Alexander R. McBirney

Properties of Some Common Igneous Rocks and Their Melts at High Temperatures

The properties of four igneous rocks (a tholeiitic and an alkali-olivine basalt, an andesite, and a rhyolite) and a synthetic lunar sample have been determined at atmospheric pressure over a range of temperatures including their melting interval. Viscosity, density, electrical and thermal conductivity, ultrasonic wave velocities, surface tension, and vesiculation rates were measured directly; these data have been used to calculate values for thermal expansion, compressibility, ultrasonic wave attenuation, and activation-energy coefficients.

Petrology and geochemistry of the Galápagos Islands: Portrait of a pathological mantle plume

motion of the Nazca plate with respect to the fixed hotspot reference frame. lsotope ratios in the Galfipagos display a considerable range, from values typical of mid-ocean ridge basalt on Genovesa (87Sr/86Sr: 0.70259, end: +9.4, 206pb/204pb: ! 8.44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, œNd: +5.2, 206pb/204pb: 20.0). La/Sm N ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galfipagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galfipagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north-south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galfipagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Sis. 0, FeB. 0, and NaB. 0 values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume-asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galfipagos Spreading Center.

Mixing and unmixing of magmas

Abstract Linear compositional variations of calc-alkaline rocks are difficult to explain in terms of mixing of two genetically unrelated magmas. At least two processes must be involved, a deep-seated one responsible for the long-term variations between suites of rocks erupted in successive episodes and a shallow-level one by which magma columns become vertically zoned during periods of repose between eruptions. The first process appears to involve resorption of differing proportions of mafic residue in a rising batch of magma. The second results from fractionation and collection of a buoyant liquid under the roof of a high-level reservoir. The effect of cooling and crystallization of a calc-alkaline intrusion is to produce a compositional change and decrease of density in the fractionated liquid that has a greater effect on the boundary layer than thermal contraction. The same effect may result from melting of felsic wall rocks or absorption of water. Many of the features that are commonly observed in calc-alkaline rocks can be explained in terms of these two processes without appealing to physically implausible mechanisms involving mixing of liquids of differing densities.

Liquid fractionation. Part I: Basic principles and experimental simulations

Abstract A possible explanation for the closely associated magmas of contrasting compositions erupted from many mature volcanic centers can be found in the large differences of density produced by relatively small compositional variations in liquids that evolve by crystallization or melting at the walls of shallow magma chambers. A mechanism of liquid fractionation in which differentiated liquids segragate gravitationally to form compositionally graded columns of magma may surmount the long-standing problem of explaining large volumes of highly evolved liquids that reach advanced degrees of differentiation in times that are too short to be consistent with conventional models of crystal fractionation based on crystal settling. In those types of magmas that decrease in density as they differentiate, a fractionated liquid next to a wall may form a buoyant compositional boundary layer that flows up the wall and accumulates as a separate zone in the upper levels of the reservoir. Magmas that increase in density as they differentiate will have the opposite behavior; they descend along the wall and pond on the floor. Both types of systems can be modeled using simple aqueous solutions and techniques similar to those developed by Chen and Turner (1980). The insights gained through experiments of this kind suggest a number of processes that may be responsible for common types of volcanic behavior and patterns of differentiation in shallow plutons.

The Skaergaard Intrusion

Abstract Thanks to its magnificent exposures and extraordinarily complete sequence of strongly differentiated rocks, the Skaergaard Intrusion has long served as a prime example of shallow magmatic differentiation and as a testing ground for a wide range of petrologic concepts. During the magmatic episode accompanying the opening of the North Atlantic about 55 Ma ago, a moderately evolved tholeiitic magma was intruded, apparently in a single, prolonged pulse, into Archean gneisses and Tertiary basalts close to the eastern edge of Greenland. Over a period of about 10,000 years, the Layered Series crystallized on the floor, while similar sequences crystallized on the walls and under the roof to form the Marginal and Upper Border Series. In all three places, the minerals follow parallel trends with plagioclase progressing steadily from basic labradorite to sodic oligoclase and olivine and Ca-rich pyroxene evolving to pure fayalite and hedenbergite. The original magma, which was unusually rich in phosphorus and titanium, followed a trend of differentiation characterized by exceptionally strong iron enrichment and relatively little increase of silica until the very latest stages of differentiation when the magma split into two liquids, one very rich in iron and the other in silica. The original compositions and textures of the rocks have been altered by late-stage metasomatism which, in extreme cases, resulted in closely associated anorthosites and pyroxenites. The principal mechanism of crystal-liquid fractionation during formation of the Layered Series was compaction, but convective fractionation seems to have become important in the late stages of evolution.

