Showing papers in "Contributions to Mineralogy and Petrology in 1972"
TL;DR: In this article, phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for H2O contents in the melt of 2 to 10 weight percent.
Abstract: Phase relations have been determined for a Paricutin Volcano andesite at pressures to 10 kilobars and for H2O contents in the melt of 2 to 10 weight percent. Runs were made under H2O-saturated and undersaturated conditions. In undersaturated runs a H2O-CO2 fluid phase was always present. Fugacity of H2O in melt, which is directly related to H2O content in the melt, was calculated from thermodynamic data. Plagioclase was found to be the liquidus phase when H2O contents in melt were less than about two percent. With more H2O, orthopyroxene, in some cases joined by olivine, assumes the liquidus. Clinopyroxene crystallizes near the liquidus only for H2O contents greater than five percent. The upper temperature stability limit of hornblende is about 950° C, well below the other silicate liquidi except at H2O-saturated conditions above 5 kb. The geometry of undersaturated liquidi and experimental phase compositions may be compared to the mode and phase compositions of the natural rock. From this comparison, megaphenocrysts of the natural rock are interpreted to have crystallized from a lava which had a water content of 2.2±0.5 percent and a temperature of 1110±40°C. Mass-balance calculations on experimental and natural phase assemblages show that the Paricutin series could not have formed by fractionation at pressures less than 10 kb; rather, it was probably derived by partial melting of subducted basaltic oceanic floor.
236 citations
TL;DR: A series of experimental runs has been conducted on a glass prepared from a natural island are calc-alkaline andesite from Fiji as discussed by the authors, and the crystallization sequence was determined for the pressure interval 9-36 kb under anhydrous conditions and with 2, 5, and 10% by weight of water carefully added.
Abstract: A series of experimental runs has been conducted on a glass prepared from a natural island are calc-alkaline andesite from Fiji. The crystallization sequence was determined for the pressure interval 9–36 kb under anhydrous conditions and with 2, 5, and 10% by weight of water carefully added. Addition of water markedly lowers the liquidus, depresses the appearance of quartz and plagioclase in the crystallization sequence, and greatly enlarges the field of garnet-clinopyroxene crystallization above 25 kb. Amphibole crystallizes in hydrous runs up to 25 kb. Electron microprobe analyses of critical phases allows calculation of controls on crystal fractionation trends. For hydrous conditions at 5–15 kb amphibole-clinopyroxene dominate fractionation and a moderate decrease in Mg/Fe and a slight increase in K/Na occurs. At 15–25 kb garnet also affects the fractionation and a moderate decrease in Mg/Fe and an increase in K/Na results. Above 25 kb garnet-clinopyroxene control the fractionation and there is a slight decrease in Mg/Fe but a significant increase in K/Na and a pronounced silica enrichment. In terms of major element chemistry, the derivation of the Fijian dacites in the second period of eruption may be satisfactorily explained by the fractionation of hydrous andesite at pressures >25 kb. Alternatively the dacites may result from lower degrees of melting of the down-going hydrous lithosphere. Similarly other members of this eruptive period may be derived according to a model of eclogite-controlled fractional melting or crystallization. Models involving amphibole fractionation at lower pressures are less satisfactory for explaining compositions in the Fijian second period of eruption, but in other environments models including amphibole-controlled fractionation may form part of a continuum of melting processes in subduction zones.
224 citations
TL;DR: In this article, the geothermal area on Reykjanes, Iceland has been investigated mineralogically and the temperature within the studied area is very variable from 30-300°C.
Abstract: The geothermal area on Reykjanes, Iceland has been investigated mineralogically. The temperature within the studied area is very variable from 30–300° C. Mineral zones corresponding to the temperature conditions in the area are found. Accidental changes in the geothermal system are also reflected in the mineralogy by formation of anhydrite. Changes in temperature conditions in the field are indicated by epidote occurrence at 40° C and retrograd formation of montmorillonite.
