I N T R O D U C T I O N One of the standard inverse problems of metamorphic petrology is to infer the thermal regime operating during a metamorphic episode from the mineral assemblages and compositions in rocks that reach the surface after the episode is finished. In fact, orogenic processes are sufficiently complicated, and petrological data sufficiently limited that no formal inverse is ever attempted; interpretation usually depends on the calculation of pressure-temperature— time {PTt) paths for model tectonic settings and on using these with metamorphic petrological data to give estimates of thermal regimes operating during metamorphism. The previous paper (England & Thompson, 1984a, hereafter called Part I) outlined some of the model pressure-temperature-time (PTt) paths that would be followed by rocks under a variety of conditions resulting from crustal thickening, and addressed the question of what class of information is recoverable from PT data obtained from metamorphic rocks. A major conclusion of Part I is that we expect the overall thermal development of a regional metamorphic terrain to depend on relatively few large scale parameters and, consequently, that if it is possible to extract information from mineral assemblages about regional PTt paths, we should be able to make estimates of the magnitudes of the parameters controlling the regional metamorphic budget—notably the total heat supply to the orogenic belt, the lengthscales of thickening, the length and time scales of erosion and the extent of magmatic involvement in the metamorphic episode. Smaller scale phenomena (intrusions, transposition of isograds by folding or faulting, etc.) will affect observations made in the field, but cannot be uniquely interpreted in terms of larger scale phenomena. I Journal of Petrology. Vol. 25. Part 4. pp. 929-955. I984| 930 A. B. THOMPSON AND P. C. ENGLAND In this paper we discuss the problems of relating petrological PT points or, more usefully, portions of PTt paths to regional thermal regimes. In principle such data, particularly in conjunction with heat flow, isotopic and structural studies, can provide valuable information on the tectonic setting and thermal budget of metamorphism. Unfortunately, it is often not possible to gather all the information necessary for a unique interpretation of a given terrain; nonetheless PTt data may be used to place constraints on metamorphic processes, and we hope that the considerations presented here will help in the interpretation of such data. 2. PT POINTS AND PTt PATHS Many of the inferences of conditions during the evolution of orogenic belts that have been made using PT data from metamorphic rocks are based on relatively simple conceptual models of the metamorphic environment and, while these models serve a useful purpose, it is important to recognize their limitations as well as their merits. In this section we consider how these conceptual models need modification in view of the nature of the PTt paths that are followed by rocks undergoing regional metamorphism. For consistency, most of the paths discussed here are taken from section 5 of Part I, but many of the arguments may readily be applied (with the appropriate adjustments) to other forms of PTt paths. The first set of problems encountered in inferring thermal and tectonic regimes from metamorphic data are concerned with deciding where on a PTt path the points that make up a PT array are recorded, and the rest of this section is largely concerned with them. We consider how the kinds of PTt paths calculated in Part I influence PT data recorded in mineral assemblages on the surface, through their effects on reaction kinetics, chemical diffusion and through the control exerted by fluids on the mineral assemblages preserved. In particular, we emphasize the means whereby information may be gained on parts of PT paths, rather than on discrete PT points. 