Experimental Petrology Research Articles

Energy-dispersive x-ray microanalysis in the earth sciences

The electron microprobe analyser (EMPA) is used extensively for the analysis of the constituent minerals in rocks and the samples generated by experimental petrology apparatus. These analyses, combined with the results of field observations and data from other analytical techniques, are used in petrogenetic studies and hence in the determination of planetary formation and evolution. In turn, this knowledge helps the mining industry in their mineral exploration programs.In the 1960s almost all geological usage of the EMPA was confined to the x-ray spectrometry of L3-K lines of elements of atomic number 11 through 30 (Na through Zn). By the end of this decade semi conductor technology had advanced so that these x-ray lines could be resolved using a lithium-drifted silicon detector working as an energy dispersive spectrometer (EDS). Quantitative EDS software was developed in the early 1970s and it became possible to perform major element analyses of silicates, oxides, carbonates and sulphides using a scanning electron microscope (SEM) fitted with an EDS.

Experimental petrology and petrogenesis of mare volcanics

Mare volcanics consist of basalts and picritic pyroclastic glasses spanning a wide range of TiO 2 concentration. The more primitive low-Ti basalts and picritic glasses have olivine alone on their lowpressure liquidi. Most of the chemical variation among the low-Ti basalts is the result of olivine fractionation in a series of parental MgO-rich liquids differing in TiO 2 concentration. With one possible exception (Apollo 17 VLT) none of the picritic compositions is a suitable parent for any of the observed low-Ti basalts. Most of the chemical variation among the high-Ti basalts is the result of a series of magmas fractionated along the low-pressure olivine + armalcolite/ilmenite cotectic. All of the picritic high-Ti glasses have olivine alone on the liquidus, but none is a suitable parent for any of the basalts. Volcanics with intermediate TiO 2 concentrations (5 to 10 wt%) are widespread in the maria, even though they are not well represented in the sample collections; however, there is no evidence either among the samples or from remote sensing studies of basalts with > 13 wt% TiO 2 that would be expected as differentiates of the picritic glasses with the highest TiO 2 concentrations. Controlled-cooling-rate crystallization studies on a variety of mare compositions have provided the basis for reconstructing the size and, in some cases, stratigraphy of mare flows. Groundmass textures, crystal size, crystal morphology, nucleation density, and zoning patterns have all been employed to quantify cooling histories of mare basalts. A single-stage linear cooling rate may produce a porphrytic texture. Rapid cooling may also cause plagioclase to crystallize after a mineral that it precedes during slow cooling. Mare basalts are highly reduced. Mineral assemblages and intrinsic oxygen fugacity measurements indicate ƒ O 2 below the wüstite-iron buffer and at or near iron metal saturation. Accordingly, experiments run in high-purity iron capsules gained or lost little iron. Most basalts are undersaturated with respect to sulphur, so reduction through sulphur volatilization cannot be invoked to explain the presence of iron in olivine phenocrysts. The low oxidation state is most likely the result of melting a reduced interior under fluid-absent conditions. The progressive reduction of Cr 3+ → Cr 2+ and Ti 4+ → Ti 3+ in lunar melts permits the elimination of the Fe 3+ that is present at iron-saturation in simple systems. Crystal-liquid partition coefficients determined from melting experiments have been used in a wide range of calculations of major and trace element evolution. The coefficient for Fe-Mg exchange between olivine and liquid apparently varies with TiO 2 concentration of the liquid and is particularly useful in assessing whether fine-grained rocks have excess olivine. Nb and Ti have excess concentrations in mare basalts relative to adjacent REE in incompatibility diagrams. These excesses or positive anomalies are consistent with ilmenite accumulation in light of measured partition coefficients and imply continuous variation of accumulated ilmenite even in the low-Ti mare source regions. Pressures of multiple saturation (olivine + pyroxene ± Cr-rich spinel ± ilmenite) are in the range of 5 to 12.5 kbar for primitive mare basalts and in the range of 18 to 25 kbar for the picritic glasses. Low-Ca pyroxene is the only pyroxene along the liquidus of the low-Ti basalts and glasses; however, augite is the pyroxene most commonly observed along the high-pressure liquidi of the high-Ti basalts: High-Ti picrites have augite in the subliquidus region at intermediate pressures where olivine is the liquidus phase, but orthopyroxene is the liquidus phase at multiple saturation. Because of the steep depth/pressure gradient in the outer portion of the Moon (20 km/kbar), these pressures imply: 1. (a) great depths of melting within the Moon. 2. (b) some means of transporting magmas hundreds of kilometers to the surface without significant chemical modification, if both olivine and pyroxene were left in the residuum. Modeling of major elements during polybaric partial melting suggests that it is possible to reproduce the composition and high-pressure signature of the low-Ti (green) picritic glasses by accumulating small degrees of melt extracted from an upwelling source region. In such a case, melting of the differentiated source must begin at > 1000 km depth (40 kbar) and cease at ~ 100 km (5 kbar). If this model is correct, then experimental determination of the pressure of multiple saturation gives an average pressure of melting: the onset of melting is at higher pressure and actual segregation of the melt from the mantle is at lower pressure.

