Helium Isotopic Signatures of Icelandic Alkaline Lavas

Iceland is widely accepted to be underlain by a plume of actively upwelling mantle. The nature of the individual mantle components within the plume however remains enigmatic. The most compelling evidence for involvement of relatively undegassed, `undepleted' mantle material are the high 3He/4He lavas (>20 RRa) that mainly extrude in the neovolcanic rift zone above the plume stem. It has been proposed that the high 3He4He mantle component is equivalent to the FOZO component on the basis of the Sr, Nd and Pb-isotopic signature of the most extreme high 3He4He sample (Selardalur, 37 RRa). The mildly alkaline centre Snaefell, which is situated close to the plume axis in SE Iceland, has Sr, Nd and Pb-isotopic ratios similar to the high 3He4He sample from Selardalur, which may indicate that Snaefell is mainly sourced by FOZO. Incompatible trace element-rich, high 87Sr/86Sr, 206Pb204Pb and low 143Nd144Nd alkaline basalts from the other off-rift zones have also been attributed to an `undepleted' plume component, which could be FOZO. However, 3He4He measurements of these alkaline lavas are sparse and are typically comparable to MORB values (Snaefellsnes & Vestmannaeyjar, 6.9-15.1 RRa, MORB, 9.13 {+/-}3.57 RRa). Furthermore, these centres have Sr, Nd and Pb- isotopic ratios, which are different from FOZO and have incompatible trace element patterns that are consistent with a high share of recycled oceanic basalt in their source. This study compares the Sr, Nd, Pb, He-isotope ratios and trace element characteristics of MgO-rich alkaline lavas from Eyjafjöll, Vestmannaeyjar, Snaefellsnes, Snaefell and Selardalur (NW Iceland) in order to assess whether the alkaline centres originate from different sources or variable mixtures thereof. Specifically, Snaefell and Selardalur are inspected in order to test whether the 3He/4He ratios and trace element characteristics are consistent with an origin from FOZO.