DISTRIBUTION OF BORON, LITHIUM AND BERYLLIUM IN OCEAN ISLAND BASALTS FROM FRENCH-POLYNESIA - IMPLICATIONS FOR THE B BE AND LI/BE RATIOS AS TRACERS OF SUBDUCTED COMPONENTS/

The study focuses on the distribution of B, Be, Li, rare earth element s (REE), high-field-strength elements (HFSE), Tn, U and Pb in fresh an d hydrothermally altered ocean island basalts (GIB) from French Polyne sia, and evaluates B/Be and Li/Be ratios as potential tracers of subdu cted components in the mantle. Hydrothermal solutions affecting the ro cks during cooling were derived from meteoric water, sea water and mag matic fluids. The concentrations of REE, HFSE, Th and Be in the OIB we re not affected by secondary processes except during advanced stages o f subaerial hydrothermal alteration where saponite was a dominant seco ndary phase. This alteration modified the contents of these elements, changed REE patterns and produced a positive Ce anomaly. The subaerial and submarine hydrothermal alteration (T similar to 70-100 degrees C) may change U concentrations in GIB, whereas Pb was only marginally re distributed during alteration. Boron was enriched during submarine and subaerial hydrothermal alteration but was not noticeably affected in basalts altered by magmatic fluids at T > 200 degrees C. Like B, the m obility of Li during the alteration varies with fluid temperature. Lit hium became enriched in the basalts during advanced stages of lower T hydrothermal alteration (<100 degrees C). However, this element was pa rtly removed from the rocks during higher T alteration (>200 degrees C ) by magmatic fluids. Boron, Be and Li behave as incompatible trace el ements in basaltic magmas. Beryllium content in primitive mantle is es timated to be 0.07 ppm. Fresh Polynesian OIB have low abundances of B and Li and low B/Be (2-5) and Li/Be (2.5-5) ratios compared with volca nic are rocks, marine sediments and altered oceanic crust. Various OIB including even those which have HIMU- and EM-affinities have similar overlapping B/Be and Li/Be ratios. Both B and Li are probably stripped from a lithospheric slab during subduction-related metamorphism and a re, thus, not involved in deep mantle recycling. The mantle-normalized trace element abundances of MORE and OIB usually display patterns cha racterized by negative B, Pb and Li anomalies. The patterns of contine ntal crust and crustal rocks have distinct positive anomalies for thes e elements whereas continental basaltic rocks have variable relative a bundances of B, Pb and Li. The anomalies of these elements in basalts can be useful in discriminating their tectonic setting and constrainin g the mantle source regions of basalts.