Element partitioning between majorite and liquid: A secondary ion mass spectrometric study

The secondary ion mass spectrometry (SIMS) was applied to measure the trace element abundances in majorite and the ultramafic liquid equilibrating at high pressure and temperature for the first time, and was proved to be useful for the high pressure experimental petrology. The analysis by SIMS was made to determine the abundance of 15 lithophile elements (Li, Be, B, Sc, Ti, V, Cr, Mn, Y, Zr, La, Sm, Gd, Yb, and Hf), one siderophile (Co), and two chalcophile elements (Cu, Zn). The electron microprobe (EPMA) analysis was also made to obtain the major element abundance. Some lithophile elements (Al, Be, and V) are enriched in majorite, whereas the lithophile elements (Li, B, Na, K, Ca, Ti, Zr, Y, La, Sm, Gd, and Hf), and siderophile and chalcophile elements (Co, Cu, Zn) are favored in the liquid. These results give important constraint for the genesis of komatiites and chemical heterogeneity in the Archean mantle.

Norman L. Bowen (1887-1956), Mit Class of 1912, First Predoctoral Fellow of the Geophysical Laboratory

One of the great pioneers in experimental petrology, Norman Levi Bowen bridged geology and physical chemistry. Trained at Queen's University, Kingston, Ontario, Canada, where he received prizes in both mineralogy and chemistry, he then studied under Reginald A. Daly at MIT. On the advice of T. A. Jaggar and with the encouragement of C. H. Warren of MIT, he undertook an experimental thesis study on the nepheline-anorthite system at the Geophysical Laboratory on the suggestion of its director. A. L. Day. Bowen was the first Predoctoral Fellow of the Laboratory. The results were accepted by MIT as partial fulfillment of the Ph. D. degree in 1912, the third Ph. D. degree to be issued by their Department of Geology. By the end of 1912, Bowen had six field seasons and eight publications to his credit. Within the next three years he laid out the physico-chemical methodology for the solution of problems in igneous petrology in "The later stages of the evolution of the igneous rocks." The greatness of Norman Levi Bowen stems from his clear exposition of physicochemical principles and their application to major complex geological field problems.

Chemical characteristics and genesis of the quartz-feldspathic rocks in the Archean crust of Greenland

