Showing papers by "University of Oxford published in 1971"

A plea for excuses

Abstract: The subject of this paper, Excuses, is one not to be treated, but only to be introduced, within such limits. It is, or might be, the name of a whole branch, even a ramiculated branch, of philosophy, or at least of one fashion of philosophy. I shall try, therefore, first to state what the subject is, why it is worth studying, and how it may be studied, all this at a regrettably lofty level; and then I shall illustrate, in more congenial but desultory detail, some of the methods to be used, together with their limitations, and some of the unexpected results to be expected and lessons to be learned. Much, of course, of the amusement, and of the instruction, comes in drawing the coverts of the microglot, in hounding down the minutiae, and to this I can do no more here than incite you. But I owe it to the subject to say, that it has long afforded me what philosophy is so often thought, and made, barren of — the fun of discovery, the pleasures of co-operation, and the satisfaction of reaching agreement.

The structure of the caudate nucleus of the cat: light and electron microscopy.

Abstract: The caudate nucleus of the cat appears to be homogeneous when examined with the light or electron microscope, except for a layer beneath the ependyma where there is a high concentration of glial cells and few neurons. In sections of brains stained with thionin the nerve cells in the caudate nucleus fall into three size groups: less than 8 $\mu m$ , 9 to 18 $\mu m$ , greater than 20 $\mu m$ . Examination of material impregnated with the Golgi technique shows that there are six cell types (one small, four medium and one large), and these are distinguishable on the basis of the size of the cell somata and the appearance and arrangement of their dendrites. One type of medium cell with many dendritic spines forms over 95% of the cell population. The large and one medium cell type are tentatively identified as the source of the efferent fibres of the nucleus. These efferent fibres and the axons of the remaining medium cell types have collateral branches. Three groups of possible afferent fibres have been identified, and these and the collateral branches of the intrinsic neurons form a dense plexus whose individual fibres cross dendrites rather than lie parallel to them. Six cell types may also be distinguished with the electron microscope, and four of these can be correlated directly with those seen in Golgi impregnated material. Several kinds of dendrites are present, the commonest having numerous spines. Fewer spines are present on other varieties of dendrite, and those dendrites which are varicose have no spines. Numerous fine, nonmyelinated axons are present.

Ketone-body utilization by adult and suckling rat brain in vivo

Abstract: 1. Ketone-body utilization in fed and starved adult and suckling rats has been investigated by measuring arterio-venous differences across the brain. Venous blood was collected from the confluence of sinuses and arterial blood from the femoral artery in adult rats and by cardiac puncture in suckling rats. 2. During starvation the arterio-venous difference of ketone bodies increased in proportion to their concentrations in the blood and reached a value of 0.16mm at 48h. At a given concentration of the respective ketone bodies the arterio-venous differences of acetoacetate were about twice those of 3-hydroxybutyrate. 3. Fed rats in which the concentrations of ketone bodies were raised by intravenous infusion of sodium acetoacetate had the same arterio-venous differences as starved rats at corresponding ketone-body concentrations. Thus the ability of the rat brain to utilize ketone bodies is independent of the nutritional state. 4. The concentrations of glucose, acetoacetate and 3-hydroxybutyrate were much lower in the brain than in the arterial blood. The measured (blood concentration)/(brain concentration) ratio was 4.4 for glucose, 4.5 for acetoacetate and 8.1 for 3-hydroxybutyrate in 48h-starved rats. 5. The mean arterio-venous difference of glucose across the brain was 0.51mm in fed rats and 0.43mm in 96h-starved rats. 6. Conversion of glucose into lactate rose from negligible values in the fed state to 0.2mm after 48h starvation and decreased to zero after 96h starvation. 7. In 16–22-day-old suckling rats the arterio-venous differences of ketone bodies across the brain were also proportional to the ketone-body concentration, but they were about 3–4 times greater than in adult rats at the same blood ketone-body concentration. 8. Arterio-venous differences of glucose were about the same in adult and suckling rats. 9. The brain of fed suckling rats formed more lactate from glucose than fed adult rats. 10. The results indicate that ketone bodies are major metabolic fuels of the brain of the suckling rat under normal conditions.

