Oceanic Lithosphere Created at a Segment of the Mid Atlantic Ridge: A 20 Million Year History

点击翻译按钮获取中文摘要

Under this funding, the PIs will study samples from the Vema lithospheric section, a flexured, uplifted sliver of oceanic lithosphere, exposed for over 300 km along a flow line near the Vema fracture zone in the central Atlantic, and representing a time interval of 20 my of accretion at a ridge axis. This effort builds on the finding that the degree of melting undergone by the upwelling mantle has changed significantly through time, with short-wavelength (~3- 4 my) oscillations superimposed on a long range (~ 20 my) steady increase, correlated with crustal thickness variations. The PIs will: (a) Determine the major and trace elements of mantle equilibrated phases orthopyroxene, clinopyroxene and spinel from peridotites to refine the pattern of variations of degree of melting with time. (b) Determine the major and trace elements composition of a set of basaltic glasses and non-glassy basalts from the upper crustal unit of the VLS to obtain an independent estimate of variations of extent of melting. (c) Complete the study of Sr-Nd-Pb isotopic composition of cpx from the peridotites and of the basaltic glasses, in order to confirm a possible relationship between degree of melting and isotopic composition and to help solve the issue of the genetic link between mantle peridotites and crustal basalts, given that preliminary data show the basalts to be isotopically more "enriched" than the peridotites. (d) Estimate the upwelling velocity of the mantle beneath the Ridge will by comparing the mantle peridotites degree of melting curve with that derived from the basalts or with a crustal thickness curve inferred from gravity. Knowledge of mantle upwelling velocity relative to spreading half-rate is critical to discriminate between "passive" models of creation of lithosphere, and models where buoyant forces play a role.