Episodes of cenozoic volcanism in the circum-pacific region

Abstract The tempo of Cenozoic volcanism on opposite sides of the Pacific Ocean has been examined by compiling the numbers of radiometric dates reported for terrestrial volcanic sequences and the numbers of volcanic ash (glass) horizons recorded in Neogene deep-sea (DSDP) sedimentary sections. Within certain limits these data are believed to provide a reliable record of extrusive and explosive volcanism. Although terrestrial and marine records for individual regions reveal important differences in the episodicity of volcanism, a correlation is found between activity in the Southwestern Pacific, Central America and the Cascade Range of western North America. Two important pulses of Neogene volcanism (the Cascadian and Columbian episodes) occurred during the Quaternary ( t = ∼ 2 m.y. to present) and within the Middle Miocene ( t = 16 to 14 m.y. ago), with less important episodes in the latest Miocene to Early Pliocene ( t = 6 to 3 m.y. ago) and Late Miocene (11 to 8 m.y. ago). The names Fijian and Andean are proposed to these episodes. Dating of terrestrial sequences indicates that these episodes of intense volcanism took place in relatively short intervals of time, separated by longer more quiescent periods. It has been suggested that synchronous episodic volcanism is related to changes in rates of sea-floor spreading and subduction. If so, volcanism must amplify these changes, because the variations in tempo of volcanism are much too great for proportional rate changes. An apparent correlation of volcanism in orogenic zones of the circum-Pacific region with world-wide changes of sea level and changes of activity in the Hawaiian-Emperor chain suggests that volcanism records fundamental tectonic changes throughout the entire Pacific region.

Paricutin re-examined: a classic example of crustal assimilation in calc-alkaline magma

Following its birth on the 20th of February 1943, the Mexican volcano Paricutin discharged a total of 1.38 km3 of basaltic andesite and andesite before the eruption came to an end in 1952. Until 1947, when 75% of the volume had been erupted, the lavas varied little in chemical or isotopic composition. All were basaltic andesites with 55 to 56% SiO2, δ18O of +6.9 to 7.0, and 87Sr/86Sr ratios close to 0.7038. Subsequent lavas were hypersthene andesites with silica contents reaching 60%, δ18O values up to +7.6, and 87Sr/86Sr of 0.7040 to 0.7043. The later lavas were enriched in Ba, Rb, Li, and K2O and depleted in MgO, Cu, Zn, Cr, Ni, Sr, and Co. The isotopic and other chemical changes, which appeared abruptly over a few months in 1947, are interpreted as the result of tapping a sharply zoned and density stratified magma chamber. Xenoliths of partially fused felsic basement rocks in the lavas have silica contents greater than 70%, δ18O of +5.6 to 9.9 and 87Sr/86Sr between 0.7043 and 0.7101. In many respects they resemble samples of basement rocks collected from nearby outcrops. Three analysed samples of the latter have silica contents of 65 to 67%, δ18O of +7.7 to 8.6, and 87Sr/86Sr between 0.7047 and 0.7056.These new data provide strong support for the original interpretations of Wilcox (1954), who explained the chemical variations by a combination of fractional crystallization and concurrent assimilation of up to 20 weight % continental crust. Except for a few trace elements, particularly Ba, Sr, and Zr, the chemical and isotopic compositions of the xenoliths and basement rocks that crop out nearby match the type of contaminant required to explain the late-stage lavas. Some of the discrepancies may be explained by postulating a contaminant that was older and richer in Ba, Sr, and Zr than those represented by the analysed xenoliths. Others can be attributed to chemical changes accompanying disequilibrium partial melting, contact metamorphism, and meteoric-hydrothermal alteration of the country rock. Many of the xenoliths show evidence of having been affected by such processes.The lavas were erupted from a zoned magma chamber that had differentiated by liquid fractionation prior to the eruption. The order of appearance of the lavas can be explained in terms of withdrawal of stratified liquids of differing densities and viscosities.

