Helium isotope evidence for off-axis degassing of the Icelandic hotspot

We report a helium and carbon survey of 16 hydrothermal localities from Vestfirdir-the northwest peninsula of Iceland. The region is situated ~250 km away from the Icelandic rift-axis, and is characterized by low-temperature (<100°C) hydrothermal activity. A mantle-derived input dominates the helium systematics: nine localies have ³He/⁴He ratios (R/R_A) values between those typical of MORB (8±1) and 30-the highest value matches the most extreme magmatic ratios reported worldwide to date. Four other localities have R/R_A~MORB ratios with the remaining three showing heavy dilution with crustal radiogenic helium (R/R_A<MORB). Carbon isotope analysis of seven of the localities reveals a wide range in δ¹³C(CO₂)(-6 to -16‰ PDB), and CO₂/³He ratios vary over 4 orders of magnitude (4×10⁶ to 2.9×10¹⁰). There appears no systematic relationship between the volatile systematics and geographic distribution or age of reservoir rocks. Stable isotope (δ¹⁸O and δD) variations are consistent with a localized meteoric origin for the hydrothermal waters-the carrier phase of the volatiles. We conclude that release of magmatic volatiles in Vestfirdir occurs predominately via incipient mantle melting with a negligible input of re-distributed volatiles from the rift zones. It is also highly unlikely that leaching of old reservoir rocks contribute to the mantle volatile flux: however, release of radiogenic helium from the uppermost crust does occur and this is accompanied by addition of isotopically light carbon most likely of organic orgin. The light carbon input is most discernible when the mantle-derived carbon signal is reduced-most probably by reaction to form calcite. Although Vestfirdir makes a discernible (off-axis) input to the degassing history of the Icelantic hotspot, its contribution is extremely minor (~0.04%) when compared to the degassing flux associated with rifting and volcanism along the Icelandic spreading zones.