U-238 TH-230 DISEQUILIBRIA, MAGMA PETROGENESIS, AND FLUX RATES BENEATH THE DEPLETED TONGA-KERMADEC ISLAND-ARC

The highly depleted intra-oceanic Tonga-Kermadec island are forms an e ndmember of are systems and a unique location in which to isolate the effects of the slab flux. High precision TIMS uranium, thorium, stront ium, neodymium, and lead isotopes, along with complete major and trace element data, have been obtained on an extensive sample set comprisin g fifty-eight lavas along the are as well as nineteen samples of the s ubducting sediments at DSDP site 204 just to the east of the Tonga-Ker madec trench. Ca/Ti and Al/Ti ratios extend from values appropriate to an N-MORB source in the southern Kermadecs to very high ratios in Ton ga interpreted to reflect increasing degrees of depletion of the mantl e wedge due to backarc basalt extraction. The isotope data emphasize t he need for four components in the petrogenesis of the lavas: (1) the mantle wedge; (2) a component with elevated Pb-207/Pb-204 towards whic h the Kermadec and southern Tongan lavas extend; (3) a component chara cterised by high Pb-206/Pb-204, Ta/Nd, and low Nd-143/Nd-144 observed only in the northernmost Tongan islands of Tafahi and Niuatoputapu; (4 ) a fluid component characterised by strong enrichments of Rb, Ba, U, K, Pb, and Sr, relative to Th, Zr, and the REE and producing large U-2 38 excesses ((Th-230/U-238) = 0.8-0.5) in the more depleted lavas. The mantle wedge (Component 1) is isotopically similar to the source of t he Lau BABE. Component 2 is average pelagic sediment on the downgoing Pacific plate as observed at DSDP sites 595/596 and in the upper secti ons of the sediment pile at DSDP site 204. Mass balance calculations i ndicate that less than 0.5% is recycled into the are lavas; essentiall y all the subducted sediment is returned to the upper mantle (similar to 0.03 km(3) yr(-1)). Exceptionally low concentrations of Ta and Nb r elative to Th and the LREE requires that this sediment component is ad ded as a partial melt which was in equilibrium with residual rutile or ilmenite. Component 3 is identified as volcaniclastics from the Louis ville Ridge which comprise the lower 44 m of the sediment section inte rsected at DSDP site 204. These volcaniclastics are spatially restrict ed to the vicinity of the Louisville Ridge and provide a unique sedime nt tracer which can be used to show that it takes similar to 4 Myr fro m the time of subduction to its first appearance in the are lava signa ture. Component 4, the fluid contribution to the lava source is inferr ed to contribute similar to 1 ppm Rb, 10 ppm Ba, 0.02 ppm U, 600 ppm K , 0.2 ppm Pb, and 30 ppm Sr. It has Sr-87/Sr-86 = 0.7035 and Pb-206/Pb -204 = 18.5 and thus it is inferred to have been derived from dehydrat ion of the subducting altered oceanic crust. U-Th isotope disequilibri a reflect the time since fluid release from the subducting slab and a reference line through the lowest (Th-230/Th-232) lavas constrains thi s to be 30,000-50,000 yr. The U-Th and Th-Ra isotope systematics are d ecoupled, and it is suggested that Th-Ra isotope disequilibria record the time since partial melting and thus indicate rapid channelled magm a ascent. Olivine gabbro xenoliths from Raoul are interpreted as cumul ates to their host lavas with which they form zero age U-Th isochrons indicating that minimal time was spent in magma chambers. The subducti on signature is not observed in lavas from the backarc island of Niuaf o'ou. These were derived from partial melting of fertile peridotite at 130-160 km depth with melt rates around 2 x 10(-4) kg m(-3) yr(-1). C opyright (C) 1997 Elsevier Science Ltd.