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 similar to 250 km away from the Icelandic rift-axis, and is characterized by low-temperature (< 100 degrees C) hydrothermal activity. A mantle-derived input dominates the helium systematics: nine localities have He-3/He-4 ratios (R/R-A) values between those typical of MORE (8 +/- 1) and 30-the highest value matches the most extreme magmatic ratios reported worldwide to date. Four other localities have R/R-A similar to 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 delta(13)C(CO2) (-6 to - 16 parts per thousand PDB), and CO2/He-3 ratios vary over 4 orders of magnitude (4 x 10(6) to 2.9 x 10(10)). There appears no systematic relationship between the volatile systematics and geographic distribution or age of reservoir rocks. Stable isotope (delta(13)O and delta D) 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 predominantly 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 origin. 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 Icelandic hotspot, its contribution is extremely minor(similar to 0.04%) when compared to the degassing flux associated with rifting and volcanism along the Icelandic spreading zones. (C) 1998 Elsevier Science B.V. All rights reserved.