Interaction Between Magma & Altered Crust on a Mature Oceanic Island: An Isotope & Trace Element Study of the Diego Hernandez Fmtn, Las Canadas Caldera, Tenerife, Canary I

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EAR-0001013Wolff/LarsonMuch of our understanding of how Earth's mantle works comes from measurements of trace element abundances and isotope ratios in ocean island basalts. Both act as conservative "flight recorders" of the mantle sources and transport histories of magma up to the point of eruption and freezing upon Earth's surface. Application of basalt geochemistry to the mantle depends on the assumption that basaltic magmas are not significantly affected by their passage through the crust en route to the surface. While most scientists would admit the possibility of some degree of host-rock assimilation by rising magma, a powerful argument in favor of ocean island basalts as faithful recorders of source chemistry is that crustal host-rocks - dominantly earlier erupted basalts - exert no chemical leverage on the magma that they may be incorporated into. We shall test this assertion, and place limits on the maximum extent of assimilation that may reasonably be expected in an ocean island setting, by studying an extreme case. In contrast to many islands, Tenerife has an extensive development of fusible felsic rocks both on the surface and within the island structure. Also, in the formation we propose to study, there is abundant evidence for intimate mingling of mantle-derived basaltic magmas with felsic magma, some of which is derived from melting of subsurface rock. The preservation of mingling textures in rapidly-quenched ignimbrites will allow us to study basaltic-felsic interactions in detail, while the compositional leverage provided by the felsic rocks will allow us to constrain the extent to which the mantle isotopic and, especially, trace-element characteristics of ocean-island basalts are compromised by crustal interactions.