Hafnium Isotope Ratio in Mid-Ocean Ridge Basalts: a Global Survey and a Focus on the Southern Mid-Atlantic Ridge (40°S - 55°S)

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