217 citations
TL;DR: In this article, rare earth elements, Rb, Sr, Ba, Ba and K have been determined in tonalite, trondhjemite, dacite, tholeiite, and graywacke from the 2700 my old Early Precambrian greenstone-granite terrane of northeastern Minnesota-northwestern Ontario, and also from the 3550 my old Morton Gneiss, southwestern Minnesota; and the Mesozoic Craggy Peak Pluton, Klamath Mountains, California.
Abstract: Rare earth elements, Rb, Sr, Ba and K have been determined in tonalite, trondhjemite, dacite, tholeiite and graywacke from the 2700 my old Early Precambrian greenstone-granite terrane of northeastern Minnesota-northwestern Ontario, and also in trondhjemite from the 3550 my old Morton Gneiss, southwestern Minnesota; and the Mesozoic Craggy Peak Pluton, Klamath Mountains, California The Early Precambrian tholeiites have trace element compositions similar to modern oceanic tholeiites, while the quartz dioritic rocks, regardless of age, have total rare earth contents lower than that of tholeiitic basalts, with near chondritic heavy rare earth contents Rb, Sr, Ba and K contents of the quartz diorites are about five times that of oceanic tholeiites, with similar alkali and alkaline earth ratios The Early Precambrian graywacke has a rare earth content intermediate between greenstone and quartz diorite, reflecting its provenance It is proposed that the analyzed quartz dioritic rocks, whether plutonic tonalite, dacite porphyry, gneissic or plutonic trondhjemite, or trondhjemite dikes had similar modes of origin, and were derived by partial melting of amphibolite or eclogite of basaltic or gabbroic composition at depths greater than thirty kilometers, leaving a residue consisting predominantly of garnet and clinopyroxene
205 citations
TL;DR: A comparison of the crystal contents of selected sieve grades from 9 ignimbrites, with the yield of crystals from crushed pumice, shows that on average a tenfold concentration of crystals has taken place in the ignimbrite.
Abstract: A comparison of the crystal contents of selected sieve grades from 9 ignimbrites, with the yield of crystals from crushed pumice, shows that on average a tenfold concentration of crystals has taken place in the ignimbrites. This is explained by the selective loss of pumice by an amount equal to at least the present bulk of the samples studied. The pumice is thought to have been lost above the vent or the moving pyroclastic flow, though there is a possibility, yet untested, that it may be contained in the distil part of the ignimbrites. This paper also further characterises the relatively well-sorted basal layer, and the pipes (interpreted as due to primary fumaroles) which cut some ignimbrites. Basal layer and pipes show an even greater concentration of crystals than the body of the ignimbrites. One corollary of this study is that doubt is placed on the value of making chemical analyses of ignimbrites, the composition of which is likely to depart appreciably from that of the eruptive magma.
155 citations
TL;DR: The eclogite on Hareidland, western Norway, is one of the largest known clogite bodies as mentioned in this paper, and it occurs as a sheet-like body folded together with the surrounding metasedimentary gneisses.
Abstract: The eclogite on Hareidland, western Norway, is among the largest eclogite bodies yet described. It occurs as a sheet-like body folded together with the surrounding metasedimentary gneisses of the basal gneiss complex of western Norway. The maximum surface extension of the body is about 6 km with the width varying between 0.2 and 1.5 km. Relations between bulk chemical variation and mineralogical variation suggest that the mafic rock now found as eclogite probably crystallized as a gabbroic rock. Field and mineralogical evidence suggests simultaneous later metamorphism of the gabbro and the surrounding gneisses at a temperature and load pressure near 625±30°C and 14 kb. The temperature estimate is based on a “thermometer” which uses the distribution coefficient,
$${\text{K}}_{{\text{ga}} - {\text{cpx}}}^{{\text{Fe}}^{{\text{2 + }}} - {\text{Mg}}}$$
. Whole rock Rb-Sr dating indicates about 1700 m.y. as the age of the metamorphism of the gneisses and therefore also the age of the eclogite. The Sr87/Sr86-ratios of a few biotite and amphibole-rich samples suggest a later partial Rb-Sr homogenization (Caledonian?) of the gneisses on Hareidland.
