A geochemical evaluation of cryptotephra from Montserrat, Lesser Antilles Arc.
Barickman, Mattison H.
- In volcanic eruptions, magmatic or phreatomagmatic fragmentation transforms the erupting magma into a gas-particle/droplet mixture that is then injected into the atmosphere as an ash plume. The material from this ash plume is then deposited on land or in the ocean as a tephra layer, an unconsolidated, fragmented material, or cryptotephra, which is a tephra layer that is invisible to the naked eye.... read moreTephra has long been used in geochemical studies of volcanic systems to assess how the magma chambers evolve in terms of their composition and volatile content. It is particularly useful as a chronostratigraphic marker that can provide precise age dates for both ice and oceanic cores. This project explores two newly documented cryptotephra deposits from core U1396-1H-4W collected on IODP expedition 340 to the Caribbean Sea, near Montserrat. The overall particle content and composition is documented through point counting. Textural analysis via scanning electron microscopy (SEM) indicates that white glass, mineral fragments, and vesicular pumice fragments are the main constituents of these cryptotephra deposits. A geochemical evaluation of glass particles in both deposits indicates that samples range in composition from rhyolitic to dacitic. The variation in composition of glass particles in core segment 11-15 cm can be attributed to one of two processes: mixing of a more silicic and a more mafic magma in the chamber which feeds the various volcanic complexes on Montserrat, or a relatively fast ascent rate of magma through the crustal column which leads to relatively less degassing until the magma reaches the subaerial environment. The low variance in composition of glass particles in core segment 41-45 cm suggests that the magma either ascended through the conduit at a slower rate and degassed as it rose, or that the magma chamber fractionated significantly before the eruption.read less