Open University Uranium-Series Laboratory  
      Earth and Environmental Sciences, The Open University, Milton Keynes, UK

Magmatic Systems  
Volcanology  

Continental basaltic magmatism reveals a compositional diversity that is greater than in the oceanic realm. This relates to the interaction of mafic magmas with the more easily fusible parts of the continental crust, but much compositional diversity has been attributed to magma sources located in the continental mantle lithosphere. Basalts from numerous CFB provinces and extensional continental regimes reveal compositional trends that are consistent with a pattern of early lithospheric melts followed by plume-related or asthenospheric magmas once the lithosphere has thinned (Perry et al., 1987, Fitton et al., 1991, Turner et al., 1996). Heating of the lithosphere may result in so-called thermal erosion during which lithospheric mantle is incorporated into the underlying asthenosphere or mantle plume where it may contribute to plume-related magmatism (Davies, 1994). Thermal erosion melting implies a dynamic system in which melt is extracted by compaction of a convecting matrix, similar to the predominant process operating beneath mid-ocean ridges (e.g. Elliott, 1997, Asmerom et al., 2000). Within the Kenya rift, mafic melts of variable composition have been generated over the past 35 Ma and reflect the interaction between the underlying mantle plume with the overlying lithosphere (Macdonald et al., 2001).

 

New mass spectrometric analyses of trace element abundances and (230Th/238U) disequilibrium from recent (<10 ka) basalts from the Kenya rift have allowed us to confirm dynamic melting processes in the source regions of basalts with lithospheric origins (Rogers et al., 2004). Northern Kenya basalts define a vertical array on the equiline diagram, with all but one of the samples lying between (238U/232Th) of 0.655 – 0.735. This array is consistent with a dynamic melting regime (Elliott, 1997). It is suggested that basalts from the northern Kenya rift represent melts derived from lithospheric material that has been thermally reactivated by the underlying mantle plume and is now behaving as convecting mantle. The minimum upwelling rates beneath the rift axis are ~1 cm yr-1 but basalts with (230Th/238U) < 1.4 may be generated either where the upwelling rate is greater or from a source with < 6% modal garnet.

 

Aserom et al 2000 Nature 406 293–296
Davies 1994 J. Geophys. Res. 99 15709 – 15722
Elliott 1997 Chem. Geol. 139 165–183
Fitton et al. 1991 J. Geophys. Res. 96 13693 – 13711
Macdonald et al. 2001 J. Petrol. 42 877–900
Perry et al. 1987 J. Geophys. Res. 92 9193–9214
Rogers et al. 2004 Journal of Petrology 45 1747-1776
Rogers et al. 2005 Chemical Geology submitted
Turner et al. 1996 J. Geophys. Res. 101 11503–11518

 
   
Magma fractionation  

A major challenge in igneous petrology is to determine the rates at which magma evolves, the factors that contribute to controlling those rates and how they relate to the thermal budgets of volcanic systems. An understanding of the evolution of magmatic systems and of the dynamics of the deeper regions of volcanoes is critical to the development of models for accurate forecasting of volcanic hazards (Sparks, 2003).

 
One method for determining rates of magma evolution involves the use of U-series disequilibrium. Element fractionation processes within sub-volcanic magma chambers prior to eruption operate on timescales similar to the half-lives U-series isotopes. There have been numerous studies of U–Th disequilibrium in volcanic rocks, both on whole-rock samples and mineral separates, and these have been extensively reviewed by Hawkesworth et al., (2000) and, more recently, by Condomines et al., (2003). Rates of fractionation relate to the rate of change of melt composition, (see Figure). We measured U-series disequilibria from a series of mafic to trachytic lavas from Longonot volcano in the Kenya rift. The lavas are related by fractional crystallization and the analyses reveal subtle variations in composition that are related to the separate and rapid evolution of distinct magma batches and their storage for significant periods of time prior to eruption. The evolution of trachytic magmas at Longonot is dominated by fractional crystallization, with a secondary contribution from magma mixing.
 

 

 
Variation of 226Ra/230Th) with Th (expressed as the initial Th content, Th0, divided by sample Th content) in lavas from Vestmanneayjar. Curve labelled Fractional crystallisation assumes no ageing during fractionation. The curve through the four data points shows how the model suggests a fractionation rate of 4.5 x 10-4 yr-1, Rogers et al., 2004.

