Ultramafic and Mafic Inclusions from Adak Island: Crystallization History, and Implications for the Nature of Primary Magmas and Crustal Evolution in the Aleutian Arc

Petrology of the Green Knobs diatreme and implications for the upper mantle below the Colorado Plateau

On November 4, 1975 in the evening, an eruption took place at Mt. Stromboli. On the following day lava flowed on the Sciara del Fuoco downward to the sea, accompanied by an intense explosive activity at the crater plane. Direct observations on the volcanic activity were carried out since November 6 while a seismic survey was made from Nov. 7 to 12. The total volume of the lava outpoured during this period of activity that lasted 21 days, was estimated to be about 104 m3. This paper reports the results of direct observations, and of the petrological, radioactive disequilibria and seismic activity studies performed for this eruption. The eruption was preceded by an insignificant change of seismic activity, which was monitored by a seismic station located about 2 km East of the crater. A shallow seismicity was strietly related to crater explosions accompanying the eruptive phenomenon. Radioactive disequilibria showed a lack of disequilibrium between228Ra and232Th explainable in terms of a fast rising of magma in the conduit. Chemical analyses of lava samples and deep seismic sounding data indicate a correspondence between the depth (10–15 km) at which crystallization pressure of phenocrysts occurs and a low velocity laver.

Stromboli and its 1975 eruption

Compositional relations among pyroxenes, amphiboles and other mafic phases in the Oslo Region plutonic rocks: Publication No. 115 in the Norwegian Geotraverse project

Nous avons analyse huit echantillons de lave et de tuff pantelleritiques et une inclusion lithique de trachyte provenant de Pantelleria, en Italie, au moyen d'une microsonde electronique. Ces echantillons revelent cinq assemblages differents de mineraux selon la presence de fayalite, aenigmatite, ilmenite. et magnetite: (1) augite + fayalite + ilmenite + magnetite, (2) augite + fayalite + ilmenite, (3) hedenbergite plus ou moins sodique + fayalite + ilmenite + aenigmatite + quartz, (4) hedenbergite sodique ou aegirine-augite + ilmenite + aenigmatite + quartz ′ ferrorichterite, et (5) aegirine-augite + aenigmatite + quartz. Un feldspath alcalin (Or 3 5 - 3 7 ) constitue la phase phenocristique dominante avec chaque assemblage. La teneur de la roche en silice et le degre d'hyperalcalinite montrent une forte correlation avec l'assemblage de ces mineraux. L'assemblage 1 caracterise l'echantillon le moins fortement hyperalcalin [indice d'agpaicite, I.A. = (Na + K)/Al, proportion molaire] et la plus faible teneur en silice (I.A. < 1..31, SiO 2 < 64.8%, poids), et s'est equilibre a 991-888°C a une fugacite d'oxygene entre 0.7 et 1.1 unites logarithmiques en dessous du tampon FMQ (FMQ - 0.7 to FMQ - 1.1). L'assemblage 2 est associe a un indice d'agpaicite et une teneur en silice plus eleves (I.A. = 1.42, SiO 2 = 67.1%) et aurait equilibre a environ 794°C et a FMQ-0.5. L'assemblage 3 est associe a indice d'agpaicite et une teneur en silice encore plus eleves (I.A. dans l'intervalle 1.55 - 1.63, 66.8 < SiO 2 < 67.8%) et aurait equilibre a 764-756°C a entre FMQ - 0.5 et FMQ - 0.2. L'assemblage 4 est associe a un indice d'agpaicite legerement plus eleve et une teneur en silice encore plus elevee (1.61 < 1.A. < 1.75, 67.6 < SiO 2 < 72.0%) et marquerait un equilibre entre 740 et 700°C a une fugacite d'oxygene soit a ou legerement sous le tampon FMQ. L'assemblage 5 est associe a l'indice d'agpaicite et la teneur en silice les plus eleves (I.A. = 1.97, SiO 2 = 69.7%) et marquerait un equilibre a <700°C a une fugacite d'oxygene depassant legerement le tampon FMQ, dans un champ sans oxyde stable. Malgre la rarete d'assemblages a deux oxydes, deux pyroxenes ou deux feldspaths, il semble donc possible de delimiter les conditions de temperature et de fugacite d'oxgene dans les roches hyperalcalines au moyen des equilibres QUI1F impliquant un assemblage de fayalite, ilmenite, et clinopyroxene a l'equilibre.

/pdf/variation-in-mineralogy-temperature-and-oxygen-fugacity-in-a-46u2v3z00k.pdf

Variation in mineralogy, temperature, and oxygen fugacity in a suite of strongly peralkaline lavas and tuffs, pantelleria, italy

The iron-magnesium exchange reaction between olivine and calcium-rich clinopyroxene is formulated as a geothermometer. It is shown that the clinopyroxene M1 site must be nonideal and it is expressed as a regular solution. The appropriate mixing parameters are calculated from a set of groundmass olivine-clinopyroxene pairs from lavas for which there are groundmass iron-titanium oxide temperatures. The pressure dependence of the geothermometer is calculated from abailable experimental work, and is approximately 5°C per kilobar. Layered gabbros from the Kap Edvard Holm Complex, East Greenland, show no significant variation of temperature with structural height in the intrusion, while those of Skaergaard give temperatures which do not have a consistent variation with height. Continued equilibration during post-crystallisation cooling is a possibility in slowly cooled intrusions. Inclusions in diamond give a pressure-temperature line consistent with formation at 1300°C at 55 kb, 1400°C 72 kb and 1500°C 90 kb. Ultrabasic xenoliths in Basutoland kimberlites have similar pressure-temperature lines. Lavas, including alkali basalts, basanites, andesites and rhyolites give temperatures from 1025°C to 890° C.

Marjorie Powell

Papers

An olivine-clinopyroxene geothermometer