Systematic differences in trace element compositions of volcanic arc versus intraplate magmas are commonly attributed to involvement of subducted sediments and/or seawater-altered oceanic crust in arc magma genesis. The exact contributions from these materials is poorly understood. Metamorphic processes may significantly modify subducted oceanic slabs before they reach subarc depths. The nature and magnitude of such compositional modifications, as exemplified by the element boron, are investigated in metamorphic suites having protoliths analogous to subducted marine sediments and basalts. Our results indicate that in addition to protolith type, B content depends on metamorphic temperature. Greenschist and amphibolite facies metasediments generally have B contents at least a factor of two lower than in equivalent unmetamorphosed sediments. Moreover, four low-pressure pelitic to semipelitic metasediment suites display progressive loss of B with increasing peak metamorphic temperature (ca. 200–750° C). Metabasalts of greenschist and higher grades also have consistently lower B contents (Most subducted B initially resides in marine sediment and altered oceanic crust (ie, within the uppermost kilometer). Our observations suggest that B may be extracted by fluids released by devolatilization reactions upon progressive heating during subduction. The relatively high B contents observed in many arc basalts imply that slab temperatures at subarc depths commonly do not exceed conditions at which B is systematically depleted. Otherwise, subducted slabs would retain insufficient quantities of B to balance its inventory in arc magmas. Using the temperature dependence of B depletion estimated from metamorphic suites, maximum temperatures are unlikely to exceed∼ 800–900° C for uppermost portions of subducted slabs beneath arcs which show typical B enrichments. In contrast, arc basalts commonly have eruptive temperatures approaching the dry liquidus (ca. 1300° C) of subducted oceanic crust. By the time such temperatures are reached in subducted slabs, it is likely that H 2 O, B and other fluid-mobile elements are strongly depleted by metamorphic processes. Thus, slab melting does not appear to be a viable process for generating typical arc basalts. A more plausible scenario that is commonly invoked for production of arc basaltic magmas involves migration of aqueous fluid and fluid-mobile elements from the slab to hotter regions in the mantle wedge. Such fluids could metasomatize portions of the mantle wedge and induce melting to produce B-rich arc magmas.