The Rise and Function of the Holy Man in Late Antiquity

Abstract: To study the position of the holy man in Late Roman society is to risk telling in one's own words a story that has often been excellently told before. In vivid essays, Norman Baynes has brought the lives of the saints to the attention of the social and religious historian of Late Antiquity. The patient work of the Bollandists has increased and clarified a substantial dossier of authentic narratives. These lives have provided the social historian with most of what he knows of the life of the average man in the Eastern Empire. They illuminate the variety and interaction of the local cultures of the Near East. The holy men themselves have been carefully studied, both as figures in the great Christological controversies of the fifth and sixth centuries, and as the arbiters of the distinctive traditions of Byzantine piety and ascetic theology.The intention of this paper is to follow well known paths of scholarship on all these topics, while asking two basic questions: why did the holy man come to play such an important role in the society, of the fifth and sixth centuries ? What light do his activities throw on the values and functioning of a society that was prepared to concede him such importance? It is as well to ask such elementary questions. For there is a danger that the holy man may be taken for granted as part of the Byzantine scene. Most explanations of his position are deceptively easy.

Transitive Inferences and Memory in Young Children

Abstract: Contrary to the conclusions of Piaget, young children can make transitive inferences if precautions are taken to prevent deficits of memory from being confused with inferential deficits.

Studies of anaesthesia in relation to hypertension ii: haemodynamic consequences of induction and endotracheal intubation

Abstract: The electrocardiographic and haemodynamic responses to the induction of anaesthesia, followed by laryngoscopy and endotracheal intubation have been studied in a group of 16 untreated hypertensive patients, and a group of 20 patients receiving antihypertensive therapy up to and including the day of operation. The influence of five different induction agents, thiopentone, methohexitone, propanidid, diazepam, and neuroleptanalgesia induced by a combination of phenoperidine and droperidol were compared. Neuroleptanalgesia caused less arterial hypotension than any of the other agents, but afforded only marginally more protection than other agents against hypertension, tachycardia and dysrhythmia associated with laryngoscopy and tracheal intubation. Both propanidid and diazepam caused dramatic but transient hypotension in a small number of patients and were not investigated further. Unlike its effects in normotensive subjects, methohexitone caused greater hypotension than thiopentone in hypertensive patients. The rationale is presented for the prophylactic blockade of beta-adrenergic receptors to prevent hypertensive crises during laryngoscopy and intubation in both treated and untreated hypertensive patients.

Thermoluminescent dating using fine grains from pottery

Abstract: Various aspects of the technique of TL dating using fine grains are discussed. Nonradiation-induced TL can be reduced by taking the glow-curve in an atmosphere of O 2 -free N 2 . U and Th contents are measured by alpha-counting as sealed and unsealed samples in order to give a measure of the amount of emanation of 222 Rn and 220 Rn from the samples. Fine grain sample preparation is described. Beta and alpha TL sensitivities are measured by exposing fine-grain samples to known doses of 90 Sr and 210 PO and responses to gamma rays is assumed to be the same as to beta rays. Included are results of studies of the relationship of the energy spectrum of the alpha par ticles to the induced TL; of the effect of ground water on the natural dose rate; and of the effect of radon emanation on the natural dose rate. The dating procedure and a discussion of results are given. -- AATA

Evaluation of the isolated perfused rat hindquarter for the study of muscle metabolism

Abstract: 1. The metabolic integrity of a new isolated rat hindquarter preparation was studied. The hindquarter was perfused with a semi-synthetic medium containing aged human erythrocytes. More than 95% of the oxidative metabolism of the preparation was due to muscle, the remainder being due to bone, adipose tissue and, where present, skin. 2. Consumption of O(2), glucose utilization, glycerol release and lactate production were similar in the presence and in the absence of the skin, indicating that the latter contributed little to the overall metabolism of the preparation. 3. After 40min of perfusion, tissue concentrations of creatine phosphate, ATP and ADP were similar to those found in muscle taken directly from intact animals. The muscle also appeared normal under the electron microscope. 4. The hindquarter did not lose K(+) to the medium during a 30min perfusion. In the presence of insulin it had a net K(+) uptake. 5. Insulin caused a sixfold increase in glucose uptake, stimulated O(2) consumption by nearly 40% and depressed glycerol release to less than half the control value. 6. Bilateral sciatic-nerve stimulation caused severalfold increases in O(2) consumption and lactate production. In the absence of insulin nerve stimulation also enhanced glucose uptake; in the presence of insulin it did not further increase the already high rate of glucose uptake. 7. Rates of lactate production and O(2) consumption of the rat hindquarter in vivo and the isolated perfused hindquarter were very similar. 8. Ketone bodies were a major oxidative fuel in vivo of the hindquarter of a rat starved for 2 days. If the acetoacetate and 3-hydroxybutyrate removed by the tissue were completely oxidized, they would have accounted for 77% of the O(2) consumption. 9. Acetoacetate accounted for 84% of the ketone bodies removed by the hindquarter in vivo even though its arterial concentration was half that of 3-hydroxybutyrate. 10. Similar rates of acetoacetate and 3-hydroxybutyrate utilization were observed in the perfused hindquarter. 11. Acetoacetate utilization by the perfused hindquarter was not diminished by the addition of either oleate or insulin to the perfusate. 12. Oxidation of glucose to CO(2) accounted for less than 4% of the O(2) consumed by the perfused hindquarter in both the presence and the absence of insulin. 13. The results indicate that the isolated perfused hindquarter is a useful tool for studying muscle metabolism. They also suggest that ketone bodies, if present in sufficient concentration, are the preferred oxidative fuel of resting muscle.

