The role of fluorine and oxygen fugacity in the genesis of the ultrapotassic rocks

The effects of H₂O, CO₂, CH₄ and HF on partial melting of a model phlogopite harzburgite mantle are considered with regard to the production of ultrapotassic magmas. Fluorine has a polymerising effect in H₂O-poor conditions, but in the presence of abundant H₂O where HF rather than F is dominant, the overall effect is depolymerisation. Methane also dissolves by forming (OH)^– groups, and so has a depolymerising effect. Group I ultrapotassic rocks (lamproites) probably originate from primary magmas with SiO₂ contents ranging from around 40 wt% to at least 52 wt%. This range can be explained by differing depths of origin from a similar source with a similar reduced H₂O-CH₄-HF volatile mixture. The formation of silica-rich initial melts from a model phlogopite harzburgite is assisted by the presence of CH₄ and HF. Dissociation of less than 0.1 wt% H₂O, driven by H₂ loss, is sufficient to cause oxidation during emplacement to observed oxidation states. Silica-poor ultrapotassic rocks could be produced at higher pressures in a reduced environment, or in an oxidised environment with high CO₂/(CO₂ + H₂O) ratios.Group II (African Rift) potassic rocks may originate in H₂O-poor conditions in which fluorine will maintain a large phlogopite phase field, so that initial melts will be magnesian and silica-undersaturated.     

The effect of fluorine on phase relationships in the system KAlSiO4-Mg2SiO4-SiO2 at 28 kbar and the solution mechanism of fluorine in silicate melts

Phase relationships in the system kalsilite-forsterite-quartz with fluorine added by direct substitution for oxygen were examined at 28 kb. A large liquidus field for fluorphlogopite exists with approx. 4 wt% F added to the system and the thermal stability of phlogopite is increased by 300° C relative to the water saturated system. Fluorine expands the phase volume of enstatite relative to forsterite so that the peritectic point PHL+EN+FO+L, a model for melting of a phlogopite harzburgite, lies in the silica-undersaturated field. Experimental phlogopites have excess Si which correlates with F content and are Al-deficient. The high Si contents indicate solid solution with an end member intermediate between tri- and di-octahedral micas. Glasses with compositions analogous to partial melts from phlogopite harzburgite were examined by infrared spectroscopy in the mid- and far-IR regions. Results show that fluorine polymerises the melt by bonding with all the network modifying cations K, Mg and Al. At higher F contents, but still less than 1 wt%, tetrahedral KAlO₂-groups are complexed by fluorine and removed from the aluminosilicate network simultaneously polymerising and increasing the Si/(Si+Al) ratio of the network. However, when HF rather than F is present, the overall effect will be to depolymerise melts due to the effect of OH released by dissolution of HF. The presence of abundant Si-F bonds is considered unlikely even in silica-rich magmas: the viscosity decrease characteristic of fluorine-bearing melts can be attributed to the formation of fluoride complexes.     

Alpha Ridge and iceland-products of the same plume?

Refraction results from the 1983 Canadian Expedition to Study the Alpha Ridge suggest a crustal model with many similarities to the region of Iceland. The overall character of the seismic sections, the derived velocity structure, the lateral dimensions of crustal transition zones and the general crustal thicknesses are comparable from the Arctic and Atlantic regions. Additional similarities are noted in the alkalic basalts, the subaerial to subaqueous, within-plate volcanic environment and the positive regional magnetic responses at satellite altitudes.Based on the geophysical semblances and the apparent evolutionary sequence of within-plate volcanism and tectonic events extending from the northern Ellesmere-Greenland area to Iceland, it is proposed that the Alpha Ridge and Iceland may have been affected by the same mantle plume.     

Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: Implications for the origin of primitive MORB glasses

Summary Anhydrous partial melting experiments on four peridotite compositions have been conducted at 10 kbar providing a relatively internally consistent set of data on the character of primary melts expected from the oceanic upper mantle in the mid-ocean ridge setting. The four peridotite compositions are: MORB pyrolite (considered to be suitable for the production of primitive (Mg#0.68) MORB glasses at 10 kbar), Hawaiian pyrolite (representative of enriched upper mantle), Tinaquillo lherzolite (representative of more depleted upper mantle), and the spinel lherzolite KLB-1 which is a suitable composition for the production of primitive MORB glasses. The equilibrium liquids were determined by sandwich experiments. The primitive MORB glass DSDP 3-18-7-1 was used in experiments using MORB pyrolite and KLB-1, while a calculated 10 kbar liquid composition fromJaques andGreen (1980) was used in experiments with Hawaiian pyrolite and Tinaquillo lherzolite. The results of the experiments are used to test a 10 kbar melt model for the generation of primitive MORB glasses, which are parental magmas to typical MORB compositions. The melt compositions from the four peridotites studied are significantly different from primitive MORB glasses in major element chemistry and plot away from the field of primitive MORB glasses in the CIPW molecular normative Basalt tetrahedron. The results indicate that primitive MORB glasses are derivative compositions lying on olivine fractionation lines from picritic parents, which themselves are primary magmas at pressures greater than 10 kbar. The results of this study are integrated with previous 10 kbar experimental studies.

