Open AccessLight Stable Isotopic Compositions of Enriched Mantle Sources: Resolving the Dehydration Paradox
Author(s) -
Dixon J. E.,
Bindeman I. N.,
Kingsley R. H.,
Simons K. K.,
Le Roux P. J.,
Hajewski T. R.,
Swart P.,
Langmuir C. H.,
Ryan J. G.,
Walowski K. J.,
Wada I.,
Wallace P. J.
Publication year - 2017
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2016gc006743
Subject(s) - geology , mantle (geology) , basalt , geochemistry , metasomatism , transition zone , subduction , eclogite , mantle wedge , partial melting , paleontology , tectonics
Abstract Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid‐oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA‐type basalts (H 2 O/Ce = 215 ± 30, δD SMOW = −45 ± 5 ‰) are similar to those in the northern Atlantic (H 2 O/Ce = 220 ± 30; δD SMOW = −30 to −40 ‰). Basalts with EM‐type signatures have regionally variable volatile compositions. Northern Atlantic EM‐type basalts are wetter (H 2 O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δD SMOW = −57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM‐type basalts are damp (H 2 O/Ce = 120 ± 10) with intermediate δD SMOW (−68 ± 2 ‰), similar to δD SMOW for Pacific MORB. Northern Pacific EM‐type basalts are dry (H 2 O/Ce = 110 ± 20) and isotopically light (δD SMOW = −94 ± 3 ‰). 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‐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 <1% carbonated eclogite ± sediment‐derived C‐O‐H‐Cl fluids to depleted mantle at 180–280 km (EM) or within the transition zone (PREMA).