Dorsale (géologie) | EPFL Graph Search

droite|vignette|upright=2.5|Plaine abyssale et rides médio-océaniques. En géologie, une dorsale, appelée aussi crête médio-océanique ou ride médio-océanique, est une chaîne de montagnes sous-marine, que l'on rencontre dans tous les bassins océaniques. Elle se distingue de la plaine abyssale, la partie plate de la zone abyssale océanique entre et de profondeur, par des profondeurs beaucoup moins marquées, typiquement aux alentours de . Une dorsale se présente sous la forme d'un système de reliefs formant une chaîne, de part et d'autre d'un rift central marqué. Le réseau de ces dorsales est continu et s’étend sous les océans sur près de 60 à . Pour la tectonique des plaques, une dorsale océanique est une frontière de divergence entre deux plaques tectoniques. En réponse à la séparation des plaques, le manteau terrestre chaud arrive en contact avec l'océan, et forme de la nouvelle lithosphère océanique. Les reliefs de la dorsale sont formés par isostasie, conséquence de la remontée du

Weekly to monthly time scale of melt inclusion entrapment prior to eruption recorded by phosphorus distribution in olivine from mid-ocean ridges

Lukas Baumgartner, Stéphane Laurent Escrig, Mélina Manzini, Anders Meibom

Melt inclusions (MIs) hosted in euhedral olivine have been proposed to represent droplets of primary melt, protected from processes occurring near Earth's surface during eruption. The complex zoning of phosphorus (P) in some olivines and the presence of a P-depleted zone around MIs indicate a complex history for the host-MI system. We analyzed P in olivine and MIs from two mid-oceanic ridge basalt (MORB) samples from the Mid-Atlantic Ridge (MAR) by electron probe microanalyzer, secondary ion mass spectrometry (SIMS), and NanoSIMS. Phosphorus dendrites in olivine suggest an initial fast olivine growth followed by a stage of slower growth. Dissolution textures around some MIs were identified and were probably caused by adiabatic decompression melting. Based on diffusion modeling of P in olivine, we infer that olivine beneath the MAR remains in the system (1) for days to weeks after crystallization of P-rich lamellae, and (2) for a few hours after recrystallization of dissolved olivine. Dissolution and reprecipitation of olivine containing boundary layers suggests that most MIs might be affected by late post-entrapment processes.

Geological Soc Amer, Inc2017

Re-Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle

Reto Frei, Anders Meibom

The geochemical composition of the Earth's upper mantle(1-3) is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re-Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt(3). Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re-Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a 'plum pudding' distribution in the upper mantle, rather than from individual melt depletion events.

2002

The statistical upper mantle assemblage

Anders Meibom

A fundamental challenge in modern mantle geochemistry is to link geochemical data with geological and geophysical observations. Most of the early geochemical models involved a layered mantle and the concept of geochemical reservoirs. Indeed, the two layer mantle model has been implicit in almost all geochemical literature and the provenance of oceanic island basalt (0113) and mid-ocean ridge basalt (MORB) [van Keken et al., Annu. Rev. Earth Planet. Sci. 30 (2002) 493-525]. Large-scale regions in the mantle, such as the 'convective' (i.e. well-stirred, homogeneous) upper mantle, sub-continental lithosphere, and the lower mantle were treated as distinct and accessible geochemical reservoirs. Here we discuss evidence for a ubiquitous distribution of small- to moderate-scale (i.e. 10(2)-10(5) m) heterogeneity in the upper mantle, which we refer to as the statistical upper mantle assemblage (SUMA). This heterogeneity forms as the result of long-term plate tectonic recycling of sedimentary and crustal components. The SUMA model does not require a convectively homogenized MORB mantle reservoir, which has become a frequently used concept in geochemistry. Recently, Kellogg et al. [Earth Planet. Sci. Lett. 204 (2002) 183-202] modeled MORB and OIB Sr and Nd isotopic compositions as local mantle averages of random distributions of depleted residues and recycled continental crustal material. In this model, homogenization of the MORB source region is achieved by convective stirring and mixing. In contrast, in the SUMA model, the isotopic compositions of MORB and OIB are the outcome of homogenization during sampling, by partial melting and magma mixing (e.g. [Helffrich and Wood, Nature 412 (2001) 501-507]), of a distribution of small- to moderate-scale upper mantle heterogeneity, as predicted by the central limit theorem. Thus, the 'SUMA' acronym also captures what we consider the primary homogenization process: sampling upon melting and averaging. SUMA does not require the participation of distinct (e.g. lower mantle) reservoirs to explain OIB compositions. The complementary methods outlined in this work and in e.g. Kellogg et al. and Helffrich and Wood, provide a strong link between geochemical data and potential geophysical observables, such as the degree of partial melting, size of the melting regime, the characteristic length scales of the chemical heterogeneity and bathymetry, seismic scattering and anisotropy. We support the view that statistical distributions of lithologic components and sampling theory should replace the concept of distinct, isolated geochemical reservoirs, and extensive convective stirring prior to sampling. (C) 2003 Elsevier B.V. All rights reserved.

2004

The statistical upper mantle assemblage

Anders Meibom

2004