Despite the high silica contents of the Longonot trachytes, fractionation rates (0.5–2×10-4/year) are comparable with those from the basaltic system at Ardoukoba, (3.3–3.6×10-4/ year, Vigier et al., 1999), but the rates may be >2×10-4/year within the trachytes themselves. A possibly slower rate of fractionation is inferred for the differentiation of hawaiite to trachyte and may reflect a two-stage process in the evolution of the Longonot lavas. Initially, hawaiitic magma differentiates in a large magma reservoir at mid-crustal levels to produce a parental trachyte magma, which is then introduced into the magma reservoir in the immediate sub-volcanic environment, in which the parental trachyte further differentiates into the range of compositions erupted as lavas at the surface.
(230Th/238U) disequilibria indicate that limited U/Th fractionation occurred during the past 10 kyr, whereas (226Ra/230Th) disequilibria reflect the effect of alkali feldspar fractionation. (226Ra/230Th) disequilibria in the trachyte lavas are interpreted using a model that combines the equations of radioactive decay and in-growth with Rayleigh crystallization to give fractionation rates of about 0.2×10-4/year for the evolution of hawaiite to trachyte, but more rapid rates of up to 3×10-4/year for fractionation within the trachyte sequence. (226Ra/230Th) from two whole-rock–alkali feldspar pairs are interpreted to show the crystals formed at 5800 y and 2800 y , implying that phenocryst formation continued almost up to the time of eruption. The results strongly indicate that fractionated magmas can be stored for periods on the order of 1000–2500 years prior to eruption, whereas other magmas were erupted as fractionation was proceeding.

 
 

Condomines et al. 2003 Reviews in Mineralogy and Geochemistry 52 125–174
Hawkesworth et al. 2000 Journal of Petrology 41 991–1006
Rogers et al. 2004 Journal of Petrology 45 1747-1776
Sparks 2003 Earth and Planetary Science Letters 210 1–15
Vigier et al. 1999 Earth and Planetary Science Letters 174 81–97
 
   
Constraining partial melting parameters  

Models of partial melting have been revolutionised by the observation that many ocean ridge, ocean island and island-arc lavas exhibit significant (230Th/238U) and (226Ra/230Th) disequilibria, and the realisation that such disequilibria are often too great to be attributed to the small degrees of partial melting (McKenzie, 1985; Gill et al., 1992). Dynamic melting models (Gill et al., 1992; Cohen and O'Nions, 1993; Bourdon et al., 1996; Thomas et al., 2000; Kokfelt et al., 2000) based on continuous interaction between melt and matrix as the melt migrates up towards the surface (Spiegelman and Elliott, 1993) seem more appropriate. Recently, Sims et al., (2002). have concluded, based on a highly detailed study of very recent lavas from the East Pacific Rise, that the ‘reactive porous flow’ model (Spiegelman and Elliott, 1993) can best explain (230Th/238U) and (226Ra/230Th) disequilibria and other radiogenic isotope data.

 

Bourdon et al., 1996, Nature, 384, 231-235.
Cohen and O'Nions, 1993, Earth and Planetary Science Letters, 120, 169-175.
Gill et al., 1992, in: Uranium-series Disequilibrium, Oxford Sci. Publ., pp. 207-258.
Kokfelt. et al. 2000, State of the Arc, abstract.
McKenzie, 1985, Earth and Planetary Science Letters 74, 81-91.
Sims et al., 2002, Geochim. Cosmochim. Acta, 66,3481-3504.
Spiegelman and Elliott, 1993, Earth and Planetary Science Letters, 118, 1-20.
Thomas et al., 2000, Geochim. Cosmochim. Acta, 63, 4081-4099.

 
Constraining fractional crystallisation
 

Recent and historic lavas from Tenerife, Canaries, show that magma differentiation has happened in two stages ( Hawkesworth et al, 2001 Thomas et al. under review). Stage one occurred at a depth of greater than 20 km , where 50% fractional crystallisation produced phonolite from primitive basanite taking in the order of 200 ky in a closed system. The second stage occurred at shallow levels at sea level and phonolite was differentiated and mixed with crustal phonolite to form a more evolved amphibole bearing phonolite by 50% fractional crystallisation. This second stage occurred some few hundred years prior to eruption.
Along destructive plate margins the rates of fluid-induced melt generation may be significantly faster than that beneath ocean ridges and ocean islands (Turner et al., 1996,1997), and it has been suggested that that may in turn be responsible for some of the distinctive trace element features of island arc rocks. Hobden et al., (2002), present geochemical data on the frequently active composite cone of Ngauruhoe, in New Zealand based on the same suite of rocks. Preliminary conclusions are that specific batches of melt evolve within a number of chambers in the system, and are tapped periodically. Each batch provides a different composition with glass matrix and feldspar separate yielding age constraints on the evolution within the chamber.
A number of studies have highlighted the different information available from U-Th-Ra analyses of separated crystals and the groundmass, or of bulk rocks, in studies of the time scales of crystallisation and of magma differentiation (e.g. Hughes and Hawkesworth, 2000). The presence of crystals with ages significantly older than the age of eruption arguably provides maximum ages for the magmatic system (Halliday et al., 1989; Condomines et al., 1995). However, in many cases the separated crystals are not in bulk equilibrium with the host rock, and so the links between the ages of crystals and the time of magma differentiation are difficult to establish (Hawkesworth et al., 2000). Nevertheless, the observation that separated minerals and groundmass from five samples from Soufrière, St. Vincent, scatter around a 50 ky isochron, has highlighted the need to develop better constrained models for magma evolution over longer time scales (Heath et al., 1998; Hawkesworth et al., 2000). Recent and historic Montserrat volcanics yielded data for glass, phenocryst and whole rocks close to secular equilibrium in U-Th- Ra systematics. Such results suggest that initial U excesses due to the slab fluids contribution in the subduction zone have decayed. The fluid signature is less that >350kyr old and this is consistent with the highly differentiated nature of these rocks, which have undergone long term magma storage and evolution in the crust (Zellmer et al 2002).