The Dynamics of a Current Collection System for an Electric Locomotive

Abstract: The motion of an overhead trolley wire, suspended at equal intervals by stiff springs, in response to a pantograph moving with constant speed is analysed. The pantograph is modelled by two discrete masses connected by springs and dampers. Away from the supports the inertia and elasticity of the pantograph can be neglected and a simple solution for the wire and pantograph displacement is obtained. Near a support this solution is not valid as it predicts discontinuities in the vertical pantograph velocity. A different first approximation is then required in which the support elasticity and the pantograph inertia and elasticity must be included. This problem is reduced to that of solving a system of four linear differential equations containing one term with a stretched argument. The numerical and asymptotic solution of such a system is discussed and results are obtained for the contact force and pantograph displacement near a support in typical operating conditions. This disturbance at the support is propagated with the wire wave speed and reflected at the subsequent support, thus interacting with the pantograph again. This interaction is analysed and a uniformly valid solution obtained for the contact force over a complete span. Some conclusions are made about possible operating conditions in which loss of contact between the pantograph and the wire may occur.

The connexions of the striatum and globus pallidus: synthesis and speculation.

Abstract: Recent findings on the afferent and efferent connexions of the striatum (the caudate nucleus and putamen) and globus pallidus have been summarized in an orderly sequence. The striatum receives afferent fibres from three main sources, the cerebral cortex, the intralaminar nuclei of the thalamus and the midbrain; the major features of each of these pathways are outlined. The striatum sends efferent fibres to the globus pallidus and the substantia nigra, and the two segments of the globus pallidus in turn project upon the subthalamic nucleus and upon the thalamus and midbrain tegmentum. Through the thalamus the major influence of the striopallidum is upon the motor area of the cerebral cortex, and it is suggested that through the midbrain tegmentum there may also be a descending influence upon the spinal cord. These findings from light microscopical investigations are synthesized with observations made in electron microscopic studies of the striatum and globus pallidus. On the basis of present knowledge of their structure and connexions attention is drawn to several marked similarities between these parts of the basal ganglia and the cerebellum.

Studies of anaesthesia in relation to hypertension i: cardiovascular responses of treated and untreated patients

Abstract: The cardiovascular responses to the induction, maintenance, and recovery from anaesthesia with thiopentone, nitrous oxide and halothane, have been studied in seven elderly normotensive patients, seven untreated hypertensive patients, and fifteen patients under treatment with a variety of anti-hypertensive drugs. In five untreated and three treated hypertensive patients, all of whom had high arterial pressures before anaesthesia, severe reduction of arterial pressure occurred during anaesthesia and was associated with electrocardiographic evidence of myocardial ischaemia. Other treated hypertensive patients, whose arterial pressures were well controlled, behaved in a similar manner to the normotensive patients, and gave no ground for concern during anaesthesia. Cardiac output fell to the same extent (30 per cent) in all three groups, and where a great reduction of arterial pressure occurred it was largely due to reduction of initially high systemic vascular resistance. Baroreflex control of heart rate was significantly depressed in hypertensive patients both before and during anaesthesia. It is concluded that untreated high arterial pressure constitutes a serious risk to patients undergoing anaesthesia and surgery, and therefore anti-hypertensive therapy should not be withdrawn prior to anaesthesia without a compelling reason.

Activities of enzymes involved in acetoacetate utilization in adult mammalian tissues.

