14C and U-series disequilibria age constraints from recent eruptions at Sete Cidades volcano, Azores
Accurate dating of young (<10 ka) volcanic rocks poses numerous challenges, yet determining eruption ages is critical to understanding magmatic evolution and hazards associated with active volcanic centers. 14C dating is the preferred technique for dating young geologic materials, but a few studies have suggested that volcanic degassing can result in erroneously old 14C dates (e.g. Pasquier-Cardin et al., 1999). Finding an isotopic system with which to accurately constrain eruptive recurrence intervals at recently active volcanic centers is critical to assessing future volcanic hazards. In this study, we have obtained 14C data from modern terrestrial gastropods on Sa~o Miguel island, Azores to assess the current impact of volcanic degassing on 14C ages near active and extinct volcanic centers, and for one active volcanic center, we have compared 14C of paleosols with 226Ra-230Th maximum eruptive ages for pumices immediately overlying the respective paleosols. 14C ages of the modern terrestrial gastropod Cornu aspersum demonstrate the effects of dilution from a range of current outgassing levels at three active trachytic stratovolcanoes on Sa~o Miguel island: Sete Cidades, Fogo, Furnas, as well as the extinct Povoac¸a~o volcano. A modern gastropod sampled at Furnas, where there is evidence of extensive present-day degassing, has a 14C age of 394 ±97 y cal BP. At Fogo volcano, also a location with active degassing, the 14C age of a modern gastropod is 1324 ±60 y cal BP. Both of these ages clearly show effects of dilution of atmospheric 14C due to volcanic degassing. In contrast, there is currently no apparent outgassing occurring at Sete Cidades or Povoac¸a~o, and modern gastropod ages from these volcanoes are post-bomb. We also obtained 14C data from paleosols and 226Ra-230Th ages associated pumices from the youngest eruptive sequence of Sete Cidades volcano, the Pepom P1-P17 deposits, the ages of which are poorly constrained but thought to be <5,000 y (Queiroz, 1997). 226Ra-230Th disequilibrium in a P1 glass separate indicates a maximum eruptive age of 4440 ±195 y. 14C ages of paleosols below P1 and P3 are older than those derived from 226Ra-230Th dating, at 7293 ±141 y cal BP and 7698 ± 172 y cal BP, respectively. We interpret the 14C ages as being erroneously old due to abundant volcanic degassing at the time of eruption. Similarly, but to a lesser extent, the 226Ra-230Th maximum eruptive age of glass separated from the P8 eruption is 2659 ±55 y, whereas the age of the paleosol sampled directly beneathP8 is older (3054 ±194 y BP). In contrast, paleosols sampled at multiple field localities from below the most recent eruptive deposit (P17) are within error of each other (476 ±56 cal y BP and 585 ±68 cal y BP), while 226Ra-230Th maximum eruptive ages in glass separates from two different P17 sampling sites are 762 ±14 y and 730 y ±15 y. This is consistent with the observation that Sete Cidades volcano is experiencing limited hydrothermal activity at the present time and likely had similar low levels of degassing just prior to the last eruption. The average eruptive recurrence interval for the P1-P17 deposits, based on 226Ra-230Th maximum eruptive ages from P1 and P8 and 14C ages from P17, is ~220 years. This recurrence interval is similar to the ~200 y eruptive recurrence interval at Furnas volcano and much shorter than the ~1200 y eruptive interval at Fogo volcano. This has important implications with respect to volcanic hazards and should be considered in any future volcanic hazard planning.