Experimental study on the production of rhyolites from basaltic sources in the bimodal Snake River Plain-Yellowstone province
|Leitung:||Dr. Almeev, Dr. Charlier, Dr. Namur, Prof. Dr. Holtz|
Tholeiitic basalts from many tectonic environments are commonly associated with silica-rich eruptive products. This association is commonly bimodal, with a noticeable dearth of intermediate compositions. Silica-rich rocks are generally enriched in iron, alkalis and incompatible elements and are referred to as A-type magmas. Two processes are usually invoked to explain their origin: (1) protracted fractional crystallization of parental basalts or (2) partial melting of a mafic source in the middle or upper crust. Silicate liquid immiscibility could also explain the bimodal character of tholeiitic provinces. However, even if immiscible textures occur in the mesostasis of tholeiitic basalts worldwide, evidence for large-scale separation of immiscible melts in volcanic setting has yet to be shown.
The Snake River Plain-Yellowstone (SRPY) province preserves a unique record of bimodal magmatism (tholeiitic basalts-rhyolites). Moreover, the ICDP HOTSPOT drilling project (2010-2012) has retrieved two deep cores, the Kimama core dominated by basaltic rocks and the Kimberley core dominated by rhyolites. These samples allow investigating the origin of A-type magmas, and their potential link with basalts in unprecedented detail.
In this project, we propose to combine complementary investigations of natural core samples and an extensive experimental work to determine whether the origin of A-type rhyolites in SRPY is best explained by fractional crystallization, silicate liquid immiscibility or partial melting in the crust. First, we will select natural samples of evolved basalts and perform a detailed mineralogical and petrographical investigation to determine their pre-eruptive conditions (thermobarometry) and adequate liquid compositions for these basalts. Second, we will carry out three different sets of experiments simulating the formation of rhyolitic liquids from basaltic source by different mechanisms: (1) equilibrium and fractional crystallization experiments to identify the liquid line of descent of these basalts; (2) immiscibility experiments to determine whether these basalts might have encountered a two-liquid immiscibility field and (3) partial melting experiments of gabbro and basalt. Experimental results will then be compared with published, unpublished (cooperation with colleagues from USA) and our own natural data, to determine which of these three processes best explains the major and trace element composition of SRPY rhyolites. This project will also add to the current understanding of phase equilibria in tholeiitic ferrobasalts, especially related to immiscibility for which phase diagrams are poorly known. It will also put new constraints on the liquids produced by partial melting of dry mafic rocks, a topic which is currently underrepresented in the experimental literature.