A total of 108 samples of meta-tonalites, metagranodiorites, granites and meta-tholeiites representing groups of Early to Late Archean age and different metamorphic history from SW and SE Greenland have been analyzed for Ca, K and 28 trace elements. There is no systematic change of the chemical composition with age observable. The results support petrologic experiments which suggest that tonalites and granodiorites (the most abundant rocks of the Archean crust) are partial melting products of a mafic lower crust. Modelling suggest that this crust consisted of garnet amphibolite derived from a source with a bulk composition resembling a slightly enriched rather than depleted mantle. The CeN/YbN ratio is above 10 in the majority of tonalites. Most samples have no Eu anomaly because of a balanced contribution from the minerals of a mafic rock (or a plagioclase-free source). The positive Eu anomaly of some granodiorites and of a minor proportion of tonalites can be explained as being caused by plagioclase accumulation during differentiation or by partial melting of plagioclase-rich fractions. Modelling with Zn excludes an origin of tonalitic melts by differentiation of basaltic to dioritic magmas. The Archean meta-diorites, meta-tonalites and meta-granodiorites from Greenland have generally lost some K and S relative to their suggested magmatic protoliths. Loss of Rb, Tl, Pb and K and relative gain of Ca, Sr, Ba and Sc connected with granulitization of meta-tonalites can be explained in the majority of cases by separation of about 25 percent granitic partial melt. High K/Rb, K/Pb, Zn/Cd and Nb/Th ratios of granulites plus low ratios of granites are almost in balance with intermediate ratios of amphibolite-facies tonalites. Retrogression of granulites into amphibolites was accompanied by introduction of Pb, Tl, Rb, Ba, Sr and K from Na-Cl-rich brines circulating on fractures. A comparison of the abundance of 24 elements (characterized by different compatibility) in the Archean crust of Greenland with the present bulk crust reflects only minor changes (Th, Nb) if at-all in the chemical composition of the continental crust since the Archean.

The Geochemistry and Experimental Petrology of Sodic Alkaline Basalts from Oatlands, Tasmania

Sodic basalts of Oligocene-Early Micene age occur within an Early Tertiary graben in the Oatlands district of Tasmania. They include olivine tholeiites, alkali olivine basalts, basanites, transitional nephelinites, nepheline hawaiites, and nepheline mugearites. They have compositional characteristics in common with sodic alkaline basalt suites from other parts of the world. With decreasing SiO2, concentrations of CaO, alkalis, P2O5, and incompatible trace elements increase. Compositional and experimental data for the basalts are consistent with their derivation by polybaric partial melting of a garnet lherzolite source enriched in P2O5, light rare earths, Nb, and other incompatible trace elements. Experimental data for a primitive nepheline basanite from the Oatlands district indicate that concentrations of H2O+CO2 need not have been more than 6·5 wt.% for the original basanite magma to have derived from an amphibole-bearing garnet lherzolite source. In the case of more SiO2-undersaturated olivine melilitites from the neighbouring Central Plateau, the experimental evidence is consistent with either higher concentrations of H2O+CO2 (approaching 14 wt.%), or higher pressures of origin (>35 kb). Petrographic and geochemical evidence suggests that the latter is the more probable of the alternatives.

Experimental petrology of the lower mantle: A natural peredottte taken to 54 GPA

The high‐pressure mineral assemblage produced from a natural peridotite sample (xMg = 0.897) has been synthesized in the laser‐heated diamond cell and identified by x‐ray diffraction at 54 GPa. Orthorhombic perovskite, (Mg,Fe)SiO3, magnesiowüstite, (Mg,Fe)O, and cubic perovskite, CaSiO3, are observed in estimated molar ratios of 1.00 : 0.46 : 0.08 (1.00 : 0.21 : 0.09 by volume) respectively, with no evidence of other phases being present. The lattice parameters measured at 54 GPa yield a density of 4.97(±0.03) Mg/m3 for the high pressure assemblage at 300 K, in good agreement with prior calculations of density for candidate lower mantle assemblages.

