Rsj Sparks

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.

Explosive volcanism on Santorini, Greece

Santorini volcanic field has had 12 major (1–10 km 3 or more of magma), and numerous minor, explosive eruptions over the last ~ 200 ka. Deposits from these eruptions (Thera Pyroclastic Formation) are well exposed in caldera-wall successions up to 200 m thick. Each of the major eruptions began with a pumice-fall phase, and most culminated with emplacement of pyroclastic flows. Pyroclastic flows of at least six eruptions deposited proximal lag deposits exposed widely in the caldera wall. The lag deposits include coarse-grained lithic breccias (andesitic to rhyodacitic eruptions) and spatter agglomerates (andesitic eruptions only). Facies associations between lithic breccia, spatter agglomerate, and ignimbrite from the same eruption can be very complex. For some eruptions, lag deposits provide the only evidence for pyroclastic flows, because most of the ignimbrite is buried on the lower flanks of Santorini or under the sea. At least eight eruptions tapped compositionally heterogeneous magma chambers, producing deposits with a range of zoning patterns and compositional gaps. Three eruptions display a silicic–silicic + mafic–silicic zoning not previously reported. Four eruptions vented large volumes of dacitic or rhyodacitic pumice, and may account for 90% or more of all silicic magma discharged from Santorini. The Thera Pyroclastic Formation and coeval lavas record two major mafic-to-silicic cycles of Santorini volcanism. Each cycle commenced with explosive eruptions of andesite or dacite, accompanied by construction of composite shields and stratocones, and culminated in a pair of major dacitic or rhyodacitic eruptions. Sequences of scoria and ash deposits occur between most of the twelve major members and record repeated stratocone or shield construction following a large explosive eruption. Volcanism at Santorini has focussed on a deep NE–SW basement fracture, which has acted as a pathway for magma ascent. At least four major explosive eruptions began at a vent complex on this fracture. Composite volcanoes constructed north of the fracture were dissected by at least three caldera-collapse events associated with the pyroclastic eruptions. Southern Santorini consists of pryoclastic ejecta draped over a pre-volcanic island and a ridge of early- to mid-Pleistocene volcanics. The southern half of the present-day caldera basin is a long-lived, essentially non-volcanic, depression, defined by topographic highs to the south and east, but deepened by subsidence associated with the main northern caldera complex, and is probably not a separate caldera.

Petrology and geochemistry of volcanic rocks of the Cerro Galan Caldera, Northwest Argentina

At least 2000 km 3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km 3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO 2 ) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87 Sr/ 86 Sr = 0.7108–0.7181; 143 Nd/ 144 Nd = 0.51215–0.51225, and δ 18 O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87 Sr/ 86 Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87 Sr/ 86 Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges ( 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga). Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO 2 = 51–57%; K 2 O = 2–3% and normative plagioclase compositions of An 37–48 , and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87 Sr/ 86 Sr ranges from 0.7055 to 0.7094 and 143 Nd/ 144 Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO 2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87 Sr/ 86 Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites. The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

The role of compositional convection in the formation of adcumulate rocks

The origins of cumulate rocks are re-examined in the light of new concepts in the fluid dynamics of crystallising systems. We propose that during the formation of adcumulate rocks, compositional convection allows continuous exchange of melt in the pores of a cumulus pile with the main magma reservoir. We consider this explanation to be physically more plausible than that proposed in the conventional cumulus theory. We present an experimental study of crystallisation in a porous medium to illustrate the process, and a theoretical analysis to demonstrate its applicability to magma chambers. We also consider factors which may inhibit convection and thus lead to the formation of meso- and orthocumulates. Finally we suggest how these ideas may be used to understand the distribution of these different rock types in layered intrusions.

Origin of modal and rhythmic igneous layering by sedimentation in a convecting magma chamber

EXPERIMENTAL investigations of convecting, particle-laden fluids show two regimes for convection driven by cooling from above1. In very dilute suspensions, convection will maintain a homogeneous distribution of particles throughout the convecting layer provided that particle fall velocities are small compared with turbulent fluid velocities. Above a critical concentration, convection is unable to keep the particles suspended, so the particles settle, leaving behind a layer of convecting fluid virtually free of particles. Here we apply these results to cooling magma chambers, in which crystallization leads to an increase in suspended crystal content with time. Discrete sedimentation events are predicted each time the concentration exceeds the critical value. For common igneous minerals, critical concentrations are very small (typically 0.002–0.03 wt%) and layers of the order of centimetres to a few metres thick will result. Because minerals of different density and size have different critical concentrations and settling velocities, complex fluctuations in sedimentation rate and mineral proportions can occur in a multi-component melt. This may lead to either regular repetitive cycles or more complex fluctuations. The process is confined to low-viscosity magmas, such as basalts, in which the crystals are able to separate from the active thermal boundary layer during convection.

Magma–cumulate mixing identified by U–Th disequilibrium dating

Magma mixing is believed to be important in magmagenesis1, particularly in volcanic arcs2, where its influence is revealed by heterogeneous phenocryst populations3 and quenched inclusions4. Nevertheless isotope studies, particularly of uranium-series dis-equilibria, have concentrated on the analysis of bulk rock samples5–9; comprehensive U–Th mineral-separate data from arc volcanics are scarce10–14. Here we present the first U–Th isotopic evidence for mixing between a crystal mush and a magma in an andesite from the island of Santorini, in the Aegean arc. The lava contains crystal populations from two sources of distinct thorium isotope composition: one from a basic cumulate; the other phenocrysts from a dacite magma. The crystal populations have statistically indistinguishable crystallization ages of 79+14−12kyr and 93+29−22kyr, and are thought to have physically intermingled just before eruption. The crystals did not re-equilibrate and hence retain the isotopic compositions and ages of the parental materials.

Future research directions on the physics of explosive volcanic eruptions

Scientific research can take unexpected, even counter-intuitive, directions because of technical innovation, the occasional brilliant idea that overturns conventional wisdom and new observations that provide previously unexpected insights into the way in which nature works. For these reasons no one is certain what the future holds in terms of breakthroughs. This chapter highlights some of the most recent developments in research on the physics of explosive volcanism. It pin-points cardinal areas of study poised for new research and anticipates major future developments. Advances in remote sensing and computational power are two examples of technical developments which are currently having dramatic impacts on understanding the physics of explosive volcanism. Such technical innovations, together with many good ideas and observations, are changing perceptions of the mechanisms of explosive volcanism. With an increasingly populated and ecologically stressed world, the potential effects of explosive volcanism are being exacerbated. Several megacities, e.g. Tokyo, Naples and Mexico City, now exist close to active volcanoes, and in many parts of the world economic development and population expansion have combined such that the risk of major volcanic disasters increases year by year. Volcanic activity has both local and global environmental effects. For example, fallout of volcanic ash from eruption plumes can disrupt air, sea, road and rail traffic, inhibit electrical communications, cause respiratory problems for people and animals, pollute water, damage crops, cause failure of building roofs and generally bring havoc to local communities. On a larger scale, volcanic aerosols from some events, such as the 1991 Pinatubo