| Summary: | Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1990 The mantle melting process is fundamental to basalt genesis and crustal accretion at mid-ocean ridges. It is believed that melts ascend more rapidly than the surrounding mantle, implying a process similar to fractional melting may be occurring, but geochemical evidence for this has been lacking. Furthermore, crustal accretion is thought to be episodic at slow spreading ridges, but sampling programs that can test this temporal variability are virtually nonexistent. This dissertation examines the trace element compositions of abyssal peridotites and discusses how they preserve details of the melting process that are not recognizable in mid-ocean ridge basalts. The results support fractional melting as the dominant melting process in the sub-ridge upper mantle. Evidence is also presented supporting non-steady state mantle melting at the Atlantis II Fracture Zone cutting the very slow spreading Southwest Indian Ridge. Trace element compositions of peridotite clinopyroxenes from fracture zones along the American-Antarctic and Southwest Indian Ridges vary as a function of proximity to hotspots. The results presented in Chapter 2 are consistent with higher degrees of melting and greater incompatible element depletion in the upper mantle near hotspots. All peridotites studied are consistent with being residues of fractional melting and inconsistent with batch melting. Some samples recovered near hotspots appear to have begun melting in the garnet stability field, deeper than samples recovered away from hotspots. Most samples show pronounced negative Zr and Ti anomalies, which increase with increasing incompatible element depletion (increased melting), on extended rare earth (spider) diagrams. The results of Chapter 2 indicated the importance of accurately knowing trace element partition coefficients between clinopyroxene and ...
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