13.2 Magma chamber evolution

Just as initial Sr isotope compositions are useful as a geochemical tracer when using the Rb)Sr method, initial Th isotope compositions are also a useful product of the U)Th dating method. One area where they have proved particularly valuable is in studying magma chamber evolution.

In the same way that Th isotope evolution in a volcanic rock can be used to date crystallisation, Th isotope evolution between successive eruptions can be used to date the residence of magma in a chamber. We can use the same equation as for the rock system, except that what we input as the ‘final composition’ on the left hand side is actually the initial Th activity ratio of a magma at the time of eruption (E), while the Th activity ratio on the right hand side of the equations is the composition of the magma in the chamber at the time of influx (I), simplistically, from the mantle. The quantity t then represents the residence time of the magma batch in the chamber (all nuclide ratios in activities):

(²³⁰Th) (²³⁰Th) ²³⁸U

())))) = ())))) e^{!8230 t} + )))) (1 ! e^{!8230 t}) [13.5]

(²³²Th)_E (²³²Th)_I ²³²Th

It is best to first rearrange equation [13.5] to gather the exponent terms in one place:

(²³⁰Th) | (²³⁰Th) ²³⁸U | ²³⁸U

())))) = | ())))) ! ))) | e^{!8230 t} + ))) [13.6]

(²³²Th)_E | (²³²Th)_I ²³²Th | ²³²Th

Allegre and Condomines (1976) preferred to refer all times to the present, introducing T as the age of influx into the chamber and t as the time of eruption. Then:

(²³⁰Th) | (²³⁰Th) ²³⁸U | ²³⁸U

())))) = | ())))) ! ))) | e^{8230 (t ! T)} + ))) [13.7]

(²³²Th)_t | (²³²Th)_T ²³²Th | ²³²Th

Just as we make a closed-system assumption in the case of rock dating, so we must assume that the magma in the chamber remains a closed system to U and Th during its evolution and eruption. This assumption should not be upset by Rayleigh crystal fractionation, since the distribution coefficients of U and Th are so low that both elements normally remain entirely in the liquid.

13.2.1 The Th isotope evolution diagram

The ²³⁰Th evolution of magmas can be shown on U)Th isochron diagrams, but it is also convenient to display isotopic evolution on a plot of Th activity ratio against time (Fig. 13.8). This plot is analogous to the evolution diagrams of Sr or Nd isotope composition against time (e.g. section 4.2), but because the half-life of ²³⁰Th is short relative to the time periods under study, the x axis must be calibrated in log time.

Fig. 13.8. Schematic diagram of Th isotope evolution against time for two closed-system magma chambers: a) displaying daughter deficiency; and b) daughter excess. Insets show U/Th fractionation and subsequent evolution on U)Th isochron diagrams. After Condomines et al. (1982).

A magma body in secular equilibrium must maintain its ²³⁰Th/²³²Th activity ratio. Hence, such evolution is described by a horizontal line in Fig. 13.8. If the system undergoes U/Th fractionation (horizontal displacement from M₀ to M or M’ on the inset diagrams) then the ²³⁰Th/²³²Th activity ratio of the magma will evolve over time to regain secular equilibrium. A magma enriched in ²³⁸U/²³⁰Th (to the right of the equiline on the inset, with activity M) defines a line of negative slope on the main diagram. Similarly, a magma depleted in ²³⁸U/²³⁰Th (activity ratio M’ on the inset) defines an evolution line of positive slope on the main diagram. If we extrapolate the growth lines to e^8t = 0, then we can see from equation [13.6] that the y ordinate in the main diagram describes the ²³⁸U/²³²Th activity ratio of the evolving magma.

The U)Th isotope system is a very powerful tool for studying magma chamber evolution because the 75.4 kyr half-life of ²³⁰Th is similar in size to the time interval between magma chamber events. However, a significant data base is needed to unravel the history of most volcanoes, which usually involve repeated magma injection and eruption events. A simple scenario of this type is illustrated in Fig. 13.9. In this case a primary mantle source in secular equilibrium supplies a series of magma batches (over a period of time) which have a constant disequilibrium ²³⁸U/²³²Th activity ratio generated by the partial melting process (see below). After a period of magma evolution in a high-level chamber (sloping line), the chamber is emptied by eruption and re-filled, causing a kick back to the starting composition. However, in the real world, mixing of magmas of different ages is likely to occur, and this will generate a more complex pattern of magma evolution.

