Differentiation Of Basalts Research Articles

Petrogenetic implications of tholeiitic basalt glasses from the East Pacific Rise and the Galápagos Spreading Center

Basalt glasses dredged from the East Pacific Rise and the Galápagos Spreading Center show a wide range of composition with FM-values [ FeO total ( FeO total + MgO ) ] from 0.49 to 0.72. Analyses for major elements of 40 unaltered glasses and of 17 of these glasses for trace elements demonstrate an analogous line of differentiation over a considerable distance in the East Pacific. Using data from the experiments of peridotite partial melting and their implication for primary melts, the composition of the initial basalt magma in equilibrium with the residual phase is extrapolated to be FM = 0.40–0.48. The importance of apatite introduced from the parental peridotite for the incompatible elements (REE, F and P 2O 5) within various basalt differentiation series is demonstrated.

Lead and strontium isotopes and related trace elements as genetic tracers in the Upper Cenozoic rhyolite‐basalt association of the Yellowstone Plateau Volcanic Field

Supported by various field geologic and petrologic data, the contents of Pb, U, Th, Rb, and Sr and the isotopic compositions of Pb and Sr for upper Cenozoic volcanic rocks of the Yellowstone Plateau volcanic field are consistent with the hypothesis of derivation of the basaltic and rhyolitic magmas by partial melting of distinct source regions in the upper mantle and lower crust, respectively. All the basalt samples analyzed but one have systematically lower values of 207Pb/204Pb and 87Sr/86Sr than the rhyolites. The values of 206Pb/204Pb are smaller, and 87Sr/86Sr are mostly larger than known values in oceanic basalts. In all but one case, the values of 207Pb/204Pb are higher than expected from an extrapolation of known values in oceanic basalts to less radiogenic values of 206Pb/204Pb. Because there are no xenoliths, phenocrysts are only moderate to sparse in abundance, REE patterns are low and flat at the radiogenic end of lead isotopic compositions, several values of Rb/Sr are low, and 80% of the basalt samples form a well‐developed secondary isochron separate from the rhyolites, we favor an interpretation for basalt genesis wherein isotopic signatures of most mafic magmas were attained in a continental ‘keel’ of mantlelike character about 2.6 b.y. old or somewhat older attached to the crust, and these signatures were unaltered by magma passage through the crust. At the very least, the current data continue to cast serious doubt as to the inevitability of crustal contamination for basaltic magma intruding the continental environment and postulate that much can be learned about the mantle under continents through the study of continental basalts. One basalt unit with an unusually low value of 207Pb/204Pb and an 87Ar/86Ar less than 0.704 may represent subcontinental ‘keel’‐derived magma that rose unaltered to the surface. Our data also are not consistent with formation of this rhyolite‐basalt association primarily by such processes as crystal fractionation, separation of immiscible silicate liquids from a common parental magma, or fractional melting of a homogeneous source. Rather as a conceptual model, we envision large mafic intrusions to have been injected into the lower crust resulting in rhyolite generation through partial anatexis of the adjacent wall rocks which probably had a 206Pb/204Pb < 17 and 87Sr/86Sr > 0.709; a model that has much in common with that proposed by Holmes (1931). All the other hypotheses listed have the necessary added complication that either the basalt or the rhyolite or both become contaminated after the two magma types separated, have problems accounting for the lack of igneous rocks of intermediate compositions or production of such large volumes of rhyolitic material (∼5000 km3), and fail to explain why rhyolitic magma is not a more common occurrence in the ocean basin. We appeal to bouyancy of rhyolites to generate a barrier for basalt magma migration and account for the great preponderance of rhyolite relative to basalt at the surface. Furthermore, the complex isotopic picture in the rhyolites indicates that many of these magmas interacted with the upper crustal geologic units that they traversed. The interactions involved diverse processes, probably including reacton with hydrothermal fluids or hydrothermally altered rocks at high levels as well as by contamination with Phanerozoic sedimentary and Precambrian crystalline rocks at deeper levels. At the very least, we feel our study adds a cautionary note to the currently increasingly popular hypothesis that differentiation of basalt or gabbro magmas to rhyolite or granite (as distinct from tonalite or dacite) is a common occurrence and is therefore an important continential building process. Models for formation of rhyolite and granite predominantly by reworking of crust (anatexis) must still be considered. The primitive Archean mantle of the region was characterized by higher Rb/Sr, U/Pb, and Th/U values than are typical of modern suboceanic mantle. The mantle residuum within the continental subcrustal lithosperic ‘keel’ that resulted from the Archean crustal differentiation event probably was depleted in Rb/Sr and U/Pb, and the crust was correspondingly enriched in these ratios. The crust probably was further differentiated by an Archean high‐grade metamorphism, during or after the primary event, into a granulitic lower crust depleted in U/Pb and Rb/Sr and a lower‐grade upper crust enriched in these ratios.

