Abstract
How subduction-related magmatism starts at convergent plate margins is still poorly understood. Here we show that boron isotope variations in early-formed boninites from the Izu-Bonin arc, combined with radiogenic isotopes and elemental ratios document rapid (~0.5 to 1 Myr) changes in the sources and makeup of slab inputs as subduction begins. Heterogeneous hornblende-granulite facies melts from ocean crust gabbros ± basalts fluxed early melting to generate low silica boninites. Hydrous fluids from slab sediments and basalts later fluxed the low silica boninites mantle source to produce high silica boninites. Our results suggest that initially the uppermost parts of the slab were accreted near the nascent trench, perhaps related to early low-angle subduction. The rapid changes in slab inputs recorded in the boninites entail a steepening subduction angle and cooling of the plate interface, allowing for subduction of slab sediment and basalt, and generating hydrous fluids at lower slab temperatures.
Highlights
How subduction-related magmatism starts at convergent plate margins is still poorly understood
Boron concentrations in Expedition 352 boninites vary by boninite type, with mean low silica boninites (LSB) at ≈4 μg/g B, and mean high silica boninites (HSB) at ≈12 μg/g B (Supplementary Data 1)
While the B concentration ranges of HSB and LSB show some overlap, there are clear distinctions with respect to MgO and TiO2, suggesting multiple magmatic sequences with distinct B abundances
Summary
How subduction-related magmatism starts at convergent plate margins is still poorly understood. We show that boron isotope variations in early-formed boninites from the Izu-Bonin arc, combined with radiogenic isotopes and elemental ratios document rapid (~0.5 to 1 Myr) changes in the sources and makeup of slab inputs as subduction begins. Hydrous fluids from slab sediments and basalts later fluxed the low silica boninites mantle source to produce high silica boninites. The rapid changes in slab inputs recorded in the boninites entail a steepening subduction angle and cooling of the plate interface, allowing for subduction of slab sediment and basalt, and generating hydrous fluids at lower slab temperatures. The Izu-Bonin-Mariana (IBM; Fig. 1) convergent plate margin is a unique natural laboratory for the study of subduction initiation, with well-preserved forearc igneous sequences that represent the first eruptive products of subduction in this region. In the boninite holes (U1439A, U1439C and U1442A), lavas < ~250 meters below the seafloor (mbsf) are dominated by high silica boninites (HSB)
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