MANTLE AND CRUSTAL CONTRIBUTION IN THE GENESIS OF RECENT BASALTS FROM OFF-RIFT ZONES IN ICELAND - CONSTRAINTS FROM TH-ISOTOPES, SR-ISOTOPES AND O-ISOTOPES

Along the two volcanic off-rift zones in Iceland, the Snaefellsnes volcanic zone (SNVZ) and the South Iceland volcanic zone (SIVZ), geochemical parameters vary regularly along the strike towards the centre of the island. Recent basalts from the SNVZ change from alkali basalts to tholeiites where the volcanic zone reaches the active rift axis, and their Sr-87/Sr-86 and Th/U ratios decrease in the same direction. These variations are interpreted as the result of mixing between mantle melts from two distinct reservoirs below Snaefellsnes. The mantle melt would be more depleted in incompatible elements, but with a higher He-3/He-4 ratio (R/Ra almost-equal-to 20) beneath the centre of Iceland than at the tip of the Snaefellsnes volcanic zone (R/Ra almost-equal-to 7.5). From southwest to northeast along the SIVZ, the basalts change from alkali basalts to FeTi basalts and quartz-normative tholeiites. The Th/U ratio of the Recent basalts increases and both (Th-230/Th-232) and delta-O-18 values decrease in the same direction. This reflects an important crustal contamination of the FeTi-rich basalts and the quartz tholeiites. The two types of basalts could be produced through assimilation and fractional crystallization in which primary alkali basaltic and olivine tholeiitic melts 'erode' and assimilate the base of the crust. The increasingly tholeiitic character of the basalts towards the centre of Iceland, which reflects a higher degree of partial melting, is qualitatively consistent with increasing geothermal gradient and negative gravity anomaly. The highest Sr isotope ratio in Recent basalts from Iceland is observed in Oraefajokull volcano, which has a He-3/He-4 ratio (R/Ra almost-equal-to 7.8) close to the MORB value, and this might represent a mantle source similar to that of Mauna Loa in Hawaii.