Magma mixing at mid‐ocean ridges: Evidence from legs 45 and 46‐DSDP

An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has yielded evidence that these moderately evolved basalts are mixtures of primitive, mantle‐derived tholeiites with more evolved magmas. The hybrid nature of these rocks is recognized on the basis of disequilibrium mineralogy. Plagioclase phenocrysts exhibit substantial diversity in composition and zoning patterns including both normally and reversely zoned grains and abundant textural evidence of resorption. Many olivine and plagioclase phenocrysts are too refractory (e.g., Fo90−88) to be in equilibrium with liquids of the host basalt composition and are in fact of a composition consistent with crystallization from a primitive, mantle‐derived basalt liquid. An estimate of this primitive melt, presumed to be parental to the observed lavas, has been derived from melt inclusions trapped in the olivine phenocrysts. These are characterized by high Ca/Al and low TiO2, two chemical features typical of primitive ocean floor basalts recovered elsewhere in the Atlantic.We suggest that subvolcanic magma chambers beneath mid‐ocean ridges receive periodic injections of this primitive melt and its attendant phenocrysts which mix with fractionated chamber‐bound magmas, resulting in observed moderately evolved lavas. Chemical modeling of ocean floor basalt differentiation assuming a combination of magma mixing and crystal fractionation alleviates some apparent anomalies encountered in previous studies (Rhodes et al., 1978).

Gravitational separation of quenching crystals: a cause of chemical differentiation in lunar basalts

The low viscosity of lunar lavas allowed skeletal quenching crystals, which formed during post-eruption cooling of the flows, to sink and accumulate despite the low gravitational acceleration. The crystals which sank were not in chemical equilibrium with the liquid. Hand specimen bulk compositions are related to each other by differential movement of the materials composing the cores and rims of the skeletal ‘phenocrysts’, and not by any equilibrium chemical process. Crystal sinking must have occurred, and cumulates must have formed. Mafic hand specimens, consisting of skeletal phenocrysts in a more feldspathic groundmass, represent the cumulus enriched material and not the original liquids. The lava compositions were much more feldspathic, and were controlled by fractional crystallization at low pressures. The effects of such fractionation must be considered and, in most cases, restored before valid petrogenetic inferences about the mineralogy and chemistry of the lunar interior can be drawn.

Inferred geologic settings and differentiation in basalts from the Deep-Sea Drilling Project