The Encyclopedia of igneous and metamorphic petrology

Abbreviations. Acid(ic) igneous rocks. Alkaline rocks-undersaturated. Alkaline rocks-undersaturated: other petrological terms. Alpine mafic magma stem. Alteration, replacement. Amphibolite. Amphibolite facies. Analcimite. Anatexis. Andesite and dacite. Andesite line. Anorthosite. Aplite. Appinite. Archaean volcanism. Ariegite. Assimilation. Astrobleme. Atoll garnet. Augitite. Aureole. Banding in igneous rocks. Basalt. Basic igneous rocks. Batholith. Breccia pipe. British Tertiary Volcanic Province. Brownstone facies. Buchite. Burial metamorphism. Bushveld Complex. Calc-alkaline rocks. Calderas and volcano-tectonic depressions. Carbonatite. Cataclastic rocks. Charnockite. Charnockite suite. Circum-Pacific plutonic and volcanic activity. Columnar jointing. Cone sheets. Contact (thermal) metamorphism. Contact metasomatism. Core. Cryptovolcanic structure. Cumulate textures. Depth zones. Deserpentinization. Diabase. Diapirism. Diatreme. Differentiation index. Diorite and tonalite. Dolerite. Double-diffusive convection. Durbachite. Dyke (dike). Eclogite. Eclogite facies. Element partitioning in petrogenesis. Emplacement-modes. Enclave. Endogenous dome. Epidote amphibolite facies. Eutectic melting. Experimental petrology. Explosive volcanic eruptions-classification. Facies of thermal metamorphism. Feldspathoidal rocks-genesis. Fenite. Flood basalt. Fluidization. Fumarole. Gabbro and norite. Geochemical modeling in petrogenesis. Geosynclinal volcanism. Geyser. Glaucophane schist facies. Gneiss. Granite. Granites and mineralization. Granites and volcanism. Granodiorite. Granophyre. Granulite. Granulite facies. Greenschist facies. Greenstone. Greisen. Harzburgite. Hauynitite. Hornfels. Hornfelsic textures. Hot ground. Hot spring. Hybridization. Hydrothermal processes. Hypabyssal rocks. Igneous intrusions-structural behavior. Igneous petrology. Igneous petrology - terms. Igneous rocks - classification and nomenclature. Igneous rocks - other terms. Igneous rocks - tectonic setting. Injection complex. Intermediate igneous rocks. Intrusion. Intrusion-layered. Intrusive igneous structures. Island arcs-petrology. Khondalite. Kimberlite. Laccolith. Lamproite. Lamprophyre. Latite. Lava and lava eruptions. Lawsonite-albite-chlorite facies. Leucitite. Lherzolite. Limburgite. Liquid immiscibility. Lopolith. Lunar petrology. Mafic and ultramafic inclusions in igneous rocks. Magma. Magmatic crystallization and paragenesis. Magmatic crystallization in metamorphic environments. Magmatic crystallization in open systems. Magmatic currents and flow. Magmatic differentiation. Magmatic-tectonic cycle. Marble. Mediterranean suite. Melilitite. Metamorphic convergence. Metamorphic facies. Metamorphic facies series. Metamorphic grade, metamorphic zone. Metamorphic mineral growth. Metamorphic petrology-use of thermodynamics. Metamorphic reactions. Metamorphic rock composition-graphical representation. Metamorphic rocks-other terms. Metamorphism. Metamorphism - controls. Metamorphism - processes. Metamorphism - types. Metasomatism. Meteorites - petrology. Mid-ocean ridge and ocean-floor petrology. Migmatite. Migmatite-origin of neosome. Migmatite-structural relationships. Mnemonics, acronyms. Mohorovicic discontinuity (Moho). Monzonite (syenodiorite). Nephelinite. Neutrality-pH and temperature variation. Noseanite. Nucleation of metamorphic minerals. Nucleation of minerals in magmas. Nuee ardente. Ocean-floor metamorphism. Ocean-island igneous rocks. Oceanic crust. Ophiolite. Orbicular rocks. Pacific and Atlantic suites. Paired metamorphic belts. Palaeovolcanology. Palagonite. Pegmatite. Pegmatitic ore deposits. Peralkaline igneous rocks. Peridotite. Petrochemical calculations. Petrographic methods. Petrographic microscope. Petrographic province. Petrological terms. Petrology-branches. Petrology-history. Phacolith. Phase rule. Phase transformation. Phenocryst. Phonolite. Phyllite. Picrite. Plutonic rocks. Plutonic rocks - classification and nomenclature. Pneumatolysis. Polymetamorphism. Polyphase metamorphic mineral growth. Porphyroblast. Porphyroclast. Porphyry. Prefixes, suffixes. Prehnite-pumpellyite facies. Ptygmatic vein. Pumpellyite-actinolite schist facies. Pyroclastic eruptions. Pyroclastic flows. Pyrolite. Pyrometamorphism. Pyrometasomatism. Pyroxenite. Rapakivi texture. Reaction principle and reaction series. Reaction rates in metamorphism. Reaction rim. Regional metamorphism. Retrograde metamorphism. Rheomorphism. Rhyolite. Rift valley volcanism. Ring dyke. Schist. Serpentinite. Serpentinization. Shock metamorphism. Silicate melting. Sill. Skaergaard intrusion. Skarn. Slate. Snowball garnet. Sodalitite. Solfatara. Sovite. Spilite. Spinifex texture. Statistics in petrology. Steinmann Trinity. Syenite. Tektite. Tephra. Tephrite, basanite. Textures of igneous rocks. Textures of metamorphic rocks. Thermal activity. Trachyandesite, trachybasalt. Trachyte. Trend surface analysis in petrology. Tuffisite. Ultrabasic igneous rocks. Ultramafic belts. Ultramafic lavas. Ultramafic rocks. Units and conversion factors. Upper mantle-experimental petrology. Volatiles. Volcanic breccia. Volcanic glass. Volcanic neck. Volcanic rocks. Volcanoes and volcanism. Water in rock melts. Xenolith. Zeolite facies.