 

Halliday et al., 1989, Earth and Planetary Science Letters, 94, 274-290.
Hawkesworth et al., 2000, State of the Arc, abstract.
Hawkesworth et al., 2001, EOS Transactions 82, 261-265.
Heath et al., 1998, Earth and Planetary Science Letters, 160, 49-63.
Hobden et al., 2002, Bull Volcanol 64, 392-409.
Hughes and Hawkesworth, 2000, State of the Arc, abstract.
Thomas et al., 2006. Earth Planet. Sci. Lett.( under review).
Turner et al., 1996, Earth and Planetary Science Letters, 142, 191-208.
Turner et al., 1997, Chem. Geol. Spec. Vol., 139, 145-164.
Zellmer et al., 2000, Earth and Planetary Science Letters, 174, 265-28.

 
   
Young zircons and super volcano eruptions
 
Large rhyolitic volcanic systems are the surface expressions of shallow level processes that are responsible for some of the largest and most violent volcanic eruptions. Rhyolitic volcanic fields also provide prodigious amounts of geothermal energy and valuable mineral deposits and it is therefore a major goal in Earth sciences to gain an understanding of the processes and dynamics of such systems. Super-volcano eruptions of rhyolitic deposits vary in volume from tens to thousands of cubic kilometers and both the processes and the time-scales over which these are active, have been extensively studied. It has been argued that a large magma chamber may underlie a volcano throughout long repose periods of > 100 ky (Halliday et al. (1989); Christensen and DePaolo (1993); Reid et al. (1997) or alternatively, that silicic magma is produced by rapid crustal anatexis in < 1000 y and transported to the surface with only the formation of a transient magma chamber e.g., (Sparks et al. (1990),
Taupo volcano, New Zealand is particularly well suited to a study of magma chamber longevity because it has an exceptionally well-preserved eruption stratigraphy (Wilson, (1993); Sutton et al., (1995)). The time-scales of rates of cooling and crystallisation can be constrained using 230Th/238U disequilibrium, particularly in zircon phenocrysts. The (230Th/232Th) - (238U/232Th) isochron diagram (Condomines et al., 1988) can be used to calculate the rate of crystallisation in a magma chamber to a precision that depends largely on the range in U/Th ratios in the separate mineral phases. During zircon crystallisation, U is preferentially sequestered over Th in the zircon crystal lattice (e.g., Fukuoka and Kigoshi (1974); Mahood (1990)). This results in a significant excess of 238U over its daughter product 230Th. The in-growth of 230Th in response to this U enrichment therefore provides a means of determining the time elapsed since crystallisation, and hence the residence times of the zircon phenocrysts (Condomines, 1997). Data for three different size fractions of zircon from the 26500 y Oruanui eruption are plotted on an equiline diagram Fig. 2. The reference lines represent 5 kyr increments older than the eruption age The first order observation is that the larger zircons took longer to grow than the smaller ones and this has led to a more sophisticated model of crystal-growth and simultaneous U-series decay (Charlier and Zellmer, 2000). Coupled with knowledge of the temperature dependence of zircon saturation in this magma type, zircon crystallisation ages offer an important insight into magmatic evolution. Zircon ages ranging from tens to thousands of years at the time of eruption indicate that, at least for Taupo volcano, there is strong evidence that magma chambers are rather short-lived. This has important implications for the heat-flow budget in this area and for volcanic hazard prediction models.
 
   
 
 
Oruanui equiline diagram (230Th/232Th) - (238U/232Th) isochron diagram for three different size fractions of zircon from the 26500y Oruanui eruption from Taupo volcano, New Zealand. The reference lines represent 5 ky increments older than the eruption age (after Charlier and Zellmer, 2000)
 

Charlier and Zellmer, 2000, Earth and Planetary Science Letters, 183, 457-469.
Christensen and DePaolo, 1993, Contrib. Mineralogy and Petrology 113, 100-114.
Condomines. 1997, Geology 25, 375-378.
Condomines et al., 1988, Earth and Planetary Science Letters, 90, 243-262.
Fukuoka and Kigoshi, 1974, Geochemical Journal 8, 117-122.
Halliday et al., 1989, Earth and Planetary Science Letters, 94, 274-290.
Mahood., 1990, Earth and Planetary Science Letters, 99, 395-399.
Reid et al., 1997, Earth and Planetary Science Letters 150, 27-39.
Sparks et al., 1990, Earth and Planetary Science Letters 99, 387-389.
Sutton et al., 1995, Journal of Volcanology and Geothermal Research 68, 153-175.
Wilson. 1993, Philosophical Transactions of the Royal Society of London 343A, 205-306.

 
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Last updated: December 23, 2011 11:40