1,721,114 research outputs found
Solid mantle upwelling rate beneath the Mid Atlantic Ridge
No full textThe upwelling velocity of the solid mantle beneath mid ocean ridges affects processes of melting and generation of the oceanic crust, and constrains models of spreading of oceanic plates. Models of passive flow require that the solid mantle rises beneath a ridge at a speed similar to the half spreading rate of the plates. However, increased buoyancy due to melt depletion and melt retention may cause the sub-ridge mantle to rise at a speed much faster than the half spreading rate. In order to constrain these models it is important to estimate the sub- ridge mantle up-welling rate. Such estimates have been attempted in a few cases by measuring in zero-age basalt disequilibria in short lived isotopes produced by the U-decay series. Melt upward migration from the sub-ridge melting zone to the crust is much faster than its parent solid mantle upward flow; therefore, a time lag is created between melt emplacement as basalt in upper lithosphere and emplacement of the parent residual peridotite in the lower lithosphere. We were able to estimate this time lag along a lithospheric sliver, generated at a 80 km long segment (EMAR) of the Mid Atlantic Ridge located between 10° N and 11°N, just south of the Vema transform. The northern edge of this lithospheric sliver exposes crustal and upper mantle units along a 320 km long spreading flowline equivalent to a 26 Myr long time interval. Comparing temporal variations of crustal thickness, inferred from the Residual Mantle Bouguer Anomaly along a flowline starting from the centre of the EMAR segment, to temporal variations of mantle degree of melting, estimated from mineral chemistry of peridotites exposed along the Vema lithospheric section, allowed us to estimate an average solid mantle rising rate of 16.5 mm/yr below the EMAR segment for a time interval from 26 to 2 Ma. This rate is slightly higher than the average half spreading rate of 15.6 mm/yr for the same period. The similarity between average up-welling rate and spreading rate at 11° N on the Mid Atlantic ridge is in line with up-welling estimates obtained by U-series disequilibria, supporting in general a mostly passive mantle flow model. However, temporal variations of spreading rate at 11° N for the last 26 Myr are decoupled from mantle degree of melting and geothermometry estimates, suggesting variations of mantle upwelling rates due to an active component
Rapid uplift of a lithospheric Sliver Near the Vema FZ (Central Atlantic) due to change in the pole of rotation
No full textThe Vema F.Z. offsets the Mid Atlantic Ridge by about 320 km at 10ø45' N. The southern flank of the transform valley is bounded by a prominent transverse ridge (VTR), reaching a minimum depth of ~500 m. Several lines of evidence indicate that the north facing wall of the VTR exposes a complete, relatively undisturbed section of uplifted oceanic lithosphere. The length of the VTR is ~300 km, similar to the length of offset of the transform. The VTR starts abruptly about 140 km from the MAR axis, in crust roughly 10 m.y. old, assuming an half spreading rate of 1.5 cm/y. This age marks the end of the uplift phase of the VTR. The time of initiation of uplift is marked probably by an abrupt change of orientation of sea floor morphostructural fabric, from parallel to the present ridge axis to ~5§ to 10§ oblique. This change of orientation took place about 12 Ma, and is related to a change in the position of the pole of rotation that caused extension across the transform and triggered flexure of a lithospheric slab and uplift of the transverse ridge. The ~4 km uplift of the VTR took place in roughly 2 m.y. with an average rate of uplift of 2 mm/y. Uplift rates appear to have been similar throughout the 300 km length of the VTR. The flexure and rapid uplift did not cause major internal deformation of the lithospheric slab
Multi-stage impregnation of the lithospheric mantle at the Andrew Bain FZ (SWIR)
No full textThe Southern ridge-transform intersection of Andrew Bain Fracture Zone (ABFZ) is interpreted as a "cold spot" in the mid-ocean ridge system being characterized by a negative thermal anomaly in the oceanic upper mantle. The negative thermal anomaly is associated to the cold-edge effect due to the great age contrast of the active ridge segments. During the oceanic expedition AB06-S23, in 2006, (organized by ISMAR-CNR, Bologna, Italy, and co-financed by PRNA, Italy) with the russian R/V N. Strakhov, several samples of abyssal peridotites have been collected. Textures and modal distribution of the samples have been investigated revealing a multistage impregnation history. Deep spinel-field impregnation assemblages (sp+cpx‚±opx‚±ol) are followed by plagioclase-field patches and mineral trails (pl+cpx‚±ol) and late shallow gabbroic pockets and veins. The major elements mineral chemistry reveals compositional trends of low-P/T subsolidus partial- to-complete re-equilibration undergone by the upper mantle during the upwelling beneath the ridge. These samples have experienced variable degrees of melting and reacted with percolating melts of possible different composition. In particular, samples showing the lowest degrees of melting have interacted with MORB-like melts and pyroxenitic-derived melts in the spinel and plagioclase stability fields. The presence of these two kinds of melts might prove the presence of enriched portions scattered in a normal depleted mantle beneath ocean ridges. MELTS-based runs provide constraints to variable extents of pyroxenitic-derived melt interaction with the mantle source and crystallization at variable depth of the products of such an interaction. Supported by MIUR-PRIN Cofin project 200
Is the Mid-Atlantic Ridge becoming hotter with time?
