Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

High 3He/4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes

“…The thermal boundary layer between upper-and lower-mantle regions is one potential reservoir for such material 26,27 , although it cannot be ruled out that the mixing occurs as the plume head disperses in the upper mantle. The dominance of the 3 He-recharged, depleted component in mantle plumes is consistent with a low mass flux from the lower mantle and explains why basalts from Iceland and Hawaii appear to have lower He concentrations than predicted for undegassed mantle 9,10 . A…”

“…Plume-derived OIB from, for example, Loihi seamount, Hawaii, have lower He concentrations than basalts from the degassed upper mantle 2 and have posed a persistent problem for a unified geochemical model of Earth structure 6 . The apparent paradox can be explained, at least in part, by more extensive degassing of ocean island basalts 9 . However, basalts from Iceland and the Hawaiian islands have linear, rather than strongly hyperbolic, He-Pb isotope mixing arrays that are consistent with relatively small He concentration contrasts between the degassed mantle and less degassed (high 3 16 .…”

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

“…Along-axis variations in trace element ratios and long-lived radiogenic isotopic ratios for the Reykjanes Ridge, Iceland, and the Kolbeinsey Ridge. Literature data shown as small black data points (O'Nions and Pankhurst, 1973;Schilling, 1975;Sun and Jahn, 1975;Cohen et al, 1980;Dupre and Allegre, 1980;Condomines et al, 1981;Schilling et al, 1983Schilling et al, , 1999Neumann and Schilling, 1984;Poreda et al, 1986;Furman et al, 1991;Mertz et al, 1991;Hemond et al, 1993;Devey et al, 1994;Hards et al, 1995;Hardarson and Fitton, 1997;Mertz and Haase, 1997;Salters and White, 1998;Stecher et al, 1999;Hanan et al, 2000;Hilton et al, 2000;Peate et al, 2001;Murton et al, 2002;Haase et al, 2003;Stracke et al, 2003aStracke et al, ,b, 2006Thirlwall et al, 2004;Blichert-Toft et al, 2005 (Figs. 3 and 4).…”

Noble gas constraints on the origin and evolution of Earth’s volatiles

“…To counter this dilemma it has been suggested that the helium loss is pre-eruptive (Hopp and Trieloff, 2008) or that hotspot lavas contain less helium because they contain more water and CO 2 , favouring more extreme degassing than in a ridge setting (Hilton et al, 2000b;Yamamoto and Burnard, 2005;Gonnermann and Mukhopadhyay, 2007).…”