123 citations
TL;DR: In this article, the relation of natural minerals to epidote stability and its stability curves was investigated. But the results were limited to the case of clinozoisite and epidote.
Abstract: Laboratory experiments have been conducted with natural minerals to determine the relation of \(f_{{\text{o}}_{\text{2}} } \) to epidote stability, and to determine stability curves for clinozoisite and epidote. Under oxidizing conditions Fe-epidote decomposes to grandite, anorthite, hematite, and quartz. Under more reducing conditions corundum becomes a stable product instead of quartz, and magnetite, and finally hercynite replace hematite. As conditions change from oxidizing to reducing the temperature of epidote breakdown decreases, epidote becomes more aluminous and the grandite produced increases in grossularite component and, to a lesser extent, in almandine.
120 citations
TL;DR: Barnes and O'Neill as discussed by the authors reported a Ca+2-OH−1 fluid of local meteoric origin, the chemical and thermodynamic properties of which are entirely controlled by olivine, orthopyroxene, brucite, and serpentine reactions.
Abstract: Fluids related to Serpentinization are of at least three types. The first reported (Barnes and O'Neil, 1969) is a fluid of local meteoric origin, the chemical and thermodynamic properties of which are entirely controlled by olivine, orthopyroxene, brucite, and serpentine reactions. It is a Ca+2-OH−1 type and is shown experimentally to be capable of reacting with albite to yield calcium hydroxy silicates. Rodingites may form where the Ca+2-OH−1 type waters flow across the ultramafic contact and react with siliceous country rock.
117 citations
TL;DR: In this paper, the trace elements Ba, Ce, La, Nb, Rb, Sr and Zr have been determined from six trachytic and pantelleritic volcanoes in the Kenyan and Ethiopian Rifts.
Abstract: Determination of the trace-elements Ba, Ce, La, Nb, Rb, Sr and Zr have been made on lavas from six trachytic and pantelleritic volcanoes in the Kenyan and Ethiopian Rifts. Consideration of these data shows that Ce, La, Nb and Zr have behaved as truly residual elements. In the peralkaline suites examined, plotting other chemical parameters against a residual-element such as Zr appears to be informative. The individual residual-element ratios are constant at any one volcano suggesting that the salic and basic lavas are geochemieally related and that the trachytes and pantellerites do not have an origin independent of the associated basalts.
114 citations
TL;DR: In this paper, lead isotopic compositions and lead and uranium concentrations have been determined for samples from the Tongan islands of Late, Fonualei, Hunga-Ha'apai and Eua and for the Kermadec islands of Raoul, Macauley, Esperance and Napier.
Abstract: Lead isotopic compositions, and lead and uranium concentrations have been determined for samples from the Tongan islands of Late, Fonualei, Hunga-Ha'apai and Eua and for the Kermadec islands of Raoul, Macauley, Esperance and Napier. The Kermadec samples form a linear array in a Pb206/Pb204 vs Pb207/Pb204 plot which if interpreted in terms of a two stage mantle model indicates a mantle fractionation of Pb from U at about 1.2×109 yr. The Tongan volcanics, except Eua, have a very narrow range of isotopic compositions, only slightly exceeding the experimental error limits. This is interpreted to be the result of efficient mixing of the mantle in response to rapid extension behind the Tongan arc. The rate of extension appears to determine whether convection mixing can occur, since the sub-Kermadec mantle which experienced slower extension is not well mixed isotopically. The isotopic compositions show no evidence for a component in the magma derived from sediments dragged down the Benioff zone. The pre-Eocene Eua volcanics have lead which is substantially more radiogenic than the younger Tongan volcanics and are interpreted as a sample of the mantle before convective mixing occurred.