Experimental petrology: Quantitative boundaries for petrogenesis

The paper is an overview of the current status on experimental petrology — its objectives and its major role in solving problems related to various earth processes. It describes how investigations related to solid-solid transitions, dehydration or decarbonation reactions, and melting studies have helped to formulate petrological models for the earth’s internal structure. It also describes how measurements of physical properties of minerals, melts and vapour under extreme conditions have provided vital information of fluid dynamics of magmatic systems. The paper narrates the role of experimental petrology in calibrating geophysical processes with petrological consequences. Model P-T-X (SiO2)-fluid systems are considered to emphasize the role of various gas species in shifting the solidus in a P-T space, in degrees of melting and composition of the melt. Synthetic models and study of whole rock systems are considered to discuss the zonation and metasomatic processes in the mantle of the earth. The paper is also concerned with mantle convection and the uprise of thermal plumes, particularly, in the oceanic environment. It discusses the petrological structures associated with the plume and shows how static petrological maps are modified by the dynamics of the plumes.

The American Countercurrent - Eastward Flow of Geologists and Their Ideas in the Late Nineteenth Century

Whereas the trans-Atlantic flow of geological knowledge previously had been overwhelmingly westward, by the mid-nineteenth century, an eastward countercurrent had begun. That flow increased rapidly after the Civil War, when geology was at the forefront in the maturation of science in America. H.D. Rogers was appointed Regius Professor at the University of Glasgow in 1855. James Hall was chosen to be Organizing President of the first International Geological Congress in Paris (1878) and the first English-speaking foreign correspondent of the Academy of Sciences of France (1884). James D. Dane was almost as well known abroad as Hall, especially for his mountain-building theory. Increasingly, American theoretical contributions had to be reckoned with in such fields as Mountain Building, Structural and Precambrian Geology, Geomorphology and Glacial Geology, and Paleontology. By the first decade of the twentieth century, America had seized the initiative on several fronts, but especially in experimental petrology and physics of the earth's interior through the creation of the Carnegie Institution of Washington.

Cryptoexplosion structures, shock deformation and siderophile concentration related to explosive venting of fluids associated with alkaline ultramafic magmas

Cryptoexplosion structures, shock deformation and siderophile concentration related to explosive venting of fluids associated with alkaline ultramafic magmas

A self‐consistent model of melting, magma migration and buoyancy‐driven circulation beneath mid‐ocean ridges

Numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle‐derived magma at mid‐ocean ridges. Our model includes the following effects: melting due to decompression, magma migration by percolation, and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge. The distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material. The calculation of densities is based on a simple petrological model in which garnet‐spinel lherzolite melts to form a basaltic liquid and a harzburgite residue. We find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis, faster than the plate velocity, is confined laterally by stably stratified residual material beneath the newly formed plates. This effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included. Our results provide a simple explanation for the narrowness of the zone of crustal formation at mid‐ocean ridges. The model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced, perhaps giving rise to along‐axis variations in the character of seafloor spreading. The narrow, rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus. This may allow the liquid at depth to segregate into macroscopic channels, which would account for the consensus from experimental petrology that the liquids parental to MORB are derived from well below the base of the crust.

