Azores mantle blob: Rare-earth evidence
TL;DR: In this paper, rare earths (RE) in basalts erupted within the rift of the Mid-Atlantic Ridge show a progressive change from light-RE enriched to depleted patterns from the Azores Platform (40°N) down to 33°30′N.
About: This article is published in Earth and Planetary Science Letters.The article was published on 1975-03-01. It has received 289 citations till now. The article focuses on the topics: Mantle (geology) & Asthenosphere.
Citations
More filters
TL;DR: In this article, the mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data.
Abstract: [1] The mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of all morb and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.
1,103 citations
01 Jan 1992
TL;DR: In this article, Niu et al. developed a method for estimating the major element compositions of mid-ocean ridge basalts (MORB) by calculating the partition coefficients for the major elements between mantle minerals and melts.
Abstract: Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al 203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km. Local variations in basalt chemistry from slowand fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts. Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical. Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements. We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched
1,011 citations
TL;DR: The mixing equation applied by Vollmer [1] to Pb and Sr isotope ratios is shown to be a general equation applicable to consideration of element and isotope ratio.
954 citations
TL;DR: In this article, a strong correlation exists between percent TiO 2 (proportional to amount of melting) and Al 2 O 3 /TiO 2, CaO/TiO2 ratios of these close to primary MORB, indicating a progressive release of Al and Ca from the mantle source.
693 citations
TL;DR: In this article, it is suggested that the observed variations are a consequence of dynamic partial melting of a homogeneous mantle source region, which may be generated by processes in the mantle during melting.
627 citations
References
More filters
01 Jan 1972
TL;DR: In this article, the authors examined more than 100 fault plane solutions for earthquakes within the Alpide belt between the Mid-Atlantic ridge and Eastern Iran and found that the deformation at present occurring is the result of small continental plates moving away from Eastern Turkey and Western Iran.
Abstract: Summary Examination of more than 100 fault plane solutions for earthquakes within the Alpide belt between the Mid-Atlantic ridge and Eastern Iran shows that the deformation at present occurring is the result of small continental plates moving away from Eastern Turkey and Western Iran. This pattern of movement avoids thickening the continental crust over much of Turkey by consuming the Eastern Mediterranean sea floor instead. The rates of relative motion of two of the small plates involved, the Aegean and the Turkish plates, are estimated, but are only within perhaps 50 per cent of the true values. These estimates are then used to reconstruct the geometry of the Mediterranean 10 million years ago. The principal difference from the present geometry is the smooth curved coast which then formed the southern coast of Yugoslavia, Greece and Turkey. This coast has since been distorted by the motion of the two small plates. Similar complications have probably been common in older mountain belts, and therefore local geological features may not have been formed by the motion between major plates. A curious feature of several of the large shocks for which fault plane solutions could be obtained for the main shock and one major aftershock was that the two often had different mechanisms.
2,378 citations
TL;DR: In this paper, the authors examined more than 100 fault plane solutions for earthquakes within the Alpide belt between the Mid-Atlantic ridge and Eastern Iran and found that the deformation at present occurring is the result of small continental plates moving away from Eastern Turkey and Western Iran.
Abstract: Summary Examination of more than 100 fault plane solutions for earthquakes within the Alpide belt between the Mid-Atlantic ridge and Eastern Iran shows that the deformation at present occurring is the result of small continental plates moving away from Eastern Turkey and Western Iran. This pattern of movement avoids thickening the continental crust over much of Turkey by consuming the Eastern Mediterranean sea floor instead. The rates of relative motion of two of the small plates involved, the Aegean and the Turkish plates, are estimated, but are only within perhaps 50 per cent of the true values. These estimates are then used to reconstruct the geometry of the Mediterranean 10 million years ago. The principal difference from the present geometry is the smooth curved coast which then formed the southern coast of Yugoslavia, Greece and Turkey. This coast has since been distorted by the motion of the two small plates. Similar complications have probably been common in older mountain belts, and therefore local geological features may not have been formed by the motion between major plates. A curious feature of several of the large shocks for which fault plane solutions could be obtained for the main shock and one major aftershock was that the two often had different mechanisms.
2,342 citations
TL;DR: The concept of crustal plate motion over mantle hotspots has been advanced to explain the origin of the Hawaiian and other island chains and the origin the Walvis, Iceland-Farroe and other aseismic ridges as discussed by the authors.
