USES OF BORON IN CONSTRAINING SUBDUCTION ZONE PROCESSES

The element boron is both an incompatible and highly water-soluble trace element. In dry magmatic systems it behaves like other incompatible elements, being strongly enriched in silicate melts. Thus, in intraplate systems (MORB, OIB) B/Nb, B/Zr, B/Be and similar ratios are near constant and representative of mantle sources. In contrast, in hydrated magmatic or metamorphic systems B can be significantly mobilized in an aqueous phase. In subduction zones (SZs) this behavior is manifest by progressive dehydration and B-loss from subducted slabs, and complementary enrichments of water and B in the overlying mantle wedge. This, volcanic arc lavas are characterized by relative enrichment of B and by large B/Nb, B/Zr, etc. ratios.

These properties can be used to evaluate changes in thermal state across and between SZs by monitoring compositions of erupted lavas. Mafic arc lavas define strong B-depletion trends toward back-arc regions, signifying progressive warming of the slab with depth. Likewise, B-enrichments in volcanic front lavas differ by nearly two orders of magnitude between arcs, and correlate with temperature-sensitive parameters (e.g., length of seismogenically active slab) or numerical estimates of slab-surface temperature.

We demonstrate these systematics for arcs having a wide range in thermal structure (e.g., Cascades, Andes, W. Pacific arcs), and speculate on the implications for magma generation and material recycling at SZs.