The hydrogen isotopic composition and water content of southern Pacific MORB: A reassessment of the D/H ratio of the depleted mantle reservoir

Abstract

International audienceIn this paper, we re-investigate the isotopic composition of hydrogen in MORB and the possible effects of contamination on δD and water content. A suite of 40 N-MORB from the Pacific-Antarctic ridge, far from any hotspot, was analyzed for chlorine content by electron microprobe and for water content and δD with silica tubes. Cl concentrations (from 29 to 2400 ppm) indicate widespread contamination, more intense with faster spreading rates, while water contents (from 840 to 7800 ppm) are mainly controlled by igneous processes. δD values range from ‑76 to ‑48‰, with an average value of ‑61‰. The lack of correlation between Cl content and either H2O/Ce or δD indicate that contamination has a negligible effect on δD for our samples, which is therefore characteristic of the mantle below the Pacific-Antarctic ridge. We suggest that the 20‰ lower δD value reported for the North Pacific and North Atlantic is highly unlikely from geodynamical arguments. We propose that the convecting mantle is characterized by a δD of ‑60±5‰, as supported by the most recent data from North Atlantic N-MORB

Hydrogen isotope fractionation and redox‐controlled solution mechanisms in silicate‐COH melt + fluid systems

“…The inferred −90 to −40‰ δD range inferred for the upper mantle from mantle‐derived magma (as summarized by Clog et al . [], for example) would then be explained by an increase in oxygen fugacity during partial melting of a COH‐bearing mantle.…”

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

“…Hydrogen isotopes have been previously studied in three southern Pacific regions: (1) Easter Microplate (EMP) and Easter Salas y Gomez Seamount Chain (ESC; supporting information Figure S2‐16 and 17) (Cheng et al, ; Kingsley et al, ; Kingsley & Schilling, ); (2) the Pacific‐Antarctic Ridge 41° to 65°S (PAR, supporting information Figure S2‐16) (Clog et al, ; Hamelin et al, ; Vlastélic et al, ); and (3) Macquarie Island (supporting information Figure S2–16) (Bindeman et al ; Kamenetsky et al, ). The overlapping radiogenic isotopic compositions for these three regions (Figure a) suggest influence by a common component with high 206 Pb/ 204 Pb similar to that for the Easter Salas y Gomez (SYG) plume, even for ridges far from hotspots.…”

Petrogenesis of ODP Hole 735B (Leg 176) Oceanic Plagiogranite: Partial Melting of Gabbros or Advanced Extent of Fractional Crystallization?

“…from the Southwest Indian Ridge (25 to 42°S) database of PetDB(Lehnert et al, 2000; see supporting informationTable S2) using Rhyolite-MELTS(Gualda et al, 2012) under conditions of 0.2 GPa, T = 1270-857°C, and quartz-fayalite-magnetite oxygen fugacity buffer with 0.2 wt% water (average water concentration of natural MORB;Clog et al, 2013, and references therein). yielded 87 Sr/ 86 Sr = 0.710257 ± 0.000019 (2σ, n = 10), 143 Nd/ 144 Nd = 0.512105 ± 0.000018 (2σ, n = 10), and 176 Hf/ 177 Hf = 0.282180 ± 0.000006 (2σ, n = 8).…”