Abstract: 1. The activities in rat tissues of 3-oxo acid CoA-transferase (the first enzyme involved in acetoacetate utilization) were found to be highest in kidney and heart. In submaxillary and adrenal glands the activities were about one-quarter of those in kidney and heart. In brain it was about one-tenth and was less in lung, spleen, skeletal muscle and epididymal fat. No activity was detectable in liver. 2. The activities of acetoacetyl-CoA thiolase were found roughly to parallel those of the transferase except for liver and adrenal glands. The high activity in the latter two tissues may be explained by additional roles of thiolase, namely, the production of acetyl-CoA from fatty acids. 3. The activities of the two enzymes in tissues of mouse, gerbil, golden hamster, guinea pig and sheep were similar to those of rat tissues. The notable exception was the low activity of the transferase and thiolase in sheep heart and brain. 4. The activities of the transferase in rat tissues did not change appreciably in starvation, alloxan-diabetes or on fat-feeding, where the rates of ketone-body utilization are increased. Thiolase activity increased in kidney and heart on fat-feeding. 5. The activity of 3-hydroxybutyrate dehydrogenase did not change in rat brain during starvation. 6. The factors controlling the rate of ketone-body utilization are discussed. It is concluded that the activities of the relevant enzymes in the adult rat do not control the variations in the rate of ketone-body utilization that occur in starvation or alloxan-diabetes. The controlling factor in these situations is the concentration of the ketone bodies in plasma and tissues.

An experimental study of the termination of the lateral geniculo-cortical pathway in the cat and monkey.

Abstract: The thalamic projection to the visual cortex has been studied in the cat and monkey by experimental light and electron microscopic techniques. After large lesions of the lateral geniculate nucleus degeneration is confined to the ipsilateral hemisphere. In the cat it is found in areas 17, 18 and 19 and in the lateral suprasylvian area, terminal degeneration occurring predominantly in layer IV, with less in layers I, III and V; fibre degeneration crossing layers VI and V towards layer IV is coarser in area 18 than elsewhere. Some fine horizontal degenerating fibres are seen in layer I. In the monkey terminal degeneration is restricted to area 17; again degenerating fibres ascend to layer IV where there is dense fragmentation, but in contrast to the cat there is also a second, less dense, but distinct, band in layer IIIb. A little fine, horizontal fibre degeneration is present in layer I and there is slight terminal degeneration in this site and in layer V. Electron microscopy shows that degenerating terminals are recognizable in the visual cortex at several stages according to survival period, but that most stages can exist simultaneously in any one site, and that all are associated with asymmetrical membrane thickenings. Mapping of electron microscopic sections confirms the laminar pattern seen with the light microscope. In area 17 of the cat and monkey and in area 19 of the cat over 80% of degenerating terminals end on dendritic spines, the rest making synaptic contact mainly with dendritic shafts, and very few with the soma of stellate cells, but in area 18 some 10% are related to stellate cell bodies. In layer IV of all areas degenerating terminals tend to occur in clusters which are separated by approximately 100 $\mu $m. Where degenerating thalamic afferents end on cell somata or varicose dendrites almost all are identifiable as derived from stellate cells. Although it is difficult to identify positively the parent dendrites bearing the spines which receive the majority of the thalamo-cortical afferents, it is suggested that some, at least, of them may also originate from stellate cells.

Surface Effects in Magnetic Crystals near the Ordering Temperature

Abstract: A form of the Landau-Ginsberg equations applicable to semi-infinite magnetic crystals is derived from the molecular-field theory. We consider both the Heisenberg antiferromagnet and the Heisenberg ferromagnet. We find simple analytic expressions that describe the temperature dependence of the order parameter in the surface region near the ordering temperature. In both cases we find that the order parameter in the surface vanishes linearly with temperature as the ordering temperature is approached from below. This result is in good agreement with the temperature variation of the sublattice magnetization in the surface of antiferromagnetic NiO inferred from the low-energy-electron-diffraction (LEED) data of Palmberg and co-workers, for the entire range of temperatures studied ($0.8{T}_{N}lTl{T}_{N}$). We find that one cannot use the existing LEED data to determine the value of the exchange constants in the surface layer without a measurement of the absolute value of the sublattice magnetization or measurements over a wider range of temperatures. This conclusion differs from that reached in an earlier study based on a numerical solution of the molecular-field equations. We also examine the behavior of the static spin correlation function $〈{S}_{z}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{l}}){S}_{z}({\stackrel{\ensuremath{\rightarrow}}{\mathrm{l}}}^{\ensuremath{'}})〉$ in the paramagnetic state, when the sites $\stackrel{\ensuremath{\rightarrow}}{\mathrm{l}}$ and/or ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{l}}}^{\ensuremath{'}}$ lie near the surface. We find that there should be no magnetic critical scattering of low-energy electrons from the surface, as the ordering temperature is approached from above. The correlation length for spins in or near the surface remains the order of a lattice constant, even at the ordering temperature for both the ferromagnet and the antiferromagnet.

