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

Wolff/Larson我们对地球地幔如何运作的理解大部分来自于对海洋岛屿玄武岩中微量元素丰度和同位素比率的测量。 两者都充当保守的“飞行记录器”，记录地幔来源和岩浆的运输历史，直到喷发和冻结在地球表面。 玄武岩地球化学在地幔中的应用依赖于这样一个假设，即玄武岩岩浆在穿过地壳到达地表的过程中没有受到明显的影响。 虽然大多数科学家都承认，上升的岩浆可能会在一定程度上同化主岩，但支持洋岛玄武岩作为源岩化学成分忠实记录者的一个强有力的论据是，地壳主岩--主要是早期喷发的玄武岩--对它们可能融入的岩浆没有化学影响。 我们将通过研究一个极端的案例来检验这一论断，并对在海岛环境中可能合理预期的最大同化程度加以限制。 与许多岛屿相比，特内里费岛在表面和岛屿结构内部都有大量可熔长英质岩石。 此外，在我们建议研究的地层中，有大量的证据表明幔源玄武岩浆与长英质岩浆密切混合，其中一些来自地下岩石的熔融。 快速淬火熔结凝灰岩中混合纹理的保存将使我们能够详细研究玄武岩-长英质相互作用，而长英质岩石提供的成分杠杆作用将使我们能够限制地幔同位素的程度，特别是洋岛玄武岩的微量元素特征受到地壳相互作用的影响。

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.