FORE-ARC BASALT TO BONINITE MAGMATISM: CHARACTERIZING THE TRANSITION FROM DECOMPRESSION TO FLUID FLUX MELTING AFTER SUBDUCTION INITIATION
Forearc basalt (FAB) is the oldest volcanic product of subduction initiation. Hole U1440B comprises a thin talus cover underlain by ~170 meters of FAB pillow lavas, sheet flows, and hyaloclastites, interlayered FAB-related dolerite and lavas, and dolerite.
FAB trace element patterns strongly resemble those of mid-ocean ridge basalts (MORB), but FAB is distinguished from MORB by a stronger depletion in incompatible trace element concentrations. Low-Si Boninites (LSB), genetically related proto-arc rocks encountered as hyaloclastites and pillow lavas at sites U1439 and U1442, regionally overlie FAB, and are distinguished by more silicious and magnesian major element compositions, enrichments in fluid mobile trace elements, and extreme depletions in rare-earth element concentrations. FAB is largely a product of decompression melting given its similarity to MORB. LSB and arc lavas have many similar geochemical traits that are often attributed to fluid-flux melting of the mantle. Discerning the origin and relative timing of this significant transition in sources and petrogenetic processes are primary goals of this research.
A large majority of FAB samples lack a distinct fluid mobile element enrichment. This observation applies to both aqueous fluid soluble trace elements such as Rb, K, and Sr in addition to sediment-melt mobile elements such as Ba and Th. Thus, FAB appears to have been generated as a result of rapid slab-rollback and upwelling of a depleted mantle source. Trace element ratios for LSB plot along trends that imply mixing between FAB and an endmember LSB composition, suggesting that the flux-melting source developed concurrently with continued decompression melting.