GeoBremen2017: Biases in the geochemical record of oceanic magmatism

At the end of summer 2017, I gave a talk at the annual meeting of the Deutsche Mineralogische Gesellschaft (DMG) at GeoBremen2017. My talk focussed on the results of my 3-kbar experiments on primtive Icelandic basalts, and how they show that depleted mantle melts are much less likely to survive being processed during their ascent through the crust than enriched melts. In other words, enriched melts are more likely to erupt at the surface and depleted melts are more likely to freeze at depth, fundamentally biasing the record of oceanic magmatism we see at the surface. You can download my slides here.

Experimental liquid lines of descent (LLD) for melts from the depleted Háleyjabunga (Hál) and enriched Stapafell (Sta) eruptions. Grey dots show Icelandic compositions from the Western Volcanic Zone and Reykjanes Peninsula (Shorttle & Maclennan, 2011). The arrow shows where compositions were resynthesised to mimic fractional crystallisation.

IAVCEI: The effect of mantle-derived variability on the mineralogy of primitive basalts: Experimental constraints from Icelandic systems

In summer 2017, I presented a poster at the excellent IAVCEI Scientific Assembly in Portland. My contribution summarised the findings of my experimental work in Hannover so far. In particular, I focussed the effects of mantle-dervied heterogeneity on the phase equilibria of primitive Icelandic basalts in the 1–7 kbar range. You can download a copy of my poster here.

False-colour backscattered electron image of the run products of an experiment on the Háleyjabunga eruption, Iceland

The effect of anorthite content and water on quartz–feldspar cotectic compositions in the rhyolitic system and implications for geobarometry

Since moving to Hannover, I have become involved in a number of exciting new projects. One project, lead by François Holtz and carried out by Sören Wilke, involved carrying out a large number of experiments to determine how variations in the anorthite (i.e. calcium) and water contents of rhyolites affects the position of quartz–feldspar cotectics. This is important because the position of quartz–feldspar cotectics can be used as a geobarometer, especially is systems lacking pressure sensitive minerals such as amphibole, but only if the effects of anorthite and water contents are appropriately accounted for.

Our experiments allowed us to define thermal minima and quartz–sanidine–plagioclase triple points on quartz–feldspar cotectics at various pressures, water contents and anorthite contents. This information was then used calibrate an empircal barometer (DEtermination of Rhyolite Pressures; DERP) to esimtate the storage pressure of rhyolitic glasses in equilibrium with quartz and at least one feldspar. DERP is calibrated in the range 50–500 MPa and for any H2O content. Importantly, our findings suggest that rhyolite-MELTS may underestimate the storage pressures of rhyolitic magmas. Bringing emprical and thermodynamic geobarometers into alignment thus represents a key next step in the investigation of rhyolitic magmas.

Publication

Wilke, S., Holtz, F., Neave, D. A. & Almeev, R. R. 2017. The effect of anorthite content and water on quartz–feldspar cotectic compositions in the rhyolitic system and implications for geobarometry. Journal of Petrology 58, 789–818.