Stephen R. Tait

Postcumulus processes in layered intrusions

During the postcumulus stage of solidification in layered intrusions, fluid dynamic phenomena play an important role in developing the textural and chemical characteristics of the cumulate rocks. One mechanism of adcumulus growth involves crystallization at the top of the cumulate pile where crystals are in direct contact with the magma reservoir. Convection in the chamber can enable adcumulus growth to occur to form a completely solid contact between cumulate and magma. Another important process may involve compositional convection in which light differentiated melt released by intercumulus crystallization is continually replaced by denser melt from the overlying magma reservoir. This process favours adcumulus growth and can allow adcumulus growth within the pore space of the cumulate pile. Calculations indicate that this process could reduce residual porosities to a few percent in large layered intrusions, but could not form pure monomineralic rocks. Intercumulus melt may also be replaced by more primitive melt during episodes of magma chamber replenishment. Dense magma, emplaced over a cumulate pile containing lower density differentiated melt may sink several metres into the underlying pile in the form of fingers. Reactions between melt and matrix may lead to changes in mineral compositions, mineral textures and whole rock isotope compositions. Another important mechanism for forming adcumulate rocks is compaction, in which the imbalance of the hydrostatic and lithostatic pressures in the cumulate pile causes the crystalline matrix to deform and intercumulus melt to be expelled. For cumulate layers from 10 to 1000 metres in thickness, compaction can reduce porosities to very low values (< 1 %) and form monomineralic rocks. The characteristic time-scale for such compaction is theoretically short compared to the time required to solidify a large layered intrusion. During compaction changes of mineral compositions and texture may occur as moving melts interact with the surrounding matrix. Both compaction and compositional convection can be interrupted by solidification in the pore spaces. Compositional convection will only occur if the Rayleigh number is larger than 40, if the residual melt becomes lower in density, and the convective velocity exceeds the solidification velocity (measured by the rate of crystal accumulation in the chamber). Orthocumulates are thus more likely to form in rapidly cooled intrusions where residual melt is frozen into the pore spaces before it can be expelled by compaction or replaced by convection.

Fluid dynamic and geochemical evolution of cyclic unit 10, Rhum, Eastern Layered Series

Lithological, major element, trace element and Sr isotope data from cyclic unit 10 of the Rhum Eastern Layered Series are presented. The lower 65 metres of the unit are peridotite, subdivided on textural and geochemical grounds into a lower homogeneous portion approximately 50 metres thick and an upper heterogeneous portion approximately 15 metres thick. The uppermost 16.5 metres of the unit are allivalite. There are steep geochemical gradients across the peridotite-allivalite boundary in Ni content of olivine and whole-rock Sr isotope composition. Calculations are presented on the geochemical evolution of a Rhum picritic liquid undergoing olivine precipitation, both when the olivines remain suspended in the residual liquid as they precipitate, and when they are continuously fractionated. Quenched groundmass and olivine compositions from the Rhum dykes and the unit 10 peridotite olivines show good agreement with the suspension model but are inconsistent with the fractionation model. The Rhum chamber is thought to have been replenished with a picritic liquid from which olivine crystallized while held in suspension; however, replenishment by a highly olivine-phyric basalt is also possible. The peridotite probably accumulated rapidly as olivines were dumped out of suspension onto the chamber floor. The lower part of the peridotite is a poikilitic adcumulate; it is suggested that this formed by convective circulation of melt in the pores of the pile of cumulus olivines. In the latter stages of adcumulus growth, more Fe-rich and isotopically contaminated magma entered the top of the cumulus pile causing cumulus olivines to re-equilibrate and giving the intercumulus plagioclase a higher Sr isotope ratio than lower down. The olivines in the allivalite show steep stratigraphic gradients in major element composition but not in their Ni content. They also show substantial variation in major element composition laterally within the allivalite. It is suggested that these features are a consequence of postcumulus re-equilibration of olivine with migrating intercumulus magma.