Fig. 13.9. Schematic illustration of the thorium isotope evolution of a periodically tapped and re-filled magma chamber with a constant magmatic U/Th ratio (defined by the intercept on the y axis). P= present day Th activity ratio. After Condomines et al. (1982).

Early studies (Allegre and Condomines, 1976; Condomines and Allegre, 1980) lacked sufficient data to resolve the magmatic history of long-lived volcanoes; hence, their results were ambiguous. However, a later study by Condomines et al. (1982) provided enough data to interpret the history of the Etna volcano in Sicily. Thirteen mineral ) whole-rock U)Th isochrons were determined, together with analyses of recent lavas. The results were plotted on a Th isotope evolution diagram (Fig. 13.10), and an isochron diagram (Fig. 13.11).

On the Th isotope evolution diagram (Fig. 13.10) the Etna data provide evidence for four episodes of eruption and magma replenishment, the last three of which (numbered) tie in with known dates of major caldera collapse events. Between these events, small magma tappings monitor Th isotope evolution in the high-level chamber. However, ²³⁰Th/²³²Th activites fall too rapidly during these periods to be explained by closed-system evolution, given the observed range of U/Th activity ratios (hatched band on the left-hand axis). Therefore, Condomines et al. invoked a magma mixing model to explain these steep trends, suggesting that the sub-horizontal evolution line represented a deep, long-lived alkali basalt reservoir which continually supplied magma to higher levels, where a low- ²³⁰Th/²³²Th tholeiitic component was added intermittently.

Fig. 13.10. Thorium isotope evolution diagram showing the history of Etna volcano. Three magma influx ) mixing events correspond to known episodes of caldera formation, possibly preceded by an earlier fourth event. Symbols indicate erupted products of different ages. After Condomines et al. (1982).

Initial ratios of lavas at the time of eruption are plotted on a U)Th isochron diagram in Fig. 13.11. The data suggest mixing between an old magma nearly in secular equilibrium and a young one substantially out of equilibrium. Condomines et al. argued that the low ²³⁰Th/²³²Th (young) component could not be a crustal contaminant, since this should be close to secular equilibrium, whereas Fig. 13.10 indicates it to be far from equilibrium. However, this does not exclude the possibility of sediment contamination of the mantle source of these magmas. The straight line (1) represents an instantaneous mixing model whereas the curved lines (2 and 3) model progressive mixing over a time interval. Present-day ratios (of old lavas) are not plotted on Fig. 13.11 because they do not yield any useful information about magma evolution.

Fig. 13.11. U)Th isochron diagram showing activity data for Etna lavas at the time of eruption. The data array implies mixing between an old U-rich magma and a young U-depleted magma. Symbols as in Fig. 13.10. After Condomines et al. (1982).

13.2.2 Short-lived species in magma evolution

The first detailed application of very short-lived species to magma evolution was made by Capaldi et al. (1976) on the volcanoes of Etna and Stromboli. However, one of the most interesting applications of these species is in studying the genesis of carbonatite magmas, as exemplified in Oldoinyo Lengai volcano, in the East African Rift of Tanzania. The 1960 and 1988 carbonatite eruptions from this volcano were studied, respectively, by Williams et al. (1986) and Pyle et al. (1991). Both of these eruptions exhibited strong disequilibrium between ²²⁸Ra (t_1/2 = 5.77 yr) and its ultimate parent, ²³²Th. The 1988 eruption also demonstrated ²²⁸Th disequilibrium (t_1/2 = 1.91 yr); however, it was not possible to test for this phenomenon in the 1960 eruption, since the samples had reached secular equilibrium in the twenty years between sampling and analysis! The so-called ‘1963’ eruption of Williams et al. (1986) is also excluded from this discussion because of uncertainty about its eruption age (Williams et al., 1988).

It is now generally agreed that carbonatites are formed by the evolution of per-alkaline magmas under conditions of strong CO₂ enrichment, probably involving the segregation of immiscible droplets of carbonate magma from a silicate magma host (e.g. Pyle et al., 1991). The discovery of ²²⁸Ra disequilibrium in the Oldoinyo carbonatites suggests that the segregation process probably occurred shortly before eruption. Over this timescale, ²²⁶Ra (t_1/2 = 1620 yr) can be treated effectively as a stable isotope. Therefore the ²²⁸Ra decay equation can be divided by ²²⁶Ra to yield an isochron relation analogous to equation [13.2] (Capaldi et al., 1976):

(²²⁸Ra) (²²⁸Ra) ²³²Th

())))) = ())))) e^{!8228 t} + )))) (1 ! e^{!8228 t}) [13.8]

(²²⁶Ra)_P (²²⁶Ra)_I ²²⁶Ra

The effects of a hypothetical Ra/Th fractionation event are shown in Fig. 13.12. A reservoir previously in secular equilibrium (e.g. a long-lived magma body) becomes enriched in radium relative to thorium, due to some form of differentiation event. After this enrichment event, excess ²²⁶Ra and ²²⁸Ra activities decay away at different rates. ²²⁸Th is initially in equilibrium with its ultimate parent (²³²Th), but subsequently builds to a peak and then decays as it approaches equilibrium with its immediate parent (²²⁸Ra).

