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

Book chapter


Phethean, Jordan, Papadopoulou, Martha and Peace, Alexander L. 2022. Dense melt residues drive mid-ocean-ridge “hotspots”. in: In the Footsteps of Warren B. Hamilton: New Ideas in Earth Science Geological Society of America.
AuthorsPhethean, Jordan, Papadopoulou, Martha and Peace, Alexander L.
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 distribu- tion 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 ther- mal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle resi- dues 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 mecha- nism 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 pro- posed 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.

KeywordsMantle dynamics; Hotspot; Atlantic Ocean; Dense residue
Year2022
Book titleIn the Footsteps of Warren B. Hamilton: New Ideas in Earth Science
PublisherGeological Society of America
ISBN9780813795539
Digital Object Identifier (DOI)https://doi.org/10.1130/2021.2553(30)
Web address (URL)http://hdl.handle.net/10545/626298
http://creativecommons.org/publicdomain/zero/1.0/
hdl:10545/626298
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Publication dates27 Jan 2022
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Deposited17 Feb 2022, 10:09
Accepted10 Jun 2021
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ContributorsUniversity of Derby, University of Leicester and McMaster University, Hamilton, Ontario L8S 4K1, Canada
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