Systematics of halogen elements and their radioisotopes in thermal springs of the Cascade Range, Central Oregon, Western USA

• Published in: Earth and planetary science letters 2005-07, Vol.235 (3), p.700-714
• Main Authors: Hurwitz, Shaul, Mariner, Robert H., Fehn, Udo, Snyder, Glen T.
• Format: Article
• Language: English
• Subjects: Cascade Range; halogens; magma degassing; subduction cycling; thermal springs
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2005.04.029

This study quantifies the cycling of halogen elements through the Cascadia subduction zone based on the chemistry of thermal springs in the Central Oregon Cascade Range and of a mineral spring in the forearc (Willamette Valley). Considerations based on mass balances, element ratios, and 36Cl/Cl and 129I/I ratios suggest that halogens discharged through the thermal springs in the Cascade Range are probably derived from magma degassing. Our results indicate that < 35% of the subducted Cl and < 20% of the subducted Br and I could be transported through arc volcanism and the thermal springs, a considerably lower percentage than estimated for other volcanic arcs along the Pacific Rim. A likely explanation for this difference is that a large fraction of the halogens is released from the slab at shallow depths into the serpentinized sub-forearc mantle because of the relatively high temperatures in the subducting Juan de Fuca plate. The small fraction of halogens subducted to depth probably also indicates a low rate of water transport, which is consistent with the observation that the Cascade Range sub-arc mantle is relatively dry and has a low degree of volcanic vigor, compared with other arcs.