Progress and challenges using 40AR/39AR geochronology in Costa Rica and Nicaragua

The extrusive flux of the Central American Arc to be better estimated, from 2002-2008, was obtained sixty one high precision 40Ar/39Ar ages on geographically well-situated lavas and tephra from Costa Rica and Nicaragua. Here, a number of observations is described during this study using four examples that well document the precision, accuracy and general reliability of the 40Ar/39Ar ages. First, low K2O values, particularly in samples from Nicaragua, has been a major limitation in or attempts to obtain reliable dates on samples under 1 My. Second, extensive weathering of samples due to the tropical climate of America Central has resulted in various levels of argon loss even when the hand sample has appeared unaltered. Third, the field and geochronological data have lead to conclude that eruptive rates have not been constant over the past 15 to 20 My, but rather appears punctuated by gaps of up to several million years. The temporal gaps are attempted to address in several ways. First, geochemical analyses were used to identify samples that may have erupted during time periods without known volcanism. For example, U/Th values in the active arc of America Central are significantly higher than those obtained from the Miocene Coyol Group except for four samples with intermediate values that were dated to determine if their ages were intermediate as well. However, all of these samples were found to be from a period with known volcanism. Second, the oldest sections of the active arc and the youngest sections of the Coyol Group are sought to locate in order to better constrain the timing and duration of the apparent gap in volcanic productivity. This approach also has failed to locate samples from periods without known volcanism. When these methods proved largely unsuccessful, the focus shifted to dating regions of minor volcanism between the active and Coyol volcanic fronts as well as between Cosiguina and San Cristobal, the longest stretch of the Frente Volcanico of America Central without active volcanism. This effort has yielded ages on samples ranging from 1.1 to 3.6 Ma and, thus, substantially reduced the apparent volcanic gap in Nicaragua. (author)