95 citations
TL;DR: The assemblage paragonite + quartz is encountered frequently in low-to medium-grade metamorphic rocks and with a rising grade of metamorphism they react mutually to yield the condensed assemblages albite + Al2SiO5 as mentioned in this paper.
Abstract: The assemblage paragonite + quartz is encountered frequently in low- to medium-grade metamorphic rocks. With rising grade of metamorphism they react mutually to yield the condensed assemblage albite + Al2SiO5.
TL;DR: In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass as mentioned in this paper, and these glasses were formed from melt produced by shock fusion of crystalline basement rocks.
Abstract: In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass. These glasses were formed from melt produced by shock fusion of crystalline basement rocks. Ejection from the crater resulted in the formation of aerodynamically shaped bombs, a few homogeneous spherules and a large mass of small glass particles which were deposited in the suevite breccia. Bombs and small particles included within chilled bottom and top layers of suevite deposits have been preserved in vitreous state, whereas glasses within the interior of the suevite devitrified, due to slower cooling rates.
TL;DR: The Cenozoic volcanic activity in northeastern Nigeria began with the intrusion into the Benue trough of a trachyte-phonolite suite of plugs 22-11 m.y. as discussed by the authors.
Abstract: The Cenozoic volcanic activity in northeastern Nigeria began with the intrusion into the Benue trough of a trachyte-phonolite suite of plugs 22-11 m.y. ago. Later activity, which was more widespread and dominantly basaltic in character, began some 7 m.y. ago and has continued until very recent times. It resulted in basaltic plugs and lava plateaux within the Benue trough, and cinder cones and lavas on the Jos Plateau.
TL;DR: In this article, the conditions under which oxidation and reduction occur during fluid flow are discussed and it is thought that progessive oxidation of the shear zones at Scourie indicates that fluid flow took place up a thermal gradient.
Abstract: Rocks from some shear zones at Scourie are highly oxidised and enriched in potash relative to adjacent undeformed granulites. Oxidation is considered to result from dissociation of water, and the extent of oxidation is evidence for massive fluid flow through the shear zones. The conditions under which oxidation and reduction occur during fluid flow are discussed and it is thought that progessive oxidation of the shear zones at Scourie indicates that fluid flow took place up a thermal gradient. It is proposed that the inverted thermal gradient resulted from overthrusting, and this is consistent with existing interpretations of the structural evolution of the area.
TL;DR: Pleistocene to recent stratovolcanoes in the Highlands of Papua New Guinea are made up of calc-alkaline to shoshonitic lava, tuff, agglomerate, ash, and lahar deposits as discussed by the authors.
Abstract: Pleistocene to Recent stratovolcanoes in the Highlands of Papua New Guinea are made up of calc-alkaline to shoshonitic lava, tuff, agglomerate, ash, and lahar deposits. The volcanic rocks are characterized by high and variable Al, high K and total alkalis, and low Fe, Mg, and Ca. There is a continuous variation between high-K calc-alkaline, low-Si high-K calc-alkaline, and shoshonitic rocks. The elements V, Rb, Sr, Zn, Nb, and Ba are high relative to general andesitic abundances, particularly in the shoshonites. The Highlands volcanic rocks originated either in the base of thick sialic crust which is undergoing stabilization after major orogeny and uplift, or more probably, in eclogite sinking through the underlying mantle. Variation in content of K and other “incompatible” elements was either inherited from the source rocks in the base of the crust, or was produced by zone refining in a thick upper mantle zone containing interstitial fluid rich in these elements. Further variation, mainly in Fe/Mg, Si, and total alkalis, was caused by minor low-pressure crystal fractionation involving olivine, clinopyroxene, and, to a lesser extent, calcic plagioclase.
