THE TRACE-ELEMENT SYSTEMATICS OF BERYLLIUM IN GEOLOGIC ENVIRONMENTS
JEFFREY G. RYAN, Department of Geology, Univ. South Florida, 4202 E. Fowler Ave., Tampa, FL 33620, ryan@chuma.cas.usf.edu
Recent analytical advances have permitted the measurement of trace-level beryllium in rocks, minerals and fluids from a variety of geologic settings. In igneous rocks, Be acts as a strongly incompatible trace element - DBe Å 0.05-0.1 in basalts, and Be behaves similarly to Nd in MORBs and OIBs. While MORB Be contents are generally low (0.2-1 ppm in all but the most evolved basalts), intraplate lavas can contain very high Be (_10 ppm Be, correlated with high LREE contents). In arc lavas Be behaves less incompatibly (similar to Zr or Gd), with concentrations similar to those of MORBs. Differences in the behavior of Be among basaltic lava types probably relate to its unique mineral-melt partitioning. Be has a small ionic radius, and can substitute for Al in mineral structures. Be plagioclase/melt partition coefficients, at _ 0.2, are markedly larger than those for clinopyroxene (at ~0.05) or olivine (~0). Covariation similarities to Nd or other elements are thus at least partly fortuitous, and place constraints on the residual mineral assemblages of basaltic melts.
In sedimentary, metamorphic and hydrothermal systems, the most distinctive characteristic of Be is its low solubility in hydrous fluids. Seawater concentrations are low and variable (pg/gram levels) and the residence time for Be in the oceans is <<1000 yrs. While higher than seawater, Be in ridgecrest hydrothermal fluids (<0.9 ng/g) is three orders of magnitude lower than in the ocean crust. Be in clay-rich seafloor sediments and marine shales are consistently 1.5-2 ppm, while carbonates and marine precipitates contain negligible Be.
Be in metamorphic rocks varies with protolith, with pelites showing higher abundances. Be is housed largely in white micas, though both amphibole and lawsonite may host significant Be. Mineral/fluid partitioning results for Be show it to be much less soluble than B or Li, and Be contents show essentially no change with increasing grade in prograde metamorphic suites. The transport of Be off slabs into arc source regions must thus occur via some variety of slab- or sediment-derived melt phase.