A geochemically consistent hypothesis for MORB generation

doi:10.1016/S0009-2541(99)00122-9

A geochemically consistent hypothesis for MORB generation

2000-01-01

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DOI：10.1016/S0009-2541(99)00122-9

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Geochemical observations of MORE including U-series disequilibria are used to examine the processes and timescales of MORE melt generation. Incompatible elements in MORE suggest that the MORE source region consists of a depleted Iherzolite matrix interspersed with chemically enriched mafic veins. Wide variations in Th/U distinguish these source variations in MORE better than Sr-87/Sr-86 and document that the relative chemical homogeneity of normal MORE reflects efficient melt mixing rather than a homogeneous source. Spinel compositional variations in MORE and in mantle solids (abyssal peridotites and dunites) reflect reactive flow of melts having significant compositional variations. High Cr# spinels result from reactive flow of chemically enriched melts derived from the mafic vein source ascending through the Iherzolite of the upper melting column. High Cr# and TiO2 contents in dunite spinels indicate that dunites form by reactive flow of enriched melts through the upper melting column. Once formed, dunites act as high permeability pathways for melt from surrounding lherzolite and are responsible for the fractional signatures observed in the major element chemistry, melt inclusions, abyssal peridotites and Lu-Hf systematics of MORE. Based on the recognition that there are two sources melting beneath ridges that have different porosity characteristics, a melting model consistent with evidence for both fractional and equilibrium porous flow melting is proposed. In this model, the presence of dunite channels affect melt generation and transport in the 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 melting occurs in the two endmember sources and suggest that melt porosities in the Iherzolite may be as low as 0.1%. Melt within Iherzolite maintains equilibrium 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 the depleted endmember melt maintains chemical equilibrium with Iherzolite until shallow mantle depths (similar to 30 km). Melt porosities in enriched heterogeneities remain below 1% for perhaps 10s of km before losing chemical equilibrium with the solid during transport in the upper melting column. Because the porosities required by the observed disequilibria are small, the 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 and significantly long for Pa-231. Thus, instantaneous transport dynamic melting models appear incompatible with the observed disequilibria even when initial melt productivities as low as 0.05%/km are used. (C) 2000 Elsevier Science B.V. All rights reserved.

Keywords:

U-series disequilibria

MORB

partial melting

spinel iherzolite

dunite

A geochemically consistent hypothesis for MORB generation

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