TL;DR: A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major and trace-element analyses of the garnet phenocrysts are presented as mentioned in this paper.
Abstract: A suite of garnet-bearing andesites and dacites from the Ordovician of N. W. England is described and major- and trace-element analyses of the garnet phenocrysts are presented. The garnets are of almandine-pyrope composition, with minor amounts of spessartine and grossular, and often show marked reversed zoning; the crystal becoming progressively enriched in pyrope towards the margin. Garnets from the dacites are consistently richer in almandine and spessartine than are those from the andesites.
TL;DR: In this paper, the equilibrium data of reaction were experimentally determined, and the equilibrium curve was characterized by the following P, T-data: 4 kb; 360±20° C; 5 kb; 375 ±20°C; 7kb;410± 20° C.
Abstract: At pressures which are expected in the earth's crust, the high temperature border of the lawsonite stability field is marked by reaction lawsonite = zoisite + kyanite/andalusite + pyrophyllite + H2O. (1a) The equilibrium data of reaction (1a) have been experimentally determined, and the equilibrium curve is characterized by the following P, T-data: 4 kb; 360±20° C; 5 kb; 375 ±20° C; 7kb;410±20° C.
TL;DR: The thermal stability of the orthorhombic pyroxene phase is strongly increased by the incorporation of Al. as mentioned in this paper showed that even at 1 kb up to 9 wt.-% Al2O3 can be substituted at 1200°C.
Abstract: Solid solubility of Al2O3 in orthorhombic enstatite by the substitution AlAl=MgSi is, in the range studied, mainly a function of temperature and not strongly pressure-dependent. Even at 1 kb up to 9 wt.-% Al2O3 can be substituted at 1200° C. The thermal stability of the orthorhombic pyroxene phase is strongly increased by the incorporation of Al. In crustal rocks the alumina content of orthopyroxene might be used as a geothermometer but not, as sometimes suggested, as a barometer.
TL;DR: The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanite⇌sapphirine solid solution +quartz was determined experimentally in the pressure range between 12 and 20 kb as discussed by the authors.
Abstract: The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanite⇌sapphirine solid solution+quartz was determined experimentally in the pressure range between 12 and 20 kb. It is defined by four reversals at 15 kb, 1140°+10°C; at 1190°C, 15.9±1 kb; at 1300°C, 17.2±1 kb; and at 1400°C, 18±1 kb. Among the coexisting phases the Al-content of the enstatites increases strongly, with rising temperatures and pressures, up to values approaching that of pyrope composition, whereas the Al-content of the sapphirine solid solution appears to increase only slightly. Concomitantly, the sillimanites, most probably of invariant composition, exhibit growing Al/Si-disorder. These compositional and structural variations, in addition to the changing stoichiometry of the reaction equation, cause the progressively decreasing positive dP/dT slope of the equilibrium curve. — The assemblage sapphirine + quartz found in natural granulites is indicative of conditions of water pressure much lower than total pressure.
TL;DR: In this article, a micro-probe was used for fluorine analysis in existing Ca-amphiboles, biotites, apatites, and sphenes, and it was shown that apatite mirrors a low activity of fluorine better than coexisting biotite.
Abstract: Coexisting Ca-amphiboles, biotites, apatites, and sphenes were analysed for fluorine by a micro-probe. Fifty samples from four areas were examined. The distribution coefficient K
D(F)
A-B
for coexisting Ca-amphibole and biotite varies between 0.5 and 1.1 and is positively correlated with increasing metamorphic grade and with increasing contents of Ti in Ca-amphibole. Samples carrying hematite have normally a higher content of fluorine in the hydrous minerals due to an increasing activity of fluorine during the oxidation. It is shown that apatite mirrors a low activity of fluorine better than coexisting biotite, which in turn is a more sensitive indicator at higher F-activities. The distribution of fluorine between sphene and biotite is also discussed.
