G. C. Brown

The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources

Geochemical variations in modern subduction-related igneous suites with respect to arc ‘maturity’ in time and space are illustrated using data for both volcanic suites (basalt, andesite and dacite) and intrusive granitoid suites (diorite, tonalite/granodiorite and granite) from circum-Pacific arcs. Using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites: (a) subduction-zone enrichment of lithospheric mantle, locally coupled with crustal assimilation allied with fractional crystallization (AFC) in zones of thickened crust, all of which yield magmas with enhanced concentrations of the large-ion lithophile (LIL) elements K, Rb, Th, U, LREEs, etc; (b) with increasing distance from the active trench, contributions from within-plate sub-continental lithosphere producing mantle-derived magmas with enhanced levels of high-field strength (HFS) elements, among which Nb, Ta, Hf and Y are particularly distinctive. Thus, even for the evolved granitoids of intrusive arc series, ratios of HFS/LIL elements not significantly affected by crystal fractionation, such as (Ta, Nb)/(K, Rb, La), may throw some light on the origin of mafic-intermediate precursor magmas. In terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites. 1. Primitive, calcic arc granitoids with low LIL and HFS element abundances. 2. Normal, calc-alkaline continental arc granitoids with enhanced LIL element abundances and low HFS/LIL ratios. 3. Mature alkali-calcic arc granitoids with high levels of LIL and HFS elements and higher HFS/LIL ratios. 4. Back-arc/anorogenic alkaline granitoids with the highest levels of HFS elements.

Gravity fields and the interpretation of volcanic structures: Geological discrimination and temporal evolution

Abstract A review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes. To a first approximation, all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice. Positive anomalies with wavelengths less than 20 km and amplitudes up to ca. 30 mGal, characterise mainly basaltic volcanoes from various tectonic settings, and are caused by a relatively dense intrusive complex/magma body which contrasts with its surroundings either because the body is more mafic than average or, more likely, because near surface, previously erupted materials are uncompacted. Negative anomalies with wavelengths up to 100 km and amplitudes up to ca. 60 mGal, occur over much larger volcanic calderas, many of which have erupted highly silicic pyroclastic ash and pumice; uncompacted silicic caldera infill, with a possible contribution from low-density magma bodies, is responsible for the observed anomalies. There is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction-related basaltic andesite to mature, high-level silicic calderas. Repeated microgravity observations over active volcanoes enable magma movements, variations in magma input and changes in magma density to be monitored closely. Dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events.

Gravity−height correlations for unrest at calderas

Abstract Calderas represent the sites of the worlds most serious volcanic hazards. Although eruptions are not frequent at such structures on the scale of human lifetimes, there are nevertheless often physical changes at calderas that are measurable over periods of years or decades. Such calderas are said to be in a state of unrest, and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors. Here we review combined gravity and elevation data from several restless calderas, and present new data on their characteristic signatures during periods of inflation and deflation. We find that unless the Bouguer gravity anomaly at a caldera is extremely small, the free-air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient, and not the theoretical gradient, use of which may introduce significant errors. In general, there are two models that fit most of the available data. The first involves a Mogi-type point source, and the second is a Bouguer-type infinite horizontal plane source. The density of the deforming material (usually a magma chamber) is calculated from the gravity and ground deformation data, and the best fitting model is, to a first approximation, the one producing the most realistic density. No realistic density is obtained where there are real density changes , or where the data do not fit the point source or slab model. We find that a point source model fits most of the available data, and that most data are for periods of caldera inflation. The limited examples of deflation from large silicic calderas indicate that the amount of mass loss, or magma drainage, is usually much less than the mass gain during the preceding magma intrusion. In contrast, deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms.

Heat flow, heat production and thermo-tectonic setting in mainland UK

New heat flow data for the United Kingdom, together with additional heat flow and heat production determinations for Caledonian-age granites, have led to a revision of the UK heat flow map and a re-examination of the relationship between heat flow (q0) and heat production (A0) for granites and basement rocks. Previously recognized broad belts of above-average heat flow are now resolved into separate zones which reflect, to a greater extent, the geological structure and tectonic history of the UK. The zones of highest heat flow are spatially associated with voluminous, high heat production granitoid batholiths in SW England, northern England and the Eastern Highlands of Scotland. A single linear correlation between q0 and A0 is no longer tenable and an analysis in terms of broad heat flow provinces, each with a characteristic upper-crustal heat production distribution and deep heat flow contribution, is also considered to be an oversimplification. On the q0–A0 plot, the data form four separate clusters; three corresponding to the granite batholiths in SW England, northern England and the Eastern Highlands of Scotland, and the fourth to the basement rocks of central England and Wales. An explanation of the q0–A0 data is proposed in terms of the crustal structure and thermo-tectonic setting of each area. In the case of the granite batholiths the data reflect the contrasting depth extent and radioelement-depth functions of the intrusions. These parameters in turn are related to the magmatic evolution and emplacement history of each batholith and the nature of the crust into which they were emplaced.

Volcano monitoring by microgravity and energy budget analysis

Microgravity monitoring of active volcanoes can provide evidence of sub-surface mass and/or density changes that precede eruptions. If, in addition, precursory increases in thermal emissions are observed, an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions. Crater-lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calotimetry. Here we summarize 10 years of gravity and thermal data from Poás volcano, Costa Rica. Mass/energy balance calculations demonstrate that, in the steady-state, the large thermal inertia of the crater lake acts as a buffer to short-term changes in the energy input from the cooling magma feeder pipe. Since February 1986, it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface. This follows from unambiguous, gravity increases, constrained by elevation control, that are coincident in time with a period of long-term increased energy input to the crater lake. Progressive reduction of the lake volume by evaporation/seepage culminated in an (April 1989) ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity.