No full textMore than 20 million years of oceanic lithosphere accretion history at a segment of the Mid-Atlantic Ridge are recorded in the Vema Lithospheric Section (VLS), a 300 km long flexured and uplifted sliver of lithosphere exposed near the Vema Fracture Zone in the Central Atlantic. Systematic sampling of the basal mantle ultramafic unit and of crustal basalts along the VLS together with geophysical surveys gave us the opportunity to study temporal changes in the processes of generation of the oceanic lithosphere at a ridge axis. The degree of melting of the mantle upwelling below the ridge axis, estimated from the chemistry of mantle-equilibrated mineral phases in the peridotites, as well as crustal thickness, inferred from shipboard and satellite gravity data, both show ~3-4 my long oscillations superimposed on long-range steady increases with time. Based on basaltic glasses elemental and isotopic chemistry, we assume the composition of the source stayed nearly constant. The steady increase with time of mantle degree of melting and of crustal thickness suggests that the mantle rising beneath the MAR became gradually hotter during the last 20 million years, even though the spreading half rate slowed significantly during this time. We offer two explanations for the increase in mantle temperature with time at the Mid-Atlantic Ridge. A first possibility, of local significance, calls for gradual lengthening of the eastern MAR segment where the VLS was created, leading to an increasing degree of melting below center of the segment as it lengthens, due to the a decreasing influence of the "cold edge effect" from the Vema transform. The second, of broader significance, calls for a gradual increase of mantle potential temperature along a significant portion of the northern MAR during the last 20 million years, resulting in an increase of melt production despite decreasing spreading rates. This second hypothesis is supported by an increase of crustal thickness towards ridge axis observed at several other locations in the northern MAR. The chemistry of basaltic glasses, collected along the VLS above the peridotites, suggests that no deep plume source is involved in the steady heating of the Ridge
26 Million Years of mantle upwelling below a segment of the Mid Atlantic Ridge: the Vema Lithosperic Section revisited
No full textAn uplifted, relatively undeformed sliver of oceanic lithospheric (Vema Lithospheric Section or VLS), exposing a >26 Myr record of lithosphere generation at a segment of the Mid Atlantic Ridge (EMAR segment, 10°-11°N, central Atlantic), provides the ideal setting to tackle the problem of temporal variations in the processes of creation of oceanic lithosphere at a slow-spreading ridge. A first study, that combined the temporal variations of the mineral chemistry of mantle peridotites sampled along the VLS and gravity measured along a flow line starting from the center of the EMAR segment, detected a steady increase of crustal thickness and of mantle degree of melting from ~20 Ma to 4 Ma before present. Additional sampling was carried out in 2005 along the VLS, extending the coverage of mantle peridotites from a lithospheric age of 26 Ma to 2 Ma. The 26 Ma to 18.5 Ma interval shows short wavelength (~4 Myr) variations of the mantle degree of melting, superimposed on a weak trend of decreasing degree of melting. In the 18.5 to 2 Ma interval a steady increase of degree of melting was observed, with superimposed 3-4 Myr oscillations. Temporal variations of RMBA and crustal thickness inferred from gravity data reveal similar trends. The older (26 to 18.5 Ma) and the younger (18.5 to 2Ma) peridotite suites differ in clinopyroxene CaO/Al2O3 ratio, suggesting that not only the thermal regime, but also the composition of the mantle source might be different in the two suites. Peridotite two-pyroxene geothermometry shows a decreasing trend of the calculated equilibration temperature from 26 Ma to 18.5 Ma ago, followed by a steady increase from 16 Ma to 2 Ma ago, suggesting that the solid mantle upwelling speed varied through time. A ~2 Myr long interval with strongly deformed ultramafic mylonites as the dominant rock type lies between the two opposite trends, as if marking a change in mantle thermal regime. It may reflect a thermal minimum in the subridge upwelling mantle, with nearly a-magmatic emplacement of the lithosphere resulting in strong deformation of the mantle rocks. When upwelling of hot and/or fertile mantle resumes a new cycle starts with injection of igneous crust
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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