Isotopic and trace element variations along the southern Mid -Atlantic Ridge (40°--50°S): An evaluation of regional plume -ridge interaction and large scale mantle heterogeneities

Jill Douglass, University of Rhode Island

Abstract

We report on Pb, Sr, and Nd isotope and parent-daughter (P/D) ratios and trace element concentrations in 66 basalts dredged along the southern Mid-Atlantic Ridge (MAR) from 40° to 55°S. Latitudinal variation of the isotope, parent-daughter, and trace element ratios reveal two along ridge distinct geochemical anomalies which are coupled with gravity and bathymetric highs and crest-like rather than rifted ridge-axis morphology. The Discovery anomaly (45–49°S), located opposite the Discovery hotspot track and the off-ridge Discovery mantle plume, is irregular and shows large local variation. The Shona anomaly (50–52.5°S) is smoother and shows less local variation. A large residual mantle bouguer anomaly is centered on the MAR at 51°S suggesting that the influential Shona mantle plume may be located on or very close to the ridge. ^ Basalts from the Discovery region are characterized by the high 87Sr/86Sr, low 143Nd/144Nd, intermediate 208Pb/204Pb, and high Ba/Nb ratios of enriched mantle (EM) plumes. In contrast, basalts from the Shona region are characterized by the high 206Pb/204Pb, high 143Nd/144Nd, low 87Sr/86Sr, and low Ba/Nb ratios of high μ (HIMU) ocean island basalts (OIBs). In isotope-isotope space, these southern MAR basalts form three distinct linear mixing arrays. The Shona mid-ocean ridge basalts (MORBs) extend from a depleted mantle region towards a relatively HIMU composition similar to Bouvet. The Discovery MORBs between 45–48.5°S extend from a depleted mantle region towards the EM type composition of an OIB from Discovery Tablemount. These two mixing arrays strongly suggest plume ridge interaction between the off-ridge Discovery mantle plume, the ridge-centered Shona mantle plume, and the southern ambient MAR mantle. The Discovery basalts between 48.5–49°S form a separate third mixing array which is offset from the main Discovery mixing array by low 206Pb/204Pb ratios. This third array is thought to be the result of three-component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low-μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet (i.e. the “C” component) and the LOW component, which progressively increases southward. (Abstract shortened by UMI.) ^

Subject Area

Geochemistry

Recommended Citation

Jill Douglass, "Isotopic and trace element variations along the southern Mid -Atlantic Ridge (40°--50°S): An evaluation of regional plume -ridge interaction and large scale mantle heterogeneities" (2000). Dissertations and Master's Theses (Campus Access). Paper AAI9999783.
http://digitalcommons.uri.edu/dissertations/AAI9999783

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