Brian H. Baker

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.

Geochemistry and petrogenesis of a basalt-benmoreite-trachyte suite from the southern part of the Gregory Rift, Kenya

In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 104 km3 during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.

Liquid fractionation. Part III: Geochemistry of zoned magmas and the compositional effects of liquid fractionation

Abstract Compositional variations of zoned magmas ranging from basalt to rhyolite and basanite to phonolite are compatible with crystal-liquid fractionation. There is no difference in the trends within zoned magmas and those of differentiated suites erupted over long times in the form of separate homogeneous extrusions. Apparent anomalies in the trace-element variations of zoned high-silica magmas can be explained by fractionation of accessory phases. The wide range of incompatible trace-element abundances in silicic magmas requires extended crystal-liquid fractionation. This cannot take place in the zoned, static, and viscous upper parts of silicic magma chambers. Most of the compositional variations must develop during flow of buoyant differentiated liquid up the walls of a magma reservoir. Distribution coefficients measured from zoned magmas cannot be used directly to model fractionation in a flowing boundary layer, because the mineral assemblage crystallizing on the wall differs from the phenocryst assemblage that grows after segregation to form a zoned magma layer under the roof of the chamber, and kinetic effects in the boundary layer tend to increase trace-element enrichments and depletions. Similar effects result from crystallization on the chamber roof, and from periodic extractions of roof-zone magma by eruption or intrusion. Laboratory experiments illustrate the roles of crystallization and diffusion during and after ascent of boundary-layer magmas. Theory suggests that the effects of fractionation and mixing will be dominant in the boundary layer at the wall and under the roof of a magma chamber, but ordinary diffusion may also be detectable in the roof zone. The compositional ranges and volumes of different igneous suites are well explained by multi-stage liquid-fractionation processes. The Soret effect cannot explain the differentiation of zoned magmas unless they are above their liquidus temperatures. Linear correlation of most incompatible elements in zoned magmas suggests that vapor phase transport is not important to their differentiation.

Liquid fractionation. Part II: Fluid dynamics and quantitative implications for magmatic systems

Abstract The processes of liquid fractionation described in part I of this report are evaluated quantitatively to determine the ranges of physical conditions in which they are likely to be effective under realistic geological conditions. Similitude analyses and numerical modeling are used to identify the conditions consistent with counterflow of a rising compositional boundary layer and a descending thermally driven flow in the interior of a cooling intrusion. Estimates are derived for the rates of accumulation of differentiated liquids in the upper levels of stratified reservoirs; these are more than adequate to account for rates inferred from field observations. Parametric studies illustrate the influence of various factors (e.g. Prandtl, Lewis, Reyleigh, and Grashof numbers, the ratio of buoyancy forces, turbulence, and viscosity contrasts between the wall and interior) for values spanning many orders of magnitude. In particular, it is shown that flow in the buoyant boundary layer is relatively unaffected by turbulence of the interior and that viscosity variations can be accounted for by an appropriately chosen effective or mean viscosity. Results are used to assess the fidelity of laboratory models outlined in Part I, and specific examples are given to illustrate the distribution of velocity, temperature, and composition at various levels within a calc-alkaline magma chamber.

Introduction—Processes of Continental Rifting

It is thought likely that thermal thinning and/or diapirism can cause the extensional stress required for rifting. The rifting, however, will not occur unless the regional tectonic regime permits the sides of the rift to diverge. Whereas passive plate extension could cause rifting in isolation, the extension and rifting are likely to be localized where the lithosphere is weakest over an existing thermal anomaly. In those cases where asthenospheric diapirism occurs, which is essentially a response to thinning of the lithosphere by thermal thinning or plate extension, the effects of diapirism may completely mask the initiating mechanism. It is believed that anomalous heat transfer into the lithosphere, diapirism, and magmatism must all figure in rifting, along with a deviatoric stress field that will permit extension in a developing rift. Even though the models are useful in permitting idealized processes to be quantified and tested, better knowledge of lithosphere properties is considered necessary, in particular knowledge of mantle viscosity and its temperature dependence.

Compositional changes during crystallization of some peralkaline silicic lavas of the Kenya rift valley

Abstract Comparison of crystalline and glassy comendites shows that compositional changes during solidification involve losses of Na, Cs, Cl, F, gain of Sr, and gains and losses of REE at near-solidus temperatures. Variation in a thick trachyte lava suggests deuteric mobility of the same elements, while variation in a trachyte-comendite suite shows that Fe loss can also occur. Al, Si, Zr, Hf, Nb, Ta, Th, U, and Rb are not significantly affected, nor are the proportions of the REE, and these elements can be relied upon in petrochemical studies of crystalline peralkaline silicic rocks.