Publication Date:
2017-09-22
Description:
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 multi-stage 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 to 280 km (EM) or within the transition zone (PREMA).
Electronic ISSN:
1525-2027
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
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