IODINE ABUNDANCES IN OCEANIC BASALTS - IMPLICATIONS FOR EARTH DYNAMICS

Iodine analyses by neutron activation have been performed on 32 oceanic basalts glasses, 1 phonolite and 3 subaerial arc basalts. The world-wide sample set encompasses all typical geodynamic settings [ridges (MORB). oceanic islands (OIB), arcs (IAB) and back-arc basins (BABB)] and the diversity of oceanic basalt types [depleted (N), intermediate (T) and enriched (P)]. Most basalts, including all N-types, all but one T-type, and some P-types, exhibit low iodine concentrations (2.5-13 ppb). Very high I concentrations (up to 363 ppb) in a small number of samples (all P-types) are interpreted to be the result of a recycled component which includes organic matter of sedimentary origin (sediment organic matter accounts for about 80% of total terrestrial iodine). Iodine appears to be the most incompatible element after the noble gases. Mass balance considerations permit the mantle iodine concentration and hence the bulk silicate abundance of the Earth to be constrained to 9-24 ppb, with a preferred value of 10 ppb. The low terrestrial iodine abundance, coupled with a chondrite-like chlorine/iodine ratio, strongly favours the late veneer model of Earth accretion. The scenario proposed to explain the terrestrial iodine distribution includes heterogeneous accretion, iodine extraction from the mantle simultaneous with (or even before) continent formation, and depleted mantle homogenization. As iodine is not recycled into the mantle by oceanic crust. heterogeneities in the mantle should result from organic sediment (C) recycling, of which iodine may be a good tracer.