Tonga

Sodium test. Treat 1 ml. of the compound with a amall thin slice of freshly cut sodium (handle with the tonga or with a penknife) in a small, dry test-tube (75 X 10 mm. or 100 X 12 mm.). Observe whether hydrogen is evolved and the sodium reacts. (If the compound is suspected to contain water, dry it first with a little anhydrous calcium or magnesium sulphate.)... 

The most important observations about U-series isotopes in arc lavas for this chapter are (1) the widespread excess of over °Th but deficit of with respect to Pa and (2) the extreme Ra enrichments in some arc lavas. We will explore the profound implications of these for magma genesis and transport at subduction zones. The conclusions apply most convincingly to the oceanic arcs where the observations are most extreme (the volcanic fronts of Tonga, Marianas, and eastern Sunda, and one or two volcanoes in some other arcs). Whether the conclusions apply elsewhere is harder to verify but there is no convincing reason with respect to U-series data to believe that they do not. 

An underlying assumption in these interpretations is that U addition by fluids is the only cause of U/Th fractionation (see Fig. 8b,c) and if the same were true of U/Pa ratios then U addition should likewise produce U-Pa arrays which record a similar time to the U-Th arrays. So far the only arc where this may be true is Tonga, where Turner et al. (1997) showed that the combined Tonga and Kermadec lava array scatters around a 50 kyr U-Th isochron (Fig. 9a). Subsequently, Bourdon et al. (1999) showed that the... 

Tonga lavas are characterized by excesses of U over Pa, and by normalizing to Nb, obtained a U addition age of 60 kyr (Fig. 9b). This result supports the interpretation that the U-Th arrays have time significance (see Section 8). In the original age calculation made by Bourdon et al. (1999) the most depleted sample (26837) was omitted because of its very high ( U)/Nb ratio which was most likely due to imprecision on its very low Nb... 

A second possibility is that the inclined U-Th arrays reflect addition of fluids containing some Th (Fig. 8c) and Pa from a source with higher U/Th than the mantle wedge (e.g., Yokoyama et al. 2002). As discussed earlier, U is significantly more fluid-mobile than Pa or Th, but the absolute magnitude remains to be confirmed. Bourdon et al. (2003) used a plot of ( Pa/ U) versus Nb/U ratios for the Tonga lavas to show that a... 

Figure 14. (a) Plot of U-Pa versus U-Th disequilibria for arc lavas. The Tonga samples are those (23ipa/235u) (230xh/ 38 j < 1 as expected from U addition by subduction zone fluids. However, all... 

One of the seemingly inescapable conclusions from the Ra- °Th disequilibrium data, at least for Tonga and the Mariana volcanic fronts, is that significantly less than 8000 years and arguably only a few half lives (ca. 1000-3000 yrs) can have elapsed since the generation of the Ra-excesses observed in the arc lavas plotted on Figure 10. 

Bourdon B, Turner S, Allegre C (1999) Melting dynamics beneath the Tonga-Kermadec island arc inferred from Pa- U systematics. Science 286 2491-2493... 

Ewart A, Hawkesworth CJ (1987) The Pleistoeene-Reeent Tonga-Kermadee are lavas Interpretation of new isotopic and rare earth data in terms of a depleted mantle somce model. J Petrol 28 495-530 Fumkawa F (1993a) Magmatic processes under arcs and formation of the volcanic front. J Geophys Res 98 8309-8319... 

Reagan MK, Sims KW, Erich J, Thomas RB, Cheng H, Edwards RL, Layne G, Ball L (2003) Timescales of differentiation from mafic parents to rhyolite in North American continental arcs. J Petrol (in press) Regelous M, Collerson KD, Ewart A, Wendt JI (1997) Trace element transport rates in subduction zones evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas. Earth Planet Sci Lett 150 291-302... 

Th disequilibria, magma petrogenesis and flux rates beneath the depleted Tonga-Kermadec island arc. Geochim Cosmochim Acta 61 4855-4884... 

Wendt Jl, Regelous M, Collerson KD, Ewart A (1997) Evidence for a contribution from two mantle plumes to island arc lavas from northern Tonga. Geology 25 611-614 Williams RW, Gill JB (1989) Effects of partial melting on the uranium decay series. Geochim Cosmochim Acta 53 1607-1619... 

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