Mid-ocean ridge

About: Mid-ocean ridge is a research topic. Over the lifetime, 4190 publications have been published within this topic receiving 262361 citations. The topic is also known as: mid-ocean ridges & ridge.

Oceanic crustal thickness from seismic measurements and rare earth element inversions

Abstract: Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot-spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow-spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow-spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

The Amount of Recycled Crust in Sources of Mantle-Derived Melts

Abstract: Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs

Abstract: Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics Their differences are evaluated within the framework of global tectonics and mantle differentiation Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (morb) leads They form linear trends on lead isotopic ratio plots Many of the trends extend toward the field of morb On plots of 207 P b / 204 Pb against 206 Pb / 204 Pb, their slopes are generally close to 01 Island arc leads in general are confined between sediment and morb type leads with slopes of ca 030 on a plot of 207 P b / 204 Pb against 206 Pb / 204 Pb Pb, Sr and Nd isotopic data of Hawaiian volcanics are closely examined Data from each island support a two-component mixing model However, there is a lack of full range correlation between islands, indicating heterogeneity in the end members This mixing model could also be extended to explain data from the Iceland-Reykjanes ridge, and from 45° N on the Atlantic Ridge The observed chemical and isotopic heterogeneity in young volcanic rocks is considered to be a result of long-term as well as short-term mantle differentiation and mixing Lead isotopic data from ocean islands are interpreted in terms of mantle evolution models that involve long-term (more than 2 Ga) mantle chemical and isotopic heterogeneity Incompatible element enriched ‘plume’-type morb have Th/U ratios ca 30 too low and Rb/Sr ratios ca 004 too high to generate the observed 208 Pb and 87 Sr respectively for long periods of time Elemental fractionation in the mantle must have occurred very recently This conclusion also applies to mantle sources for ocean island alkali basalts and nephelinites Depletion of incompatible elements in morb sources is most probably due to continuous extraction of silicate melt and/or fluid phase from the low-velocity zone throughout geological time Data on Pb isotopes, Sr isotopes and trace elements on volcanic rocks from island arcs are evaluated in terms of mixing models involving three components derived from (1) sub-arc mantle wedge, (2) dehydration or partial melting of subducted ocean crust, and (3) continental crust contamination In contrast to the relation between 87 Sr/ 86 Sr and 143 Nd / 144 Nd ratios of ocean volcanics, there is a general lack of correlation between Pb and Sr isotopic ratios except that samples with very radiogenic Pb ( 206 Pb / 204 Pb > 195) have low 87 Sr/ 87 Sr ratios (07028- 07035) These samples also have inferred source Th/U ratios (30-35) not high enough to support long-term growth of 208 Pb Data suggest that their mantle sources have long-term integrated depletion in Rb, Th, U and light ree High 238 U / 204 Pb (y a)values required by the Pb isotopic data are most probably due to depletion of Pb by separation of a sulphide phase Relations between Pb, Sr and Nd isotopic ratios of young volcanic rocks could be explained by simultaneous upward migration of silicate and/or fluid phase and downward migration of a sulphide phase in a differentiating mantleration of a sulphide phase in a differentiating mantle

The mean composition of ocean ridge basalts

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.