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Wasserfreie partielle aufschmelzung von MORB pyrolit und andere peridotit-zusammensetzungen bei 10 kbar: bedeutung für die entstehung primitiver MORB gläser

Zusammenfassung Vier Peridotit-Zusammensetzungen wurden bei 10 kbar unter wasserfreien Bedingungen partiell aufgeschmolzen, und die Ergebnisse mit möglichen primitiven Schmelzen Mittel-Ozeanischer Rücken verglichen.Die folgenden perioditischen Zusammensetzungen wurden untersucht: MORB pyrolite [mögliche Ausgangszusammensetzung für primitive (Mg# > 0.68) MORB-Glaszusammensetzungen bei 10 kbar], Hawaiian pyrolite (representativ für angereicherten Oberen Mantel); Tinaquillo lherzolite (representativ für verarmten' Oberen Mantel) und spinel lherzolite, KLB-1 (im Gleichgewicht mit primitiver MORB-Glaszusammensetzung). Die Schmelzen im Gleichgewicht mit diesen Ausgangszusammensetzungen wurden mittels Sandwich-Experimenten ermittelt.Die primitive MORB-Glaszusammensetzung DSDP 3-18-7-1 wurde mit MORB pyrolite und KLB-1 equilibriert, während eine Modell-Zusammensetzung vonJaques and Green (1980) in Verbindung mit Hawaiian pyrolite und Tinaquillo lherzolite vermischt wurde. Die Resultate der Experimente werden mit einem 10 kbar Aufschmelzungsmodell zur Entstehung primitiver MORB-Gläser verglichen. Die Schmelzen im Gleichgewicht mit den vier Peridotit-Ausgangszusammensetzungen unterscheiden sich wesentlich von primitiven MORB-Gläsern, sowohl hinsichtlich ihrer Hauptelemente als auch ihrer Plot-Parameter im Basalttetraeder. Primitive MORB-Glaszusammensetzungen stellen keine primären Schmelzen dar, sondern sind durch Olivinfraktionierung von primitiven Magmen abzuleiten. Die Resultate dieser Untersuchungen werden mit früheren 10 kbar Experimenten verglichen.

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With 10 Figures     

Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions

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.     

On the occurrence of eclogite in Western Norway

Recent experimental data show that eclogites may form in the crust under conditions where total pressure exceeds water pressure. The regional distribution of eclogites in Western Norway and their association with crustal rocks makes their formation in the crust the most attractive hypothesis. Before a mantle origin should be assigned to any rock within this eclogite and garnet peridotite area it should be demonstrated that their country rocks are in a metamorphic state incompatible with the load pressures required for eclogite stability. Criteria for ascertaining eclogite formation in the crust are discussed.Publication no. 8 in the Norwegian geotraverse project.     

Fluorine geochemistry of quaternary volcanic rocks from Southwestern British Columbia: Some petrogenetic implications

Fluorine contents in 38 Quaternary volcanic rocks, representing calc-alkaline andesite eruptive groups from the Garibaldi Lake area, were determined by a selective ion-electrode method. A close relationship is evident between F abundance and the type of ferromagnesian phenocrysts present in the andesitic rocks. Hypersthene andesites have the lowest F contents (142–212 ppm), whereas hornblende-biotite andesites exhibit the highest F values (279–368 ppm); hornblende andesites have intermediate F contents (238–292 ppm). The hornblende-free Desolation Valley basaltic andesite has a lower F content than the hornblende-bearing Sphinx Moraine basaltic andesite (122 ppm versus 317–333 ppm).Different eruptive suites can be grouped on the basis of F differentiation patterns into (1) a hornblende-free lava series in which the F content of basaltic andesite is less than that of andesite, and (2) a hornblende-bearing lava series in which F contents remain constant or decrease slightly from basaltic andesite through dacite. Fluorine variation in the former series was controlled largely by fractionation of anhydrous minerals, whereas that in the latter was influenced by crystallization of amphibole, biotite and apatite.The distinctive F variation patterns of the two lava series appear to represent real differences in the volatile contents of Garibaldi Lake magmas. These different volatile concentrations may reflect varying degrees of magma-wallrock interaction, differences in the initial volatile contents of the primary magmas, or some combination of these factors.