Abstract: THE concept of crustal plate motion over mantle hotspots has been advanced1 to explain the origin of the Hawaiian and other island chains and the origin of the Walvis, Iceland-Farroe and other aseismic ridges. More recently the pattern of the aseismic ridges has been used in formulating continental reconstructions2. I have shown3 that the Hawaiian-Emperor, Tuamotu-Line and Austral-Gilbert-Marshall island chains can be generated by the motion of a rigid Pacific plate rotating over three fixed hotspots. The motion deduced for the Pacific plate agrees with the palaeomagnetic studies of seamounts4. It has also been found that the relative plate motions deduced from fault strikes and spreading rates agree with the concept of rigid plates moving over fixed hotspots. Fig. 1 shows the absolute motion of the plates over the mantle, a synthesis which satisfies the relative motion data and quite accurately predicts the trends of the island chains and aseismic ridges away from hotspots.
2,277 citations
1,797 citations
TL;DR: In this article, a comprehensive study of the observations of seismology provides widely based strong support for the new global tectonics which is founded on the hypotheses of continental drift, sea-floor spreading, transform faults and underthrusting of the lithosphere at island arcs.
Abstract: A comprehensive study of the observations of seismology provides widely based strong support for the new global tectonics which is founded on the hypotheses of continental drift, sea-floor spreading, transform faults, and underthrusting of the lithosphere at island arcs. Although further developments will be required to explain certain part of the seismological data, at present within the entire field of seismology there appear to be no serious obstacles to the new tectonics. Seismic phenomena are generally explained as the result of interactions and other processes at or near the edges of a few large mobile plates of lithosphere that spread apart at the ocean ridges where new surficial materials arise, slide past one another along the large strike-slip faults, and converge at the island arcs and arc-like structures where surficial materials descend. Study of world seismicity shows that most earthquakes are confined to narrow continuous belts that bound large stable areas. In the zones of divergence and strike-slip motion, the activity is moderate and shallow and consistent with the transform fault hypothesis; in the zones of convergence, activity is normally at shallow depths and includes intermediate and deep shocks that grossly define the present configuration of the down-going slabs of lithosphere. Seismic data on focal mechanisms give the relative direction of motion of adjoining plates of lithosphere throughout the active belts. The focal mechanisms of about a hundred widely distributed shocks give relative motions that agree remarkably well with Le Pichon's simplified model in which relative motions of six large, rigid blocks of lithosphere covering the entire earth were determined from magnetic and topographic data associated with the zones of divergence. In the zones of convergence the seismic data provide the only geophysical information on such movements.
Two principal types of mechanisms are found for shallow earthquakes in island arcs: The extremely active zone of seismicity under the inner margin of the ocean trench is characterized by a predominance of thrust faulting, which is interpreted as the relative motion of two converging plates of lithosphere; a less active zone in the trench and on the outer wall of the trench is characterized by normal faulting and is thought to be a surficial manifestation of the abrupt bending of the down-going slab of lithosphere. Graben-like structures along the outer walls of trenches may provide a mechanism for including and transporting sediments to depth in quantities that may be very significant petrologically. Large volumes of sediments beneath the inner slopes of many trenches may correspond, at least in part, to sediments scraped from the crust and deformed in the thrusting.
Simple underthrusting typical of the main zone of shallow earthquakes in island arcs does not, in general, persist at great depth. The most striking regularity in the mechanisms of intermediate and deep earthquakes in several arcs is the tendency of the compressional axis to parallel the local dip of the seismic zone. These events appear to reflect stresses in the relatively strong slab of down-going lithosphere, whereas shearing deformations parallel to the motion of the slab are presumably accommodated by flow or creep in the adjoining ductile parts of the mantle. Several different methods yield average rates of underthrusting as high as 5 to 15 cm/yr for some of the more active arcs. These rates suggest that temperatures low enough to permit dehydration of hydrous minerals and hence shear fracture may persist even to depths of 700 km. The thickness of the seismic zone in a part of the Tonga arc where very precise hypocentral locations are available is less than about 20 km for a wide range of depths. Lateral variations in thickness of the lithosphere seem to occur, and in some areas the lithosphere may not include a significant thickness of the uppermost mantle.
The lengths of the deep seismic zones appear to be a measure of the amount of under thrusting during about the last 10 m.y. Hence, these lengths constitute another ‘yardstick’ for investigations of global tectonics. The presence of volcanism, the generation of many tsunamis (seismic sea waves), and the frequency of occurrence of large earthquakes also seem to be related to underthrusting or rates of underthrusting in island arcs. Many island arcs exhibit a secondary maximum in activity which varies considerably in depth among the various arcs. These depths appear, however, to correlate with the rate of underthrusting, and the deep maxima appear to be located near the leading (bottom) part of the down-going slab. In some cases the down-going plates appear to be contorted, possibly because they are encountering a more resistant layer in the mantle. The interaction of plates of lithosphere appears to be more complex when all the plates involved are continents or pieces of continents than when at least one plate is an oceanic plate. The new global tectonics suggests new approaches to a variety of topics in seismology including earthquake prediction, the detection and accurate location of seismic events, and the general problem of earth structure.
1,335 citations