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 repor ted for twenty samples from fourteen Holocene volcanic centers in Papu a 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 1 00 to 400 km deep. Samples from the Western Bismarck Arc are from diff erent segments of an eastward-propagating zone of collision between Pa pua New Guinea and the arc. Measured ratios of (Th-230)/(Th-232), Be-1 0/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 positivel y Correlated (Th-230)/(Th-232), Be-10/Be-9, and B/Be ratios. The posit ive 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 large r fluxes of uranium than thorium, indicate that uranium was added to t he 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 betwee n 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 f ront differ from those at the front but are still indicative of slab i nput. The subduction signature is absent only where the slab is 400 km deep. Lavas from the Western Bismarck Arc share most of the character istics 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-l ife) to stable boron in the lavas can be used to ''date'' the collisio n. Results suggest that suturing began in the westernmost segment in t he 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 w here the slab now beneath the volcanoes may have had a long (>3 Ma) re sidence 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 si milar data set for southern Chile where the elapsed time is inferred t o be less than 20 Ka.