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 (H2O/Ce = 215 ± 30, δDSMOW = −45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = −30 to −40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = −57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (−68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = −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).
Bibliographical noteFunding Information:
J.E.D. is grateful to the University of South Florida for granting a 1 week nano-sabbatical at the University of Oregon in March 2015 and a 2 month mini-sabbatical in March–April 2016 at the Carnegie Institution of Washington Department of Terrestrial Magnetism (DTM), where she was fortunate to serve as a Tuve Fellow. Work by R. Kingsley, J. Dixon, and T. Rau Hajewski was supported by NSF/OCE-0351149. This work brings together studies of MORB suites from both C. Langmuir and J.-G. Schilling funded over the years by NSF/OCE-9302574 (1993 to J. Dixon and C. Langmuir), NSF/OCE- 9530373 (1995 to J. Dixon and J.-G. Schilling), NSF/OCE-0351149 (2004 to J. Dixon, R. Kingsley, and J.-G. Schilling), NSFOCE-0351125 (2006 to R. Kingsley and J.-G. Schilling for the D/H and O isotopes along the Arctic ridges), and NSF/EAR-844772 (2009 to I. Bindeman for D/H analysis of Atlantic and north Pacific samples). Ideas were refined during many helpful discussions with Erik Hauri, Peter van Keken, Steve Shirey, and Rick Carlson. The first author is extremely grateful for the thoughtful and thorough review by A. Pietruszka. The manuscript also benefited from reviews by R. Stern and an anonymous reviewer. The data used are listed in Figures 2–8 are provided in Tables (1–4) and archived in IEDA:EarthChem [http://www. earthchem.org/library].
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- mantle geochemistry
- oceanic basalt
- stable isotopes