The enrichment of boron relative to similarly incompatible elements, such as Be, in arc volcanic rocks has been used as a proxy for the involvement of slab flux in petrogenesis. New ion microprobe analyses of single glass shards in tephra layers recovered by the Ocean Drilling Program (ODP) in the Izu and NE Japan forearc basins now allow the temporal variation in slab flux to be charted since 7 and 5 Ma, respectively. B/Be ratios are typically <70 in NE Japan and <130 in Izu, with no single grain exceeding 200. Although moderate to high for modern arcs, these values are much less than those recorded in the Marianas and Tonga at 3–4 Ma, shortly after the start of rifting of their back-arc basins. This observation suggests that the peak B/Be values seen in Tonga and the Marianas are related to the tectonics of slab roll-back and basin opening, rather than changes in the dynamics of the Pacific Plate. There is no temporal trend to enrichment in the high field strength elements (HFSE) or rare earth elements (REE) in either Izu or NE Japan since 7 Ma, although the two elemental groups do show clear positive correlation. A lack of correlation between REE, HFSE and B/Be suggests that slab flux is not the only control on melting in these arcs. 11B values measured in the NE Japan glasses ranged from +4·1 to -8·3, with significant scatter over short periods of time and even between different grains within single tephra layers. Such variations are interpreted to reflect short-term changes in the degree of sediment subduction, as well as heterogeneity in the magma chamber prior to eruption. Greater than 80% of the NE Japan boron budget appears to derive from altered oceanic crust, with additional involvement from continental trench sediments in NE Japan and carbonates in Izu. There is no evidence to support the idea of slab melting in either Izu or NE Japan arcs.