Two nodular websterite inclusions (3-4 cm in diameter) were identified in the predominantly harzburgitic core in Hole 920D. These inclusions and a representative harzburgite from the same drill hole have been analyzed for their Sr-, Nd-, and Pb-isotope ratios along with major-element mineral compositions to better understand their origin and significance. Elevated Sr- and Pb-isotope ratios provide abundant evidence for seawater interaction; 208Pb/204Pb ratios are higher for a given 206Pb/204Pb than those of normal (i.e., away from plume/hotspot influences) mid-ocean ridge basalt (N-MORB) from the North Atlantic and are consistent with a model for alteration involving a component of pelagic sediment. In contrast, Nd isotopes are largely unaffected by alteration processes. The pyroxenes separated from the Site 920 peridotites have higher 143Nd/144Nd ratios than most North Atlantic N-MORB, and higher than any measured abyssal peridotite to date. Nodular websterites have slightly higher Nd-isotope ratios than the analyzed harzburgite. Both websterites and harzburgite are isotopically distinct from all magmatic rocks in the Mid-Atlantic Ridge south of the Kane Fracture Zone (MARK) area, including gabbros, diabase dikes, and recent basalts. Thus, in terms of their isotopic compositions, the analyzed peridotites are not representative of the mantle source of the MARK magmatic rocks. High aluminum, iron, and titanium contents in the websterite pyroxenes suggest a vein origin, possibly as a result of decompression melting in an upwelling diapir. These veins preceded the more numerous clinopyroxenite veinlets and gabbroic dikes observed throughout the Site 920 core and probably formed in the asthenosphere. These inclusions may thus be analogous to Type I/Type I composite relationships found in alpine peridotites and alkali-basalt hosted xenolith suites. They provide evidence that interaction between basaltic melts and peridotite residues is a common and varied occurrence in the genesis of oceanic lithosphere.
Date of initial receipt: 1 August 1995
Date of acceptance: 22 March 1996
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