Low-pressure Fractionation Research Articles

Petrological aspects and evolution of the leucite bearing Lavas from Bufumbira, south west Uganda

Lavas ocouring in the western Rift of Central Africa are characterized by the presence of leucite both as phenocrysts and in the groundmass. Samples studied from the Bufumbira Bay area of the Birunga volcanic field, in south-west Uganda, form two distinct chemical series diverging from basanite compositions. One differentiates through phonolitictephrites to a trachyte and the other differentiates through leueitites to a phonolite. The low-pressure fractionation controls of both series are discussed in terms of the coexisting phases and ultramafic inclusions observed in the lavas. Equilibrium fusion of a mantle phlogopite-bearing peridotite combined with subsequent fractionation is the favoured petro-genetic process to produce a potash-rich basanite liquid, which is considered a likely parent of the Bufumbira lavas.

A Carboniferous Continental Andesite--Rhyolite Association from the 'Nychum' Area, North Queensland, Australia

The Nychum Volcanics, which were emplaced on the continental side of the Tasman Geosyncline immediately after the mid-Carboniferous orogeny, are composed mostly of acid pyroclastics and lavas, together with subordinate andesite and minor basalt. They are intruded by closely associated plugs of andesite and rhyolite, and a dyke swarm. The volcanics show calc-alkaline characteristics, except for two andesites that have tholeiite-like mineralogy and major-element chemistry. It is suggested that the volcanics are derived from three magma types, and have been affected by both high-pressure and low-pressure fractionation.

Strontium, Rubidium, Potassium, and Calcium Variations in Quaternary Lavas, Crater Lake, Oregon, and Their Residual Glasses

Strontium and rubidium contents of 22 whole-rock specimens and 15 residual glasses from Crater Lake, Oregon, range from 650 to 725 ppm and 23 to 33 ppm, respectively, for the less differentiated hypersthene andesites and basaltic andesites; to 150 to 400 ppm and 50 to 80 ppm for the more salic whole-rock and residual glass specimens. The behavior of these elements largely reflects the separation of large amounts of phenocrystic plagioclase. Sr content closely follows that of calcium. More sodic plagioclase exhibits a greater absolute affinity and a slightly greater relative affinity for Sr than more calcic compositions. K/Rb decreases progressively from about 440 at a K 2 O content of 1.2 weight percent to 330 at 3.4 percent K 2 O as a result of separation of plagioclase with a K/Rb ratio several times that of the associated magmatic liquid. Two factors support the hypothesis that the late dacites and rhyodacites of Mount Mazama were derived by low-pressure crystal fractionation of intermediate magma(s) that are very similar in composition to the much more voluminous andesites of the volcano. They are (1) the coincidence of the whole-rock–residual liquid tie lines, with general variation trends defined by the combined whole-rock and residual liquid data; and (2) the similarity of the coherent interelement variations, with trends inferred from data on elemental distribution between coexisting phenocryst minerals and magmatic liquid. Removal of 45 to 65 percent crystalline material from andesitic magma with 25 to 30 ppm Rb is required to produce the more salic rocks containing 50 to 65 ppm Rb.

Proceedings

15 November 1972 Volcanic Studies Group Meeting Current vulcanological research. Chairman: Professor I. G. Gass 1. Magmatic evolution of Mauritius . A. N. Baxter, Sir John Cass College, London. Mauritius is entirely volcanic with three distinct series. (1) Older Series. Early activity constructed a shield volcano with alternating lavas and agglomerates. The lavas form oceanites, arrkaramites, olivine basalts, basalts, hawaiites, mugearites and feldsparphyric basalts, with ultramafic and anorthositic inclusions and a related radial dyke swarm. A slight erosional unconformity within the series, separates the shield phase from a later group of feldsparphyric basalts, hawaiites, mugearites and trachytic endogenous domes. The basic lavas show a marked transitional character and the differentiated nature of the series is reflected in well defined chemical trends controlled by fractionation of observed phenocryst phases. The shield was highly incised by erosion, following cessation of activity. (2) Intermediate Series. Activity renewed with the eruption of small volumes of alkali olivine basalts and basanitoids, in the SW of the island. Major element and some trace element trends are poorly defined, but the ‘incompatible’ elements K, P, Ti, Ba, Rb, Sr, Zr, show pronounced enrichment trends clearly unrelated to any likely low pressure fractionation scheme, and attributed here to eclogite fractionation at elevated pressures. (3) Younger Series. A further pause was termin- ated by eruption of a voluminous alkaline olivine basalt from a median NNE-SSW line of 21 very low angled shield volcanoes. Major and trace elements show trends compatible with the fractionation of olivine, the dominant phenocryst phase in

Le volcanisme tertiaire des monts du Forez (Massif central français) : basanites à analcime, à leucite et néphélinites à mélilite

The oligocene-miocene volcanics of the Forez region include : melilite nephelinites, leucite and analcite basanites, limburgites and basanitoids. On the basis of twenty new analysis, it is suggested that nephelinites and basanites form two groups of distinct origin ; the variations in alkali content cannot explain alone the presence of analcite or leucite. The alumina content of several salites indicates a high temperature and low pressure of cristallisation. The origin of the lavas is to be sought for in the upper mantle, low pressure fractionation being probably of minor importance.