Causes of microgravity change at Poás volcano, Costa Rica: an active but non-erupting system

Microgravity changes with time, not always consistent with the Bouguer-corrected free air gradient, have been recorded and associated with cyles of eruptive activity at Krafla, Kilauea, Pacaya and Etna volcanoes. In contrast, over the non-erupting yet active fumarolic vents at Poás (Costa Rica), real-time gravity observations over three periods during the years 1983–1985 have identified ca. 140µGal amplitude, cyclic gravity variations. Their decrease in amplitude with distance from the active crater, coupled with a ‘static’ sub-surface structural model, have allowed the effects of a variety of possible causative dynamic phenomena to be evaluated. It is concluded that cyclic changes in the average density (by ca. 0.03 Mg m−3) in the magma pipe at depths below 500 m, rather than variation in magma chamber or water table geometry, are responsible for the observed gravity variations. Specifically, anaverage of 1 % fluctuation in the volume of gas in crystal-free magma, a process driven by thermal convection cycles, probably accounts for the density/gravity change.

New light on caldera evolution—Askja, Iceland

The large multiple-caldera volcanic system of Askja, central Iceland, is composed principally of subglacial basaltic hyaloclastite-pillow-lava formations and postglacial basaltic scoria and flows. Traditionally, such calderas are believed to be formed by downfaulting and ring-fracture collapse. Whereas this certainly applies to the smaller A.D. 1875 caldera, the older main caldera may have developed positive relief during subglacial construction of laterally confined hyaloclastite ridges above erupting fractures. This is supported by the evidence of a large negative gravity anomaly that reaches minima over the marginal low-density ridges but which is less negative within the caldera, where relatively dense postglacial lavas are believed to cover a more limited hyaloclastite succession beneath the caldera floor.

Space-time variations in British Caledonian Granites: some geophysical correlations

Abstract The characteristics of 30 “granites” from the postulated Caledonian (Iapetus Ocean) suture zone of mainland Britain are discussed. Geophysical (gravity, aeromagnetic), geochemical (U/Pb and initial 87 Sr/ 86 Sr) and isotopic age data indicate that these British Caledonian intrusions (390–600 Ma) can be divided into two distinct groups temporally, each of which is further subdivided spatially. The temporal division applies throughout the British province and separates a pre-Silurian (group 1) suite of low-volume, low-mobility magmas, which were intruded under compressive conditions, from a Siluro-Devonian (group 2) suite of large volume, mobile magmas intruded under tensional conditions. The spatial subdivisions of groups 1 and 2 are made between intrusions emplaced to either side of the postulated ENE-WSW Iapetus Ocean suture which runs through the Solway Firth. First, the group 1 granites northwest of the suture probably were produced by partial fusion involving Proterozoic continental crust (group 1N) whereas those to the southeast have isotopic characteristics simply indicating a mantle or ocean crustal source (group 1S). Second, the Siluro-Devonian granites, which were all derived largely at the expense of Caledonian mantle, have different aeromagnetic expressions depending on their position in the northwest (2N) or southeast (2S) Caledonides. These aeromagnetic characteristics are probably related to the differences in basement structure recently identified by seismic surveys and they provide further evidence for the former existence and Siluro-Devonian closure of Iapetus. Another significant implication may be that Proterozoic basement is lacking from beneath the region immediately adjacent to and southeast of the suture — southern Scotland and most of England.

Dynamics of a geyser eruption

Geyser action is characterised by the eruption of a mixture of steam, water and, in some cases, mud to substantial heights above surface level. The role of vaporisation in the ejection process is investigated with the aid of a mathematical model which treats geyser eruptions as isolated, steady state, one-dimensional phenomena; they are treated from an Eulerian point of view. It is found that large scale boiling of water occurs close to the surface, even though the instabilities that initiate the cycle may occur much lower down. Somewhat different models characterise superficially similar geyser eruptions from narrow confined tubes and from pools such as crater lakes. In the latter case the model predicts a root zone for steam flash at a depth of a few metres below the water surface, compared to a depth of tens of metres below the exit plane when eruption is from a confining tube, for normal eruptions ten to one hundred metres high. Relationships are obtained between geyser height, water content, depth of vaporisation, and such parameters as temperature and pressure at the onset of vaporisation. These relationships are used to provide a simple model of an eruption of Old Faithful, which reflects its main characteristics. They are also used to discuss geysering in the crater lake of Poas volcano in Costa Rica. A triggering mechanism which depends on progressive compression of a volume of steam is considered briefly. The system ultimately becomes unstable and results in the ejection of a volume of fluid.

Magma chamber below Poàs volcano, Costa Rica

Direct field evidence for links between volcanic rocks and contemporaneous subvolcanic intrusions are difficult to establish. Such links might, however, be inferred by making physical measurements on a youthful active volcano. Poás is a small composite volcano (elevation 2700 m, diameter 20 km) which has evolved to its present form by a sequence of caldera- and crater-forming episodes, possibly during the last 5 × 104 y. There has been a high level of historic activity from the active crater, which is 1 km in diameter and 300 m deep, and which contains a hot lake. Lavas and dominant pyroclastic rocks exposed in the active crater calc-alkaline basaltic andesites and andesites. Poás volcano is therefore typical of those in island arcs and young continental margins. Gravity measurements show that the volcano has a regional negative Bouguer anomaly, of amplitudes 100–200 gu, upon which is superimposed a closed positive anomaly, in the active crater area, about 2 km in diameter and with a maximum amplitude of 100 gu. This is thought to indicate a 1 km radius cylinder of solid rock of basaltic andesite or andesite composition with a shallow upper surface and extending to a depth of several kilometers. The concept of a large shallow magma chamber, with a diameter similar to that of the volcano, appears not to be appropriate for this volcano.