OBSERVATIONS OF PA-231 U-235 DISEQUILIBRIUM IN VOLCANIC-ROCKS/

We present here the first survey of (P-231/U-235) ratios in volcanic r ocks; such measurements are made possible by new mass spectrometric te chniques. The data place new constraints on the timing and extent of m agma source and evolutionary processes, particularly due to the sensit ivity of the Pa-231-U-235 pair and its intermediate time scale (Pa-231 t(1/2) = 33 ky). (Pa-231/U-235) is found to vary widely, from 0.2 in carbonatites to 1.1-2.9 in basalts and 0.9-2.2 in arcs. Substantial Pa enrichment is nearly ubiquitous, suggestive of the relative incompati bility of Pa, qualitatively consistent with available partitioning dat a. The level of Pa-231-U-235 disequilibrium typically far exceeds that of Th-230-U-238 and is comparable to Ra-226-Th-230. The high (Pa-231/ U-235) ratios in MORB and other basalts reflect a large degree of disc rimination between two incompatible elements, posing challenges for mo delling of melt generation and migration. Fundamental differences in ( Pa-231/U-235) among different basaltic environments are likely related to contrasts in melting zone conditions (e.g., melting rate). Strong (Pa-231/U-235) disequilibria in continental basalts, for which (Th-230 /U-238) disequilibria are small or absent, demonstrate that Pa-U fract ionation is possible in both garnet and spinel mantle stability fields . In arcs, correlation of (Pa-231/U-235) and (Th-230/U-238) is consist ent with U enrichment via slab-derived fluids, a process which is addi tional to the still dominant Pa enrichment. An important new constrain t is provided by the observation that the near-equilibrium (Th-230/U-2 38) common to arcs and continental basalts is not typically accompanie d by near-equilibrium (Pa-231/U-235), arguing against the influence of long magma history, crustal material, or equilibrium mantle sources i n affecting decay-series ratios. Small sample sets from two silicic ce nters illustrate: (1) recent, rapid U enrichment in the magma chamber (El Chichon); and (2) the failure of substantial magma H2O-CO2 degassi ng to effect U-Th-Pa fractionation (Mono Craters).