Light Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox

Jacqueline E. Dixon, I. N. Bindeman, R. H. Kingsley, Jeffrey G. Ryan

Research output: Contribution to journalArticlepeer-review

Abstract

<p> Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid&hyphen;oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA&hyphen;type basalts (H2O/Ce&thinsp;=&thinsp;215&thinsp;&plusmn;&thinsp;30, &delta;DSMOW&thinsp;=&thinsp;&minus;45&thinsp;&plusmn;&thinsp;5 &permil;) are similar to those in the northern Atlantic (H2O/Ce&thinsp;=&thinsp;220&thinsp;&plusmn;&thinsp;30; &delta;DSMOW&thinsp;=&thinsp;&minus;30 to &minus;40 &permil;). Basalts with EM&hyphen;type signatures have regionally variable volatile compositions. Northern Atlantic EM&hyphen;type basalts are wetter (H2O/Ce&thinsp;=&thinsp;330&thinsp;&plusmn;&thinsp;30) and have isotopically heavier hydrogen (&delta;DSMOW&thinsp;=&thinsp;&minus;57&thinsp;&plusmn;&thinsp;5 &permil;) than northern Atlantic MORB. Southern Atlantic EM&hyphen;type basalts are damp (H2O/Ce&thinsp;=&thinsp;120&thinsp;&plusmn;&thinsp;10) with intermediate &delta;DSMOW (&minus;68&thinsp;&plusmn;&thinsp;2 &permil;), similar to &delta;DSMOW for Pacific MORB. Northern Pacific EM&hyphen;type basalts are dry (H2O/Ce&thinsp;=&thinsp;110&thinsp;&plusmn;&thinsp;20) and isotopically light (&delta;DSMOW&thinsp;=&thinsp;&minus;94&thinsp;&plusmn;&thinsp;3 &permil;). A multistage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration, and re&hyphen;equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of</p>
Original languageAmerican English
JournalGeochemistry, Geophysics, Geosystems
Volume18
DOIs
StatePublished - Nov 1 2017

Keywords

  • volatiles
  • stable isotopes
  • mantle geochemistry
  • subduction
  • metasomatism
  • oceanic basalt

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