OPEN SYSTEM MELTING AND TEMPORAL AND SPATIAL VARIATION OF PERIDOTITE AND BASALT AT THE ATLANTIS-II FRACTURE-ZONE

In situ ion microprobe analysis of trace and rare earth elements in discrete diopsides in abyssal peridotites from nine transform dredge hauls from the Atlantis II Fracture Zone (AII FZ) shows that these samples have a wide range of trace element contents close to the total range found for the entire Southwest Indian Ridge. Though the spread in analyses is large, the average composition of the peridotites is close to that reported for the AII FZ by Johnson et al. (1990) and lies at the relatively undepleted end of the spectrum for SW Indian Ridge residual mantle peridotites. A sharp break in peridotite diopside composition and modal mineralogy occurs across the transform, suggesting that it acts as a boundary for different melting regimes and initial mantle compositions. The difference in peridotite compositions is mirrored in spatially associated basalts, which lie on separate parallel liquidus trends in the normative ternary pyroxene-olivine-plagioclase. Basalts from the east side of the transform have higher normative plagioclase contents, indicating that they may be products of lower degrees of mantle melting than basalts from the western side, consistent with greater depletion of peridotites from the western wall or a more depleted initial composition. Basalts from the eastern wall also have consistently lower Fe8.0 and higher Na8.0 than basalts from the western wall and lie parallel to the global along-ridge Fe8.0 - Na8.0 trend (Klein and Langmuir, 1987) and orthogonal to the local melting paths of Klein and Langmuir (1989). Our data provide strong evidence for segmentation of the melting regime, with major mantle discontinuities occurring at transform offsets at slow spreading ridges. Peridotites analyzed along the eastern wall of the fracture zone also show a systematic change in composition with latitude and, with the older peridotites from the median tectonic ridge, define a systematic change in the degree of melting of the mantle occurring beneath the paleoridge axis over the last 11 m.y. Emplacement of mantle showing the lowest degree of melting, or the least depleted parental mantle composition, corresponds roughly to the time of crystallization of Ocean Drilling Program site 735B gabbros. Melting is modeled as a non-steady state, discontinuous process with 0.1-0.5 vol % aggregated melt retained in the porous residue (open system melting). The range in degree of open system melting for the combined suite of AII FZ peridotites is 8-20%. Such a large systematic variation would appear to require a dynamically significant change with time, either in the initial temperature and/or a large compositional difference of the mantle beneath the paleoridge axis. This in tum suggests that in the relative reference frame of the ridge axis, mantle flow was non-steady state. This could reflect episodic mantle diapirism beneath the ridge axis or, alternatively, that the ridge axis has moved over a zone of enhanced upflow in the underlying mantle that was fixed in the absolute hotspot mantle reference frame.