Depth‐Integrated Boron Inventories of Subducting Oceanic Plates: New Insights From Sediments and Altered Oceanic Crust at ODP Sites 1179, 1173, and DSDP Site 443

Boron (B) is a highly fluid‐mobile tracer of slab‐derived fluids in subduction zones; however, the quantitative comparisons of B input among different oceanic plates remain poorly constrained. Here, we present a geochemical study of the sediments, chert, and altered oceanic crust (AOC) from three DSDP/ODP Sites (443, 1173, and 1179) representing the young Philippine Sea and old Pacific Plates. Downhole variations in B showed that lithology and alteration history exert first‐order control on B distribution, with sediments containing B concentrations 1–2 orders of magnitude higher than basalts, and AOC showing variable enrichment related to plate age and alteration style. The chert at Site 1179 exhibited distinct geochemical characteristics and could not be treated as a low‐B sediment. By integrating the B concentrations with the lithological thicknesses and densities, we estimated the depth‐integrated boron inventories (B mass per unit area) for each plate. Total B inventories are highest at Site 1173 (∼162 kg m −2 ), followed by Site 443 (∼117 kg m −2 ) and Site 1179 (∼88 kg m −2 ), reflecting primarily differences in sediment thickness and lithological proportions rather than absolute concentrations. Although the AOC at Site 1179 showed elevated B, its contribution to slab B budgets was subordinate to that of the sediments. Slab B inventories did not show a simple correlation with the B content in the arc basalts. This decoupling supports models in which the thermal structure of the slab–mantle system and the depth of slab dehydration, rather than the initial slab inventory, govern B transfer to arc magma sources.