Temporal Evolution of Boron Flux in the Izu and Honshu Arcs Measured by Ion Microprobe from the Forearc Tephra Record

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 Ocean Drilling Program (ODP) in the Izu and Honshu 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 Honshu 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 backarc 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 (HFSE) or rare earth elements (REEs) in either Izu or Honshu Arcs since 7 Ma, although the two elemental groups do show clear positive correlation. A lack of correlation between REEs, HFSEs and B/Be suggests that slab flux is not the only control on melting in these arcs. δ¹¹B values measured in the Honshu 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. >80% of the Honshu boron budget appears to derive from altered oceanic crust, with additional involvement from continental trench sediments in Honshu and carbonates in Izu. There is no evidence to support the idea of slab melting in either Izu or Honshu Arcs.