Origin of geochemical heterogeneity in mid-ocean ridge upper mantle, ODP Leg 209

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Ocean Drilling Program (ODP) Leg 209 expects drilling six to seven, 100 m scale cores of mantle peridotite along the Mid Atlantic Ridge (MAR) north and south of the 15°20'N (Capo Verde) Fracture Zone. Global scale dredging has demonstrated the influence of hot spots and spreading rate on mantle peridotite chemistry. The importance of mantle anomalies away from such hot spots is just beginning to be fully realized. The extent of melt rock reaction as associated with melt transport remains, however, fiercely debated. Diverse models, in part stimulated by the work of Leg 209 co-chief Peter Kelemen, range from fractional melting linked with isolated (channeled) melt transport to global porous flow, the latter implying that all peridotites have been affected by melt-rock reaction. Variations in the abundance of pyroxenites, dunites, shear zones and modal phase variation in peridotite hosts can be well determined in drill core and are the prime candidates to indicate variable extents of melt-rock reaction. We propose to select two suitable sections of core (length scale 20-30 m) showing variations in lithology and strain and request their detailed sampling. By obtaining a complete geochemical data set on these samples, we expect to better constrain the extent of, and the reactions governing, melt rock reaction and porous flow and to constrain what processes control the typical "noise" or background variation in a section of in-situ oceanic mantle. Importantly, the suggested scale of investigation bridges a current gap in the investigations of oceanic peridotite and the tectonic situation of the chosen site is, in contrast to any ophiolitic section, well known.