A phenomenological model for precursor volcanic eruptions

Intense explosions of relatively short duration frequently precede large explosive and effusive volcanic eruptions—by as much as weeks to months in the case of very viscous magmas. In some cases, such pre-eruption activity has served as a sufficient warning to those living in the vicinity to evacuate and avoid calamity. Precursor events seem to be related to the formation of a magma pathway to the surface, but their precise interpretation is a long-standing puzzle. It has been inferred from theoretical studies that exsolution of volatiles might create an almost separate gas pocket at the tip of a propagating dyke. Here we explain the role that such a process may have, using a laboratory study of the transient propagation of a liquid-filled crack with a gas pocket at its tip that grows with time. We show that once the gas pocket acquires sufficient buoyancy to overcome the fracture resistance of the host solid the dynamics of the gas pocket, rather than those of the liquid, determine the velocity of the crack tip. Furthermore, we find that the gas can ultimately separate from the liquid. We propose that fast-moving, gas-rich pockets reaching the surface ahead of the main liquid-filled fissure could be the origin of many precursor eruptions.

Convective exchange between pore fluid and an overlying reservoir of denser fluid: a post-cumulus process in layered intrusions

Abstract We describe experiments on buoyancy-driven exchange between fluid in a porous bed of glass balls and an overlying reservoir of denser fluid. “Fingers” of the dense fluid penetrated downwards into the porous medium. When the two fluids had equal viscosities, the rate of convective exchange was proportional to permeability and the density difference between the fluids, and was inversely proportional to viscosity. When the fluids had different viscosities, the rate further depended on the viscosity ratio raised to the power 0.4. If a layer of dense new melt is emplaced above a porous cumulus pile forming the floor of a magma chamber an analogous process may occur, as the new melt can percolate down into the pore space and expel the less dense intercumulus liquid. Convective exchange may proceed at rates of metres per year to tens of metres per year in basic to ultrabasic magmatic systems. This conclusion strongly depends on the porosity of the cumulus pile and is less sensitive to the magmatic viscosities. Chemical and textural effects will result from this process, such as re-equilibration and resorption of cumulus crystals. When intercumulus melt exchange is incomplete, lateral geochemical gradients may be preserved in cumulate rocks.

Isotopic and geochemical investigation of unit 10 from the Eastern Layered Series of the Rhum Intrusion, Northwest Scotland

Sr and Nd isotopic compositions together with whole rock Sr and Nd abundances and olivine compositions are presented for cumulates from unit 10 of the Eastern Layered Series of the Rhum intrusion, Northwest Scotland. The results show an Nd–Sr isotopic correlation consistent with contamination by high 87 Sr/ 86 Sr, low 143 Nd/ 144 Nd crust. There is a strong correlation between the degree of contamination, Sr and Nd whole rock abundance and Fe enrichment in cumulus olivine. The allivalites are the most contaminated, while the basal peridotites are uncontaminated. This suggests that the cumulates crystallized from a primitive uncontaminated magma and the allivalites from an evolved contaminated magma. Subsequently, movement of contaminated feldspathic intercumulus liquid has perturbed the isotopic, trace element and olivine composition of the upper peridotites.

Experimental Modelling of Interstitial Melt Convection in Cumulus Piles

Fluid dynamic processes occur during the formation of cumulate rocks and thus model experiments and theory are a valuable counterpart to geological work in studying magma chamber processes. We describe experiments on crystallisation in porous media. Crystal growth from saturated aqueous solutions produced light, chemically-depleted fluid which could potentially convect out of the porous medium and be replaced by additional saturated solution from above. As long as the convective velocity in the pore space remained greater than the rate of solidification, the pore space became enriched in chemical components entering the growing crystals relative to the initial solution. This convection is a possible mechanism of adcumulus crystal growth in magma chambers, and might produce cumulate rocks with low trapped melt fraction in large mafic and ultramafic intrusions. When the porous media had low initial permeabilities, interstitial melt motion was greatly reduced or completely suppressed. An impermeable crystal layer grew on top of the porous medium in these circumstances. Trapping of dynamically-unstable melt beneath this layer occurred in the experiments and may be important geologically. If during magma chamber replenishment, dense melt is emplaced above a porous cumulus pile, it can sink into the pore space and replace the less dense intercumulus liquid. Experiments indicate that convective exchange takes the form of downward penetrating “fingers”. Quantitative results suggest that exchange may proceed at rates of metres to tens of metres per year in mafic to ultramafic magma chambers — this conclusion strongly depends on cumulus pile porosity and is less sensitive to magma viscosity. Re-equilibration and resorption of cumulus crystals could result from this process. When intercumulus melt exchange is incomplete, lateral geochemical gradients may be preserved in cumulate rocks. We interpret geological data from cyclic unit 10 of the Rhum Layered Intrusion in the light of these two experimental studies.