2.1. Geotherms, facies series and PT paths The material of these papers concerns the perturbing and relaxation of temperatures in continental lithosphere, and the PT conditions experienced by rocks moving relative to these evolving geotherms. As the word geotherm is used in at least two distinct senses in the petrological literature it is worth including definitions here to avoid possible ambiguities. A geotherm is simply the relation between temperature and depth within the earth at any one time; this relation may be a steady state one or a transient one, but in either case its FIG. 1. Metamorphic mineral assemblages at the erosion surface in (a) are assigned through experimentally calibrated mineral reactions to metamorphic fades series (6) and fades (c). The equilibrium data in (b) are shown as examples only; the sources are given by Thompson & Thompson (1976, fig. 13) and by Thompson & Tracy (1979, fig. 2) and transferred to this figure using a crustal density of 2-8 Mg m~. The equilibria include two versions of the relative stability of Al2SiOj polymorphs (from Holdaway, 1971 and Richardson el a/., 1969; where andalusite = AND; kyanite = KYA; sillimanite = SIL), and the breakdown of albite (ALB) to jadeite (JAD) plus quartz (QTZ). The dehydration reactions produdng almost pure H2O (vapor = V) are experimentally investigated in the presence of excess water (onp x 1). Other abbreviations: muscovite (MUS), analdme (ANL), pyrophyllite (PYP), potash-feldspar (KSP). Mineral assemblages assigned to mineral facies (c) at the erosion surface in (a) are shown with normal type to distinguish them from facies representing present day conditions in the underlying crust, shown by italic type. The P-T locations of metamorphic facies (c) and facies series (6) are based upon correlation with experimental studies of many more mineral equilibria than shown here (for example see Fyfe el a/., 1978, fig. 6.8). Because of the transitional nature of mineral assemblages in the field and the displacement of calibrated reactions through crystalline solution in natural minerals, the P-T boundaries of facies and fades series are approximate and gradational. One common practice is the assigning of mineral assemblages at the erosion surface (solid points in (a)) to a steady-state geotherm, shown by the heavy curves in (6) and (c). The dashed lines crossing the geotherm in (c) represent schematic PTt paths experienced by rocks during exhumation (see Part I). •1 3 JO m C C C jo m I H m m C m m H X P R E S S U R E , kb < p » 2 .8 g c m \" 3 ) ? * * * PR ES SU R E , kb ( p -2 .8 g c m \" 3 ) 932 A. B. THOMPSON AND P. C. ENGLAND reference level is the earth's surface. A transient geotherm may be the result of transient heat sources, of motions due to erosion, overthrusting, rifting etc., or of thermal relaxation from a previously perturbed regime; all three of these cases occur during the evolution of the systems discussed in this paper. Individual PT points obtained from mineral assemblages are often interpreted in terms of geothermal gradients (Fig. 1); it should be clear from Part I that while an individual PT point may have been set on a geotherm, its relation to the heat source distribution giving rise to that geotherm is usually unclear. The problem becomes worse if an array of PT points from one terrain is to be interpreted; such arrays have been referred to, misleadingly, as 'metamorphic geotherms' (e.g. England & Richardson, 1977). In fact these arrays bear no relation to any one geotherm that existed during metamorphism, because they consist of a set of PT conditions inferred from assemblages that were preserved at different times and different places during metamorphism (e.g. England & Richardson, 1977, fig. 1; section 3.1 below). We prefer the designation 'piezothertnic array' (Richardson & England, 1979) for the set of peak metamorphic conditions experienced in a thickened pile, or 'PT array' for a suite of metamorphic conditions preserved in a metamorphic terrain. Certain rock types and minerals are often regarded as characteristic of distinct pressure (/>) and temperature (T) regimes, and the indications these give of changing metamorphic grade on a regional scale lie behind the facies series concept of Miyashiro (1961). The three principal facies series, glaucophane-jadeite (Gla—Jad), kyanite—sillimanite (Kya-Sil) and andalusite-sillimanite (And—Sil) are taken to be representative of, respectively, high P— low T, intermediate T and P, and high T-low P metamorphism. In addition, several workers (e.g. Heitanen, 1967) specify intermediate facies series (see Fig. 1). In principle this approach acknowledges only that the rocks involved have passed through some region of PT space that is characterized by the stability fields of the diagnostic minerals, although the facies series concept, in its application, is often used to infer a tectonic setting for the metamorphism. We shall refer in this paper to facies, but we