TL;DR: The rate of potassium self-diffusion in pure microcline was measured using K40 as a tracer and the large difference in activation energies suggests that the atomic mechanisms for sodium and potassium diffusion are different.
Abstract: The rate of potassium self-diffusion in pure microcline was measured between 600° and 800° C using K40 as a tracer. Transport of K40 by processes other than volume diffusion was insignificant or minimal. Isotropic diffusion coefficients were calculated assuming spherical grains. The data are well fit by the Arrhenius relation and yield a pre-exponential factor (D0) of 133.8 cm2/sec and an activation energy (Q) of 70 kcal/mole. Similar experiments on the self-diffusion of Na22 in a pure low-albite (exchanged microcline) yield D0 of 2.31×10−6 cm2/sec and Q of approximately 19 kcal/mole for the temperature interval from 200° to 600° C. The large difference in these activation energies suggests that the atomic mechanisms for sodium and potassium diffusion are different.
TL;DR: In this article, the authors investigated the relationship between compositional variation and the sequence of phenocryst phases available for fractionation in basaltic rocks from the New Georgia Group and found that the discrepancy appeared explicable in terms of two low-pressure crystal fractionation mechanisms.
Abstract: Study of the data provided by Stanton and Bell (1969) for certain basaltic rocks from the New Georgia Group reveals an apparent discrepancy between compositional variation and the sequence of phenocryst phases available for fractionation. The discrepancy none-the-less appears explicable in terms of two low-pressure crystal fractionation mechanisms. The first of these we term compensated crystal settling, a process which, it is postulated, allows a substantial amount of magma undergoing crystal settling to maintain its overall composition since crystals settling from it are continually replaced by compositionally similar crystals which settle into it from higher levels. The second process involves selective fractionation of phases sinking at different rates. Slow sinking of plagioclase relative to ferromagnesian minerals is believed to produce cumulus enrichment in plagioclase in the upper part of the chamber, the resultant magmas being erupted as highly porphyritic, high-alumina, basaltic andesites.
TL;DR: In this paper, the δ18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2
Abstract: The (δ18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2‰ relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8‰. The 18O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of 18O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.
TL;DR: The peridotitic and gabbroic rocks described in this article occur as a tectonically emplaced layered body in Piton des Neiges volcano, and as blocks in basaltic agglomerate, Piton Des Neige and Piton de la Fournaise volcanoes.
Abstract: The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons.
TL;DR: In this article, the structure and optical properties in anorthites of different origin are analyzed with the petrographic microscope, U-stage methods, X-ray single crystal analysis and high voltage electron microscopy.
Abstract: Variations of structure and optical properties in anorthites (An 93–97%) of different origin are analyzed with the petrographic microscope, U-stage methods, X-ray single crystal analysis and high voltage electron microscopy. No significant variation has been found in the orientation of the indicatrix and of the lattice constants. But c-type reflections (h + k even, ι odd) are strong and sharp in anorthites from slowly cooled rocks and diffuse in anorthites of identical chemical composition from quenched igneous rocks. Large type c-antiphase domains (5000–10000 A) are found in the slowly cooled rocks, c-domains in volcanic rocks are small (100 A) or could not be imaged. The presence of only b-domains in lunar basalt 14310 indicates quenching of this rock. Large c-domains in the Apollo 15 genesis rock (15415, Lally et al., 1972) indicate slow cooling similar to terrestrial metamorphic rocks.
TL;DR: In this paper, the major and trace element analyses for 7 kaersutites from basic alkaline rocks are presented for 7 different classes of basic alkalinized rocks and the ratios of K/Rb ratios lie between 1209 and 4276.