総説 オフィオライトの起源とエンプレイスメント

A brief review of the study on ophiolite is given. 165 years have passed already since a first use of the term “ophiolite” by BRONGNIART (1813), but still have not yet obtained a broadly satisfying solution on its origin and emplacement. However, the rapidly increased data set during the last 15 years on both on-land ophiolite and oceanfloors clearly indicate the strong constraints on its origin and emplacement.The period during 1813-1927 was a time of description of ophiolite. BRONGNIART (1827) classified ophiolite into a group of igneous rocks, since then began a debate whether ophiolitic peridotite is igneous or the other in origin. SUESS (1909) had noticed that ophiolites appear characteristically in orogenic belts. It was STEINMANN (1927) who had first recognized a close association of peridotite, gabbro, diabase-spilite, and radiolarian chert suggesting a deep sea origin of ophiolite. The significance of his finding has never been looked back until the revolutional period of plate tectonics in the late' 60s.The second period of 1927-1949 was the time of debate on igeneous origin. BOWEN and his coworkers insisted igneous origin based on experimental petrology for the ultramafic rocks in general. But if so, an abnormally high temperature ca. 1, 900°C was necessary to explain the occurrence of dunite. BENSON (1926) pointed out that if BOWEN'S idea is true, the country rocks of ophiolite must be subjected a high-temperature contact metamorphism, but not in the field. HESS (1939) has given a new idea of serpentinite magma to solve the problem, but its possibility had completely been disproved by the experiment of MgO-SiO2-H2O by BOWEN and TUTTLE (1949).The third period (1949-1959) began by a break-through idea of DE ROEVER (1957), who speculated that ophiolitic peridotite is a piece of mantle material, which was brought into an orogen by a tectonic process.The fourth period (1959-1973) started by BRUNN (1959) who compared ophiolite with the rocks in the Mid-Atlantic Ridge. This period (1959-1973) was the time of plate tectonics. During the early' 60s the ocean-floor spreading theory was proposed by HESS and DIETZ, and both thought that the layer 3 is composed of serpentinite oreclogite. The year 1969 was a memorial year, when both MOORES and DAVIES distinguished cumulate peridotite from the underlying residue tectonite, the latter of which is a refractory mantle after the formation of oceanic crust by partial fusion of mantle peridotite. The best example of ophiolite was the Troodos massif in Cyprus, where the extensive-scale of parallel dike swarm develops indicating ocean-floor spreading. Thereafter an ophiolite boom has come out, and flood of papers appeared to regard ophiolite to be of mid-oceanic ridge in origin. However, several geologists have doubted mid-oceanic ridge origin by the facts of much thinner crust, more silicic volcanic composition, and frequent occurrence of phenocrystic augite in ophiolites. MIYASHIRO (1973) solved such problems, and concluded that Troodos was formed in an island-arc setting. This paper was very shocking for geologists who wanted to establish the basic framework of orogeny by plate tectonics in those days, but epoch-making on the study of ophiolite, and corresponding to the time, when the method of study has changed to be modernized and more interdisciplinary.

The formation of continental crust: Part 1. A review of some principles; Part 2. An application to the Proterozoic evolution of southern North America

Understanding of the evolution of continental crust has been dominated by work in tectonics, experimental petrology and geochemistry, and geochronology and isotope geochemistry. In part 1, these approaches are discussed in the light of keynote articles, most published by the Geological Society of America, in an attempt to show how new data and new thinking have influenced understanding of the formation and modification of continental crust. In part 2, the evolution of continental crust in southern North America from 1.8 Ga until 1.3 Ga is discussed as an application of ideas generated during the past 30 years.

