Drilling on the western wall of the Mid-Atlantic Ridge south of the Kane Fracture Zone at MARK (23°20'N and 23°30'N) recovered plutonic rocks <1 Ma in age. A selection of these rocks has been analyzed for their Sr-, Nd-, Pb-, and O-isotope compositions. O-isotope analyses of gabbro mineral separates have near primary magmatic values. Delta18Ocpx vs. delta18Oplag systematics support only limited fluid-rock interaction, predominantly at moderate to low temperatures (200°-300°C). Combined Sr- and O-isotope data indicate that, even where fluids have found ingress into the gabbroic crust, they have had limited compositional consequences, presumably because seawater was rapidly modified by interaction with the overlying basalt crust. Pb- and Sr-isotope compositions of leachates further demonstrate that, for most samples, seawater was the principal component in hydrothermal fluids; in a few cases, however, sediments were also involved.
Sr-, Nd-, and Pb-isotope systematics of leached gabbro mineral separates indicate that, to a first-order approximation, the sub-MARK mantle is isotopically homogeneous from near the middle of the spreading cell (Sites 921-924) to its boundary in the south (Site 920). Trace-element concentrations of Sr, Nd, and Pb (and, by implication, other incompatible trace elements) must also be homogeneous. Thus, variations in trace-element concentrations in gabbros and calculated parental magmas must be the product of the melting regime and subsequent fractionation processes in the magma chamber(s) and not variations in the mantle source composition.
Superimposed on this homogeneity is smaller scale, within-sample, isotope disequilibrium between coexisting plagioclase and clinopyroxene. Such heterogeneities provide evidence of complex magma chamber processes and are most likely to be preserved at the beginning of magmatic cycles. Unusual isotopically enriched compositions are found in the MARK area as diabase and amphibolitized microgabbros that crosscut the peridotite section. Restriction of these distinctive compositions in the peridotite sections at Site 920 may result from the lower magma supply rate expected near the boundary of a spreading cell.
The average isotopic composition within the MARK area has changed over a period of <1 m.y., becoming more radiogenic in its Pb-isotope composition but less radiogenic in its Nd composition, implying that the source is currently less depleted than that available 750,000 yr earlier.
Date of initial receipt: 3 August 1995
Date of acceptance: 14 December 1995
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