Date of Award

2001

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Jean-Guy Schilling

Abstract

Using high-precision MC-ICP-MS, 176Hf/ 177Hfwas measured in 28 MORB as part of a global survey. The goal of the survey is to establish the full range EHf in EMO RB and NM ORB from sections · of the mid-ocean ridges which are within or immediately adjacent to areas of mantle plume influence. In addition, 176Hf/177Hfwas measured in 64 MORB from the Mid-Atlantic Ridge between 40°S and 55°S and in l basalt from the Discovery Tablemount in order to study mantle mixing processes beneath the southern South Atlantic Ocean.

EHf ranges from -2.8 to + 11.5 in EMORB included in the global survey and from + 12.6 to 24.2 in NM ORB from the global survey. This range of EHf in MORB completely overlaps the range that is observed in OIB. Most of the basalts from the global survey fall on the well-established EHf-ENd mantle array. Only the depleted basalt from the Mohns Ridge falls above this array to higher EHf (+24.2) for a given ENd (9.1). EHf versus 206Pb/204Pb and EHf versus 87Sr/86Sr of basalts from the MidAtlantic Ridge (MAR) confirm the existence of the "IDMU province" from 24°S - 34°N which is likely due to the pollution of the upper mantle by plume-head restites from a family ofIDMU-type plumes. In addition to this broad upper mantle pollution roughly centered beneath the equator, there is a longwavelength gradient in EHf along the MAR from + 24.2 in the Arctic down to + 14. 7 in the southern South Atlantic. This EHf gradient, which correlates positively with ENd and negatively with 207Pb/204Pb and delta 207Pb/204 Pb, may be due to a lower average, time-weighted mean degree of melting, , of the Atlantic upper mantle towards the south. It is also possible that the observed isotope gradients are due to a gradient in the onset of melting, T, in the Atlantic upper mantle. This model requires that T decreases to the south. Alternatively, the gradients may be due to pollution of the upper mantle by a Dupal-type component. In this case, the concentration of pollutant must increase to the south.

EHf measured in basalts from the southern South Atlantic (40°S to 55°S) range from -2.8 to + 16.6. Large-scale pollution of the mantle beneath the southern South Atlantic Ocean can be modeled by a three component mixture of "normal" upper mantle (DM) with recycled ancient oceanic crust (OC) and recycled ancient pelagic sediment (SED). This pollution model requires that less than 5% recycled oceanic crust and less than 1% recycled pelagic sediment mix with normal depleted upper mantle to produce ambient upper mantle (ADM). In addition to this large-scale pollution, the South Atlantic is a region oflocalized plume-ridge interactions. The 176Hf/177Hffrom this study confirms the interaction of the Discovery and Shona plumes with the MAR as has been previously proposed based on the Pb, Sr and Nd isotope ratios in these basalts. 176Hf/177Hfversus 206Pb/204Pb for basalts from the Shona anomaly indicate that a heterogeneous plume is mixing with a homogeneous ambient upper mantle. Tue Shona anomaly basalts are successfully modeled as three component mixtures of ambient upper mantle (ADM) with a recycled component consisting of oceanic crust and pelagic sediment. The proportion of sediment in the recycled component mixing with ambient upper mantle in the basalts increases from 0.7% to l. 1 % to 1.6% indicating that the plume is not well-mixed. While pelagic sediment is probably not a component in the Discovery plume, delaminated subcontinental lithosphere or lower crust may be.

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