Felix S. Genske

Oxygen isotopes in the Azores islands: Crustal assimilation recorded in olivine

Oxygen isotope ratios of olivine have become a widely used tool for the study of magmatic systems, especially in the interpretation of source heterogeneities in mantle plume–derived ocean island basalts. The underlying assumption is that fresh minerals provide a better guide to magma δ 18 O than bulk rock analyses and that olivine is also likely to be a major phenocryst phase in primitive magmas. However, distinctions between source compositions and the effects of subsequent magma evolution have not always been thoroughly scrutinized. For the Azores samples investigated here, we can demonstrate that the δ 18 O variation (+4.84‰ to +5.25‰ Vienna standard mean ocean water) observed in the olivine phenocryst population is closely linked to evolution in the host magmas during ascent to the surface. We observe a linear, positive correlation between forsterite (Fo) content and δ 18 O in all of the individual island lava suites. This forces us to conclude that the low oxygen isotope ratios result from combined assimilation and fractional crystallization processes, the assimilant being hydrothermally (temperature > 250 °C) altered, lower oceanic crust. Linear regression of the measured δ 18 O olivine values to Fo 89 suggests a homogeneous mantle source with δ 18 O =

Where to Go? A Selection and Short Description of Geological Highlights in the Azores

Volcanism in the Azores is heterogeneous in terms of composition, frequency, style and duration. Additionally, they differ significantly in their geomorphological characteristics, accentuated by climate as well as the time that volcanism lasted for and the time that has passed by since the last eruption. This is responsible for costal cliffs several hundred meters high or flat lava deltas, large volcanic edifices and those that underwent significant vertical subsidence, exposed intrusive rocks and incomplete deposit sequences, lush rain forests or arid areas. This chapter is a selection of geological outcrops or landmarks from each island that we have selected to be worthwhile visiting. This list will significantly differ from the must-see/must-do lists of many guidebooks. Albeit incomplete in terms of what the islands have to offer geologically, the presented sites allow for grasping the plethora of geologic features in the Azores. This chapter is structured in alphabetic order of the nine islands.

Geophysical characterisation of a blind I-type pluton emplaced within the Bundarra Suite S-type granites of the New England Batholith

Geophysical data are presented that characterise a blind pluton, the Mountain Home Pluton (MHP), which intrudes the southern portion of the Bundarra Suite (BS), 30 km northeast of Bendemeer, New South Wales. A positive magnetic anomaly within the non-magnetic granites of the BS (Banalasta and Pringles Monzogranites) was previously identified as a sub-surface intrusion. Interpretation of new gravity data and analysis of aeromagnetic data are used to infer the depth, size, density, magnetic susceptibility and likely petrology of the pluton. The best-fit model indicates that the MHP is very similar to the Looanga Monzogranite, a felsic member of the Moonbi Suite of the New England Batholith (NEB) that intrudes the BS 5–7 km southeast of the MHP. The top of the MHP is inferred to lie about 1 km beneath the surface and the pluton extends to a depth of at least 6 km. Our model furthermore suggests that the southwestern margin of the MHP is subvertical, whereas a shallower dip (<45°) towards the north is proposed for the northeastern surface of the pluton. A north-trending dyke swarm, identified on the basis of linear positive magnetic anomalies, may be related to the MHP. This swarm of more than 20 relatively magnetic dykes extends out to about 10 km north from the pluton. Magnetic modelling of the dykes indicates that susceptibility values of the dykes are probably very similar to the range of the MHP, and also suggests the width of individual dykes (also not known to be exposed at the surface) to be at most a few tens of metres. A petrographic examination of the intruded BS granites at the surface suggests that metamorphic zoning as seen in mineralogical characteristics may be related to the underlying pluton.