Hydrogen and Oxygen Stable Isotope Ratios in MORBs From Arctic Mid-Atlantic Ridge Segments Near the Jan Mayen Hotspot

Hydrogen (D/H) and oxygen (18O/16O) stable isotope ratios in 22 fresh mid-ocean ridge basalt (MORB) glasses dredged from 3 contiguous Arctic MAR ridge segments were measured. The Jan Mayen platform is bounded by the northern terminus of the Kolbeinsey Ridge segment and the western terminus of the Mohns Ridge segment, and the hotspot influences both of these segments geochemically. The most northern segment is the Knipovich Ridge, which parallels the nearby Svalbard continental shelf break. Hydrogen isotope ratios (as delta Deuterium per mil) range from a typical MORB value of -78.1 just north of the Spar FZ on the Kolbeinsey Ridge to a deuterium-enriched high of -28.6 on the Knipovich Ridge. The northernmost Kolbeinsey Ridge sample, which is closest to the Jan Mayen hotspot is the most deuterium-rich sample measured from this ridge segment at -45.9. Previous studies of basalts from the Mohns Ridge on Pb-Sr-Nd-Hf isotope ratios confirm a strong binary mixing relationship along this ridge segment grading from the Jan Mayen platform (plume-like) eastward to the Greenland FZ where the Mohns Ridge meets the Knipovich Ridge where more depleted basalts are erupted. This mixing scenario is also present in the hydrogen isotope variation, which is evident in the strong positive correlation of D/H ratios with Pb isotope ratios in Mohns Ridge glasses. The Mohns Ridge sample closest to the Jan Mayen hotspot exhibits the highest D/H ratio (-32.7). Oxygen isotope ratios (as delta 18O per mil) exhibit a very narrow range of 5.39 to 5.65 across the three ridge segments. Values for all these samples fall within the range found by Eiler et al. 2000 in a study of N-MORB glasses from the Mid Atlantic Ridge, East Pacific Rise and the Indian Ocean ridges of 5.37 to 5.81. No clear trends of oxygen isotope ratios are evident with proximity to the Jan Mayen hotspot, or radiogenic isotope ratios, trace element ratios, or major element variations in this region. Jan Mayen is a HIMU-like low 3He/4He hotspot (mantle plume). This study confirms the suggestion by Poreda et al. 1986 that its source is also deuterium-rich. The D/H ratio correlation with HIMU-like plume indicators such as Pb and Sr isotope ratios, minor element ratio of K/Ti, and trace element ratios such as La/Sm along with their low 3He/4He characteristics, indicate the origin of the enriched end-member (i.e. Jan Mayen mantle plume) for the mixing evident along the Mohns Ridge is recycled crustal material. However, any recycling model must consider the fact that the oxygen isotopes in the E-MORB and N-MORB show no such variations. Solutions to this paradox where, at some point in the past, the trace element (H) was affected by surface processes and apparently the major element (O) was not might include fluid/melt metasomatism as an enrichment process, or some combination of upper and lower oceanic crust as the material recycled (e.g. Eiler et al. 1997).