Beryllium Systematics in Young Volcanic Rocks: Implications for \u3csup\u3e10\u3c/sup\u3eBe

Beryllium is an incompatible trace element that closely parallels neodymium in its geochemical behavior. Be analyses conducted on well-characterized oceanic and arc volcanic rock suites, as well as on marine sediments, suggest a bulk solid/liquid distribution coefficient of 0.03–0.06 for melting of the mantle and crystallization of basalts. The Be/Nd ratio for many volcanic rocks from diverse tectonic environments is approximately .05, similar to the ratio in chondrites. Be data for samples from volcanic arcs show that there are significant variations in 10Be/9Be among different arcs, and that variations in 10Be are not due to variations in Be concentration alone. For at least one volcano (Bogoslof), the 10Be/9Be ratio is constant for samples that vary by a factor of three in both their Be and 10Be concentrations, suggesting that 10Be is an inherited magmatic signature and not simply a result of contamination near the surface. In addition, the Be, Nd and Pb isotope systems for this volcano are all consistent with a model in which small amounts of sediment were incorporated into the Bogoslof source region—provided the mantle wedge has the isotopic characteristics of depleted MORB. Since 10Be exists only in the uppermost tens of meters of oceanic sediments, the data suggest an efficient return flux of sediment to the mantle at subduction zones