Re-fertilization of abyssal peridotites by ultra-depleted melts at the Doldrums Fracture Zone transform system (7-8ºN, Mid Atlantic Ridge).

Abstract:

Abyssal peridotites from mid-ocean ridges are commonly thought to represent residues of adiabatic decompression melting occurred at the ridge axis. However, most of these rocks are collected at the walls of transform valleys, which expose uplifted sections of oceanic lithosphere with a long-term tectono-magmatic history. The Doldrums Fracture Zone (DFZ), at 7-8 °N in the Equatorial region of the Mid Atlantic Ridge (MAR), represents a tangible laboratory for the study of the evolution of the abyssal mantle, here exposed as a result of the tectonic forces active in a large transform system (630 km length and 100 km wide). The peculiarity of the DFZ is the presence of four intra-transform spreading centers (ITRs) that are bounded by five transform faults. An extensive sampling of the north-western part was conducted by two R/V A. N. Strakhov expeditions (S06 and S09) in the 80’. Residual harzburgites were mainly sampled along the northern Doldrums transform valley, whereas plagioclase-bearing peridotites characterize the ITR domain. Observation on major and trace elements compositions allow to defining three distinct families of peridotites: : I) residual mantle peridotites, having a variably depleted geochemical signature such as locally high Cr#[Cr/(Cr+Al) mol%] and low TiO² content in spinel, and clinopyroxene variously depleted in LREE; II) ‘plagioclase impregnated’ peridotites; showing variable Cr# but high TiO² content in spinel, high REE in Cpx but strong LREE depletion and strong Eu and Sr anomalies; III) ‘refertilized’ peridotites, differing from the latter for the lower TiO² content in spinel, and for having clinopyroxene with nearly flat REE patterns but very low M-REE contents. Geochemical models are used demonstrate that residual peridotites underwent up to 12% of partial melting, while the rocks with evidence of melt-rock interactions were modified by the percolation of melts produced by an ultra-depleted source. On this basis, we infer that the complex multi-stage evolution of the Doldrums FZ allowed the residual mantle to melt for a second time, after being transposed from the ridge axis towards the ITR domain. This process produced an ultra-depleted melt, which is here considered the main agent of the re-fertilization process.