Mantle melting variation and refertilization beneath the Dragon Bone amagmatic segment (53°E SWIR): Major and trace element compositions of peridotites at ridge flanks

Abstract

The evolution of the mantle melting regime and the process of refertilization beneath cold lithosphere remain ambiguous at ultraslow spreading ridges. Few previous studies were concerned with the temporal variations of mantle melting indicated by peridotites on the flank of a single ridge segment. Here, we present in-situ major and trace element analyses of harzburgites from the Dragon Bone ridge segment both from near the spreading axis and from rift mountains up to tens of kilometers from the ridge axis. We propose that the Dragon Bone mantle had been depleted anciently at higher pressures, prior to the recent ascent under the ridge at lower pressures, and only a limited volume of melt was generated during recent melting beneath the ridge. This interpretation is consistent with the high degree of depletion of the Dragon Bone harzburgites with little trapped melt, and thin discontinuous igneous crust on the seafloor. Comparison of peridotites from near-ridge locations with those further out on the flanks indicates that the melting regime has progressively shrunk over time creating at present a nearly amagmatic segment. Peridotites from segment ends are seldom affected by late-stage refertilization, while those from the segment center are highly depleted mantle residues (resembling those from fast spreading ridges) that sustained syn-melting metasomatism.