Pb isotope analyses combined with U-series disequilibrium in young volcanic rocks is a powerful tool with which to study the processes and timescales of melt generation in subduction-related rocks. Pb and Th isotope measurements can be used to infer the source U/Th over very different timescales.
We present additional Pb and U-Th disequilibrium data (measured by TIMS) for a suite of samples from the Tonga-Kermadec Arc in the western Pacific. Samples are from the length of the arc, from L'Esperance (southern Kermadec) to Tafahi (northern Tonga), and include low-K tholeiites, andesites and dacites, for which Sr, Nd, Pb, Th, U isotope and ICPMS trace element data already exist studies (Regelous et al., 1995; Ewart et al., 1995).
In conventional 206Pb/204Pb vs. 207Pb/204Pb diagrams most Tonga lavas plot close to the radiogenic end of the field defined by Indian Ocean MORB and Lau back-arc basin basalts. Lavas from the northernmost Tonga island of Tafahi have more radiogenic compositions compared to more southern islands, reflecting the influence of the Samoa or Louisville plume components. The Kermadec lavas have more variable Pb isotope compositions, and higher 207Pb/204Pb relative to Tonga lavas, extending towards values for Tonga trench sediments. 208Pb*/206Pb* for Tonga and Kermadec range from 0.937 to 0.948 and from 0.950 to 0.975 respectively, corresponding to time-integrated source Th/U values (kPb) of 3.807-3.849 and 3.828-3.959.
Tonga lavas have high (238U/232Th) and (230Th/232Th) relative to the Kermadec lavas and in fact, most other subduction-related rocks. All Tonga lavas analysed display excess U. Lavas from the islands of Hunga, Metis, Late, Tofua and Fonualei have (230Th/238U) between 0.59-0.73. Kermadec lavas are closer to equilibrium or have small Th excess, with 230Th/238U between 0.73 and 1.02.
Variations in Pb and Th between Tonga and Kermadec lavas, and between individual islands in the Tonga arc, partly reflect differences in relative amounts of U, Th and
Pb derived from sources in the subducting slab and the
overlying mantle wedge, which in turn can be linked to differences in the degree of depletion of the mantle wedge underlying the arc. Mantle wedge depletion increases northwards along Tongan arc, probably also along Kermadec arc. However, converging rates also increase northwards, presumably correlative with increasing slab flux. The chemistry of the slab-derived component differs between the Tonga and Kermadec arc segments. In Tonga, strong U enrichment relative to Th of the mantle wedge over a period of time has led to high 238U/232Th ratios, while in the Kermadec lavas, U and Th have both been added from the slab to the mantle.
In addition, a suite of lavas from the back-arc island of Niua fo'ou have been analysed, and in conjunction with further Pb, Nd, Sr, Th and ICPMS data from sediments and the Louisville seamount chain this expanded study will allow a more complete refinement of the mixing fluxes identified in the previous studies (Regelous et al., 1995; Ewart et al., 1995).
Ewart, A., Collerson, K.D., Niu, Y. & Wendt, I., Eos, Trans. A.G.U. 76, F654 (1995).
Regelous, M., Collerson, K.D., Ewart, A & Wendt, I., Eos, Trans. A.G.U. 76, F654 (1995).