A geochemically consistent hypothesis for MORB generation

Geochemical observations of MORE including U-series disequilibria are used to examine the processes and timescales of MORE melt generation. Incompatib le elements in MORE suggest that the MORE source region consists of a deple ted Iherzolite matrix interspersed with chemically enriched mafic veins. Wi de variations in Th/U distinguish these source variations in MORE better th an Sr-87/Sr-86 and document that the relative chemical homogeneity of norma l MORE reflects efficient melt mixing rather than a homogeneous source. Spi nel compositional variations in MORE and in mantle solids (abyssal peridoti tes and dunites) reflect reactive flow of melts having significant composit ional variations. High Cr# spinels result from reactive flow of chemically enriched melts derived from the mafic vein source ascending through the Ihe rzolite of the upper melting column. High Cr# and TiO2 contents in dunite s pinels indicate that dunites form by reactive flow of enriched melts throug h the upper melting column. Once formed, dunites act as high permeability p athways for melt from surrounding lherzolite and are responsible for the "f ractional signatures" observed in the major element chemistry, melt inclusi ons, abyssal peridotites and Lu-Hf systematics of MORE. Based on the recogn ition that there are two sources melting beneath ridges that have different porosity characteristics, a melting model consistent with evidence for bot h fractional and equilibrium porous flow melting is proposed. In this model , the presence of dunite channels affect melt generation and transport in t he Iherzolite matrix, suggesting that mantle heterogeneity may be critical to the physical aspects of melting and melt transport in the mantle beneath mid-ocean ridges. U-series disequilibria provide information on how meltin g occurs in the two endmember sources and suggest that melt porosities in t he Iherzolite may be as low as 0.1%. Melt within Iherzolite maintains equil ibrium with the coexisting solid while it ascends porously. Primitive MORE with high Mg# consistently have low Th-230 excesses or deficits with major element chemical signatures of equilibration near 1.0 GPa suggesting that t he depleted endmember melt maintains chemical equilibrium with Iherzolite u ntil shallow mantle depths (similar to 30 km). Melt porosities in enriched heterogeneities remain below 1% for perhaps 10s of km before losing chemica l equilibrium with the solid during transport in the upper melting column. Because the porosities required by the observed disequilibria are small, th e transition to porosities large enough to form "veins" of melt must occur over a timescale which is very long in comparison to the Ra-226 half-life a nd significantly long for Pa-231. Thus, instantaneous transport dynamic mel ting models appear incompatible with the observed disequilibria even when i nitial melt productivities as low as 0.05%/km are used. (C) 2000 Elsevier S cience B.V. All rights reserved.