Hydrogen isotopes in hornblendes and biotites from Quaternary volcanic rocks of the Kamchatka-Kurile arc

Abstract

The hydrogen isotopic composition of hornblendes (Hb) and biotites (Bi) from Quaternary volcanic rocks along the Kamchatka-Kurile volcanic arcs was measured. Volcanoes from three different tectonic environments were sampled: The Kurile chain, East Kamchatkan volcanic belt and Sredinny Ridge. Large local and regional variations were observed. There is no correlation between δD values of minerals and their chemical composition, including Cl and F contents. All observed trends are caused by regional variations in the major element chemistry (Hb) or Cl and F contents (Bi). Minerals from the Kuriles and East Kamchatka are most enriched in D, have a low F content, and a high Cl content, the latter 500-1000 ppm. Hb and Bi from Sredinny Ridge are D-depleted, enriched in F (up to 1.7 wt.% in Bi) and depleted in Cl. Based on the data set for Shiveluch and Avacha volcanoes, δD variations for Hb are caused mainly by degassing of the erupted material; the most degassed rocks contained the most D-depleted Hb. We believe that the maximum δD values correspond to isotopic equilibrium of OH-bearing minerals with a “parent magmatic water”, dissolved in the melt prior to eruption. Magmatic water for the Kurile volcanoes with δD in the range of -40 to -25‰ could be derived mainly from subducted oceanic sediments. Magmatic water for the East Kamchatka volcanoes is slightly depleted in D (-46 to -36%) and corresponds to a mixture of “devolved” water from subducted sediments and water released from altered basalts of oceanic crust. The water source of the Sredinny Ridge magmas with δD values in the range of -70 to -80‰ may not be related to oceanic slab-derived fluids; rather, they may preserve a depleted mantle isotopic signature. The regional variation of maximum D/H ratios for Hb and Bi agrees well with trends of the Sr isotopic composition and the ¹⁰Be content in volcanic rocks throughout the modem Kurile-Kamchatka arc.