Release of noradrenaline from sympathetic endings.

Abstract: There is evidence that in the transmission of certain nervous impulses, acetylcholine stimulates the release of noradrenaline which then acts as the transmitter substance.

The site of termination of afferent fibres in the caudate nucleus

Abstract: An electron microscopic study has been made of the axon terminal degeneration in the caudate nucleus in the cat after lesions in either the cerebral cortex, the thalamus, the cerebral cortex and the thalamus, the midbrain or within the caudate nucleus. Degenerating axon terminals can be recognized after a survival period of 4 days as dark, shrunken profiles with indistinct vesicles. After shorter survival periods the degenerating terminals contain swollen vesicles and have pale cytoplasm. After lesions in all the above sites there is degeneration of fine myelinated and nonmyelinated fibres. The degenerating terminals of all the afferent fibres to the caudate nucleus have asymmetrical membrane thickenings and end mainly on dendritic spines with a small proportion in contact with peripheral dendrites; after damage of the cerebral cortex or thalamus a few of the degenerating terminals also end upon main stem dendrites and cell bodies. The projection from the ipsilateral cerebral cortex is greater than that from the thalamus, which in turn is heavier than that from the contralateral cortex or midbrain. After lesions within the caudate nucleus degenerating terminals with symmetrical membrane thickenings are found in a region extending approximately 450 $\mu m$ from the damaged part of the nucleus. These terminals make contact with nerve cell somata, main stein and peripheral dendrites and the initial segments of axons. After such a lesion of the caudate nucleus degenerating axon terminals with symmetrical membrane thickenings are also seen in the globus pallidus and the substantia nigra.

Mechanism of absorption of phosphate from soil by Endogone mycorrhizas.

Abstract: ECTOTROPHIC mycorrhizas have often been shown to benefit the growth of their host plants, and similar results have also been found for endotrophic mycorrhizas formed by Endogone species1,2 which are widespread on agricultural crops and are of considerable importance. Several workers have concluded that these growth responses are caused by an increased uptake of phosphate from the soils and possible mechanisms have been suggested2 to account for this increased uptake. The purpose of our work was to distinguish between some of these possibilities, and we report preliminary results here.

Sexual dimorphism in the preoptic area of the rat.

Abstract: A quantitative evaluation of the relative distribution of synapses on dendritic shafts and spines serves to differentiate the neuropil of the preoptic area from that of the ventromedial hypothalamic nucleus; it also shows that the neuropil of the preoptic area is sexually dimorphic.

The termination of fibres from the cerebral cortex and thalamus upon dendritic spines in the caudate nucleus: a study with the Golgi method.

Abstract: The termination of fibres from the cerebral cortex and thalamus upon the dendritic spines of the medium spiny cell of the caudate nucleus has been studied with the Golgi method. Lesions were placed in the cerebral cortex, thalamus or cerebral cortex and thalamus of adult cats and kittens. After survival periods of between 6 and 52 weeks the animals were perfused with a mixture of formaldehyde and glutaraldehyde and the caudate nuclei impregnated by a Golgi technique. The distribution of spines along the dendrites of the medium spiny cell was determined in normal material by counting them over 20 $\mu m$ lengths of the dendrites, and was compared with their number and distribution after the various lesions. The density of spines on the dendrites varies with the distance from the cell body. The first 20 $\mu m$ length of dendrite is spine free, but thereafter the number increases to a peak between 60 and 80 $\mu m$ from the cell body after which the number per 20 $\mu m$ length decreases. The distribution pattern does not alter after any of the lesions, although the overall number of spines decreases. The decrease after lesions in the cerebral cortex or thalamus is the same, and after a combined lesion of thalamus and cortex is twice as great indicating that the fibres from both these regions end upon spines of the same cells. Statistical analysis shows that these results are significant.