Petrographic and geochemical studies of basalt glass and related rocks from the Deep-Sea Drilling Project (DSDP) provide a random sampling of magmas which have crystallized to form sea floor basement rocks. The samples range in age from 155 m.y. to less than 4 m.y. and represent a variety of geologic settings within the three major ocean basins. The majority show the mineralogy, textures, and large-ion lithophile (LIL) element-depleted characteristics of modern basalts from mid-ocean spreading ridges (group I). A smaller but compositionally diverse group shows distinctive mineralogy and LIL enrichment (group II). Group II magmas from spreading ridges are very similar to group I magmas in terms of major element variations but are more enriched in LIL elements. Group II magma from seamounts and off-ridge centers tends to be olivine normative and enriched in normative plagioclase, while that from aseismic ridges tends to be iron-enriched and quartz normative. The diversity among DSDP rocks is only slightly greater than that shown at a single well-sampled mid-Atlantic ridge (MAR) site (FAMOUS). There are a few unusual Fe- or Ti-rich samples which so far are unique to the Pacific and Indian oceans. Compositional diversity in the Indian Ocean is especially striking. Preliminary petrogenetic modeling suggests that the samples as a whole are not products of a single high-level fractionation sequence. Glasses which project on a pseudobinary cotectic in the Di-An-Fo system may differ significantly in FeO*/MgO, TiO2, and LIL element enrichment at comparable points on the co-tectic. These liquids probably represent several compositionally distinct mantle-derived partial melts and their fractionation products. Within the data set, individual fractionation sequences can be defined, in which most major elements are adequately explained by simple crystal-melt equilibria. However, even in these sequences, inferred residual liquids tend to show excess LIL element enrichment which is not yet understood. Most DSDP basement rocks can be related back to a spreading ridge enviroment if due account is taken of apparent ‘plume-related‘ geochemical variations along modern ridges. Unusual LIL element-enriched sea floor basalts may be related to areas such as the one at 45°N on the MAR where there is little evidence of structural or bathymetric anomalies associated with a ‘plume’. We see no evidence for geochemical or mineralogical features diagnostic of variations in spreading rate, nor is there any obvious relation between composition and age.

Crystallization and Fractionation Trends in the System Andesite-H2O-CO2-O2 at Pressures to 10 Kb

Phase relations of a Mount Hood andesite, which has the composition of an average orogenic andesite, have been determined as a function of O 2 fugacity at 1 atm and of H 2 O fugacity to pressures of 10 kb, at O 2 fugacities of the quartz-fayalite-magnetite (QFM) buffer. All runs contained either a H 2 O or H 2 O–CO 2 fluid phase; melts in runs with a H 2 O–CO 2 fluid phase were H 2 O undersaturated. The H 2 O contents of the melts and H 2 O fugacities were calculated from NaAlSi 3 O 8 –H 2 O thermo-dynamic data on the assumption of ideal mixing in the system H 2 O–CO 2 . One-atmosphere runs show that melting relations of silicates are little affected by f o 2 but that both ilmenite- and magnetite-out temperatures are raised by higher f o 2 . Ilmenite precipitates at higher temperature than magnetite. In these runs and in all runs at high pressure with H 2 O and H 2 O–CO 2 fluid phases, oxides were not stable at temperatures of the silicate liquidus. Oxides might be stable on the silicate liquidus if f o 2 rose two or more log units above the Ni–NiO (NNO) buffer. However, calculations indicate that in natural magmas, those processes which might change f o 2 —crystal-liquid equilibria or exchange of H 2 , or H 2 and H 2 O with the wall rocks—cannot raise f o 2 by that magnitude. Because differentiation of basalt melts to andesite must involve iron-rich oxide phase subtraction, such fractionation models appear unreasonable. For the Mount Hood andesite composition, plagioclase is the liquidus phase under H 2 O–saturated conditions to 5 kb and under H 2 O–undersaturated conditions at 10 kb when the H 2 O content of the melt is less than 4.7 wt percent. For higher H 2 O contents, either orthopyroxene or, at H 2 O saturation at pressure greater than 8 kb, amphibole assumes the liquidus. In all cases, clinopyroxene crystallizes at lower temperature than orthopyroxene. Melting curves in the H 2 O–under-saturated region may be contoured either as percent H 2 O in melt or as P e H 2 O ; in either case, the topology of the various silicate melting curves is different from the case of H 2 O–saturated melting. Therefore, melting relations determined at H 2 O–saturated conditions cannot be used successfully to predict melting relations in the H 2 O–undersaturated region. Amphibole melting relations were studied isobarically at 5 kb as a function of temperature and fluid-phase composition. Amphibole has a maximum stability temperature of 940 ± 15°C for fluid compositions of 100 to 44 mole percent H 2 O; for fluids containing more CO 2 than 56 percent (or, equivalently, less than 4.4 wt percent H 2 O in melt), the melting temperature is lower. The same relations would be seen if CO 2 were not present and the melt were H 2 O undersaturated. These rather low melting temperatures, relative to other silicate phases, preclude andesite generation by basalt fractionation involving amphibole at pressures less than 10 kb.

