scholarly journals Numerical model of crustal accretion and cooling rates of fast-spreading mid-ocean ridges

2013 ◽  
Vol 6 (5) ◽  
pp. 1659-1672
Author(s):  
P. Machetel ◽  
C. J. Garrido
Keyword(s):  
Numerical Model ◽  
Dynamic Properties ◽  
Ridge Axis ◽  
Crustal Accretion ◽  
Cooling Rates ◽  
Temperature Range ◽  
Mid Ocean Ridge ◽  
Fast Spreading ◽  
Thermal Histories ◽  
Ocean Ridge

Abstract. We designed a thermo-mechanical numerical model for fast-spreading mid-ocean ridge with variable viscosity, hydrothermal cooling, latent heat release, sheeted dyke layer, and variable melt intrusion possibilities. The model allows for modulating several accretion possibilities such as the "gabbro glacier" (G), the "sheeted sills" (S) or the "mixed shallow and MTZ lenses" (M). These three crustal accretion modes have been explored assuming viscosity contrasts of 2 to 3 orders of magnitude between strong and weak phases and various hydrothermal cooling conditions depending on the cracking temperatures value. Mass conservation (stream-function), momentum (vorticity) and temperature equations are solved in 2-D cartesian geometry using 2-D, alternate direction, implicit and semi-implicit finite-difference scheme. In a first step, an Eulerian approach is used solving iteratively the motion and temperature equations until reaching steady states. With this procedure, the temperature patterns and motions that are obtained for the various crustal intrusion modes and hydrothermal cooling hypotheses display significant differences near the mid-ocean ridge axis. In a second step, a Lagrangian approach is used, recording the thermal histories and cooling rates of tracers travelling from the ridge axis to their final emplacements in the crust far from the mid-ocean ridge axis. The results show that the tracer's thermal histories are depending on the temperature patterns and the crustal accretion modes near the mid-ocean ridge axis. The instantaneous cooling rates obtained from these thermal histories betray these discrepancies and might therefore be used to characterize the crustal accretion mode at the ridge axis. These deciphering effects are even more pronounced if we consider the average cooling rates occurring over a prescribed temperature range. Two situations were tested at 1275–1125 °C and 1050–850 °C. The first temperature range covers mainly the crystallization range that is characteristic of the high temperature areas in the model (i.e. the near-mid-oceanic-ridge axis). The second temperature range corresponds to areas in the model where the motion is mainly laminar and the vertical temperature profiles are closer to conductive. Thus, this situation results in less discriminating efficiency among the crustal accretion modes since the thermal and dynamic properties that are described are common to all the crustal accretion modes far from the ridge axis. The results show that numerical modeling of thermo-mechanical properties of the lower crusts may bring useful information to characterize the ridge accretion structure, hydrothermal cooling and thermal state at the fast-spreading ridges and may open discussions with petrological cooling rate results.

scholarly journals A numerical model of hydrothermal cooling and crustal accretion at a fast spreading mid-ocean ridge

2003 ◽  
Vol 4 (9) ◽  
pp. n/a-n/a ◽  
Author(s):  
Abdellah S. M. Cherkaoui ◽  
William S. D. Wilcock ◽  
Robert A. Dunn ◽  
Douglas R. Toomey
Keyword(s):  
Numerical Model ◽  
Crustal Accretion ◽  
Mid Ocean Ridge ◽  
Fast Spreading ◽  
Ocean Ridge

scholarly journals Evolutionary ecology during the rise of dioxygen in the Earth's atmosphere

2008 ◽  
Vol 363 (1504) ◽  
pp. 2651-2664 ◽  
Author(s):  
Norman H Sleep ◽  
Dennis K Bird
Keyword(s):  
Ferrous Iron ◽  
Evolutionary Ecology ◽  
Granitic Rocks ◽  
Oxygenic Photosynthesis ◽  
Ridge Axis ◽  
Anoxygenic Photosynthesis ◽  
Arc Volcanism ◽  
Oxidized Iron ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Pre-photosynthetic niches were meagre with a productivity of much less than 10 −4 of modern photosynthesis. Serpentinization, arc volcanism and ridge-axis volcanism reliably provided H 2 . Methanogens and acetogens reacted CO 2 with H 2 to obtain energy and make organic matter. These skills pre-adapted a bacterium for anoxygenic photosynthesis, probably starting with H 2 in lieu of an oxygen ‘acceptor’. Use of ferrous iron and sulphide followed as abundant oxygen acceptors, allowing productivity to approach modern levels. The ‘photobacterium’ proliferated rooting much of the bacterial tree. Land photosynthetic microbes faced a dearth of oxygen acceptors and nutrients. A consortium of photosynthetic and soil bacteria aided weathering and access to ferrous iron. Biologically enhanced weathering led to the formation of shales and, ultimately, to granitic rocks. Already oxidized iron-poor sedimentary rocks and low-iron granites provided scant oxygen acceptors, as did freshwater in their drainages. Cyanobacteria evolved dioxygen production that relieved them of these vicissitudes. They did not immediately dominate the planet. Eventually, anoxygenic and oxygenic photosynthesis oxidized much of the Earth's crust and supplied sulphate to the ocean. Anoxygenic photosynthesis remained important until there was enough O 2 in downwelling seawater to quantitatively oxidize massive sulphides at mid-ocean ridge axes.

Axial summit trough of the East Pacific Rice 9°-10°N: Geological characteristics and evolution of the axial zone on fast spreading mid-ocean ridge

1998 ◽  
Vol 103 (B5) ◽  
pp. 9827-9855 ◽  
Author(s):  
Daniel J. Fornari ◽  
Rachel M. Haymon ◽  
Michael R. Perfit ◽  
Tracy K. P. Gregg ◽  
Margo H. Edwards
Keyword(s):  
Axial Zone ◽  
Geological Characteristics ◽  
East Pacific ◽  
Mid Ocean Ridge ◽  
Fast Spreading ◽  
Ocean Ridge

scholarly journals Dense melt residues drive mid-ocean-ridge “hotspots”

2022 ◽  
Author(s):  
Jordan J.J. Phethean ◽  
Martha Papadopoulou ◽  
Alexander L. Peace
Keyword(s):  
Latent Heat ◽  
Elevated Temperatures ◽  
Thermal Expansivity ◽  
Alternative Mechanism ◽  
Mantle Plumes ◽  
Mantle Dynamics ◽  
Mid Ocean Ridge ◽  
Fast Spreading ◽  
Spreading Centers ◽  
Ocean Ridge

ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.

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