Deactivation of Cholera Toxin by Ganglioside

Abstract: cholerae leads to the profuse secretory activity seen in choleraic diarrhea. Lipidcontaining extracts of various tissues were found to inhibit the activity of cholera toxin; the highest amounts of the inhibitory factor were found in brain. The inhibitor has properties similar to those of gangliosides. The effect of cholera toxin on intestinal loops of rabbits and epididymal fat cells of rats was neutralized by purified ganglioside. The results suggested that fixation to gangliosides may play a role in the binding of cholera toxin to cell membranes.

Homotransplantation of Isolated Epiphyseal and Articular Cartilage Chondrocytes into Joint Surfaces of Rabbits

Abstract: THE limited capacity for repair of mature articular cartilage damaged by arthritis, trauma, infection or tumour has stimulated many clinical and experimental investigations into methods of replacing joint cartilage with different types of homotransplants.

Quantitative Determination of Mononucleotide Conformations in Solution using Lanthanide Ion Shift and Broadening NMR Probes

Abstract: A computer treatment is developed for determining the conformations of flexible molecules, such as nucleotides, in aqueous solution.

Preliminary Observations on Tickling Oneself

Abstract: WHY is it that most people cannot tickle themselves? Darwin observed that “from the fact that a child can hardly tickle itself, or in a much less degree than when tickled by another person, it seems that the precise point to be touched must not be known”1. (There may be differences between man and other primates. The Kelloggs observed that their chimpanzee Gua was “frequently observed in the process of tickling herself and laughing as a result”2.) But this hypothesis seems prima facie incorrect, or at least inadequate, as most children can be tickled even when they know where and when the tickle stimulus is to be applied. No experimental work appears to have been reported on this question, and Darwin's interest was primarily centred on the biological value of ticklishness and laughter. The problem is perhaps not trivial; evidently knowledge of some sort is necessary for the cancellation of the ticklish sensation, and it has been shown that cancellation of other signals (for example, those that arise from voluntary movement of the eyes) seems to be produced by self-generated “command” signals rather than by external feed-back3,4. Does a similar mechanism exist for the tactile system ? Another question is whether pain is subject to the same type of control; if so, there could be practical clinical applications.

Activities of enzymes of ketone-body utilization in brain and other tissues of suckling rats

Abstract: 1. The activities of 3-hydroxybutyrate dehydrogenase and 3-oxo acid CoA-transferase in rat brain at birth were found to be about two-thirds of those of adult rat brain, expressed per g wet wt. The activities rose throughout the suckling period and at the time of weaning reached values about three times higher than those for adult brain. Later they gradually declined. 2. At birth the activity of acetoacetyl-CoA thiolase in rat brain was about 60% higher than in the adult. During the suckling period there was no significant change in activity. 3. In rat kidney the activities of the three enzymes at birth were less than one-third of those at maturity. They gradually rose and after 5 weeks approached the adult value. Similar results were obtained with rat heart. 4. The activity of glutamate dehydrogenase (a mitochondrial enzyme like 3-hydroxybutyrate dehydrogenase and 3-oxo acid CoA-transferase) also rose in brain and kidney during the suckling period, but at no stage did it exceed the adult value. 5. Throughout the suckling period the total ketone-body concentration in the blood was about six times higher than in adult fed rats, and the concentration of free fatty acids in the blood was three to four times higher. 6. It is concluded that the rate of ketone-body utilization in brains of suckling rats is determined by both the greater amounts of the key enzymes in the tissue and the high concentrations of ketone bodies in the blood. In addition, the low activities of the relevant enzymes in kidney and heart of suckling rats may make available more ketone bodies for the brain.

Garnet-peridotite stability and occurrence in crust and mantle

Abstract: The boundary between the spinel- and garnet-lherzolite mineral facies is strongly curved between 1300° C and 1450° C; below 1200° C it lies almost parallel to the temperature axis of the T-P diagram. Pressure of at least 12 kb is required to stabilise garnet-peridotite on the geothermal gradient and the depth at which the boundary is encountered is not sensitive to variations in the geotherm. Garnet-peridotite is metastable with respect to spinel-peridotite in normal continental crust. Natural occurrences are mantle derived and have either suffered rapid upward transit in diatremes or, in orogenic zones, either they were emplaced after upward tectonic transport through tens of kilometres, or have originated by in situ metamorphism of pre-existing crustal peridotite in an orogenic root downfolded to depths of at least 40 km.