Hydration of an active shear zone: interactions between deformation, metasomatism and magmatism—the spinel-lherzolites from the Montferrier (southern France) Oligocene basalts

Geochemical and textural investigations have been simultaneously performed on spinel-lherzolite xenoliths from the Oligo-Miocene alkali basalts of Montferrier (southern France).All the investigated samples have undergone a deformation very particular by intense shearing under high stresses (up to 1.75 kbar), low temperatures ( 900°C) and strain rates of about 10⁻¹⁸ to 10⁻¹⁵ s⁻¹.Mineral chemistry reveals that the Montferrier lherzolites are fragments of an undepleted relatively shallow upper mantle level located at a depth of 50 km (15 kbar). Moreover, Na and Ti enrichment in diopside would reflect a metasomatic event, also emphasized by the common occurrence of pargasite in 50–70% of the investigated samples.Crystallization of this amphibole is attributed to a hydrous infiltration which is related in time and space to the deformation. Indeed, amphibole is preferentially concentrated in strongly deformed zones and in kink-band boundaries of orthopyroxene porphyroclasts. Moreover, the grain boundaries were used by the pervasive agent to percolate into the lherzolite: significant chemical variations (increase in MgO: 15% and decrease in Al₂O₃: 55%) are observed within the range of 7–5 μm adjacent to the grain boundary.Finally, Sr isotopic data (⁸⁷Sr/⁸⁶Sr) demonstrate that the amphibole, i.e. the metasomatic agent, is genetically related to the host lava of the xenoliths. Thus, the hydrous silicate liquid from which the amphibole has crystallized may be an early percolation of the ascending alkali magma.This silicate liquid hydrated the shear zone, located at a depth of 50 km, induced the hydraulic fracturation of the lherzolite and the magmatic conduit opening. Subsequently, the alkali magma sampled some fragments of this strongly deformed and metasomatized undepleted upper mantle level and brought them to the surface.     

Investigation of upper mantle structure under young regions of the southeast Pacific using long-period Rayleigh waves

Long-period Rayleigh wave group velocity dispersion curves are presented for paths across the east Pacific Ocean. The records are from the IPG site at Pamatai (Tahiti) and the IDA station at Nana. For the first time, direct-path observations of group velocities up to 300 s have been obtained. This study shows that for young oceanic regions group velocities are low even at long periods. The observations are interpreted in terms of an S-wave velocity model by a generalized inversion scheme. In the models for young ages, the low-velocity zone under the asthenosphere lid is well developed, with a strong velocity gradient at the bottom of this zone, followed at larger depths by a plateau representing a lower-velocity zone, and a marked gradient at 400 km.     

Differentiation processes in FeO‐rich asteroids revealed by the achondrite Lewis Cliff 88763

Olivine‐dominated (70–80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO‐rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Δ¹⁷O value of ?1.19 ± 0.10‰, and low bulk‐rock Mg/(Mg+Fe) (0.39), similar to the FeO‐rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk‐rock major‐, minor‐, and trace‐element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and ¹⁸⁷Os/¹⁸⁸Os (0.1262), implies a FeO‐ and volatile‐rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe‐Ni‐S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal‐rich partial melts from FeO‐rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to <<5%). As such, LEW 88763 represents the least‐modified FeO‐rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe,Ni‐FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO‐rich meteorites, such as brachinites, brachinite‐like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.     

Volume and surface sources due to laterally varying boundaries of elastic homogeneous layers

Summary. Various methods have been applied to solve propagation problems in media composed of homogeneous layers, the boundaries of which have slight lateral variations. In this paper, volume sources, introduced in a previous paper to describe the effects of lateral irregularities, are replaced by an equivalent body force and by surface sources concentrated on the irregular boundaries. The surface sources we find are similar to those obtained by Kennett for a two-dimensional structure and we generalize his results to a three-dimensional structure and to the case when an external body force is present.