Geochemistry of the highly depleted peridotites drilled at ODP Sites 1272 and 1274 (Fifteen-Twenty Fracture Zone, Mid-Atlantic Ridge): Implications for mantle dynamics beneath a slow spreading ridge

摘要: During ODP Leg 209, a magma-starved area of the Mid-Atlantic Ridge (MAR) was drilled in vicinity Fifteen-Twenty Fracture Zone (FZ) that offsets one slowest portions spreading ridge. We present here results bulk rock multi-elemental study 27 peridotites at Sites 1272 and 1274 (to south north FZ, respectively). The comprise mainly harzburgites with minor dunites. Clinopyroxene (Cpx), which is interstitial interpreted as secondary, observed Site peridotites. have low Al2O3 contents (≤ 1 anhydrous wt.%), high Mg# (> 91.5), trace elements compositions mostly below 0.1 × primitive mantle (PM). These peridotites, particular represent most depleted yet sampled slow Their indicate degrees partial melting melt extraction. A single open-system event (melting plus percolation melts produced within upwelling mantle) can explain their highly linear chondrite-normalized REE patterns, characterized by steady depletion from HREE to LREE. Late melt-rock reactions precipitation Cpx explains slightly less Hence, differences composition between do not provide evidence for regional variations FZ. The occurrence refractory suggests we more actively convecting than generally supposed centers.