BORON GEOCHEMISTRY OF THE CENTRAL-AMERICAN VOLCANIC ARC - CONSTRAINTSON THE GENESIS OF SUBDUCTION-RELATED MAGMAS

Boron contents were measured in representative Quaternary lavas from t he Central American Volcanic Arc to evaluate along-strike variations i n subduction processes. Despite the significant range in B concentrati ons (approximately 2-37 ppm) in the mafic lavas, B/La ratios vary in a systematic fashion along the arc; higher values (>1) are typical betw een Guatemala and northern Costa Rica, whereas low values (most <0.5) typify central Costa Rica and western Panama. B/La is highly correlate d with Be-10/Be-9 (r2 = 0.94, excluding one sample) and appears to be a useful indicator of subduction contributions to the magma sources. I f enrichments of both B and Be-10 are proportional to the flux of subd ucted sediment, along-strike variations in B/La suggest at least a two fold variation in this flux with maximum values below western Nicaragu a and minimum values below Costa Rica and western Panama where the Coc os Ridge is being subducted. These data may also reflect significant d ifferences in thermal state of the descending slab, which in turn diff erentially affects release patterns of fluids and fluid-mobile trace e lements, and possibly melting processes beneath different parts of the arc. The following scenario is suggested to explain the geochemical r esults. Beneath the northwestern part of the arc steep subduction of o lder, relatively cold slab favors more efficient subduction of fluid c omponents to depths beneath the volcanic front. The released fluids ca rry fluid-mobile elements to the overlying mantle, which upon melting produces calcalkalic magmas. Shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid-related metasomatism of sub-arc mantle beneath the volca nic front. Under such conditions, B/La and Ba/La ratios in the sub-arc mantle vary little from values seen in oceanic island basalts. Magmas in this part of the arc nevertheless display the highest La/Yb and lo west Ba/La and B/La ratios, which are consistent with prior light rare -earth-element enrichment in the source, significantly lower degrees o f melting, or a combination thereof. Because some of the largest volca noes occur in Costa Rica, and magma flux there is nearly an order of m agnitude higher than elsewhere in the arc, source enrichment is consid ered to be the more plausible explanation. It is proposed that Quatern ary magma production below Costa Rica involved lithospheric sources co ntaining trapped or stored melt components, but this enrichment proces s is unlikely to have involved typical arc magmas or subduction fluids because we see no B-enrichment.