HELIUM ISOTOPE EVIDENCE FOR OFF-AXIS DEGASSING OF THE ICELANDIC HOTSPOT

We report a helium and carbon survey of 16 hydrothermal localities fro m Vestfirdir-the northwest peninsula of Iceland. The region is situate d similar to 250 km away from the Icelandic rift-axis, and is characte rized by low-temperature (< 100 degrees C) hydrothermal activity. A ma ntle-derived input dominates the helium systematics: nine localities h ave 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 ratio s reported worldwide to date. Four other localities have R/R-A similar to MORB ratios with the remaining three showing heavy dilution with c rustal radiogenic helium (R/R-A < MORB). Carbon isotope analysis of se ven 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 re lationship between the volatile systematics and geographic distributio n or age of reservoir rocks. Stable isotope (delta(13)O and delta D) v ariations are consistent with a localized meteoric origin for the hydr othermal waters-the carrier phase of the volatiles. We conclude that r elease of magmatic volatiles in Vestfirdir occurs predominantly via in cipient mantle melting with a negligible input of re-distributed volat iles from the rift zones. It is also highly unlikely that leaching of old reservoir rocks contribute to the mantle volatile flux: however, r elease of radiogenic helium from the uppermost crust does occur and th is is accompanied by addition of isotopically light carbon most likely of organic origin. The light carbon input is most discernible when th e mantle-derived carbon signal is reduced-most probably by reaction to form calcite, Although Vestfirdir makes a discernible (off-axis) inpu t to the degassing history of the Icelandic hotspot, its contribution is extremely minor(similar to 0.04%) when compared to the degassing fl ux associated with rifting and volcanism along the Icelandic spreading zones. (C) 1998 Elsevier Science B.V. All rights reserved.