Genetic relations of volcanic rocks of the Pacific Ocean

The volcanic rocks of Hawaii form two distinct series, one of basalts that become increasingly alkaline and undersaturated with silica and a second group of differentiated rocks that become progressively more felsic. Although most petrologists agree that the second group is the product of differentiation of alkaline basalts, there is less agreement on the origin of compositional variations among the basalts.

長崎県・北松浦玄武岩類の分化と成因について : 日本火山学会1962年度秋季大会講演要旨

長崎県・北松浦玄武岩類の分化と成因について : 日本火山学会1962年度秋季大会講演要旨

Differentiation of Hawaiian basalts; Trends of Mauna Loa and Kilauea historic magma

New analyses strengthen the conclusion that the historic lavas of Mauna Loa and Kilauea are derived from different magmatic batches. The presence or absence of hypersthene in the lavas seems to be related to the silica and lime contents.

Differentiation of Hawaiian Basalts: Some Variants in Lava Suites of Dated Kilauean Eruptions

Differentiation of Hawaiian Basalts: Some Variants in Lava Suites of Dated Kilauean Eruptions

The chemistry and mineralogy of the Kinkell tholeiite, Stirlingshire

The tholeiite of Kinkell (11 km. north of Glasgow G.P.O.) was described by the first author (Walker, 1930) and an analysis of its residual glass was given in a later publication (Walker, 1935). Since unaltered and separable residual glasses of basaltic rocks are rare, the Kinkell example has been referred to frequently in petrological literature. E. A. Vincent (1950) has made a detailed comparison between the Kinkell glass and a similar one from a tholeiite from Kap Daussy, east Greenland. In view of this and of the important part played by such glasses in discussions on basaltic differentiation, all the major constituents of the Kinkell rock have been isolated and analysed chemically and spectrographically. They are described below together with a fuller account of the field relations of the rock and some comments upon the significance of the new data.

Some aspects of magmatic evolution

Summary The address is concerned with some aspects of magmatic evolution in volcanic successions, and includes particularly a discussion of the nature and genetic relations of primary and derivative basic and intermediate magmas. Among the volcanic associations of the oceanic areas, those of the Hawaiian archipelago provide data of critical importance in the study of basaltic evolution. The established chronological succession of magmas in these islands makes it probable that the alkali-poor tholeiites to be recognized in the lavas of the primitive shield volcanoes form the closest approach to the primary magma of the ocean basins, and from them derive by fractional crystallization processes the Hawaiian alkali magma series of alkali olivine-basalt, mugearite (so-called andesite) and trachyte. The course of volcanic variation there exhibited provides a new orientation to the study of the genetic evolution of the Tertiary igneous Brito-Icelandic province, and leads to the conception that the Hebridean alkali magma series derives through tholeiites as an alternative line of descent. The course of fractional crystallization of tholeiite magma in the great layered intrusion with its final product of granophyre is illustrative of the great flexibility in line of descent which fractionation processes permit. Derivation by crystallization differentiation in a convecting system comparable to that demonstrated in layered intrusions seems, indeed, the most likely mechanism for the genesis of the primitive sialic layer of the crust itself. The problem of intermediate magma in the volcanic successions of the orogenic belts is critical in petrogenic theory. Though fractionation processes appear most competent to give a liquid line of descent through andesite to rhyolite, normal andesite magma does not reflect in its chemistry the course of basaltic (tholeiitic) differentiation. Consideration is devoted to the conception that andesites, though ultimately of basaltic parentage, have derived in part by sialic contamination. The testing of the genetic relations of assemblages within the basalt-andesite range of the volcanic suites of the orogens remains one of the most important problems of volcanic petrogenesis. Systematic attack through intimate phase analysis and geochemical study may provide the data which will go far to solve, not only the status of the voluminous andesite, but of intermediate magma in general in igneous rock evolution.