Chemical Disequilibria, Lithospheric Thickness, and the Source of Ocean Island Basalts

摘要: We examine REE (Rare-Earth Element) and isotopic (Sr–Hf–Nd–Pb) signatures in OIB (Ocean Island Basalts) as a function of lithospheric thickness show that the data can be divided into thin- ( 12 Ma) sub-sets. Comparison to geophysically constrained thermal plate models indicates demarcation age (∼12 Ma) corresponds about 50 km. Thick-plate incompatible element enrichments, whereas thin-plate lavas MORB-like or slightly enriched values. argue thick-plate originate from low-degree melting at depths below dry solidus, while depleted MORB are indicative higher-degree melting. tested quantitative explanations systematics using for homogeneous fertile peridotite. Using experimental partition coefficients major upper mantle minerals, our equilibrium not able explain data. However, new grain-scale disequilibrium model same lithology explained. Disequilibrium by reducing amount partitioning low degree melts. To explore levels detail phenomena, we employ Monte-Carlo Potts characterize textural evolution microstructure undergoing coarsening phase transformation processes simultaneous with diffusive trace elements among solid phases melt decompressing mantle. further inverse methods study thermochemical properties required Both theory erupted on spreading ridges contain close values, although E-MORB provides best fit. This produced compositionally indistinguishable sources, indicate source is heterogeneous. Also, posteriori distributions found temperature thermomechanical lithosphere-asthenosphere boundary (T_(LAB)), (T_(p, oib)) extent during (i.e. grain size). T_(LAB) has been 1200–1300°C T_(p, oib) <1400°C. consider constraints description all provisional, because it statistical inference global dataset. Exceptional islands island groups may exist, such classical ‘hotspots’ (Hawaii, Reunion, etc) these hot sources. On other hand, arguments their origins anomalously hydrated instead. Mean size 1–5 mm, this also provisional due strong dependence knowledge coefficients, ascent rate function. perform an inversion which were allowed vary In inversions unchanged, but realizations when differ substantially investigated bulk compositions models. Corrections crystallization effects provided ambiguous confirmations previously proposed compositions, providing poorest fits. did include isotopes models, briefly evaluate effect isotopes. Although do require lithologically heterogeneous source, minor component disproportionately contributes OIB, diluted high-degree generation MORB.