TIMESCALE FOR PRODUCING THE GEOCHEMICAL SIGNATURE OF ISLAND-ARC MAGMAS - U-TH-PO AND BE-B SYSTEMATICS IN RECENT PAPUA-NEW-GUINEA LAVAS

U-series activities, beryllium isotopes and boron abundances are reported for twenty samples from fourteen Holocene volcanic centers in Papua New Guinea, spanning several tectonic settings. Samples from centers on and behind the volcanic front in the Eastern Bismarck Arc are from a region of normal subduction. The volcanoes overlie a seismic zone 100 to 400 km deep. Samples from the Western Bismarck Arc are from different segments of an eastward-propagating zone of collision between Papua New Guinea and the arc. Measured ratios of (Th-230)/(Th-232), Be-10/Be-9, and B/Be in basalts and andesites from all settings are 1.2-2.9, 2-28, and 8-160, respectively. Rocks from the Eastern Bismarck Arc are characterized by approximately 10% excess uranium and by positively Correlated (Th-230)/(Th-232), Be-10/Be-9, and B/Be ratios. The positive correlation of Be-10/Be-9 With (Th-230)/(Th-232) rather than with (U-238)/(Th-230) ratios, and the association of high values with larger fluxes of uranium than thorium, indicate that uranium was added to the arc source and stored there for longer than 10(5) y prior to magma genesis. There was a time lag of hundreds of thousands of years between slab dehydration and melt formation. Lavas from volcanoes up to 230 km above the slab have the same U-series relationships as those at the volcanic front. Beryllium isotope and B/Be relationships behind the front differ from those at the front but are still indicative of slab input. The subduction signature is absent only where the slab is 400 km deep. Lavas from the Western Bismarck Arc share most of the characteristics of those at the eastern Bismarck volcanic front except that Be-10 concentrations decrease from low to unmeasurable levels westward in the direction of earlier collision. The ratio of Be-10 (1.5 Ma half-life) to stable boron in the lavas can be used to ''date'' the collision. Results suggest that suturing began in the westernmost segment in the early Pliocene and within the last million years at 148-degrees-E, the site of the present triple junction. Large fluxes of slab-derived elements, including excess U-238 which must have developed within the last approximately 350 ka, are seen in the collision zone lavas even where the slab now beneath the volcanoes may have had a long (>3 Ma) residence time in the mantle. The U-series data from the entire Bismarck volcanic front suggest approximately 200 Ka between slab dehydration and magma formation and eruption. This is in striking contrast to a similar data set for southern Chile where the elapsed time is inferred to be less than 20 Ka.