Maria Lavas, Mascons, Layered Complexes, Achondrites and the Lunar Mantle

Experimental data show Apollo 11 and 12 lava compositions to be controlled by fractional crystallization close to the lunar surface, in a process which yields achondrite-like igneous rocks as underlying complementary crystal accumulates. Volatilization losses during eruption can account for most other chemical differences between lunar lavas and common terrestrial magmas. No specific hypotheses of the composition, mineralogy, or origin of lunar interior can be sustained until the extent of these processes is known. A terrestrial upper-mantle-type lunar interior cannot yet be excluded. The assumption that maria surface lavas are primary partial melts is unjustified and leads to a postulated lunar interior with too low Mg/Mg+Fe to serve as a source for Apollo 14 and other igneous liquids. Other workers' uncontrolled visual estimates of crystallinity in experimental charges, purporting to show that maria lavas were not modified by low pressure fractionation, are irreconcilable with the chemistry of the residual liquids developed in our ‘reversed’ equilibrium experiments. The undesirability of using glass as a starting material for this type of experiment is re-emphasized.

Basalts from Madeira: A petrochemical contribution to the genesis of oceanic alkali rock series

Petrological and geochemical investigations have been conducted on the little studied Neogene basaltic rocks of the Madeiran Islands. The Madeiran suite of minor intrusives and lavas consists of parental, unusually soda rich, alkali olivine basalts with hawaiite, mugearite and essexite derivatives. Olivine and clinopyroxene are dominant phenocryst and cumulus nodule phases. Low pressure fractionation of the parental magma by precipitation of these minerals gave rise to the hawaiitic trend. That olivine settling precedes clinopyroxene in the fractionation process can be deduced from Ca and Ni variations in the analysed rocks and phenocryst separates. Late stage feldspar flotation in a hawaiitic derivative liquid led to extrusive mugearites and an intrusive essexite.

Apollo 11 and 12 Mare Basalts and Gabbros: Classification, Compositional Variations, and Possible Petrogenetic Relations

On the basis of composition, it is possible to distinguish three major groups of Apollo 12 basaltic rocks: olivine-pigeonite basalts and gabbros, ilmenite-bearing basalts and gabbros, and feldspathic basalts. Two major groups of Apollo 11 basalts are also distinguishable: ophitic ilmenite basalts and intersertal ilmenite basalts. Compositional variations between samples within groups are generally dominated by MgO variations, whereas differences between groups are primarily inverse variations of TiO 2 and SiO 2 or Al 2 O 3 and FeO. Results of fractionation calculations indicate that the MgO variation trends are explained principally by low-pressure fractionation of early-crystallized olivine ± pigeonite ± chrome spinel. The Al 2 O 3 versus FeO trend in the basalts might possibly be explained by near-surface fractionation, but the TiO 2 versus SiO 2 trend is not explainable in this way. Investigations of the latter trend in terms of possible processes of high-pressure fractional melting or fractional crystallization indicate that the compositional variations cannot be the products of simple variations in depth or degree of fractionation. Our data are consistent with the view that the mafic magmas formed by partial melting in the lunar interior, and that near-surface fractionation, with the exception of removal or addition of olivine, has not been extensive.

One-atmosphere melting and crystallization relations of lavas from Anjouan, Comores Archipelago, Western Indian Ocean

Basaltic and differentiated lavas from Anjouan, Comores Archipelago, may be divided into three groups: hypersthene-normative, alkalic and basanitic, the latter two showing chemical trends to trachyte and phonolite respectively. The experimental melting relations at one atmosphere of samples from each magmatic lineage have been investigated. Olivine is a liquidus phase in all the mafic lavas. It is accompanied or rapidly followed by clinopyroxene, whilst plagioclase precipitates at consistently lower temperatures. Low-pressure fractionation of the basaltic magmas was clearly dominated by olivine and clinopyroxene separation. In contrast, their protracted in situ crystallization, with minimal differentiation movement of crystals and liquid, is considered responsible for the development of numerous ankaramitic lavas. A genetic association between the principal magma types appears to require fractionation mechanisms at high pressure.

Phase relations of a simulated lunar basalt as a function of oxygen fugacity, and their bearing on the petrogenesis of the Apollo 11 basalts

A glass of Apollo 11 basalt composition crystallizing at 1 atm at low ƒ 0 2 showed the following crystallization sequence; ferropseudobrookite at 1210°C, olivine at 1200°C, ilmenite and plagioclase at 1140°C, clinopyroxene at 1113°C. Ferropseudobrookite and olivine have a reaction relation to the melt. This sequence agrees with that assumed on textural grounds for some Apollo 11 basalts. It also indicates that the Apollo 11 basalts cannot have been modified by low-pressure fractionation.

Hawaiite of High Pressure Origin from North-eastern New South Wales

ALTHOUGH it is now widely accepted that the source of “primary” basaltic magmas is located within the Earth's upper mantle, only rarely has a high pressure origin been proposed for the more evolved members of alkali volcanic lineages1,2 and then partly to overcome the problems posed by the relative abundances of mafic and felsic lavas in some volcanic provinces. Current petrological opinion mostly attributes the origin of the series (alkali basalt)→hawaiite→mugearite→benmoreite→alkali trachyte (compare the Hebridean and Hawaiian sodic alkali successions) to low pressure fractionation in shallow level magma chambers3,4. Certainly this hypothesis finds support in the chemistry of felsic veins and schlieren in differentiated alkaline mafic iritrusives, and of vitric groundmasses in rapidly cooled alkali basaltic lavas. The compositions of these “residua” indicate that low pressure fractionation of alkali basaltic liquids can produce mugearitic5, ben-moreitic6, alkali trachytic and phonolitic differentiates7,8. We report here, however, the discovery of a hawaiite containing lherzolite xenoliths, and an abundance of large pyroxene crystals (“megacrysts”). The latter are considered to be high pressure cognate phases of the hawaiite, and the occurrence of both megacrysts and xenoliths indicates that comparatively “evolved” alkalic liquids may also be generated at high pressures.