Mechanisms of Formation of Igneous Layering

Abstract Layering is a common, almost ubiquitous, feature of gabbroic and syenitic intrusions. Individual layers, or layered sequences, however, vary greatly in such features as thickness and length, the nature of layer boundaries, internal vertical and lateral variations within layers, and the relationships to other nearby layers. Their modal proportions, grain-sizes, mineral compositions, whole-rock compositions, and textures present in layers and their surrounding host rock, are also quite varied. Given the wide range of these characteristics, it is unlikely that any single layer-forming mechanism can explain all or even most of the known occurrences of igneous layering. A wide variety of layer-forming mechanisms has been proposed. Some operate during the initial filling of a magma chamber, as a result of the settling of crystals carried in suspension, flow segregation during magma transport, magma chamber recharge, or magma mixing. Other proposed mechanisms operate in response to continuous, intermittent, or double-diffusive convection. Layering may also form as the result of mechanical processes, such as gravity settling, crystal sorting by magma currents, magmatic deformation, compaction, seismic shocks, or tectonic deformation. Variations of intensive parameters and kinetic factors, such as fluctuations of rates of nucleation and growth of crystals, oxygen fugacity, pressure, and rates of separation of immiscible liquids, may also be responsible for certain types of layering. During late-stage crystallization and cooling, layering may form in response to porous flow of interstitial liquids, metasomatism, constitutional zone refining, solidification contraction, Ostwald ripening, or contact metamorphism. The simple concept of a magma chamber undergoing differentiation as a result of early-formed crystals settling out of the magma and accumulating in layers on the floor of the chamber, has been discarded by most petrologists in favor of models involving in situ crystallization, in which magma chambers are thought to have the general form of a central mass of nearly crystal-free magma, that gradually loses heat and crystallizes inwards from its margins. The transition from crystal-free magma in the central part of the chamber to completely solidified rock in the outer parts is thought to occur through a marginal zone of crystal-liquid mush. As magmas crystallize and differentiate, components included in early-crystallizing minerals are depleted, while those excluded from these phases are enriched. It is unclear, however, how the latter are effectively transferred through the crystal mush zone, so that crystallization at margins results in differentiation of the body as a whole. It is also not clear what non-steady-state or non-equilibrium processes are responsible for the formation of layering during the crystallization process. Because these two problems are interrelated, an understanding of the formation of igneous layering should eventually lead to a better understanding of the processes responsible for igneous differentiation. The time scales and length scales involved in the formation of igneous layering preclude direct experimentation on silicate melts at magmatic temperatures, and as a result, the origin of these features must be largely deduced from field observations and theoretical considerations. The challenge for the igneous petrologist is to determine which features of igneous layering are diagnostic of a particular mechanism, which reflect subsequent compositional or textural modifications, and which can best discriminate between the plethora of possible mechanisms that have been proposed.

Relations of oceanic volcanic rocks to mid-oceanic rises and heat flow

Abstract Oceanic volcanic rocks are far from uniform; distinct magmatic provinces can be defined in terms of large-scale structural features. Petrochemical differences are most apparent in strongly differentiated rocks which accentuate characteristics that are only weakly developed in undifferentiated basalts. The degree of silica saturation of basalts and differentiated rocks of oceanic islands can be correlated with the position of an island on the crest or flanks of oceanic rises. In both the Atlantic and Pacific, strongly oversaturated rocks are found on or near the crests of the East Pacific Rise and Mid-Atlantic Ridge, whereas undersaturated alkaline rocks appear on the flanks and in areas far removed from rises. A close relation to heat flow from the ocean floor suggests that the more siliceous rocks near the crests are produced by melting at shallow depths while more alkaline magmas erupted far down the flanks are produced at greater depths. The relations have possible implications bearing on hypotheses of sea floor spreading and the origin of oceanic rises.

The differentiation of the Skaergaard Intrusion: A discussion of Hunter and Sparks (Contrib Mineral Petrol 95:451?461)

ConclusionsWe find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.