Abstract: New major and trace element analyses are presented for 7 kaersutites from basic alkaline rocks. K/Rb ratios lie between 1209 and 4276. Rb is low, averaging 6 ppm. Sr ranges from 532 to 1060 ppm and Ba from 181 to 701 ppm. Zr averages 109 ppm, Nb 44 ppm and V 390 ppm. There is a moderate enrichment in light REE. Zn correlates with FeO + Fe2O3 but the concentrations of Ni, Co, Cr, Cu and Pb are variable. Large variations in trace element concentration in kaersutites reflect only small variations in the melt when the distribution coefficient for a given element strongly favours the amphibole.
TL;DR: Experimental data obtained for dry melting of muscovite + quartz show that the stability field of this assemblage is extended to higher temperatures compared with the corresponding melting reactions with excess vapor which have been determined by Storre and Karotke as mentioned in this paper.
Abstract: Experimental data obtained for dry melting of muscovite + quartz show that the stability field of this assemblage is extended to higher temperatures compared with the corresponding melting reactions with excess vapor which have been determined by Storre and Karotke (1971, 1972).
TL;DR: In this article, the results of an electron microprobe were analyzed in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii.
Abstract: Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO2-K2O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K2O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.
TL;DR: In this paper, the chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities, suggesting that partial pressure of fluorine might modify the stability of chloritoid from that determined in pure H2O.
Abstract: Ninety-seven mineral phases consisting of ten chloritoids, fifteen epidotes, sixteen garnets, four sphenes, seven rutiles, seven pyroxenes, thirteen blue amphiboles, two green amphiboles, eleven phengites, two paragonites, a mariposite, seven chlorites, and two specimens of albite were obtained from the metamorphic rocks of Ile de Groix, and their chemical, physical, optical and X-ray properties determined. The chloritoids are all optically positive, monoclinic polymorphs with large 2V, moderate refractive indices and characterized by high densities. Their fluorine contents have been used to propose a new upper limit for OH→F substitution in the chloritoid structure, suggesting that partial pressure of fluorine might modify the stability of chloritoids from that determined in pure H2O. The epidotes belong to the Al-Fe epidote series and are “epidote” sensu stricto. The almandine-rich garnets and the chloromelanites are metastable relics in the glaucophane schists. The grossular contents of the calcareous schist garnets are believed to have become depressed under high CO2 pressure and the low Tschermak's contents of the pyroxenes are to be explained by equilibria involving epidote at high \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) and low temperature when the Tschermak's components will break down to epidote group minerals. The sphenes contain appreciable amounts of combined water, fluorine substituting for oxygen and aluminium substituting for silicon and titanium. The presence of H3O+ is suspected in a specimen of blue amphibole. The barroisite has a composition between glaucophane and hornblende. On account of its high Fe3+ content it is believed to have formed under higher PO2 than the blue amphiboles. The paragonites which occur in the ohloritoid veins are unstable in the potassium-rich aluminous schists. The phengites show a tendency towards sericitic composition due to post-glaucophanisation readjustments under the lower pressure conditions of the greenschist facies. Some of the Fe3+ contents of the chlorites are interpreted as due to oxidation of ferrous iron, e.g. 2 [Fe(OH)2]→2FeOOH + H2. The minerals show strong chemical control of the host rock and their Mn contents are directly related to those of the minerals from which they have evolved through retrogression.
TL;DR: In an external vacuum and at temperatures between 900° C and 1650° C internal graphitisation takes place on or around mineral inclusions in diamond, and appears to be responsible for similar features previously reported in diamond from several localities as discussed by the authors.
Abstract: In an external vacuum and at temperatures between 900° C and 1650° C internal graphitisation takes place on or around mineral inclusions in diamond, and appears to be responsible for similar features previously reported in diamond from several localities. Several mechanisms are proposed and discussed for internal graphitisation at temperatures as low as 900° C: it is proposed that at low external pressures CO2 exsolves from inclusions and causes internal graphitisation. The results also indicate that immediately after pressure release arising from volcanic breakthrough of kimberlites of different geological ages at several localities in West and South Africa, the temperature was not in excess of 800° C to 900° C in certain regions of the diatremes.