IMPLICATIONS OF LAVA COMPOSITIONS FROM THE CENTRAL AMERICAN AND IBM ARCS FOR THE EXTENTS OF REE RECYCLING IN SUBDUCTION ZONES

Nb and Ta concentrations are low compared to of rare-earth elements (REE) in arc lavas, resulting in La/Nb values that typically range from 2-8. This contrasts with lavas from ocean island and or mid-ocean ridge settings, which typically have La/Nb < 2. The high La/Nb values for arc lavas commonly are attributed to lower solubilities of Nb and Ta with respect to REE in fluids derived from the subducting lithosphere. However, the proportions of REE from the subducting slab required to explain the high La/Nb ratios of arc magmas typically are significantly greater than the proportion of slab-derived REE needed to explain Ce anomalies and Nd isotope values. We have geochemically analyzed suites of lavas from the Central American and Izu-Bonin-Mariana arcs with wide-ranging La/Nb values to further investigate the origin of Nb-Ta anomalies. Turrialba volcano in Costa Rica has erupted lavas with La/Nb < 2 and OIB-like trace elements together with basalts with Ba/La = 20-21 and La/Nb >2. The OIB-like lavas appear to have resulted from decompression melting alone, whereas those with high Ba/La were produced by fluxed melting in the presence of a residual high-Nb mineral. Most Nicaraguan basalts have La/Nb > 2, whereas rare basalts have La/Nb=0.3-1.4. The latter appear to have been generated by melting the residual mantle phase that held back Nb during generation of the former. Boninites from Guam in the IBM arc have La/Nb ranging from 1.1-2.5. These values negatively correlate with measures of fluxed melting, implying that a phase with a relatively high-D for Nb melts out at the highest degrees of melting. These observations imply that a slab-fluid is necessary to stabilize high DNb phases such as ilmenite (Grove et al., 2006) or amphiboles in the mantle wedge during arc magma generation. Small degrees of melting in the cool region above the subducting slab generates incompatible-element and water enriched melts that flux further melting to generate the parental arc basalts. When melting degrees become too high, these phases melt out and Nb-Ta anomalies disappear. If this origin for Nb-Ta anomalies in arcs is commonplace, then recycling of REE and fluid soluble trace elements from subducting slabs to overlying plates may be less than is commonly believed.