Fig. 13.12. Schematic illustration of the effects of a Ra/Th fractionation event on the activity ratios of a system previously in secular equilibrium. After Capaldi et al. (1976).

Williams et al. (1986) preferred to use the alternative isochron diagram on which ages are defined by the intercept on the left hand axis, using the following equation, which is analogous to [13.4]:

(²²⁸Ra)

())))) = 1 ! e^{!8228 t} [13.9]

(²³²Th)_P

This activity ratio is plotted in Fig. 13.13a against ²²⁶Ra/²³²Th activity. The zero-age line passes through the origin and represents a Ra/Th fractionation line. The slope of this line is the ²²⁸Ra/²²⁶Ra activity ratio, which is equal in turn to the ²³²Th/²³⁸U activity ratio, since ²²⁶Ra and ²²⁸Ra are in separate decay chains. For Oldoinyo Lengai, this ratio was found to be unity. Radioactive decay of a system located on the fractionation line causes it to move vertically downwards towards the equiline and permits an age to be assigned.

The initial activity of the 1960 carbonatite is plotted on Fig. 13.13a. If this magma is attributed to a single instantaneous event which caused Ra/Th fractionation, it follows that this event occurred seven years before eruption. However, more complex models are also possible. For example, if segregation occurred in two events, ²²⁸Ra formed in the first event might decay before the second event (Fig. 13.13a). In this case, the time from the second event to eruption is less than seven years. Alternatively, carbonatite segregation might have occurred over a period of time. In Fig. 13.13b, this process is modelled by drawing Ra/Th growth curves for different rates of Ra enrichment in the carbonatite magma (relative to the conjugate silicate liquid). These growth curves are then calibrated by determining the time necessary for enrichment of the effectively stable ²²⁶Ra isotope. A simple model of constant enrichment rate yields a calculated duration of this process for the 1960 magma of ca. 18 years, if this was occurring immediately prior to eruption (Fig. 13.13b).

Fig. 13.13. Ra)Ra)Th isochron diagrams showing alternative models to explain the 1960 carbonatite of Oldoinyo Lengai. a) involving one or two discrete Ra/Th fractionation events. b) involving a continuous radium enrichment process of specified intensity and duration. After Williams et al. (1986).

The addition of ²²⁸Th data can potentially allow selection between short-term differentiation models such as those outlined above, because of its dependence on very recent events. These data were collected by Pyle et al. (1991) for the 1988 carbonatite of Oldoinyo Lengai, but due to analytical difficulties, Ra/Th activity ratios could not be measured. Therefore, the advantage of the combined systems was lost.

Capaldi et al. (1976) attempted to use the ²²⁸Ra and ²²⁸Th systems to study the recent eruptive history of Mount Etna. However, Condomines (1995) showed that these systems were in secular equilibrium in all Etna lavas. They also found that ²¹⁰Pb was in secular equilibrium with its parent (²²⁶Ra) in all Etna lavas except the 1991 and 1992 flows. However, Gauthier and Condomines (1999) found significant ²¹⁰Pb disequilibrium in lavas from Merapi volcano in Java, Indonesia. Between 1984 and 1992, the initial ²¹⁰Pb/²²⁶Ra activity ratio of erupted products decreased from unity (secular equilibrium) to a value of 0.75 (Fig. 13.14). This decrease was attributed to degassing of radon, which is the immediate daughter of ²²⁶Ra, and is the only gaseous species in the ²²⁶Ra) ²¹⁰Pb decay chain. This species was therefore able to escape from a shallow magma chamber, under otherwise closed-system conditions. A major eruption in 1992 marked the end of this period of closed-system evolution, as new magma was added to the chamber and gradually mixed with its contents, bringing them progressively back to secular equilibrium.

Fig. 13.14. Evolution of ²¹⁰Pb/²²⁶Ra activity ratios as a function of time in Merapi volcano, Java. After Gauthier and Condomines (1999).

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