Deformation, Annealing, Melt‐Rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

Abstract

We analyzed a set of mantle xenoliths from the Fernando de Noronha archipelago to constrain the roles of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1,000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber‐[100] tendency; these patterns imply deformation by dislocation creep with dominant activation of the [100](010) slip system. Olivine fiber‐[010] patterns are less common; they probably result from recrystallization or melt‐rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post‐deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid‐Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt‐rock interaction. An early refertilization of the base of the lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the Fernando de Noronha mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.

Textural and Compositional Changes in the Lithospheric Mantle Atop the Hawaiian Plume: Consequences for Seismic Properties

“…These processes are best recorded in those samples where recrystallization is concentrated in vein‐like domains (Figures 3g, 3h, 6a, and 6b), but peridotites with distributed recrystallization also often show a clear spatial association between pyroxene enrichment and recrystallization (Figure S1). Similar partial recrystallization of initially coarse‐porphyroclastic microstructures has been described in peridotite xenoliths from Fernando de Noronha Island in the Equatorial Atlantic (Liu et al, 2019). In the latter, decrease in Mg# and Ni content and increase in Ca content in olivine limited to a vein‐like recrystallized domain has been documented, corroborating the role of reactive melt transport in the development of the recrystallized domains.…”

“…In (a), labels indicate the samples with CPO presented in Figure 12. J and BA indexes for mantle xenoliths from Fernando de Noronha (Liu et al, 2019) and Canary Islands (Vonlanthen et al, 2006) in the Atlantic Ocean and French Polynesia in the Pacific Ocean (Tommasi et al, 2004) recalculated using with the same CPO analysis parameters as in the present study are shown for comparison in (a).…”

“…Partial annealing would occur during the cooling stage; its intensity should therefore increase with depth (longer time at high temperature). Consistently, coarse‐porphyroclastic microstructures also predominate among mantle xenoliths from French Polynesia and Ontong Java in the Pacific and from the Canaries and Fernando de Noronha in the Atlantic (Liu et al, 2019; Tommasi et al, 2004; Tommasi & Ishikawa, 2014; Vonlanthen et al, 2006).…”

“…(a) Maximum S wave polarization anisotropy versus maximum P wave propagation anisotropy. For comparison, data for mantle xenoliths from French Polynesia in the Pacific Ocean (Tommasi et al, 2004) and Fernando de Noronha in the equatorial Atlantic (Liu et al, 2019) are presented. (b) Maximum propagation anisotropy of S2 waves and epsilon ( ξ ) values, which correspond to the azimuthal anisotropy of Rayleigh waves and the polarization anisotropy of surface waves, respectively, if the flow direction and the flow plane are horizontal, as expected in a “normal” oceanic lithospheric mantle.…”

Grain‐Boundary Diffusion Creep of Olivine: 2. Solidus Effects and Consequences for the Viscosity of the Oceanic Upper Mantle

“…Using the former method, we obtained d values that are about 1.6 times larger than values given by the latter method from the same microstructures (see Figure A4 in Yabe et al, 2020). Although their size ratio is a function of the grain‐size distribution, we can reasonably consider a grain size of 1 mm in Equation 20a based on the oceanic lherzolite described by Liu et al (2019). Obviously, it is premature to conclude the predominance of a millimeter‐scale grain size in the mantle with such limited observations, leaving the issue for future studies.…”