Christopher Scarfe, 1941–1988

Tragedy of major proportion befell the family of Chris Scarfe and the University of Alberta, Canada, at 8 a.m. on July 20, 1988, when an errant car killed Chris instantly while he was out jogging on his way to work.Born in England, Chris graduated at the University of Durham, beginning his career in experimental petrology with Peter Wyllie at the University of Chicago. Returning to England, he completed a Ph.D. at the University of Leeds, assisting in the development of a high‐pressure laboratory with Peter Harris. Appointed at the Univeristy of Alberta in 1972, he steadily developed a new facility, expanding the Department of Geology's embryonic high‐pressure laboratory with equipment capable of pressures to 40 kbar and 2000°C. He also supervised research on basalts in the Atlantic Ocean, British Columbia, and the Northwest Territories. He spent 1987–1980 in the Geophysical Laboratory, where he met Eiichi Takahashi, establishing a friendship and a most fruitful working partnership. Quickly realizing t h e significance of very high‐pressure equipment, Chris strenuously fought for a major equipment grant from the Natural Sciences and Engineering Research Council of Canada and secured it in time to have a Superpress delivered in February 1988, also utilizing support from the University. Quickly assembling a team of researchers, he brought the Superpress into immediate operation, producing diamonds within a month of start‐up. Major discoveries concerning the range of stability of carbonates and on the petrogenesis of komatiites are well under way at pressures up to 200 kbar.

マグマ成因論小史 過去三十年間における実験岩石学の展開

マグマ成因論小史 過去三十年間における実験岩石学の展開

Sub-solidus dehydration of amphiboles in an andesitic magma

Several petrologic experiments have demonstrated that in igneous and metamorphic reactions amphibole minerals can break down by a subsolidus dehydration reaction, but evidence for the reaction in natural rocks has been lacking. Evidence for the breakdown of an edenite-pargasite amphibole by a subsolidus dehydration reaction has now been found in an andesite flow from Garner Mountain, southern Cascase Range. The andesite contains one modal percent of ‘crystal clots’ formed of crystallites of opx, cpx, plag, K-spar, opaque and quartz. The crystal clots retain the original amphibole morphology and intra-clot pyroxenes are aligned with crystallographic c parallel to c in the amphibole precursor; these conditions would not be duplicated by a melting reaction. Microprobe analyses of the bulk clot and the intra-clot minerals suggest the solid-state reaction: 100 amph+10 SiO2=>55 cpx+33 plag+22 opx+ 1 opq+1 kspar Pyroxene thermometry of the andesite groundmass pyroxenes and the intra-clot pyroxenes demonstrates that the amphibole dehydration reaction occurred in the xenocrystic amphiboles as a result of heating by the near-solidus andesite magma.

Experimental petrology of normal MORB near the Kane Fracture Zone: 22�?25� N, mid-Atlantic ridge

Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in “normal” mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.

Second international symposium on experimental mineralogy, petrology, and geochemistry

Second international symposium on experimental mineralogy, petrology, and geochemistry

Experimental petrology of alkalic lavas: constraints on cotectics of multiple saturation in natural basic liquids

Experimental study of natural alkalic lava compositions at low pressures (pO2∼QFM) reveals that crystallization of primitive lavas often occurs in the sequence olivine, plagioclase, clinopyroxene, nepheline without “obvious” reaction relation. Pseudoternary liquidus projections of multiply saturated liquids coexisting with plagioclase (±olivine±clinopyroxene±nepheline) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Use of (TiAl2)(MgSi2)−1 and Fe3+ (Al)−1 exchange components is a key aspect of the projection procedure which is succesful in reducing a wide range of compositions to a systematic graphical representation. These projections, and the experiments on which they are based, show that low pressure fractionation plays a significant role in the petrogenesis of many alkalic lava suites from both continental and oceanic settings. However, the role of polybaric fractionation is more evident in the major element chemistry of these lava suites than in many tholeiitic suites of comparable extent. For example, the lavas of Karisimbi, East Africa, show a range of compositions reflecting a polybaric petrogenesis from primitive picrites at 1360° C/18 kb and leading to advanced low pressure differentiates. Evolved leucite-bearing potassic members of this and other suites may be treated in a nepheline-diopside-kspar (+olivine+leucite) projection. Compositional curvature on the plagioclase+clinopyroxene+olivine+leucite cotectic offers a mechanism to explain resorption of plagioclase in alkalic groundmass assemblages and the incompatibility of albite and leucite. This projection is useful for evaluating the extent of assimilation of the alkalic portions of crustal granulites. Assimilation appears to have played some role in the advanced differentiates from Karisimbi.