Trace element discrimination diagrams for the tectonic interpretation of granitic rocks

Abstract: Granites may be subdivided according to their intrusive settings into four main groups—ocean ridge granites (ORG), volcanic arc granites (VAG), within plate granites (WPG) and collision granites (COLG)—and the granites within each group may be further subdivided according to their precise settings and petrological characteristics. Using a data bank containing over 600 high quality trace element analyses of granites from known settings, it can be demonstrated using ORG-normalized geochemical patterns and element-SiO2 plots that most of these granite groups exhibit distinctive trace element characteristics. Discrimination of ORG, VAG, WPG and syn-COLG is most effective in Rb-Y-Nb and Rb-Yb-Ta space, particularly on projections of Y-Nb, Yb-Ta, Rb-(Y + Nb) and Rb—(Yb + Ta). Discrimination boundaries, though drawn empirically, can be shown by geochemical modelling to have a theoretical basis in the different petrogenetic histories of the various granite groups. Post-collision granites present the main problem of tectonic classification, since their characteristics depend on the thickness and composition of the lithosphere involved in the collision event and on the precise timing and location of magmatism. Provided they are coupled with a consideration of geological constraints, however, studies of trace element compositions in granites can clearly help in theelucidation of post-Archaean tectonic settings.

An internally consistent thermodynamic data set for phases of petrological interest

Abstract: The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral–fluid equilibria to 100 kbar pressure or higher. A pressure-dependent Landau model for order–disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha–beta quartz transition to be handled more satisfactorily. Several melt end-members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data.

A geochemical classification for granitic rocks

Abstract: This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

Tectonic Implications of the Composition of Volcanic Arc Magmas

Abstract: Volcanic arc magmas can be defined tectonically as magmas erupting from volcanic edifices above subducting oceanic lithosphere. They form a coherent magma type, characterized compositionally by their enrichment in large ion lithophile (LlL) elements relative to high field strength (HFS) elements. In terms of process, the predominant view is that the vast majority of volcanic arc magmas originate by melting of the underlying mantle wedge, which contains a component of aqueous fluid and/or melt derived from the subducting plate. Recently, opinions have converged over the key aspects of the physical model for magma generation above subduction zones (Davies & Stevenson 1992), namely: 1. that the mantle wedge experiences subduction-induced corner flow (e.g. Spiegelman & MacKenzie 1987); 2. that the subduction component reaches the fusible part of the mantle wedge by the three-stage process of (i) metasomatism of mantle lithosphere, followed by (ii) aqueous fluid release due to breakdown of hydrous minerals at depth (e.g. Wyllie 1983, Tatsumi et al 1983) and (iii) aqueous fluid migration, followed by hydrous melt migration, to the site of melting; 3. that slab-induced flow may be locally reversed beneath the arc itself, allowing mantle decompression to contribute to melt generation (e.g. Ida 1983). The simplified model in Figure 1 highlights the physical and chemical processes that have been invoked as being important in controlling the composition of volcanic arc magmas. Magma compositions (coupled with experimental data on element behavior) can help us gain further understanding of these physical and chemical processes. In this review, we first summarize knowledge of the behavior of elements in the subduction system. We then focus on compositional evidence for the processes illustrated in Figure 1, which we group as follows: 1. derivation of the subduction component, 2. transport of the subduction component to the melting column, 3. depletion and enrichment of the mantle wedge, and 4. processes in the melting column.

An internally consistent thermodynamic data set for phases of petrological interest

Abstract: The thermodynamic properties of 154 mineral end-members, 13 silicate liquid end-members and 22 aqueous fluid species are presented in a revised and updated data set. The use of a temperature-dependent thermal expansion and bulk modulus, and the use of high-pressure equations of state for solids and fluids, allows calculation of mineral-fluid equilibria to 100 kbar pressure or higher. A pressure-dependent Landau model for order-disorder permits extension of disordering transitions to high pressures, and, in particular, allows the alpha-beta quartz transition to be handled more satisfactorily. Several melt end- members have been included to enable calculation of simple phase equilibria and as a first stage in developing melt mixing models in NCKFMASH. The simple aqueous species density model has been extended to enable speciation calculations and mineral solubility determination involving minerals and aqueous species at high temperatures and pressures. The data set has also been improved by incorporation of many new phase equilibrium constraints, calorimetric studies and new measurements of molar volume, thermal expansion and compressibility. This has led to a significant improvement in the level of agreement with the available experimental phase equilibria, and to greater flexibility in calculation of complex mineral equilibria. It is also shown that there is very good agreement between the data set and the most recent available calorimetric data. kinetics which apply to determining directly the greatest majority of such equilibria in the laboratory, for forming solid solutions, and inclusion of aqueous and silicate melt species), and provides uncertainties especially at lower temperatures, as well as the diYculty of establishing reversals of reactions involving solid allowing the likely uncertainties on the results of thermodynamic calculations to be estimated. This is a solutions. The levels of precision and accuracy required of thermodynamic data in order to be able to forward- critical issue in that calculations using data sets should always involve uncertainty propagation to help evalu- model synthetic and natural mineral assemblages mean that the continuing upgrading and expansion of the ate the results. Because the experimental phase equilib- ria involve overlapping subsets of compositional space, data set by incorporation of new phase equilibrium constraints, calorimetry and new measurements of the derived thermodynamic data are highly correlated, and it is only the inclusion of the correlations which molar volume, thermal expansion and compressibility are more than justified. enables the reliable calculation of uncertainties on mineral reactions to be performed. Earlier work on mineral thermodynamic data sets for rock-forming minerals includes compilations of The thermodynamic data extraction involves using weighted least squares on the diVerent types of data