Revista del Centro de Investigaciones Geoscientificas (CIGEO), Nicaragua

During drilling in the Caribbean Sea on Leg 165 of the Ocean Drilling Program (ODP) a well-preserved record of explosive volcanic eruptions was found as ashfall layers within the deep-sea Cenozoic sediments. The ash layers define two major episodes of volcanism, one in the late Eocene, the other in mid-Miocene. Much of Central America is blanketed in thick ignimbrite sheets, extending from southern Mexico into Costa Rica. Evidence from atmospheric flow regimes suggests the Central American ignimbrite province as the source for these deep-sea ash layers. A comparative geochemical analysis (laser ICPMS, XRF and electron microprobe) of glass particles from the Leg 165 deep-sea ash layers and of glasses from unaltered basal vitrophyres and obsidian clasts in the Central American ignimbrites has been carried out. Glass shards in the deep-sea ash layers of the Caribbean Sea show a comparable compositional range with all but five of the analyzed ash samples falling within a range between dacite and rhyolite. Unaltered glasses in the Central American ignimbrites also show a limited range in composition with all but one falling in the same range--from dacite to rhyolite. However samples from both the ignimbrites and the ashfall layers are predominately rhyolitic. To test the hypothesis that the deep-sea ash layers are derived as distal co-ignimbrite ash fallout from the ignimbrite volcanism of Central America we have made a quantitative comparison using a "similarity coefficient" algorithm. This algorithm gives a correlation value between 0.00 and 1.00 that is derived from the average ratios and standard deviations of two compared samples. The closer the value is to 1.00, the more likely the samples are related. The results for major element data indicate good correlations (greater than 88% confidence level) between glass compositions of Leg 165 ash layers, between several ignimbrite land samples, and between numerous ocean and land samples.